The Late Miocene Ocean Carbon Cycle Changes: New Evidence From the South Atlantic IODP Expeditions

Abstract The late Miocene ocean carbon cycle changes can be characterized by the secular 405‐kyr long‐eccentricity cycles in ocean dissolved inorganic carbon isotopic composition (C), superimposed by a negative shift in ocean C, namely, the Late Miocene Carbon Isotope Shift. During this epoch, the Earth experienced a long‐term cooling, especially in high‐latitude oceans. Comparatively, atmospheric reconstructions do not show a paralleled decline as the late Miocene cooling, suggesting a decoupling of radiative forcing and Earth's climate. Analysis on ocean carbon cycle and circulation changes can provide insights into the late Miocene cooling. Here we present new benthic foraminiferal isotopes from International Ocean Discovery Program Site U1560 in Hole A (U1560A) in the South Atlantic. Together with existing benthic foraminiferal C records, we examine the pattern of global deep ocean C over the late Miocene. Our analysis shows a step‐wise increase in the benthic C difference between the Atlantic and Indian‐Pacific Oceans. The increased basinal deep ocean C gradient agrees with a positive shift in the benthic O in Hole U1560A, which may indicate deep ocean cooling or enhanced Antarctic glaciation. As supported by lines of geological evidence and a diagnostic carbon cycle box‐model developed in this study, sluggish or weakening of ocean meridional overturning circulation due to enhanced Antarctic glaciation can be a driver for the increased C gradient. Such a change in ocean circulation may reduce the polarward ocean heat transport, leading to deepened mixed layer depth in the tropical oceans.