E.T. Brown

In situ produced 10Be measurements at great depths: Implications for production rates by fast muons

In situ cosmogenic 10Be values have been used to investigate a Brazilian quartz vein from the surface to a depth of 15 m. At depths greater than 1000 g/cm2, deep enough for neutron-induced reactions to be insignificant, there is only a slight decrease in 10Be concentration with increasing depth. Our results are consistent with deep production of 10Be by a mechanism, presumably induced by fast muons, with an attenuation length of 5300±950 g/cm2 and a contribution of 0.65±0.25% to the total surface production. Results are compared with values from the literature and implications of this re-evaluation are discussed.

Late Pleistocene to Holocene slip rates for the Gurvan Bulag thrust fault (Gobi-Altay, Mongolia) estimated with 10Be dates

[1] We surveyed morphotectonic markers along the central part of the Gurvan Bulag thrust, a fault that ruptured with the Bogd fault during the Gobi-Altay earthquake (1957, M 8.3), to document climatic and tectonic processes along the fault for the late PleistoceneHolocene period. The markers were dated using 10 Be produced in situ. Two major periods of alluviation ended at 131 ± 20 and 16 ± 4.8 ka. These appear to be contemporaneous with global climatic changes at the terminations of marine isotope stages (MIS) 6 and 2. The vertical slip rates, determined from offset measurements and surfaces ages, are 0.14 ± 0.03 mm/yr over the late Pleistocene-Holocene and between 0.44 ± 0.11 and 1.05 ± 0.25 mm/yr since the end of the late Pleistocene. The higher of these slip rates for the last � 16 kyr is consistent with paleoseismic investigations along the fault [Prentice et al., 2002], and suggests that, at the end of late Pleistocene, the fault evolved from quiescence to having recurrence intervals of 4.0 ± 1.2 kyr for surface ruptures with � 4 m vertical offset (similar to that of 1957). The inferred recurrence interval is comparable to that of the Bogd fault (3.7 ± 1.3 kyr) suggesting that the two faults may have ruptured together also earlier during the last � 16 kyr. INDEX TERMS: 7221 Seismology: Paleoseismology; 1208 Geodesy and Gravity: Crustal movements—intraplate (8110); 1824 Hydrology: Geomorphology (1625); 7230 Seismology: Seismicity and seismotectonics; 8107 Tectonophysics: Continental neotectonics; KEYWORDS: Late Pleistocene, Holocene, thrust fault, slip rate, 10Be dating, Mongolia

Application of in situ-produced cosmogenic 10Be and 26Al to the study of lateritic soil development in tropical forest: theory and examples from Cameroon and Gabon

Abstract Depth profiles of in situ-produced cosmogenic nuclides, including 10 Be ( T 1/2 =1.5×10 6 years) and 26 Al ( T 1/2 0.73×10 6 years), in the upper few meters of the Earths crust may be used to study surficial processes, quantifying denudation and burial rates and elucidating mechanisms involved in landform evolution and soil formations. In this paper, we discuss the fundamentals of the method and apply it to two lateritic sequences located in African tropical forests.

African laterite dynamics using in situ-produced 10Be

Abstract We have investigated the development of iron crust lateritic systems and rain forest soils in tropical environments using in situ-produced 10Be in quartz veins and cobbles. The variability of its concentration as function of subsurface depth in soil profile provides criteria for distinguishing between allochthonous (elsewhere formation) and autochthonous (in situ formation) processes and hence for determining whether colluvial transport or in situ chemical weathering dominates soil development in these environments. In the stable West African Craton in Southwest Burkina Faso at Larafella, the exponential decrease in 10Be concentration observed along a quartz vein from the lowland lateritic system corresponds to the decrease in its production rate associated with attenuation of cosmic rays and thus indicates an autochthonous development. Models of the 10Be depth profile in this unit suggest that this surface has been subject to erosion at a mean rate of ∼2 m · My−1 and that ∼1.5% of the surface 10Be production rate is associated with muon-induced reactions. In a contrasting environment, tropical rain forest (Malemba, Congo), 10Be concentrations have been measured in quartz vein and in round and angular quartz cobbles incorporated in a downslope “stone-line.” Comparison with data from Larafella suggests that the angular cobbles have been transported a few meters from the quartz veins by downslope lateral creeping, whereas the round cobbles have an allochthonous origin. Models of 10Be distributions as this site yield an erosion rate of ∼12 m · My−1 and a rate of lateral creeping on the order of 60 m · My−1.

Beryllium isotope systematics of submarine hydrothermal systems

Abstract Both 10 Be and 9 Be concentrations have been measured in a hydrothermal fluid from the sediment-starved East Pacific Rise at 21°N as well as in fluids and associated sediments from the sediment covered Guaymas Basin and Escanaba Trough hydrothermal systems. The results show that while the 9 Be concentrations in fluids from both types of systems are comparable, about three orders of magnitude higher than that of the seawater supplying the systems, their 10 Be concentrations are significantly different. The 10 Be concentration in the solution emanating from the studied sediment-starved hydrothermal system is at least two times lower than that of the Pacific bottom water. By contrast, the 10 Be concentration in hydrothermal solutions from sediment-hosted environments are enriched over ambient seawater and are at least one order of magnitude higher than that measured in the 21°N fluid. The sedimentary cover being the only available source of 10 Be in these systems, these data demonstrate that 10 Be is an unambiguous tracer of interactions between hydrothermal fluids and recent sediments. As an example, this study examines hydrothermal solutions from the sediment-starved Mariana Trough back-arc basin which have compositions similar in many respects to those from sediment-hosted environments. However, these fluids are depleted in 10 Be relative to ambient seawater, which precludes interaction with recent sediment as the cause of their peculiar chemical composition.