An osmium isotope excursion associated with the late Paleocene thermal maximum: Evidence of intensified chemical weathering

In the latest Paleocene an abrupt shift to more negative delta C-13 values has been documented at numerous marine and terrestrial sites [Bralower et al., 1997; Cramer ef al., 1999; Kaiho et al., 1996; Kennett and Stott, 1991; Koch et al., 1992; Stott et al., 1996; Thomas and Shackleton, 1996; Zachos et al., 1993]. This carbon isotope event (CIE) is coincident with oxygen isotope data that indicate warming of surface waters at high latitudes of nearly 4 degrees -6 degreesC [Kennett and Stott, 1991] and more moderate warming in the subtropics [Thomas ct al., 1999]. Here we report Os-187/Os-188 isotope records from the North Atlantic and Indian Oceans which demonstrate a >10% increase in the Os-187/Os-188 ratio of seawater coincident with the late Paleocene CIE. This excursion to higher Os-187/Os-188 ratios is consistent with a global increase in weathering rates. The inference of increased chemical weathering during this interval of unusual warmth is significant because it provides empirical evidence supporting the operation of a feedback between chemical weathering rates and warm global climate, which acts to stabilize Earth's climate [Walker ct nl, 1981]. Estimates of the duration of late Paleocene CIE [Bains et al., 1999; Bralower et al., 1997; Norris and Rohl, 1999; Rohl et al., 2000] in conjunction with the Os isotope data imply that intensified chemical weathering in response to warm, humid climates can occur on timescales of 10(4)-10(5) years. This interpretation requires that the late Paleocene thermal maximum Os isotope excursion be produced mainly by increased Os flux to the ocean rather than a transient excursion to higher Os-187/Os-188 ratios in river runoff. Although we argue that the former is more likely than the latter, we cannot rule out significant changes in the Os-187/Os-188 ratio of rivers.