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 C3-dominated landscapes east of the range have δ13Ccarbonate that is enriched by approximately 4‰ relative to carbonates formed in sub-tropical C3-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 δ13Ccarbonate record shows a gradual increase of 5‰ from approximately − 10‰ in older pedogenic carbonates to − 5‰ in younger ones whereas δ18Ocarbonate record indicates a decrease of approximately 4‰ in the late Neogene. In addition, the Quaternary δ13Ccarbonate and δ18Ocarbonate records indicate the highest δ13C and δ18O among the samples during the Pleistocene and significant fluctuation over the time period. The gradual increase in the δ13Ccarbonate and the decrease in the δ18Ocarbonate 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.