Microstructural Mapping of Arctica islandica Shells Reveals Environmental and Physiological Controls on Biomineral Size

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.

Sr/Ca in shells of laboratory-grown bivalves (Arctica islandica) serves as a proxy for water temperature – implications for (paleo)environmental research?

“…Most likely, the more uniform ultrastructure in conjunction with larger/more elongated BMUs of laboratory-grown specimens was the main reason why shell Sr/Ca was more strongly coupled with temperature than Sr/Ca of wild A. islandica. As illustrated by many previous studies, some element chemical properties, specifically for Sr and Mg, are strongly tied to the prevailing shell nanoarchitecture (e.g., Schöne et al, 2011;Karney et al, 2012;Schöne et al, 2013;Shirai et al, 2014;Höche et al, 2022). Accordingly, if the shell ultrastructure properties change as the result of physiological or environmental variations (other than temperature) (compare Höche et al, 2022), the relationship between shell Sr/Ca and temperature will break down.…”

“…For example, aside from HOM (oOSL) and CA (iOSL), no other ultrastructure type was formed under laboratory conditions after the acclimatization period. Specifically, fine complex cross-lamellar ultrastructure was absent, which was often reported in conjunction with stressful environmental conditions (Höche et al, 2022). Likewise, no ISP was produced, which typically occurs during annual growth line formation, i.e., during times of very slow growth (Dunca et al, 2009;Schöne et al, 2011;Karney et al, 2012;Schöne et al, 2013).…”

“…The right valves of the Baltic Sea (BS) specimens and the left valves of the Gulf of Maine (GOM) shells were mounted on acrylic glass cubes using a plastic welder (WIKO Multi Power 3). The subsequent preparation was done in accordance with methods described in Höche et al (2022). Briefly, a protective layer of metal epoxy resin (WIKO 05) was applied to the shell surfaces along the planned cutting axis.…”

“…The other polished cross-section of each specimen was used for ultrastructural properties analysis by means of SEM, following the method reported by Höche et al (2022). An ultrafine chemomechanical polishing, i.e., superficial oxidation of the intercrystalline organics and smoothing of the carbonate phase, was performed on the cross-sections to better distinguish individual biomineral units (BMUs).…”

“…Sr islandica shell growth rate differs significantly between populations and sampling localities (e.g., Begum et al, 2010;Höche et al, 2022). Therefore, data detrending for shell growth-related effects was performed independently between BS and GOM specimens.…”