EVIDENCE FOR HOLOCENE ENVIRONMENTAL-CHANGE FROM C N RATIOS, AND DELTA-C-13 DELTA-N-15 VALUES IN SWAN LAKE-SEDIMENTS, WESTERN SAND HILLS, NEBRASKA/

Profiles of percent carbon and nitrogen, carbon/nitrogen (C/N) ratios and stable carbon (delta(13)C), and nitrogen (delta(15)N) isotopic rat ios in organic matter from an 11.6 m core were used to reconstruct env ironments of deposition in the Swan Lake basin during the past 5300 YB P. The upper 6.5 m consisted of gyttja containing variable amounts of reddish brown-colored fine organic matter and calcium carbonate. It wa s followed by a 0.5 m sandy silt, which was followed by a 3.6 m reduce d layer characterized by large quantities of black organic plant remai ns, sapropel, and then by another sapropel layer consisting mainly of well-sorted sapropelic sand with relatively low organic matter content . The C-and N-contents in the organic matter in the sediment profile r anged from 0.5 to 23% and from 0.02 to 2%, respectively. Carbon conten t were positively correlated to both N and clay content while carbon c ontent was negatively correlated to sand content. Two major environmen tal phases in Swan Lake were apparent from large differences in the C and N data of the sediment organic matter. These include the sapropel (marsh) stage that stretched from approximately 5330 to 3930 YBP, and the following gyttja (open water stage). During the sapropel marsh pla nts identified in a previous pollen study as cattails and sedges proli ferated and produced copious amounts of well-preserved organic matter. C/N ratios, delta(13)C values, and delta(15)N values in the sapropel were significantly different from those that characterized organic mat ter in the gyttja. During the gyttja delta(13)C values indicated that deep primary producers have dominated lake biomass. By utilizing bicar bonate as their C-source, the accumulating biomass became relatively e nriched delta(13)C values. The presence of high sediment CaCO3 content s indicated more alkaline and deeper water conditions prevailed during the gyttja. Further refinement of the data suggested that each major phase initially contained an identifiable transition stage. During the sapropelic (initial marsh stage) which occurred before 5330 YBP, sand content gradually decreased as organic matter increased. As reflected by high C/N ratios and slightly enriched delta(13)C values, these san ds appear to have contained sufficient permeability to promote partial mineralization of accumulated organic-N containing compounds. A short initial gyttja transition period from about 3930-3830 YBP occurred in which the sediment silt content was anomalously high relative that me asured in the surrounding layers. The silt content suggests that this turbid transition layer can not be completely explained by sediment mi xing via bioturbation. The silts appeared to have been associated with the sharp climate change that resulted in higher water-table conditio ns during the gyttja stage.