Silica depletion in the thermocline of the glacial North Pacific: corollaries and implications

Over much of the tropical ocean, late Quaternary productivity patterns show maxima associated with glacial periods, minima during warm times. Also, biogenic opal deposition tends to parallel the accumulation rates of organic carbon and related productivity proxies. This pattern is found in the eastern equatorial Pacific, for example. However, in the western equatorial Pacific, while productivity is normally high during glacials, opal deposition is reduced. In Santa Barbara Basin, both organic matter supply and opal supply are reduced during the last glacial period, showing a reversed productivity pattern. Silicate/phosphate ratios at 100 m depth in Pacific waters and elsewhere suggest that present circulation greatly favors silicate enrichment of the thermocline in the North Pacific, presumably through large-scale deep upwelling, which prevents segregation of silicate from phosphate through partial recycling in the uppermost water column. If so, it appears that the opal supply to Santa Barbara Basin may indicate the nutrient content of the North Pacific thermocline. We suggest that during the last glacial period the nutrient content of the thermocline was low, and large-scale deep upwelling was absent. The records of organic matter supply and laminations show patterns that call for additional factors, presumably changing wind strength, changing shelf exposure, and local feedback effects.