MIXING AND PARTICLE SCAVENGING RATES IN SURFACE WATERS OF THE GULF OF MEXICO USING RN-222 AND TH-234

Quantifying the magnitude and spatial variability of the flux of particulate matter from the surface to the deep ocean is important for understanding carbon and nutrient cycling and for assessing the fate and transport of metals and potential contaminants introduced into the ocean basin. As part of a larger study, Surface Water Iron Speciation Study (SWISS), we are using a combination of natural tracers to examine particle scavenging and removal and surface water mixing rates. During April and August 2001, we occupied several stations in the southern and northeastern Gulf of Mexico representative of different water masses: open Gulf, coastal waters, and an intermediate water mass. At each station, in situ measurements were made (temperature, salinity, fluorescence, PAR, and turbidity) and water samples were collected at varying depths in the water column, focusing on surface waters and the pycnocline. Water samples were analyzed for dissolved and particulate 234Th activities and 222Rn activities. These two natural tracers have been used extensively in the past to provide insight into particle residence times and surface water mixing rates. Th-234 (t1/2=24.1 days) residence times in the water column provide insight as to the efficiency and rate of removal processes. While Rn-222 (t1/2=3.85 days) residence times provide insight into the time scale of mixing processes in the water column. The deficiency of radon in near surface waters resulting from loss to the atmosphere can be used to define both the rate of vertical mixing and the rate of gas exchange between the ocean and the atmosphere. Preliminary estimates of these surface water processes, as well as spatial and temporal relationships between the two cruises will be discussed.