Quantifying Carbonate Dissolution in Shallow Pelagic Seas: A New Aragonite Dissolution Proxy

The preservation of calcium carbonate in deep sea sediments reflects the global ocean’s ability to regulate atmospheric carbon dioxide concentration over millennia. Many studies use shells of foraminifera to trace calcite dissolution; but foraminifera are often less useful indicators of whole ocean carbonate chemistry in pelagic environments shallower than the calcite saturation horizon. We performed a core top calibration study with pteropod shells to quantify the increase in aragonite dissolution with water depth and seawater carbonate ion concentration. This is an important issue particularly in the Atlantic Ocean because the aragonite compensation depth (ACD) is quite deep (~2800m) compared to the Pacific (~500m). We counted fragments and whole shells from three distinct pteropod morphotypes in core tops from the Rio Grande Rise (1562 to 4177m). Then we developed a new pteropod fragmentation index quantifying aragonite dissolution with water depth (and carbonate ion concentration) above the ACD where most planktonic foraminifera shells are well preserved. We also counted whole foraminifer shells from two species in our core tops. Globigerinoides rubers are known to be more susceptible to dissolution than Globorotalia menardiis due to their porous shell structure. We find that the ratio of whole G. ruber shells to whole G.menardii shells in our samples increases with water depth between the aragonite saturation horizon and the ACD. This result is counter-intuitive but we believe it is caused by carbonate ion added to seawater through pteropod dissolution at these depths. Our G. menardii fragmentation index data (MFI) also corroborate the increase of calcite preservation with depth above the ACD.