Microstructural mapping of Arctica islandica shells reveals environmental and physiological controls on biomineral size
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Abstract
The shells of long-lived bivalves record environmental variability in their geochemical
signatures and are thus used extensively in marine high-resolution paleoclimate
studies. To possibly overcome the limitations of the commonly employed temperature
proxy, the δ18Oshell value, which requires knowledge of the seawater δ18O signature and is
prone to diagenetic overprint, the shell microstructures and the morphological properties
of individual biomineral units (BMUs) recently attracted research interest as an alternative
paleoclimate proxy. In shells of A. islandica, one of the most extensively used and best
studied sclerochronological archives, the size of the BMUs increases in warmer
temperatures under laboratory circumstances. This study assesses whether this
relationship persists under natural growth conditions or whether additional
environmental and physiological factors control the BMU size and bias temperature
reconstructions. For this purpose, shells from the surface waters of NE Iceland and the
Baltic Sea, as well as from deeper waters of the North Sea (100 and 243 m) were analyzed
by means of SEM. The BMU sizes were measured by means of image processing
software. Results demonstrate a strong effect of temperature on the BMU size at NE
Iceland and in the North Sea at 100 m depth. At 243 m depth, however, temperature
variability was likely too low (1.2°C) to evoke a microstructural change. At the Baltic Sea, the
BMUs remained small, possibly due to physiological stress induced by low salinity and/or
hypoxia. Thus, the size of BMUs of A. islandica shells only serves as a relative temperature
indicator in fully marine habitats, as long as seasonal temperature amplitudes exceed ca.
1°C. Furthermore, BMU size varied through lifetime with the largest units occurring during
age seven to nine. This pattern is possibly linked to the shell growth rate or to the amount of
metabolic energy invested in shell growth.
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Frontiers in Earth Science, 9, Frontiers Media, Lausanne, 2021, https://doi.org/10.3389/feart.2021.781305