Atomistic simulation of the mechanisms of noble gas incorporation in minerals

Atomistic simulations have been carried out to investigate the mechanisms of noble gas incorporation in minerals using both the traditional two-region approach and the “supercell” method. The traditional two-region approach has been used to calculate defect energies for Ne, Ar, Kr and Xe incorporation in MgO, CaO, diopside and forsterite in the static limit and at one atmosphere pressure. The possibilities of noble gas incorporation via both substitution and interstitial mechanisms are studied. The favored mechanism varies from mineral to mineral and from noble gas to noble gas. In all minerals studied, the variation of the solution energies of noble gas substitution with atomic radius appears approximately parabolic, analogous to those for 1⁺, 2⁺, 3⁺ and 4⁺ trace element incorporation on crystal lattice sites. Noble gas solution energies thus also fall on a curve, similar to those previously observed for cations with different charges, but with much lower curvature.The “supercell” method has been used to investigate the pressure dependence of noble gas incorporation in the same systems. Results indicate a large variation of the solubility of the larger noble gases, Kr and Xe with pressure. In addition, explicit simulation of incorporation at the (0 0 1) surface of MgO shows that the solubility of the heavier noble gases may be considerably enhanced by the presence of interfaces.     

Characterization and cycling of atmospheric mercury along the central US Gulf Coast

Concentrations of atmospheric Hg species, elemental Hg (Hg^°), reactive gaseous Hg (RGM), and fine particulate Hg (Hg-PM_2.5) were measured at a coastal site near Weeks Bay, Alabama from April to August, 2005 and January to May, 2006. Mean concentrations of the species were 1.6 ± 0.3 ng m⁻³, 4.0 ± 7.5 pg m⁻³ and 2.7 ± 3.4 pg m⁻³, respectively. A strong diel pattern was observed for RGM (midday maximum concentrations were up to 92.7 pg m⁻³), but not for Hg^° or Hg-PM_2.5. Elevated RGM concentrations (>25 pg m⁻³) in April and May of 2005 correlated with elevated average daytime O₃ concentrations (>55 ppbv) and high light intensity (>500 W m⁻²). These conditions generally corresponded with mixed continental-Gulf and exclusively continental air mass trajectories. Generally lower, but still elevated, RGM peaks observed in August, 2005 and January–March, 2006 correlated significantly (p < 0.05) with peaks in SO₂ concentration and corresponded to periods of high light intensity and lower average daytime O₃ concentrations. During these times air masses were dominated by trajectories that originated over the continent. Elevated RGM concentrations likely resulted from photochemical oxidation of Hg^° by atmospheric oxidants. This process may have been enhanced in and by the near-shore environment relative to inland sites. The marine boundary layer itself was not found to be a significant source of RGM.     

Instrument calibration of ocean bottom seismographs

To increase the accuracy of measuring sea floor motion with ocean bottom seismometers, we calibrate the seismometer system on the ocean floor. Data from the sea floor calibration, augmented with electronic and land calibration data, enables us to find the OBS transfer function to an accuracy of 0.5% in the frequency range of 0.1 to 32 Hz. We are able to distinguish between temperature, instrument and OBS ground coupling effects, all of which alter the transfer function. This paper reviews our method of calibration and discusses the effects of temperature and some of the instrument design features on the vertical seismometer transfer function.     

Trace metals in the Western Pacific: temporal and spatial variability in the concentrations of Cd, Cu, Mn and Ni

Profiles of total dissolvable Cd, Cu, Mn and Ni are reported for samples collected from the southwest Pacific in 1989, from the western equatorial Pacific along 155°E at 5°S, 0° and 5°N in 1990 and 1993, and along the equator from 143°E to 152°E and in the Bismarck Sea in 1997 and 2000. Profiles of Cd along 155°E in 1990 and along the equator were essentially the same but, in 1993, Cd values at 5°N were higher by a factor of about 1.5–2 than at 5°S over the depth range 500–1500 m. Similar, but less pronounced, differences were observed for PO₄ and Ni. Cd and Ni were both strongly correlated with PO₄, and an even stronger correlation was found between Ni and Cd. The concentration of Ni did not fall below ≈2 nmolkg⁻¹, even in the nitrate-depleted waters of the western equatorial Pacific, where primary production is strongly dependent on recycled nitrogen (mainly ammonia and urea). It is proposed that this residual Ni is not bioavailable and that Ni could be biolimiting, since the metabolism of urea requires the nickel-containing enzyme urease. The impact of the Sepik River on Cd, Cu and Ni concentrations was small but elevated concentrations of Mn were observed near the Sepik River and close to the coast suggesting that the rivers and sediments on the north coast of New Guinea are a significant local source of Mn to the Bismarck Sea. Simple mass balance calculations show that the elevated levels of Mn observed in the Equatorial Undercurrent cannot be due to input from the rivers of New Guinea and they were attributed to the trapping of particulate matter due to strong current shear. A strong hydrothermal source of Mn was observed in the central Bismarck Sea.     

Phytoplankton and iron: validation of a global three-dimensional ocean biogeochemical model

The JGOFS program and NASA ocean-color satellites have provided a wealth of data that can be used to test and validate models of ocean biogeochemistry. A coupled three-dimensional general circulation, biogeochemical, and radiative model of the global oceans was validated using these in situ data sources and satellite data sets. Biogeochemical processes in the model were determined from the influences of circulation and turbulence dynamics, irradiance availability, and the interactions among four phytoplankton functional groups (diatoms, chlorophytes, cyanobacteria, and coccolithophores) and four nutrients (nitrate, ammonium, silica, and dissolved iron).Annual mean log-transformed dissolved iron concentrations in the model were statistically positively correlated on basin scale with observations (P<0.05) over the eight (out of 12) major oceanographic basins where data were available. The model tended to overestimate in situ observations, except in the Antarctic where a large underestimate occurred. Inadequate scavenging and excessive remineralization and/or regeneration were possible reasons for the overestimation.Basin scale model chlorophyll seasonal distributions were positively correlated with SeaWiFS chlorophyll in each of the 12 oceanographic basins (P<0.05). The global mean difference was 3.9% (model higher than SeaWiFS).The four phytoplankton groups were initialized as homogeneous and equal distributions throughout the model domain. After 26 years of simulation, they arrived at reasonable distributions throughout the global oceans: diatoms predominated high latitudes, coastal, and equatorial upwelling areas, cyanobacteria predominated the mid-ocean gyres, and chlorophytes and coccolithophores represented transitional assemblages. Seasonal patterns exhibited a range of relative responses: from a seasonal succession in the North Atlantic with coccolithophores replacing diatoms as the dominant group in mid-summer, to successional patterns with cyanobacteria replacing diatoms in mid-summer in the central North Pacific. Diatoms were associated with regions where nutrient availability was high. Cyanobacteria predominated in quiescent regions with low nutrients.While the overall patterns of phytoplankton functional group distributions exhibited broad qualitative agreement with in situ data, quantitative comparisons were mixed. Three of the four phytoplankton groups exhibited statistically significant correspondence across basins. Diatoms did not. Some basins exhibited excellent correspondence, while most showed moderate agreement, with two functional groups in agreement with data and the other two in disagreement. The results are encouraging for a first attempt at simulating functional groups in a global coupled three-dimensional model but many issues remain.