How does rising atmospheric CO2 affect marine organisms?

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Three Estuaries off the Atlantic Coast of France
Chaumillon, E., Tessier, B., Weber, N., Tesson, M. and Bertin, X. 2004. Buried sandbodies within present-day estuaries (Atlantic coast of France) revealed by very high resolution seismic surveys. Marine Geology 211: 189-214.

The authors examined very high resolution seismic profiles from three estuary locations on the Atlantic coast of France (~ 48.95N, 1.23W; 45.2N, 1.43W; 46.25N, 1.43W). Common to all three locations was a "very sharp" sedimentary transition from basal sand to muddier sediments, from which two shells were recovered and radiocarbon dated to AD 720 and 1100. Speaking of this transition, they say "it could correspond to the transition between the cold period of the Dark Age (AD 400-900) and the Medieval warm period (AD 900-1400)," as warmer temperatures brought about higher rainfall and more frequent floods that resulted in an increase of estuarine mud supply to the region.