Cosmogenic Noble Gas Paleothermometry ∗ Marissa M

Earth and Planetary Science Letters 400 (2014) 195–205 Contents lists available at ScienceDirect Earth and Planetary Science Letters www.elsevier.com/locate/epsl Cosmogenic noble gas paleothermometry ∗ Marissa M. Tremblay a,b, , David L. Shuster a,b, Greg Balco b a Department of Earth and Planetary Science, University of California, Berkeley, 307 McCone Hall #4767, Berkeley, CA 94720-4767, USA b Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, USA a r t i c l e i n f o a b s t r a c t Article history: We present a theoretical basis for reconstructing paleotemperatures from the open-system behavior of Received 30 March 2014 cosmogenic noble gases produced in minerals at Earth’s surface. Experimentally-determined diffusion Received in revised form 22 May 2014 3 21 kinetics predicts diffusive loss of cosmogenic He and Ne from common minerals like quartz and Accepted 23 May 2014 feldspars at ambient temperatures; incomplete retention has also been observed empirically in field Available online 10 June 2014 studies. We show that the theory of simultaneous production and diffusion that applies to radiogenic Editor: T.M. Harrison noble gases in minerals—the basis of thermochronology—can also be applied to cosmogenic noble gases Keywords: to reconstruct past surface temperatures on Earth. We use published diffusion kinetics and production 3 21 cosmogenic nuclide rates for He in quartz and Ne in orthoclase to demonstrate the resolving power of cosmogenic noble gas noble gas paleothermometry with respect to exposure duration, temperature, and diffusion domain size. 21 diffusion Calculations indicate that, when paired with a quantitatively retained cosmogenic nuclide such as Ne paleoclimate 10 3 or Be, observations of cosmogenic He in quartz can constrain temperatures during surface exposure in surface processes 21 polar and high altitude environments. Likewise, Ne retention in feldspars is sensitive to temperatures at lower latitudes and elevations, expanding the potential geographic applicability of this technique to 3 10 most latitudes. As an example, we present paired measurements of He and Be in quartz from a suite 3 of Antarctic sandstone erratics to test whether the abundances of cosmogenic He agree with what is predicted from first principles and laboratory-determined diffusion kinetics. We find that the amounts 3 of cosmogenic He present in these samples are consistent with the known mean annual temperature ◦ (MAT) for this region of Antarctica between −25 and −30 C. These results demonstrate the method’s ability to record paleotemperatures through geologic time. © 2014 Elsevier B.V. All rights reserved. 1. Introduction in carbonates have circumvented this limitation (Eiler, 2007; Ghosh et al., 2006). Nevertheless, temperatures calculated from carbonate Reconstructing past surface temperatures from continental set- clumped isotope measurements can be anomalously high (Quade tings is important for understanding how climatic processes, such et al., 2011) and vary as a function of seasonal parameters like as glacial cycles, and tectonic processes, such as orogenic-driven precipitation (Peters et al., 2012)precluding simple interpreta- elevation change, have shaped terrestrial environments and land- tion in some cases. Leaf physiognomy (e.g. Forest et al., 1999; forms. The most widely applied methods for reconstructing past Gregory and McIntosh, 1996) and pollen distribution (e.g. Bartlein surface temperatures on continents involve measuring stable iso- et al., 2011) have also been used to estimate terrestrial paleotem- topes of oxygen in carbonates or ice. However, in the case of peratures. However, these bioclimatic proxies depend on numer- carbonates, traditional stable isotope methods require that the oxy- ous additional climate parameters, including precipitation, plant- gen isotopic composition of water from which carbonate formed is available moisture, seasonality, and length of the growing sea- known, which is rarely the case (e.g. Eiler, 2011; Kim and O’Neil, son. All existing terrestrial paleotemperature proxies are limited 1997; Rowley and Garzione, 2007). In the past decade, measure- by the abundance of specific minerals/fossils in the rock record ments of multiply substituted isotopologues (“clumped isotopes”) and can suffer from poor preservation and diagenetic alteration (Eiler, 2011). Thus quantitative reconstruction of past terrestrial temperatures remains a major challenge despite the development * Corresponding author at: Department of Earth and Planetary Science, Univer- of several paleotemperature proxies over the past few decades. sity of California, Berkeley, 307 McCone Hall #4767, Berkeley, CA 94720-4767, USA. In this paper we describe a new paleotemperature proxy based Tel.: +1 603 203 4976. E-mail addresses: mtremblay@berkeley.edu (M.M. Tremblay), on the open-system behavior of cosmogenic noble gases in com- 3 21 dshuster@berkeley.edu (D.L. Shuster), balcs@bgc.org (G. Balco). mon minerals. He and Ne were the first in situ-produced http://dx.doi.org/10.1016/j.epsl.2014.05.040 0012-821X/© 2014 Elsevier B.V. All rights reserved. 196 M.M. Tremblay et al. / Earth and Planetary Science Letters 400 (2014) 195–205 cosmogenic nuclides to be unambiguously observed in terrestrial used to quantify past temperatures at Earth’s surface. We utilize materials (Craig and Poreda, 1986; Kurz, 1986; Marti and Craig, published production rates and diffusion kinetics of these cosmo- 1987). Since then, measurements of cosmogenic nuclides in min- genic noble gas–mineral pairs, although future paleothermome- erals have transformed how Earth surface processes are studied try applications will warrant tighter constraints on both of these 3 21 and quantified (Granger et al., 2013). He and Ne are partic- variables. We show that with relatively simple models based on 3 ularly attractive for studying surface processes. Since He and first principles we can predict the observed abundances of cos- 21 3 Ne are both stable, they can be used to quantify exposure his- mogenic He in an example set of quartz samples from Antarctica 3 tories that are long relative to the half-lives of cosmogenic ra- with simple Holocene exposure histories. This work focuses on He 10 26 3 21 21 dionuclides like Be and Al. Cosmogenic He and Ne also and Ne in quartz and feldspars, respectively, but the theoretical have high production rates in terrestrial materials, minimizing framework presented here can be extended to any cosmogenic no- the amount of material needed for analysis, and noble gas mea- ble gas–mineral system and to other planetary bodies and higher surements are faster and cheaper than measurements of cosmo- temperature regimes (e.g. Shea et al., 2012; Shuster et al., 2010; genic radionuclides by accelerator mass spectrometry. Cosmogenic Suavet et al., 2013). Further, this cosmogenic nuclide-based pa- 21 Ne in quartz, olivine, pyroxene, and amphibole and cosmogenic leothermometer, which is characterized by fundamentally differ- 3 He in the latter three phases have been extensively used to ent principles and assumptions from preexisting paleotemperature study geomorphic and geologic processes (e.g. review by Dunai, proxies, may be particularly useful for studying surface processes 2010). Empirical observations (Bruno et al., 1997; Cerling, 1990; where pre-existing proxies cannot be applied, or provide important Ivy-Ochs et al., 2007; Schäfer et al., 1999), and in some cases mea- tests for internal consistency in cases where they can. sured diffusion kinetics (Cherniak et al., 2014; Gourbet et al., 2012; Shuster and Farley, 2005; Trull et al., 1991), indicate that these cos- 2. Theoretical background mogenic noble gas–mineral pairs exhibit quantitative retention at Earth surface temperatures. 2.1. Simultaneous production and diffusion 3 21 In contrast, cosmogenic He and Ne experience significant diffusive loss over geologic time at Earth’s surface in quartz and The theory and mathematics describing the open-system be- feldspars, respectively. As a result, these cosmogenic noble gas– havior of noble gases in minerals have been extensively developed mineral pairs have been almost entirely avoided in surface process in the field of noble gas thermochronology; we present a brief re- 3 studies. Incomplete retention of cosmogenic He was first inferred view of fundamental concepts here. The concentration of a noble 3 10 26 from comparisons of He-based exposure ages with Be or Al gas M simultaneously being produced and diffusing from a min- 3 ages from the same quartz samples. In diverse settings, the He eral changes as a function of time t according to: ages were systematically younger (Brook and Kurz, 1993; Brook ∂ M D(T ) 2 et al., 1993; Cerling, 1990; Trull et al., 1995). Experimentally- = ∇ M + P M (1) ∂t a2 determined diffusion kinetics of 3He in quartz broadly agrees with ∇2 these observations and indicates that significant proportions of where D is diffusivity, is the second-order spatial derivative 3 He can be lost from quartz over time, even at subzero temper- or Laplacian, a is the dimension of the diffusion domain, and atures (Shuster and Farley, 2005). Although one study showed that P M is the mineral-specific production rate of M. Eq. (1) requires 3 He could be quantitatively retained in very large quartz grains that diffusion of noble gases in the mineral is Fickian (i.e., atoms with <100 ka of exposure in Antarctica (Brook et al., 1995), the move from regions of high to low concentration with a flux pro- 3 only published cosmogenic

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