The anoxic aquatic environment is a mass of water so depleted in oxygen that virtually all aerobic biologic activity has ceased. Anoxic conditions occur where the demand for oxygen in the water column exceeds the supply. Oxygen demand relates to surface biologic productivity, whereas oxygen supply largely depends on water circulation, which is governed by global climatic patterns and the Coriolis force. Organic matter in sediments below anoxic water is commonly more abundant and more lipid-rich than under oxygenated water mainly because of the absence of benthonic scavenging. The specific cause for preferential lipid enrichment probably relates to the biochemistry of anaerobic bacterial activity. Geochemical-sedimentologic evidence suggests that potential oil source beds are and have been deposited in the geologic past in four main anoxic settings as follows. 1. Large anoxic lakes: Permanent stratification promotes development of anoxic bottom water, particularly in large lakes which are not subject to seasonal overturn, such as Lake Tanganyika. Warm equable climatic conditions favor lacustrine anoxia and nonmarine oil source bed deposition. Conversely, lakes in temperate climates tend to be well oxygenated. 2. Anoxic silled basins: Only those landlocked silled basins with positive water balance tend to become anoxic. Typical are the Baltic and Black Seas. In arid-region seas (Red and Mediterranean Seas), evaporation exceeds river inflow, causing negative water balance and well-oxygenated bottom waters. Anoxic conditions in silled basins on oceanic shelves also depend upon overall climatic and water-circulation patterns. Silled basins should be prone to oil source bed deposition at times of worldwide transgression, both at high and low paleolatitudes. Silled-basin geometry, however, does not automatically imply the presence of oil source beds. 3. Anoxic layers caused by upwelling: These develop only when the oxygen supply in deep water cannot match demand owing to high surface biologic productivity. Examples are the Benguela Current and Peru coastal upwelling. No systematic correlation exists between upwelling and anoxic conditions because deep oxygen supply is often sufficient to match strongest demand. Oil source beds and phosphorites resulting from upwelling are present preferentially at low paleolatitudes and at times of worldwide transgression. 4. Open-ocean anoxic layers: These are present in the oxygen-minimum layers of the northeastern Pacific and northern Indian Oceans, far from deep, oxygenated polar water sources. They are analogous, on a reduced scale, to worldwide "oceanic anoxic events" which occurred at global climatic warmups and major transgressions, as in Late Jurassic and middle Cretaceous times. Known marine oil source bed systems are not randomly distributed in time but tend to coincide with periods of worldwide transgression and oceanic anoxia. Geochemistry, assisted by paleogeography, can greatly help petroleum exploration by identifying paleoanoxic events and therefore widespread oil source bed systems in the stratigraphic record. Recognition of the proposed anoxic models in ancient sedimentary basins should help in regional stratigraphic mapping of oil shale and oil source beds.

Anoxic Environments and Oil Source Bed Genesis

The ferrozine method revisited: Fe(II)/Fe(III) determination in natural waters

Environmental analysis of paleoceanographic systems based on molybdenum–uranium covariation

Pyritization of trace metals in anoxic marine sediments

The chemistry of the hydrogen sulfide and iron sulfide systems in natural waters

A 400-square-kilometer depression in the continental slope of the northern Gulf of Mexico (approximately 27 degrees N, 91 degrees W) has been found to contain anoxic, hypersaline ( approximately 250 grams per kilogram) water in the bottom 200 meters. The interface between the brine and overlying seawater acts as a midwater seismic reflector similar to those seen in the Red Sea. The bulk chemical composition of the brine is similar to that from the Red Sea, but differences between the two in both heat content and geomorphological setting indicate different modes of origin.

https://science.sciencemag.org/content/sci/196/4297/1443.full.pdf

Anoxic, Hypersaline Basin in the Northern Gulf of Mexico

Permeability of a large number of natural marine sediment samples from the Gulf of Mexico was determined through the use of laboratory consolidation tests. The samples were divided into the following groups: Group 1, sediment consisting of more than 80% clay (material 2 μm or less in size); Group 2, sediment containing from 60 to 80% clay‐size material; Group 3, silty clays with less than 60% clay; and Group 4, silts and clays that have a significant sand‐size fraction present (more than 5% sand). The permeabilities of the groups ranged from 10−5 to 10−10 cm/s with 35% normal seawater being used as the saturating fluid.

A statistical analysis of the natural log of permeability versus porosity was used to develop the permeability prediction equation for each of the groups listed. The equation for Group 1 is k =en(15.05)‐27.37. for Group 2, k=en(14. 18)‐26.50. for Group 3, k= en(15.59)‐26.65. for Group 4 k=en(17.51)‐26.93.and for all data, k = en(14.30)‐26.30; wherc n is the porosity (in decimals) and k is the coefficient of permeability. These equations are useful for predicting changes in permeability with depth in fine‐grained sediments of the Gulf of Mexico. The ability to predict permeability in a continuous sequence, where the deposition history is known, may explain the large variations that we see in the physical properties in sediments similar in grain size and mineralogy.

Permeability of Unconsolidated and Consolidated Marine Sediments, Gulf of Mexico

Consolidation tests performed on a large number of marine sediments from the Gulf of Mexico indicate that high void ratio marine clay sediments exhibit a linear void ratio-pressure relation in contrast to the non-linear relation as ordinarily observed in clay soils. The use of this linear relation will provide a more accurate evaluation of preconsolidation pressures of marine sediments.

Consolidation of Marine Clays and Carbonates

The Orca basin is an intraslope depression on the continental slope, northwestern Gulf of Mexico, which contains an uncommon anoxic, saline brine. The 400-sq-km basin slopes at angles ranging between 3 and 14° from surrounding water depths of 1,700 to 1,900 m, to over 2,400 m. A 700-j sparker seismic survey shows conformably dipping strata indicative of postdepositional movement, as well as chaotic zones beneath the basin seafloor. The basin appears to be the result of salt tectonism. Sediments from the deep parts of the basin consist of dark anoxic clays with interstitial salinities of 250 ppt. Brine fills the bottom 200 m and provides a seismic-interface reflector at its surface similar to that of the Red Sea. Oxygen values are reduced to zero within the 260-g/kg sa ine brine and temperatures are slightly greater than the directly overlying bottom water. The bulk chemical composition of the brine is similar to that from the Red Sea whereas differences between the two in both heat content and geologic setting indicate different modes of origin.

Orca Basin, Anoxic Depression on the Continental Slope, Northwest Gulf of Mexico: 5. Intraslope Basins

The importance of determining consolidation characteristics and associated geotechnical properties of sediments obtained by the Deep Sea Drilling Project is twofold: first, these properties provide an additional tool for the understanding of depositional processes and the history of sediment accumulation within ocean basins; secondly, the data provide engineering criteria for the design of foundations, sediment-bearing capacity, and slope-stability problems. In recent years the geologist, studying the change of soft deep-sea sediment to indurated marine sedimentary rock, has allied himself with the field of soil mechanics to better understand certain of these processes (Hamilton, 1959). Prior to drilling the experimental Mohole off Guadalupe Island in 1961, studies of the geotechnical properties of marine sediments were limited to those recovered from shallow sampling depths permitted by standard gravity-coring devices. Project Mohole permitted such studies as those by Hamilton (1964) and Moore (1964) of the mass physical properties of marine sediments to a depth of 170 meters. With the advent of DSDP in 1968, the possibility of retrieving samples up to 1000 meters beneath the sea floor became a reality. Although the project has elucidated many aspects of sea-floor spreading, stratigraphy, and sedimentology, very little work has been published on mass physical properties and even less on consolidation characteristics. Samples from DSDP Leg 10 (Trabant, 1972) and Leg 16 (Keller and Bennett, 1973) had been evaluated for their consolidation characteristics and associated geotechnical properties prior to the work described here.

/pdf/consolidation-characteristics-of-sediments-from-leg-31-of-bx667e9yv7.pdf

Peter K. Trabant

Papers

Anoxic, Hypersaline Basin in the Northern Gulf of Mexico

Permeability of Unconsolidated and Consolidated Marine Sediments, Gulf of Mexico

Consolidation of Marine Clays and Carbonates

Orca Basin, Anoxic Depression on the Continental Slope, Northwest Gulf of Mexico: 5. Intraslope Basins

Consolidation Characteristics of Sediments from Leg 31 of the Deep Sea Drilling Project