How does rising atmospheric CO2 affect marine organisms?

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Lake Korttajarvi, Central Finland
Tiljander, M., Karhu, J.A. and Kauppila, T. 2006. Holocene records of carbon and hydrogen isotope ratios of organic matter in annually laminated sediments of Lake Korttajarvi, central Finland. Journal of Paleolimnology 36: 233-243.

The authors studied the isotopic composition of carbon (δ13C) and hydrogen (δD) in organic matter found in two sediment cores retrieved from the center of the southern basin of Lake Korttajarvi (6220'N, 2541'E). In doing so, they found that "the Medieval Warm Period in AD 980-1250 is associated with a local maximum in δD, lending support for a significant warming during that time," as can be seen in the figure below, since researchers generally find, in their words, "δD increasing at warmer environmental conditions."

Fifteen-hundred-year history of Lake Korttajarvi sediment organic-matter δD. Adapted from Tiljander et al. (2006).