Trace metals and natural radionuclides as tracers of ocean productivity.

摘要

The theme unifying the two chapters of this thesis is the search for proxies which record the bioproductivity of the surface ocean (and changes in bioproductivity) in the underlying sediments, such that the sedimentary record of these proxies may be used to reconstruct surface ocean bioproductivity in past times.; In the first chapter, sediment trap and core-top samples from the Mid-Atlantic Bight are studied to test the utility of trace metal proxies of export production. With possible exceptions of (a) organic carbon and Cu and, (b) Ba and opal, consistent (Redfield-like) ratios between the trace metals studied and major biogenic phases were not observed. Ba, Cu and Ni are all too labile to serve as quantitative proxies of export production. Authigenic Cu and Ni are almost entirely remineralized during early diagenesis at the sediment-water interface whereas 80 {dollar}pm{dollar} 10% of authigenic Ba is remineralized during this process.; In the second chapter, radionuclide and trace metal proxies are used to constrain the changes in bioproductivity in the South Atlantic sector of the Southern Ocean over glacial - interglacial timescales. The zone of maximum productivity in the South Atlantic region moved northward, from south of present-day position of Antarctic Polar Front (APF), to north of it during glacial periods. There was a large increase ({dollar}sim{dollar}5X) in the flux of aeolian dust to the South Atlantic during glacials as well, resulting in enhancement of Fe available to phytoplankton. Increased Fe supply (and nutrient availability, due to northward transport of nutrients unutilized further south) resulted in significant enhancement of export production in the Subantarctic region of South Atlantic, consistent with the "Fe hypothesis". This increase in the Subantarctic export production was sufficient to compensate for, and exceed, the decrease further south in the Antarctic region. Thus, the net export production of organic carbon (C{dollar}sb{lcub}rm org{rcub}{dollar}) in the South Atlantic was higher during glacials, consistent with the "polar nutrient hypothesis". Moreover, the ratio of C{dollar}sb{lcub}rm org{rcub}{dollar} flux to opal burial was higher during glacials than during the Holocene. Due to this nonlinearity between opal burial and C{dollar}sb{lcub}rm org{rcub}{dollar} fluxes, opal burial may not be used to estimate changes in C{dollar}sb{lcub}rm org{rcub}{dollar} fluxes.