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1. A database of Holocene nearshore marine mollusc shell geochemistry from the Northeast Pacific

   https://doi.org/10.1594/PANGAEA.932671  

   Palmer, Hannah M ; Padilla Vriesman, Veronica ; Banker, Roxanne M ; Bean, Jessica R ( January 2021 , PANGAEA - Data Publisher for Earth & Environmental Science)

   We compiled a database of previously published oxygen and carbon isotope data from archaeological, archival, and modern marine molluscs from the North American coast of the Northeast Pacific (32oN to 50oN). This database includes oxygen and carbon isotope data from over 550 modern, archaeological, and sub-fossil shells from 8880 years before present (BP) to the present, from which there are 4,845 total δ¹³C and 5,071 total δ¹⁸O measurements. Database includes the following parameters: paper of original publication, publication year, unique shell identification, unique subsample identification, sample number (given by original authors), subsample number (given by original author), number of subsamples per shell (added here), age in years before present, species, source (midden or modern), latitude, longitude, calculated sea surface temperature (only if published by original authors), tidal height, life mode, habitat, archaeological trinomial (when applicable), oxygen isotope value, and carbon isotope value. Shell dating and sampling strategies vary among studies (1-118 samples per shell) and vary significantly by journal discipline. Data are from various bivalves and gastropod species, with Mytilus spp. being the most commonly analyzed taxon. This novel database can be used to investigate changes in nearshore sea surface conditions including warm-cool oscillations, heat waves, and upwelling intensity, and provides nearshore calcite δ¹³C and δ¹⁸O values that can be compared to the vast collections of offshore foraminifera calcite δ¹³C and δ¹⁸O data from marine sediment cores. By utilizing previously published geochemical data from midden and museum shells rather than sampling new specimens, future scientific research can reduce or omit the alteration or destruction of culturally valued specimens and sites. 

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2. Preservation of stable isotope signatures of amino acids in diagenetically altered Middle to Late Holocene archaeological mollusc shells

   https://doi.org/10.1016/j.gca.2023.05.005  

   Vokhshoori, N.L. ; Rick, T.C. ; Braje, T.J. ; McCarthy, M.D. ( July 2023 , Geochimica et Cosmochimica Acta)

   Stable isotope proxies measured in the proteinaceous fraction of archaeological mollusc shell represents an increasingly important archive for reconstructing past ecological and biogeochemical conditions of nearshore environments. A major issue, however, is understanding the impact of diagenetic alteration in sub-fossil shell isotope values. “Bulk” stable isotope values of nitrogen (δ15N), and especially carbon (δ13C) often shift strongly with increasing C/N ratios in degraded shell, resulting in unreliable data. Here, we examine preservation of an entirely new set of shell paleo-proxies, compound-specific isotopes of amino acids (CSI-AA). We examine carbon (δ13CAA) and nitrogen (δ15NAA) patterns and values from the organic fraction of California mussel (Mytilus californianus) shells from the California Channel Islands. Archaeological shell samples ranging in age from ca. 6,100 to 250 cal BP exhibiting a wide range of degradation states were collected from varied depositional environments (e.g., exposed coastal bluff, buried strata, etc.), and were directly compared to modern shells of the same species and region. Our results indicate organic matter C/N ratios as the best bulk diagnostic indicator of the relative degradation state of shell organic fraction, including changes at the molecular level. Modern shell organic C/N ratios ranged from 2.8 to 3.5, while those in archaeological shell were substantially elevated (3.4–9.5), exhibiting strong and significant negative correlations with bulk δ13C values, weight %C, and weight %N, and a significant but weaker correlation with δ15 N values. An additional “cleaning” step using weak NaOH helped to remove possible exogenous contaminants and improved bulk values of some samples. However, relative molar AA abundances revealed that some AAs, especially the two most abundant, Glycine and Alanine, progressively decreased with increasing C/N ratio. The loss of these amino acids permanently alters bulk isotope values regardless of removal of contaminants. Modeling the bulk isotope change expected due to amino acid molar composition showed major and predictable shifts in bulk δ13C values from selected AA loss, and similarly large but far more variable impacts from exogenous contaminants. In contrast to bulk data, key CSI-AA values and patterns remained almost entirely unaltered, even in the most degraded shell samples, closely matching expected biosynthetic isotope patterns in modern mussel shell. AA isotope proxies for “baseline” (δ15N-Phenylalanine and average δ13C-Essential AAs) and planktonic trophic structure (δ15N-Glutamic Acid and δ15N-Phenylalanine) were not statistically altered with degradation in any sample. Overall, we conclude that while bulk isotopes, particularly δ13C, are very likely to be unreliable in archaeological or subfossil shell with C/N ratios higher than ∼4.0, CSI-AA proxies can still be used to reconstruct past climatic and ecological conditions of the nearshore marine environment. 

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3. The Effects of Heating on δ18O and δ13C in Mytilus californianus Shell Carbonate: Implications for Paleoenvironmental Reconstruction and Season of Harvest

   Jazwa, Christopher S. ; Jantz, Samuel ( December 2019 , Journal of California and Great Basin anthropology)

   The use of stable oxygen (δ18O) and carbon (δ13C) isotopic ratios of marine shell carbonate is a powerful tool for reconstructing past sea surface temperatures (SST) and estimating season of harvest for shells from coastal archaeological sites. While methods for sampling shells and analyzing the resulting data are established, less is known about the effects of anthropogenic activity on the geochemistry of the shells. Through an experimental study in which we heat carbonate powder from Mytilus californianus shells, we show that mussels cooked by boiling or steaming were unlikely to have their isotopic composition altered by the process. Shells heated over coals, however, show evidence of both visible and structural changes and in some cases are depleted in δ13C and/or δ18O. This indicates that archaeologists should use caution in interpreting past SST or site seasonality from burned shells and should instead test intact, unburned shells. 

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4. 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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5. The Link Between Southern Portuguese Climate and the East Atlantic Mode from Highly Resolved Speleothem Records

   Chormann, A. ( January 2022 , PAGES Open Science Meeting)

   Climate in the Iberian Peninsula is impacted by both internal and external climate modes, which are expected to shift in position and intensity due to anthropogenic climate change. Examples of such modes include the North Atlantic Oscillation (NAO) and the East Atlantic mode (EA). Changes in the behavior in these regional climate modes could significantly alter water availability in the Iberian Peninsula, a region identified by model projections as particularly sensitive to future warming scenarios. There has been extensive research and paleoclimate reconstructions of the NAO and its impacts on Iberian climate. However, to date few paleoclimate records have been developed to evaluate the behavior of the EA over the late Holocene and into the present. The development of highly resolved regional paleoclimate records from Iberia is critical for improving the predictive capability of regional climate models under future warming scenarios and to determine the extent to which different teleconnection patterns are influencing climate. Here we present a near annually resolved stable carbon isotope (δ13C) and oxygen (δ18O) isotope time-series from three stalagmites from the Algarve region of southern Portugal from two caves within 2.3 km of each other. The southern coast of Portugal offers an ideal location to study the behavior of the EA due to the modulation of storm tracks coming across the North Atlantic Ocean into Iberia associated with the EA. U/Th dating indicates that our composite record spans the last millennia continuously through 2018 CE. Two stalagmites (GIA-19-1 and C-18-1) stopped growing around 1600 CE, during a dry interval, and sample GIA-19-2 grew continuously since the 15th century. GIA-19-2, with sub-annual resolution, is compared to modern instrumental records to evaluate the influence of specific environmental controls, including temperature and precipitation amounts. Isotope data from all three stalagmites exhibit substantial multidecadal variability indicating relatively wet and dry intervals. Based on our initial results, it is likely that both temperature and precipitation amount effects are the dominant controls on isotopic variability in these stalagmites. Comparison of the GIA-19-2 oxygen isotope time-series with the instrumental index (1950 to present) and reconstructed index (1650 CE to present) of the EA mode shows strong coherence with both index records. Hence, multidecadal variability observed in our stalagmite isotope time series may provide insight into the historical behavior of the EA mode and its resulting impacts on southern Portuguese climate. 

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