d18O Seasonality of Planktonic Foraminifera from Southern Ocean Sediment Traps: Latitudinal Gradients and Implications for Paleoclimate Reconstructions

The oxygen isotopic record obtained from Globigerina bulloides, Globorotalia inflata, and Neogloboquadrina pachyderma (s.) was analysed for 5 sediment traps moored in the Southern Ocean and Southwest Pacific. The traps extend from Subtropical to the Polar Frontal environments, providing the first analysis of seasonal foraminiferal d18O records from these latitudes. Comparison between the foraminiferal records and various equations for predicted d18O of calcite reveals that the predicted d18O is best captured by the equations of Epstein et al. (1953) [Epstein, S., Buchsbaum, R., Lowenstam, H.A., Urey, H.C., 1953. Revised carbonate-water isotopic temperature scale. Geological Society of America Bulletin 64, 1315-1326.] and Kim and O'Neil (1997) [Kim, S.-T., O'Neil, J.R., 1997. Equilibrium and non-equilibrium oxygen isotope effects in synthetic carbonates. Geochimica et Cosmochimica Acta 61, 3461-3475.]. The Epstein equation shows a constant offset from the -18O of G. bulloides and N. pachyderma (s.) across the full range of latitudes. The seasonal range in -18O values for these two species implies a near-surface habitat across all sites, while G. inflata most likely dwells at 50 m depth. A significant finding in this study was that offsets from predicted -18O for G. bulloides do not correlate to changes in the carbonate ion concentration. This suggests that [CO32-] in and of itself may not capture the full range of carbonate chemistry conditions in the marine system. This sediment trap deployment also reveals distinct seasonal flux patterns for each species. Comparison between flux-weighted isotopic values calculated from the sediment traps and the isotopic composition of nearby surface sediments indicates that the sedimentary records retain this seasonal imprint. At the 51°S site, G. bulloides has a spring flux peak while N. pachyderma (s.) is dominated by summer production.

本研究对布放于南大洋及西南太平洋的5个沉积物捕集器（sediment traps）中的样品进行了分析，样品取自布氏球抱球虫（Globigerina bulloides）、膨胀拟抱球虫（Globorotalia inflata）以及厚壳新单角虫左旋型（Neogloboquadrina pachyderma (s.)）的壳体氧同位素（δ¹⁸O）记录。本次布设的捕集器覆盖了从亚热带至极锋带的多种海洋环境，为本研究首次针对该纬度带的有孔虫δ¹⁸O季节记录开展分析。将有孔虫同位素记录与多种方解石δ¹⁸O预测方程进行对比后发现，Epstein等（1953）[Epstein, S., Buchsbaum, R., Lowenstam, H.A., Urey, H.C., 1953. "修订的碳酸盐-水同位素温度标度". 美国地质学会公报, 64, 1315-1326.] 与Kim和O'Neil（1997）[Kim, S.-T., O'Neil, J.R., 1997. "合成碳酸盐中的平衡与非平衡氧同位素效应". 地球化学与宇宙化学学报, 61, 3461-3475.] 提出的方程能够最佳拟合预测的δ¹⁸O值。Epstein方程在全纬度范围内，与布氏球抱球虫和厚壳新单角虫左旋型的δ¹⁸O值均呈现恒定偏移。这两个物种的δ¹⁸O值季节变化范围表明，它们在所有采样点均栖息于近表层水体，而膨胀拟抱球虫则大概率栖息于50米水深的水层。本研究的一项重要发现为，布氏球抱球虫的δ¹⁸O预测值偏移量与碳酸根离子浓度的变化并无关联。这表明，仅以碳酸根离子（[CO₃²⁻]）本身的浓度，或许无法全面反映海洋系统中碳酸盐化学环境的全部变化范围。本次沉积物捕集器的布设结果还显示，各有孔虫物种均存在独特的季节性通量模式。将沉积物捕集器计算得到的通量加权同位素值，与邻近海域表层沉积物的同位素组成进行对比后发现，沉积记录保留了上述季节性同位素印记。在南纬51°的采样点，布氏球抱球虫的通量峰值出现在春季，而厚壳新单角虫左旋型的壳体产出则以夏季为主。