Trace element distribution in latest Pliocene sapropels in the Mediterranean Sea

Compositions of productivity- and redox-sensitive trace-metals in two Upper Pliocene sapropel sequences from western Mediterranean ODP Sites 974 and 975 and eastern Mediterranean Sites 964, 969, and 967 provide information about the conditions leading to sapropel deposition. Except at Site 975, closest to the Atlantic Ocean, all sapropels display elevated element/Al ratios for Ba, Re, Cd, Mo, U, Cr, Cu, V, and Zn. High total organic carbon (TOC) concentrations and Ba/Al ratios point to increased primary productivity during sapropel deposition. Enrichments of redox-sensitive trace elements and elevated Re/Mo ratios indicate watercolumn oxygen depletion during sapropel formation. Anoxia likely was caused by increased fluxes of organic matter to the seafloor that would have enhanced oxygen demand at depth. One of the sapropel sequences investigated in this study, insolation-cycle 178, contains a zone having low TOC, low Ba/Al ratios, and low trace-metal contents. These features identify this interval as an interruption of the increased productivity associated with sapropels and of the sub-oxic/anoxic conditions that are fully-developed in the upper half of this sapropel layer. Significant differences in trace-metal enrichments between the eastern and western Mediterranean indicate that the eastern basin was a focal point for sapropel initiation. In contrast, Site 975 appears to have been heavily influenced by the Atlantic Ocean during i-cycles 178 and 180 and was not strongly influenced by the paleoceanographic changes affecting the rest of the Mediterranean.

针对西地中海大洋钻探计划（Ocean Drilling Program, ODP）974、975站位以及东地中海964、969、967站位所获的两宗上新世晚期腐泥层序列，对其中生产力敏感型与氧化还原敏感微量元素的组成进行分析，所得数据可为腐泥层沉积的形成条件提供科学参考。除距离大西洋最近的975站位外，其余所有站位的腐泥层均呈现出Ba、Re、Cd、Mo、U、Cr、Cu、V及Zn的元素/铝比值显著升高的特征。高总有机碳（Total Organic Carbon, TOC）含量与Ba/Al比值均指示腐泥层沉积时期的初级生产力水平显著提升。氧化还原敏感微量元素的富集现象以及偏高的Re/Mo比值，则表明腐泥层形成过程中水柱环境发生了氧含量亏损。缺氧环境大概率由海底有机质输入通量增加所引发，该过程会加剧深部水体的氧消耗需求。本研究涉及的其中一宗腐泥层序列——日照周期178（insolation-cycle 178）——存在一处总有机碳、Ba/Al比值及微量元素含量均偏低的层段。上述特征表明，该层段属于腐泥层关联的高生产力过程以及该腐泥层上段完全发育的次氧化/缺氧环境的中断事件。东、西地中海之间的微量元素富集程度存在显著差异，这说明东地中海盆地是腐泥层形成的触发核心区域。与之相反，975站位在日照周期178与180期间明显受到大西洋的强烈影响，并未受到其余地中海区域古海洋变化的显著作用。