Strontium Storage and Release During Deposition and Diagenesis of Marine Carbonates Related to Sea-Level Variations

Abstract

By virtue of their high content of aragonite, that contains ∼5,000×10−6 g/g strontium, biogenic carbonates that accumulate as shallow-water reef and shelf limestones serve as a major sink and source of strontium in the ocean. In contrast, deep-sea pelagic carbonates are almost entirely calcite that contains ∼1400×10−6 g/g strontium. The strontium sink represented by present-day aragonitic shallow-water carbonate deposition sequesters ∼3.3×1012 g/yr whereas the deep-sea pelagic carbonate strontium sink accounts for ∼2.0×1012 g/yr; a total of 5.3×10−12 g/yr of strontium is being stored now in all marine carbonate sediments. The riverine input of strontium from the continents amounts to ∼2.2×1012 g/yr. This present-day annual deficit of ∼3.1×1012 g/yr of strontium in the continent-oceanic carbonate sediment system is a short-term one because reef and shelf limestones store strontium during periods of rising sea level, such as the modern interglacial episode. During periods of falling sea level, reef and shelf limestones are exposed to fresh-water diagenesis which results in the dissolution of strontium-rich aragonite and the consequent return of strontium to the sea; thus short-term strontium deficits that accumulate while sea level is rising are partially paid back during periods of falling sea level at the rate of ∼1.8×1012 g/yr from shallow-water carbonate sediments. Medium-term dissolution of pelagic carbonates at the sea floor returns strontium to the sea water at a rate of ∼1.0×1012 g/yr and long-term burial diagenesis of pelagic carbonates returns ∼0.5×1012 g/yr. Therefore, the long-term storage rate of strontium in reef, shelf and pelagic carbonates of ∼2.0×1012 g/yr derived in this analysis is roughly in balance with the present-day riverine strontium input of ∼2.2×1012 g/yr determined by Brass (1976) and by Martin and Meybeck (1979). Prolonged periods of reef and shelf carbonate accumulation during high sea-level stands result in significant changes in the Sr/Ca ratio in the oceanic reservoir which are detectable in the geological record as discussed by Graham et al. (1982).