Event-dominated transport, provenance, and burial of organic carbon in the Japan Trench

T Schwestermann,T.I Eglinton,N Haghipour,A.P Mcnichol,K Ikehara,M Strasser

doi:10.1016/j.epsl.2021.116870

Abstract

The delivery of organic carbon (OC) to the ocean's deepest trenches in the hadal zone is poorly understood, but may be important for the carbon cycle, contain crucial information on sediment provenance and event-related transport processes, and provide age constraints on stratigraphic sequences in this terminal sink. In this study, we systematically characterize bulk organic matter (OM) and OC signatures (TOC/TN, δ13C, 14C), as well as those from application of serial thermal oxidation (ramped pyrolysis/oxidation) of sediment cores recovered along an entire hadal trench encompassing high stratigraphic resolution records spanning nearly 2000 years of deposition. We analyze two cores from the southern and northern Japan Trench, where submarine canyon systems link shelf with trench. We compare results with previously published data from the central Japan Trench, where canyon systems are absent. Our analyses enable refined dating of the stratigraphic record and indicate that event deposits arise from remobilization of relatively surficial sediment coupled with deeper erosion along turbidity current pathways in the southern and central study site and from canyon flushing events in the northern study site. Furthermore, our findings indicate deposition of predominantly marine OC within hemipelagic background sediment as well as associated with event deposits along the entire trench axis. This implies that canyon systems flanking the Japan Trench do not serve as a short-circuit for injection of terrestrial OC to the hadal zone, and that tropical cyclones are not major agents for sediment and carbon transfer into this trench system. These findings further support previous Japan Trench studies interpreting that event deposits originate from the landward trench slope and are earthquake-triggered. The very low terrestrial OC input into the Japan Trench can be explained by the significant distance between trench and hinterland (>180 km), and the physiography of the canyons that do not connect to coast and river systems. We suggest that detailed analyzes of long sedimentary records are essential to understand OC transfer, deposition and burial in hadal trenches.

Highlights

It has been more than half a century since the pioneering work of sampling and analyzing organic carbon (OC) content of surface sediments in deep-water hadal trenches (water depths of >6000 m; Jumars and Hessler (1976); Bartlett (2009)), the processes involved in the transport, deposition and burial of OC in oceanic trenches remain poorly understood
Thick event deposits are absent from the lower part (>720.0 cm) of the core, and instead this interval contains only thin deposits resulting from minor sediment remobilization processes
Applying corrected mean age offsets in the northern study area (i.e., 149 years younger than estimated by Kioka et al (2019b)), the refined age of the thick event deposit is 1.77 (+0.49/-0.31) bulk OC 14C kyr BP, potentially coeval with tsunami deposits along the Iwate coast that suggest large earthquake in this area during the 2nd or 3rd Century AD (Takada et al, 2016). This investigation is, to the best of our knowledge, the first to systematically characterize OC signatures in sediments that have been deposited along the axis of an entire hadal oceanic trench system (Japan Trench (JT)) over the past 2 millennia

Summary

Introduction

It has been more than half a century since the pioneering work of sampling and analyzing organic carbon (OC) content of surface sediments in deep-water hadal trenches (water depths of >6000 m; Jumars and Hessler (1976); Bartlett (2009)), the processes involved in the transport, deposition and burial of OC in oceanic trenches remain poorly understood. An important and frequently proposed trigger mechanism along subduction trenches is earthquake, which can remobilize clastic materials and OC from the open adjacent slope and along canyons incising the slope (e.g., Bao et al, 2018b; Kioka et al, 2019a,b; Migeon et al, 2017). Tropical cyclones serve as important trigger mechanisms to initiate sediment gravity flow and associated OC transport, especially when canyons connect trench basins with the coast and fluvial systems (e.g., Kao et al, 2010; Pope et al, 2017). Studies in the New Britain Trench (NBT; Luo et al, 2019; Xiao et al, 2020) show overall a high deposition of terrestrial OC in the trench (comprising 61±1% of the TOC) due to the steep landward trench slope (∼8◦) and the proximity to land (∼60 km)

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Discussion

Conclusion