Geochemical investigations of continental and hydrothermal inputs to the ocean.

Abstract

Continental and hydrothermal materials in marine sedimentary sequences are geochemically investigated to evaluate provenance, transport processes, and proxies for seawater chemistry and biological productivity in paleoceanographic studies. The detrital material in 33 surface samples from the North Pacific is extracted, analyzed for trace element compositions, and subjected to Q-mode factor analysis to verify a two end-member mixing scheme. Linear modeling is used estimate the relative contents of Asian eolian continental material and Pacific-rim volcanic detritus. The spatial pattern shows a maximum in eolian contents in the central North Pacific at approximately 40$\sp\circ$N with increasingly significant volcanic detritus around the margins of the North Pacific. This technique will improve paleoceanographic studies of eolian continental dust. North Pacific eolian material has a different composition than loess in the Asian source region. China loess samples are separated into five grainsize fractions, an aliquot of each is subjected to a detrital extraction procedure, and all are analyzed for trace elements. Dissolution of phosphate phases during the extraction procedure causes a decrease in the middle rare earth elements and Th in the fine grained fraction (2-3 $\mu$m) which is identical to eolian material extracted from North Pacific sediment. The late Miocene-early Pliocene biogenic bloom is evident from higher biological productivity and phosphorous accumulation in Indo-Pacific upwelling areas. Central Indian Ocean samples are analyzed for phosphorous accumulation and show a decrease during the same interval. This suggests a redistribution in productivity which intensified in upwelling regions in the Indo-Pacific region at the expense of low productivity convergent regions where a strengthened thermocline prevailed. The elements As, P, Sb and V (which form oxyanions in seawater) are linked to marine biological processes, and are coprecipitated with mid-ocean ridge hydrothermal particles in proportion to their concentration in seawater. Four hydrothermal sequences are evaluated for the long term integrity of this potential proxy for ancient seawater chemistry. There is no post-depositional uptake for the elements As, P, and V, whereas Sb is equivocal. Such hydrothermal sequences may record information about the composition of ancient seawater if problems with biogenic components of As and P can be overcome.