Mono Lake, CA

Paleoclimate and Geochronology Studies in the Mono Basin

It has been known for more than a century that the internally drained lake basins of the western Great Basin have been much more extensive in the past (e.g., Russell, 1889 for the Mono Basin) and it has been known for at least 50 years, since shortly after radiometric dating came to be, from Wally Broecker’s C-14 analyses that the most recent of the wet periods in the western Great Basin occurred during the most recent Northern Hemisphere glacial cycle, with the most extreme high levels at the deglacial transition. Additionally, correlatives to the D-O events in the Santa Barbara Basin (Behl and Kennett, 1996; Hendy and Kennett, 1999; Hendy and Kennett, 2000) as well as demonstrated millennial scale variability of paleoclimate proxies in the Great Basin (e.g., Benson et al., 1998; Lin et al., 1998), suggest that the terrestrial climate of the American West may also provide important information about these millennial events. However, difficulties in dating the records at high resolution and precision have limited their use for global correlations.

For the Mono Basin we have taken a significant step to overcome the challenge of obtaining a high resolution time scale by correlation of a new relative paleomagnetic intensity record from the Wilson Creek Formation to the GLOPIS global paleointensity stack (Laj et al., 2004), on the GISP2 (Greenland Ice Sheet Project) timescale (Zimmerman et al., 2006). This has allowed us to constrain the base of the Wilson Creek Formation to be 67 ka, and to create a high resolution time scale for the record. This correlation is consistent with all existing chronological data, but is not without dissenters. A critical aspect of the time scale, and one that is at the center of disagreements is whether or not the “Mono Lake Excursion” (a paleomagnetic excursion split by Wilson Creek Ash 15) is the Mono Basin expression of the Laschamp excursion as suggested by Kent et al. (2002). Therefore, additional geochronological constraints are necessary in order to be confident about the time scale and thus fully realize the potential of this record. An unequivocal age on Ash 15 would settle the debate.

Zimmerman’s (2006) application of the paleointensity-based age model to a published record of carbonate concentration in the late Pleistocene Mono Lake sediments has revealed variability at D-O pacing at Mono Lake. The overarching goal of our field project is to obtain a better understanding of the chronology of climate change in the Mono Basin. The exact goal of the project shifted due to a combination of failure to tap organic rich sediments as old as Ash 15 and discovery of how dynamic and interesting the (more readily tractable) deglacial part of the Wilson Creek Formation is.

Summary of Project Results so far: Although we still have much to accomplish, we have made some significant progress in our overarching goal of documenting the paleo-hydrological changes in and around the Mono Basin. Sidney and Gary Hemming spent 3 weeks in 2007 and 5 weeks in 2009 in the Mono Basin. Susan Zimmerman was able to join for 5 days in 2009, and has contributed a lot of C-14 data to the project as well as expanding our resources during the 2009 field season with internal funding from LLNL.

Coring and sampling of Wilson Creek Formation at Mono Lake: The paleointensity time scale we generated represented a significant improvement, but the importance of the accuracy of correlation motivated the work we proposed. During the first field season in summer 2007, we were accompanied for part of the time by Wally Broecker as well as staff from the LaCore Facility at University of Minnesota. Wally Broecker had the goal of studying the C-14 reservoir changes in the lake by comparing terrestrial plant macrofossils with materials formed in the lake. We were also hopeful that a core penetrating the top of the Wilson Creek Fm. could be achieved. In the Mono Lake coring, we tried two different locations: the western embayment (37.99°N, 119.106°W) and Horse Meadows sub-basin (37.96°N, 119.07°W). Both were at approximately 30 m water depth. In both cases, beautifully laminated sediments were found in the uppermost part of the cores, but below 300 years B.P. (the time of formation of Paoha Island), the sediments were badly disturbed (photos of the cores can be found at http://www.geo.umn.edu/people/researchers/noren021/CRAD/). The metadata for the cores can be found in the excel file “CRAD2007metadata.xls”.

Based on the results from 2007, we recognized that our strategy would not lead to a complete record of the Wilson Creek Formation ashes from cores achievable with equipment and resources available to us; however, we became very hopeful that we could achieve a good deglacial record from the Mono Basin, possibly contributing to the growing database of abrupt climate variations during the “mystery interval” (Denton et al., 2006) and Younger Dryas. In 2007 the Younger Dryas was on our minds as we had just attended the spring AGU session, led by Jim Kennett and colleagues, in Acapulco. We had been reading the Davis (1999) report on his core from Post Office Creek in which he had noted the presence of in situ thinolite crystals as well as abundant sequoiadendron pollen in the Younger Dryas. Thinolite crystal form has been recognized in tufas of the Great Basin lakes for over 100 years (e.g., Russell, 1889), and has been interpreted as a psuedomorph of the cold water mineral ikaite (e.g., Shearman et al., 1989). In the Wilson Creek sediments, thinolite crystals are always found just above Ash 1, and never anywhere else in the stratigraphy. The Davis age model for the Post Office Creek core is based on tephra layers and C-14 from plant macrofossils. We knew that the apparent age of basaltic Ash 2 of the Wilson Creek Fm. was ~13.3 C-14 kyr (reported in Benson et al., 1998), based on measurements of tufa coatings without leaching to remove modern carbon and without correction for reservoir age). And we knew that thinolite fans are found just a few cm above Ash 1 in the Wilson Creek Fm. The Benson et al. (1998) estimate for this age of Ash 1 is 14 ka. Davis’ core extended to 7.5 meters and ended in a series of sand layers within or just below the Younger Dryas (~13 ka). We assumed that the thinolite crystals recognized by Davis might be the same stratigraphic level as the thinolite crystals found in the Wilson Creek Fm. In this case, the bottom of Davis’ core barely missed the top of the Wilson Creek Fm. Unfortunately this important core has been misplaced and needs to be replicated. During the 2009 field season we obtained and modified a pontoon boat in order to attempt to replicate Davis’ core. In addition to our coring efforts described below, we also spent 4 days out on the lake with Steve Colman who made a CHIRP survey that including the area of Owen Davis’ core. His survey clearly identified the area where the sediments are disturbed from the Paoha Island event (including the locations of our 2007 cores), and also clearly identified some substantial tracts where the sediments appear to be well layered. The two areas where the sediments are undisturbed are both found at 20 meters and shallower in the western embayment as well as on the eastern side of the lake. The survey is pretty extensive and will be an important contribution to studies of Mono Lake, and Steve Colman plans to write a paper promptly on this topic.

In addition to the CHIRP survey, we attempted to reproduce Owen Davis’ lost Post Office Creek core. We spent three days attempting to core with a vibracore setup but were unsuccessful. The vibracore simply did not penetrate the sediments, and the water depth made the operation more challenging. At the time of Davis’ coring (1986), the water depth at this site was about 2 meters. Now it is 6 meters. We found a way to stabilize our position in the lake using anchors on either side of the back and using the engine to pull against the anchors, and eventually ended up successfully coring at 6 meters by pounding PVC pipe into the sediments using a post driver. We glued together enough pipe to stick above a derrick on the boat platform and we stood up on the derrick and pounded until the pipe was down near the deck. Then we glued another joint on and continued pounding until we could not make the pipe penetrate any further. We took a second core in shallower water (1.5 meters) near Post Office Creek. We stopped again when we could not penetrate further into the sediment.

We obtained two cores from 6 and 1.5 m water depth, near the north side of the Post Office Creek Delta. Both were stopped by a very hard layer that we think is from a rock fall- we found gravel in the core catcher of both and they were both stopped at approximately the same age sediment based on correlation of distinctive ashes in the two cores. The stratigraphy of the deeper core is remarkably similar to the description of Davis’ core and by comparison with Davis’ stratigraphy we penetrated to 8.8 C-14 ky at our two sites. Photographs of the 2009 Mono Lake cores can be seen at (http://www.ldeo.columbia.edu/~sidney/CRAD2/). A query to Davis (after the fact) revealed that his core was from the south side of Post Office Creek delta. It is our opinion that it should be possible to get high quality cores in the western embayment south and east of Post Office Creek that will penetrate at least through the Younger Dryas (that has already been demonstrated by Owen Davis) and it is reasonable to expect that we could penetrate the entire Wilson Creek Formation there. The LacCore group is working to obtain a new coring device that will allow greater penetration and they are enthusiastic about applying it to this problem.

Fossil tufa towers of the Mono Basin: Our excitement about the context of the thinolites led us to sample a large number of tufa towers in the Mono Basin in 2007.  Pleistocene towers can be found at elevations from near the present lake level to above 6800 ft.  Above 6700 ft, the towers all have abundant thinolite texture.  In addition to the thinolite texture, centimeter- to decimeter-scale dropstones were observed cemented in the towers, attesting to at least seasonally cold conditions.  Previously published C-14 ages and elevations of tufa towers (Benson et al., 1990) found that the highest elevation tufas had a limited range of apparent ages, between 11.8 and 14.1 kyr (6 of 8 are between 12.0 and 13.4 kyr).  Given the uncontrolled reservoir ages of the water as well as the porous nature of the thinolite, making modern contamination an issue, the relatively limited age range is somewhat surprising.  Xianfeng Wang has recently discovered that it is possible to get high quality U-Th ages on the outermost clean carbonate coatings on the tufa towers, and the age he has obtained from one tower at 6788 ft is 14.5±0.1 kyr (average of 4 analyses).  Additionally, the work that Scott Stine and Susan Zimmerman are doing on modern tufas is leading to some positive strategies for sampling.  They have documented a very large range in the apparent ages of modern tufa precipitating at and near the lakeshore.  The texture of this material is fluffy, and Scott Stine refers to it as physico-chemical precipitate.  He also finds tufa of a slightly later generation that is dense and laminated, that he calls algal tufa.  This tufa gives C-14 ages that are similar to the known reservoir age of the lake.  In attempting to get reliable ages on ancient tufa towers, their results point to the need to sample the dense laminated form of tufa.  We think the fine laminated coating that Xianfeng analyzed for U-Th may be analogous to this, and Susan Zimmerman will be measuring C-14 on the same materials.

Two undergraduate projects were done on tufa tower samples in the summer of 2009. Sverre LeRoy, an intern at LLNL and a senior at Mills College in Oakland, made a number of radiocarbon measurements of tufas. She first measured the highest elevation tower sampled and the lowest elevation tower sampled. The lowest elevation tower is layered, and she measured several layers in the first round. Following the first round of measurements, Sverre made a much larger set of measurements on the different layers of the mound. Her results were presented at the 2009 fall AGU.

The second undergraduate project was done with Paul Tomascak on his new ICP-MS at SUNY Oswego. Ellen Wilcox dissolved a batch of tufa samples (including the ones dated by Sverre) and measured the rare earth elements, U and Th, as well as other elements (not examined yet). Rare earth element concentrations and patterns as well as U/Th ratios appear to vary systematically with elevation of the tufa towers. The U/Th ratio varied from less than 0.1 to 20, with the highest ratios occurring in the highest elevation samples. Our interpretation is that these variations are related to the alkalinity of the water from which the tufas precipitated, because of changes in lake level and/or stratification. Troy Rasbury (SUNY Stony Brook) is going to examine the petrography of the tufas in order to map the internal distribution of U and Th and seek the best samples for U/Th dating.

Chronology of the Wilson Creek ashes: In addition to the work on cores and tufa, S. Hemming went back to the outcrops of ashes within the Wilson Creek Fm. with intern Guleed Ali as well as Research Assistant Stephen Cox, and cleaned them off carefully, photographed them, and took samples of Ashes 1, 3, 4 and 15 at Mill Creek, and Ashes 15 and 17 at the South Shore outcrop. S. Hemming also sampled with Berkeley Geochronology Center geochronologist Al Deino, helping him to sample several ashes within Ash Packages C, B and A at South Shore and Warm Springs (Ashes1 and 4) as well as several at the Wilson Creek type section and Mill Creek. Deino is in the process of setting up a new generation noble gas machine that will allow very rapid analysis of samples, and he is eager to make many measurements of sanidines from Mono Lake ashes. We will share these samples with him, in particular he will provide us with the fine heavy mineral fractions. Stephen Cox will begin a Ph.D. at Caltech in fall 2010, and is considering U-Th/He dating of Mono ashes as a potential project.

We are particularly interested in obtaining a precise and accurate time scale for the deglacial interval, which is essentially Ash Package A. Chen et al. (1996) demonstrated that ashes 5 and 12 (Lajoie’s original numbering) gave a range of 40Ar/39Ar ages, but the youngest population of ages is consistent with C-14 based ages for these layers. In our efforts to constrain the age of the Mono Lake Excursion, we used the same approach on ashes 8, 15 and 16 (Kent et al., 2002). Of these ashes, 8 gave a simple population of ~860 ky , but its depositional age is about 30 ky. Ash 15 gave a range of ages between 49 and 90 ky. Ash 16 gave a simple population that is probably pretty close to the depositional age (see reinterpretation in Zimmerman et al., 2006). We had become pretty discouraged about the prospects of Ar-Ar dating of the ages, but we decided to try again and have made progress on Ash 4. This ash is so young that the biggest source of analytical uncertainty is the correction for the blank. So we set up to measure 5 blank runs before each unknown measurement. We had 37 sanidines, coarser than 0.8 mm, and we began by fusing each crystal individually in a single step. After the first 13 crystals, we realized that it was not going to be possible to obtain a reliable age by this method. Examination of the population on an isochron plot led to the conclusion that at least part of the problem was that the assumption that the initial isotope composition of the Ar was atmospheric was in error. So we decided to measure each crystal in two steps- a low Watt step to release as much unradiogenic Ar as possible and a higher Watt step to fuse the crystal. This approach yielded some very interesting results, including a population of 6 crystals that yielded a result we think might be close to the depositional age (16.6±0.7 ka). The two-step heating approach was applied to 23 crystals and yielded four populations: 1) the good one- ages are young and initials are pretty high; 2) inherited grains- pretty good looking results with initials close to atmosphere but in the mid 20 to mid 30 ka range; 3) signals were so low that the blank correction swings things around way too much- they tended to be near zero or even negative ages; 4) samples where the first step was more radiogenic than the second, both are very unradiogenic, the apparent age is very old, and the apparent initial is less than atmospheric.

Population 1 consisted of 6 crystals. It is a pretty good population, so we can say with reasonable confidence that the ash is not older than 16.6 ka. This result requires that the age of anything above ash 4 cannot be older than 16.6, and this includes the thinolite fans discussed above. It will be really interesting to see what the ostracods through package A yield for Elena Steponaitis (she recently completed her senior thesis on getting a good chronology for events within package A and is still working on it), as well as any other age constraints we can get.  

Collaborators: 

LDEO: Sidney Hemming, Gary Hemming (LDEO/Queens College), Xinfeng Wang, Wally Broecker, Dorothy Peteet (GISS/LDEO) 

Elsewhere: Susan Zimmerman (Lawrence Livermore), Anders Noren, Christina Brady and Mark Shapley (LacCore), Scott Stine (Cal State Eastbay), Owen Davis (U of AZ), Steve Colman (LLO-UMD), Dan Davis (SUNY Stony Brook), Sarah Feakins (USC), Paul Tomascak (and undergraduate student, SUNY Oswego), Troy Rasbury (SUNY Stony Brook), Al Deino (Berkeley Geochronology Center), Brent Turrin (Rutgers), Tom Guilderson (LLNL), Neil Tibert (and undergraduate student, Mary Washington University), Elena Steponaitis (MIT), Stephen Cox (CALTech), Rahul Sahajpal (Queens College), Guleed Ali (U of AZ), Sverre LeRoy (Mills College/LLNL), Ellen Wilcox (SUNY Oswego), Chris Lowery (Mary Washington University),

Selected Papers and Abstracts

Kent, D. V., Hemming, S. R., and Turrin, B. D., 2002, Laschamp Excursion at Mono Lake?, Earth and Planetary Science Letters, 197, 151-164.

Lin, J. C., Broecker, W. S., Anderson, R. F., Rubenstone, J. L., Hemming, S., and Bonani, G. (1996). New 230Th/U and 14C ages from Lake Lahonton carbonates, Nevada, USA, and a discussion of the origin of initial thorium. Geochimica et Cosmochimica Acta, v. 60, p. 2817-2832.

Lin, J. C., Broecker, W. S., Hemming, S. R., Hajdas, I., Anderson, R. F., Smith, G. I., Kelley, M. Bonnani, G. (1998). A reassessment of U-Th and C-14 ages for late-glacial high-frequency hydrological events at Searles Lake, California. Quaternary Research, v.49, p. 11-23.

Zimmerman*, S. R. H., Hemming, S. R., Kent, D. V., and Searle**, S. V., 2006, Revised chronology for late Pleistocene Mono Lake sediments based on paleointensity correlation to the global reference curve, Earth and Planetary Science Letters, 252, 94-106.