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 13C depleted organic carbon in marine sediments or enhanced carbon burial in peat/lignite deposits on land. Here we test the hypothesis that the marine δ 13C 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 δ 13C of bulk sedimentary organic matter, and dissolution indices. We compare these proxies to benthic foraminiferal δ 13C values measured on the same samples. Our results indicate that marine paleoproductivity in the Atlantic Ocean is not related to the benthic foraminiferal δ 13C excursion. A numerical box model confirms that marine productivity cannot account for the δ 13C maximum. The model shows that sequestration of 1.5 × 1018 mol C in the terrestrial realm over a period of 3 Ma leads to a 0.9‰ δ 13C 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 13C, which is consistent with coeval lignite deposits worldwide. The δ 13C values of bulk sedimentary organic matter parallel the δ 13C of dissolved inorganic carbon as reflected by benthic foraminiferal δ 13C values suggesting no significant change in atmospheric pCO2 levels over the investigated period
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