Distribution of Florisphaera profunda and Helicosphaera carteri in down core and surface sediments

A detailed assessment of the respective roles of production, export, and subsequent preservation of organic carbon (Corg) in the eastern Mediterranean (EMED) sediments during the formation of sapropels remains elusive. Here we present new micropaleontological results for both surface samples taken at several locations in the EMED and last interglacial sapropel S5 from core LC21 in the southeastern Aegean Sea. A strong exponential anticorrelation between relative abundances of the lower photic zone coccolithophore Florisphaera profundain the surface sediments and modern concentrations of chlorophyll a (Chl-a) at the sea surface suggests thatF. profunda percentages can be used to track past productivity changes in the EMED. Prior to S5 deposition, an abrupt and large increase of F. profunda percentages in LC21 coincided (within the multidecadal resolution of the records) with the marked freshening of EMED surface waters. This suggests a strong coupling between freshwater-bound surface to intermediate water (density) stratification and enhanced upward advection of nutrients to the base of the photic zone, fuelling a productive deep chlorophyll maximum (DCM) underneath a nutrient-starved surface layer. Our findings imply that (at least) at the onset of sapropel formation physical and biogeochemical processes likely operated in tandem, enabling high Corg accumulation at the seafloor.目前针对东地中海（eastern Mediterranean, EMED）沉积物中腐泥层（sapropels）形成时期，有机碳（Corg）的生产、输出与后续保存各自所扮演的角色，仍不甚明晰。本研究针对东地中海多个站位的表层沉积物样品，以及爱琴海东南部LC21岩芯中的末次间冰期腐泥层S5，提供了全新的微古生物学分析结果。表层沉积物中透光带下层颗石藻——深水圆盘球藻（Florisphaera profunda）的相对丰度，与海表叶绿素a（Chl-a）的现代浓度之间存在显著的指数负相关关系，这表明可通过F. profunda的百分含量来追踪东地中海过去的生产力变化。在腐泥层S5沉积之前，LC21岩芯中F. profunda的百分含量出现突发性大幅升高，这与东地中海表层海水的显著淡化在数十年分辨率的沉积记录范围内完全吻合。这表明，受淡水调控的表层至中层水密度层化过程，与营养盐向上平流至透光带底部的增强过程存在紧密耦合，进而在营养盐匮乏的表层水体下方催生了高产的深层叶绿素最大值（DCM）。本研究结果表明，至少在腐泥层形成的起始阶段，物理过程与生物地球化学过程大概率协同运作，从而实现了海底高有机碳埋藏量。