Cosmogenic isotope exposure dating

Cosmogenic exposure dating. cosmo. Text by Carrie Patterson, MGS. Cosmogenic isotopes are created when elements in the atmosphere or earth are .
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To obtain the true age of a moraine deposit, one needs to collect several samples that not only greatly increases the budget of the project but also is time consuming. To overcome this problem, we developed a new sampling strategy for dating moraine surfaces by cosmogenic nuclides. Fourteen to 32 boulders were chosen for sampling every 20 m. All rock chips were amalgamated to make one sample. The age results from the two Late Pleistocene moraines and one rock glacier surface show no difference in terms of boulder exposure ages.

This work is supported by the U. Determining timing of Alaska Range exhumation and glaciation through cosmogenic nuclide burial dating. The Alaska Range is a transpressional orogen with modern exhumation initiating 6 Ma. The stratigraphic record of unroofing and uplift of the foreland basin is largely preserved along the northern flank of the Alaska Range in the Pliocene-Pleistocene aged Nenana Gravel, an extensive alluvial fan and braidplain deposit.

Chronometric control on the Nenana Gravel is largely lacking, with the limited available age control based on a single Ar-Ar tephra date in an underlying unit and via stratigraphic inferences for the upper portions. Higher-resolution dating of the Nenana Gravel unit is imperative in order to quantify deposition rates and the timing of uplift and deformation of the foreland basin. Furthermore, a glacial unit has been found to lie unconformably on top of the unit at Suntrana Creek and may represent the initiation of glacial advances in the Alaska Range. Three samples from the lower Nenana Gravel yield an isochron burial age of 4.

Two samples collected from the middle of the Nenana Gravel unit produced an average simple burial age of 2. Two samples from the upper-most portion of the Nenana Gravel yielded an average simple burial age of 1. In addition, the age of the glacial unit provides a minimum age for inception of foreland basin uplift and abandonment of the Nenana Gravel in this region. Isotopic abundances of the noble gases were measured in the following Martian meteorites: These glass samples, when combined with literature data on a separate single glass inclusion from EET and a glass vein from Zagami, permit examination of the isotopic composition of Ne, Ar, Kr, and Xe trapped from the Martian atmosphere in greater detail.

The isotopic composition of Martian Ne, if actually present in these glasses, remains poorly defined. Martian atmospheric Kr appears to be enriched in lighter isotopes by approx. Cosmogenic gases indicate space exposure ages of The space exposure age for Y is 4. Using cosmogenic nuclides to date the stabilisation age of relict rockglaciers.

Active rockglaciers are periglacial landforms which are creeping down mountain slopes due to plastic deformation of the interstitial ice. The occurrence of active rockglaciers is an indicator of Alpine permafrost. Relict rockglaciers are not moving anymore because the ice melted, but they give evidence for the earlier existence of permafrost. In the Alps, relict rockglaciers can often be found below today's tree line raising the question of when these landforms have last been active.

Judging from the present position of the relict rockglaciers, the lower permafrost limit during the time of their activity must have been hundreds of meters lower than it is today. Already in the early days of rockglacier research, the potential of relict rockglaciers as a paleoclimate proxy was recognised Barsch , Haeberli However, obtaining absolute ages on relict rockglaciers has always been a major difficulty.

Lately it has been shown that with cosmogenic nuclides it is possible to date the stabilisation age of relict rockglaciers, but it has been applied only in a few cases Ivy-Ochs et al.

Cosmogenic nuclide dating

According to Reitner , the lowest relict rockglaciers of the eastern Alps, the Tandl rockglaciers, are located in the Province of Carinthia Austria. The Tandl rockglaciers are a complex series of rockglaciers spanning from around m down to m a. Therefore, samples from the entire rockglacier series were taken for 10Be exposure dating. Furthermore, the close proximity of the rockglaciers to moraines associated to the Gschnitz stadial allow comparing the dating results to equilibrium line depression reconstructions. Less than 10 km to the southwest, a second rockglacier series, the Norbert rockglaciers, was sampled.

Surface exposure dating

In contrast to the. Glacier retreat in polar areas has major implications at a local, regional and even planetary scale. Global average sea level rise is the most obvious and socio-economically relevant, but there are others such as the arrival of new fauna to deglaciated areas, plant colonisation or permafrost formation and degradation. Ice-cap glacier retreat chronology will be revealed by the use of cosmogenic isotopes mainly 36Cl on glacially originated sedimentary and erosive records.

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Cosmogenic dating will be complemented by other dating methods C14 and OSL , which will permit the validation of these methods in regions with cold-based glaciers. Given the geomorphological evidences and the obtained ages, a deglaciation calendar will be proposed and we will use a GIS methodology to reconstruct the glacier extent and the ice thickness. The results emerging from this project will allow to assess whether the high glacier retreat rates observed during the last decades were registered in the past, or if they are conversely the consequence and evidence of the Global Change in Antarctica.

Recent summer warming has now raised the equilibrium line above almost all ice caps on Baffin Island, resulting in surface lowering and marginal recession everywhere. As cold-based ice recedes it frequently exposes in situ tundra plants that were living at the time ice expanded across the site.

Radiocarbon dates for each plant records when cold summers dropped regional snowline below the site, killing the plants, and snowline remained below the site until the collection date. The kill dates also represent the last time that the climate was warm enough to expose the sampling location. Radiocarbon dead sites north of the Penny Ice cap experienced significantly more exposure over their lifetimes than their counterparts east of the Penny Ice cap, suggesting significant differences in local and regional land ice fluctuations over the last 2 million years. Accessing the application of in situ cosmogenic 14C to surface exposure dating of amorphous SiO2.

We assess the feasibility and utility of in situ cosmogenic 14C as a geochronometer for landforms composed of amorphous SiO2 through the comparison of 14C surface exposure ages to independently determined eruption ages on Obsidian Dome, California. Landforms composed of amorphous SiO2 phases are difficult to date by conventional cosmogenic nuclide methods due to several complications that may arise e.

The onset of an increased understanding of production rates and analytical measurement of in situ 14C in SiO2 provides an opportunity to address this limitation. Obsidian Dome is a year-old phreatomagmatic dome of the Mono-Inyo Craters located in Inyo County, California, and consists of vesicular pumice, obsidian, and rhyolite. Measurement of 14C concentrations in shielded, subsurface samples will assist in isolating the atmospheric 14C component and aid in correcting the surface exposure ages.

Hydrological travel times are studied on scales that span six orders of magnitude, from daily event water in stream flow to pre-Holocene groundwater in wells. Groundwater vulnerability to contamination, groundwater surface water interactions and catchment response are often focused on "modern" water that recharged after the introduction of anthropogenic tritium in precipitation in Shorter residence times are expected in smaller catchments, resulting in immediate vulnerability to contamination.

Tritium samples 1 L are analyzed by noble gas mass spectrometry after helium-3 accumulation. The observed variability of deposition rates complicates direct estimation of stream water age fractions. Estimates of subsurface storage are in agreement with estimates from geophysical studies. In order to determine the age of the relatively younger fans, the erosion rate of the boulders and the cosmogenic nuclide inheritance from exposure prior to deposition in the fan were established. Cosmogenic nuclide inheritance values that range between 8.

Boulder erosion rate, ranging between 17 and mm k.

How can we date rocks?

Since the boulders on the fans represent the most resistant rocks in this environment, we used the lowest rate for the age calculations. Monte Carlo simulations were used to determine ages of 16?? Older fans older than ka were dated by analyzing 10Be and 26Al concentrations in buried sand samples. The ages of the three oldest fans range between ?? Although fan age determinations are accompanied by large uncertainties, the results of this study show a clear trend of increasing fan ages with increasing distance from the source near Little Rock Creek and provide a long-term slip rate along this section of the San Andreas fault.

Slip rate along the Mojave section of the San Andreas fault for the past k. The average slip rate calculated from the individual fan ages is 4. A linear regression through the data points implies a slip rate of 3. A most probable slip rate of 3. Terrestrial cosmogenic nuclide surface exposure dating of the oldest glacial successions in the Himalayan orogen: Ladakh Range, northern India. Terrestrial cosmogenic nuclide surface exposure dating of moraine boulders and alluvial fan sediments define the timing of five glacial advances over at least the last five glacial cycles in the Ladakh Range of the Transhimalaya.

The glacial stages that have been identified are: The exposure ages of the Indus Valley moraines are the oldest observed to date throughout the Himalayan orogen. We observe a pattern of progressively more restricted glaciation during the last five glacial cycles, likely indicating a progressive reduction in the moisture supply necessary to sustain glaciation. Alternatively, this pattern of glaciation may reflect a trend of progressively less extensive glaciation in mountain regions that has been observed globally throughout the Pleistocene. Deglacial history of the Pensacola Mountains, Antarctica from glacial geomorphology and cosmogenic nuclide surface exposure dating.

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The retreat history of the Antarctic Ice Sheet is important for understanding rapid deglaciation, as well as to constrain numerical ice sheet models and ice loading models required for glacial isostatic adjustment modelling. There is particular debate about the extent of grounded ice in the Weddell Sea embayment at the Last Glacial Maximum, and its subsequent deglacial history.

Here we provide a new dataset of geomorphological observations and cosmogenic nuclide surface exposure ages of erratic samples that constrain the deglacial history of the Pensacola Mountains, adjacent to the present day Foundation Ice Stream and Academy Glacier in the southern Weddell Sea embayment. We show there is evidence of at least two glaciations, the first of which was relatively old and warm-based, and a more recent cold-based glaciation. During the most recent glaciation ice thickened by at least m in the Williams Hills and at least m on Mt Bragg.

Progressive thinning from these sites was well underway by 10 ka BP and ice reached present levels by 2. The thinning history is consistent with, but does not mandate, a Late Holocene retreat of the grounding line to a smaller-than-present configuration, as has been recently hypothesized based on ice sheet and glacial isostatic modelling.

cosmogenic isotope dating: Topics by

The data also show that clasts with complex exposure histories are pervasive and that clast recycling is highly site-dependent. These new data provide constraints on a reconstruction of the retreat history of the formerly-expanded Foundation Ice Stream, derived using a numerical flowband model. Cosmic ray exposure dating with in situ produced cosmogenic 3He: In an effort to determine the in situ production rate of spallation-produced cosmogenic 3He, and evaluate its use as a surface exposure chronometer, we have measured cosmogenic helium contents in a suite of Hawaiian radiocarbon- dated lava flows.

The lava flows, ranging in age from to 13, years, were collected from Hualalai and Mauna Loa volcanoes on the island of Hawaii. Because cosmic ray surface-exposure dating requires the complete absence of erosion or soil cover, these lava flows were selected specifically for this purpose. The 3He production rate, measured within olivine phenocrysts, was found to vary significantly, ranging from 47 to atoms g-1 yr-1 normalized to sea level. Although there is considerable scatter in the data, the samples younger than 10, years are well-preserved and exposed, and the production rate variations are therefore not related to erosion or soil cover.

Data averaged over the past years indicate a sea-level 3He production rate of ?? The longer record suggests a minimum in sea level normalized 3He production rate between and years 55?? The minimum in production rate is similar in age to that which would be produced by variations in geomagnetic field strength, as indicated by archeomagnetic data.