Upwelling

Abstract

Upwelling currents are driven by the wind and increase in strength as the wind does. They bring up nutrients from below, stimulating productivity in surface waters where winds blow parallel to the coast and where winds blow persistently over the open ocean, especially at the equator and around Antarctica. Upwelling became more prominent in the Late Cenozoic as the world cooled and winds strengthened. It was prominent in the Miocene and has continued to the present, fluctuating from stronger in glacial times to weaker in interglacial times, with a tendency to increase at times of transition from glacial to interglacial. Signs of upwelling are found further offshore during times when sea level was low. The oxygen minimum zone beneath upwelling centers may become significantly depleted in oxygen, permitting more preservation oforganic matter; this is less true of well-oxygenated basins like the Atlantic and more true of poorly oxygenated basins like the North Pacific and Arabian Sea. Upwelling led to the deposition of abundant marine organic matter along the margins of northwest Africa and Guyana during the Cenomanian-Turonian, most likely due to an influx of nutrient-rich and oxygen-poor water from the South Atlantic as Africa and South America moved apart. The deep ocean black shales of oceanic anoxic events are not directly related to upwelling, but may contain abundant organic-rich turbidites derived from continental margins beneath upwelling currents. A by-product of decomposition beneath upwelling centers is the release of phosphate to precipitate as calcium carbonate-fluorapatite or to phosphatize preexisting carbonates, which may progress to the point of forming phosphorites. A further by-product is the deposition ofsediments rich in the remains of siliceous diatoms, provided that surface waters have not become depleted in silica. Upwelling is thus frequently recognized in the geological record from an association between organic-rich shales or mudstones, diatomite, and phosphorite, the “signature triad.” The absence of organic enrichment may not mean that upwelling was absent – it may reflect either low productivity, due to an inadequate supply of nutrients, or low preservation, due to either an abundance of oxygen or the absence of conditions suitable for the accumulation of fine-grained sediment. The richness of productivity associated with upwelling depends as well upon the source of nutrients in the intermediate depth waters of the adjacent ocean. Predictions of upwelling occurrence and organic enrichment based purely on past wind directions are likely to be overly simplistic.