Potential Influences of Pacific Meridional Overturning Circulation on Climate Change Across the Mid Eocene Climatic Optimum (MECO)

The mid to late Eocene (49—34 Ma) is characterized as a long-term cooling (7℃) transition from greenhouse to icehouse conditions. However, this long-term cooling period was interrupted by a rapid warming event: the Mid Eocene Climatic Optimum (MECO; 40.6 Ma). During this episode of transient warming, Southern Ocean temperatures slowly rose 4°C over ~500 ka, and then warmed ~1.5°C over a brief period at the peak of the MECO. After the peak warming, rapid cooling occurred over ~200 ka returning the Southern Ocean waters to pre-MECO temperatures. Such prominent and transient warming may have affected (or been influenced) by a change in deep-water formation and circulation, particularly in the Southern Ocean. Here we investigate the potential relationship between the meridional overturning circulation (MOC) and climate change during the MECO using high-resolution records of water mass Neodymium (Nd) and Lead (Pb) isotopes. Neodymium and Pb seawater isotope signatures are widely used water mass and weathering input tracers. They are useful as water mass tracers due to their short residence times relative to oceanic mixing (Nd ~1000 years; Pb ~50-200 years; ocean mixing ~1500 years). Dissolved Nd and Pb compositions in seawater are influenced by weathering inputs from local continental rocks. The use of the two tracers together can provide changes in ocean circulation rates because of their different residence times. High resolution seawater Nd and Pb isotope records from Pacific Ocean sediment cores at Sites 464, 596, 865B and 883B across the MECO indicate seawater Nd isotope values increase by ~.5 epsilon units from 42.6Ma to 41Ma, and then decrease back to -4.8 by 40.3 Ma at North Pacific Site 464. Corresponding seawater Pb isotope values indicate a 206Pb/204Pb decrease from 18.96 to 18.67 from 41.8 to 41 Ma but no change in 207,208Pb/204Pb values. At North Pacific Site 883B, ɛNd seawater values also increase by ~.5 epsilon units from 41.8 to 40.9 Ma, then decrease by .5 epsilon units by 39.7 Ma. The Nd and 206Pb isotope changes that coincided with the MECO are consistent with either 1) a transient increase in the influence of radiogenic deep waters from the north (North Pacific Deep Water; NPDW), or 2) a transient change in the weathering inputs to the NPDW source region. Both are consistent with warming documented during the MECO.