Sequence stratigraphy of the Precambrian     

O. Catuneanu  P.G. Eriksson   《Gondwana Research》2007,12(4):560-565

The method of sequence stratigraphy requires the application of the same workflow and principles irrespective of the age of strata under analysis. In that respect, its application to Precambrian successions is similar to the approach used for Phanerozoic case studies. Differences, however, are recorded in terms of the preservation potential and the amount of data available for analysis; the rates and intensities of the allogenic controls on sedimentation; the environmental conditions and related physical processes; and the evolution of competing groups of organisms and associated biogenic processes. The combined effect of these contrasting aspects accounts for differences in the architecture of depositional sequences, particularly with respect to the relative contributions of various systems tracts to the makeup of a sequence.

The application of sequence stratigraphy to Precambrian basins has considerably enlarged the perspective on the fundamental principles governing the processes of sedimentary basin formation and the mechanisms controlling stratigraphic cyclicity in the rock record. These first-order principles are perhaps the most important contribution of Precambrian research to sequence stratigraphy. At the broader scale of Earth's geological history, the tectonic regimes governing the formation and evolution of sedimentary basins are shown to have been much more erratic in terms of nature and rates than originally inferred solely from the study of the Phanerozoic record. This provides important clues with respect to the criteria that should be involved in the hierarchy system of classification of stratigraphic sequences and bounding surfaces.  相似文献   

Interpreting the hydrological history of a temporary pond from chemical and microscopic characterization of siliceous microfossils     

Evelyn E. Gaiser  Mark J. Brooks  William F. Kenney  Claire L. Schelske  Barbara E. Taylor 《Journal of Paleolimnology》2004,31(1):63-76

The hydrological history of a temporary pond in South Carolina was inferred from a 5500-year record of siliceous microfossils, including diatoms, freshwater sponge spicules, chrysophyte cysts, plates of testate amoebae and plant phytoliths. Microfossil abundance was estimated by microscopic quantification of siliceous particles and by chemical extractions of silica. Diatom, sponge and mineral particle volumes were correlated with silica concentrations attributable to these fractions. Both techniques suggested a sequence of four distinct community types. Basal sediments (4630–5520 ¹⁴C YBP) containing phytoliths and sponge spicules indicative of a wetland community were covered by sediments dominated by the remains of planktonic protists (3750–4630 ¹⁴C YBP) suggesting a transition from a vegetated marsh to an open-water, permanently flooded pond. Microfossil assemblages above this zone indicate the return of a wetland community ca. 3750 YBP that persisted until recently, when pond water levels stabilized as a result of seepage from a reservoir constructed nearby in 1985. This study suggests that the suite of siliceous microfossils commonly found in pond sediments can be used to infer historical alternations between macrophyte and plankton-dominated states in shallow basins. Regional climate inferences from this record include a mid-Holocene hydrological maximum and the onset of the modern climate ca. 3500 YBP.  相似文献   

Settling velocities and entrainment thresholds of biogenic sands (shell fragments) under unidirectional flow   总被引：3，自引：0，他引：3  

D. Paphitis  M. B. Collins  L. A. Nash†  & S. Wallbridge‡ 《Sedimentology》2002,49(1):211-225

Settling velocities and entrainment thresholds of biogenic sedimentary particles, under unidirectional flow conditions, are derived on the basis of settling tower and laboratory flume experiments. Material consisting predominantly of equant blocks (shell fragments of Cerastoderma edule , density, ρ _s=2800 kg m⁻³) or of mica-like flakes and elongate rods ( Mytilus edulis fragments, ρ _s=2720 kg m⁻³) are used in separate series of experiments. Differences in the measured settling and threshold properties are related primarily to particle shape. The selection of a characteristic length scale for non-spherical grains is investigated by comparing two approaches used to define the grain size ( D ) of the sediment samples: grain settling and sieve analysis that are used to derive data for the threshold criteria, in terms of the Shields and Movability diagrams. The empirical curves effectively predict the threshold conditions for any grain shape, provided that grain size is defined in terms of grain settling velocity. However, a functional distinction is made between the characteristic `hydraulic' grain size, defined by grain settling for grain transport applications, and the actual (physical) grain size defined by sieve analysis.  相似文献   

Shrinking Sphere Kinetics for Batch Dissolution of Mixed Particles of a Single Substance at High Under-Saturation: Validation with Sodium Chloride, but with Biogenic Silica in Mind     

Victor W. Truesdale 《Aquatic Geochemistry》2008,14(4):359-379

The cubic equation recently derived for the increase in concentration of a solute with time, as the solid dissolves in batch according to the shrinking sphere model at high under-saturation, is extended to dissolutions of mixtures of differently sized particles. This problem needs to be solved if batch dissolutions are to play their part in the proposed amelioration of global warming and associated climate change by accelerated ‘re-burial’ of excess CO₂ in ocean sediment. The upgraded model was tested using sodium chloride dissolved in 50% aqueous propanone, whence the model fitted two separate runs with 500 and 212 μm, and 212 and 38 μm, diameter crystals, respectively. The key to simulating dissolution in this way lies in the dissolutions being independent of each other. It is further shown that although this condition was implicit in the recent derivation of the cubic equation, it was not recognised at the time. The work should be applicable to any batch dissolution of mixed particles at high under-saturation, and hence, may find use in many industrial and laboratory dissolutions. Simulations show how agglomerated mixtures can yield a straight line on the plot of ln(1 − C/C _T) versus time, as was reported to occur recently with sodium chloride taken ‘straight from the bottle’. It is shown that this probably explains why exponential dissolutions may have seemed appropriate to the dissolution of biogenic silica in earlier literature. This study suggests that a new round of biogenic silica dissolutions, but with sieved samples, would be worthwhile, with the likelihood that shrinking sphere behaviour might well be found to characterise the kinetics. The opportunity is taken to investigate a number of aspects of the shrinking sphere model not generally discussed before, e.g. the graph for the change in surface area with time. The limitations of using cubic salt crystals with the shrinking sphere model are discussed.  相似文献   

Bitumen biomarkers in the Mid-Proterozoic Ilímaussaq intrusion, Southwest Greenland - A challenge to the mantle gas theory     

T. Laier  H.P. Nytoft 《Marine and Petroleum Geology》2012,30(1):50-65

GC and GC/MS/MS analysis on rock extracts has shown that the bitumen in the peralkaline Ilímaussaq intrusion, previously assumed to be abiogenic, is biotic in origin. A biotic origin is in accordance with previously published stable carbon isotopic data on bituminous matter in the rocks. The biomarker distribution in the bitumen, including the less common bicadinanes, resembles that of oil seeps on the central West Greenland coast 2200 km farther north, whose source rocks and migration history are relatively well established. We use a recent re-construction of the subsidence and later exhumation of the West Greenland coastal region during the Mesozoic-Cenozoic (Japsen et al., 2006a, b) to anticipate that hydrocarbons migrated from deeper parts of the basin offshore west of Greenland. The rocks of Ilímaussaq were probably more deeply invaded than the surrounding granites due to their higher proportion of corroded minerals, which may explain why bitumen has not been observed elsewhere in the area.Hydrocarbon gases (C1-C5) present in fluid inclusions were also analysed, after having been released by treatment with hydrochloric acid that resulted in an almost complete disintegration of the Ilímaussaq intrusion rocks. The acid extraction method proved generally more efficient than the crushing procedure applied by others, but gave similar results for the chemical composition of the gas (CH₄: 88-97%) and isotopic ratios (δ¹³C₄CH: −1.6 to −5.0‰; δ¹³CC₂H₆: −9.2 to −12.5‰), with the exception of hydrocarbons hosted in quartz, which showed significantly lower isotopic values for methane (Graser et al., 2008). Previous researchers have suggested an abiotic origin for these hydrocarbon gases, but we suggest a biotic origin for the majority of them, not just those in quartz, assuming that the isotopic ratio of the constituents have changed due to loss of gas by diffusion. The assumption of gas loss via diffusion is supported by published studies on micro-fissures in minerals typical of the Ilímaussaq and field investigations showing diffusive loss of gas from the peralkaline Khibina and Lovozero massifs on the Kola Peninsula, Russia, which are, in many respects of mineralogy and hydrocarbon content, similar to the Ilímaussaq intrusion.Both the hydrocarbon gases and bitumen in the Khibina and Lovozero massifs have been cited as prime examples of a deep mantle source, although the carbon isotopic ratio of the bitumen clearly pointed to an organic origin. The trends in carbon isotopic ratio of methane released with time from freshly exposed rocks also supports our hypothesis of ¹³C enrichment of the methane remaining within the rock. Thus, there is good evidence that the hydrocarbons in the Kola alkaline massifs are mostly biotic in origin, in which case the probability of finding economic hydrocarbon accumulations from a deep mantle source seems exceedingly small.  相似文献   

A 2000 year record of climatic change at Ongoke Lake,southwest Alaska     

Melissa L. Chipman  Gina H. Clarke  Benjamin F. Clegg  Irene Gregory-Eaves  Feng Sheng Hu 《Journal of Paleolimnology》2009,41(1):57-75

We analyzed sediments of the past 2000 years from Ongoke Lake, southwest Alaska, for organic carbon, organic nitrogen, biogenic silica (BSi), and diatom assemblages at decadal to centennial resolution to infer limnological changes that may be related to climatic variation in southwestern Alaska. The chronology is based on a ²¹⁰Pb profile from bulk sediments and nine AMS ¹⁴C ages from terrestrial plant macrofossils. Four of the ¹⁴C ages span a core depth interval of 60.5 cm but are statistically indistinguishable from one another with a mean of ~1300 AD, which compromises the determination of temporal trends at Ongoke Lake and comparison with other paleoclimate records. The diatom record suggests changes in the duration of ice cover and strength of thermal stratification that are probably related to temperature variation. This variation includes a cold interval around the first millennium cooling (FMC) and a warm interval spanning the medieval climate anomaly (MCA). However, the lake-sediment record shows no clear signals of temperature variation for the period of the Little Ice Age (LIA) or the twentieth century. Climatic changes during these periods may have been manifested through effective-moisture (precipitation minus evapotranspiration) variation in the Ongoke Lake area. We estimate water depths and infer effective-moisture fluctuations by applying a regional transfer function to our diatom record. Together with inferences from diatom autecologies, this water-depth reconstruction suggests that effective moisture increased steadily from 50 BC to 350 AD, which was followed by relatively dry conditions between 550 and 750 AD and relatively wet conditions between 750 and 1450 AD. Effective moisture was low from ~1450 to 1850 AD, coinciding with the LIA; an alternative age model places this interval between ~1315 and 1850 AD. During the past 150 years, effective moisture increased, with estimated water depths reaching peak values in the second half of the twentieth century. This study offers the first paleolimnological record for inferring centennial-scale climatic variation over the past two millennia from southwestern Alaska.

Feng Sheng HuEmail:

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Magnetic properties of marine magnetotactic bacteria in a seasonally stratified coastal pond (Salt Pond, MA, USA)     

Bruce M. Moskowitz  Dennis A. Bazylinski  Ramon Egli  Richard B. Frankel  Katrina J. Edwards 《Geophysical Journal International》2008,174(1):75-92

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Potential limitations of biogenic silica as an indicator of abrupt climate change in Lake Baikal, Russia   总被引：2，自引：0，他引：2  

George E. A. Swann  Anson W. Mackay 《Journal of Paleolimnology》2006,36(1):81-89

Biogenic Silica (BSi) has been one of the most important proxies for determining the palaeoclimate from Lake Baikal over glacial-interglacial cycles. Concentrations (calculated through a 1% Na₂CO₃ wet-alkaline digestion) at a site in the north basin, however, reveal consistently low values during MIS 3 compared to greater than tenfold changes in diatom concentrations and biovolumes from c. 53.3–51.5 kyr BP. With similar glacial trends present at other low sedimentation sites, we suggest that significant amounts of BSi are removed from diatoms during glacials due to a relative increase in diatom dissolution at the sediment–water interface. This contrasts with existing results from other, higher sedimentation, sites such as those within the Selenga Delta, which display a strong relationship between diatoms and BSi. Site selection is therefore essential when searching for Heinrich and other glacial millennial-scale events in Lake Baikal, and we recommend that both BSi and diatom concentrations be calculated together in future studies.  相似文献   

Research Advance of Changing Biogenic Substance Cycling in River Systems by Damming*   总被引：1，自引：0，他引：1  

Haojun Deng  Zhen Tao  Quanzhou Gao  Ling Yao  Yong Feng  Yinhua Li 《地球科学进展》2018,33(12):1237-1247

River damming transforms allotropic natural rivers into autotrophic 'impound river' (referred to "reservoir"), which changes the processes of river biogenic substance cycle and the matter properties as well as export flux from land to ocean, thus becoming one of the key problems of element biogeochemical cycle. Due to the different behavior of biogenic substances (C, N, P, Si) in biological processes, biogenic substances cycle efficiency is different, in turns, Silicon (Si)>Organic Carbon (OC)>Phosphorus (P). The migration and transformation processes of C and Si are significantly affected by phytoplankton and water retention time. Nitrogen (N) and P are mainly affected by water pH, temperature, Dissolved Oxygen (DO) and retention time. The retention efficiency of biogenic substances is shown as N>C>P>Si at the global scale. Besides, the sedimentation and burial processes of reservoirs constitute the net sink of OC in rivers. River damming alters the stoichiometric characteristics of water elements, nutrient constraints, aquatic communities composition and the coupling effect of C/N/P/Si. The stable isotopic compositions of C, N and Si can effectively trace the source, migration and transformation of biogenic matter. A combination of elements stoichiometric characteristics and stable isotopic composition could effectively indicate the change of source materials in reservoirs. With the increasing demand for clean energy, the intensity of river damming and reservoir construction will increase. Thus, a series of scientific problems including changing law of biogenic substance migration and transformation dynamic, as well as accumulation effect of ecological environment in watershed systems by river cascade damming, should need to be concerned in the biogeochemistry cycle study.  相似文献   

Assessment of metal contamination in surface sediments from Zhelin Bay,the South China Sea     

《Marine pollution bulletin》2013,77(1-2):383-388

Metals and biogenic elements were analyzed from surface sediments collected from Zhelin Bay in the South China Sea in December 2008. The high concentrations of TOC, TN and BSi indicate the high nutrient level and diatom productivity in Zhelin Bay. The concentrations of metals were generally far lower than the effects-range-low (ERL) values that define pollutant levels. Enrichment factors (EF) and geoaccumulation indices (I_geo) suggest there are pollution levels of Cd, Cu and Zn at some stations. As, Cu, and Pb are potentially biotoxic in some stations. Correlation and principal component analyses indicate that most of the metals primarily originate from natural sources, and from maricultural activities as well. Mariculture contributes considerable Cd and Cu contamination. As and Pb comes primarily from combustion of gasoline and diesel fuel by ships.  相似文献