J. Kirk Cochran
Ph.D., 1979, Yale University
Marine geo-chemistry, use of radionuclides as
geochemical tracers; diagenesis of marine sediment
(Global Research Projects: 1, 2, 3)
My research group and I are using natural radionuclides, as well as those produced by human activities such as atomic weapons testing, the nuclear fuel cycle and nuclear medicine, to study Earth surface processes. The fact that different chemical elements are represented in the suite of radioactive nuclides permits studies of chemical behavior, and the property of radioactivity provides a clock with which to measure rates. Much of my recent research has focused on using naturally occurring thorium isotopes to determine rates of particle cycling and particulate organic carbon fluxes in the open ocean. This work has been carried out in the North Atlantic, Equatorial Pacific and Southern Oceans, as well as the Mediterranean Sea The thorium isotopes provide a means of determining the export of particulate organic carbon from the upper ocean and provide information on this important aspect of the carbon cycle. My group also has used natural radionuclides to characterize carbon export and transport in the Arctic in the Northeast Water and North Water Polynyas (seasonally ice-free areas) and on the Mackenzie River shelf as part of the Canadian Arctic Shelf Exchange Study (CASES). We have also used natural and anthropogenic radionuclides to study the transport of sediments and associated contaminants by sea ice in the Arctic.
In coastal waters and estuaries, naturally occurring particle-reactive radionuclides provide tracers to determine rates of removal of contaminants from the water column and the subsequent transport and deposition of sediments and associated contaminants. Radionuclides that tend to remain in solution, such as the Ra isotopes, serve as tracers of submarine groundwater discharge to the coastal ocean. These applications are part of ongoing projects involving salt marshes and subtidal sediments in New York’s coastal lagoons (Jamaica Bay, Great South Bay).
Krishnaswami, S. and J. K. Cochran. 2008. U-Th Series Nuclides in Aquatic Systems, Elsevier, New York, 458 pp.
Cochran, J. K., J. C. Miquel, R. Armstrong, S. Fowler, P. Masqué, B. Gasser, D. Hirschberg, J. Szlosek, A. M. Rodriguez y Baena, E. Verdeny, and G. Stewart (2009) Time-series measurements of 234Th in water column and sediment trap samples from the northwestern Mediterranean Sea. Deep-Sea Research II, doi:10.1016/j.dsr2.2008.12.034.
Cochran, J. K., D. J. Hirschberg and H. Feng. 2006. Reconstructing sediment chronologies in the Hudson River Estuary. In: The Hudson River Estuary (J. S. Levinton, J. R. Waldman eds.) Cambridge University Press, pp. 65-78.
Savoye, N., C. Benitez-Nelson, A. B. Burd, J. K. Cochran, M. Charette, K. O. Buesseler, G. A. Jackson, M. Roy-Barman, S. Schmidt and M. Elskens. 2006. 234Th sorption and export models in the water column: A review. Mar. Chem. 100, 224-249.
Cochran, J.K. and P. Masqué. 2003. Short-lived U/Th series radionuclides in the ocean: Tracers for scavenging rates, export fluxes and particle dynamics. In: Uranium Series Geochemistry (B.P. Bourdon, G. Henderson, C.C. Lundstrom, S.P. Turner eds.), Reviews in Mineralogy and Geochemistry, vol. 52, Mineralogical Society of America, pp. 461-492.
Amiel, D., J.K. Cochran and D.J. Hirschberg. 2002. 234Th/238U disequilibrium as an indicator of the seasonal export flux of particulate organic carbon in the North Water. Deep-Sea Res. II, 49, 5191-5209.