Abstract: The oceanic meridional overturning circulation (MOC) is a crucial component of the climate system, impacting heat and nutrient transport, and global carbon cycling. Changes in the pattern and strength of the MOC are considered to have played a large role in past climate variations. However, the role of the MOC in greenhouse climate states presents a paradox - basic physical arguments suggest that MOC strength is proportional to density gradients, which were much lower during greenhouse climates. Equally paradoxical is the role of MOC in global carbon cycling during warm intervals. Mechanisms proposed to explain periods of high organic carbon burial during the Late Cretaceous invoke either a sluggish or a rapid MOC. However, organic carbon burial during the equally warm early Eocene was anomalously low perhaps implying a more rapid MOC. Data that constrain the pattern and strength of the MOC are required to resolve these conflicting hypotheses.
Neodymium isotopes are one of the most robust tracers of ancient water mass composition, widely applied to timescales spanning the past ~400 million years. In this study, we pair new Nd isotope analyses with numerical simulations using general circulation models. A synthesis of new Nd isotope data from South Pacific Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) Sites 323, 463, 596, 865, and 869 with previously published data indicate an MOC characterized by vigorous sinking in the South and North Pacific during the greenhouse climate interval of the latest Cretaceous through early Paleogene (~70 to 30 Mya). This indicates an MOC characterized by separate overturning in the Atlantic and Pacific. Convection occurred in the North Pacific, the Ross Sea region, as well as the Atlantic sector of the Southern Ocean.
To investigate the relationship between this mode of MOC and global heat transport we compare the data with Nd-tracer enabled numerical ocean circulation and coupled climate model simulations. This comparison confirms that a bipolar Pacific deep convection mode was a robust feature of past greenhouse climates. Using the ocean-only GCM, we find that simulations with enhanced deep-ocean vertical mixing produced the best data-model match. Strong deep-ocean mixing enables models to achieve enhanced pole-ward ocean heat transport, and may resolve the paradox of warmer worlds with reduced temperature gradients.