Processes influencing dissolved iron distributions below the surface at the Atlantic Ocean–Celtic Sea shelf edge     

Florence Ndlec  Peter J. Statham  Matt Mowlem 《Marine Chemistry》2007,104(3-4):156-170

Shelf break systems are highly dynamic environments. However little is known about the influence that benthic interactions and water mass mixing may have on vertical distributions of iron in these systems. Dissolved Fe (< 0.4 μm) concentrations were measured in samples from nine vertical profiles across the upper slope (150–2950 m water depth) at the Atlantic Ocean–Celtic Sea shelf break. Dissolved iron concentrations varied between less than 0.2 and 5.4 nM, and the resulting detailed section showed evidence of a range of processes influencing the Fe distributions. The near sea floor data were interpreted in terms of release and removal processes. The concentrations of dissolved Fe present in near seabed waters were consistent with release of Fe from in situ remineralisation of particulate organic matter at two upper slope stations, and possibly release from pore water upon resuspension on shelf. Lateral transport of dissolved iron was evident from elevated Fe concentrations in an intermediate nepheloid layer and its advection along isopycnals. Surface waters at the shelf break also showed evidence of vertical mixing of deeper iron-rich waters. These waters contained macronutrients that sustained primary productivity in these otherwise nutrient-depleted surface waters. The data also suggest some degree of stabilisation of relatively high concentrations of iron, presumably through ligand association or as colloids. This study supports the view that lateral export of dissolved iron to the interior of the ocean from shelf and coastal zones and may have important implications for the global budget of oceanic iron.  相似文献   

Field investigation of ridge–runnel dynamics on an intertidal mudflat     

Jon J. Williams  Paul A. Carling  Carl L. Amos  Charlie Thompson 《Estuarine, Coastal and Shelf Science》2008

Flow-parallel linear ridge–runnel (R–R) bedforms composed of mainly cohesive sediments have been investigated at an intertidal site located at Hills Flats in the Severn Estuary, UK. It is argued that the sequences of sand–mud laminae in the ridge sediments indicate strongly that R–R are depositional bedforms. Faint R–R topography scoured in the underlying soft bedrock and parallel rows of coarser particle now found at the base of ridges are evidence of secondary vortices that may have provided a catalyst for mud ridge growth. Bed shear stress remains below the erosion threshold for all but the most recent and weak surficial mud deposits which are removed by evorsion [mechanical erosion by turbulent flows that may also carry sand and/or gravel] during Spring tidal flow ca. 60 cm/s. Differences in flow characteristics between ridges and runnel are minor. Corrasion [to erode or be eroded by abrasion] by sparse coarse sediment largely prevents net deposition in the runnels. Over time a slight imbalance between mud deposition and erosion rates allows slow growth of the ridges suggesting that these mudflats may be especially sensitive to slight changes in the local hydrodynamic regime and/or sediment supply.  相似文献   

Provenance and pathways of late Quaternary turbidites in the deep-water Agadir Basin,northwest African margin     

Michael Frenz  Russell B. Wynn  Aggeliki Georgiopoulou  Vera B. Bender  Gayle Hough  Douglas G. Masson  Peter J. Talling  Bryan T. Cronin 《International Journal of Earth Sciences》2009,98(4):721-733

A series of individual turbidites, correlated over distances >100 km, are present in the recent fill of the Agadir Basin, offshore northwest Africa. The aim here is to unravel multiple turbidite source areas and flow pathways, and show how turbidite provenance studies contribute to interpretation of flow processes. Agadir Basin turbidites are sourced from four main areas, with the majority originating from the siliciclastic Morocco Shelf; their sand-mud distribution is strongly controlled by flow sediment volume, with relatively low-volume flows dying out within the Agadir Basin and large-volume flows bypassing significant sediment volumes to basins further downslope. Two large-volume volcaniclastic turbidites are attributed to a Canary Islands landslide source, while several small mud-dominated turbidites are interpreted to be locally sourced from hemipelagic-draped seamounts (e.g. Turbidite AB10). Finally, Turbidite AB1 (∼1 ka) is only present in the western Agadir Basin, and is linked to recent “re-activation” of the Sahara Slide headwall. The muddy suspension clouds of three large-volume flows, all linked to large-scale landslides, have covered huge areas of seafloor and flowed along or even slightly upslope for long distances. It is proposed that northeastwards-flowing bottom currents have aided transport of these dilute flow fractions into and across the Agadir Basin.  相似文献   

Heat shock protein expression pattern (HSP70) in the hydrothermal vent mussel Bathymodiolus azoricus     

Pruski AM  Dixon DR 《Marine environmental research》2007,64(2):209-224

We previously reported evidence of increased levels of DNA damage in the hydrothermal mussel Bathymodiolus azoricus, which suggested that the species was not fully resistant to the natural toxicity of its deep-sea vent environment. In the present study, HSP70 was used as a biomarker of sub-cellular stress. Differences in HSP70 expression pattern were observed between vent sites, typified by different depths/toxicity profiles, and between different mussel tissue types. A comparison of specimens collected by remote operated vehicle (ROV) and acoustically-operated cages showed that less stress (as indicated by changes in HSP70 levels) was induced by the faster cage recovery method. Therefore alternatives to ROV collection should be considered when planning experiments involving live deep sea organisms. Significantly, a positive correlation was found between the levels of DNA strand breakage, as measured using the Comet assay, and HSP70 expression pattern; evidence was also obtained for the constitutive expression of at least one HSP isoform which was located within the cell nucleus.  相似文献   

Extended orbitally forced palaeoclimatic records from the equatorial Atlantic Ceara Rise     

Heiko Plike  Julia Frazier  James C. Zachos 《Quaternary Science Reviews》2006,25(23-24):3138

We extend existing high-resolution Oligocene–Miocene proxy records from Ocean Drilling Program (ODP) Leg 154. The extended record spans the time interval from 17.86 to 26.5 Ma. The data are age calibrated against a new astronomical solution that affords a re-evaluation of the intricate interaction between orbital (“Milankovitch”) forcing of the climate and ocean system, and the fidelity with which this forcing is recorded in oxygen and carbon stable isotope measurements from benthic foraminifera, and associated lithological proxy records of magnetic susceptibility, colour reflectance, and the measured sand fraction. Our records show a very strong continual imprint of the Earth's obliquity cycle, modulate in amplitude every , a very strong eccentricity signal in the carbon isotope records, and a strong, but probably local, imprint of climatic precession on the coarse fraction and magnetic susceptibility records. Our data allowed us to evaluate how the interaction of long, multi-million year beats in the Earth's eccentricity and obliquity are implicated in the waxing and waning of ice-sheets, presumably on Antarctica. Our refined age model confirms the revised age of the Oligocene–Miocene boundary, previously established by analysis of the lithological data, and allows a strong correlation with the geomagnetic time scale by comparison with data from ODP Site 1090, Southern Ocean.  相似文献   

First-order reversal curve (FORC) diagrams of magnetic mixtures: Micromagnetic models and measurements     

《Physics of the Earth and Planetary Interiors》2006,154(3-4):308-322

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Response of the meridional overturning circulation to variable buoyancy forcing in a double hemisphere basin     

Marc A. Lucas  J. J.-M. Hirschi  J. Marotzke 《Climate Dynamics》2010,34(5):615-627

We consider how a highly idealized double-hemisphere basin responds to a zonally constant restoring surface temperature profile that oscillates in time, with periods ranging from 0.5 to 32,000 years. In both hemispheres, the forcing is similar but can be either in phase or out of phase. The set-up is such that the Northern Hemisphere always produces the densest waters. The model’s meridional overturning circulation (MOC) exhibits a strong response in both hemispheres on decadal to multi-millennial timescales. The amplitude of the oscillations reaches up to 140% of the steady-state maximum MOC and exhibits resonance-like behaviour, with a maximum at centennial to millennial forcing periods. When the forcing is in phase between the Northern and Southern Hemispheres, there is a marked decrease in the amplitude of the MOC response as the forcing period is increased beyond the resonance period. In this case the resonance-like behaviour is identical to the one we found earlier in a single-hemisphere model and occurs for the same reasons. When the forcing is out of phase between the Northern and Southern Hemispheres, the amplitude of the MOC response is substantially greater for long forcing periods (millennial and longer), particularly in the Southern Hemisphere. This increased MOC amplitude occurs because for an out of phase forcing, either the northern or the southern deep water source is always active, leading to generally colder bottom waters and thus greater stratification in the opposite hemisphere. This increased stratification in turn stabilises the water column and thus reduces the strength of the weaker overturning cell. The interaction of the two hemispheres leads to response timescales of the deep ocean at half the forcing period. Our results suggest a possible explanation for the half-precessional time scale observed in the deep Atlantic Ocean palaeo-temperature record.  相似文献   

Climate change mitigation: trade-offs between delay and strength of action required     

Naomi E. Vaughan  Timothy M. Lenton  John G. Shepherd 《Climatic change》2009,96(1-2):29-43

Climate change mitigation via a reduction in the anthropogenic emissions of carbon dioxide (CO₂) is the principle requirement for reducing global warming, its impacts, and the degree of adaptation required. We present a simple conceptual model of anthropogenic CO₂ emissions to highlight the trade off between delay in commencing mitigation, and the strength of mitigation then required to meet specific atmospheric CO₂ stabilization targets. We calculate the effects of alternative emission profiles on atmospheric CO₂ and global temperature change over a millennial timescale using a simple coupled carbon cycle-climate model. For example, if it takes 50 years to transform the energy sector and the maximum rate at which emissions can be reduced is ?2.5% $\text{year}^{-1}$ , delaying action until 2020 would lead to stabilization at 540 ppm. A further 20 year delay would result in a stabilization level of 730 ppm, and a delay until 2060 would mean stabilising at over 1,000 ppm. If stabilization targets are met through delayed action, combined with strong rates of mitigation, the emissions profiles result in transient peaks of atmospheric CO₂ (and potentially temperature) that exceed the stabilization targets. Stabilization at 450 ppm requires maximum mitigation rates of ?3% to ?5% $\text{year}^{-1}$ , and when delay exceeds 2020, transient peaks in excess of 550 ppm occur. Consequently tipping points for certain Earth system components may be transgressed. Avoiding dangerous climate change is more easily achievable if global mitigation action commences as soon as possible. Starting mitigation earlier is also more effective than acting more aggressively once mitigation has begun.  相似文献   

GEC special edition – Reframing adaptation pathways     

《Global Environmental Change》2014

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Historical analogues of the recent extreme minima observed in the Atlantic meridional overturning circulation at 26°N     

Adam T. Blaker  Joël J.-M. Hirschi  Gerard McCarthy  Bablu Sinha  Sarah Taws  Robert Marsh  Andrew Coward  Beverly de Cuevas 《Climate Dynamics》2015,44(1-2):457-473

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