sign in sign up
<<
Home , Harper Lake, Lake Manix, Lake Manly, Lake Mojave

Changes in the water budget of the Mojave and the drying out of the Southwest: New constraints from the study of ancient deposits

John Mering, UCLA Department of Earth and Space Science

During the next several centuries, anthropogenic global warming is expected to result in aridification in mid- latitude desert regions such as the Southwestern , further reducing hydrologic budgets (Wetherald and Manabe, 2002). In the Mojave region of Eastern this is problematic, where surface and resources are already overdrawn to sustain population centers and agriculture. In order to accurately model potential aridification in the Mojave, a thorough understanding of regional climate history is essential. Specifically data on past variability in temperature and hydrology can be used to study regional climate processes and assess the accuracy of computer models. I propose to apply new geochemical methods to deposits from sites in the Mojave in order to study the regional climate history and establish the magnitude of change in summertime temperature, and in the source(s) of moisture in the region, to changing greenhouse gas forcing. Data from paleolakes (Fig. 1) have been used to develop climate reconstructions for the , including the Last Glacial Maximum (LGM) interval, 23,000 to 19,000 years before present (Lyle et al., 2012). Many workers have hypothesized a southward shift of the Polar during this time as major sources for precipitation feeding in eastern and southern California at that time, with aridification being driven by the jet shifting northwards and a resultant decrease in winter storms (Oviatt, 1999; Reheis et al., 2012). More recently, it has been suggested that an enhanced summer monsoon may have been the source of precipitation and that a weakening of the monsoon has driven a drying out of the southwestern USA (Lyle et al., 2012). The balance between winter storms, carried by the jet stream, and summer monsoons is poorly constrained and controversial. Figure 1: The distribution of ancient lakes in Southeast California. Over a century of scientific work at paleolakes in the Mojave and elsewhere across the intermontane west has enabled researchers to identify oscillations in levels throughout the Pleistocene. This figure shows the shorelines that have been reconstructed for the last (~20,000 years ago), when atmospheric CO2 levels were significantly (~100 ppmv) lower than pre-industrial levels. Since then, the region has experienced aridificiation. The low point within the Mojave System was (V), situated in . I. , II. China Lake, III. ; IV. Panamint Lake; V. Lake Manly; VI. Rosamond/ Rogers Lake; VII. ; VIII. ; IX.

I propose that new isotopic measurements of lacustrine carbonate should provide a direct route to reconstruct summer water and air temperatures, and can be used to identify the source of precipitation feeding lakes. Comparison of isotope-based temperature data from this ancient interval, when conditions were cooler and wetter, to the arid modern regime, is useful towards refining predictions for future environmental change in the Mojave. These results will be compared to model simulations for the region from CMIP5 (Coupled Model Intercomparison Project Phase 5). This work is directly applicable to the mission of the La Kretz Center, to improve conservation science in California, by enhancing understanding of lakes in the Mojave, while providing a framework to begin to consider the impacts that global warming may have on the water cycle, regionally.

Research Plan The proposed work will involve sample collection of shallow-water carbonate (CaCO3) phases, including shells from gastropods and bivalves, that grew in Mojave lakes during the Last Glacial Maximum interval (Fig. 1). Sampling will focus on Lake Manix, which is established to have experienced a highstand between 21.8 and 19.7 thousand years before present (Lyle et al., 2012; Reheis et al. 2012). The Manix site is ideal because this highstand shoreline is unambiguous, and contains abundant carbonate material; data collected here may readily be correlated with climate proxies and contemporaneous models elsewhere (Reheis et al., 2012). Following field sample collection, ³clumped´ isotope temperature data will be collected using the Thermo MAT 253 isotope ratio mass spectrometer in the lab of Prof. Aradhna Tripati (UCLA ESS/AOS/IoES). Clumped isotopes are a measure of the abundance of 13C-18O bonds in a sample, which is thermally dependant, and can be related to the temperature of the surrounding lake water at the time of carbonate crystallization (Csank et al; Tripati et al., 2010). Water temperature estimates will be applied to constrain air temperatures (Hren and Sheldon, 2012). Carbonate oxygen isotope (18O/16O) ratios will simultaneously be measured and can be combined with temperature data to calculate water 18O/16O values for a lake (Vasconcelos et al., 2005), which in turn will constrain whether moisture was principally derived from high-latitude sources (i.e., winter storms), low-latitude sources (i.e., summer monsoonal precipitation), or a mixture. Precipitation sourced from higher latitude areas will have lower 18O/16O values, and will have a noticeable effect on the oxygen isotope ratio of lake water. Ultimately these data will be compared to results from lakes from other sites in the Southwestern US that I will sample over the course of my thesis, enabling me to constrain source region changes accurately and quantify mixing ratios of the two sources, if appropriate. In order to constrain sample ages, and exclude material from outside the proposed window of 19-23 thousand years before present, radiocarbon dating will be applied to field- collected samples. Samples will also be correlated relative to key stratigraphic markers from volcanic eruptions and floods. A NSF-funded ion microprobe facility at UCLA is available for measurements of ash ages during the course of this research. This project provides a framework to better understand the sensitivity of the hydrologic cycle in Eastern California to changes in atmospheric conditions. The results of this work will be compared to Coupled Model Intercomparison Project Phase V (CMIP5) simulations through interactions with faculty members at UCLA (Prof. Aradhna Tripati, David Neelin, Alex Hall, Jonathan Mitchell), and will be made available to CMIP5 group members. CMIP is an international collaboration of 20 climate modeling groups. Furthermore, the data collected in our study could be used to adjust the boundary conditions for regional-scale simulations involving California.

Funds and Key Personnel: I am a Geochemistry M.Sc. student working in the lab of Aradhna Tripati, and plan to continue for a Ph.D. The Tripati lab group at UCLA pursues the development and application of innovative geochemical paleoclimate proxies, including carbonate clumped isotopes. This study is an extension of ongoing work for my thesis to collect climate data from paleolakes Bonneville (Utah), and Lahontan (NV). I am collaborating with individuals at Kansas State, BYU, Stanford, and Pomona College on these efforts. Field sample collection will occur during the and summer of 2013. I request funds to cover field costs for this unfunded project, including vehicle rental, gas, and lodging, totaling no more than $1,000. Funding is also requested cover the cost of radiocarbon dating of sample material, totaling no more than $1,000. Analysis is ~$500 per sample.

References: Csank, A. Z., et al., 2011, Estimates of Arctic land surface temperatures during the early from two novel proxies. Earth and Planetary Science Letters, v. 304, p. 291-299. Hren, M. T., and N. D. Sheldon, 2012, Temporal variations in lake water temperature: Paleoenvironmental implications of lake carbonate į18O and temperature records. Earth and Planetary Science Letters, v. 337, p. 77-84. Lyle, M., et al., 2012, Out of the Tropics: The Pacific, Lakes, and Late Pleistocene Water Cycle in the Western United States. Science, v. 337, p. 1629-1633. Oviatt, C.G., et al., 1999, Reinterpretation of the Burmester Core, Bonneville Basin, Utah. Quaternary Research, v. 52, p. 180-184. Reheis, Marith C., et al., 2012, A Half-Million-Year Record of Paleoclimate from the Lake Manix Core, , California. Palaeogeography, Palaeoclimatology, Palaeoecology, v. 365-366, p. 11-37. Tripati, A.K., et al., 2010, 13C-182LVRWRSHVLJQDWXUHVDQGµFOXPSHGLVRWRSH¶WKHUPRPHWU\LQIRUDPLQLIHUDDQG coccoliths. Geochimica et Cosmochimica Acta, v. 74, p. 5697-5717. Vasconcelos, C., McKenzie, J.A., Warthmann, R., and Bernasconi, S.M., 2005, Calibration of the G18O paleothermometer for dolomite precipitated in microbial cultures and natural environments: Geology, v.33, p. 317±320. Wetherald, R.T., and S. Manabe, 2002, Simulation of hydrologic changes associated with global warming. Journal of Geophysical Research, v. 107, D19, p. ACL 7-15.