Mid-Miocene paleoproductivity in the Atlantic Ocean and implications for the global carbon cycle

A prominent, middle Miocene (17.5-13.5 Ma) carbon isotope excursion ubiquitously recorded in carbonate sediments has been attributed to enhanced marine productivity and sequestration of C-13 depleted organic carbon in marine sediments or enhanced carbon burial in peat/lignite deposits on land. Here we test the hypothesis that the marine delta C-13 record reflects a change in productivity with proxy records from three Atlantic Ocean sites (Deep Sea Drilling Program Site 608 and Ocean Drilling Program Sites 925 and 1265). Our multiproxy approach is based on benthic foraminiferal accumulation rates, elemental ratios (Ba/Al and P/Al), the delta C-13 of bulk sedimentary organic matter, and dissolution indices. We compare these proxies to benthic foraminiferal delta C-13 values measured on the same samples. Our results indicate that marine paleoproductivity in the Atlantic Ocean is not related to the benthic foraminiferal delta C-13 excursion. A numerical box model confirms that marine productivity cannot account for the delta C-13 maximum. The model shows that sequestration of 1.5 x 10(18) mol C in the terrestrial realm over a period of 3 Ma leads to a 0.9 parts per thousand delta C-13 increase in the deep ocean, which is near the observed records. Therefore, an increase in continental organic carbon sequestration is the most plausible way to enrich the ocean's carbon pool with C-13, which is consistent with coeval lignite deposits worldwide. The delta C-13 values of bulk sedimentary organic matter parallel the delta C-13 of dissolved inorganic carbon as reflected by benthic foraminiferal delta C-13 values suggesting no significant change in atmospheric pCO(2) levels over the investigated period.