Cosmogenous sediments

Cosmogenic Sediment component(s) derived from outside the Earth system. 

The geochemistry of marine sediments is a major source of information about the past environment. Of the many measurements that provide such information, those of the U-series nuclides are unusual in that they inform us about the rate and timescales of processes. Oceanic processes such as sedimentation, productivity, and circulation, typically occur on timescales too short to be assessed using parent-daughter isotope systems such as Rb-Sr or Sm-Nd. So the only radioactive clocks that we can turn to are those provided by cosmogenic nuclides (principally or the U-series nuclides. This makes the U-series nuclides powerful allies in the quest to understand the past ocean-climate system and has led to their widespread application over the last decade. 

The unique chemical composition of cosmogenous debris has provided some insight into why approximately 70% of the species of organisms on Earth were driven extinct over a relatively short time interval approximately 66 million years ago. Evidence for this mass extinction has been observed in marine sediments throughout all the ocean basins. In a contemporaneous layer deposited at the end of the Cretaceous period, the hard parts of many species of marine plankton abruptly vanished from the sedimentary record. This sedimentary layer is also characterized by a large enrichment in the rare element iridium. 

In addition to cosmic ray spallation also produces many other radioactive nuclides. °Be is another example. Once cosmogenically produced, atoms of °Be are rapidly removed from the atmosphere by meteoric precipitation, and are absorbed onto surfaces of solid particles such as clay minerals. Hence, newly formed marine sediment contains some initial concentration of °Be. After removal from the atmosphere, the concentration of °Be in sediment decays away by p-decay to °B with a half-life of 1.51 million years (and a decay constant of 4.59 X 10 yr ). 

Frank M., Schwaz B., Baumann S., Kubik P.W., Suter M., and Mangini A. (1997) A 200 kyr record of cosmogenic radionuclide production rate and geomagnetic field intensity from lOBe in globally stacked deep-sea sediments. Earth Planet Sci. Lett. 149, 121-129. 

There are two sources that give rise to minor components in the marine sediments. Cosmogenous material is that derived from an extra-terrestrial source. Such material tends to comprise small i.e., <0.5 mm) black... 

Cosmogenic radionuclides applicable for dating are listed in Table 16.1. The radionuclides are produced at a certain rate by the interaction of cosmic rays with the components of the atmosphere, mainly in the stratosphere. If the intensity of cosmic rays (protons and neutrons) can be assumed to be constant, the production rate of the radionuclides listed in Table 16.1 is also constant. The cosmogenic radionuclides take part in the various natural cycles on the surface of the earth (water cycle, CO2 cycle) and they are incorporated in various organic and inorganic products of these cycles, such as plants, sediments and glacial ice. If no exchange takes place, the activity of the radionuclides is a measure of the age. 

The rate of cosmogenic production of Al is still lower than that of Be, and in fresh sediments the ratio A1 A1 is of the order of 10 ", whereas the ratio Be Be is of the order of 10. This makes the determination of A1 in terrestrial samples very difficult. On the other hand, the production rate of A1 in meteorites and samples from the surface of the moon is comparable with that of Be, because in these samples low-energy protons from the sun contribute appreciably to the production of Al. Measurement of the A1 Be ratio in extraterrestrial samples provides information about their history. 

This chapter focuses on subduction zone processes and their implications for mantle composition. It examines subduction contributions to the shallow mantle that may be left behind in the wedge following arc magma genesis, as well as the changing composition of the slab as it is processed beneath the fore-arc, volcanic front and rear arc on its way to the deep mantle. Much of this chapter uses boron and the beryllium isotopes as index tracers boron, because it appears to be completely recycled in volcanic arcs with little to none subducted into the deep mantle, and cosmogenic e, with a 1.5 Ma half-life, because it uniquely tracks the contribution from the subducted sediments. 

Valentine R., Morris J., Zheng S. H., and Cardace D. Sediment accretion, erosion and subduction along the Costa Rica convergent margin constraints provided by cosmogenic Be. J. Geophys. Res. (in revision). 

Soil production is a function of sod depth (Heimsath et al., 1997), parent material, and environmental conditions (Heimsath et al.,1999). As soil thickens, the rate of the conversion of the underlying rock or sediment to sod decreases. This has been shown using field observations of the relation between soil thickness and the abundance of cosmogenic nuclides ( °Be and A1) in the quartz grains at the rock-soil interface (Figure 24). From this work, soil production can be described by... 

Certain smaller suboxic environments within larger estuarine systems also show unbioturbated sections of sediments. Because of their generally rapid rate of accumulation, the cosmogenic nuclide Be (53 d half-life) may be used in addition to °Pb for sediment accumulation rate assessment for the more recently deposited sediments. Similarly the pattern of bomb-produced Cs in sediments can serve as a chronometer. In addition, radiocarbon ages commonly can be obtained from calcareous fractions in the accumulating sediment. All these approaches were used at the FOAM site in Long Island Sound by Krishnaswami et al. (1984). 

U/Th and other age dating of speleothems Isotopic profiles through speleothems Cosmogenic isotope dating of clastic sediments High-resolution records (One year or less)... 

Schaller, M., von Blanckenburg, F., Kubik, P. Kramers, J. D. 1999. Catchment-wide denudation rates from cosmogenic nuclides in river sediment. Jourrml of Conference Abstracts, 4, 444. 

Complex exposure histories may also show up as a disturbed pattern of cosmogenic nuclide concentration versus depth. Particular care must be taken when dating soil or alluvial deposits, which often experience varying sedimentation rates, sudden burial, or bioturbation (i.e., soil mixing by living organisms) of the uppermost layers (e.g., Phillips et al. 1998 Braucher et al. 2000). In such studies it is extremely important that depth profiles are taken. 

Brown et al. (1995b) and Bierman and Steig (1996) developed a method to estimate mean erosion rates for entire river basins from cosmogenic nuclide concentrations in river sediment. Under the assumptions that production rates within the catchment area do not vary much, that cosmogenic nuclide concentrations are in erosion equilibrium, and that sediments are not stored for long times on the valley floor, the basin-wide erosion rate is given by... 

Libarkin et al. (2002) measured cosmogenic Ne in quartz from the 28 Ma old Fish Canyon Tuff (Colorado), which is covered by later-emplaced tuff and sediment layers. 

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