Heterogeneous Mg isotopic composition of the early Carboniferous limestone: implications for carbonate as a seawater archive

Abstract

Carbonate precipitation and hydrothermal reaction are the two major processes that remove Mg from seawater. Mg isotopes are significantly (up (1) micrite samples ranged from -2.86% to -2.97%; (2) pure marine cements varied from -3.40% to -3.54%, while impure cement samples containing small amount of Rugosa coral skeletons showed a wider range (-3.27% to -3.75%); (3) values for the mixture component were -3.17% and -3.49%; and (4) 26 Mg sw was only slightly affected by the faunal composition of carbonate-secreting organisms, even though biogenic carbonate accounts for more than 90% of marine carbonate production in Phanerozoic oceans and there is a wide range (0.2%-4.8%) of fractionation during biogenic carbonate formation.