Stable Isotope Investigations of Two Components of the Cryosphere: Snow and Icicle Formation

Transcription

1 Western Michigan University ScholarWorks at WMU Master's Theses Graduate College Stable Isotope Investigations of Two Components of the Cryosphere: Snow and Icicle Formation Thomas C. Brubaker Western Michigan University Follow this and additional works at: Part of the Geology Commons Recommended Citation Brubaker, Thomas C., "Stable Isotope Investigations of Two Components of the Cryosphere: Snow and Icicle Formation" (2016). Master's Theses This Masters Thesis-Open Access is brought to you for free and open access by the Graduate College at ScholarWorks at WMU. It has been accepted for inclusion in Master's Theses by an authorized administrator of ScholarWorks at WMU. For more information, please contact

2 STABLE ISOTOPE INVESTIGATIONS OF TWO COMPONENTS OF THE CRYOSPHERE: SNOW AND ICICLE FORMATION by Thomas C. Brubaker A thesis submitted to the Graduate College in partial fulfillment of the requirements for the degree of Master of Science Department of Geosciences Western Michigan University April 2016 Thesis Committee: R.V. Krishnamurthy, Ph.D., Chair Duane Hampton, Ph.D. Alan Kehew, Ph.D.

3 STABLE ISOTOPE INVESTIGATIONS OF TWO COMPONENTS OF THE CRYOSPHERE: SNOW AND ICICLE FORMATION Thomas C. Brubaker, M.S. Western Michigan University, 2016 Understanding the processes that govern the cryosphere is necessary to understand the water budget within an area that receives significant winter precipitation. This research investigates two components of the cryosphere, namely snow and icicle formation, through the application of the stable isotopes of oxygen and hydrogen. Stable isotope measurements from precipitation collected throughout the winter of are presented. The measured isotope values are in discord with known isotope effects. This discrepancy hints at a previously unexplored atmospheric phenomenon, where the enriched oxygen isotope signature of atmospheric ozone was incorporated, via photochemical reactions, into water vapor which subsequently fell as precipitation. Furthermore, this work uses stable isotope measurements to tests the theoretical model suggested for icicle formation and examines icicle evolution. This is the first systematic stable isotope study of icicles. It is proposed that icicles grow according to a growth-cessation-growth model, where a cessation period occurs between growth periods.

4 Copyright by Thomas C. Brubaker 2016

5 ACKNOWLEDGEMENTS I would like to begin by acknowledging my advisor Dr. R.V. Krishnamurthy for his support and guidance throughout this project. This project greatly benefited as a result of his illuminating ideas and constant help. I also thank the members of my graduate committee, Dr. Duane Hampton and Dr. Alan Kehew, for taking the time to review my work. A special thank you to my peers, Chanse Ford, Ben Hinks, and Jeff Hudson, for your discussion and helpful insights. Lastly, I would like to thank my family, Briana and Marshall, for your endless support and understanding as I worked to complete this project. Thomas C. Brubaker ii

6 TABLE OF CONTENTS ACKNOWLEDGMENTS... ii LIST OF TABLES...v LIST OF FIGURES... vi CHAPTER I. INTRODUCTION...1 Overview of stable isotope hydrology...1 Snow condensation...4 Icicle formation...5 II. RESULTS AND DISCUSSION...7 Photochemical imprint on the oxygen isotope ratios in mid-latitude winter precipitation...7 Methods...17 Stable isotope investigations of icicle formation and evolution...18 Abstract...18 Introduction...19 Isotope fractionation in the hydrologic cycle...20 Methods...22 Results and interpretation...24 Conclusion...28 iii

7 Table of Contents continued III. SIGNIFICANCE OF THIS WORK APPENDICES A. Complete isotope data for winter precipitation B. Complete data for all icicles BIBLIOGRAPHY iv

8 LIST OF TABLES 1. Data for 22 samples exhibiting unusual isotope effects, arranged by decreasing 18 O Fractionation factors for each icicle...27 v

9 LIST OF FIGURES 1. Schematic of icicle growth as described by Makkonen (1988) Map showing the study area, Kalamazoo, Michigan, USA Plot of 17 O vs. 18 O for (A) samples with unusually high 18 O and (B) samples with 18 O values within the expected range Plot of d-excess vs. 18 O for all samples Plot of 2 H vs. 18 O for (A) samples with 18 O within the expected range and (B) samples with unusual 18 O values Apparatus used to collect stepwise fractions of icicles in the laboratory Results of isotopic analysis from all icicle samples Regression plot of 17 O vs. 18 O showing no...27 major deviations from the GMWL 9. Regression plot of 2 H vs. 18 O...28 vi

10 1 CHAPTER I INTRODUCTION Overview of stable isotope hydrology The measurement of isotopic ratios in water has become a routine component of hydrogeological studies. The term isotope describes an atom with the same number of protons, but different number of neutrons and thus different atomic mass. Oxygen has three stable isotopes ( 16 O, 17 O, 18 O) with 16 O accounting for greater than 99.7% of the natural abundance of oxygen. Hydrogen has two stable isotopes ( 1 H, 2 H or Deuterium) with 1 H accounting for greater than 99.9% of the natural abundance (Sharp, 2007). The water molecule is comprised of these two elements, which makes them ideal tracers to study the processes that govern the hydrologic cycle (Clark and Fritz, 1997; Machavaram and Krishnamurthy, 1995). Isotopic ratios are not measured in absolute terms, but are expressed with respect to an international standard. Isotopic ratios are reported in delta per mil notation, which is defined as: δ = ( R sample R standard 1) 1000 Where R represents the ratio of heavy to light isotopes, i.e. 18 O/ 16 O and 2 H/ 1 H. The standard for oxygen and hydrogen is known as V-SMOW (Vienna Standard Mean Oceanic Water) and has a δ-value of 0 for oxygen and hydrogen. This notation provides a convenient way to express small relative differences between samples and standards, whereas precise absolute isotopic ratios of a sample are often difficult to obtain and not in fact required in geochemical studies (Clark and Fritz, 1997; Sharp, 2007).

11 2 The isotopic composition of meteoric waters are primarily governed by multiple kinetic and equilibrium fractionations. This fractionation primarily occurs during phase changes such as evaporation of the source water and condensation of water vapor into precipitation (Sharp, 2007). Thus, the isotopic composition of precipitation varies from the source water that was its initial form. In evaporative processes, lighter isotopes evaporate faster than the heavier ones, leaving the source water enriched in the heavy isotopes. The degree of isotopic enrichment is dependent on the humidity and temperature of the atmosphere (Dansgaard, 1964). During condensation, it is assumed that the process occurs under isotopic equilibrium conditions; fractionation is controlled by temperature and is opposite to what happens during evaporation. As the vapor source continues to condense the isotopic compositions of the precipitation will become progressively more depleted in heavy isotopes. The rain out process can be modeled as a Rayleigh Distillation, which can be explained by the equation: R = R 0 f α 1 Where R is the isotopic composition of the precipitation, Ro is the original isotopic composition of the water vapor, f is the fraction of vapor remaining, and α is the equilibrium fractionation factor (Dansgaard, 1964). Furthermore, numerous experimental studies (Jouzel and Merlivat, 1984; Umera, et al., 2005; Luz, et al., 2009; Deshpande, et al., 2013) have suggested that kinetic isotope fractionation may occur during the condensation of vapor to ice. In addition to phase changes resulting in isotopic fractionation due to temperature and the Rayleigh Effect, factors such as, altitude, latitude, and amount of precipitation have distinct effects on the isotopic composition of meteoric waters (Dansgaard, 1964).

12 3 As a result of the temperature effect, isotopic ratios will often possess strong seasonal variations in precipitation where strong seasonality exists. Previous research has confirmed this seasonality in the present locality of Kalamazoo, Michigan. Machavaram and Krishnamurthy (1995) reported that the average δ 18 O and δ 2 H of the summer and winter precipitations respectively are δ 18 O = -5.7 and and δ 2 H = -30 and The loci of the global distribution of isotopes in meteoric waters can be described by the so-called Global Meteoric Water Line (GMWL), defined by the empirical relationship 2 H = 8 18 O + 10 (Craig, 1961). The intercept of this line is further defined by the deuterium excess parameter, where d = 2 H 8 18 O (Dansgaard, 1964). The value of 10 is fixed at the source of global precipitation, equatorial oceans. Secondary processes, such as evaporation, will cause this value to change, making it a widely useful tool in interpreting the hydrologic cycle and past climate (Merlivat and Jouzel, 1979). While d-excess has proven to be an invaluable tool in many studies, recent experimental works (Fisher, 1991; Umera, et al., 2005; Luz, et al., 2009; Deshpande, et al., 2013) have confirmed the presence of a kinetic isotope effect during condensation of vapor to ice and liquid. The presence of kinetic isotope effects help to explain the isotopic interactions involved in snow formation when the typical Rayleigh model fails to satisfy observed isotopic results (Jouzel and Merlivat, 1984). The kinetic isotope effect typically results in unusually high d-excess values that increase under supercooled and supersaturated conditions (Luz, et al., 2009). Recently, the relationship between δ 17 O and δ 18 O has received significant interest. Extensive analysis of the 17 O vs. 18 O relationship in meteoric waters has shown that

13 4 17 O is expected to be enriched by times that of 18 O; however, this distribution may be altered by relative humidity during condensation (Barkan and Luz, 2007). Thus, comparisons of 17 O to 18 O may yield additional information on hydrologic processes, in part because it is less sensitive to temperature effects than the traditional d-excess. Snow condensation Stable isotope measurements have been widely used to examine the climatological processes involved in condensation. Within the Great Lakes region of the United States, numerous studies (Burnett, et al., 2004; Machavaram and Krishnamurthy, 1995; Gat, et al., 1994; Iqbal, 2008) have utilized isotope measurements in precipitation, typically to examine the degree to which large bodies of surface water contribute moisture flux to the atmosphere that will eventually condense into precipitation. These investigations into how meteoric condensates are influenced by evaporative flux from surface water bodies largely rely on the deuterium excess parameter. Water vapor that contains some constituent of evaporative flux will produce condensate with a higher d- excess, and thus deviate from the GMWL. It is noteworthy that these studies mostly focused on summer precipitation. This likely arises because the interpretation of d-excess in winter condensate is less straightforward than that for summer, largely because of the kinetic effects associated with condensation of snow. A better understanding of the isotopic processes involved in snow formation can strengthen the hydrological assessment of a region. Furthermore, this can complement recent air borne studies that are exploring the isotopic composition of atmospheric water vapor at varying altitudes (Keith, 2000; Johnson, et al, 2001; Sayres, et al, 2010).

14 5 Icicle formation Icicles are wintery features which form whenever cold water, typically snowmelt, continuously flows over an overhang under subfreezing temperatures. These elongate structures typically possess a slightly convex, carrot-like form that is distinct from a cone and can bear a striking resemblance to the stalactites that form in caves (Short, et al, 2006). Despite their frequent occurrence, relatively little scientific attention has been paid to the physical processes that lead to the formation and subsequent growth of icicles. The basic processes involved in icicle growth have been well explained (Makkonen, 1988; Walker, 1988). When water supply is low and temperatures are well below freezing, icicles may form from the advancement of a uniform, horizontal freezing front. Hatakeyama and Nemoto (1958) observed this process during the infant stage of icicle formation. The root of a pendant drop freezes, creating the uniform, horizontal freezing front. Subsequently, water flows down to the tip of the icicle producing another pendant drop. This drop freezes entirely and lengthens the icicle, with each successive drop lowering the freezing boundary. When there is sufficient meltwater available to maintain a continuous pendant drop, icicles grow as the sides of the drop freeze, producing a thin ice shell (Maeno and Takahashi, 1984). This growth model is shown schematically in Figure 1. A thin film of liquid water encompasses the ice shell and flows downward, continuously supplying the pendant drop with water. The thickness of this liquid film varies with supply rate, but is typically less than 0.1 mm. The diameter of the icicle increases as portions of the liquid film freeze during downward flow. The icicle lengthens as the sides of the drop continue to freeze, simultaneously producing a narrow cylinder. This cylinder, usually about 5 mm. in diameter, captures unfrozen water and

15 6 extends several centimeters into the core of a growing icicle. The encompassed water column remains unfrozen until meltwater is no longer available and icicle growth ceases. At this time, the liquid column can freeze, trapping many air bubbles within the core of the icicle. Figure 1: Schematic of icicle growth as described by Makkonen (1988). This latter method of icicle growth has received the majority of attention, in that few theoretical approaches have attempted to describe the mechanisms of growth, namely Maeno, et al. (1994), Neufeld, et al. (2010), and Short, et al. (2006). However, the evolution of icicles has not been studied from the perspective of stable isotope effects during the freezing process. A better understanding of the melting and refreezing processes involved in icicle growth can advance our broader knowledge of ice dynamics.

16 7 CHAPTER II RESULTS AND DISCUSSION The results from this study are presented below as two separate papers which have been submitted for publication to two different peer reviewed international journals. The following papers have been slightly modified from their initial submission for continuity within this thesis. The first paper, entitled Photochemical imprint on the oxygen isotope ratios in mid-latitude winter precipitation, investigates unusual isotopic values from precipitation collected from December, 2013 through March, In this paper, it is hypothesized that photochemical reactions involving upper tropospheric and lower stratospheric ozone and resulting in the formation of 18 O enriched water vapor and its mixing with the typical near ground-level tropospheric water contributed to the observed isotope ratios. Theoretical interpretations are presented as lines of evidence to support this conclusion. The second paper, entitled Stable isotope investigations of icicle formation and evolution, presents the results of the systematic stable isotope study of icicle formation. In this paper, 2 H, 17 O, and 18 O measurements are presented as support for the theoretical models which have been proposed for icicle growth where the freezing is rapid. Further, the isotope data suggests that there is a cessation period between growth periods, a process that has not been suggested in previous icicle studies. Photochemical imprint on the oxygen isotope ratios in mid-latitude winter precipitation Stable isotope measurements in precipitation are a powerful tool to interpret past and present hydrologic processes and climate. Isotopic controls have typically been

17 8 explained by equilibrium and non-equilibrium isotope effects during condensation and evaporation by the Rayleigh distillation model (Dansgaard, 1964). Recent studies demonstrate situations in which the Rayleigh model fails to sufficiently explain the observed isotope values (Jouzel and Merlivat, 1984; Uemura, et al., 2005). Here we report unique isotopic results in precipitation collected in Kalamazoo, Michigan ( N, W), throughout the winter of The isotopic values of a number of samples are in discord with previous investigations (Machavaram and Krishnamurthy, 1995) and the isotope distribution that is expected, insofar as these samples indicate enriched 18 O ( 18 O), without the concomitant enrichment in 2 H ( 2 H), and very low d- excess. These samples (22) have 18 O values ranging from -4.5 to 13.3 and 2 H from to -16. The rest of the samples have 18 O and 2 H values ranging from -27 to -5 and -199 to -100, respectively. These later values are consistent with previous work. The 18 O values of these select samples do not follow known thermodynamic principles, thus alternative explanations are required. We hypothesize the incorporation of water vapor with high 18 O resulting from photochemical reactions involving ozone in the stratosphere and upper troposphere. The winter of was among the most severe winters on record across many parts of North America and was characterized by a shift in the arctic polar vortex (Gronewold, 2015). It is conceivable that this shift in the polar vortex catalyzed this process. Mass balance calculations require only minor amounts of photochemically derived water vapor to explain the high 18 O in these samples. The majority of the samples exhibit very low deuterium excess values compared to the expected values. This also hints at atmospheric processes not common to the study site. Our results suggest a hitherto unexplored atmospheric phenomenon.

18 9 Thermodynamically driven effects can satisfactorily explain the distribution of oxygen and hydrogen isotopes in the hydrologic cycle. Evaporation is considered a kinetic effect accompanied by a faster rate of evaporation of the lighter isotopes. Condensation is treated as an equilibrium process which is primarily controlled by temperature and results in the heavier isotopes getting removed in the condensate. Additionally, a Rayleigh distillation model accounts for the progressive depletion of the heavy isotopes in successive precipitation along the traverse of an air mass 1. A straight line, the so-called Global Meteoric Water Line, defined by the empirical equation 2 H = 8 18 O + 10, represents the loci of all fresh water systems that have not been subjected to post-condensation processes (Craig, 1961). The intercept of this line further defines the deuterium excess (d-excess) parameter, where d = 2 H 8 18 O, and the value of 10 is fixed at the source of global precipitation, being equatorial oceans (Dansgaard, 1964; Merlivat and Jouzel, 1979). The d-excess has been widely used in paleo-climatic studies and other hydrological applications. In this study, samples of precipitation were collected for stable oxygen and hydrogen isotope analysis from Kalamazoo, Michigan, USA (Figure 2). The study area is characterized by temperate climate with distinct winter and summer seasons. In total, 108 samples were collected between December, 2013 and March, 2014, collectively representing the winter precipitation. The complete results of our isotopic analysis are given in Appendix B. The striking observation is that several samples exhibit unusually high 18 O, with a single sample giving up a value of 13.3, without the concomitant increase in 2 H (Table 1). Previous extensive study of winter precipitation from the study area give an average value of -100 and for 2 H and 18 O, respectively

19 10 (Machavaram and Krishnamurthy, 1995). These authors also defined the Local Meteoric Water Line (LMWL) for winter as 2 H = O and the average winter deuterium excess as Significant research has been conducted to examine the causes of why the observed isotopic ratios in winter precipitation sometimes fail to satisfy the results predicted by the Rayleigh distillation model (Jouzel and Merlivat, 1984; Uemura 2005). These studies have attributed the presence of a thermodynamically driven kinetic isotope effect due to differences in molecular diffusivities of isotopes during condensation of vapor to ice or water. This results in unusually high d-excess. Our results exhibit unusually low d-excess and thus cannot be attributed to this effect. It has been suggested that evaporative processes that occur during deposition may result in low d-excess in Figure 2: Map showing the study area, Kalamazoo, Michigan, USA.

20 Sample ID Table 1. Data for 22 samples exhibiting unusual isotope effects, arranged by decreasing 18 O % Ozone Date δ 18 O δ 2 H δ 17 O d- influence excess ( 18 OO3 = 100 ) % Ozone influence ( 18 OO3 = 400 ) 74 12/31/ ± ± ± /21/ ± ± ± /25/ ± ± ± /22/ ± ± ± /28/ ± ± ± /5/ ± ± ± /20/ ± ± ± /10/ ± ± ± /30/ ± ± ± /19/ ± ± ± /24/ ± ± ± /10/ ± ± ± /11/ ± ± ± /27/ ± ± ± /27/ ± ± ± /25/ ± ± ± /11/ ± ± ± /19/ ± ± ± /1/ ± ± ± /25/ ± ± ± /6/ ± ± ± /20/ n precipitation (Froehlich, et al., 2001). However, these processes do not result in the degree of 18 O enrichment, unaccompanied by a similar enrichment in 2 H, observed in our results. To our knowledge, no such isotope effect, as reported here, has been demonstrated in winter precipitation, especially from mid-latitudes. In recent years numerous experimental, mostly based on air borne measurements, and theoretical models have been provided to explain the isotope values of upper tropospheric and stratospheric water vapor (Taylor, 1972; Guo, et al., 1989; Kaye, 1990; Moyer, et al., 1996; Keith, 2000; Johnson, et al., 2001; Sayres, et al., 2010). The main

21 12 focus has been on hydrogen isotopes with a few also dealing with oxygen. At high altitude, water vapor has been shown to be greatly depleted in heavy isotopes due to a variety of factors. Upper tropospheric measurements of 2 H and 18 O are approximately and -50, respectively. These 2 H depletions are expected to be modestly reduced in the lower stratosphere due to the admixture of slightly enriched water vapor created from methane oxidation (Moyer, at al., 1996). These authors argue in favor of kinetic effects and processes such as ice lofting that control the isotopic composition in the stratosphere. These processes affect oxygen and hydrogen simultaneously. Our results, on the other hand, appear to specifically point to processes that affected oxygen with no or negligible impact on hydrogen. We argue that photochemical reactions in the stratosphere transferred the enriched oxygen isotope signature of ozone to water vapor, which was subsequently circulated into the troposphere and mixed with low altitude water vapor. Enrichments of 18 O in ozone have been typically reported at , but some observations have measured 18 O enrichments of up to 400 (Johnson and Thiemens, 1997; Mauersberger, 1987; Mauersberger, et al., 2001; Mauersberger, et al., 2003; Krankowsky, et al., 2000; Sato, et al., 2014). Tropospheric ozone appears to have a similar isotopic enrichment, as enrichments of have been reported from three extratropical locations in North America (Johnson and Thiemens, 1997). The prime reactions involved in the formation of stratospheric water vapor appear to be: OH + HNO4 H2O + NO2 + O2 (reaction 1) OH + HNO3 H2O NO3 (reaction 2)

22 13 OH + HO2 H2O + O2 (reaction 3) If the OH present in these reactions formed as a product of ozone dissociation, where: H + O3 OH + O2 (reaction 4) then the water vapor formed in reactions 1, 2, and 3 could impart the enriched 18 O signature of ozone. There is contention as to how much this transfer can enrich the oxygen that would be ultimately transferred to water vapor and a full understanding is still elusive (Guo, 1989; Kaye, 1990). Given that we do see evidence of 18 O enriched values in the snow samples, it has to be assumed that the above reactions do transfer 18 O enriched oxygen to be incorporated in the water vapor. It must be stressed that the 18 O enriched samples were not all derived from the stratosphere. Obviously, a stratosphere derived fraction mixed with the normal tropospheric water vapor. Simple mass balance considerations can be applied to examine this degree of mixing. For example, use can be made of the mass balance expression: 18 Oozone derived vapor (x) + 18 O average Kalamazoo snow (1-x) = 18 Omeasured sample, where x is the proportion of ozone influenced water vapor required. For this, we separate the samples in the light of previous published work. The range of 18 O values for precipitation collected for the same period (December-March) was -4.5 to We consider our samples that fall in the same range to be the normal ones. An average 18 O of normal samples gave a value of This is not much different from the value of -15 quoted by the previous work based on samples collected over a period of three years (Machavaram and Krishnamurthy, 1995). We argue that the samples with high 18 O values shown in Table 1 are the result of mixing by stratospheric water vapor with tropospheric vapor with a normal value. Taking the often reported value of 100 and the upper extreme

23 14 of 400 for stratospheric ozone the required mixing ranges from %. Note that the relatively high 24% is just for one sample with a 18 O value of This sample as well as many others were analyzed multiple times (denoted by column n in Table 1). There is irrefutable evidence that stratospheric ozone exhibits non-mass dependent fractionation (Johnson and Thiemens, 1997). This will yield a slope of 1 or nearly so on a 17 O vs 18 O plot. Terrestrial systems, on the other hand, are subject to mass dependent fractionation that results in a slope of 0.5. In fact, detailed analysis of global meteoric waters give a slope of (Barkan and Luz, 2007). Whether the stratospheric non-mass dependence in ozone will be transferred to the water vapor derived from it is debatable (Lyons, 2001; Lin, et al., 2013). However, it is interesting to look at the 17 O vs 18 O plot for the samples with unusual 18 O enrichment and compare it with the normal samples. As shown in Figure 3a-b, the 18 O enriched samples fall on a line with a slope of compared to a slope of for the normal samples. It is premature to emphasize on this slight difference, but the observation warrants further investigation. While the Brewer Dobson circulation is implicated as an agent in transporting ozone to the middle latitudes (Brewer, 1949; Plumb & Eluskiewicz, 1999), our samples represent a season when the arctic polar vortex was very active and shifted to lower latitudes, especially the interior of the United States, generating record snow fall. It is therefore imperative to examine the influence of the polar vortex. The polar vortex has been suggested as an efficient agent in ozone destruction in the Polar Regions (Wilmouth, et al., 2006). Although the destruction is much more severe in the South Pole, the southward shift from the Arctic pole can be expected to have an impact in mid latitudes

24 15 Figure 3: Plot of 17 O vs. 18 O for (A) samples with unusually high 18 O and (B) samples with 18 O values within the expected range. (Pyle, 1995; Müller, et al., 2005), as perhaps happened during the winter of Above the mid-latitudes, this southward shift could have catalyzed the ozone dissociation necessary to transfer the 18 O enrichment to water vapor. Interestingly, recent studies have also observed higher concentrations of water vapor in the stratosphere of midlatitude regions of the United States leading to significant ozone destruction via chlorine radicals (Anderson, et al., 2012). The presence of higher concentrations of water vapor adds further plausibility to our interpretations.

25 16 The unusually low d-excess of majority of the samples, in contrast to expectations and previous work, also requires explanation (Figure 4). This is most likely associated with extreme kinetic effects under super cooled conditions. One possibility is rapid sublimation of solid ice in the upper troposphere and lower stratosphere. This can leave Figure 4: Plot of d-excess vs. 18 O for all samples. the residual ice, the source of snow on the ground, with low d-excess values. The slope of 5.4 on the 2 H vs. 18 O plot (Fig 5a) seems to support this possibility. Lowering of d- excess can also arise from a lesser contribution of moisture from the Great Lakes to the atmosphere. This is not unlikely, given that during the winter of the extent of ice cover on the Great Lakes was the broadest it has been in over 40 years (Gronewold, 2015). The low slope of the 2 H vs. 18 O plot (Fig 5b), and the poor correlation (R 2 =0.22), representing the unusually 18 O enriched samples confirm non-thermodynamic processes responsible for the isotope effect. A photochemical influence is indeed worthy of consideration.

26 17 Figure 5: Plot of 2 H vs. 18 O for (A) samples with 18 O within the expected range and (B) samples with unusual 18 O values. Methods Samples of precipitation were collected for isotopic analysis from December, March, 2014, in Kalamazoo, Michigan ( N, W). Samples were collected in commercially available glass jars. When the sampling interval was complete, the jar was sealed to prevent evaporation and the snow was allowed to melt. Samples were then transferred to 5 milliliter glass vials and sealed for storage. Date, time, and nature of precipitation were recorded during sampling. Meteorological records of total daily precipitation, relative humidity, and air temperature were attained to compare with isotopic measurements. A total of 108 samples were collected for the present report.

27 18 Sample fractions were analyzed for their δ 17 O, δ 18 O, and δ 2 H values using a Los Gatos Research Triple Liquid Water Isotope Analyzer in the Stable Isotope Laboratory at Western Michigan University. Measurements are expressed in the conventional δ - notation and referenced to the international standard VSMOW, where: δ = ( R sample R standard 1) x1000 R represents the ratio of heavy to light isotopes, i.e. 18 O/ 16 O and 2 H/ 1 H. Results are presented with an analytical precision of 0.05, 0.1, and 1 for 17 O, 18 O, and 2 H, respectively, based on the repeated analysis of an internal standard. Some samples yielding unexpected isotopic values were analyzed multiple times for quality assurance. The -values for these samples are accompanied by a ± value representing one standard deviation. The number of times a sample was analyzed is denoted in the n column of Table 1. Stable isotope investigations of icicle formation and evolution Abstract Icicles are elongate structures formed when water flows over an overhang and crystallizes in sub-freezing conditions. While a widely accepted theoretical model describes their formation, experimental evidence is primarily based on simulated or laboratory based studies. First studies dealing with stable isotope measurements of icicles that were sampled from a natural setting and melted stepwise into fractions are reported. The 2 H values are presented here as support for the laboratory based studies that suggest the rapid freezing of icicles. Icicles exhibit minimal to no fraction-to-fraction isotope

28 19 variation, suggesting insufficient time for isotope equilibrium to be achieved during phase changes. An equilibrium process results in an isotopic enrichment of the solid phase. Deviations from the winter Local Meteoric Water Line, based on 2 H and 18 O, indicate that post-depositional processes, possibly sublimation, are likely to occur throughout the freezing process. The δ 17 O vs. δ 18 O plots are consistent with what has been observed for meteoric waters. Isotopic evidence lends support to a growthcessation-growth process as a proposed method of icicle formation, where a cessation period occurs between periods of freezing during icicle growth. Introduction Icicles are a picturesque feature commonly seen in regions that experience cold winter weather. These wintery features form whenever cold water, typically snowmelt, continuously flows over an overhang under subfreezing temperatures. The basic processes involved in icicle growth have been well explained by the pioneering work of Makkonen (1988) and others (Maeno and Takahashi, 1984; Walker, 1988; Maeno, et al., 1994; Short, et al., 2006; and Neufeld, et al. 2010). The growth model is shown schematically above in Figure 2. However, experimental studies have been limited to laboratory based investigations. A better understanding of the mechanisms of natural icicle formation can advance our knowledge of the general process of ice formation and melting of ice. It has also been suggested that they may add to the understanding of the growth of hailstones and ground water hydrology (Geer, 1981; Reesman, 1973). Furthermore, the isotopic interactions involved in the melting and subsequent refreezing of ice may provide some insight into the dynamic processes involved in the basal ice of a glacier (Jouzel and Souchez, 1982).

29 20 The present paper reports the first stable isotope measurements from several natural icicles that formed during the winter of 2015 (February and March) in Kalamazoo ( N, W) in South West Michigan USA. Our experimental data complement the mechanisms suggested by the models referred to previously. Furthermore, our data is in agreement with laboratory experiments which describe the isotopic fractionation that occurs during freezing, where diffusion induced kinetic fractionation occurs through a boundary layer between the advancing ice front and remaining water. The thickness of this boundary layer has been suggested be to an important parameter in controlling the kinetic effects and rate of growth of freezing liquids (Souchez, et al., 1987). Isotope fractionation in the hydrologic cycle The relative abundance of the stable isotopes of oxygen and hydrogen in precipitation varies spatially and seasonally due to a variety of factors (Dansgaard, 1964). Winter precipitation is generally more depleted in heavy isotopes when compared to summer precipitation. In Kalamazoo, Michigan, the site of the present study, earlier work has confirmed this seasonality (Machavaram and Krishnamurthy, 1995). The average δ 18 O and δ 2 H of precipitation collected during the present sampling period (February- March, 2015) are and , respectively. Isotopic fractionation typically occurs during phase changes in water. Jouzel and Souchez (1982) and Moser and Stichler (1980) suggest that isotopic fractionation does not occur during the melting of compact ice due to the low diffusion coefficient in ice. Búason (1972) proposed that fractionation may occur during the melting process due to the partial recrystallization of snow and change in crystal size. During freezing, isotope

30 21 fractionation does take place, such that 18 O and 2 H are preferentially incorporated into the solid phase, leaving the residual liquid depleted in heavy isotopes (Michel, 1985). Isotopic equilibrium processes largely govern the fractionation during freezing. Under equilibrium conditions, the isotope fractionation factors (α) for oxygen and hydrogen, as reported by Suzuoki and Kimura (1973), are and , respectively. Thus, the δ 18 O and δ 2 H values of ice are 2.8 and 20.6 more positive, respectively, than the water (Michel, 1985). Arnason (1969) reported a similar fractionation factor of for hydrogen. However, non-equilibrium effects, such as diffusion through a boundary layer and the trapping of liquid water during crystal growth, do alter the distribution of isotopes in the freezing process. These kinetic effects usually result in lesser isotope fractionation than equilibrium effects (Posey and Smith, 1957; Suzuoki and Kimura, 1973; Souchez, et al., 1987). Furthermore, [i]f the rate of freezing is too great, the ice does not fractionate isotopes with respect to the water (O Neil, 1968). If the present icicles formed in isotopic equilibrium, the first ice that formed would be enriched with heavy isotopes relative to the snowmelt from which it formed. Subsequent ice crystallization would result in continuous depletion of the snowmelt reservoir and forming ice. This process can be described by the Rayleigh distillation equation as: R = R 0 f α 1 Where R is the isotopic composition of the ice formed, R0 is the original isotopic composition of the snowmelt prior to refreezing, f is the fraction of icicle formed, and α is the equilibrium fractionation factor (Dansgaard, 1964; Michel, 1985). It should be noted that icicles freeze through three mechanisms, as observed by Maeno and Takahashi

31 22 (1984) and Maeno, et al. (1994), but we hypothesize that there will be no isotopic distinction between these mechanisms. Moreover, the rate of freezing would be reflected in the amount of isotope fractionation, as rapid freezing restricts fractionation. Methods Icicles were sampled throughout February-March, 2015, from the roof of a building on the campus of Western Michigan University, Kalamazoo, Michigan, where they were growing adjacent to each other on the structures of an external ventilation system. Dates of sample collection are available in the supplementary materials. This is analogous to a poorly insulated roof of a building, the most common place of icicle formation. The icicles sampled varied in length from 25 to 50 cm. and in diameter from 2-4 cm. Analysis of oxygen and hydrogen isotope ratios ( 17 O, 18 O and 2 H) was performed by melting the icicles at room temperature (25 C + 1.6) into several fractions using the device shown in Figure 6 and analyzing each fraction separately. It can be assumed that the step wise melting process is the reverse of freezing since it has been shown that no isotope fractionation occurs during melting of ice (Moser and Stichler, 1980; Jouzel and Souchez, 1982; Souchez and Jouzel, 1984). As melting proceeded several water fractions were captured from below in 15mL plastic vials as it dripped from the hanging icicle. Each of the vials was capped and sealed to prevent evaporation. Samples were melted in fractions of varying volumes until the icicle was completely melted. Fractions collected per icicle ranged from 5 to 19 fractions; however, only samples with at least 10 fractions are presented in this work. Selection of these samples was dictated by the need to create regression plots and a rule of thumb suggesting at least 10 samples be used (Van Belle, 2008). Aliquots of snowfall from a nearby location

32 23 Figure 6: Apparatus used to collect stepwise fractions of icicles in the laboratory. in Kalamazoo, Michigan, were periodically collected throughout the icicle sampling period and analyzed. Their isotope values were presumed to be the starting value of snow out of which the icicles were forming. Sample fractions were analyzed for their δ 17 O, δ 18 O, and δ 2 H values using a Los Gatos Research Triple Liquid Water Isotope Analyzer in the Stable Isotope Laboratory at Western Michigan University. Measurements are expressed in the conventional δ - notation and referenced to the international standard VSMOW, where: δ = ( R sample R standard 1) 1000

33 24 R represents the ratio of heavy to light isotopes, i.e. 18 O/ 16 O and 2 H/ 1 H. Results are presented with an analytical precision of 0.1 and 1 for oxygen and hydrogen, respectively, based on the repeated analysis of an internal standard. Results and interpretation Results of our analyses are presented in Figures All of the isotope data are presented in Appendix B. We prefer to use the 2 H record of the samples analyzed since it is more sensitive than 18 O. Based on our isotope studies we look at two plausible models of icicle growth. Model 1: In this model which we refer to as the growth-cessation-growth model, under favorable conditions the collected basal snow melts and as it falls under gravity gets frozen through laminar heat flow (Neufeld, et al., 2010) since the ambient temperature is very low. In this scenario, our data agrees with previous lab based studies that there is no fractionation during the melting of the basal snow (Jouzel and Souchez, 1982; Moser and Stichler, 1980) and there is no fractionation when the snow melt freezes too rapidly (O Neil, 1968). This is exhibited by all our samples except Icicle B. Icicle B exhibits the largest inter fraction variations in δ 2 H ( ± 2.8 ) of all the icicles. While the spread is outside of the analytical precision we believe it is still within a narrow range, making this model a likely scenario. Interestingly this model does not contradict the theoretical models that suggest rapid growth of icicles. Model 2: In this model, it is proposed that the entire icicle formed out of one reservoir of snow melt. In such a scenario it can be examined if a Rayleigh type process controlled their formation. The 2 H of this reservoir is taken to be the average 2 H of the snow that fell during the sampling period. Fractionation factors for each icicle

34 25 fraction were calculated using the Rayleigh distillation equation, expressed using the delta notation, where: (α 1) ln f = ln( δ 2 Hice) ln( δ 2 Hsnowmelt) The value used for snowmelt was ± 27.3, which was derived from the average values for all the snow collected during the sampling period. It is presumed that this is the snow out of which the icicles were formed. Figure 7 presents the results of this analysis. The Δ 2 H term on the y-axis denotes ln ( Hice) ln ( Hsnowmelt). The slope of the graphs is equal to -1. Table 2 presents the fractionation factor for individual icicles. The mean fractionation factor (α) for all icicle samples is ± It is noteworthy that when the standard deviation is applied to the average snowmelt (-169 to 114 ) the fractionation factor does not change statistically significantly. Alternatively, if the ice formed under equilibrium conditions, a fractionation factor of around is expected (Arnason, 1969; Suzuoki and Kimura, 1973). Our results lend support to the contention that an isotopic equilibrium is not achieved. Figure 8 shows the relationship between δ 17 O and δ 18 O of each icicle fraction. The slope of 0.57 calculated for the present analysis closely resembles the expected slope of 0.528, as previously defined for meteoric waters (Miller, 2002; Luz and Barkan, 2010). Figure 9 represents the δ 2 H vs. δ 18 O regression plot of all the icicle fractions analyzed. The slope (6.54) and intercept (-18.26) of this plot deviate from the winter Local Meteoric Walter Line defined by Machavaram and Krishnamurthy (1994) as δ 2 H = 7.5(δ 18 O) This deviation suggests post-depositional processes, possibly sublimation may have occurred after freezing. Ambient diurnal temperature differences make this a likely scenario. This lends support to the growth-cessation-growth model,

35 26 in that the occurrence of sublimation is a possibility during the cessation period of icicle growth. However, whether the isotopic changes are caused by post depositional processes, such as sublimation, requires further investigation. Figure 7: Results of isotopic analysis from all icicle samples.

36 27 Table 2. Fractionation factore for each icicle Icicle Slope α A B C D E F G H Average Figure 8: Regression plot of 17 O vs. 18 O showing no major deviations from the GMWL.

37 28 Figure 9: Regression plot of 2 H vs. 18 O. Conclusion The results of the δ 17 O, δ 18 O, and δ 2 H analyses on natural icicles suggest that icicles form according to the theoretical model proposed by Makkonen (1988) for the formation of icicles. Minimal isotope fractionation observed in melted icicle fractions suggests the absence of equilibrium isotope effects, as well as, the presence of diffusion processes during freezing, which are expected to impart significant isotope effects. Rate of freezing has been shown to be a major limiting factor on equilibrium effects, suggesting that the phase change is often rapid. The slope of the δ 2 H and δ 18 O regression line shows deviation from the previously defined winter Local Meteoric Water Line. This data suggests that post-depositional processes occurred after freezing, possibly sublimation. The combination of this evidence indicates that icicles may follow according to the growth-cessation-growth model described above, where a cessation period

38 occurs between periods of freezing. Lack of isotope equilibrium also lends support to the rapid growth of icicles. 29

39 30 CHAPTER III SIGNIFICANCE OF THIS WORK This study aimed to use isotopic ratios of oxygen and hydrogen to investigate two components of the cryosphere, namely snow and icicle formation, common to southwest Michigan. An improved understanding of the processes controlling these components is important in informing the hydrologic assessment of this region. This study introduces a previously unexplored atmospheric phenomenon involved in snow formation. The unusual results from the isotopic analysis of precipitation collected during the winter of do not agree with known isotope effects and thus, call for alternative explanations. It is hypothesized that photochemical reactions involving upper tropospheric and lower stratospheric ozone and resulting in the formation of 18 O enriched water vapor, which was subsequently mixed with typical near groundlevel water vapor, contributed to the observed isotope ratios. This conclusion complements other research which has aimed to describe the processes involved when known isotope models fail to satisfy the observed results. A better understanding of the mechanisms of icicle formation can advance our knowledge of the general process of ice formation and melting of ice. The isotopic interactions involved in the melting and refreezing of ice may also provide some insight into the dynamic processes involved in the basal ice of a glacier. Although few studies have presented theoretical approaches for icicle formation, these studies have examined icicles that were grown artificially in a laboratory. This study represents the first systematic study of icicles that were grown in a natural setting. Furthermore, this is the first study to utilize stable isotope measurements in icicles.

40 31 Appendix A Complete isotope data for winter precipitation

41 32 Appendix A Complete isotope data for winter precipitation Sample ID Date Start Time End Time 18 O 2 H 17 O d- excess n 18 12/3/13 3: /7/13 1:00 10: /8/13 19:00 23: /8/13 13:00 19: /8/13 Night /9/13 8:00 17: /9/13 Night /10/13 13:00 18: /10/13 Night 10: /11/13 10:00 12: /11/13 10:00 18: /11/13 Night /14/13 1:00 9: /14/13 10:30 11: /14/13 11:30 12: /14/13 11:30 13: ± ± ± /14/13 13:30 14: ± ± ± /14/13 14:30 15: /14/13 15:30 16: /14/13 16:30 18: ± ± ± /14/13 18:30 20: ± ± ± /14/13 20:30 23: /15/13 2:00 10: ± ± ± /15/13 10:00 11: /15/13 11:30 12: /15/13 12:30 13:

42 33 Appendix A continued Sample ID Date Start Time End Time 18 O 2 H 17 O d- excess n 44 12/16/13 2:00 9: /17/13 1:00 6: /17/13 10:00 11: /17/13 13:00 16: /18/13 1:00 8: /20/13 13:00 15: /20/13 6:00 18: /21/13 13:00 16: ± ± ± /21/13 16:00 20: /22/ ± ± ± /22/ /22/13 13:00 19: /22/13 11:00 20: ± ± ± /23/13 1:00 9: ± ± ± /24/13 1:00 10: /25/ /25/13 10:00 19: /26/13 1:00 10: /28/13 0:00 10: ± ± ± /30/13 0:00 10: ± ± ± /30/13 10:00 14: /31/13 10:00 12: ± ± ± /1/14 1:00 10: /1/14 10:00 11: ± ± ± /1/14 11:30 13: /1/14 13:00 14: /1/14 14:00 16: /1/14 16:00 21: