Nd isotopic structure of the Pacific Ocean 70–30 Ma and numerical evidence for vigorous ocean circulation and ocean heat transport in a greenhouse world

Abstract

The oceanic meridional overturning circulation (MOC) is a crucial component of the climate system, impacting heat and nutrient transport, and global carbon cycling. Past greenhouse climate intervals present a paradox because their weak equator-to-pole temperature gradients imply a weaker MOC, yet increased poleward oceanic heat transport appears to be required to maintain these weak gradients. To investigate the mode of MOC that operated during the early Cenozoic, we compare new Nd isotope data with Nd tracer-enabled numerical ocean circulation and coupled climate model simulations. Assimilation of new Nd isotope data from South Pacific Deep Sea Drilling Project and Ocean Drilling Program Sites 323, 463, 596, 865, and 869 with previously published data confirm the hypothesized MOC characterized by vigorous sinking in the South and North Pacific~70 to 30 Ma. Compilation of all Pacific Nd isotope data indicates vigorous, distinct, and separate overturning circulations in each basin until~40 Ma. Simulations consistently reproduce South Pacific and North Pacific deep convection over a broad range of conditions, but cases using strong deep ocean vertical mixing produced the best data-model match. Strong mixing, potentially resulting from enhanced abyssal tidal dissipation, greater interaction of wind-driven internal wave activity with submarine plateaus, or higher than modern values of the geothermal heat flux enable models to achieve enhanced MOC circulation rates with resulting Nd isotope distributions consistent with the proxy data. The consequent poleward heat transport may resolve the paradox of warmer worlds with reduced temperature gradients.

Constraints on ocean circulation at the Paleocene–Eocene Thermal Maximum from neodymium isotopes

“…Lacan et al, 2012) due to mixing, which is a similar magnitude to the offset observed in our data (∼ 1 ε Nd unit). Our interpretation of these δ 13 C and ε Nd data supports changes in areas of convection that are consistent with simulations of the PETM with coupled climate models (Lunt et al, 2012;Thomas et al, 2014) and with a comprehensive climate model (Winguth et al, 2010). It is impossible to interpret from the Nd isotope data alone whether there was reduced or increased overturning associated with carbon release, as these data reflect water mass geometries and not rates of overturning.…”

“…Such data provide high-resolution records of changes in past deep-water mass mixing and are used to reconstruct deep-ocean circulation (Böhm et al, 2015;Martin et al, 2010Martin et al, , 2012Piotrowski et al, 2004Piotrowski et al, , 2005Piotrowski et al, , 2008Thomas et al, 2014). Here we compare deep-water Nd isotope data obtained from leachates to fish teeth records and carbon isotope data from the same cores, which allows us to elucidate the causes and controls of δ 13 C variations in the past deep oceans (Piotrowski et al, 2005).…”

“…The previously proposed formation of low-latitude Tethyan deep water (e.g., Cope and Winguth, 2011;Huber and Sloan, 2001) or slower overturning circulation (Winguth et al, 2010) is unlikely to have generated such a range and similarity in the evolution of ε Nd signatures at these three sites. Furthermore, numerical simulations indicate that strong overturning circulation with multiple deep convection sites best explains the ε Nd record (Thomas et al, 2014).…”

Climate sensitivity and meridional overturning circulation in the late Eocene using GFDL CM2.1

“…Nd tracer-enabled ocean simulations that are able to reproduce the observed pattern of Eocene Nd data have vigorous bipolar Pacific sinking, with southern rather than North Pacific bottom water dominating (Thomas et al, 2014). Furthermore, idealized GCM simulations show that salt advection feedbacks 20 create a competition between North Pacific and North Atlantic deep water formation (Wolfe and Cessi, 2014), so that the onset of North Atlantic sinking may imply a shutdown of North Pacific sinking and vice versa.…”

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“…The operating mode of the MOC was significantly different during the Late Cretaceous and early Paleogene (e.g., Thomas et al 2014). In the modern oceans, Antarctica is surrounded by the world's strongest ocean current, the Antarctic Counter Current (ACC), which flows around Antarctica and reaches from the surface to abyssal depths.…”