CO2-induced climate forcing on the fire record during the initiation of Cretaceous oceanic anoxic event 2

Sarah J Baker,Richard S Barclay,Claire M Belcher,Bradley B Sageman,Jiří Laurin,Stephen P Hesselbo

doi:10.1130/b35097.1

Abstract

Abstract Cretaceous oceanic anoxic event 2 (OAE2) is thought to have been contemporary with extensive volcanism and the release of large quantities of volcanic CO2 capable of triggering marine anoxia through a series of biogeochemical feedbacks. High-resolution reconstructions of atmospheric CO2 concentrations across the initiation of OAE2 suggest that there were also two distinct pulses of CO2 drawdown coeval with increased organic carbon burial. These fluctuations in CO2 likely led to significant climatic changes, including fluctuations in temperatures and the hydrological cycle. Paleofire proxy records suggest that wildfire was a common occurrence throughout the Cretaceous Period, likely fueled by the estimated high atmospheric O2 concentrations at this time. However, over geological time scales, the likelihood and behavior of fire are also controlled by other factors such as climate, implying that CO2-driven climate changes should also be observable in the fossil charcoal record. We tested this hypothesis and present a high-resolution study of fire history through the use of fossil charcoal abundances across the OAE2 onset, and we compared our records to the estimated CO2 fluctuations published from the same study sites. Our study illustrates that inferred wildfire activity appears to relate to changes in CO2 occurring across the onset of OAE2, where periods of CO2 drawdown may have enabled an increase in fire activity through suppression of the hydrological cycle. Our study provides further insight into the relationships between rapid changes in the carbon cycle, climate, and wildfire activity, illustrating that CO2 and climate changes related to inferred wildfire activity can be detected despite the estimated high Cretaceous atmospheric O2 concentrations.

Highlights

Periods of oceanic anoxia have occurred many times in Earth’s history, and they were common throughout the Cretaceous Period (Jenkyns, 2010)
We found that the abundance of charcoal per g/total organic matter (TOM) at the study site is generally low up to an estimated age of 94.436 Ma (Fig. 4), averaging 189 particles per g/TOM
This period is marked as phase 2 (P2) on Figure 4, and it represents the onset of oceanic anoxic event 2 (OAE2), which is commonly recognized by a positive shift in carbonate and organic δ13C values (e.g., Jarvis et al, 2011, and references therein; Du Vivier et al, 2014, 2015; Jenkyns et al, 2017)

Summary

Introduction

Periods of oceanic anoxia have occurred many times in Earth’s history, and they were common throughout the Cretaceous Period (Jenkyns, 2010) These anoxic events are evidenced in the geological record by common occurrences of widespread black shales and excursions in the organic and carbonate δ13C record, indicating significant disruption to the global carbon (C) cycle (Jarvis et al, 2011). OAE2 is hypothesized to have been triggered by an influx of volcanically derived CO2, driving increases in global temperatures, enhancing the hydrological cycle, and increasing continental weathering rates (Du Vivier et al, 2015; Jenkyns et al, 2017). These changes are hypothesized to have preconditioned the oceans in the runup to the event, creating lowoxygen conditions. During the initiation, enhanced preservation of organic matter is hypothesized to have taken place in euxinic hotspots (Owens et al, 2016), which constituted a small proportion of the global seafloor area (

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