How does rising atmospheric CO2 affect marine organisms?

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Lake Hvítárvatn, Central Iceland
Larsen, D.J., Miller, G.H., Geirsdottir, A. and Thordarson, T. 2011. A 3000-year varved record of glacier activity and climate change from the proglacial lake Hvítárvatn, Iceland. Quaternary Science Reviews 30: 2715-2731.

Larsen et al. used a suite of environmental proxies -- varve thickness, varve thickness variance, ice-rafted debris, total organic carbon (mass flux and bulk concentration), and the C:N ratio of sedimentary organic matter) -- that they obtained from annually-laminated sediments laid down by a proglacial lake (Hvítárvatn) in the central highlands of Iceland, the thickness of which layers is controlled by the rate of glacial erosion and efficiency of subglacial discharge from the nearby Langjokull ice cap, which facts allowed them to reconstruct histories of regional climate variability and glacial activity over the past 3000 years. This work revealed that "all proxy data reflect a shift toward increased glacial erosion and landscape destabilization from ca 550 AD to ca 900 AD and from ca 1250 AD to ca 1950 AD, separated by an interval [900 AD to 1250 AD] of relatively mild conditions," and they say that "the timing of these intervals coincides with the well-documented periods of climate change commonly known as the Dark Ages Cold Period, the Medieval Warm Period and the Little Ice Age." And in speaking of the MWP, they note that "varve thickness decreases after 950 AD and remains consistently low through Medieval time with slightly thinner annual laminations than for any other multi-centennial period in the past 3000 years," which suggests that the MWP was the warmest period of the past three millennia in that particular part of the world.