Dating silica sinter (geyserite): A cautionary tale

https://doi.org/10.1016/j.jvolgeores.2020.106991Get rights and content

Highlights

  • We identify challenges in determining reliable ages of hydrothermal sinter deposits.

  • We apply radiocarbon, uranium-series and cosmogenic beryllium dating methods.

  • Radiocarbon ages can be biased by microbial fixation of dissolved inorganic carbon.

  • Uranium-series uncertainties are too large for precise dating of Holocene sinter.

  • Beryllium-10 exposure ages cannot be used for dating sinter from the Holocene.

Abstract

We describe a new effort to date hydrothermal silica sinter deposits (geyserite) from the Upper Geyser Basin of Yellowstone National Park using 14C of co-deposited organic matter, U-series and cosmogenic 10Be methods. A majority of the samples were collected from stratigraphic sections, mainly at Riverside, Giant, and Castle Geysers. Ages obtained from 41 14C analyses range from modern to 12.1 cal ka BP. Nearly all the 14C ages show inconsistencies with their stratigraphic positions, and several replicate 14C analyses from the same sample result in significantly different ages. The δ13C values of the organic material in the sinter range from −26.6‰ to −12.7‰. The more enriched values are attributed to microbial fixation of dissolved inorganic carbon (DIC), which has heavier δ13C values and is 14C-depleted relative to atmospheric CO2, leading to an apparent older age. U-series analyses on 4 samples yielded ages between 2.2 and 7.4 ka. Large 230Th/U age uncertainties in the sinter, due to low uranium concentrations along with elevated 232Th and associated initial 230Th, make these ages imprecise for use on Holocene deposits. A single cosmogenic 10Be exposure age of 596 ± 18 ka is considerably older than the age of underlying rhyolite and is thus unreliable. This apparent old age results from contamination by meteoric 10Be trapped in the opal that overprints the very small amount of cosmogenic 10Be. By presenting the problems we encountered and discussing their probable cause, this paper highlights the difficulty in obtaining reliable, high-precision geochronological data necessary to use sinter deposits as paleoenvironmental and paleo-hydrothermal archives.

Introduction

The hydrothermal system in Yellowstone National Park (YNP) consists of more than 10,000 diverse thermal features, including geysers and non-erupting springs and pools (Hurwitz and Lowenstern, 2014). A significant proportion of Yellowstone's iconic geysers are in the Upper Geyser Basin (UGB; Fig. 1). Thermal waters currently discharging in the UGB have elevated concentrations of chloride, sodium, and silica (Hurwitz et al., 2012). As thermal waters cool following discharge, the solubility of silica (SiO2) decreases (Fournier, 1985), leading to precipitation of opal-A and deposition of siliceous sinter that forms geyser cones, domal mounds, and terraces (e.g., Jones and Renaut, 2003; Lynne, 2012; Campbell et al., 2015a), collectively termed geyserite. Repeated wetting and evaporation of surfaces and capillary effects are the main controls on the deposition, morphology, and microstructure within sinter deposits. As the sinter deposits evolve, particles of rock, plant matter, charcoal, pollen, and microbial filaments can be trapped (Lowe and Braunstein, 2003; Guidry and Chafetz, 2003; Lynne et al., 2017).

Most geysers worldwide formed following the last glaciation, as inferred by stratigraphy and the few radiometric ages of sinter deposits (Hurwitz and Manga, 2017). In Yellowstone, it is assumed that all geyser deposits post-date the Pinedale deglaciation of the Yellowstone Plateau (ca. 15 ka; Licciardi and Pierce, 2018), as older deposits are expected to have been removed by glacial erosion beneath the >1 km thick ice cap. Therefore, the organic material trapped in the sinter deposited by these geysers can provide abundant information on post-glacial environmental changes, if deposits can be successfully dated. The original goal of this study was to establish temporal correlations between hydrothermal activity in the UGB and post-glacial regional climate. We also wanted to address questions such as: How long does it take for a large geyser cone to form; how old are some of the largest geysers in the UGB; do any of the sinter deposits in the UGB pre-date the Pinedale deglaciation? To establish age correlations and address these questions, we applied radiocarbon (14C) dating methods on a large number of silica sinter samples. We also applied U-series and cosmogenic 10Be dating methods to a small subset of those samples. We have focused our sampling on three of the largest geysers in the UGB: Riverside, Giant and Castle Geysers (Fig. 1b).

We were motivated by prior studies that applied Accelerator Mass Spectrometry (AMS) 14C measurements to date organic material trapped in silica sinter samples from thermal areas worldwide (Lutz et al., 2002; Campbell et al., 2004; Lynne et al., 2005, Lynne et al., 2008; Lynne, 2012; Campbell and Lynne, 2006; Foley, 2006; Howald et al., 2014; Lowenstern et al., 2016; Slagter et al., 2019; Muñoz-Saez et al., 2020). Uranium-thorium (Sharp et al., 2003; Paces et al., 2004, Paces et al., 2010; Maher et al., 2007), and uranium-lead (Neymark et al., 2002; Amelin and Back, 2006; Nemchin et al., 2006; Neymark and Paces, 2013) methods have been used to successfully date non-hydrothermal opal and chalcedony deposits associated with pedogenic or deeper vadose-zone environments, and electron spin resonance (ESR) methods have been used to date siliceous sinter (Chen et al., 1993). To the best of our knowledge, no previous attempts have been made to date sinter deposits using cosmogenic 10Be exposure dating methods.

Despite the large number of samples that we dated and the multiple dating methods used, resulting ages scatter widely and are unreliable given their stated uncertainties and stratigraphic positions, or, in the case of U-series, have precisions too poor to allow detailed paleoenvironmental reconstruction. The goals of this paper are to present the data that we consider unreliable and communicate the complicating issues we faced in dating sinter using these geochronologic methods. All of the data collected during this study are summarized in condensed form here (Table 1) and are documented in more detail along with descriptions of the methods used for collecting, processing, and analyzing the sinter samples in a U.S. Geological Survey (USGS) data release (Churchill et al., 2020). It is our hope that highlighting the issues we encountered, and their probable causes, will provide a cautionary tale that helps inform and guide future studies that intend to use sinter deposits as paleoenvironmental and paleo-hydrothermal archives.

Section snippets

Methods

Forty-seven approximately fist-sized (400–800 g) sinter samples were collected from UGB geysers in April 2018, November 2018, and April 2019 at 9 separate locations (Fig. 1b). At Riverside (Fig. 2), Giant (Fig. 3), and Castle (Fig. 4) Geysers, samples were collected at approximately even spacings following the stratigraphy from the shield (old sinter) to the cone (young sinter). At Castle Geyser, the oldest exposed stratigraphic levels were mapped as “old sinter” (Fig. 1b; Muffler et al., 1982

Results

Complete datasets used to derive age estimates are available in a USGS data release (Churchill et al., 2020) with the resulting age estimates summarized in Table 1. Including sample replicates, where we dated different size fractions of the same sample, we report forty-one radiocarbon ages, four 230Th/U ages, and one 10Be age (Table 1). Age estimates are all reported in thousands of years (ka) with analytical uncertainties reported at the 95% confidence level (±2σ). The radiocarbon data are

Discussion

Most previous studies using 14C to date post-glacial sinter deposits either relied on a limited number of samples, were restricted by relatively small sample sizes which did not allow for δ13C analysis of the organic material, or both (e.g., Lutz et al., 2002; Lynne et al., 2005; Foley, 2006; Lynne et al., 2008; Lowenstern et al., 2016). Furthermore, replicate 14C analyses from individual specimens were not attempted and well-defined stratigraphic information allowing comparison of relative

Conclusions

Based on the analysis of many silica sinter samples using 14C, along with complementary U-series and cosmogenic 10Be dating methods, we demonstrate that there are significant challenges to determining the timing of major post-glacial geologic events using these hydrothermal deposits. Each of the methods we applied was hampered by a different cause. In the case of 14C, we propose that the most significant problem is incorporation of microbial mats that metabolize old carbon present in thermal

CRediT authorship contribution statement

Dakota M. Churchill:Investigation, Writing - original draft, Data curation, Visualization. Michael Manga:Conceptualization, Investigation, Validation, Resources, Writing - original draft, Supervision, Funding acquisition. Shaul Hurwitz:Conceptualization, Validation, Investigation, Resources, Writing - original draft, Supervision, Funding acquisition, Project administration. Sara Peek:Investigation, Validation, Data curation, Writing - review & editing. Joseph M. Licciardi:Investigation,

Acknowledgements

This study was conducted under Research Permits YELL-2018-SCI-8030 and YELL-2018-SCI-5910. We thank Annie Carlson, Jeff Hungerford, Erin White, Behnaz Hosseini and Bill Keller from the Yellowstone Center for Resources for help in the field and with logistics. We thank Alan Hidy at LLNL-CAMS, Avriel Schweinsberg at the University at Buffalo, and Debra Driscoll at SUNY College of Environmental Science and Forestry for help with data acquisition and interpretation. We thank the KCCAMS staff for

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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