How does rising atmospheric CO2 affect marine organisms?

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Shark River Slough, Florida Everglades, USA
Sanchez, C., Gaiser, E.E., Saunders, C.J., Wachnicka, A.H., Oehm, N. and Craft, C. 2013. Challenges in using siliceous subfossils as a tool for inferring past water level and hydroperiod in Everglades marshes. Journal of Paleolimnology 49: 45-66.

Sanchez et al. examined diatom, plant and sponge silico-sclerids found in soil cores from the central Everglades marshes, which they collected from three locations centered at 25.6279°N, 80.7246°W that were "characterized by 'ridge and slough' micro-topography," finding that "changes in subfossil quality and abundances at centennial time-scales were associated with mid-Holocene climate events including the Little Ice Age and Medieval Warm Period." More specifically, they report that a shift from diatom- to sponge-dominance from 1400 to 1200 years before present (YBP) was "coincident with the onset of the Medieval Warm Period (MWP)," and they state that the absence of subfossils at the transition to 1000 YBP "indicates the culmination of a sustained dry period roughly marking the duration of the MWP." Hence, they identify the MWP as holding sway from approximately AD 750 to AD 950.