Cosmogenic Nuclide Dating

BURIAL DATING OF LATE-CENOZOIC DEPOSITS USING IN-SITU PRODUCED COSMOGENIC NUCLIDES

by Florence ReSeT

Andrea Ciampalini, Cristina Persano, Derek Fabel, Marco Firpo

Dating the Miocene to Pleistocene deposition of fluvial and deltaic sediments is often difficult because of the... more Dating the Miocene to Pleistocene deposition of fluvial and deltaic sediments is often difficult because of the absence of suitable biostratigraphic markers. Temporal limits on sediment deposition, however, are fundamental constraints in many geological, geomorphological, stratigraphical and archeological studies. In particular a dated stratigraphy of sedimentary basins provides information about timing of evolution of the sediment source and routing system due to tectonics, sea-level and/or climate changes. Traditional techniques, such as luminescence, radiocarbon dating of organic material, etc. only permit us to date sediments that are no older than ~250,000 years. Recently a method for dating sediment burial using the radioactive decay of cosmogenic nuclides 26Al and 10Be has been developed. 26Al and 10Be are produced by the continuous bombarding by high-energy cosmic rays of the Earth surface where they penetrate into rocks and sediments at a depth that varies, depending on rock density, between 2 and 3 m. The long half-life of the 26Al (7.05X105 yr) and 10Be (1.5X106 yr) make them optimal for dating sediments that were deposited over the past five million years, as long as the sediments were at the surface and exposed to cosmic rays before sedimentation and burial. The sediments that can be dated using the cosmogenic isotopes technique need to have the following characteristics: (1) quartz needs to be present and have been exposed at the surface for a time necessary to accumulate measurable cosmogenic radionuclides concentration (i.e., depending
on latitude and altitude, at least ~ 200 years); (2) transport time needs to have been negligible (3) burial was rapid and deep (>10 m) to avoid cosmogenic nuclides production after deposition, otherwise a correction needs to be introduced. This method is useful in many Italian areas where deltaic or fluvial Miocene to Pleistocene deposits crop out.

Last Glacial Maximum and the Gschnitz stadial in the Maritime Alps according to 10Be cosmogenic dating

by Adriano Ribolini

BURIAL DATING OF LATE-CENOZOIC DEPOSITS USING IN-SITU PRODUCED COSMOGENIC NUCLIDES

by Andrea Ciampalini

A. Ciampalini, C. Persano, D. Fabel, M. Firpo, Il Quaternario Italian Journal of Quaternary Sciences
23(1), 2010 - 125-136,

Dating the Miocene to Pleistocene deposition of fluvial and deltaic sediments is often difficult because of the... more Dating the Miocene to Pleistocene deposition of fluvial and deltaic sediments is often difficult because of the absence of suitable biostratigraphic markers. Temporal limits on sediment deposition, however, are fundamental constraints in many geological, geomorphological, stratigraphical and archeological studies. In particular a dated stratigraphy of sedimentary basins provides information about timing of evolution of the sediment source and routing system due to tectonics, sea-level and/or climate changes. Traditional techniques, such as luminescence, radiocarbon dating of organic material, etc. only permit us to date sediments that are no older than ~250,000 years. Recently a method for dating sediment burial using the radioactive decay of cosmogenic nuclides 26Al and 10Be has been developed. 26Al and 10Be are produced by the continuous bombarding by high-energy cosmic rays of the Earth surface where they penetrate into rocks and sediments at a depth that varies, depending on rock density, between 2 and 3 m. The long half-life of the 26Al (7.05X105 yr) and 10Be (1.5X106 yr) make them optimal for dating sediments that were deposited over the past five million years, as long as the sediments were at the surface and exposed to cosmic rays before sedimentation and burial. The sediments that can be dated using the cosmogenic isotopes technique need to have the following characteristics: (1) quartz needs to be present and have been exposed at the surface for a time necessary to accumulate measurable cosmogenic radionuclides concentration (i.e., depending on latitude and altitude, at least ~ 200 years); (2) transport time needs to have been negligible (3) burial was rapid and deep (>10 m) to
avoid cosmogenic nuclides production after deposition, otherwise a correction needs to be introduced. This method is useful in many Italian areas where deltaic or fluvial Miocene to Pleistocene deposits crop out.

Exposure age dating and Equilibrium Line Altitude reconstruction of an Egesen moraine in the Maritime Alps, Italy

by Adriano Ribolini

Spatial distribution of denudation in Eastern Tibet and regressive erosion of plateau margins

by Vincent Godard

Godard, V.; Lavé, J.; Carcaillet, J.; Cattin, R.; Bourlès, D. & Zhu, J.  Tectonophysics, 2010, 491, 253-274

The Longmen Shan range is one of the major topographic and structural features of the eastern margin of the Tibetan... more The Longmen Shan range is one of the major topographic and structural features of the eastern margin of the Tibetan Plateau. With an impressive topographic gradient and low convergence rates across the range this region has raised important questions concerning the dynamics of plateau margin settings, such as the long-term mechanisms of topographic evolution. The investigation of the distribution in space and time of denudation can provide critical insight into such dynamics and shed light on still unresolved controversies. For that purpose, we present a new dataset that documents the intensity and distribution of denudation processes across this plateau margin through field survey of fluvial incision markers, quantitative geomorphology and cosmogenic nuclide derived basin-wide erosion rates. Erosion is 0.5 mm/year in the frontal region of the Longmen Shan and between 0.5 and 1 mm/year further west, with a gradual decrease when reaching the northern headwaters of the Min Jiang watershed, adjacent to the beginning of the Tibetan Plateau. The spatial distribution of denudation inferred from the various methods we use suggests that most of the differential uplift in the Central Longmen Shan is accommodated by the Beichuan Fault and frontal structures located in the foothills. The denudation pattern seems also to reflect the large-scale propagation of erosion from the Sichuan Basin toward the Plateau. This suggests that the Longmen Shan range is submitted to the combined influences of slow thrusting activity on the frontal structures and progressive westward regressive erosion as a probable response to a pulse of uplift of the Tibetan Plateau Eastern margin, that started 10 Ma ago.

A note of caution on the use of boulders for exposure dating of depositional surfaces

by Silke Schmidt

Schmidt, S., Hetzel, R., Kuhlmann, J., Mingorance, F. & Ramos, V.A. (2011) - Earth and Planetary Science Letters 302, 60-70, doi:10.1016/j.epsl.2010.11.039.

Effects of terrain smoothing on topographic shielding correction factors for cosmogenic nuclide-derived estimates of basin-averaged denudation rates

by veerle Vanacker

Norton and Vanacker (2009) ESPL

Estimation of spatially averaged denudation rates from cosmogenic nuclide concentrations in sediments depends on the... more Estimation of spatially averaged denudation rates from cosmogenic nuclide concentrations in sediments depends on the surface production rates, the scaling methods of cosmic ray intensities, and the correction algorithms for skyline, snow and vegetation shielding used to calculate terrestrial cosmogenic nuclide production.

While the calculation of surface nuclide production and application of latitude, altitude and palaeointensity scaling algorithms are subjects of active research, the importance of additional correction for shielding by topographic obstructions, snow and vegetation is the subject of ongoing debate. The derivation of an additional correction factor for skyline shielding for large areas is still problematic. One important issue that has yet to be addressed is the effect of the accuracy and resolution of terrain representation by a digital elevation model (DEM) on topographic shielding correction factors.

Topographic metrics scale with the resolution of the elevation data, and terrain smoothing has a potentially large effect on the correction of terrestrial cosmogenic nuclide production rates for skyline shielding. For rough, high-relief landscapes,the effect of terrain smoothing can easily exceed analytical errors, and should be taken into account. Here we demonstrate the effect
of terrain smoothing on topographic shielding correction factors for various topographic settings, and introduce an empirical model for the estimation of topographic shielding factors based on landscape metrics.

Constraining landscape development of the Sri Lankan escarpment with cosmogenic nuclides in river sediment

by veerle Vanacker

Vanacker et al. (2007) EPSL

Escarpments are prominentmorphological features along high-elevation passive margins. Recent studies integrating... more Escarpments are prominentmorphological features along high-elevation passive margins. Recent studies integrating geomorphology,thermochronology, and cosmogenic nuclide-based denudation rate estimates suggest a rapid phase of denudation immediately after the earliest stages of seafloor spreading, and subsequent slow denudation rates since. To constrain the geomorphic evolution of passive margins, we have examined the development of the Sri Lankan escarpment.

Cosmogenic nuclide data on river sediment along a north–south transect across the southern escarpment reveal that the landscape is eroding ten times more rapidly in the escarpment zone (26 to 71 mm kyr−1) than in the high-elevation plateau above it and in the lowland plain beneath it (2.6 to 6.2 mm kyr−1). Unlike these low denudation rate areas, the escarpment denudation is strongly and linearly hill slope-dependent. This shows that denudation and retreat are tightly interlinked within the escarpment, which suggests that the escarpment is evolving by rift-parallel retreat, rather than by escarpment downwearing.

Supporting evidence is provided by themorphology of rivers draining the escarpment zone. These have steep bedrock channels which show sharp and prominent knickpoints along their longitudinal profiles. It appears that fluvial processes are driving escarpment retreat, as rivers migrate headwards were they incise into the high-elevation plateau. However, the average catchment-wide denudation rates of the escarpment zone are low compared to the denudation rates that are estimated for constant escarpment retreat since rifting. In common with other escarpments worldwide, causes for this slow down can be tectonic change related to flexural bending of the lithosphere, climate change that would vary the degree of precipitation focused into the escarpment, or the decrease in the contributing catchment area, which would reduce the stream power available for fluvial erosion.