Threshold of carbonate saturation state determined by CO2 control experiment

[摘要] Acidification of the oceans by increasing anthropogenic CO₂ emissionswill cause a decrease in biogenic calcification and an increase in carbonatedissolution. Previous studies have suggested that carbonate dissolution willoccur in polar regions and in the deep sea where saturation state withrespect to carbonate minerals (Ω) will be <1 by 2100. Recentreports demonstrate nocturnal carbonate dissolution of reefs, despite aΩ_a (aragonite saturation state) value of >1. This isprobably related to the dissolution of reef carbonate (Mg-calcite), which ismore soluble than aragonite. However, the threshold of Ω for thedissolution of natural sediments has not been clearly determined. Wedesigned an experimental dissolution system with conditions mimicking thoseof a natural coral reef, and measured the dissolution rates of aragonite incorals, and of Mg-calcite excreted by other marine organisms, underconditions of Ω_a > 1, with controlled seawater pCO₂. Theexperimental data show that dissolution of bulk carbonate sediments sampledfrom a coral reef occurs at Ω_a values of 3.7 to 3.8. Mg-calcitederived from foraminifera and coralline algae dissolves at Ω_avalues between 3.0 and 3.2, and coralline aragonite starts to dissolve whenΩ_a = 1.0. We show that nocturnal carbonate dissolution of coralreefs occurs mainly by the dissolution of foraminiferans and coralline algaein reef sediments.

[学科分类] 地球化学与岩石