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1. The CoralHydro2k database: a global, actively curated compilation of coral δ ¹⁸ O and Sr ∕ Ca proxy records of tropical ocean hydrology and temperature for the Common Era

   https://doi.org/10.5194/essd-15-2081-2023  

   Walter, Rachel M.; Sayani, Hussein R.; Felis, Thomas; Cobb, Kim M.; Abram, Nerilie J.; Arzey, Ariella K.; Atwood, Alyssa R.; Brenner, Logan D.; Dassié, Émilie P.; DeLong, Kristine L.; et al (January 2023, Earth System Science Data)

   Abstract. The response of the hydrological cycle to anthropogenic climatechange, especially across the tropical oceans, remains poorly understood due to the scarcity of long instrumental temperature and hydrological records. Massive shallow-water corals are ideally suited to reconstructing past oceanic variability as they are widely distributed across the tropics,rapidly deposit calcium carbonate skeletons that continuously record ambient environmental conditions, and can be sampled at monthly to annualresolution. Climate reconstructions based on corals primarily use the stable oxygen isotope composition (δ18O), which acts as a proxy for sea surface temperature (SST), and the oxygen isotope composition ofseawater (δ18Osw), a measure of hydrological variability. Increasingly, coral δ18O time series are paired with time series of strontium-to-calcium ratios (Sr/Ca), a proxy for SST, from the same coral to quantify temperature and δ18Osw variabilitythrough time. To increase the utility of such reconstructions, we presentthe CoralHydro2k database, a compilation of published, peer-reviewed coral Sr/Ca and δ18O records from the Common Era (CE). The database contains 54 paired Sr/Ca–δ18O records and 125 unpaired Sr/Ca or δ18O records, with 88 % of these records providing data coverage from 1800 CE to the present. A quality-controlled set of metadata with standardized vocabulary and units accompanies each record, informing the useof the database. The CoralHydro2k database tracks large-scale temperatureand hydrological variability. As such, it is well-suited for investigationsof past climate variability, comparisons with climate model simulationsincluding isotope-enabled models, and application in paleodata-assimilation projects. The CoralHydro2k database is available in Linked Paleo Data (LiPD) format with serializations in MATLAB, R, and Python and can be downloaded from the NOAA National Center for Environmental Information's Paleoclimate Data Archive at https://doi.org/10.25921/yp94-v135 (Walter et al., 2022). 

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

   https://doi.org/10.1029/2021PA004323  

   Cheung, Anson H.; Cole, Julia E.; Thompson, Diane M.; Vetter, Lael; Jimenez, Gloria; Tudhope, Alexander W. (December 2021, Paleoceanography and Paleoclimatology)

   Abstract Coral 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 modernPorites lobatafrom 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 dataafterthe 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. 

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3. A Multi-Proxy Speleothem Approach to Understanding the South American Monsoon System During the Last Glacial Cycle

   Parmenter, D; Olson, E J; Velasquez, H; Clavel, A; Verheyden, A; Piccirillo, L; Rodbell, DT; Gillikin, DP; Edwards, RL (December 2023, American Geophysical Union)

   For the past few decades, many researchers have sought to understand how tropical hydroclimate responds to climate change via lakes, marine sediments, and speleothems records. Speleothem δ18O records throughout South America have shown that regional rainfall responds to Northern Hemisphere forcing on the millennial scale. Areas under the influence of the South Atlantic Convergence Zone (SACZ) have also shown a close relationship with local insolation on longer timescales. However, apart from the Cruz et al. (2007) record in Southern Brazil, long-term speleothem records throughout the continent have relied primarily on stable oxygen isotopes and are therefore limited to describing large-scale regional variability in rainfall. As such, many areas in South America still lack long-term records of local hydroclimate, which is critical to understanding how different components of the monsoon system respond to orbital and millennial-scale climate change. One proxy that has gained more attention in recent years is trace metal-to-calcium ratios (TM/Ca). Sr, Mg, and Ba to Ca ratios in speleothems are known in certain situations to respond to the degree of Prior Calcite Precipitation (PCP) above a drip site, a phenomenon directly tied to local aridity. In this study, we have obtained high-resolution TM/Ca measurements to pair with stable isotopes from samples spanning 23 to 66 ka from Huagapo Cave in the Peruvian Andes (11.27°S; 75.79°W). TM/Ca ratios in these samples are strongly correlated (R2>0.89), making them suitable for use as PCP proxies. We see that decreases in δ18O during Heinrich events are accompanied by a drop in TM/Ca. The period defined by the MIS 4/3 transition is accompanied by a simultaneous increase in TM/Ca and δ18O. TM/Ca and δ18O negatively correlate with local insolation for the entire record. Interestingly, the Paraíso Cave record from the Amazon Basin shows no correlation between regional or local hydroclimate and insolation during the last glacial period. The discrepancy between the two records and the close relationship between TM/Ca, δ18O, and local insolation in Huagapo samples, may call for a revised interpretation of Andes speleothem δ18O variability, which was originally thought to reflect rainout over the Amazon Basin. 

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4. Late Holocene increase of winter precipitation in mid-continental North America from a seasonally resolved speleothem record

   https://doi.org/10.1130/G50096.1  

   Batchelor, Cameron J.; Marcott, Shaun A.; Orland, Ian J.; Kitajima, Kouki (April 2022, Geology)

   Abstract Subannual climate reconstructions of the Holocene are rare despite the ability of such records to provide a better understanding of the underlying factors that drive subannual climate variability. We used specialized confocal laser fluorescent microscope imaging and automated secondary ion mass spectrometry microanalysis to resolve a seasonal oxygen isotope (δ18O) record of a late Holocene–aged (2.7–2.1 ka) speleothem from mid-continental North America. We did this by measuring intra-band δ18O variability (Δ18O) within 117 annual bands over a 600 yr span of the late Holocene. We interpret a change in Δ18O values after 2.4 ± 0.1 ka to reflect an increase in the amount of winter precipitation. Our study produced direct measurements of past seasonality, offers new insights into shifting seasonal precipitation patterns that occurred during the late Holocene in central North America, and adds a new tool for understanding the complex precipitation and temperature histories of this region. 

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5. Synthetic and practical reconstructions of SST and seawater pH using the novel multiproxy SMITE method

   https://doi.org/10.1371/journal.pone.0305607  

   Hughes, Hunter P; Thompson, Diane; Foster, Gavin L; Lees, Jonathan; Surge, Donna; Standish, Christopher D (June 2024, PLOS ONE)

   Pokkathappada, Abdul Azeez (Ed.)

   Geochemical proxies of sea surface temperature (SST) and seawater pH (pH_sw) in scleractinian coral skeletons are valuable tools for reconstructing tropical climate variability. However, most coral skeletal SST and pH_swproxies are univariate methods that are limited in their capacity to circumvent non-climate-related variability. Here we present a novel multivariate method for reconstructing SST and pH_swfrom the geochemistry of coral skeletons. Our Scleractinian Multivariate Isotope and Trace Element (SMITE) method optimizes reconstruction skill by leveraging the covariance across an array of coral elemental and isotopic data with SST and pH_sw. First, using a synthetic proxy experiment, we find that SMITE SST reconstruction statistics (correlation, accuracy, and precision) are insensitive to noise and variable calibration period lengths relative to Sr/Ca. While SMITE pH_swreconstruction statistics remain relative toδ¹¹B throughout the same synthetic experiment, the magnitude of the long-term trend in pH_swis progressively lost under conditions of moderate-to-high analytical uncertainty. Next, we apply the SMITE method to an array of seven coral-based geochemical variables (B/Ca,δ¹¹B, Li/Ca, Mg/Ca, Sr/Ca, U/Ca & Li/Mg) measured from two BermudanPorites astreoidescorals. Despite a <3.5 year calibration period, SMITE SST and pH_swestimates exhibit significantly better accuracy, precision, and correlation with their respective climate targets than the best single- and dual-proxy estimators. Furthermore, SMITE model parameters are highly reproducible between the two coral cores, indicating great potential for fossil applications (when preservation is high). The results shown here indicate that the SMITE method can outperform the most common coral-based SST and pH_swreconstructions methods to date, particularly in datasets with a large variety of geochemical variables. We therefore provide a list of recommendations and procedures for users to begin implementing the SMITE method as well as an open-source software package to facilitate dissemination of the SMITE method. 

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