Showing papers on "Varve published in 1991"

Low organic carbon accumulation rates in black sea sediments

Abstract: THE Black Sea, the world's largest anoxic marine basin, is frequently used as a modern analogue for the formation of organic-rich sediments and carbonaceous rocks1–3, on the widely held assumption that anoxic conditions promote the preferential preservation of organic matter in sediments. Data for testing this hypothesis have so far been equivocal4–7, but here we use radiocarbon ages obtained using accelerator mass spectrometry for the organic fraction of recent Black Sea sediments to estimate the organic carbon accumulation rates. These range from 0.69 to 2.09 g C m−2 yr−1 and are significantly lower than earlier estimates based on varve counting6. Depending on the value taken for the rate of primary production in the Black Sea4,8, between 0.7 and 2.1% of the organic carbon is preserved in the bottom sediments. When compared with carbon accumulation rates in equivalent oxygenated environments9, these results indicate that the modern Black Sea is not a site of anomalously high organic carbon accumulation. This suggests that anoxic conditions in the water column may not be a prerequisite for the preservation of organic matter in marine sediments, and that models of the origin of carbonaceous facies in the geological record may therefore need to be modified.

Enso and Sunspot Cycles in Varved Eocene Oil Shales from Image Analysis

Abstract: On the premise that sequential changes in varve thickness offer a proxy for climatic variations, we investigated varve thickness in three core segments from the distal lacustrine oil shales (Tipton and Laney members) of the Green River Formation, by means of an image analysis program. Of two strong bimodal periodicities one, at 4.8-5.6 years, is interpreted as an El Nino type (ENSO) phenomenon of atmospheric dynamics, while the other, at 10.4-14.7 years, is interpreted as the sunspot cycle, originally recognized in this formation by Bradley (1929, 1931). Weaker periodicities may exist at ca. 8 and 33 years--the latter also recognized by Bradley. Taken in conjunction with the work of Bradley (1929, 1931) and of Crowley et al. (1986), this suggests that some but not all of the oil shale of the Green River Formation is truly varved and can be used to infer climatic time-series. Automated instrumental scans have greatly extended the possibilities of varve studies as approaches to climatic variations in time, but careful selection of facies in their geological framework is necessary.

Seasonal signals in lake Malawi sediments

Abstract: The sedimentary record of Lake Malawi, one of the large rift valley lakes of east Africa, contains thick sequences of biologically undisturbed, finely laminated muds and silts. Laminated sequences were recovered in cores from the extensive diatom ooze and silty diatom clay facies of the lake basin that is permanently anoxic below 250 m. The sequences consist of light-dark laminae couplets with average thickness of 1 mm. The light laminae are completely dominated by the diatom genus Melosira with a relatively minor contribution from the genus Stephanodiscus. The dark laminae consist of 50% diatoms (equal proportions of Melosira and Stephanodiscus, few Nitzschia, and occasional littoral diatom genera) and 50% terrestrial plant debris, mineral grains, and fine clay and organic material. Based on a 210Pb and 14C calculated sedimentation rate of 1 mm yr−1 for Lake Malawi basin sediments and the well-defined seasonal variations in fluvial input and diatom productivity levels, our data suggest that the lamination couplets are annual varves, representing biannual sedimentation sequences. The light laminae result from diatom blooms occurring during the dry, windy season (April–October) when the upper 250 m of the lake is well-mixed, localized upwelling occurs, and the epilimnion becomes enriched with nutrients. The dark laminae represent sedimentation in the rainy, low-wind velocity season (November–March) when runoff and allochthonous terrestrial input are at a maximum and primary productivity is at a minimum.

The Bude Formation (Lower Westphalian), SW England: siliciclastic shelf sedimentation in a large equatorial lake

Abstract: The 1300-m-thick turbiditic Bude Formation was deposited in a lake, Lake Bude, but disagreement persists over whether the environment was a deltaic or deep-water fan. The tectonic setting of the lake was the northern flank of a northerly advancing Variscan foreland basin, close to the Westphalian palaeo-equator. Palaeocurrents indicate sediment sourcing from all quadrants except the south. There is a dm-m scale cyclicity, whereby sandstone bodies comprising amalgamated event beds alternate with mudstone intervals containing non-amalgamated event beds. The ‘ideal’ cycle is a symmetrical coarsening-up/fining-up cycle, consisting of three facies (I, 2 and 3) arranged in 12321 order. Facies 3, in the middle of the cycle, is an amalgamated sandstone body up to 10 m thick which interfingers laterally with thin (cm) mudstone layers. The sandstone body comprises amalgamated beds of very fine sandstone which are largely massive and up to 0.4 m thick. Channels are absent except for scours up to 0.2 m deep which truncate the interfingering mudstone layers. Sandstone bodies are inferred to be tongue-shaped in three dimensions. Facies 1 and 2, completing the 12321 cycle, are respectively dark-grey fine and light-grey coarse, varved(?) mudstone containing thin ( < 0.4 m) sandstone event beds. Fossils and burrows indicate that facies 1 and 2 were deposited, respectively, in brackish (rarely marine) and fresh water. Hence, the ideal cycle (12321) reflects an upward decrease then increase in salinity (brackish-fresh-brackish); this is attributed to the lake sill being periodically overtopped by the sea, due to glacio-euitatic sea-level oscillations. The resulting oscillations in lake depth produced the coarsening-up/fining-up (regressivetransgressive) cyclicity, the central sandstone body representing the regressive maximum. Event beds are interpreted as river-fed turbidites deposited during catastrophic storm-floods. Combinedflow ripples and other wave-influenced structures occur in event beds throughout the ideal cycle, suggesting deposition of the entire Bude Formation above storm wave base. The proposed environment is a shelf, of continental-shelf dimensions, but lacustrine instead of marine. Sandstone bodies are interpreted to be river-connected tongues or lobes. The absence of cycles containing nearshore or emergent facies is attributed to: (i) the lake sill preventing the water level from falling below sill level, thereby insulating the lake floor from eustatically forced emergence; and (ii) relatively distal deposition, beyond the reach of shoreline progradations. The lack of palaeoflow from the south is attributed to a (now eroded?) deep-water trough lying to the south, in front of the northerly advancing orogen. Some facies 2 laminated mudstone beds grade laterally into massive and/or contorted beds, interpreted as in-situ seismites (Facies 4), consistent with an active foreland basin setting. Development of seismites was possibly facilitated by gas bubbles and/or weak cohesion in the (fresh water) bottom mud. The late Quaternary Black Sea, with its broad northwestern shelf, is probably a good physiographical analogue of Lake Bude, and was likewise fresh at times.

Absolute dating of late Quaternary Lacustrine sediments by high resolution varve chronology

Abstract: A high precision absolute timescale has been developed from annually laminated lake sediments from lakes in the Eifel area, West Germany. Calibration of relative dating methods (palynology, paleomagnetism) was carried out successfully. In addition palecological and astronomical information was obtained from varve thickness measurements, and the composition of annual layers.

Varve Formation in the Central Gulf of California: A Reconsideration of the Origin of the Dark Laminae from the 20th Century Varve Record: Chapter 30: Part V. Physical Oceanography, Primary Productivity, Sedimentology

Abstract: Formation of the dark "summer" laminae of the varved sediments from the anaerobic slopes of the central Gulf of California has previously been considered a response to the summer-fall peak in runoff from the major rivers of Sonora. Reconstruction of a 20th century varve chronology for an area off Guaymas, Sonora now provides a well constrained temporal framework for direct comparison of river discharge data to the mass accumulation of material in the dark laminae. A continuous record of dark lamina deposition from 1934 through 1966 shows no indication that construction of dams across the major Sonoran rivers, begun during the 1940s, has had any effect on the accumulation of these laminae. No relationship exists between the mass flux values and the "controlled" discharge o the rivers below the dams. However, a weak relationship was found to exist between mass flux and the "free," undammed summer-fall river discharge of the Sonora, Yaqui and Mayo Rivers compiled from stations above the dams or for periods prior to dam construction. Based on these results we argue that the transfer of terrigenous material from the mainland occurs principally through eolian processes by removal and transport of desert dust associated with convective summer thunderstorms.

Sediment characteristics in relation to cultivation history in two varved lake sediments from East Finland

Abstract: Changes in agricultural land-use patterns was studied with canonical correspondence analysis (CCA). The data sets consisted of sediment quality variables and pollen analysis data of two varved lake sediments in East Finland. The strong correlations between the sediment variables and pollen taxa reflect the simultaneous reactions to land-use changes of both vegetation and sediment quality; reactions which are reflected more drastically in the sediment quality of small Heinalampi than the larger Lake Suurjarvi.

Reconstruction of a 20th Century Varve Chronology from the Central Gulf of California: Chapter 29: Part V. Physical Oceanography, Primary Productivity, Sedimentology

Abstract: The retrieval of useful environmental information from the annual layers of natural high-resolution records such as tree rings, coral banding, or marine varves first requires reconstruction and validation of the chronological sequence in which this information has been preserved. This chapter describes the detailed reconstruction of a varve chronology from the central Gulf of California which encompasses most of the 20th century. The chronology was developed from a set of five box cores taken from the anaerobic slope off Guaymas, Sonora, and consists of a unique sequence of lamina couplets identifiable throughout the study area stretching more than 20 km along the slope. The reconstruction was initially hampered by the fact that each core site has suffered a history of sm ll-scale mass movement that has disrupted the stratigraphic continuity of its lamina sequence. Because these breaks generally do not occur at the same level in all of the cores, a continuous varve stratigraphy could be developed by piecing together short lamina sequences from four of the five box cores available. The chronostratigraphy commences with the winter/spring lamina of the year 1908. This lower boundary is imposed by a stratigraphic discontinuity that cuts across all five End_Page 603------------------------ core sites and is inferred to be a sediment gravity flow triggered by a nearby large-magnitude earthquake in the autumn of 1907. The surface lamina is dated from the time of collection in 1978. Independent 210Pb dating of specific lamina intervals indexed to the lamina stratigraphy from all five cores strongly supports our interpretation that the reconstructed varved interval represents 70.5 years of continuous sedimentation.

Diatomeen-, pollen- und sedimentmikrostratigraphische Untersuchungen zur anthropogenen Beeinflussung des Höllerer Sees (Oberösterreich)

Abstract: A profundal sediment core from Hollerer See frozen in situ was studied by means of diatom, microstratigraphical tape peel and pollen analyses. Varved sequences reflect eutrophication during mediaeval settlement and according to varve counts a recent eutrophic phase since the 1970's. Mesotrophication in-between coincides with regrowth of forests near the lake.

Geochemical evidence for enhanced upwelling and organic productivity during the late Quaternary on the continental margin of northern California

Abstract: EXTRACT (SEE PDF FOR FULL ABSTRACT): Laminated sediments are preserved in upper Pleistocene sections of cores collected on the continental slope at water depths within the present oxygen-minimum zone from at least as far north as the Klamath River and as far south as Point Sur. Comparison of sediment components in the laminae with those delivered to sediment traps as pelagic marine "snow" show the dark/light lamination couplets are indeed annual (varves). ... The presence of carbon-, sulfur-, and metal-rich sediments, as well as lack of bioturbation, all support the theory that the oxygen-minimum zone in the northeastern Pacific Ocean was more intense - in fact, anoxic - during the late Pleistocene in response to greater coastal upwelling and higher organic productivity.

Stratigraphic and geochronologic analyses of the quaternary deposits in the three-lake region of qaidam basin

Abstract: The Qaidam Basin is a larger sedimentary basin in western China where the thickness of the Quaternary deposits is in the range of 2000-3000 m at its subsiding center. This paper first dates palaeomagnetically the cores obtained from the five continuously sampled intervals and those from a typical profile, determineds the age of the index fossil zones, and calculates the deposition rates for the varve deposits based on the sedimentological analysis. As a result, we can calculate the ages of the various characteristic beds (including fossil zones, seismic reflection beds, and marker layers of electrical properties) for any given borehole-section when it was divided into facies, and check the validity of the chronology. According to all those studies, the stratigraphic and geochronologic analysis program of the Quaternary deposits in the Qaidam Basin has permitted a detailed discussion to be made in this paper.

Stratigraphic and geochronologic analyses of the quaternary deposits in the three-lake region of qaidam basin

Abstract: The Qaidam Basin is a larger sedimentary basin in western China where the thickness of the Quaternary deposits is in the range of 2000-3000 m at its subsiding center. This paper first dates palaeomagnetically the cores obtained from the five continuously sampled intervals and those from a typical profile, determineds the age of the index fossil zones, and calculates the deposition rates for the varve deposits based on the sedimentological analysis. As a result, we can calculate the ages of the various characteristic beds (including fossil zones, seismic reflection beds, and marker layers of electrical properties) for any given borehole-section when it was divided into facies, and check the validity of the chronology. According to all those studies, the stratigraphic and geochronologic analysis program of the Quaternary deposits in the Qaidam Basin has permitted a detailed discussion to be made in this paper.