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1.
We present a quantification of total and partial (divided by time slices) sedimentary volumes in the Neogene basins of the Betic-Rif orogen. These basins include the Alboran Sea, the intramontane basins, the Guadalquivir and Rharb foreland basins and the Atlantic Margin of the Gibraltar Arc. The total volume of Neogene sediments deposited in these basins is ~ 209,000 km3 and is equally distributed between the internal (Alboran Basin and intramontane basins) and the external basins (foreland basins and Atlantic Margin). The largest volumes are recorded by the Alboran Basin (89,600 km3) and the Atlantic Margin (81,600 km3). The Guadalquivir and Rharb basins amount 14,000 km3 and 14,550 km3, respectively whereas the intramontane basins record 9235 km3. Calculated mean sediment accumulation rates for the early-middle Miocene show an outstanding asymmetry between the Alboran basin (0.24 mm/yr) and the foreland basins (0.06-0.07 mm/yr) and the Atlantic Margin (0.03 mm/yr). During the late Miocene, sedimentation rates range between 0.17 and 0.18 mm/yr recorded in the Alboran Basin and 0.04 mm/yr in the intramontane basins. In the Pliocene-Quaternary, the highest sedimentation rates are recorded in the Atlantic Margin reaching 0.22 mm/yr. Sedimentary contribution shows similar values for the inner and outer basins with a generalized increase from late Miocene to present (from 3500 to 6500 km3/My). Interestingly, the Alboran Basin records the maximum sedimentary contribution during the late Miocene (5500 km3/My), whereas the Atlantic Margin does during the Pliocene-Quaternary (6600 km3/My). The spatial and time variability of the sediment supply from the Betic-Rif orogen to basins is closely related to the morphotectonic evolution of the region. The high sedimentation rates obtained in the Alboran Basin during the early-middle Miocene are related to active extensional tectonics, which produced narrow and deep basins in its western domain. The highest sedimentary contribution in this basin, as well as in the foreland and intramontane basins, is recorded during the late Miocene due to the uplift of wide areas of the Betics and Rif chains. The analysis of the sedimentary supply also evidences strong relationships with the post-Tortonian crustal thickening and coeval topographic amplification that occurred in the central Betics and Rif with the concomitant evolution of the drainage network showing the fluvial capture of some internal basins by rivers draining to the Atlantic Ocean (the ancestral Guadalquivir).  相似文献   

2.
The supply and accumulation of silica in the marine environment   总被引：4，自引：0，他引：4  
Rivers and submarine hydrothermal emanations supply 6.1 × 1014g SiO2/yr to the marine environment. Approximately two-thirds of the silica supplied to the marine environment can be accounted for in continental margin and deep-sea deposits. Siliceous deep-sea sediments located beneath the Antarctic Polar Front (Convergence) account for over a fourth (1.6 × 1014g SiO2/yr) of the silica supplied to the oceans. Deep-sea sediment accumulation rates beneath the Polar Front are highest in the South Atlantic with values as large as 53cm/kyr during the last 18.000 yr. Siliceous sediments in the Bering Sea, Sea of Okhotsk, and Subarctic North Pacific accumulate 0.6 × 1014g SiO2/yr or 10% of the dissolved silica input to the oceans. The accumulation of biogenic silica in estuarine deposits removes a maximum of 0.8 × 1014g SiO2/yr. Although estuarine silica versus salinity plots indicate extensive removal of riverine silica from surface waters, the removal rates must be considered as maximum values because of dissolution of siliceous material in estuarine sediments and bottom waters. Siliceous sediments from continental margin upwelling areas (e.g. Gulf of California, Walvis Bay, or Peru-Chile coast) have the highest biogenic silica accumulation rates in the marine environment (69 g SiO2 cm2/kyr). Despite the rapid accumulation of biogenic silica, upwelling areas account for less than 5% of the silica supplied to the marine environment because they are confined laterally to such small areas.  相似文献   

3.
A spatio-temporal analysis based on the data of eleven repeated levellings around the Tangshan region prior to the 1976 earthquake indicates that an uplift lasting for 2 years, from 1968 through 1969. with a magnitude of 50 mm, occurred in the epicentral area.Aseismic creep superimposed on the accumulated strain has been found in the vicinity of Tangshan and Baodi along both the Tangshan and the Jiyunhe faults.Assuming uniform strain accumulation and elastic dislocation, theoretical values of displacement at the various dislocation sites have been calculated and, using the least squares method, the optimal values of strain accumulation and the parameters of the creep faults in different years have been determined.The creep fault under Tangshan, a right-lateral normal fault, strikes N47°E and dips S87°E. and is 8 km long and 6 km wide. The upper boundary of the fault lies 2 km deep. The strike-slip and dip-slip offsets are, respectively, 104 cm and 8cm. The average rate of strain accumulation amounts to 0.9 × 10−7/yr. Creep at the fault amounted to 18.6 cm/yr and 1.4 cm/yr, respectively, in the strike and dip directions over the period 1969–1975. The Jiyunhe fault, although of smaller dimensions, has experienced a greater rate of creep than the Tangshan fault.A correlation of the above-mentioned uplift and creep with that of the Tangshan earthquake suggests that the uplift might have been a manifestation of the early development of the earthquake and that aseismic creep may be one of the precursory phenomena of shallow earthquakes. The sequence of processes preceding the Tangshan earthquake may be described as: strain accumulation-land upliftaseismic creep-inverse land deformation (or decrease in creep rate)-earthquake.  相似文献   

4.
We measured 10Be and 26Al in 29 sediment samples to infer the history and millennial-scale rates of change down a low-gradient piedmont, a common but enigmatic landform that dominates the Mojave Desert. Nuclide data suggest that a large volume of sediment was deposited on the proximal East Range Road piedmont in Fort Irwin, California, ∼ 75,500 yr ago. Since then, this material has been stable or eroding slowly. In contrast, on the distal piedmont (3.5 km from the upland source basins) soil stratigraphy suggests that there have been alternating periods of surface stability, erosion, and deposition over the last 70,000 yr. Nuclide data from samples amalgamated along cross-piedmont transects suggest that long-term average down-gradient sediment speeds range from 9 cm yr− 1 near the uplands to 22 cm yr− 1 6 km down-piedmont. These speeds are similar to 10Be-estimated sediment speeds down three other piedmonts in the Mojave Desert, suggesting that piedmont surface morphologies dominated by shallow migrating channels have similar sediment transport rates. The timing of surface process change down the East Range Road piedmont is determined by a combination of sediment available in the source basins, sediment transport rates, and the size of the piedmont.  相似文献   

5.
Geological and seismic profiling data (more than 25000 km of seismic profiles and about 1000 sediment sampling stations) collected during the last 30 yr by research vessels of the Shirshov Institute of Oceanology, Russian Academy of Sciences are summarized. Seismic records are directly correlated with sediment cores. The distribution map (scale 1 : 500000) of Quaternary lithofacial complexes corresponding to certain stages of the Baltic Sea evolution is compiled. The following four complexes are distinguished (from the base to the top): (I) moraine, with maximum thicknesses 60 and 170 m in valleys and ridges respectively: (II) varved clay of periglacial basins and from the Baltic Ice Lake (BIL), up to 25 m thick in depressions; (III) lacustrinemarine homogeneous clay with a thickness up to 4–8 m in depressions; (IV) marine sediments (mud, aleurite, coarse-grained deposits) accumulated in environments with intense bottom currents activity (thickness 2–4 m in the Gotland Basin, 4–6 m in the Gdansk Basin, and 10–20 m in fans and prodeltas). The Quaternary sequence is cut through by inherited valleys, where the thickest Holocene sediments are noted. Today, these valleys serve as routes of sediment transport to slope bases and central parts of basins. Outblows of deep gas (through faults and fractures) and diagenetic gas (from sediments) to the bottom surface also occur in the valleys. Sedimentation rates are higher in the Gdansk Basin (up to 100–120 cm/ka). Thick sand, aleurite, and mud bodies are accumulated here (about 15–20 m in the Visla River prodelta). The sedimentation rate is slower in the Gotland Basin (up to 50–60 cm/ka), where thin (2–4 m) sections of more fine-grained mud occur  相似文献   

6.
Many small estuaries are influenced by flow restrictions resulting from transportation rights-of-way and other causes. The biogeochemical functioning and history of such systems can be evaluated through study of their sediments. Ten long and six short cores were collected from the length of Jordan Cove, Connecticut, a Long Island Sound subestuary, and analyzed for stratigraphy, radionuclides (14C, 210Pb, 226Ra, 137Cs, and 60Co), and metals (Ag, Cd, Cu, Pb, Zn, Fe, and Al). For at least 3,800 yr, rising sea level has gradually inundated Jordan Cove, filling it with mud similar to that currently being deposited there. Long-term sediment accumulation in the cove averaged close to 0.1 cm yr−1 over the last three millennia. Recent sediment accumulation rates decrease inland from 0.84 cm yr−1 to 0.40 cm yr−1, and are slightly faster than relative sea-level rise at this site (0.3 cm yr−1). Similarity of depth distributions of trace metals was used to confirm relative sediment accumulation rates. 60Co and Ag are derived from sources outside the cove and its watershed, presumably the Millstone nuclear power plant and regional contaminated sediments, respectively. The combined data suggest that Long Island Sound is an important source of sediment to the cove; a minor part of total sediment is supplied from the local watershed. Trace metal levels are strongly correlated with Fe but not with either organic matter or Al. Sediment quality has declined in the cove over the past 60 yr, but only slightly. Cu, Pb, and Zn data correlate strongly with Fe but not with either organic matter or aluminum. Ratios of Ag to Fe and to trace metals suggest that Ag in the cove is derived almost entirely from Long Island Sound. This result supports the notion that Fenormalized Ag can serve as a better tracer of some kinds of contamination than more common and abundant metals, like Cu, Pb, and Zn. *** DIRECT SUPPORT *** A01BY085 00008  相似文献   

7.
Water quality and criculation in Florida Bay (a shallow, subtropical estuary in south Florida) are highly dependent upon the development and evolution of carbonate mud banks distributed throughout the Bay. Predicting the effect of natural and anthropogenic perturbations on carbonate sedimentation requires an understanding of annual, seasonal, and daily variations in the biogenic and inorganic processes affecting carbonate sediment precipitation and dissolution. In this study, net calcification rates were measured over diurnal cycles on 27 d during summer and winter from 1999 to 2003 on mud banks and four representative substrate types located within basins between mud banks. Substrate types that were measured in basins include seagrass beds of sparse and intermediate densityThalassia sp., mud bottom, and hard bottom communities. Changes in total alkalinity were used as a proxy for calcification and dissolution. On 22 d (81%), diurnal variation in rates of net calcification was observed. The highest rates of net carbonate sediment production (or lowest rates of net dissolution) generally occurred during daylight hours and ranged from 2.900 to −0.410 g CaCO3 m−2d−1. The lowest rates of carbonate sediment production (or net sediment dissolution) occurred at night and ranged from 0.210 to −1.900 g CaCO3 m−2 night−1. During typical diurnal cycles, dissolution during the night consumed an average of 29% of sediment produced during the day on banks and 68% of sediment produced during the day in basins. Net sediment dissolution also occurred during daylight, but only when there was total cloud cover, high turbidity, or hypersalinity. Diurnal variation in calcification and dissolution in surface waters and surface sediments of Florida Bay is linked to cycling of carbon dioxide through photosynthesis and respiration. Estimation of long-term sediment accumulation rates from diurnal rates of carbonate sediment production measured in this study indicates an overall average accumulation rate for Florida Bay of 8.7 cm 1000 yr−1 and suggests that sediment dissolution plays a more important role than sediment transport in loss of sediment from Florida Bay.  相似文献   

8.
Sediment core samples from the center of Lake Kasumigaura, Japan, were collected from 1979 to 2007 at intervals of 1 month to 5 yr. We evaluated the degree of modification in N isotope composition during sedimentation and diagenesis. We estimated the degree of isotope discrimination during diagenesis by comparing historical changes in N isotope composition of the surface sediment (top 2 cm) against the vertical profile of the isotope composition of sediment core samples (15 cm depth). The degree of 15N enrichment during sedimentation appeared to be significant under the preferential N decomposition that occurred in the periods with low C/N ratio values of suspended particulate organic matter. We documented 15N depletion in sediment deeper than approximately 3 cm during diagenesis. The contrasting directions of N isotope discrimination during sedimentation and diagenesis suggest changing mechanisms of isotopic shift across an oxidation-reduction boundary.  相似文献   

9.
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10.
Kinetics of microbial sulfate reduction in estuarine sediments   总被引：2，自引：0，他引：2  
Kinetic parameters of microbial sulfate reduction in intertidal sediments from a freshwater, brackish and marine site of the Scheldt estuary (Belgium, the Netherlands) were determined. Sulfate reduction rates (SRR) were measured at 10, 21, and 30 °C, using both flow-through reactors containing intact sediment slices and conventional sediment slurries. At the three sites, and for all depth intervals studied (0-2, 2-4, 4-6 and 6-8 cm), the dependence of potential SRR on the sulfate concentration followed the Michaelis-Menten rate equation. Apparent sulfate half-saturation concentrations, Km, measured in the flow-through reactor experiments were comparable at the freshwater and marine sites (0.1-0.3 mM), but somewhat higher at the brackish site (0.4-0.9 mM). Maximum potential SRR, Rmax, in the 0-4 cm depth interval of the freshwater sediments were similar to those in the 0-6 cm interval of the marine sediments (10-46 nmol cm−3 h−1 at 21 °C), despite much lower in situ sulfate availability and order-of-magnitude lower densities of sulfate-reducing bacteria (SRB), at the freshwater site. Values of Rmax in the brackish sediments were lower (3.7-7.6 nmol cm−3 h−1 at 21 °C), probably due to less labile organic matter, as inferred from higher Corg/N ratios. Inflow solutions supplemented with lactate enhanced potential SRR at all three sites. Slurry incubations systematically yielded higher Rmax values than flow-through reactor experiments for the freshwater and brackish sediments, but similar values for the marine sediments. Transport limitation of potential SRR at the freshwater and brackish sites may be related to the lower sediment porosities and SRB densities compared to the marine site. Multiple rate controls, including sulfate availability, organic matter quality, temperature, and SRB abundance, modulate in situ sulfate-reducing activity along the estuarine salinity gradient.  相似文献   

11.
Owing to the hypercontinental location of Western Nubia, secular fluctuations of climate have been filtered and wet phases can be considered as representative of conditions throughout the southeastern Sahara. The study area is crossed by the 20-mm isohyet; between 9300 and about 4000 yr B.P., however, there were widespread lake and swamp environments with freshwater molluscs, ostracods, and diatoms, and a species-rich savanna mammal fauna. The center of the West Nubian Basin (approx. 18°N), an area of about 20,000 km2, was occupied by a semiaquatic landscape which was situated at the same latitude as Paleolake Chad. From extensive lake carbonates up to about 4 m thick, a long-term rise of the groudwater table is inferred. Environments developed that now exist at about latitude 13°N. Radiocarbon dates from lake sediment sequences cluster between 30,000 and 21,000 yr B.P., indicating a Pleistocene wet phase. A gap in radiocarbon dates between 21,000 and 11,000 yr B.P. signals a phase of hyperaridity, similar to the present hyperarid phase, with eolian deflation and deposits of sand being the dominant forms of erosion and accumulation.  相似文献   

12.
Sediments from Rapid Lake document glacial and vegetation history in the Temple Lake valley of the Wind River Range, Wyoming over the past 11,000 to 12,000 yr. Radiocarbon age determinations on basal detrital organic matter from Rapid Lake (11,770 ± 710 yr B.P.) and Temple Lake (11,400 ± 630 yr B.P.) bracket the age of the Temple Lake moraine, suggesting that the moraine formed in the late Pleistocene. This terminal Pleistocene readvance may be represented at lower elevations by the expansion of forest into intermontane basins 12,000 to 10,000 yr B.P. Vegetation in the Wind River Range responded to changing environmental conditions at the end of the Pleistocene. Following deglaciation, alpine tundra in the Temple Lake valley was replaced by a Pinus albicaulis parkland by about 11,300 14C yr B.P. Picea and Abies, established by 10,600 14C yr B.P., grew with Pinus albicaulis in a mixed conifer forest at and up to 100 m above Rapid Lake for most of the Holocene. Middle Holocene summer temperatures were about 1.5°C warmer than today. By about 5400 14C yr B.P. Pinus albicaulis and Abies became less prominent at upper treeline because of decreased winter snowpack and higher maximum summer temperatures. The position of the modern treeline was established by 3000 14 C yr B.P. when Picea retreated downslope in response to Neoglacial cooling.  相似文献   

13.
Recent sediment accumulation rates were measured in Moon Lake, a large (10.1 km2) Mississippi River oxbow lake in northwestern Mississippi. Moon Lake, which receives channeled inflow from an intensively cultivated soybean, rice, and cotton watershed (166 km2) and limited overland flow from surrounding lands, exhibited depositional patterns that were associated with (1) points of inflow, (2) flow patterns, and (3) lake morphology. From 1954 to 1965, 70 percent of the lake bottom experienced accumulation rates greater than 2 cm/yr. Accumulation rates exceeded 4 cm/yr in areas of delta formation. Changes in cropping systems during the 1960s, from cotton to soybeans and rice which require less cultivation, resulted in significantly (a = 0.01) less sediment accumulation during the period 1965- 1982 when 86 percent of the lake averaged less than 2 cm/yr sediment deposition. If current sediment accumulation rates continue, open water habitat in the lake will be reduced by only 3 to 7 percent during the next 50 years.Contribution of the Sedimentation Laboratory, Agricultural Research Service, U.S. Department of Agriculture, Oxford, MS 38655  相似文献   

14.
We measured the vertical distributions of Mo, Fe, Mn, sulfide, sulfate, organic carbon, major ions, and pH in sediment porewater from one perennially oxic and three seasonally anoxic lacustrine basins in Eastern Canada, as well as those of Mo, acid volatile sulfide, Fe, Mn, Al, organic C, 210Pb and 137Cs in sediment cores from the same sites. The only input of anthropogenic Mo to these lakes comes from atmospheric deposition.The relatively monotonous distribution of Mo in the porewater of three seasonally anoxic basins suggests that Mo is not redistributed in the sedimentary column during periods of anoxia. In contrast, porewater Mo profiles obtained at three sampling dates in a perennially oxic basin display sharp Mo peaks below the sediment-water interface, indicating redistribution subsequent to deposition. Modeling of these latter porewater Mo profiles with a diagenetic reaction-transport equation coupled to comparisons among the various porewater and solid phase profiles reveal that Mo is released at 1-2 cm depth as a consequence of the reductive dissolution of Fe oxyhydroxides and scavenged both at the vicinity of the sediment-water interface, by re-adsorption onto authigenic Fe oxyhydroxides, and deeper in the sediments where dissolved sulfide concentrations are higher. The estimated rate constant for the adsorption of Mo onto Fe oxyhydroxides is 36 ± 45 cm3 mol−1 s−1.Diagenetic modeling indicates that authigenic Mo in sediments of the perennially oxic basin represents about one-third of the total solid phase Mo in the first cm below the sediment-water interface and only one tenth below this horizon. If we assume that no authigenic Mo is accumulated in the seasonally anoxic lake sediments we conclude that the sediment Mo concentrations, which are up to 3-16 times higher than the average lithogenic composition, depending on the lake, are mainly due to atmospheric deposition of anthropogenic Mo and not to the formation of authigenic Mo phases. Reconstructed historical records of the atmospheric Mo deposition indicate maximum values in the 1970s and 1980s and significant decreases since then. Emissions to the atmosphere associated with the smelting of non-ferrous ores and coal combustion appear to be the most important sources of anthropogenic Mo.  相似文献   

15.
Subduction zones with deep seismicity are believed to be associated with the descending branches of convective flows in the mantle and are subordinated to them. Therefore, the position of subduction zones can be considered as relatively fixed with respect to the steady-state system of convective flows. The lithospheric plate overhanging a subduction zone (as a rule of continental type) may:
1. (1) either move away from the subduction zone; or
2. (2) move onto it. In the first case extensional conditions originate behind the subduction zone and the new oceanic crust of back-arc basins forms. In the second case active Andean-type continental margins with thickening of the crust and lithosphere are observed.
Behind the majority of volcanic island-arcs, along the boundary with marginal-sea basins, independent shallow seismicity belts can be traced. They are parallel to the main seismicity belts coinciding with the Benioff zones. The seismicity belts frame island-arc microplates. Island-arc microplates are assumed to be a frame of reference to calculate relative movements of the consuming and overhanging plates. Using slip vector azimuths for shallow seismicity belts in the frontal parts of the Kurile, Japan, Izu-Bonin, Mariana and Tonga—Kermadec arcs, the position of the pole of rotation of the Pacific plate with respect to the western Pacific island-arc microplates was computed. Its coordinates are 66.1°N, 119.2°W. From the global closure of plate movements it has been determined that for the past 10 m.y. the Eurasian and Indian plates have been moving away from the Western Pacific island-arc system, both rotating clockwise, around poles at 31.1°N, 164.2°W and 1.3°S, 157.5°W, respectively. This provides for the opening of the back-arc basins. At the same time South America is moving onto the subduction zone at the rate of 4 cm/yr. Some “hot spots”, such as Hawaiian, Tibesti, and those of the South Atlantic, are moving relative to the island-arc system at a very low rate, viz. 0.5–0.7 cm/yr. Presumably, the western Pacific subduction zone and “hot spots” form a single frame of reference which can generally be used for the analysis of absolute motions.  相似文献   

16.
This paper compares archaeological evidence of Aboriginal occupation inside rock shelters and outside in adjacent sand sheets, focusing on two locations in the Keep‐River region, northwestern Australia. Luminescence and radiocarbon dating reveal that occupation sequences inside rock shelters are generally younger ( < 10,000 yr B.P.) than outside ( < 18,000 yr B.P.). Differences in occupation chronology and artifact assemblages inside and outside rock shelters result from depositional and postdepositional processes and shifts in site function. An increase in regional sedimentation rate from 10 cm/ka − 1 in the Pleistocene to 20 cm/ka − 1 in the Holocene may account for late buildup of sediments within rock shelters, increased artifact accumulation, and reduced postdepositional disturbance in some settings. More intense use of rock shelters in the Late Holocene is indicated from a change in hunting technology and greater production of rock art. The results indicate that some cultural interpretations might be flawed unless archaeological evidence from rock‐shelter and open‐site excavations is integrated. © 2006 Wiley Periodicals, Inc.  相似文献   

17.
Lago Verde is a fresh-water maar found on the lower slopes of San Martin volcano, at the Sierra de Los Tuxtlas, Mexico, currently the northernmost remnant of the tropical rain forest in America. 210Pb and 137Cs analyzed in a sediment core were used to reconstruct the historical fluxes of Ag, Cd, Cu, Pb, Hg and Zn to the site during the last ∼ 150 yr. The 210Pbxs-derived sediment accumulation rates, the magnetic susceptibility, C/N ratios and δ13C data evidenced background conditions at the lake until 1960s, when enhanced erosion related to the clearing of large forested areas at Los Tuxtlas promoted higher accumulation rates of a heavier and more magnetic sedimentary material. Recent sediments from Lago Verde were found enriched by Pb (26-fold natural concentration level [NCLs]) and moderately enriched by Cd > Cu > Zn and Hg (6-, 5-, 4- and 4-fold corresponding NCLs, respectively). The fluxes of Cu, Hg, Pb and Zn have significantly increased since 1940s, with peak ratios of total modern to pre-industrial fluxes of 11, 11, 19 and 49, respectively. The lake occupies a relatively pristine, non-industrialized basin, and therefore the increased metal fluxes might be related to long-distance aeolian transport of trace metals.  相似文献   

18.
We measured the amount of arsenic, chromium, copper, lead, nickel, vanadium, and zinc accumulated over a five-year period from 1997 to 2002 in surface sediments of seven salt marshes along the New Brunswick coast of the Bay of Fundy, Canada. Study sites extended from outer to inner Bay, spanning a gradient in tidal range (6–12 m) and mean sediment deposition rate (0.27–1.76 cm yr−1). In each study site, metal concentrations were measured in low and high marsh areas. Concentrations of chromium, nickel, and zinc appear to be within their natural range, while arsenic, lead, and vanadium are enriched in some sites. Calculated sediment metal loadings rates showed variability among marsh sites that closely followed sediment deposition patterns, suggesting sediment deposition rate is the driving factor of short-term metal accumulation in Fundy marshes. The value of salt marshes as a sink for metals may be enhanced by high sedimentation rates.  相似文献   

19.
Bulk dissolution rates for sediment from ODP Site 984A in the North Atlantic are determined using the 234U/238U activity ratios of pore water, bulk sediment, and leachates. Site 984A is one of only several sites where closely spaced pore water samples were obtained from the upper 60 meters of the core; the sedimentation rate is high (11-15 cm/ka), hence the sediments in the upper 60 meters are less than 500 ka old. The sediment is clayey silt and composed mostly of detritus derived from Iceland with a significant component of biogenic carbonate (up to 30%).The pore water 234U/238U activity ratios are higher than seawater values, in the range of 1.2 to 1.6, while the bulk sediment 234U/238U activity ratios are close to 1.0. The 234U/238U of the pore water reflects a balance between the mineral dissolution rate and the supply rate of excess 234U to the pore fluid by α-recoil injection of 234Th. The fraction of 238U decays that result in α-recoil injection of 234U to pore fluid is estimated to be 0.10 to 0.20 based on the 234U/238U of insoluble residue fractions. The calculated bulk dissolution rates, in units of g/g/yr are in the range of 4 × 10−7 to 2 × 10−6 yr−1. There is significant down-hole variability in pore water 234U/238U activity ratios (and hence dissolution rates) on a scale of ca. 10 m. The inferred bulk dissolution rate constants are 100 to 104 times slower than laboratory-determined rates, 100 times faster than rates inferred for older sediments based on Sr isotopes, and similar to weathering rates determined for terrestrial soils of similar age. The results of this study suggest that U isotopes can be used to measure in situ dissolution rates in fine-grained clastic materials.The rate estimates for sediments from ODP Site 984 confirm the strong dependence of reactivity on the age of the solid material: the bulk dissolution rate (Rd) of soils and deep-sea sediments can be approximately described by the expression Rd ≈ 0.1 Age−1 for ages spanning 1000 to 5 × 108 yr. The age of the material, which encompasses the grain size, surface area, and other chemical factors that contribute to the rate of dissolution, appears to be a much stronger determinant of dissolution rate than any single physical or chemical property of the system.  相似文献   

20.
The end of the Pleistocene in North America was marked by a wave of extinctions of large mammals, with the last known appearances of many species falling between ca. 11,000–10,000 14C yr BP. Temporally, this period overlaps with the Clovis Paleoindian cultural complex (11,190–10,530 14C yr BP) and with sudden climatic changes that define the beginning of the Younger Dryas chronozone (ca. 11,000–10,000 14C yr BP), both of which have been considered as potential proximal causes of this extinction event. Radiocarbon dating of enamel and filtered bone collagen from an extinct American Mastodon (Mammut americanum) from northern Indiana, USA, by accelerator mass spectrometer yielded direct dates of 10,055 ± 40 14C yr BP and 10,032 ± 40 14C yr BP, indicating that the animal survived beyond the Clovis time period and into the late Younger Dryas. Although the late survival of this species in mid-continental North America does not remove either humans or climatic change as contributing causes for the late Pleistocene extinctions, neither Clovis hunters nor the climatic perturbations initiating the Younger Dryas chronozone were immediately responsible for driving mastodons to extinction.  相似文献   

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