Carbon mineralization and carbonate preservation in modern cold-water coral reef sediments on the Norwegian shelf

L M Wehrmann,C Wild,V Unnithan,T G Ferdelman,H Pirlet,N J Knab

doi:10.5194/bg-6-663-2009

Abstract

Abstract. Cold-water coral ecosystems are considered hot-spots of biodiversity and biomass production and may be a regionally important contributor to carbonate production. The impact of these ecosystems on biogeochemical processes and carbonate preservation in associated sediments were studied at Røst Reef and Traenadjupet Reef, two modern (post-glacial) cold-water coral reefs on the Mid-Norwegian shelf. Sulfate and iron reduction as well as carbonate dissolution and precipitation were investigated by combining pore-water geochemical profiles, steady state modeling, as well as solid phase analyses and sulfate reduction rate measurements on gravity cores of up to 3.25 m length. Low extents of sulfate depletion and dissolved inorganic carbon (DIC) production, combined with sulfate reduction rates not exceeding 3 nmol S cm−3 d−1, suggested that overall anaerobic carbon mineralization in the sediments was low. These data showed that the coral fragment-bearing siliciclastic sediments were effectively decoupled from the productive pelagic ecosystem by the complex reef surface framework. Organic matter being mineralized by sulfate reduction was calculated to consist of 57% carbon bound in CH2O groups and 43% carbon in -CH2- groups. Methane concentrations were below 1 μM, and failed to support the hypothesis of a linkage between the distribution of cold-water coral reefs and the presence of hydrocarbon seepage. Reductive iron oxide dissolution linked to microbial sulfate reduction buffered the pore-water carbonate system and inhibited acid-driven coral skeleton dissolution. A large pool of reactive iron was available leading to the formation of iron sulfide minerals. Constant pore-water Ca2+, Mg2+ and Sr2+ concentrations in most cores and decreasing Ca2+ and Sr2+ concentrations with depth in core 23–18 GC indicated diagenetic carbonate precipitation. This was consistent with the excellent preservation of buried coral fragments.

Highlights

Marine surface sediments constitute an important element of the global carbonate cycle
The findings from Integrated Ocean Drilling Program (IODP) Expedition 307 indicate a tight coupling between the sulfur, carbon and iron cycles in sediments associated with cold-water coral reef environments
Existing models suggest that the production of hydrogen sulfide and bicarbonate in reactive iron-poor sediments can lead to carbonate dissolution depending on the magnitude of organoclastic sulfate reduction (Ben-Yaakov, 1973; Gardner, 1973; Walter and Burton, 1990)

Summary

Introduction

Marine surface sediments constitute an important element of the global carbonate cycle. They link carbonate production in the surface ocean with the deep subsurface sedimentary environment (Ridgwell and Zeebe, 2005), and provide an important record of the ocean’s response to changing climate conditions in the past (Broecker and Clark, 2001, 2003; Crowley, 1983). Cold-water corals are widespread at a range of water depths (30–4000 m) along continental margins, seamounts and banks (Wheeler et al, 2007).

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