Age model of ODP Hole 107-654A

It has long been speculated that glacio-eustatic sea level drop may have caused or contributed to the isolation and consequent desiccation of the Mediterranean in the late Miocene (the 'Messinian salinity crisis'). Ocean Drilling Program site 654 on the Sardinia margin of the Tyrrhenian Sea is the first deep-sea drill site to penetrate through upper Messinian evaporites into lower Messinian/upper Tortonian open marine sediments, and thus offers a unique opportunity to date the onset of the salinity crisis. A reexamination of the magnetostratigraphic, biostratigraphic, and stable isotope-stratigraphic constraints on the preevaporite sediments of site 654 has yielded two possible ages for the contact between salinity crisis sediments and the underlying normal marine sediments. One magnetostratigraphic interpretation plus the biostratigraphically determined position of the Tortonian/Messinian boundary imply a date of about 6.2 Ma for the youngest presalinity crisis sediments. An alternative magnetostratigraphic interpretation plus the carbon isotope stratigraphy imply a date of about 5.2 Ma. The younger of these dates coincides with a delta18O spike in open ocean sediments [Keigwin, 1987 doi:10.1029/PA002i006p00639], which is attributed to increased ice volume.长期以来，学界推测冰川驱动的全球海平面下降（glacio-eustatic sea level drop）可能引发或促成了中新世晚期地中海的孤立与后续干涸事件——即“墨西拿盐度危机（Messinian salinity crisis）”。位于第勒尼安海（Tyrrhenian Sea）撒丁岛陆缘的大洋钻探计划（Ocean Drilling Program）654号站位，是首个穿透上墨西拿蒸发岩、进入下墨西拿（Messinian）/上托尔托尼亚（Tortonian）阶开阔海相沉积物的深海钻探站位，因此为确定盐度危机的起始时间提供了独一无二的研究契机。本次对654号站位蒸发岩前沉积物的磁性地层学、生物地层学与稳定同位素地层学约束条件的重新分析，得出了盐度危机沉积层与下伏正常海相沉积物之间接触界面的两种可能年代。其中一种磁性地层学解释结合生物地层学确定的托尔托尼亚/墨西拿阶界线位置，表明最新的盐度危机前沉积物的年代约为6.2 Ma。另一种磁性地层学解释结合碳同位素地层学结果，则得到约5.2 Ma的年代。这两个年代中较年轻的一个，与开阔海相沉积物中的δ¹⁸O（delta18O）峰值[Keigwin, 1987 doi:10.1029/PA002i006p00639]相吻合，该峰值被归因于冰量增加。