Continental composition

This chapter examines climatic and tectonic controls on erosion in the tropics and the implications of these observations regarding the composition of erosion products in general. The role of glaciations in continental denudation will then be examined and contrasted with tropical conditions. Finally, we will briefly examine human effects. 

Bryant, J.D., Luz, B. and Froelich, P.N. 1994 Oxygen isotopic composition of fossil horse tooth phosphate as a record of continental paleoclimate. Palaeogeography, Palaeoclimatology, Palaeoecology 107 303-316. 

Suggestions that phosphatic minerals in mammals could be used, however, revived the interest in climate reconstruction in continental interiors. Aquatic, cold-blooded animals like fish have body temperatures and body water oxygen isotopic compositions that are directly dependent on the water in which they live. For these animals, a commonly used equation describes the relationships among temperature, water oxygen isotopic composition and phosphate oxygen isotopic composition as (Longinelli and Nuti 1973 verified by Kolodny et al. 1983, among others) ... 

Price, T.D. and Kavanagh, M. 1982 Bone composition and the reconstruction of diet examples from the midwestem United States. Mid-Continental.Journal of Archaeology 7 61-70. Quilter, J. 1989 Life and Death at Paloma. Iowa City, University of Iowa Press. 

Rubin KH, Wheller GE, Tanzer MO, MacDougall JD, Vame R, Finkel R (1989) decay series systematics of young lavas from Batur volcano, Sunda arc. J Volcanol Geotherm Res 38 215-226 Rudnick RL, Fountain DM (1995) Nature and composition of the continental cmst a lower cmstal perspective. Rev Geophys 33 267-309... 

Fritz P, Drimmie RJ, Frape SK, O Shea K (1987) The isotopic composition of precipitation and groundwater. In Canada International Symposium on the Use of Isotope Techniques in Water Resources Development. IAEA Symposium, Vienna 299 539-550 Frumkin A, Ford DC, Schwarcz HP (1999) Continental oxygen isotopic record of the last 170,000 years in Jerasalem. (JuatRes 51 317-327... 

Taylor S.R., McLennan S.M. The continental crust Its composition and evolution, Oxford Blackwell, 1985. 

Wedepohl K.H. The composition of the continental cmst. Geochim Cosmochim Acta 1995 59 1217-1232. 

Yan, M.C. Chi, Q.H. 2005. The Chemical Compositions of the Continental Crust and Rocks in the Eastern Part of China. Beijing Science Press. 171 p. 

Let us first introduce some important definitions with the help of some simple mathematical concepts. Critical aspects of the evolution of a geological system, e.g., the mantle, the ocean, the Phanerozoic clastic sediments,..., can often be adequately described with a limited set of geochemical variables. These variables, which are typically concentrations, concentration ratios and isotope compositions, evolve in response to change in some parameters, such as the volume of continental crust or the release of carbon dioxide in the atmosphere. We assume that one such variable, which we label/ is a function of time and other geochemical parameters. The rate of change in / per unit time can be written... 

L. Lampert, B. Queguiner, T. Labasque, A. Pichon and N. Lebreton, Spatial variability of phytoplankton composition and biomass on the eastern continental shelf of the Bay of Biscay (north-east Atlantic Ocean). Evidence for a bloom of Emiliana huxleyi (Prymnesiophyceae) in spring 1998. Coni. Shelf Res. 22 (2002) 1225-1247. 

Xenoliths from Siberian continental lithosphere, with Archean model ages, had b Li as low as +0.5 (Eouman et al. 2000). If these values accurately represent the Archean mantle, they suggest the potential for Li isotopic evolution in the Earth, from lighter compositions in the ancient mantle to what is seen in present-day MORE. In spite of the analytical challenges presented by ultramafic rocks, more data from these materials are crucial to an understanding of Li in the mantle, and in resolving questions about the appropriateness of the accepted MORE mantle range. 

Regardless of the ultimate sources of these compositions, these results clearly show that strongly isotopically fractionated Li from crustal sources plays a role in the mantle. Processes active in subduction zones appear to be cardinal in the control of the Li isotopic composition of different parts of the mantle. The results to date imply that both isotopically enriched (8 Li > MORE) and depleted (5T i < MORE) material are available for deep subduction, and that areas of the continental lithosphere may retain these records on long time scales. 

Figure 14. Plot of Li isotopic composition vs. concentration of thermal waters from the continents and the oceans (see text for references). Differences between the isotopic range of marine versus non-marine fluids emphasizes the variability in 5 Li of continental rocks compared to oceanic basalt. The dilution of the continental fluids goes along with their lower temperatures vent fluids are the only truly geothermal samples here, with temperatures in excess of 300°C.

Combinations of mineral reactions at lower temperatures and mixing with more dilute fluids are likely to result in the variations in concentration and isotopic composition in many of the continental thermal spring waters but not seen in their marine relatives. The extreme manifestation of this difference may have been generated in the dilute hot spring waters from around Lake Baikal, whose heavy isotopic compositions required extensive re-equilibration at temperatures 100-150°C (Falkner et al. 1997). 

---