Can we tell the difference between carbonate chemistry of the surface ocean and the sediments over the last 21,000 years? Tools, proxies and caveats

Discriminating between surface ocean and sediment pore-water carbonate chemistry is important for understanding the regulation of atmospheric CO2 by marine carbonates over millennia. While the preservation of planktonic foraminifer tests is used to interpret sedimentary carbonate chemistry, the size normalized shell weight (SNSW) of many species is largely affected by [CO32-] in habitat waters. However, results from 43 core tops show that both the fragmentation and SNSW trend of Globorotalia menardii tests trace dissolution in sediments well and are both minimally affected by the [CO32-] of habitat waters. Additionally, pteropod fragmentation trends from 38 core tops trace well aragonite dissolution in sediments above the calcite saturation horizon. Combining the dissolution trend of G. menardii tests and their SNSW with the SNSW of other foraminifer species and the fragmentation of pteropod shells yields the possibility of distinguishing carbonate chemistry among surface waters, and deep and shallow water sediments. Down core results from over 35 cores from the Atlantic, Pacific and Indian Oceans show that the expected deglacial preservation maximum is missing in most cores for both calcite and aragonite. While the reasons for the lack of a calcite preservation maximum vary with location and include variations in the ratio of organic carbon to CaCO3 flux, low sedimentation rates and bioturbation, and increased northward penetration of Antarctic Bottom Water during deglaciation in the Atlantic; the cause for the lack of a deglacial aragonite preservation maximum in high sedimentation rate cores from the western equatorial Pacific is difficult to explain.