Paleoclimate and Flooding History in the Southern San Joaquin Valley, California

Overview of Research Project Sediments in terminal lake basins in the Southern San Joaquin Valley including Buena Vista Lake and Kern Lake preserve a record of the regional climate history and flooding of the Kern River and other streams feeding the basin. We propose to collect a series of cores from these lakes and use established geophysical, geochemical, and lithologic techniques to unravel the regional paleoclimate and the history of flooding in the area.

Both goals are important on a societal level. Paleoclimatological studies have made great strides towards the understanding of natural climate change, a necessary starting point toward the characterization and mitigation of anthropogenic global warming. Despite these advances, there is a need for detailed regional climate records to test a growing family of computer-generated global and regional climate models.

Flooding is a common problem in Kern County as it is in most semi-arid environments. In fact, many of the students and teachers involved in this research will have friends or relatives who have been directly affected by such flooding within the past decade, or have even been affected themselves. Knowledge of natural flooding frequencies would better our understanding of the effect of urbanization, agriculture, and the alteration of watercourses on the modern flooding regime. Furthermore, a time-dependent look at flooding frequencies, particularly in the context of a paleoclimate record, would aid in the prediction of flood frequency in light of predicted global warming.

Finally, research on both regional paleoclimate and the temporal and spatial distribution of flooding would provide essential information toward the understanding of deposition on the Kern River Alluvial Fan. This geomorphologic feature is the site of the world's most effective water banking projects.

This project draws on our expertise in Paleoclimatology and Geochemistry and takes advantage of CSUB’s already existing research facilities including laboratories for trace-element analysis and environmental magnetism, and the CSUB Geotechnology Center.  The project also offers numerous opportunities for student recruitment and learning at multiple levels particularly due to the multidisciplinary approaches of our methodology.

Research Objectives We will achieve a better understanding of regional paleoclimate and flooding histories through the objectives listed below (Note: identified in this list are the corresponding methods which are each discussed in more detail in a following section).

·         Acquisition and Sampling of Sediments. (cores from terminal lake basins, upstream flood deposits)

·         Age Control and Correlation of Cores. (¹⁴C, tephrochronology, paleomagnetism, environmental magnetism)

·         Acquisition of Climate Records (ostracode paleontology, geochemistry, lithologic/petrographic description, environmental magnetism).

·         Reconstruction of Flooding History (lithologic/petrographic description, granulometry, environmental magnetism)

·         Provenance Determination (lithologic/petrographic description, geochemistry of clastic fraction, environmental magnetism)

Background The southern San Joaquin Basin is an active sedimentary basin located in south-central California at the southern end of the Great Valley (Fig. 1). Its Quaternary sediments are dominated by the Kern River Alluvial Fan and its terminal lakes, Kern Lake and Buena Vista Lake. This is presently a closed system. However, in extremely wet years prior to the construction of the modern irrigation system, Buena Vista Lake overflowed into Tulare Lake and the San Joaquin River system.

The Quaternary deposits in this system are reviewed in Hoots et al. (1954), Wood et al. (1964), Dale et al. (1966), Croft (1972), Page (1986) and Bartow (1991). Briefly, the geological setting is as follows. Although the dominant source of sediment are weathered crystalline basement and batholithic rocks from the southern Sierra Nevada catchment areas of the Kern River and Caliente Creek, the terminal lakes also receive input from the Coast Ranges to the west and the San Emigdio Mountains to the south. Clasts from these latter sources dominantly consist of weathered marine sediments. Thus the sediments from these source area are, in principle, distinguishable using the methods described below.

With few exceptions (e.g., Atwater et al., 1986; Davis, 1999) regional paleoclimate studies have been restricted to areas outside of the southern San Joaquin Valley (e.g., Mehringer, 1986; Scuderi, 1993; Smith et al., 1997; Benson, 1999; Clark et al., 1999; Hendy and Kennett, 1999; Rose and Wigand, 1999; Whitlock and Grigg, 1999; Negrini, in press). Consistent with these studies, we expect to acquire paleoclimate records from Kern and Buena Vista lakes which reflect the high amplitude changes associated with Milankovitch-scale (>10,000 yr periods) global climate change and millennial scale global climate change (1,00-10,000 yr periods) and possibly, lower amplitude changes associated with more subdued Holocene changes in climate (e.g., the Little Ice Age, the Medieval Warm Period).

We also expect to recover a record of flooding events, perhaps reflecting a dynamic character in the frequency of flooding due to climate change. For example, higher temperatures observed in western Pacific waters in the middle Holocene (Gagan et al., 1998) suggests more powerful El Niño events during this time of generally warmer global climate. Furthermore, the northward advancement of summer monsoonal storms expected during the middle Holocene would have enhanced flooding in the southern San Joaquin Valley. Regardless of the mechanism, if increased flooding is observed during the middle Holocene interval of slightly elevated temperature, this would predict local effects of global warming of particular interest to local high school students and teachers involved in this project.

Hypotheses 

1. Sediment properties will be mainly governed by climate change. We anticipate observing changes in these properties resulting from the following:

            a. Long-term Milankovitch-scale climate change (e.g., OIS stages 1-5 will be detected).

            b. Higher frequency, millennial-scale climate change reflecting GISP2 archetypical record of global climate change. This may possibly extend                    throughout entire record including Holocene though it should be more pronounced in Pleistocene when amplitudes of change were higher.

2. Flooding history will be uncovered and will also be governed by climate change. If flooding is more related to summer monsoonal storms, then expect higher frequency during mid-Holocene. If not, then higher frequency flooding will follow stadial conditions.

Methods 

Acquisition of Cores and Flood Deposits. Core sites will be selected primarily on scientific merit but also based on availability of access. Over the course of this project we plan to acquire at least six cores. One core will come from near the depocenter of each lake, one from near the margin of the lake toward each provenance source (e.g., toward and away from the Sierra Nevada). For coring we will use a Giddings 25-SCT (Model HD65RPST) coring rig purchased with funds from this grant. One of the investigators (RN) has experience with one of these rigs. They are capable of acquiring oriented 4-inch core in lacustrine sediments to depths of 70 feet.   Transparent plastic liners will be used in all cases. Drillers notes will be taken on site including top and bottom depths for each drive, percent recovery, position of recovered core relative to enclosing plastic liner, and visual description of lithology as possible through core sleeve. Core segments will be sealed with plastic caps, labeled with name and depths, and shipped to holding facility (refrigerated room at CSUB). A full suite of analyses will be done on core samples as described in the subsequent segments of the method section.  

Sediment Magnetism. Because the measurement of sediment magnetism is a non-destructive process, it will be done first. One set of boxed samples will be measured a Spinner magnetometer at the CSUB paleomagnetics laboratory after cleaning in that facility's Molspin AF demagnetizer. This procedure will likely result in a record of the earth's paleomagnetic field vector for this site. Because this information is well constrained for western North America for the time interval represented by our cores (Negrini and Davis, 1992; Lund, 1996; Bradley, 1999), this record potentially constrains age control. The magnetism of this suite of samples will be remeasured after being subjected to various anysteretic and isothermal remnant magnetizations (e.g., ARM, SIRM, IRM, backfield IRM). This latter set of procedures will constrain the concentration, mineralogy, and grain-size of the magnetic carrier grains which have proven to be useful parameters regarding the environment of sediment deposition and, hence, the detection of paleoclimate change and flooding history (e.g., Oldfield and Thompson, 1986; Peck et al., 1994; Negrini et al., 2000). The environmental magnetism parameters will also be useful in the correlation of cores taken from nearby sites.

Ostracode Analysis. A split of one set of boxed samples will be used for ostracode paleontological and paleoecological analyses as per the procedures outlined in Palacios-Fest et al. (1993). First, ostracodes will be separated from the sediments and then the relative percentages of taxa will be determined and the condition of the shells will be described (e.g., degree of encrustation) for each sample. The former set of data will be expressed in terms of a salinity index wherein a ratio is calculated which compares weighted abundances of salinophile species with weighted abundances of species which prefer fresh water. This parameter has proven useful in determining lake salinity which, in turn, is often closely related to lake depth (e.g., Cohen et al., 2000).

Granulometry and Petrographic Description. Another split of one set of boxed samples will be analyzed for grain size. The coarse fraction will be removed in a sieve and described under petrographic microscope in order to help constrain provenance. The remaining fine fraction will be passed through a particle size analyzer recently purchased by CSUB. The grain-size spectrum will help us to constrain lake depth and "event" input (e.g., pulses of coarse sediment from floods and/or turbidity flows).

Geochemical Analysis. Another split of one set of boxed samples will be analyzed for total organic (TOC) and total inorganic (TIC) carbon using the loss-on-ignition method. The remaining fraction of sediment after this procedure will be dissolved in a microwave digester and analyzed for major and trace-element abundances in CSUB's Perkin-Elmer Elan 6100 ICP-MS instrument. Finally, elemental abundances of ostracode will also be measured in the ICP-MS. This set of geochemical data should allow us to constrain several environmental conditions at the time of deposition including depth, salinity, and temperature of water, and also, provenance and diagenetic history.

 

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