Late Holocene climate change inferred from the lithology and chemical stratigraphy of sediment cores from Swan Lake, Nebraska Sandhills, USA

Continuous sediment cores H1 (11.0 m long) and H2 (8.5 m long), collected from Swan Lake, in the Nebraska Sandhills (USA), contain a sandy silt interval between overlying lake gyttja and underlying marsh peat. The marsh peat is a 3-m-thick layer in core H1 and a 1.5-m-thick layer in core H2, with basal ages of ca. 4790 and 4710 cal yr BP, respectively. Peat samples contain up to ~ 61% organic matter (OM) and lack CaCO₃ and opal phases. Gyttja extends from 7.6 to 0 m in core H1 and from 6.6 to 0 m in core H2, with estimated basal ages of 3480 and 3530 cal yr BP, respectively. Gyttja samples contained stratigraphically fluctuating relative amounts of OM, precipitated CaCO₃, biogenic (ostracod) CaCO₃, and diagenetic mineralogical aggregates with CaCO₃-opal-A-opal-CT-quartz or opal-A-quartz. OM content in the gyttja ranged from 15.1 to 32.1% and bulk CaCO₃ content ranged from 13.5 to 34.6%. Mineralogical aggregates appeared after ~ 2630 cal yr BP in core H1 and after ~ 2590 cal yr BP in core H2, whereas ostracods were found only after ~ 1250 and ~ 1840 cal yr BP in the two cores, respectively. The discrepancy in the timing of ostracod occurrence may be related to environmental differences at the two locations, which are ~ 100 m apart. The sandy silt interval (0.4 m thick) between the peat and overlying gyttja accumulated between ~ 3700 and 3500 cal yr BP. It has relatively low OM (10.3–15.1%) and calcite content (0–6.1%) and no ostracods or mineralogical aggregates. The sandy silt interval possessed a high clastic content (~ 88%), with up to 3.2% Fe, 0.25% Ti, 108 ppm Rb, 105 ppm Zr, 65.5 ppm Cr, 36.6 ppm La, 10 ppm Th, and 2.6 ppm U. The clastic particles and associated metals were deposited by eolian processes during a dry and windy climate episode at the onset of the late Holocene in the Sandhills region.