Mg/Ca and d18O measurements of planktonic foraminifera from Sargasso Sea

Paired Mg/Ca and d18O measurements on planktonic foraminiferal species (G. ruber white, G. ruber pink, G. sacculifer, G. conglobatus, G. aequilateralis, O. universa, N. dutertrei, P. obliquiloculata, G. inflata, G. truncatulinoides, G. hirsuta, and G. crassaformis) from a 6-year sediment trap time series in the Sargasso Sea were used to define the sensitivity of foraminiferal Mg/Ca to calcification temperature. Habitat depths and calcification temperatures were estimated from comparison of d18O of foraminifera with equilibrium calcite, based on historical temperature and salinity data. When considered together, Mg/Ca (mmol/mol) of all species, except two, show a significant (r = 0.93) relationship with temperature (T °C) of the form Mg/Ca = 0.38 (±0.02) exp 0.090 (±0.003)T, equivalent to a 9.0 ± 0.3% change in Mg/Ca for a 1°C change in temperature. Small differences exist in calibrations between species and between different size fractions of the same species. O. universa and G. aequilateralis have higher Mg/Ca than other species, and in general, data can be best described with the same temperature sensitivity for all species and pre-exponential constants in the sequence O. universa > G. aequilateralis = G. bulloides > G. ruber = G. sacculifer = other species. This approach gives an accuracy of ±1.2°C in the estimation of calcification temperature. The 9% sensitivity to temperature is similar to published studies from culture and core top calibrations, but differences exist from some literature values of pre-exponential constants. Different cleaning methodologies and artefacts of core top dissolution are probably implicated, and perhaps environmental factors yet understood. Planktonic foraminiferal Mg/Ca temperature estimates can be used for reconstructing surface temperatures and mixed and thermocline temperatures (using G. ruber pink, G. ruber white, G. sacculifer, N. dutertrei, P. obliquiloculata, etc.). The existence of a single Mg thermometry equation is valuable for extinct species, although use of species-specific equations will, where statistically significant, provide more accurate evaluation of Mg/Ca paleotemperature.

本数据集基于马尾藻海（Sargasso Sea）为期6年的沉积物捕集器（sediment trap）时间序列，对12种浮游有孔虫（planktonic foraminifera）开展了镁钙比（Mg/Ca）与氧同位素δ¹⁸O（d18O）的配对测量，具体物种包括：白氏红拟抱球虫（G. ruber white）、粉红红拟抱球虫（G. ruber pink）、囊球拟抱球虫（G. sacculifer）、团聚拟抱球虫（G. conglobatus）、平切拟抱球虫（G. aequilateralis）、万能圆球虫（O. universa）、杜氏新抱球虫（N. dutertrei）、斜室抱球虫（P. obliquiloculata）、膨胀拟抱球虫（G. inflata）、截锥抱球虫（G. truncatulinoides）、多毛抱球虫（G. hirsuta）以及厚壳抱球虫（G. crassaformis），以此明确有孔虫Mg/Ca对钙化温度的响应敏感性。研究通过将有孔虫的δ¹⁸O与平衡方解石的δ¹⁸O进行对比，并结合历史温度与盐度数据，估算了各物种的栖息深度与钙化温度。整体而言，除两个物种外，其余所有物种的Mg/Ca（单位：mmol/mol）均与温度（T，单位：°C）呈现显著相关关系（相关系数r=0.93），其拟合公式为：Mg/Ca = 0.38(±0.02)exp[0.090(±0.003)T]，对应温度每变化1°C，Mg/Ca比值变化9.0±0.3%。不同物种之间，以及同一物种的不同粒级组分之间，其校准关系存在小幅差异。万能圆球虫（O. universa）与平切拟抱球虫（G. aequilateralis）的Mg/Ca比值高于其余物种；总体而言，所有物种可采用统一的温度响应敏感性参数，而预指数常数的排序为：O. universa > G. aequilateralis = G. bulloides > G. ruber = G. sacculifer = 其余物种。该方法估算钙化温度的精度可达±1.2°C。其对温度的9%响应敏感性与已发表的培养实验及岩芯顶部（core top）校准研究结果相近，但预指数常数的数值与部分文献报道存在差异。这种差异可能与不同的样品清洗方法、岩芯顶部溶解作用带来的人为偏差，以及尚未明确的环境因素有关。浮游有孔虫的Mg/Ca温度估算结果可用于重建表层海水温度、混合层温度及温跃层温度（可通过粉红红拟抱球虫、白氏红拟抱球虫、囊球拟抱球虫、杜氏新抱球虫、斜室抱球虫等物种实现）。尽管针对特定物种的校准方程在统计学上显著时，可更精准地评估Mg/Ca古温度（paleotemperature）重建结果，但通用的Mg温度计方程对于已灭绝物种的研究具有重要价值。