Minimum Exposure Ages Research Articles

Fault kinematics and active tectonics at the southeastern boundary of the eastern Alborz (Abr and Khij fault zones): Geodynamic implications for NNE Iran

The Alborz is a region of active deformation within the Arabia–Eurasia collision zone. The Abr and the Khij Faults are two NE-trending left-lateral strike-slip faults in the eastern Alborz that correspond to the Shahrud fault system extended through an area of about 95 km × 55 km. Tectonic landforms typically associated with active strike-slip faults, such as deflected stream channels, offset ridges and fault scarps are documented along the mentioned faults. Detailed analyses of satellite images and digital topographic data accompanied by field surveys allowed us to measure horizontal offsets of about 420 ± 50 m and 400 ± 50 m for the Abr and Khij Faults, respectively. A total of 8 quartz-rich samples were sampled and dated from two different fan surfaces using in situ-produced 10Be cosmogenic dating method. Minimum exposure ages for the abandonment of the alluvial fan surfaces of 115 ± 14 kyr along the Abr Fault and of 230 ± 16 kyr along the Khij Fault imply that both faults are active with slip rates of about 3–4 mm yr −1 and 1–3 mm yr −1, respectively. The results of our study provide the first direct quantitative geological estimates of slip rate along these two active faults and place a new constraint on slip distribution between the faults in the eastern Alborz. Fault kinematic studies (from fault slip data) indicate a N35°E-trending maximum stress axis comprising a dominant strike-slip regime in agreement with the geomorphological analyses. The left-lateral strike-slip faulting along the Abr and Khij Faults and their associated fault zones in the eastern Alborz can be due to the westward component of motion of the South Caspian Basin with respect to Eurasia and Central Iran.

Cosmogenic radionuclide chronology of pre-last glacial cycle moraines in the Western Arthur range, Southwest Tasmania

Cosmogenic 10Be and 26Al exposure ages of 12 boulders from two moraine complexes in the Western Arthur Range of southwest Tasmania, which previously were considered to have been deposited during the Last Glacial Maximum, predate the Last Glacial Cycle. Zero-erosion minimum exposure ages range from 95 ka to 232 ka based on weighted mean 10Be and 26Al ages per boulder. For a reasonable choice of erosion rates, 10Be boulder ages range from 105 to 326 ka, respectively. Although a direct association of moraine construction to a specific marine isotope stage (MIS) glaciation is not definitive, erosion-corrected exposure ages indicate glacial advances commensurate with MIS-6 and -10, although the latter advance may be attributable to MIS-8 if the erosion rate correction is an over-estimate. Given the relatively close proximity of the dated moraines to their source cirques, this new cosmogenic glacial chronology implies a very limited extent for any younger glacial advances that occurred in the Western Arthur Range during the Last Glacial Cycle (post MIS-5) including the global LGM.

Minimum Bedrock Exposure Ages and Their Implications: Larsemann Hills and Neighboring Bolingen Islands, East Antarctica

Abstract:Considerable controversy exists over whether or not extensive glaciation occurred during the global Last Glacial Maximum (LGM) in the Larsemann Hills. In this study we use the in situ produced cosmogenic nuclide 10Be (half life 1.51 Ma) to provide minimum exposure ages for six bedrock samples and one erratic boulder in order to determine the last period of deglaciation in the Larsemann Hills and on the neighboring Bolingen Islands. Three bedrock samples taken from Friendship Mountain (the highest peak on the Mirror Peninsula, Larsemann Hills; ∼2 km from the ice sheet) have minimum exposure ages ranging from 40.0 to 44.7 ka. The erratic boulder from Peak 106 (just at the edge of the ice sheet) has a younger minimum exposure age of only 8.8 ka. The minimum exposure ages for two bedrock samples from Blundell Peak (the highest peak on Stornes Peninsula, Larsemann Hills; ∼2 km from the ice sheet) are about 17 and 18 ka. On the Bolingen Islands (southwest to the Larsemann Hills; ∼10 km from the ice sheet), the minimum exposure age for one bedrock sample is similar to that at Friendship Mountain (i.e., 44 ka). Our results indicate that the bedrock exposure in the Larsemann Hills and on the neighboring Bolingen Islands commenced obviously before the global LGM (i.e., 20–22 ka), and the bedrock erosion rates at the Antarctic coast areas may be obviously higher than in the interior land.

Age of Unstable Bedrock Landforms Southwest of Yucca Mountain, Nevada, and Implications for Past Ground Motions

We determine minimum exposure ages for unstable outcrops at three sites in Amargosa Desert, southwestern Nevada, including a site at the southern end of Yucca Mountain. Varnish microlamination dating techniques provide minimum exposure ages of 12.5–36 k.y. for the unstable outcrops of welded tuff, including a 24 k.y. age for the south Yucca Mountain site. The youngest exposure age (12.5 k.y.) is found at the site located only 10 km from the Death Valley–Furnace Creek fault, suggesting outcrops may be more frequently modified when close to major active earthquake sources. A simplistic stability assessment of the south Yucca Mountain outcrops suggests peak ground accelerations (PGAs) may not have exceeded about 1 g (uncertainty bounds 0.5–2 g ) in at least 24 k.y. A PGA of 1 g is consistent with the predicted 24 k.y. return period PGAs from the near decade-old Yucca Mountain probabilistic seismic hazard (PSH) model, except for the ninety-fifth percentile and above. We gain confidence in our interpretations by additionally observing: (1) minimal damage to the south Yucca Mountain outcrops from a recent moderate earthquake that is estimated to have produced a PGA of less than 0.1 g there (i.e., motions less than 0.5 g do not significantly damage the outcrops); and (2) severe damage to similar volcanic outcrops associated with PGAs of the order 0.5–1 g near a nuclear blast site from the 1960s. These observations support our suggestion that PGAs greater than 0.5–1 g have not occurred at the south Yucca Mountain site for a time period of at least 24 k.y. Significant seismic events that substantially modify the outcrops and produce associated rubble fields may therefore occur on longer time scales.

Cosmogenic 21Ne concentrations and exposure ages of summit bedrocks in the Grove Mountains, Antarctica

Various sources of 21Ne and 22Ne exist in surface rocks: cosmogenic, in situ nucleogenic from internal U and Th, trapped crustal nucleogenic and trapped atmospheric. This paper reports the first measurement, in China, of cosmogenic 21Ne and 22Ne in surface bedrocks. We developed a unique sample pre-treatment procedure that effectively removed inclusions inside quartz grains, and thus maximally reduced nucleogenic contributions of 21Ne and 22Ne. Step-heating experiments show that concentrations of cosmogenic 21Ne and 22Ne in summit bedrock samples R9202 and R9203 from Grove Mountains, Antarctica, are (3.83±0.87)×108 and (5.2280.51)×108 atoms/g, respectively. The corresponding minimum exposure ages are 2.2±0.5 and 3.0±0.3 Ma. This indicates that the ice sheet in East Antarctica was uncovered the crest of Mount Harding, a typical nunatak in Grove Mountains, since at least mid-Pliocene.

10Be exposure ages of a rock avalanche and a late glacial moraine in Alta Valtellina, Italian Alps

A rock avalanche deposit was investigated in order to understand the chronological evolution of geological hazards and to evaluate the interaction of the triggering geodynamic processes in the valley Val Viola, Italian Alps. The deposit is situated west of the Alpe Dosdé, in a permafrost area with deep-seated gravitational deformations (DSGD) along a tectonic line. Based on its geomorphologic context, the rock avalanche was first interpreted as a result of slope stress release without exact timing. This hypothesis was tested by measuring the 10Be exposure date of quartz from one boulder from the rock avalanche. The age of 7430±460 years places the event in the early Holocene. The timing of the last deglaciation was constrained using the inner late glacial moraine of a moraine doublet in the valley Alpe Dosdé situated at an altitude between 2140 and 2120 m a.s.l. west of the rock avalanche. The 10Be concentrations of quartz yield minimum exposure ages of 11,480±670 and 10,850±820 years. Different proposals for potential triggering factors of the rock avalanche include (a) melting of the local valley glacier and slope stress release in the Val Viola, likely to play a minor role as trigger, because of the time delay between the deglaciation and the rock avalanche event. More likely are (b) enhanced crustal seismicity induced by post-glacial regional isostatic glacial rebound coupled with tectonic stress or/and (c) climate conditions with higher temperatures around 7430±460 years, resulting in an upwards movement of the permafrost limit and destabilization of the rock walls.

Cosmogenic 10Be, 21Ne and 36Cl in sanidine and quartz from Chilean ignimbrites

Our initial results indicate that three cosmogenic nuclides: 10Be, 21Ne and 36Cl can be analyzed in sanidine. To uncover complex exposure histories or marked changes in denudation rates over time several nuclides with different half-lives (or stable) must be measured. Because of its shorter half-life, the combination of 36Cl and a long-lived nuclide 10Be or stable nuclide 21Ne will provide more information than the pairs 10Be and 26Al or 10Be and 21Ne (in quartz). Sanidine (alkali feldspar) is a common high temperature mineral and often dominates the phenocryst assemblage in silicic to intermediate volcanic rocks. Bedrock surfaces studied come from the Oxaya (erupted 19–23Ma) and Lauca (erupted 2.7Ma) ignimbrites of northern Chile. Quartz and sanidine phenocrysts coexist; therefore, we can check the viability of sanidine through direct comparison with nuclide concentrations in quartz. In addition, as quartz has no target for 36Cl in significant abundance we show that the unique power of sanidine is that 36Cl can be measured. We have obtained very good agreement between 10Be and 21Ne concentrations measured in sanidine and coexisting quartz. No meteoric 10Be was apparent in these sanidines. Concentrations of all three nuclides in mineral separates from rock sample CN309 from the Lauca ignimbrite in the Western Cordillera agree well and correspond to minimum exposure ages of 30–50ka. 10Be and 21Ne measured in both sanidine and quartz from three rock samples from the Oxaya ignimbrite (CN19, CN23, CN104a) in the Western Escarpment record low average landscape modification rates (<0.70m/Ma) over the last several million years. In contrast, 36Cl data from sanidine in CN23 seem to indicate shorter minimum exposures and more rapid maximum erosion rates.

Determining eruption ages and erosion rates of Quaternary basaltic volcanism from combined U-series disequilibria and cosmogenic exposure ages

Research Article| May 01, 2007 Determining eruption ages and erosion rates of Quaternary basaltic volcanism from combined U-series disequilibria and cosmogenic exposure ages Kenneth W.W. Sims; Kenneth W.W. Sims 1Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA Search for other works by this author on: GSW Google Scholar Robert P. Ackert, Jr.; Robert P. Ackert, Jr. 2Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA Search for other works by this author on: GSW Google Scholar Frank C. Ramos; Frank C. Ramos 3Department of Geological Sciences, Central Washington University, Ellensburg, Washington 98926, USA Search for other works by this author on: GSW Google Scholar Robert A. Sohn; Robert A. Sohn 4Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA Search for other works by this author on: GSW Google Scholar Michael T. Murrell; Michael T. Murrell 5Nuclear and Radiochemistry, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA Search for other works by this author on: GSW Google Scholar Donald J. DePaolo Donald J. DePaolo 6Department of Earth and Planetary Science, MC4767, University of California, Berkeley, California 94720-4767, USA Search for other works by this author on: GSW Google Scholar Geology (2007) 35 (5): 471–474. https://doi.org/10.1130/G23381A.1 Article history received: 20 Sep 2006 rev-recd: 26 Dec 2006 accepted: 04 Jan 2007 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Kenneth W.W. Sims, Robert P. Ackert, Frank C. Ramos, Robert A. Sohn, Michael T. Murrell, Donald J. DePaolo; Determining eruption ages and erosion rates of Quaternary basaltic volcanism from combined U-series disequilibria and cosmogenic exposure ages. Geology 2007;; 35 (5): 471–474. doi: https://doi.org/10.1130/G23381A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract We present 238U-230Th -226Ra disequilibria and cosmogenic 3He and 36Cl data for the Bluewater flow of the Zuni-Bandera volcanic field in western New Mexico. The 238U-230Th disequilibria measured on separated groundmass phases yield an internal isochron age of 68 ka (+24/–20 ka; 2σ). This value cannot be directly compared with surface exposure ages unless erosion rates are known. The apparent (zero erosion) ages determined from both the 3He concentration (47.5 ± 5 ka; 2σ) and the 36Cl concentration (41.2 ± 8.8 ka; 2σ) are significantly younger than the U-Th isochron age. When minimum estimates of surface erosion based on flow morphology are considered, the 3He concentrations indicate a minimum exposure age of 60 ka, in good agreement with the U-Th isochron age, with a minimum erosion rate of 1.7 mm/k.y. and an erosion rate as high as 5 mm/k.y. in other locations. Correcting for erosion has little effect on the model 36Cl age and, as a result, the 36Cl age is significantly younger than the U-Th isochron age and erosion-corrected 3He ages; this discordance is attributed to a lack of closed-system behavior in the 36Cl system. These new ages have local significance for the geochronology of the Zuni-Bandera volcanic field; however, their larger significance is in their applicability to dating Quaternary basalts and quantifying erosion rates. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

“Sarsen Stones” at German Hill, Central Otago, New Zealand, and Their Potential for In Situ Cosmogenic Isotope Dating of Landscape Evolution

Abstract 10Be concentrations measured in silica‐cemented Tertiary sandstones in Central Otago have yielded minimum exposure ages of up to 1400 ka, some of the oldest ever recorded outside the dry valleys of Antarctica and some arid deserts. The silica‐cemented sandstones outcrop as boulders in a region where their exposure is caused by the growth of anticlinal ridges above blind reverse faults. Initial studies using a combination of in situ cosmogenic isotope measurements and geomorphological observations on one range have demonstrated the potential of this technique to investigate Late Cenozoic fault growth and erosion rates. The best‐exposed and preserved occurrence of the quartz‐rich boulders is at German Hill on North Rough Ridge, where their stratigraphic context and their sedimentary and diagenetic origin, together with their method of emplacement and preservation on the modern land surface, can be studied in detail. These are all features that affect their suitability for cosmogenic isotope studies...

Growth of South Rough Ridge, Central Otago, New Zealand: Using in situ cosmogenic isotopes and geomorphology to study an active, blind reverse fault

In situ cosmogenic 10Be and 26Al measurements on sedimentary quartzites, together with analysis of abandoned river terraces, can be used to show how drainage responded to the lateral propagation of a late Quaternary anticline forming above a blind reverse fault in Central Otago, New Zealand. A close link between the progression of cosmogenic ages and the tectonic geomorphology allows us to confirm that uplift and propagation rates on the anticline are in the ranges 0.08–0.12 mm yr−1 and 0.8–1.5 mm yr−1, respectively, over the last 550,000 years. The agreement between the isotopic and geomorphological evidence in turn requires that minimum 10Be exposure ages as great as 660 ka are not in steady state with respect to erosion. This is a geochemical result of significance, as it opens the possibility of a more regional analysis of landscape evolution in this region, using the same rocks. On an adjacent anticline, thought to be even older on geomorphological arguments, 10Be concentrations give minimum exposure ages in the range 750–1400 ka. These extremely old minimum ages are rare worldwide, as they are usually limited to smaller values by erosion. They are attributable to the very resistant nature of the quartz‐rich boulders in which the measurements were made, which in places are shown to have erosion rates lower than 0.5 mm kyr−1.

Exposure ages deduced from cosmogenic 10Be and 26Al produced in situ: application to granite domes and tors in Korea

The cosmogenic radionuclides 10Be and 26Al produced in situ were analyzed to determine the exposure age and to estimate the landform evolution of a granite dome and tor in Korea. Since the tor of Mt Sapae has no joints, and the 26Al/10Be ratio has a singular point near the steady state island, the sample is supposed to have undergone steady-state erosion over a long period. In the dome on Mt Bukhan, many sheeting joints having thickness 20–70 cm develop on the slope surface, to make step-like sheet structures by exfoliation. Since the 26Al/10Be ratio of samples from Mt Bukhan is plotted into the lower area apart from the steady erosion curve, we work with a two-stage exposure model. Results show that (1) the minimum surface exposure ages of the two landforms based on the 10Be model are about 8 and 71 kyr respectively, (2) the sample from Mt Sapae was taken to have undergone steady-state erosion at rate of 7.0×10−4 cm/yr, and (3) a two-stage exposure model on Mt Bukhan suggests that the samples were irradiated by cosmic radiation in the past several thousand years, after having being shielded at the depth of 300–400 cm.

Cosmogenic nuclides 10Be and 26Al imply limited Antarctic Ice Sheet thickening and low erosion in the Shackleton Range for &gt;1 m.y.

Concentrations of the cosmogenic nuclides 1 0 Be and 2 6 Al on bedrock surfaces in the Shackleton Range, Antarctica, indicate minimum exposure ages between 3.0 ′ 03 and 1.16 ′ 0.10 Ma. The isotope data indicate that the maximum long-term erosion rate is 0.10-0.35 m/m.y., and the ratios suggest no prolonged periods of burial by cold-based ice. The findings are important because of the location of the massif close to the junction of the East Antarctic Ice Sheet and the landward extent of the Filchner Ice Shelf. These results point to three conclusions. First, the massif has not been overridden by the East Antarctic Ice Sheet during the late Quaternary. Rather, moraines 200-340 m above outlet glaciers are likely to represent the maximum thickening of the Filchner-Ronne Ice Shelf during the Quaternary. Second, the high radionuclide concentrations on bedrock surfaces, one of which is striated, suggest that some glacial landforms were created in at least Pliocene or more likely Miocene time. Third, the exceptionally low erosion rates imply that the modern cold, arid climate has persisted for millions of years. These findings provide evidence of old, stable landscapes over a wider area of Antarctica than the Mc-Murdo Dry Valleys.

Mineralogy and noble-gas signatures of the carbonate-rich lithology of the Tagish Lake carbonaceous chondrite: evidence for an accretionary breccia

The carbonate-rich lithology of the Tagish Lake carbonaceous chondrite was characterized by noble-gas mass spectrometry, synchrotron X-ray diffraction analysis, and transmission and scanning electron microscopy. Noble-gas analysis was performed on two samples and the results showed that primordial noble gases are abundant and solar noble gases are absent in the samples of carbonate-rich lithology. The concentrations of Ne-A2 and -E in both samples are at the maximum level observed for CI and CM chondrites, suggesting high abundances of presolar diamonds and SiC/graphite, respectively. The cosmic-ray exposure age cannot be determined precisely, because the shielding depth of our Tagish Lake samples is unknown, but the minimum exposure age was determined to be 5.5±0.7 Myr on the basis of cosmogenic 21Ne concentrations and the highest 21Ne production rate. X-ray and electron-microscopic study showed that the carbonate-rich lithology is dominated by loosely packed porous matrix that consists mainly of fine-grained saponite and ferromagenesian carbonate. The matrix contains very few chondrules, but many fine-grained clasts having angular shape with longest dimensions up to 1 mm. The clasts differ from host matrix in both texture and mineralogy. They are massive, compacted material with porosity much lower than matrix and contain abundant magnetite and a coherent intergrowth of serpentine and saponite that is rare in matrix. The presence of texturally and mineralogically distinct clasts indicates that the carbonate-rich lithology is a breccia, but the absence of solar noble gases and impact-induced deformational features in host matrix distinguish it from an asteroid regolith breccia. Our results instead indicate that it is an accretionary breccia formed by simultaneous accretion of diverse objects in a massive dust cloud. The clasts often enclose chondrules and anhydrous silicate fragments such as low-iron-manganese-enriched olivines. This observation and their highly compacted, angular morphology suggests that the clasts derive from earlier-formed chondritic materials which have experienced compression and fragmentation due to mild impacts and from which they were excavated and incorporated into the final rock. It is thus suggested that, in the course of planetesimal formation, agglomeration and disagglomeration of small parent bodies occurred repeatedly in a dense dust cloud where solar wind from the infant sun was shielded. The fine-grained ferromagnesian carbonates must have precipitated from aqueous solutions after the assembly of the Tagish Lake asteroid from recycled materials from previous bodies, because the carbonates coat the surfaces of pores and fill in veins in the clasts.

Fault growth and landscape development rates in Otago, New Zealand, using in situ cosmogenic 10Be

We use in situ cosmogenic 10Be measurements in quartzites to examine the growth and propagation rates of a Late Quaternary anticline forming above a blind reverse fault in Central Otago, New Zealand. We obtain average uplift and propagation rates of 0.10–0.15 mm yr −1 and 1.0–2.0 mm yr −1 respectively over the last 450 000 yr, though it is probable that growth occurred episodically, with more rapid development in the periods 600–400 kyr and 200–100 kyr and relative quiescence between about 400 and 200 kyr ago. These results quantify and confirm a qualitative analysis of the fault evolution based on geomorphology and drainage patterns. We obtain minimum 10Be exposure ages of up to 750 000 yr for the quartzites, some of the oldest ages obtained by this method outside Antarctica, which are made possible by the extremely low erosion rates of less than 0.6 mm kyr −1 in the resistant quartzites. These rocks consequently provide an opportunity to study geomorphic and tectonic processes in this region over unusually long time periods.

A relict landscape in the centre of Fennoscandian glaciation: cosmogenic radionuclide evidence of tors preserved through multiple glacial cycles

The presence of well-developed tors, boulder fields, and weathering mantles in the Parkajoki area of northeastern Sweden, near the centre of Fennoscandian glaciation, has been used to suggest that these landscapes were preserved during all glacial cycles since ice-sheet initiation in the late Cainozoic. This implies that all successive large-scale glaciations must have had frozen bed conditions across this area to allow for subglacial landscape preservation. Cosmogenic 10Be and 26Al data from three tors and a meltwater channel in the Parkajoki area were used to test this hypothesis of landscape preservation through multiple glacial cycles. Apparent exposure ages of tor summit bedrock surfaces ranging between 79 and 37 ka in an area deglaciated at ∼11 ka are consistent with the interpretation of these features as relict landforms that have survived glaciation with little or no erosion. Single nuclide minimum exposure age data require that the tors have survived at least two complete glacial cycles. This estimate is based on (i) the approximate duration of periods of ice sheet cover versus ice free conditions as deduced from the DSDP 607 marine benthic foraminifer oxygen isotope record, in conjunction with (ii) a record of Fennoscandian ice sheet flow traces (and hence, ice sheet extent), and (iii) noting that cosmogenic nuclides are accumulated only during ice free periods. In addition, mean cosmogenic 26Al/ 10Be concentration ratios from two of the sites indicate a minimum model total history of 605 ka and a maximum erosion rate of 1.6 m Ma −1. Thus, the numerical ages confirm the overall qualitative interpretation of landscape preservation through multiple glacial cycles.

Constraints for the latest glacial advance on Wrangel Island, Arctic Ocean, from rock surface exposure dating

During an expedition to Wrangel Island in 1997, three samples of quartz veins were collected for surface exposure dating. Two were bedrock surface samples from different parts of the island, one was from a local boulder. The samples were collected where former coverage by valley glaciers can be excluded. Especially, the bedrock surface samples were expected to provide new constraints for the chronology of glaciations on the island and on the arctic continental shelf in the Beringian region. Minimum exposure age estimates for these bedrock samples, calculated on the basis of 10Be concentrations, are 64,600±6400 and 26,400±2100 years. These estimates suggest that no major glaciations have affected Wrangel Island or the adjacent shelf area after 64,600±6400 years. Specifically, these dates seem to rule out any major glaciation during the Last Glacial Maximum (LGM). The estimates should however be regarded as preliminary, as only two bedrock surface samples were analysed, but are compatible with the absence of glacial morphology and sediments, and with radiocarbon dates from mammoth remains.

10Be dating of Younger Dryas Salpausselkä I formation in Finland

Boulders of the Younger Dryas Salpausselkä I (Ss I) formation west of Lahti, southern Finland, were sampled for surface exposure dating. The 10Be concentrations, determined by accelerator mass spectrometry, yield minimum exposure ages of 11 930 ± 950, 11 220 ± 890, 11 050 ± 910 and 11 540 ± 990 years, using recently published production rates scaled for latitude and elevation. This includes a correction to the production rate resulting from postglacial uplift of the Fennoscandian lithosphere (i.e. changing elevation) during the time of exposure. The error‐weighted mean exposure age of 11 420 ± 470 years of the analysed boulders agrees with previous varve dates of Ss I, which range from 11 680 to 11 430 calendar years BP. However, erosion has to be taken into account as a process affecting rock surfaces and therefore influencing exposure ages. Available information suggests an erosion rate of 5 mm/kyr, which increases the error‐weighted mean exposure age to a value of 11 610 ± 470 years. Within the errors, the formation of Ss I in the Vesala area west of Lahti falls into the Younger Dryas time bracket, as defined by the GRIP and GISP 2 ice core (Greenland).

The oldest ice on Earth in Beacon Valley, Antarctica: new evidence from surface exposure dating

Beacon Valley, Antarctica, contains unique remnants of glacier ice underneath a till layer covering the valley floor. To constrain the age and evolution of this important indicator of Antarctic paleoclimate, we analyzed two dolerite erratics from the till surface and one from within the ice for cosmogenic helium and neon. A conservative minimum exposure age of the older surface sample is 2.3 Ma, but taking into account erosion, the true exposure age of this boulder is likely to be considerably higher. The buried sample contains more than 20 times less cosmogenic noble gases than the old surface sample, although its current shielding would imply only a three times lower production rate. This indicates that the ice level has slowly been lowered by sublimation at the rate of a few m/Ma. The high exposure age of the surface sample as well as the very low sublimation rate of the relict ice both support the conclusion that the remnant ice in Beacon Valley was deposited many million years ago [Sugden et al., Nature 376 (1995) 412–414] and has never been thinner than at present. In addition, we found that cosmogenic helium and neon are released quantitatively from pyroxene at temperatures of <900°C and >1000°C, respectively.

Noble gases in interplanetary dust particles, I: The excess helium‐3 problem and estimates of the relative fluxes of solar wind and solar energetic particles in interplanetary space

Abstract— We report mass‐spectrometric measurements of light noble gases pyrolytically extracted from 28 interplanetary dust particles (IDPs) and discuss these new data in the context of earlier analyses of 44 IDPs at the University of Minnesota. The noble gas database for IDPs is still very sparse, especially given their wide mineralogic and chemical variability, but two intriguing differences from isotopic distributions observed in lunar and meteoritic regolith grains are already apparent. First are puzzling overabundances of 3He, manifested as often strikingly elevated 3He/4He ratios—up to &gt;40x the solar‐wind value—‐and found primarily but not exclusively in shards of some of the larger IDPs (“cluster particles”) that fragmented on impact with the collectors carried by high‐altitude aircraft. It is difficult to attribute these high ratios to 3He production by cosmic‐ray‐induced spallation during estimated space residence times of IDPs, or by direct implantation of solar‐flare He. Minimum exposure ages inferred from the 3He excesses range from ∼50 Ma to an impossible &gt;10 Ga, compared to Poynting‐Robertson drag lifetimes for low‐density 20–30 μm particles on the order of ∼0.1 Ma for an asteroidal source and ∼10 Ma for origin in the Kuiper belt. The second difference is a dominant contribution of solar‐energetic‐particle (SEP) gases, to the virtual exclusion of solar‐wind (SW) components, in several particles scattered throughout the various datasets but most clearly and consistently observed in recent measurements of a group of individual and cluster IDPs from three different collectors. Values of the SEP/SW fluence ratio in interplanetary space from a simple model utilizing these data are ∼1% of the relative SEP/SW abundances observed in lunar regolith grains, but still factors of approximately 10–100 above estimates for this ratio in low‐energy solar particle emission.

Exposure age and erosional history of an upland planation surface in the US Atlantic Piedmont

The upland planation surface in the Piedmont of central New Jersey consists of summit flats, as much as 130 km2 in area, that truncate bedding and structure in diabase, basalt, sandstone, mudstone and gneiss. These flats define a low-relief regional surface that corresponds in elevation to residual hills in the adjacent Coastal Plain capped by a fluvial gravel of late Miocene age. A Pliocene fluvial sand is inset 50 m below the upland features. These associations suggest a late Miocene or early Pliocene age for the surface. To assess exposure age and erosional history, a 4·4 m core of clayey diabase saprolite on a 3 km2 remnant of the surface was sampled at six depths for atmospherically produced cosmogenic 10Be. The measured inventory, assuming a deposition rate of 1·3 × 106 atoms cm−2 a−1, yields a minimum exposure age of 227 000 years, or, assuming continuous surface erosion, a constant erosion rate of 10 m Ma−1. Because the sample site lies about 60 m above the aggradation surface of the Pliocene fluvial deposit, and itself supports a pre-Pliocene fluvial gravel lag, this erosion rate is too high. Rather, episodic surface erosion and runoff bypassing probably have produced an inventory deficit. Reasonable estimates of surface erosion (up to 10 m) and bypassing (up to 50 per cent of total precipitation) yield exposure ages of as much as 6·4 Ma. These results indicate that (1) the surface is probably of pre-Pleistocene age and has been modified by Pleistocene erosion, and (2) exposure ages based on 10Be inventories are highly sensitive to surface erosion and runoff bypassing. Copyright © 2000 John Wiley & Sons, Ltd.