How does rising atmospheric CO2 affect marine organisms?

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Rockall Trough, Northeast Atlantic
Copard, K., Colin, C., Henderson, G.M., Scholten, J., Douville, E., Sicre, M.-A. and Frank, N. 2012. Late Holocene intermediate water variability in the northeastern Atlantic as recorded by deep-sea corals. Earth and Planetary Science Letters 313-314: 34-44.

Working with pristine aragonite fragments of fossil deep-sea corals of the species Lophelia pertusa taken by gravity core from the southwestern flank of Rockall Trough in the Northeast Atlantic Ocean (55°31.17'-55°32' N, 15°39.08'-15°40'W), the authors extracted the rare earth element neodymium (Nd) and calculated its isotopic composition (ɛNd) according to the relationship ɛNd = ([(143Nd/144Nd)sample/0.512638]-1)x10,000, as per Jacobsen and Wasserburg (Earth and Planetary Science Letters 50: 139-155). This work revealed, in the words of Copard et al. that "the warm Medieval Climate Anomaly (1000-1250 AD) was characterized by low ɛNd values (-13.9 to -14.5) ... while the Little Ice Age (around 1350-1850 AD) was marked by higher ɛNd values." After the end of the LIA, however, ɛNd once again declines; but according to the author's Figure 5d, it never quite reaches the -13.9 value that defines the boundary condition (ɛNd = -13.9) of the beginning and end of the MWP. And because the ɛNd value of modern seawater recirculating in the northern North Atlantic at surface and intermediate depths is only -13.1, it can cautiously be concluded that ocean temperatures during the Current Warm Period have not eclipsed those experienced during Medieval times.