Many black shales, phosphorites, and cherts that formed at low paleolatitudes on the North American continent during the late Paleozoic were a direct consequence of equatorial upwelling and an equatorial undercurrent in an ocean that spanned much of the globe. In equatorial parts of modern oceans, wind stress divergence leads to high surface productivity. Sinking organic matter is remineralized in the strong, eastward-flowing equatorial undercurrent. The undercurrent thus acts as a "nutrient-trap” that becomes progressively oxygen poor and nutrient rich as it moves eastward. The late Paleozoic global ocean was 60% to 80% wider than the modern Pacific Ocean ∼24 000 km total width). The nutrient-trapping equatorial current system of this globe-encircling ocean was probably anoxic and may have been sulfate reducing. Nutrient-rich, anoxic water from the undercurrent would have had direct consequences for the genesis of black-shale facies in Devonian and Pennsylvanian epicontinental seaways as well as possibly providing the source water for coastal upwelling in settings such as the Phosphoria sea.

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