Strong Coupling between Biomineral Morphology and Sr/Ca of Arctica islandica (Bivalvia)—Implications for Shell Sr/Ca-Based Temperature Estimates

Abstract

Bivalve shells serve as powerful high-resolution paleoclimate archives. However, the number of reliable temperature proxies is limited. It has remained particularly difficult to extract temperature signals from shell Sr/Ca, although Sr is routinely employed in other biogenic aragonites. In bivalves, Sr/Ca is linked to the prevailing microstructure and is sometimes affected by kinetics. Here, the hypothesis is tested that temperature can be reconstructed from shell Sr/Ca once microstructure and/or growth-rate-related bias has been mathematically eliminated. Therefore, the relationship between Sr/Ca and increment width, as well as biomineral unit size, has been studied in three different shell portions of field-grown Arctica islandica specimens. Subsequently, microstructure and/or growth-rate-related variation was removed from Sr/Ca data and residuals compared to temperature. As demonstrated, the hypothesis could not be verified. Even after detrending, Sr/Ca remained positively correlated to water temperature, which contradicts thermodynamic expectations and findings from inorganic aragonite. Any temperature signal potentially recorded by shell Sr/Ca is overprinted by other environmental forcings. Unless these variables are identified, it will remain impossible to infer temperature from Sr/Ca. Given the coupling with the biomineral unit size, a detailed characterization of the microstructure should remain an integral part of subsequent attempts to reconstruct temperature from Sr/Ca.

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

“…In the outer portion of the outer shell layer (oOSL), annual growth increments consist predominantly of homogeneous (HOM) ultrastructure, whereas in the inner portion of the outer shell layer (iOSL) crossed-acicular (CA) and finecomplex crossed-lamellar ultrastructures prevail (Ropes, 1984). As recently demonstrated (Brosset et al, 2022), the shell Sr/Ca values in annual increments (reflecting the main growing season) of juvenile field-grown A. islandica specimens from NE Iceland are weakly positively correlated to water temperature. After mathematical elimination of growth rate and/or ultrastructure-related bias, only a slightly stronger positive correlation was observed implying that other environmental variables exert a strong control on shell Sr/Ca (Brosset et al, 2022).…”

“…As recently demonstrated (Brosset et al, 2022), the shell Sr/Ca values in annual increments (reflecting the main growing season) of juvenile field-grown A. islandica specimens from NE Iceland are weakly positively correlated to water temperature. After mathematical elimination of growth rate and/or ultrastructure-related bias, only a slightly stronger positive correlation was observed implying that other environmental variables exert a strong control on shell Sr/Ca (Brosset et al, 2022). Notably, the positive relationship between Sr/Ca and temperature differs from such found in synthetic aragonite and scleractinian corals (negative correlation) but agrees with thermodynamic expectations and the lattice strain model (Gaetani and Cohen, 2006).…”

“…The trace element analysis of the shells of the laboratory-grown specimens was done by LA-ICP-MS at the Institute of Geosciences, University of Mainz, following the same method and settings used in Brosset et al (2022) for field-grown specimens. Briefly, the system consisted of a 193 nm ArF Eximer laser (ESI NWR 193; repetition rate = 10 Hz; energy density = approx.…”

“…Subsequently, these models were employed to compute temperature prediction intervals (1s) based on detrended and undetrended shell Sr/Ca. These intervals were then compared with data gathered from field-grown A. islandica specimens from NE Iceland (Brosset et al, 2022). Alternatively, for all analyses, models using specimen-specific medians or non-weighted median values, as well as linear and/or natural logarithm detrending and regressions to water temperature for both experiments combined or The regression curves were examined in the outer and inner portions of the outer shell layer (oOSL and iOSL, respectively) for undetrended AR and EL as well as growth rate (GR)-detrended AR and EL data.…”

“…While Sr/Ca and Mg/Ca values are routinely used in many nonmolluscan biogenic carbonates to reconstruct temperature histories (Beck et al, 1992;Rosenheim et al, 2004;Corrège, 2006), their use in bivalve shells (Dodd, 1965;Stecher et al, 1996;Zhao et al, 2017) is controversially debated, even in the same species. Reports on A. islandica range from positive (Hart and Blusztajn, 1998;Toland et al, 2000;Brosset et al, 2022) to negative (Schöne et al, 2013;Yan et al, 2013) to no correlation (Wanamaker and Gillikin, 2019) between shell Sr/Ca or Mg/Ca and temperature. Partly, these controversial findings are related to the experimental design, e.g., averaging Sr/Ca data from very slow and fast-growing shell portions, growth lines and growth increments, respectively (Schöne et al, 2013).…”