GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 249-6
Presentation Time: 9:00 AM-6:30 PM


WAN, Jiamin1, TOKUNAGA, Tetsu1, DONG, Wenming1, WILLIAMS, Kenneth1, HOBSON, Chad1, KIM, Yongman1, CONRAD, Mark E.2, BILL, Markus2, LONG, Philip E.3 and HUBBARD, Susan S.3, (1)Energy Geosciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, (2)Earth Sciences Division, Lawrence Berkeley National Laboratory, Mailstop 70A-4418, Berkeley, CA 94720, (3)Earth Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720,

Understanding of terrestrial carbon cycling relies primarily on studies of topsoils that are typically shallower than 0.3 m and seldom deeper than 1 m. Much less is known about carbon fluxes through deeper strata where about half of the Earth’s terrestrial carbon inventory resides. This is especially true in semi-arid and arid regions, which represent about 40% of the Earth’s land surface. Unique field instrumentation was installed within a semi-arid floodplain of the Colorado River to understand carbon inventories and carbon and water fluxes. Measurements were made over a 2 year period along a transect extending laterally for 250 m and vertically for 7 m through the unsaturated zone into the underlying aquifer. Year-round vadose zone pore-water sampling revealed the existence of a high dissolved organic carbon (DOC) influx from the rhizosphere infiltrating into the deeper vadose zone. The field and associated laboratory derived seasonal- and depth-resolved CO2 fluxes and respiration rates combined with vadose zone and groundwater flow rates led to the important discoveries that 1) about 30% of the CO2 emitted from the land surface is originating from depths below 1 m and above the water table, although the current global scale land models (CLM/ALM) show practically no CO2 emission from below 1 m depth. 2) The CO2 efflux to the atmosphere accounts for ~85% of the total floodplain carbon export, with only ~15% of the total carbon as DIC (the characteristics of the semi-arid climate) and < 1% of total DOC export to the aquifer and river, despite being along the river. To our knowledge, these analyses of a novel dataset are the first to quantify the relative contributions to carbon exports from different subsurface compartments into the atmosphere and river at a semi-arid floodplain. While studies of carbon cycling in other semi-arid region floodplains are needed, a similar hierarchy for magnitudes of carbon exports is expected in other semi-arid flood plains because of efficient OC utilization in soils and the deep vadose zone, and relatively low flow rates and DOC concentrations in groundwater.