Friday, 10 October 2014

Searching for Tsunamis in the Aleutians... by Chris Vane

Chris Vane takes us to the unhinhabited island of Sanak, Alaska in search of the unique sandy fingerprints of old tsunamis. The team's mission is to improve our knowledge of past events so we better understand the hazard of future tsunamis... 

Over the past decade coastal communities around the world have been subject to a number of tsunamis that occur at seismically active subduction zones such as those in Sumatra-Andaman (2004) and Tohoku-Oki, Japan (2011). Globally we are unprepared for such catastrophic events such as the Boxing Day tsunami and know relatively little about their frequency. Although historical records of tsunamis do exist these are patchy and only in a few places such as Cascadia and perhaps Japan do they extend back far enough in time to allow some insight into the hazard from tsunami impact. 


From left to right: Location of Sanak in the Aleutians. Beautiful scenery of Sanak. Me with a GPS station on Sanak.
Reconstruction of large earthquakes and their associated tsunami requires the detailed inspection, collection and analysis of coastal sediments that, as well as dating and sedimentary evidence, may contain the unambiguous chemical and or biological fossil signature of a tsunami. Earlier international collaborations in Oregon and Japan suggested that a suite of organic geochemical compounds such as alkanes, hydrocarbons and glycerol dialkyl glycerol tetraethers (GDGT) from Archea and Bacteria could, when used together, help distinguish a tsunami origin from other extreme events such as storm over-wash and or fluvial flood. This cutting edge research led to Alaska to head up the geochemical portion of a grant aimed at tracking past tsunamis in the Aleutian Islands on the boundary between the Pacific and Bearing Sea. This area is of interest to geoscientists because it frequently generates earthquakes and tsunamis due to multiple cycles of strain accumulation and release on the megathrust faults of the Pacific plate subduction zones.


Example of a GDGT marker compound for
marine and soil organic matter

In early August I joined a multi-disciplinary team of six US scientists from USGS and one from University of Rhode Island to track ancient tsunamis in sediments. We lived and worked on the uninhabited Island of Sanak which is situated mid-way along eastern Alaska- Aleutian arc that extends 1600 km from southeast Alaska westward to Kamchatka. In reality it’s four flights and an eight hour trawler boat ride from Cold Bay, AK. The first few days in the field were spent gouge coring and ‘calibrating’ everyone’s sediment descriptions so that the team could break into tsunami hunting ‘pairs’ so that the island could be fully surveyed using a consistent terminology. We hiked around 20 km each day for the next 10 days gathering up geochemical, lithologic and stratigraphic samples from south facing valleys. We revealed multiple tsunami sand layers sandwiched between tephra (possibly from the ever looming Shishaldin, Isanotski and Pavlof Volcanoes) by coring through peats using a ‘Russian auger’ or digging trail pits. Additional evidence of a recent historical tsunamis was provided by fugitive logs stranded high above cliffs (the island has no trees). 

From left to right: The trawler from Cold Bay. Tsunami sand layer. Wild horses of Sanak.

I left the island with not only 50 kg of sediments samples but amazing memories of the landscape, incredible wildlife which included thousands of spawning salmon, nesting eagles, wild horses and the occasional sea otter. The samples now at BGS Keyworth are excellent examples of multiple tsunami sands and myself and the organic geochemistry team have already begun preparing them for molecular level analyses which will tell us about the contribution of marine as compared to terrestrial organic matter inputs to the sediments.

Chris

Below are some papers I used to inform my blog and which you may find interesting further reading:

KHAN, N. S., HORTON, B. P., MCKEE, K. L., JEROLMACK, D., FALACINI, F., ENACHE, M. D. & VANE, C. H. 2013. Tracking sedimentation from the historic A.D. 2011 Mississippi River flood in the deltaic wetlands of Louisinana, USA. Geology, 41, 391-394.
NIKITINA, D. L., KEMP, A. C., HORTON, B. P., VANE, C. H., VAN DE PLASSCHE, O. & ENGELHART, S. E. 2014. Storm erosion during the past 2000 years along the north shore of Delaware Bay, USA. Geomorphology, 208, 160-172.
PILARCZYK, J. E., HORTON, B. P., WITTER, R. C., VANE, C. H., CHAGUÉ-GOFF, C. & GOFF, J. 2012. Sedimentary and foraminiferal evidence of the 2011 Tōhoku-oki tsunami on the Sendai coastal plain, Japan. Sedimentary Geology, 282, 78-89.


* Editor - i've boldly paraphrased from the mightly father of modern geology, Mr James Hutton, who actually said "from what has actually been, we have data for concluding with regard to that which is to happen thereafter."

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