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Measurements of particle size-fractionated POC/ 234 Th ratios and 234 Th and POC fluxes were conducted using surface-tethered, free-floating, sediment traps and large-volume in-situ pumps during four cruises in 2004 and 2005 to the oligotrophic eastern Mediterranean Sea and the seasonally productive western Mediterranean and northwest Atlantic. Analysis of POC/ 234 Th ratios in sediment trap material and 10, 20, 53, 70, and 100 μm size-fractionated particles indicate, for most stations, decreasing ratios with depth, a weak dependence on particle size, and ratios that converge to ∼1–5 μmol dpm −1 below the euphotic zone (∼100–150 m) throughout the contrasting biogeochemical regimes. In the oligotrophic waters of the Aegean Sea, 234 Th and POC fluxes estimated using sediment traps were consistently higher than respective fluxes estimated from water-column 234 Th– 238 U disequilibrium, observations that are attributed to terrigenous particle scavenging of 234 Th. In the more productive western Mediterranean and northwest Atlantic, 234 Th and POC fluxes measured by sediment trap and 234 Th– 238 U disequilibrium agreed within a factor of 2–4 throughout the water column. An implication of these results is that estimates of POC export by sediment traps and 234 Th– 238 U disequilibrium can be biased differently because of differential settling speeds of POC and 234 Th-carrying particles.

Seasonal changes in Th scavenging and particle aggregation were determined along two shelf-basin transects in the western Arctic Ocean during the spring (May–June) and summer (July–August) of 2002 and 2004. Measurements of dissolved and particulate 234 Th and 228 Th activities were used to quantify Th residence times and reversible rates of Th sorption and particle aggregation. Prior to the spring bloom in 2002, 234 Th and 228 Th residence times were equal and Th scavenging was concordant, indicating predominately steady-state conditions. In contrast, scavenging of 234 Th and 228 Th in the summer of 2002 and the spring and summer of 2004 was discordant, indicating a departure in scavenging rates from steady-state conditions during periods of seasonally high biological activity and particle export. Rates of particle aggregation and disaggregation were calculated using a one-dimensional reversible exchange model and 234 Th and 228 Th activities in small (1–53 μm) and large (>53 μm) particles. Maximum rates were determined coincident with the chlorophyll maximum (25–100 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 might increase the magnitude of the particulate organic carbon (POC) flux in this Arctic regime.

Measurements of 234 Th/ 238 U disequilibria and particle size-fractionated (1, 10, 20, 53, 70, 100mm) organic C and 234 Th were made to constrain estimates of the export flux of particulate organic C (POC) from the surface waters of the Ligurian, Tyrrhenian and Aegean Seas in March–June 2004. POC exported from the surface waters (75–100 m depth) averaged 9.2 mmol m � 2 d � 1 in the Ligurian and Tyrrhenian Seas (2.370.5–14.973.0 mmol m � 2 d � 1 ) and 0.9 mmol m � 2 d � 1 in the Aegean Sea. These results are comparable to previous measurements of 234 Th-derived and sediment-trap POC fluxes from the upper 200 m in the Mediterranean Sea. Depth variations in the POC/ 234 Th ratio suggest two possible controls. First, decreasing POC/ 234 Th ratios with depth were attributed to preferential remineralization of organic C. Second, the occurrence of maxima or minima in the POC/ 234 Th ratio near the DCM suggests influence by phytoplankton dynamics. To assess the accuracy of these data, the empirical 234 Th-method was evaluated by quantifying the extent to which the 234 Thbased estimate of POC flux, PPOC, deviates from the true flux, FPOC, defined as the p-ratio (p-ratio ¼ PPOC/FPOC ¼ STh/ SPOC, where S ¼ particle sinking rate). Estimates of the p-ratio made using Stokes’ Law and the particle size distributions of organic C and 234 Th yield values ranging from 0.93–1.45. The proximity of the p-ratio to unity implies that differences in the sinking rates of POC- and 234 Th-carrying particles did not bias 234 Th-normalized POC fluxes by more than a factor of two.

An analysis of the 234 Th method for determining the export flux of particulate organic carbon ( P POC ) from the upper ocean using in situ pumps or water bottles shows that the accuracy of the method (the ratio of the experimental value of P POC divided by the true POC flux, F POC ), defined as the p -ratio, is equal to the mean settling velocity of particulate 234 Th divided by that of POC. Therefore, P POC is equivalent to the true POC flux ( F POC ) and the p -ratio is equal to unity if, and only if, POC and particulate 234 Th have the same mean sinking velocities. A simple particle settling model is discussed that invokes Stokes’ Law settling velocities, volume:surface area ( V :SA) fractionation of C: 234 Th and two assemblages of particles having different sizes and densities. The model is used to illustrate the ranges of parameter values that conform to values of the p -ratio sufficiently close to unity that the experimentally determined POC flux can be considered to be an accurate representation of the true POC flux. Despite the over-simplification of real systems implicit in the model, the results suggest that p -ratios 234 Th on particles is governed by the volume to surface area ( V :SA) ratio of the particles. This results in a greater proportion of 234 Th compared to C being associated with the smaller, more slowly settling particles and, as a result, normalization of the POC flux to the 234 Th flux provides an under-estimate of the former quantity. However, when the smaller particle assemblage dominates the vertical flux, as could occur in open ocean regimes having high aeolian inputs of dense, rapidly settling, inorganic particles, then the p -ratio could exceed unity ( p -ratio >1) resulting in an over-estimate of the POC flux using the 234 Th method. High levels of flocculation associated with phytoplankton blooms in productive regions of the ocean are likely to produce p -ratios approaching unity, because flocs would tend to preserve the V :SA partitioning of the original particle size distribution and thereby minimize differences in the mean settling speeds of POC and particulate 234 Th (Waite, A.W., Hill, P.S., 2006. Flocculation, phytoplankton and the accuracy of 234 Th-based estimates of the vertical flux of particulate organic carbon in the sea. Marine Chemistry in press). Selective sampling of the large particle fraction using, for example, 53-μm screens can produce a more accurate estimate of the true POC flux, but may not entirely shift the p -ratio to a value of unity.

Measurements of the naturally occurring radionuclides, 210Po, 210Pb, 228Ra and 226Ra were conducted on water and sediment samples collected during the Arctic Ocean Section (1994) and Beaufort Sea (1995) expeditions and are used to assess circulation and shelf/slope scavenging in the Western Arctic Ocean. 228Ra concentrations are higher over the continental shelves of the Chukchi and Beaufort Seas compared to interior waters, consistent with a shelf source for 228Ra. However, 228Ra/226Ra ratios for Pacific-origin water have a lower end member value (ca. 1.0) compared to literature values of 3.6 for end member 228Ra/226Ra ratios for Atlantic-origin water passing across the shelf of the Laptev Sea. The shift during the 1990s in the position of the axis of the Trans-Polar Drift and the consequent change in the pathways for the flow of freshwater from the Russian continental shelves is reflected in a shift of elevated 228Ra concentrations in surface water from the Lomonosov Ridge to the Mendeleyev Ridge. 210Pb water column distributions in the Eurasian Basin are similar to those measured in the North Atlantic and indicate scavenging residence times of 15–20 yr. 210Pb concentrations are lower in intermediate waters of the Makarov and Canada Basins, because of enhanced scavenging at the continental margin. The lowest 210Pb concentrations (