Iron mineral structure, reactivity, and isotopic composition in a South Pacific Gyre ferromanganese nodule over 4 Ma

Deep-sea ferromanganese nodules accumulate trace elements from seawater and underlying sediment porewaters during the growth of concentric mineral layers over millions of years. These trace elements have the potential to record past ocean geochemical conditions. The goal of this study was to determine whether Fe mineral alteration occurs and how the speciation of trace elements responds to alteration over ∼3.7 Ma of marine ferromanganese nodule (MFN) formation, a timeline constrained by estimates from 9Be/10Be concentrations in the nodule material. We determined Fe-bearing phases and Fe isotope composition in a South Pacific Gyre (SPG) nodule. Specifically, the distribution patterns and speciation of trace element uptake by these Fe phases were investigated. The time interval covered by the growth of our sample of the nodule was derived from 9Be/10Be accelerator mass spectrometry (AMS). The composition and distribution of major and trace elements were mapped at various spatial scales, using micro-X-ray fluorescence (μXRF), electron microprobe analysis (EMPA), and inductively coupled plasma mass spectrometry (ICP-MS). Fe phases were characterized by micro-extended X-ray absorption fine structure (μEXAFS) spectroscopy and micro-X-ray diffraction (μXRD). Speciation of Ti and V, associated with Fe, was measured using micro-X-ray absorption near edge structure (μXANES) spectroscopy. Iron isotope composition (δ56/54Fe) in subsamples of 1-3 mm increments along the radius of the nodule was determined with multiple-collector ICP-MS (MC-ICP-MS). The SPG nodule formed through primarily hydrogeneous inputs at a rate of 4.0 ± 0.4 mm/Ma. The nodule exhibited a high diversity of Fe mineral phases: feroxyhite (δ-FeOOH), goethite (α-FeOOH), lepidocrocite (γ-FeOOH), and poorly ordered ferrihydrite-like phases. These findings provide evidence that Fe oxyhydroxides within the nodule undergo alteration to more stable phases over millions of years. Trace Ti and V were spatially correlated with Fe and found to be adsorbed to Fe-bearing minerals. Ti/Fe and V/Fe ratios, and Ti and V speciation, did not vary along the nodule radius. The δ56/54Fe values, when averaged over sample increments representing 0.25 to 0.75 Ma, were homogeneous within uncertainty along the nodule radius, at -0.12 ± 0.07 ‰ (2sd, n=10). Our results indicate that the Fe isotope composition of the nodule remained constant during nodule growth and that mineral alteration did not affect the primary Fe isotope composition of the nodule. Furthermore, the average δ56/54Fe value of -0.12 ‰ we find is consistent with Fe sourced from continental eolian particles (dust). Despite mineral alteration, the trace element partitioning of Ti and V, and Fe isotope composition, do not appear to change within the sensitivity of our measurements. These findings suggest that Fe oxyhydroxides within hydrogenetic ferromanganese nodules are out of geochemical contact with seawater once they are covered by subsequent concentric mineral layers. Even though Fe-bearing minerals are altered, trace element ratios, speciation and Fe isotope composition are preserved within the nodule.

A revised L-band radio-brightness sensitivity to extreme winds under tropical cyclones: The 5 year SMOS-Storm database

Five years of SMOS L-band brightness temperature data intercepting a large number of tropical cyclones (TCs) are analyzed. The storm-induced half-power radio-brightness contrast (ΔI) is defined as the difference between the brightness observed at a specific wind force and that for a smooth water surface with the same physical parameters. ΔI can be related to surface wind speed and has been estimated for ~ 300 TCs that intercept with SMOS measurements. ΔI, expressed in a common storm-centric coordinate system, shows that mean brightness contrast monotonically increases with increased storm intensity ranging from ~ 5 K for strong storms to ~ 24 K for the most intense Category 5 TCs. A remarkable feature of the 2D mean ΔI fields and their variability is that maxima are systematically found on the right quadrants of the storms in the storm-centered coordinate frame, consistent with the reported asymmetric structure of the wind and wave fields in hurricanes. These results highlight the strong potential of SMOS measurements to improve monitoring of TC intensification and evolution. An improved empirical geophysical model function (GMF) was derived using a large ensemble of co-located SMOS ΔI, aircraft and H*WIND (a multi-measurement analysis) surface wind speed data. The GMF reveals a quadratic relationship between ΔI and the surface wind speed at a height of 10 m (U10). ECMWF and NCEP analysis products and SMOS derived wind speed estimates are compared to a large ensemble of H*WIND 2D fields. This analysis confirms that the surface wind speed in TCs can effectively be retrieved from SMOS data with an RMS error on the order of 10 kt up to 100 kt. SMOS wind speed products above hurricane force (64 kt) are found to be more accurate than those derived from NWP analyses products that systematically underestimate the surface wind speed in these extreme conditions. Using co-located estimates of rain rate, we show that the L-band radio-brightness contrasts could be weakly affected by rain or ice-phase clouds and further work is required to refine the GMF in this context.

Modelling the biogeochemical cycles of elements limiting primary production in the English channel .2. Sensitivity analyses

A general framework for an ecological model of the English Channel was described in the first of this pair of papers. In this study, it was used to investigate the sensitivity of the model to various factors: model structure, parameter values, boundary conditions and forcing variables. These sensitivity analyses show how important quota formulation for phytoplankton growth is, particularly for growth of dinoflagellates. They also stress the major influence of variables and parameters related to nitrogen. The role played by rivers and particularly the river Seine was investigated. Their influence on global English Channel phytoplanktonic production seems to be relatively low, even though nutrient inputs determine the intensity of blooms in the Bay of Seine. The geographical position of the river Seine's estuary makes it important in fluxes through the Straits of Dover. Finally, the multi-annual study highlights the general stability of the English Channel ecosystem. These global considerations are discussed and further improvements to the model are proposed.