Particle flux characterisation and sedimentation patterns of protistan plankton during the iron fertilisation experiment LOHAFEX in the Southern Ocean     

《Deep Sea Research Part I: Oceanographic Research Papers》2014

The taxonomic composition and types of particles comprising the downward particle flux were examined during the mesoscale artificial iron fertilisation experiment LOHAFEX. The experiment was conducted in low-silicate waters of the Atlantic Sector of the Southern Ocean during austral summer (January–March 2009), and induced a bloom dominated by small flagellates. Downward particle flux was low throughout the experiment, and not enhanced by addition of iron; neutrally buoyant sediment traps contained mostly faecal pellets and faecal material apparently reprocessed by mesozooplankton. TEP fluxes were low, ≤5 mg GX eq. m⁻² d⁻¹, and a few phytodetrital aggregates were found in the sediment traps. Only a few per cent of the POC flux was found in the traps consisting of intact protist plankton, although remains of taxa with hard body parts (diatoms, tintinnids, thecate dinoflagellates and foraminifera) were numerous, far more so than intact specimens of these taxa. Nevertheless, many small flagellates and coccoid cells, belonging to the pico- and nanoplankton, were found in the traps, and these small, soft-bodied cells probably contributed the majority of downward POC flux via mesozooplankton grazing and faecal pellet export. TEP likely played an important role by aggregating these small cells, and making them more readily available to mesozooplankton grazers.  相似文献   

Asphalt mounds and associated biota on the Angolan margin     

《Deep Sea Research Part I: Oceanographic Research Papers》2014

Release of hydrocarbons from sediments is important in increasing habitat heterogeneity on deep ocean margins. Heterogeneity arises from variation in abiotic and biotic conditions, including changes in substratum, geochemistry, fluid flow, biological communities and ecological interactions. The seepage of heavy hydrocarbons to the seafloor is less well studied than most other cold seep systems and may lead to the formation of asphalt mounds. These have been described from several regions, particularly the Gulf of Mexico. Here, we describe the structure, potential formation and biology of a large asphalt mound province in Block 31SE Angola. A total of 2254 distinct mound features was identified by side-scan sonar, covering a total area of 3.7 km² of seafloor. The asphalt mounds took a number of forms from small (<0.5 m diameter; 13% observations) mounds to large extensive (<50 m diameter) structures. Some of the observed mounds were associated with authigenic carbonate and active seepage (living chemosynthetic fauna present in addition to the asphalt). The asphalt mounds are seabed accumulations of heavy hydrocarbons formed from subsurface migration and fractionation of reservoir hydrocarbons primarily through a network of faults. In Angola these processes are controlled by subsurface movement of salt structures. The asphalt mounds were typically densely covered with epifauna (74.5% of mounds imaged had visible epifauna) although individual mounds varied considerably in epifaunal coverage. Of the 49 non-chemosynthetic megafaunal taxa observed, 19 taxa were only found on hard substrata (including asphalt mounds), 2 fish species inhabited the asphalt mounds preferentially and 27 taxa were apparently normal soft-sediment fauna. Antipatharians (3.6±2.3% s.e.) and poriferans (2.6±1.9% s.e.) accounted for the highest mean percentage of the observed cover, with actinarians (0.9±0.4% s.e.) and alcyonaceans (0.4±0.2% s.e.) covering smaller proportions of the area. Asphalt mounds represent a common and important habitat on several margin systems globally and should be recognised in future environmental assessment and management of these areas.  相似文献   

On how thin submarine flows transported large volumes of sand for hundreds of kilometres across a flat basin plain without eroding the sea floor     

Christopher J. Stevenson  Peter J. Talling  Esther J. Sumner  Douglas G. Masson  Micheal Frenz  Russell B. Wynn 《Sedimentology》2014,61(7):1982-2019

Submarine gravity currents, especially long run‐out flows that reach the deep ocean, are exceptionally difficult to monitor in action, hence there is a need to reconstruct how these flows behave from their deposits. This study mapped five individual flow deposits (beds) across the Agadir Basin, offshore north‐west Africa. This is the only data set where bed shape, internal distribution of lithofacies, changes in grain size and sea floor gradient, bed volumes, flow thickness and depth of erosion into underlying hemipelagic mud are known for individual beds. Some flows were 30 to 120 m thick. However, flows with the highest fraction of sand were less than 5 to 14 m thick. Sand was most likely to be carried in the lower 5 to 7 m of these flows. Despite being relatively thin, one flow was capable of transporting very large volumes of sediment (ca 200 km³) for large distances across very flat sea floor. These observations show that these relatively thin flows could travel quickly enough on very low gradients (0·02° to 0·05°) to suspend sand several metres to tens of metres above the sea floor, and maintain those speeds for up to 250 km across the basin. Near uniform hemipelagic mud interval thickness between beds, and coccolith assemblages in the mud caps of beds, suggest that the flows did not erode significantly into the underlying sea floor mud. Simple calculations imply that some flows, especially in the proximal part of the basin, were powerful enough to have eroded hemipelagic mud if it was exposed to the flow. This suggests that the flows were depositional from the moment they arrived at a basin plain location, and that deposition shielded the underlying hemipelagic mud from erosion. Reproducing the field observations outlined in this exceptionally detailed field data set is a challenge for future experimental and numerical models.  相似文献   

Growth of the Afanasy Nikitin seamount and its relationship with the 85°E Ridge,northeastern Indian Ocean   总被引：1，自引：0，他引：1  

K S KRISHNA  J M BULL  O ISHIZUKA  R A SCRUTTON  S JAISHANKAR  V K BANAKAR 《Journal of Earth System Science》2014,123(1):33-47

The Afanasy Nikitin seamount (ANS) is a major structural feature (400 km-long and 150 km-wide) in the Central Indian Basin, situated at the southern end of the so-called 85°E Ridge. Combined analyses of new multibeam bathymetric, seismic reflection and geochronological data together with previously described magnetic data provide new insights into the growth of the ANS through time, and its relationship with the 85°E Ridge. The ANS comprises a main plateau, rising 1200 m above the surrounding ocean floor (4800 m), and secondary elevated seamount highs, two of which (lie at 1600 and 2050 m water depths) have the morphology of a guyot, suggesting that they were formed above or close to sea-level. An unbroken sequence of spreading anomalies 34 through 32n.1 identified over the ANS reveal that the main plateau of the ANS was formed at 80–73 Ma, at around the same time as that of the underlying oceanic crust. The ⁴⁰Ar/³⁹Ar dates for two basalt samples dredged from the seamount highs are consistent, within error, at 67 Ma. These results, together with published results of late Cretaceous to early Cenozoic Indian Ocean plate reconstructions, indicate that the Conrad Rise hotspot emplaced both the main plateau of the ANS and Conrad Rise (including the Marion Dufresne, Ob and Lena seamounts) at 80–73 Ma, close to the India–Antarctica Ridge system. Subsequently, the seamount highs were formed by late-stage volcanism c. 6–13 Myr after the main constructional phase of the seamount plateau. Flexural analysis indicates that the main plateau and seamount highs of the ANS are consistent with Airy-type isostatic compensation, which suggest emplacement of the entire seamount in a near spreading-center setting. This is contrary to the flexural compensation of the 85°E Ridge further north, which is interpreted as being emplaced in an intraplate setting, i.e., 25–35 Myr later than the underlying oceanic crust. Therefore, we suggest that the ANS and the 85°E Ridge appear to be unrelated as they were formed by different mantle sources, and that the proximity of the southern end of the 85°E Ridge to the ANS is coincidental.  相似文献   

Geochemistry and mineralogy of REY-rich mud in the eastern Indian Ocean     

《Journal of Asian Earth Sciences》2014

Deep-sea sediments in parts of the Pacific Ocean were recently found to contain remarkably high concentrations of rare-earth elements and yttrium (REY) of possible economic significance. Here we report similar REY-rich mud in a core section from Deep Sea Drilling Project Site 213 in the eastern Indian Ocean. The sediments consist mainly of siliceous ooze, with subordinate zeolitic clay that contains relatively high REY concentrations. The maximum and average total REY (ΣREY) contents of this material are 1113 and 629 ppm, respectively, which are comparable to those reported from the Pacific Ocean. The REY-rich mud at Site 213 shows enrichment in heavy rare-earth elements, negative Ce anomalies, and relatively low Fe₂O₃/ΣREY ratios, similar to those in the Pacific Ocean. In addition, the major-element composition of the Indian Ocean REY-rich mud indicates slight enrichment in lithogenic components, which probably reflects a contribution from southern African eolian dust. A volcaniclastic component from neighboring mid-ocean ridges or intraplate volcanoes is also apparent. Elemental compositions and X-ray diffraction patterns for bulk sediment, and microscopic observation and elemental mapping of a polished thin section, demonstrate the presence of phillipsite and biogenic apatite, such as fish debris, in the REY-rich mud. The strong correlation between total REY content and apatite abundance implies that apatite plays an important role as a host phase of REY in the present deep-sea sediment column. However, positive correlations between ΣREY and elements not present in apatite (e.g., Fe₂O₃, MnO, and TiO₂) imply that the REY-rich mud is not formed by a simple mixture of REY-enriched apatite and other components.  相似文献   

Constraining input and output fluxes of the southern-central Chile subduction zone: water,chlorine and sulfur     

David Völker  Heidi Wehrmann  Steffen Kutterolf  Karthik Iyer  Wolfgang Rabbel  Jacob Geersen  Kaj Hoernle 《International Journal of Earth Sciences》2014,103(7):2129-2153

In this paper, we constrain the input and output fluxes of H₂O, Cl and S into the southern-central Chilean subduction zone (31°S–46°S). We determine the input flux by calculating the amounts of water, chlorine and sulfur that are carried into the subduction zone in subducted sediments, igneous crust and hydrated lithospheric mantle. The applied models take into account that latitudinal variations in the subducting Nazca plate impact the crustal porosity and the degree of upper mantle serpentinization and thus water storage in the crust and mantle. In another step, we constrain the output fluxes of the subduction zone both to the subcontinental lithospheric mantle and to the atmosphere–geosphere–ocean by the combined use of gas flux determinations at the volcanic arc, volume calculations of volcanic rocks and the combination of mineralogical and geothermal models of the subduction zone. The calculations indicate that about 68 Tg/m/Ma of water enters the subduction zone, as averaged over its total length of 1,480 km. The volcanic output on the other hand accounts for 2 Tg/m/Ma or 3 % of that input. We presume that a large fraction of the volatiles that are captured within the subducting sediments (which accounts for roughly one-third of the input) are cycled back into the ocean through the forearc. This assumption is however questioned by the present lack of evidence for major venting systems of the submarine forearc. The largest part of the water that is carried into the subduction zone in the crust and hydrated mantle (accounting for two-thirds of the input) appears to be transported beyond the volcanic arc.  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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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   

Changes in tourmaline composition during magmatic and hydrothermal processes leading to tin-ore deposition: The Cornubian Batholith,SW England     

《Ore Geology Reviews》2017

To investigate the potential of tourmaline as a geochemical monitor, a comprehensive dataset on major, minor and trace element concentrations as well as Fe³⁺/ΣFe ratios of tourmaline is presented. The dataset includes samples from five plutonic complexes related to diverse magmatic to hydrothermal stages of the Cornubian Batholith (SW England). Tourmaline composition found in barren and cassiterite-bearing samples include all three primary tourmaline groups and tourmaline species with the general endmembers schorl, dravite, elbaite, uvite, feruvite, foitite and Mg-foitite.Based on textures and compositions, it is possible to distinguish not only between late-magmatic and hydrothermal tourmaline, but also between several formation stages. Hence, tourmaline monitors late-magmatic processes and the partitioning of elements during exsolution of an aqueous phase. For example, in hydrothermal tourmaline Sn is strongly enriched, while Ti, Cr, V and Sc are depleted compared to late-magmatic tourmaline of the same sample. Several tourmaline generations that precipitated from magmatic fluids can be distinguished with differing major and minor elements and REE patterns depending on the composition of the melt from which they were expelled from. Strongly zoned tourmaline allows for unraveling the hydrothermal history of a distinct location including ore precipitation. The precipitation of SnO₂ in the study area was probably caused by mixing between acidic, reduced, Sn-bearing magmatic fluids and oxidized meteoric fluids, which is in agreement with London and Manning (1995) and Williamson et al. (2000). Hence, the ability of tourmaline composition to monitor changes in Sn concentration and redox conditions in hydrothermal fluids has potential as an exploration tool.  相似文献   

Paleomagnetic study of ferromanganese crusts recovered from the northwest Pacific — Testing the applicability of the magnetostratigraphic method to estimate growth rate     

《Ore Geology Reviews》2017

Previous studies have shown that ferromanganese crusts in the Pacific Ocean commonly record paleomagnetic reversals and that the reversal patterns can be used to estimate growth rates. In order to investigate the applicability of the magnetostratigraphic method, we conducted paleomagnetic measurements of crust samples recovered from five locations in the northwest Pacific. A series of thin slices, with thicknesses of 0.5–1.0 mm, was prepared for each sample, and a paleomagnetic polarity was determined for each slice. In all five samples, we found a consistent reversal pattern of N1–R1–N2–R2–N3 from the surface to the inner part of the crust. In three samples, another polarity interval (R3) was recognized below the N3 section of the crust. These data suggest that ferromanganese crusts in the northwest Pacific recorded paleomagnetic reversals and that reversal patterns can be used for ocean-scale correlations. The magnetostratigraphic method suggests constant growth rates of 1.49, 2.54, 3.56 and 3.67 mm/Ma for four samples, three of which are consistent with those estimated using ¹⁰Be/⁹Be dating at the 2σ (standard deviation) level.  相似文献