Sea Surface Temperature Calibration Research Articles

Sea surface temperature reconstruction in the Pacific Ocean using multi-elemental proxy in Porites and Diploastrea corals: Application to Palau Archipelago

Massive reef-building Porites corals are commonly studied to obtain long-term reconstructions of past sea surface temperature (SST) using temperature-sensitive elemental proxies, such as Sr/Ca or Li/Mg ratios. Most recently, a multi-proxy approach combining these two ratios (e.g. D'Olivo et al., 2018) and the SrU method (DeCarlo et al., 2016) have proved to be more robust to reconstruct paleo-temperatures. To date, no study has been carried out on the application of these new approaches on the Diploastrea heliopora coral, another massive reef-building genus that can potentially provide longer temperature records. Moreover, only a few studies have assessed coral SST calibrations at the scale of the Indo-Pacific basin and compared SST reconstructions obtained from two massive coral genera from the same site.In this study, we investigated the elemental composition of the topmost portion of 34 modern tropical Porites and 6 Diploastrea colonies collected during the Tara Pacific expedition (2016–2018) from various hydrological contexts in the Pacific Ocean. We derived and discussed annual Sr/Ca, Li/Mg, combined Sr/Ca-Li/Mg and Sr/Ca-Li/Ca-Mg/Ca and SrU vs. SST calibrations as well as potential intra-colonial and genus specific effects and evaluated the use of these basin-wide calibration equations. Overall, multi-ratio and multi-genera SST calibrations perform better than single-ratio calibrations and seem to improve temperature reconstructions.These new SST calibrations were applied to two colonies of Porites and Diploastrea collected from the same site in the North-West of the Palau archipelago located in the western Pacific Ocean to evaluate the applicability of universal calibrations based on different proxies and their combination. Coral-based SST records spanning the last 141 years show decadal changes and recent warming episodes that are related to major El Niño Southern Oscillation events. However, differences in reconstruction remain between both genera in the long-term trends, amplitudes, and absolute temperatures, depending on which genus or temperature proxy is considered.

A Simple Monte Carlo Approach to Estimate the Uncertainties of SST and δ18Osw Inferred From Coral Proxies

AbstractThe skeletons of massive corals inhabiting tropical and subtropical oceans grow over centuries. The chemical and isotopic compositions of their skeletons reflect changes in the marine environment and provide an archive of sea surface temperature (SST) and salinity at a high temporal resolution spanning decades to centuries. Stable oxygen isotope ratios in coral skeletons (δ18Oc) are an excellent proxy for SST and the stable oxygen isotopic composition in seawater (δ18Osw). Variations in δ18Osw serve as a hydrological proxy and are routinely estimated by subtracting the SST component from δ18Oc. The SST component of δ18Oc is inferred from coral Sr/Ca (SST proxy). Both Sr/Ca and δ18Oc are converted to SST units based on Sr/Ca‐ and (δ18Oc–δ18Osw)‐SST calibrations. However, the exact slope values of the Sr/Ca‐SST and (δ18Oc–δ18Osw)‐SST calibrations are challenging to assess, which hampers the reliable estimation of SST, δ18Osw, and their uncertainties. Here, we present a simple Monte Carlo approach to estimate variations in SST and δ18Osw together with their confidence intervals. Our approach improves on previous error propagation methods by including intercolonial differences of calibration slopes in addition to analytical errors. In addition, Monte Carlo experiments using adjusted errors suggest that uncertainties in the calibration slopes have a minor influence on estimates of SST and δ18Osw.

Fidelity of the Coral Sr/Ca Paleothermometer Following Heat Stress in the Northern Galápagos

AbstractCoral Sr/Ca records have been widely used to reconstruct and understand past sea surface temperature (SST) variability in the tropical Pacific. However, in the eastern equatorial Pacific, coral growth conditions are marginal, and strong El Niño events have led to high mortality, limiting opportunities for coral Sr/Ca‐based SST reconstructions. In this study, we present two ∼25‐year Sr/Ca and Mg/Ca records measured on modern Porites lobata from Wolf and Darwin Islands in the northern Galápagos. In these records, we confirm the well‐established relationship between Sr/Ca and SST and investigate the impact of heat stress on this relationship. We demonstrate a weakened relationship between Sr/Ca and SST after a major (Degree Heating Months 9°C‐months) heat stress event during the 1997–1998 El Niño, with a larger response in the Wolf core. However, removing data that covers the 1997–1998 El Niño from calibration does not improve reconstruction statistics. Nevertheless, we find that excluding data after the 1997–1998 El Niño event from the calibration reduces the SST reconstruction error slightly. These results confirm that coral Sr/Ca is a reliable SST proxy in this region, although it can respond adversely to unusual heat stress. We suggest that noise in Sr/Ca‐SST calibrations may be reduced by removing data immediately following large heat extremes.

Assessing multiproxy approaches (Sr/Ca, U/Ca, Li/Mg, and B/Mg) to reconstruct sea surface temperature from coral skeletons throughout the Great Barrier Reef

Due to the increasing concerns of global warming and short instrumental records of sea surface temperature (SST), coral-based proxies, such as δ18O, Sr/Ca, U/Ca, and Li/Mg have been developed and applied to reconstruct SST in paleoclimate studies. However, these proxies are not universally applicable in different environments, because they are affected by coral physiology and various environmental factors. In this study, seven long-lived Porites corals were collected from the southern sector of the Great Barrier Reef (GBR) off the coast of Gladstone and the central sector of the GBR within the Whitsunday Islands in 2017 and 2018. Coral sites were selected to cover a wide latitudinal range with different annual temperature ranges. Century-long geochemical records (Li/Ca, B/Ca, Mg/Ca, Sr/Ca, and U/Ca) were generated using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at weekly resolution. This study has tested the robustness of two traditional temperature proxies (Sr/Ca and U/Ca), a recently developed temperature proxy (Li/Mg), and an additional potential temperature proxy (B/Mg). U/Ca was found to be the most robust and stable temperature proxy for corals from the GBR over long-term timescales. Sr/Ca is a close second, however due to the lower response of Sr fractionation per 1 °C, it is more sensitive to analytical methods and less sensitive to annual SST changes than U/Ca. Li/Mg and B/Mg have clearer periodicity compared to Li/Ca and B/Ca. Both Li/Mg and B/Mg are strongly correlated with SST, which is due to the cancellation of temperature-independent commonality. Empirical calibrations established from this multi-proxy approach increase the certainty of temperature reconstruction when a single proxy does not perform well. These site- and colony-specific SST calibrations also provide an opportunity to revisit the universal multi-trace element calibration of sea surface temperatures (UMTECS) model, which does not require the knowledge of local SST for calibration.

Coral Record of Younger Dryas Chronozone Warmth on the Great Barrier Reef

AbstractThe Great Barrier Reef (GBR) is an internationally recognized and widely studied ecosystem, yet little is known about its sea surface temperature (SST) evolution since the Last Glacial Maximum (LGM) (~20 kyr BP). Here, we present the first paleo‐application of Isopora coral‐derived SST calibrations to a suite of 25 previously published fossil Isopora from the central GBR spanning ~25–11 kyr BP. The resultant multicoral Sr/Ca‐ and δ18O‐derived SST anomaly (SSTA) histories are placed within the context of published relative sea level, reef sequence, and coralgal reef assemblage evolution. Our new calculations indicate SSTs were cooler on average by ~5–5.5°C at Noggin Pass (~17°S) and ~7–8°C at Hydrographer's Passage (~20°S) (Sr/Ca‐derived) during the LGM, in line with previous estimates (Felis et al., 2014, https://doi.org/10.1038/ncomms5102). We focus on contextualizing the Younger Dryas Chronozone (YDC, ~12.9–11.7 kyr BP), whose Southern Hemisphere expression, in particular in Australia, is elusive and poorly constrained. Our record does not indicate cooling during the YDC with near‐modern temperatures reached during this interval on the GBR, supporting an asymmetric hemispheric presentation of this climate event. Building on a previous study (Felis et al., 2014, https://doi.org10.1038/ncomms5102), these fossil Isopora SSTA data from the GBR provide new insights into the deglacial reef response, with near‐modern warming during the YDC, since the LGM.

Tropical Atlantic Cooling and Freshening in the Middle of the Last Interglacial From Coral Proxy Records

AbstractThe last interglacial (LIG; Marine Isotope Substage 5e, ~127–117 ka) experienced globally warmer than modern temperatures; however, profound differences in regional climate occurred that are relevant to the assessment of future climate change scenarios. Tropical Atlantic sea surface temperature (SST) and hydrology are intrinsic to the spatiotemporal evolution of past and future climate. We present eight monthly resolved coral Sr/Ca and δ18O records (130–118 ka) to reconstruct mean western tropical Atlantic SST and seawater δ18O changes during the LIG. Cooler and fresher than modern surface waters are indicated for the middle of the LIG at ~126 ka. This was followed by a rapid transition to modern‐like SSTs and salinities that characterized the remaining part of the LIG. Our results, which account for differences found among corals, proxies, and SST calibration uncertainties, agree with western tropical Atlantic sediment records. Together, they suggest that an oceanic regime existed that differed from today.

Mid‐Holocene, Coral‐Based Sea Surface Temperatures in the Western Tropical Atlantic

AbstractThe Holocene is considered a period of relative climatic stability, but significant proxy data‐model discrepancies exist that preclude consensus regarding the postglacial global temperature trajectory. In particular, a mid‐Holocene Climatic Optimum, ~9,000 to ~5,000 years BP, is evident in Northern Hemisphere marine sediment records, but its absence from model simulations raises key questions about the ability of the models to accurately simulate climate and seasonal biases that may be present in the proxy records. Here we present new mid‐Holocene sea surface temperature (SST) data from the western tropical Atlantic, where twentieth‐century temperature variability and amplitude of warming track the twentieth‐century global ocean. Using a new coral thermometer Sr‐U, we first developed a temporal Sr‐U SST calibration from three modern Atlantic corals and validated the calibration against Sr‐U time series from a fourth modern coral. Two fossil corals from the Enriquillo Valley, Dominican Republic, were screened for diagenesis, U‐series dated to 5,199 ± 26 and 6,427 ± 81 years BP, respectively, and analyzed for Sr/Ca and U/Ca, generating two annually resolved Sr‐U SST records, 27 and 17 years long, respectively. Average SSTs from both corals were significantly cooler than in early instrumental (1870–1920) and late instrumental (1965–2016) periods at this site, by ~0.5 and ~0.75 °C, respectively, a result inconsistent with the extended mid‐Holocene warm period inferred from sediment records. A more complete sampling of Atlantic Holocene corals can resolve this issue with confidence and address questions related to multidecadal and longer‐term variability in Holocene Atlantic climate.

Quantifying Uncertainty in Sr/Ca‐Based Estimates of SST From the Coral Orbicella faveolata

AbstractThe strontium to calcium ratio (Sr/Ca) in aragonitic skeletons of massive corals provides a proxy for sea surface temperature (SST) that can be used to reconstruct paleoclimates across decades, centuries, and, potentially, millennia. Determining the reproducibility of Sr/Ca records among contemporaneous coral colonies from the same region is critical to quantifying uncertainties associated with the Sr/Ca‐SST proxy. We evaluated both intracolony and intercolony variability in Sr/Ca using five modern colonies of Orbicella faveolata collected live from the Dry Tortugas National Park, FL. We regressed all available Sr/Ca‐SST data pairs from the five O. faveolata colonies against the Advanced Very High Resolution Radiometer gridded SST data set to produce a new Sr/Ca‐SST calibration equation (Sr/Ca = −0.049 × SST + 10.460), which we suggest can be applied to O. faveolata colonies collected throughout the Gulf of Mexico/Caribbean region. We estimated total uncertainty by calculating the root‐mean‐square of the intracolony, intercolony, and analytical error terms. Our (1σ) uncertainty estimates of 0.082 mmol/mol (1.66 °C) for subannual Sr/Ca‐SST and 0.070 mmol/mol (1.43 °C) for mean annual Sr/Ca‐SST represent conservative error terms that can be applied to individual data points in single‐colony Sr/Ca‐SST reconstructions. We illustrate how these uncertainties can be significantly reduced by generating multicolony reconstructions and/or through replication of sampling within individual coral colonies. Although the uncertainties on absolute Sr/Ca‐based SST are likely too large to allow researchers to evaluate subdecadal temperature variability, we show that the O. faveolata paleothermometer can reliably detect changes of ~2 °C across decadal timescales and ~1 °C over multidecadal timescales.

Spatial and Temporal Robustness of Sr/Ca‐SST Calibrations in Red Sea Corals: Evidence for Influence of Mean Annual Temperature on Calibration Slopes

AbstractSr/Ca ratios recorded in the aragonite skeleton of massive coral colonies are commonly used to reconstruct seasonal‐ to centennial‐scale variability in sea surface temperature (SST). While the Sr/Ca paleothermometer is robust in individual colonies, Sr/Ca‐SST relationships between colonies vary, leading to questions regarding the utility of the proxy. We present biweekly‐resolution calibrations of Sr/Ca from five Porites spp. corals to satellite SST across 10° of latitude in the Red Sea to evaluate the Sr/Ca proxy across both spatial and temporal scales. SST is significantly correlated with coral Sr/Ca at each site, accounting for 69–84% of Sr/Ca variability (P ≪ 0.01). Intercolony variability in Sr/Ca‐SST sensitivities reveals a latitudinal trend, where calibration slopes become shallower with increasing mean annual temperature. Mean annual temperature is strongly correlated with the biweekly‐resolution calibration slopes across five Red Sea sites (r2 = 0.88, P = 0.05), while also correlating significantly to Sr/Ca‐SST slopes for 33 Porites corals from across the entire Indo‐Pacific region (r2 = 0.26, P < 0.01). Although interannual summer, winter, and mean annual calibrations for individual Red Sea colonies are inconsistently robust, combined multicoral calibrations are significant at summer (r2 = 0.53, P ≪ 0.01), winter (r2 = 0.62, P ≪ 0.01), and mean annual time scales (r2 = 0.79, P ≪ 0.01). Our multicoral, multisite study indicates that the Sr/Ca paleothermometer is accurate across both temporal and spatial scales in the Red Sea and also potentially explains for the first time variability in Sr/Ca‐SST calibration slopes across the Indo‐Pacific region. Our study provides strong evidence supporting the robustness of the coral Sr/Ca proxy for examining seasonal to multicentury variability in global climate phenomena.

Refining Globigerinoides ruber Mg/Ca paleothermometry in the Atlantic Ocean

The Mg/Ca ratio of the planktonic foraminifera Globigerinoides ruber (white) has become a widely used proxy for reconstructing sea surface temperature (SST), as numerous studies have shown that temperature is the primary control on Mg/Ca ratios in foraminiferal calcite. However, a recent study of core-top sediments across an Atlantic meridional transect suggests that salinity might have a stronger control on foraminiferal Mg/Ca ratios than previously thought. By analyzing Mg/Ca ratios and δO18 values in G. ruber (white), Arbuszewski et al. (2010) found a 27% increase in G. ruber Mg/Ca ratios per 1 salinity unit increase for seawater salinities above 35.5. Here, we use shell weight analyses and SEM images from a subset of the core-tops used in the Arbuszewski et al. (2010) study across a narrow depth range (3197–3733 m) to show that G. ruber shells from the equatorial region are highly dissolved compared to those from the subtropical North and South Atlantic gyres, significantly impacting their Mg/Ca-SSTs. Shell weights from the higher-productivity equatorial region of the Atlantic are on average 20% and 15% lower than those from the oligotrophic North and South Atlantic gyres, respectively. Given the large preservation gradient along the transect studied by Arbuszewski et al. (2010), application of a single dissolution-corrected Mg/Ca:SST calibration equation (Dekens et al., 2002) on cores from the subtropical gyres and the equatorial region is not appropriate. When regional differences in preservation are considered, as well as realistic habitat depths and calcification seasons for G. ruber in temperate latitudes, we find a strong correlation between observational SSTs and calculated G. ruber Mg/Ca-SSTs in core-top samples spanning 43°N to 25°S in the Atlantic. In addition, our re-calibrated Mg/Ca-SSTs are more strongly correlated with isotopic calcification temperatures across the transect than originally reported by Arbuszewski et al. (2010). This study provides evidence that regional differences in preservation, rather than salinity, significantly affects G. ruber Mg/Ca-SSTs in the Atlantic, and supports culturing experiments that found salinity only has a minor influence on foraminiferal Mg/Ca ratios.

Sr/Ca–sea surface temperature calibration in the coral Porites lutea from subtropical northern South China Sea

Strontium to calcium (Sr/Ca) ratios in annual bands of Porites corals are commonly used as past sea surface temperature proxy in tropical oceans, whereas studies that refer to Sr/Ca–SST calibration in high-latitude corals are rather rare in general. Here we present a 50-year (1961–2010) near-fortnightly Sr/Ca record in two Porites lutea corals living near the northern limit of reef-building corals in the subtropical northern South China Sea, where winter temperatures are extreme for hermatypic corals. The Sr/Ca time series shows a clear annual cyclicity, with marked sharp/narrow winter peaks. Comparison with an instrumental SST record (since 1961) indicates that this Sr/Ca record is chiefly modulated by SST. Two different methods are discussed for calibrating Sr/Ca with SST. The correlation between interpolated Sr/Ca and SST is significant, but the coefficient is relatively low, mainly attributed to extreme cold winter SSTs (<~16°C) which cause a significant decrease or even cessation of coral skeletal extension. Another modified method using only annual low Sr/Ca values which could clearly be matched with the same features in the instrumental SST record is more reliable. The modified regression equation does not encompass SSTs below 24°C. The fidelity of the coral-based Sr/Ca–SST calibration was assessed and the precision of the method is estimated to be 0.31±0.62°C based on the calibration–verification exercise. The modified method is suggested in marginal coral areas with extreme intra-annual SST variation (e.g., from 13 to 30°C).

A skeletal Sr/Ca record preserved in Dipsastraea (Favia) speciosa and implications for coral Sr/Ca thermometry in mid‐latitude regions

A core (900 mm long) of the scleractinian coral Dipsastraea (Favia) speciosa was collected from Iki Island (∼33°48′N), Japan, one of the highest latitude coral reefs known to exist at present, where winter monthly mean sea surface temperature (SST) drops to 13°C. The Sr/Ca profile was constructed using a bulk sampling method for the uppermost 280 mm interval of the core, which grew between 1966 and 2007, to test whether it could act as a suitable proxy for SST in a harsh environmental setting where reef‐building coral do not usually survive. The Sr/Ca‐SST relationship derived from the annual Sr/Ca and SST extremes predicted the observed monthly averaged summer SST extremes within an error range of ±1.1°C (1 s.d., n = 40). The obtained Sr/Ca‐SST calibration was also found to be valid for subtropical Dipsastraea (Favia) corals, proving its broad applicability. However, low‐amplitude winter peaks were observed in the slow‐growing intervals, which we confirmed (using individual spot analysis along a continuous growth line) result from the mixing of theca grown at different times. Our bulk sampling approach, across multiple growth lines in the skeleton of D. (F.) speciosa, led to the mixing of asynchronous skeletal part. At the study site, D. (F.) speciosa grows continuously, even during the cold season, suggesting that the skeletal Sr/Ca obtained from specimens of D. (F.) speciosa can be used as an SST proxy in the northwest Pacific marginal seas.

Sensible Heat Flux in Near-Neutral Conditions over the Sea

Abstract The variation of the sea surface sensible heat flux is investigated using data from the Gulf of Tehuantepec Experiment (GOTEX) and from eight additional aircraft datasets representing a variety of surface conditions. This analysis focuses on near-neutral conditions because these conditions are common over the sea and are normally neglected, partly because of uncertain reliability of measurements of the small air–sea temperature difference. For all of the datasets, upward heat flux is observed for slightly stable conditions. The frequency of this “countergradient” heat flux increases with increasing wind speed and is possibly related to sea spray or microscale variations of surface temperature on the wave scale. Upward area-averaged sensible heat flux for slightly stable conditions can also be generated by mesoscale heterogeneity of the sea surface temperature (SST). Significant measurement errors cannot be ruled out. The countergradient heat flux for weakly stable conditions is least systematic for weaker winds, even though it occurs with weak winds in all of the datasets. In an effort to reduce offset errors and different SST processing and calibration procedures among field programs, the authors adjusted the SST in each field program to minimize the countergradient flux for weak winds. With or without this adjustment for the combined dataset, the extent of the upward heat flux for weakly stable conditions increases with increasing wind speed.

A coral Sr/Ca calibration and replication study of two massive corals from the Gulf of Mexico

This study examined the variations in the ratio of strontium-to-calcium (Sr/Ca) for two Atlantic corals ( Montastraea faveolata and Siderastrea siderea) from the Dry Tortugas National Park (centered on 24.7°N, 82.8°W) in the Gulf of Mexico. Cores from coral colonies in close proximity (10s of meters) and with the same environmental conditions (i.e., depth and water chemistry) were micro-sampled with approximately monthly resolution and the resulting Sr/Ca variations were calibrated with local sea surface temperature (SST) records. Replication tests for coral Sr/Ca variations found high agreement between intra-colony variations and between individual colonies of S. siderea (a single M. faveolata colony was sampled). Regression analysis of monthly variations in coral Sr/Ca and local SST revealed significant correlation on monthly and inter-annual timescales. Verification of the calibration on different timescales found coral Sr/Ca–SST reconstructions in S. siderea were more accurate than those from M. faveolata, especially on inter-annual timescales. Sr/Ca–SST calibration equations for the two species are significantly different (cf., Sr/Ca = − 0.042 SST + 10.070, S. siderea; Sr/Ca = − 0.027 SST + 9.893, M. faveolata). Mean linear extension for M. faveolata is approximately twice that of S. siderea (4.63, 4.31, and 8.31 mm year −1, A1, F1, and B3, respectively); however, seasonal Sr/Ca variability in M. faveolata is less than S. siderea (0.323, 0.353, and 0.254 mmol mol −1, A1, F1, and B3, respectively). The reduced slope for M. faveolata is attributed to physical sampling issues associated with complex time-skeletal structure of M. faveolata, i.e., a sampling effect, and not a growth effect since the faster growing M. faveolata has the reduced Sr/Ca variability.

Merging late Holocene molecular organic and foraminiferal‐based geochemical records of sea surface temperature in the Gulf of Mexico

A molecular organic geochemical proxy (TEX86) for sea surface temperature (SST) is compared with a foraminifera‐based SST proxy (Mg/Ca) in a decadal‐resolution marine sedimentary record spanning the last 1000 years from the Gulf of Mexico. We assess the relative strengths of the organic and inorganic paleoceanographic techniques for reconstructing high‐resolution SST variability during recent climate events, including the Little Ice Age (LIA) and the Medieval Warm Period (MWP). SST estimates based on the molecular organic proxy TEX86 show a similar magnitude and pattern of SST variability to foraminiferal Mg/Ca‐SST estimates but with some important differences. For instance, both proxies show a cooling (1°C–2°C) of Gulf of Mexico SSTs during the LIA. During the MWP, however, Mg/Ca‐SSTs are similar to near‐modern SSTs, while TEX86 indicates SSTs that were cooler than modern. Using the respective SST calibrations for each proxy results in TEX86‐SST estimates that are 2°C–4°C warmer than Mg/Ca‐SST throughout the 1000 year record. We interpret the TEX86‐SST as a summer‐weighted SST signal from the upper mixed layer, whereas the Mg/Ca‐SST better reflects the mean annual SST. Downcore differences in the SST estimates between the two proxies (ΔT = TEX86 − Mg/Ca) are interpreted in the context of varying seasonality and/or changing water column temperature gradients.

A 5 million year comparison of Mg/Ca and alkenone paleothermometers

Geochemical sea surface temperature (SST) proxies such as the magnesium to calcium ratio (Mg/Ca) in foraminifera and the alkenone unsaturation index (UK′37) are becoming widely used in pre‐Pleistocene climate records. This study quantitatively compares previously published Mg/Ca and UK′37 data from Ocean Drilling Program (ODP) site 847 in the eastern equatorial Pacific to assess the utility of these proxies to reconstruct tropical SST over the last 5 Ma. Foraminiferal Mg/Ca–SST calibrations that include a dissolution correction are most appropriate at this location because they provide SST estimates for the youngest sample that are close to modern mean annual SST. The long‐term trends in the two records are remarkably similar and confirm a ∼3.5°C cooling trend from the early Pliocene warm period to the late Pleistocene noted in previous work. Absolute temperature estimates are similar for both proxies when errors in the dissolution correction used to estimate SST from Mg/Ca are taken into account. Comparing the two SST records at ODP site 847 to other records in the region shows that the eastern equatorial Pacific was 2–4°C warmer during the early Pliocene compared to today.

Interpreting sea surface temperature from strontium/calcium ratios in Montastrea corals: Link with growth rate and implications for proxy reconstructions

We analyzed strontium/calcium ratios (Sr/Ca) in four colonies of the Atlantic coral genus Montastrea with growth rates ranging from 2.3 to 12.6 mm a−1. Derived Sr/Ca–sea surface temperature (SST) calibrations exhibit significant differences among the four colonies that cannot be explained by variations in SST or seawater Sr/Ca. For a single coral Sr/Ca ratio of 8.8 mmol mol−1, the four calibrations predict SSTs ranging from 24.0° to 30.9°C. We find that differences in the Sr/Ca–SST relationships are correlated systematically with the average annual extension rate (ext) of each colony such that Sr/Ca (mmol mol−1) = 11.82 (±0.13) – 0.058 (±0.004) × ext (mm a−1) – 0.092 (±0.005) × SST (°C). This observation is consistent with previous reports of a link between coral Sr/Ca and growth rate. Verification of our growth‐dependent Sr/Ca–SST calibration using a coral excluded from the calibration reconstructs the mean and seasonal amplitude of the actual recorded SST to within 0.3°C. Applying a traditional, nongrowth‐dependent Sr/Ca–SST calibration derived from a modern Montastrea to the Sr/Ca ratios of a conspecific coral that grew during the early Little Ice Age (LIA) (400 years B.P.) suggests that Caribbean SSTs were &gt;5°C cooler than today. Conversely, application of our growth‐dependent Sr/Ca–SST calibration to Sr/Ca ratios derived from the LIA coral indicates that SSTs during the 5‐year period analyzed were within error (±1.4°C) of modern values.

Recovery of temperature records from slow‐growing corals by fine scale sampling of skeletons

We used laser ablation inductively coupled plasma mass spectrometry (LA ICP‐MS) to analyze Sr/Ca ratios in 5 colonies of the Atlantic corals, Diploria labyrinthiformis and Montastrea franski, each growing less than 5 mm yr−1. By targeting the centers of septa we avoided thickening deposits to achieve an analytical sampling resolution of 5‐10 days. The sensitivity of Sr/Ca to temperature (−0.096 mmol/mol/°C) is ∼3 times higher than previously reported for these species and equivalent to that exhibited by fast‐growing Porites corals from the Indo‐Pacific. The Sr/Ca‐sea surface temperature (SST) calibrations derived from these corals were not statistically different and were independent of colony growth rate over the period studied. Data from 4 D. labyrinthiformis colonies were pooled to produce a single Sr/Ca‐SST calibration with a calculated standard error on the predicted ocean temperature of ±0.51°C. Applying our calibration to Sr/Ca analyses of D. labyrinthiformis skeleton deposited in the late 18th century indicated that average annual sea surface temperatures around Bermuda were ∼1°C cooler than today.

A multicoral calibration method to approximate a universal equation relating Sr/Ca and growth rate to sea surface temperature

Combining strontium‐to‐calcium ratios (Sr/Ca) with mean annual growth rates in Bermuda Diploria labyrinthiformis (brain corals) is shown to improve sea surface temperature (SST) calibrations relative to instrumental data. Growth‐corrected Sr/Ca–SST calibrations based on single‐coral colonies over the same calibration interval, however, are found to be poorly suited for application to data from different coral colonies. This raises concerns about the accuracy of SST reconstructions from fossil coral measurements that involve extrapolation beyond the range of values seen during the calibration period. Here we pursue a novel approach to this problem by incorporating data from multiple coral colonies into a single growth‐corrected Sr/Ca–SST calibration equation, effectively expanding the range of modern values constraining the model. The use of a multiple‐colony calibration model for reconstructing SST yields greater precision and accuracy relative to instrumental data than single‐colony models, providing greater confidence for applications to fossil coral samples.

Sr/Ca and δ18O in a fast‐growing Diploria strigosa coral: Evaluation of a new climate archive for the tropical Atlantic

This study provides the first monthly resolved, 41‐year record of geochemical variations (δ18O and Sr/Ca) in a fast‐growing Diploria strigosa brain coral from Guadeloupe, Caribbean Sea. Linear regression yields a significant correlation of coral Sr/Ca (δ18O) with instrumental sea surface temperature (SST) on both monthly and mean annual scales (e.g., r = −0.59 for correlation between Simple Ocean Data Assimilation (SODA) SST and Sr/Ca, and r = −0.66 for δ18O; mean annual scale, p &lt; 0.0001). The generated coral Sr/Ca (δ18O)‐SST calibration equations are consistent with each other and with published equations using other coral species from different regions. Moreover, a high correlation of coral Sr/Ca and δ18O with local air temperature on a mean annual scale (r = −0.78 for Sr/Ca; r = −0.73 for δ18O; p &lt; 0.0001) demonstrates the applicability of geochemical proxies measured from Diploria strigosa corals as reliable recorders for interannual temperature variability. Both coral proxies are highly correlated with annual and seasonal mean time series of major SST indices in the northern tropical Atlantic (e.g., r = −0.71 for correlation between the index of North Tropical Atlantic SST anomaly and Sr/Ca, and r = −0.70 for δ18O; mean annual scale, p &lt; 0.001). Furthermore, the coral proxies capture the impact of the El Niño–Southern Oscillation on the northern tropical Atlantic during boreal spring. Thus fast‐growing Diploria strigosa corals are a promising new archive for the Atlantic Ocean.