We show that the isotopic composition of oxygen (δ 18 O) in dissolved inorganic phosphate (P i ) reveals the balance between P i transport and biological turnover rates in marine ecosystems. Our δ 18 Op of P i (δ 18 Op) measurements herein indicate the importance of cell lysis in the regeneration of P i in the euphotic zone. Depth profiles of the δ 18 Op in the Atlantic and Pacific Oceans are near a temperature-dependent isotopic equilibrium with water. Small deviations from equilibrium below the thermocline suggest that P remineralization in the deep ocean is a byproduct of microbial carbon and energy requirements. However, isotope effects associated with phosphohydrolase enzymes involved in P remineralization are quite large and could potentially lead to significant disequilibration of P i oxygen. The observed near equilibration of deep water P i likely calls for continued slow rates of microbial uptake and release of P i and/or extracellular pyrophosphatase-mediated oxygen exchange between water and P i along the deep water flow path.
The osmium (Os) concentration and187Os/186Os ratio of several recent, marine, organic-rich sediment samples from three widely separated sites have been measured. Os concentrations range from 0.095 to 0.212 ppb and187Os/186Os ratios range from 8.2 to 8.9. The calculated fraction of hydrogenous Os exceeds 78% in all samples. Thus, the187Os/186Os ratio of these samples reflects Os isotopic composition of seawater. The small range in measured187Os/186Os ratio indicates that the Os isotopic composition at these sites is fairly homogeneous. The large magnitude of the Os burial flux at these sites indicates the Os burial in association with organic-rich sediments is an important sink in the marine cycle of Os. These data also suggest that ancient organic-rich sediments may provide a record of past variations in the Os isotopic composition of seawater.
Five groundwater samples taken from different Hydrogeologie settings in Connecticut were analyzed for major cation chemistry and the concentration of U and Th decay series nuclides 238 U, 234 Th, 226 Ra, 222 Rn, 210 Pb, 210 Po, 232 Th, 228 Ra, 228 Th, and 224 Ra. The concentration of 222 Rn in the waters ranged between 10 3 and 10 4 dpm l −1 and was three to four orders of magnitude greater than that of the short‐lived alpha daughters 224 Ra, 228 Ra, and 234 Th, even though the rates of supply of these four nuclides to solution are expected to be similar. We infer that sorption removes radium and thorium from these groundwaters on a time scale of 3 minutes or less. The ( 224 Ra/ 228 Ra) and ( 234 Th/ 228 Th) activity ratios in these waters indicate that desorption of these nuclides occurs on a time scale of a week or less and that equilibrium between solution and surface phases is established. In situ retardation factors for radium, thorium, and lead may therefore be calculated directly from the isotopic data; values range from 4,500 to 200,000. Neither sorption time scales nor retardation factors are strongly dependent on the nuclide or on hydrogeology of the aquifer. Since our study includes nuclides with diverse chemical properties, we suggest that other uncomplexed heavy metals and transuranic elements will also behave in a manner similar to those measured here. The approach presented here should therefore find application in developing site‐specific models of the transport of radioactive or stable elemental waste through water‐saturated media.
Geochemical data show that radioactive heat production in the crust plus upper mantle (which is defined seismically to terminate at a depth of 415 km) cannot account for the heat escaping from the Earth. Deeper sources must be invoked, and a number of qualitative models of the variation of radioactive heat generation with depth are suggested. Preferred models involve a narrow zone of high heat production about halfway between the crust and the core.
