COMPARISONS BETWEEN SEISMIC PROFILES AND DRILLINGThe site survey cruise (EW9709) continuously collected data along two transects of the northern tropical Pacific. Segments of these transects are combined to reconstruct two cross sections: one at ~40 Ma (late middle Eocene) and the other at ~56 Ma (late Paleocene) (Fig. 1). These transects were planned to follow the 57-Ma (Chron An25r) or 41-Ma (Chron An18r) ridge crest (where carbonate sediments will be better preserved). They span the time of maximum warmth and extend through the cool down of the "hothouse world" into the time of initial Antarctic glaciation. We chose the crustal age to be ~1 m.y. older than the age of the first sediments of interest to avoid sediments with the largest hydrothermal component.
The collection of high-resolution seismic reflection data along the two EW9709 transects has given us valuable insights into the character of Paleogene deposition in the equatorial region and has led us to speculate that the patterns of sediment accumulation and biogenic flux were markedly different in the early Paleogene (Moore et al., 1999). We have correlated the seismic signature of the equatorial Pacific section in data collected during our recently completed cruise with that developed by Mayer and his co-workers (Mayer et al., 1985, 1986; Knappenberger, 2000). This correlation is based on the seismic character of the reflections themselves and is checked against the age of surficial sediments recovered in piston cores during our cruise (Fig. 4, Fig. 5) and in nearby DSDP drill sites. The seismic stratigraphy of Mayer et al. (1985, 1986) covers the Pleistocene to the uppermost Oligocene. We have tentatively extended this stratigraphy to the base of the sections imaged in our transects.
Our extension of this seismic stratigraphy and the exact ages of the reflecting horizons we have identified await verification by drilling. However, assuming that the stratigraphic horizons and the ages we have assigned to them are even approximately correct, we can make a few rather startling observations:
1.The equatorial mound of the lower Miocene sediments (as defined by Mayer et al., 1985)
can be clearly seen in the seismic data; however, the "upper-middle Eocene" (M-E1) and
"middle-early Eocene" (E1-acoustic basement) sedimentary packages show a very
different pattern: (a) the M-E1 package shows only a hint of thickening in the equatorial
region and (b) the E1-basement package actually appears to be thicker 5°-10° north of the
hotspot-estimated positon of the equator than it does at the equator.
2.Cores taken on cruises to the tropical North Pacific, DSDP Site 40, and Site PAT-13C, have recovered middle Eocene radiolarian oozes at hotspot-estimated paleolatitudes of 7° 8°N. Throughout the Neogene and into the Quaternary, sections at comparable paleolatitudes are typically devoid of siliceous microfossils or contain only sparse, highly corroded specimens.
3.Given that our assigned ages are approximately correct, the accumulation rates of sediments in the thicker lower Eocene sections of the 56-Ma transect are similar to average accumulation rates calculated for Neogene and Quaternary sections.
These observations, if substantiated by the proposed coring, require a new oceanographic paradigm for the tropics of the early Paleogene. Our stratigraphic interpretations must be verified by drilling; however, if they prove to be approximately correct, they will necessitate a revolution in our thinking about wind-driven circulation and productivity in the tropical oceans during times of extremely warm climates. The sections recovered at Leg 199 drill sites will help to establish the patterns of biogenic sediment flux, the distribution patterns of planktonic assemblages, the accumulation patterns, size variation, and sources of wind-blown dust, and the isotopic compositions of benthic and planktonic (deep- and shallow-living) organisms. With these data we should be able to develop a clearer understanding of tropical atmospheric and oceanic circulation during the extremely warm climate of the early Paleogene.
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