Paleoceanographic significance of Late Paleocene dysaerobia at the shelf/slope break around New Zealand

A geochemical and micropaleontological study of the Waipawa Formation in Ne w Zealand was undertaken to investigate its depositional setting and possib le relationship to major changes in the marine environment. The organic-ric h rich formation was deposited in the Late Paleocene, during a third-order, eustatic sea-level rise. Precise age control is lacking, but deposition ca n be placed between a climatic cooling at 59.1 Ma and a thermal maximum at 55.5 Ma, during the transition from cold to warm-saline deep-water circulat ion in the South Pacific. Deposition occurred around most of the land mass, generally near the shelf break, and microfaunal evidence suggests dysaerob ic conditions. There appears to have been no contemporaneous organic-rich s edimentation on the shelf, so the widespread high phytoplanktonic productiv ity, which is reflected in positive excursions in kerogen delta(13)C to -20 parts per thousand, was probably caused by regional upwelling. However, th e paleodistribution of the formation cannot be readily accounted for by win d-driven upwelling alone. There is strong positive correlation between orga nic content, kerogen delta(13)C values and unusually high abundance of 24-n -propylcholestanes (20-55% of C-27-C-30 regular steranes). Surface-water co nditions favoured a particular group of phytoplankton that preferentially s ynthesised the precursor C-30 sterols, and there is microfaunal evidence fo r warming. Carbonate oxygen isotopic data ape also consistent with warming, but may be affected by diagenetic alteration. The Waipawa Formation may re sult from regional upwelling of warm, saline, oxygen-depleted, nutrient-ric h deep water, in what appears to be an earlier and longer-term, but geograp hically more restricted, event than that responsible for the benthonic exti nction event at 55.5 Ma. (C) 2000 Elsevier Science B.V. All rights reserved .