Thermogenic Methane release as a Cause for the Long Duration of the PETM

Abstract:

The Paleocene-Eocene Thermal Maximum (PETM; 55 Ma) was a ~170 kyr period of global warming associated with rapid and massive injections of ¹³C-depleted carbon into the ocean and atmosphere, reflected in sedimentary components as a negative carbon isotope excursion (CIE). Carbon cycle modelling has indicated that the shape of the CIE, which consists of a rapid onset, a prolonged phase of stable low δ¹³C and subsequent recovery, is best explained by an initial large pulse (3,000 Pg), followed by ~50 kyr of slow continuous release of ¹³C-depleted carbon (1,480 Pg). Suggested sources include submarine methane hydrates, terrigenous organic matter and thermogenic methane and CO₂ from hydrothermal vent complexes in the Norwegian Sea. Here, we test the latter hypothesis by dating the active phase of a hydrothermal vent relative to the CIE through dinoflagellate cyst and carbon isotope stratigraphy. We find that activity in this vent system post-dates the onset of the PETM, excluding the possibility that it triggered the PETM. However, our record indicates the vent system was active during the ~60 kyr long “body” phase and thus represents the first actual proof of PETM carbon release from a particular reservoir.

To test whether the pulsed release of carbon from these vent systems may have caused the long duration of the body of the PETM, we conduct a suite of experiments using a simple carbon cycle box model (LOSCAR). Our experiments indicate that pulsed carbon input from several vent systems over a prolonged period as suggested from the vent-literature (4-12 pulses within 60 kyr; total volume of 1,480 Pg) matches the CIE and deep ocean carbonate dissolution as recorded in sediment records. We therefore conclude that methane and CO₂ from the Norwegian Sea vent complexes may have been the main source of carbon during the PETM, following its dramatic onset.