Abstract As part of the VERTIGO project (VERtical Transport In the Global Ocean) we used Thorium-234 ( 234 Th) 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 234 Th 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 234 Th for a wide variety of sinking and suspended materials. Results to date indicate a rather large difference in 234 Th distributions at ALOHA and K2. At ALOHA, the 234 Th: 238 U disequilibrium is small, and hence 234 Th fluxes on particles are low, on the order of dpm m -2 d -1. Variability in space and time within a 200 km 2 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 234 Th 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, and >53 micron pore sized filters. Interestingly, the µm fraction was closest in C/Th ratio to the sinking material caught in traps. At K2, the 234 Th: 238 U disequilibrium was much larger, with total activities as low as 1 dpm L -1 within the mixed layer. By sampling with up to point vertical resolution in the upper 300m, we can see a regular "excess" 234 Th feature at about m at the base of the subsurface Chlorophyll maximum, and in many cases a small disequilibrium between m. 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 m, consistent with sediment traps during our first deployment. Trap fluxes drop off during the cruise to values closer to dpm m -2 d -1, and the average 234 Th disequilibrium drops slightly, but there is significant spatial and temporal variability at K2. Final chemical yields from K2 will be needed to finalize this 234 Th data set, but early indications are that the increase in 234 Th over time will lead to a lower predicted 234 Th export using a non steady-state approach to modeling the changing 234 Th activities over the 3 week observation period.   Water Column Activity Time/Space VariationPOC/Th Ratios in Traps and Pumps Measured and Calculated Fluxes In VERTIGO, we measured total 234 Th 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 234 Th via beta counting (followed by 230 Th yield recovery corrections in the lab). Shown here are time series 234 Th 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 234 Th (t 1/2 = 24.1 days) relative to its longer lived and conservative parent, 238 U (dotted line). 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 234 Th deficit is smaller, there is a general trend towards lower 234 Th, i.e. high particle fluxes, during our VERTIGO study of particle flux and remineralization in At K2, overall activities are much lower and there is an increase in 234 Th over time during VERTIGO, consistent with the large decrease in total sediment trap fluxes for 234 Th and other elements seen during our occupation of this site (see Flux panels here and Manganini et al. poster). Also important in quantifying flux variability are not just changes with time, but spatial variations in 234 Th activity. Shown here are a series of spatial maps of 234 Th for all stations averaged at selected depths. While overall 234 Th activities at ALOHA are higher than K2, there are significant spatial differences in 234 Th, suggesting low, but variable fluxes. K2 shows more coherence in these 234 Th activity maps, but lower 234 Th overall, and a temporal trend that hidden in these spatial maps (see Flux panels). With continued improvements in 234 Th 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 234 Th 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 234 Th 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 234 Th “excess” peak, which is indicative of shallow remineralization. From the water column distribution of total 234 Th, one can calculate 234 Th 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 234 Th, 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 234 Th flux is quantified by the difference in total 234 Th and 238 U, and as this difference decreases, errors increase in our ability to quantify 234 Th export. This variability may also reflect temporal and physical impacts on the 234 Th 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. A common application of 234 Th is its use as a tracer of upper ocean carbon fluxes. In this case, the 234 Th 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. Interestingly, in both profiles, there is a sub-mixed layer deficit between and m. 234 Th 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 234 Th sampling. This opens up the possibility of new applications of 234 Th towards understanding particle sources and sinks in association with physical and biological stratified systems in the Twilight Zone. A conceptual view of the impact of various biogeochemical processes on C/ 234 Th 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/ 234 Th 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/ 234 Th, while consumption of particles by zooplankton would result in preferential assimilation losses of carbon and hence a decrease in C/ 234 Th 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/ 234 Th ratios, respectively. 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 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 234 Th as proxy for the flux of C and other elements in the ocean. Schematic of the 234 Th flux approach. Three scenarios are shown for differing conditions of 234 Th: 238 U disequilibria, 234 Th flux, sinking particle C/ 234 Th and the impact on calculated C flux. The magnitude of 234 Th flux is proportional to the 234 Th: 238 U activity ratio (here <1 in surface waters, where 234 Th solid line < 238 U dotted line). In panel a, the 234 Th flux of 1000 and a sinking particle C/ 234 Th ratio of 1/4 results in a calculated POC flux of 250. Panel b shows the impact of a doubling of the C/ 234 Th ratio for the same 234 Th flux (C flux doubles). Panel c shows how a 50% reduction in Th flux for the same C/ 234 Th 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 234 Th flux, mmol dpm-1 for C/ 234 Th, and C flux in mmol m -2 d -1. One dpm = 1/60th Bq. Calculating 234 Th flux from 234 Th activities d 234 Th/dt = ( 238 U Th) * l - P Th + V where l = decay rate P Th = 234 Th export flux V = sum of advection & diffusion low 234 Th = high flux Th>U indicates remineralization need to consider non-steady state and physical transport Carbon flux = 234 Th flux  [C/ 234 Th] sinking part. A High Resolution Study of Particle Export Using Thorium-234 in the N. Central Pacific and NW Pacific as Part of the VERTIGO Project Steve Pike *1, John Andrews 1, Tom Trull 2 and Ken Buesseler 1 1 Woods Hole Oceanographic Institution, MS #25, Falmouth, MA United States ( * corresponding author) 2 University of Tasmania, Institute of Antarctic and Southern Ocean Studies, Hobart, Tas 7001 Australia