1. Tephrochronology BY SUZANNE UBICK

2. What is tephrochronology?  “Tephrochronology as used here is the study of tephra layers – volcanic ash beds and tuffs – for purposes of correlation and dating of sediments, rocks, and structures.”  Sarna-Wojcicki, A.M. and J.O. Davis. 1991. Quaternary tephrochronology. In The Geology of North America, Vol. K-2: 93.

3. Tephrochronology subdisciplines  Tephrostratigraphy: correlation of tephra layers by physical and chemical characteristics and stratigraphic sequences  Tephrochronometry: numerical age determination of tephra layers  Used in tandem for correlation of layers and indirect dating

4. What is tephra? (Tuff is cemented or otherwise consolidated tephra)  “A collective term for all clastic volcanic materials which during an eruption are ejected from a crater or some other kind of vent and transported through the air; includes volcanic dust, ash, cinders, lapilli, scoria, pumice, bombs, and blocks.”  American Geological Institute. 1962. Dictionary of Geological Terms. New York: Dolphin Books.  Sarna-Wojcicki and Davis (1991) expand the definition to include all pyroclastic material ejected from a volcanic vent, including pyroclastic flow material.

5. Types of tephra

6. Tephra: pyroclastic flow

7. Tephra components  Pumice  Glass shards  Crystalline minerals and crystal fragments  Lithic fragments  Clasts, bioclasts, organic sediments, chemical precipitates, cementing materials

8. Tephra generator Undersea volcano, Tonga, March 2009

9. Tephra Generator Mount St. Helens, 1980

10. Tephra Generator Mount Montserrat, Caribbean, 1997

11. What is the Quaternary?

12. What is the scientific basis for tephrochronology?  “Tephra layers have unique characteristics, physical or chemical, by which they can be distinguished one from another, and by which they can be consistently and correctly identified.” Sarna-Wojcicki and Davis, 1991a.  Example on next slide: Layered scoria, pumice, and ash strata in an alpine meadow near Williwakas Glacier, on the southeast flank of Mt. Rainier. The dark reddish scoria and pumice, and the dark brownish ash layers, are all from Mt. Rainier eruptions. The paler ash deposits come from at least one other volcano.

14. Why are tephras good tools?  Each eruption of each volcano produces a different tephra  Because each magma chamber has different proportions of constituents  Because each eruption, by removing some minerals - following Bowen’s reaction series - changes the composition of the magma left behind, and increases the viscosity  Because the physical properties of each successive magma affect the kind of tephra that is produced  Because the chemical composition of each successive magma affects the tephra it will produce (e.g. color, reactivity)  Each volcano’s successive eruptions have smaller differences than those of different volcanos of the same age and region

15. Bowen’s reaction series

16. Important US Quaternary tephras  Mazama ash bed: 6850 BP  Rockland ash bed: 0.4 Ma  Lava Creek B: 0.62 Ma  Bishop Ash Bed: 0.74 Ma  Huckleberry Ridge ash bed: 2.0 Ma

17. What laboratory techniques are used for preparation of tephra samples?  Removal of organic material  Sieving  Heavy liquid flotation to extract shards ranging from sand-size to microscopic cryptotephra  Source: Tephra Lab. 2015.

18. What techniques are used for analysis?  Physical determination of characteristics, e.g. microscopic examination of ash to determine the shapes and sizes of glass shards

19. What techniques are used for analysis?  Microscopic examination to identify  Crystal fragments  Crystalline minerals, e.g. feldspars, quartz, biotite, apatite  Lithic fragments of rock chambers  Inclusions: clasts, bioclasts, organic sediments, chemical precipitates and cementing materials incorporated into tephra during transportation, deposition, and reworking of tephra

20. The polarizing microscope

21. Mineral components in tephra: aegerine, unique to Tuhuara Tephra, New Zealand

22. Using birefringence to identify minerals: pyroxene and feldspar in glassy matrix

23. What techniques are used for analysis?  Precise chemical analyses of glass and mineral grains by  Electron microprobe  X-ray fluorescence  Atomic absorption  Plasma atomic emission spectrometry  Chemical fingerprints of tephras can be produced in this way

24. What techniques are used for analyzing tephra?  Dating techniques, including  Laser-fusion 40Ar/39Ar analysis  Fission-track dating of zircons and glass shards  Hydration  Thermoluminescence  Uranium-series dating

25. What are the analytical uncertainties – precision (laboratory)?  Failure to recognize leaching, Aeolian and hydraulic sorting, and chemical and physical weathering during collection  Failure to use large enough samples of young tephras for dating; young tephras have accumulated only small quantities of radiogenic argon

26. What are the analytical uncertainties – accuracy (field)?  Tephras can be easily reworked, making it difficult to distinguish layers close to the eruptive source. Care must be taken to ensure homogeneity of the layer before specimen collection  Contamination during collection (sloppy field techniques!)  Physical, mineralogic and chemical characteristics can change with distance from source, depositional environment, and time

27. Which materials are most and least useful for this technique?  Tephras from Plinian eruptions are most useful, as very large amounts of material are produced, which may be carried far and deposited widely  Tephras containing large amounts of potassium-rich minerals: sanidine, biotite, honblende, and plagioclase feldspar  Small eruptions of lava are least useful, as they are highly localized

28. What are the strengths of tephrochronology?  Given that tephras are easily dated, materials or structures positioned above or below the tephra can be set into relation with an absolute date  Materials or structures positioned between tephra layers can be placed between two absolute dates  The source of the tephra need not be known in order to carry out correlations  Tephras can be collected from ice cores, varves, and deepsea oozes to identify deep time events

29. What are the weaknesses of tephrochronology?  Larger ejecta are not carried long distances  Tephra is usually carried downwind, so that upwind areas must be dated by correlation with other techniques  Tephra composition can change (by sorting and contamination) with distance from source, place of deposition, and age  Most airfall tephras only accurate to 200km from source

30. What age ranges are covered by tephrochronology?  Depending on the age of the tephra, and the dating method used, the potential age range is from the present to 4.6Ga.

31. What are tephrochronology’s applications?  To provide stratigraphic and age control when synthesizing tectonic events and determining regional stratigraphy  To assist in determining crustal motions  To calibrate other dating methods  To correlate continental and marine faunal and isotopic stages  To provide relative ages, calibrate and correlate events in human history and prehistory by anthropologists and archaeologists  *Very important: precise correlations between deep-ocean, uplifted onshore marine, and continental stratigraphic sequences. No other technique is so good.  Potential global correlations as techniques improve for detection and identification of very fine tephra in sediments worldwide

32. Case study 1: Sacramento-San Joaquin Delta, CA  Maier et al. (2014) used four tephras to correlate subsurface facies at 27 sites in the Sacramento-San Joaquin Delta  The tephras were Wildcat Grade, Loleta, Rockland, and Hood  Three facies were recognized and described  Wildcat Grade is not yet fully interpreted

33. Sacramento-San Joaquin Delta, tephra correlation panel

34. Future work in Sacramento-San Joaquin Delta project  The chronostratigraphy of the region can be mapped in fine detail  Buried sediments can be assigned reliable ages  Assessment of geotechtonic risks can be carried out

35. Case Study 2: Hey Fatty Boom Boom!

36. Dating an hypothesized microevolutionary event in modern humans: Ubick 2014.  Something happened to Homo sapiens before their second great dispersal from Africa 60,000 years ago  This event gave them an adaptive edge that allowed them to replace, displace, or absorb all other human species on Earth  I argue that an environmentally-driven microevolutionary event increased Homo sapiens female fertility by increasing their ability to gain body fat  The lower time boundary can be inferred by tephrochronology: ash from the Mt. Toba eruption of ~72,000 years ago has been recovered from East Africa – the homeland of all modern humans on Earth today.

37. Mt. Toba eruption ashfall range

38. Image sources  Tonga volcano: http://i.telegraph.co.uk/multimedia/archive/03163/tonga- volcano_3163001b.jpg http://i.telegraph.co.uk/multimedia/archive/03163/tonga- volcano_3163001b.jpg  Mount Monserrat: http://iavceicev.clas.asu.edu/images/montserrat.jpg http://iavceicev.clas.asu.edu/images/montserrat.jpg  Mount St. Helens: https://geohazards.community.uaf.edu/files/2014/03/738-567-mt-st- helens.jpg https://geohazards.community.uaf.edu/files/2014/03/738-567-mt-st- helens.jpg  Types of tephra: http://image.slidesharecdn.com/13398107- volcanoes-a-presentation-140321050223-phpapp01/95/13398107- volcanoesapresentation-12-638.jpg?cb=1395391971http://image.slidesharecdn.com/13398107- volcanoes-a-presentation-140321050223-phpapp01/95/13398107- volcanoesapresentation-12-638.jpg?cb=1395391971

39. Image sources continued  Pyroclastic flow: http://9.thumbs.scribol.com/4/sites/default/files/images/scaryflow.jpg?v =1 http://9.thumbs.scribol.com/4/sites/default/files/images/scaryflow.jpg?v =1  Quaternary Period: http://www.buzzle.com/images/infographics/quaternary-period- timescale.jpg http://www.buzzle.com/images/infographics/quaternary-period- timescale.jpg  Mt. Rainier tephra layers: http://pubs.usgs.gov/bul/1326/report.pdfhttp://pubs.usgs.gov/bul/1326/report.pdf  Dating chart  http://www.geocraft.com/WVFossils/DatingMethods.html http://www.geocraft.com/WVFossils/DatingMethods.html  Aegirine  http://www.teara.govt.nz/en/photograph/8732/minerals-from-mayor- islandhttp://www.teara.govt.nz/en/photograph/8732/minerals-from-mayor- island

40. Yet more image sources  Pyroxene and feldspar: Geosec slides. http://www.geosecslides.co.uk/index.php?s=Viewing http://www.geosecslides.co.uk/index.php?s=Viewing  More minerals: Geosec slides. http://www.geosecslides.co.uk/minerals/MIAindex0.4.html http://www.geosecslides.co.uk/minerals/MIAindex0.4.html  Pompeii victims: https://encrypted- tbn0.gstatic.com/images?q=tbn:ANd9GcTbMnZHw4ntqLR0aQUnK- Qx3c__dstf48H49hZIgrRT-hm5hy3n2whttps://encrypted- tbn0.gstatic.com/images?q=tbn:ANd9GcTbMnZHw4ntqLR0aQUnK- Qx3c__dstf48H49hZIgrRT-hm5hy3n2w

41. Bibliography  American Geological Institute. 1962. Dictionary of Geological Terms. New York: Dolphin Books.  Tephra Lab. 2015. Tephrachronology - extracting and preparation of samples. http://www.uib.no/en/earthlab/81502/tephrachronology-extracting-and-preparation- samples  Maier, K.L., et al., Quaternary tephrochronology and deposition in the subsurface Sacramento-San Joaquin Delta,California, U.S.A., Quaternary Research (2015), http://dx.doi.org/10.1016/j.yqres.2014.12.007 http://dx.doi.org/10.1016/j.yqres.2014.12.007  Mullineaux, D.R. 1974. Pumice and other pyroclastic deposits in Mt. Rainier National Park, Washington. Geological Survey Bulletin 1326. http://pubs.usgs.gov/bul/1326/report.pdfhttp://pubs.usgs.gov/bul/1326/report.pdf  Sarna-Wojcicki, A.M. and J.O. Davis. 1991. Quaternary tephrochronology. In The Geology of North America, Vol. K-2: 93-116  Ubick, S. 2014. Grey cells or fat cells? A speculative hypothesis for the success of Homo sapiens. Unpublished senior honors thesis. https://www.academia.edu/15771416/Grey_cells_or_fat_cells_A_speculative_hypothesis_f or_the_success_of_Homo_sapiens

42. A nice quick stop site for dating methods: http://datingthepast.org/