John Meriwether's Research Page
 
 

Research in my laboratory focuses on the measurement of natural and induced radioactivity to determine properties of natural systems. There is a close working relationship between my students and me and we spend a lot of time in the field.  With me in the field are, from left to right, Hsing-chong Chang, Ming Xie, James Sheu, and Xin Wang.

As a result of the atmospheric testing on nuclear weapons in the 1950's and early 1960's, the earth was blanketed with radioactive fallout. There was a strong maximum in the amount of fallout (from a lot of testing) in 1963, just before the signing of the Limited Test Ban Treaty by the U.S., the U.K., and the U.S.S.R. One of the long-lived components of this fallout is 137Cs (t1\2 33 years) is bound quite strongly on clays and other natural ion-exchangers. It is thus a rather stable geologic horizon which one can often locate. In undisturbed forest soils, all of the 137Cs is in the upper few centimeters of the surface. If the horizon is diffuse or found deeper than the surface, it is a rather good sign that the soils have been disturbed in the last 35 years.

In Lakes and marshes subsequent sedimentation buries the 137Cs horizon. If the bio-and mechanical turbation is small, the horizon is stable and its depth is a measure of the sedimentation since 1963.

Below on the left James Sheu lifts a core from the sediments in Fearman Lake, Louisiana. As shown in the graph on the right, based on data taken in 1990, eleven centimeters of sediment had been added above the 137Cs horizon in White Lake, Louisiana.

Dr. Jim Beck from Nicholls State U. and I have been conducting research together for several years. Harish Dhurvasula worked with Cary Hardaway, one of Jim's students on a joint project. Shown below are Jim, Harish, and Cary on a trip to collect sediment core samples in North Lake on Redfish Point off Vermilion Bay.


 

North Lake is a shollow, quiet, brackish lake that has deep, soft, little-disturbed sediments. The students have correlated the concentrations of various elements and other chemical parameters, such as pH and Eh with depth and date of deposition.
 
 
 

Much of our recent field work has taken place in the Gulf of Mexico. The sea grass beds in the lagoon to the landward of the Chandeleur Islands (off the southeast tip of Louisiana) have been studied by Tommy Michot and Hilary Neckels of National Wetland Research Center. We joined with them to study the rate of sediment deposition in the lagoon. Harish Dhurvasula, using both 137Cs and 210Pb dating techiniques, wrote his masters t hesis on this problem. Shown below is an aerial view of the island which I took on a flight with Tommy Michot in a fish and Wildlife Service plane. The lagoon is to the right in the photograph.

Radon-222 is a member of the naturally occurring 238U decay series (shown below), a direct decay product of 228Ra. Gaseous radon effuses from soils and breaks the equilibrium in the decay series. It soon decays in the atmosphere (3.85 day half-life) as does its short lived grand daughters to produce 210Pb which has a 22.3-year half life. The 210Pb "falls out", usually attached dust particles. That which reaches water bodies and is incorporated into the sediments is in excess of 210Pb in equilibrium with 226Ra in situ. A determination of the "excess" or "non-equilibrium" 210Pb, decay with its own half-life, as a function depth allows one to establish a chronology of deposition. If the sediments are relatively undisturbed, one can, from the analysis of a core, determine sedimentation rates and correlate other paramenters with date of deposition.

C(x) = C(0)e-l t, where C(0) and C(x) are the non-equilibrium 210Pb concentrations at the surface and at depth x, and l is

the decay constant. Often, through bioturbation or resuspension processesm there has been some disturbance, and one must invoke various models to interpret the non-equilibrium 210Pb depth profile. One common model assumes that there is a constant rate of supply (CRS model) of 210Pb from the atmosphere. The residual amount of non-equilibrium 210Pb below the depth x is defined : A(x) = S C(x)rDx where r is the density and Dx is the thickness of a core segment. The age 't' before the present is determined from the expression A(x) = A(0)e-l t.

Sediment from cores extracted from sediment samples taken from the Mississippi Canyon in the Gulf of Mexico have been analyzed to determine 137Cs, 210Pb, and the concentration of various chemical elements. The sediment samples were lifted from the Gulf bottom using a 1 m2 box corner (below left) dropped and recovered from the research vessel R.V.Pelican. The Pelican (below right) is the largest in the fleet of vessels operated by the Louisiana Universities Marine Consortium (LUMCON). The LUMCON research laboratory and port is at Cocodrie, LA.


 
 
 

Greg Booth, a recent LSU Ph.D., whose work was done at LUMCON and in our spectroscopy laboratory, is shown on the right, collecting samples from Barataria Bay from which he determined the amount of 7Be (53.3 day half-life), a cosmogenically produced radionuclide, in the sediments. Greg's dissertation focused on the redistribution and movement of sediments in the Bay.