Abstract:
During December and January of 1997/98 the R/V Ewing surveyed a series of potential Ocean Drilling Program (ODP) drillsites that form a latitudinal transect across the Paleogene equator at a present longitude of about 140 degrees W. Seismic reflection profiles from this cruise when dated using pre-existing seismic stratigraphy, cores recovered on the cruise, and ties to extant DSDP sites, show a remarkable change in the character of equatorial sedimentation that occurs near the Eocene-Oligocene boundary. The younger sedimentary accumulations show the distinct equatorial bulge, but the Eocene and older horizons show no such bulge, rather we find sedimentary layers of relatively uniform thickness from the equatorial zone as far as the profiles extend to the north and south. The lack of an equatorial sediment bulge implies the absence of an equatorial zone of high biological productivity. We interpret this change in the nature of equatorial deposition, first outlined by van Andel and others nearly 25 years ago, as representing a very different kind of equatorial oceanographic, thus also atmospheric, circulation in Eocene and older times. Today, dust of shale bulk composition transported from Asia dominates the North Pacific east most of the way to North America and south to the ITCZ. Dust of andesite bulk composition from America occurs in the easternmost Pacific and south of the ITCZ. We can plot the downcore transition from Asian to American dust, a proxy for the position of the ITCZ, in several North Pacific cores, and find that that the Eocene ITCZ may have lain north of 20 degrees N. Other work indicates that wind intensities were much reduced in the Eocene. These observations of sluggish winds, significantly asymmetric atmospheric circulation, and no unusual productivity at the equator provide the basis for a scenario of Eocene atmospheric and oceanic circulation. This scenario is characterized by the Pacific ITCZ lying at 20 degrees to 25 degrees N, a broad region of southwest tradewinds and underlying equatorial counter current between the ITCZ and the equator, and no equatorial divergence.