31 research outputs found
Evidence for oxygenic photosynthesis half a billion years before the Great Oxidation Event
The early Earth was characterized by the absence of oxygen in the oceanâatmosphere system, in contrast to the well-oxygenated conditions that prevail today. Atmospheric concentrations first rose to appreciable levels during the Great Oxidation Event, roughly 2.5â2.3 Gyr ago. The evolution of oxygenic photosynthesis is generally accepted to have been the ultimate cause of this rise, but it has proved difficult to constrain the timing of this evolutionary innovation. The oxidation of manganese in the water column requires substantial free oxygen concentrations, and thus any indication that Mn oxides were present in ancient environments would imply that oxygenic photosynthesis was ongoing. Mn oxides are not commonly preserved in ancient rocks, but there is a large fractionation of molybdenum isotopes associated with the sorption of Mo onto the Mn oxides that would be retained. Here we report Mo isotopes from rocks of the Sinqeni Formation, Pongola Supergroup, South Africa. These rocks formed no less than 2.95 Gyr ago in a nearshore setting. The Mo isotopic signature is consistent with interaction with Mn oxides. We therefore infer that oxygen produced through oxygenic photosynthesis began to accumulate in shallow marine settings at least half a billion years before the accumulation of significant levels of atmospheric oxygen
Mantle Pb paradoxes : the sulfide solution
Author Posting. © Springer, 2006. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Contributions to Mineralogy and Petrology 152 (2006): 295-308, doi:10.1007/s00410-006-0108-1.There is growing evidence that the budget of Pb in mantle peridotites is largely
contained in sulfide, and that Pb partitions strongly into sulfide relative to silicate melt. In
addition, there is evidence to suggest that diffusion rates of Pb in sulfide (solid or melt)
are very fast. Given the possibility that sulfide melt âwetsâ sub-solidus mantle silicates,
and has very low viscosity, the implications for Pb behavior during mantle melting are
profound. There is only sparse experimental data relating to Pb partitioning between
sulfide and silicate, and no data on Pb diffusion rates in sulfides. A full understanding of
Pb behavior in sulfide may hold the key to several long-standing and important Pb
paradoxes and enigmas. The classical Pb isotope paradox arises from the fact that all
known mantle reservoirs lie to the right of the Geochron, with no consensus as to the
identity of the âbalancingâ reservoir. We propose that long-term segregation of sulfide
(containing Pb) to the core may resolve this paradox. Another Pb paradox arises from the fact that the Ce/Pb ratio of both OIB and MORB
is greater than bulk earth, and constant at a value of 25. The constancy of this âcanonical
ratioâ implies similar partition coefficients for Ce and Pb during magmatic processes
(Hofmann et al. 1986), whereas most experimental studies show that Pb is more
incompatible in silicates than Ce. Retention of Pb in residual mantle sulfide during
melting has the potential to bring the bulk partitioning of Ce into equality with Pb if the
sulfide melt/silicate melt partition coefficient for Pb has a value of ~ 14. Modeling shows
that the Ce/Pb (or Nd/Pb) of such melts will still accurately reflect that of the source, thus
enforcing the paradox that OIB and MORB mantles have markedly higher Ce/Pb (and
Nd/Pb) than the bulk silicate earth. This implies large deficiencies of Pb in the mantle
sources for these basalts. Sulfide may play other important roles during magmagenesis:
1). advective/diffusive sulfide networks may form potent metasomatic agents (in both
introducing and obliterating Pb isotopic heterogeneities in the mantle); 2). silicate melt
networks may easily exchange Pb with ambient mantle sulfides (by diffusion or
assimilation), thus âsamplingâ Pb in isotopically heterogeneous mantle domains
differently from the silicate-controlled isotope tracer systems (Sr, Nd, Hf), with an
apparent âde-couplingâ of these systems.Our intemperance
should not be blamed on the support we gratefully acknowledge from NSF: EAR-
0125917 to SRH and OCE-0118198 to GAG
The working alliance in supported employment for people with severe mental health problems
Aims: This study examined the utility of the Working Alliance Inventory Short form (WAI-S) for measuring the working alliance between supported employment service providers and service users with severe mental health problems
Characterising the nickel isotopic composition of organic-rich marine sediments
New Ni stable isotope data (ÎŽ60Ni) determined by double-spike MC-ICP-MS for two geologically distinct suites of organic-rich marine sediments from the Sinemurian-Pliensbachian (S-P) Global Stratotype Section and Point (GSSP; Robin Hood's Bay, UK) and the Devonian-Mississippian Exshaw Formation (West Canada Sedimentary Basin) is presented herein. These sediments yield ÎŽ60Ni values of between 0.2â° and 2.5â°, and predominantly have Ni isotopic compositions that are heavier than those of abiotic terrestrial and extraterrestrial samples (0.15â° and 0.27â°), and in some cases present-day seawater (1.44â°) and dissolved Ni from riverine input (0.80â°). In addition, the observed degree of isotopic fractionation in the marine sediments is far greater than that of these other sample matrices. However, a strong similarity is exhibited between the ÎŽ60Ni values of the organic-rich sediments studied here and those of ferromanganese crusts (0.9-2.5â°), suggesting that factors ubiquitous to the marine environment are likely to play a key role in the heightened level of isotopic fractionation in these sample matrices.A lack of correlation between the Ni stable isotope compositions of the organic-rich sediments and Ni abundance suggests that isotopic fractionation in these sediments is not controlled by incorporation or enrichment of Ni during sediment accumulation. Further, no relationship is observed between ÎŽ60Ni and TOC concentrations or bottom-water redox conditions, indicating that the organic carbon reservoir and levels of oxygenation at the sediment-water interface do not exert a primary control on Ni isotope fractionation in marine sediments. Following examination of these relationships, it is therefore more likely that the heavy Ni isotope compositions of marine sediments are controlled by the weathering environment and the dominant sources of dissolved Ni into the global ocean reservoir.</p
The biogeochemical balance of oceanic nickel cycling
Nickel is a biologically essential element for marine life, with the potential to influence diverse processes, including methanogenesis, nitrogen uptake and coral health, in both modern and past oceans. However, an incomplete view of oceanic Ni cycling has stymied understanding of how Ni may impact marine life in these modern and ancient oceans. Here we combine data-constrained global biogeochemical circulation modelling with culture experiments and find that Ni in oligotrophic gyres is both chemically and biologically labile and only minimally incorporated into diatom frustules. We then develop a framework for understanding oceanic Ni distributions, and in particular the two dominant features of the global marine Ni distribution: the deep concentration maximum and the residual pool of approximately 2 nM Ni in subtropical gyres. We suggest that slow depletion of Ni relative to macronutrients in upwelling regions can explain the residual Ni pool, and reversible scavenging or slower regeneration of Ni compared with macronutrients contributes to the distinct Ni vertical distribution. The strength of these controls may have varied in the past ocean, impacting Ni bioavailability and setting a fine balance between Ni feast and famine for phytoplankton, with implications for both ocean chemistry and climate state