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1. In Situ Geochemical Analysis of Organics in Growth Lines of Antarctic Scallop Shells: Implications for Sclerochronology

   https://doi.org/10.3390/min10060529  

   Pérez-Huerta, Alberto ; Walker, Sally E. ; Cappelli, Chiara ( June 2020 , Minerals)

   Bivalve shells are extensively used as bioarchives for paleoclimate and paleoenvironmental reconstructions. Proxy calibrations in recent shells are the basis for sclerochronology and the applications of geochemistry data to fossils. Shell geochemical information, however, could be altered with the disappearance of intercrystalline organic matrix components, including those linked to shell growth increments, during early diagenesis. Thus, an evaluation of the chemistry of such organics is needed for the correct use of sclerochronological records in fossil shells. Here, we use atom probe tomography (APT) for in situ geochemical characterization of the insoluble organic matrix in shell growth increments in the Antarctic scallop, Adamussium colbecki. We confirm the presence of carboxylated S-rich proteoglycans, possibly involved in calcite nucleation and growth in these scallops, with significant concentrations of magnesium and calcium. Diagenetic modification of these organic components could impact proxy data based on Mg/Ca ratios, but more importantly the use of the δ15N proxy, since most of the shell nitrogen is likely bound to the amide groups of proteins. Overall, our findings reinforce the idea that shell organics need to be accounted for in the understanding of geochemical proxies. 

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2. Documenting Environmental and Oceanic Variability in the Down East Coastal Region of the Gulf of Maine using the long-lived bivalve Arctica islandica

   Walton, A. ; Griffin, S.M. ; Wanamaker, A.D. ; Jarosinski, L. ( April 2022 , PAGES Open Science Meeting)

   The Gulf of Maine (GoM) is currently undergoing rapid environmental and ecological changes and this system is projected to become increasingly stressed in the coming decades. Thus understanding past spatial and temporal conditions of this region is key to understanding how future environmental changes and extreme events may impact fisheries and ecosystem dynamics in the GoM. Changes in the physical and chemical variations in the shells of mollusks can be used as a powerful proxy for studying past climates and environments. In this study, we used the growth and geochemistry signatures in the long-lived marine bivalve Arctica islandica collected from the Down East coastal region in the Gulf of Maine (Jonesport, ME) to evaluate past climatic and hydrographic variability in the northwestern North Atlantic Ocean. The recent collection of shells extends a previously developed master shell growth chronology by 11 years and now spans from 1954 to 2020 CE. Based on visual crossdating techniques, shell growth variability is highly coherent among the population indicating that environmental conditions are driving growth. Variability in annually resolved shell growth increments and stable oxygen isotope values are largely related to sea surface temperatures (SSTs) and water mass properties of the Eastern Maine Coastal Current. This master shell growth chronology and annually-resolved isotope series will fill data gaps prior to instrumental records and allow us to better understand the spatial oceanographic variability in the GoM. 

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3. Nitrogen and oxygen isotopes in the shell of the Antarctic scallop Adamussium colbecki as a proxy for sea-ice cover in Antarctica.

   https://doi.org/10.1130/abs/2019AM-338279  

   Camarra, S. ; Puhalski, E. ; Gillikin, D. ; Cronin, K. ; Walker, S.E. ( October 2019 , Geological Society of America Abstracts with Programs)

   null (Ed.)

   Adamussium colbecki is a large thin-shelled scallop common in Antarctic waters and well represented in the fossil record. Shell oxygen (δ18Os) and nitrogen isotopes in carbonate bound organic matter (δ15NCBOM) have the potential to record sea ice state over time. To test this hypothesis we will analyze A. colbecki shells from Explorers Cove and Bay of Sails, western McMurdo Sound, Antarctica. These sites have different sea ice states: persistent (multiannual) sea ice at Explorers Cove and annual sea ice (that melts out every year) at Bay of Sails. Six adults shells collected at these sites in 2008 (3 from each site) and two juveniles collected in 2016 from Explorers Cove will be serially sampled for δ18Os values from the growing shell margin to the umbo. We hypothesize that melting glacial ice will pulse freshwater with low δ18O values into the system, which will be recorded in as larger amplitude pulses in shells from the Bay of Sails, but as dampened pulses in Explorers Cove. Carbonate bound organic material will be sampled for δ15NCBOM values. Recent studies illustrated that δ15NCBOM values provide a similar proxy as soft tissue δ15N values (Gillikin et al., 2017, GCA, 200, 55–66, doi: 10.1016/j.gca.2016.12.008). The organic content of the shell is low and the shells are thin, so δ15NCBOM values will be more time averaged than δ18Os values. Nevertheless, sea-ice organic N should have higher δ15N values compared to open water organics due to nitrate draw down in the ice (Fripiat et al., 2014, Global Biogeochem. Cycles, 28, 115–130, doi:10.1002/2013GB004729). Thus we expect large differences between Explorers Cove with persistent sea ice cover and Bay of Sails where the sea ice melts out every year. We posit that oxygen and nitrogen isotopes in A. colbeckishells have a high potential to record sea ice cover. 

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4. Stable isotopes in bivalves as indicators of nutrient source in coastal waters in the Bocas del Toro Archipelago, Panama

   https://doi.org/10.7717/peerj.2278  

   Graniero, Lauren E. ; Grossman, Ethan L. ; O’Dea, Aaron ( August 2016 , PeerJ)

   To examine N-isotope ratios (¹⁵N/¹⁴N) in tissues and shell organic matrix of bivalves as a proxy for natural and anthropogenic nutrient fluxes in coastal environments,Pinctada imbricata,Isognomon alatus, andBrachidontes exustusbivalves were live-collected and analyzed from eight sites in Bocas del Toro, Panama. Sites represent a variety of coastal environments, including more urbanized, uninhabited, riverine, and oceanic sites. Growth under differing environmental conditions is confirmed byδ¹⁸O values, with open ocean Escudo de Veraguas shells yielding the highest averageδ¹⁸O (−1.0‰) value and freshwater endmember Rio Guarumo the lowest (−1.7‰). At all sites there is no single dominant source of organic matter contributing to bivalveδ¹⁵N andδ¹³C values. Bivalveδ¹⁵N andδ¹³C values likely represent a mixture of mangrove and seagrass N and C, although terrestrial sources cannot be ruled out. Despite hydrographic differences between end-members, we see minimalδ¹⁵N andδ¹³C difference between bivalves from the river-influenced Rio Guarumo site and those from the oceanic Escudo de Veraguas site, with no evidence for N from open-ocean phytoplankton in the latter. Populated sites yield relative¹⁵N enrichments suggestive of anthropogenic nutrient input, but lowδ¹⁵N values overall make this interpretation equivocal. Lastly,δ¹⁵N values of tissue and shell organic matrix correlate significantly for pterioideansP. imbricataandI. alatus. Thus for these species, N isotope studies of historical and fossil shells should provide records of ecology of past environments.

    

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5. Correlative chemical and elemental nano-imaging of morphology and disorder at the nacre-prismatic region interface in Pinctada margaritifera

   https://doi.org/10.1038/s41598-023-47446-5  

   O’Callahan, Brian T. ; Larsen, Amy ; Leichty, Sarah ; Cliff, John ; Gagnon, Alex C. ; Raschke, Markus B. ( December 2023 , Scientific Reports)

   Understanding biomineralization relies on imaging chemically heterogeneous organic–inorganic interfaces across a hierarchy of spatial scales. Further, organic minority phases are often responsible for emergent inorganic structures from the atomic arrangement of different polymorphs, to nano- and micrometer crystal dimensions, up to meter size mollusk shells. The desired simultaneous chemical and elemental imaging to identify sparse organic moieties across a large field-of-view with nanometer spatial resolution has not yet been achieved. Here, we combine nanoscale secondary ion mass spectroscopy (NanoSIMS) with spectroscopic IRs-SNOM imaging for simultaneous chemical, molecular, and elemental nanoimaging. At the example ofPinctada margaritiferamollusk shells we identify and resolve ~ 50 nm interlamellar protein sheets periodically arranged in regular ~ 600 nm intervals. The striations typically appear ~ 15 µm from the nacre-prism boundary at the interface between disordered neonacre to mature nacre. Using the polymorph distinctive IR-vibrational carbonate resonance, the nacre and prismatic regions are consistently identified as aragonite ($${\overline{\nu }}_{a}=860$$${\overline{\nu }}_a=860$cm⁻¹) and calcite ($${\overline{\nu }}_{c}=880$$${\overline{\nu }}_c=880$cm⁻¹), respectively. We observe previously unreported morphological features including aragonite subdomains encapsulated in extensions of the prism-covering organic membrane and regions of irregular nacre tablet formation coincident with dispersed organics. We also identify a ~ 200 nm region in the incipient nacre region with less well-defined crystal structure and integrated organics. These results show with the identification of the interlamellar protein layer how correlative nano-IR chemical and NanoSIMS elemental imaging can help distinguish different models proposed for shell growth in particular, and how organic function may relate to inorganic structure in other biomineralized systems in general.

    

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