H.J. Hofmann

Boundary scavenging in the Pacific Ocean: a comparison of 10Be and 231Pa

Concentrations of U, Th, 231Pa and 10Be 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 10Be from the Pacific Ocean. Deposition rates of 10Be 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 10Be 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 10Be.

PARTICULATE FLUXES OF 230TH, 231PA, AND 10BE IN THE NORTHEASTERN PACIFIC OCEAN

Abstract Measurements of 230Th, 231Pa, and 10Be in sediment trap samples and Holocene sediments collected along a transect in the northeastern Pacific indicate that the removal of these nuclides from the ocean is greatly influenced by boundary scavenging. More than 35% of the 230Th, at least 70% of the 231Pa, and as much as 85% of the 10Be fluxes collected by sediment traps at a nearshore site (located ~120 km off shore) are supplied by lateral transport of the dissolved nuclides from the open ocean. These findings are consistent with the hypothesis that boundary scavenging is proportional to the oceanic residence times of the nuclides (10–40 y for 230Th, ~ 100 y for 231 Pa, and ~ 1000 y for 10Be). Results of this study indicate that particle flux is a principal factor influencing scavenging of 230Th, 231Pa, and 10Be from the water column. The marine geochemistry of 10Be in ocean-margins is fundamentally different from that in the open ocean. Scavenging of 10Be occurs throughout the water column at the margin area in this study, in contrast to the pattern of 10Be being removed only in the surface waters and then remineralized at depth in the open ocean. Aluminosilicate may be a major phase removing 10Be from the water column in the open ocean but not at ocean-margins. Nuclide accumulation rates in the underlying Holocene sediments are all higher than the corresponding fluxes collected by sediment traps. Several possibilities that could account for this observation include: downslope sediment transport; deployment of the traps during a period when the particle flux was not representative of long-term average conditions; and scavenging of the nuclides from near bottom waters. The high accumulation rate of authigenic Mn in some Holocene sediments, which can be attributed to redox cycling of Mn, may have caused near bottom scavenging of 230Th and 231Pa as well as certain other trace elements (such as Ba, Cu, and Zn).

Transport and burial rates of 10Be and 231Pa in the Pacific Ocean during the Holocene period

An ocean-wide study of the rates of removal of10Be and231Pa in the Pacific Ocean has identified intensified scavenging of the10Be and231Pa in several ocean margin areas, including the Northeastern and Northwestern Pacific, the Bering Sea, the Eastern Equatorial Pacific and the South Pacific Ocean. Scavenging rates of10Be and231Pa are clearly correlated to particle flux. Principal component analysis further suggests that scavenging of10Be and231Pa may be related to opal productivity in surface waters. A simple box model was constructed to partition the deposition of230Th,231Pa and10Be between open ocean and ocean margin sediments. Model parameters were constrained using measured values of230Th and231Pa, which have a common source, and then applied to10Be. An average Holocene10Be deposition rate for the entire Pacific Ocean is estimated to be ∼ 1.5 × 106 atoms/cm2 yr−1, with ∼ 70% of the total10Be supplied to the Pacific being deposited in margin sediments underlying only 10% of the ocean. The short residence times of10Be in ocean margin regions (from < 100 to ∼ 200 yr) compared to the long10Be residence time in the central open Pacific Ocean ( ∼ 1000 yr) reflects the intensified scavenging of10Be in ocean margin waters. The results of this study suggest that the Pacific Ocean acts as a relatively closed basin with respect to the transport and burial of10Be; therefore, the average10Be deposition rate in the Pacific Ocean can be used as an estimate of the global average production rate of10Be in the atmosphere during the Holocene period.

10Be and9Be in South Atlantic DSDP Site 519: Relation to geomagnetic reversals and to sediment composition

A high-resolution10Be profile has been measured on DSDP Site 519 (central South Atlantic) in order to search for changes in the flux of10Be into a pelagic sediment during geomagnetic reversals as compared with times of constant magnetic polarity. The decay corrected10Be concentrations range from 1 to 9 × 108 at/g. The average10Be flux rate is 5 × 108 [at cm−2 kyr−1].9Be and CaCO3 were measured on the same samples. The10Be vs. depth curve is overall similar to the9Be vs. depth curve, while the CaCO3 vs. depth curve anticorrelates the beryllium isotope curves closely, implying that Be is carried mainly by the non-CaCO3 phases of the sediment. The positive correlation between10Be and9Be along with the fact that the average 10Be/9Be ratio of 5.5 × 10−8 is within the range of 10Be/9Be ratios reported for deep ocean water leads us to conclude (1) that the beryllium isotopes come close to an equilibrium in the open ocean, and (2) that the 10Be/9Be in deep sea sediments with a low detrital component monitors the 10Be/9Be of deep ocean water through time. Relative increases of the 10Be/9Be ratio are observed in the vicinity of the geomagnetic reversals, but the relationship between reversals and higher 10Be/9Be remains doubtful. Other mechanisms which could alter the 10Be/9Be ratio are discussed (input of10Be by melting of glacial ice; changing input of9Be due to changes in continental erosion; changing deep ocean circulation; changing scavenging activity at ocean margins), but as yet no conclusive explanation for the observed behavior of the beryllium isotopes can be given.

Fractionation, precision and accuracy in 14C and 13C measurements

Abstract During the past ten years, AMS has become a powerful tool in radiocarbon dating. In some applications, an overall accuracy comparable to that of conventional high precision low level counting is required. To achieve this accuracy, fractionation during sample preparation and measurement has to be constant. Comparison of series of 13 C 12 C ratios measured with AMS and a conventional mass spectrometer indicates that systematic errors in the carbon isotope ratio measurements are in the order of 0.1 to 0.2%. With mg size samples prepared by catalytic reduction on iron, delivering beam currents of about 12 μA for at least 1 h, statistical uncertainties for 14C of 0.3% and a total error of 0.4 to 0.5% for the 14 C 12 C ratio (including the calibration error and uncertainty in the background subtraction) for modern carbon has been obtained.

Precision measurements of 14C in AMS — some results and prospects

Abstract Some of the factors affecting the precision in AMS measurements will be discussed and the specific developments undertaken to reduce the errors at the ETH facility are described. Based on a large number of 14C measurements we show the present limitations of our system and consider what improvements might be possible. Further, a comparison is made between high precision measurements of 14C and current achievements relating to Be, Al and Cl.

The camp century 10Be record: Implications for long-term variations of the geomagnetic dipole moment

Abstract 10 Be concentrations measured in ice samples from Camp Century, Greenland, show short term variations which in general correspond to the 100–200 year “wiggles” in the 14 C tree ring record. There is, however, no evidence for a long term variation over the last 5000 years. This constancy is in contrast to the approximately sinusoidal variation of the atmospheric 14 C concentration which has generally been attributed to a changing geomagnetic dipole moment. This discrepancy implies that the 14 C trend might stem from other causes such as changes of oceanic circulation processes or from higher production rates during the Wisconsin rather than from variation in the geomagnetic field.

36Cl studies at the ETH/SIN-AMS facility

Abstract 36 Cl has been measured in polar ice from Dye 3 (65° 11 N, 43° 50 W) and Camp Century (77° 11 N, 82° 08W) in Greenland. In an exploratory study 4 series consisting of about 20 samples each have been selected covering approximately the following time periods: 12000-10000 BP (transition Wisconsin-Holocene), 1530–1730 a.d. (Maunder minimum), 1942–1977 a.d. (nuclear bomb pulse) and 1978 (seasonal variations). The results are compared with 10 Be data of the same cores which were measured previously. The variations of the 10 Be 36 Cl ratio are generally larger than expected from the experimental errors indicating that this ratio is not suitable for dating of old ice.

14C/12C-ratios in organic matter and hydrocarbons extracted from dated Lake sediments

Abstract Extractable organic matter (lipids) and hydrocarbons isolated from lake sediments were investigated by AMS by directly evaporating the extracts onto targets. Carbon-14 ages obtained for the postglacial sediments of Lake Baldegg (Switzerland) are in good agreement with results from pollen stratigraphy. The hydrocarbons contained in modern sediments of Lake Constance were found to be largely of fossil origin.

Detection of 32Si and 36Cl with the ETH/SIN EN-Tandem

Abstract Improvements made at the Zurich AMS-facility in the detecting technique of heavier cosmogenic radioisotopes are described. It is possible to perform now 36Cl measurements of natural samples with isotope ratios of greater than 10−14. The reproducibility of 36Cl measurements with a beam energy of 48 MeV is discussed. In the case of 32Si successful efforts were made to determine absolute isotope ratios of artificially enriched silicon samples.