Cold War Tree Rings: Do trees archive uranium?

by Kelly McHugh, Miami University, Ohio

Most folks recognize tree rings as being excellent climate records. However, tree rings are also used in archaeology, ecology, geomorphology, and geochemistry. Lead (Pb) and other metals have been demonstrated to be stored in tree rings on a species-dependent basis. My research focuses on testing whether trees store uranium (U) in their growth rings, and utilizing U isotopes to track non-natural uranium in the trees. Understanding how tree rings interact with U could have applications to nuclear forensics. So how does one test whether certain tree species archive U?

At the height of the Cold War, the Fernald Feed Materials Production Center (FFMPC) in southwest Ohio was feeding the Manhattan project. This U purification facility was known to process natural U ore and recycled U from the nuclear fuel and weapons cycle, and has a well-known contamination history (1951 – 1989), making it an optimal test site. . The first challenge with this project became finding multiple trees of different species that were old enough to capture pre-FFMPC ring compositions. We were able to survey trees in the surrounding area with an increment borer to find the oldest trees, collect 1-cm cores with a battery-powered drill for dendro-chemistry, and in an extreme case, collect a cross-sectional slab of tree for chemistry.

An ash tree is cored with an increment borer to see how old it is (left) before collaborator Henry Spitz (University of Cincinnati) takes a sample for chemistry (center). Another tree is cut down by Miami University staff member Kendall Hauer to collect a cross section (right).

An ash tree is cored with an increment borer to see how old it is (left) before collaborator Henry Spitz (University of Cincinnati) takes a sample for chemistry (center). Another tree is cut down by Miami University staff member Kendall Hauer to collect a cross section (right).

I dated the cores by cross correlation with the help of Greg Wiles at the College of Wooster Tree Ring Lab. In preparation for isotopic analysis, the cores were sectioned into 3-5 ring samples, ashed in a furnace at 550° C, and dissolved in acid.

A 1-cm diameter slippery elm tree core dated by ring counting and width measuring

A 1-cm diameter slippery elm tree core dated by ring counting and width measuring

The dissolved wood was then passed through anion exchange columns to separate U from the remaining elements. Now, the isotopes of U can be measured by a Thermal Ionization Mass Spectrometer (TIMS). In the most basic terms, a TIMS measures U isotopes by heating a sample to form a cloud of charged ions that is then pulled down a flight tube where a magnet separates the U isotopes by mass so that they can be counted separately.

The life cycle of a tree in the isotope geochemistry lab: woody material is collected (upper left), sectioned (upper center, black walnut core segment), ashed (upper right, walnut segment post-ashing), and U separated by column chemistry (lower, multiple samples are passed through columns at once).

The life cycle of a tree in the isotope geochemistry lab: woody material is collected (upper left), sectioned (upper center, black walnut core segment), ashed (upper right, walnut segment post-ashing), and U separated by column chemistry (lower, multiple samples are passed through columns at once).

To date, we have measured U concentrations and isotopic compositions in sugar maple, slippery elm, and black walnut from the area surrounding FFMPC. All of these trees contain non-natural U, but only the sugar maple may be storing the U. It is likely that U, a fluid mobile element, is being transported by tree sap across annual growth rings in the slippery elm and black walnut.

The TIMS at Miami University where U concentrations and isotopes are measured in tree rings.

The TIMS at Miami University where U concentrations and isotopes are measured in tree rings.

To find out more about this project, please visit me Monday, November 2, 2015 from 4:30 – 6:30 PM at GSA Annual Meeting Poster Booth Number 22 (or at my other poster about Hayes Volcano glass chemistry at the same time but different place, Poster Booth Number 333). E-mail me with any questions: mchughkc@miamioh.edu.

References:

FEMP (1998) Fernald Management Project. Introduction to Fernald: Fernald Environmental Management Project (Fernald, Ohio). FEMP 20900-RP-0001 rev. 0. Fernald Environmental Management Project (U.S.). Print

McHugh, KC et al. (2015). Analysis of a sugar maple tree core for monitoring environmental uranium contamination. Journal of Radioanalytical and Nuclear Chemistry. DOI: 0.1007/s10967-015-4369-x.

Speer, JH (2010). Fundamentals of Tree Ring Research. The University of Arizona Press, Tuscon. Print.

 

 

One response to “Cold War Tree Rings: Do trees archive uranium?

  1. Trees contain a lot of different materials including water and some gasses. Uranium (U) can also be stored effectively in trees without affecting the productivity and growth of the tree ring.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s