Abstract
Recent geochemical and sedimentological evidence constrains the response of seawater chemistry to carbon injection during the Paleocene-Eocene Thermal Maximum (PETM): foraminiferal boron-based 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 pCO2 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 pCO2 relative to pre-event 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 13C-depleted C in response to initial warming followed by its subsequent sequestration during the cooling phase.
Highlights
The Paleocene-Eocene Thermal Maximum (PETM, ∼56 Ma) involved the geologically rapid release of thousands of petagrams of 12C-enriched carbon (Dickens et al 1997, Panchuk et al 2008, Zeebe et al 2009, Cui et al 2011) into the ocean-atmosphere system, resulting in global warming (Dunkley-Jones et al 2013), ocean acidification (Zachos et al 2005, Penman et al 2014, Babila et al 2016, Babila et al 2018), and varied yet pronounced impacts on marine and terrestrial biota (Thomas and Shackleton 1996, Wing et al 2005, Gibbs et al 2016)
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
The event is often considered a geologic point of comparison to current anthropogenic CO2 release, offering the opportunity to improve our understanding of the response of climate, biota, and the carbon cycle to rapid carbon injection (Alley 2016)
Summary
The Paleocene-Eocene Thermal Maximum (PETM, ∼56 Ma) involved the geologically rapid release of thousands of petagrams of 12C-enriched carbon (Dickens et al 1997, Panchuk et al 2008, Zeebe et al 2009, Cui et al 2011) into the ocean-atmosphere system, resulting in global warming (Dunkley-Jones et al 2013), ocean acidification (Zachos et al 2005, Penman et al 2014, Babila et al 2016, Babila et al 2018), and varied yet pronounced impacts on marine and terrestrial biota (Thomas and Shackleton 1996, Wing et al 2005, Gibbs et al 2016). Zeebe et al (2009) and Panchuk et al (2008) used records of sedimentary CaCO3 reduction during the acidification phase to estimate the carbon release mass, whereas Gutjahr et al (2017) used single-site boron isotopes and δ13C to constrain scenarios of carbon emissions and sequestration in an Earth system model.
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