A paleoclimate record for the past 250,000 years from Summer Lake, Oregon,USA: I. Chronology and magnetic proxies for lake level

This study presents the age control and environmental magnetism components of a new, late Pleistocene paleoclimate record for the Great Basin of weste rn North America. Two new cores from the Summer Lake sub-basin of pluvial L ake Chewaucan, Oregon, USA are correlated to basin margin outcrops on the b asis of tephrochronology, lithostratigraphy, sediment magnetism and paleoma gnetic secular variation. Eleven tephra layers were found in the cores that correlate to tephra identified previously in the outcrop. The Olema ash wa s also found in one of the cores; its stratigraphic position, relative to 3 dated tephra layers, indicates that its age is 50-55 ka, somewhat younger than has been previously reported. The Summer Lake sediments are divided in to deep and shallow lake lithosomes based on sedimentary features. The stra tigraphic position of these lithosomes support the tephra-based correlation s between the outcrop and the cores. These sediments contain a well resolve d record of the Mono Lake Excursion (MLE) and an earlier paleomagnetic excu rsion as well as a high quality replication of the paleosecular variation i mmediately above the MLE. Relative sedimentation rates increased dramatically toward the depocenter d uring intervals of low-lake level. In contrast, during intervals of high-la ke level, relative sedimentation rates were comparable along the basin axis from the basin margin to the depocenter. The magnetic mineralogy of the Su mmer Lake sediments is dominated by pseudo-single domain (titano)magnetite and intervals of high/low magnetite concentration coincide with lithosomes that indicate high/low lake levels. Magnetic grain size also varies in acco rd with bulk sediment grain size as indicated by the silt/clay ratio. To a first order, variations in magnetic parameters, especially those attributab le to the concentration of magnetic minerals, correlate well with global gl acial/interglacial oscillations as indicated by marine oxygen isotope stage s. This relationship can be explained by increased dissolution of (titano)m agnetite minerals as lake level dropped and the lake became more productive biologically. This inference is supported by a correspondence between lowe r concentrations of magnetite with higher levels of total organic carbon an d vice-versa.