Late Miocene to Pleistocene paleoceanographic records from the Feni and Gardar Drifts: Pliocene reduction in abyssal flow

Magnetic fabric analyses from two North Atlantic drift deposits provide proxies for determining relative variations in the strength of abyssal flow over the last 10 my. The data show a cessation of current-controlled sedimentation at the shallower Feni Drift (2417 m) at the time of onset of Northern Hemisphere glaciation (2.6 Ma). Drift formation ended nearly 2 my earlier (4.2 Ma) at the deeper Gardar Drift (3220 m), implying stepwise reduction in deep-water flow. Relatively light delta18O values at the deeper Gardar Drift indicate a warmer, thus also more salty, water mass site prior to 6 Ma. We interpret this as representing Mediterranean Sea water, which flowed north at depths greater than that of the Feni Drift Site. The supply of Mediterranean Water to the North Atlantic was shut off as the Gibraltar Straits closed, causing the Messinian salinity crisis, and never returned to that position in the water column after the Mediterranean opened again.对两处北大西洋漂移沉积体（North Atlantic drift deposits）的磁组构（magnetic fabric）分析，可为反演过去1000万年间深渊洋流（abyssal flow）强度的相对变化提供代用指标（proxy）。数据显示，在北半球冰期启动（2.6 Ma）之时，水深较浅的费尼漂移体（Feni Drift，2417米）处的洋流主控沉积作用宣告终止。而水深更深的加达尔漂移体（Gardar Drift，3220米）处的漂移体形成则在约早200万年的4.2 Ma时便已停止，这表明深水环流呈现出逐步减弱的趋势。在6 Ma之前的时段内，加达尔漂移体处相对偏轻的δ18O（delta18O）值，指示当时该站位存在温度更高、盐度亦更大的水团。我们将该现象解释为来自地中海的海水——这类海水曾以高于费尼漂移体站位的水深向北流动。随着直布罗陀海峡（Gibraltar Straits）闭合，地中海向北大西洋的海水补给被切断，进而引发了墨西拿盐度危机（Messinian salinity crisis）；即便之后地中海再次连通，该类海水也再未回到其此前在水柱中的所处深度。