6.5 Cosmogenic nuclide dating

Cosmogenic nuclide dating is a powerful tool in isotope geochemistry for determining surface exposure ages and erosion rates. This technique relies on measuring isotopes produced when cosmic rays interact with Earth's atmosphere and surface materials, providing crucial insights into landscape evolution.

The method involves careful sampling, precise measurement of trace isotopes, and complex age calculations. By accounting for factors like latitude, elevation, and shielding, scientists can date glacial landforms, fault scarps, and quantify long-term erosion rates across various geomorphic settings.

Principles of cosmogenic nuclides

• Cosmogenic nuclides form key components in isotope geochemistry used to date surface exposure and erosion rates
• Understanding cosmic ray interactions with Earth's atmosphere and surface materials underpins cosmogenic dating techniques
• Cosmogenic nuclide production varies with latitude, elevation, and other factors requiring careful calibration

Formation of cosmogenic nuclides

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• Cosmic rays interact with atoms in Earth's atmosphere and surface to produce cosmogenic nuclides
• Secondary cosmic ray cascades generate neutrons and muons that produce nuclides in rock minerals
• Spallation reactions break apart target nuclei to form lighter cosmogenic isotopes
• Thermal neutron capture produces some cosmogenic nuclides like 36Cl in certain minerals

Common cosmogenic isotopes

• 10Be forms primarily in quartz and has a half-life of 1.39 million years
• 26Al also produced in quartz with 0.71 million year half-life enables paired dating with 10Be
• 36Cl forms in calcite and feldspar with 0.30 million year half-life
• 14C has 5,730 year half-life allowing dating of younger surfaces
• 3He and 21Ne stable noble gas isotopes accumulate over time without decay

Cosmic ray flux variations

• Primary cosmic ray flux varies with the 11-year solar cycle
• Geomagnetic field strength affects cosmic ray flux reaching Earth's surface
• Long-term variations in cosmic ray flux require calibration of production rates
• Galactic cosmic ray flux considered relatively constant over past few million years

Surface exposure dating

• Measures accumulation of cosmogenic nuclides to determine how long a surface has been exposed
• Enables dating of geomorphic surfaces like glacial moraines, lava flows, and fault scarps
• Requires assumptions about initial nuclide concentrations and erosion rates

Accumulation of cosmogenic nuclides

• Nuclide concentration increases with exposure time following exponential saturation curve
• Accumulation rate depends on production rate and radioactive decay for unstable isotopes
• Stable noble gas isotopes accumulate linearly without reaching saturation
• Nuclide concentration reaches equilibrium when production balances radioactive decay

Erosion rate effects

• Surface erosion removes cosmogenic nuclides and reduces apparent exposure age
• Steady state erosion results in constant nuclide concentration at surface
• Erosion rates can be calculated from nuclide concentrations assuming steady state
• Complex erosion histories require multi-nuclide approaches to resolve

Burial dating applications

• Measures decay of previously accumulated cosmogenic nuclides after burial
• Allows dating of sediments and cave deposits shielded from cosmic rays
• Requires initial nuclide ratio assumptions based on surface production rates
• Paired isotopes like 26Al/10Be enable determination of complex exposure-burial histories

Sampling strategies

• Proper sample collection and preparation crucial for accurate cosmogenic dating results
• Site selection and sampling methods aim to minimize geological uncertainties
• Sample preparation isolates target minerals and removes meteoric components

Site selection criteria

• Choose stable, well-preserved surfaces with simple exposure histories
• Avoid areas with significant erosion, burial, or prior shielding
• Select multiple samples to assess reproducibility and spatial variability
• Consider bedrock vs boulder samples based on geomorphic context

Sample collection methods

• Collect from upper few centimeters of rock surface to maximize cosmogenic signal
• Use hammer and chisel or rock saw to obtain ~500g of rock per sample
• Record sample location, elevation, topographic shielding, and site characteristics
• Photograph sample site and surrounding landscape for documentation

Sample preparation techniques

• Crush and sieve samples to isolate target grain size fraction (typically 250-500 μm)
• Perform mineral separation to isolate quartz or other target minerals
• Acid etching removes meteoric 10Be and atmospheric contaminants
• Carrier addition and chemical processing to extract and purify target isotopes

Measurement techniques

• High-sensitivity methods required to measure trace amounts of cosmogenic nuclides
• Different analytical approaches used for various cosmogenic isotope systems
• Advances in measurement precision enable dating of younger surfaces and lower concentrations

Accelerator mass spectrometry

• Separates and counts individual atoms based on mass and charge
• Enables measurement of rare long-lived radionuclides like 10Be, 26Al, and 36Cl
• Accelerates ions to MeV energies to eliminate molecular interferences
• Achieves isotope ratio precision of 2-5% for typical cosmogenic dating samples

Noble gas mass spectrometry

• Measures concentrations of stable cosmogenic noble gases (3He, 21Ne)
• Static vacuum systems with high-sensitivity detectors achieve sub-femtomole precision
• Step-heating extracts gases and separates cosmogenic component from other sources
• Enables dating of very old surfaces and determination of complex exposure histories

Isotope ratio analysis

• Measures relative abundances of different isotopes of same element
• Thermal ionization mass spectrometry used for some radiogenic systems
• Multicollector inductively coupled plasma mass spectrometry enables high-precision ratios
• Internal standardization and sample-standard bracketing improve measurement accuracy

Age calculation methods

• Convert measured nuclide concentrations to exposure ages or erosion rates
• Account for variations in production rate due to location and sample characteristics
• Apply appropriate scaling factors and corrections to derive final ages

Production rate determination

• Site-specific production rates calibrated using independently dated surfaces
• Global production rate databases compiled from multiple calibration sites
• Time-dependent production rate models account for geomagnetic field variations
• Muogenic production becomes significant for deeply buried samples

Scaling factors for latitude

• Cosmic ray flux increases with latitude due to geomagnetic field effects
• Scaling factors derived from neutron monitor data and theoretical models
• Latitude scaling more pronounced at low elevations
• Time-dependent scaling accounts for paleomagnetic field variations

Scaling factors for elevation

• Cosmic ray flux increases with elevation due to less atmospheric shielding
• Exponential increase in production rate with atmospheric depth
• Scaling factors based on atmospheric pressure rather than elevation
• Local hypsometry affects production rates in high-relief landscapes

Topographic shielding corrections

• Surrounding topography blocks portion of cosmic ray flux reaching sample
• Shielding calculated from horizon measurements or digital elevation models
• Corrections typically <10% for most samples but can be significant in deep valleys
• Self-shielding within sample accounted for in production rate depth profiles

Applications in geomorphology

• Cosmogenic nuclide dating provides powerful tool for quantifying landscape evolution
• Enables direct dating of landforms and determination of long-term erosion rates
• Applications span wide range of geomorphic processes and timescales

Glacial landform dating

• Dates exposure of glacially transported boulders on moraines
• Reconstructs ice retreat chronologies and paleoclimate records
• Accounts for complex exposure histories of reworked glacial deposits
• Combines with other dating methods to constrain glacial-interglacial cycles

Fault scarp dating

• Measures exposure ages of bedrock fault scarps or offset alluvial fans
• Determines timing and recurrence intervals of past earthquakes
• Accounts for gradual scarp degradation and erosion over time
• Combines with fault slip rates to assess seismic hazards

Landscape evolution studies

• Quantifies long-term erosion rates in various geomorphic settings
• Determines sediment generation rates and catchment-averaged denudation
• Assesses relative stability of different landscape elements
• Provides input for numerical landscape evolution models

Limitations and uncertainties

• Various factors introduce uncertainties in cosmogenic exposure dating results
• Understanding and quantifying sources of error crucial for data interpretation
• Ongoing research aims to refine methods and reduce uncertainties

Inheritance issues

• Prior exposure of sample results in overestimation of true exposure age
• Particularly problematic for surfaces with complex exposure histories
• Addressed through careful site selection and sampling of multiple clasts
• Depth profile sampling can help quantify inherited nuclide component

Erosion rate assumptions

• Unknown erosion history introduces uncertainty in exposure age calculations
• Steady state erosion assumption may not apply in all geomorphic settings
• Erosion rates can be constrained using multi-nuclide approaches
• Sensitivity analysis assesses impact of erosion rate uncertainty on ages

Atmospheric pressure variations

• Long-term changes in atmospheric pressure affect cosmogenic nuclide production
• Paleoclimate variations can alter effective elevation of sample sites
• Corrections based on independent paleoclimate proxies introduce additional uncertainty
• Effect most significant for high-elevation sites and old exposure ages

Multi-nuclide approaches

• Measurement of multiple cosmogenic nuclides in same sample provides additional constraints
• Enables resolution of complex exposure histories and erosion rates
• Requires consideration of different production rates and decay constants

Paired isotope dating

• Compares ratios of two nuclides with different half-lives (26Al/10Be)
• Identifies samples with simple exposure histories vs complex burial-exposure
• Burial dating possible for sediments shielded from cosmic rays
• Graphical methods using two-isotope diagrams aid data interpretation

Depth profile analysis

• Measures nuclide concentrations at multiple depths below surface
• Constrains both exposure age and erosion rate simultaneously
• Helps identify inherited nuclide component in depositional settings
• Requires careful sampling and consideration of deposit characteristics

Complex exposure histories

• Combines multiple nuclides to resolve periods of exposure and burial
• Identifies samples affected by past cover by ice, soil, or volcanic deposits
• Enables dating of surfaces with intermittent exposure histories
• Requires sophisticated modeling approaches to interpret data

Recent advances

• Ongoing research expands applications and improves accuracy of cosmogenic dating
• New analytical techniques enable measurement of additional cosmogenic nuclides
• Refined production rate models and scaling factors reduce systematic uncertainties

In situ vs meteoric nuclides

• In situ nuclides form within mineral lattices of surface rocks
• Meteoric nuclides delivered by precipitation and dust accumulate on surfaces
• Meteoric 10Be in soil profiles used to quantify erosion rates
• Combining in situ and meteoric approaches provides complementary information

Cosmogenic paleothermometry

• Measures cosmogenic 3He diffusion in quartz to reconstruct thermal histories
• Enables quantification of surface temperature changes over time
• Requires careful calibration of 3He diffusion kinetics in quartz
• Combines with other thermochronometers to constrain landscape thermal evolution

Developments in data interpretation

• Bayesian statistical approaches incorporate multiple sources of uncertainty
• Monte Carlo simulations assess impact of input parameter uncertainties
• Machine learning algorithms aid in complex multi-nuclide data interpretation
• Open-source software tools standardize data reduction and age calculations

Integration with other techniques

• Combining cosmogenic dating with other geochronological methods provides robust age constraints
• Multi-method approaches help validate assumptions and identify discrepancies
• Integrating different techniques enables comprehensive reconstruction of landscape histories

Luminescence dating comparison

• Optically stimulated luminescence dates burial of sediments
• Complements cosmogenic burial dating of cave deposits and terraces
• Provides independent check on cosmogenic erosion rate estimates
• Enables dating of younger surfaces beyond range of some cosmogenic systems

Radiocarbon dating vs cosmogenic

• 14C dating applies to organic materials up to ~50,000 years old
• Cosmogenic 14C in rock surfaces allows dating of younger exposures
• Combining radiocarbon and longer-lived cosmogenic nuclides constrains recent histories
• Helps identify recent erosion or burial events affecting older surfaces

Thermochronology applications

• Low-temperature thermochronology constrains rock cooling and exhumation histories
• Combining with cosmogenic data provides erosion rates at different timescales
• Apatite (U-Th)/He dating complements cosmogenic 3He in quartz
• Integrating methods helps reconstruct long-term landscape evolution trajectories