Eolian deposition in the northeast Pacific Ocean: Cenozoic history of atmospheric circulation

Abstract

The mass accumulation rate and grain size distribution of the total eolian component of North Pacific pelagic clays have been used to evaluate changes in eolian sedimentation and in the intensity of atmospheric circulation that have occurred during the past 70 m.y. Eolian deposition was relatively low, approximately 12 mg/cm2/1,000 yr, in the Late Cretaceous and Paleocene, reflecting an increasingly humid environment. A peak in accumulation rates across the Paleocene-Eocene boundary may be the result of episodic volcanism in Mexico. A sharp decrease in grain size across the Paleocene-Eocene boundary is not readily interpreted. Fine eolian grain size and the low accumulation rates, averaging 8 mg/cm2/1,000 yr, in the Eocene and early Oligocene are in agreement with low early Tertiary thermal gradients and less vigorous atmospheric circulation. Large increases in grain size between 37 and 30 m.y. ago, 20 and 10 m.y. ago, and 5 m.y. ago to the present suggest that atmospheric circulation intensified by a factor of 1.5, 1.8, and 1.4, respectively, at these times. Eolian accumulation rates increased from 8 mg/cm2/1,000 yr at the Eocene-Oligocene boundary to approximately 25 mg/cm2/1,000 yr at the end of the Miocene, with significant changes at 25 and 13 m.y. ago. Increased rates about 25 m.y. ago probably reflect movement of the core site into the influence of the westerlies, and the increase 13 m.y. ago appears to be a response to steepening thermal gradients and intensifying atmospheric circulation resulting from major expansion of Antarctic ice in the mid-Miocene along with a general increase in global aridity in the late Cenozoic. An order of magnitude increase in eolian accumulation rates (25 to 226 mg/cm2/1,000 yr) in Pliocene-Pleistocene time coincides with the onset of Northern Hemisphere glaciation and consequent climatic deterioration. The large increase in accumulation rates in Pliocene-Pleistocene time coincides with a relatively smal! increase in the intensity of atmospheric circulation compared to that in Oligocene and Miocene time, suggesting that the increased deposition rates were the result of an increase in global aridity in glacial times, rather than a further increase in the vigor of the global wind systems.