Chapter 5
Organic Matter Preservation in the Ocean:
Lipid Behavior from Plankton to Sediments
S.G. Wakeham
5.1
Introduction
Marine sediments preserve a record of past oceanic environmental conditions. This
record, however, may be compromised and difficult to interpret if there is qualitative
and quantitative uncoupling between the water column and the sediments. In the case
of organic matter, this bias is severe since most organic matter produced in surface
waters is degraded in the water column and only a tiny fraction is buried in the sediments. Valid reconstruction of the sediment record requires a firm understanding of
the processes that occur in the ocean and how they affect the behavior of organic
matter.
The oceanic carbon cycle involves a complex interaction between physical, biological
and chemical processes. The bulk of particulate organic carbon (POC) in the ocean is
biosynthesized in surface waters by photo autotrophic plankton (see review by
Wakeham and Lee 1993), with rivers and aeolian sources (rain fall and dry fallout)
adding a minor fraction. Once produced, most POC is consumed by heterotrophic
organisms in the upper ocean, leading to changes in the particle size-spectrum by
continuous aggregation/disaggregation ("repackaging") processes, alteration of the
chemical composition of particles, and ultimately remineralization of most (>90%)
of the POC pool. A small fraction, perhaps 5-10%, of POC produced in surface waters
escapes into the deep ocean. Organic materials associated with fast-sinking, large particles such as marine snow aggregates or faecal pellets may reach the seafloor relatively quickly and with minimal degradation. Compounds associated with slowly sinking fine particles may undergo intense chemical alteration during long transit times.
Regardless of the transport mechanism and its time scale, only <1% of the primary
production reaches the sea floor, and this in a highly altered state. Further degradation by benthic macro- and microfauna in surface sediments removes most of any remaining organic matter, with the final burial of only'" 0.1% of primary production.
Interpreting the sedimentary record is thus confounded by the fact that very little of
the signal originally produced in surface waters is preserved in the sediments.
Examples from recent Joint Global Ocean Flux Study OGOFS) experiments in the
equatorial Pacific Ocean (EqPac; Murray et al.1992, 1995) and the Arabian Sea (Lee et al.
1998) are presented below to highlight the processes affecting preservation of organic
matter in marine sediments. The goal of these studies was to evaluate the efficiency
and dynamics of the carbon pump in these oceanic systems, and to establish how water
column biogeochemical processes affect the burial and preservation of biogenic components in sediments.
Organic Matter Preservation in the Ocean:
Lipid Behavior from Plankton to Sediments
S.G. Wakeham
5.1
Introduction
Marine sediments preserve a record of past oceanic environmental conditions. This
record, however, may be compromised and difficult to interpret if there is qualitative
and quantitative uncoupling between the water column and the sediments. In the case
of organic matter, this bias is severe since most organic matter produced in surface
waters is degraded in the water column and only a tiny fraction is buried in the sediments. Valid reconstruction of the sediment record requires a firm understanding of
the processes that occur in the ocean and how they affect the behavior of organic
matter.
The oceanic carbon cycle involves a complex interaction between physical, biological
and chemical processes. The bulk of particulate organic carbon (POC) in the ocean is
biosynthesized in surface waters by photo autotrophic plankton (see review by
Wakeham and Lee 1993), with rivers and aeolian sources (rain fall and dry fallout)
adding a minor fraction. Once produced, most POC is consumed by heterotrophic
organisms in the upper ocean, leading to changes in the particle size-spectrum by
continuous aggregation/disaggregation ("repackaging") processes, alteration of the
chemical composition of particles, and ultimately remineralization of most (>90%)
of the POC pool. A small fraction, perhaps 5-10%, of POC produced in surface waters
escapes into the deep ocean. Organic materials associated with fast-sinking, large particles such as marine snow aggregates or faecal pellets may reach the seafloor relatively quickly and with minimal degradation. Compounds associated with slowly sinking fine particles may undergo intense chemical alteration during long transit times.
Regardless of the transport mechanism and its time scale, only <1% of the primary
production reaches the sea floor, and this in a highly altered state. Further degradation by benthic macro- and microfauna in surface sediments removes most of any remaining organic matter, with the final burial of only'" 0.1% of primary production.
Interpreting the sedimentary record is thus confounded by the fact that very little of
the signal originally produced in surface waters is preserved in the sediments.
Examples from recent Joint Global Ocean Flux Study OGOFS) experiments in the
equatorial Pacific Ocean (EqPac; Murray et al.1992, 1995) and the Arabian Sea (Lee et al.
1998) are presented below to highlight the processes affecting preservation of organic
matter in marine sediments. The goal of these studies was to evaluate the efficiency
and dynamics of the carbon pump in these oceanic systems, and to establish how water
column biogeochemical processes affect the burial and preservation of biogenic components in sediments.