New constraints on massive carbon release and recovery processes during the Paleocene-Eocene Thermal Maximum

Abstract

Recent geochemical and sedimentological evidence constrains the response of seawater chemistry to carbon injection during the Paleocene-Eocene Thermal Maximum (PETM): foraminiferal boronbased proxy records constrain the magnitude and duration of surface ocean acidification, while new deep sea records document a carbonate compensation depth (CCD) over-shoot during the recovery. Such features can be used to more tightly constrain simulations of the event within carbon cycle models, and thus test mechanisms for carbon release, buffering, and sequestration. We use the LOSCAR carbon cycle model to examine first the onset of, and then recovery from the PETM. We systematically varied the mass, rate, and location of C release along with changes in ocean circulation patterns as well as initial conditions such as pre-event pCO 2 and the strength of weathering feedbacks. A range of input parameters produced output that successfully conformed to observational constraints on the event's onset. However, none of the successful scenarios featured surface seawater aragonite or calcite undersaturation at even peak PETM conditions (in contrast to anthropogenic acidification projections), and most runs featured approximately a doubling of pCO 2 relative to preevent conditions (suggesting a high PETM climate sensitivity). Further runs test scenarios of the body and recovery of the PETM against records of sustained acidification followed by rapid pH recovery in boron records, as well as the timing and depth of the CCD overshoot. Successful scenarios all require a sustained release of carbon over many tens of thousands of years following the onset (comparable to the mass released during the onset) and removal of carbon (likely as burial of organic carbon in addition to elevated chemical weathering rates) during the recovery. This sequence of events is consistent with a short-lived feedback involving the release of 13 C-depleted C in response to initial warming followed by its subsequent sequestration during the cooling phase.

Paleoenvironmental Changes at ODP Site 702 (South Atlantic): Anatomy of the Middle Eocene Climatic Optimum

“…Warming during the MECO should have favored the arrival and proliferation of this species, as documented for other warm water/oligotrophic taxa (Agnini et al, ). However, its increase during the post‐MECO at Site 702 points to an intermittent carbonate oversaturation of sea surface waters, triggered by increased alkalinity due to accelerated terrestrial chemical weathering, as suggested for the PETM (Dickens et al, ; Penman & Zachos, ; Zeebe & Zachos, ). The record of these changes well after the climax of the MECO contributes to the MECO conundrum (Sluijs et al, 2013).…”

Delays, Discrepancies, and Distortions: Size‐Dependent Sediment Mixing and the Deep‐Sea Record of the Paleocene‐Eocene Thermal Maximum From ODP Site 690 (Weddell Sea)

“…The CIE and OIE marking the PETM were first documented at Site 690 (Kennett & Stott, 1991), and this record has since been treated as a type of reference section to which other PETM records are compared (e.g., Bains et al, 1999; Kelly et al, 2010; Penman & Zachos, 2018; Zachos et al, 2005; Zhang et al, 2020). However, the depth of the CIE/OIE onset in the δ 13 C bulk and δ 18 O bulk records is ~14 cm above where the onsets of these same isotopic excursions occur in the acarininid δ 13 C and δ 18 O records (Figures 2b and 2c).…”

Atlantic Deep‐Sea Cherts Associated With Eocene Hyperthermal Events

“…As a result, modern silica burial primarily occurs in the Southern Ocean and the equatorial and North Pacific Ocean. The connection between opal burial and circulation patterns is particularly important in the case of hyperthermal events because it has been argued on the basis of global ocean physics modeling (Bice & Marotzke, ) and spatial trends in the extent of deep‐sea carbonate dissolution (Penman & Zachos, ; Zeebe et al, ; Zeebe & Zachos, ) that there was a transient reversal in the deep‐water aging gradient during the PETM, consistent with a switch from southern‐ to northern‐sourced overturning. Such a large‐scale circulation reversal could significantly affect regional SiO 2 burial patterns by the same mechanisms by which modern circulation focuses SiO 2 burial into the eastern equatorial Pacific and Southern Ocean.…”

“…We performed a suite of atmospheric CO 2 release and circulation change experiments using LOSiCAR (Figure ) in which we varied the mass of carbon released from 2,000 to 10,000 Gt C to cover a range of possible C release for the PETM (e.g. Gutjahr et al, ; Penman & Zachos, ) and the smaller Eocene hyperthermals (e.g. Kirtland Turner et al, ; Sexton et al, ).…”

“…These events range in size from the Paleocene‐Eocene Thermal Maximum (PETM, ~56 Ma, a >2 permil (‰) CIE lasting ~150,000 years; McInerney & Wing, ) to smaller (<1‰ CIE) orbitally paced events occurring semiperiodically throughout the early Eocene Climatic Optimum (50–53 Ma) (Kirtland Turner et al, ; Littler et al, ; Sexton et al, ; Westerhold et al, ). Coupled with evidence for temperature rise (often in the form of coincident negative oxygen isotope excursions) and deep‐sea sedimentary carbonate dissolution, hyperthermals are interpreted to be associated with the geologically rapid (<10 4 years) release of thousands of gigatons (Gt C) of 13 C‐depleted carbon into the exogenic carbon cycle (Gutjahr et al, ; Panchuk et al, ; Penman & Zachos, ; Sexton et al, ; Zeebe et al, ). Modern understanding of the carbon cycle posits that an important negative feedback to such perturbation is the response of silicate weathering to climate: During the warm, high p CO 2 conditions immediately following carbon release, silicate weathering rates are thought to increase, consuming excess atmospheric CO 2 and cooling climate (Berner et al, ; Walker et al, ).…”