271
8
Sulfur Cycling and Methane Oxidation
BO BARKER JØRGENSEN AND SABINE KASTEN
This chapter deals with the biogeochemical transformations of sulfur and methane in marine
sediments during early diagenesis. The term ‘early
diagenesis’ refers to the whole range of postdepositional processes that take place in aquatic
sediments and are coupled either directly or
indirectly to the degradation of organic matter. We
focus on the processes that drive sulfate reduction together with the manifold associated biotic
and abiotic reactions that make up the sedimentary sulfur cycle – including the various pathways
of sulfide oxidation. Furthermore, we give an
overview of the quantitative significance of
microbial sulfate reduction for the mineralization
of organic matter and oxidation of methane in
different depositional environments and discuss
the different approaches to quantify sulfate reduction through radiotracer measurements or modeling of pore-water concentration profiles. As
sedimentary pyrite represents the most important
sink for seawater sulfate, the mechanisms of pyrite
formation are discussed. The sulfidization of
sediment organic matter is another sink for sulfur
in the modern ocean, but will not be discussed
here. For an overview of the processes and
pathways involved in the incorporation of sulfur
into organic matter, we refer to Orr and White
(1990), Krein and Aizenshtat (1995), Schouten et
al. (1995), and Werne et al. (2004).
Due to the profound alteration of the primary
sediment geochemistry across and below the
sulfate/methane transition (SMT, also called the
sulfate/methane interface), where the process of
anaerobic oxidation of methane (AOM) takes
place, we also dedicate a part of this chapter to
the processes of mineral dissolution and precipitation occurring at and below the SMT. We will
demonstrate that sulfate reduction driven by
AOM significantly alters primary mineral
associations accompanied by strong perturbations of mineralogical, isotopic and rock
magnetic signatures. These diagenetic processes
have a profound impact on the preservation of
numerous paleoceanographic proxy variables and
are therefore relevant for the interpretation of the
geological record.
Reviews of the sulfur cycle from a biogeochemical or microbiological perspective have
recently been presented, e.g. by Amend et al.
(2004) and Canfield et al. (2005). The latter authors
describe the relationship between the different
microorganisms metabolizing sulfur compounds
and their environment and emphasize the organisms as well as the biogeochemical processes.
8.1
Introduction
With 1.3 · 10
9
Tg (teragram = megaton = 10
12
g) of
sulfur present as sulfate, the oceans represent one
of the largest sulfur pools on earth (Vairavamurthy
et al. 1995). The main influx of sulfur to the oceans
occurs via river water carrying the products of
mechanical and chemical weathering of continental rocks. Relative to this fluvial input, the
atmospheric transport of sulfur is of minor
importance. It mainly consists of recycled oceanic
sulfate from seaspray, volcanic sulfur gases, H 2 S
released by sulfate-reducing bacteria, organic Sbearing compounds released into seawater and
subsequently into the atmosphere by phytoplankton, and anthropogenic emissions of sulfur
dioxide. Due to the oxic conditions that prevail in
the world’s oceans, the dominant sulfur species in
seawater is by far the sulfate ion (SO 4
2). Sulfate is
the second most abundant anion next to chloride
and has a concentration of 29 mM (2.71 g/kg) in
ocean water.
Marine sediments are the main sink for seawater sulfate which demonstrates that the sedimentary sulfur cycle is a major component of the
global sulfur cycle. The most important mechanisms for removing sulfate from the oceans to the
8
Sulfur Cycling and Methane Oxidation
BO BARKER JØRGENSEN AND SABINE KASTEN
This chapter deals with the biogeochemical transformations of sulfur and methane in marine
sediments during early diagenesis. The term ‘early
diagenesis’ refers to the whole range of postdepositional processes that take place in aquatic
sediments and are coupled either directly or
indirectly to the degradation of organic matter. We
focus on the processes that drive sulfate reduction together with the manifold associated biotic
and abiotic reactions that make up the sedimentary sulfur cycle – including the various pathways
of sulfide oxidation. Furthermore, we give an
overview of the quantitative significance of
microbial sulfate reduction for the mineralization
of organic matter and oxidation of methane in
different depositional environments and discuss
the different approaches to quantify sulfate reduction through radiotracer measurements or modeling of pore-water concentration profiles. As
sedimentary pyrite represents the most important
sink for seawater sulfate, the mechanisms of pyrite
formation are discussed. The sulfidization of
sediment organic matter is another sink for sulfur
in the modern ocean, but will not be discussed
here. For an overview of the processes and
pathways involved in the incorporation of sulfur
into organic matter, we refer to Orr and White
(1990), Krein and Aizenshtat (1995), Schouten et
al. (1995), and Werne et al. (2004).
Due to the profound alteration of the primary
sediment geochemistry across and below the
sulfate/methane transition (SMT, also called the
sulfate/methane interface), where the process of
anaerobic oxidation of methane (AOM) takes
place, we also dedicate a part of this chapter to
the processes of mineral dissolution and precipitation occurring at and below the SMT. We will
demonstrate that sulfate reduction driven by
AOM significantly alters primary mineral
associations accompanied by strong perturbations of mineralogical, isotopic and rock
magnetic signatures. These diagenetic processes
have a profound impact on the preservation of
numerous paleoceanographic proxy variables and
are therefore relevant for the interpretation of the
geological record.
Reviews of the sulfur cycle from a biogeochemical or microbiological perspective have
recently been presented, e.g. by Amend et al.
(2004) and Canfield et al. (2005). The latter authors
describe the relationship between the different
microorganisms metabolizing sulfur compounds
and their environment and emphasize the organisms as well as the biogeochemical processes.
8.1
Introduction
With 1.3 · 10
9
Tg (teragram = megaton = 10
12
g) of
sulfur present as sulfate, the oceans represent one
of the largest sulfur pools on earth (Vairavamurthy
et al. 1995). The main influx of sulfur to the oceans
occurs via river water carrying the products of
mechanical and chemical weathering of continental rocks. Relative to this fluvial input, the
atmospheric transport of sulfur is of minor
importance. It mainly consists of recycled oceanic
sulfate from seaspray, volcanic sulfur gases, H 2 S
released by sulfate-reducing bacteria, organic Sbearing compounds released into seawater and
subsequently into the atmosphere by phytoplankton, and anthropogenic emissions of sulfur
dioxide. Due to the oxic conditions that prevail in
the world’s oceans, the dominant sulfur species in
seawater is by far the sulfate ion (SO 4
2). Sulfate is
the second most abundant anion next to chloride
and has a concentration of 29 mM (2.71 g/kg) in
ocean water.
Marine sediments are the main sink for seawater sulfate which demonstrates that the sedimentary sulfur cycle is a major component of the
global sulfur cycle. The most important mechanisms for removing sulfate from the oceans to the