9000 years of geochemical evolution of lithogenic major and trace elements in the sediment of an alpine lake – the role of climate, vegetation, and land-use history

A 9000cal. year record of geochemistry was analysed in a sediment core obtained from a Swiss alpine hard-water lake (1937 ma.s.l.) that is located at the present-day tree-line. Geochemical stratigraphies are compared to changes in mineralogy, grain-size, pollen, and macrofossil records. This allows the reconstruction of the effects of changes in vegetation and of 3500 years of land-use in the catchment area on sediment geochemistry. Using principal component analysis, two major geochemical groups are distinguished: (i) Changes in concentrations of Rb, Ti, Zr, Fe, As, and Pb are closely related to corresponding changes in the concentrations of quartz and clay. They are thus considered to represent the silicate fraction which shows an increase from the oldest to the youngest core section. (ii) In contrast, Ca and Sr concentrations are positively correlated with changes in silt, sand, and calcite. They are therefore considered to represent the carbonate fraction which gradually decreased. Based on constrained cluster analysis, the core is divided into two major zones. The oldest zone (A; 9000–6400 cal.BP) is characterised by high concentrations of detrital carbonates. The more open catchment vegetation at that time promoted the physical weathering of these carbonates. The second major zone (B, 6400 cal.BP–1996 AD) is divided into four subsections with boundaries at ca. 3500, 2400, and 160cal. BP. The lower part of this zone, B1, is characterized by a gradual decrease in the carbonate-silt fraction and a pronounced increase in the silicate-clay fraction. This is concurrent with the expansion of Picea in the catchment area, which probably stabilized the soil. The middle part, B2 and B3 (3500–160cal. BP), comprises pronounced fluctuations in all elements, especially Ca, Sr, Mn, and Rb, but also in clay and silt. These changes are related to varying intensities of alpine farming. In the same section, Mn/Fe ratios are highly variable, suggesting changes in the mixing regime of the lake with phases of anoxic bottom water. The uppermost section, B4 (since 160cal. BP), is characterized by a steep decline in the silicate fraction and an increase in Ca and Sr. Despite the decrease in the silicate fraction, Pb increases, due to elevated atmospheric input resulting from early metal pollution, are masked by the high natural variability. Generally, changes in vegetation, which correspond to climate changes in the early Holocene and to human activities since ca. 3700cal. BP, are the controlling factor for variations in the geochemical composition of the sediment of Sägistalsee.     

Classification of lacustrine sediments based on sedimentary components

This paper introduces a flexible sediment classification scheme for modern and ancient lake sediments employed at the Limnological Research Center, University of Minnesota. Our classification scheme emphasizes the macroscopic structure and microscopic components of the sedimentary matrix (clastic, chemical and biological) and can be applied to a wide range of lacustrine settings. Such a classification scheme is necessary to i) facilitate communication and collaboration between the highly interdisciplinary community of scientists that study lacustrine archives, ii) create a structure for comparative lacustrine sedimentological studies, and iii) take greater advantage of the great potential information recorded by changing depositional environments. Such a development is needed in light of current and planned Global Lake Drilling efforts and to expedite the creation of on-line, searchable databases of global lacustrine archives. A more comprehensive treatment of the topics discussed in this paper is available at: http://lrc.geo.umn.edu/services/handbook/sedclass.html.     

Reconstructing 20thcentury lead pollution andsediment focusing in a peat land pool (Kempen, Belgium), via 210Pbdating

Pb-210 dating of two metal-polluted organic sedimentcores obtained near a former pyrometallurgical zinc smelter in Lommel, Belgiumhave been used to reconstruct atmospheric lead deposition rates during the20^th century. Independent knowledge concerning historical pollutionevents and ¹³⁷Cs fall-out profiles has allowed a criticalevaluation of the CRS, CIC and CF-CS models for the ²¹⁰Pb ageinterpretation. Resulting ages for the three models suggest that, in this case,the CIC model gives the most accurate interpretation of historical pollutionevents and atmospheric lead fall-out. The ²¹⁰Pbwater-sediment flux was estimated at 141–1158Bq·m^–2·yr^–1 for one site and62–106 Bq·m^–2·yr^–1 at theother site, during the last century. The large difference illustrates thatsediment focusing was important on a small spatial scale (10 m).The direction of focusing correlates with the predominant wind direction.Maximum atmospheric lead deposition rates were found to be 1.63 ± 0.59g·m^–2·yr^–1 around 1968 AD,which is 2 orders of magnitude larger than the Belgian average in 1980 AD, and5 orders larger than Holocene atmospheric lead deposition.     

Sediment yield variability in Spain: a quantitative and semiqualitative analysis using reservoir sedimentation rates

An existing dataset of area-specific sediment yield (SSY) for 60 catchments in Spain that was retrieved from sediment deposition rates in reservoirs [Avendaño Salas, C., Sanz Montero, E., Rayán, C., Gómez Montaña, 1997. Sediment yield at Spanish reservoirs and its relationship with the drainage basin area. In: Proceedings of the 19th Symposium of Large Dams, Florence, 1997. ICOLD (International Committee on Large Dams), pp. 863–874] reveals that catchment area alone explains only 17% of the variability in SSY. In this study, an attempt to explain the remaining variability in SSY was made using a quantitative and a semiqualitative approach for 22 catchments. During a field survey, the 22 selected catchments were characterised by topography, vegetation cover, lithology, shape and the presence of gullies in the broad vicinity of the reservoir. This information was used to develop a factorial scoring index model that provides a fairly accurate and reliable prediction of SSY. A classical multiple regression model using climatic, topographic and land use properties derived from regional datasets could not explain as much variance as the qualitative index model, nor did it appear to be as reliable. The same conclusion could be drawn when using the CORINE soil erosion risk map of southern Europe. The low prediction capability of the multiple regression models and the CORINE soil erosion risk map could be attributed mainly to the fact that these methods do not incorporate gully erosion and that the land cover data are not a good representation of soil cover. Both variables have been shown to be of great importance during the field surveys. Future assessments of SSY could be quickly and efficiently made using the proposed factorial scoring index model. In comparison with other models, which demand more data, the index model offers an alternative prediction tool.     

Advances in Holocene mountain geomorphology inspired by sediment budget methodology

The sediment budget, which links sediment sources to sediment sinks with hydroclimatic and weathering processes mediating the response, is applied to the analysis of sediments in three alpine lakes in British Columbia. We provide two ways of using the sediment budget as an integrating device in the interpretation of mountain geomorphology. These approaches differ in their resolution and ability to budget the major components of the fine-sediment cascade in glaciated environments. Taken together, they provide an integrated index of landscape change over the Holocene. The first example compares the hydroclimatic controls of lake sedimentation for the last 600 years (A.D. 1370–1998) preserved in varved sediments from two of the lake basins. This hydroclimatological approach incorporates contemporary monitoring, air photo analysis, and detailed stratigraphy of sedimentation events within a single varve to infer the timing, sources, and preferred pathways of fine-grained sediments reaching the lake basins. The results indicate that glaciers, hillslope, and channel instability within the major subbasins are the principal sediment sources to the lake basins. Transitory sediment storage of glacially derived sediments within the channels is believed to modulate the episodic and more frequent delivery of sediments from adjacent hillslope and fluvial storage sites and direct routing of glacial rock flour during years of prolonged glacial melt. The second example, relying on the phosphorus geochemistry of sediments in an alpine lake basin, considers the evolution of phosphorus forms (from mineral to occluded and organic fractions) as a function of the soil development, inherent slope instability, and repeated cycles of glaciation and neoglaciation over the Holocene. This geochemical approach demonstrates that both neoglaciation and full glaciation have essentially zeroed the system in such a way that a high proportion of mineral phosphorus remains in the present lake sediments and the bioavailability of phosphorus (a key to ecosystem development) is low. Both examples illustrate the importance of variable sediment sources; the seasonality, frequency, and magnitude of sediment transfers; and the profound influence of ice cover over contemporary, neoglacial and Pleistocene time scales. They also signal the value of including both clastic and dissolved components in the sediment budget.     

INFLUENCE OF SEDIMENT TRANSPORT ON ARMORED SURFACES

1NFLU'ENCE OF SEDmeNT TRANSPORT ON ARMORED SURFACESKatinka KOLLl bo Antheas DrmCHlAbstract:LaboratOry exWnts have been cAned ou to stUdy the chateristics of stwen tZ'ansport inSteeP tw. The question is discussed wether an annM sMs is deStroyed due tD thebosport of material, because addihonal wises are tw on iL or the laye is shehe by thetranSN medal, beause a part of the stream poWer is used to move the material. Accordin tothe resultS of the PresN exH' in Wh the Me…     

MODELING OF SEDIMENT AND NONPOINT SOURCE POLLUTANT YIELD

1 INTRODUCT1ONWaer bodies commonly receive both point soure and nonpoint source pollutans. NonPoint pollution ischaracterized by occasional high loads and high concotions (PfatO, et al, l990; Shen, et al., l992;Novotuy and Olem,l994; Li and Shen, l996). It is very difficult to measure, to colltrOl and to trCat. FornonPoint source pollution control, the most productive research area is probably the develOPmnt andtesting of better watershed water quality models (Haith, l998). Measureme…     

Prediction of sand transport rates by waves and currents in the coastal zone

The predictions of a sand transport research model and Bijker's (J. Waterways, Harbours Coastal Eng. Div. ASCE 97 (WW4) (1971) 687) engineering model are compared with data obtained in wave-current conditions at three field sites. A key element in the present study is that the bed roughness at the three sites has been estimated from predictions of the sand ripple dimensions. The comparisons between suspended sand concentrations and transport rates show that a considerable amount of uncertainty (factor ±5 or more) arises when individual predictions are compared with the measurements. However, the overall bias in each set of comparisons is smaller than this, with overall agreement being within a factor of ±2 in most cases. While the results demonstrate that research models, adapted for field application, may be used to make practical sand transport computations with as much accuracy as engineering formulations, the true benefit of research models lies in the improved understanding of transport processes that they provide. This is illustrated with reference to the mechanism of grain size sorting caused by oblique incidence of waves on a current.     

Growth of large-scale bed forms due to storm-driven and tidal currents: a model approach

An idealized morphodynamic model is used to gain further understanding about the formation and characteristics of shoreface-connected sand ridges and tidal sand banks on the continental shelf. The model consists of the 2D shallow water equations, supplemented with a sediment transport formulation and describes the initial feedback between currents and small amplitude bed forms. The behaviour of bed forms during both storm and fair weather conditions is analyzed. This is relevant in case of coastal seas characterized by tidal motion, where the latter causes continuous transport of sediment as bed load.The new aspects of this work are the incorporation of both steady and tidal currents (represented by an M₂ and M₄ component) in the external forcing, in combination with dominant suspended sediment transport during storms. The results indicate that the dynamics during storms and fair weather strongly differ, causing different types of bed forms to develop. Shoreface-connected sand ridges mainly form during storm conditions, whereas if fair weather conditions prevail the more offshore located tidal sand banks develop. Including the M₄ tide changes the properties of the bed forms, such as growth rates and migration speeds, due to tidal asymmetry. Finally a probabilistic formulation of the storm and fair weather realization of the model is used to find conditions for which both types of large-scale bed forms occur simultaneously. These conditions turn out to be a low storm fraction and the presence strong tidal currents in combination with strong steady currents during storms.     

Impact of hillslope-derived sediment supply on drainage basin development in small watersheds at the northern border of the central Alps of Switzerland

This paper explores the effects of hillslope mobility on the evolution of a 10-km² drainage basin located at the northern border of the Swiss Alps. It uses geomorphologic maps and the results of numerical models that are based on the shear stress formulation for fluvial erosion and linear diffusion for hillslope processes. The geomorphic data suggest the presence of landscapes with specific cross-sectional geometries reflecting variations in the relationships between processes in channels and on hillslopes. In the headwaters, the landscape displays parabolic cross-sectional geometries indicating that mass delivered to channels by hillslope processes is efficiently removed. In the trunk stream portion, the landscape is (i) V-shaped if the downslope flux of mass is balanced by erosion in channels (i.e. if mass delivered to channels by hillslope processes is efficiently removed) and (ii) U-shaped if in-channel accumulation of hillslope-derived material occurs. This latter situation indicates a non-balanced mass flux between processes in channels and on hillslopes.Information about the spatial pattern of the postglacial depth of erosion allows comparative estimates to be made about the erosional efficiency for the various landscapes that were mapped in the study area. The data suggest that the erosional potential and sediment discharge are reduced for the situation of a non-balanced mass flux between processes in channels and on hillslopes. These findings are also supported by the numerical model. Indeed, the model results show that high hillslope mobility tends to reduce the hillslope relief and to inhibit dissection and formation of channels. In contrast, stable hillslopes tend to promote fluvial incision, and the hillslope relief increases. The model results also show that very low erosional resistance of bedrock promotes backward erosion and steepening of channel profiles in headwaters. Beyond that, the model reveals that sediment discharge generally increases with decreasing erosional resistance of bedrock, but that this increase decays exponentially with increasing magnitudes of fluvial and hillslope mobilities. Very high hillslope diffusivities even tend to reduce the erosional potential of the whole watershed. It appears that besides rates of base-level lowering, factors limiting sediment discharge might be the nonlinear relationships between processes in channels and on hillslopes.