Abstract

1. Paper Number OS26A-07 A High Resolution Study of Particle Export Using Thorium-234 in the N. Central Pacific and NW Pacific as Part of the VERTIGO ProjectSteve Pike*1, John Andrews1, Tom Trull2 and Ken Buesseler11Woods Hole Oceanographic Institution, MS #25, Falmouth, MA 02543 United States (* corresponding author)2University of Tasmania, Institute of Antarctic and Southern Ocean Studies, Hobart, Tas 7001 Australia Abstract As part of the VERTIGO project (VERtical Transport In the Global Ocean) we used Thorium-234 (234Th) as a natural proxy for particle export. As developed in recent years, application of a small volume sampling method allowed for relatively high resolution total 234Th sampling in both space and time, during 3 week occupations of two contrasting flux sites off Hawaii (ALOHA) and in the NW Pacific (K2). The higher vertical resolution allows us to constrain not only particle export out of the euphotic zone, but export and remineralization processes below. We have made extensive use in the VERTIGO project of different sediment traps as well as large volume pumping systems for size fractionated filtration, and thus can compare for export calculations of C, N, bSi and PIC, the ratios of these elements to particulate 234Th for a wide variety of sinking and suspended materials. Results to date indicate a rather large difference in 234Th distributions at ALOHA and K2. At ALOHA, the 234Th:238U disequilibrium is small, and hence 234Th fluxes on particles are low, on the order of 300-500 dpm m-2 d-1. Variability in space and time within a 200 km2 area is also small and hence a 1-D steady-state model is a good approximation of the flux conditions. The predicted fluxes are consistent within errors with simultaneously measured 234Th trap fluxes. As developed in prior studies, we can apply the ratio of X/Th on particles to convert from Th to other elemental fluxes, such as C, N, bSi and PIC. At ALOHA C/Th on size fractionated particles decreased with depth, and increased with size for 1-10, 10-53 and >53 micron pore sized filters. Interestingly, the 10-53 µm fraction was closest in C/Th ratio to the sinking material caught in traps. At K2, the 234Th:238U disequilibrium was much larger, with total activities as low as 1 dpm L-1 within the mixed layer. By sampling with up to 20-24 point vertical resolution in the upper 300m, we can see a regular "excess" 234Th feature at about 100-120m at the base of the subsurface Chlorophyll maximum, and in many cases a small disequilibrium between 100-250m. This deeper feature may be an indication of repackaging of suspended material into sinking particles. Fluxes calculated from a 1-D SS model thus increase from about 1700 to 2200 dpm m-2 d-1 between 100-300m, consistent with sediment traps during our first deployment. Trap fluxes drop off during the cruise to values closer to 500-700 dpm m-2 d-1, and the average 234Th disequilibrium drops slightly, but there is significant spatial and temporal variability at K2. Final chemical yields from K2 will be needed to finalize this 234Th data set, but early indications are that the increase in 234Th over time will lead to a lower predicted 234Th export using a non steady-state approach to modeling the changing 234Th activities over the 3 week observation period. Water Column Activity Time/Space Variation POC/Th Ratios in Traps and Pumps In VERTIGO, we measured total 234Th activities on more than 19 profiles at ALOHA and 25 profiles at K2. This high resolution data set was made possible by development of a new 4L method with rapid processing and ship board analyses of 234Th via beta counting (followed by 230Th yield recovery corrections in the lab). Shown here are time series 234Th profiles for a smaller subset of central stations, which represent particle source conditions during the experiments. Thorium-234 activities are significantly lower at K2, due to higher particle flux conditions which remove the particle reactive natural radionuclide 234Th (t1/2 = 24.1 days) relative to its longer lived and conservative parent, 238U (dotted line). A common application of 234Th is its use as a tracer of upper ocean carbon fluxes. In this case, the 234Th derived from the water column distributions (see Flux panels), is simply scaled to the C/Th ratio of sinking particles to estimate particulate carbon export. Shown here is the C/Th ratio on sinking particles collected by two types of sediment traps employed during VERTIGO (see poster by Andrews et al.), and for ALOHA, a comparison to size fractionated particles collected using an in situ large volume pumping system. Measured and Calculated Fluxes From the water column distribution of total 234Th, one can calculate 234Th export fluxes on sinking particles. We have used the most simple 1-D steady state model to illustrate the range of fluxes predicted vs. depth for 234Th, and compare these fluxes to the average trap activities. At both sites, the range in predicted fluxes is significant, reflecting temporal and spatial variability in particle export not caught in the traps. Remember too that the 234Th flux is quantified by the difference in total 234Th and 238U, and as this difference decreases, errors increase in our ability to quantify 234Th export. This variability may also reflect temporal and physical impacts on the 234Th activity budget that are not quantified in this simple 1-D SS flux model. Overall, the fluxes measured by our traps are consistent with the range of fluxes measured at each site, and the overall decease in flux between deployments early and late at K2. This decrease with depth is evident during both low and high flux conditions, and the absolute value of this ratio is similar at two sites with widely differing particle characteristics. When compared to the filtered particles, an interesting pattern at ALOHA (K2 data not yet processed) is that the 1-10 and 10-53 µm size classes are more similar to the sinking material caught in the traps than the >53 µm fraction. This may be due to the few and rare zooplankton caught on the larger screens, that are known to carry significantly higher C/Th than detrital aggregates, pellets and other sinking debris. Such comparisons vs. size and sinking are critical to a more accurate use of 234Th as proxy for the flux of C and other elements in the ocean. Thorium-234 data are shown as a color contour time series plot for the central VERTIGO stations and compared to evolving temperature, salinity and fluorescence fields for ALOHA and K2. At ALOHA, where the 234Th deficit is smaller, there is a general trend towards lower 234Th, i.e. high particle fluxes, during our VERTIGO study of particle flux and remineralization in 2004. At K2, overall activities are much lower and there is an increase in 234Th over time during VERTIGO, consistent with the large decrease in total sediment trap fluxes for 234Th and other elements seen during our occupation of this site (see Flux panels here and Manganini et al. poster). Carbon flux = 234Th flux  [C/234Th]sinking part. Schematic of the 234Th flux approach. Three scenarios are shown for differing conditions of 234Th:238U disequilibria, 234Th flux, sinking particle C/234Th and the impact on calculated C flux. The magnitude of 234Th flux is proportional to the 234Th:238U activity ratio (here <1 in surface waters, where 234Th solid line <238U dotted line). In panel a, the 234Th flux of 1000 and a sinking particle C/234Th ratio of 1/4 results in a calculated POC flux of 250. Panel b shows the impact of a doubling of the C/234Th ratio for the same 234Th flux (C flux doubles). Panel c shows how a 50% reduction in Th flux for the same C/234Th ratio as in b results in a decrease in C flux by 50%. Units are not needed in these examples, but are commonly dpm m-2 d-1 for 234Th flux, mmol dpm-1 for C/234Th, and C flux in mmol m-2 d-1. One dpm = 1/60th Bq. Also important in quantifying flux variability are not just changes with time, but spatial variations in 234Th activity. Shown here are a series of spatial maps of 234Th for all stations averaged at selected depths. While overall 234Th activities at ALOHA are higher than K2, there are significant spatial differences in 234Th, suggesting low, but variable fluxes. K2 shows more coherence in these 234Th activity maps, but lower 234Th overall, and a temporal trend that hidden in these spatial maps (see Flux panels). With continued improvements in 234Th methods, it is now possible to obtain very highly resolved vertical profiles of this particle flux tracer. Two examples are shown here with 20 point vertical resolution for total 234Th from K2, with comparisons to CTD based sensors which indicate layering of large particles (scatter), biomass (flu), small particles (transmission), and significant stratification of density and low oxygen at relatively shallow depths. Most of the 234Th removal takes place within the mixed layer down through the base of the fluorescence maximum. Immediately below the mixed layer and chlorophyll maximum, there is a 234Th “excess” peak, which is indicative of shallow remineralization. • Calculating 234Th flux from 234Th activities • d234Th/dt = (238U - 234Th) * l - PTh + V • where l = decay rate • PTh = 234Th export flux • V = sum of advection & diffusion • low 234Th = high flux • Th>U indicates remineralization • need to consider non-steady state and physical transport A conceptual view of the impact of various biogeochemical processes on C/234Th ratios and particle sizes. As thorium associates principally with surface sorption sites and organic carbon is dominated by pools internal to cells, one might expect C/234Th ratios to increase as particle size increases, with the volume to surface area (V:SA) ratios of spheres representing the upper limit for the relationship (all other cell/particle shapes have lower V:SA trends with size). Particle sizes in real marine systems tend to increase as a result of complex biological processes, however, including aggregation of small, neutrally buoyant cells into larger sinking particles and the generation of fecal material. Rapid aggregation of small particles alone without loss of mass would probably yield no change in V:SA ratios and hence no change in C/234Th, while consumption of particles by zooplankton would result in preferential assimilation losses of carbon and hence a decrease in C/234Th ratios in larger fecal pellets. Processes that affect the Th side of the ratio (Th speciation), are not likely to be linked to particle size in a general way. These include increases in dissolved and particulate Th-binding ligands or sorption sites, which would increase or decrease C/234Th ratios, respectively. Interestingly, in both profiles, there is a sub-mixed layer deficit between 100-150 and 250-300m. 234Th trap fluxes increase between 150, 300 and 500m, consistent with a process at depth that enhances particle export (perhaps zooplankton driven- a topic of ongoing investigation in VERTIGO). Both features would be missed with traditional 234Th sampling. This opens up the possibility of new applications of 234Th towards understanding particle sources and sinks in association with physical and biological stratified systems in the Twilight Zone.