Variations In Sedimentation Rate Research Articles

Origin of apparent magnetic excursions in deep‐sea sediments from Mendeleev‐Alpha Ridge, Arctic Ocean

Arctic deep‐sea sediments often record intervals of negative inclination of natural remanence that are tens of centimeters thick, implying magnetic excursions with durations of tens of thousand years that far exceed excursion durations estimated elsewhere, and the lack of tight age control usually provides excessive freedom in the labeling of Arctic excursions. Fortuitous variations in sedimentation rate have been invoked to explain the “amplified” excursions. Alternating field demagnetization of natural remanent magnetization (NRM) of sediment cores 08JPC, 10JPC, 11JPC, and 13JPC recovered by the Healy Oden Trans‐Arctic Expedition in August 2005 (HOTRAX05) to the Mendeleev‐Alpha Ridge yields apparent magnetic “excursions” in sediments deposited in the Brunhes Chron. Thermal demagnetization of the NRM, however, implies multiple magnetization components with negative inclination components usually “unblocked” below ∼350°C. Analysis of isothermal remanent magnetization acquisition curves from magnetic extracts indicates two magnetic coercivity components superimposed on one another. Magnetic experiments conducted under high and low temperatures show features that are characteristic for (titano)magnetite and titanomaghemite. Presence of the two magnetic phases is further confirmed by elemental mapping on a scanning electron microscope equipped for X‐ray energy dispersive spectroscopy (EDS) and by high‐resolution X‐ray diffraction (XRD). It is unlikely that anomalously thick intervals of negative inclination in these Brunhes‐aged sediments are caused by unusual behavior of the magnetic field in the Arctic area. We therefore attribute low and negative NRM inclinations in these cores to partially self‐reversed chemical remanent magnetizations, apparently carried by titanomaghemite and acquired during the oxidation of detrital (titano)magnetite grains. The high Ti contents and high oxidation states indicated by EDS and XRD data provide the conditions required for partial self‐reversal by ionic reordering during diagenetic maghemitization, and this process appears to have affected all HOTRAX05 cores collected from the Mendeleev‐Alpha Ridge.

Spatial and temporal variations of sedimentation rate in the Timah Tasoh Water Reservoir, Perlis, Malaysia

Spatial and temporal variations of sedimentation rate in the Timah Tasoh Water Reservoir, Perlis, Malaysia Human activities such as logging, land conservation, road construction and other disturbances in watersheds will increase erosion rate and thus the amount of sediment transported into the river that reaches lakes, reservoirs and sea. Accelerated sedimentation rate in the reservoir can significantly reduce a reservoir's surface area, eliminating wetland area surrounding the reservoir and shallow the areas near the shore of the reservoir. A study of sedimentation rate in the Timah Tasoh water reservoir was carried out from May 2001 to April 2002 by means of sediment traps. The aim of this study is to determine the spatial and temporal pattern of sedimentation rates in the reservoir. The sediment traps were installed at five different locations in the reservoir and replaced every month. Gross sedimentation rates measured in the sediment traps vary from about an average of 1.4 kg/m2/month or 16.3 kg/m2/yr in the deepest part of the reservoir to about 79.2 kg/m2/yr (monthly average of 6.6 kg/m2/month) and 47.1 kg/m2/yr (monthly average of 3.9 kg/m2/month) near the inlet of the Pelarit River and the Tasoh River respectively. The sedimentation rate near the Pelarit River and the Tasoh River inflow is correlated with the water and suspended sediment discharge in the river, owing to its proximity to the rivers and the flooding effect, mainly during the wet season. The sedimentation rate decreases southward along the reservoir, as a result of increasing distance from the river mouth. The total suspended sediment load of the Upper Pelarit River and the Jarum River flowing into the reservoir is 11.4 × 103 ton/year and 5.41 × 103 ton//year respectively.

High-frequency palaeoenvironmental changes on a shallow carbonate platform during a marine transgression (Late Oxfordian, Swiss Jura Mountains)

Based on a well-established bio- and sequence-stratigraphic framework, a narrow time window in the Bimammatum ammonite zone (Late Oxfordian) is investigated in six Swiss Jura sections representing a shallow-water carbonate platform. From the detailed facies and microfacies analysis of oncoid-rich (Hauptmumienbank Member) and ooid-rich (Steinebach Member) limestones, a microfacies classification is established, depositional environments are interpreted, and a depositional model for the Swiss Jura platform is proposed. The sequence- and cyclostratigraphic interpretation has been performed for the transgressive part of the medium-scale sequence Ox6+, independently for each section, with a very high time resolution at the scale of elementary depositional sequences. The good correlation of the elementary and small-scale sequences between the six studied sections and the similar number of elementary sequences in all sections strongly suggest that allocyclic processes were involved in their formation. The hierarchically stacked depositional sequences (small-scale and elementary sequences) result from orbitally controlled sea-level changes with periodicities of 100 and 20 kyr, respectively. Thickness variations in the correlated small-scale and elementary sequences imply variable sedimentation rates, probably resulting from differential subsidence due to the activity of tectonic blocks. The tectonically controlled platform morphology contributed significantly to the general pattern of depositional environments and, combined with high-frequency sea-level fluctuations, created a complex facies distribution in time and space on the Swiss Jura carbonate platform.

U-Pb zircon ages from the southwestern Karoo Basin, South Africa--Implications for the Permian-Triassic boundary

Research Article| August 01, 2009 U-Pb zircon ages from the southwestern Karoo Basin, South Africa—Implications for the Permian-Triassic boundary Andrea Fildani; Andrea Fildani 1Chevron Energy Technology Company, 6001 Bollinger Canyon Road, San Ramon, California 94583, USA Search for other works by this author on: GSW Google Scholar Amy Weislogel; Amy Weislogel 2Department of Geological Sciences, University of Alabama, Tuscaloosa, Alabama 35487, USA Search for other works by this author on: GSW Google Scholar Nicholas J. Drinkwater; Nicholas J. Drinkwater 3Chevron Southern African Business Unit, 1400 Smith Street, Houston, Texas 77002, USA Search for other works by this author on: GSW Google Scholar Tim McHargue; Tim McHargue 1Chevron Energy Technology Company, 6001 Bollinger Canyon Road, San Ramon, California 94583, USA Search for other works by this author on: GSW Google Scholar Anthony Tankard; Anthony Tankard 471 Lake Crimson Close SE, Calgary, Alberta T2J 3K8, Canada Search for other works by this author on: GSW Google Scholar Joe Wooden; Joe Wooden 5Stanford–U.S. Geological Survey Micro-Analysis Center, Stanford University, Stanford, California 94305, USA Search for other works by this author on: GSW Google Scholar David Hodgson; David Hodgson 6STRAT Group, Department of Earth and Ocean Sciences, University of Liverpool, Liverpool L69 3GP, UK Search for other works by this author on: GSW Google Scholar Stephen Flint Stephen Flint 6STRAT Group, Department of Earth and Ocean Sciences, University of Liverpool, Liverpool L69 3GP, UK Search for other works by this author on: GSW Google Scholar Geology (2009) 37 (8): 719–722. https://doi.org/10.1130/G25685A.1 Article history received: 20 Nov 2008 rev-recd: 16 Mar 2009 accepted: 30 Mar 2009 first online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Andrea Fildani, Amy Weislogel, Nicholas J. Drinkwater, Tim McHargue, Anthony Tankard, Joe Wooden, David Hodgson, Stephen Flint; U-Pb zircon ages from the southwestern Karoo Basin, South Africa—Implications for the Permian-Triassic boundary. Geology 2009;; 37 (8): 719–722. doi: https://doi.org/10.1130/G25685A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract U-Pb ages determined using sensitive high-resolution ion microprobe-reverse geometry on 205 single-grain zircons from 16 ash beds within submarine fan deposits of the Ecca Group provide the first evidence of a marine Permian-Triassic (P-T) boundary in the Karoo Basin of South Africa. These U-Pb ages provide an objective basis for correlating the deep-marine sediments of the southwest Karoo Basin with fluvial-deltaic deposits in the central and eastern parts of the basin where the P-T boundary is recorded in a diverse macrofauna. Furthermore, these new zircon ages and their correlation imply asymmetric subsidence and variable sedimentation rates across the basin. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Synchronization of late-glacial vegetation changes at Crystal Lake, Illinois, USA with the North Atlantic Event Stratigraphy

Late-glacial (17–11 cal ka BP) pollen records from midwestern North America show similar vegetation trends; however, poor dating resolution, wide-interval pollen counts, and variable sedimentation rates have prevented the direct correlation with the North Atlantic Event Stratigraphy as represented in the Greenland ice-core records, thus preventing the understanding of the teleconnections and mechanisms of late-Quaternary events in the Northern Hemisphere. The widespread occurrence of late-glacial vegetation and climates with no modern analogs also hinders late-glacial climate reconstructions. A high-resolution pollen record with a well-controlled age model from Crystal Lake in northeastern Illinois reveals vegetation and climate conditions during the late-glacial and early Holocene intervals. Late-glacial Crystal Lake pollen assemblages, dominated by Picea mariana and Fraxinus nigra with lesser amounts of Abies and Larix, suggest relatively wet climate despite fluctuations between colder and warmer temperatures. Vegetation changes at Crystal Lake are coeval with millennial-scale trends in the NGRIP ice-core record, but major shifts in vegetation at Crystal Lake lag the NGRIP record by 300–400 yr. This lag may be due to the proximity of the Laurentide ice sheet, the ice sheet's inherent slowness in response to rapid climate changes, and its effect on frontal boundary conditions and lake-effect temperatures.

Assessment of multiple sources of anthropogenic and natural chemical inputs to a morphologically complex basin, Lake Mead, USA

Lakes with complex morphologies and with different geologic and land-use characteristics in their sub-watersheds could have large differences in natural and anthropogenic chemical inputs to sub-basins in the lake. Lake Mead in southern Nevada and northern Arizona, USA, is one such lake. To assess variations in chemical histories from 1935 to 1998 for major sub-basins of Lake Mead, four sediment cores were taken from three different parts of the reservoir (two from Las Vegas Bay and one from the Overton Arm and Virgin Basin) and analyzed for major and trace elements, radionuclides, and organic compounds. As expected, anthropogenic contaminant inputs are greatest to Las Vegas Bay reflecting inputs from the Las Vegas urban area, although concentrations are low compared to sediment quality guidelines and to other USA lakes. One exception to this pattern was higher Hg in the Virgin Basin core. The Virgin Basin core is located in the main body of the lake (Colorado River channel) and is influenced by the hydrology of the Colorado River, which changed greatly with completion of Glen Canyon Dam upstream in 1963. Major and trace elements in the core show pronounced shifts in the early 1960s and, in many cases, gradually return to concentrations more typical of pre-1960s by the 1980s and 1990s, after the filling of Lake Powell. The Overton Arm is the sub-basin least effected by anthropogenic contaminant inputs but has a complex 137Cs profile with a series of large peaks and valleys over the middle of the core, possibly reflecting fallout from nuclear tests in the 1950s at the Nevada Test Site. The 137Cs profile suggests a much greater sedimentation rate during testing which we hypothesize results from greatly increased dust fall on the lake and Virgin and Muddy River watersheds. The severe drought in the southwestern USA during the 1950s might also have played a role in variations in sedimentation rate in all of the cores.

Cenozoic basin-forming processes along the northeastern margin of Eurasia: Constraints determined from geophysical studies offshore of Hokkaido, Japan

The Cenozoic evolution of a sedimentary basin on a long-lasting deformation zone along the northeastern Eurasian margin is investigated using geophysical studies. Analysis of seismic reflection data with detailed age assignments reveals that an offshore Paleogene and early Miocene basin around northwestern Hokkaido developed as a typical pull-apart depression at the right-stepping part of N–S trending dextral faults. Numerical modeling estimates the total slip on the Paleogene strike-slip faults is ca. 12 km, which matches the actual shape and volume of the basin. The total displacement resulted from quite active (∼1.4 mm/year) dextral motions along the fault zone during the Paleogene and early Miocene. Deformation fields around the fault step also indicate a complicated pattern of vertical axis rotation. The emergence of contraction tectonics in the middle Miocene is suggested from the foreland-type basin configuration observed. The temporal variations of sedimentation rate through the Cenozoic are estimated utilizing the depth-converted seismic record, and they confirm an accelerated subsidence, implying a forearc–foreland transition upon the Eurasian margin.

Migration of 137Cs in tributaries, lake water and sediment of Lago Maggiore (Italy, Switzerland) – analysis and comparison with Lago di Lugano and other lakes

This paper describes the behaviour of 137Cs in Lago Maggiore and other pre-alpine lakes as a consequence of atmospheric nuclear weapons testing fallout and the fallout from the nuclear accident in Chernobyl. It presents data on the 137Cs distribution in tributaries, lake water, bottom sediments and reveals the role of 137Cs as a marker of the sedimentation processes. The run-off of 137Cs from the watershed to the lake is described with a simple compartment model. Measurements of the activity concentration of 137Cs in sediments are compared with the output of a model (diffusion–convection type) which describes the input of 137Cs into and its vertical distribution within the sediment. Varying sedimentation rates (0.05–0.90 g (cm 2 y) −1) in Lago Maggiore are compared with data of other authors. Sedimentation rates and total distribution coefficients (of about 10 5 L kg −1) in Lago Maggiore are discussed and compared with those of Lago di Lugano, Lake Constance, and Lake Vorsee.

Summer sedimentation in six shallow arctic lakes

The movement of sediment between the lake bottom and water column of shallow lakes can be sizeable due to the large potential for resuspension in these systems. Resuspended sediments have been shown to alter phytoplankton community composition and elevate water column production and nutrient concentrations. We measured the summer sedimentation rates of two lakes in 2003 and six lakes in 2004. All lakes were shallow and located in the Alaskan Arctic. In 2004, turbidity, light attenuation, total sediment:chlorophyll a mass in the sediment traps, and thermal stratification were also measured in each of the lakes. The sediment:chlorophyll a mass was much greater than if the sediment was derived from phytoplankton production in all of the lakes, indicating that the source of the sedimenting material was resuspension and allochthonous inputs. Consistent with these findings, the temporal variation in sedimentation rate was synchronous between most lakes, and sedimentation rate was positively related to wind speed and rainfall suggesting that sedimentation rate was strongly influenced by landscape-scale factors (e.g., wind and rain events). Two of the lakes are located on deposits of loess that accumulated during past glacial periods. These two lakes had sedimentation rates that were significantly greater and more variable than any of the other lakes in the study, as well as high turbidity and light attenuation. Our results indicate that sedimentation in these shallow arctic lakes is supported primarily by allochthonous inputs and resuspension and that landscape-scale factors (e.g., weather and geology) impact on the transport of materials between the lake bottom and water column.

A generic, process‐based model of microbial pollution in aquatic systems

Based on a comprehensive synthesis of data available within the literature, a new process‐based model of microbial pollution is presented, which is applicable for surface and coastal waters. The model is based on a generic set of parameterisations that describe the dynamics of most protozoan, bacterial and viral organisms of interest, including pathogens and microbial indicator organisms. The parameterisations dynamically account for the effects of temperature, salinity, pH, dissolved oxygen, sunlight, nutrients and turbidity on the growth and mortality of enteric organisms. Parameters for a range of organisms are also presented which are based on collation of literature data. The model has been implemented within an aquatic ecology model, Computational Aquatic Ecosystem Dynamics Model (CAEDYM), which can couple to multidimensional hydrodynamic models. Without adjustment of the literature derived parameter values, a 3‐D implementation is validated against observed data from three freshwater systems that differ in their climatic zone, trophic status and operation. The simulations highlight the spatial and temporal variability that may be encountered by operators. Additionally, large differences in the fate and distribution of different species originate from variable rates of growth, mortality and sedimentation and it is emphasized that the use of surrogates for quantifying risk is problematic. The model can be used to help design targeted monitoring programs, explore differences between species, and to support real‐time decision‐making. Areas where insufficient understanding and data exist are discussed.

Sediment dynamics in an upland temperate catchment: changing sediment sources, rates and deposition

We examine sediment dynamics in an upland, temperate lake system, Lake Bassenthwaite (NW England), in the context of changing climate and land use, using magnetic and physical core properties. Dating and analysis of the sedimentary records of nine recovered cores identify spatially variable sedimentation rates across the deep lake basin. Mineral magnetic techniques, supported by independent geochemical analyses, identify significant variations both in sediment source and flux over the last ∼2100 years. Between ∼100 years BC and ∼1700 AD, sediment fluxes to the lake were low and dominated by material sourced from within the River Derwent sub-catchment (providing 80% of the hydraulic load at the present day). Post-1700 AD, the lake sediments became dominantly sourced from Newlands Beck (presently providing ∼10% of the lake’s hydraulic load). Three successive, major pulses of erosion and increased sediment flux appear linked to specific activities within the catchment, specifically: mining activities and associated deforestation in the mid-late nineteenth century; agricultural intensification in the mid-twentieth century and, within the last decade, the additional possible impact of climate change. These results are important for all upland areas as modifications in climate become progressively superimposed upon the effects of previous and/or ongoing anthropogenic catchment disturbance.

Thermal segmentation along the N. Ecuador–S. Colombia margin (1–4°N): Prominent influence of sedimentation rate in the trench

Abstract Along the deformation front of the North Ecuador–South Colombia (NESC) margin, both surface heat flow and trench sediment thickness show prominent along-strike variations, indicating significant spatial variations in sedimentation rate. Investigating these variations helps us address the important question of how trench sedimentation influences the temperature distribution along the interplate contact and the extent of the megathrust seismogenic zone. We examine this issue by analysing 1/ a new dense reflection data set, 2/ pre-stack depth migration of selected multichannel seismic reflection lines, 3/ numerous newly-identified bottom-simulating reflectors and 4/ the first heat probe measurements in the region. We develop thermal models that include sediment deposition and compaction on the cooling oceanic plate as well as viscous corner flow in the mantle wedge. We estimate that the temperature from 60–150 °C to 350–450 °C, commonly associated with the updip and downdip limits of the seismogenic zone, extends along the plate interface over a downdip distance of 160 to 190 ± 20 km. We conclude that the updip limit of the seismogenic zone for the great megathrust earthquake of 1979 is associated with low-temperature (60–70 °C) processes. Our models also suggest that 60–70% of the two-fold decrease in measured heat flow from 3°N to 2.8°N is related to an abrupt southward increase in sedimentation rate in the trench. Such a change may potentially induce a landward shift of the 60–150 °C isotherms, and thus the updip limit of the seismogenic zone, by 10 to 20 km.

Empirical models for describing recent sedimentation rates in lakes distributed across broad spatial scales

Over the last 20 years there has been a surge of interest in paleolimnology and as a result a large accumulation of lake sedimentation records. This emerging archive has allowed us to develop empirical models to describe which variables explain significant variation in sedimentation rates over the past ∼150 years across large spatial scales. We hypothesized that latitude would be a significant explanatory variable of profundal zone lake sedimentation rates across a temperate to polar gradient. We further hypothesized that along a more longitudinally-constrained dataset (i.e. east coast of North America), latitude would explain a greater proportion of the variance. To test these hypotheses, we collated data from 125 natural, average-sized lakes (with surface area <500 km2) by recording authors’ estimates of sedimentation rates (measured as mm/year) or by digitizing recent sediment profiles and calculating sedimentation rates over the past ∼150 years. We found that, at both scales, latitude was the strongest predictor of lake sedimentation rates (full dataset: r2 = 0.28, P = 0.001, n = 125; east coast dataset: r2 = 0.58, P < 0.001, n = 43). By conducting a multiple linear regression analysis, we found that 70% of the variance in sedimentation rates from the east coast transect was explained by latitude and elevation alone. This latter model is of sufficient strength that it is a robust predictive tool. Given that climate and land-use strongly co-vary with latitude and that both of these factors have previously been shown to influence lake sedimentation rates, it appears that latitude is a surrogate measure for climate and land-use changes. We also show support for land-use as an important variable influencing sedimentation rates by demonstrating large increases in recent versus Holocene accumulation rates. These results indicate that it is possible to make generalizations about sedimentation rates across broad spatial scales with even limited geographic data.

On the identification of magnetostratigraphic polarity zones

Polarity zones of sedimentary sections reflect a pattern of alternating polarity of the geomagnetic field recorded by the remanent magnetization of rocks. Unfortunately, this pattern can have been modified by the variable sedimentation rate, which complicates the identification of polarity zones against the reference geomagnetic polarity time scale. To avoid this obstacle, the present paper suggests a transform applied to both the sequence of levels of polarity reversal horizons and the sequence of ages of polarity reversals before computing their cross-correlation. This transform usually reduces the impact of the variable sedimentation rate so that a sequence of more than eight polarity reversal horizons may be identified without biostratigraphic constraints. Numerical experiments involving random processes to simulate both the duration of polarity reversals and the sedimentation rate proved, however, that not all the parts of a hypothetical stratigraphic section spanning the past 165 Ma would be equally suitable for dating by magnetic polarity stratigraphy.

Sedimentación en comunidades arrecifales de Bahías de Huatulco, Oaxaca, México

Although coral reef sedimentation is important because it modifies processes like symbioses, reproduction, recruitment and coral growth, Mexican Pacific studies are lacking. On this regard, spatio-temporal variations in sedimentation rate were investigated in six coral reef communities from Oaxaca. During February 2006-January 2007 (excluding April, July and August) two sediment structures, with four sediment traps each, were randomly installed. Sediment traps were replaced with a mean periodicity of 38 days, and the sediments were washed, filtered, dried and weighted in order to calculate sedimentation rate. Sedimentation rate was heterogeneous among localities (F,36 = 7.06, P < 0.01). It was high at Isla Montosa (653.31 kg m(-2) year(-1)) and Isla Cacaluta (450.09 kg m(-2) year(-1)), intermediate at San Agustin, Jicaral-Chachacual and Dos Hermanas (155.18-92.53 kg m(-2) year(-1)) and low at La Entrega (14.33 kg m(-2) year(-1)). Sedimentation rate was homogeneous through time (F7,34 = 0.85, P > 0.5); nonetheless, during the dry season (November-March) sedimentation rate in the area oscillated between 6.8-73.5 mg cm(-2) day(-1), whereas during the rainy season (May-October) the values were 141-1088 % higher (74.5-147.6 mg cm(-2) day(-1), Mann-Whitney U = 137, n = 42, P = 0.03). There was a significant relationship between sedimentation rate and pluvial precipitation (Spearman R = 0.83, n = 8, P = 0.009), suggesting that the amount of sediment reaching coral communities is closely tied to regional precipitation. Sedimentation rates recorded at Isla Montosa (366.64 mg cm(-2) day(-1)) and Isla Cacaluta (366.03 mg cm(-2) day(-1)) during the rainy season can be considered lethal-sublethal considering sediment tolerance and rejection efficiency of stony corals. The absence of coral mortality during the rainy season may result from: a) high efficiency of active sediment removal, b) increased physiological tolerance to sediments, and c) a high degree of passive sediment removal via turbulence. Nonetheless, the combined effect of natural and anthropogenic induced sedimentation may cause a shift in coral community structure and eventually a loss of the reef areas in Bahiás de Huatulco.

Pyrite framboids interpreted as microbial colonies within the PermianZoophycosspreiten from southeastern Australia

AbstractTwo types of pyrite framboids (PF, probably sulphate-reducing bacteria) have been found within theZoophycosspreiten, hosted in the Guadalupian (Middle Permian) glaciomarine greywacke of the Westley Park Sandstone Member within the Broughton Formation from the southern Sydney Basin of southeastern Australia. They are composed of non-sheathed (PF1) and sheathed (PF2) sub-micron balls, respectively. Chemically, the sub-micron balls consist of iron, sulphur, carbon and oxygen. Both PF1 and PF2 occur in rhythmic alternation within the thick, light-grey and thin, dark-grey minor lamellae ofZoophycosspreiten. The framboids from the minor lamellae are highly abundant and occur in an orderly arrangement of equal density and in a good state of preservation. WithinZoophycosspreiten no homogeneous filling, fecal pellets, or any sign of re-exploitation of the minor lamellae have been recognized. No similar framboids have been observed outsideZoophycosspreiten. Therefore, the framboids are interpreted as the pyritized remains of microbial colonies withinZoophycosspreiten. The traceZoophycoswould be a multifunctional garden that may have been carefully constructed by theZoophycosmaker, where different microbial colonies were orderly and carefully planted and cultured within different minor lamellae. Further, it is proposed that theZoophycosmaker had a symbiotic relationship with microbial colonies on the mutual basis of food supply and redox conditions. The fact that the overlying spreiten cut the underlying ones indicates that theZoophycosfrom the study area is of an upward construction. The rhythmic alternation of both the thick, light-grey and thin, dark-grey minor lamellae withinZoophycosspreiten may be suggestive of a gardening manner of theZoophycosmaker responding to the warm and cold changes, food supply in pulses and variations of sedimentation rate for planting and culturing microbial colonies under the conditions of a glaciomarine environment at the high latitudes.

Paleoenvironmental interpretation of multiple soil–loess sequence at Milford Reservoir, northeastern Kansas

Abstract A loess/soil section near the Republican River in northeastern Kansas includes three superposed soils/paleosols in at least three superposed loess units, including Peoria Loess (PL), the Gilman Canyon Formation (GCF), and Loveland Loess (LL), The GCF coincides with a composite soil with a 1.3 m thick, buried organic-rich (A) horizon that yields AMS radiocarbon ages from 30.3 to 22.4 ka (or 35.7 to 26.9 ka when calibrated) that overlies a 1.6 m oxidized B horizon complex. The B subhorizons stratigraphically and morphologically resemble the “reddish pedocomplex” described elsewhere in Kansas and Illinois that dates between about 69 and 35 ka. The upper LL yields a TL age of about 136 and 130 + 30 ka. Thus, PL, the GCF, and LL were probably deposited mainly during the late Wisconsinan, middle Wisconsinan, and Illinoian (∼ Oxygen Isotope Stages (OIS) 2, 3, and 6, or about 12 to 24, 24 to 60, and 130 to 188 ka, respectively). Soils 1, 2, and 3, then formed mainly during OIS stages 1, 3, and 5, respectively, or about 12 to 12, 60 to 60, and 73 to 130 ka. In comparison with other dated stratigraphic sections in the Great Plains, soils/paleosols have relatively simple, cumulic horizon sequences, as is typical at sites distal from principle loess source regions. Nonetheless, they record a complex pedosedimentary history that reflects climatically-driven variations in rates of sedimentation and pedogenesis. Soil 1 is an Udic Argiustoll with 21-cm A (mollic)/69-cm Bt (argillic)/60-cm Bk horizons, that formed under modern-type grassland and climate. Soil 3, the Sangamon paleosol, includes a 3-m thick, highly rubified and clay-rich argillic horizon that, based on its morphological properties, probably formed under a relatively warm, moist, equable climate and deciduous-type forest. Soil 2 coincides with the GCF, and as such is a pedolithostratigraphic unit that includes a 1.3-m mollic horizon and to 1.6-m oxidized 3Btb and 3BCb horizons. Whereas the oxidized B horizons apparently represent relatively warm, moist climatic conditions between about 65 to 35 ka, the upper cumulic (mollic) A horizons of the GCF probably formed under progressively cooling climate and grassland between about 35.7 and 26.9 (calibrated radiocarbon) ka. Estimated mean accumulation rates (MARs) of loess range from 0.12 m/k yr for the GCF to about 0.5 m/k yr for PL.

Late Quaternary changes in ecosystems and carbon cycling on Mt. Kenya, East Africa: a landscape-ecological perspective based on multi-proxy lake-sediment influxes

The degree to which different lakes within a landscape respond coherently (in unison) to external drivers such as climate change and soil development is uncertain. Presentation of multi-proxy, geochemical and palaeoecological data from individual lakes in the form of fluxes minimizes distortions resulting from variable sedimentation rates and changes in sediment composition. We use the accumulation rates of magnetic minerals, total organic C and N, terrestrial and aquatic biomarkers, graminoid epidermis, pollen, green algae, diatoms and diatom C in four small lakes, situated between 2350 and 4595 m a.s.l. on the NE flank of Mt. Kenya, East Africa, to reconstruct changes in C cycling over the last 38 ka. The results conflict with earlier models of landscape and lake development, showing: (1) that glacial–interglacial changes in vegetation cannot be interpreted as simple, altitudinal shifts in the modern vegetation belts; and (2) that limnological changes were not coherent. Rapid variations in climate, water level, erosion and nutrient input overwhelmed long-term, successional trends in lake sedimentation and C accumulation since the Last Glacial Maximum. The results also reveal previously unrecognized features of the palaeoenvironmental record, such as the rapid degradation of organic matter in diatoms and the occurrence of a productive, fire-prone montane grassland during the highly seasonal, monsoonal climate of the Lateglacial and early Holocene.

Last millennia sedimentary record on a micro-tidal, low-accumulation prodelta (Têt NW Mediterranean)

Statistical sequential analysis was performed on a series of sediment cores collected from the Têt prodelta in the Gulf of Lions, northwestern Mediterranean Sea, between October 2003 and October 2004. Seabed changes during that period were correlated to hydrodynamic conditions (waves and currents) and river discharge. Low sediment supply prevents full preservation of new sediment strata on this low-accumulation prodelta located on a microtidal, storm-dominated inner shelf. Severe meteorological events caused a rapid succession of erosion and deposition phases. For example, the December 2003 flood and storm produced a flood layer deposit that persisted for 2 months with only slight transformations due to early diagenesis and/or bioturbation, until a new storm event eroded this layer. A typical sedimentary sequence was observed for the secular deposits composed of a 10-cm-thick sandy layer overlaying siltyclayey layers. These characteristic features were used to analyse the last millennia sedimentary record of the Têt prodelta. The low preservation of freshly deposited sediments and variable sedimentation rates during the last millennia period yield a sedimentary sequence formed by the outcropping of muddy prodeltaic units intersected by heterogeneous siltysandy units similar to those formed under present day hydrodynamic conditions. No flood layer was found related to catastrophic flooding of the last century in the sedimentary record. The Little Ice Age (∼ 1550–1850 AD) probably favoured the formation of a well-developed muddy prodelta in the mouth of the Têt River. Later on, the decrease of sediment supply by rivers due to climate change and/or human activities (damming, irrigation), and the increase of the number of high-energy storms reaching the coast, induced a coarsening of the top sediment layer on this prodelta. This modern change of the substrate is also observed in the composition of benthic biota found in the substrate.

Variable 10Be fluxes in lacustrine sediments from Tristan da Cunha, South Atlantic: a solar record?

A 650-yr-long sediment sequence from a crater lake on Tristan da Cunha, South Atlantic, was analysed for its 10Be content. Based on 14C dating, and sedimentary, geochemical, magnetic and palynological records, the period between 900 and 1450 AD appears to have been unusually stable in terms of sedimentation and vegetation and therefore this period was chosen for analysis of the 10Be content. During this period of highly organic sedimentation and closed vegetation, the pattern of 10Be flux variations follows the 10Be fluctuations from the GRIP ice core and estimated 14C production rates well. However, before and after this stable period, variable sedimentation rates have to be accounted for to obtain results that are comparable to the established 10Be/ 14C records. Our data show not only the possibility of obtaining detailed enough 10Be flux data from sedimentary sequences to reconstruct past solar forcing but also how sensitive this type of record is to sedimentary and environmental changes. If suitable archives can be found, they have the potential to improve reconstructions of solar activity far back in time.