Radiocarbon-dated ferricrete provides a record of natural acid rock drainage and paleoclimatic changes

Ancient deposits of ferricrete (stratified iron oxyhydroxide or elastic sed iment cemented by stratified iron oxyhydroxide) are present in headwater st reams draining an historic mining district in the Rocky Mountains, USA. The se deposits, which form under conditions of low pH and high metal concentra tions, are present in streams draining many other mineralized areas. Radioc arbon dating of wood fragments entombed within the ferricrete indicates dat es of deposition as old as 8840 +/- 50 years before present (b.p.). The fer ricrete deposits provide evidence that natural, metal-rich acid rock draina ge has been occurring in this mineralized region for thousands of years, du e to the weathering and oxidation of exposed or near-surface massive sulfid e ore deposits. The dating and chemical composition of the ferricrete depos its have applications to the environmental remediation of historic mining s ites and to environmental issues related to natural background chemistry. I n addition, the radiocarbon dates of the ferricrete deposits correlate clos ely with warm-wet periods in the Holocene record, showing that ferricrete d eposits may represent a previously untapped source of paleoclimatic informa tion.