The Evolution of Pacific Meridional Overturning Circulation through the Paleogene

Abstract:

The Paleogene operating mode of Meridional overturning circulation (MOC) consisted of a bipolar Pacific MOC that circulated independently from the Atlantic MOC. Reconstructions based on Nd isotopes and numerical modeling confirm a Paleogene MOC mode significantly different from the modern. But the outstanding questions of the timing and cause of the transition to the “modern” global mode of MOC remain. Furthermore, how did the strength of the MOC vary through the Paleogene and during the transition to the modern mode?

In order to address these questions, we need a way to estimate rates of deep-water advection. Here we explore the use of paired seawater Nd and Pb isotopes as a qualitative gauge of water mass advection rate. The premise is that as a water mass moves further from the region of convection, the initial dissolved Nd and Pb fingerprint will become increasingly decoupled due to increasing contributions of dissolved Pb from dust or ash dissolution in the water column.

South Pacific DSDP Site 596 and North Pacific ODP Site 883 were well positioned to monitor the evolution of South Pacific Deep Water (SPDW) and North Pacific Deep Water (NPDW). At Site 596 the Nd and ²⁰⁶Pb/²⁰⁴Pb isotopes were relatively constant from ~60 to 38 Ma, but then varied significantly from ~38 Ma to 10 Ma. However, the corresponding ^207,208Pb/²⁰⁴Pb values varied significantly until ~48 Ma, and them remained relatively constant to 10 Ma. At Site 883 the Nd values covaried with all three Pb isotope tracers from ~60 to 50 Ma - an increase in Nd coincided with a decrease in Pb values, followed by a decrease in Nd/increase in Pb. After 50Ma, Nd values remained constant (~-2 epsilon units) until ~35Ma, however during this interval all three Pb isotopes gradually increased. This decoupling of Pb and Nd either resulted from a change in the composition of Pb weathering into the NPDW source region, or the overprinting of Pb during water mass advection southward. The latter is consistent with a diminished flux of NPDW from ~50 to 35Ma, potentially enabling dust and ash dissolution to overprint the original NPDW Pb signature, while the Nd signature was preserved due to the longer residence time. As the flux of NPDW waned, the flux of SPDW remained nearly constant until ~38Ma.