Tephra Deposition and Bonding With Reactive Oxides Enhances Burial of Organic Carbon in the Bering Sea

Jack Longman,Martin R Palmer,Thomas M Gernon,Hayley R Manners

doi:10.1029/2021gb007140

Abstract

AbstractPreservation of organic carbon (OC) in marine sediments exerts a major control on the cycling of carbon in the Earth system. In these marine environments, OC preservation may be enhanced by diagenetic reactions in locations where deposition of fragmental volcanic material called tephra occurs. While the mechanisms by which this process occurs are well understood, site‐specific studies of this process are limited. Here, we report a study of sediments from the Bering Sea (IODP Site U1339D) to investigate the effects of marine tephra deposition on carbon cycling during the Pleistocene and Holocene. Our results suggest that tephra layers are loci of OC burial with distinct δ13C values, and that this process is primarily linked to bonding of OC with reactive metals, accounting for ∼80% of all OC within tephra layers. In addition, distribution of reactive metals from the tephra into non‐volcanic sediments above and below the tephra layers enhances OC preservation in these sediments, with ∼33% of OC bound to reactive phases. Importantly, OC‐Fe coupling is evident in sediments >700,000 years old. Thus, these interactions may help explain the observed preservation of OC in ancient marine sediments.

Highlights

The preservation of organic carbon (OC) in marine sediments exerts a controlling influence on the carbon cycle, providing a link between the active pools and the inactive, long-term carbon pools, such as those within sedimentary rocks (Arndt et al, 2013; Burdige, 2007; LaRowe et al, 2020)
Our work investigates the changing chemistry above, below and within, tephra layers deposited throughout the Quaternary Period (2.6 Ma to present), helping to improve our understanding of the impact tephra deposition has on marine sedimentary organic carbon
The OC in the tephra layers is primarily associated with reactive metal phases, with an average fOC-Fe value of 77% in tephra

Summary

Introduction

The preservation of organic carbon (OC) in marine sediments exerts a controlling influence on the carbon cycle, providing a link between the active pools (e.g. oceans, atmosphere and terrestrial environments) and the inactive, long-term carbon pools, such as those within sedimentary rocks (Arndt et al, 2013; Burdige, 2007; LaRowe et al, 2020). About 0.5% of all organic matter produced in the oceans is preserved in the sedimentary record, with the remainder remineralised and reintroduced into active carbon pools (Hedges & Keil, 1995). Tephra enters the oceans through rapid erosion of newly created volcanic deposits (Cashman et al, 2013). This material eventually settles to the seafloor, and is deposited in the sedimentary record as tephra layers (Dingwell et al, 2012; Pyle, 1989). Tephra may derive from submarine eruptions, such that tephra is thought to represent as much as 25% of marine sediments in the Pacific Ocean (Scudder et al, 2009; Straub & Schmincke, 1998)

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Discussion

Conclusion