Kirman Lake: A Record of California Aridity

The most notable feature of the 3.1 m sediment stratigraphy from Kirman Lake is a multi-millennial arid period from ~8 to 3 ka. During this time diatom-inferred salinity and depth indicate that Kirman Lake was shallow and characterized by increased salinity (Fig. 2). High abundances of the diatom Gomphonema angustum (see Supplementary Fig. S4) support an interpretation of expanded marsh. Gomphonema angustatum is observed growing on Scirpus acutus var. occidentalis (common tule), the dominant plant in the marsh of Kirman Lake. Replacement of open water by marsh is also consistent with the elemental and isotopic records. The increased sediment organics, δ13C values (−275‰ to −32‰), C:N values (~15 to 20) and depleted δ15N during this period are consistent with algae combined with increasing vascular marsh plants as the primary carbon source to the sediments (Fig. 2)13,14. Mollusks disappear during this interval, which is expected because of loss of non-vegetated benthic habitat and decreased preservation in more organic, lower pH sediments (Fig. 2). A surface sample transect across Kirman Lake shows high organic content and low mollusk abundance in shallow marsh fringes of the lake relative to the deeper, open water area (Supplementary Information and Supplementary Fig. S2). The fossil pollen and charcoal records also indicate enhanced aridity between 8 to 3 ka (Fig. 2). The depression of Pinus:Artemisia suggests a decrease of mesic pine woodlands in favor of more xerophytic sagebrush. The high background values and peaks in charcoal suggest fires of greater spatial extent and/or intensity relative to prior and subsequent fire regimes. In addition, all proxies suggest decreased amplitude in hydroclimatic variability during the 8 to 3 ka period (Fig. 2). However, a notable and discrete excursion to greater lake depths and lower salinity occurs ~4.2 ka (Figs 2 and 3).

Figure 2 General sediment stratigraphy, organic content as represented by loss-on-ignition at 550 °C (LOI), δ13C, δ15N and C:N stratigraphies, sediment mollusk concentrations, diatom-inferred depth and salinity, Pinus:Artemisia, and charcoal accumulation rates. Dry mid-Holocene indicated with light grey shading, 4.2 ka wet interval indicated by dark grey shading. Medieval Climate Anomaly (MCA) dry interval is indicated with tan shading. Full size image

Figure 3 North American Ice cover39, orbitally induced solar forcing40, eastern tropical Pacific (ETP - green) and western tropical Pacific (WTP - orange) sea surface temperatures (SST’s)9,10 from foraminifera, ENSO events per century inferred from South American lake sediments41, record of eastern tropical Pacific precipitation inferred from Galapagos Island lake sediment silt content and of ENSO frequency inferred from Galapagos Island lake sediment sand content25, alkenone based reconstruction of eastern North Pacific SST’s from ODP101911 (Fig. 1), Kirman Lake Pinus-Artemisia pollen ratio, LOI and diatom-inferred depth and salinity. Dry mid-Holocene period indicated with light grey shading, 4.2 ka wet interval indicated by dark grey shading. Medieval Climate Anomaly (MCA) dry interval is indicated with tan shading. Heavier overlay lines represent LOESS (span 0.10) smoothed series. Full size image

Supporting evidence for aridity in the Sierra Nevada and adjacent areas between ~8 to 3 ka can be found at a number of other sites (Fig. 1)15. Lowstands occur at Pyramid Lake, Nevada and Owens Lake, California beginning ~8 ka with a shift towards moister conditions between ~5 and 3 ka. Walker Lake, located between Kirman Lake and Pyramid Lake, shows evidence of desiccation between at least 5.5 and 2.7 ka. Drier conditions between 8 and 3 ka have been reconstructed from drowned forest at Lake Tahoe15. Fossil pollen and charcoal data from the Sierra Nevada indicate drier, more open forests in the early through middle Holocene and moister, more closed forest with decreased fire frequency following 3 ka16. Increased sand and mud cracks in sediment records from Silver Lake, California between ~7.5 and 4 ka and lowstands at Tulare Lake, California between 7.8 to 5.5 ka also indicate a prolonged period of mid-Holocene aridity17,18. Warmer temperatures occurred between 8 to 3 ka and could have played a role in depressing effective moisture. Chironomid-inferred air temperatures for the Sierra Nevada and Great Basin indicate summer air temperatures between 8-3 ka were 1 to 2 °C warmer than present19, which is consistent with elevated treeline observed in the Sierra Nevada and nearby White Mountains at this time20. Warmer mid-Holocene temperatures are also supported by higher δ18O in a speleothem record from nearby Levithan Cave, Nevada21.

Although aridity returns after the 4.2 ka event, in the period between 4 and 3 ka there is a gradual shift to moister conditions at Kirman Lake as evidenced by increasing lake levels and declining salinity (Figs 2 and 3). After ~3 ka conditions are generally wetter and also more variable than during the mid-Holocene. This interpretation is supported by decreased sediment organics and increased mollusk content, which would result from a reduction in marsh conditions as lake levels rose and a decrease in C:N values indicating an increase in algae relative to emergent vascular plants contributing to organic sediments13. Increasing moisture in the Kirman Lake region is also suggested by a decrease in Artemesia pollen relative to Pinus and decreased charcoal accumulation (Fig. 2). A variety of paleo-hydroclimatic proxies similarly show that between ~4 ka to 3 ka there was a shift to wetter and cooler conditions at sites across California and in the adjacent Great Basin (Fig. 1)8,17,18,21,22.

Our lake level and salinity data show that the generally wetter period following ~3 ka was punctuated by markedly drier periods that persisted for hundreds of years. Such events were also reported in a high temporal resolution record from Zaca Lake in southern California8. One of these distinct dry periods is coincident with the Medieval Climate Anomaly (MCA ~1 ka to 0.7 ka)23, which is observed in many California paleoclimate records24. Kirman Lake experienced high salinity, decreased depth and increased organic sedimentation during the MCA (Figs 2, 3 and 4) and a reversal of these conditions during the subsequent Little Ice Age (LIA ~0.7 ka to 0.16 ka) (Fig. 4). Numerous lines of evidence demonstrate that generally increased aridity and higher temperatures during the MCA extended over California, the Southwest and the Colorado River basin during the MCA (Fig. 4)6,24.

Figure 4 Volcanic activity and solar and greenhouse gas and particulate radiative forcing42, modeled response of Nino343, eastern equatorial Pacific sea surface temperature (SST)44, Pacific Decadal Oscillation (PDO) index45, Santa Barbara Channel SST from ODP893A46 (Fig. 1), Sierra Nevada Palmer Drought Severity Index (PDSI)47, estimate of eastern California mega-droughts based on submerged tree-stumps48, Kirman Lake loss-on-ignition (LOI), diatom-inferred depth and salinity and charcoal accumulation. Medieval Climate Anomaly (MCA) dry interval is indicated with tan shading. Heavier overlay lines represent LOESS (span 0.10) smoothed series. Full size image

Connections between California Aridity and the Pacific Ocean

Evidence of the Pacific Ocean as a forcing agent contributing to prolonged aridity between ~8 ka to 3 ka, the excursion to wetter conditions during the 4.2 ka event and greater variability during the late Holocene can be identified in Pacific Ocean SST variations (Fig. 3)25,26. The period of 8 to 3 ka is typified by depression of tropical Pacific SST’s in the east and elevated SST’s in the west. This is analogous to a multi-millennial La Niña conducive to enhanced aridity in California. Pacific SST’s along the California coastline were also depressed between 8 to 3 ka and provide evidence of an extratropical extension of eastern Pacific cooling (Fig. 3). The extratropical extension of eastern Pacific cooling is similar in nature to a prolonged negative state of the Pacific Decadal Oscillation (PDO) and would have been additionally conducive to enhanced aridity in California27.

The lake deepening and freshening during the 4.2 ka event is contemporaneous with evidence of a discrete oceanic event typified by warming in the eastern Pacific and corresponding cooling in the western Pacific (Fig. 3). Hydrological changes reflecting increased aridity during the 4.2 ka event have been widely observed in the Middle East, India and China and have been correlated with cooling of the western Pacific and warming of the eastern Pacific and associated with boreal cooling and moister conditions in western North America26,28,29. California and India generally display an opposite response to El Niño-Southern Oscillation (ENSO) variations, and increased moisture in California at the same time that aridity increases in India provides strong evidence for the importance of the Pacific Ocean in the hydroclimatological changes associated with the 4.2 ka event in the two regions26. Similar to the 4.2 ka event, the last 3 ka have been generally wetter than the mid-Holocene, to which warmer eastern Pacific SST’s would have contributed (Fig. 3)22.

The higher amplitude hydroclimatic excursions of the past 3 ka at Kirman Lake correspond to similar changes reported from Zaca Lake and increased variability in ENSO (Fig. 3) which is also manifest in higher amplitude hydroclimatic variability at the opposite latitudinal dipole in India8,26. High-resolution analysis of the past 1,000 years and the impacts of the MCA at Kirman Lake and from other California and Pacific Ocean paleorecords provide further evidence of the sensitivity of eastern Pacific SST’s and California hydroclimatology to warming (Fig. 4). The MCA drying widely experienced in California is contemporaneous with apparent cooling in the eastern equatorial Pacific and eastern North Pacific coastal waters and a negative state of the PDO (Fig. 4)24,30,31.