Isotopic evidence for eolian recycling of pedogenic carbonate and variations in carbonate dust sources throughout the southwest United States

Using isotopic ratios of Sr, C and O, we trace calcium carbonate through su rface systems across a wide region of semi-arid terrain in Arizona, southwe stern USA, in order to evaluate the contribution of cations from silicate w eathering to soil carbonate. We present Sr-87/Sr-86 ratios of soil carbonat e, parent rock, dry river course silts, floodplain, playa, dust and rain sa mples, as well as delta(18)O and delta(13)C values of selected samples. Res ults show that both parent rock and dust are important sources of cations f or soil carbonate in this inland setting where bedrock is dominated by sili cate lithologies. Dust in southeast Arizona has higher Sr-87/Sr-86 ratios ( 0.7100-0.7123) than Phanerozoic sea water (0.7070-0.7096). These high ratio s derive ultimately from silicate rocks. Our delta(18)O and delta(13)C data show clearly that the dominant source of carbonate dust is eroded older so il carbonate, not bedrock limestone. Because dust contributes significantly to newly-forming soil carbonate, some products of silicate weathering may reside in soil carbonate two or more times before being removed from the re gion, and this recycling retards the rate at which the products of silicate weathering enter the sedimentary system. Comparison of the Sr-87/Sr-86 rat ios of carbonate dust from southeast Arizona with those from surrounding re gions shows that dust Sr-87/Sr-86 ratios, and by inference carbonate dust s ources, vary on a scale of 200-300 km in the southwest United States. Copyr ight (C) 2000 Elsevier Science Ltd.