Oxygen Isotope Composition of Arctica islandica Aragonite in the Context of Shell Architectural Organization: Implications for Paleoclimate Reconstructions

Abstract

The analysis of the stable isotopic composition of bivalve shells provides the data needed to construct climate records at high temporal resolution. Yet, the reproducibility of the results and the effect of microstructural organization on the isotopic signature and measurements have not been extensively studied. Here, we examine the architectural changes within Arctica islandica shells, specifically if samples from microstructurally different shell layers differ in respect to stable oxygen isotope values. The oxygen isotope profiles of two microstructurally different shell layers, each sampled at different temporal resolution, were compared to each other. Our results show that aragonite, collected from the layer that is dominated by cross‐acicular/lamellar structures, tends to be enriched in heavier oxygen isotopes compared to samples from portions of the outer shell layer dominated by homogeneous microstructure. In some cases, this difference exceeded 0.3 ‰, which can significantly affect the interpretation of a recorded environmental signal. Observed differences in stable oxygen isotope data may be associated with the physiology of the mollusk and the physical and chemical composition of studied shell layers.

Interspecific and Intrashell Stable Isotope Variation Among the Red Sea Giant Clams

“…In other bivalve taxa, different biomineralization processes between the outer crossed‐lamellar and inner prismatic shell layers may cause different isotopic fractionation. Trofimova et al (2018) found lower δ 18 O values in the outermost homogenous prismatic/granular portion of the outer shell layer of Arctica islandica relative to the crossed‐acicular/lamellar inner portion of the outer layer. This opposes the pattern in Tridacna , which have lower δ 18 O values in their crossed‐lamellar shell layer.…”

“…The lower oxygen isotope ratios recorded in the outer shell layer versus the inner layer suggests attention should be paid to the shell regions used when comparing the results of separate reconstructions. Additional investigations of intrashell oxygen isotope variability will be needed for other bivalve taxa, which are paleoclimate proxy targets (Cusack et al, 2016; Trofimova et al, 2018). We also found that shell carbon isotope ratios did not conform with known variations in photosymbiotic activity between closely related giant clam species, suggesting that different proxies should be prioritized in future efforts to identify photosymbiosis in fossil bivalves.…”

“…Any photosymbiotic effect on the extrapallial fluid of the animal might manifest as a systematic offset between δ 13 C values in the inner and outer shell layers. Physiologically driven offsets have been found in heterotrophic bivalves (Trofimova et al, 2018), and we wish to test the hypothesis that differing levels of photosymbiosis will introduce an interspecies spectrum within shell δ 13 C. An isotopic signal from photosymbiosis could reveal whether ancient reef-building bivalves, such as rudists, lithiotids, and megalodontids, were aided by symbiotic algae, as hypothesized based on their shell morphology (Vermeij, 2013).…”

“…Additionally, aside from the work of Pätzold et al and Elliot et al, these investigations typically use transects from the inner or outer shell layers of giant clams to reconstruct temperature time series, but not both. However, because an increasing number of studies have identified offsets in paleotemperature estimates constructed from differing shell layers in bivalves (Cusack et al, 2016; Trofimova et al, 2018), a population‐wide comparison of inner and outer layers of many individuals would help identify whether such an offset exists for Tridacna .…”

Temperature-induced microstructural changes in shells of laboratory-grown Arctica islandica (Bivalvia)

“…For example, the homogeneous microstructure (HOM) in the outer portion of the outer shell layer of A. islandica gradually merges into crossed-acicular (CA) microstructure toward the myostracum (Fig 1A-1D) [27,28]. In addition, the thickness of the HOM portion increases with ontogenetic age [29]. Yet, fine complex crossed-lamellar (FCCL) microstructure occurs in the inner portion of the outer shell layer and in the inner shell layer (ISL) [28,30].…”