Ratios of particulate organic carbon (POC) to particulate 234Th activities (POC/234Th) in the ocean are used to determine the POC export flux, and thus, the power of the biological pump. In order to understand the main reasons why this ratio frequently varies as a function of depth, size, and kind (suspended vs. sinking particles), we measured vertical profiles in a cold core ring and warm core ring in the Gulf of Mexico in May 2001. Here we show that particulate 234Th/POC ratios in the Gulf of Mexico are positively correlated to the content of different Th(IV)‐binding polysaccharide fractions (uronic acids, total acid polysaccharides, total polysaccharides) in both suspended and sinking particles as well as to prymnesiophyte abundance, but negatively correlated to bacterial production. Variations in acid polysaccharide compounds, produced by both algae and bacteria, but degraded only by bacteria, can account for observed variations in POC/234Th ratios seen in the ocean.

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At AECL -Chalk River Laboratories, Ontario, Canada solid, low-level radioactive wastes from industrial, academic and medical applications have been stored in trenches above unconsolidated sandy glacial tills and permeable very-fine to fine-grained sands overlying crystalline bedrock. The sandy aquifer system drains into a swamp comprised of approximately 3 m of sphagnum peat. A comprehensive field and analytical program, involving measurements of total iodine, 129 I, tritium, 14 C and 13 C/ 12 C ratios in groundwater and geologic materials (sands and peats), was initiated at this site to examine the partitioning of 127 I and 129 I amongst the various reservoirs in this system and the controlling factors.The maximum iodine concentration and 129 I inside the groundwater contaminant plume at the recharge and discharge sites were 67.0 ng/ml and ∼ 8.3 × 10 11 atoms/liter, and 32.4 ng/ml and ∼ 2.9 × 10 11 atoms/liter, respectively, with positive correlations between iodine, 14 C (0.82), and tritium (0.87). Maximum total iodine concentrations for in-plume, rechargesite sands and discharge-site peats were 190.1 µg/kg and 14 100 µg/kg, respectively. 129 I analyses on these same samples showed concentrations of 2.3 × 10 7 and 6.4 × 10 9 atoms/g of soil, respectively.K D values (concentration on porous medium/concentration in co-existing water) calculated from the contaminant plume data for 127 I and 129 I were 1.3 and 1.6 l/kg, respectively, at the recharge site and 486 and 93 l/kg, respectively, at the discharge area, indicating that both stable and radio-iodine are preferentially sorbed to the organic rich, aquifer materials at the discharge sites. Incremental leach experiments on these same geologic materials have borne out these differences, with 127 I being more strongly sorbed than 129 I, probably as the result of kinetically controlled sorption mechanisms and the differing residence times of stable and radio-iodine in this hydrologic regime.