The Lodo Formation (central CA), records the carbon isotope excursion (CIE) associated with the Paleocene Eocene Thermal Maximum (PETM) in a continental margin setting, where increased sedimentation rates - attributed to an enhanced hydrological cycle - provide a means for significant carbon drawdown after the peak of this hyperthermal event (John et al. 2008). This study investigates variations in grain size associated with the recovery from peak CIE values to better constrain the potential mechanism of the apparent increase in sedimentation rates, which can shed light on climate dynamics and feedbacks associated with other warming events.
We collected bulk samples at ~10 cm increments over a stratigraphic interval that encompasses the previously documented CIE onset, peak, and recovery phases. We used a laser diffraction particle size analyzer on disaggregated samples to permit detection of variation within clay and silt sediment size fractions (2-63 microns) in addition to sand-size grains. Results show that, while the onset and peak CIE is associated with abundant clays, the recovery interval preserves a marked shift in median grain size to coarse silt and very fine sand. Ongoing work indicates that the grain size increase is not associated with an increase in foraminiferal abundance.
Enhanced terrigenous sediment delivery during the CIE recovery could explain both the increase in grain size (this study) and sedimentation rates (previous work). This increased sediment supply could be attributed to (1) increased seasonality and storm intensity during the hyperthermal, (2) marine regression (i.e. basinward shift in shoreline) associated with decreasing rates of sea level rise during sea level highstand, or some combination of these factors. Future work to document compositional and preservational changes in sediments and foraminifera over this time interval will aid in teasing apart the relative influence of potential drivers of change associated with this event.