OBSIDIAN AS A TIME KEEPER: AN INVESTIGATION IN ABSOLUTE AND RELATIVE DATING by Kimberly Jo Tremaine A thesis submitted to Sonoma State University in partial fulfillment of the requirements for the degree of MASTERS OF ARTS in cultural Resources Management Dr. David A. Fredrickson, Chairman ~~~i~ July 28, 1989 .. Date Copyright 1989 By Kimberly Jo Tremaine ii AUTHORIZATION FOR REPRODUCTION OF MASTER'S THESIS I grant permission for the reproduction of this thesis in its entirety, without further authorization from me, on the condition that the person or agency requesting reproduction absorb the cost and provide proper acknowledgment of authorship. Dated ~/~}gi 148 Wilson Hill Rd. (Address) Petaluma. California 95942 iii OBSIDIAN AS A TIME KEEPER: AN INVESTIGATION IN ABSOLUTE AND RELATIVE DATING Thesis by Kimberly Jo Tremaine ABSTRACT Purpose of the Study: Over the last 30 years, obsidian has served archaeologists by providing a means to estimate age in one capacity or another. An ultimate objective of this thesis was to reexamine both absolute and relative dating approaches. Procedure: Two lines of action were pursued in this endeavor. First, an extensive review of studies on glass surface reactions was undertaken, with the intent to synthesize the information gathered, and assess both its pertinence to the obsidian hydration process and its implications regarding dating issues. Secondly, accelerated hydration experiments were conducted, reacting California North Coast Ranges obsidians, with the intent to evaluate the application of the experimental method for both absolute and relative dating purposes. Findings: From the review of studies on glass surface reactions, it was found that glass scientists have made considerable advances recently in their understanding of glass weathering kinetics. Many factors, working synergetically, have been shown to affect both mechanisms and rates of reaction, reflecting the complexity of glass weathering processes. As the literature search indicates, such factors as solution pH, solution composition, glass surface area-to-solution volume (SA/V), relative humidity, and temperature, all affect experimentally determined rates of reaction. The extent of their significance to naturally hydrating obsidian, in archaeological site contexts, needs to be assessed to fully evaluate implications to obsidian dating in its absolute chronometric capacity. From accelerated hydration experiments, designed primarily to investigate the replicability of test results, it was found that some variability between results occurred. This variability was attributed to small differences in condition such as non-standardized flake sizes (affecting SA/V ratios), and differences in solution composition (deionized water vs. silica saturated solutions). Although it was determined that test results appear to be reproducible if all factors are held constant, the appropriateness of applying condition-specific rate constants (derived from these high-temperature essentially closed­ system experiments) to field conditions was questioned. iv Conclusions: I conclude, based on the review of glass surface reaction studies and accelerated hydration experiments, that the absolute dating of obsidian remains less than secure. From a more practical standpoint, until problematic issues are resolved, perhaps a relative dating approach has more merit. An investigation was conducted exploring the potential of accelerated hydration experiments to enhance this approach. A proposed application is suggested to obtain relative calibrations of hydration measurements between sources, allowing temporal ordering and providing a means to make cross-source comparisons of obsidians exposed to similar temperature histories and conditions. ~'QA~.~ . Chairperson: .L...c .~ - i' f'..?- M.A. Program: Cultural Resources Management Sonoma State University !L8 X'-'YI'7B cr Date v ACKHONLEDGMENTS During the course of this project, I have received advice, encouragement, assistance, and support from many individuals. First, and foremost, lowe the inspiration for this project to Dr. David A. Fredrickson, my committee chair, whose inquiries prompted me to seek answers. Without his very active involvement and enthusiasm, I'm sure I would not have had the courage to proceed. His support and counsel throughout this ordeal has been invaluable. To Dave, my deepest gratitude. lowe the rigour displayed in this thesis to Dr. Mildred Dickemann, who intellectually challenged me and forever changed my perception of things. Although she was not directly involved, I recognize her influence and will always be grateful. Another person instrumental in this endeavor has been Dr. Christopher stevenson. I have benefited from his helpfulness and open communication. Thankyou, Chris. I thank the Anthropology Department and Dr. David Peri for their efforts in obtaining the pressure reaction vessel needed to conduct accelerated hydration experiments. I thank the Geology Department and Natural Sciences for their generosity in providing the space to conduct these experiments. I am indebted to the Sonoma state Obsidian Hydration Lab for making their facilities available to me, and especially to Thomas Origer for his assistance in measuring hydration slides. I would also like to express thanks to committee member, Dr. Thomas L. Jackson, who offered constructive criticism of my first draft, vi pointing out important omissions; to committee member, Dr. Stephen Harwick, for answering questions on the basics in physics and chemistry (his interest and encouragement were also appreciated); and Dr. Art White for his willingness to review and comment on a prelimdnary version of Chapter 2 (an overview of glass surface reaction studies). I would like to give special thanks to~all my friends and collegues who helped in countless ways throughout this undertaking: Rob Jackson, for his willingness to part with his preciously quarried obsidian cobbles and knap them into suitable flake samples; Mick Hayes, for his patient assistance with computer problems; Ron Leu, Mike Tatros, Vic Reindahl, steve Anderson, John Carpenter, and Dr. Rolfe Ericson, for their assistance and advice regarding technical matters with the pressure reactor; Allan Bramlette, for the thought provoking discussions we have had over the yea~s; and John Lopez, for his help with last minute graphs and moral support. Finally, the support my family has shown me, and their acceptance of my long absences, has meant so much to me. Thanks to Mom, Dad, and Jen. vii 'l'ABLE or coarDTS Abstract.••............................ ..iv Acknowledgements .. • ••• to ••••••••• • Vl Table of Contents. ..vii List of Tables .. .................. .ix List of Figures .. • •x 1 OBSIDIAN DATING. ..1 Introduction. .1 Trends in Use. • ••••••••••••••••• 3 Present status.••......•.....•.•..•••• • ••••••••••••••••• 5 statement of Purpose .••••..•.••.••..•••••••.••...••.•••..••..•9 2 OVERVIEW OF GLASS SURFACE REACTION STUDIES .•. ............. .. 11 Background. ••...•.•.........•...•••••. .•..•••.••••.•11 Gl ass Weathering Reactions........... •.•...•.•.•••.13 Factors Affecting Glass Reactions •••••••••......•.••••••••..•15 Solution pH 16 So1ution Composi tion ••.•..••.•.•••••••••.••••.••.•...••••• 20 Glass Surface Area-to-Solution Volume.. •••••••.•..•••.•. 23 ReI ative Humidity...................... ......•.••••.••• 25 Tempera t ure. •.........••........ ....•.......•••.•...•••. 27 Glass Weathering Models .•.•••••••••••.•....••.••••••••..•.35 Implications for Obsidian Dating.. ••..•••••.•...•...•.•... 42 3 RESEARCH DESIGN FOR ACCELERATED HYDRATION EXPERIMENTS 46 Background 46 Objectives.. • •••••••••••••••••••• 49 Methods ..... 50 4 RESULTS •.....••••••••••...••.••••.••••••••••..•••••••••••••••••. 53 Findings of Accelerated Hydration Experiments ••.••..•.......•53 Implications of Accelerated Experiments for Absolute Dating.• 61 Application of Accelerated Experiments for Relative Dating ... 64 Case Exampl e..........•...••.•.•.••...•.......•...•..•....70 5 SUMMARy .......•••..• II ••••••••••••••••••••••••••••••••••••••••••• 75 Bibl iography 80 Appendix: Measurements of Induced Obsidian Hydration ..•••......•.•• 93 viii List of Tables 1 Example of Classified Glass Surface Reaction Studies.••.••..•••••.14 2 Factors Affecting Glass Durability..•••.•••••.••••.•.•.•.........•15 3 Transition Times from tl/2 to t Kinetics •.••..••.••.•.......•••...39 4 Initial Experimental Design ......•........••••••••.........••••...51 5 Average Hydration Measurements for Accelerated Experiment Set 1 53 6 Michels' Measurements: Accelerated Hydration•••••.••......•..•...•54 7 Differences between Experiment Set 1 and Michels· ••••...•••.•••.••54 8 Average Hydration Measurements for Accelerated Experiment Set 2 56 9 Differences between Repeated Experiments: ~ets 1 and 2••.•.••••.•.57 10 Solution pH for Experiment Set 2••.••••.•••••••••••••••••••••.•••57 11 Average Hydration Measurements for Accelerated Experiment ~ Set 3 58 12 Differences between Repeated Experiments: Sets 2 and 3.•.•••••••.59 13 Weights of Obsidian Used in Experiment Sets 1, 2, 3, and 4•••••••59 14 Average Hydration Measurements for Accelerated Experiment Set 4 60 15 Differences between Repeat Experiments: Sets 3 and 4..•.•...••.•.63 16 Solution pH for Experiment Set 4 63 17 Ratios of Temporally Equivalent Hydration Measurements: Comparisons within and between Experiment Sets 1 and 2••.•.••••••66 18 Ratios of Temporally Equivalent Hydration Measurements According to Experiment Set ....••.•..•••..••.•••••.••.•.•.••.•..•68