Anomalous AMS radiocarbon ages for foraminifera from high-deposition-rate ocean sediments

Radiocarbon ages on handpicked foraminifera from deep-sea cores are revealing that areas of rapid sediment accumulation are in some cases subject to hiatuses, reworking and perhaps secondary calcite deposition. We present here an extreme example of the impacts of such disturbances. The message is that if precise chronologies or meaningful benthic planktic age differences are to be obtained, then it is essential to document the reliability of radiocarbon ages by making both comparisons between coexisting species of planktomc foraminifera and detailed down-core sequences of measurements

Updated Nucleosynthesis Constraints on Unstable Relic Particles

We revisit the upper limits on the abundance of unstable massive relic particles provided by the success of Big-Bang Nucleosynthesis calculations. We use the cosmic microwave background data to constrain the baryon-to-photon ratio, and incorporate an extensively updated compilation of cross sections into a new calculation of the network of reactions induced by electromagnetic showers that create and destroy the light elements deuterium, he3, he4, li6 and li7. We derive analytic approximations that complement and check the full numerical calculations. Considerations of the abundances of he4 and li6 exclude exceptional regions of parameter space that would otherwise have been permitted by deuterium alone. We illustrate our results by applying them to massive gravitinos. If they weigh ~100 GeV, their primordial abundance should have been below about 10^{-13} of the total entropy. This would imply an upper limit on the reheating temperature of a few times 10^7 GeV, which could be a potential difficulty for some models of inflation. We discuss possible ways of evading this problem.Comment: 40 pages LaTeX, 18 eps figure

Anomalous ams radiocarbon ages for Foraminifera from high-deposition-rate ocean sediments

Radiocarbon ages on handpicked foraminifera from deep-sea cores are revealing that areas of rapid sediment accumulation are in some cases subject to hiatuses, reworking and perhaps secondary calcite deposition. We present here an extreme example of the impacts of such disturbances. The message is that if precise chronologies or meaningful benthic planktic age differences are to be obtained, then it is essential to document the reliability of radiocarbon ages by making both comparisons between coexisting species of planktomc foraminifera and detailed down-core sequences of measurements.This material was digitized as part of a cooperative project between Radiocarbon and the University of Arizona Libraries.The Radiocarbon archives are made available by Radiocarbon and the University of Arizona Libraries. Contact [email protected] for further information.Migrated from OJS platform February 202

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

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

Increased production of cosmogenic ¹⁰Be during the last glacial maximum

BERYLLIUM-10 (half-life 1.5 Myr) is produced by spallation of nitrogen and oxygen atoms by cosmic rays in the upper atmosphere. Its production rate is proportional to the flux of cosmic rays, which is modulated by solar activity and the strength of the Earth's magnetic field 1,2. Weakening of the magnetic field allows more cosmic rays to impinge on the Earth's atmosphere, thereby increasing Be-10 production. Here we report that the ocean-wide average accumulation rate of Be-10 in Pacific sediments, which reflects the global average production rate of Be-10 (ref. 3), was at least 25% greater during the height of the most recent ice age (approximately 24,000-16,000 yr ago) than during the Holocene (the past 10,000 yr). The higher production rate of Be-10 records the lower intensity of the geomagnetic field during that period and is consistent with the hypothesis developed to explain the younger C-14 ages of fossil corals compared with ages obtained by U-Th dating 4. These results also point to a more general need to consider variations in production rate in geochronological studies using other cosmogenic nuclides

New evidence from the South China Sea for an abrupt termination of the last glacial period

Abrupt changes in climatic conditions have been seen at high latitudes in the North Atlantic and the Antarctic at 13 kyr BP. It is important to determine whether this abrupt change was confined to high-latitude regions or whether it was global. Here we present results demonstrating an abrupt change in the rate and character of sedimentation in the South China Sea at the close of the last glacial period. Radiocarbon dating and its position in the oxygen isotope shift suggest that this change may be coincident with the changes found at high latitudes

The Influence of CaCO3 Dissolution on Core Top Radiocarbon Ages for Deep-Sea Sediments

Radiocarbon ages on CaCO3 from deep-sea cores offer constraints on the nature of the CaCO3 dissolution process. The idea is that the toll taken by dissolution on grains within the core top bioturbation zone should be in proportion to their time of residence in this zone. If so, dissolution would shift the mass distribution in favor of younger grains, thereby reducing the mean radiocarbon age for the grain ensemble. We have searched in vain for evidence supporting the existence of such an age reduction. Instead, we find that for water depths of more than 4 km in the tropical Pacific the radiocarbon age increases with the extent of dissolution. We can find no satisfactory steady state explanation and are forced to conclude that this increase must be the result of chemical erosion. The idea is that during the Holocene the rate of dissolution of CaCO3 has exceeded the rain rate of CaCO3. In this circumstance, bioturbation exhumes CaCO3 from the underlying glacial sediment and mixes it with CaCO3 raining from the sea surface

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. © 1990

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 of 10Be and 231Pa in the Pacific Ocean has identified intensified scavenging of the 10Be and 231Pa 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 of 10Be and 231Pa are clearly correlated to particle flux. Principal component analysis further suggests that scavenging of 10Be and 231Pa may be related to opal productivity in surface waters. A simple box model was constructed to partition the deposition of 230Th, 231Pa and 10Be between open ocean and ocean margin sediments. Model parameters were constrained using measured values of 230Th and 231Pa, which have a common source, and then applied to 10Be. An average Holocene 10Be deposition rate for the entire Pacific Ocean is estimated to be ∼ 1.5 × 106 atoms/cm2 yr-1, with ∼ 70% of the total 10Be supplied to the Pacific being deposited in margin sediments underlying only 10% of the ocean. The short residence times of 10Be in ocean margin regions (from < 100 to ∼ 200 yr) compared to the long 10Be residence time in the central open Pacific Ocean ( ∼ 1000 yr) reflects the intensified scavenging of 10Be 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 of 10Be; therefore, the average 10Be deposition rate in the Pacific Ocean can be used as an estimate of the global average production rate of 10Be in the atmosphere during the Holocene period. © 1992