LATEST PALEOCENE BENTHIC FORAMINIFERAL EXTINCTION AND ENVIRONMENTAL-CHANGES AT TAWANUI, NEW-ZEALAND

A major extinction of intermediate-water (500-1000 m) benthic foramini feral species coincided with a major decrease in delta(13)C (2.8 parts per thousand) of terrestrial organic matter (n-C-29 alkane) and delta (34)S (20 parts per thousand) of whole rock sulfide in a continuous si ltstone sequence in the Tawanui Section (4S degrees S paleolatitude) a long the Akitio River, southeastern North Island, New Zealand, in the middle part of the uppermost Paleocene nannofossil zone (CP8). The ben thic extinction (25% of species) occurred over similar to 3 kyr at sim ilar to 55.5 Ma. Increases in kaolinite/illite and kaolinite/smectite ratios and in terrestrial organic carbon percentages started similar t o 3 kyr before the major benthic extinctions, lasted over similar to 4 0 kyr, and probably reflect warmer climate and increased rainfall. The productivity of planktonic foraminifera and calcareous nannoplankton decreased similar to 3 kyr prior to the major extinctions and recovere d at the time of benthic extinctions. These events that started simila r to 3 kyr before the extinction can be best explained by warming, inc reased rainfall, reduced salinity of surface waters, and increased inf luence of warm saline deep water (WSDW). Benthic foraminiferal oxygen indices indicate a strong decrease in dissolved oxygen levels within t he intermediate water from low oxic (1.5-3.0 mL/L O-2) to suboxic (0.3 -1.5 mL/L O-2) conditions coinciding with the benthic extinctions. Inc reases in total organic carbon (TOC) and in the hydrocarbon-generating potential of kerogen (measured as the hydrogen index (HI)) agree with the interpretation of decreased dissolved oxygen levels of the interm ediate water. The lowest oxygen conditions lasted similar to 40 kyr an d coincided with a decrease in calcareous benthic foraminiferal produc tivity, highest TOC levels, and lowest delta(13)C of terrestrial organ ic carbon. Dominant formation of WSDW or sluggish intermediate-water c irculation caused by warming and high rainfall in high-latitude areas most likely led to the 3-kyr time lag between events on land and in su rface waters preceeding the extinction and the development of dysaerob ia in the sea, coinciding with the major benthic extinction and decrea se in delta(13)C and delta(34)S in New Zealand. Global warming of deep and intermediate waters may have caused decomposition of methane hydr ate in sediments, resulting in a strongly decreased delta(13)C of mari ne carbonates, promoting dysaerobia in the ocean, and warming global c limate by increased methane concentrations in the atmosphere. Upwellin g of WSDW, occurring soon after it became dominant in high-latitude ar eas, is likely responsible for the recovery of normal salinity and the concomitant recovery of planktonic foraminifera and calcareous nannop lankton productivity in high-latitude surface waters. Minor benthic fo raminiferal extinctions (9% of species) occurred similar to 40 kyr aft er the major extinctions, lasted less than or equal to similar to 6 ky r, and coincided with the initiation of environmental recovery.