Please join us for the first of two Chow Series Lectures from Professor Daniel M. Sigman of Princeton University! His first talk, titled "The Southern Ocean during the ice ages", is geared towards a more general audience.
Pizza will be served at noon!
Abstract: In the effort to identify the cause of the lower atmospheric CO2 concentration during ice ages, the potential impact of the Southern Ocean has long been recognized. I will describe an increasing body of nitrogen isotope evidence that both the (lower latitude) Subantarctic and (higher latitude) Antarctic Zones of the Southern Ocean played roles in lowering the atmospheric concentration of CO2 during the ice ages. In the Subantarctic, the data indicate dust-driven iron fertilization of phytoplankton during the peak ice age conditions. In the ice age Antarctic, the area-normalized exchange of water between the surface and subsurface was apparently reduced, a state that I summarize here as “isolation” of the Antarctic surface. This surface isolation would have stemmed the leak of biologically stored CO2 that occurs there today. Antarctic surface isolation does not arise spontaneously in physical climate model simulations of the last ice age, neither is the opposite (increased surface-deep exchange) predicted by models for global warming. Thus, if our diagnosis of ice age Antarctic isolation is correct, then we must identify a mechanism and explain why it is not captured by climate models. I will compare data from the Antarctic with those from the subarctic North Pacific to argue for an equatorward shift and overall weakening in the upwelling caused by the westerly winds, occurring in both hemispheres. This interpretation leads to a set of critical follow-on questions. In particular, in the face of reduced Antarctic upwelling, how did the Subantarctic maintain its nutrient supply, and how did the North Atlantic meridional overturning circulation maintain its water supply? As a possible answer to these questions, I will propose a large scale interior circulation for the ice age ocean that resembles a view of the modern ocean circulation developed by Scripps oceanographer Walter Munk.
The cost of the series is supported by the Chow Fund.