An extraterrestrial ~3He-based timescale for the Paleocene-Eocene thermal maximum (PETM) from Walvis Ridge, IODP Site 1266

摘要

In the deep-sea, the Paleocene-Eocene Thermal Maximum (PETM) is often marked by clay-rich condensed intervals caused by dissolution of carbonate sediments, capped by a carbonate-rich interval. Constraining the duration of both the dissolution and subsequent cap-carbonate intervals is essential to computing marine carbon fluxes and thus testing hypotheses for the origin of this event. To this end, we provide new high-resolution helium isotope records spanning the Paleocene-Eocene boundary at ODP Site 1266 in the South Atlantic. The extraterrestrial ~3He, ~3He_(ET), concentrations replicate trends observed at ODP Site 690 by Farley and Eltgroth (2003). By assuming a constant flux of ~3He_(ET) we constrain relative changes in accumulation rates of sediment across the PETM and construct a new age model for the event. In this new chronology the zero carbonate layer represents 35kyr, some of which reflects clay produced by dissolution of Paleocene (pre-PETM) sediments. Above this layer, carbonate concentrations increase for ～165kyr and remain higher than in the latest Paleocene until 234 ~(+48)/_(-34)kyr above the base of the clay. The new chronology indicates that minimum δ~(13)C values persisted for a maximum of 134 ~(+27)/_(-19)kyr and the inflection point previously chosen to designate the end of the CIE recovery occurs at 217 ~(+44)/_(-31)kyr. This allocation of time differs from that of the cycle-based age model of R?hl et al. (2007) in that it assigns more time to the clay layer followed by a more gradual recovery of carbonate-rich sedimentation. The new model also suggests a longer sustained δ~(13)C excursion followed by a more rapid recovery to pre-PETM δ~(13)C values. These differences have important implications for constraining the source(s) of carbon and mechanisms for its subsequent sequestration, favoring models that include a sustained release of carbon after an initial pulse.