![Locations of sites used by Broecker [1986] to compile mean G/H alSO changes for single species of planktonic foraminifera. The map has been modified from that in Broecker [1986]. The values represent the](https://www.researchgate.net/profile/Lowell-Stott/publication/241060985/figure/fig1/AS:668976807944195@1536507908063/Locations-of-sites-used-by-Broecker-1986-to-compile-mean-G-H-alSO-changes-for-single_Q320.jpg)
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Reassessment of foraminiferal-based tropical sea surface ??18O paleotemperatures
Abstract and Figures
The possibility exists that the magnitude of Glacial/Holocene δ18O change in the tropical oceans was previously under-estimated due to the dampening effect of bioturbation on bulk foraminiferal δ18O measurements. We have investigated this possibility by isotopically analyzing suites of individual planktonic foraminifera from box core samples from the equatorial Atlantic. We present oxygen isotopic results for individual specimens of two surface-dwelling planktonic foraminifera Globigerinoides ruber (white) and Globigerinoides sacculifer (no final sac) from glacial-age box core samples from seven sites located between about 15°N and 15°S in the tropical Atlantic. These data are used to constrain the mimimum glacial temperatures recorded by tropical surface-dwelling planktonic foraminifera during the last glacial maximum. The individual foraminiferal δ18O temperatures reveal minimum sea surface temperatures (SSTs) that ranged from 1°C colder to as much as 5° to 6°C colder than present-day summer SSTs after correcting for an “ice volume effect”. However, these minimum temperatures represent less than 10% of the foraminiferal population measured from glacial horizons. Approximately 80% of the glacial specimens record temperatures that were within 2°C of the mean Holocene SST. We conclude that either the tropical Atlantic was not pervasively colder during the Glacial or that there was a systematic negative shift in the isotopic composition of the surface waters that offset a more signficant temperature change.
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... Our results reveal a high degree of intraspecific variation in both δ 13 C and δ 18 O values of R. fructicosa, P. palpebra, and P. acervulinoides and somewhat less variation in P. multicamerata (Figs. 5 and 6; Table 2), but the variation observed here is generally similar to that observed in previous studies of living and fossil foraminifera. Billups and Spero (1996) and Stott and Tang (1996) document δ 13 C and δ 18 O intraspecific variability in Orbulina universa, Globigerinoides sacculifer, and Globigerinoides ruber (white) using single specimen analyses. Orbulina universa, collected from Equatorial Atlantic cores, exhibits variation between individuals from the same size interval up to approximately 2.0‰ for both δ 13 C and δ 18 O values (Billups and Spero, 1996). ...
... Orbulina universa, collected from Equatorial Atlantic cores, exhibits variation between individuals from the same size interval up to approximately 2.0‰ for both δ 13 C and δ 18 O values (Billups and Spero, 1996). Similarly, Stott and Tang (1996) report G. sacculifer individuals to vary by up to approximately 2.0 and 1.7‰ for δ 13 C and δ 18 O values, respectively; and, in G. ruber (white) δ 13 C and δ 18 O values vary by up to 2.0 and 2.5‰. D'Hondt and Arthur (1995) report intraspecific δ 13 C variation from specimens of a single size interval and sample of generally 0.0 to 0.3‰, or slightly higher, for a total of 30 Maastrichtian species from Sample 390A-13-1, 99-100 cm based on multi-specimen whole-test analyses containing between five and fifteen specimens. ...
The serial test dissection and sieve fraction methods for determining the pattern of size-related change in oxygen and carbon isotopic ratios are compared using four Late Cretaceous planktic foraminifer species (Racemiguembelina fructicosa, Planoglobulina acervulinoides, Planoglobulina multicamerata, and Pseudoguembelina palpebra) from a subtropical site in the North Atlantic (DSDP Hole 390A). Despite the extra labor required, we identify several clear advantages of the dissection method, including: (1) it provides a means of obtaining size-dependent changes in isotopic signatures that are unequivocally ontogenetic, whereas isotopic variation observed from sieve-separated size fractions could be ontogenetic or ecotypic; (2) the taxonomic identity of smaller sized specimens using the dissection method is unequivocal, whereas species identification is increasingly ambiguous in smaller size fractions using the sieve method; (3) it reveals a greater total range and a greater complexity in the pattern of ontogenetic change in stable isotopic values, whereas the sieve method averages the isotopic signal across the entire ontogenetic range preserved within the whole tests that are used. Our results from serial dissections demonstrate that among the species analyzed, R. fructicosa and P. acervulinoides yield relatively negative adult delta18O values, a large size-related change in delta13C values (1.32 and 2.05?, respectively), and virtually no correlation between size-related delta13C and delta18O values. On this basis we suggest that these were photosymbiotic species that inhabited relatively shallow surface waters. Evidence for photosymbiosis is not as compelling for P. palpebra, as this species yields a 1.06? shift in delta13C and relatively negative delta18O values in adult chambers, but much stronger correlation between size-related delta13C and delta18O values (r square = 0.4) than in R. fructicosa and P. acervulinoides. Planoglobulina multicamerata yields the most positive adult delta18O values of the species studied, a strong covariance between size-related delta13C and delta18O values (r square = 0.77), and a 0.97? shift in d13C composition during ontogeny. We conclude that this species lacked photosymbionts and migrated to a deeper surface water paleohabitat as it increased in size. Single specimen analyses of tightly constrained size fractions reveal a high degree of intraspecific variation. Delta13C and delta18O values vary by up to 0.70 and 0.28? in R. fructicosa, 1.41 and 0.80? in P. acervulinoides, 0.66 and 0.82? in P. palpebra, and 0.18 and 0.33? in P. multicamerata, respectively. Such a range of isotopic variation has been observed in modern day planktic foraminifer assemblages, and likely results from growth of individuals during different phases of the seasonal cycle and/or the kinetic effect of intraspecific variation in shell calcification rates. As suggested by other investigators, large sample sizes should be analyzed to provide the most reliable correlation of stable isotopic stratigraphic records.
... For instance, the seasonality of species may play an important role in the observed stable isotope composition of a temporally integrated sediment sample (G. M. Ganssen et al., 2011;King & Howard, 2005), as does the intermixing of sedimented shells due to bioturbation (Billups & Spero, 1996;Stott & Tang, 1996). The interpretation of geochemical compositions of shells may even be further complicated by intra-shell variations of stable isotope compositions (Vetter et al., 2013) and ontogenetic changes in shell geochemistry (Rohling et al., 2004;Shackleton et al., 1985). ...
Planktonic Foraminifera are widely used for environmental reconstructions through measurements of their shell's geochemical characteristics, including its stable oxygen and carbon isotope composition. Using these parameters as unbiased proxies requires a firm knowledge of all potential confounding factors influencing foraminiferal shell geochemistry. One such parameter is the shell calcification intensity (shell weight normalized for shell size) that may influence the shell δ18O value either bioenergetically (by reducing energy available and required for equilibrium isotope fractionation during faster calcification) or kinetically (by influencing calcification depth through the shell's density contrast with seawater). Specimens from the Globigerinoides ruber/elongatus compound from a sediment trap in the North Atlantic have been used to quantify the influence of shell calcification intensity on shell δ18O values. Shell calcification intensity was found to have a significant effect on the shell stable oxygen isotope composition in all species. Through model fitting, it is suggested that the effect size may be in a range of 1 to 2‰ (depending on species, depth migration, and local oceanographic conditions). We show that the confounding effect of shell calcification intensity on stable oxygen isotope composition can be of importance, depending on the anticipated precision of the derived reconstructions. A framework is provided to quantify this effect in future studies.
... This was linked to varying water depths of calcification for different specimens and used by Schiffelbein and Hills (1984) to estimate the uncertainty of analyses commonly performed on pooled specimens. Many studies then investigated additional factors that can impact variations in single shell δ 18 O, including bioturbation (Billups & Spero, 1996;Stott & Tang, 1996), interspecific shell ontogeny (Spero & Williams, 1990), photosymbiont influences (Houston et al., 1999;Spero & Lea, 1993), seasonal salinity changes (Spero & Williams, 1990;Tang & Stott, 1993), discrepancies between living and recently fossilized individuals (Waelbroeck et al., 2005), or genetic differences within the same morphospecies (Morard et al., 2016;Sadekov et al., 2016). More recently, it was suggested that this variation within the same sample can be linked to shorter timescale climate variability related to El Niño-Southern Oscillation (ENSO; Khider et al., 2011;Koutavas et al., 2006;Leduc et al., 2009;Rustic et al., 2015; ©2019. ...
Foraminifera are commonly used in paleoclimate reconstructions as they occur throughout the world's oceans and are often abundantly preserved in the sediments. Traditionally, foraminifera‐based proxies like δ¹⁸O and Mg/Ca are analyzed on pooled specimens of a single species. Analysis of single specimens of foraminifera allows reconstructing climate variability on timescales related to El Niño–Southern Oscillation or seasonality. However, quantitative calibrations between the statistics of individual foraminifera analyses (IFA) and climate variability are still missing. We performed Mg/Ca and δ¹⁸O measurements on single specimens from core top sediments from different settings to better understand the signal recorded by individual foraminifera. We used three species of planktic foraminifera (Globigerinoides ruber (s.s.), T. sacculifer, and N. dutertrei) from the Indo‐Pacific Warm Pool and one species (G. ruber (pink)) from the Gulf of Mexico. Mean values for the different species of Mg/Ca versus calculated δ¹⁸O temperatures agree with published calibration equations. IFA statistics (both mean and standard deviation) of Mg/Ca and δ¹⁸O between the different sites show a strong relationship indicating that both proxies are influenced by a common factor, most likely temperature variations during calcification. This strongly supports the use of IFA to reconstruct climate variability. However, our combined IFA data for the different species only show a weak relationship to seasonal and interannual temperature changes, especially when seasonal variability increases at a location. This suggests that the season and depth habitat of the foraminifera strongly affect IFA variability, such that ecology needs to be considered when reconstructing past climate variability.
... However, other data seem to support the CLIMAP values, for example, temperature calculations applying the alkenone method (e.g. Schneider et al. 1995), and re-evaluations of foraminifersl statistics (Prell 1985) and foraminifersl δ 18 O values (Stott and Tang 1996). In any case, the geographic patterns are more complicated than those presented by the CLIMAP group (Sarnthein et al. 1998;Gersonde et al. 1998). ...
Paleoceanographic proxies provide infonnation for reconstructions of the past, including climate changes, global and regional oceanography, and the cycles of biochemical components in the ocean. These prox ies are measurable descriptors for desired but unobservable environmental variables such as tempera ture, salinity, primary productivity, nutrient content, or surface-water carbon dioxide concentrations. The proxies are employed in a manner analogous to oceanographic methods. The water masses are first characterized according to their specific physical and chemical properties, and then related to particular assemblages of certain organisms or to particular element or isotope distributions. We have a long-standing series of proven proxies available. Marine microfossil assemblages, for instance, are employed to reconstruct surface-water temperatures. The calcareous shells of planktonic and benthic microorgan isms contain a wealth of paleoceanographic information in their isotopic and elemental compositions. Stable oxygen isotope measurements are used to detennine ice volume, and MglCa ratios are related to water temperatures, to cite a few examples. Organic material may also provide valuable infonnation, e. g. , about past productivity conditions. Studying the stable carbon isotope composition of bulk organic matter or individual marine organic components may provide a measure of past surface-water CO 2 conditions within the bounds of certain assumptions. Within the scope of paleoceanographic investigations, the existing proxies are continuously evolving and improving, while new proxies are being studied and developed. The methodology is improved by analysis of samples from the water column and surface sediments, and through laboratory experiments.
... However, other data seem to support the CLIMAP values, for example, temperature calculations applying the alkenone method (e.g. Schneider et al. 1995), and re-evaluations of foraminifersl statistics (Prell 1985) and foraminifersl δ 18 O values (Stott and Tang 1996). In any case, the geographic patterns are more complicated than those presented by the CLIMAP group (Sarnthein et al. 1998;Gersonde et al. 1998). ...
... However, other data secni to support the CLIMAP values, for example, temperature calculations applying tlie V. alkenone method (e.g. Schneider et al. 1995), and re-evaluations of foraminifersl stalislics (Prell 1985) and foraminifersl 5"'0 values(Stottand Tang 1996). In any case, the geographic patterns arc more complicated than Ihosc presented by the CLIMAP group (Samthein et al. 1998; Gersonde et al. 1998). ...
The reconstruction of ocean history employs a large variety of methods with origins in the biological, chemical, and physical sciences, and uses modern statistical techniques for the interpretation of extensive and complex data sets. Various sediment properties deliver useful information for reconstructing environmental parameters. Those properties that have a close relationship to environmental parameters are called “proxy variables” (“proxies” for short). Proxies are measurable descriptors for desired (but unobservable) variables. Surface water temperature is probably the most important parameter for describing the conditions of past oceans and is crucial for climate modelling. Proxies for temperature are: abundance of microfossils dwelling in surface waters, oxygen isotope composition of planktic foraminifera, the ratio of magnesium or strontium to calcium in calcareous shells or the ratio of certain organic molecules (e.g. alkenones produced by coccolithophorids). Surface water salinity, which is important in modelling of ocean circulation, is much more difficult to reconstruct. At present there is no established method for a direct determination of this parameter. Measurements associated with the paleochemistry of bottom waters to reconstruct bottom water age and flow are made on benthic foraminifera, ostracodes, and deep-sea corals. Important geochemical tracers are δ13C and Cd/Ca ratios. When using benthic foraminifera, knowledge of the sediment depth habitat of species is crucial. Reconstructions of productivity patterns are of great interest because of important links to current patterns, mixing of water masses, wind, the global carbon cycle, and biogeography. Productivity is reflected in the flux of carbon into the sediment. There are a number of fluxes other than those of organic carbon that can be useful in assessing productivity fluctuations. Among others, carbonate and opal flux have been used, as well as particulate barite. Furthermore, microfossil assemblages contain clues to the intensity of production as some species occur preferentially in high-productivity regions while others avoid these. One marker for the fertility of sub-surface waters (that is, nutrient availability) is the carbon isotope ratio within that water (I3C/12C, expressed as δ13C). Carbon isotope ratios in today’s ocean are negatively correlated with nitrate and phosphate contents. Another tracer of phosphate content in ocean waters is the Cd/Ca ratio. The correlation between this ratio and phosphate concentrations is quite well documented. A rather new development to obtain clues on ocean fertility (nitrate utilization) is the analysis of the 15N/14N ratio in organic matter. The fractionation dynamics are analogous to those of carbon isotopes. These various ratios are captured within the organisms growing within the tagged water. A number of reconstructions of the partial pressure of CO2 have been attempted using δ13C differences between planktic and benthic foraminifera and δ13C values of bulk organic material or individual organic components. To define the carbon system in sea water, two elements of the system have to be known in addition to temperature. These can be any combination of total CO2, alkalinity, or pH. To reconstruct pH, the boron isotope composition of carbonates has been used. Ba patterns have been used to infer the distribution of alkalinity in past oceans. Information relating to atmospheric circulation and climate is transported to the ocean by wind or rivers, in the form of minerals or as plant and animal remains. The most useful tracers in this respect are silt-sized particles and pollen.
... conglobatus have revealed a large intraspecific range of δ 13 C and δ 18 O within assemblages from the same intervals in deep sea cores as well as in sediment traps (Killingley et al. 1981;Schiffelbein and Hills 1984;Oba 1990;Billups and Spero 1995;Billups and Spero 1996;Stott and Tang 1996) (Fig. 3). Although δ 18 O variability can be explained by differences in seasonal or mixed layer and upper thermocline calcification temperatures and salinity (Spero and Williams 1989;Spero and Williams 1990;Schweitzer and Lohmann 1991;Ravelo and Fairbanks 1992;Wolff et al. 1998), it is more difficult to explain the large range of δ 13 C given that δ 13 C ∑CO2 is relatively constant throughout the year and across the habitat range. ...
Application of planktonic foraminifera to micropaleontological, paleoceanographic and paleoclimatic research has enjoyed more than 150 years of activity. During the first century, foraminifera were used primarily for biostratigraphic analysis. Although fossil shells were recognized from beach sands and deep sea sediments as early as 1826 (d'Orbigny, 1826; Parker and Jones, 1865), it wasn't until Owen (1867) and the scientific results of the Challenger expedition (Brady, 1884) that the planktonic life habitat of these marine protozoans was clearly established. By the early 20th century, researchers were studying the biology of planktonic foraminifera at the cellular level (Rhumbler, 1901; Le Calvez, 1936), and linking their distributional patterns to regions of the ocean surface (Lohmann, 1920; Schott, 1935).
“VALUE,” WROTE John Ruskin (1862), “is the life-giving power of anything; cost, the quantity of labor required to produce it; price, the quantity of labor which its possessor will take in exchange for it”. These distinctions see obvious enough. Yet in the bustle of everyday modern life in a highly materialistic society, it seems increasingly difficult to separate “value” from “cost” and “price”. How do we — as individuals, groups, or a society — assign a value to something? What, in fact, do we value? A glance at television or a popular magazine offers some clues. We value things economic, those associated with “making a living”, with solving the everyday problems of making one's way in the world. We value things that enhance our position or status in society, or that make our lives easier or give us pleasure or diversion. We value things that make our lives meaningful. We do not tend to necessarily value what's good for us, at least not simply because someone tells us it is.
Foraminiferal tests are a common component of many marine sediments. The oxygen isotope ratio (δ 18 O) of test calcite is frequently used to reconstruct aspects of their life environment. The δ 18 O depends mainly on the isotope ratio of the water it is precipitated from, the temperature of calcifica-tion, and, to a lesser extent, the carbonate ion concentration. Foraminifera and other organisms can potentially preserve their original isotope ratio for many millions of years, although diagenetic processes can alter the ratios. Work on oxygen isotope ratios of foraminifera was instrumental in the discovery of the orbital theory of the ice ages and continues to be widely used in the study of rapid climate change. Compilations of deep sea benthic foraminifer oxygen isotopes have revealed the long history of global climate change over the past 100 million years. Planktonic foraminifer oxygen isotopes are used to investigate the history of past sea surface temperatures, revealing the extent of past 'greenhouse' warming and global sea surface temperatures.
El Niño-Southern Oscillation (ENSO) is a major source of global interannual variability, but its response to climate change is uncertain. Paleoclimate records from the Last Glacial Maximum (LGM) provide insight into ENSO behavior when global boundary conditions (ice sheet extent, atmospheric partial pressure of CO2) were different from those today. In this work, we reconstruct LGM temperature variability at equatorial Pacific sites using measurements of individual planktonic foraminifera shells. A deep equatorial thermocline altered the dynamics in the eastern equatorial cold tongue, resulting in reduced ENSO variability during the LGM compared to the Late Holocene. These results suggest that ENSO was not tied directly to the east-west temperature gradient, as previously suggested. Rather, the thermocline of the eastern equatorial Pacific played a decisive role in the ENSO response to LGM climate.
Copyright © 2015, American Association for the Advancement of Science.
Biological carbonates are built largely from COâ, which diffuses across the skeletogenic membrane and reacts to form HCOââ». Kinetic discrimination against the heavy isotopes ¹â¸O and ¹³C during COâ hydration and hydroxylation apparently causes most of the isotopic disequilibrium observed in biological carbonates. These kinetic isotope effects are expressed when the extracytosolic calcifying solution is thin and alkaline, and HCOââ» precipitates fairly rapidly as CaCOâ. In vitro simulation of the calcifying environment produced heavy isotope depletions qualitatively similar to, but somewhat more extreme than, those seen in biological carbonates. Isotopic equilibration during biological calcification occurs through COâ exchange across the calcifying membrane and by admixture ambient waters (containing HCOââ») into the calcifying fluids. Both mechanisms tend to produce linear correlations between skeletal δ¹³C and δ¹â¸O.
Carbon isotopic records of nutrient-depleted surface water place constraints on the past fertility of the oceans and on past atmospheric pCO2 levels. The best records of nutrient-depleted δ13C are obtained from planktonic foraminifera living in the thick mixed layers of the western equatorial and tropical Atlantic Ocean. We have produced a composite, stacked Globigerinoides sacculifer δ13C record from the equatorial Atlantic, which exhibits significant spectral power at the 100,000- and 41,000-year Milankovitch periods, but no power at the 23,000-year period. Similar to the record presented by Shackleton and Pisias [1985], surface-deep ocean Δδ13C produced with the G. sacculifer record leads the δ18O ice volume record. However, the glacial-interglacial amplitudes of Δδ13C differ between our record and Shackleton and Pisias [1985] record. Although large changes in Δδ13C occur in the equatorial Atlantic during early stages of the last three glacial cycles, surface-deep Δδ13C at glacial maxima (18O stage 2, late stage 6, and late stage 8) was only about 0.2‰ greater than during the subsequent interglacial. Our results imply that nutrient-driven pCO2 changes account for about one third of the pCO2 decrease observed in ice cores, and consequently, Δδ13C should not be used as a proxy pCO2 index. Enough variance in the ice core pCO2 records remains to be explained that conclusions about pCO2 and ice volume phase relationships should also be reexamined. As much as 40 ppm pCO2 change still has not been accounted for by models of past physics and chemistry of the ocean.
Biological carbonates are built largely from CO 2 , which diffuses across the skeletogenic membrane and reacts to form HCO - 3 . Kinetic discrimination against the heavy isotopes 18 O and 13 C during CO 2 hydration and hydroxylation apparently causes most of the isotopic disequilibrium observed in biological carbonates. These kinetic isotope effects are expressed when the extracytosolic calcifying solution is thin and alkaline, and HCO - 3 precipitates fairly rapidly as CaCO 3 . In vitro simulation of the calcifying environment produced heavy isotope depletions qualitatively similar to, but somewhat more extreme than, those seen in biological carbonates. Isotopic equilibration during biological calcification occurs through CO 2 exchange across the calcifying membrane and by admixture ambient waters (containing HCO - 3 ) into the calcifying fluids. Both mechanisms tend to produce linear correlations between skeletal 13 C and 18 O.
Small live individuals of Globigerinoides sacculifer which were cultured in the laboratory reached maturity and produced garnets. Fifty to ninety percent of their skeleton weight was deposited under controlled water temperature (14° to 30°C) and water isotopic composition, and a correction was made to account for the isotopic composition of the original skeleton using control groups. Comparison of. the actual growth temperatures with the calculated temperature based on paleotemperature equations for inorganic CaCO 3 indicate that the foraminifera precipitate their CaCO 3 in isotopic equilibrium. Comparison with equations developed for biogenic calcite give a similarly good fit. Linear regression with (1965) equation yields: where t is the actual growth temperature and Is the calculated paleotemperature. The intercept and the slope of this linear equation show that the familiar paleotemperature equation developed originally for mollusca carbonate, is equally applicable for the planktonic foraminifer G. sacculifer . Second order regression of the culture temperature and the delta difference ( 18 Oc - 18 Ow ) yield a correlation coefficient of r = 0.95: and 18 Ow are the estimated temperature, the isotopic composition of the shell carbonate and the sea water respectively. A possible cause for nonequilibnum isotopic compositions reported earlier for living planktonic foraminifera is the improper combustion of the organic matter.
A six-year series of sediment-trap samples from the deep Sargasso Sea provides data on seasonal stable-isotope and flux variations of the tests of planktonic foraminifera. -from Author
The order of succession in the course of the calendar year was Pulleniatina obliquiloculata, Globorotalia truncatulinoides, Globigerina bullolides, Globorotalia inflata, Globorotalia hirsuta, Globigerinita glutinata, Neogloboquadrina dutertrei, Globorotalia carassaformis, Globigerinoides sacculifer, Globigerinoides ruber (pink, var.), Globigerina rubescens, and Globigerinoides conglobatus. All 12 species occurred in significant numbers for 2-6 months each year and were very rare or completely absent during the remaining months. For paleoceanographic purposes, different species can be used as indicators of hydrographic conditions (mainly temperature). -from Authors
A project to objectively analyze historical ocean temperature, salinity,
oxygen, and percent oxygen saturation data for the world ocean has
recently been completed at the National Oceanic and Atmospheric
Administration's (NOAA) Geophysical Fluid Dynamics Laboratory,
Princeton, New Jersey. The results of the project are being made
available through distribution of the Climatological Atlas of the World
Ocean (NOAA Professional Paper No. 13), and through distribution of
magnetic tapes containing the objective analyses.The sources of data
used in the project were the Station Data, Mechanical Bathythermograph,
and Expendable Bathythermograph files of the National Oceanographic Data
Center (NODC) in Washington, D.C., updated through 1977-1978. The raw
data were subjected to quality control procedures, averaged by
one-degree squares, and then used as input to an objective analysis
procedure that fills in one-degree squares containing no data and
smooths the results. Due to the lack of synoptic observations for the
world ocean, the historical data are composited by annual, seasonal, and
(for temperature) monthly periods.
Eight box cores from the tropical Atlantic were studied in detail with regard to foraminiferal oxygen isotopes, radiocarbon, and Globorotalia menardii abundance. A standard Atlantic oxygen-isotope signal was reconstructed for the last 20,000 yr. It is quite similar to the west-equatorial Pacific signal published previously. Deglaciation is seen to occur in two steps which are separated by a pause. Onset of deglaciation is after 15,000 yr B.P. The pause is centered between 11,000 and 12,000 yr B.P., but may be correlative with the Younger Dryas (10,500 yr B.P.) if allowance is made for a scale shift due to mixing processes on the sea floor. Step 2 is centered near 10,000 yr B.P. and is followed by a brief excursion toward light oxygen values. This excursion (the M event) may correlate with the Gulf of Mexico meltwater spike.
The fact that frequencies measured in climate records are the same as those predicted by the astronomical theory of climate change is undisputed (Hays, Imbrie & Shackleton 1976). However, the mechanisms by which these small changes in seasonal insolation are amplified into glacial cycles remain a fundamental mystery of the Earth’s climate system. The Barbados postglacial sea-level record is sufficiently detailed to resolve, for the first time, the rates as well as the magnitude of continental ice melting (Fairbanks 1989, 1990) (Fig 30.1A). The Barbados meltwater discharge curve is not smooth but pulsed, with peaks at 12,000 14C years1 and 9500 14C years (Fig 30.1B). Sea level rose more than 24 m during each of these pulses, with annual rates of sea-level rise exceeding 3 cm/yr. These enormous pulses must mark the ice-sheet response to a change in one or more of the climate amplifiers (eg, greenhouse gases and oceanic heat transports). The suspected amplifiers have different time constants and different regional sensitivities. Therefore, the discovery of both the pulsed deglaciation itself and the geographic origin of the pulses may help pinpoint the factors responsible for the timing of the large sea-level change associated with the last deglaciation, as well as the cause of previous “terminations” which recur every 100,000 14C years during the late Pleistocene Epoch (Broecker 1984).