Showing papers by "Peter W. Kubik published in 1994"

Anomalous boundary scavenging in the Middle Atlantic Bight: evidence from 230Th, 231Pa, 10Be and 210Pb

Abstract: Boundary scavenging processes result in the deposition of particle-reactive chemical substances in ocean margin sediments at rates much greater than their average rate of deposition over the entire ocean. Sediments and samples collected by time-series sediments traps were studied to examine the nature and extent of boundary scavenging in the Middle Atlantic Bight (MAB) region of the NW Atlantic. Here, new results are presented for 230Th, 231Pa and 10Be and compared to older findings, based on studies of 210Pb, where it appeared that the normal enhancement of particle-reactive trace-metal deposition at ocean margins was absent. In contrast to the behavior of 210Pb, deposition rates of 230Th, 231Pa and 10Be exceed their local rates of supply, in some cases by more than an order of magnitude. Enhanced deposition of these tracers is clearly occurring in the MAB. An unusual feature of boundary scavenging, however, is that the pattern of enhanced deposition follows the order Be > Th > Pa > Pb, whereas the expected order be Be > Pb ≥ Pa > Th. Furthermore, unsupported 231Pa/230Th activity ratios are consistently less than their production ratio of 0.093, in contrast to ratios typically found at other ocean margins in the range of 0.2–0.3. Export from the region of fine-grained manganese-rich particles is postulated to explain the low fluxes of 210Pb and the low 231Pa/230Th ratios. Other trace elements which have high affinities for adsorption to Mn oxides may, like Pb and Pa, have much lower rates of deposition in the MAB than in other, more typical, ocean-margin environments.

Beryllium 10, thorium 230, and protactinium 231 in Galapagos microplate sediments: Implications of hydrothermal activity and paleoproductivity changes during the last 100,000 years

Abstract: Biogenic particle fluxes from highly productive surface waters, boundary scavenging, and hydrothermal activity are the main factors influencing the deposition of radionuclides in the area of the Galapagos microplate, eastern Equatorial Pacific. In order to evaluate the importance of these three processes throughout the last 100 kyr, concentrations of the radionuclides 10Be, 230Th, and 231Pa, and of Mn and Fe were measured at high resolution in sediment samples from two gravity cores KLH 068 and KLH 093. High biological productivity in the surface waters overlying the investigated area has led to 10Be and 231Pa fluxes exceeding production during at least the last 30 kyr and probably the last 100 kyr. However, during periods of high productivity at the up welling centers off Peru and extension of the equatorial high-productivity zone, a relative loss of 10Be and 231Pa may have occurred in these sediment cores because of boundary scavenging. The effects of hydrothermal activity were investigated by comparing the 230Thex concentrations to the Mn/Fe ratios and by comparing the fluxes of 230Th and 10Be which exceed production. The results suggest an enhanced hydrothermal influence during isotope stages 4 and 5 and to a lesser extent during isotope stage 1 in core KLH 093. During isotope stages 2 and 3, the hydrothermal supply of Mn was deposited elsewhere, probably because of changes in current regime or deep water oxygenation. A strong increase of the Mn/Fe ratio at the beginning of climatic stage 1 which is not accompanied by an increase of the 230Thex concentration is interpreted to be an effect of Mn remobilization and reprecipitation in the sediment.

Origin of groundwater salinity in the Lac du Bonnet granite, southeastern Manitoba, from 36Cl measurements

Abstract: The results of analyses of the 36Cl content of groundwaters in the Lac du Bonnet granite batholith are described and compared with previous modelling calculations and 36Cl content of rock matrix solutions obtained from a borehole leaching experiment. The results are used to provide support for the proposal that groundwater salinity is largely derived from dissolution of soluble salts in the rock matrix.