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1. Shark teeth zinc isotope values document intrapopulation foraging differences related to ontogeny and sex

   https://doi.org/10.1038/s42003-023-05085-6  

   McCormack, Jeremy; Karnes, Molly; Haulsee, Danielle; Fox, Dewayne; Kim, Sora L. (July 2023, Communications Biology)

   Abstract Trophic ecology and resource use are challenging to discern in migratory marine species, including sharks. However, effective management and conservation strategies depend on understanding these life history details. Here we investigate whether dental enameloid zinc isotope (δ⁶⁶Zn_en) values can be used to infer intrapopulation differences in foraging ecology by comparing δ⁶⁶Zn_enwith same-tooth collagen carbon and nitrogen (δ¹³C_coll, δ¹⁵N_coll) values from critically endangered sand tiger sharks (Carcharias taurus) from Delaware Bay (USA). We document ontogeny and sex-related isotopic differences indicating distinct diet and habitat use at the time of tooth formation. Adult females have the most distinct isotopic niche, likely feeding on higher trophic level prey in a distinct habitat. This multi-proxy approach characterises an animal’s isotopic niche in greater detail than traditional isotope analysis alone and shows that δ⁶⁶Zn_enanalysis can highlight intrapopulation dietary variability thereby informing conservation management and, due to good δ⁶⁶Zn_enfossil tooth preservation, palaeoecological reconstructions. 

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2. Probing the Ecology and Climate of the Eocene Southern Ocean With Sand Tiger Sharks Striatolamia macrota

   https://doi.org/10.1029/2020PA003997  

   Kim, Sora L.; Zeichner, Sarah S.; Colman, Albert S.; Scher, Howie D.; Kriwet, Jürgen; Mörs, Thomas; Huber, Matthew (December 2020, Paleoceanography and Paleoclimatology)

   Abstract Many explanations for Eocene climate change focus on the Southern Ocean—where tectonics influenced oceanic gateways, ocean circulation reduced heat transport, and greenhouse gas declines prompted glaciation. To date, few studies focus on marine vertebrates at high latitudes to discern paleoecological and paleoenvironmental impacts of this climate transition. The Tertiary Eocene La Meseta (TELM) Formation has a rich fossil assemblage to characterize these impacts;Striatolamia macrota, an extinct (†) sand tiger shark, is abundant throughout the La Meseta Formation. Body size is often tracked to characterize and integrate across multiple ecological dimensions. †S. macrotabody size distributions indicate limited changes during TELMs 2–5 based on anterior tooth crown height (n = 450, mean = 19.6 ± 6.4 mm). Similarly, environmental conditions remained stable through this period based on δ¹⁸O_PO4values from tooth enameloid (n = 42; 21.5 ± 1.6‰), which corresponds to a mean temperature of 22.0 ± 4.0°C. Our preliminaryε_Nd(n = 4) results indicate an early Drake Passage opening with Pacific inputs during TELM 2–3 (45–43 Ma) based on single unit variation with an overall radiogenic trend. Two possible hypotheses to explain these observations are (1) †S. macrotamodified its migration behavior to ameliorate environmental changes related to the Drake Passage opening, or (2) the local climate change was small and gateway opening had little impact. While we cannot rule out an ecological explanation, a comparison with climate model results suggests that increased CO₂produces warm conditions that also parsimoniously explain the observations. 

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3. Shelf Ecosystems Along the U.S. Atlantic Coastal Plain Prior to and During the Paleocene‐Eocene Thermal Maximum: Insights Into the Stratigraphic Architecture

   https://doi.org/10.1029/2022PA004475  

   Doubrawa, Monika; Stassen, Peter; Robinson, Marci M.; Babila, Tali L.; Zachos, James C.; Speijer, Robert P. (October 2022, Paleoceanography and Paleoclimatology)

   Abstract The Paleocene‐Eocene Thermal Maximum (PETM) is the most pronounced global warming event of the early Paleogene related to atmospheric CO₂increases. It is characterized by negative δ¹⁸O and δ¹³C excursions recorded in sedimentary archives and a transient disruption of the marine biosphere. Sites from the U.S. Atlantic Coastal Plain show an additional small, but distinct δ¹³C excursion below the onset of the PETM, coined the “pre‐onset excursion” (POE), mimicking the PETM‐forced environmental perturbations. This study focuses on the South Dover Bridge core in Maryland, where the Paleocene‐Eocene transition is stratigraphically constrained by calcareous nannoplankton and stable isotope data, and in which the POE is well‐expressed. The site was situated in a middle neritic marine shelf setting near a major outflow of the paleo‐Potomac River system. We generated high‐resolution benthic foraminiferal assemblage, stable isotope, trace‐metal, grain‐size and clay mineralogy data. The resulting stratigraphic subdivision of this Paleocene‐Eocene transition is placed within a depth transect across the paleoshelf, highlighting that the PETM sequence is relatively expanded. The geochemical records provide detailed insights into the paleoenvironment, developing from a well‐oxygenated water column in latest Paleocene to a PETM‐ecosystem under severe biotic stress‐conditions, with shifts in food supply and temperature, and under dysoxic bottom waters in a more river‐dominated setting. Environmental changes started in the latest Paleocene and culminated atthe onset of the PETM, hinting to an intensifying trigger rather than to an instantaneous event at the Paleocene‐Eocene boundary toppling the global system. 

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4. Seasonally Variable Aquifer Discharge and Cooler Climate in Bermuda During the Last Interglacial Revealed by Subannual Clumped Isotope Analysis

   https://doi.org/10.1029/2020PA004145  

   Zhang, Jade Z.; Petersen, Sierra V.; Winkelstern, Ian Z.; Lohmann, Kyger C. (May 2021, Paleoceanography and Paleoclimatology)

   Abstract Faunal analog reconstructions suggest that Last Interglacial (MIS 5e) sea surface temperatures were cooler around Bermuda and in the Caribbean than modern climate. Here we describe new and revised clumped isotope measurements ofCittarium picafossil shells supporting previous findings of cooler than modern temperatures in Bermuda during the Last Interglacial. We resolve temperature and δ¹⁸O_wdifferences between two closely located and apparently coeval sites described in Winkelstern et al. (2017),https://doi.org/10.1002/2016pa003014through reprocessing raw isotopic data with the updated Brand/IUPAC parameters. New subannual‐resolution clumped isotope data reveal large variations in δ¹⁸O_wout of phase with seasonal temperature changes (i.e., lower δ¹⁸O_wvalues in winter). Supported by modern δ¹⁸O_wmeasurements identifying similar processes occurring today, we suggest past variations in coastal δ¹⁸O_wwere driven by seasonally variable freshwater discharge from a subterranean aquifer beneath the island. Taken together, our results emphasize the importance of δ¹⁸O_win controlling carbonate δ¹⁸O, and suggest that typical assumptions of constant δ¹⁸O_wshould be made cautiously in nearshore settings and can contribute to less accurate reconstructions of paleotemperature. 

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5. Seasonal SIMS δ18O record in Astarte borealis from the Baltic Sea tracks a modern regime shift in the NAO

   https://doi.org/10.3389/fmars.2023.1293823  

   Hughes, Hunter P; Surge, Donna; Orland, Ian J; Zettler, Michael L; Moss, David K (December 2023, Frontiers in Marine Science)

   IntroductionAstarte borealisholds great potential as an archive of seasonal paleoclimate, especially due to its long lifespan (several decades to more than a century) and ubiquitous distribution across high northern latitudes. Furthermore, recent work demonstrates that the isotope geochemistry of the aragonite shell is a faithful proxy of environmental conditions. However, the exceedingly slow growth rates ofA. borealisin some locations (<0.2mm/year) make it difficult to achieve seasonal resolution using standard micromilling techniques for conventional stable isotope analysis. Moreover, oxygen isotope (δ¹⁸O) records from species inhabiting brackish environments are notoriously difficult to use as paleoclimate archives because of the simultaneous variation in temperature and δ¹⁸O_watervalues. MethodsHere we use secondary ion mass spectrometry (SIMS) to microsample anA. borealisspecimen from the southern Baltic Sea, yielding 451 SIMS δ¹⁸O_shellvalues at sub-monthly resolution. ResultsSIMS δ¹⁸O_shellvalues exhibit a quasi-sinusoidal pattern with 24 local maxima and minima coinciding with 24 annual growth increments between March 1977 and the month before specimen collection in May 2001. DiscussionAge-modeled SIMS δ¹⁸O_shellvalues correlate significantly with bothin situtemperature measured from shipborne CTD casts (r² = 0.52, p<0.001) and sea surface temperature from the ORAS5-SST global reanalysis product for the Baltic Sea region (r² = 0.42, p<0.001). We observe the strongest correlation between SIMS δ¹⁸O_shellvalues and salinity when both datasets are run through a 36-month LOWESS function (r² = 0.71, p < 0.001). Similarly, we find that LOWESS-smoothed SIMS δ¹⁸O_shellvalues exhibit a moderate correlation with the LOWESS-smoothed North Atlantic Oscillation (NAO) Index (r² = 0.46, p<0.001). Change point analysis supports that SIMS δ¹⁸O_shellvalues capture a well-documented regime shift in the NAO circa 1989. We hypothesize that the correlation between the SIMS δ¹⁸O_shelltime series and the NAO is enhanced by the latter’s influence on the regional covariance of water temperature and δ¹⁸O_watervalues on interannual and longer timescales in the Baltic Sea. These results showcase the potential for SIMS δ¹⁸O_shellvalues inA. borealisshells to provide robust paleoclimate information regarding hydroclimate variability from seasonal to decadal timescales. 

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