Particle dynamics and shelf -basin interactions in the western Arctic Ocean investigated using radiochemical tracers

A fundamental question in Arctic carbon cycling is the extent to which particulate organic carbon produced over the shelves is exported to the slope and interior basins. In this dissertation, the particle-reactive radionuclides 234Th (t1/2 = 24.1 d), 228Th (t1/2 = 1.9 y), and 210Pb (t1/2 = 22.3 y) were used as tracers of particulate organic carbon (POC) export fluxes, rates of particle aggregation, and sediment focusing. Samples were collected as part of the Shelf-Basin Interactions (SBI) Phase II field program in the shelf-slope region of the Chukchi Sea along a series of shelf-basin transects during the spring and summer of 2002 and 2004. POC export fluxes were estimated using measurements of 234Th- 238U disequilibria and the POC/234Th ratio on large (\u3e53-μ) particles collected using in situ pumps. POC fluxes were similar during the spring (average = 19.7 ± 24.8 mmol C m-2 d-1) and summer (average = 20.6 ± 14.0 mmol C m -2 d-1). A shelf-slope POC budget revealed that during periods of high productivity, POC export fluxes exceeded respiration and burial in underlying sediments. However, a water column-sediment budget of 234Th indicated that this excess POC was retained in shelf sediments. Rates of particle aggregation and disaggregation were calculated using a 1-dimensional reversible exchange model and 234Th and 228Th activities in small (1--53μm) and large (\u3e53-μm) particles. Maximum rates were determined coincident with the chlorophyll maximum (25--75 m) and increased by an order of magnitude between periods of low and high productivity. These Th measurements provide evidence that seasonally enhanced rates of particle aggregation may increase the magnitude of the POC flux in this Arctic regime. Distributions of 210Pb and suspended particulate matter indicate efficient removal of 210Pb over the shelf by particle scavenging. Water mass transport rates from the shelf to the interior basin estimated from 210Pb-226Ra disequilibria in the slope and basin halocline ranged from 0.1 cm s-1 to 5 cm s-1. Additionally, a water column-sediment budget of 210Pb indicates that particles and associated organic matter produced in the Chukchi Sea are retained in shelf sediments over a decadal timescale