Incorporation of Mg and Sr in calcite of cultured benthic foraminifera (Heterostegina depressa and Ammonia tepida) and seawater carbonate chemistry, 2010

We investigated the effect of the calcium concentration in seawater and thereby the calcite saturation state (omega) on the magnesium and strontium incorporation into benthic foraminiferal calcite under laboratory conditions. For this purpose individuals of the shallow-water species Heterostegina depressa (precipitating high-Mg calcite, symbiont-bearing) and Ammonia tepida (low-Mg calcite, symbiont-barren) were cultured in media under a range of [Ca2+], but similar Mg/Ca ratios. Trace element/Ca ratios of newly formed calcite were analysed with Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) and normalized to the seawater elemental composition using the equation DTE=(TE/Cacalcite)/(TE/Caseawater). The culturing study shows that DMg of A. tepida significantly decreases with increasing omega at a gradient of -4.3x10-5 per omega unit. The DSr value of A. tepida does not change with omega, suggesting that fossil Sr/Ca in this species may be a potential tool to reconstruct past variations in seawater Sr/Ca. Conversely, DMg of H. depressa shows only a minor decrease with increasing omega, while DSr increases considerably with omega at a gradient of 0.009 per omega unit. The different responses to seawater chemistry of the two species may be explained by a difference in the calcification pathway that is, at the same time, responsible for the variation in the total Mg incorporation between the two species. Since the Mg/Ca ratio in H. depressa is 50-100 times higher than that of A. tepida, it is suggested that the latter exhibits a mechanism that decreases the Mg/Ca ratio of the calcification fluid, while the high-Mg calcite forming species may not have this physiological tool. If the dependency of Mg incorporation on seawater [Ca2+] is also valid for deep-sea benthic foraminifera typically used for paleostudies, the higher Ca concentrations in the past may potentially bias temperature reconstructions to a considerable degree. For instance, 25 Myr ago Mg/Ca ratios in A. tepida would have been 0.2 mmol/mol lower than today, due to the 1.5 times higher [Ca2+] of seawater, which in turn would lead to a temperature underestimation of more than 2 °C.

本研究在实验室条件下，探究了海水钙浓度及其由此产生的方解石饱和状态（Ω，omega）对镁与锶向底栖有孔虫方解石中掺入量的影响。为此，我们选取两种浅水底栖有孔虫物种进行培养：携带共生体的高镁方解石沉积种异盖虫（Heterostegina depressa）与无共生体的低镁方解石沉积种卷转虫（Ammonia tepida），培养介质的钙浓度[Ca²+]梯度覆盖范围较广，但Mg/Ca比值保持一致。采用激光剥蚀电感耦合等离子体质谱法（Laser Ablation Inductively Coupled Plasma Mass Spectrometry，LA-ICP-MS）对新生方解石的微量元素/Ca比值进行分析，并通过公式D_TE=(TE/Ca)方解石/(TE/Ca)海水以海水元素组成为基准进行归一化处理。培养实验结果显示，卷转虫的镁分配系数D_Mg随Ω值升高呈显著下降趋势，变化斜率为-4.3×10^-5每Ω单位。而卷转虫的锶分配系数D_Sr则不受Ω值变化影响，这表明该物种化石的Sr/Ca比值可作为重建古海水Sr/Ca变化的潜在代用指标。与之相反，异盖虫的D_Mg仅随Ω值升高出现小幅下降，但其D_Sr则随Ω值升高显著上升，变化斜率为0.009每Ω单位。两种有孔虫对海水化学特征的响应差异，可通过其钙化途径的差异加以解释——该差异同时也是导致两者总镁掺入量存在分化的核心原因。由于异盖虫的Mg/Ca比值是卷转虫的50~100倍，我们推测卷转虫具备调控钙化液Mg/Ca比值的生理机制，而高镁方解石沉积物种则不具备该生理调控功能。若镁掺入量与海水[Ca²+]的相关性同样适用于古气候研究中常用的深海底栖有孔虫，则地质历史时期更高的海水钙浓度可能会在较大程度上导致温度重建结果出现偏差。例如，2500万年前的海水[Ca²+]浓度为现今的1.5倍，据此推算卷转虫的Mg/Ca比值会比现今低0.2 mmol/mol，进而导致温度重建结果被低估超过2℃。