Caldera resurgence during magma replenishment and rejuvenation at Valles and Lake City calderas

A key question in volcanology is the driving mechanisms of resurgence at active, recently active, and ancient calderas. Valles caldera in New Mexico and Lake City caldera in Colorado are well-studied resurgent structures which provide three crucial clues for understanding the resurgence process. (1) Within the limits of ⁴⁰Ar/³⁹Ar dating techniques, resurgence and hydrothermal alteration at both calderas occurred very quickly after the caldera-forming eruptions (tens of thousands of years or less). (2) Immediately before and during resurgence, dacite magma was intruded and/or erupted into each system; this magma is chemically distinct from rhyolite magma which was resident in each system. (3) At least 1?km of structural uplift occurred along regional and subsidence faults which were closely associated with shallow intrusions or lava domes of dacite magma. These observations demonstrate that resurgence at these two volcanoes is temporally linked to caldera subsidence, with the upward migration of dacite magma as the driver of resurgence. Recharge of dacite magma occurs as a response to loss of lithostatic load during the caldera-forming eruption. Flow of dacite into the shallow magmatic system is facilitated by regional fault systems which provide pathways for magma ascent. Once the dacite enters the system, it is able to heat, remobilize, and mingle with residual crystal-rich rhyolite remaining in the shallow magma chamber. Dacite and remobilized rhyolite rise buoyantly to form laccoliths by lifting the chamber roof and producing surface resurgent uplift. The resurgent deformation caused by magma ascent fractures the chamber roof, increasing its structural permeability and allowing both rhyolite and dacite magmas to intrude and/or erupt together. This sequence of events also promotes the development of magmatic–hydrothermal systems and ore deposits. Injection of dacite magma into the shallow rhyolite magma chamber provides a source of heat and magmatic volatiles, while resurgent deformation and fracturing increase the permeability of the system. These changes allow magmatic volatiles to rise and meteoric fluids to percolate downward, favouring the development of hydrothermal convection cells which are driven by hot magma. The end result is a vigorous hydrothermal system which is driven by magma recharge.     

The influence of a season of extreme wet weather events on exposure of the World Heritage Area Great Barrier Reef to pesticides

The 2010-2011 wet season was one of extreme weather for the State of Queensland, Australia. Major rivers adjacent to the Great Barrier Reef (GBR) were discharging at rates 1.5 to >3 times higher than their long term median. Exposure to photosystem II herbicides has been routinely monitored over a period of up to 5 years at 12 inshore GBR sites. The influence of this wet season on exposure to photosystem II herbicides was examined in the context of this long-term monitoring record and during flood plume events in specific regions. Median exposures expressed as diuron equivalent concentration were an average factor of 2.3 times higher but mostly not significantly different (p<0.05) to the median for the long-term monitoring record. The herbicides metolachlor and tebuthiuron were frequently detected in flood plume waters at concentrations that reached or exceeded relevant water quality guidelines (by up to 4.5 times).     

Bioreduction of hematite nanoparticles by the dissimilatory iron reducing bacterium Shewanella oneidensis MR-1

We examined the reduction of different size hematite (α-Fe₂O₃) nanoparticles (average diameter of 11, 12, 30, 43, and 99 nm) by the dissimilatory iron reducing bacteria (DIRB), Shewanella oneidensis MR-1, to determine how S. oneidensis MR-1 may utilize these environmentally relevant solid-phase electron acceptors. The surface-area-normalized-bacterial Fe(III) reduction rate for the larger nanoparticles (99 nm) was one order of magnitude higher than the rate observed for the smallest nanoparticles (11 nm). The Fe(III) reduction rates for the 12, 30, and 43 nm nanoparticles fell between these two extremes. Whole-cell TEM images showed that the mode of Fe₂O₃ nanoparticle attachment to bacterial cells was different for the aggregated, pseudo-hexagonal/irregular and platey 11, 12, and 99 nm nanoparticles compared to the non-aggregated 30 and 43 nm rhombohedral nanoparticles. Due to differences in aggregation, the 11, 12, and 99 nm nanoparticles exhibited less cell contact and less cell coverage than did the 30 and 43 nm nanoparticles. We hypothesize that S. oneidensis MR-1 employs both indirect and direct mechanisms of electron transfer to Fe(III)-oxide nanoparticles and that the bioreduction mechanisms employed and Fe(III) reduction rates depend on the nanoparticles’ aggregation state, size, shape and exposed crystal faces.     

Basinal setting and origin of thick (1·8 km) mass‐flow dominated Grand Conglomérat diamictites,Kamoa, Democratic Republic of Congo: Resolving climate and tectonic controls during Neoproterozoic glaciations

The Kamoa sub‐basin, in the south‐eastern part of the Democratic Republic of Congo, is a rift basin that hosts a world‐class stratiform copper deposit at the base of a very thick (1·8 km) succession of matrix‐supported conglomerates (diamictite) (Grand Conglomérat Formation) that has been interpreted by some as the product of deposition in the aftermath of a planet‐wide glaciation. Newly available subsurface data consisting of more than 300 km of drill core throws new light on the origin of diamictite and associated facies types, and their tectonic, basinal and palaeoclimatic setting. Initiation of rifting is recorded by a lowermost subaqueous succession of fault‐related mass flow conglomerates and breccias (the ‘Poudingue’) with interdigitating coeval and succeeding sandstone turbidites (Mwashya Subgroup). Overlying diamictites of the Grand Conglomérat were deposited as subaqueous debrites produced by mixing and homogenization of antecedent breccias and gravel from the Poudingue and Mwashya sediments with basinal muds. Failure of over‐steepened basin margins and debris flow was likely to be triggered by faulting and seismic activity, and was accompanied by syn‐depositional subaqueous basaltic magmatism recorded by peperites and pillow lavas within diamictites. The thickness of diamictites reflects recurring phases of faulting, volcanism and rapid subsidence allowing continued accommodation of rapidly deposited resedimented facies well below wave base. A distal or indirect, glacial influence in the form of rare dropstones and striated clasts is evident, but tectonically‐driven mass flow destroyed any primary record of glacial climate originally present in basin margin sediments. Such basin margin settings were common during Rodinia rifting and their stratigraphy and facies record a dominant tectonic, rather than climatic, control on sedimentation. Deposition occurred on tectonic timescales inconsistent with a Snowball Earth model for Neoproterozoic diamictites involving a direct glacial contribution to deposition.     

Quantifying Site-Specific Physical Heterogeneity Within an Estuarine Seascape

Quantifying physical heterogeneity is essential for meaningful ecological research and effective resource management. Spatial patterns of multiple, co-occurring physical features are rarely quantified across a seascape because of methodological challenges. Here, we identified approaches that measured total site-specific heterogeneity, an often overlooked aspect of estuarine ecosystems. Specifically, we examined 23 metrics that quantified four types of common physical features: (1) river and creek confluences, (2) bathymetric variation including underwater drop-offs, (3) land features such as islands/sandbars, and (4) major underwater channel networks. Our research at 40 sites throughout Plum Island Estuary (PIE) provided solutions to two problems. The first problem was that individual metrics that measured heterogeneity of a single physical feature showed different regional patterns. We solved this first problem by combining multiple metrics for a single feature using a within-physical feature cluster analysis. With this approach, we identified sites with four different types of confluences and three different types of underwater drop-offs. The second problem was that when multiple physical features co-occurred, new patterns of total site-specific heterogeneity were created across the seascape. This pattern of total heterogeneity has potential ecological relevance to structure-oriented predators. To address this second problem, we identified sites with similar types of total physical heterogeneity using an across-physical feature cluster analysis. Then, we calculated an additive heterogeneity index, which integrated all physical features at a site. Finally, we tested if site-specific additive heterogeneity index values differed for across-physical feature clusters. In PIE, the sites with the highest additive heterogeneity index values were clustered together and corresponded to sites where a fish predator, adult striped bass (Morone saxatilis), aggregated in a related acoustic tracking study. In summary, we have shown general approaches to quantifying site-specific heterogeneity.     

The geomorphology and evolution of a large barrier spit: Farewell Spit,New Zealand

Farewell Spit is a 25 km long barrier spit that marks the end of a littoral drift system, almost 1000 km in length that runs along South Island, New Zealand. The spit is composed of barchan dunes over 20 m high, sand sheets over 1 km wide and vegetated linear dunes. Analysis of aerial photography indicates a rapid colonization of the spit by vegetation which has expanded in area by 75% since 1950. Vegetation colonization preferentially occurs on the southern side of the spit, with its northern margin characterized by barchan dunes which migrate at rates of up to 64 m/yr. Sand sourced from longshore drift appears to be the primary source of beach sediment, which is then transported into the dune field by the persistent westerly winds of the Roaring 40s. While there has been significant dune roll‐over on the surface of the spit, its overall area has remained much the same for the past 54 years. Occasional cyclone events cause erosion, but this is balanced by aeolian sediment transport. It would appear that extension of the subaerial portion of the spit is related to the development of shells banks at its downdrift end which are periodically welded to the main spit by dune extension. Farewell Spit therefore provides an ideal example of a barrier environment where longshore sediment supply and aeolian transport dominates geomorphic evolution. This differentiates the study site from other barrier environments where overwash or tidal inlet development often characterizes recent landform evolution. Copyright © 2010 John Wiley & Sons, Ltd.     

Oxidative dissolution potential of biogenic and abiogenic TcO2 in subsurface sediments

Technetium-99 (Tc) is an important fission product contaminant associated with sites of nuclear fuels reprocessing and geologic nuclear waste disposal. Tc is highly mobile in its most oxidized state and less mobile in the reduced form [Tc(IV)O₂·nH₂O]. Here we investigate the potential for oxidation of Tc(IV) that was heterogeneously reduced by reaction with biogenic Fe(II) in two sediments differing in mineralogy and aggregation state; unconsolidated Pliocene-age fluvial sediment from the upper Ringold (RG) Formation at the Hanford Site and a clay-rich saprolite from the Field Research Center (FRC) background site on the Oak Ridge Site. Both sediments contained Fe(III) and Mn(III/IV) as redox active phases, but FRC also contained mass-dominant Fe-phyllosilicates of different types. Shewanella putrefaciens CN32 reduced Mn(III/IV) oxides and generated Fe(II) that was reactive with Tc(VII) in heat-killed, bioreduced sediment. After bioreduction and heat-killing, biogenic Fe(II) in the FRC exceeded that in RG by a factor of two. More rapid reduction rates were observed in the RG that had lower biogenic Fe(II), and less particle aggregation. EXAFS measurements indicated that the primary reduction product was a TcO₂-like phase in both sediments. The biogenic redox product Tc(IV) oxidized rapidly and completely in RG when contacted with air. Oxidation, in contrast, was slow and incomplete in the FRC, in spite of similar molecular scale speciation of Tc compared to RG. X-ray microprobe, electron microprobe, X-ray absorption spectroscopy, and micro X-ray diffraction were applied to the whole sediment and isolated Tc-containing particles. These analyses revealed that non-oxidizable Tc(IV) in the FRC existed as complexes with octahedral Fe(III) within intra-grain domains of 50-100 μm-sized, Fe-containing micas presumptively identified as celadonite. The markedly slower oxidation rates in FRC as compared to RG were attributed to mass-transfer-limited migration of O₂ into intra-aggregate and intraparticle domains where Tc(IV) existed; and the formation of unique, oxidation-resistant, intragrain Tc(IV)-Fe(III) molecular species.     

The daily‐scale entrance dynamics of intermittently open/closed estuaries

Intermittently open/closed estuaries (IOCE) are a dynamic form of estuary characterised by periodic entrance closure to the ocean. Entrance closure is a function of the relative balance between on and offshore sediment transport with closures occurring during periods of low fluvial discharge whereby the estuary ebb‐tidal prism is reduced. Although the broad scale processes of entrance closure are becoming better understood, there remains limited knowledge on channel morphodynamics during an individual closure event. In this study, the entrance dynamics of three IOCE on the coast of Victoria, Australia, were monitored over a daily timescale following both artificial and natural openings. The influence of changing marine and fluvial conditions on the relative sedimentation rate within the entrance channel was examined. IOCE in Victoria showed two distinct modes of entrance closure: (a) lateral accretion, whereby the estuary gradually closes by longshore drift‐driven spit growth during low river flows; and (b) vertical accretion, where the channel rapidly aggrades under high (> 2 m), near‐normal waves. During storms, when fluvial discharge and wave heights simultaneously increase, large swells will not always close the mouth due to an increase in the ebb‐tidal prism. The estuary water depth and the maximum channel dimensions following opening were not proportional to the opening duration, with this being a function of the wave and fluvial conditions occurring following lagoon drainage. Based on the findings of this work, implementing a successful artificial entrance opening is dependent on reduced onshore sedimentation rates which occur when wave energy is low (< 2 m Hs) relative to river flow. Copyright © 2017 John Wiley & Sons, Ltd.     

Amounts,forms, and management of nitrogen and phosphorus export from agricultural peatlands

Peatlands provide a setting that is well suited for cranberry agriculture in the Northeastern United States. However, misconceptions exist about the amounts and forms of nitrogen (N) and phosphorus (P) export from cranberry farms. In this study, we report inorganic and organic forms of N and P export from five peatlands cultivated for cranberry production in southeastern, Massachusetts, United States. We then compare N loading rates among cranberry farms in southeastern Massachusetts, row crop farms in the Midwestern United States, and uncultivated peatlands in the United States and United Kingdom. Based on a fluvial mass balance analysis, we find that nonriparian cranberry farms export 2.56 kg of P ha⁻¹ year⁻¹of total P and 12.1 kg of N ha⁻¹ year⁻¹of total N. Total N export from riparian or “flow through” farms is two times higher than nonriparian farms due to less retention of N fertilizer in the vadose zone of riparian farms. Gross total N export from riparian and nonriparian cranberry farms consists of 35% particulate organic N, 26% dissolved organic N, 31% ammonium (NH₄⁺), and 8% nitrate (NO₃⁻). The low proportions of NO₃⁻ export (13% of total dissolved N [TDN]) for cranberry farms differ from NO₃⁻ export for row crop farms (75% of TDN; p < .001) but not for uncultivated peatlands (17% of TDN; p = .61). Despite being highly modified by fertilizers and artificial drainage, low NO₃⁻ export (2.2 kg of N ha⁻¹ year⁻¹) from cranberry farms is consistent with field measurements of rapid N turnover in uncultivated peatlands. This finding suggests that state-funded wetland restoration efforts to restore denitrification in retired cranberry farms may be limited by NO₃⁻ rather than soil moisture or organic matter.