Geochemistry and morphometry on planktonic foraminifera@en

About one third of the anthropogenic carbon dioxide (CO2) released into the atmosphere in the past two centuries has been taken up by the ocean. As CO2 invades the surface ocean, carbonate ion concentrations and pH are lowered. Laboratory studies indicate that this reduces the calcification rates of marine calcifying organisms, including planktic foraminifera. Such a reduction in calcification resulting from anthropogenic CO2 emissions has not been observed, or quantified in the field yet. Here we present the findings of a study in the Western Arabian Sea that uses shells of the surface water dwelling planktic foraminifer Globigerinoides ruber in order to test the hypothesis that anthropogenically induced acidification has reduced shell calcification of this species. We found that light, thin-walled shells from the surface sediment are younger (based on 14C and d13C measurements) than the heavier, thicker-walled shells. Shells in the upper, bioturbated, sediment layer were significantly lighter compared to shells found below this layer. These observations are consistent with a scenario where anthropogenically induced ocean acidification reduced the rate at which foraminifera calcify, resulting in lighter shells. On the other hand, we show that seasonal upwelling in the area also influences their calcification and the stable isotope (d13C and d18O) signatures recorded by the foraminifera shells. Plankton tow and sediment trap data show that lighter shells were produced during upwelling and heavier ones during non-upwelling periods. Seasonality alone, however, cannot explain the 14C results, or the increase in shell weight below the bioturbated sediment layer. We therefore must conclude that probably both the processes of acidification and seasonal upwelling are responsible for the presence of light shells in the top of the sediment and the age difference between thick and thin specimens.过去两个世纪间排放至大气的人为源二氧化碳（anthropogenic carbon dioxide, CO₂）中，约有三分之一被海洋吸收。当CO₂侵入表层海洋后，海水的碳酸根离子浓度与pH值均会下降。实验室研究表明，该过程会降低海洋钙化生物（包括浮游有孔虫）的钙化速率。然而，由人为CO₂排放引发的这种钙化速率下降现象，目前尚未在野外被观测或量化。 本研究以西阿拉伯海为研究区域，利用表层水域栖息的浮游有孔虫——红拟抱球虫（Globigerinoides ruber）的壳体，验证“人为诱导的海洋酸化已降低该物种壳体钙化程度”这一假说，现将研究结果呈现如下。研究发现，基于碳十四（¹⁴C）与碳十三（δ¹³C，原文标注为d13C）测量结果，表层沉积物中的轻质薄壳壳体比厚重厚壳壳体更为年轻；位于上层生物扰动沉积物层中的壳体，其重量显著低于该层下方的壳体。上述观测结果与“人为诱导的海洋酸化降低了有孔虫的钙化速率，进而形成轻质壳体”的情形相符。 另一方面，本研究表明该区域的季节性上升流同样会影响有孔虫的钙化作用，以及壳体所记录的稳定同位素（δ¹³C、δ¹⁸O，原文标注为d13C、d18O）信号。浮游生物拖网与沉积物捕集器数据显示，上升流时期壳体更轻，非上升流时期壳体更重。然而，仅靠季节性因素无法解释¹⁴C的观测结果，也无法解释生物扰动沉积物层下方壳体重量的增加现象。因此我们可以得出结论：海洋酸化与季节性上升流这两个过程，可能共同造成了沉积物表层轻质壳体的出现，以及厚薄壳体之间的年龄差异。