Carbonate system data on the Molokai reef flat

The severity of the impact of elevated atmospheric pCO2 to coral reef ecosystems depends, in part, on how seawater pCO2 affects the balance between calcification and dissolution of carbonate sediments. Presently, there are insufficient published data that relate concentrations of pCO2 and CO3**2- to in situ rates of reef calcification in natural settings to accurately predict the impact of elevated atmospheric pCO2 on calcification and dissolution processes. Rates of net calcification and dissolution, CO3**2- concentrations, and pCO2 were measured, in situ, on patch reefs, bare sand, and coral rubble on the Molokai reef flat in Hawaii. Rates of calcification ranged from 0.03 to 2.30 mmol CaCO3/m**2/h and dissolution ranged from -0.05 to -3.3 mmol CaCO3/m**2/h. Calcification and dissolution varied diurnally with net calcification primarily occurring during the day and net dissolution occurring at night. These data were used to calculate threshold values for pCO2 and CO3**2- at which rates of calcification and dissolution are equivalent. Results indicate that calcification and dissolution are linearly correlated with both CO3**2- and pCO2. Threshold pCO2 and CO3**2- values for individual substrate types showed considerable variation. The average pCO2 threshold value for all substrate types was 654±195 µatm and ranged from 467 to 1003 µatm. The average CO3**2- threshold value was 152±24 µmol/kg, ranging from 113 to 184 µmol/kg. Ambient seawater measurements of pCO2 and CO3**2- indicate that CO3**2- and pCO2 threshold values for all substrate types were both exceeded, simultaneously, 13% of the time at present day atmospheric pCO2 concentrations. It is predicted that atmospheric pCO2 will exceed the average pCO2 threshold value for calcification and dissolution on the Molokai reef flat by the year 2100.

大气二氧化碳分压（pCO₂）升高对珊瑚礁生态系统的影响严重程度，在一定程度上取决于海水二氧化碳分压如何调控碳酸盐沉积物的钙化与溶解过程之间的平衡。当前，已发表的研究数据尚不足以将二氧化碳分压与碳酸根离子（CO₃²⁻）浓度，与自然环境中珊瑚礁的原位（in situ）钙化速率建立关联，因此无法准确预测大气二氧化碳分压升高对钙化与溶解过程的影响。
本研究在夏威夷莫洛凯礁坪的斑块礁、裸沙与珊瑚砾石区域开展了原位测量，获取了净钙化速率、溶解速率、碳酸根离子浓度与二氧化碳分压的实测数据。其中，钙化速率的范围为0.03至2.30 mmol CaCO₃/m²/h，溶解速率的范围为-0.05至-3.3 mmol CaCO₃/m²/h。
钙化与溶解过程呈现显著的昼夜节律：净钙化主要发生于日间，而净溶解多出现于夜间。
本数据集被用于推导钙化与溶解速率相等时的二氧化碳分压与碳酸根离子浓度阈值。研究结果显示，钙化与溶解速率分别与碳酸根离子浓度和二氧化碳分压呈线性相关。不同基底类型对应的二氧化碳分压与碳酸根离子浓度阈值存在明显差异：所有基底类型的平均二氧化碳分压阈值为654±195 µatm，波动区间为467至1003 µatm；平均碳酸根离子浓度阈值为152±24 µmol/kg，波动区间为113至184 µmol/kg。
对当前环境海水的二氧化碳分压与碳酸根离子浓度的实测结果表明，在当前大气二氧化碳分压水平下，所有基底类型的碳酸根离子浓度与二氧化碳分压阈值同时被突破的情况占比为13%。
据预测，到2100年，大气二氧化碳分压将超过莫洛凯礁坪钙化与溶解过程的平均二氧化碳分压阈值。