Lake Sediment Records of Late Holocene Climate Change in the Khanuy Valley, Arkhangai Aimag, North-Central Mongolia and Implications for Local Bronze and Iron Age Cultural Histories

Late Holocene geochemical records from three lakes in the Khanuy Valley, Arkhangai Aimag, north-central Mongolia document centennial and millennial-scale climate variability within central Asia. The oldest sediments of the shallow Tsegeen Nuur basin (49.09°N, 101.86°E) and nearby Sharga Nuur (48.92°N, 101.96°E) indicate lake filling and open water deposition by at least 3600 years before present (yrs. B.P.). Minimum oxygen isotope (δ18O) values, suggesting high water levels and relatively wet conditions, are preserved in biogenic carbonates (ostracod shells) from both lakes during the same time interval. Mid-core sedimentary sequences in Tsegeen Nuur and Sharga Nuur (~2200 to 400 yrs. B.P.) exhibit weak blocky and columnar structures. These lithologic changes indicate lake drying, sub-aerial exposure, and some soil development. However, these soil-like horizons contain sparse shell remains of both ostracods and bivalves and therefore suggest that both lakes held water at least periodically during this time. Ostracod shells from this interval record the greatest δ18O values and support the inference for low lake stage. No evidence exists for lake desiccation at a deeper and higher elevation lake core site (Doroo Tsagaan Nuur, 49.02°N, 101.20°E) located approximately eighty kilometers west of the Tsegeen and Sharga Nuur basins. This also suggests that although late Holocene moisture availability was significantly reduced, aridification was likely not extreme. At Tsegeen and Sharga Nuur, wet conditions appear to have returned after 400 yrs. B.P., as indicated by a decrease in δ18O values and the resumption of lacustrine deposition. It should be noted that archaeological evidence suggests that the Khanuy Valley was greatly populated between ~3200 and 2700 yrs. B.P., roughly coinciding with the wettest period recorded within these lake core sites. Human occupation of the Khanuy Valley may therefore have been linked, at least in part, to regional climate changes.