Ancient meteorite finds and the earth's surface environment

The flux of meteorites to the Earth over the last 50,000 yr has remained ap proximately constant. Most meteorites that fall in temperate or tropical ar eas are destroyed on a time scale which is short compared to the rate of in fall; however, in arid regions (both "hot" deserts and the "cold" desert of Antarctica) weathering is slower and accumulations of meteorites may occur , The initial composition for many meteorite groups is well known from mode rn falls, and terrestrial ages may be established from analyses of the abun dance of cosmogenic radionuclides, providing an absolute chronology for rec ording terrestrial processes. As samples are falling constantly, and are di stributed approximately evenly over the Earth, meteorites may thus be thoug ht of as an appropriate "standard sample" for studying aspects of the terre strial surface environment, Studies involving C-14 and Cl-36 terrestrial ag es of meteorites, Fe-57 Mossbauer spectroscopy (to quantify the degree of o xidation in samples), stable isotopes, and determination of halogen abundan ces are yielding information on the terrestrial history of meteorites: (i) terrestrial age and oxidation-frequency distributions for populations of sa mples allow the ages of surfaces to be estimated; (ii) differences in the w eathering rate of samples between sites allows constraints to be imposed on the effect of climate on rock weathering rates; (iii) carbon isotopic comp ositions of generations of carbonate growth within meteorites allows, in so me cases, temperatures of formation of carbonates to be estimated; (iv) str ucture in the oxidation-terrestrial age distribution for meteorites from so me arid accumulation sites (specifically, the Nullarbor of Australia) appea rs to be linked to previous humid/arid cycles; (v) meteorite accumulations in Antarctica have been used to constrain aspects of the Quaternary evoluti on of the ice sheet, and terrestrial age and oxidation data have been used to constrain ice flow. (C) 2000 University of Washington.