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Improving SST reconstructions from coral Sr/Ca records: Multiple corals from Tahiti (French Polynesia)
Abstract and Figures
We reconstruct SST from coral Sr/Ca ratios measured at three coral cores taken from the lagoon of Tahiti (French Polynesia).
Two coral cores were drilled from the same coral colony (one horizontally and one vertically), and a third core was drilled
vertically from another coral growing at a different site. We evaluate several Sr/Ca records as proxies for regional SST variations:
(1) the three single-core records from Tahiti, (2) an average Sr/Ca record computed from the two cores drilled from the same
coral colony, (3) an average Sr/Ca record computed from all three Tahiti cores, and (4) an average Sr/Ca record computed from
the three Tahiti cores and a fourth core taken from a different island (Rarotonga). On a monthly scale, the average Sr/Ca
record including the four coral cores from Tahiti and Rarotonga shows the best correlation with regional SST. The variance
of the SST reconstruction is very realistic and the residual SST is low. This suggests that reconstructing SST from average
proxy records gives a better representation of regional SST variations. Of the three Tahiti cores, the one that was drilled
horizontally shows the best correlation with grid-SST on an annual mean scale. All three Tahiti corals show much larger interannual
SST variations than that indicated by grid-SST.
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... Within a region, there are many reports of inter-colony differences in Sr/Ca variation and its relationship to T (Abram et al., 2007;Alpert et al., 2016;Cahyarini et al., 2009;Fallon et al., 2003;Felis et al., 2004;Gagan et al., 1998;Linsley et al., 2004;Marshall & McCulloch, 2002;Pfeiffer et al., 2009). In cases where Sr/Ca could not be solely explained by local T (Linsley et al., 2004) or environmental stress (Fallon et al., 2003;Marshall & McCulloch, 2002), the inter-colony variability is often attributed to vital effects (Alpert et al., 2016;Felis et al., 2004;Grove et al., 2013;Pfeiffer et al., 2009). ...
... Composites of multiple coral Sr/Ca have been used to reduce intrinsic variance due to small-scale spatial environment heterogeneity or vital effects. Averaging the multiple-core data reduces the error with instrumental "regional" temperature (Cahyarini et al., 2009;Pfeiffer et al., 2009). However, lack of understanding of actual in situ T, and its relationship to Sr/Ca makes accurate interpretation of how well the composite T reflects local seawater temperature difficult (Grove et al., 2013). ...
The Sr/Ca ratio of modern coral skeletons can record local seawater temperature (T) and is an important tool for reconstructing past environments. However, site‐specific calibrations are required to ensure accurate temperature reconstructions. Here, we examine three modern coral skeletons collected at contrasting sites on the island of Oahu, Hawaii to establish the first accurate calibrations for this region and investigate site specific influences on the calibration process. Satellite T data, which is used for many calibrations, may not be able to derive an accurate thermometer. For our shallow lagoonal sites, satellite T had smaller seasonal T ranges, which resulted in significantly higher slopes of Sr/Ca‐T compared to using in situ T. The traditional age model based on aligning only min/max values can lead to errors in the Sr/Ca‐T calibration due to variable growth rates. An enhanced age model which adds midpoint alignments between the min/max peak values can account for seasonal changes in growth rate and reduce the error. On the same island, site‐ and time period specific conditions can cause notable differences in the Sr/Ca‐T calibrations. The coral from an estuarine embayment showed a high Sr/Ca offset, likely due to high Sr/Ca in ambient seawater. For corals which experienced thermal stress, lower slopes were observed probably due to elevated Sr/Ca values during the period of thermal stress.
... Within a region, there are many reports of inter-colony differences in Sr/Ca variation and its relationship to T (Abram et al., 2007;Alpert et al., 2016;Cahyarini et al., 2009;Fallon et al., 2003;Felis et al., 2004;Gagan et al., 1998;Linsley et al., 2004;Marshall & McCulloch, 2002;Pfeiffer et al., 2009). In cases where Sr/Ca could not be solely explained by local T (Linsley et al., 2004) or environmental stress (Fallon et al., 2003;Marshall & McCulloch, 2002), the inter-colony variability is often attributed to vital effects (Alpert et al., 2016;Felis et al., 2004;Grove et al., 2013;Pfeiffer et al., 2009). ...
... Composites of multiple coral Sr/Ca have been used to reduce intrinsic variance due to small-scale spatial environment heterogeneity or vital effects. Averaging the multiple-core data reduces the error with instrumental "regional" temperature (Cahyarini et al., 2009;Pfeiffer et al., 2009). However, lack of understanding of actual in situ T, and its relationship to Sr/Ca makes accurate interpretation of how well the composite T reflects local seawater temperature difficult (Grove et al., 2013). ...
The Sr/Ca ratio of modern coral skeleton can record local seawater temperature (T) and is an important tool to reconstruct past environments. However, site-specific calibrations are required to ensure accurate temperature reconstructions. Here, we examine three modern coral skeletons collected at contrasting sites on the island of Oahu, Hawaii to establish the first accurate calibrations for this region and investigate site specific influences on the calibration process. Satellite T data, which is used for many calibrations, may not be able to derive an accurate thermometer. For our shallow lagoonal sites, satellite T had smaller seasonal T ranges which resulted in significantly higher slopes of Sr/Ca-T compared to using in situ T. Traditional age model based on aligning only min/max values can lead to errors in the Sr/Ca-T calibration due to variable growth rates. An enhanced age model which add midpoints alignments between the mim/max peak values can account for seasonal changes in growth rate and reduce the error. On the same island, site and time period specific conditions can cause notable differences in the Sr/Ca-T calibrations. The coral from an estuarine embayment showed a high Sr/Ca offset, likely due to high Sr/Ca in ambient seawater. For corals which experienced thermal stress, lower slopes were observed probably due to elevated Sr/Ca values during the period of thermal stress.
... Coral Sr/Ca is convinced as sea surface temperature proxies [16], in some coral which is closed to the mainland, Sr/Ca also records air temperature [15]. Coral Sr/Ca is then commonly used for past temperature reconstruction, which can picture the temperature variation in seasonal resolution. ...
... To indicate the influence of IOD/ENSO to the coral SST, Nino 3.4 and DMI index are used. The coral SST shows that both climate event (ENSO/IOD) influence the temperature which is recorded in coral Sr/Ca [8] while in Seribu IOD is influenced stronger than the ENSO [16]. ...
Climate phenomena which is happened in the Pacific and Indian oceans i.e. El Niño Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) influence Indonesian climate. To understand behaviour of climate phenomena and its impact to Indonesian climate it is required paleoclimate data which provide long time series climate data back to hundred till thousands year. Sea surface temperature (SST) is one of important climate parameter. Coral Sr/Ca is convinced as coral paleo-thermometer. In this study, the available coral Sr/Ca is used to reconstruct past SST variability (Coral SST). Univariate linear regression between coral Sr/Ca and SST is used to reconstruct past SST variability. To understand how the climate phenomena influence the SST variability, the coral Sr/Ca is correlated with the Nino 3.4 and IOD indices. The result shows that the influence of IOD and ENSO is difference between sites. At Seribu island coral SST shows that SST is influenced by IOD rather than ENSO, while from Timor coral SST shows that Indian Ocean Dipole (IOD) and ENSO influences SST.
... The highest δ 15 N values were observed in temperate (<20 • C) and warm (>28 • C) waters with Chl-a over 0.1 mg m − 3 , which probably reflected the different SST found in other regions where yellowfin tuna have the highest δ 15 N -French Polynesia and the California Current system. French Polynesia has a SST range value from 25.4 • C to 29.4 • C (Cahyarini et al., 2009), while in the California Current system the SST range was completely different from ca. 12-17 • C in the central area of the California Current (Mauzole et al., 2020). These differences in SST range could be correlated with high δ 15 N values found in the different areas. ...
Stable isotope (δ¹³C and δ¹⁵N) analysis is an ecological tracer technique that reveals information on links between trophic levels, migratory species movement patterns, or geographic variations due to the different oceanic processes affecting them. Tuna species are ecologically important and can be used as proxies to generate information on how different variables –spatio-temporal, environmental and biological– can affect ecological tracers such as stable isotopes at the basin level. Thus, the main objective of this study is to examine how spatiotemporal, biological, baseline isotopic values and environmental drivers impact stable isotope distribution in three tuna species –albacore (Thunnus alalunga), bigeye (T. obesus) and yellowfin (T. albacares) in the Pacific Ocean (obtained from the Global data set for nitrogen and carbon stable isotopes of tunas) using Generalized Additive Modeling (GAM). The results suggest that the three species δ¹⁵N and δ¹³C values are mainly affected by the spatial and size or spatial and physical (depth) predictors. The highest isotopic values in the three species were found to be associated with hydrological and oceanographic conditions (Island Mass Effect), generating more productive areas and serves as a feeding ground for various commercially and ecologically important species. Spatial predictions (best-fitted GAM) of δ¹⁵N suggest three regions with the highest values: 1) Western Central Pacific (Solomon Island, 160-190°E); 2) Central Pacific (French Polynesia, 200-230°E); 3) Eastern Pacific (Southern California Current; 240°E, 20°N). For δ¹³C, the same three regions were observed with some differences in centroids –southwestward, northward, and southeastward, respectively. This knowledge can be used to infer ecosystem productivity, using isotope signatures at an ocean basin scale.
... Moreover, they incorporate different isotopic and geochemical tracers within their skeleton that vary based on various environmental factors such as SST when the skeleton was formed (Gagan et al., 2000;Ellis et al., 2019;Tao et al., 2021). During the past decades, Sr/Ca is considered the most reliable tracer for SST reconstruction due to the established negative correlation of [Sr] to SST (Beck et al., 1992;Cardinal et al., 2001;Cahyarini et al., 2009). Another standard tracer is coralline δ 18 O. ...
Massive coral species are essential archives of past sea surface temperature (SST) records as they incorporate biological and geochemical tracers that reflect temperature variations of their living marine environment. Existing methods of reconstructing past SST data involve analyzing elemental or isotopic ratios (e.g., Sr/Ca, coralline δ18O) that are known functions of seawater temperature. The potential of annual density bands of coral skeletons as an SST tracer has received less attention so far. 3D X-ray computed tomography (3DXCT) allows quick imaging of coral density bands with minimal to no sample preparation to extract relevant climate data as gray values (GV) – a measure of total X-ray absorption of the sample. Given the known influence of temperature on annual growth density bands and the GV profiles of corals, a novel method of SST reconstruction using GV from 3DXCT analysis, is presented. Two Porites spp. coral core samples (Baler 2 and 3) from Baler, Aurora, Philippines were analyzed using 3DXCT to obtain their GV profiles. GV profiles were matched with existing SST data (optimally interpolated SST). Comparisons showed significant positive linear correlation with equations SST = 0.3645 GV + 10.414 (r2 = 0.6389) and SST = 0.462 GV – 1.8888 (r2 = 0.7845) for Baler 2 and 3, respectively. SST reconstructed using these linear equations had mean absolute errors of 3.4 and 2.9% compared with OISST for Baler 2 and Baler 3, respectively. These findings showed the potential of 3DXCT analysis of coral cores as a relatively easy, quick non-destructive and precise method for SST reconstruction.
... are common in the Indo-Pacific and known for preserving reliable paleoclimate information. Although some factors (e.g., changes in coral vital effects or Sr/Ca of sea water) have been identified as possible sources of non-temperature influences on the coral Sr/Ca paleothermometer (e.g., (de Villiers et al., 1995;Shen et al., 1996;de Villiers, 1999;Cohen et al., 2001;Cohen et al., 2002;Allison and Finch, 2004;Alibert and Kinsley, 2008;Grove et al., 2013;Alpert et al., 2016;Kuffner et al., 2017), many studies have shown success in accurately reconstructing SST from coral Sr/Ca (McCulloch et al., 1994;Marshall and McCulloch, 2001;Goodkin et al., 2005;DeLong et al., 2007;Goodkin et al., 2007;Cahyarini et al., 2009;Pfeiffer et al., 2009;DeLong et al., 2011;DeLong et al., 2013;Wu et al., 2013;Bolton et al., 2014;Ramos et al., 2017;Pfeiffer et al., 2019). Also, a recent study investigating variability in Sr/Ca-SST calibrations across large SST gradients showed that the Sr/Ca-SST slopes do not change randomly but vary systematically with mean SST (Murty et al., 2018). ...
Constraining past variability in ocean conditions in the Western Pacific Warm Pool (WPWP) and examining how it has been influenced by the El-Niño Southern Oscillation (ENSO) is critical to predicting how these systems may change in the future. To characterize the spatiotemporal variability of the WPWP and ENSO during the past three decades, we analyzed climate proxies using coral cores sampled from Porites spp. from Kosrae Island (KOS) and Woleai Atoll (WOL) in the Federated States of Micronesia. Coral skeleton samples drilled along the major growth axis were analyzed for oxygen isotopes (δ18 Oc ) and trace element ratios (Sr/Ca), used to reconstruct sea surface salinity and temperature (SSS and SST). Pseudocoral δ18O time series (δ18 Opseudo ) were calculated from gridded instrumental observations and compared to δ18 Oc , followed by fine-tuning using coral Sr/Ca and gridded SST, to produce age models for each coral. The thermal component of δ18 Oc was removed using Sr/Ca for SST, to derive δ18O of seawater (δ18O sw ), a proxy for SSS. The Sr/Ca, and δ18 O sw records were compared to instrumental SST and SSS to test their fidelity as regional climate recorders. We found both sites display significant Sr/Ca-SST calibrations at monthly and interannual (dry season, wet season, mean annual) timescales. At each site, δ18Osw also exhibited significant calibrations to SSS across the same timescales. The difference between normalized dry season SST (Sr/Ca) anomalies from KOS and WOL generates a zonal SST gradient (KOSWOL SST), capturing the east-west WPWP migration observed during ENSO events. Similarly, the average of normalized dry season δ18Osw anomalies from both sites produces an SSS index (KOSWOL SSS) reflecting the regional hydrological changes. Both proxy indices, KOSWOL SST and KOSWOL SSS , are significantly correlated to regional ENSO indices. These calibration results highlight the potential for extending the climate record, revealing spatial hydrological gradients within the WPWP and ENSO variability back to the end of the Little Ice Age.
... Sr/Ca, Mg/Ca and U/Ca in the coral skeleton will be largely determined by the temperature dependent distribution coefficients of Sr/Ca, Mg/Ca and U/Ca between aragonite and sea water 12 . Since these ratios are independent of the oxygen isotopic composition of the sea water, analysis of trace metal concentrations has been widely used to reconstruct the SST [13][14][15][16][17][18][19][20][21][22][23] . On the other hand, paired analysis of the oxygen isotopes and trace metal abundance provide a means to estimate the salinity of the ocean surface 12,24 . ...
Reef building corals are known to provide high resolution records of ocean-atmospheric variables for the last several centuries. The most important parameter is the sea surface temperature, which is accurately recorded by the oxygen isotopic composition as well as by a few trace elemental ratios such as, Sr/Ca, Mg/Ca, U/Ca, etc. Determination of the sea surface temperature has enabled the palaeocenographers to study a variety of oceanic processes, such as the El Niño and Southern Oscillation, ocean circulation, air-sea gas exchange, Indian Ocean dipole, etc. Monsoon is an important atmospheric process in which sea surface temperature plays an active role in governing the moisture production and modulation of the wind circulation. Apart from temperature reconstruction, the corals have also been used to study the monsoon processes by some investigators, viz. the estimation of rainfall. In this article, we review the available coral records, specifically for studying the Indian summer monsoon rainfall variability. The study reveals that the coral records only from a few specific regions show promise in this endeavour.
Sea surface temperature (SST) is a crucial parameter in climate change studies, and it can be obtained from various sources, including in-situ measurements, satellite observations, reanalysis, and models. However, due to the limited availability of in-situ data, researchers often rely on satellite observations and models, all of which are susceptible to errors. This study aims to improve the accuracy of SST data from satellite observations and models by evaluating and calibration the data through comparison with in-situ SST data using time series analysis. The study areas are Panjang Island, Banten, with a period from April 15, 2022, to February 7, 2023. The data was collected from satellite (OISST V2.1), model (Hycom GOFS V3.0-3.1), and in-situ (loggers HOBO U-24 and TidBit V2.1). The results show that satellite-derived SST values perform better than the model based on higher satellite correlation coefficient values (r = 0.853) compared to the model (r = 0.685). In addition, satellite data also have lower error rate ( x ¯ b i a s = 0.445°C; σ bias = 0.304°C; RMSE = 0.538°C) than model ( x ¯ b i a s = 0.472°C; σ bias = 0.32°C; RMSE = 0.57°C). The variance tests show no significant difference for satellite (p-value = 0.785) and model (p-value = 0.346) data against in-situ data, while the mean tests reveal disparities (p-value < 0.001). The calibration process reduces error values for both satellite ( x ¯ b i a s = 0.276°C; σ bias = 0.225°C; RMSE = 0.356°C) and model data ( x ¯ b i a s = 0.398°C; σ bias = 0.298°C; RMSE = 0.397°C), but the correlation coefficients remain stable at 0.853 (satellite) and 0.685 (model). In conclusion, satellite observations provide a more precise picture of the SST conditions on Panjang Island, Banten and the calibration process can improve the quality of satellite model data.
A period that lasted from 900-1300 AD has been known as the Medieval Climate Anomaly. This period has been indicated as the warming period of the earth’s temperature. However, this warming phenomenon is still the subject of debate today, whether global or regional warming. Several studies concluded that the Medieval warm period is a global phenomenon and an important warm period, although external forcing is mainly similar to the present day. Several climate archives from Indonesia i.e. lake sediment, marine sediment and speleothem show a warming trend during this period. In this study, the seasonal warming trend during the Medieval climate is resolved from Porites coral from Lampung Bay, Indonesia. However, the seasonal temperature magnitude during the Medieval period is lower than today. This study confirms the coral medieval climate records from Mentawai islands. The result suggests that a warming trend occurred in Indonesia during the Medieval climate anomaly.
This special section is dedicated to the memory of Prof. Rengaswamy Ramesh, who untimely passed away on 2 April 2018 at the age of 62 years. He had an illustrious career at Physical Research Laboratory, Ahmedabad for almost 40 years and later at NISER, Bhubaneshwar as Senior Professor in the School of Earth and Planetary Sciences. Ramesh was a true polymath and made an immense contribution, especially to the study of past climate variability of the Indian subcontinent using various isotopic and geochemical proxies from both marine and terrestrial archives. We hope that this special
section having articles reviewing different proxies and their applications would be immensely useful for the palaeoclimate research community especially young researchers.
We measured delta18O and delta13C in Porites lutea collected in the Moorea lagoon where the instrumental records show that an El Niño-Southern Oscillation (ENSO) event implies both a cloud cover decrease and a weak sea surface temperature (SST) increase. Two proxies allow an ENSO record to be reconstructed at Moorea: the annual delta13C anomaly, associated with the South Pacific Convergence Zone motion, and an annual delta18O anomaly showing increased SST. The frequency bands exhibited by the singular spectral analysis (SSA) and the multitaper method of the annual delta18O and delta13C records are centered on 2.5 and 5.2 years and 2.4 and 3.2 years, respectively, which are the main ENSO modes. We used SSA to reconstruct ENSO events at Moorea over the past 137 years. Results indicate that climate variability at this site is strongly affected by ENSO events.
Sea surface salinity (SSS) and temperature (SST) data collected from voluntary observing ships over 25 years (1976-2000) are analyzed in the Southwestern Tropical Pacific (10°S-24°S/160°E-140°W). This region lies under the South Pacific Convergence Zone (SPCZ), at the southern edge of the western Pacific warm pool between Tahiti and Darwin, the two places whose atmospheric sea level pressure difference is used to define the Southern Oscillation Index (SOI). Complementary data such as precipitation are used to assist in the analysis. The mean and seasonal variations of these parameters are described. An empirical orthogonal function (EOF) analysis of low-pass filtered time series is then performed to extract the interannual variability. All parameters show an interannual signal which correlates well with the SOI. The Southwestern Tropical Pacific Ocean is saltier and colder during El Niño than during La Niña events. In the southwestern part, there is a shortage (excess) in precipitation during El Niño (La Niña) events. The greatest anomalies appeared during the last La Niña, in 1996 and 1999 as regards SST and in 1999 and 2000 as regards SSS. SST and precipitation El Niño-Southern Oscillation (ENSO)-related anomalies are an order of magnitude smaller than seasonal anomalies, while the SSS ENSO-related signal is twice as strong as the seasonal signal. These facts reflect the northeastward (southwestward) shift of the SPCZ during El Niño (La Niña) events. While consistent with precipitation changes, the ENSO-related variability in SSS can also be partly explained by the displacement of the salinity front that separates fresh, warm pool waters from salty subtropical waters. Computation of surface geostrophic current anomalies from Geosat (1987-1988) and TOPEX/Poseidon (1993-2000) indicates that westward current anomalies developed during the 1987/88 and 1997/98 El Niño and are linked to the displacement of the salinity front. The Southwestern Tropical Pacific salinity front moves westward (eastward) in contrast to the equatorial salinity front which moves eastward (westward) during an El Niño (La Niña) event.
Sea surface salinity (SSS) and sea surface temperature (SST) in the western Pacific warm pool (130-180°E 10°N-10°S) are analyzed for the period 1992-2000 taking advantage of complementary data from the ship of opportunity program and the Tropical Atmosphere-Ocean (TAO)-Triangle Trans-Ocean Buoy Network (TRITON) array of moored buoys. Covariability of these variables with surface wind stress, surface zonal currents, evaporation, precipitation, and barrier layer thickness is also examined. These fields all go through large oscillations related to the El Niño Southern Oscillation (ENSO) cycle, most notably during the record breaking 1997-1998 El Niño and subsequent strong 1998-2000 La Niña. East of about 160°E, during El Niño, precipitation minus evaporation increases in the equatorial band, in conjunction with anomalous increases in westerly winds, eastward surface currents, SST, and decreases in SSS. Opposite tendencies are evident during La Niña. Peak to peak 2°N-2°S averaged variations reached as much as 1.2 m s-1 for zonal currents and 1.5 practical salinity units (psu) for SSS. West of about 160°E, SST cools during El Niño and warms during La Niña, opposite to what occurs further east. To understand these SST tendencies west of 160°E, a proxy indicator for barrier layer formation is developed in terms of changes in the zonal gradient of SSS (∂S/∂x). Zonal SSS gradients have been shown in modeling studies to be related to barrier layer formation via subduction driven by converging zonal currents in the vicinity of the salinity front at the eastern edge of the warm pool. Correlation between changes in ∂S/∂x and changes in SST a few degrees longitude to the west is significantly nonzero, consistent with the idea that increased barrier layer thickness is related to warmer SSTs during periods of westward surface flow associated with La Niña, and vice versa during El Niño. Direct evidence of barrier layer thickness variations in support of this hypothesis is also presented.
The oxygen isotopic composition of a banded coral from the western equatorial Indian Ocean provides a 150-year-long history
of the relation between the El Niño–Southern Oscillation (ENSO) phenomenon and the Asian monsoon. Interannual cycles in the
coral time series were found to correlate with Pacific coral and instrumental climate records, suggesting a consistent linkage
across ocean basins, despite the changing frequency and amplitude of the ENSO. However, decadal variability that is characteristic
of the monsoon system also dominates the coral record, which implies important interactions between tropical and midlatitude
climate variability. One prominent manifestation of this interaction is the strong amplitude modulation of the quasi-biennial
cycle.
This paper provides an evaluation of two of the most likely pitfalls of Sr / Ca thermometry, i.e., the effect of biogenic cycling of Sr vs. Ca in the surface ocean and the effect of variable extension rate on Sr incorporation in coralline aragonite. We also report calibration of the Sr / Ca -temperature relationship for three coral species, Porites lobata, Pocillopora eydouxi and Pavona clavus , collected from the Hawaiian and Galapagos islands. Analyses of seawater samples show significant spatial and depth variability in the Sr:Ca ratio. The uncertainty introduced by this effect is estimated to be
An improved SST reconstruction for the 1854-1997 period is developed. Compared to the version 1 analysis, in the western tropical Pacific, the tropical Atlantic, and Indian Oceans, more variance is resolved in the new analysis. This improved analysis also uses sea ice concentrations to improve the high-latitude SST analysis and a modified historical bias correction for the 1939-41 period. In addition, the new analysis includes an improved error estimate. Analysis uncertainty is largest in the nineteenth century and during the two world wars due to sparse sampling. The near-global average SST in the new analysis is consistent with the version 1 reconstruction. The 95% confidence uncertainty for the near-global average is 0.4°C or more in the nineteenth century, near 0.2°C for the first half of the twentieth century, and 0.1°C or less after 1950.
Understanding the relationship between sea surface temperature (SST) and precipitation is a significant challenge for climate models, particularly for the tropics. Here we present a new monthly coral Sr/Ca record from the tropical Indian Ocean (Chagos Archipelago) that extends from 1950 to 1995. The coral Sr/Ca ratio shows a stationary relationship with local SST, and documents a warming of 0.3 °C since 1950. Previous work has shown that the delta18O values measured in the same coral core provide a proxy record of precipitation in the tropical Indian Ocean. The coral delta18O record shows a nonstationary relationship with local SST, and a correlation between delta18O and SST only emerges in the 1970s. It was proposed that this nonstationary behavior is due to an increase in mean SSTs in the tropical Indian Ocean. During the 1970s, SSTs reached a critical threshold (28.5 °C) beyond which small SST anomalies can have a significant impact on atmospheric convection. As a result, the covariance between SST and precipitation in the tropical Indian Ocean increased. Our new Sr/Ca data confirm that the warming of the Indian Ocean during the late twentieth century affects atmospheric convection and rainfall variability. Moreover, our proxy data show that the relationship between SST and precipitation is nonlinear and characterized by threshold behavior.
A method for rapid determination of high-precision Sr/Ca ratios in
scleractinian corals is presented. Using an inductively coupled plasma
atomic emission spectrophotometer, samples are corrected for instrument
drift using a reference solution, similar to the approach used for
analysis of stable isotopes using gas-source mass spectrometry. Further
correction for variation of the Sr/Ca ratio with Ca concentration is
accomplished using internal standards. The precision, once all
corrections have been made, is better than 0.1% (relative standard
deviation, 1σ) for samples of similar Ca concentration and better
than 0.2% for samples with variable Ca concentrations. This method
increases the sample throughput by approximately a factor of 20 relative
to thermal ionization mass spectrometry and significantly reduces
instrument and per sample costs. Comparison of Sr/Ca data for a coral
from the Galapagos Islands with an instrumental temperature record shows
excellent agreement and demonstrates the potential for application of
this technique to samples of modern and fossil scleractinian corals and
other marine carbonates, including foraminifera.