A Holocene record of Pacific Decadal Oscillation (PDO)-related hydrologic variability in Southern California (Lake Elsinore, CA)
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High-resolution terrestrial records of Holocene climate from Southern California are scarce. Moreover, there are no records of Pacific Decadal Oscillation (PDO) variability, a major driver of decadal to multi-decadal climate variability for the region, older than 1,000 years. Recent research on Lake Elsinore, however, has shown that the lake’s sediments hold excellent potential for paleoenvironmental analysis and reconstruction. New 1-cm contiguous grain size data reveal a more complex Holocene climate history for Southern California than previously recognized at the site. A modern comparison between the twentieth century PDO index, lake level change, San Jacinto River discharge, and percent sand suggests that sand content is a reasonable, qualitative proxy for PDO-related, hydrologic variability at both multi-decadal-to-centennial as well as event (i.e. storm) timescales. A depositional model is proposed to explain the sand-hydrologic proxy. The sand-hydrologic proxy data reveal nine centennial-scale intervals of wet and dry climate throughout the Holocene. Percent total sand values >1.5 standard deviation above the 150–9,700 cal year BP average are frequent between 9,700 and 3,200 cal year BP (n = 41), but they are rare from 3,200 to 150 cal year BP (n = 6). This disparity is interpreted as a change in the frequency of exceptionally wet (high discharge) years and/or changes in large storm activity. A comparison to other regional hydrologic proxies (10 sites) shows more then occasional similarities across the region (i.e. 6 of 9 Elsinore wet intervals are present at >50% of the comparison sites). Only the early Holocene and the Little Ice Age intervals, however, are interpreted consistently across the region as uniformly wet (≥80% of the comparison sites). A comparison to two ENSO reconstructions indicates little, if any, correlation to the Elsinore data, suggesting that ENSO variability is not the predominant forcing of Holocene climate in Southern California.
KeywordsPDO Grain size Holocene Lake sediment Southern California
Southern California is home to over 18,000,000 people (ca. AD 2000) with a projected increase in population to nearly 25,000,000 by AD 2030 (CDWR 2005). As part of the South Coast hydrologic region, Southern California meets 23% of its combined agricultural and urban water demands directly from its own groundwater basins (Swartz and Hauge 2003). Therefore, future drying trends will produce a severe water demand and availability predicament (Seager et al. 2007). The region is characterized by an arid, Mediterranean climate (cool, wet winters and hot, dry summers) and faces a perennial freshwater availability crisis (Beuhler 2003). It is well known that the availability of freshwater to, and within, Southern California is controlled, fundamentally, by climate variability, which likely includes recent human-caused climate change (Barnett et al. 2008). Climate models suggest that future global warming will lead to increased aridity in Southern California (Seager et al. 2007). These model results present a serious challenge to water management and usage in Southern California. Critical to this challenge is the placement of modern and predicted climate change in the context of geologically recent climate change. Reconstructions of past climate provide a common method for assessing modern and future climate trends and predictions, particularly terrestrially-based reconstructions from the region (i.e. Southern California) of interest. The prehistoric record (>150 years) of climate variability in Southern California is sparsely documented, and limited to Mission diaries, tree-ring studies, some palynology, and a few lake studies (see Kirby et al. 2007 for reference summary). Building on these previous studies, there is an on-going project that focuses specifically on the rare, but valuable lacustrine archives of Southern California (Kirby et al. 2004, 2005, 2006, 2007; Bird et al. 2010).
To investigate the importance of higher frequency/sub-orbital-scale climate change not addressed in Kirby et al. (2007), the authors measured 1-cm contiguous sediment grain size data from core LEGC03-3. The working hypothesis for this new data is straightforward: differences in grain size, particularly the very fine-to-fine sand, reflect changes in run-off dynamics as coupled to changes in atmospheric circulation (i.e. climate). Our hypothesis builds on the observations of: (1) Inman and Jenkins (1999) who show a strong positive relationship between sediment flux in the rivers of Southern California and intervals of increased precipitation during the twentieth century; and, (2) Cayan and Peterson (1989), Brito-Castillo et al. (2003), and Hanson et al. (2006), who show that higher streamflow/precipitation in southwestern North America is associated with a preferred mode of atmospheric circulation akin to the positive/warm phase of the PDO. Our paleo-run-off hypothesis is assessed through comparison to the twentieth century PDO index, Lake Elsinore lake level, San Jacinto River discharge, and sediment grain size over the past 100 years. Results indicate that sediment grain size, specifically percent very fine-to-fine sand, is a reasonable proxy for hydrologic change at a range of time scales. This modern relationship is used to develop a qualitative reconstruction of PDO-related, Holocene hydrologic variability. Results are compared to regional climate records and ENSO reconstructions. A depositional model is developed to explain the grain size proxy.
Present-day precipitation variability across southern California is a winter season occurrence. The amount of precipitation for the region is linked to the mean position of the winter season polar front, which is modulated by changes in the position of the eastern Pacific subtropical high (Cayan and Peterson 1989; Hanson et al. 2006). Under modern conditions, dry winters in Southern California are linked to a strong high-pressure ridge off the west coast of the United States. This configuration directs storms over the northwestern United States. Wet winters are related to a weakening of the subtropical high, causing a southward shift in the winter season storm track (Cayan and Peterson 1989). The large-scale atmospheric patterns that control storm trajectories are influenced by Pacific Ocean sea-surface conditions (Trenberth and Hurrell 1994). Interannual precipitation variability across Southern California is related to the El Niño-Southern Oscillation (ENSO hereafter; El Niño = higher precipitation in Southern CA and vice versa in northern CA), whereas inter-decadal variability is linked to the Pacific Decadal Oscillation (PDO hereafter; +PDO similar to El Niño effects) (Castello and Shelton 2004; Hanson et al. 2006; Wise 2010).
An examination of the relationship between twentieth century lake level at Lake Elsinore and regional precipitation indicates a strong positive correlation (Kirby et al. 2004, 2007). A similar comparison using the PDO also shows a positive relationship to lake level (Kirby et al. 2007). Together, these analyses indicate that large-scale ocean–atmosphere interactions are recorded at our study site and that Lake Elsinore responds to a broad range of spatial and temporal hydrologic change.
The contribution of summer-fall precipitation to Southern California is small under present conditions, though the effects can be severe, generating localized flooding, landslides, and lightning-formed forest fires (Tubbs 1972; Adams and Comrie 1997). Today, summer precipitation is a product generally of an expanded North American monsoon, which enhances local atmospheric convection and its associated thunderstorms, or waning tropical cyclones (Tubbs 1972). Between AD 1900 and AD 1997, there have been over 39 years with measurable precipitation attributed to waning tropical cyclones in Southern California (Williams 2005). From a paleoclimatological perspective, both Bird et al. (2010) and Kirby et al. (2005, 2007) argue that a wet early Holocene in southern California was, in part, caused by a regionally expanded and more intense North American Monsoon (NAM). Records from the Mojave Desert to the east of Lake Elsinore suggest a similar effect of the NAM on early Holocene climate (Enzel et al. 1992; Li et al. 2008). It is, however, very unlikely that summer precipitation outweighed winter precipitation in terms of total annual hydrologic budget at any time in the Holocene in southern California.
Lake Elsinore is located along the northern Elsinore Fault zone, 120 km SE of Los Angeles, California (Fig. 1). Fault step-over from the Wildomar Fault to the Glen Ivy North Fault generates the Lake Elsinore pull-apart basin (Hull 1990). The Lake Elsinore Basin is 11 km long, 3.5 km wide, and less than 2 million years old (Hull 1990). As of March 2007, water occupies only 5.7 km × 2.8 km of the total basin surface area, though the lake’s surface area can change dramatically from year to year (Kirby et al. 2004, 2007). The lake is surrounded by a combination of predominantly igneous and metamorphic rocks (Hull 1990). It is constrained along its southern edge by the steep, deeply incised Elsinore Mountains that rise to more than 900 m above lake level. The Elsinore Mountains likely provide a local sediment source during heavy precipitation years and/or wet climates (Kirby et al. 2004, 2007). The lake’s drainage basin is relatively small (<1,240 km2) from which the San Jacinto River flows (semi-annually) into and terminates within the lake’s basin (Fig. 1) (Kirby et al. 2004). Lake Elsinore has overflowed to the northwest through Walker Canyon very rarely, only three times in the twentieth century and 20 times since AD 1769, according to mission diaries (Kirby et al. 2004, 2007). Each overflow event lasted for a short period of time, demonstrating that Lake Elsinore is essentially a closed-basin lake system, at least over the past few hundred years (Kirby et al. 2004, 2007). Conversely, Lake Elsinore has dried completely on only four occasions since AD 1769 (Kirby et al. 2004, 2007). Unexpectedly, only sediments younger than 450 calendar years before present (hereafter cal year BP) in core LEGC03-3 contain mudcracks. Because core LEGC03-3 is from the lake’s deepest basin, the lack of mudcracks older than 450 cal year BP suggests that whole lake desiccation is a relatively recent phenomenon in Lake Elsinore’s Holocene history. In addition, this finding argues that sedimentation at core site LEGC03-3 was probably uninterrupted over the age interval addressed in this paper. Recently acquired seismic reflection data from Lake Elsinore support the latter statements (Pyke et al., pers. commun.).
Lake Elsinore is a shallow, polymictic lake (13 m maximum depth based on historic records) (Anderson 2001a). The hypolimnion is subject to short periods (i.e. days to weeks) of anoxia (Anderson 2001a); however, frequent mixing of oxygen-rich epilimnetic waters into the hypolimnion precludes permanent, sustained anoxia, at least during the period of observation. Over a 24-month period between April 2007 and 2009, surface salinity ranged from a high of 2,850 EC (μS/cm) to a low of 2,170 EC (μs/cm) (J. Noblet, pers. commun.). Highest surface salinities occur generally in late fall-early winter before the onset of winter rain. Evaporation accounts for >1.4 m/year water loss. Consequently, water residence time in Lake Elsinore is short at all times and shorter during drought periods (Anderson 2001a).
Core information (Water Depth as of November 2003)
Water depth (cm)
Core length (cm)
16′6′′ (5.0 m)
16′ (4.9 m)
13′ (4.0 m)
Pollen age and radiocarbon analyses
Core 3 equivalent
14C Age (BP)
Calendar years BP
0 (pollen: Kirby et al. 2004)
18a (event layer?)
19a (event layer?)
Core sedimentology and grain size
Core 3 sedimentology is based on a combination of visual description, grain size analysis, environmental magnetic susceptibility, LOI 550°C, and LOI 950°C (see Kirby et al. 2007 for latter three analytical methods). Core 3 grain size was determined on approximately 0.1–0.5 cm3 of sediment at 1-cm contiguous intervals. Samples were boiled in DI water and pre-treated with at least 30 ml of 30% H2O2 to remove organics. Biogenic silica (i.e. diatoms) is almost entirely absent in the lake sediments (A. Bloom, pers. commun. 2007); therefore, we did not process the sediments to remove biogenic silica. The sediments were not pre-treated with HCl because carbonate microfossils (e.g. ostracods [gastropods are completely absent]) are extremely rare and poorly preserved in the Holocene section. Furthermore, we opted to include the micron-size, chemically precipitated CaCO3 (Anderson 2001a, b) as a part of the lake’s total inorganic, minerogenic size fraction and therefore did not acidify the sediments. Prior to grain size analysis, but after organic removal, samples were split using a TFE fluoro-carbon plastic riffle splitter with 2,000-μ slots. Samples were split, if necessary, to achieve an obscuration of 8–14% (Malvern Instruments 1999).
All samples were run on a Malvern Mastersizer 2000 laser diffraction grain size analyzer coupled to a Hydro 2000G. At the beginning of each measurement day, a tuff standard (TS2) with a known distribution between 1.0 and 16.0 μ (avg. 4.54 μ ± 0.07; n = 3,194) was measured twice and compared to past measurements to assess the equipment’s accuracy and repeatability. Thereafter, TS2 was run every 10 samples to verify analytical repeatability and stability and once at the end of the day’s analyses for a final assessment. TS2 results are compared to values obtained by measuring known Malvern standards as an additional measure of stability. The measurement principle used is the Mie Scattering principle. Sample measurement time was 30 s with 30,000 measurement snaps per single sample aliquot averaged per 10,000 snaps. The final three measurements (30,000 measurements/10,000 snaps = 3 time-averaged measurements) were compared for internal consistency per sample. All data are reported as volume percent and divided into 10 grain-size intervals as well as d(0.1), d(0.5), d(0.9), %clay, %silt, %sand, and mode.
The total percent sand data were standardized by subtracting the mean of the distribution (as calculated between 150 and 9,700 cal year BP) from each observation, and dividing the value by the standard deviation (as calculated between 150 and 9,700 cal year BP). The standardization process creates a mean of zero with deviations from the mean in units of standard deviation. The standardized data were also binned into 50-year intervals to assess multi-decadal- to centennial-scale variability. The anthropogenic interval (−53 to 150 cal year BP or AD 2003 to AD 1800 [0 cal years BP = AD 1950]) was not included in the standardization calculation because of the much higher-than-Holocene average, post-settlement sand values. From AD 1900 to AD 2003, the sand data were standardized using the AD 1900 to AD 2003 average and standard deviation. These modern data were binned into 10-year averages (e.g. AD 1910–1919, AD 1920–1929, etc.) for comparison to lake level, river discharge, and PDO data.
Meteorological indices and lake level data
Meteorological data were obtained from the National Climatic Data Center weather observation station records (lwf.ncdc.noaa.gov/oa/climate/stationlocator.html). Lake level data for Lake Elsinore were obtained from the United States Geological Survey, the Elsinore Valley Municipal Water District monitoring program (www.evmwd.com), and Berry et al. (1953). San Jacinto River discharge data were obtained from the USGS Water Data for the Nation website (nwis.waterdata.usgs.gov/nwis). PDO data were obtained from Mantua et al. (1997; [http://jisao.washington.edu/pdo/PDO.latest]). The PDO, lake level, and discharge data were binned into 10-year averages (e.g. AD 1910–1919, AD 1920–1929, etc.) so that their time intervals correspond to that of the typical sand datum. Intervals of missing lake level data were substituted with values interpolated between points of known data using a linear interpolation (i.e. AD 1998–1999, 1991, 1989–1984, 1972–1971, 1968, 1966, 1963–1960) based on the relationship between precipitation and the predictable lake level response in an arid environment. Binning the sand, PDO, discharge, and lake level data by mid-decade (i.e. AD 1905–1914) does not change the relationships between the various data.
Core LESS02-11ab and Core LEGC03-3 age-depth correlations
Depth (cm) Core LESS02-11aba
Depth (cm) Core LEGC03-3
cal year BP
Exotic pollen (Eucalyptus)
Exotic pollen (Erodium)
Core sedimentology and grain size
Development and assessment of a grain size PDO-related hydrological proxy
Inman and Jenkins (1999) examined the relationship between twentieth century climate along the central and southern Californian coasts and sediment flux in the region’s rivers. Their study revealed the strong positive coupling between periods of wet climate and enhanced river sediment flux. Conversely, dry climates reduce the flux of sediment in the region’s rivers. Cayan and Peterson (1989), Brito-Castillo et al. (2003), and Hanson et al. (2006) examined the relationship between patterns of atmospheric circulation and streamflow/precipitation in western North America. Their combined results indicate that higher streamflow is associated with preferred modes of atmospheric circulation. For the southwestern United States, this preferred mode is similar to the positive, or warm phase of the Pacific Decadal Oscillation (Cayan and Peterson 1989; Brito-Castillo et al. 2003; Hanson et al. 2006). Together, these results indicate that (1) wet/high streamflow years in Southern California are associated with a preferred (predictable?) mode of atmospheric circulation akin to the positive/warm phase of the PDO, and (2) wet/high streamflow years in Southern California result in higher river competence/capacity. Using these two results, we developed a general working hypothesis for our paper, which states that differences in grain size reflect changes in run-off dynamics as coupled to changes in atmospheric circulation (i.e. climate).
Inman and Jenkins (1999) have already shown that river sediment flux and regional climate are positively correlated in Southern California. Building on this relationship, we hypothesize that differences in grain size reflect changes in run-off dynamics as coupled to changes in atmospheric circulation. In other words, wetter climates increase run-off, increase river competence/capacity, and increase the average grain size transported to, and deposited in, Lake Elsinore. The idea of using grain size as a proxy for climate is well-documented. Recently, Parris et al. (2010) reconstructed a history of storm activity in the NE USA using changes in grain size. Conroy et al. (2008) used grain size to infer changes in lake level and El Niño-Southern Oscillation variability in a small lake in the Galapagos Islands. Anderson (1977, 2001b) demonstrated that enhanced river discharge and changes from dry to wet climates increase the sand content in arid-environment lakes. Anderson also illustrated the importance of mechanisms, such as density overflow during high river discharge, for transporting coarse-grain sediment far into the lake basin. Benson et al. (1991) used grain size to infer changes in the size of Walker Lake, though their grain size data conflicted with other proxy data at times. Of note, Yair and Kossovsky (2002) and Dearing (1991) caution against simple climate-sediment flux models, suggesting that surface properties and disturbance (e.g. human history, fires, changes in surface properties) may act as the primary controls on changes in the flux of sediment from a lake’s drainage basin. For arid environments, however, sediment flux may be tied more simply to climate change than for other environments (Hunt and Wu 2004; Collins and Bras 2008).
Depositional model for grain size proxy
Our results indicate that percent total sand, especially very fine sand, increases during twentieth century highstands (and high discharge) and decreases during lowstands (and low discharge) (Figs. 8, 9). As with most, if not all lake basins, Lake Elsinore is characterized by a pronounced grain size gradient from coarse grains in the littoral zone to fine grains in the profundal zone (Anderson 2001a). In most lake basins, sediment focusing is the usual explanation for this gradient (Davis and Ford 1982). Sediment focusing is a process that removes fine grain sediment from the littoral zone and re-deposits it in the profundal zone via wave action/winnowing, near-shore currents, and/or littoral migration associated with changes in lake level. Here, we present a depositional model to explain the coarse-highstand/fine-lowstand relationship observed in Lake Elsinore using the concept of sediment focusing as an important depositional process.
A 10,000-year PDO-related record of hydrologic variability and regional comparisons
The age model for LEGC03-3 is supported by two independently dated cores also from Lake Elsinore—core LESS02-5 (Kirby et al. 2004) and LEGC03-4 (Fig. 1). As shown in Kirby et al. (2004), the lake edge core LESS02-5 contains sedimentologic and isotopic evidence for a high stand centered on ca. 3,400 cal year BP and ca. 1,800 cal year BP, with a period of inferred low lake level in between. The timing of this high-low–high lake level cycle fits with the LEGC03-3 sand proxy, which shows a wet climate ca. 3,400–3,250, a dry climate from 3,250 to 1,850, and brief return to a wet climate ca. 1,800 cal year BP (Fig. 12). Core LEGC03-4 is from a slightly shallower depth than LEGC03-3, though the depth difference between the two cores has decreased over time (0.90 m today vs. 3.80 m 8,900 cal year BP) due to infilling. Mudcracks in core LEGC03-4 are bracketed by dates of 3,260 and 1,710 cal year BP, indicating a period of low level at the same time as that inferred from the sand proxy in core LEGC03-3 (Fig. 12). Together, three independently dated core stratigraphies indicate a wet-dry-wet cycle ca. 3,400–1,700 cal year BP, which lends support to our LEGC03-3 age model, at least for the late Holocene.
A comparison to paleo-ENSO reconstructions
We note that Kirby et al. (2005) used low-resolution, littoral sediment cores from Lake Elsinore to postulate the onset of modern El Niño activity in Southern California. Our new results (i.e. this paper) do not support the interpretation of Kirby et al. (2005) regarding the ENSO hypothesis. The higher-resolution, profundal sediment core data combined with the correlation between recent sand data, the PDO index, river discharge data, and Lake Elsinore lake level (Fig. 9), support a re-interpretation of the Kirby et al. (2005) data in terms of PDO-related, not ENSO, variability.
This apparent decoupling between ENSO activity and the paleo-records of Southern California, despite the modern relationship, is not entirely unexpected. Kirby et al. (2006) used sediments from Baldwin Lake in Southern California, spanning the last glacial period to demonstrate that periods of supposed super-ENSO activity [i.e. preferred El Niño-like conditions (Stott et al. 2002)] were associated with lake level lowstands at Baldwin Lake and elsewhere in western North America (Benson et al. 2003). The new Lake Elsinore data seem to support the conclusions of Kirby et al. (2006) that ENSO’s forcing on the climate of Southern California is decoupled at longer timescales than that captured in the modern/historical record. It remains unclear why there is a decoupling between that observed in the modern climate system and that reconstructed in the paleoclimate system for Southern California. Clearly, there is a need for additional high-resolution, terrestrial records from the region as well as focused GCM work to address this issue, which is critical to understanding present and future hydrologic variability in the over-populated, water-poor region of Southern California.
Percent sand, especially very fine sand, shows a strong correlation with twentieth century PDO variability, Lake Elsinore lake level, and San Jacinto River discharge. Consequently, we used percent total sand as a qualitative proxy for PDO-related hydrologic variability over the past 9,700 cal year BP.
As an independent assessment of our Lake Elsinore PDO-related sand proxy, we compared our sand data to the MacDonald and Case (2005), 1,000-year PDO reconstruction. The comparison reveals excellent correlations at decadal to multi-decadal time scales, with the exception of the LIA. The MacDonald and Case (2005) PDO reconstruction is weakly negative (cold PDO phase) during the peak stage of the LIA. The Elsinore PDO-related sand proxy indicates a wet climate during the peak stage of the LIA, which is supported by other evidence across western North America.
A depositional model is proposed to explain the coarse (fine), wet (dry) relationship observed in the twentieth century Lake Elsinore proxy calibration. The model focuses on the impact of the asymmetrical lake level changes characteristic of arid-environment lakes (i.e. rapid transgressions vs. slow regressions) on sediment depositional processes.
Using the PDO-related sand proxy, nine intervals of sustained wet climate are recorded during the past 9,700 cal year BP: from 9,700 to 9,500, 9,100 to 8,850, 6,900 to 6,350, 4,550 to 4,100, 3,700 to 3,550, 3,350 to 3,200, 1,550 to 1,350, 1,200 to 1,050, and 600 to 150 calendar years before present (cal year BP). A comparison between the new Lake Elsinore PDO-related sand proxy and the ten regional records reveals that six out of the nine Lake Elsinore “wet” intervals correspond across >50% of the comparison sites. This comparison suggests that for the broader region of Southern California, there is some uniformity of climate across the region at multi-decadal to centennial intervals through the Holocene.
A comparison to two well known ENSO reconstructions (Moy et al. 2002; Conroy et al. 2008) shows almost no relationship between increased frequency or occurrence of El Niño over the Holocene despite the well documented impact of ENSO variability in the modern Southern California climate system. Future regional GCMs are required to explain this observation for both the lack of an ENSO signal and the presence of a PDO signal.
The lack of a strong ENSO signal coupled with the presence of a strong PDO signal over the past 9,700 cal year BP suggests that future predictive models should focus on the PDO for predicting decadal-scale hydrologic variability in the over-populated, water-poor region of Southern California.
This research was funded by the National Science Foundation (EAR-0602269-01) to MEK and SPL. Additional funds were provided by a Lake Elsinore-San Jacinto Water Authority (LESJWA) contract to MEK and MAA and the American Chemical Society-Petroleum Research Fund (ACS-PRF: Grant #41789-GB8) to MEK. Funds from Cal-State Fullerton Faculty-Student Creative Research Grants provided summer stipends for several students. Special thanks to the City of Lake Elsinore, particularly Mr. Patrick Kilroy (Lake Manager) for access to the lake; Mr. David Ruhl (LESJWA) for contract management; Gregg Drilling and Testing, Inc. for exceptional quality service; Drs. John Southon and Guaciara dos Santos (Univ. of Cal. Irvine) for radiocarbon dating; Dr. James Noblet (CSUSB) and his students for modern limnological data; and Ms. Jennifer Schmidt for careful lab analyses. Excellent reviews by two anonymous referees helped improve the paper’s content and clarity.
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