Carbon contents and isotopic compositions from the ODP Hole 161-974C near the center of the Tyrrhenian Basin (Table 1)

The modern Mediterranean Sea is oligotrophic, yet its sediment record contains layers of organic-carbon-rich sapropels at 21 ky (precessional) spacing that imply periods of elevated paleoproductivity that approached the high productivities of modern upwelling systems. Resolution to this paradox is provided by lines of evidence suggesting that the mode of primary productivity changed from one dominated by algae to one during times of sapropel deposition in which photosynthetic bacteria were important. We have made a high-resolution comparison of the organic carbon and nitrogen isotopic compositions of three sapropels and their background sediments in a 3-m sequence that corresponds to 1001 to 946 ka. Organic d13C values systematically increase from -26 per mil to -21 per mil and d15N values systematically decrease from ~4 per mil to <0 per mil as organic carbon mass accumulation rates increase in the sapropel layers. The increase in carbon isotope values mirrors the increases in primary productivity and associated organic matter export indicated by the increased mass accumulation rates. The decrease in nitrogen isotope values implies major contributions of nitrogen-fixing cyanobacteria to the total marine productivity. The precessional minima with which sapropels coincide were times of wetter climate that stratified the surface Mediterranean Sea, increased delivery of soil-derived phosphorus, and evidently amplified microbial primary production. Our high-resolution study reveals several relatively rapid excursions into and out of the high-productivity mode that suggest that sapropel deposition was a climate-sensitive surface-driven phenomenon that was not accompanied by basin-wide stagnation.

现代地中海属于贫营养（oligotrophic）海域，但其沉积记录中保存有以21千年（ky）为岁差周期出现的富有机碳腐泥层（sapropel），这表明曾存在古生产力抬升的阶段，其生产力水平可媲美现代上升流系统的高产能状态。这一悖论的破解线索来自多项证据：初级生产力的模式在腐泥层沉积期发生了转变，从以藻类为主导的模式，变为光合细菌发挥关键作用的模式。我们对一段对应地质年代为1001至946千年（ka）的3米长沉积序列中的三层腐泥层及其背景沉积物开展了高分辨率对比分析。结果显示，随着腐泥层内有机碳质量堆积速率升高，有机碳δ¹³C值从-26‰系统性升高至-21‰，而δ¹⁵N值则从约4‰系统性降低至0‰以下。碳同位素值的升高趋势，与质量堆积速率增加所指示的初级生产力提升及相关有机质输出量增加的规律一致；而氮同位素值的降低，则表明固氮蓝细菌（nitrogen-fixing cyanobacteria）对海洋总生产力做出了核心贡献。与腐泥层同步出现的岁差极小值时期，气候更为湿润，使得地中海表层海水形成稳定分层，陆源磷的输入量显著增加，进而大幅放大了微生物初级生产力。我们的高分辨率研究还揭示了数次相对快速的高生产力模式切换事件，这表明腐泥层沉积是一类对气候敏感的表层驱动现象，并未伴随全盆地的水体停滞。