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Seismic and geologic evidence for Early Paleogene deepwater circulation in the western North Atlantic


  • Gregory S. Mountain,

  • Kenneth G. Miller


Previous studies of paleogeography, global climate, and foraminiferal stable isotopes have suggested that Paleogene deepwater circulation was significantly different from that of today. Extracting details of deepwater history from the western North Atlantic has been hampered by Oligocene erosion and the lack of ideally located drill holes. We report the discovery of a seismic disconformity on the SW Bermuda Rise that we have tied to a late Paleocene erosional event in Deep Sea Drilling Project (DSDP) boreholes from this region. Distribution of this new seismic marker, Reflector Ab, indicates that it was formed by cyclonically circulating bottom water entering the western North Atlantic from the south. We attribute this vigorous deep circulation to a Southern Ocean source, analogous to the effects of modern Antarctic Bottom Water; this interpretation is supported by isotopic comparisons that show the Paleocene Southern Ocean was enriched in 13C relative to the Pacific. We distinguish Reflector Ab from the overlying Reflector Au on the basis of superposition and ages obtained at DSDP sites and piston cores; using one of the cores, we have estimated that the youngest age for Reflector Au is earliest Oligocene (36.6–34.0 Ma), while its oldest age has been previously established as late Eocene. Laminated sediments beneath Reflector Ab are enriched in organic carbon, indicating that the early Paleocene deep western North Atlantic was occasionally anoxic. These low oxygen levels may have been the result of sluggish deep circulation and/or low-latitude production of Warm Saline Deep Water (WSDW). Sediments immediately above Reflector Ab show that deposition resumed on the western Bermuda Rise (Site 387) by the latest Paleocene to earliest Eocene (Biochron NP10; 57.2±0.6 Ma). While deposition resumed on the central Bermuda Rise (Site 386) during the late Paleocene, erosion/nondeposition continued across much of the Bermuda Rise. Widespread resumption of pelagic deposition in the early Eocene is consistent with δ13C evidence for reduced Southern Ocean influence at this time. However, lower Eocene sediments record well-oxygenated conditions, and δ13C and δ18O comparisons indicate that there were two early Eocene deepwater sources: the Southern Ocean and WSDW (Pak and Miller, this issue).

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