Strontium isotopic ratios of evaporites of the Tyrrhenian Sea

Strontium isotopic ratios of gypsums recovered from upper Miocene (Messinian) evaporites at ODP Leg 107 Holes 652A, 653B, and 654A (Tyrrhenian Sea) are lower than expected. The values for the Messinian balatino-like gypsum, single gypsum crystals, and anhydrites range from 0.70861 to 0.70886 and are approximately 25 * 10**-5 less than would be expected for evaporites precipitated from Messinian seawater (0.70891-0.70902). Pre-evaporitic planktonic foraminifers from Hole 654A show variable degrees of dolomitization and 87Sr/86Sr values that irregularly decrease upward from normal marine values approximately 81m below the lowest evaporite occurrence. This suggests diagenetic alteration by advecting interstitial water with a low 87Sr/86Sr ratio or that the lower Sr isotopic ratios for the Messinian evaporites could have resulted from a greater influence of fresh water on the Sr isotopic composition of the desiccating Tyrrhenian Sea. Fluctuations of the 87Sr/86Sr-ratio for evaporites in the sedimentary cycles recognized for Holes 653B and 654A, the generally low Sr isotopic ratio of river water entering the Mediterranean Sea, and the presence of dwarf marine microfossils suggest that the 87Sr/86Sr ratio of the evaporites responded to hydrologic variations in a very restricted basin with variable rates of marine and fresh water input. The strontium isotopic ratios of the Messinian anhydrites from the proposed lacustrine sequence at Hole 652A fall in the same range as the marine evaporites from Holes 654A and 653B. This suggests a common or similar origin of the brines at the three locations. […]从大洋钻探计划（Ocean Drilling Program, ODP）第107航次652A、653B与654A井（第勒尼安海（Tyrrhenian Sea））的上新世晚期（墨西拿期（Messinian））蒸发岩中回收的石膏的锶同位素比值（strontium isotopic ratios）低于预期值。墨西拿期巴拉蒂诺型石膏、单石膏晶体及硬石膏（anhydrites）的比值范围为0.70861~0.70886，较墨西拿期海水沉积的蒸发岩理论值（0.70891~0.70902）低约25×10^-5。取自654A井的蒸发岩沉积前浮游有孔虫（planktonic foraminifers）显示出不同程度的白云岩化（dolomitization）作用，其87Sr/86Sr比值从最低蒸发岩产出层位下方约81米处的正常海水值向上呈不规则降低趋势。这表明两种潜在成因：一是受低87Sr/86Sr比值的对流运移孔隙水（interstitial water）引发的成岩蚀变（diagenetic alteration）；二是干涸的第勒尼安海的锶同位素组成受淡水影响程度更高，进而导致墨西拿期蒸发岩的锶同位素比值偏低。653B与654A井沉积旋回中蒸发岩的87Sr/86Sr比值波动、流入地中海的河水普遍偏低的锶同位素比值，以及小型海洋微化石的产出，均表明该受限盆地（restricted basin）的蒸发岩锶同位素比值响应于水文变化（hydrologic variations），且海水与淡水的输入速率存在差异。652A井推测湖相序列（lacustrine sequence）中的墨西拿期硬石膏的锶同位素比值，与654A、653B井的海相蒸发岩处于同一范围，表明三处的卤水（brines）成因具有共性或相似性。[...]