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

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 (δ13 ${{\updelta }}^{13}$C), superimposed by a negative shift in ocean δ13 ${{\updelta }}^{13}$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 CO2 ${\text{CO}}_{2}$ reconstructions do not show a paralleled decline as the late Miocene cooling, suggesting a decoupling of CO2 ${\text{CO}}_{2}$ 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 δ13 ${{\updelta }}^{13}$C records, we examine the pattern of global deep ocean δ13 ${{\updelta }}^{13}$C over the late Miocene. Our analysis shows a step‐wise increase in the benthic δ13 ${{\updelta }}^{13}$C difference between the Atlantic and Indian‐Pacific Oceans. The increased basinal deep ocean δ13 ${{\updelta }}^{13}$C gradient agrees with a positive shift in the benthic δ18 ${{\updelta }}^{18}$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 δ13 ${{\updelta }}^{13}$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. Plain Language Summary: The oxygen and carbon isotopes (δ18 ${{\updelta }}^{18}$O and δ13 ${{\updelta }}^{13}$C) of benthic foraminifer can record the isotopic signatures of seawater and dissolved inorganic carbon in deep ocean. A higher benthic δ18 ${{\updelta }}^{18}$O indicates cooler deep water and/or that more ice with lower δ18 ${{\updelta }}^{18}$O is trapped in land. Benthic δ13 ${{\updelta }}^{13}$C can reflect the transfer or cycling of carbon between the biosphere, hydrosphere, atmosphere, and lithosphere reservoirs. In this study, we present new benthic foraminifer δ18 ${{\updelta }}^{18}$O and δ13 ${{\updelta }}^{13}$C records from the marine sediments which are deposited during the late Miocene period, approximately 5–10 million years ago, at an ocean drilling site in the South Atlantic. By comparing our data with other published late Miocene benthic δ13 ${{\updelta }}^{13}$C, we find a generally increased deep ocean δ13 ${{\updelta }}^{13}$C gradient between the Atlantic and Pacific Oceans over the late Miocene. This phenomenon can reveal a reorganization of global deep ocean circulation. Key Points: This study presents new late Miocene benthic foraminiferal δ13 ${{\updelta }}^{13}$C and δ18 ${{\updelta }}^{18}$O in the South AtlanticThere is an intensified basinal gradient in the deep ocean δ13 ${{\updelta }}^{13}$C over the late MioceneThe intensified basinal δ13 ${{\updelta }}^{13}$C gradient can result from sluggish or weakening of ocean overturning circulation [ABSTRACT FROM AUTHOR]