A rapid sedimentary response to the Paleocene-Eocene Thermal Maximum hydrological change: New data from alluvial units of the Tremp-Graus Basin (Spanish Pyrenees)

Abstract

A massive emission of light carbon about 56 Ma, recorded in marine and terrestrial sediments by a negative carbon isotope excursion (CIE), caused a short-lived (similar to 170 kyr) global warming event known as the Paleocene-Eocene Thermal Maximum (PETM). The onset and core of this event is represented in the south Pyrenean Tremp-Graus Basin by two successive alluvial units, the Claret Conglomerate (CC) and the Yellowish Soils, which represent laterally juxtaposed depositional environments. It is generally agreed that these units record a dramatic increase in seasonal rain and an increased intra-annual humidity gradient during the PETM, but the timing of the sedimentary response to this hydrological change is a matter of debate. Some authors maintain that the CC was developed during the early, most intense phase of the carbon emission, others that its formation lagged by 16.5 +/- 7.5 kyr behind the onset of the PETM. The latter claim was mainly based on the assumption that in two sections of this basin, Claret and Tendrui, the onset of the CIE occurs 3 and 8 m below the base of the CC, respectively. Here we show that in some zones between these two sections the CC was not deposited and the Yellowish Soil unit rests directly and conformably on the underlying Esplugafreda Formation. New delta C-13(org) data from this intervening zone provide evidence that the onset of the CIE is situated just similar to 1 m below the Yellowish Soils. In adjacent sections the CC erosional base cuts down deeper than 5 m, thus removing or obscuring the chemostratigraphically-defined base of the PETM. A tentative estimate based on averaged sedimentation rates indicates that similar to 3.8 kyr, or less, may have elapsed from the onset of the CIE to the arrival of PETM alluvium into the Claret-Tendrui study area, less than half of the lowest estimate of previous authors. Since the study area was situated about 15 km from the source area, our new estimate supports a rapid response of the sedimentary system to the hydrological change at the onset of the PETM.