Findings of oceanic anoxia or global oceanic anoxia are speculatively vague and wildly misleading. Sedimentary investigations cannot be used in support of global oceanic anoxia. They can however, be used in support of a shift in biological productivity. Such claims more often than not reflect the sloppy verbal skills of investigators more concerned with the mathematical precision of their isotopic findings. Sedimentary studies can only indicate the respirational conditions prevailing within a specific sedimentary environment and NO more.

Unnecessarily, sedimentologists confuse everybody when they make oceanic anoxia claims. Incorrectly, they globally apply their findings within the sediments of a particular basin. If such conditions prevail in numerous basins, there are probably tectonic and accompanying biological explanations for such major shifts between an oxygenated and deoxygenated sedimentary environment. The oil shales of Wyoming are a notable example. As I mentioned previously, the oceanic photion can be oversaturated with mixed diatomic oxygen from photosynthesis while a sedimentary environment below the 38 degree Fahrenheit isotherm enriched with organic materials is very deoxygenated. This condition is not uncommon in large, deep storage water reservoirs during periods of limited inflows and warm temperature. My graduate limnology professor at Southwest Texas State University made a career of studying such conditions within the subtropical impoundments constructed on the Guadalupe and Colorado (the Texas river, not the Arizona river of the same name) Rivers of central Texas.

Once established, sedimentary anoxia can occur just millimeters away from a highly oxygenated environment unless disturbed by the oxygenating activities of worms. The bacteria truly do not lose their anaerobic environment. They gain a better one inside the worms. Our own thermally regulated digestive guts are simple proof of the invagination of anaerobic environments within aerobic organisms. Talk about hydrogen sulfide! Lacking sunlight our guts cannot usefully employ photosynthetic anoxic bacteria to consume all those eggs we eat. Remarkably, anaerobic bacteria obligated to energetically inefficient anaerobic respiration must find other sources of hydrogen to hydrogenate carbon compounds, consequently some bacteria use the hydrogen sulfide respirationally released by other bacteria. Why does this happen? Because the sedimentary environment or the waters within a tectonically strangulated basin like the Black Sea or Cariaco Basin offshore of Venezuela have been enriched with hydrogenated organic materials in excess of the mass of diatomic oxygen available for aerobic metabolic stabilization.

Lacking adequate water circulation, these anoxic sedimentary environments are perennially maintained until tectonic extension alleviates the strangulation of a biologically productive, coastal, sublittoral basin. Tectonic activity can also release previously buried organic materials within a sedimentary basin. The Black Sea and the Cariaco are both submarinally influenced by hydrocarbon discharges. Subaerially, the La Brea Tar Pits in downtown Los Angeles are a testament to the tectonic release of asphaltic hydrocarbons tortuously trapping thirsty animals till the present day. Undoubtedly, sedimentary investigations are being funded by companies and countries exploring for more oily hydrocarbons for industrial recovery. Additionally, tectonic extension can disrupt epilithic salt domes and karst, aqueously releasing hypersaline solutions of halite from the domes and calcium, magnesium, potassium, or sodium sulfates from the cavernous karst system.

Consequently, as you have correctly noted, the bacterial marker's presence in the particular boundary layers mark a major ecosystemic shift from an energetically efficient, oxygenated sediment to a less efficient, deoxygenated, anaerobic respirational environment. Undoubtedly, major metamorphic tectonic episodes, marked by volcanism, can abundantly fertilize an aquatic environment with oxidizable materials, enabling the expansion of anaerobic respirational sedimentary environments. Simultaneously, volcanic emissions of sulfuric aerosols can cause photometrically depress primary production as a terrestrial and oceanic photic environments respond to changes in sunlight and temperature. As aerosols and aquasols of sulfur subaerially or submarinally settle, primary productivity will substantially increase as light and temperature increase, abundantly fertilized by volcanically released material from the growing Earth. Silicaceous growth enabling carbonaceous growth and vice versa.

Also be wary of isotopic studies by sedimentologists or paleontologists. They, too, can be based upon the expectations of scientists studying fossilized environments who are poorly grounded in the aquatic physiology of living ecosystems. Clearly, sedimentologists and paleontologists studying carbonate precipitating micro-organisms such as the anabolic, photosynthetic golden algae, or coccoliths, and the catabolic cellular protoctists called foraminifers have little understanding of the living descendants and their water quality protective activity physiology of these remarkable organisms. But, that's a whole different story.

Tidal, lagunal flats, blackwater freshwater swamps, and organically overloaded streams commonly have anaerobic sediments. If as James Maxlow has suggested that oceanic basin expansion is accompanied by the retreat of shallow waters from continental shelves, formerly well-oxygenated reef environments can be replaced by muddy, organically enriched environments that demand oxygen in excess of its immediate availability. I think James is correct in this observation. During the Mesophytic Era, sedimentary isotopic studies of carbon may actually reflect that increased demand for carbon by terrestrial ecosystems has they expanded into shallow brackish and freshwater environments, reestablishing lagunal botanical habitats, but this time with evergreen, broadleaf, flowering angiosperms. These reef and lagunal environments wax and wane over time as the crustal shell is tectonically extended and enlarged. Sedimentologists fail to note, that once established, anaerobic bacteria can very quickly and effectively seal their environment from their oxygenated surroundings. Photosynthetic purple-green bacteria are especially good at building semi-permeable membranes of sticky, waxy hydrocarbons, that prevent the transmission of oxygenated waters into their environment.

In a bacterially maintained, anoxic, sedimentary habitat, water is more to important as a source of plasma medium to deliver nutrients and maintain osmotic pressures. Consequently, purple-green bacteria use hydrogen sulfide as a source of hydrogen. These bacteria are photosynthetic meaning they use photons to split hydrogen away from the central sulfur atom. Obviously, they can only do this in illuminated sedimentary environments or in the lowest illuminate, non-mictic stratified waters of strangulated basins like the Black Sea or Cariaco Basin, all the while doing so below an oxygenated upper photion well oxygenated activities of blue-green bacteria. Thus the environments studied by sedimentologist and paleontologists are truly oceanic but coastal sedimentary environments that alternate between aquatic and terrestrial habitats or a combination of the two environments.

Personally, I think the megosaurs ate themselves into a corner of oblivion. Their gigantism, like that of elephants, was sexually forced by females upon males to establish dominance and harem privileges. That kind of gigantic existence is fraught with peril, especially if tectonic or meteorological conditions suddenly shift. Dinosaurs became huge because they had a huge terrestrial habitat supporting their existence. Tectonically, it began to fragment into smaller habitats during the late Mesophytic Era. The K-T metamorphic event marks the end of the continent of Pacifica and its dispersal around the Pacific Rim. Good-bye huge, tropical habitat. Good-bye big lizards with huge appetites and multiple females to please. A Mesophtic Era version of sex, drugs, and rock 'n' roll ended forever.