Chapter 6
Diagenesis of Organic Matter at the
Water-Sediment Interface
S.Wakeham
6.1
Introduction
The water-sediment interface is an intense heterotrophic reactor through which organic matter must pass if it is to be preserved in sediments. The term "water-sediment
interface" is not a simple descriptor. Strictly speaking, it implies a geometric surface
between the water column and the sediments, but to a biogeo chemist it is the "zone in
which organic matter is first accumulated from the water column and is initially metabolized by the sediment heterotrophic community" (Mayer 1993). The depth of this
zone may range from millimetres to a metre or more, depending on the perspective of
the biogeochemist and the processes involved.
The physical character of the interfacial zone is dictated by a combination of physical
and chemical factors (Rhodes 1974). It is characterized by high water content, various
forms of biological aggregations of mineral grains (faecal pellets, burrows etc.), horizontal and vertical mixing, resuspension, and elevated biological activity. Compared
to underlying sediments, the interface is usually enriched in labile organic matter that
has recently been delivered from the water column but has not yet been degraded. It
is a very patchy environment on both macro- and microscales, and both chemically
and biologically. Resuspension and bioturbation alter the topography and sedimentological character on a variety of time scales by mixing and redistributing particles.
Living organisms are present at higher concentrations (on a volume basis) than in any
other marine zone (Mayer 1993). Bacteria are present at lO3_lO4 times greater numbers and activities than in the overlying water column (Deming and Baross 1993). Most
organisms in marine sediments are heterotrophs that rely on the settling of particulate organic matter from the water column and its accumulation at the interface. Feeding by macrofauna physically mixes sediments, and the degradation of organic matter by bacteria using various electron acceptors produces the marked biogeochemical
zonation and gradients (redox, organic carbon content, chemical composition etc.)
commonly observed in surface sediment layers. Biogeochemical processes at the water-sediment interface consume >90% of the organic carbon (DC) that rains down upon
it, leaving <0.1% of the DC produced in the water column to be preserved in sediments.
Chemical gradients are steep in the interfacial zone. The composition of the preserved
material can be markedly different from that which is delivered.
Most changes in the concentration and composition of sedimentary organic matter take place in this interfacial zone, and it is the combined processes affecting these
changes that is termed "diagenesis". Most remineralization and transformation reactions that accompany diagenesis occur because the processes yield energy for benthic
organisms. Organic matter diagenesis is rapid on the geological scale, with most de-
Diagenesis of Organic Matter at the
Water-Sediment Interface
S.Wakeham
6.1
Introduction
The water-sediment interface is an intense heterotrophic reactor through which organic matter must pass if it is to be preserved in sediments. The term "water-sediment
interface" is not a simple descriptor. Strictly speaking, it implies a geometric surface
between the water column and the sediments, but to a biogeo chemist it is the "zone in
which organic matter is first accumulated from the water column and is initially metabolized by the sediment heterotrophic community" (Mayer 1993). The depth of this
zone may range from millimetres to a metre or more, depending on the perspective of
the biogeochemist and the processes involved.
The physical character of the interfacial zone is dictated by a combination of physical
and chemical factors (Rhodes 1974). It is characterized by high water content, various
forms of biological aggregations of mineral grains (faecal pellets, burrows etc.), horizontal and vertical mixing, resuspension, and elevated biological activity. Compared
to underlying sediments, the interface is usually enriched in labile organic matter that
has recently been delivered from the water column but has not yet been degraded. It
is a very patchy environment on both macro- and microscales, and both chemically
and biologically. Resuspension and bioturbation alter the topography and sedimentological character on a variety of time scales by mixing and redistributing particles.
Living organisms are present at higher concentrations (on a volume basis) than in any
other marine zone (Mayer 1993). Bacteria are present at lO3_lO4 times greater numbers and activities than in the overlying water column (Deming and Baross 1993). Most
organisms in marine sediments are heterotrophs that rely on the settling of particulate organic matter from the water column and its accumulation at the interface. Feeding by macrofauna physically mixes sediments, and the degradation of organic matter by bacteria using various electron acceptors produces the marked biogeochemical
zonation and gradients (redox, organic carbon content, chemical composition etc.)
commonly observed in surface sediment layers. Biogeochemical processes at the water-sediment interface consume >90% of the organic carbon (DC) that rains down upon
it, leaving <0.1% of the DC produced in the water column to be preserved in sediments.
Chemical gradients are steep in the interfacial zone. The composition of the preserved
material can be markedly different from that which is delivered.
Most changes in the concentration and composition of sedimentary organic matter take place in this interfacial zone, and it is the combined processes affecting these
changes that is termed "diagenesis". Most remineralization and transformation reactions that accompany diagenesis occur because the processes yield energy for benthic
organisms. Organic matter diagenesis is rapid on the geological scale, with most de-