Paleoceanography http://dx.doi.org/10.1029/2011PA002163 (in the press)

The middle of the Cretaceous period, beginning about 110 million years ago, was dominated by hot climate conditions. An analysis of carbon in marine rocks reveals that the marine carbon cycle was strongly influenced by changes in the eccentricity of the Earth's orbit during this greenhouse interval.

Martino Giorgioni of ETH Zurich and colleagues measured the carbon isotope composition of carbonates within the mid-Cretaceous marine rocks of the Apennines in Central Italy. They found clear variations on a 400,000-year timescale, in line with long-term variations in the eccentricity of the Earth's orbit. The high eccentricity periods are associated with the deposition of black, organic-rich shales.

The researchers speculate that the long-period eccentricity cycle induced variations in the geographic distribution and intensity of monsoon systems, as strong monsoons deliver more organic carbon from the continents to the oceans. Orbital forcing could have also affected marine circulation patterns and the storage of carbon in terrestrial wetlands.