The anoxic, aquatic environment is a mass of water so depleted in oxygen that virtually all aerobic biologic activity has ceased. Oxygen demand relates to surface biologic productivity, while 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 oxic water mainly because of the absence of benthic scavenging and bioturbation. Geochemical and sedimentologic evidence suggests that oil source beds are and have been deposited in four main anoxic settings.

1. Large anoxic lakes. Permanent stratification promotes development of anoxic bottom water, particularly in lakes not subject to seasonal overturn such as Lake Tanganyika. Warm, equable, paleoclimatic conditions favored lacustrine anoxic settings.

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, such as the Red Sea and the Mediterranean Sea, evaporation exceeds river inflow, causing negative water balance and well-oxygenated bottom waters. Hence, silled basins do not necessarily imply the presence of oil source beds.

3. Anoxic layers caused by upwelling. These develop when the oxygen supply in deep water cannot match demand due to high surface biologic productivity. Examples are the Benguela current and Peru upwellings. No systematic correlation exists between upwelling and anoxic conditions because deep oxygen supply can commonly match strongest demand. Anoxic sediments resulting from upwelling are found preferentially at low paleolatitudes.

4. Open-ocean anoxic layers. These are found in the oxygen-minimum layers of the Pacific and northern Indian Oceans, far from deep, oxygenated, polar water sources. They are analogous to worldwide "oceanic anoxic events" during global climatic warm-ups and major transgressions, as in Late Jurassic and middle Cretaceous times.

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