Lake Sediments as Evidence of Natural and Human-Induced Environmental Change from California and Nevada

This study focuses on the history of natural and human-induced environmental change as recorded in the sediments of two lakes: Mountain Lake in the Presidio National Park, San Francisco, California and Big Soda Lake, near Fallon, Churchill County, Nevada. The records of these lakes examined in this study cover approximately the last 2,000 years. Sediment cores from the lakes were dated with radiocarbon, lead-210, plutonium 240/239, tephrachronology, and the first appearance of non-native pollen types. The cores were analyzed to determine changes in stable isotopes (carbon and oxygen), sediment chemistry, fossil pollen, magnetic susceptibility, organic content, and brine shrimp cyst concentrations.Big Soda Lake has been the subject of scientific investigation since the 19th Century and two famous scientists have previously worked at the site. First, the geologist, Israel Russell explored the lake in 1882 as part of his work on Pleistocene Lake Lahontan and provided the first scientific report on the lake. Later in 1933, Evelyn Hutchinson, the famous Yale limnologist, provided the first detailed limnological report for the lake. More recently in the 1980's, the lake has been studied by scientists from the United States Geological Survey. However, prior to the research reported on here, very little was known of the history of the lake or to what extent its sediments contained a useful record of environmental change.The sediments of Big Soda Lake provide clear evidence for both natural and human-induced environmental change during the past 1600 years. The climate record developed from the analyses of stable isotopes of oxygen and carbon, sediment chemistry, and the concentrations of brine shrimp cysts show several significant shifts in climate. The early part of the record from A.D. 400-850 is period marked by a fluctuating climate, with alternating wet/dry phases each lasting several decades each (40-60 years). During the period known as the Medieval Climate Anomaly (MCA)(A.D. 850-1400), we observe at least two relatively dry periods from A.D. 850-1150 and A.D. 1260-1400. Between the two dry phases, there is a pronounced wet period from A.D. 1150-1260. This wet period matches fairly well with evidence presented in other paleoenvironmental studies in the western Great Basin. During the Little Ice Age (LIA), the evidence indicates that the Big Soda Lake area was not always colder and/or wetter, but that it was in fact drier and perhaps warmer from A.D. 1400-1700 than it had been in the previous millennium. Pronounced dry phases were observed around A.D. 1400, A.D. 1500 and A.D. 1650. The wettest period during the LIA came between A.D. 1750-1800.The human impact record at Big Soda Lake developed from the analyses of stable isotopes of oxygen and carbon, sediment chemistry, and the concentrations of brine shrimp cysts show several dramatic changes in and around the lake since Anglo American settlement of the area began, in the 1850's. Several human impacts have been identified, including regional mining activity, soda salt extraction from the lake, and irrigation induced rising groundwater levels in the last century. Two of these events have dramatically impacted the lake in that time. Firstly, the development of a commercial soda manufacturing and processing facility at the lake beginning in 1875 until the early 20th century; and secondly, the development of irrigation agriculture which led to an 18 m rise in lake level in the first few decades of the twentieth century.The sediments at Mountain Lake provide evidence of unprecedented heavy metal contamination at the San Francisco Presidio during the past 60 years. The lake evidence is consistent with local land-use changes initiated by the arrival of Europeans in the area after 1776 and the construction of California State Highway 1 adjacent to the lake in the late 1930's. The study shows how small water bodies alongside roads can concentrate heavy metals and demonstrates the need for careful scientific investigation of sediments earmarked for dredging to determine what if any contaminants are present. A key outcome of the Mountain Lake research carried out as part of this dissertation was that in the Fall of 2011 a Federal judge ordered the California Department of Transportation (Caltrans) to pay 13.5 million dollars to the Presidio Trust so that the contaminated sediments could be removed and further run-off from the road be prevented from entering the lake