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 surface systems across a wide region of semi-arid terrain in Arizona, southwestern USA, in order to evaluate the contribution of cations from silicate weathering to soil carbonate. We present ⁸⁷Sr/⁸⁶Sr ratios of soil carbonate, parent rock, dry river course silts, floodplain, playa, dust and rain samples, as well as δ¹⁸O and δ¹³C values of selected samples. Results show that both parent rock and dust are important sources of cations for soil carbonate in this inland setting where bedrock is dominated by silicate lithologies. Dust in southeast Arizona has higher ⁸⁷Sr/⁸⁶Sr ratios (0.7100-0.7123) than Phanerozoic sea water (0.7070-0.7096). These high ratios derive ultimately from silicate rocks. Our δ¹⁸O and δ¹³C data show clearly that the dominant source of carbonate dust is eroded older soil 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 region, and this recycling retards the rate at which the products of silicate weathering enter the sedimentary system. Comparison of the ⁸⁷Sr/⁸⁶Sr ratios of carbonate dust from southeast Arizona with those from surrounding regions shows that dust ⁸⁷Sr/⁸⁶Sr ratios, and by inference carbonate dust sources, vary on a scale of 200-300 km in the southwest United States. Copyright (C) 2000 Elsevier Science Ltd.