Evolution of the Global Overturning Circulation since the Last Glacial Maximum based on marine authigenic neodymium isotopes
Jianghui Du, Brian A. Haley, Alan C Mix
Abstract
The Global Overturning Circulation is linked to climate change on glacial-interglacial and multi-millennial timescales. The understanding of past climate-circulation links remains hindered by apparent conflicts among proxy measures of circulation. Here we reconstruct circulation changes since the Last Glacial Maximum (LGM) based on a global synthesis of authigenic neodymium isotope records (ε Nd ). We propose the bottom-up framework of interpreting seawater and authigenic ε Nd considering not only conservative watermass mixing, but also the preformed properties and the non-conservative behavior of ε Nd , both subject to sedimentary influences. We extract the major spatial-temporal modes of authigenic ε Nd using Principal Component Analysis, and make a first-order circulation reconstruction with budget-constrained box model simulations. We show that during the LGM, the source region of North Atlantic overturning shifted southward, which led to more radiogenic preformed ε Nd of glacial Northern Source Water (NSW). Considering this preformed effect, we infer that glacial deep Atlantic had a similar proportion of NSW as today, although the overall strength of glacial circulation appears reduced from both North Atlantic and Southern Ocean sources, which increased the relative importance of non-conservative behavior of ε Nd and may have facilitated accumulation of respired carbon in the deep ocean. During the deglaciation , we find that Southern Ocean overturning increased, which offset suppressed North Atlantic overturning and resulted in a net stronger global abyssal circulation . Faster global scale deglacial circulation reduced the relative importance of non-conservative effects, resulting in Atlantic-Pacific convergence of abyssal ε Nd signatures. Variations of Southern Ocean overturning likely drove a significant fraction of deglacial changes in atmospheric CO 2 and oceanic heat budget.