Antarctic bivalve Limatula hodgsoni as a proxy for ambient oceanographic conditions at McMurdo Sound
|Affiliation(s)||1 : Scripps Institution of Oceanography, USA|
|Keyword(s)||bivalve, paleontology, stable isotopes, trace elements|
Bivalves have been used as proxies to reconstruct ancient oceanographic conditions based on the assumption that their shell calcite is precipitated in near isotopic equilibrium with ambient seawater. Few studies, however, have tested the presumption of equilibrium precipitation for specimens from polar environments. Furthermore, reported isotopic values for polar specimens are largely indicative of disequilibrium precipitation, leading to the conclusion that bivalves living in extreme environments may be poor recorders of ambient oceanography. An analysis of shell chemistry of the Antarctic bivalve Limatula hodgsoni is compared to the local oceanographic data at Cape Armitage, McMurdo Sound, to assess the suitability of extreme environment bivalves as environmental proxies. Results reveal that significant kinetic fractionation occurs during primary-layer shell secretion, resulting in whole-shell isotopic compositions that do not reflect equilibrium with ambient seawater. Secondary-layer shell calcite, however, is less affected by biological fractionation and exhibits isotopic compositions that fall within the range of predicted equilibrium values. Additionally, whole-shell concentrations of elements including Ba, Cd, Cr, Fe, Mn, Na, Sr, and Zn exhibit trends that are interpreted to reflect their relative concentrations in ambient seawater. Concentrations of Mg and B, however, are found to be largely controlled by physiological processes related to bivalve growth rate. While this study concludes that the shell chemistry of L. hodgsoni does reflect the local oceanographic conditions, the reliability of extreme-environment bivalves should be assessed on a species basis as differences in bivalve physiology and microstructure can significantly influence the degree of equilibrium reflected in shell calcite.
The 18O/16O and 13C/12C ratios of shell calcite were determined by reacting approximately 0.1 mg of powdered shell material with 100% anhydrous phosphoric acid at 72°C for 4 hours. The CO2 released was analyzed using a Thermo-Finnigan Gas Bench coupled to a Thermo-Finnigan DeltaPlus XP Continuous-Flow Isotope-Ratio Mass Spectrometer (CF-IRMS). All isotope ratios are reported using the delta (δ) notation in per mil, relative to Vienna Pee Dee Belemnite (VPDB) international standard.
Major-element and trace-element concentrations were measured for 5-20 mg of whole- shell material sampled from the posterior and anterior portions of both valves for all specimens. Samples were powdered and digested in high-purity 2% HNO3 solution. Calcium concentration was analyzed using a Thermo Scientific iCAP 6500 Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) with analytical precision better than 2.3 %. Magnesium, Na, Sr, and trace - element concentrations (Al, B, Ba, Cd, Cr, Fe, Mn, Pb and Zn) were measured using a Thermo Scientific ELEMENT 2 High Resolution Inductively Coupled Plasma Mass Spectrometer (XR HR- ICP-MS). Precision (measured though duplicate analyses) is better than 3.8 % for Mg, 2.7 % for Na and 4.3 % for Sr