R. Middleton

Sediment incorporation in island-arc magmas: Inferences from 10Be

Abstract The radioisotope 10Be has been used as a tracer to evaluate subduction and recycling of sediments in island arcs. As a cosmogenic isotope strongly enriched in oceanic sediments, it is especially suitable for monitoring sediment subduction. We report here 10Be results for 106 arc volcanic rocks and 33 basalts from mid-ocean ridges, oceanic islands, continental rifts, and continental flood basalt provinces. The 33 basalts from non-arc environments all contain Conceptually, four models may be used to explain the incorporation of 10Be in island arcs. Model 1 is the limiting case calculation presented in Brown et al. (1982). Physically, it corresponds to an assumption that all 10Be in the uppermost sediment layers is carried to depth, the 10Be is mechanically decoupled from the sediment pile and is added to the arc magma source region. This model results in the highest calculated 10Be contents in arc lavas. In the more physically reasonable Model 2, the 10Be-rich upper sediments are assumed to mix with the deeper, 10Be-poor, sediments and the sediments are subsequently incorporated into the source region. Model 3 assumes 10Be to be incorporated from sedimentary layers encountered during magma ascent. In Model 4 only sediments contained within grabens in the downgoing slab may be subducted. As yet insufficient data exist to permit conclusive evaluation of these models, but correlations between 10Be contents in arc volcanic rocks and recent sedimentation rates and sediment thickness, measured outboard of the trench, suggest that Model 2 may be the most important mechanism for 10Be incorporation in many island arcs.

Beryllium-10 Dating of the Duration and Retreat of the Last Pinedale Glacial Sequence

Accurate terrestrial glacial chronologies are needed for comparison with the marine record to establish the dynamics of global climate change during transitions from glacial to interglacial regimes. Cosmogenic beryllium-10 measurements in the Wind River Range indicate that the last glacial maximum (marine oxygen isotope stage 2) was achieved there by 21,700 � 700 beryllium-10 years and lasted 5900 years. Ages of a sequence of recessional moraines and striated bedrock surfaces show that the initial deglaciation was rapid and that the entire glacial system retreated 33 kilometers to the cirque basin by 12,100 � 500 beryllium-10 years.

Precise cosmogenic 10Be measurements in western North America: Support for a global Younger Dryas cooling event

The Inner Titcomb Lakes moraine in the Wind River Mountains, Wyoming, has been dated with a precision of 5% using accelerator mass spectrometric measurements of cosmogenic 10Be in surface samples from nine boulders resting on its crest. The age of this moraine is 11.4 ± 0.5 to 13.8 ± 0.6 ka; the range in age is due to the uncertainty in the 10Be production rate and uncertainties in rock erosion rate and shielding of cosmic rays by snow. Additional exposure ages on bedrock and boulder surfaces that are stratigraphically younger and older than the Inner Titcomb Lakes moraine conform with this age. The exposure ages of the Inner Titcomb Lakes moraine overlap the North Atlantic Younger Dryas event, 12.8 ± 0.2 to 11.5 ± 0.3 ka.

10Be analysis of a Quaternary weathering profile in the Virginia Piedmont

Samples from a residual weathering profile in the Virginia Piedmont have been analyzed for cosmogenic 10Be. Concentrations are highest in clay-rich soil and decrease exponentially to a depth of about 15 m. Despite uncertainties about the processes by which 10Be may be intercepted before entering the solum and eroded after incorporation, a minimum age may be calculated for the regolith. This calculation is based on the delivery rate of 10Be and its decay rate and suggests that this residual profile developed during a period no shorter than 8 × 105 yr. The calculated minimum age may be within a factor of 2 of maximum-age estimates based on surface lowering by erosion and on the rate of rock weathering to saprolite. The vertical distribution of 10Be in the profile could result from a steady-state balance of deposition, weathering, radioactive decay, and erosion.

10Be distribution in soils from Merced River terraces, California

Abstract The distribution and residence time of cosmogenic 10 Be in clay-rich soil horizons is fundamental to understanding and modelling the migration of 10 Be on terrestrial sediments and in groundwater solutions. We have analyzed seven profiles of clay-rich soils developed from terrace sediments of the Merced River, California. The terraces and soils of increasing age are used to compare the 10 Be inventory with a simple model of accumulation, decay and erosion. The data show that the distribution of 10 Be varies with soil horizon clay content, that the residence time of 10 Be in these horizons exceeds 10 5 years, and that to a rough approximation the inventory of 10 Be in a thoroughly sampled soil profile fits the equation: N = ( q − Em )(1 − e − λι )/ λ where q is delivery rate, E is erosion rate, m is the concentration of 10 Be in the eroding surface layer, λ is the decay constant, and t is the age of the depositional unit from which the soil has developed. The general applicability of this model is uncertain and warrants further testing in well-calibrated terrace sequences.

Revealing histories of exposure using in situ produced (super 26) Al and (super 10) Be in Libyan Desert glass.

We present the results of measurements of 26 Al and 10 Be produced in situ in 12 samples of Libyan Desert Glass by cosmic rays during the last ten million years. Based on the variability of the concentrations of 10 Be and of the 26 Al/ 10 Be ratios we measured, we conclude that individual fragments of glass have experienced different exposure histories, implying several major redistributions of the glass within the past 10 6 years. The 26 Al and 10 Be concentrations are inconsistent with the theoretical estimates of the rates of in situ production. We estimate minimum production rates of 70 atoms g -1 yr -1 and 10 atoms g -1 yr -1 for 26 Al and 10 Be, respectively, produced in quartz at sea level between 60–90° latitude. Despite the present uncertainty in the rates of production, we feel that these results show clearly the effectiveness of in situ produced 26 Al and 10 Be in studying earth-surface processes.

Atmospheric deposition of 7Be and 10Be

Abstract Measurements of 10 Be in precipitation taken in Hawaii, Illinois and New Jersey over a period of five years are reported. The problem of contamination by the isotope being resuspended on wind blown soil that is also collected is addressed. Rain collected at Mauna Loa, Hawaii has such low values of dust contamination that it has been taken as clean, and the data from Illinois and New Jersey are evaluated on that assumption. The conclusion is that the deposition in a given amount of rain for the non-resuspended component is the same for all three stations, and we propose that the annual rate for mid-latitude locations having moderate rainfall is proportional to the local rainfall. 7 Be, which is probably negligibly contributed to the measurements by soil contamination, was measured for individual rains in Illinois and found to have a deposition of 1.4 × 10 4 atom/cm 3 . We have found that concentration variations between precipitation events greater than a factor of 20 exist for both isotopes and that relatively rare, high concentration events dominate deposition, thereby requiring long periods of observation to avoid significant error. Based on our own and other data we conclude that the best value for 10 Be deposition is 1.5 × 10 4 atom/cm 3 , uncertain by 20%, and for 7 Be is 1.2 × 10 4 atom/cm 3 , uncertain by 25%. A global average deposition rate cannot be inferred directly for either isotope from these kinds of data; however, the theoretical global deposition rate for 10 Be is shown to be consistent with the deposition reported here, if the concentration in equatorial rain is about 3300 atom/g.

On 10Be standards and the half-life of 10Be

Using accelerator mass spectrometry, we have made absolute measurements of the ratios of 10Be to 9Be in the National Institute of Standards and Technology (NIST) standard reference material SRM 4325, and in material produced by ICN. We obtain for SRM 4325 a value that is 14% higher than that claimed by the NIST (30.6 ± 1.4 × 10−12 instead of 26.8 ± 1.4 × 10−12, 2σ uncertainty). Our absolute values for the ratios of 10Be to 9Be, and the activity measurements of the NIST and ICN, yield values for the half-life of 10Be of 1.53 ± 5% and 1.48 ± 5% Ma, respectively.

10Be accumulation in a soil chronosequence

We have measured the concentration of the cosmogenic isotope10Be in soil samples from various horizons at six sites, including three independently dated Rappahannock River terraces and a previously undated Piedmont soil to which we have assigned an age. All of the incident10Be can be accounted for in one of these soils and a second is within a factor of two. In three soils, whose concentrations vary widely with depth, a significant fraction of the incident10Be cannot be accounted for. Incomplete sampling, and enhanced Be mobility caused by organic components, are the probable reasons for the low inventory of Be from these three soils. Overall, the data from these six sites indicate that10Be accumulation could be used to assign ages to soils if Be is not mobilized and lost from the soil profile.

A review of ion sources for accelerator mass spectrometry

Abstract Accelerator mass spectrometry places unusually difficult demands on ion source design and possible ways of meeting these are discussed. A brief review of the state-of-the-art methods of generating negative ion beams of beryllium, carbon, aluminum, chlorine and calcium are presented. A new method of producing 20–25 μA of 12C− ions from CO2 gas is described in detail with emphasis on accelerator 14C dating. Ionization efficiency is high (∼ 7.7%) and it may be possible to date samples containing 100 μg of 12C (i.e. ∼ 0.2 at. cm3 of CO2). Accelerator measurements with contemporary CO2 followed by CO2 from anthracite enabled the age of the latter to be determined to be > 39000 years without background subtraction.