Neogene history of the south Pacific tradewinds: Evidence for hemispherical asymmetry of atmospheric circulation

Eolian dust grains extracted from southeast Pacific pelagic sediments provide a Neogene record of southern hemisphere atmospheric processes. The mass accumulation rate of dust has been low and generally constant at 1-4 mg/cm2 103 y since the late Oligocene. Eolian grainsize data show an increase from 8.2[phi] (3.40 [mu]) in older sediments to 7.2[phi] (6.8o [mu]) in younger material. This shift occurred about 10.5 m.y. ago and reflects a significant increase in the intensity of atmospheric circulation then. There is no obvious response to the onset of northern hemisphere glaciation 2.5 m.y. ago in these eolian records from the South Pacific. Comparison with existing information from the northern hemisphere suggests that the southern hemisphere has had more intense atmospheric circulation throughout the Neogene and that this asymmetry reached its greatest extent between 10.5 and 2.5 Ma.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26165/1/0000242.pd

(Table 1) Age, porocity, density, mass accumulation rates and grain size of the eolian component of sediments from DSDP Leg 92

Eolian dust preserved in deep-sea sediments provides a record of atmospheric circulation throughout geologic time. This eolian component has been isolated to determine its mass accumulation rate (MAR) and grain size in post-middle Oligocene sediments from DSDP Sites 597 to 602, which lie along 19°S on the East Pacific Rise. The rate of eolian deposition has been very low (about 2 to 3 mg/(cm**2 x 10**3 yr) and reasonably consistent at all the sites, except that the MAR of the eolian component at Site 597 increases to an average of about 8 mg/(cm**2 x 10**3 yr) between 17 and 19 Ma. Eolian grain size at Site 598 changes dramatically at 10.5 Ma from 7.2 phi in younger sediments to 8.2 phi in older material and may provide evidence of an intensification of zonal winds that took place in response to Antarctic ice formation just prior to that time. Pliocene to Pleistocene samples from Leg 92 show no evidence of the severalfold increase in dust accumulation and grain size recorded at Northern Hemisphere locations at the time corresponding to the onset of Northern Hemisphere glaciation. The low and constant flux of eolian material indicates that South America was consistently vegetated from post-middle Oligocene time to the present