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Measurements of cosmic-ray-produced beryllium-10, neon-21, and helium-3 in quartz in a soil profile from a forested landscape in the Oregon Coast Range show that the cosmogenic noble gases 21Ne and 3He are depleted relative to 10Be in the shallow subsurface. The noble gases are mobile in mineral grains via thermally activated diffusion and 10Be is not, implying that noble gas depletion is the result of surface heating by wildfires and subsequent mixing of partially degassed quartz downward into the soil. Cosmogenic noble gas depletion by wildfire heating of soils is a potential means of estimating wildfire intensity and/or frequency over pre-observational timescales.
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Cosmogenic 3 He paleothermometry on post-LGM glacial bedrock within the central European Alps
Abstract. Diffusion properties of cosmogenic 3He in quartz at Earth surface temperatures offer the potential to directly reconstruct the evolution of pastin situ temperatures from formerly glaciated areas, which is important information for improving our understanding of glacier–climateinteractions. In this study, we apply cosmogenic 3He paleothermometry to rock surfaces gradually exposed from the Last Glacial Maximum(LGM) to the Holocene period along two deglaciation profiles in the European Alps (Mont Blanc and Aar massifs). Laboratory experiments conducted onone representative sample per site indicate significant differences in 3He diffusion kinetics between the two sites, with quasi-linearArrhenius behavior observed in quartz from the Mont Blanc site and complex Arrhenius behavior observed in quartz from the Aar site, which weinterpret to indicate the presence of multiple diffusion domains (MDD). Assuming the same diffusion kinetics apply to all quartz samples along eachprofile, forward model simulations indicate that the cosmogenic 3He abundance in all the investigated samples should be at equilibrium withpresent-day temperature conditions. However, measured cosmogenic 3He concentrations in samples exposed since before the Holocene indicate anapparent 3He thermal signal significantly colder than today. This observed 3He thermal signal cannot be explained with a realisticpost-LGM mean annual temperature evolution in the European Alps at the study sites. One hypothesis is that the diffusion kinetics and MDD modelapplied may not provide sufficiently accurate, quantitative paleo-temperature estimates in these samples; thus, while a pre-Holocene 3Hethermal signal is indeed preserved in the quartz, the helium diffusivity would be lower at Alpine surface temperatures than our diffusion modelspredict. Alternatively, if the modeled helium diffusion kinetics is accurate, the observed 3He abundances may reflect a complexgeomorphic and/or paleoclimatic evolution, with much more recent ground temperature changes associated with the degradation of alpine permafrost.
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- Award ID(s):
- 1935945
- PAR ID:
- 10436689
- Date Published:
- Journal Name:
- Geochronology
- Volume:
- 4
- Issue:
- 2
- ISSN:
- 2628-3719
- Page Range / eLocation ID:
- 641 to 663
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract. We report new cosmogenic 21Ne in quartz exposure ages from 18 samples on three distinct moraines deposited in the Lost Creek drainage, approximately 3–7 km down-valley from Lassen Peak in Lassen Volcanic National Park. Although measuring 21Ne in quartz is generally straightforward, accurate 21Ne exposure dating of deposits of late Pleistocene is rarely possible due to the significant quantities of non-cosmogenic 21Ne present in most lithologies. Young quartz-bearing volcanic rocks have been observed to be an exception. We take advantage of moraine boulders sourced from the ∼ 28 ka dacite of Lassen Peak to generate a chronology of alpine deglaciation in Lassen Volcanic National Park. Ages from three distinct moraines are in stratigraphic order at 22.1 ± 3.8, 20.2 ± 2.4, and 15.3 ± 3.8 ka and generally agree with other terminal and some recessional moraine ages across the Cascade Range and Sierra Nevada of the western United States. To date, these are among the youngest surfaces ever dated using cosmogenic 21Ne and provide a cost-effective proof-of-concept approach to dating moraines where applicable.more » « less
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Abstract. Measurements of multiple cosmogenic nuclides in a single sample are valuable for various applications of cosmogenic nuclide exposure dating and allow for correcting exposure ages for surface weathering and erosion and establishing exposure–burial history. Here we provide advances in the measurement of cosmogenic 10Be in pyroxene and constraints on the production rate that provide new opportunities for measurements of multi-nuclide systems, such as 10Be/3He, in pyroxene-bearing samples. We extracted and measured cosmogenic 10Be in pyroxene from two sets of Ferrar Dolerite samples collected from the Transantarctic Mountains in Antarctica. One set of samples has 10Be concentrations close to saturation, which allows for the production rate calibration of 10Be in pyroxene by assuming production–decay equilibrium. The other set of samples, which has a more recent exposure history, is used to determine if a rapid fusion method can be successfully applied to samples with Holocene to Last Glacial Maximum exposure ages. From measured 10Be concentrations in the near-saturation sample set we find the production rate of 10Be in pyroxene to be 3.74 ± 0.10 atoms g−1 yr−1, which is consistent with 10Be/3He paired nuclide ratios from samples assumed to have simple exposure. Given the high 10Be concentration measured in this sample set, a sample mass of ∼ 0.5 g of pyroxene is sufficient for the extraction of cosmogenic 10Be from pyroxene using a rapid fusion method. However, for the set of samples that have low 10Be concentrations, measured concentrations were higher than expected. We attribute spuriously high 10Be concentrations to failure in removing all meteoric 10Be and/or a highly variable and poorly quantified procedural blank background correction.more » « less
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Cosmogenic isotopes of helium and neon are produced at the Earth’s surface and exhibit a wide range of thermal sensitivities in common minerals. We can take advantage of this range of thermal sensitivities to reconstruct past near surface thermal conditions using cosmogenic noble gas observations. For example, cosmogenic noble gases have been used to study past ambient temperatures, changes in snow cover duration, and wildfire histories. Interpreting cosmogenic noble gas observations requires a model of both production and diffusion that predicts cosmogenic noble gas concentrations for different thermal histories. Additionally, models that characterize the diffusion kinetics of helium or neon in a particular mineral sample are often needed, as laboratory-based diffusion experiments demonstrate that helium and neon diffusion kinetics are sample specific and often complex. At present, various codes are available that can carry out pieces of the modeling, but they are generally not interoperable and are often highly specific to a particular past application, limiting the codes’ use for future applications. Here we present progress on creating a general forward modeling framework for inferring thermal histories using cosmogenic noble gas observations, structured around the concept of proxy system modeling. We will describe the architecture of this model framework as well as provide examples of applying it to new and existing cosmogenic noble gas datasets.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/gchron-4-641-2022
