Southward shift of the Intertropical Convergence Zone due to Northern Hemisphere cooling at the Oligocene-Miocene boundary

Abstract

The Mi-1 glaciation (ca. 23 Ma), which marks the Oligocene-Miocene boundary, was an aberrant cooling event that led to a build-up of the Antarctic ice sheet, which reached the near-modern volume (or greater) from its ephemeral or partial existence. An increase of similar to 1% in the delta O-18 of benthic foraminifera during this interval has been attributed to the development of Antarctic ice sheets and deep-water cooling. Without definitive evidence, Northern Hemisphere (NH) glaciation has not been a material consideration for the delta O-18 increase. Here we investigate the interhemispheric temperature contrast during Mi-1, with the movement of the Intertropical Convergence Zone (ITCZ) at a site (10 degrees 31'N) in the East Pacific (Integrated Ocean Drilling Program Site U1333), to understand NH cooling and the possibility of NH glaciation. The measured Nd-143/Nd-144, Sr-87/Sr-86, and clay mineral compositions of eolian dust fractions indicate unequivocally the deposition of Asian dust during Mi-1, and of Central American and South American dust before and after Mi-1. This is attributed to the southward displacement of the ITCZ over Site U1333 during Mi-1. The ITCZ shifts toward the warmer hemisphere. Thus our results suggest that the cooling during Mi-1 was more significant in the NH than in the Southern Hemisphere, which underwent a sudden expansion of continental ice sheets. Our data call for a forcing mechanism to drive significant NH cooling during this episode. Based on the available data, we propose that the widespread growth of NH ice sheets and/or changes in the production of North Atlantic-origin deep water could be possible causes of the NH cooling at that time.