FAST REACTIONS OF METASTABLE CARBONATE MINERALS UNDER RISING PCO₂

Recently, coral reef communities and calcifying organisms in general, such as corals, coralline algae, coccolithophoridae and foraminifera have received increased attention concerning their ability to calcify under conditions of rising atmospheric pCO₂ and consequent decreasing surface water carbonate saturation state. Much less attention has been given to the effects on the carbonate chemistry and mineralogy of the sediment-pore water system in carbonate dominated regions and relatively few data exist on how calcifying ecosystems may be affected as a whole. Here, we present observational data from Bermuda demonstrating significant changes in calcium carbonate precipitation and dissolution as a function of pCO₂ and carbonate saturation state driven by changes in primary production and decomposition of organic matter on a diurnal cycle in a semi-enclosed basin associated with a barrier reef as well as in a seasonally stratified water column. Substantial dissolution of metastable carbonate minerals, mainly high magnesian calcites, occurs when decomposition of organic matter is at its maximum resulting in pCO₂ levels significantly higher than what is observed in today's atmosphere. As atmospheric CO₂ concentration continues to increase owing to human activities, the average mineral composition and stability of contemporary carbonate sediments may change, favoring carbonate minerals with a lower magnesium content and consequently higher stability. Furthermore, the magnesium content of skeletal hard parts produced by calcifying organisms, which may contain more than 20 mol% MgCO₃, may also decrease owing to decreasing surface water carbonate saturation state.