Pedogenic carbonate carbon isotopic constraints on paleoprecipitation: Evolution of desert in the Pacific Northwest, USA, in response to topographic development of the Cascade Range

The Cascade Range in the northwestern U.S. acts as an orographic barrier to the easterly movement of humid air masses from the Pacific Ocean, producing a massive rain shadow with arid to semi-arid climate conditions on the leeward side of the range. Pedogenic carbonates formed in the C-3-dominated landscapes east of the range have delta C-13(carbonate) that is enriched by approximately 4 parts per thousand relative to carbonates formed in sub-tropical C-3-dominated ecosystems. We measured stable isotopic compositions of pedogenic carbonates preserved in the Quaternary and Neogene sedimentary rock units in the inland Pacific Northwest, and examined the impact of long-term aridification and paleohydrology on their isotopic compositions. The determined delta C-13(carbonate) record shows a gradual increase of 5 parts per thousand from approximately -10 parts per thousand in older pedogenic carbonates to -5 parts per thousand in younger ones whereas delta O-18(carbonate) record indicates a decrease of approximately 4 parts per thousand in the late Neogene. In addition, the Quaternary delta C-13(carbonate) and delta O-18(carbonate) records indicate the highest delta C-13 and delta O-18 among the samples during the Pleistocene and significant fluctuation over the time period. The gradual increase in the delta C-13(carbonate) ate and the decrease in the delta O-18(carbonate) in the late Neogene are interpreted by the development of the rain shadow as a result of topographic development of the Cascade Range. Changes in the Quaternary isotope records suggest a decrease in regional precipitation and isotopically depleted winter precipitation, resulting from changes in large-scale atmospheric circulation patterns due to advance and retreat of northern hemisphere continental ice sheet. The isotope record from pedogenic carbonates provides a direct record of climatic change, suggesting that long-term carbon isotope data can provide useful constraints on timing and intensity of regional desertification, and its associated climate and topographic interaction. (C) 2010 Elsevier B.V. All rights reserved.