Timescales for the development of methanogenesis and free gas layers in recently-deposited sediments of Arkona Basin (Baltic Sea)

J M Mogollón,H Fossing,P Regnier,A W Dale

doi:10.5194/bg-9-1915-2012

J M Mogollón, H Fossing + Show 2 more

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https://doi.org/10.5194/bg-9-1915-2012

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Abstract

Abstract. Arkona Basin (southwestern Baltic Sea) is a seasonally-hypoxic basin characterized by the presence of free methane gas in its youngest organic-rich muddy stratum. Through the use of reactive transport models, this study tracks the development of the methane geochemistry in Arkona Basin as this muddy sediment became deposited during the last 8 kyr. Four cores are modeled each pertaining to a unique geochemical scenario according to their respective contemporary geochemical profiles. Ultimately the thickness of the muddy sediment and the flux of particulate organic carbon are crucial in determining the advent of both methanogenesis and free methane gas, the timescales over which methanogenesis takes over as a dominant reaction pathway for organic matter degradation, and the timescales required for free methane gas to form.

Highlights

Methane, a potent greenhouse gas, is ubiquitously present within marine sediments in dissolved, gaseous and hydrate form
The depositional rate model is based on steady state compaction and the assumption that the accumulation of organic rich sediment during the last 8 kyr occurred under a constant particulate organic carbon (POC) flux to the sediment surface
Arkona Basin is a seasonally-hypoxic marine system in the southwestern Baltic Sea characterized by an organic-rich Holocene mud layer which increases in thickness toward the center of the depression

Summary

Introduction

A potent greenhouse gas, is ubiquitously present within marine sediments in dissolved, gaseous and hydrate form. A broad array of evidence suggests that the masses and fluxes of methane in seafloor sediments can vary significantly over time. Several studies and models have focused on the sources and sinks of methane in the slope (Davie and Buffett, 2001; Jørgensen et al, 2001; Haeckel et al, 2004; Jørgensen et al, 2004) and on the shelf (Dale et al, 2008a,b; Mogollon et al, 2009; Mogollon et al, 2011; Regnier et al, 2011). The focus of this study, is to trace back the development of the methane cycle in shelf sediments of the Baltic Sea. Except for high-latitude regions, shelf systems are generally not favorable for gas hydrate accumulation; they are much more sensitive to fluctuating conditions in the water-column such as temperature or bottom-water hypoxia. It is expected that their dynamics might differ significantly from sediments on the slope

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