Abstract:
Gypsum is not usually found as a constituent of deep marine sediments, reflecting the typical condition that sea water is undersaturated with respect to gypsum. In fact, the classic model for the origin of marine gypsum requires an arid climate and a semi-restricted basin, such as the Mediterranean Sea during the Messinian salinity crisis and the nearshore environments of the Persian Gulf today. Nevertheless, compositional analyses of coarse sand-sized grains from two deep-water ODP sites, Site 918 off the SE Greenland margin and Site 887 in the Gulf of Alaska, have unexpectedly identified gypsum as a minor constituent. Furthermore, gypsum crystals with sizes, shapes, and surface textures strikingly similar to those recovered from the Gulf of Alaska and the SE Greenland margin have also been identified in shallow cores of Holocene sediments from two marine embayments in southern Alaska.The occurrence of gypsum in these marine cores is both unusual and intriguing, especially considering their cool climate, mid- to high latitude settings. Of primary importance in understanding these gypsum occurrences is determining the origin of the gypsum, as it could be either detrital or authigenic in nature. In this study, physical grain characteristics and sulfur isotope measurements are used to explain the occurrence and origin of coarse-grained gypsum. Although initial morphologic and textural observations suggested a complex system in which the gypsum may have had more than one origin, simple quantitative sulfur isotope analyses of the gypsum provide clear and direct evidence of its detrital nature. The physical and isotopic evidence, in combination with the onshore geology, the environmental setting, and the physical characteristics of the gypsum-bearing marine localities, lead us to propose that the ultimate source of the gypsum is precipitation from freeze-induced soil brines, and that its subsequent occurrence in glacimarine sediments results from ice-rafting (by either icebergs or sea ice.