Terrestrial gypsum from Alaska and Greenland in glacially influenced marine sediments

Abstract

Abstract Gypsum grains were identified in Miocene–Pleistocene sediment cores from two deep-water ODP sites, Site 918 off the SE Greenland margin and Site 887 in the Gulf of Alaska, and in Holocene sediment cores from shallow-water localities in Disenchantment Bay and Muir Inlet in southern Alaska. Although initial morphologic and textural observations suggested a complex system in which the gypsum may have had more than one origin, quantitative sulfur isotope analyses of the gypsum provide evidence of its detrital nature. δ34S values in gypsum from southern Alaska range between +0.0 and +7.1‰. Gypsum has δ34S values between −27.1 and −27.5‰ in the Gulf of Alaska and values between −28.5 and +0.2‰ off the SE Greenland margin. All of these isotopic signatures are too highly depleted in δ34S to have precipitated from seawater, present or past. In addition there is no significant change in δ34S values for gypsum crystals with differing physical characteristics (abraded vs. unabraded) from the same stratigraphic horizon, suggesting all the gypsum is detrital regardless of the degree of abrasion. The isotopic and physical evidence, in combination with the onshore geology the environmental setting, and site characteristics of the gypsum-bearing marine localities, lead us to propose that the ultimate source of the gypsum is precipitation from freeze-induced terrestrial sediment or soil brines. Furthermore the combined evidence suggests that the subsequent occurrence of gypsum in glacimarine sediments results from ice-rafting (by icebergs or sea ice) of the frozen regolith and/or, in the proximal glacimarine setting of southern Alaska, very rapid burial via turbidity currents.