Stable sulfur isotopes in the water column of the Cariaco Basin

Previous geochemical and microbiological studies in the Cariaco Basin indicate intense elemental cycling and a dynamic microbial loop near the oxic-anoxic interface. We obtained detailed distributions of sulfur isotopes of total dissolved sulfide and sulfate as part of the on-going CARIACO time series project to explore the critical pathways at the level of individual sulfur species. Isotopic patterns of sulfate (δ³⁴S_SO4) and sulfide (δ³⁴S_H2S) were similar to trends observed in the Black Sea water column: δ³⁴S_H2S and δ³⁴S_SO4 were constant in the deep anoxic water (varying within 0.6‰ for sulfide and 0.3‰ for sulfate), with sulfide roughly 54‰ depleted in ³⁴S relative to sulfate. Near the oxic-anoxic interface, however, the δ³⁴S_H2S value was ∼3‰ heavier than that in the deep water, which may reflect sulfide oxidation and/or a change in fractionation during in situ sulfide production through sulfate reduction (SR). δ³⁴S_H2S and Δ³³S_H2S data near the oxic-anoxic interface did not provide unequivocal evidence to support the important role of sulfur-intermediate disproportionation suggested by previous studies. Repeated observation of minimum δ³⁴S_SO4 values near the interface suggests ‘readdition’ of ³⁴S-depleted sulfate during sulfide oxidation. A slight increase in δ³⁴S_SO4 values with depth extended over the water column may indicate a reservoir effect associated with removal of ³⁴S-depleted sulfur during sulfide production through SR. Our δ³⁴S_H2S and Δ³³S_H2S data also do not show a clear role for sulfur-intermediate disproportionation in the deep anoxic water column. We interpret the large difference in δ³⁴S between sulfate and sulfide as reflecting fractionations during SR in the Cariaco deep waters that are larger than those generally observed in culturing studies.     

Geochemical and mineralogical evidence for Sahara and Sahel dust additions to Quaternary soils on Lanzarote,eastern Canary Islands,Spain

Africa is the most important source of dust in the world today, and dust storms are frequent on the nearby Canary Islands. Previous workers have inferred that the Sahara is the most important source of dust to Canary Islands soils, with little contribution from the Sahel region. Soils overlying a late Quaternary basalt flow on Lanzarote, Canary Islands, contain, in addition to volcanic minerals, quartz and mica, exotic to the island’s bedrock. Kaolinite in the soils also likely has an exotic origin. Trace‐element geochemistry shows that the soils are derived from varying proportions of locally derived basalt and African dust. Major‐element geochemistry, clay mineralogy and interpretation of satellite imagery suggest that dust additions to the Canary Islands come not only from the Sahara Desert, but also from the Sahel region.     

The Role of One- and Two-Electron Transfer Reactions in Forming Thermodynamically Unstable Intermediates as Barriers in Multi-Electron Redox Reactions

In the aquatic geochemical literature, a redox half-reaction is normally written for a multi-electron process (n > 2); e.g., sulfide oxidation to sulfate. When coupling two multi-electron half-reactions, thermodynamic calculations indicate possible reactivity, and the coupled half-reactions are considered favorable even when there is a known barrier to reactivity. Thermodynamic calculations should be done for one or two-electron transfer steps and then compared with known reactivity to determine the rate controlling step in a reaction pathway. Here, thermodynamic calculations are presented for selected reactions for compounds of C, O, N, S, Fe, Mn and Cu. Calculations predict reactivity barriers and agree with one previous analysis showing the first step in reducing O₂ to O₂ ^? with Fe²⁺ and Mn²⁺ is rate limiting. Similar problems occur for the first electron transfer step in these metals reducing NO₃ ^?, but if reactive oxygen species form or if two-electron transfer steps with O atom transfer occur, reactivity becomes favorable. H₂S and NH₄ ⁺ oxidation in a one-electron transfer step by O₂ is also not favorable unless activation of oxygen can occur. H₂S oxidation by Cu²⁺, Fe(III) and Mn(III, IV) phases in two-electron transfer steps is favorable but not in one-electron steps indicating that (nano)particles with bands of orbitals are needed to accept two electrons from H₂S. NH₄ ⁺ oxidation by Fe(III) and Mn(III, IV) phases is generally not favorable for both one- and two-electron transfer steps, but their reaction with hydroxylamine and hydrazine to form N₂O and N₂, respectively, is favorable. The anammox reaction using hydroxylamine via nitrite reduction is the most favorable for NH₄ ⁺ oxidation. Other chemical processes including photosynthesis and chemosynthesis are considered for these element–element transformations.     

Analysis and Simulation of Propagule Dispersal and Salinity Intrusion from Storm Surge on the Movement of a Marsh–Mangrove Ecotone in South Florida

Coastal mangrove–freshwater marsh ecotones of the Everglades represent transitions between marine salt-tolerant halophytic and freshwater salt-intolerant glycophytic communities. It is hypothesized here that a self-reinforcing feedback, termed a “vegetation switch,” between vegetation and soil salinity, helps maintain the sharp mangrove–marsh ecotone. A general theoretical implication of the switch mechanism is that the ecotone will be stable to small disturbances but vulnerable to rapid regime shifts from large disturbances, such as storm surges, which could cause large spatial displacements of the ecotone. We develop a simulation model to describe the vegetation switch mechanism. The model couples vegetation dynamics and hydrologic processes. The key factors in the model are the amount of salt-water intrusion into the freshwater wetland and the passive transport of mangrove (e.g., Rhizophora mangle) viviparous seeds or propagules. Results from the model simulations indicate that a regime shift from freshwater marsh to mangroves is sensitive to the duration of soil salinization through storm surge overwash and to the density of mangrove propagules or seedlings transported into the marsh. We parameterized our model with empirical hydrologic data collected from the period 2000–2010 at one mangrove–marsh ecotone location in southwestern Florida to forecast possible long-term effects of Hurricane Wilma (24 October 2005). The model indicated that the effects of that storm surge were too weak to trigger a regime shift at the sites we studied, 50 km south of the Hurricane Wilma eyewall, but simulations with more severe artificial disturbances were capable of causing substantial regime shifts.     

An analysis and comparison of observed Pleistocene South Carolina (USA) shoreline elevations with predicted elevations derived from Marine Oxygen Isotope Stages

Geological maps of South Carolina, covering > 6800 km², confirm the existence of eight preserved Pleistocene shorelines above current sea level: Marietta (+ 42.6 m), Wicomico (+ 27.4 m), Penholoway (+ 21.3 m), Ladson (+ 17.4 m), Ten Mile Hill (+ 10.7 m), Pamlico (+ 6.7 m), Princess Anne (+ 5.2 m), and Silver Bluff (+ 3 m). Current geochronologic data suggest that these eight shorelines correlate with Marine Oxygen Isotope Stages (MIS) as follows: Marietta—older than MIS 77; Wicomico—MIS 55–45; Penholoway—MIS 19 or 17; Ladson—MIS 11; Ten Mile Hill—MIS 7; Pamlico—MIS 5; Princess Anne—MIS 5; and Silver Bluff—MIS 5 or 3. Except for the MIS 5e Pamlico, and possibly the MIS 11 Ladson, the South Carolina elevations are higher than predicted by isotope proxy-based reconstructions. The < 4 m of total relief from the Pamlico to the Silver Bluff shoreline in South Carolina, for which other reconstructions suggest an expected relief of ~ 80 m, illustrates the lack of match. Our results suggest that processes affecting either post-depositional changes in shoreline elevations or the creation of proxy sea-level estimates must be considered before using paleo sea-level position on continental margins.     

Hubble Space Telescope imaging polarimetry of Mars during the 2003 opposition

We report results of polarimetric imaging observations of Mars with the Hubble Space Telescope during the 2003 opposition. Through careful calibration, the observations with the ACS camera allow measurements of the polarization degree with an absolute accuracy better than 0.5% and detection of features with polarization degree contrast as small as 0.2%. The general distribution of linear polarization parameters over the Mars disk and their dependence on phase angle and wavelength are well explained qualitatively by a combination of scattering separately by the martian surface and atmosphere. We have discovered transient polarization phenomena interpreted as clouds that are best observed in ultraviolet light. These clouds are optically thin but strongly polarizing, and their origin may be related to atmospheric ice condensation processes.     

The Fossil Starburst in M82

We present high-resolution optical and near-infrared HST observations of two adjacent regions in the fossil starburst region inM82, M82 B1 and B2. The presence of both the active and the fossil starburst in M82 provides a unique physical environment to study the stellar and dynamical evolution of star cluster systems. The cluster population in B2 is more heavily affected by internal extinction than that in B1, amounting to an excess extinction in B2 of A_V,excess≃1.1±0.3 mag. Preliminary age estimates date the cluster population in the fossil starburst between ∼2× 10⁸ and ∼ 10⁹ years. The radial luminosity profiles of the brightest clusters are more closely approximated by power laws than by a Gaussian model, in particular in their wings, which favours a slow star formation scenario. This revised version was published online in July 2006 with corrections to the Cover Date.     

Improved reliability of ENSO hindcasts with multi-ocean analyses ensemble initialization

Currently, ensemble seasonal forecasts using a single model with multiple perturbed initial conditions generally suffer from an “overconfidence” problem, i.e., the ensemble evolves such that the spread among members is small, compared to the magnitude of the mean error. This has motivated the use of a multi-model ensemble (MME), a technique that aims at sampling the structural uncertainty in the forecasting system. Here we investigate how the structural uncertainty in the ocean initial conditions impacts the reliability in seasonal forecasts, by using a new ensemble generation method to be referred to as the multiple-ocean analysis ensemble (MAE) initialization. In the MAE method, multiple ocean analyses are used to build an ensemble of ocean initial states, thus sampling structural uncertainties in oceanic initial conditions (OIC) originating from errors in the ocean model, the forcing flux, and the measurements, especially in areas and times of insufficient observations, as well as from the dependence on data assimilation methods. The merit of MAE initialization is demonstrated by the improved El Niño and the Southern Oscillation (ENSO) forecasting reliability. In particular, compared with the atmospheric perturbation or lagged ensemble approaches, the MAE initialization more effectively enhances ensemble dispersion in ENSO forecasting. A quantitative probabilistic measure of reliability also indicates that the MAE method performs better in forecasting all three (warm, neutral and cold) categories of ENSO events. In addition to improving seasonal forecasts, the MAE strategy may be used to identify the characteristics of the current structural uncertainty and as guidance for improving the observational network and assimilation strategy. Moreover, although the MAE method is not expected to totally correct the overconfidence of seasonal forecasts, our results demonstrate that OIC uncertainty is one of the major sources of forecast overconfidence, and suggest that the MAE is an essential component of an MME system.