EVIDENCE FOR RAPID CLIMATE-CHANGE IN NORTH-AMERICA DURING THE LATEST PALEOCENE THERMAL MAXIMUM - OXYGEN-ISOTOPE COMPOSITIONS OF BIOGENIC PHOSPHATE FROM THE BIGHORN BASIN (WYOMING)

Oxygen isotope records of Cenozoic sea water temperatures indicate tha t a rapid warming event known as the Latest Paleocene Thermal Maximum (LPTM) occurred during the otherwise gradual increase in world tempera tures during the Late Paleocene and Early Eocene. Oxygen isotope analy sis of the carbonate and phosphate components of hydroxyapatite found in mammalian tooth enamel and body scales of river-dwelling fish from the Bighorn Basin in Wyoming were made to investigate corresponding ch anges in the terrestrial climate. A comparison of carbonate and phosph ate isotope data from modern and fossil material indicates that some d iagenetic alteration of the fossil material has occurred, although sys tematically larger intra-tooth ranges in the oxygen isotope compositio n of carbonate indicate that it is more likely to have been affected t han phosphate. Carbonate and phosphate from the ecologically diverse m ammals and fishes both record a shift to higher oxygen isotope ratios at the same time and of the same duration as the LPTM. These shifts re flect a change in the isotopic composition of regional precipitation, which in turn provides the first evidence for continental climate chan ge during the LPTM. Assuming the present-day relation between the oxyg en isotope composition of precipitation and temperature applies to con ditions in the past, and that animal physiology and behavior is relati vely invariant over time, the isotopic shift is equivalent to an incre ase of surface temperature in western North America of several degrees . This result is consistent with the magnitude of high-latitude ocean warming, and provides a basis for relating marine and terrestrial oxyg en isotope records to records of terrestrial biotic change. (C) 1998 E lsevier Science B.V. All rights reserved.