Chemoautotrophy as the driver of decoupled organic and carbonate carbon isotope records at the onset of the Hangenberg (Devonian-Carboniferous Boundary) Oceanic Anoxic Event

Highlights

• First high-resolution paired δ13C record from the Hangenberg in North America.

• Demonstration of apparent decoupling of carbon records at the onset of Hangenberg.

• First identification of Hangenberg Black Shale equivalent in study area

Abstract

New high-resolution organic (δ¹³C_org) and carbonate (δ¹³C_carb) carbon isotope data from middle shelf deposits in southeastern Iowa demonstrate decoupled signals during the onset of the Hangenberg Event and across the Devonian-Carboniferous Boundary. High-resolution sampling captures a transient negative excursion in δ¹³C_org during the initiation of rising δ¹³C_carb values at the onset of the Hangenberg Event that ends prior to the onset of the final rise in δ¹³C_carb to values greater than +6.0‰ in the Louisiana Limestone. This negative excursion in δ¹³C_org is coincident with a significant increase in Total Organic Carbon (TOC) content in the underlying English River Formation, which likely corresponds to the well-known Hangenberg Black Shale of the classical European sections. The complex behavior of the carbon isotope record recovered here, combined with recently published geochemical data from classical European sections, demonstrate that a succession of geochemical events took place during the initiation of this global biogeochemical event that include a negative excursion in both δ¹³C_carb and δ¹³C_org prior to the major positive carbon isotope excursion, and that the role of organic carbon burial in this Devonian Oceanic Anoxic Event (OAE) extends well beyond the depositional interval of the Hangenberg Black Shale.

Introduction

The Devonian-Carboniferous Boundary (DCB) interval contains one of the largest extinction events and perturbations to the global carbon cycle of the Phanerozoic (Kaiser et al., 2016; Becker et al., 2016; Cramer and Jarvis, 2020). This biogeochemical event, known as the Hangenberg Event (or Hangenberg Crisis) had a generic extinction rate surpassing 45% and is considered to be a first-order extinction event comparable to the Frasnian-Famennian boundary (Walliser, 1996; Kaiser et al., 2016). The Hangenberg Crisis affected both marine and terrestrial organisms including conodonts, bivalves, ammonoids, trilobites, vertebrates, and land plants (see review by Kaiser et al., 2016), and was clearly an interval of profound global biogeochemical change (Pisarzowska et al., 2020; Rakociński et al., 2020). Recent high-precision radioisotopic dating of volcanic tephra suggests that the majority of extinctions took place in less than 100 kyr (Myrow et al., 2014), mostly during deposition of the Hangenberg Black Shale in the classical European sections (Kaiser et al., 2016; Rakociński et al., 2020).

Global change during the Hangenberg Crisis was not limited to biology and the DCB interval marks the onset of a major transition from the Devonian green-house world to what would become the Carboniferous ice-house world. This transition was punctuated by a major perturbation to the global carbon cycle, which was one of the largest positive carbon isotope excursions of the Phanerozoic Era and persisted into the Mississippian well past the end of the extinction interval with δ¹³C_carb values reaching +6‰ (Kaiser et al., 2006; Cramer et al., 2008; Myrow et al., 2011; Cramer and Jarvis, 2020; Stolfus et al., 2020). Whereas the Hangenberg δ¹³C excursion has been documented globally, many sections only permit discontinuous sampling across the interval, or are stratigraphically condensed, or have only been sampled for one of the carbon isotope systems (δ¹³C_org or δ¹³C_carb). As a result, this has limited our ability to evaluate fully the global carbon cycle and its relationship to extinction during the Hangenberg Crisis. Here we produced both δ¹³C_org and δ¹³C_carb data using two cores from southeastern Iowa (H-28 and H-32) that contain one of the most expanded records of the Hangenberg Event and excursion ever recovered. The two cores were chosen because conodont biostratigraphy across the DCB interval was already available from these cores (Day et al., 2019; Stolfus et al., 2020), and additionally, the use of two cores acts as a data quality check to demonstrate the validity of our results. For this study we analyzed 199 carbonate carbon and 172 organic carbon samples from the H-28 core and 396 samples of both carbonate carbon and organic carbon samples from the H-32 core that provide the highest resolution continuous data ever produced through the DCB interval. The paired data permit direct comparison of the δ¹³C_org and δ¹³C_carb systems, which provides unique insight into the global carbon cycle during the Hangenberg Crisis.