Peaks in equatorial Pacific export production during the Middle Miocene Climate Transition

The Middle Miocene Climate Transition (MMCT) is one of three major benthic foraminiferal oxygen isotope (δ¹⁸O) events during the Cenozoic reflecting key changes in Earth’s climate. The MMCT marks expansion of the Antarctic Ice Sheet (AIS) at ~13.8 million years ago (Ma), and global cooling. Concurrent with this cooling step is a globally recognized long positive carbon isotope excursion seen in benthic and planktonic foraminifera with shorter carbon isotope maxima (CM) events linking hypotheses for global cooling at this time with changes in the carbon cycle. Associated with this time period is a pulse of evolutionary turnover in terrestrial and marine biota as well as other physical oceanographic changes. In order to test whether export production, a variable linked to marine ecosystem change, in the eastern equatorial Pacific is related to the largest CM event coincident with expansion of the AIS, a high resolution (<5 kyr) record of export production at Integrated Ocean Drilling Program Site U1337 was produced. Export production was calculated from marine pelagic barite accumulation rates in the deep sea sediment. Elevated export production of more than twice present day values with peaks of up to 10 times more, not seen in any other time period previously investigated, suggests that export production in this region fostered global cooling. We suggest that that the extreme peaks in export production, which are not orbitally paced, provide evidence for a dramatic change in biological efficiency associated with a reorganization of nutrients supplied to the equatorial Pacific in the Miocene. Furthermore, the lack of Milankovitch cycling within the record of export production suggests that it is responding more to critical thresholds in the Earth system than being paced by orbital periodicity.