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1.
《Basin Research》2018,30(2):169-186
Long‐term (106–7 yr) clastic sedimentary fluxes to the ocean provide first‐order constraints on the response of continental surfaces to both tectonic and climatic forcing as well as the supply that builds the stratigraphic record. Here, we use the dated and regionally correlated relict lateritic landforms preserved over Sub‐Saharan West Africa to map and quantify regional denudation as well as the export of main catchments for three time intervals (45–24, 24–11 and 11–0 Ma). At the scale of West Africa, denudation rates are low (ca. 7 m Myr−1) and total clastic export rate represents 18.5 × 103 km3 Myr−1. Export rate variations among the different drainage groups depend on the drainage area and, more importantly, rock uplift. Denuded volumes and offshore accumulations are of the same magnitude, with a noticeably balanced budget between the Niger River delta and its catchment. This supports the establishment of the modern Niger catchment before 29 Ma, which then provided sufficient clastic material to the Niger delta by mainly collecting the erosion products of the Hoggar hotspot swell. Accumulations on the remaining Equatorial Atlantic margin of Africa suggest an apparent export deficit but the sediment budget is complicated by the low resolution of the offshore data and potential lateral sediment supply from the Niger delta. Further distortion of the depositional record by intracontinental transient storage and lateral input or destabilization of sediments along the margin may be identified in several locations, prompting caution when deducing continental denudation rates from accumulation only.  相似文献   

2.
Total organic carbon (TOC), total nitrogen (TN), stable carbon and nitrogen isotopes (δ13C, δ15N), total phosphorus (TP) and organic phosphorus (OP) were measured in surface sediments and two short cores (DU-3 and WS-4) from Lake Nansihu, China to infer historical changes in anthropogenic nutrient inputs and corresponding shifts in lake primary productivity. Results indicate that organic matter preserved in the sediments is mainly autochthonous and that analyzed sediment variables were affected little by post-burial diagenesis. Increasing TOC, TN, OP and TP concentrations since the 1940s reflect increased P loading and elevated lake productivity. The δ13C values varied from ?21.5 to ?26.6‰ in the two sediment cores. Values were relatively more negative before the 1940s, but thereafter increased until the mid-1980s, reflecting elevated lake productivity. Since the mid-1980s, δ13C values remained relatively constant in core WS-4 and decreased in core DU-3, perhaps reflecting a change in the phytoplankton community. The δ15N values ranged from ?0.5 to 1.3‰ in core DU-3 and from 1.2 to 2.5‰ in core WS-4 before the mid-1980s, and increased to between 2.1 and 8.0‰ and 5.2 and 7.8‰, respectively, thereafter. Topmost sediments in the two cores display δ15N values similar to those recorded in the surface sediments (5.5–7.5‰). Higher δ15N values in recent deposits correspond to greater nitrogen concentration in water, and likely indicate anthropogenic nitrogen input, mainly from human and animal wastes.  相似文献   

3.
We propose a palaeolimnological method for inferring past total phosphorus (TP) concentrations in lake water from spectrophotometrically-measured sedimentary pigments, particularly total carotenoids (TC). Our approach is based on a highly significant statistical correlation (P < 0.0001) between pigment concentrations (total carotenoids) in the surface sediment of 28 Italian lakes (subalpine, large, deep, shallow, volcanic) and TP concentrations measured in these lakes at overturn when the core was collected. A transfer function was developed from this “training” set, and used to estimate past TP concentrations from pigment concentrations in sediment cores. The results generally agreed with TP values as measured by long-term water quality monitoring programs. Contrasting results were obtained by a comparison with diatom-inferred TP. While the diatom model showed a tendency to overestimate TP values higher than 100 μg l−1, the pigment model correctly estimated TP in lakes when TP was <100 μg l−1, but not when lakes were rich in macrophytes. In fact, lakes with extensive populations of aquatic submersed macrophytes and epiphytes are outliers in terms of the TC versus TP relationship. The root mean square error of prediction of the pigment model is lower than those derived from certain diatom—based inference models. The predicted and residual values are not related to the estimated values and their average is not statistically different from zero. Errors were estimated via a ‘leave-one-out’ re-sampling technique. The proposed method permits rapid and relatively inexpensive determination of reference trophic conditions.  相似文献   

4.
Reconstructions of ancient delta systems rely typically on a two-dimensional (2D) view of prograding clinothems but may miss their three-dimensional (3D) stratigraphic complexity which can, instead, be best documented on modern delta systems by integrating high-resolution geophysical data, historical cartography, core data and geomorphological reconstructions offshore. We quantitatively compare three precisely positioned, high-resolution multi-beam bathymetry maps in the delta front and pro delta sectors (0.3 to 10 m water depth) of Po di Pila, the most active of the modern Po Delta five branches. By investigating the detailed morphology of the prograding modern Po Delta, we shed new light on the mechanisms that control the topset to foreset transition in clinothems and show the temporal and spatial complexity of a delta and its pro delta slope, under the impact of oceanographic processes. This study documents the ephemeral nature of the rollover point at the transition between sandy topset (fluvial, delta plain to mouth-bar) and muddy seaward-dipping foreset deposits advancing, in this case, in >20 m of water depth. Three multibeam surveys, acquired between 2013 and 2016, document the complexity in space and time of the topset and foreset regions and their related morphology, a diagnostic feature that could not be appreciated using solely 2D, even very high-resolution, seismic profiles. In addition, the comparison of bathymetric surveys gathered with one-year lapses shows the migration of subaqueous sand dunes on the clinothem topset, the formation of ephemeral cut-and-fill features at the rollover point (few m below mean sea level), the presence of collapse depressions derived by sagging of sediments and fluid expulsion (possibly induced by storm waves) on the foreset, and splays of sand likely reflecting gravity flows on the lower foreset. Though the modern Po Delta is anthropogenic in many respects, its subaqueous clinothem can be studied as a scale model for ancient clinothems that are less resolved geometrically and far less constrained chronologically.  相似文献   

5.
在中国第 1 8次南极科学考察期间 ( 2 0 0 2年 1月至 3月 )于南极普里兹湾进行了 3次铁加富培养浮游植物实验。共在三个站位 (St.Ⅰ 1 ,Ⅶ 1和Ⅶ 5 )的 2 0m水深处采水进行培养实验(分别标记为实验 1 ,2和 3)。三个站位水样的营养盐浓度都很高 (NO3 N 2 2- 2 9μM ,PO4 P1 1- 1 7μM ,SiO3 Si35- 60 μM ) ,而叶绿素a (Chla)浓度却变化很大 ,三个站位的浓度分别为 0 1 3- 0 1 7,0 2 0- 0 2 6和 2 35- 2 65 μg·L- 1 。St.Ⅰ 1和Ⅶ 1是典型的HNLC(high nu trient,low chlorophyllaconcentration ,高营养盐 ,低叶绿素a浓度 )海区特征。结果表明 ,在实验1和 2 ,添加铁大大促进了浮游植物的生长和营养盐的消耗。在实验 1中 ,添加 2 0 ,40和 80nM铁的培养瓶中的Chla浓度指数生长 ,在第 1 7d达到最高 ( 4 0- 43μg·L- 1 ) ,生长率为 0 .36- 0 .38d- 1 。添加 1 0nM铁的培养瓶中Chla浓度增长缓慢 ,在第 2 3d达到最高 ( 1 9.35 μg·L- 1 ) ,生长率为 0 .2 7d- 1 。在对照瓶和添加 5nM铁的培养瓶中 ,Chla浓度的生长率分别为 0 .1 3和0 .1 6d- 1 。在实验 2中 ,对照瓶和添加 1 0和 40nM铁的培养瓶中的Chla浓度的生长率分别为0 .1 3,0 .32和 0 .40d- 1 。在实验 3,对照瓶和添加铁的培养瓶中  相似文献   

6.
《Basin Research》2018,30(1):5-19
The Chatham Rise is located offshore of New Zealand's South Island. Vast areas of the Chatham Rise are covered in circular to elliptical seafloor depressions that appear to be forming through a bathymetrically controlled mechanism, as seafloor depressions 2–5 km in diameter are found in water depths of 800–1100 m. High‐resolution P‐Cable 3D seismic data were acquired in 2013 across one of these depressions. The seafloor depression is interpreted as a mounded contourite. Our data reveal several smaller buried depressions (<20–650 m diameter) beneath the mounded contourite that we interpret as paleo‐pockmarks. These pockmarks are underlain by a complex polygonal fault system that deforms the strata and an unusual conical feature results. We interpret the conical feature as a sediment remobilization structure based on the presence of stratified reflections within the feature, RMS amplitude values and lack of velocity anomaly that would indicate a nonsedimentary origin. The sediment remobilization structure, polygonal faults and paleo‐depressions are the indicators of the past subsurface fluid flow. We hypothesize that the pockmarks provided the necessary topographic roughness for the formation of the mounded contourites thus linking fluid expulsion and the deposition of contouritic drifts.  相似文献   

7.
A decade of widespread increases in surface water concentrations of total organic carbon (TOC) in some regions has raised questions about longer term patterns in this important constituent of water chemistry. This study uses near-infrared spectroscopy (NIRS) to infer lake water TOC far beyond the decade or two of observational data generally available. An expanded calibration dataset of 140 lakes across Sweden covering a TOC gradient from 0.7 to 24.7 mg L−1 was used to establish a relationship between the NIRS signal from surface sediments (0–0.5 cm) and the TOC concentration of the water mass. Internal cross-validation of the model resulted in an R 2 of 0.72 with a root mean squared error of calibration (RMSECV) of 2.6 mg L−1. The TOC concentrations reconstructed from surface sediments in four Swedish lakes were typically within the range of concentrations observed in the monitoring data during the period represented by each sediment layer. TOC reconstructions from the full sediment cores of four lakes indicated that TOC concentrations were approximately twice as high a century ago.  相似文献   

8.
Atmospheric deposition of reactive nitrogen (Nr) has enriched oligotrophic lakes with nitrogen (N) in many regions of the world and elicited dramatic changes in diatom community structure. The lakewater concentrations of nitrate that cause these community changes remain unclear, raising interest in the development of diatom-based transfer functions to infer nitrate. We developed a diatom calibration set using surface sediment samples from 46 high-elevation lakes across the Rocky Mountains of the western US, a region spanning an N deposition gradient from very low to moderate levels (<1 to 3.2 kg Nr ha−1 year−1 in wet deposition). Out of the fourteen measured environmental variables for these 46 lakes, ordination analysis identified that nitrate, specific conductance, total phosphorus, and hypolimnetic water temperature were related to diatom distributions. A transfer function was developed for nitrate and applied to a sedimentary diatom profile from Heart Lake in the central Rockies. The model coefficient of determination (bootstrapping validation) of 0.61 suggested potential for diatom-inferred reconstructions of lakewater nitrate concentrations over time, but a comparison of observed versus diatom-inferred nitrate values revealed the poor performance of this model at low nitrate concentrations. Resource physiology experiments revealed that nitrogen requirements of two key taxa were opposite to nitrate optima defined in the transfer function. Our data set reveals two underlying ecological constraints that impede the development of nitrate transfer functions in oligotrophic lakes: (1) even in lakes with nitrate concentrations below quantification (<1 μg L−1), diatom assemblages were already dominated by species indicative of moderate N enrichment; (2) N-limited oligotrophic lakes switch to P limitation after receiving only modest inputs of reactive N, shifting the controls on diatom species changes along the length of the nitrate gradient. These constraints suggest that quantitative inferences of nitrate from diatom assemblages will likely require experimental approaches.  相似文献   

9.
The spatial distribution of Calanus species was examined near Franz Josef Land archipelago in August 2006 and 2007. Surface and bottom water temperatures exceeded the average multiannual values. Calanus species dominated the total mesozooplankton abundance and biomass, accounting for 818 ± 178 individuals m−3 (mean ± SE) and 803 ± 163 mg wet weight (WW) m−3, respectively, in 2006. In 2007, the values were much lower (153 ± 29 individuals m−3 and 192 ± 17 mg WW m−3, respectively), reflecting the weaker influence of warm Atlantic water that year. Calanus glacialis dominated the Calanus populations, contributing 95% and 60% of the biomass in 2006 and 2007, respectively. Older copepodite stages (CIV–CV) predominated in the C. finmarchicus (69% and 76%, respectively) and C. hyperboreus populations (80% and 77%, respectively), whereas young copepodites (CI–CIII) were predominant in C. glacialis (45% and 59%, respectively). A clear negative relationship between the average water temperature and the total Calanus biomass was observed in 2006, whereas a positive correlation between these parameters was observed in 2007. The distributions of three Calanus species off Franz Josef Land were mainly associated with hydrological conditions and circulation patterns.  相似文献   

10.
The flux of biologically-effective ultraviolet radiation (UV) is diminished by atmospheric absorption which may cause physiological and morphological phenotypic differences among plant populations at different elevations. We examined UV-screening effectiveness of leaves of Artemisia tridentata ssp wyomingensis (Wyoming big sagebrush) along an 800-m elevation gradient in North-central Wyoming with a pulse-amplitude modulated fluorometer. Adaxial epidermal transmittance of UV increased at lower elevations; means ranged from 2.3% (high elevation) to 10.2% (low elevation). To provide a proximate explanation for this relationship, we collected leaves across the gradient and estimated concentrations of bulk-soluble UV-absorbing compounds (λ = 300 and 365 nm) and the density of trichomes on the adaxial surface. Concentrations of UV-absorbing compounds decreased with elevation and ranged from 0.64–2.25 A300 cm−2 and 0.43–1.35 A365 cm−2. Trichome density increased from 14,400 cm−2 at low elevation to 22,500 cm−2 at high elevation. Because the distance along the elevation gradient was only 18 km, gene flow likely prevents ecotypic differentiation; the ultimate cause of the ecocline in screening effectiveness is likely the evolution of environmentally-induced phenotypic plasticity in both biochemical and anatomical properties of leaves in response to variation in UV irradiance.  相似文献   

11.
Central Eurasia underwent significant palaeoclimatic and palaeogeographic transformations during the middle to late Miocene. The open marine ecosystems of the Langhian and Serravallian seas progressively collapsed and were replaced in the Tortonian by large endorheic lakes. These lakes experienced major fluctuations in water level, directly reflecting the palaeoclimatic conditions of the region. An extreme lowstand of the Eastern Paratethys lake (?300 m) during the regional Khersonian stage reveals a period of intensely dry conditions in Central Eurasia causing a fragmentation of the Paratethys region. This period of “Great Drying” ended by a climate change towards more humid conditions at the base of the Maeotian stage, resulting in a large transgressive event that reconnected most of the Paratethyan basins. The absence of a robust time frame for the Khersonian–Maeotian interval hampers a direct correlation with the global records and complicates a thorough understanding of the underlying mechanisms. Here we present a new chronostratigraphic framework for the Khersonian and Maeotian deposits of the Dacian Basin of Romania, based on integrated magneto‐biostratigraphic studies on long and continuous sedimentary successions. We show the dry climate conditions in the Khersonian start at 8.6–8.4 Ma. The Khersonian/Maeotian transition is dated at 7.65–7.5 Ma, several million years younger than previous estimates. The Maeotian transgression occurs later (7.5–7.4 Ma) in more marginal and shallower basins, in agreement with the time transgressive character of the flooding. In addition, we date a sudden water level drop of the Eastern Paratethys lake, the Intra‐Maeotian Event (IME), at 6.9 Ma, and hypothesize that this corresponds to a reconnection phase with the Aegean basin of the Mediterranean. Finally, we discuss the potential mechanisms explaining the particularities of the Maeotian transgression and conclude that the low salinity and the seemingly “marine influxes” most likely correspond to episodes of intrabasinal mixing in a gradual and pulsating transgressive setting.  相似文献   

12.
Stratigraphic data from petroleum wells and seismic reflection analysis reveal two distinct episodes of subsidence in the southern New Caledonia Trough and deep‐water Taranaki Basin. Tectonic subsidence of ~2.5 km was related to Cretaceous rift faulting and post‐rift thermal subsidence, and ~1.5 km of anomalous passive tectonic subsidence occurred during Cenozoic time. Pure‐shear stretching by factors of up to 2 is estimated for the first phase of subsidence from the exponential decay of post‐rift subsidence. The second subsidence event occured ~40 Ma after rifting ceased, and was not associated with faulting in the upper crust. Eocene subsidence patterns indicate northward tilting of the basin, followed by rapid regional subsidence during the Oligocene and Early Miocene. The resulting basin is 300–500 km wide and over 2000 km long, includes part of Taranaki Basin, and is not easily explained by any classic model of lithosphere deformation or cooling. The spatial scale of the basin, paucity of Cenozoic crustal faulting, and magnitudes of subsidence suggest a regional process that acted from below, probably originating within the upper mantle. This process was likely associated with inception of nearby Australia‐Pacific plate convergence, which ultimately formed the Tonga‐Kermadec subduction zone. Our study demonstrates that shallow‐water environments persisted for longer and their associated sedimentary sequences are hence thicker than would be predicted by any rift basin model that produces such large values of subsidence and an equivalent water depth. We suggest that convective processes within the upper mantle can influence the sedimentary facies distribution and thermal architecture of deep‐water basins, and that not all deep‐water basins are simply the evolved products of the same processes that produce shallow‐water sedimentary basins. This may be particularly true during the inception of subduction zones, and we suggest the term ‘prearc’ basin to describe this tectonic setting.  相似文献   

13.
Organic arsenical herbicides, which include monosodium methylarsonate (MSMA), have been applied to golf courses and lawns throughout Florida, USA, since the 1950s. These products convert rapidly to inorganic forms of arsenic (As) in soils and are mobilized readily. Leachates have been known to contaminate groundwater and surface waters, although past studies have not examined whether use of these products has led to significant As accumulation in lake sediments. We used paleolimnological methods to document the depositional history and inventories of total As in sediments and porewaters of Little Lake Jackson in Florida, which is adjacent to three golf courses. Six sediment cores, four of which were 210Pb dated, showed porewater total As concentrations as high as 435 μg l−1, and dry-sediment total As concentrations as high as 148 mg kg−1. Approximately 537 kg of total As is present in >19,000 metric tons of sediment (dry mass), and an additional 18 kg of As is dissolved in 10.8 × 104 m3 of porewaters. Total As content in surface sediments (mean = 47.3 mg kg−1) exceeds the consensus-based sedimentary concentration for probable toxicity effects in freshwater benthic fauna. Surface and subsurface waters flow to the lake from topographically higher areas to the west, where golf courses and residential areas are located. Total As concentrations were elevated highly in monitoring wells and in a stream that flows between the golf courses and lake, but As was below detection limits in wells that were located at the distal perimeter of the golf courses. Subsurface and surface waters exit the lake towards topographically lower areas to the east. Nearly all As in sediments remains bound in the solid phase, indicating that As sedimentary profiles largely reflect depositional history. Sedimentary As concentrations are correlated strongly with aluminum and iron, which suggests that As was scavenged from lake waters during the past. Sedimentary As concentrations increased until the 1980s, then declined somewhat to the present time. Dissolved As was scavenged efficiently from the water column when hypolimnetic waters were oxygenated persistently, but after eutrophication led to a seasonally anoxic hypolimnion in the 1980s, apparently less As was co-precipitated, and more was lost to hydrological outflow. Arsenic accumulation in sediments might be common in areas where As derived from organic arsenical herbicide applications is directed by shallow water tables towards adjacent lakes.  相似文献   

14.
The central and southern Perth Basin in southwestern Australia has a geological history involving multiple regional unconformity‐forming events from the Permian to Recent. This study uses sonic transit time analysis to quantify the magnitudes of net and gross exhumation for four stratigraphic periods from 43 wells. Most importantly, we quantify gross exhumation of the Permian–Triassic, Triassic–Jurassic, Valanginian break‐up and post‐Early Cretaceous events. Post‐Early Cretaceous gross exhumation averages 900‐m offshore and 600‐m onshore. Up to 200 m of this exhumation may be attributed to localized fault block rotation during extension in the Late Cretaceous and/or reverse fault re‐activation due to the compressive stresses in Australia in the last 50 Ma. The remainder is attributed to regional exhumation caused by epeirogenic processes either during the Cenozoic or at the Aptian–Albian boundary. Maximum burial depths prior to the Valanginian unconformity‐forming event were less than those reached subsequently, so that the magnitude of Valanginian break‐up exhumation cannot be accurately quantified. Gross exhumation prior to the break‐up of Gondwana was defined by large magnitude differences (up to 2500 m) between adjoining sub‐basins. At the end of Triassic, exhumation is primarily attributed to reverse re‐activation of faults that were driven by short‐wavelength inversion and exhumation at the end Permian is likely caused by uplift of rotated fault blocks during extension. The evidence from quantitative exhumation analysis indicates a switch in regime, from locally heterogeneous before break‐up to more regionally homogeneous after break‐up.  相似文献   

15.
《Basin Research》2018,30(3):426-447
Integration of detrital zircon geochronology and three‐dimensional (3D) seismic‐reflection data from the Molasse basin of Austria yields new insight into Oligocene‐early Miocene palaeogeography and patterns of sediment routing within the Alpine foreland of central Europe. Three‐dimensional seismic‐reflection data show a network of deep‐water tributaries and a long‐lived (>8 Ma) foredeep‐axial channel belt that transported Alpine detritus greater than 100 km from west to east. We present 793 new detrital zircon ages from 10 sandstone samples collected from subsurface cores located within the seismically mapped network of deep‐water tributaries and the axial channel belt. Grain age populations correspond with major pre‐Alpine orogenic cycles: the Cadomian (750–530 Ma), the Caledonian (490–380 Ma) and the Variscan (350–250 Ma). Additional age populations correspond with Eocene‐Oligocene Periadriatic magmatism (40–30 Ma) and pre‐Alpine, Precambrian sources (>750 Ma). Although many samples share the same age populations, the abundances of these populations vary significantly. Sediment that entered the deep‐water axial channel belt from the west (Freshwater Molasse) and southwest (Inntal fault zone) is characterized by statistically indistinguishable age distributions that include populations of Variscan, Caledonian and Cadomian zircon at modest abundances (15–32% each). Sandstone from a shallow marine unit proximal to the northern basin margin consists of >75% Variscan (350–300 Ma) zircon, which originated from the adjacent Bohemian Massif. Mixing calculations based on the Kolmogorov–Smirnoff statistic suggest that the Alpine fold‐thrust belt south of the foreland was also an important source of detritus to the deep‐water Molasse basin. We interpret evolving detrital zircon age distributions within the axial foredeep to reflect a progressive increase in longitudinal sediment input from the west (Freshwater Molasse) and/or southwest (Inntal fault zone) relative to transverse sediment input from the fold‐thrust belt to the south. We infer that these changes reflect a major reorganization of catchment boundaries and denudation rates in the Alpine Orogen that resulted in the Alpine foreland evolving to dominantly longitudinal sediment dispersal. This change was most notably marked by the development of a submarine canyon during deposition of the Upper Puchkirchen Formation that promoted sediment bypass eastward from Freshwater Molasse depozones to the Molasse basin deep‐water axial channel belt. The integration of 3D seismic‐reflection data with detrital zircon geochronology illustrates sediment dispersal patterns within a continental‐scale orogen, with implications for the relative role of longitudinal vs. transverse sediment delivery in peripheral foreland basins.  相似文献   

16.
Calculated indicators are suggested, which make it possible to assess the size of maritime water areas with a different relationship between streamflow and aerial depositions (dissolved forms of nitrogen, phosphorus, Fe, Mn, Zn, Cu, Pb, and Cd). The role of streamflow decreases for the Fe, Mn, P > Cu, N > Zn > Pb > Pd series. Furthermore, the aerial input of Pb and Cd is dominant throughout most of all the stretches of water used in the study. By comparing the resulting assessments with the size of actual water areas, it is possible to carry out a zoning of the offshore waters as regards the intensity of supply and the contributors of matter from land surface.  相似文献   

17.
《Polar Science》2014,8(4):370-384
An anomalous phytoplankton bloom was recorded in the Indian Ocean sector of the Antarctic Zone (AZ) of the Southern Ocean (SO) during the austral summer, 2011. Possible mechanisms for the triggering of such a large bloom were analyzed with the help of in situ and satellite data. The bloom, which formed in January 2011, intensified during February and weakened by March. High surface chlorophyll (Chl) concentrations (0.76 mg m−3) were observed in the area of the bloom (60°S, 47°E) with a Deep Chlorophyll Maximum (DCM) of 1.15 mg m−3 at a depth of 40–60 m. During 2011, both the concentration and spatial extent of sea ice were high on the western side of the bloom, between 0°E and 40°E, and enhanced freshwater influx was observed in the study area as a result of melting ice. A positive Southern Annular Mode (SAM) (with a resultant northward horizontal advection) and an intense La Niña during 2010–2011 are possible reasons for the high sea-ice concentrations. The enhanced Chl a observed in the study region, which can be attributed to the phytoplankton bloom, likely resulted from the influx of nutrient-laden freshwater derived from melting sea ice.  相似文献   

18.
Sediment supplied by continental sources is commonly suspected to have exerted a strong influence on the development of canyons and other morphological features on the continental slopes, but rarely is the sediment supply known sufficiently quantitatively to test this link. Here, we outline an area where offshore morphology, in the western Ionian Sea, may be linked to estimated sediment fluxes produced by subaerial erosion in NE Sicily and SW Calabria. Shelves in this area are narrow (<1 km), and the bathymetry shows that rivers and adjacent submarine channels are almost directly connected with each other. Integrated topographic analyses were performed on a merged digital elevation model (DEM) of ASTER data for subaerial topography and multibeam sonar data for submarine bathymetry. Spatial variations in sediment fluxes from onshore erosion were assessed using a variety of methods, namely: long‐term sediment flux from Pleistocene uplift rates, decadal sediment flux from landslide occurrences and published long‐term exhumation rates from 10Be cosmogenic nuclide concentrations. Submarine channels associated with rivers delivering larger sediment fluxes have broad channels, high relief and smooth concave‐upward longitudinal profiles. Conversely, submarine channels that lie offshore small‐flux rivers have straight longitudinal profiles, low relief and steep gradients. Where river catchments supply a greater sediment flux offshore, shelves tend to be wider (ca. 400 m) and submarine channels have gentler gradients. In contrast, where catchments supply less sediment flux, shelves are narrow (250–300 m) and offshore channel gradients are steeper. The variation of submarine morphology with tectonic uplift rate was also studied, but we find that, unlike onshore terrains where tectonics is commonly an important factor influencing channel morphology, in the submarine landscapes, sediment flux appears to dominate here.  相似文献   

19.
Fine‐grained Palaeogene–early Neogene strata of the South Caspian basin, specifically the Oligocene–Lower Miocene Maikop Series, are responsible for the bulk of hydrocarbon generation in the region. Despite the magnitude of oil and gas currently attributed to the source interval offshore, geochemical evaluation of 376 outcrop samples from the northern edge of the Kura basin (onshore eastern Azerbaijan) indicates that depositional conditions in these proximal strata along the basin margins were dominantly oxic to mildly suboxic/anoxic throughout three major depositional stages: the Palaeocene–Eocene, Oligocene–early Middle Miocene and late Middle–Late Miocene. Palaeocene–Eocene samples have low average total organic carbon (TOC) values (0.3%), with higher total inorganic carbon (TIC) values (average=2.6%), extremely low sulphur content (0.2%) and relatively high detrital input as indicated by Fe/Al and Ti/Al ratios. C–S–Fe associations, along with relatively lower concentrations of redox‐sensitive trace elements (e.g. V, Ni, Mo, U) indicate dominantly oxic environments of deposition during much of the Palaeocene–Eocene. A pronounced geochemical shift occurred near the Eocene–Oligocene boundary, and continued through the Early Miocene. Specifically, this interval is characterized by a distinct increase in TOC (ranging from 0.1 to 6.3% with an average of 1.5%), C–S–Fe associations that reveal an abrupt relative increase of carbon and sulphur with respect to iron‐dominated Palaeocene–Eocene samples, and higher concentrations of redox‐sensitive trace metals. These changes suggest that a shift away from unrestricted marine conditions and towards more variable salinity conditions occurred coincident with the initial collision of the Arabian plate and partial closure of the Paratethys ocean. Despite periodic basin restriction, the majority of Upper Eocene–Lower Miocene strata in the northern Kura basin record oxic to slightly dysoxic conditions.  相似文献   

20.
《Basin Research》2018,30(Z1):65-88
Mass wasting is an important process in the degradation of deep‐water fold‐and‐thrust belts. However, the relationship between mass‐transport complex (MTC) emplacement and the timing and spatial progression of contractional deformation of the seabed have not been extensively studied. This study uses high‐quality, 3D seismic reflection data from the southern Magdalena Fan, offshore Colombia to investigate how the growth of a deep‐water fold‐and‐thrust belt (the southern Sinú fold belt) is recorded in the source, distribution and size of MTCs. More than nine distinct, but coalesced MTCs overlie a major composite basal erosion surface. This surface formed by multiple syn‐ and post‐tectonic mass‐wasting events and is thus highly diachronous, thereby recording a protracted period of tectonism, seascape degradation and associated sedimentation. The size and source location of these MTCs changed through time: the oldest ‘detached’ MTCs are relatively small (over 9–100 km2 in area) and sourced from the flanks of growing anticlines, whereas the younger ‘shelf‐attached’ MTCs are considerably larger (more than 200–300 km2), are sourced from the shelf, and post‐date the main phase of active folding and thrusting. Changes in the source, distribution and size of MTCs are tied to the sequential nucleation, amplification and along‐strike propagation of individual structures, showing that MTCs can be used to constrain the timing and style of contractional deformation, and seascape evolution in time and space.  相似文献   

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