USGS visual identity mark and link to main Web site

Core OL-92 from Owens Lake, southeast California

Sediment pore-waters of Owens Lake Drill Hole OL-92

James L. Bischoff
Jeffrey P. Fitts
U.S. Geological Survey, Menlo Park, California
Kirsten M. Menking
Earth Sciences Board, University of California, Santa Cruz

Contents:

Abstract

 The salinity-depth profile of pore-waters from the Owens Lake drill hole OL-92 has been drastically smoothed by post-depositional diffusion of dissolved salts and ground water flow, such that the present pore water composition bears little relationship to past climate. Water content varies erratically down the core, generally decreasing from about 60 wt % at the top to about 20 wt % at 240 m, but water content increases sharply at levels below 240 m to between 40-60 wt %, indicating significant undercompaction for the lowermost 100m. The pore waters are alkaline (pH 8-10) with anionic compositions of HCO3»Cl>SO4 which is similar to the chemistry of the modern lake before diversion of the source waters. The pore water salinity ranges from 0.4 to 5 wt % TDS. Salinity varies with depth in a smooth pattern with a minimum at 30 m, gradually increasing to a single broad maximum at about 150 meters depth, and declining sharply thereafter to steady low values at 210 meters and below where ground water is apparently flowing at present.

Introduction

 During the spring of 1992, a 323 m core (7.6 -cm diameter) was recovered from Owens Lake in order to obtain a record of climate history and natural climate variation. Drilling was carried out in three sub sections: from the surface to 7.16 m (OL-92-3), from 5.49 to 61.37 m (OL-92-1) and from 61.26 to 322.86 m (OL-92-2), together representing about 800 kyr of sedimentation (Smith, 1993). Sediments were analyzed for grain size, major elements, carbonate, organic carbon, cation-exchange capacity, radiocarbon, and pore-water composition, in order to monitor climatic changes and place them in a temporal context. We report here on the pore water content and composition of samples from the core. The companion article by Friedman et al (1993) reports deuterium/hydrogen ratios on the same samples. Results for other sediment-parameters are reported in other companion articles (Bischoff et al 1993a and 1993b; Menking et al 1993a and 1993b)

Sampling procedures

 Samples of fresh wet sediment (60 cc) for the determination of water content and pore water chemistry were taken during the drilling operations at every 2-3 m from clay-rich horizons for the entire length of the core (120 samples). The samples were taken in the field from the freshly split core within minutes of exposure of the fresh sediment. Each sample was trimmed of disturbed sediment adjacent to the core liner, and immediately sealed within a 75 ml air tight glass bottle and kept refrigerated at 5°C until laboratory processing some three months later.

Laboratory Procedures

 For the determination of water content, 1 to 2 cc splits of the fresh sediment were weighed into ceramic crucibles and their weight loss recorded after heating for two days at 100°C. Water content as weight per cent is simply the percentage of weight loss of the sediment. The remaining sample was transferred into a stainless steel cylindrical squeezer (modified after that of Manheim, 1966) which was then pressurized with a simple laboratory hydraulic press (12 ton capacity). The pore water squeezed from the sediment passed through three layers of filter paper and into a polyethylene syringe. Squeezing for 10 to 30 minutes yielded from 3 to 25 ml of pore water, depending on the water content of the sample. The sample was then passed through a swinney-mounted membrane filter (0.45 µm pore size). Two drops were used for immediate measurement of refractive index for salinity, a 1 ml aliquot was used for immediate pH determination by micro electrode, 1-2 ml were injected into a septum- capped evacuated blood tube for isotopic analyses, and the remainder stored in a tightly-capped polyethylene bottle for further chemical analyses. Ct (total dissolved inorganic carbon) was determined by an infrared CO2-analyzer. Cl was determined by potentiometric titration with an auto-titrator, and sulfate concentration was determined by ion chromatography. The remaining sediment cake was retained for analyses of organic C, carbonate, and grain size as reported in the companion reports by Menking et al (1993a) and Bischoff et al (1993a).

Data and discussion

 Results are presented in Table 1. Water content, the measure of compaction, varies erratically down the core, generally decreasing from about 60 wt % at the top to about 20 wt % at 240m (Table 1, Figure 1). Below 240 meters and to the bottom of the core the water content sharply increases to between 40 and 60 percent by weight. This zone is characterized by an abundance of sandy units (Smith, 1993). Salinity varies with depth in a smooth pattern (Table 1, Fig. 1) with a minimum at 30 m, gradually increasing to a single broad maximum at about 150 meters depth, and sharply declining thereafter to steady low values at 210 meters and below. The salinity of the modern lake (1872) prior to agricultural activity in the water shed, was about 9% (Gale, 1914). Assuming that similarly elevated salinity characterizes the various interglacial times when Owens Lake was the terminus, and that fresh waters must have characterized the glacial periods of intense overflow, one might expect about 8 salinity oscillations during the 800 kyr time span of the core. Such cycles are seen in the solid components of the sediments, particularly for carbonate and organic carbon content (Bischoff et al, 1993a) indicating the lake did indeed experience such changes. The salinity-depth profile, therefore, has been drastically smoothed by post- depositional diffusion of dissolved salts. Remnant waters of the last interglacial are the likely explanation only for the first salinity minimum seen at 30-40 m depth. The smooth and gradual increase of salinity in the older sediments below this depth to a maximum at about 150 m is likely the result of diffusional smoothing of older cycles. Diffusion should have had more than sufficient time, therefore, to smooth salinity gradients even lower in the core. The abrupt and erratic decrease of salinity from 150 to 210 m depth, and the erratic and generally low salinities from 210 to the bottom of the hole points to an open system for the basal pore fluids. The most likely explanation for this pattern is that fresher waters are actively moving through the sandy units below 200 m, diffusionally harvesting salt from the overlaying fine- grained sediments in the process. This ground water is moving at different velocities in the varyingly permeable sandy units, and diffusional steady state and smoothing of the salinity gradients has not been achieved.

The pre-1872 water of Owens Lake was characterized by an anionic composition of Cl:Ct:SO4 about 47:47:6 (mole basis), and Na was the only significant cation (Gale, 1914). With a salinity of about 9% the lake was alkaline and must have had a pH on the order of 10. The average pore water of the sediment, on the other hand, has pH between 8 and 10, a salinity of only 2.7%, and the anionic proportions are Ct:Cl:SO4 of 52:45:3. The pore water, therefore, is similar in Ct:Cl to the modern lake, but has a lower salinity and a reduced proportion of SO4, a consequence of the activity of sulfate reducing bacteria that produce the abundant iron monosulfides which blacken the fine grained sediments A depth plot of Ct and Cl (Fig. 2) shows that the relative proportions of the two change with depth. The extremely high salinity of the top 20 meters is not a reflection of the modern (pre-1872) lake. Rather, it is a consequence of almost complete desiccation of the lake in 1912 and the downward migration of the more dense residual-brines. As the lake dried Na carbonate minerals precipitated resulting in the 1 to 2 m thick salt bed at the surface today, and the residual brine became relatively enriched in Cl. This residual brine then percolated downward by gravity displacement and ionic diffusion, affecting the pore water composition down to about 15 m and explaining the reversal in the Ct/Cl ratio as observed in Fig. 2. There is neither sedimentological nor mineralogical evidence in the entire length of the core that such concentrations of brine and precipitation of saline minerals had been attained before. Below about 50 meters depth both Ct and Cl increase in a smooth pattern. At 40 m, Ct and Cl are about equal as they were in the pre-1872 lake. Below this depth the Cl pattern is more spread out than Ct, and in the region of the salinity maximum at 150 m, Ct exceeds Cl, while in the low salinity region below 240 m Cl exceeds Ct. The pattern is most readily explained by relative diffusivity of Cl and HCO3. With a decreasing salinity gradient in both directions from the maximum at 150 meters, Cl and HCO3 are diffusing from this maximum both downward and upwards. The coefficient of ionic diffusion for Cl is twice that for HCO3 (2.3 x10-5 versus 1.18 x 10-5 cm2 sec-1; Li and Gregory, 1974) so it is to be expected that Cl transport away from the salinity maximum will be about twice the rate of HCO3 transport.

The present pore water composition, therefore, bears little relationship to past climate because depth profile has clearly been smeared by post- depositional diffusion of dissolved salts, and modern flow of ground waters. Deuterium/hydrogen ratios in the pore-waters led Friedman et al (1993) to essentially the same conclusions.

Tables

  1. Water content and pore water composition of sediments from OL-92.
    Water content and salinity in wt %. Cl, Ct (total dissolved CO2), and SO4 in millimolal (mm). Absence of data entry indicates component was not analyzed.

Figures
  1. Sediment water-content and pore-water salinity variation with depth in Owens Lake drill hole OL-92. (PostScript version)

  2. Pore-water Cl and Ct (total dissolved carbonate species) concentration with depth in Owens Lake drill hole OL-92. (PostScript version)

References

U.S. Department of Interior, U.S. Geological Survey
URL of this page: https://pubs.usgs.gov/openfile/of93-683/4-geochem/2-pore-water/pore-water.html
Maintained by: Eastern Publications Group Web Team
Last modified: 03.01.01 (krw)