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 δ 13 C values has been documented at numerous marine and terrestrial sites [ Bralower et al., 1997; Cramer et 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°–6°C [ Kennett and Stott, 1991] and more moderate warming in the subtropics [ Thomas et al., 1999]. Here we report 187 Os/ 188 Os isotope records from the North Atlantic and Indian Oceans which demonstrate a >10% increase in the 187 Os/ 188 Os ratio of seawater coincident with the late Paleocene CIE. This excursion to higher 187 Os/ 188 Os 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 et al., 1981]. Estimates of the duration of late Paleocene CIE [ Bains et al., 1999; Bralower et al., 1997; Norris and Röhl , 1999; Röhl 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 187 Os/ 188 Os ratios in river runoff. Although we argue that the former is more likely than the latter, we cannot rule out significant changes in the 187 Os/ 188 Os ratio of rivers.