Microbial processes have the potential to affect the mobility of radionuclides, including U in radioactive wastes. A range of geochemical, molecular biological and mineralogical techniques were ...applied to investigate stable element biogeochemistry and U solubility in the simulated “near-field” (or local environment) of a low-level radioactive waste (LLW) repository. The experiments used a microbial inoculum from the trench disposal area of the UK LLW repository at Drigg, Cumbria, England, in combination with a synthetic trench leachate representing the local environment at the Drigg site. In batch culture experiments in the absence of U, a classic redox progression of terminal electron accepting processes (TEAPs) occurred in the order
NO
3
-
, Fe(III) and
SO
4
2
-
reduction. When 126
μM U was added to the system as U(VI)
aq, up to 80% was reduced to U(IV) by the indigenous microbial consortium. The U(IV) was retained in solution in these experiments, most likely by complexation with citrate present in the experimental medium. No U(VI)
aq was reduced in sterile cultures, confirming that U(VI)
aq reduction was microbially mediated. Interestingly, when U(VI)
aq was present, the progression of TEAPs was altered. The rate of Fe(III) reduction slowed compared to experiments without U(VI)
aq, and SO
4 reduction occurred at the same time as U(VI) reduction. Finally, an experiment where
SO
4
2
-
-reducing microorgansisms were inhibited by Na molybdate showed no ingrowth of sulfide minerals, but U(VI) reduction continued in this experiment. This suggested that sulfide minerals did not play a significant role in abiotically reducing U(VI) in these systems, and that metal-reducing microorganisms were dominant in mediating U(VI) reduction. Bacteria closely related to microorganisms found in engineered and U-contaminated environments dominated in the experiments. Denaturing gradient gel electrophoresis (DGGE) on 16SrRNA products amplified from broad specificity primers showed that after incubation, differences in diversity and abundance of the microbial culture were observed between U and non-U experiments. These results indicate that the biogeochemistry of the LLW repository near-field stimulates reduction of U(VI)
(aq) to U(IV) under anaerobic conditions and that the fate of reduced U(IV) will depend on the complexants present in LLW systems.
Biomarkers in receptor species can be used to assess and monitor ecosystem health. This study aimed to develop mRNA biomarkers in the New Zealand triplefin Forsterygion capito and validate their ...induction in response to common estuarine contaminants. The targeted genes were cytochrome P450 1A (cyp1a), metallothionein (mt), vitellogenin (vtg), and cytochrome P450 aromatase (cyp19b). Fish were treated with intraperitoneal injections of benzoapyrene (BaP), zinc, and 17β-oestradiol (E
2
). Levels of cyp1a were upregulated in the livers of fish exposed to BaP, and the vtg gene was up-regulated in male fish exposed to E
2
. These results indicated that these two biomarkers in F. capito can be used to assess exposure to contaminants exerting similar mechanisms of toxicity.
The amplitude of climatic change, as recorded in the benthic oxygen isotope record, has varied throughout geological time. During the late Pleistocene, changes in the atmospheric concentration of ...carbon dioxide (CO2) are an important control on this amplitude of variability. The contribution of CO2 to climate variability during the pre‐Quaternary however is unknown. Here we present a new boron isotope‐based CO2 record for the transition into the middle Miocene Climatic Optimum (MCO) between 15.5 and 17 Myr that shows pronounced variability between 300 ppm and 500 ppm on a roughly 100 kyr time scale during the MCO. The CO2 changes reconstructed for the Miocene are ~2 times larger in absolute terms (300 to 500 ppm compared to 180 to 280 ppm) than those associated with the late Pleistocene and ~15% larger in terms of climate forcing. In contrast, however, variability in the contemporaneous benthic oxygen isotope record (at ~1‰) is approximately two thirds the amplitude of that seen during the late Pleistocene. These observations indicate a lower overall sensitivity to CO2 forcing for Miocene (Antarctic only) ice sheets than their late Pleistocene (Antarctic plus lower latitude northern hemisphere) counterparts. When our Miocene CO2 record is compared to the estimated changes in contemporaneous δ18Osw (ice volume), they point to the existence of two reservoirs of ice on Antarctica. One of these reservoirs appears stable, while a second reservoir shows a level of dynamism that contradicts the results of coupled climate‐ice sheet model experiments given the CO2 concentrations that we reconstruct.
Key Points
The middle Miocene is characterized by CO2 variability between 300 and 500 ppmThe high‐amplitude CO2 variability is matched by the changes in the paleorecordsTwo regimes of ice volume‐CO2 variability identified in middle Miocene
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