Comment on cp-2021-83

Gabriel Bowen

doi:10.5194/cp-2021-83-rc2

Abstract

Abstract. Paleogene hyperthermals, including the PaleoceneâEocene Thermal Maximum (PETM) and several other smaller events, represent global perturbations to Earth's climate system and are characterized by warmer temperatures, changes in floral and faunal communities, and hydrologic changes. These events are identified in the geologic record globally by negative carbon isotope excursions (CIEs), resulting from the input of isotopically light carbon into Earth's atmosphere. Much about the causes and effects of hyperthermals remains uncertain, including whether all hyperthermals were caused by the same underlying processes, how biotic effects scale with the magnitude of hyperthermals, and why CIEs are larger in paleosol carbonates relative to marine records. Resolving these questions is crucial for a full understanding of the causes of hyperthermals and their application to future climate scenarios. The primary purpose of this study was to identify early Eocene hyperthermals in the Fifteenmile Creek area of the south-central Bighorn Basin, Wyoming, USA. This area preserves a sequence of fluvial floodplain sedimentary rocks containing paleosol carbonates and an extensive record of fossil mammals. Previous analysis of faunal assemblages in this area revealed two pulses of mammal turnover and changes in diversity interpreted to correlate with the ETM2 and H2 hyperthermals that follow the PETM. This was, however, based on long-distance correlation of the fossil record in this area with chemostratigraphic records from elsewhere in the basin. We present new carbon isotope stratigraphies using micrite Î´13C values from paleosol carbonate nodules preserved in and between richly fossiliferous mammal localities at Fifteenmile Creek to identify the stratigraphic positions of ETM2 and H2. Carbon isotope results show that the ETM2 and H2 hyperthermals, and possibly the subsequent I1 hyperthermal, are recorded at Fifteenmile Creek. ETM2 and H2 overlap with the two previously recognized pulses of mammal turnover. The CIEs for these hyperthermals are also somewhat smaller in magnitude than in more northerly Bighorn Basin records. We suggest that basin-wide differences in soil moisture and/or vegetation could contribute to variable CIE amplitudes in this and other terrestrial records.

Highlights

IntroductionThe early Paleogene is punctuated by numerous short duration (lasting 50–200 kyr) warming events known as “hyperthermals” (Westerhold et al, 2018; Barnet et al, 2019; Thomas et al, 2000)
1.1 Paleogene hyperthermalsThe early Paleogene is punctuated by numerous short duration warming events known as “hyperthermals” (Westerhold et al, 2018; Barnet et al, 2019; Thomas et al, 2000)
Proposed sources of carbon for the Paleocene-Eocene Thermal Maximum (PETM) include destabilized methane clathrates (Dickens et al, 1995, 1997; Katz et al, 1999), volcanism associated with the North Atlantic Igneous Province (NAIP) (Gutjahr et al, 2017), thermogenic methane released from organic-rich sediments during NAIP emplacement (Svensen et al, 2004; Frieling et al, 2016), Antarctic permafrost thaw (DeConto et al, 2010, 2012), wildfires burning Paleocene peat deposits (Kurtz et al, 2003; Moore and Kurtz, 2008), and evaporation of epicontinental seas leading to the oxidation of organic matter (Higgins and Schrag, 2006), geological evidence supporting the last two hypotheses is weak

Summary

Introduction

The early Paleogene is punctuated by numerous short duration (lasting 50–200 kyr) warming events known as “hyperthermals” (Westerhold et al, 2018; Barnet et al, 2019; Thomas et al, 2000) These hyperthermals are characterized by their rapid onset (estimates for some range from less than 10 kyr to ~20 kyr) and are associated with significant negative isotope excursions in δ13C and δ18O in marine sedimentary records, and negative δ13C excursions in terrestrial records, worldwide (e.g., Kennett and Stott, 1991; Koch et al, 1992; Zachos et al, 2001, 2008; Abels et al, 2012, 2016; Westerhold et al, 2020). The largest and best known of these hyperthermals is the Paleocene–Eocene Thermal Maximum (PETM) at ~56 Ma, during which time global temperatures increased between 5–8° C (McInerney and Wing, 2011) Along with this warming came deep-ocean acidification and carbonate dissolution, and the extinction of 30–50% of benthic foraminifera (Thomas, 1998, 2007; Zachos et al, 2005; Speijer et al, 2012). Understanding the effects of these hyperthermals on the terrestrial ecosystem requires their recognition in richly fossiliferous strata, where both plant and vertebrate fossils occur in abundance

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