Boundary scavenging in the Pacific Ocean: a comparison of10Be and231Pa

Abstract

Concentrations of U, Th, 231pa and 1°Be were measured in Holocene sediments from two cores collected off the west coast of South America, two cores from the East Pacific Rise, two from the equatorial Pacific and one from the south Pacific central gyre. Our results, together with data from 5 cores reported in the literature, show that boundary scavenging plays a major role in the removal of 1°Be from the Pacific Ocean. Deposition rates of 1°Be at three margin sites are more than an order of magnitude greater than at sites of red clay accumulation in the deep central Pacific. Deposition of 231pa is 4 to 5-fold greater at the margin sites. The residence time of 1°Be with respect to chemical scavenging, defined as its inventory in the water column divided by its rate of removal to the sediments, varies regionally from > 1000 years at the red-clay sites in the deep central Pacific to -100 years at the margin sites. Different factors control boundary scavenging of Pa and Be. For example, scavenging of 231pa is enhanced by metal-oxide coatings of particles, whereas this seems to have little influence on the scavenging of l°Be.

Transport and burial rates of10Be and231Pa in the Pacific Ocean during the Holocene period

“…The results of previous studies [9,10] and our work [23][24][25] have shown that l°Be is also preferentially removed from the oceans in some margin areas. In order to evaluate the source of l°Be, and to understand its transport within the oceans, we need to conduct an ocean-wide study: (1) to estimate the extent to which l°Be deposition is enhanced in different ocean margin regions; and (2) to examine what factors influence the scavenging of l°Be from seawater to sediments.…”

“…The initial (i.e., decay corrected) unsupported concentrations of 23°Th and 231pa (des-230 231~ ignated as xs Tho and xs rao, respectively; Table 2), were calculated based methods described previously [23,24]. The basic principle of our approach is to use 23°Th, a nuclide that is produced uniformly throughout the ocean by decay of dissolved 234U and deposited to the sea floor at a rate nearly equal to its production rate in the overlying water column, as a tracer against which 231pa and l°Be are normalized so that 231pa/Ea°Th and l°Be/23°Th ratios in the sediments can be used as indicators of the intensity of scavenging of 231pa and l°Be (see [23] for a description of the normalization and the assumptions implicit therein; and also discussions in Section 3 below).…”

“…Since the rate of removal of 23°Th from seawater is relatively uniform throughout the ocean [18,19,22], a Pa/Th ratio in the sediment greater than the production ratio of 0.093 indicates that the site is a sink for 231pa (relative to 23°Th), receiving an extra amount of 231pa in addition to the 23apa produced in the overlying water column. Conversely, if the ratio is less than the production ratio, the site acts as a source with a certain fraction of 231pa produced in the overlying water column being laterally exported (relative to 230Th) to other area(s) in the ocean [23,24].…”

“…Similar to Pa/Th ratios, Be/Th ratios can be used as indicators of the intensity of scavenging of l°Be relative to 23°Th [23].…”

Opal burial in the equatorial Atlantic Ocean over the last 30 ka: Implications for glacial‐interglacial changes in the ocean silicon cycle

“…Thorium normalization works on the assumption that the flux of particulate 230 Th sinking to the ocean floor is approximately equal to its known rate of production from 234 U decay in the water column because the residence time of 230 Th in the water column is relatively short (on the order of a few decades) [Anderson et al, 1983b] compared to the timescale for lateral mixing in deep ocean basins [Anderson et al, 1990].…”