Marine barite, a recorder of oceanic chemistry, productivity, and circulation.

摘要

Marine barite is a ubiquitous minor component of particulate matter and pelagic sediments which is relatively insoluble, thus can be used as a recorder of a variety of seawater chemical characteristics and applied to a variety of paleoceanographic studies.; The goal of the work was to evaluate the potential of marine barite as a reliable monitor of paleo-chemistry and productivity of the ocean.; After modifying a method for the quantitative separation of marine barite from sediments the research proceeded in five directions: (1) Barite samples separated from core tops as well as from 1.5 to 35 Myr old sediments record the Sr isotopic composition of contemporaneous seawater. These results indicate that marine barite could be used for the reconstruction of seawater Sr isotopic composition and for Sr isotopes stratigraphy.; Barite samples from hydrothermally active environments are morphologically distinguishable from barite which precipitates in association with biological activity. The Sr isotope ratios of hydrothermal barite samples fall on a mixing line between seawater and the hydrothermal fluid.; (2) The {dollar}sp{lcub}226{rcub}{dollar}Ra activities of marine barite (dpm/gm) separated from box cores exponentially decays down-core and old barite samples ({dollar}>{dollar}8kyr) have no detected {dollar}rm sp{lcub}226{rcub}RAsb{lcub}exs{rcub}{dollar} activity. These results indicate that barite behaves as a closed system in sediments. The decay of excess {dollar}sp{lcub}226{rcub}{dollar}Ra in barite samples is useful for estimating Holocene sedimentation rates.; (3) The degree of remineralization of Ba at the sediment-seawater interface was estimated by measuring the pore water Ba concentrations. The calculated benthic flux of Ba from the sediment is higher than previously estimated for the open ocean. Similar to the biological productivity in this area the magnitude of the benthic Ba flux is highest close to the equator.; (4) An empirical correlation between the barite accumulation rate and the primary productivity of the overlying water column from two equatorial Pacific transects, over which biological primary productivity gradients exist was obtained. This relationship was used for evaluating glacial to interglacial productivity changes in the equatorial Pacific. Results indicate that the productivity during glacial periods was almost twice as high as during interglacials.; (5) {dollar}varepsilon{dollar}Nd values of marine barite separated from central equatorial Pacific and equatorial Atlantic core-top sediments were determined and related to the water column Nd isotope ratios profile. Results suggest that marine barite records deep water {dollar}varepsilon{dollar}Nd values and could be used for paleo-circulation studies.