10-Be exposure dating

Quaternary tectonic and denudation rates are investigated for an actively growing basement anticline: the Sierra Pie de Palo range, which belongs to the Andean foreland of Northwestern Argentina (28 • S-33 • S). In this study, a detailed morphometric analysis of the topography is combined with in situ-produced cosmogenic 10 Be concentrations measured in (1) surface boulders abandoned on alluvial terraces affected by fault activity (along the north bounding fault) and growth of the basement fold (along the southeastern border), (2) bedrock outcrops corresponding to an exhumed and folded, regional erosion surface, and (3) fluvial sediments sampled at the outlets of several watersheds. Along the eastern and northern borders of the range, incision and uplift rates have been estimated at approximately 0.5 and 1 mm/yr when integrated on Holocene and Pleistocene time scales, in close agreement with both long-term (structural and basin evolution data) and short-term (GPS-derived velocity field) analyses. Cosmogenic-derived denudation and uplift rates combined with geomorphic characteristics of watersheds and river channels allows estimating the onset of the uplift at 4-6 Ma, followed by a more recent period of topographic rejuvenation at roughly 1-2 Ma, probably synchronous with steepening of the eastern and northern flanks of the anticline.

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inherited nuclide component amalgamation approach fluvial terraces Andean Precordillera Exposure dating of boulders has been widely applied to determine the age of depositional surfaces under the assumption that the pre-depositional nuclide component in most boulders is negligible. Here we present a case study on fluvial terraces at the active mountain front of the eastern Andes, where this assumption is clearly invalid, because sandstone boulders (n = 13) from terraces at two sites contain a highly variable inherited 10 Be component and have apparent 10 Be ages that exceed the age of the respective surface by up tõ 90 ka. Likewise, boulders from active stream channels (n = 5) contain a substantial inherited 10 Be component, equivalent to 5-48 ka of exposure. The age of the fluvial terraces is well determined by two approaches that allow to correct for the pre-depositional nuclide component: 10 Be dating of amalgamated pebbles and 10 Be depth profiles on sand samples. At site 1, three terraces have 10 Be ages of 3-5 ka (T 2), 11-13 ka (T 3), and 16-20 ka (T 4), which are consistent with the terrace stratigraphy. The age of terrace T 3 is confirmed by a calibrated 14 C age of 12.61 ± 0.20 ka BP obtained from a wood sample. At site 2, terrace T 3 has a 10 Be age of 13-16 ka. The average inherited 10 Be concentration of sand grainsdetermined from depth profiles and stream sedimentsis small and equivalent to 1-3 ka of exposure. In contrast, the mean inheritance of pebbles and boulders is higher and equivalent to exposure times of~10 ka and~30 ka, respectively. These differences in the pre-depositional nuclide component are related to the different provenance and transport history of sand, pebbles, and boulders. The sand is derived from rapidly eroding Miocene sediments exposed near the mountain front, whereas the pebbles and boulders originate from Triassic sandstones in the internal part of the fold-and-thrust belt. On their way to the mountain front, boulders and pebbles were temporarily stored and irradiated in alluvial fans that are currently reworked. As sediment deposition in intramontane basins and their subsequent excavation is common in the Andes and other fold-and-thrust belts, the presence of pre-depositional nuclide components should be evaluated when applying exposure dating at active mountain fronts.

Ice sheet reconstructions from diverse ice margin settings, spanning multiple millennia, are needed to assess the reaction of the Greenland Ice Sheet (GrIS) to millennial-scale climatic forcing and to place historical records in a longer-term context. Here we present 18 new cosmogenic 10 Be exposure ages and five new radiocarbon ages that constrain the early Holocene retreat of the GrIS in the Disko Bugt region in both a marine and a land-based setting. Results indicate similar rates of early Holocene retreat of $40-50 m a À1 from transects in Torsukattak fjord (marine setting) and the Naternaq area (land-based setting). We compile seven previously published chronologies of deglaciation from West Greenland, which yield early Holocene retreat rates ranging from 10 to 65 m a À1 , similar to those determined for our two study areas. This work demonstrates that when averaged on millennial timescales, retreat rates were remarkably similar along the western GrIS margin. Furthermore, the retreat rates calculated here demonstrate that terrestrial sectors of ice sheets can retreat at net rates comparable to their marine counterparts.

inherited nuclide component amalgamation approach fluvial terraces Andean Precordillera Exposure dating of boulders has been widely applied to determine the age of depositional surfaces under the assumption that the pre-depositional nuclide component in most boulders is negligible. Here we present a case study on fluvial terraces at the active mountain front of the eastern Andes, where this assumption is clearly invalid, because sandstone boulders (n = 13) from terraces at two sites contain a highly variable inherited 10 Be component and have apparent 10 Be ages that exceed the age of the respective surface by up tõ 90 ka. Likewise, boulders from active stream channels (n = 5) contain a substantial inherited 10 Be component, equivalent to 5-48 ka of exposure. The age of the fluvial terraces is well determined by two approaches that allow to correct for the pre-depositional nuclide component: 10 Be dating of amalgamated pebbles and 10 Be depth profiles on sand samples. At site 1, three terraces have 10 Be ages of 3-5 ka (T 2), 11-13 ka (T 3), and 16-20 ka (T 4), which are consistent with the terrace stratigraphy. The age of terrace T 3 is confirmed by a calibrated 14 C age of 12.61 ± 0.20 ka BP obtained from a wood sample. At site 2, terrace T 3 has a 10 Be age of 13-16 ka. The average inherited 10 Be concentration of sand grainsdetermined from depth profiles and stream sedimentsis small and equivalent to 1-3 ka of exposure. In contrast, the mean inheritance of pebbles and boulders is higher and equivalent to exposure times of~10 ka and~30 ka, respectively. These differences in the pre-depositional nuclide component are related to the different provenance and transport history of sand, pebbles, and boulders. The sand is derived from rapidly eroding Miocene sediments exposed near the mountain front, whereas the pebbles and boulders originate from Triassic sandstones in the internal part of the fold-and-thrust belt. On their way to the mountain front, boulders and pebbles were temporarily stored and irradiated in alluvial fans that are currently reworked. As sediment deposition in intramontane basins and their subsequent excavation is common in the Andes and other fold-and-thrust belts, the presence of pre-depositional nuclide components should be evaluated when applying exposure dating at active mountain fronts.

Exposure dating of boulders has been widely applied to determine the age of depositional surfaces under the assumption that the pre-depositional nuclide component in most boulders is negligible. Here we present a case study on fluvial terraces at the active mountain front of the eastern Andes, where this assumption is clearly invalid, because sandstone boulders (n = 13) from terraces at two sites contain a highly variable inherited 10 Be component and have apparent 10 Be ages that exceed the age of the respective surface by up tõ 90 ka. Likewise, boulders from active stream channels (n = 5) contain a substantial inherited 10 Be component, equivalent to 5-48 ka of exposure. The age of the fluvial terraces is well determined by two approaches that allow to correct for the pre-depositional nuclide component: 10 Be dating of amalgamated pebbles and 10 Be depth profiles on sand samples. At site 1, three terraces have 10 Be ages of 3-5 ka (T 2 ), 11-13 ka (T 3 ), and 16-20 ka (T 4 ), which are consistent with the terrace stratigraphy. The age of terrace T 3 is confirmed by a calibrated 14 C age of 12.61 ± 0.20 ka BP obtained from a wood sample. At site 2, terrace T 3 has a 10 Be age of 13-16 ka. The average inherited 10 Be concentration of sand grainsdetermined from depth profiles and stream sedimentsis small and equivalent to 1-3 ka of exposure. In contrast, the mean inheritance of pebbles and boulders is higher and equivalent to exposure times of~10 ka and~30 ka, respectively. These differences in the pre-depositional nuclide component are related to the different provenance and transport history of sand, pebbles, and boulders. The sand is derived from rapidly eroding Miocene sediments exposed near the mountain front, whereas the pebbles and boulders originate from Triassic sandstones in the internal part of the fold-and-thrust belt. On their way to the mountain front, boulders and pebbles were temporarily stored and irradiated in alluvial fans that are currently reworked. As sediment deposition in intramontane basins and their subsequent excavation is common in the Andes and other fold-and-thrust belts, the presence of pre-depositional nuclide components should be evaluated when applying exposure dating at active mountain fronts.