David Elmore

Scaling factors for the rates of production of cosmogenic nuclides for geometric shielding and attenuation at depth on sloped surfaces

Abstract The decrease in rates of production of cosmogenic nuclides occurs because of shielding of cosmic rays by mountains, sloped surfaces, and local rock formations that block them. When a large part of the sky is blocked, this correction is large and requires detailed model calculations. This paper considers three geometries: a rectangular obstruction, a triangular obstruction, and a sloped surface. Other geometries can be considered as a combination of these. The results are presented in terms of formulas and graphs so that the reader can easily apply them to common field situations. Any use of cosmogenic nuclides in the study of geomorphic processes or forms must consider factors that introduce variations in the production of nuclides.

Landscape preservation under Fennoscandian ice sheets determined from in situ produced 10Be and 26Al

Some areas within ice sheet boundaries retain pre-existing landforms and thus either remained as ice free islands (nunataks) during glaciation, or were preserved under ice. Differentiating between these alternatives has significant implications for paleoenvironment, ice sheet surface elevation, and ice volume reconstructions. In the northern Swedish mountains, in situ cosmogenic 10Be and 26Al concentrations from glacial erratics on relict surfaces as well as glacially eroded bedrock adjacent to these surfaces, provide consistent last deglaciation exposure ages (∼8–13 kyr), confirming ice sheet overriding as opposed to ice free conditions. However, these ages contrast with exposure ages of 34–61 kyr on bedrock surfaces in these same relict areas, demonstrating that relict areas were preserved with little erosion through multiple glacial cycles. Based on the difference in radioactive decay between 26Al and 10Be, the measured nuclide concentration in one of these bedrock surfaces suggests that it remained largely unmodified for a minimum period of 845−418+461 kyr. These results indicate that relict areas need to be accounted for as frozen bed patches in basal boundary conditions for ice sheet models, and in landscape development models. Subglacial preservation also implies that source areas for glacial sediments in ocean cores are considerably smaller than the total area covered by ice sheets. These relict areas also have significance as potential long-term subglacial biologic refugia.

In vivo degradation of 14C-labeled small intestinal submucosa (SIS) when used for urinary bladder repair.

The rate of in vivo degradation was determined for a naturally occurring biomaterial derived from the extracellular matrix of the small intestinal submucosa (SIS). The SIS was labeled by giving weekly intravenous injections of 10 microCi of 14C-proline to piglets from 3 weeks of age until the time of sacrifice at 26 weeks. The resultant SIS prepared from these pigs contained approximately 10(3) fold more 14C than unlabeled tissues. The labeled SIS was used to repair experimental defects in the urinary bladder of 10 dogs. The animals were sacrificed at post-operative times ranging from 3 days to 1 year and the remodeled urinary bladder tissue was harvested for evaluation of 14C by a combination of liquid scintillation counting and accelerator mass spectrometry. The remodeled tissue contained less than 10% of the 14C (disintegrations per minute/gram tissue wet weight) at 3 months post-surgery compared to the SIS biomaterial that was originally implanted. The SIS scaffold was replaced by host tissue that resembled normal bladder both in structure and function. After implantation, 14C was detected in highest concentrations in the blood and the urine. The SIS bioscaffold provides a temporary scaffold for tissue remodeling with rapid host tissue remodeling, degradation, and elimination via the urine when used as a urinary bladder repair device.

In vivo absorption of aluminium-containing vaccine adjuvants using 26Al

Aluminium hydroxide (AH) and aluminium phosphate (AP) adjuvants, labelled with 26Al, were injected intramuscularly (i.m.) in New Zealand White rabbits. Blood and urine samples were collected for 28 days and analysed for 26Al using accelerator mass spectrometry to determine the absorption and elimination of AH and AP adjuvants. 26Al was present in the first blood sample (1 h) for both adjuvants. The area under the blood level curve for 28 days indicates that three times more aluminium was absorbed from AP adjuvant than AH adjuvant. The distribution profile of aluminium to tissues was the same for both adjuvants (kidney > spleen > liver > heart > lymph node > brain). This study has demonstrated that in vivo mechanisms are available to eliminate aluminium-containing adjuvants after i.m. administration. In addition, the pharmacokinetic profiles of AH and AP adjuvants are different.

A preliminary study of the dermal absorption of aluminium from antiperspirants using aluminium-26

Aluminium chlorohydrate (ACH), the active ingredient in many antiperspirants, was labeled with the radioisotope 26Al. The labeled ACH was then fractionated into about 100 samples using gel filtration chromatography. Each fraction was analyzed for 26Al and total aluminium content. Aluminium-26 was only detected in the fractions that also contained aluminium, which verified that the ACH was uniformly labeled. 84 mg of the labeled ACH was then applied to a single underarm of two adult subjects with blood and urine samples being collected over 7 weeks. Tape-stripping and mild washings of the skin were also collected for the first 6 days. Results indicate that only 0.012% of the applied aluminium was absorbed through the skin. At this rate, about 4 microg of aluminium is absorbed from a single use of ACH on both underarms. This is about 2.5% of the aluminium typically absorbed by the gut from food over the same time period. Therefore, a one-time use of ACH applied to the skin is not a significant contribution to the body burden of aluminium.

Variable responses of western U.S. glaciers during the last deglaciation

Cosmogenic 10 Be exposure ages from moraines in the Wallowa Mountains, Oregon, identify two maximal late Pleistocene glaciations at 21.1 6 0.4 ka and 17.0 6 0.3 ka and a minor glacial event at 10.2 6 0.6 ka. Our new high-resolution chronology, integrated with other well-dated glacial records from the western United States, demonstrates sub- stantial differences in the synoptic responses of western U.S. glaciers to climate forcing associated with the global Last Glacial Maximum and subsequent millennial-scale events originating in the North Atlantic region. These variable synoptic glacier responses identify large changes in the relative contributions of regional to global controls on the climate of the western United States that accompanied the deglaciation.

Development of 36Cl standards for AMS

Abstract Large-quantity dilutions of the National Institute of Standards and Technology (NIST) 36Cl standard (SRM 4943) to 36 Cl Cl ratios of 5.000 × 10−13, 1.600 × 10−12, 5.003 × 10−12 and 1.000 × 10−11 have been prepared for AMS standards. Dilutions were made with an estimated uncertainty better than 1% and were then checked in three separate runs using the AMS facility at the University of Rochester. The results show excellent agreement between the gravimetric dilution factors and AMS measurements.

Cosmogenic 36Cl accumulation in unstable landforms: 2. Simulations and measurements on eroding moraines

Cosmogenic 36Cl surface exposure ages obtained for multiple boulders from single landforms are usually characterized by a variance larger than that of the analytical methods employed. This excessive boulder-to-boulder variability, progressively more profound with increasing age of landforms, is due to removal of soil and gradual exposure of boulders at the surface. In our gradual exposure model, boulders are initially buried in moraine matrix. With time, erosion lowers the moraine surface and the boulders are gradually exposed to cosmic rays. Because the cosmic ray intensity changes with depth, the boulders are subjected to variable production rates of the cosmogenic 36Cl. Initial depth of boulders and their chemical composition are variable, which results in different amounts of the accumulated cosmogenic 36Cl and thus different apparent ages of boulders. The shape of the resulting distribution of the apparent ages and the coefficient of variation depend on the erosion depth, while the first moment is a function of the true surface age and the erosion depth. These properties of the apparent age distributions permit calculation of the surface age, the erosion depth, and also the average erosion rate. We tested the model calculations using 26 boulders from a late Pleistocene moraine at Bishop Creek, Sierra Nevada, California. The set exhibited a bimodal distribution of the 36Cl surface exposure ages. We interpreted the older mode as the result of gradual exposure and the younger one as the result of surficial processes other than soil removal. The 10 samples that constitute the older mode produced a distribution which closely matches the modeled distribution calculated using an age of 85 kyr and erosion depth of 570 g cm−2. This age is the same as an independent estimate obtained from cation ratio studies, and the calculated erosion depth is very close to the erosion depth of 600 g cm−2 based on a simple analytical model of soil erosion. These results indicate that our statistical model adequately describes effects of soil erosion on accumulation of cosmogenic 36Cl. The approach can be used to simultaneously obtain the true landform age and the erosion rate from apparent 36Cl ages and thereifore may help in evaluation of surface exposure ages of eroding landforms.

Cosmogenic 3He and 10Be chronologies of the late Pinedale northern Yellowstone ice cap, Montana, USA

Cosmogenic 3 He and 10 Be ages measured on surface boulders from the moraine sequence deposited by the northern outlet glacier of the Yellowstone ice cap indicate that the outlet glacier reached its terminal position at 16.5 6 0.4 3 He ka and 16.2 6 0.3 10 Be ka, respectively. Concordance of these ages supports the scaled production rates used for 3 He (118.6 6 6.6 atoms · g 21 ·y r 21 ) and 10 Be (5.1 6 0.3 atoms · g 21 ·y r 21 )( 62s at high latitudes at sea level). Two recessional moraines upvalley from the terminal moraine have mean ages of 15.7 6 0.5 10 Be ka and 14.0 6 0.4 10 Be ka, respectively, and a late-glacial flood bar was deposited at 13.7 6 0.5 10 Be ka. These cosmogenic chronologies identify a late Pinedale glacial maximum in northern Yellowstone that is significantly younger than previously thought, and they suggest deglaciation of the Yellowstone plateau by ;14 10 Be ka.

Glacial geology and chronology of Bishop Creek and vicinity, eastern Sierra Nevada, California

The valley of Bishop Creek, which drains part of the eastern flank of the Sierra Nevada, California, contains an unusually well-preserved set of middle to late Quaternary moraines. These deposits have been mapped by previous investigators, but they have not been quantitatively dated. We used the accumulation of cosmogenic 36 Cl to assign a chronology to the maximal glacial positions mapped in the valley. Our results indicate that the terminal moraines mapped by previous investigators as Tahoe were all deposited between ca. 165 and ca. 135 ka, during marine isotope stage (MIS) 6. Moraines mapped as Tioga were deposited between 28 and 14 ka, during MIS 2. These can be subdivided into Tioga 1 (28–24 ka), Tioga 3 (18.5–17.0 ka), and Tioga 4 (16.0–14.5 ka) advances (no moraines dated to Tioga 2 [21–19 ka] were found, presumably because the Tioga 3 advance either overrode or fluvially eroded them). At 15.0–14.5 ka, the Tioga 4 glacier retreated abruptly to the crest of the range. This was followed by the brief and fairly minor Recess Peak advance at ca. 13.4 ka. No Holocene advances extended beyond the very restricted limits of ice during the Matthes (Little Ice Age) advance. All preserved terminal moraines at lower elevations were deposited during either the Tahoe or Tioga stades. The Tahoe terminal moraines are extensive and voluminous, whereas the Tioga moraines are relatively narrow and have small volumes. However, this notable difference may be more a result of idiosyncrasies in the local glacial history than the result of differences in the length or intensity of glaciation between the two glacial episodes. The history of glacial advances at Bishop Creek exhibits a strong correspondence to global climate cycles, and to paleoclimate events in the North Atlantic in particular.