How does rising atmospheric CO2 affect marine organisms?

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Northeastern Slope of the Cariaco Basin, Venezuela Coast
Black, D. E., Thunell, R. C., Kaplan, A., Peterson, L. C. and Tappa, E. J. 2004. A 2000-year record of Caribbean and tropical North Atlantic hydrographic variability. Paleoceanography 19, PA2022, doi:10.1029/2003PA000982.

High-resolution δ18O records generated from seasonally representative planktic foraminifera were obtained from two ocean sediment cores extracted from the Cariaco Basin off the coast of Venezuela (~ 10.65N, 64.66W) to produce a temperature/salinity reconstruction in this region of the Caribbean/tropical North Atlantic over the last 2000 years. Results indicate a general trend toward cooler and perhaps more saline waters over the length of the record. Because of this trend, the authors describe discussion of the Medieval Warm Period and Little Ice Age as "complicated," but they nonetheless acknowledge their record reveals "an interval of warmer [sea surface temperatures] prior to ~ A.D. 1600-1900" where the δ18O data "correctly sequence the relative temperature change between the so-called MWP and LIA." In viewing the authors' graph of G. bulloides δ18O (25-year mean, reproduced below), and their stated relationship that a δ18O change of 1.0 is equivalent to a 4.2C change in temperature, we calculate the difference in peak warmth between the MWP and CWP to be 1.05C, with the MWP being the warmer of the two periods.