The CCD of the Paleogene tropical Pacific - results of ODP Leg 199 (abstract of paper presented at AGU Fall Meeting, San Francisco, 6-10 Dec 2002)

Abstract

Sites drilled by Leg 199 form a latitudinal transect across the Paleogene equator, extending across paleolatitudes of 4.5°S to 18.5°N. Drill sites were explicitly located atop sea-floor anomaly 25, approximately 56 million years old, ensuring recovery of early Eocene carbonate sediment. Many of the earlier DSDP and ODP efforts with regard to Paleogene paleoceanography were confounded by the common Eocene cherts. The drilling strategy adopted by Leg 199 obviated some of these problems, with the result that we have increased amount of information regarding the Paleogene low-latitude CCD several fold.To reconstruct the CCD we have assumed that sea-floor subsidence follows the simple square root of age law, and that the depth of the Paleogene East Pacific Rise axis was 2750 meters, in agreement with earlier studies. Bulk density considerations suggest that sediment loading depresses the sea floor by an amount equal to half the sediment so any complete determination of age-depth histories requires an unloading step. Results of these determinations for the Leg 199 sites are such that paleodepths are comparable among the sites to ±50 meters, and the absolute values of the paleodepths depend on how good the ridge axis paleodepth assumption is. Results show a shallow early Eocene CCD of 3200 meters at the equator, deepening to 3600 meters at 10°N latitude. In the modern ocean the CCD always deepens towards the equator, so something quite different has occurred in the early Eocene. The CCD remained at these shallow levels until the end of the Eocene, with the exception of a sudden sharp excursion (drop/rise) of about 700 meters at 41 Ma. The large drop of the CCD at the end of the Eocene has been known since the earliest CCD studies, but completely recovered sections across this boundary have been few or none. Leg 199 recovered complete records of this event at all 8 drillsites. At the time of the Eocene/Oligocene boundary the CCD in the low-latitude Pacific fell by over 1000 meters to 4350m, the largest change in ocean paleochemistry in the entire Cenozoic. This drop occurred in two steps, each one occurring as rapidly as in thousands of years. A preliminary look at the oxygen isotopic record of these materials at Site 1218 shows the CCD drop is in phase with the long recognized O-18 shift at the E/O boundary. The Neogene CCD in sub-tropical North Pacific lies at about 4600 m, about 500 meters deeper than the CCD of the sub-tropical South Pacific.

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ORCID for P.A. Wilson: orcid.org/0000-0001-6425-8906

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