Radionuclides in the water column at stations near the Antarctic Polar Front in the South Atlantic

Vertical profiles of dissolved and particulate 230Th and 231Pa were obtained across the Antarctic Circumpolar Current (ACC) in the southern Atlantic. North of the Polar Front, dissolved and total 230Th increase with depth in conformity with published scavenging models. There is no depletion of 230Th or 231Pa in the water column south of the Polar Front, thought to be an area of enhanced biological productivity. 230Th concentrations increase three-fold to the Weddell Sea across the ACC. Dissolved and total 231Pa concentrations are relatively constant below 500 m depth at about 0.3 dpm m**-3, and change little with depth or latitude. The results from the Weddell Gyre are explained by a mixing-scavenging model that takes into account the input of lower Circumpolar Deep Water through upwelling, which is the main source of water in the Weddell Gyre and is enriched in 230Th but not in 231Pa. 230Th accumulates in the Weddell Gyre as a result of a reduction in the scavenging rate and by ingrowth from 234U. Ingrowth is more significant for 230Th than for 231Pa because the residence time of water in the gyre (about 35 years) is similar to the scavenging residence time of Th in the south Atlantic (29 years) but shorter than that of Pa (120 years). It is argued that changes in 230Th accumulation in the past may reflect changes in water residence time and in the formation rate of Weddell Sea Deep Water.

Uranium radionuclides and organic biomarkers in sediment core RC27-42

Uranium series radionuclides and organic biomarkers, which represent major groups of planktonic organisms, were measured in western Arabian Sea sediments that span the past 28 ka. Variability in the past strength of the southwest and northeast monsoons and its influence on primary productivity, sea surface temperature (SST), and planktonic community structure were investigated. The average alkenone-derived SST for the last glacial period was ~3°C lower than that measured for the Holocene. Prior to the deglacial, the lowest SSTs coincide with the highest measured fluxes of organic biomarkers, which represent primarily a planktonic suite of diatoms, coccolithophorids, dinoflagellates, and zooplankton. We propose that intensification of winter northeast monsoon winds during the last glacial period resulted in deep convective mixing, cold SSTs and enhanced primary productivity. In contrast, postdeglacial (<17 ka) SSTs are warmer during times in which biomarker fluxes are high. Associated with this transition is a planktonic community structure change, in which the ratio of the average cumulative flux of diatom biomarkers to the cumulative flux of coccolithophorid biomarkers is twice as high during the deglacial and Holocene than the average ratio during the last glacial period. We suggest that this temporal transition represents a shift from a winter northeast monsoon-dominated (pre-17 ka) to a summer southwest monsoon-dominated (post-17 ka) wind system.

Radionuclides measured on water bottle samples

Actinium is one of the rarest naturally occurring elements on earth. We measured its longest-lived isotope 227Ac (half-life 21.77 yr) for the first time in the water column of the Southeast Pacific, the Central Arctic, the Antarctic Circumpolar Current (ACC) and the Weddell Gyre (WG). Besides the profile in the Southeast Pacific, which confirms earlier findings about the role of diapycnal mixing for 227Ac distribution, we found three other different types of vertical profiles. These profiles point to a prominent role of advection for 227Ac distribution, especially in the Southern Ocean. Depending on the type of profile found, 227Ac is proposed as a tracer for different oceanographic questions. In the Southern Ocean, up to 4.93±0.32 dpm/m**3 227Ac is found close to the sea floor, which is the highest concentration ever observed in the ocean. Close to the sea surface in the WG, 0.46±0.05 dpm/m**3 227Acex (227Ac in excess of its progenitor 231Pa) is detected. We use 227Acex there to determine the upwelling velocity in the Eastern WG to be about 55 m/yr. In the ACC, Upper and Lower Circumpolar Deep Water (UCDW and LCDW) are found to differ clearly in their 227Acex activity. High 227Acex activities are therefore a promising tracer for recent inputs of LCDW to the sea surface, which may help to understand the role of deep upwelling for iron inputs into Antarctic surface waters. The expected release of 227Ac is compared with 228Ra to make sure that the large near-surface excess in the water column of the Southern Ocean is not due to lateral inputs by isopycnal mixing. Data from the Central Arctic and from a transect across the ACC confirm that 228Ra and 227Acex differ strongly in their sources. The first measurements of 227Ac on suspended matter (less than 1.7% of total 227Ac close to the sea floor) indicate that the particle reactivity of 227Ac is negligible in the open ocean, in agreement with earlier findings [Y. Nozaki, Nature 310 (1984) 486-488]. Despite the extremely low concentrations of 227Ac, new measurement techniques [W.S. Moore, R. Arnold, J. Geophys. Res. 101 (1996) 1321-1329] point to a comfortable and comparably simple determination of 227Ac in the future. Finally, 227Acex may become a widely used deep-sea specific tracer.

Radionuclides measured in surface water samples from the South Atlantic

In the nutrient-rich Southern Ocean, Fe is a vital constituent controlling the growth of phytoplankton. Despite much effort, the origin and transport of Fe to the oceans are not well understood. In this study we address the issue with geochemical data and Nd isotopic compositions of suspended particle samples collected from 1997 to 1999 in the South Atlantic Sector of the Southern Ocean. Al, Th, and rare earth element (REE) concentrations as well as 143Nd/144Nd isotopic ratios in acetic acid-leached particle samples representing the lithogenic fraction delineate three major sources: (1) Patagonia and the Antarctic Peninsula provide material with eNd > -4 that is transported toward the east with the polar and subpolar front jets, (2) the south African shelf, although its influence is limited by the circumpolar circulation and wind direction, can account for material with eNd of -12 to -14 adjacent to South Africa, and (3) East Antarctica provides material with eNd of -10 to -15 to the eastern Weddell Sea and adjacent Antarctic Circumpolar Current. For this region we interpret the Nd isotopic evidence in combination with oceanographic/atmospheric constraints as evidence for supply of significant amounts of terrigenous detritus by icebergs.

Gamma-emitting radionuclides in tuna samples from the Tokyo Central Fish Market, 1962.

Mode of access: Internet.

Release and movement of radionuclides in soils contaminated with fallout materials from an underground thermonuclear detonation /

Project Sedan.

Behavior of certain radionuclides released into fresh-water environments; annual report.

Public Health Service publication no. 999-RH.

A review of potential radionuclides produced in weapons detonations.

Mode of access: Internet.

Metabolism of selected radionuclides fed in various physical forms to animals; final report,

AD626320.

Radionuclides and sedimentation rates of sediment cores from the eastern South Atlantic

High-resolution records of the natural radionuclide 230Th were measured in sediments from the eastern Atlantic sector of the Antarctic circumpolar current to obtain a detailed reconstruction of the sedimentation history of this key area for global climate change during the late Quaternary. High-resolution dating rests on the assumption that the 230Thex flux to the sediments is constant. Short periods of drastically increased sediment accumulation rates (up to a factor of 8) were determined in the sediments of the Antarctic zone during the climate optima at the beginning of the Holocene and the isotope stage 5e. By comparing expected and measured accumulation rate of 230Thex, lateral sediment redistribution was quantified and vertical particle rain rates originating from the surface water above were calculated. We show that lateral contributions locally were up to 6.5 times higher than the vertical particle rain rates. At other locations only 15% of the expected vertical particle rain rate were deposited.

Radionuclides, Nd isotopes and biogenic Opal in Atlanic Ocean sediments

Reconstructing past modes of ocean circulation is an essential task in paleoclimatology and paleoceanography. To this end, we combine two sedimentary proxies, Nd isotopes (epsilon-Nd) and the 231Pa/230Th ratio, both of which are not directly involved in the global carbon cycle, but allow the reconstruction of water mass provenance and provide information about the past strength of overturning circulation, respectively. In this study, combined 231Pa/230Th and epsilon-Nd down-core profiles from six Atlantic Ocean sediment cores are presented. The data set is complemented by the two available combined data sets from the literature. From this we derive a comprehensive picture of spatial and temporal patterns and the dynamic changes of the Atlantic Meridional Overturning Circulation over the past ~25 ka. Our results provide evidence for a consistent pattern of glacial/stadial advances of Southern Sourced Water along with a northward circulation mode for all cores in the deeper (>3000 m) Atlantic. Results from shallower core sites support an active overturning cell of shoaled Northern Sourced Water during the LGM and the subsequent deglaciation. Furthermore, we report evidence for a short-lived period of intensified AMOC in the early Holocene.