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Paleoceanographic proxy archives encode information about the marine environment, which can yield key insights into past climate variability. In particular, marine calcifiers' stable oxygen isotopic composition (δ18Ocarb ${{\delta }^{18}\mathrm{O}}_{\mathrm{carb}}$) tells us about seawater temperature and oxygen isotope composition. Here, we use a proxy system model (PSM) framework to systematically evaluate the drivers of skeletal/shell δ18Ocarb ${{\delta }^{18}\mathrm{O}}_{\mathrm{carb}}$ in three taxa of fast‐growing marine calcifiers (crustose coralline algae, bivalves, and sclerosponges) from disparate locations, including high latitudes and deeper waters. We evaluate the impact of the quality of environmental data, the recording season in which the calcifier might document the environmental variability, and the importance of uncertainties on the PSM. Whereas the overall PSM‐modeled δ18Opseudocarb ${{\delta }^{18}\mathrm{O}}_{\mathrm{pseudocarb}}$ captured the measured δ18Ocarb ${{\delta }^{18}\mathrm{O}}_{\mathrm{carb}}$ well at some locations, local environmental variability derived from a reanalysis product and chronological uncertainties limit the ability to effectively model δ18Ocarb ${{\delta }^{18}\mathrm{O}}_{\mathrm{carb}}$ at other locations. Using the PSM approach we highlight the complexity of interpreting δ18Ocarb ${{\delta }^{18}\mathrm{O}}_{\mathrm{carb}}$ as seawater temperature and oxygen isotope composition in these remote locations. Plain Language Summary: Marine stony algae, clams, and sponges, form hard skeletons or shells and can live for hundreds of years, making them important recorders of their environment. Chemical measurements of the hard parts of these marine organisms capture changes in seawater temperature and how water cycles between the ocean and atmosphere as rainfall, both of which are changing due to human activities. By measuring the chemistry throughout the lifespan of the organisms, we can understand environmental variability before and since these human pressures. We test a simple model that relates the environmental changes to the chemical composition recorded in the hard parts of these marine organisms across a geographical range of ocean environments. We evaluate the importance of the quality of the environmental data, biological information about the growth of the organism, and uncertainty in the measurement itself on the model's effectiveness. We find that the model performs well at some locations, supporting its applications to these diverse types of marine life from different locations. Key Points: Proxy system modeling captures oxygen isotopic composition of marine calcifiersQuality of environmental data, calcifier growth characteristics, and age uncertainties impact model performanceAnalysis emphasizes the importance of strong environmental signals [ABSTRACT FROM AUTHOR]

An increased number of climate proxy records and more refined interpretation of proxy data are crucial to improve projections of future climate at high latitudes, where internal feedbacks amplify warming and established high-resolution climate archives are especially sparse. Encrusting coralline algae are being developed as a mid- to high-latitude marine climate archive. These long-lived algae form a solid high-Mg calcite skeleton with annual growth bands similar to those of trees and tropical corals. The oxygen isotope ratio of the algal skeleton (δ18Oalg) records local environmental and climatic factors, notably sea surface temperature and seawater δ18O. Here we assess the δ18Oalg–climate relationship in diverse environments across the algal habitat range utilizing two species of coralline algae from the genus Clathromorphum. Clathromorphum is widely distributed from the cold-temperate North Atlantic and Pacific to the Arctic Ocean and has recently yielded numerous climate reconstructions of up to 650 years in length. In this study, we calibrate δ18Oalg of four specimens to gridded temperature and salinity data, the latter a proxy for seawater δ18O. These specimens were collected from a variety of algal growth environments across the high-latitude Northern Hemisphere: two specimens from the Aleutian Archipelago, one from the Canadian Arctic, and one from the Gulf of Maine. Low winter temperatures and insolation restrict the months when algae record local climate in the δ18O of their skeletons; we therefore determine these response seasons by correlating monthly temperature and salinity anomalies with annual δ18Oalg anomalies at each site. We then average gridded data over months that correlate significantly (95% confidence interval) for regression with δ18Oalg. While the timing and nature of the climate signal vary across sites, we find significant relationships between δ18Oalg and either temperature or salinity averaged over the response season at three sites. Variation in local climatology among the four sites provides a physical explanation for calibration differences, compounded by uncertainties stemming from the proxy chronology, biological variability, temporal coverage, and sparse historical climate data. This work takes an essential step toward reconstructing high-latitude marine climate patterns with coralline algal δ18O and developing algae proxy system models.

The geochemistry of tropical coral skeletons is widely used in paleoclimate reconstructions. However, sub‐aerially exposed corals may be affected by diagenesis, altering the aragonite skeleton through partial dissolution, or infilling of secondary minerals like calcite. We analyzed the impact of intra‐skeletal calcite on the geochemistry (δ18O, Sr/Ca, Mg/Ca, Li/Mg, Li/Ca, U/Ca, B/Ca, Ba/Ca, and Mn/Ca) of a sub‐aerially exposed Porites sp. coral. Each micro‐milled coral sample was split into two aliquots for geochemistry and X‐ray diffraction (XRD) analysis to quantify the direct impact of calcite on geochemistry. We modified the sample loading technique for XRD to detect low calcite levels (1%–2%; total uncertainty = 0.33%, 2σ) in small samples (∼7.5 mg). Calcite content ranged from 0% to 12.5%, with higher percentages coinciding with larger geochemical offsets. Sr/Ca, Li/Mg, Li/Ca, and δ18O‐derived sea‐surface temperature (SST) anomalies per 1% calcite were +0.43°C, +0.24°C, +0.11°C, and +0.008°C, respectively. A 3.6% calcite produces a Sr/Ca‐SST signal commensurate with local SST seasonality (∼1.5°C), which we propose as the cut‐off level for screening calcite diagenesis in paleo‐temperature reconstructions. Inclusion of intra‐skeletal calcite decreases B/Ca, Ba/Ca, and U/Ca values, and increases Mg/Ca values, and can therefore impact reconstructions of paleoclimate and the carbonate chemistry of the semi‐isolated calcifying fluid in corals. This study emphasizes the importance of quantifying fine‐scale calcite diagenesis to identify coral preservation levels and assure robust paleoclimate reconstructions. Key Points: The modified sample loading technique adopted for X‐ray diffraction significantly reduced the uncertainty of calcite measurement down to 0.33%Temperature reconstructions based on Sr/Ca, Li/Mg, and Li/Ca are significantly affected by low levels of intra‐skeletal calcite (1%–2%)Impact of low levels of intra‐skeletal calcite on δ18O, Ba/Ca, Mn/Ca, and U/Ca is not significant [ABSTRACT FROM AUTHOR]

Obtaining dispersal estimates for a species is key to understanding local adaptation and population dynamics and to implementing conservation actions. Genetic isolation‐by‐distance (IBD) patterns can be used for estimating dispersal, and these patterns are especially useful for marine species in which few other methods are available. In this study, we genotyped coral reef fish (Amphiprion biaculeatus) at 16 microsatellite loci across eight sites across 210 km in the central Philippines to generate fine‐scale estimates of dispersal. All sites except for one followed IBD patterns. Using IBD theory, we estimated a larval dispersal kernel spread of 8.9 km (95% confidence interval of 2.3–18.4 km). Genetic distance to the remaining site correlated strongly with the inverse probability of larval dispersal from an oceanographic model. Ocean currents were a better explanation for genetic distance at large spatial extents (sites greater than 150 km apart), while geographic distance remained the best explanation for spatial extents less than 150 km. Our study demonstrates the utility of combining IBD patterns with oceanographic simulations to understand connectivity in marine environments and to guide marine conservation strategies. [ABSTRACT FROM AUTHOR]

Tropical Pacific trade-wind behavior is linked to the El Niño-Southern Oscillation and Pacific Decadal Variability, which modulate the rate of climate change. Despite their importance, high-resolution trade-wind observations span only the past 30–40 years and are too sparse to assess decadal wind variability and long-term trends. Previous work demonstrated that reef-building corals growing at the tropical Pacific island of Tarawa (2°N, 165°E) exhibit spikes in the manganese-to-calcium ratio (Mn/Ca) of their skeleton in response to a reversal of trade winds (i.e., westerly winds). Records of Mn/Ca from long-lived corals therefore hold great promise as indicators of past trade-wind variability. However, at other nearby islands with west-facing lagoons, there is a lag between westerly winds and coral Mn/Ca spikes and a significant difference in the magnitude of spikes between corals. To address uncertainties in how winds are recorded by coral Mn/Ca, we assess the reservoirs of Mn in the sediment, sediment pore spaces (porewater), and water column of Kiritimati's lagoon and inland lakes (1.9°N, 157.5°W). We find that insoluble dustborne Mn, once buried in lagoon sediments, becomes reduced and more soluble, leading to its accumulation in the sediment porewater. This Mn reservoir is then released into the water column when strong westerly wind events cause sufficient water-column mixing to reach lagoonal sediments. While this mechanism is consistent with what was previously proposed at Tarawa, the concentration of dissolved porewater Mn at Tarawa is nearly 20 times greater than at Kiritimati. We attribute this difference to the water depth of the sediment core from which porewater was sampled and the time elapsed between the most recent westerly wind event and core sampling, which both influence the "recharge time" of the porewater Mn reservoir. As such, lagoon bathymetry and morphology modulate lagoon water and sediment porewater Mn concentrations, which impact how westerly winds imprint their signal onto coral Mn/Ca. Armed with an improved understanding of the mechanism behind this coral Mn/Ca-trade wind relationship, we can better assess the reliability of this coral proxy through space and time and identify optimal sites for Mn/Ca-based wind reconstructions, paving the way for critical new insights into the role of winds in future climate change. [ABSTRACT FROM AUTHOR]

Reconstructions of global hydroclimate during the Common Era (CE; the past ~ 2000 years) are important for providing context for current and future global environmental change. Stable isotope ratios in water are quantitative indicators of hydroclimate on regional to global scales, and these signals are encoded in a wide range of natural geologic archives. Here we present the Iso2k database, a global compilation of previously published datasets from a variety of natural archives that record the stable oxygen (δ18O) or hydrogen (δ²H) isotopic composition of environmental waters, which reflect hydroclimate changes over the CE. The Iso2k database contains 756 isotope records from the terrestrial and marine realms, including: glacier and ground ice (205); speleothems (68); corals, sclerosponges, and mollusks (145); wood (81); lake sediments and other terrestrial sediments (e.g., loess) (158); and marine sediments (99). Individual datasets have temporal resolutions ranging from sub-annual to centennial, and include chronological data where available. A fundamental feature of the database is its comprehensive metadata, which will assist both experts and non-experts in the interpretation of each record and in data synthesis. Key metadata fields have standardized vocabularies to facilitate comparisons across diverse archives and with climate model simulated fields. This is the first global-scale collection of water isotope proxy records from multiple types of geological and biological archives. It is suitable for evaluating hydroclimate processes through time and space using large-scale synthesis, model-data intercomparison and (paleo)data assimilation. The Iso2k database is available for download at: https://doi.org/10.6084/m9.figshare.11553162 (McKay and Konecky, 2020). [ABSTRACT FROM AUTHOR]

Of the rise in global atmospheric temperature over the past century, nearly 30% occurred between 1910 and 1940 when anthropogenic forcings were relatively weak. This early warming has been attributed to internal factors, such as natural climate variability in the Atlantic region, and external factors, such as solar variability and greenhouse gas emissions. However, the warming is too large to be explained by external factors alone and it precedes Atlantic warming by over a decade. For the late twentieth century, observations and climate model simulations suggest that Pacific trade winds can modulate global temperatures, but instrumental data are scarce in the early twentieth century. Here we present a westerly wind reconstruction (1894-1982) from seasonally resolved measurements of Mn/Ca ratios in a western Pacific coral that tracks interannual to multidecadal Pacific climate variability. We then reconstruct central Pacific temperatures using Sr/Ca ratios in a coral from Jarvis Island, and find that weak trade winds and warm temperatures coincide with rapid global warming from 1910 to 1940. In contrast, winds are stronger and temperatures cooler between 1940 and 1970, when global temperature rise slowed down. We suggest that variations in Pacific wind strength at decadal timescales significantly influence the rate of surface air temperature change. [ABSTRACT FROM AUTHOR]