Regional Hydrological Variations Along the California Coast During the PETM

As a past analog for the future, the Paleocene-Eocene Thermal Maximum (PETM) is a unique case study for assessing the sensitivity of climate models to greenhouse forcing partially because of the extensive evidence for intensification of the global hydrologic cycle. In theory, however, the expression of this intensification should have varied significantly from region to region (dryer or wetter) thus necessitating additional climate records, particularly for regions that might be highly susceptible to large shifts in precipitation patterns/intensity.

Here, we present new observational constraints from the Paleocene-Eocene Lodo Formation in central California (paleolatitude ~42°N) where previous work (John et al., 2008) indicates increased sedimentation rate hinting at a modest regional hydrological response during the PETM. The Lodo, an outer shelf facies, consists mainly of siltstones (with low abundances of calcareous microfossils) truncated by thin glauconitic sand layers. To better characterize local hydrological changes, we developed detailed records of clay mineralogy, bulk organic carbon (δ¹³C_bulk), and terrestrial plant n-alkane carbon (δ¹³C_alk) and hydrogen (δD_alk) isotopes. Preliminary clay assemblage data show a slight increase in kaolinite content that is delayed relative to the PETM onset as marked by the carbon isotopic excursion (CIE), suggesting a subtle mode shift in regional hydrology, either increased chemical weathering and/or enhanced exhumation of Cretaceous laterites. The CIE is recorded in the δ¹³C_alk with a pronounced negative shift nearly 4‰, consistent with the global average. δD_alk abruptly becomes more negative by 25‰ just prior to the CIE onset and remains relatively invariable throughout the CIE, punctuated with brief intervals of more negative values. Our new observations along with previous work suggest an overall dry climate throughout but punctuated by occasionally extreme precipitation events during the PETM. We also compare observations against modeling output from high-resolution (~25 km) atmosphere simulations and isotope-enabled coupled simulations of the PETM in the Community Earth System Model (CESM1.2).