BLACK TURBIDITES: EXPORTING OCEANIC ANOXIC EVENTS TO DEEP WATER

The concept of an oceanic anoxic event (OAE) is based on the widespread occurrence of coeval black shales deposited in a wide range of environments and water depths. These black shales typically only have in common a high organic matter content; many are hemipelagites that form thick shallow-water successions, whereas others are deep-sea pelagites, such as black cherts and limestones, and fine-grained turbidites that never exceed tens of centimeters in thickness. We propose that coeval neritic hemipelagic black shales and abyssal black turbidites deposited during oceanic anoxic events may represent not ocean-wide episodes of anoxia, but a sedimentary link between global shallow-water biochemical signals and the deep ocean.

We studied the texture and composition of Aptian–Albian (OAE 1b) and Cenomanian–Turonian (OAE 2) deep-water black shales from Newfoundland Basin, Canada. Although part of the biogenous sediment settled vertically through the water column, most was reworked from shallower continental margin areas where there was an increase in the deposition of organic matter from marine and terrestrial sources. The sediments were transported to slope and basin sites by turbidity currents. These currents were triggered by tectonic activity, storms, and sediment destabilization as a consequence of gas generation in pore water. The preservation of organic matter, particularly in oxidizing environments, is favored by processes, such as turbidity currents, that deliver large quantities of sediment to the seafloor, including metabolizable organic matter, in a short period of time. Low oxygen levels in pore water and low predator pressure in deep-water environments result in the absence of deep burrowing, which also favors the preservation of organic matter in black turbidites. These deep-sea black shales result from local variations in the rate of supply of reworked organic matter.