Chapter 5
Particulate Organic Matter Composition and Fluxes
in the Sea
C.Lee
5.1
Introduction
For at least 40 years, the rain of particulate organic matter falling through the ocean
has been known to exist and to constitute a source of food for organisms on the sea
floor (Honjo 1990, 1996). Sinking of particulate material from the surface to deeper
waters and the sea floor is one of the major pathways for the transport of carbon and
other bioelements within the ocean, and a variety of biological, physical and chemical
processes alter the organic and inorganic composition of particles as they sink. In the
past four decades, marine geochemists have learned much about sinking particulate
organic matter in the sea, yet many questions remain. This chapter reviews what is
known about the quantity and quality of sinking particulate organic matter in the sea,
describes how it varies in time and space, and considers several larger questions yet
to be answered.
Organic compounds are synthesized from inorganic carbon in the sunlit surface
layer of the world ocean, where primary production by phytoplankton is clearly the
largest source of organic carbon. Heterotrophic consumption (or respiration) by zooplankton and bacteria removes most of the organic matter in the surface waters before
it is exported below the euphotic zone. On a global average, only a small fraction
(5-10%) of the total primary production sinks below the euphotic zone, and a small
fraction of that (l-lO%) survives transport to the sea floor to be preserved in sediments. Thus, the majority of exported organic matter is remineralized (returned to
inorganic form) on its way to the sea floor. Despite this great loss, the surface productivity signal can extend to the deep-sea floor and into the sediments, and the composition of organic matter in the deep ocean reflects its phytoplankton source. We can
use organic biomarkers to investigate the sources of organic mater and specific diagenetic indicator compounds to ascertain the extent of its degradation. Complicating this picture, however, is the fact that the fraction of chemically uncharacterizable
organic matter increases in importance with depth and makes up most of the bulk
carbon in sediments. Explaining the fates of different classes of organic compounds,
including the relationship between molecularly characterizable and uncharacterizable
fractions of organic matter, is a central problem that will require the development and
application of new indicators of source and degradation as well as new analytical tools.
A second question concerns the interaction between organic matter and minerals.
Material exported from the euphotic zone leaves as large, fast-sinking particles
(McCave 1975). Sinking particles include the biogenic mineral phases of planktonic
diatoms, radiolaria, foraminifera, coccolithophorids, and pteropods and lithogenic
minerals (e.g. from dust) (Honjo 1996); these minerals may serve as dense ballast to
Particulate Organic Matter Composition and Fluxes
in the Sea
C.Lee
5.1
Introduction
For at least 40 years, the rain of particulate organic matter falling through the ocean
has been known to exist and to constitute a source of food for organisms on the sea
floor (Honjo 1990, 1996). Sinking of particulate material from the surface to deeper
waters and the sea floor is one of the major pathways for the transport of carbon and
other bioelements within the ocean, and a variety of biological, physical and chemical
processes alter the organic and inorganic composition of particles as they sink. In the
past four decades, marine geochemists have learned much about sinking particulate
organic matter in the sea, yet many questions remain. This chapter reviews what is
known about the quantity and quality of sinking particulate organic matter in the sea,
describes how it varies in time and space, and considers several larger questions yet
to be answered.
Organic compounds are synthesized from inorganic carbon in the sunlit surface
layer of the world ocean, where primary production by phytoplankton is clearly the
largest source of organic carbon. Heterotrophic consumption (or respiration) by zooplankton and bacteria removes most of the organic matter in the surface waters before
it is exported below the euphotic zone. On a global average, only a small fraction
(5-10%) of the total primary production sinks below the euphotic zone, and a small
fraction of that (l-lO%) survives transport to the sea floor to be preserved in sediments. Thus, the majority of exported organic matter is remineralized (returned to
inorganic form) on its way to the sea floor. Despite this great loss, the surface productivity signal can extend to the deep-sea floor and into the sediments, and the composition of organic matter in the deep ocean reflects its phytoplankton source. We can
use organic biomarkers to investigate the sources of organic mater and specific diagenetic indicator compounds to ascertain the extent of its degradation. Complicating this picture, however, is the fact that the fraction of chemically uncharacterizable
organic matter increases in importance with depth and makes up most of the bulk
carbon in sediments. Explaining the fates of different classes of organic compounds,
including the relationship between molecularly characterizable and uncharacterizable
fractions of organic matter, is a central problem that will require the development and
application of new indicators of source and degradation as well as new analytical tools.
A second question concerns the interaction between organic matter and minerals.
Material exported from the euphotic zone leaves as large, fast-sinking particles
(McCave 1975). Sinking particles include the biogenic mineral phases of planktonic
diatoms, radiolaria, foraminifera, coccolithophorids, and pteropods and lithogenic
minerals (e.g. from dust) (Honjo 1996); these minerals may serve as dense ballast to