Ca Cycle Constraints from the Ca Isotope Composition of Precambrian Sedimentary Carbonates

Abstract:

The geochemical cycle of Ca in seawater is relatively straightforward – Ca is ultimately sourced from the weathering of silicates and largely removed as carbonates. Most of these carbonates are then recycled through subsequent uplift and weathering, but some are metamorphosed or returned to the mantle via subduction. Ca isotopes in sedimentary marine carbonates can provide new insights into the global Ca (and by corollary, C) cycle by constraining the flux and isotopic composition of the recycled and subducted Ca sinks on billion-year timescales.

Precambrian applications of Ca isotopes have so far been limited to relatively small datasets which cover unusual, dynamic intervals of the Proterozoic. In order to address questions about long-timescale Ca cycling, Ca isotopes were measured on an extensive suite of Precambrian carbonates (n > 300) which represent environments, textures, and morphologies that are typical for their age, and whose carbon isotope values have been interpreted as reflecting precipitation from seawater. Because marine carbonates constitute the dominant geological sink for Ca, this representative sample set places specific limits on the evolution of the sedimentary and crustal Ca reservoirs and suggests further applications for Ca isotopes in Precambrian time.