State Water Resources Research Institute Program

Project ID: 2008CA243B
Title: Climate Variability of the Sierra Nevada over the last Millennium: Reconstructions from Annually Laminated Sediments in Swamp Lake, Yosemite National Park, California
Project Type: Research
Start Date: 3/01/2008
End Date: 2/28/2009
Congressional District: 43
Focus Categories: Geomorphological Processes, Climatological Processes
Keywords:
Principal Investigator: Cayan, Daniel R.
Federal Funds: $ 11,817,
Non-Federal Matching Funds: $ 30,228,
Abstract: Severe drought and floods have the potential for devastating and long-lasting environmental and socioeconomic impacts in California. Severe drought plagued the State during the late 1920?s-mid 1930?s, in 1976-1977, in 1987-1992, and most recently from 1999-2002. Conversely, flooding, such as from the "New Year's flood" of 1997, have had enormous consequences in the California Central Valley, and in particular have threatened the State's precarious levee systems. The quality, quantity and management of water supply to California and the western United States depends heavily on storage of freshwater in mountain snowpack and the release of that freshwater during periods of peak snowmelt and runoff. The extent of this storage can vary greatly, and extreme drought and flood events have the potential to drain or overwhelm existing water storage facilities. Understanding the nature of these events is crucial to predicting and preparing for such conditions. But, because the events with the gravest consequences are very rare, the instrumental period of the last 100-150 years is in many ways inadequate to inform how often, how persistent, and what mechanisms drive these events. The research proposed here is an attempt to extract an unparalleled, high resolution record of hydroclimatic variability from the west slope of the Sierra Nevada from ~1000 year-plus cores of varved sediments that were recently recovered from Swamp Lake, Yosemite National Park.

Proxy climate records provide the most informative descriptions of interdecadal to centennial climate variability in the western United States. Highlighting the need for water resource preparedness is evidence from paleoclimate reconstructions that, during the early part of the last millennium, the region experienced droughts of greater duration and severity than any in recorded history. Other reconstructions from the last several thousand years describe numerous flood events in the southwestern United States of greater magnitude than any experienced in the recent past. Nonetheless, while these reconstructions provide a broad scale picture of hydrological variations in the West during the past, questions remain regarding the timing, direction and regional distribution of those variations, and the extent to which evidence of that variability is preserved in geological proxy records. Furthermore, few calibrations exist between the instrumental record of the last few decades and proxy measurement from geological archives, although such calibrations are crucial in assessing the quality and coherency with which climate signals are preserved in these archives.

In fall 2006, a set of cores of the upper layer of varved sediment were recovered from Swamp Lake, on the west slope of the Sierra Nevada in Yosemite National Park. Cores retrieved from an expedition in 2001, led by Dr. R. Scott Anderson of Northern Arizona University, have demonstrated that seasonally alternating mineralogenic and biogenic/organic rich sediment layers (i.e. varves) have been deposited in Swamp Lake throughout the Holocene. This sedimentary structure was confirmed during the recent October 2006 field expedition to Swamp Lake, organized by L. Roach and D. Cayan of Scripps Institution of Oceanography. Six frozen sediment wedges were extracted at this time using a freeze corer, four of which captured sections of the upper sedimentary layers in their undisturbed form, including from the sediment-water interface. The high quality sediment cores from Swamp Lake provide a rare opportunity to document hydrological variability of the region during the last millennium. Protected within some of the most remote confines of one of the country's oldest National Parks, the lake has experienced negligible human impact, its sediment input undisturbed throughout the Holocene. Although localized storms bring rain in the summer, the primary source of moisture to the Swamp Lake basin is snow pack at high elevations. Fluctuations in annual precipitation in the mountain range are linked to regional-larger basin scale climate anomalies over the Pacific Ocean, including the El Nino/Southern Oscillation and the Pacific Decadal Oscillation. These hydroclimatic variations, and other still longer term variations, are believed to be chronicled in sediment cores obtained from the bottom of Swamp Lake.

In light of the scientific and societal issues and the opportunities provided by these sediment cores, we propose to 1) analyze the uppermost sediment layers and compare the results to the modern instrumental record in order to facilitate translation of archival proxy data into quantitative climatological information, 2) test the hypotheses that conditions of severe aridity prevailed in the drainage basin surrounding Swamp Lake during the MWP and that the region has been affected, intermittently, by significant floods, and 3) determine whether warm or cool temperature prevailed in the Sierra Nevada during the MWP dry periods and during periods of significant flooding

Now that we have collected annually laminated, organic rich sedimentary records, spanning approximately one thousand years of deposition extending through to the present day, we intend to measure varve thicknesses, determine hydrogen isotope ratios, and create a chronology of these variations to be calibrated against nearby instrumental records and used to compare the climatic variations recorded in the deeper sediments to the deeper past. Numerous studies from high latitude lacustrine settings have demonstrated a correlation between varve thickness and hydrological variables such as snow pack, spring snowmelt discharge and annual runoff. The organic rich nature of the sediments will allow us to measure concentrations of deuterium, a stable isotope of hydrogen, in specific isolated organic compounds in order to reconstruct concurrent temperature fluctuations. Although this technique has only recently been employed in paleoclimatological investigations, it has proven a promising indicator of past temperature change in recent limnological studies.

In constructing an age-depth relationship, we will first establish anchor points by measuring the radiocarbon (14C) abundance in organic material deposited concomitantly with surrounding sediment. Careful counting of the varve couplets between these dates will facilitate a complete chronology. Taking advantage of the successful retrieval of the upper-most sedimentary layers, we will compare the climatological information extracted from these layers with corresponding instrumental measurements spanning the same time. In doing so we can assess how local and regional climate variability is manifested in the lacustrine sedimentary archive.

We expect hydrologic reconstructions to mimic, at least in part, the variability observed in other proxy records across the western United States, namely, dry conditions in the early part of the millennium, followed by wetter conditions culminating in the pluvial in the early 20th century, and finally drier conditions, including dry spells during 1928-1935, 1976-1977 and 1987-1992. We also hope to recover a record indicating the frequency and possibly the intensity of high flows (floods) at Swamp Lake. In both cases, we anticipate being able to better determine a multivariate (temperature as well as precipitation) climate signature unique to Swamp Lake and the surrounding region, allowing differences between the Sierra Nevada climate and larger scales to be determined

While the climatological record extracted by this study will reflect directly upon trends and variability experienced within the local and regional setting surrounding Swamp Lake, it will hold implications that extend across the entire western United States. This record will add to a network of observations from the region. The research will also provide graduate student training, in the form of a PhD topic carried out by Ms. Roach. Conclusions from this study will contribute to our understanding of the regional scale coherency of temperature and hydrological variability and thus will provide insight into the reach of climate impacts that may be experienced in the future.

Progress/Completion Report, PDF

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