Seawater carbonate chemistry and net dissolution rate for experiment of Shiraho reef

Acidification of the oceans by increasing anthropogenic CO2 emissions will cause a decrease in biogenic calcification and an increase in carbonate dissolution. Previous studies have suggested that carbonate dissolution will occur in polar regions and in the deep sea where saturation state with respect to carbonate minerals (Omega) will be <1 by 2100. Recent reports demonstrate nocturnal carbonate dissolution of reefs, despite a Omega a (aragonite saturation state) value of >1. This is probably related to the dissolution of reef carbonate (Mg-calcite), which is more soluble than aragonite. However, the threshold of Omega for the dissolution of natural sediments has not been clearly determined. We designed an experimental dissolution system with conditions mimicking those of a natural coral reef, and measured the dissolution rates of aragonite in corals, and of Mg-calcite excreted by other marine organisms, under conditions of Omega a > 1, with controlled seawater pCO2. The experimental data show that dissolution of bulk carbonate sediments sampled from a coral reef occurs at Omega a values of 3.7 to 3.8. Mg-calcite derived from foraminifera and coralline algae dissolves at Omega a values between 3.0 and 3.2, and coralline aragonite starts to dissolve when Omega a = 1.0. We show that nocturnal carbonate dissolution of coral reefs occurs mainly by the dissolution of foraminiferans and coralline algae in reef sediments.随着人为二氧化碳（CO₂）排放量增加引发的海洋酸化，将导致生物钙化作用减弱，并加剧碳酸盐溶解过程。此前研究指出，至2100年，极地海域与深海区域的碳酸盐矿物饱和状态参数（Omega）将低于1，届时这些区域将出现碳酸盐溶解现象。近期研究报告显示，即便珊瑚礁海域的文石（aragonite）饱和状态参数Ω_a大于1，夜间仍会发生碳酸盐溶解。该现象或与珊瑚礁碳酸盐中的镁方解石（Mg-calcite）溶解相关——镁方解石比文石更易溶解。然而，天然沉积物溶解对应的Omega临界值尚未被明确界定。本研究设计了一套模拟天然珊瑚礁环境的溶解实验系统，在可控海水二氧化碳分压（pCO₂）且Ω_a>1的条件下，分别测定了珊瑚体内文石以及其他海洋生物分泌的镁方解石的溶解速率。实验数据表明，从珊瑚礁采集的块状碳酸盐沉积物的溶解发生在Ω_a为3.7至3.8的区间内；源自有孔虫（foraminifera）与珊瑚藻（coralline algae）的镁方解石，其溶解发生在Ω_a为3.0至3.2的区间内；而珊瑚文石则在Ω_a等于1.0时开始溶解。本研究证实，珊瑚礁夜间发生的碳酸盐溶解主要源于礁体沉积物中有孔虫与珊瑚藻的溶解过程。