TWO PULSES OF CARBON RELEASED DURING THE ONSET OF PALEOCENE-EOCENE THERMAL MAXIMUM (Invited Presentation)

Global climate during the late Paleocene and early Eocene is punctuated by multiple rapid hyperthermal warming events that are associated with negative carbon isotope excursions (CIEs) and widespread dissolution of deep-sea carbonates. The most well-studied hyperthermal, the Paleocene-Eocene Thermal Maximum (PETM), occurred ~56 million years ago and is identified by a globally resolved 3-5‰ negative CIE signifying a rapid carbon injection into exogenic reservoirs. The mechanism, total mass and tempo for the release of carbon continues to be debated. Moreover, evidence of both pre-event warming and possible CO₂ release(s) are identified in a few localities, although the evidence for a global signal is equivocal. Continental shelf sites are advantageous as their relatively high sedimentation rates and limited carbonate dissolution yield expanded marine records, specifically over the CIE onset. δ¹³C records (bulk carbonate and foraminifera) from the South Dover Bridge and Cambridge-Dorchester cores, Maryland, reveal two discrete negative δ¹³C excursions. The older CIE, herein referred to as the pre-onset excursion (POE) is characterized by an abrupt negative excursion of 1.0-1.5‰. Surface ocean temperatures warmed by ~2ºC (derived by Mg/Ca paleothermometry) and returned to pre-event values immediately prior to the CIE onset. Here, we present independent supporting data from the novel application of laser ablation-MC-ICP-MS, which allows for single specimen boron isotope (δ¹¹B) analysis of benthic foraminifera, which documents a pH decline at the POE that is comparable in magnitude to the CIE itself. The coupled warming, acidification and POE signifies a rapid injection of ¹³C-depleted carbon that resulted in elevated atmospheric CO₂ levels prior to the main CIE onset. Given the transient nature of the anomaly, the mass of C would have to be on the order of just a few hundred Pg. The rapidity and severity of the ocean acidification during the POE raises several questions regarding the nature of carbon input during this event, the volume of carbon added and the cause of the apparent recovery. Regardless of the source, our records present evidence for a pre-CIE carbon injection that likely acted as an early harbinger of global carbon cycle instability.