Chapter 2
Marine Organic Geochemistry: A General Overview
S. Pantoja· S. Wakeham
2.1
Introduction
Organic geochemistry developed traditionally from the petroleum geologists' need to
find and extract petroleum, since a thorough understanding of the chemical principles
involved in the origin, migration, accumulation and alteration of petroleum would
greatly aid in its discovery. Early on it became clear that organic matter that has been
transformed into petroleum and gas was largely of marine origin and was deposited
in marine sediments. A more fundamental understanding of the processes of petroleum and gas generation required a better understanding of the cycling of organic
matter in the ocean. Simultaneously, there has been increasing awareness of the important role of the ocean in mediating global-scale processes, notably global climate
change through the ocean's buffering capacity for atmospheric carbon dioxide. Further, marine sediments hold the record of past environments, and realistic interpretations of past earth history hinge on understanding the behavior of organic
matter in the ocean. It thus becomes important to better characterize the biogeochemical cycles that influence the production and preservation of organic matter in the sea.
The term biogeochemistry emphasizes the close linkage between biology, geology, and
chemistry into a cross-disciplinary science that strives to define the relationship between the biosphere and the geosphere, and between living and non-living organic
matter.
Dynamic biogeochemical cycles link the distributions of the major chemical elements, C, N, S, P, and 0, between living and non-living organic matter and various inorganic reservoirs (reviewed by Summons 1993). The current view of organic biogeochemical cycles in the ocean is illustrated in Fig. 2.1. Primary organic matter is
biosynthesized from inorganic nutrients by the photosynthetic plankton, using
light as the major energy source. This particulate organic matter (POM) is subject to
grazing by heterotrophic bacteria and zooplankton. Grazing, excretion, cell lysis and
enzymatic hydrolysis of cellular material transfer POM to the dissolved organic matter (DOM) pool. Much of the DOM becomes refractory, but a significant fraction feeds
the microbial loop involving bacteria and protozoa. Whereas conventional wisdom
was that most POM was grazed by zooplankton, it is now becoming clear that organic matter flux though the microbial loop may process half of oceanic primary production (Azam 1998). A very small fraction of primary production becomes part of
the sinking flux, by which POM is transferred from the upper ocean to deep-sea sediments.
Marine Organic Geochemistry: A General Overview
S. Pantoja· S. Wakeham
2.1
Introduction
Organic geochemistry developed traditionally from the petroleum geologists' need to
find and extract petroleum, since a thorough understanding of the chemical principles
involved in the origin, migration, accumulation and alteration of petroleum would
greatly aid in its discovery. Early on it became clear that organic matter that has been
transformed into petroleum and gas was largely of marine origin and was deposited
in marine sediments. A more fundamental understanding of the processes of petroleum and gas generation required a better understanding of the cycling of organic
matter in the ocean. Simultaneously, there has been increasing awareness of the important role of the ocean in mediating global-scale processes, notably global climate
change through the ocean's buffering capacity for atmospheric carbon dioxide. Further, marine sediments hold the record of past environments, and realistic interpretations of past earth history hinge on understanding the behavior of organic
matter in the ocean. It thus becomes important to better characterize the biogeochemical cycles that influence the production and preservation of organic matter in the sea.
The term biogeochemistry emphasizes the close linkage between biology, geology, and
chemistry into a cross-disciplinary science that strives to define the relationship between the biosphere and the geosphere, and between living and non-living organic
matter.
Dynamic biogeochemical cycles link the distributions of the major chemical elements, C, N, S, P, and 0, between living and non-living organic matter and various inorganic reservoirs (reviewed by Summons 1993). The current view of organic biogeochemical cycles in the ocean is illustrated in Fig. 2.1. Primary organic matter is
biosynthesized from inorganic nutrients by the photosynthetic plankton, using
light as the major energy source. This particulate organic matter (POM) is subject to
grazing by heterotrophic bacteria and zooplankton. Grazing, excretion, cell lysis and
enzymatic hydrolysis of cellular material transfer POM to the dissolved organic matter (DOM) pool. Much of the DOM becomes refractory, but a significant fraction feeds
the microbial loop involving bacteria and protozoa. Whereas conventional wisdom
was that most POM was grazed by zooplankton, it is now becoming clear that organic matter flux though the microbial loop may process half of oceanic primary production (Azam 1998). A very small fraction of primary production becomes part of
the sinking flux, by which POM is transferred from the upper ocean to deep-sea sediments.