An official website of the United States government
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.
more »
« less
A proxy system model framework for reconstructing past environmental conditions with cosmogenic noble gases
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
- Award ID(s):
- 1935945
- PAR ID:
- 10650749
- Publisher / Repository:
- EGU General Assembly 2024
- Date Published:
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Cosmic ray exposure (CRE) ages are used to constrain the orbital and impact history of meteorites and identify their parent body or source region. CRE ages of enstatite (E) chondrites obtained from measurements of 3He are often much younger than 21Ne CRE ages measured in the same meteorite, which is often attributed to diffusive loss of 3He via solar heating during orbit. With knowledge of the diffusion kinetics of 3He in the major minerals making up E chondrites, we can leverage this discrepancy in CRE ages to infer a meteorite’s recent thermal history. To this end, we performed stepwise degassing experiments on fragments of albite, enstatite and kamacite, the dominant minerals in E chondrites, that were irradiated with protons to produce 3He. We find albite displays simple, Arrhenius-dependent 3He diffusion behavior, whereas enstatite and kamacite exhibit somewhat more complex diffusion behavior. We find that cosmogenic 3He will be readily lost from albite in the space environment, enstatite can exhibit significant 3He loss if exposed to high temperatures characteristic of low perihelion on million year time scales, and kamacite is highly retentive of 3He and unlikely to experience direct diffusive loss. These diffusion kinetics parameters can also be used to understand the exposure and thermal histories of other meteorite classes, terrestrial cosmogenic 3He applications, and mantle noble gas systematics.more » « less
-
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.more » « less
-
Heat transfer in the solid Earth drives processes that modify temperatures, leaving behind a clear signature that we can measure using noble gas thermochronology. This allows us to record the thermal histories of rocks and obtain the timing, rate, and magnitude of phenomena such as erosion, deformation, and fluid flow. This is done by measuring the net balance between the accumulation of noble gas atoms from radioactive decay and their loss by temperature-activated diffusion in mineral grains. Together with knowledge about noble gas diffusion in common minerals, we can then use inverse models of this accumulation–diffusion balance to recover thermal histories. This approach is now a mainstream method by which to study geodynamics and Earth evolution.more » « less
-
Abstract. The multiple-diffusion domain (MDD) model empirically describes the diffusive behavior of noble gases in some terrestrial materials and has been commonly used to interpret 40Ar/39Ar stepwise degassing observations in K-feldspar. When applied in this manner, the MDD model can be used to test crustal exhumation scenarios by identifying the permissible thermal paths a rock sample could have undergone over geologic time, assuming the diffusive properties of Ar within the mineral are accurately understood. More generally, the MDD model provides a framework for quantifying the temperature-dependent diffusivity of noble gases in minerals. However, constraining MDD parameters that successfully predict the results of step-heating diffusion experiments is a complex task, and the assumptions made by existing numerical methods used to quantify model parameters can bias the absolute temperatures permitted by thermal modeling. For example, the most commonly used method assumes that no domains lose more than 60 % of their gas during early heating steps (Lovera et al., 1997). This assumption is unverifiable, and we show that the Lovera et al. (1997) procedure may bias predicted temperatures towards lower values when it is violated. To address this potential bias and to provide greater accessibility to the MDD model, we present a new open-source method for constraining MDD parameters from stepwise degassing experimental results, called the “MDD Tool Kit” (https://github.com/dgorin1/mddtoolkit, last access: 11 October 2024). This software optimizes all MDD parameters simultaneously and removes any need for user-defined Ea or regression fitting choices used by other tools. In doing so, this new method eliminates assumptions about the domain size distribution. To test the validity of our thermal predictions, we then use the MDD Tool Kit (https://github.com/dgorin1/mddtoolkit) to interpret 40Ar/39Ar results from the Grayback Fault, AZ, USA. Although the resulting thermal histories are consistently ∼ 60–75 °C higher than those found in previous studies, they agree with independent observations from apatite fission track, zircon fission track, and (U-Th)/He.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Posted Content:
- https://doi.org/10.5194/egusphere-egu24-5971
