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Getting to the bottom of anoxia: does it matter or not?

Lookup NU author(s): Dr Richard Tyson


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The lack of consensus between oceanographers and petroleum geologists concerning the mechanism(s) responsible for the formation of organic-rich sediments and petroleum source rocks relates largely to the role played by dissolved oxygen. Most geologists consider the occurrence of dysoxic to anoxic conditions an essential prerequisite for the accumulation of oil-prone organic matter rich sediments, while many oceanographers consider that dissolved oxygen has little or no effect on either the preservation of organic matter (OM) or the total organic carbon (TOC) content of marine sediments, instead preferring carbon flux (primary productivity plus water depth), as the main control on both TOC and dissolved oxygen. However, what can be said with some certainty from the empirical geological evidence, is that high TOC contents (>3-5%) and good preservation of oil-prone planktic OM, generally required strongly reducing and commonly dysoxic-anoxic bottom water conditions, regardless of whether or not such conditions were themselves primarily the result of increased carbon flux to the sea floor. Much of the controversy has probably arisen from the confusion of 4 distinct but inter-related questions: 1) What controls the TOC of marine sediments? 2) What controls OM preservation? 3) What controls the TOC accumulation rate (i.e. the OCAR)? 4) What controls the formation of petroleum source rocks (i.e. results in high TOC and oil-prone OM over large areas)? The TOC content of sediments reflects only the balance between, and not the absolute magnitude, of 3 major variables: OM input, OM preservation, and OM dilution by mineralic material (including autodilution by planktic skeletal material). Geological data indicate most source rock facies are associated with relatively slow sedimentation (transgressive to early highstand systems tracts); at such low levels of dilution, simple calculations show that even moderate levels of productivity and preservation can explain the TOC range seen in ancient sediments. There are a number of different ways to enhance the preservation of oil-prone planktic organic matter, all of which appear to require that the organic matter is rapidly transferred to a reducing environment. This can be achieved in 3 ways: 1) Increasing the supply of OM (thus raising the oxygen demand and making the porewaters, and perhaps even the lower water column, go rapidly anoxic). 2) Higher rates of sediment deposition (more of the OM supplied degrades inside the sediment, making the porewater rapidly anoxic), or 3) Reducing oxygen resupply to levels near or below the oxygen demand (via watermass stratification), resulting in dysoxia-anoxia extending into the lower water column. Because of their relative volumes, it is much easier to deoxygenate porewaters than it is to deoxygenate the bottom water. However, the second mechanism, although often producing high OCAR values, is inherently associated with increasing dilution and often a greater terrestrial OM input, and thus lower source rock potential. The first mechanism may be locally important, but is often associated with high autodilution. The third mechanism was probably the most important in epeiric shelf seas. Carbon flux (OM input) is seldom limiting in shelf settings, meaning that the key variables are probably preservation and dilution. As the bottom water volume (and thus oxygen reservoir) of epeiric seas was comparatively low relative to their surface area, regional deoxygenation was probably a common feature, especially during warm equable climatic episodes. These considerations suggest that the most important controls on the majority of petroleum source rocks were the combination of low sediment (dilution) accumulation rates and high preservation due to dysoxia or anoxia. This accounts for why source rock deposition can be successfully related to sea level and sequence stratigraphy, provided that the palaeoxygenation history of the basin is known.

Publication metadata

Author(s): Tyson RV

Publication type: Conference Proceedings (inc. Abstract)

Publication status: Published

Conference Name: Norwegian Meeting in Organic Geochemistry

Year of Conference: 1996

Notes: As with the Tyson 1994 abstract, this is included as a record of the development of my thoughts on this topic, and because it will not be accessible elsewhere.