Mid-Pleistocene Mg/Ca, temperature and stable isotopes from Globigerinoides ruber white and Globorotalia inflata of sediment core GeoB3801-6 from the subtropical South Atlantic

The glacial marine isotope stage 14 (MIS 14) appears in many climate records as an unusually warm glacial. During this period an almost monospecific, up to 1.5 m thick, laminated layer of the giant diatom Ethmodiscus rex has been deposited below the South Atlantic Subtropical Gyre. This oligotrophic region is today less favorable for diatom growth with sediments typically consisting of calcareous nannofossil oozes. We have reconstructed temperatures and the stable oxygen isotopic compositions of sea surface and thermocline water (d18Ow) from planktonic foraminiferal (Globigerinoides ruber and Globorotalia inflata) Mg/Ca and stable oxygen isotopes to test whether perturbations in surface ocean conditions contributed to the deposition of the diatom layer at ~530 kyr B.P. Temperatures and d18Ow values reconstructed from this diatom ooze interval are highly variable, with maxima similar to interglacial values. Since the area of the Ethmodiscus oozes resembles the region where Agulhas rings are present, we interpret these hydrographic changes to reflect the varying influence of warm and saline water of Indian Ocean origin that entered the Subtropical Gyre trapped in Agulhas rings. The formation of the Ethmodiscus oozes is associated with a period of maximum Agulhas leakage and a maximum frequency of Agulhas ring formation caused by a termination-type position of the Subtropical Front during the unusual warm MIS 14. The input of silica through the Agulhas rings enabled the shift in primary production from calcareous nannoplankton to diatoms, leading to the deposition of the massive diatom oozes.冰期海洋同位素阶段14（MIS 14）在诸多气候记录中呈现为异常温暖的冰期。此时期，南大西洋亚热带环流（South Atlantic Subtropical Gyre）下方沉积了一层近乎单优种、厚度可达1.5米的纹层状大型硅藻——埃氏筛盘硅藻（Ethmodiscus rex）沉积层。如今这片寡营养海域的硅藻生长条件并不优越，沉积物通常以钙质超微化石软泥为主。 我们通过浮游有孔虫（Globigerinoides ruber与Globorotalia inflata）的Mg/Ca比值及稳定氧同位素数据，重建了海水表层及温跃层水的温度与稳定氧同位素组成（δ¹⁸O_w），以检验距今约530 kyr B.P.的表层海洋环境扰动是否促成了该硅藻沉积层的形成。从该硅藻软泥层段重建得到的温度与δ¹⁸O_w数值波动剧烈，其最大值与间冰期数值相近。 由于埃氏筛盘硅藻软泥的分布区域与现今阿古拉斯环（Agulhas rings）的发育区域高度重合，我们将这些水文学变化解释为：源自印度洋的暖盐水团受阿古拉斯环裹挟进入亚热带环流后，其影响程度随时间发生波动。在此次异常温暖的MIS 14冰期阶段，受亚热带锋面（Subtropical Front）处于冰期-间冰期旋回终止型位置的影响，阿古拉斯输运量与阿古拉斯环形成频率均达到峰值，埃氏筛盘硅藻软泥的沉积便与此时期相对应。阿古拉斯环输入的硅质物质使得初级生产从钙质超微浮游生物转向硅藻，最终促成了大规模硅藻软泥的沉积。