GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 124-5
Presentation Time: 9:00 AM-6:30 PM


DIX, Monica1, HECHT, Zoe1, BERMUDEZ, Emily A.1, SCHMIDT, Amanda H.1, BIERMAN, Paul2, CAMPBELL, Mae Kate3, DETHIER, David P.4, RACELA, Jason5, PERDRIAL, Julia6, MASSEY-BIERMAN, Marika Eden5, SIBELLO HERNÁNDEZ, Rita Y.7, CARTAS AGUILA, Hector A.7, GUILLÉN ARRUEBARRENA, Aniel7, GARCÍA MOYA, Alejandro7 and ALONSO-HERNÁNDEZ, Carlos7, (1)Geology, Oberlin College, 52 West Lorain Street, Oberlin, OH 44074-1044, (2)Geology Department and Rubenstein School of the Environment and Natural Resources, University of Vermont, Burlington, VT 05405, (3)Department of Geology, The University of Vermont, 180 Colchester Ave., Burlington, VT 05405, (4)Department of Geosciences, Williams College, Williamstown, MA 01267, (5)Williams College, Center for Environmental Studies/Geosciences, 59 Lab Campus Drive, Thompson Bio, Williamstown, MA 01267, (6)Geology, University of Vermont, Delehanty Hall, 180 Colchester Avenue, Burlington, VT 05405, (7)Centro de Estudios Ambientales de Cienfuegos, Apartado Postal 5, Cienfuegos, 59350, Cuba

Cuba has a long and varied history of land use, undergoing a nationwide agricultural transition in the 1990s from mechanized sugar monoculture to more sustainable agriculture using limited resources available after the fall of the Soviet Union. This step function transition in land use is an ideal natural experiment to understand the long term effects of organic agriculture on soil conservation.

We measured the activity of short-lived fallout radionuclides (7Be, 137Cs, and 210Pbex) in river sediment collected in August 2018 from 26 sites in central Cuba. Most of the 26 detrital sediment samples analyzed in two grain sizes (n = 20/26 for <63 µm, n = 19/26 for 250-850 µm) have detectable activity of at least one isotope. 20 sites have detectable 210Pbex and 21 have detectable 137Cs, but only two sites have detectable 7Be, in coarse and/or fine grained fractions. This suggests that sediment is sourced from near the ground surface, but perhaps below the penetration depth of 7Be (5 cm). The absence of 137Cs in 5 watersheds samples suggests that sites may have experienced deeper and/or more rapid erosion in the past. Four of these watersheds have detectable 210Pbex, suggesting that erosion has slowed since the time of 137Cs deposition. Overall our data suggest that erosion in the region is slow and shallow at the present time in many locations.

210Pbex activity is directly correlated with elevation and agricultural land use and inversely correlated with basin slope but 137Cs activity is not with any basin metrics. Agricultural land use and slope are significantly and inversely related, thus we cannot determine whether slope or land use is driving observed relationships. Watersheds with 210Pbex but not 137Cs in the coarse-grained fraction have higher elevations, higher slopes, and lower agricultural land use than samples with detectable 137Cs. However, because agricultural land use is strongly associated with low elevations and slopes, it is hard to untangle relative importance of these terms. Fine-grained samples do not have significant differences between categories with different detectable isotopes. Overall our data suggest that land use change may have affected isotopic concentrations in detrital sediments but that we cannot conclusively distinguish between human and natural controls on erosion.