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Rapid U-series dating of young fossil corals by laser ablation MC-ICPMS
Abstract
We demonstrate the utility of uranium-series age dating using laser ablation multi-collector inductively coupled plasma mass spectrometry (LA MC-ICPMS) to ‘range find’ a large suite of fossil corals in order to select those for subsequent detailed and high resolution paleoclimatological analyses. The high abundance of unaltered and long-lived (100–200 years) microatolls on Kiritimati Island, Kiribati, offers the exciting prospect of constructing a millennial, if not continuous, coral paleoclimate record for the central Pacific spanning the period from the mid-Holocene to present. However, with a multitude of collected corals and Kiritimati sample sites, we required an efficient, cost-effective and reliable analytical method to optimize selection of quality coral belonging to a prescribed age. Two modern and 41 fossil microatoll corals from Kiritimati were analysed by the range-finding LA MC-ICPMS technique with a subset also analysed by high-precision solution MC-ICPMS and AMS radiocarbon to asse
... Finally, considerable progress has been made on the development and application of rapid-screening techniques to date large quantities of fossil coral samples. Several approaches have considerably increased sample throughput, including U/Th dating by laser ablation multi-collector inductively coupled plasma mass spectrometry (LA MC-ICPMS) (Eggins et al., 2005;McGregor, Hellstrom, et al., 2011;Potter et al., 2005) or inductively coupled plasma-quadruple mass spectrometry (ICP-QMS) (Douville et al., 2010), and direct high-precision 14 C dating of carbonate powders via accelerator mass spectrometry (AMS) (Bush et al., 2013;Grothe et al., 2016). Although the LA MC-ICPMS and 14 C-AMS techniques have proven successful at accurately dating large quantities of fossil corals (Grothe et al., 2016;McGregor, Hellstrom, et al., 2011), these methods suffer from comparatively large uncertainties and greater susceptibility to diagenetic alteration, relative to high-precision U/Th dating by MC-ICP-MS. ...
... Several approaches have considerably increased sample throughput, including U/Th dating by laser ablation multi-collector inductively coupled plasma mass spectrometry (LA MC-ICPMS) (Eggins et al., 2005;McGregor, Hellstrom, et al., 2011;Potter et al., 2005) or inductively coupled plasma-quadruple mass spectrometry (ICP-QMS) (Douville et al., 2010), and direct high-precision 14 C dating of carbonate powders via accelerator mass spectrometry (AMS) (Bush et al., 2013;Grothe et al., 2016). Although the LA MC-ICPMS and 14 C-AMS techniques have proven successful at accurately dating large quantities of fossil corals (Grothe et al., 2016;McGregor, Hellstrom, et al., 2011), these methods suffer from comparatively large uncertainties and greater susceptibility to diagenetic alteration, relative to high-precision U/Th dating by MC-ICP-MS. For example, U/Th dating by LA MC-ICPMS may produce prohibitive dating uncertainties of up to 15% (2σ) for samples older than 3000 years old, and errors as large as 33% (2σ) or more for younger samples (Eggins et al., 2005;McGregor, Hellstrom, et al., 2011). ...
... Although the LA MC-ICPMS and 14 C-AMS techniques have proven successful at accurately dating large quantities of fossil corals (Grothe et al., 2016;McGregor, Hellstrom, et al., 2011), these methods suffer from comparatively large uncertainties and greater susceptibility to diagenetic alteration, relative to high-precision U/Th dating by MC-ICP-MS. For example, U/Th dating by LA MC-ICPMS may produce prohibitive dating uncertainties of up to 15% (2σ) for samples older than 3000 years old, and errors as large as 33% (2σ) or more for younger samples (Eggins et al., 2005;McGregor, Hellstrom, et al., 2011). Furthermore, McGregor, Hellstrom, et al. (2011) found that subtle diagenetic alteration may not be detected using this method, and therefore recommend that it is paired with other screening methods (i.e., thin section, SEM, and XRD analysis). ...
Hundreds of coral paleoclimate records have been developed over the past several decades, significantly extending the instrumental record and improving our understanding of tropical climate variability and change in otherwise data‐poor regions. Coral “proxy” records measure the change in skeletal geochemistry or growth as a function of ocean conditions at the time of calcification. Over the past decade (since 2010), new syntheses have identified coherent patterns of warming and variability that are unique within the paleo record (albeit not yet unprecedented). In turn, ocean warming and acidification have had a detrimental impact on coral growth, with reduced extension and increased stress banding. Methodological advances have constrained uncertainties and improved our understanding of the processes by which climate information is archived in coral skeletons. Models that describe these processes have been developed to facilitate proxy‐model comparisons, identify sources of uncertainties, and provide a benchmark upon which forced changes may be detected within a highly variable climate system. Finally, several innovative new proxies have expanded the climate and environmental information that may be obtained from corals, including: seawater pH, aragonite saturation, anthropogenic nitrogen, runoff, and trade winds. Further extending established and novel proxies should remain a priority, along with seawater monitoring and density measurements with which to screen and calibrate these records. As this critical climate archive is increasingly threatened by warming and ocean acidification, the community must work closely together to collect this invaluable climate data in an ecologically and culturally sensitive manner, before it is too late.
This article is categorized under: Paleoclimates and Current Trends > Paleoclimate
... Recently, Douville et al. [2010] simplified tedious chemical separations and simultaneously measured U and Th atoms on an inductively coupled plasma-quadruple mass spectrometry (ICP-QMS), achieving 50 dates per day with precision levels of less than 2%. Additional rapid U/Th dating methods have been developed using laser ablation (LA) MC-ICPMS [Eggins et al., 2005;Potter et al., 2005;McGregor et al., 2011] and are capable of analyzing 50 dates per day with only one day of sample preparation [Spooner et al., 2016], but uncertainties of ±33% (2σ) [McGregor et al., 2011] or more [Potter et al., 2005] on late-Holocene corals are too large for many paleoclimate applications. ...
... Recently, Douville et al. [2010] simplified tedious chemical separations and simultaneously measured U and Th atoms on an inductively coupled plasma-quadruple mass spectrometry (ICP-QMS), achieving 50 dates per day with precision levels of less than 2%. Additional rapid U/Th dating methods have been developed using laser ablation (LA) MC-ICPMS [Eggins et al., 2005;Potter et al., 2005;McGregor et al., 2011] and are capable of analyzing 50 dates per day with only one day of sample preparation [Spooner et al., 2016], but uncertainties of ±33% (2σ) [McGregor et al., 2011] or more [Potter et al., 2005] on late-Holocene corals are too large for many paleoclimate applications. ...
... Radiocarbon ages (yrs BP = years before 1950 AD) were converted to calibrated calendar ages (yrs AD for <1000 years old and cal yrs BP for samples >1000 years old) using the Calib 7.1 software [Stuiver et al., 2005] and MARINE13 calibration datasets [Reimer et al., 2013]. Additionally, we used a local radiocarbon marine reservoir correction (ΔR) of 39 ±56yrs (1σ) [McGregor et al., 2011]. ...
Time-consuming and expensive radiometric dating techniques limit the number of dates available to construct absolute chronologies for high-resolution paleoclimate reconstructions. A recently developed rapid-screen 14C dating technique reduces sample preparation time and per sample costs by 90%, but its accuracy has not yet been tested on shallow-water corals. In this study, we test the rapid-screen 14C dating technique on shallow-water corals by comparing 44 rapid-screen 14C dates to both high-precision 14C dates and U/Th dates from mid- to late-Holocene fossil corals collected from the central tropical Pacific (2-4°N, 157-160°W). Our results show that 42 rapid-screen 14C and U/Th dates agree within uncertainties, confirming closed-system behavior and ensuring chronological accuracy. However, two samples that grew ∼6500yrs ago have calibrated 14C ages ∼1000yrs younger than the corresponding U/Th ages, consistent with diagenetic alteration as indicated by the presence of 15-23% calcite. Mass balance calculations confirm that the observed dating discrepancies are consistent with 14C addition and U removal, both of which occur during diagenetic calcite recrystallization. Under the assumption that aragonite-to-calcite replacement is linear through time, we estimate the samples' true ages using the measured 14C and U/Th dates and percent calcite values. Results illustrate that the rapid-screen 14C dates of Holocene-aged fossil corals are accurate for samples with less than 2% calcite. Application of this rapid-screen 14C method to the fossil coral rubble fields from Kiritimati Island reveal significant chronological clustering of fossil coral across the landscape, with older ages farther from the water's edge. This article is protected by copyright. All rights reserved.
... Rapid dating methods include reconnaissance radiocarbon dating (Burke et al., 2010;McIntyre et al., 2011), simplified ID techniques (Adkins and Boyle, 1999;Douville et al., 2010) and laser ablation (LA) multi-collector (MC)-ICPMS (Eggins et al., 2005;Hoffmann et al., 2009;McGregor et al., 2011;Potter et al., 2005). Radiocarbon methods have been used to investigate Southern Ocean cold-water coral paleo-biogeography (Margolin et al., 2014;Thiagarajan et al., 2013) but do not provide calendar ages unless the initial marine reservoir carbon isotope ratios are known, and are only useful for dating materials younger than~35 ka (thousand years old). ...
... g of the material used (although cutting samples may be less straightforward than drilling powders for some speleothem samples, because of their slower growth rate and requirement for extensive sectioning to reveal internal stratigraphy). Published precisions on calendar ages for coral specimens obtained using variations of this method range from 7% in corals of~100 ka to 33% in recent corals of <1 ka (Eggins et al., 2005;McGregor et al., 2011;Potter et al., 2005). Here, rather than attempting to improve upon these precisions, we focussed on developing a simplified, high-throughput LA-MC-ICPMS method applicable for a wide range of cold-water corals, warm-water corals and speleothems, that could be adapted in any laboratory with a laser system and MC-ICPMS. ...
... Standards should ideally be 1) matrix-matched to the samples, 2) isotopically and chemically homogeneous at the scale of sampling and 3) have relatively high (>1mg/g) concentrations of 238 U. In previous LA-MC-ICPMS studies standard materials have included in-house corals (McGregor et al., 2011;Potter et al., 2005), secular equilibrium calcite (Hoffmann et al., 2009) and NIST glass (Eggins et al., 2005). We tested two aragonite standards formed in contrasting natural settings (Table 2). ...
Uranium-series dating is a critical tool in quaternary geochronology, including paleoclimate work, archaeology and geomorphology. Laser ablation (LA) methods are not as precise as most isotope dilution methods, but can be used to generate calendar ages rapidly, expanding the range of dating tools that can be applied to late Pleistocene carbonates. Here, existing LA methods are revisited for corals (cold- and warm-water) and speleothems spanning the last 343 thousand years (ka). Measurement of the required isotopes (238 U, 234 U, 230 Th and 232 Th) is achieved by coupling a laser system to a multi-collector inductively-coupled-plasma mass spectrometer (MC-ICPMS) using a combination of a single central ion counter and an array of Faraday cups. Each sample analysis lasts for ∼ 4.3 minutes, and fifty samples can be measured in 12 hours with an automated set up, after a day of sample preparation. The use of different standard materials and laser systems had no significant effect on method accuracy. Uncertainty on the measured (230 Th/238 U) activity ratios ranges from 5.4 % to 7.6 % for (230 Th/238 U) ratios equal to 0.7 and 0.1 respectively. Much of this uncertainty can be attributed to the heterogeneity of the standard material (230 Th/238 U) at the length scale of LA. A homogeneous standard material may therefore improve measurement uncertainty but is not a requirement for age-screening studies. The initial (234 U/238 U) of coral samples can be determined within ∼ 20 ‰, making it useful as a first indicator of open-system behaviour. For cold-water corals, success in determination of (232 Th/238 U) - which can affect final age accuracy - by LA depended strongly on sample heterogeneity. Age uncertainties (2 sigma) ranged from < 0.8 ka at 0-10 ka, ∼ 1.5 ka at 20 ka to ∼ 15 ka at 125 ka. Thus, we have demonstrated that U-series dating by LA-MC-ICPMS can be usefully applied to a range of carbonate materials as a straightforward age-screening technique.
... Expressed as 2 standard deviations (SD) of the mean of the multiple analyses, these range from 20.8 % to 1.2 % for ( 230 Th/ 238 U) equal to 0.03 and 1.25 respectively, and are ≤2 % for ( 234 U/ 238 U). External reproducibility at 95% confidence of calculated U-Th ages range from ±0.5 kyr for an age of 2.8 ka BP (16.8 %) to ±26.8 kyr for an age of 316.3 ka BP (8.6%); comparable to previous studies using laser ablation MC-ICP-MS (Eggins et al., 2005;McGregor et al., 2011;Spooner et al., 2016). External reproducibility of the δ 234 U i is typically better than 25‰ (2 SD). ...
... Calculated age uncertainties (at 95% confidence) give values of ~0.7 ka for ages of 0-10 ka, ~0.9 ka for ages of ~15 ka, ~5 ka for ages of 100 ka, and ~19 ka for ages of ~350 ka. These are comparable to other studies that have used laser ablation MC-ICP-MS (Eggins et al., 2005;McGregor et al., 2011;Spooner et al., 2016). The uncertainty of samples >400 ka could not be characterised following this approach and thus, such ages were not considered in detail here. ...
Scleractinian cold-water coral reefs are considered to be key hotspots of benthic biodiversity in the deep ocean. Due to their relevant ecological role and susceptibility to anthropogenic disturbances protection and conservation measures have been applied to these habitats, even though they are far from being completely understood. Throughout the last two decades several studies have quantitatively described the biodiversity of Atlantic cold-water coral reefs, finding considerable differences among biogeographic regions. In contrast, and probably owed to the scarcity of these habitats in the Mediterranean Sea, the knowledge related to coral reef biodiversity in this basin remains modest and almost purely qualitative. On a different note, when coral reefs are under persistent suitable environmental conditions and have a sufficient sediment input, they can develop and form large geomorphic structures known as coral mounds. The latter are sensitive to changes in climate and capable of recording such variations in the chemical composition of the coral skeletons. Numerous surveys in the Atlantic have associated coral mound development patterns to environmental variations caused by glacial-interglacial cycles. Within the Mediterranean, coral mound formation studies have been so far limited to the Alboran Sea and to the last 15 kyr, due to the lack of gravity cores encompassing longer periods of time. In this thesis a wide range of techniques, including ROV video-analysis, multivariate statistics, U-series dating, computed tomography and geochemical analyses were applied to acquire a better understanding of the spatiotemporal distribution of Mediterranean cold-water coral reefs and the processes controlling their evolution into mounds during the last 400 kyr. More precisely, the present study aimed to (1) quantify the density of uncommonly thriving coral reefs and accompanying megabenthic species within the Cabliers Coral Mound Province, and describe their distribution along it; and (2) explore which are the main environmental variables and paleoclimatic events that have controlled coral mound formation in Cabliers and in the newly discovered Tunisian Coral Mound Province. The research presented here revealed the densest and most flourishing cold-water coral reefs witnessed so far in the Mediterranean Sea and brought further insight into their distribution along the crests of ridge-like coral mounds. This thesis also contributed to increase our knowledge on the main species associated to Mediterranean coral reefs and their relative abundances, which showed considerable differences to those found in Atlantic reefs. In regards to coral mound formation, this work has expanded the current knowledge outside the Alboran Sea and back to 400 ka BP. Almost opposite development patterns were observed between the Cabliers and Tunisian coral mound provinces, with the former mainly developing throughout deglaciations and temperate interstadial periods and the latter during glacial periods. Nonetheless, both provinces seem to depend on a high surface productivity and an appropriate depth of the interface between Atlantic and Levantine Intermediate Waters for the coral mounds to develop. Lastly, the oceanographic alterations caused in the Eastern Mediterranean Basin during Sapropel events also seem to have had detrimental effects for coral mound formation in the Western basin.
... Here, we provide the first paleoecological surveys of emergent fossil reefs in the central Pacific, for two expansive and well-preserved reef sites on Kiritimati (Christmas) Island, Republic of Kiribati. Kiritimati Island is a partially filled-bucket atoll with the atoll interior characterized by a reticulate network of fossil reefs (McGregor et al., 2011;) that impound >500 brackish to hypersaline lakes . The excellent preservation of Holocene reefs at Kiritimati Island provides paleo-coral community context to infer geomorphic and physio-chemical changes in the reticulate lagoon system. ...
... This is particularly important given the prominence of Kiritimati Island in paleoclimate reconstructions (Cobb et al., 2013;Grothe et al., 2020;Woodroffe & Gagan, 2000). To date, existing age data from interior fossil microatolls of Kiritimati Island show no clear spatial-age pattern (McGregor et al., 2011; and are therefore inconclusive regarding the timing and nature of reticulate lagoonal reef growth and shutdown. Our findings present new spatiotemporal constraints on late Holocene changes in lagoon biophysical conditions consistent with discreet episodes of reticulate lagoon shutdown. ...
Plain Language Summary
Fossil coral reefs include the skeletal remains of corals and are rarely preserved in good condition above modern sea level. Such reefs are rich archives that hold information about past environmental and ecological conditions at the time of reef development. A rare and expansive example of fossil reefs grew during the period 7,000 to 1,000 years ago at Kiritimati Island, Kiribati. Surveys and radiometric dating were used to reveal, for the first time, the age, elevation, and coral ecology of two of the most well‐preserved fossil reefs on Kiritimati Island (between 4,000 and 2,000 years ago). Results show that the reefs flourished during different time periods and were dominated by different coral taxa, before dying off. We argue that reef die‐off is attributed to changing lagoon seawater conditions that became intolerable for coral growth. These changing water conditions were the result of storm‐induced changes at the atoll rim that enclosed the lagoon and reduced flushing. As tropical cyclones are projected to increase in frequency and severity under global climate change, our results are critical for understanding the future responses of mid‐oceanic atolls and their supporting ecologies.
... A recent innovative application of Uranium series dating used laser ablation multi-collector inductively coupled plasma mass spectrometry (LA MC-ICPMS) to determine the approximate age of fossil corals (Potter et al. 2005, McGregor et al. 2011. LA MC-ICPMS is a rapid (40-50 samples per day) and inexpensive analytical method to optimise selection of quality coral material belonging to a required age range. ...
... LA MC-ICPMS is a rapid (40-50 samples per day) and inexpensive analytical method to optimise selection of quality coral material belonging to a required age range. In the study by McGregor et al. (2011), the LA MC-ICPMS ages of the investigated corals ranged between 6400 and 900 years, and are generally constrained by solution uranium series and radiocarbon dating. However, for particular samples laser ablation gave results conflicting with solution MC-ICPMS, implying the method may not always able to detect subtle geochemical alteration. ...
We present an overview on different environmental zones within coastal areas and summarise the physical basis behind the three most important methods that are available to date Holocene coastal sediments. Besides radiocarbon and uranium series dating, Optically Stimulated Luminescence (Osl) has increasingly been applied for dating in coastal settings over the past decade. This is illustrated by a number of case studies showing that Osl can be applied to sediments from almost any kind of coastal environment, covering a potential dating range from some years up to several hundred thousand years. Osl dating may hence be the method of choice for deciphering natural environmental change along coasts as well as the presence and the impact of human occupation in such areas. In addition, we briefly show how and where these dating methods could be applied to constrain the palaeo-environmental context of an archaeological site at Vohemar in north-eastern Madagascar.
... These advances now provide the ability to extract various age and isotopic tracer information at lmscale resolutions. For example, in situ U-series measurements ( 234 U/ 238 U and 230 Th/ 238 U) can be obtained at spots with a spatial resolution of <100 lm in carbonate samples that contain ppm-level of U concentrations (Eggins et al., 2005;Potter et al., 2005;McGregor et al., 2011;Spooner et al., 2016). Despite that the LA-MC-ICPMS method does not provide the highest precision and accuracy of U-series isotope measurements (typical uncertainties of LA-MC-ICPMS measurements are percentage level), LA offers several unique advantages over the conventional solution-based MC-ICPMS method including: (1) increased spatial resolution of the distribution of U-series isotopes at scales <100 lm; ...
... (2) minimal sample preparation time (no sample digestion or column chemistry needed); (3) rapid isotope measurements that allow for high sample throughput and large number of data points during analysis; and (4) non-destructive (or minimal destruction) sample analysis (e.g., Eggins et al., 2005;Potter et al., 2005;McGregor et al., 2011;Spooner et al., 2016). Here, we report in situ measurements of U-series disequilibria with LA-MC-ICPMS on two weathering rind samples collected from the tropical island of Basse-Terre Island in French Guadeloupe ( Fig. 1). ...
Weathering of basaltic rocks plays an important role in many Earth surface processes. It is thus of great interest to quantify their weathering rates. Because of their well-documented behaviors during water-rock interaction, U-series isotopes have been shown to have utility as a potential chronometer to constrain the formation rates of weathering rinds developed on fresh basaltic rocks. In this study, U-series isotopes and trace element concentrations were analyzed in a basaltic andesite weathering rind collected from the Bras David watershed, Guadeloupe. From the clast, core and rind samples were obtained by drilling along a 63.8 mm linear profile across a low curvature segment of the core-rind boundary. Trace element concentrations reveal: significant loss of REE, Y, Rb, Sr, and Ba in the weathering rind; conservative behaviors of Ti and Th; and external addition of U into the rind during clast weathering. Measured (234U/238U) activity ratios of the rind samples are much higher than the core samples and show excess 234U. Measured (238U/232Th) and (230Th/232Th) activity ratios of the core and rind samples increase gradually from the core into the weathering rind. The observed depletion profiles for the trace elements in the clast suggest that the earliest chemical reaction that creates significant porosity is dissolution of plagioclase, consistent with the previous study [Sak et al., 2010, CG, in press]. The porosity growth within the rind allows for an influx of soil solution that carries dissolved U with (234U/238U) activity ratios >1 into the clast. The deposition of U in the rind is most likely associated with precipication of secondary minerals during clast weathering. Such a continuous U addition is responsible for the observed gradual increase of (238U/232Th) activity ratios in the rind. Subsequent production of 230Th in the rind over time from the decay of excess 234U accounts for the observed continuous increase of (230Th/232Th) activity ratios. The U-series activity ratios in the clast were modeled with a weathering advance rate of ~0.3 mm kyr-1. This represents the rind advance rate at the low curvature segment of the core-rind boundary under tropical climate. This rate is consistent with the previously estimated formation rates of basaltic rinds under similar tropic conditions in Costa Rica [Sak et al., 2004, GCA 68, 1453; Pelt et al., 2008, EPSL 276, 98]. This rate is about one order of magnitude greater than those in temperate regions, documenting the important control of temperature on basalt weathering. This work illustrates that the weathering advance rates of rinds can be successfully estimated by U-series isotopes, demonstrating their great potential as dating tools for Earth surface processes. Furthermore, U-series chronometry provides a suitable method for independently testing the hypothesis that rind advance rates around an individual clast increase with increasing interfacial curvature.
... Modern advances in this dating method relate to the relatively new application of laser ablation multi-collector ICPMS (LA-MC-ICPMS) techniques, which can sample very small sample sizes (Eggins et al., 2005). ICPMS allows precise micro-sampling of individual rings or layers in dense soil carbonate (Oerter et al., 2016), speleothems (Lin et al., 2017), bone and fossil teeth (Duval et al., 2011;Grün et al., 2014) and coral (Potter et al., 2005;McGregor et al., 2011). This type of sampling increases the reliability of the dates by, for example, establishing the growth history of the carbonate or bone, or identification of periods of dissolution in soil carbonate that relate to changes in climate or water flux, and testing of open-systems or re-precipitation of carbonate. ...
... Modern advances in this dating method relate to the relatively new application of laser ablation multi-collector ICPMS (LA-MC-ICPMS) techniques, which can sample very small sample sizes (Eggins et al., 2005). ICPMS allows precise micro-sampling of individual rings or layers in dense soil carbonate (Oerter et al., 2016), speleothems (Lin et al., 2017), bone and fossil teeth (Duval et al., 2011;Grün et al., 2014) and coral (Potter et al., 2005;McGregor et al., 2011). This type of sampling increases the reliability of the dates by, for example, establishing the growth history of the carbonate or bone, or identification of periods of dissolution in soil carbonate that relate to changes in climate or water flux, and testing of open-systems or re-precipitation of carbonate. ...
Oscillation of an ice sheet can be accompanied by earthquakes due to local reactivation of pre-existing faults related to the ice loading. A sufficiently large magnitude of an earthquake can trigger seismic waves that may strongly deform susceptible sediment layers and can cause the development of soft-sediment deformation structures (SSDS). Morphological and structural features of SSDS within a glaciolacustrine succession exposed at the coastal cliff on Gnitz Peninsula (Usedom Island) in NE Germany indicate that they must have developed due to glacial isostatic adjustment, which was suggested earlier by Hoffmann and Reicherter (2012).
Here we present detailed micro- and meso-scale SSDS within internally deformed layers interpreted as seismites, liquefaction and re-liquefaction sedimentological imprints on Gnitz Peninsula. New optically stimulated luminescence dating results indicate that the most probable time span of corresponding earthquake occurrence is between 23.2 and 14.6 ka. The interpretation of SSDS ‘trapped’ in layers as seismites is strongly supported by modelling of glacially induced Coulomb failure stress changes in this region. Our results point to a set of probably pre-Quaternary faults which were locally reactivated in the area of Gnitz Peninsula during the last glacial maximum.
... ka for ages of~15 ka, 5 ka for ages of 100 ka, and~19 ka for ages of~350 ka. These are comparable to other studies that have used laser ablation MC-ICP-MS (Eggins et al., 2005;McGregor et al., 2011;Spooner et al., 2016). The uncertainty of samples >400 ka could not be characterised following this approach and thus, such ages were not considered in detail here. ...
Cold-water corals are key species of benthic ecosystems, sensitive to changes in climate and capable of recording them in the chemical composition of their skeletons. The study of cold-water coral mound development in relation to palaeoceanographic variations during the Pleistocene and Holocene stages in the Mediterranean Sea has mainly been focussed in the Alboran Sea (Western Mediterranean). The present study describes the coral deposits and corresponding ages of 3 gravity cores, acquired from the newly discovered Tunisian Coral Mound Province (Central Mediterranean), which comprises several ridge-like mounds. All the cores acquired displayed dense coral deposits, dominated by Desmophyllum pertusum fragments embedded within a muddy sediment matrix. Overall, 64 coral samples have been dated with the UTh laser ablation MC-ICP-MS method, revealing corals of mostly Pleistocene age ranging from ~MIS 11 to 8.4 ka BP. Although coral mound formation was reduced for most of the last 400 kyr, a main stage of pronounced mound formation occurred during the last glacial period, which contrasts to the findings previously published for coral mounds in other regions of the Mediterranean Sea. Coral mound formation during the last glacial was most likely associated with a colder seawater temperature than the one observed in the present-day, an increased surface productivity and an appropriate depth of the interface between Atlantic Waters and Levantine Intermediate Waters. The combination of the data acquired here with that of previous mound formation studies from the Alboran Sea also suggests that cold-water coral mounds located at greater depths develop at slower rates than those found in shallower settings.
... However, the location in an atoll with a substrate of coral reef limestone [27] suggests that the 14 C signal could be affected by a marine reservoir and local hardwater reservoir effects, as typically occurs in atoll settings [31,32], which was not taken into account by Blumenberg et al. [30]. Studies that considered the marine reservoir effect in Kiritimati showed that a 14 C correction factor of 39 ± 56 years produces good correlations with U/Th ages [33,34]. In addition, the distance of the studied Kiritimati Lake 2 from the coast ( Figure 1) and its persistent hypersalinity indicate disconnection from seawater, allowing equilibration of C reservoir in lake water with atmospheric CO 2 [29]. ...
Microbial mats represented the earliest complex ecosystems on Earth, since fossil mineralized examples (i.e., microbialites) date back to the Archean Eon. Some microbialites contain putative remains of organic matter (OM), however the processes and pathways that lead to the preservation of OM within microbialite minerals are still poorly understood. Here, a multidisciplinary study is presented (including petrographic, mineralogical and organic geochemical analyses), focusing on a modern calcifying mat from a hypersaline lake in the Kiritimati atoll (Central Pacific). The results show that this mat has a complex history, with two main growth phases under hypersaline conditions, separated by an interruption caused by desiccation and/or freshening of the lake. The mineral precipitates of the mat are predominantly aragonitic and two contrasting precipitation modes are observed: the main growth phases of the mat were characterized by the slow formation of irregular micritic particles with micropeloidal textures and subspherical particles, linked to the degradation of the exopolymer (EPS) matrix of the mat; whereas the interruption period was characterized by the rapid development of a thin yet laterally continuous crust composed of superposed fibrous aragonite botryoids that entombed their contemporaneous benthic microbial community. These two precipitation modes triggered different preservation pathways for the OM of the mat as the thin crust shows a particular lipid biomarker signature, different from that of other layers and the relatively rapid precipitation of the crust protecting the underlying lipids from degradation, causing them to show a preservation equivalent to that of a modern active microbial community, despite them being >1100 years old. Equivalent thin mineral crusts occur in other microbialite examples and, thus, this study highlights them as excellent targets for the search of well-preserved biomarker signatures in fossil microbialites. Nevertheless, the results of this work warn for extreme caution when interpreting complex microbialite biomarker signatures, advising combined petrographic, mineralogical and geochemical investigations for the different microbialite layers and mineral microfabrics.
... External reproducibility at 95% confidence of calculated UeTh ages range from±0.5 ka for an age of 2.8 ka (16.8%) to±26.8 ka for an age of 316.3 ka (8.6%); comparable to previous studies using laser ablation MC-ICP-MS (Eggins et al., 2005;McGregor et al., 2011;Spooner et al., 2016). External reproducibility of the d 234 U i is typically better than 25‰ (2 SD). ...
Cold-water coral mounds are key hot-spots of deep ocean biodiversity and also important archives of past climatic conditions. Nonetheless, the paleo-oceanographic setting in which coral mounds developed in the Mediterranean Sea during the last 500 ka still needs to be properly understood. This study describes the coral deposits and corresponding ages of two on-mound gravity cores acquired from opposite sectors of the newly discovered Cabliers Coral Mound Province (CMP, Alboran Sea, W Mediterranean). U eTh data revealed Pleistocene-aged corals covering mound formation periods from >389 to 9.3 ka BP and from 13.7 to 0.3 ka BP in the southern and northern mounds respectively. The coral-rich deposits of the cores were mainly dominated by Desmophyllum pertusum, although in some sections concurrent with the Middle Pleistocene and the Holocene, other corals such as Dendrophyllia cornigera and Madrepora oculata also appeared as dominating species. Coral mound formation stages generally occurred during deglacials and temperate interstadial (3.5e4.1 d 18 O‰) periods, whereas during interglacials (<3.5 d 18 O‰) coral mound formation only occurred in the northern and shallower mound. We interpret this to indicate that the shoaling of the interface between Atlantic (AW) and Levantine Intermediate Waters (LIW) during interglacial periods prevented the corals in the southern CMP from acquiring sufficient food supply, thus causing periods of coral mound stagnation. Similarly, the interruption in LIW formation throughout sapropel events also coincides with coral mound stagnation phases. This suggests that sapropel-derived processes, which originated in the eastern Mediterranean, likely affected the entire Mediterranean basin and further supports the role of LIW as a conveyor belt facilitating cold-water coral growth in the Mediterranean Sea. Overall, we show that these coral mounds yield important insights into how local changes in oceanographic conditions can influence coral mound development.
... The methods applied in these papers require only minimal effort in sample preparation and less time of AMS measurement, allowing quick and inexpensive analysis of many more samples within a given project budget for a number of different purposes. They include survey of deep-sea coral ages for their population dynamics and past bio-geography (Longworth et al. 2013), age distribution of fossil corals collected at a study site (McGregor et al. 2011;Bush et al. 2013;Grothe et al. 2016), high-resolution and continuous records of young-speleothem 14 C (Welte et al. 2016) and 14 C-based calibration of amino acid racemization (AAR) ages (Domignuez et al. 2016;Kosnik et al. 2017). ...
We have investigated the possibility of direct accelerator mass spectrometry radiocarbon (AMS ¹⁴ C) measurement of carbonate samples at ANSTO using the STAR 2 MV tandem accelerator. Each carbonate sample was powdered, mixed with metal powder and pressed into an aluminum cathode for direct carbonate measurement by AMS ¹⁴ C. Of the three high-purity metal powders (Fe, Nb, and Ag) used in our investigation, Nb was found to be the best metal, which delivered higher carbon beam currents and lower background. Beam currents for targets containing the optimal carbonate mass of 1.5–2.0 mg were ∼8% of those obtained from graphite targets of standard size (>0.5 mg C). Typical measured blank for Carrara marble (IAEA-C1) of ∼40 ka was obtained. Background-corrected ¹⁴ C values of carbonate targets agreed well with their associated values obtained from high-precision analysis of graphite targets within 2σ uncertainties. Typical precision of this rapid AMS analysis was ∼1% for samples <8 ka. Despite lower precision for carbonate target ages (compared to standard graphite target ages), these ages are useful for palaeobiological applications where a large number of dates are required, such as time-averaging studies.
... Previous studies that reported 230 Th dates for coral abraders from Pacific Island stratified sequences used macroscopic appearance and concordance of dates for sub-samples to assess the reliability of 230 Th dates (Burley et al., 2015(Burley et al., , 2012. However, other analytical screening tests, including testing for preservation of primary mineralogy and Useries isotopic systematics, are commonly applied to 230 Th coral dates from subaerial contexts, such as in uplifted reefs widely exploited for paleoceanographic and paleoenvironmental studies (e.g., Cobb et al., 2003;Stirling and Andersen, 2009;McGregor et al., 2011;Hibbert et al., 2016;Muhs et al., 2017). Here, we apply these screening criteria to corals from buried contexts. ...
... The recent development of the measurement of 230 Th-234 U-238 U isotope ratios by laser ablation (LA) multi-collector (MC) ICPMS [1] allows the accurate in situ dating of quaternary material, thus overcoming sub-sampling problems and drastically reducing sample preparation processes. A number of different materials may be investigated [2], but so far most studies have focused on calcium carbonates [3][4][5]. ...
... AAR could make the most valuable contribution, however, in studies that are attempting to extend coral-based climate reconstructions using recently dead and fossil coral colonies cross-dated with samples extracted from living colonies (e.g. McGregor et al., 2011). In the field, it is often very difficult to assess likely age of such material which can contain mixed populations of recent to centuries and millennia since death. ...
... AAR could make the most valuable contribution, however, in studies that are attempting to extend coral-based climate reconstructions using recently dead and fossil coral colonies cross-dated with samples extracted from living colonies (e.g. McGregor et al., 2011). In the field, it is often very difficult to assess likely age of such material which can contain mixed populations of recent to centuries and millennia since death. ...
... It is particularly powerful when dating corals (e.g. Edwards et al., 1987a,b;Bard et al., 1998;Andersen, 2009a, 2009b;McGregor et al., 2011) and speleothems (e.g. Beck et al., 2001;Scholz and Hoffmann, 2011;Drysdale et al., 2012). ...
... Although cores from these corals can provide multi-decadal to multicentury records, palaeoclimate reconstructions tend to be restricted to the last several hundreds of years (Grottoli and Eakin, 2007;Lough, 2010;Neukom and Gergis, 2012) with access to older corals more limited owing to a general lack of exposure (i.e., coverage by younger living corals) and the deleterious influence of diagenetic alteration. Microatolls that are thousands of years old may be preserved on reef flats (Yu and Zhao, 2010;McGregor et al., 2011b;Woodroffe et al., 2012), but even live collected reef flat coral samples may be severely affected by early marine diagenesis (Enmar et al., 2000;Hendy et al., 2007;McGregor and Abram, 2008;Nothdurft and Webb, 2009;Sayani et al., 2011) owing to the high levels of physical-chemical and biological activity in the intertidal and very shallow water zones . Therefore, ancient corals stranded on the reef flat over thousands of years may have been subjected to significant marine diagenesis even in the absence of major meteoric diagenesis, thus altering the original geochemical signatures of the skeleton (Enmar et al., 2000;McGregor and Gagan, 2003;Lazar et al., 2004;Allison et al., 2005;Sayani et al., 2011). ...
Understanding the natural variability of the Earth’s climate system and accurately identifying potential anthropogenic influences requires long term, geographically distributed records of key climate indicators, such as temperature and precipitation that extend prior to the last 400 years of the Holocene. Reef corals provide an excellent source of high resolution climate records, and importantly represent the tropical marine environment where palaeoclimate data are urgently required. Recent decades have seen significant improvement in our understanding of coral biomineralisation, the associated uptake of geochemical proxies and methods of identifying and understanding the effects of both early and late, post depositional diagenetic alteration. These processes all have significant implications for interpreting geochemical proxies relevant to palaeoclimatic reconstructions. This paper reviews the current ‘state of the art’ in terms of coral based palaeoclimate reconstructions and highlights a key remaining problem. The majority of coral based palaeoclimate research has been derived from massive colonies of Porites. However, massive Porites are not globally abundant and may not provide material of a particular age of interest in those regions where they are present. Therefore, there is great potential for alternate coral genera to act as complimentary climate archives. While it remains critical to consider five key factors – vital effects, differential growth morphologies, geochemical heterogeneity in the skeletal ultrastructure, transfer equation selection and diagenetic screening of skeletal material – in order to allow the highest level of accuracy in coral palaeoclimate reconstructions, it is also important to develop alternate taxa for palaeoclimate studies in regions where Porites colonies are absent or rare. Currently as many as nine genera other than Porites have proven at least limited utility in palaeothermometry, most of which are found in the Atlantic/Caribbean region where massive Porites do not exist. Even branching taxa such as Acropora have significant potential to preserve environmental archives. Increasing this capability will greatly expand the number of potential geochemical archives available for longer term temporal records of palaeoclimate.
... Despite its strong potential as a geochronological tool for various igneous processes, such as cooling, recycling, and eruption dynamics, UeTh dating of zircon is seldom applied due to the time-consuming and inherent difficulty of dissolving zircons. While these kind of restrictions provide a perfect framework for the flourishing of UeTh zircondating by microbeam techniques, most efforts of in-situ U/Th dating have been mostly focused for the analysis of (bio)inorganic carbonates (Potter et al., 2005;Hoffmann et al., 2009;Mertz-Kraus et al., 2010;McGregor et al., 2011), and only few attempts to date other mineral phases, such as iron oxides (Bernal et al., 2005), and titanite (Koornneef et al., 2012) have been reported, despite the fact that in-situ UeTh analysis of zircons has been demonstrated feasible since the early days of LA-MCICPMS (Stirling et al., 2000). Thus, with only a recent exception (Ito et al., 2013), SIMS is currently used for such purposes Zou et al., 2010;Schmitt, 2011). ...
... Corals represent ideal candidates for calibrating R values due to their ability to fix dissolved inorganic carbon (DIC) from the surrounding water column (Grottoli and Eakin 2007 ). The 14 C signatures of their carbonate skeletons reflect the oceanographic conditions integrated over time and are a good proxy for oceanic circulation and climatic processes at subannual to millennial scales (Grottoli and Eakin 2007; McGregor et al. 2011). This paper presents a local marine reservoir correction for the southernmost coral reef complex in the Indian Ocean and evaluates this in context with R values calculated for the source waters of influential currents including local values in the broader region. ...
... Corals represent ideal candidates for calibrating R values due to their ability to fix dissolved inorganic carbon (DIC) from the surrounding water column (Grottoli and Eakin 2007 ). The 14 C signatures of their carbonate skeletons reflect the oceanographic conditions integrated over time and are a good proxy for oceanic circulation and climatic processes at subannual to millennial scales (Grottoli and Eakin 2007; McGregor et al. 2011). This paper presents a local marine reservoir correction for the southernmost coral reef complex in the Indian Ocean and evaluates this in context with R values calculated for the source waters of influential currents including local values in the broader region. ...
High-precision analysis using accelerator mass spectrometry (AMS) was performed upon known-age Holocene and modern, pre-bomb coral samples to generate a marine reservoir age correction value (Delta R) for the Houtman-Abrolhos Archipelago (28.7 degrees S, 113.8 degrees E) off the Western Australian coast. The mean Delta R value calculated for the Abrolhos Islands, 54 +/- 30 yr (1 sigma) agrees well with regional Delta R values for Leeuwin Current source waters (N-NW Australia-Java) of 60 +/- 38 yr. The Abrolhos Islands show little variation with Delta R values of the northwestern and north Australian coast, underlining the dominance of the more equilibrated western Pacific-derived waters of the Leeuwin Current over local upwelling. The Abrolhos Islands Delta R values have remained stable over the last 2884 cal yr BP, being also attributed to the Leeuwin Current and the El Nino Southern Oscillation (ENSO) signal during this period. Expected future trends will be a strengthening of the teleconnection of the Abrolhos Islands to the climatic patterns of the equatorial Pacific via enhanced ENSO and global warming activity strengthening the Leeuwin Current. The possible effect upon the trend of future Delta R values may be to maintain similar values and an increase in stability. However, warming trends of global climate change may cause increasing dissimilarity of Delta R values due to the effects of increasing heat stress upon lower-latitude coral communities.
... Recently, deviations from secular equilibrium have been successfully measured in ancient carbonates with the aim of age-scan samples prior to high-precision U-series dating (Douville et al., 2010). Alternative methodologies for agescanning have been proposed earlier (Potter et al., 2005b;McGregor et al., 2011). These are based on laser-ablation microsampling and, in addition to the requirement of very specialized instrumentation, significant method optimization prior to sample analysis is required. ...
We describe an analytical methodology to quantify 238U–234U and 230Th using isotope-dilution systematics and inductively coupled plasma-quadrupole mass spectrometry (ICP-QMS) for geochronological purposes. This required characterization of the mass spectrometer detector performance over a large dynamic range, including a careful calibration between pulse and analogue ion-counting modes. Our methodology gives an analytical precision for 234U/238U and 230Th/238U of 4‰ and 7‰ (2 × standard error), respectively, and yields 230Th-ages with 4–5% uncertainty from 500 ng of 238U. We tested our methodology by analyzing carbonate samples (corals and speleothems) which were previously dated using MC-ICPMS; excellent agreement between both techniques was obtained. Our results indicate that ICP-QMS is suitable to obtain valuable meaningful geochronological data, but it is limited to dating samples older than 1 ka, and younger than 400 ka. Our methodology thus represents an attractive alternative to obtain geochonological data to select the most interesting samples prior to high-precision analysis. Moreover, such data would also valuable to build preliminary age-models, which can be further complemented with high-precision analyses.
... The precision of Free Asx D/L geochronology is, therefore, within that obtained for difficult-to-date samples using other techniques and so would be a useful complementary and independent estimate of age. AAR could make the most valuable contribution, however, in studies that are attempting to extend coral-based climate reconstructions using recently dead and fossil coral colonies cross-dated with samples extracted from living colonies (e.g. McGregor et al., 2011). In the field, it is often very difficult to assess likely age of such material which can contain mixed populations of recent to centuries and millennia since death. ...
Over 500 Free Amino Acid (FAA) and corresponding Total Hydrolysed Amino Acid (THAA) analyses were completed from eight independently-dated, multi-century coral cores of massive Porites sp. colonies. This dataset allows us to re-evaluate the application of amino acid racemization (AAR) for dating late Holocene coral material, 20 years after Goodfriend et al. (GCA 56 (1992), 3847) first showed AAR had promise for developing chronologies in coral cores. This re-assessment incorporates recent method improvements, including measurement by RP-HPLC, new quality control approaches (e.g. sampling and sub-sampling protocols, statistically-based data screening criteria), and cleaning steps to isolate the intra-crystalline skeletal protein. We show that the removal of the extra-crystalline contaminants and matrix protein is the most critical step for reproducible results and recommend a protocol of bleaching samples in NaOCl for 48 h to maximise removal of open system proteins while minimising the induced racemization. We demonstrate that AAR follows closed system behaviour in the intra-crystalline fraction of the coral skeletal proteins. Our study is the first to assess the natural variability in intra-crystalline AAR between colonies, and we use coral cores taken from the Great Barrier Reef, Australia, and Jarvis Island in the equatorial Pacific to explore variability associated with different environmental conditions and thermal histories. Chronologies were developed from THAA Asx D/L, Ala D/L, Glx D/L and FAA Asx D/L for each core and least squares Monte Carlo modelling applied in order to quantify uncertainty of AAR age determinations and assess the level of dating resolution possible over the last 5 centuries. AAR within colonies follow consistent stratigraphic aging. However, there are systematic differences in rates between the colonies, which would preclude direct comparison from one colony to another for accurate age estimation. When AAR age models are developed from a combined dataset to include this natural inter-colony variability THAA Asx D/L, Glx D/L and Ala D/L give a 2σ age uncertainty of ±19, ±38 and ±29 year, for the 20th C respectively; in comparison 2σ age uncertainties from a single colony are ±12, ±12 and ±14 year. This is the first demonstration of FAA D/L for dating coral and following strict protocols 2σ precisions of ±24 years can be achieved across different colonies in samples from the last 150 years, and can be ±10 years within a core from a single colony. Despite these relatively large error estimates, AAR would be a valuable tool in situations where a large number of samples need to be screened rapidly and cheaply (e.g. identifying material from mixed populations in beach or uplift deposits), prior to and complementing the more time-consuming geochronological tools of U/Th or seasonal isotopic timeseries.
Holocene relative sea-level (RSL) data from far-field islands in the mid-Pacific have been used to validate the ice-melting histories of glacial isostatic adjustment (GIA) models. However, a lack of quality control in the reconstruction of RSL hinders the understanding of regional variability that can constrain ice-equivalent sea-level changes. Here, we present a standardised database of Holocene RSL data from five regions in the mid-Pacific (Cook Islands, Tuamotu Islands, Christmas (Kiritimati) Island, Gilbert Islands and Fiji). We categorised the data as high or low quality based on the susceptibility of samples to age and/or elevation errors. Of the 614 data points that were reviewed, 25% were rejected and 100 sea-level index points (SLIPs) were reinterpreted as limiting data. The new database consists of 141 SLIPs and 262 marine and 56 terrestrial limiting data points reconstructed from a variety of sea-level indicators (e.g., coral microatolls, mangrove peat, beachrock, and beach ridges), of which 71% provide high-quality constraints on RSL. The early to mid Holocene RSL evolution in the Cook Islands, Gilbert Islands and Fiji are poorly constrained due to a lack of high-quality SLIPs and limiting data during this period. The Tuamotu Islands provided the only record of early to mid Holocene evolution of RSL, indicating rapid RSL rise from between À22.9 m and À15.2 m at~9.0 ka to between À0.2 m and 0.5 m by~6.5 ka, at rates as high as 9.8 ± 5.1 mm/a, with a slowdown in the rate of RSL rise sometime between~8.2 ka and~6 ka. The Christmas (Kiritimati) Island record indicates stable RSL within~1.5 m of present-day levels over the past 6.6 ka. In the late Holocene, the Cook Islands record suggests a gradual fall in RSL over the past~2.9 ka at rates below 0.1 ± 4.3 mm/a. SLIPs at Fiji also indicate a slight fall in RSL at rates of less than 0.5 ±~4.4 mm/a at~4 ka, following which RSL fell from above 0.9 m at~3 ka to between À0.3 m and 0.6 m by~2.5ẽ 2.1 ka. We highlight the importance of standardisation and quality control to critically evaluate the processes controlling RSL and validate GIA models. Indeed, the new standardised database has implications for the timing of the mid-Holocene highstand, which has been used to support the ICE-4G and ICE-7G_NA models. Due to the poor constraints of data in the mid-Pacific islands, particularly in the early Holocene, there remains no unique solution for a global ice-melting history.
Uranium, thorium, and protactinium radionuclides in marine sediments are important proxies for understanding the earth's environmental evolution. Conventional solution-based methods, which typically involve isotope spike preparation, concentrated acid sample digestion, column chemistry, and mass spectrometry, allow precise but time-consuming and costly measurements of these nuclide concentrations (i.e., 230Th and 231Pa). In this work, we have established an efficient method for 230Th and 231Pa measurement of marine sediments down to the picogram-per-gram level without purification and enrichment. Our method first transforms a small amount of thermally decomposed sediments (∼0.1-0.2 g) to homogeneous silicate glass using a melt quenching technique and then analyzes the glass with laser ablation multicollector inductively coupled plasma-mass spectrometry. Standard sample bracketing with isotope-spike-calibrated glass standards prepared in this study was used to correct for instrumental fractionation during measurement. It is demonstrated that our method can accurately determine the U-Th-Pa concentrations of typical marine sediments in the late Pleistocene with precision of a few percent. Compared with the conventional solution-based methods, the turnover time of sample preparation and measurement with our established protocol is greatly reduced, facilitating future application of U-series radionuclides in reconstructing oceanic processes at high temporal and spatial resolution.
To quantify chemical weathering processes, it is essential to develop and utilize new geochemical tools that can provide information about chemical weathering in the field. U-series isotopes have emerged as a useful chronometer to directly constrain the rates and duration of chemical weathering. However, the conventional solution-based MC-ICPMS method involves a long and expensive sample processing procedure that restricts the numbers of measurements of samples by U-series analysis that can be completed. Here, we report in situ measurements of U-series disequilibria obtained with laser ablation (LA)-MC-ICPMS on weathering rinds collected from the tropical island of Basse-Terre in the archipelago of French Guadeloupe. We characterized two weathering rinds for U-series isotope compositions and elemental distributions with LA-MC-ICPMS and LA-Q-ICPMS. The in situ measurements of U-series disequilibria were consistent with the previous bulk measurements obtained by conventional solution MC-ICPMS despite the larger analytical uncertainties. The LA technique allowed a greater number of measurements that accelerated sample throughput and improved spatial resolution of measurement. The rind formation age, weathering rates, and U-series mobility parameters modeled in this study are comparable to the results from previous studies conducted on the same clasts, and also reveal new insights on rind formation such as the impact of micro-fractures on weathering history and U-series ratios. The improved spatial resolution available with LA Q-ICPMS helps distinguish between linear and power law rind thickness-age relationships that were unresolvable using conventional solution-based MC-ICPMS. In situ measurements with LA-Q-ICPMS in these weathering rinds also elucidates the sequences of mineral reactions during chemical weathering. The LA-Q-ICPMS maps of major and trace elements and elemental ratios reveal details about the rind formation processes at the weathering interfaces of clasts such as dissolution of primary phases, formation of new phases, development of porosity, and mobility behavior of U. This study demonstrates a new analytical method for determining weathering rates in rinds rapidly and accurately that can be used in a large number of rinds, providing key information at the clast scale.
Fossil corals provide valuable data for reconstructing past sea levels, as they are often well preserved in the fossil record and can be dated with U-series methods. Here we present a global and internally consistent database of Usingle bondTh dated fossil coral sea-level indicators, including full consideration of all (known) associated uncertainties (both vertical and chronological). We include carefully determined taxon-specific depth distributions, rather than blanket depth uncertainty terms as used in most previous work. This is based on a synthesis of extensive modern ecological information on depth ranges. These ranges are found to be spatially variable (between ocean basins, between regions, and on sub-regional scales) because depth itself is not limiting – instead, depth distributions arise from complex physical, chemical, and biological interactions with coral-reef growth, distribution, and composition. One of the main causes for recognition of the greater depth-variability of coral taxa has been the routine inclusion of deep-diving and ROV surveys in coral ecological studies over the past few decades, which has broken through the “shallow-water” bias of early surveys by adding frequent observations on deeper occurrences (although more are needed). It is also clear from our assessment that coral habitat-depth distributions must be determined on the species level to reduce uncertainties in reconstructions of past sea levels, and that application to sea-level studies then requires these studies also to identify fossil corals to the species level. Samples identified only to the genus level give rise to wide uncertainties in habitat depth and, hence, sea level. Our database contains extensive metadata to assist evaluations of dating quality, as well as geomorphic and stratigraphic metadata. We demonstrate with examples how such metadata can help to evaluate sea-level reconstructions, for example by identifying outlier points. One example discusses the Last Interglacial (LIG), where we use the available data with their uncertainties to assess probabilistically the time at which local sea levels exceed that of the present, which yields a mean age of 124.6 ka with 95% probability bounds at 118.5 and 129.5 ka. We conclude with identification of key outstanding issues relating to: (i) current incomplete understanding of tectonic setting (including the current lack of independent verification of uplift/subsidence rates and reliance of somewhat unsatisfactory, and circular, use of the elevation of Last Interglacial deposits); (ii) the depth-distributions of coral taxa and; (iii) the complete documentation of stratigraphic, geomorphological and other contextual information, with suggestions for strategies to address these issues.
Coral microatolls are valuable paleo-sea-level indicators not only because of their vertical precision but also because individual coral colonies grow and respond to sea level for decades to centuries: unlike shorter-lived organisms, they contain filtered records of sea level over sustained periods. In tectonic settings, microatolls record both gradual (interseismic and postseismic) and sudden (coseismic) vertical changes. This chapter describes how to use microatolls to reconstruct relative sea-level histories. We discuss the utility and limitations of microatolls; site and sample selection; field techniques; documentation strategies; slab processing, analysis, and interpretation; dating; vertical accuracy; sources of error; and indicative meaning.
There has been geographical variation in sea level since rapid postglacial melting of polar ice ceased similar to 6 k.y. ago, reflecting isostatic adjustments of Earth and ocean surfaces to past (and ongoing) redistribution of ice and water loads. A new data set of over 100 fossil microatolls from Christmas (Kiritimati) Island provides a Holocene sea-level record of unparalleled continuity. Living reef-flat corals grow up to a low-tide level. Adjacent fossil microatolls, long-lived Porites corals up to several meters in diameter, occur at similar elevations (+/-0.1 m), and extensive fossil microatolls in the island interior are at consistent elevations within each population. Collectively, they comprise an almost continuous sequence spanning the past 5 k.y., indicating that, locally, sea level has been within 0.25 m of its present position, and precluding global sea-level oscillations of one or more meters inferred from less stable locations, or using other sea-level indicators. This mid-Pacific atoll is tectonically stable and far from former ice sheets. The precisely surveyed and radiometrically dated microatolls indicate that sea level has not experienced significant oscillations, in accordance with geophysical modeling, which implies that the eustatic contribution from past ice melt and the isostatic adjustment of the ocean floor to loading largely cancel each other at this site.
Based on the concepts (a) that the stable C and O isotopes combined with the Sr isotope ratios of fracture fills should reflect the source groundwater from which the solid phases precipitated and (b) that U-series disequilibria (USD) enable the calculation of residence time for the U by using Fe oxides as the best candidate, an “isotopic toolbox” was applied to fracture fill from the crystalline basement of the Vienne district. The fracture fills are formed mainly of carbonates, clays and Fe oxides. The isotope data indicate two main generations of carbonate that originated from hydrothermal circulation and equilibrium with present-day groundwaters but the Sr isotope ratios highlight another component with a higher 87Sr/86Sr ratio reflecting the complexity of the water–rock interactions.For the USD, the Fe-hydroxides located at 207 m depth yield an age of 102 ± 5 ka (St. Germain I interglacial stage), whereas those located at 277 m and 300 m yield respective ages of 173 ± 15 ka and 181 ± 10 ka. These corresponding to the transition between the penultimate glacial period (isotopic stage 6) and the end of the preceding interglacial stage (isotopic sub-stage 7a). Investigating water–rock interaction (87Sr/86Sr, 18O, 13C, USD) in the fracture-fill minerals from the crystalline basement has shown that such an approach is relevant to developing an understanding of how the groundwater system has changed over time.
The potential of femtosecond laser ablation multi-collector inductively coupled plasma mass spectrometry (fs-LA-MC-ICPMS) for in situ analysis of U–Th disequilibria in titanite was investigated. The aim of the study was to resolve spatial variations in (230Th/238U) ratios (where parentheses denote activity) in titanite from slowly cooled magma bodies. An in-house titanite glass (TG2), determined to be in secular equilibrium by solution mode MC-ICPMS (i.e. (230Th/238U) = 1), was used to correct for U–Th elemental fractionation by sample standard bracketing. The effect of instrument operating conditions on the accuracy and reproducibility of (230Th/238U), (232Th/238U) and (230Th/232Th) ratios was studied by analyses of titanite minerals with known composition and a secondary titanite glass standard. The (230Th/232Th) data were found to be accurate and reproducible, independent of the instrument setting used, suggesting that corrections made for SEM-Faraday gain and abundance sensitivity were appropriate. However, plasma conditions, laser ablation mode, laser energy and wavelength, and titanite material properties were all found to variably influence the U–Th elemental fractionation and compromise the accuracy of the (230Th/238U) data to different extents. Hot plasma conditions significantly reduce the fractionation between U and Th. A drift in elemental fractionation was observed during single spot analyses using NIR laser ablation and results in errors of up to 29% on the (230Th/238U) data. The magnitude of the drift in the elemental fractionation was different for different laser wavelengths and energies. Ablation using the UV single spot mode was significantly less affected by variable elemental fractionation compared to NIR spot analyses, but precision was limited by lower sample uptake. Scanning mode analyses were not compromised by temporal variation of the U–Th intensity ratios but the degree of elemental fractionation was variable between analyses of different materials (e.g. glass versus minerals). This observation suggests material-dependent differences in U–Th fractionation even for near identical titanite compositions. Analyses of the secondary titanite glass standard TG1 bracketed by TG2 yield the most reproducible and accurate (230Th/238U) data, indicating more adequate correction for elemental fractionation when the calibration standard is matched in terms of material composition and structure.
Understanding the influence of climatic and non-climatic factors on geochemical signals in corals is critical for assessing coral-derived records of tropical climate variability. Porites microatolls form large, disk-shaped colonies constrained in their upward growth by exposure at or close to mean spring low water level, and occur on Indo-Pacific reefs. Microatolls appear suitable for paleoclimate reconstruction, however the systematics of the microatoll chemistry-climate relationship are yet to be characterized. In this study, the δ18O signal in Porites microatolls from well-flushed reef flats on Kiritimati (Christmas) Island, central Pacific was investigated for intra-coral (growth aspect and extension rate effects) and between-coral effects, and to explore the climate signal contained within their skeletons. Samples for δ18O analysis were taken from six individual transects from different positions within Porites microatoll XM22. The results show that: (1) the average standard deviation for the mean δ18O values of transects that represent the same time periods is 0.03‰, and is within measurement error for a single analysis (0.04‰); (2) the average standard deviation for time-equivalent, near-monthly samples along the transects within the same microatoll is 0.07‰ and; (3) comparison of the average δ18O values of records for different microatolls from across Kiritimati Island show only a small between-coral differences of 0.04‰ and 0.11‰ for different time periods. These differences in mean δ18O are within the range for intra- and inter-colony differences in seasonal and interannual δ18O reported for dome-shaped Porites. Based on these results, a stacked microatoll δ18O record was constructed for the period 1978-2007 for comparison with published coral δ18O records for nearby dome-shaped Porites. There is a systematic offset between the two types of records, which is probably due to variations in δ18O seawater across Kiritimati Island. Despite the offset, all records show similar amplitudes for the seasonal-cycle of δ18O, and there is a strong correlation (r=-0.71) between microatoll δ18O and local sea surface temperature (SST). The δ18O-SST slope relationship for microatolls is -0.15‰/°C, very similar to that reported for fast-growing domed corals (-0.18‰ to -0.22‰/°C). Statistical analysis of the stacked microatoll δ18O record shows that it is correlated with both local and large-scale climate variables (primarily SST) at semiannual, annual and interannual timescales. Our results show that the signal reproducibility and fidelity of skeletal δ18O in coral microatolls is comparable to that observed for more conventional coral growth forms. Longer-lived, and fossil, Porites microatolls, where they have grown in suitably flushed environments, are likely to contain δ18O signals that can significantly extend instrumental records of tropical climate variability.
We present corrections to uncertainty estimates of late 20th century
(1972-1998) coral-based sea-surface temperature (SST) and salinity (SSS)
trends from Palmyra, Fanning and Christmas Islands of the central
tropical Pacific reported in Nurhati et al. [2009] (Geophysical Research
Letters, 36, L21606, doi:10.1029/2009GL040270). In our original paper we
had assigned relatively large error bars to coral Sr/Ca-based warming
trends observed at all three islands, rendering those trends
statistically insignificant with respect to the large errors. However,
those large uncertainties originate from large errors associated with
quantifying absolute SST at any given point, whereas the uncertainties
for SST trends should reflect errors in quantifying relative changes,
which are much lower. Late 20th century warming trends of
0.94-1.65°C at the three islands are statistically significant
with the corrected error ranges of ±0.19-0.37°C
(1σ originally ±5.73-6.57°C). Freshening trends
inferred from coral-based oxygen isotopic composition of seawater
(δ18OSW) of -0.32‰ and
-0.12‰ (1σ) at Palmyra and Fanning, respectively, are
associated with corrected error ranges of ±0.07‰ and
±0.08‰ (1σ originally ±0.08‰ for
both), respectively. Christmas experienced an insignificant +0.03
± 0.11‰ (1σ originally also ±0.11‰)
trend in δ18OSW over this period. With the
new, lower uncertainty estimates, the coral-based SST and SSS trends are
statistically significant, thereby strengthening Nurhati et al.'s (2009)
conclusions that robust warming and freshening have occurred in the
central tropical Pacific over the late 20th century. Our detailed
uncertainty calculations presented below will be of value to the
paleoclimate community, as the correct calculation of absolute errors
and relative errors represents a key component of paleoclimatic
reconstruction.
In Quebrada de los Burros in coastal southern Peru (Tacna department), human settlements containing shells and charcoal deposits have been excavated since 1995. The sea surface 14C reservoir ages, estimated by calculating the dif- ference of 14C age between marine shells and terrestrial organic materials, exhibit high values during the lower and middle Holocene and decrease abruptly after 4000 cal BP. The increase of reservoir age at around 7000-8000 cal BP suggests an enhancement of Peruvian coastal upwelling intensity and changes in ocean circulation at intermediate depth.
Skeletal oxygen isotope ratios in Holocene Porites corals from northern Papua New Guinea record decreases in sea surface temperature (SST) and rainfall during El Niño events. Threshold analysis of seven fossil coral delta18O records spanning the period 7.6-5.4 ka (thousand years ago) shows 8-12 El Niño events/century, significantly less than the 23 events/century recorded by the NINO3.4 Index. The coral reconstructions also show a 15% reduction in El Niño event amplitude for 7.6-5.4 ka, compared to today, which is greater than the suppression given by model studies. In contrast, large and protracted El Niño events are identified for 2.5-1.7 ka. Taken together, the results indicate a non-linear atmospheric response to Holocene changes in El Niño SST anomalies. We propose that small changes in tropical SST gradients, the positioning of the Intertropical Convergence Zone, and the Pacific tradewind climatology modify the impact of El Niño events on western Pacific rainfall.
In situ Sr-isotope analysis by laser ablation multi-collector ICP-MS is a potentially powerful tracer technique with widespread application to many fields of study. The usefulness of the method, however, depends very strongly upon the quality of data that can be obtained (compared with conventional ‘solution-based’ analyses), and the spatial resolution, particularly in samples with strong compositional zonation or fine-scale growth banding. In this contribution we show that highly accurate (50 ppm) and precise (external precision 125 ppm) analyses of carbonate materials can be obtained in situ and further demonstrate that, by utilising the aperture-imaging optics of an excimer laser system with appropriate time-resolved software, isotopic variations on the scale of tens of micrometres can be resolved. An example is shown using relatively small (500 μm diameter) otoliths from a diadromous fish species, Galaxias maculatus.
Diagenesis is now recognized as a potentially major source of error in
paleoclimatic reconstructions from fossil and modern coral geochemical
records. Key to avoiding spurious results caused by diagenesis is
thorough screening of coral material prior to geochemical analysis. In
this data brief we present color images from thin sections of fossil and
modern Porites corals and demonstrate the effectiveness of thin sections
in detecting low levels of diagenesis. The images presented here cover a
range of coral preservation levels from pristine aragonite to 100%
calcite. We particularly focus on samples containing around 1%
diagenetic material, a level known to create artifacts in key climate
parameters such as sea surface temperature, and close to the detection
limits of other screening methods such as X-ray diffraction (XRD). A
qualitative scheme is also presented to rate the degree of diagenesis in
a coral, where XRD results are not available or where secondary
aragonite is present. Overall, this collection of images is designed as
a starting point, in combination with other techniques, to assist in
identifying and screening corals for diagenesis.
Global climate models and analyses of instrumental datasets provide a wide range of scenarios for future tropical Pacific climate change, limiting the accuracy of regional climate projections. Coral records provide continuous reconstructions of tropical Pacific climate trends that are difficult to quantify using the short, sparse instrumental datasets available from the tropical Pacific. Here, we present coral-based reconstructions of late 20th century sea-surface temperature and salinity trends from several islands in the central tropical Pacific. The coral data reveal warming trends that increase towards the equator, implying a decrease in equatorial upwelling in the last decades. Seawater freshening trends on the southern edge of the Inter-Tropical Convergence Zone suggest a strengthening and/or an equatorward shift of the convergence zone. Together, the new coral records support a late 20th century trend towards "El Niño-like" conditions in the tropical Pacific, in line with the majority of coupled global climate model projections.
230Th, 232Th, 234U and 238U compositions of several deep-sea solitary corals, mainly the species Desmophyllum cristagalli, were determined by thermal ionization mass spectrometry (TIMS). It is possible to obtain high precision ages on modern pristine corals that have low [232Th] (5 to a few hundred ppt). However, because older deep-sea corals tend to have higher [232Th] compared to surface corals, and the initial 230Th/232Th ratio is uncertain, older deep-sea corals have larger age uncertainties (±several hundred years for samples with a few thousand ppt 232Th). Therefore, the key hurdle for precise U-Th dating is to remove or account for contaminants which contain elevated 232Th and associated 230Th not due to closed system decay within the coral lattice. A modification of the trace metal cleaning methods used for foraminifera and surface corals can significantly reduce this contamination. By counting the visible growth bands and measuring the mean age of a single septum, the extension rate of D. cristagalli was estimated to be between 0.1 and 3.1 mm/year. In a mean sense, bands appear to be precipitated annually, but this estimate has a large uncertainty. If appropriate tracer calibrations can be established, these corals are therefore suitable to record decadal or sub-decadal oceanographic changes over the course of their lifetime. The δ234U values of all modern samples from different localities and different depths are similar (mean 145.5 ± 2.3‰) and indistinguishable from the data obtained from surface corals. At a precision of about ±2‰, we find no structure in the oceanic profile of δ234U ratios over the top 2000 m of the water column.
Elemental fractionation effects during analysis are the most significant impediment to obtaining precise and accurate U-Pb ages by laser ablation ICPMS. Several methods have been proposed to minimize the degree of downhole fractionation, typically by rastering or limiting acquisition to relatively short intervals of time, but these compromise minimum target size or the temporal resolution of data. Alternatively, other methods have been developed which attempt to correct for the effects of downhole elemental fractionation. A common feature of all these techniques, however, is that they impose an expected model of elemental fractionation behavior; thus, any variance in actual fractionation response between laboratories, mineral types, or matrix types cannot be easily accommodated. Here we investigate an alternate approach that aims to reverse the problem by first observing the elemental fractionation response and then applying an appropriate (and often unique) model to the data. This approach has the ver
New radiocarbon calibration curves, IntCal04 and Marine04, have been constructed and internationally ratified to replace the terrestrial and marine components of IntCal98. The new calibration data sets extend an additional 2000 yr, from 0–26 cal kyr BP (Before Present, 0 cal BP = AD 1950), and provide much higher resolution, greater precision, and more detailed structure than IntCal98. For the Marine04 curve, dendrochronologically-dated tree-ring samples, converted with a box diffusion model to marine mixed-layer ages, cover the period from 0–10.5 cal kyr BP. Beyond 10.5 cal kyr BP, high-resolution marine data become available from foraminifera in varved sediments and U/Th-dated corals. The marine records are corrected with site-specific 14C reservoir age information to provide a single global marine mixed-layer calibration from 10.5–26.0 cal kyr BP. A substantial enhancement relative to IntCal98 is the introduction of a random walk model, which takes into account the uncertainty in both the calendar age and the 14C age to calculate the underlying calibration curve. The marine data sets and calibration curve for marine samples from the surface mixed layer (Marine04) are discussed here. The tree-ring data sets, sources of uncertainty, and regional offsets are presented in detail in a companion paper.
The El Niño-Southern Oscillation (ENSO) is the most potent source of interannual climate variability. Uncertainty surrounding the impact of greenhouse warming on ENSO strength and frequency has stimulated efforts to develop a better understanding of the sensitivity of ENSO to climate change. Here we use annually banded corals from Papua New Guinea to show that ENSO has existed for the past 130,000 years, operating even during "glacial" times of substantially reduced regional and global temperature and changed solar forcing. However, we also find that during the 20th century ENSO has been strong compared with ENSO of previous cool (glacial) and warm (interglacial) times. The observed pattern of change in amplitude may be due to the combined effects of ENSO dampening during cool glacial conditions and ENSO forcing by precessional orbital variations.
Any assessment of future climate change requires knowledge of the full range of natural variability in the El Niño/Southern Oscillation (ENSO) phenomenon. Here we splice together fossil-coral oxygen isotopic records from Palmyra Island in the tropical Pacific Ocean to provide 30-150-year windows of tropical Pacific climate variability within the last 1,100 years. The records indicate mean climate conditions in the central tropical Pacific ranging from relatively cool and dry during the tenth century to increasingly warmer and wetter climate in the twentieth century. But the corals also document a broad range of ENSO behaviour that correlates poorly with these estimates of mean climate. The most intense ENSO activity within the reconstruction occurred during the mid-seventeenth century. Taken together, the coral data imply that the majority of ENSO variability over the last millennium may have arisen from dynamics internal to the ENSO system itself.
The last interglacial period (about 125,000 years ago) is thought to have been at least as warm as the present climate. Owing to changes in the Earth's orbit around the Sun, it is thought that insolation in the Northern Hemisphere varied more strongly than today on seasonal timescales, which would have led to corresponding changes in the seasonal temperature cycle. Here we present seasonally resolved proxy records using corals from the northernmost Red Sea, which record climate during the last interglacial period, the late Holocene epoch and the present. We find an increased seasonality in the temperature recorded in the last interglacial coral. Today, climate in the northern Red Sea is sensitive to the North Atlantic Oscillation, a climate oscillation that strongly influences winter temperatures and precipitation in the North Atlantic region. From our coral records and simulations with a coupled atmosphere-ocean circulation model, we conclude that a tendency towards the high-index state of the North Atlantic Oscillation during the last interglacial period, which is consistent with European proxy records, contributed to the larger amplitude of the seasonal cycle in the Middle East.
The Indian Ocean Dipole (IOD)--an oscillatory mode of coupled ocean-atmosphere variability--causes climatic extremes and socio-economic hardship throughout the tropical Indian Ocean region. There is much debate about how the IOD interacts with the El Niño/Southern Oscillation (ENSO) and the Asian monsoon, and recent changes in the historic ENSO-monsoon relationship raise the possibility that the properties of the IOD may also be evolving. Improving our understanding of IOD events and their climatic impacts thus requires the development of records defining IOD activity in different climatic settings, including prehistoric times when ENSO and the Asian monsoon behaved differently from the present day. Here we use coral geochemical records from the equatorial eastern Indian Ocean to reconstruct surface-ocean cooling and drought during individual IOD events over the past approximately 6,500 years. We find that IOD events during the middle Holocene were characterized by a longer duration of strong surface ocean cooling, together with droughts that peaked later than those expected by El Niño forcing alone. Climate model simulations suggest that this enhanced cooling and drying was the result of strong cross-equatorial winds driven by the strengthened Asian monsoon of the middle Holocene. These IOD-monsoon connections imply that the socioeconomic impacts of projected future changes in Asian monsoon strength may extend throughout Australasia.
We present routine methods of target preparation for radiocarbon analysis at the ANTARES Accelerator Mass Spectrometry (AMS) Centre, as well as recent developments which have decreased our procedural blank level and improved our ability to process small samples containing less than 200 μg of carbon. Routine methods of 14C sample preparation include sample pretreatment, CO2 extraction (combustion, hydrolysis and water stripping) and conversion to graphite (graphitization). A new method of cleaning glassware and reagents used in sample processing, by baking them under a stream of oxygen, is described. The results show significant improvements in our procedural blanks. In addition, a new graphitization system dedicated to small samples, using H2/Fe reduction of CO2, has been commissioned. The technical details of this system, the graphite yield and the level of fractionation of the targets are discussed. © 2001 by the Arizona Board of Regents on behalf of the University of Arizona.
Oxygen isotope ratios in Porites microatolls from Christmas Island in the central Pacific provide high-resolution proxy records of El Niño-Southern Oscillation (ENSO) variability since 3.8 thousand years ago (ka). Compared with modern microatolls, reconstructions from fossil microatolls imply that interannual variations in ENSO sea-surface temperature and precipitation were less intense 3.8-2.8 ka, but more pronounced at 1.7 ka. Amplification of ENSO at ~2 ka is consistent with precessional changes in insolation seasonality, but exceeds model predictions and may reflect stronger rainfall teleconnections through enhanced interaction between the Southern Oscillation and the Pacific Intertropical Convergence Zone.
Microatolls are discoid corals that have grown laterally because
vertical growth is constrained by exposure at lowest tides. We
demonstrate that a modern reef-flat Porites microatoll from Christmas
(Kiritimati) Island preserves an oxygen isotope record of substantial
sea surface temperature variations related to El Niño-Southern
Oscillation (ENSO). We also show that a late Holocene fossil microatoll
from the centre of the island contains interannual oxygen isotope
variations over an approximate 20-year period. Three pronounced negative
isotope anomalies attributed to warm El Niño events are
superimposed on an annual cycle. El Niño events similar to those
seen in recent decades appear to have been a feature of mid-Pacific
climate in the late Holocene. Analysis of further microatolls offers a
source of pre-instrumental El Niño tropical climate data. It may
be possible to extend the NINO-C proxy to indicate intensity and
frequency of ENSO over the past three millennia.
A weekly 18 spatial resolution optimum interpolation (OI) sea surface temperature (SST) analysis has been produced at the National Oceanic and Atmospheric Administration (NOAA) using both in situ and satellite data from November 1981 to the present. The weekly product has been available since 1993 and is widely used for weather and climate monitoring and forecasting. Errors in the satellite bias correction and the sea ice to SST conversion algorithm are discussed, and then an improved version of the OI analysis is developed. The changes result in a modest reduction in the satellite bias that leaves small global residual biases of roughly 20.038C. The major improvement in the analysis occurs at high latitudes due to the new sea ice algorithm where local differences between the old and new analysis can exceed 18C. Comparisons with other SST products are needed to determine the consistency of the OI. These comparisons show that the differences among products occur on large time- and space scales with monthly rms differences exceeding 0.58C in some regions. These regions are primarily the mid- and high-latitude Southern Oceans and the Arctic where data are sparse, as well as high- gradient areas such as the Gulf Stream and Kuroshio where the gradients cannot be properly resolved on a 18 grid. In addition, globally averaged differences of roughly 0.05 8C occur among the products on decadal scales. These differences primarily arise from the same regions where the rms differences are large. However, smaller unexplained differences also occur in other regions of the midlatitude Northern Hemisphere where in situ data should be adequate.
Changes in oceanic radiocarbon (14C) reservoir ages through the deglaciation and Holocene can provide important information on ocean circulation as Earth's climate warmed. Here, we present reservoir ages for the western tropical Pacific that span the mid-Holocene transition from less to more frequent El Niño events. Reservoir ages were calculated using paired U–Th and conventional 14C dating of eight individual fossil coral samples from Koil and Muschu Islands, northern coastal Papua New Guinea (PNG). AMS 14C and MC-ICPMS U–Th dating of additional samples from six of the fossil corals were used to confirm the TIMS U–Th and conventional 14C ages. The combined results show average reservoir ages of 185±30 14C yr (n=4) for 7220–5850 yr BP compared to 420 14C yr for a modern coral from Muschu Island. From 5850 to 5420 yr BP reservoir ages increase to modern values. The relatively young reservoir ages from 7220 to 5850 yr BP are best explained by greater influx of well-equilibrated sub-tropical water from the southern branch of the South Equatorial Current (SEC). This is consistent with strengthening trade winds (facilitating air–sea exchange) and a more northerly position of the Intertropical Convergence Zone thought to have occurred at this time. The transition to more modern-like reservoir ages from 5850 to 5420 yr BP suggests modern oceanic circulation patterns were established during this interval. The onset of modern El Niño activity around this time would have served to enhance the intrusion of 14C-depleted equatorial waters via the south equatorial branch of the SEC. Overall, the changes in reservoir age presented here for the western tropical Pacific suggest that Holocene changes in the El Niño–Southern Oscillation state of the tropical Pacific resulted in reorganisation of oceanic circulation in this region.
The in situ U-series dating of corals has been achieved using laser-ablation multiple-collector inductively coupled plasma mass spectrometry (LA MC-ICPMS). Utilising a 193 nm excimer laser coupled to a Nu Instruments Nu Plasma equipped with multiple ion counters, percent-level analytical precision for 234 U/ 238 U and 230 Th/ 238 U is obtained. Approximately 70 g of carbonate is consumed during each analysis, containing approximately 0.2 ng of 238 U, ∼10 pg of 234 U and less than 3 fg of 230 Th. A two-cycle, fast magnet jumping procedure is adopted. In the first cycle 238 U is measured in a Faraday collector while the low-abundance isotopes 234 U, 232 Th, and 230 Th are measured simultaneously in ion counters. In the second cycle 235 U is measured in an ion counter. All samples are bracketed with an isotopically characterised sample to correct for electron multiplier gain and variable elemental fractionation. U-series ages are then calculated from the measured 234 U/ 238 U and 230 Th/ 238 U. Here, we demonstrate the applicability of this technique by analysing a series of coral samples that have been previously characterised by conventional thermal ionisation mass spectrometry (TIMS) and solution-nebulisation multiple-collector ICP mass spectrometry. Excellent agreement is obtained with independent age estimates. The samples have varying degrees of preservation and ages ranging between <10 and 600 thousand years (ka). The age uncertainties achieved are ±1 ka at 3 ka and ±7 ka at 125 ka. Therefore the technique can be used to distinguish between major climatic sub-stages throughout the last 140,000 years spanning the last glacial-interglacial cycle, as recorded in coral reef terraces. The laser-ablation U-series results are independent of coral species. U-series age uncertainties are comparable to those attained using -spectrometry but require ∼10 4 times less sample and no chemical preparation.
Procedures for accurate determinations of ²³⁰Th-²³⁴U-²³⁸U isotope ratios using in situ laser ablation (LA) and micromill techniques for carbonates such as speleothems and corals are presented. For LA analyses we use a New Wave Research UP193HE laser and a new multiple ion counting detector system available for the ThermoFinnigan Neptune MC-ICPMS. Multiple ion counting increases the efficiency of low level ion beam collection by allowing simultaneous collection of all ion beams and also circumvents problems associated with unstable, transient beams. We present results of LA U-series measurements on a U-rich (10–168 µg/g) speleothem from Spannagel Cave (Austria) and of small sub-samples from the same speleothem section prepared using a New Wave Research MicroMill and processed through separation and purification solution chemistry. Using a secular equilibrium speleothem with U concentration of 0.25 µg/g we demonstrate that LA measurements yield accurate results on speleothem samples with less than 0.5 µg/g U. For LA results, we currently obtain ²³⁰Th/²³⁸U results with precisions of 2% (95% confidence limit) for single isotope ratio measurements on a U-rich sample. On the low U (0.25 µg/g U) sample we achieve a precision between 10 and 20% for single ²³⁰Th/²³⁸U measurements. Precisions of 1% and 5% (2 σm) can be achieved for high and low U concentration samples, respectively, using 10–15 measurements on coeval sub-samples. 0.4 mg micromill sub-samples of the low U concentration sample yield ²³⁰Th/²³⁸U isotope ratios with precisions of 2%. Accuracy of LA and micromill measurements is assessed by comparison with MC-ICPMS and TIMS measurements on larger sub-samples.
Laser ablation ICP-MS represents a promising new development for the acquisition of elemental and isotopic images from a variety of different materials. Compared to existing methods for imaging, it offers relatively rapid throughput, very wide dynamic range, a relatively clean mass spectrum, utility at a variety of scales (from mu m to cm) and multi-element/isotopic capability. Although developing rapidly in the biological sciences, the method has not yet seen widespread application to geological materials. This contribution documents some preliminary experiments aimed at understanding the fundamental aspects of elemental and isotopic image acquisition using laser ablation ICP-MS. In particular, we note that ablation cell designs must be optimised to promote rapid system response, in contrast to the signal-smoothing that is often preferred for simple spot analyses. Furthermore, experimental parameters must be carefully evaluated on a system-by-system basis to avoid the blurring effects of re-sampling phenomena. With careful attention to these details, and development of appropriate data processing software, laser ablation ICP-MS imaging has much to offer workers in the Earth and environmental sciences.
Christmas (Kiritimati) Island is an unusually large coral atoll, of which a large proportion of the surface is presently
subaerial. Extensive outcrops of in situ branching Acropora corals, together with Porites microatolls, Tridacna, and other shallow marine biota, indicate that the present low-lying area of interconnecting lakes in the island interior
formed as a reticulate lagoon. Radiocarbon dating indicates that these lagoonal reefs flourished between 4500 and 1500 radiocarbon
years BP, and surveying confirms that sea level was 0.5–1.0 m above present at that time, with subaerial exposure resulting
from Late Holocene emergence. Boreholes undertaken for a water resources survey of the island penetrated near-surface Pleistocene
limestones on the northern, southern, and eastern sides of the island. These are highly weathered and fractured, and although
aragonitic clasts are preserved, U-series dating indicates a Middle Pleistocene or older age. At one location flanking the
Bay of Wrecks, an outcrop of limestone, with an erosional notch, 1–2 m above present sea level, yielded a U-series age of
130 ka, and is interpreted as Last Interglacial in age. In contrast to previous interpretations which have suggested that
Christmas Island comprised an atoll superstructure that is entirely Holocene, or the layer-cake interpretation appropriate
for many mid-ocean atolls, Christmas Island appears to have had a form similar to its present in the Middle Pleistocene or
earlier. It has undergone karstification during lowstands. Interglacials, particularly the Last Interglacial and the Holocene,
appear to have resulted in only a minor veneer of coral over older limestone surfaces. Christmas Island is considered characteristic
of an atoll that has not experienced significant subsidence through the Late Quaternary.
This paper presents an overview of ANTARES operations, describing (1) technical upgrades that now allow routine 0.3–0.4% 14C precision for 1 mg carbon samples and 1% precision for 100 micrograms, (2) proficiency at 236U measurements in environmental samples, (3) new developments in AMS of platinum group elements and (4), some major application projects undertaken over the period of the past three years. Importantly, the facility is poised to enter into a new phase of expansion with the recent delivery of a 2 MV 14C tandem accelerator system from High Voltage Engineering (HVE) and a stable isotope ratio mass spectrometer from Micromass Inc. for combustion of organic samples and isotopic analysis.
The capabilities and potential applications of in situ dating of Quaternary materials using laser ablation-MC-ICPMS are explored. 234U/238U and 230Th/234U can be measured with precision sufficient for dating at a spatial resolution of 100 μm or better in samples that contain as a little as 1 ppm uranium. Moreover, U and Th concentrations and U-series isotope ratios can be continuously profiled to determine changes in age that occur with sample growth (e.g. in speleothems). These capabilities additionally permit the dating of bones, teeth and possibly molluscs, which are subject to post-mortem open-system behaviour of U-series isotopes, and can be employed to elucidate processes of U-series migration during weathering and diagenesis. A drawback of laser ablation-MC-ICPMS is that it cannot in general provide U-series age estimates with the high precision and accuracy of conventional TIMS or solution MC-ICPMS methods. However, sample preparation is straightforward, the amount of sample consumed negligible, and it can be used to rapidly characterise or screen and select samples from which more precise and accurate dates can be obtained using conventional methods. Given further instrumental developments and the establishment of suitable matrix-matched standards for carbonates and other materials, we foresee that laser ablation-MC-ICPMS will play an increasingly important role in Quaternary dating research.
Absolutely dated records of past sea-level change are extremely important for understanding the advance and retreat of the large ice sheets. When combined with other complementary climate archives and climate models, such records offer the potential to gain an improved understanding of Earth's natural climate cycles, providing a firmer basis for assessing the role of anthropogenic effects, such as greenhouse gas emissions, in modifying climate. The U-series dating of fossil coral reefs has been widely utilized to provide absolutely dated records of past sea-level change and numerous observations now exist for the past 130,000 years spanning the last glacial cycle. Despite this, controversies still exist regarding the exact timing and character of sea-level events within this time interval, and extending the sea-level history further back in time on the basis of robust and independent age constraints for older fossil reefs remains very elusive. This is primarily due to a progressive loss in the resolution of the U-series chronometer as one goes further back in time, coupled to a lack of well-preserved, dateable coral in older fossil reefs. To overcome these limitations, the primary challenges are three-fold. First, new analytical protocols are required to improve the resolution of the U-series chronometer. Enhanced analytical precision must be coupled to accuracy through continued refinement of the U-series decay constant determinations and via the implementation of rigorous inter-laboratory calibration exercises. Second, efforts should continue to be focussed on gaining an improved understanding of the mechanisms controlling open-system exchange of the U-series isotopes in fossil reef systems. This will allow the number of ‘reliable’ U-series observations to be extended. Third, alternative dateable archives of past sea-level change must continue to be emphasized to further complement the coral reef database. These limitations are discussed in the context of current developments that further advance the application of U-series chronology to older fossil reef systems formed prior to the last glacial cycle.
Although it was demonstrated 20 years ago that mass spectrometric U–Th methods are capable of high-precision dating of young corals, the use of this approach to decipher recent environmental, climatic and archaeological records is still restricted and its potential has not yet been widely recognised. U–Th methods are typically used to determine the ages of carbonate materials such as speleothem and coral. Dating young carbonates of this sort is challenging. Their extremely low 230Th content necessitates stricter instrumental and laboratory conditions than those required for dating older samples. Moreover, analyses must be corrected for the presence of non-radiogenic 230Th, which is proportionally far more significant than in older samples. Nevertheless, 2σ precisions of around ±1–10 years are readily achievable for pristine coral samples dating from the last 500 years. Although the range of analytical precisions for speleothems may vary more widely depending on U concentrations and levels of non-radiogenic 230Th correction, published studies reveal 2σ precisions of around ±10–80 years for typical speleothems. This paper demonstrates how the U–Th method may be applied to establish the causes of recent coral mortality, to determine the recurrence interval of extreme wave events, to investigate earthquake frequency and neotectonic uplift, to reconstruct recent climatic history, and to understand settlement patterns and sociopolitical changes in Polynesia prior to European contact.
Skeletons of the scleractinian coral Porites are widely utilized as archives of geochemical proxies for, among other things, sea surface temperature in paleoclimate studies. Here, we document live-collected Porites lobata specimens wherein as much as 60% of the most recently deposited skeletal aragonite, i.e., the part of the skeleton that projects into the layer of living polyps and thus is still in direct contact with living coral tissue, has been bored and replaced by calcite cement. Calcite and aragonite were identified in situ using Raman microspectroscopy. The boring-filling calcite cement has significantly different trace element ratios (Sr/Ca(mmol/mol) = 6.3 ± 1.4; Mg/Ca(mmol/mol) = 12.0 ± 5.1) than the host coral skeletal aragonite (Sr/Ca(mmol/mol) = 9.9 ± 1.3; Mg/Ca(mmol/mol) = 4.5 ± 2.3). The borings appear to have been excavated by a coccoid cyanobacterium that dissolved aragonite at one end and induced calcite precipitation at the other end as it migrated through the coral skeleton. Boring activity and cement precipitation occurred concomitantly with coral skeleton growth, thus replacing skeletal aragonite that was only days to weeks old in some cases. Although the cement-filled borings were observed in only ∼20% of sampled corals, their occurrence in some of the most recently produced coral skeleton suggests that any corallum could contain such cements, irrespective of the coral’s subsequent diagenetic history. In other words, pristine skeletal aragonite was not preserved in parts of some corals for even a few weeks. Although not well documented in coral skeletons, microbes that concomitantly excavate carbonate while inducing cement precipitation in their borings may be common in the ubiquitous communities that carry out micritization of carbonate grains in shallow carbonate settings. Thus, such phenomena may be widespread, and failure to recognize even very small quantities of early cement-filled borings in corals used for paleoclimate studies could compromise high resolution paleotemperature reconstructions. The inability to predict the occurrence of cement-filled borings in coralla combined with the difficulty in recognizing them on polished blocks highlights the great care that must be taken in vetting samples both for bulk and microanalysis of geochemistry.
We have re-determined the and half-lives to be 245,250±490 years (2σ) and 75,690±230 years (2σ), respectively. Using high precision thermal ionization mass spectrometric (TIMS) methods, we measured and atomic ratios in 4 different materials that were likely to have behaved as closed systems for 106 years or more: zircons with concordant , , and ages, Iceland Spar, Table Mountain Latite, and aliquots of a solution of Harwell uraninite (HU-1). We calibrated the TIMS multipliers using U-500, U and Th gravimetric standards, and U double spike. Consistent values for all measured materials and consistent values for all materials with the exception of our HU-1 solution support the secular equilibrium status. The new half-lives agree within error with previously determined values; however, errors in our values are generally smaller than those in the earlier determinations. Our half-life is about 3‰ higher than that commonly used in dating laboratories and our half-life is about 4‰ higher. ages calculated with the new half-lives are generally older than those calculated with the previously used half-lives. The difference in age, though, is small throughout the age range because our revised and half-lives are offset from earlier values in the same sense (both to higher values). In the case of dating materials older than 350 ka in laboratories that rely solely on gravimetric standardization procedures, use of our decay constants and their associated errors will considerably reduce the errors in age arising from uncertainty in the decay constants.
Coral proxy records of sea surface temperature (SST) and hydrological balance have become important tools in the field of tropical paleoclimatology. However, coral aragonite is subject to post-depositional diagenetic alteration in both the marine and vadose environments. To understand the impact of diagenesis on coral climate proxies, two mid-Holocene Porites corals from raised reefs on Muschu Island, Papua New Guinea, were analysed for Sr/Ca, δ18O, and δ13C along transects from 100% aragonite to 100% calcite. Thin-section analysis showed a characteristic vadose zone diagenetic sequence, beginning with leaching of primary aragonite and fine calcite overgrowths, transitional to calcite void filling and neomorphic, fabric selective replacement of the coral skeleton. Average calcite Sr/Ca and δ18O values were lower than those for coral aragonite, decreasing from 0.0088 to 0.0021 and −5.2 to −8.1‰, respectively. The relatively low Sr/Ca of the secondary calcite reflects the Sr/Ca of dissolving phases and the large difference between aragonite and calcite Sr/Ca partition coefficients. The decrease in δ18O of calcite relative to coral aragonite is a function of the δ18O of precipitation. Carbon-isotope ratios in secondary calcite are variable, though generally lower relative to aragonite, ranging from −2.5 to −10.4%. The variability of δ13C in secondary calcite reflects the amount of soil CO2 contributing 13C-depleted carbon to the precipitating fluids. Diagenesis has a greater impact on Sr/Ca than on δ18O; the calcite compositions reported here convert to SST anomalies of 115°C and 14°C, respectively. Based on calcite Sr/Ca compositions in this study and in the literature, the sensitivity of coral Sr/Ca-SST to vadose-zone calcite diagenesis is 1.1 to 1.5°C per percent calcite. In contrast, the rate of change in coral δ18O-SST is relatively small (−0.2 to 0.2°C per percent calcite). We show that large shifts in δ18O, reported for mid-Holocene and Last Interglacial corals with warmer than present Sr/Ca-SSTs, cannot be caused by calcite diagenesis. Low-level calcite diagenesis can be detected through X-ray diffraction techniques, thin section analysis, and high spatial resolution sampling of the coral skeleton and thus should not impede the production of accurate coral paleoclimate reconstructions.
Laser ablation ICP–MS is a uniquely powerful technique for the spatially resolved analysis of geological materials (e.g., Sinclair et al. 1998, Treble et al. 2005, Desmarchelier et al. 2006, Woodhead et al. 2007), but has been hindered in this application by the absence of suitably flexible tools for data processing and visualization. Here we describe and release Iolite, our in-house application which has been purpose-developed for spatially resolved LA–ICP–MS.
Understanding the influence of climatic and non-climatic factors on geochemical signals in corals is critical for assessing coral-derived records of tropical climate variability. Porites microatolls form large, disk-shaped colonies constrained in their upward growth by exposure at or close to mean spring low water level, and occur on Indo-Pacific reefs. Microatolls appear suitable for paleoclimate reconstruction, however the systematics of the microatoll chemistry-climate relationship are yet to be characterized. In this study, the δ18O signal in Porites microatolls from well-flushed reef flats on Kiritimati (Christmas) Island, central Pacific was investigated for intra-coral (growth aspect and extension rate effects) and between-coral effects, and to explore the climate signal contained within their skeletons. Samples for δ18O analysis were taken from six individual transects from different positions within Porites microatoll XM22. The results show that: (1) the average standard deviation for the mean δ18O values of transects that represent the same time periods is 0.03‰, and is within measurement error for a single analysis (0.04‰); (2) the average standard deviation for time-equivalent, near-monthly samples along the transects within the same microatoll is 0.07‰ and; (3) comparison of the average δ18O values of records for different microatolls from across Kiritimati Island show only a small between-coral differences of 0.04‰ and 0.11‰ for different time periods. These differences in mean δ18O are within the range for intra- and inter-colony differences in seasonal and interannual δ18O reported for dome-shaped Porites. Based on these results, a stacked microatoll δ18O record was constructed for the period 1978-2007 for comparison with published coral δ18O records for nearby dome-shaped Porites. There is a systematic offset between the two types of records, which is probably due to variations in δ18O seawater across Kiritimati Island. Despite the offset, all records show similar amplitudes for the seasonal-cycle of δ18O, and there is a strong correlation (r=-0.71) between microatoll δ18O and local sea surface temperature (SST). The δ18O-SST slope relationship for microatolls is -0.15‰/°C, very similar to that reported for fast-growing domed corals (-0.18‰ to -0.22‰/°C). Statistical analysis of the stacked microatoll δ18O record shows that it is correlated with both local and large-scale climate variables (primarily SST) at semiannual, annual and interannual timescales. Our results show that the signal reproducibility and fidelity of skeletal δ18O in coral microatolls is comparable to that observed for more conventional coral growth forms. Longer-lived, and fossil, Porites microatolls, where they have grown in suitably flushed environments, are likely to contain δ18O signals that can significantly extend instrumental records of tropical climate variability.
U-series dating of fossil reef corals is a well established and widely applied technique in paleoclimate research. Many fossil corals, however, show evidence for post-depositional diagenetic alteration, and it is generally accepted that the accuracy of U-series coral ages is more limited due to coral diagenesis than analytical precision. In recent years, three models have been published that try to correct the effects of diagenesis and allow the calculation of model ages [Thompson W. G., Spiegelmann M. W., Goldstein S. L., and Speed R. C. (2003) An open-system model for U-series age determinations of fossil corals. Earth and Planetary Science Letters 210, 365–381; Villemant B., and Feuillet N. (2003) Dating open systems by the ²³⁸U–²³⁴U–²³⁰Th method: application to Quaternary reef terraces. Earth and Planetary Science Letters 210, 105–118; Scholz D., Mangini A., and Felis T. (2004) U-series dating of diagenetically altered fossil reef corals. Earth and Planetary Science Letters 218, 163–178].
Using the parallel ion-counting capability of an appropriately configured multi-collector ICP-MS it is possible to simultaneously measure the activity ratios Th-230/U-234 and U-234/U-238 in a single solution. A method has been developed which fully internalises the measurement of elemental fractionation, instrument mass bias and ion counter gain, allowing on-line calculation of U/Th ages on a cycle-by-cycle basis during data acquisition. Because of the high efficiency of this technique very small sample sizes can be used, with permil-level precision possible on both Th-230/U-238 and U-234/U-238 from analyses of a few tens of nanograms of uranium. As U and Th fractions do not need to be collected and analysed separately sample throughput is very high, using a simple TRU-Spec ion exchange resin procedure. Repeat analyses of Harwell Uraninite, NIST SRM 960 and two carbonate age standards show excellent agreement with results from other laboratories.
The reliabilities of the radiocarbon and uranium series inequilibrium methods of absolute age determination as applied to lacustrine carbonates have been evaluated. Fifty samples of gastropods, chara, tufa, marl, and ostracods from the Pleistocene deposits of Lakes Lahontan and Bonneville were analyzed for C , Us, U , Th m, Th ø, and Ra ø. The results indicate that samples in which Ra ø is at equilibrium with Th 2ø and where the Thconcen- tration is not unusually large, satisfactory agreement between the two methods generally exists. Whereas a common thorium correction is required, evidence is given that for the basins studied the Thø-Th TM ratio has a nearly constant value of 1.70. Where disagreement between the two methods occurs, the lower age appears to be in error. The anomalies appear to re- fleet U addition (low Thage) or contamination with young carbon (low Cage). Samples beyond the range of radiocarbon yield Th ø ages consistent with stratigraphic information, offering the possibility of extending the absolute chronology of the fluctuation in the size of these lakes back to 200,000 years ago. Independent age estimates based on the disappearance of the excess LIincorporated into these samples are clearly invalid. Samples with Th ø ages of more than 100,000 years show excesses of Llcomparable to or even greater than those for samples less than 20,000 years old. Support for Attree's (1962) estimate of 75,200 years for the Th ø half-life has been obtained (opposed to the previously accepted estimate of 80,000 years). Introduction. The primary objective of this work is to provide a means for evaluating the reliability o,f radiocarbon and uranium series disequilibrium ages for carbonate samples. To this end 50 samples (marl, ostracod, gastropods, chara, and tufa) from the deposits of the Great Basin pluvial lakes Lahontan and Bonneville were analyzed. Whereas these samples were so selected that the majority fall within the 10,000 to 20,000 year age range where cross-checks can be made with the maximum sensitivity, in- cluded also were samples less than 5000 years old and samples more than 30,000 years old. The principle of the C ' method is by now fa- miliar to almost all earth scientists. Based on the decay of the 5730-year half-life (Godwin, 1962), cosmic-ray-produced isotope of carbon, it is routinely applied to the age determination of samples in the 0- to 25,000-year range. Whereas the analytical capability has been ex- tended to 65,000 years (DeVries, 1959), the question of contamination by recent carbon has hindered its application in this extended range.
. Central equatorial Pacific sea surface temperature (SST) anomaly is a critical predictor of basin-wide oceanographic and atmospheric effects of the El Ni~no-Southern Oscillation (ENSO) phenomenon. We employ two geochemical thermometers measured on coralline aragonite to reconstruct an independent proxy-based measure of central equatorial Pacific sea surface temperature anomaly. In addition, we assess the observational error associated with extraction of large-scale SST anomalies from ffi 18 O and Sr/Ca measurements. On the basis of paired data for the 1981--1987 period, RMS error for the estimation of SST from Kiritimati coral ffi 18 O is about 0.4 o C, assuming no seawater ffi 18 O influence; for Sr/Ca, the observational error is about 0.5 o C. Singular spectrum analysis of the ffi 18 O time series suggests that 1/3 of the variance is explained by SST anomaly and that this variance may be separated from other signals in the frequency domain. The interannual ...
An improved in situ and satellite SST analysis for climate How precise are U-series coral ages? Uranium-series dating of fossil coral reefs: extending the sea-level record beyond the last glacial cycle
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Spatial distribution of coral ages shown in Fig. 5. A) All coral ages for Kiritimati Island. B) Late Holocene ages for one reef complex along the Carver Way, C) Ages of Teraaka Lagoon microatolls
- Fig
Fig. 6. Spatial distribution of coral ages shown in Fig. 5. A) All coral ages for Kiritimati Island. B) Late Holocene ages for one reef complex along the Carver Way, C) Ages of Teraaka
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