George I. Smith

Stable isotope composition of waters in southeastern California 1. Modern precipitation

Over a 7-year period from April 1982 to April 1989, integrated samples of rain and snow were collected at 32 sites by oil-sealed storage gage stations in (and adjoining) the southeast California desert; station elevations ranged from −65 m to 2280 m, and the collection network covered an area measuring about 400 km in each dimension. Deuterium (δD) analysis of 406 samples shows that the average δD of summer precipitation was −56 per mil (‰) whereas winter values averaged −78‰, averaged annual values were close to −69‰ because most of the area is in a winter-dominated precipitation regime. We found no correlation between wetness or dryness of a season and the δD of its precipitation. The δ18O versus δD plots show that rain samples define a line of slope 6.5, less than the 8 of the Meteoric Water Line, whereas snow samples define a line of slope 9.2. These differences in slope are the result of isotopic fractionation which occurred during evaporation of raindrops but not during sublimation of snow. Trajectory plots of 68 of the major storm events show that all of the winter storms originated in the Pacific, and passed over high mountains before reaching our collection stations. However, 21 of the 30 summer storms had trajectories that originated either over the Gulf of Mexico or the subtropical Pacific and traveled either west or north to reach our stations, without traversing high mountains. The difference in δD between winter and summer precipitation is due to different air flow patterns during those seasons.

Paleohydrologic regimes in the southwestern Great Basin, 0–3.2 my ago, compared with other long records of “gobal” climate

Abstract Nine distinct paleohydrologic regimes in the southwestern Great Basin over the last 3.2 my are recorded by the lacustrine deposits in KM-3, a 930-m core from Searles Lake, California. These are characterized as being “wet,” “intermediate,” or “dry” (like today). Excepting the present incomplete regime, each lasted 0.12 to 0.76 my. Major regime changes 0.01, 0.13, 0.6, and 2.5 my ago appear to coincide with recognized changes in global ice-sheet histories as represented by 18 O and other records from marine sediments, but comparable changes 0.3, 1.0, 1.3, and 2.0 my ago do not appear to coincide closely with comparable perturbations in ice-sheet histories. However, all regime boundaries (during the last 1.75 my) coincide closely in time with changes in sea-surface temperatures in the tropical Atlantic, and many coincide with other deep-sea and continental paleoclimatic boundaries.The average duration of these paleohydrologic regimes was about 0.4 my (standard deviation, 0.2 my or less, depending on assumptions), and it is suggested that the regime boundaries reflect times of change in global(?) sea-surface temperatures, possibly controlled in part by the Earths 413,000-yr orbital eccentricity cycle. During the wettest and driest regimes in the Searles Lake area, lake levels were not sufficiently affected by the 23,000-, to 42,000-, or 100,000-yr climate cycles related to high-latitude ice-sheet fluctuations to produce changes in the lacustrine sediment character. During intermediate regimes, however, when lacustrine sedimentation in this area was more sensitive to climate, the sediments, in KM-3, record lake fluctuations with average frequencies near those of the ice sheets. This seems to indicate that the high-latitude ice-sheet fluctuations caused local climatic perturbations but did not dominate the hydrologic component of climate in this area. Other lacustrine deposits in the southwestern Great Basin of California and Nevada have ages comparable in part to those of the wet to intermediate regimes indicated by KM-3, and they may all be products of finite periods when lake expansion, alluvial fan growth, increased spring discharge, and fluvial deposition were promoted in this area by widespread wet climates. Glacier expansion in the Sierra Nevada may also have been primarily an expression of, and in phase with, these wet regimes.

Areal Distribution of Deuterium in Eastern California Precipitation, 1968–1969

Abstract Rain and snow were collected during major storms at 26 stations in California and Nevada during the exceptionally wet 1968–69 season. The deuterium/hydrogen ratios (δD, in per mil, VSMOW) of the samples varied according to the elevation of the sample site and its position relative to the Sierra Nevada. Eighteen storms were sampled at one or more stations. Eleven of them were sampled at almost all of 13 stations that lie east and southeast of the Sierra Nevada which allowed a storm-by-storm comparison of the influence of the topographic setting of the station and the synoptic meteorology of the storm on the δD characteristics of the precipitation. A model is presented that illustrates possible changes in δD values as a storm moves inland. It is calculated using probable starting values for moist air masses arriving from the Pacific and follows the meteorological and isotopic changes that should occur during adiabatic uplift, cooling and condensation. The model agrees satisfactorily with data from ...

Deuterium Content of Snow Cores from Sierra Nevada Area

The relative deuterium content was measured on 37 snow cores collected in April 1969 in the Sierra Nevada. The deuterium content varies inversely with altitude of collection (approximately 40 per mil per 1000 meters) but is unrelated to latitude. The altitude relationship is particularly well defined west of the crest of the range but is not well defined east of the crest. However, samples from east of the crest tend to be depleted by about 10 to 15 per mil relative to samples collected at the same elevation west of the crest. We propose that the deuterium content of snow cores, collected so as to include the total winters precipitation, can be used as a climatic indicator to compare the climate of one winter with that of another.

A 36Cl chronology of lacustrine sedimentation in the Pleistocene Owens River system

We have used 36 Cl to date climatically sensitive saline-lake sediments from hydrologically closed basins in southeastern California, primarily sediments from Searles and Panamint basins. During wet periods of the past 2.0 m.y., lakes that formed in the closed basins fluctuated in size in direct response to the balance between runoff and evaporation. We have compiled a chronology for the KM-3 core from Searles Lake, based on ages determined by 36 Cl for the evaporites, together with those determined by 14 C and U-Th series for younger sediments, and by magnetostratigraphy for older sediments. This chronology, along with other criteria, such as correlations between Searles and Panamint basins, the chloride budget, and sedimentology, is used to reconstruct the history of lake fluctuations in the paleo-Owens River system. We infer that Searles Lake desiccated at most twice since 600 ka: during the interval around 290 ka, and from 10 ka to the present. The lake history curve shows that the Holocene Epoch is anomalously arid. Major overflows from Searles to Panamint occurred during the intervals between 1.3 Ma and 1.0 Ma, 750 ka and 600 ka, 500 ka and 400 ka, and 150 ka and 120 ka. Comparing the lake-fluctuation chronology to the δ 18 O record of marine foraminifera, we infer that the strongest similarity is in the periodicities of the cycles-40 to 50 kyr before the Matuyama/Brunhes magnetic reversal (730 ka) and 100 kyr thereafter. We find, however, that at Searles Lake this fluctuation in the lake chronology is modulated by longer-term cycles of aridity and humidity. Thus, although the mid-latitude Quaternary climate record reflects the mid- to high-latitude ice-volume fluctuations that dominate the marine 18 O record, it also contains evidence for climatic forcing of a different type.

Interstadial climatic cycles: A link between western North America and Greenland?

During the interval 33.6 to 26.1 ka, Searles Lake in southeastern California went through six major cycles of expansion and contraction. A comparison of U/Th ages for these events with the chronologies for quasi-cyclic interstadial episodes in the ice core from Summit, Greenland, suggests that the episodes of low water at Searles Lake are synchronous with the interstadial episodes at Summit. The two phenomena may be linked by variations in the strength of the global hydrological cycle, driven by oscillations in the Atlantic Ocean thermohaline circulation.

Stable isotope composition of waters in southeastern California: 2. Groundwaters and their relation to modern precipitation

We compare isotopic data on modern groundwater recharge, estimated from a 7-year collection of precipitation samples, with the isotopic compositions of 82 samples from wells and perennial springs collected in southeastern California. Over half the samples represent recharge that is considerably more depleted in deuterium (δD) than the lightest (winter) precipitation δD values now precipitating in nearby areas. Wells in the vicinity of irrigation canals fed from the Colorado River contain water whose δD resembles that of the irrigation canals, and wells adjacent to the Mojave River are recharged by Mojave River water. The remainder of the samples from the 17 hydrologic units discussed in this paper include one or more whose δD values are depleted in deuterium by at least 10‰ compared to recharge estimated from modern winter precipitation. A plot of δ18O versus δD for 56 wells and 22 springs shows substantial scatter (R2=0.66) and defines a slope of 4.9, which is lower than the slope of the Meteoric Water Line. This slope and the scatter of the data suggest these diverse waters were evaporated to different degrees before recharge, and they may also have exchanged oxygen with solid mineral phases. We interpret the fact that many groundwaters in this region have lower δD values than modem precipitation to indicate that recharge of many of the groundwater basins occurred during an earlier period, probably the late Pleistocene, when winters were colder, evaporation rates were lower, and precipitation was quantitatively greater and isotopically lighter.

Deuterium content of snow as an index to winter climate in the Sierra Nevada area

The winter of 1968-69 produced two to three times the amount of precipitation in the Sierra Nevada area, California and Nevada, as the winter of 1969-70. The deuterium content in snow cores collected at the end of each winter at the same sites, which represents the total snowfall of each interval, shows a depletion in 1968-69 of approximately 20 per mil. The higher snowfall in 1968-69 and the depletion of deuterium can be explained by an uncommonly strong westward flow of cold air over and down the western slopes of the Sierras, which interacted with an eastward flow of moist Pacific air that overrode and mixed with the cold air; this resulted in precipitation that occurred in greater than normal amounts and at a lower than normal temperature. Pluvial periods of the Pleistocene may have had the same shift in air-mass trajectory as the wet 1968-69 year. Snow cores collected in the norrmal 1970-71 winter have deuterium concentrations that resemble those of the normal 1969-70 winter. Small and nonsystematic differences in samples from these two normal winters are due to variations in climatic character as well as to factors inherent in the sampling sites.

Studies of quaternary saline lakes—II. Isotopic and compositional changes during desiccation of the brines in Owens Lake, California, 1969–1971

Abstract Owens Lake is an alkaline salt lake in a closed basin in southeast California. It is normally nearly dry, but in early 1969, an abnormal runoff from the Sierra Nevada flooded it to a maximum depth of 2·4 m. By late summer of 1971, the lake was again nearly dry and the dissolved salts recrystallized. Changes in the chemistry, pH, and deuterium content were monitored during desiccation. During flooding, salts (mostly trona, halite, and burkeite) dissolved slowly from the lake floor. Their concentration in the lake waters increased as evaporation removed water and salts again crystallized, but winter temperatures caused precipitation of some salts and the following summer warming caused their solution, resulting in seasonal variations in the concentration patterns of some ions. The pH values (9·4–10·4) changed with time but showed no detectable diurnal pattern. The deuterium concentration increased during evaporation and appeared to be in equilibrium with vapor leaving the lake according to the Rayleigh equation. The effective α(D/H in liquid/D/H in vapor) decreased as salinity increased; the earliest measured value was 1·069 [as total dissolved solids (TDS) of lake waters changed from 136,200 to 250,400 mg/1]and the last value (calc.) was 1·025 (as TDS changed from 450,000 to 470,300 mg/1). Deuterium exchange with the atmosphere was apparently small except during late desiccation stages when the isotopic contrast became great. Eventually, atmospheric exchange, combined with decreasing α and lake size and increasing salinity, stopped further deuterium concentration in the lake. The maximum contrast between atmospheric vapor and lake deuterium contents was about 110%.

Studies of quaternary saline lakes—I. Hydrogen isotope fractionation in saline minerals☆

Borax, gaylussite, nahcolite and trona were synthesized in aqueous solution at temperatures ranging from 8° to 35°C. Except for borax, deuterium was always depleted in these hydrated minerals relative to the solutions from which they were crystallized. In borax, no significant fractionation was found. The fractionation factor of DH for the trona-water system exhibited a marked temperature dependence. By combining the deuterium contents of trona and the solution from which trona was crystallized, the following thermometer scale was obtained: In(DH) trona(DH)water = 1.420 × 104T2 + 23.56T (1) An attempt to establish a geothermometer based on C13C12 fractionation between carbonate minerals and carbonate ions in aqueous solution was not successful.