An abyssal carbonate compensation depth overshoot in the aftermath of the Palaeocene–Eocene Thermal Maximum

Abstract

The Paleocene-Eocene Thermal Maximum (PETM; ~56 Ma) represents one of the largest and most abrupt greenhouse warming events in Earth history. Marine and terrestrial records document a global >2.5‰ negative carbon isotope excursion (CIE) [1][2][3] coincident with global mean surface ocean warming of >4°C 4 and geochemical and sedimentological evidence for ocean acidification 5,6 . Collectively, these lines of evidence suggest a rapid (10 3 -10 4 years) and massive (~3,000-10,000 PgC) release of 13 C-depleted carbon into the ocean-atmosphere system 7-9 . The PETM thus offers the opportunity to examine the response and recovery of the global carbon cycle and seawater carbonate chemistry to an ancient CO2 release similar in magnitude to ongoing anthropogenic fossil fuel combustion 10 . Nonetheless, some models 9,10,20 predict that a testable facet of the recovery process from massive carbon cycle perturbation involves an over-deepening of the CCD, and the location of these sites above the pre-PETM CCD means that they cannot directly test for this predicted CCD over-deepening. Direct observational evidence sites deep enough to test for a post-PETM CCD overshoot has thus far remained elusive.Here we present lithology, CaCO3 content, and carbon isotope (δ 13 C) records from recently recovered sediment cores in the North Atlantic (IODP Sites U1403, PETM 6 paleodepth ~4374m and U1409, paleodepth ~2913m 30 ) that provide important constraints on the evolution of the CCD through the PETM, including the first evidence for CCD over-deepening during the PETM recovery. To explore the broader implications of these records for PETM carbon emissions scenarios, we present new carbon release experiments using two carbon cycle models -LOSCAR 9,31 and cGENIE 32,33 and discuss uncertainties in the representation of geological carbon cycling in current models.At Site U1409, the PETM CIE occurs in an interval of variously silicified sediments (siliceous claystones, siliceous limestones and cherts) at 178.9-179.2 mcd ( Figure 1A), contrasting with the nannofossil chalk that characterizes much of the Paleogene at this site 30 . Although likely somewhat condensed, the δ 13 C carb record bears the typical 27 PETM CIE pattern of an abrupt decrease (here of ~2‰) followed by a plateau of low values and then gradual recovery (Fig. 1). Bulk δ 13 Ccarb over the PETM CIE interval sampled a heterogeneous mixture of lithology (clay, carbonate-rich burrows within that clay, and siliceous sediments), with all three lithologies revealing significantly lower δ 13 C within the CIE than pre-event values. The integrity of the bulk δ 13 Ccarb record is also supported by the close structural similarity between it and the equivalent bulk δ 13 Ccarb records from the Southern Ocean and Walvis Ridge (Fig. 1, 2A). The Site U1409 (Figure 2A), so we confidently assign the onset of carbonate sedimentation at Site U1403 to early in the PETM recovery phase.We construct age models by correlating the δ 13 Ccarb records to a compilation of bulk and fine-fraction δ 13 Cc...

Implementation of methane cycling for deep-time global warming simulations with the DCESS Earth system model (version 1.2)

“…6g). In combination with decreasing pCO 2 and associated increasing ocean [CO 2− 3 ], the biogenic CaCO 3 production increase drives an overshoot of the LL CCD to depths greater than the pre-event depth, in agreement with theory and sediment core data (Dickens et al, 1997;Leon-Rodriguez and Dickens, 2010;Penman et al, 2016). This overshoot, which is even more pronounced at high latitudes, peaks by 35-45 kyr into the simulation (Fig.…”

Orbital Signals in Carbon Isotopes: Phase Distortion as a Signature of the Carbon Cycle

“…Short‐ and intermediate‐term effects (kiloyear to tens of kiloyears) involve responses of carbonate saturation and dissolution to changes in atmospheric CO 2 concentrations and related changes in pH and temperature of ocean water. On longer time scales (>100 kyr), changes in weathering rates of silicate and carbonate rocks can buffer and potentially “overshoot” the initial change in carbonate saturation (e.g., Penman et al, ; Walker et al, ; Zeebe et al, ). Both processes can provide positive or negative feedbacks to the primary δ 13 C 0 response to carbon flux perturbations (Table and Figures and ), although the effect would be small due to the similar δ 13 C composition of seawater DIC and of the marine carbonate that contributes to the lithospheric inventory.…”

Size-dependent response of foraminiferal calcification to seawater carbonate chemistry

“…Foster et al (2013) found that small benthic foraminifera became more heavily calcified during the PETM, when ocean pH was lower (Penman et al, 2014). By contrast, during the high pH "carbonate overshoot" in the aftermath of the event (Penman et al, 2016), these foraminifera displayed thinner walls . Diverse small benthic foraminifera assemblages have also been found living in highly undersaturated oligohaline conditions (Flako-Zaritsky et al, 2011).…”