GLOBAL PACING OF PLIOCENE CLIMATE BY NORTHERN HEMISPHERE PRECESSION: AN ENIGMA (Invited Presentation)

The origin of paleoclimatic variations in the precessional band has plagued students of orbital forcing for decades. Where precessional power is present, the problem can be “solved” by positing the dominance of one hemisphere and the existence of a significant non-linearity that translates seasonal forcing into a time-averaged effect on climate. Where precessional power is weak to non-existent, one can posit offsetting processes in the northern versus southern hemisphere that lead to cancellation. An unfortunate further complication comes from the difficulty in constraining the phase of a precessional response, since age model uncertainties on the order of 10 kyr (one half precessional cycle) will lead to completely ambiguous results in assigning a paleoclimatic signal to the hemisphere of origin.

We will present here a potential, but puzzling, result of constraining the phase of ocean temperature variations and global ice volume/deep ocean temperature responses at the precessional wavelength in the mid-to-late Pliocene. During this time, essentially all the Earth’s ice resided in the Southern Hemisphere, so one might expect a Southern Hemisphere phasing of precessional signals. In order to “clock” the Pliocene precession cycles, we anchor our analyses in the Mediterranean pelagic sequence at Punta Piccola (Sicily). Here, the timing of ocean temperature variations (“SST”) can be reconstructed very accurately via the regional hydrological signal (northern African monsoon) that is expressed in cyclic marl-carbonate alternations. We show that this signal can be exported to the open ocean via magnetostratigraphy and correlation to North Atlantic temperature records. Oxygen isotope stratigraphy then allows us to demonstrate that this temperature pattern is to first order synchronous globally- crucially, in Southern as well as Northern Hemisphererecords. We find that, at the precessional wavelength, global temperatures and ice volume minima follow northern hemisphere summer insolation, despite the absence of northern hemisphere cryospheric amplification mechanisms in the Pliocene. The evidence thus suggests that the Northern Hemisphere can dictate global climate changes even in a minimally glaciated condition, perhaps through carbon cycle processes.