Reconstructing Mediterranean Overflow Plume Properties

海洋在调节气候、从赤道向两极重新分配热量和吸收大气二氧化碳方面发挥着关键作用。这些过程在一定程度上受到推动海洋环流的大洋两端的温度和盐度差异的控制。像地中海这样的海洋，由于蒸发损失的淡水比流入地中海的河流获得的淡水更多，是稠密咸水的重要来源。地中海水溢出了直布罗陀海峡的一个狭窄而浅的基岩，形成了一股羽流，沿着西班牙和葡萄牙周围的海底轮廓，向北流向北极。今天，流入北大西洋的最稠密的水的来源是地中海。然而，在过去，这种水的密度至少是现在的三倍。五六百万年前，海洋中约6%的盐以盐巨人的形式沉淀在地中海海底。这极大地提高了地中海的盐度，改变了地中海溢出水的化学和物理性质。要评估这些变化对区域和全球气候的影响，首先必须确定溢出的性质。其中一些性质，如古代羽流的大小和位置，可以从溢流产生的沉积物的分布中绘制出来。其他的，如温度、盐度、密度和水的化学成分，不能直接测量，但必须从这些特性的替代品中重建，这些特性作为溢出沉积物的一部分被保存下来。IODP探险401将恢复盐巨星形成期间大西洋溢出羽流沉积的沉积物。将测量这些沉积物中颗粒的大小，以确定其流动速度。生活在海底的微生物从它们所居住的水中建造它们的外壳。这些贝壳在生物体死亡后很久才作为化石保存下来，它们的化学成分在一定程度上反映了水的温度、盐度和化学成分。通过在显微镜下观察沉积物，可以用精细的画笔挑选出这些微小的化石，然后进行分析。所产生的记录将提供地中海溢流水的演变及其与大西洋相互作用的定量图景。这是评估蒸发岩形成和由此产生的高盐度溢出对海洋化学造成的扰动对气候的影响的关键的第一步。

The ocean plays a critical role in regulating climate, redistributing heat from the equator to poles and absorbing atmospheric CO2. These processes are partly controlled by the temperature and salinity differences across the ocean that drive ocean circulation. Seas like the Mediterranean that lose more fresh water by evaporation than they gain from rivers flowing into them, are an important source of dense, salty water. Mediterranean water overflows a narrow, shallow sill in the Gibraltar Strait, forming a plume that follows seafloor contours around Spain and Portugal, flowing northwards to the Arctic. Today, the source of the most dense water flowing into the North Atlantic is the Mediterranean. In the past however, this water was at least three times more dense. Five to six million years ago, around 6% of all the salt in the ocean precipitated on the Mediterranean sea floor as a salt giant. This raised Mediterranean salinity dramatically, changing both the chemical and physical properties of the Mediterranean's overflow water. To evaluate the impact of these changes on regional and global climate, the overflow's properties have first to be determined. Some of these properties such as the size and location of the ancient plume can be mapped from the distribution of the sediment that the overflow produced. Others, like temperature, salinity, density and the chemistry of the water cannot be measured directly, but must be reconstructed from proxies of these properties that are preserved as part of the overflow sediment.IODP Expedition 401 will recover the sediment deposited by the overflow plume in the Atlantic during salt giant formation. The size of the particles in these sediments will be measured to determine the speed it was flowing. Microscopic organisms that live at the bottom of the sea, build their shells from the water in which they dwell. These shells are preserved as fossils long after the organism has died and their chemical composition reflects, in part, the temperature, salinity and chemistry of the water. By viewing the sediments under a microscope, these tiny fossils can be picked out with a fine paintbrush and then analysed. The records generated will provide a quantitative picture of the evolution of both Mediterranean overflow water and its interaction with the Atlantic. This is a critical first step towards evaluating the climatic impact of perturbations to ocean chemistry as a consequence of evaporite formation and the resulting high salinity overflows.