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, bi ogenic opal deposition tends to parallel the accumulation rates of organic carbon and related productivity proxies. This pattern is found in the easte rn equatorial Pacific, for example. However, in the western equatorial Paci fic, while productivity is normally high during glacials, opal deposition i s reduced. In Santa Barbara Basin, both organic matter supply and opal supp ly are reduced during the last glacial period, showing a reversed productiv ity pattern. Silicate/phosphate ratios at 100 m depth in Pacific waters and elsewhere suggest that present circulation greatly favors silicate enrichm ent of the thermocline in the North Pacific, presumably through large-scale deep upwelling, which prevents segregation of silicate from phosphate thro ugh partial recycling in the uppermost water column. If so, it appears that the opal supply to Santa Barbara Basin may indicate the nutrient content o f the North Pacific thermocline. We suggest that during the last glacial pe riod 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 st rength, changing shelf exposure, and local feedback effects. (C) 1998 Elsev ier Science Ltd. All rights reserved.