Strong biological controls on Sr/Ca ratios in aragonitic marine bivalve shells

Abstract

[1] It is well known that skeletal remains of carbonate secreting organisms can provide a wealth of information about past environments. Sr/Ca ratios have been successfully used as a temperature proxy in corals and sclerosponges. Previous work on aragonitic bivalve shells has not been conclusive but suggests a major control of growth rate on Sr/Ca ratios. As many studies have used bivalve growth rates to determine temperature, we tested if Sr/Ca ratios could predict temperature through its relationship with growth rate. Shells from the two species of clams from the same family (veneroidea) studied here, Saxidomus giganteus and Mercenaria mercenaria, show vastly different seasonal Sr/Ca profiles. A strong relationship between average annual Sr/Ca ratios and annual growth rate was found in S. giganteus shells from both Washington (R 2 = 0.87) and Alaska (R 2 = 0.64), USA, but not in M. mercenaria shells from North Carolina, USA. Furthermore, the Sr/Ca-growth rate relationship was also evident upon a more detailed inspection of subannual growth rates in S. giganteus (R 2 = 0.73). Although there were significant positive correlations between Sr/Ca ratios and temperature in S. giganteus shells, the correlations were weak (0.09 < R 2 < 0.27), and thus Sr/Ca ratios cannot be used as a reliable temperature proxy in these species of aragonitic bivalves. It is clear from this study that Sr/Ca ratios are not under thermodynamic control in either clam species, since thermodynamics predict a negative correlation between Sr/Ca ratios and temperature in aragonite. This points toward dominance of biological processes in the regulation of Sr 2+ . This is also reflected by the largely differing Sr/Ca partition coefficients (D Sr ) in these shells (D Sr % 0.25), when compared to inorganic, coral, and sclerosponge studies (D Sr % 1), all of which show a negative dependence of Sr/Ca on temperature. We suggest that caution be taken when using Sr/Ca in any biogenic aragonite as a temperature proxy when the D Sr greatly deviates from one, as this indicates the dominance of biological controls on Sr/Ca ratios.

Past daily light cycle recorded in the strontium/calcium ratios of giant clam shells

“…However, available reports of Sr/Ca measurements on bivalve shells suggest no temperature control 20,21 . Also, a decreasing trend of water temperature observed from 1-13 October (Fig.…”

“…Elemental fractionation that acts on these steps can cause variability in Sr/Ca ratio in aragonite skeleton. In the first step, two mechanisms have been hypothesized to cause Sr/Ca fractionation in bivalve shells 20,21 . They are the Ca-ATPase hypothesis and the Ca-channel hypothesis.…”

Increased seasonality in the Western Mediterranean during the last glacial from limpet shell geochemistry

“…Freitas et al, 2008;Gillikin et al, 2005;Vander Putten et al, 2000). Without such an independent palaeothermometer, considerable uncertainties can be introduced to δ 18 O-derived palaeo-SST estimates due to the necessity of assuming a seawater δ 18 O value.…”

Barium uptake into the shells of the common mussel (Mytilus edulis) and the potential for estuarine paleo-chemistry reconstruction

“…It can 540 generally be considered that when the partition coefficient of a particular element (Weber, 1973), which is similar to abiogenic aragonite (Kinsman and Holland, 545 1969), whereas in aragonitic bivalve shells the D Sr is around 0.25 and there is no link with SST (Gillikin et al, 2005a) …”

“…Unlike corals and foraminifera, much of the bivalve data presented suggests that many of these elemental profiles (e.g., Sr, Mn, Pb, U), which often largely differ from expected concentrations based on inorganic and other biogenic carbonates, cannot be used as proxies of environmental conditions (e.g., Stecher et al, 1996;Purton et al, 1999;Vander Putten et al, 2000; 70 Takesue and van Geen, 2004;Freitas et al, 2005;Gillikin et al, 2005a;Gillikin, 2005). There have been some promising reports of bivalve shell Mg/Ca ratios as a proxy of sea surface temperature (SST) (Klein et al, 1996), but other reports illustrate that this is not always the case, and is apparently strongly species specific (Vander Putten et al, 2000;Takesue and van Geen, 2004;Freitas et al, 2005;Gillikin, 2005;75 Lorrain et al, in press).…”