Holocene alluvial morphopedosedimentary record and environmental changes in the Bardenas Reales Natural Park (NE Spain)

A Holocene alluvial morphostratigraphy, based on geomorphological mapping, sedimentological analysis, soil development and radiocarbon chronology is presented for the semiarid Blanca basin in the Bardenas Reales Natural Park (NE, Spain). Four morphopedosedimentary alluvial units (MU1, MU2, MU3 and MU4) were differentiated, which may correspond to two palaeogeographical stages. The first one includes the morphopedosedimentary unit MU1 and is the most extensive in the area. It is made up of a complex aggradation (fill up) sequence comprising a braided channel system, a sand–mud flat (9200–7900 cal BP) and a playa-lake environment related to a nearly closed depression. An intensive fluvial entrenchment period is recognized, leading the opening of the Blanca basin, before the second stage that includes the units MU2, MU3 and MU4. This set of units forms a sequence of cut-and-fill and nested fill terraces recording the alluvial activity in the Blanca basin during the last 6 ka. The MU2 unit, 5900–3300 cal BP in age, represents a period of high geomorphic instability with rapid alluvial sedimentation/channel incision oscillating changes. On the other hand, the MU3 unit has an age of 1050–500 cal BP and appears as a thin extensive alluvial cover overlying previous units. Finally, the MU4 unit comprises superposition of numerous flood depositional sequences, centimetric in scale, dated from 450–10 cal BP. The established soil chronosequence supports the numerical chronology of the four units. These accumulation periods (MU1, MU2, MU3, MU4) are associated with dry phases and/or high hydrological variability (floods and droughts) at a regional scale. Changes on the North Atlantic atmospheric circulation mode over Southern Europe may provide the key to explain the nature of alluvial activity during Holocene in Bardenas Reales.     

Landscape sensitivity to rapid environmental change—a Quaternary perspective with examples from tropical areas

A millennial scale of ‘rapid’ change during the Quaternary is recorded in GRIP and GISP2 ice-core records, and is also found in tropical oceans. Rapid warming episodes followed by gradual cooling, associated with sub-Milankovich cycles implies an asymmetry in the behaviour of climate that is likely to be reflected in landscape responses. Slope failures, floods, colluvial/alluvial sedimentation may reflect short-term changes in the record, but reorganisation of slope and fluvial systems involves significant time lags or delays, often on a millennial scale, and requiring changes to the vegetation cover. While some records of sedimentation indicate major landscape instability during last 20 ky, or in the early Holocene, others indicate pulses of activity throughout the Last Glacial Cycle (LGC). These differences may reflect regional patterns of climate change, but also illustrate the importance of landscape sensitivity to our understanding of the impacts of rapid environmental change. Methodological problems arise in attempting correlations between site-specific records of sedimentation and integrated signals of hydrologic change found in deltaic and off-shore fans.     

Contribution to the paleolimnology of lake Valencia, Venezuela: Seismic stratigraphy

Lake Valencia is situated in a graben at 402 m a.s.l. in northern South America. The lake is slightly larger than 300 km² and the basin area is approximately 2750 km². Observations during the last two centuries suggest that the lake level has oscillated significantly. It is thought that the lake originated during the Quaternary, but its age may be as old as the Late Tertiary. Acoustic reflection profiles (UNIBOOM) indicate that, to the depth reached, the subsurface seismic stratigraphy consists of three units (Units I, II and III) separated by prominent reflectors. A 7.43 m core was obtained in the central part of the lake which reached approximately the base of Unit I. Sedimentological, palynological, and radiocarbon analyses indicate that Unit I is a Holocene lacustrine deposit, and that in the Late Pleistocene the lake either did not exist or was greatly reduced. Units II and III probably represent similar older lacustrine units. The stratigraphy is interrupted by Late Pleistocene-Holocene faulting. Clastic lenses were found near the base of Unit I, above the reflector which separates it from Unit II, in the eastern seismic profiles. These are interpreted as Early Holocene fluvio-lacustrine sediments which were deposited subaerially or as deltas.     

Response of alluvial fan systems to the late Pleistocene to Holocene climatic transition: contrasts between the margins of pluvial Lakes Lahontan and Mojave, Nevada and California, USA

Dated shorelines of late Pleistocene pluvial Lakes Lahontan (Great Basin Desert, northwest Nevada) and Mojave (Mojave Desert, eastern California) provide timelines for the assessment of alluvial fan sedimentation at the lake margins during the late Pleistocene to early Holocene. Two sets of alluvial fan systems have been mapped: the Stillwater fans, feeding Lake Lahontan; and the Zzyzx fans, feeding Lake Mojave. Their contrasting morphologies suggest different responses of the two fan systems to late Pleistocene to early Holocene climatic change. At the time the Stillwater fan systems underwent minimal sedimentation, with the catchment hillslopes apparently stable. The Zzyzx fans experienced major changes in water and sediment supply from the catchment hillslopes. There was a major phase of hillslope debris-flow activity, followed by fanhead trenching and distal fan progradation. Both areas were wetter and colder in the late Pleistocene than they are today, but during the transition to the Holocene the Zzyzx area was more likely to experience intense rains associated with the monsoonal penetration of warm moist tropical air into the Southwest. Vegetation reconstructions for the late Pleistocene to the early Holocene suggest that catchment hillslopes in the Mojave supported a desert shrub vegetation, but those in the Stillwaters supported juniper woodland and grasses at low elevations and pine at higher elevations. Contrasts in hillslope vegetation cover together with storm activity may account for the different responses of the alluvial fans to climatic change during the Pleistocene to Holocene climatic transition. After the falls in lake levels of Lakes Lahontan and Mojave in the early Holocene, both areas underwent aridification, resulting in reductions in hillslope vegetation cover. Increased storm runoff led to fanhead trenching and distal progradation of the alluvial fans. Variations in fan style at that time may relate primarily to base-level conditions resulting from different gradients on the exposed lake shores.     

Trends of the natural evolution of chernozems on the East European Plain

A generalized chronosequence of changes in the morphometric parameters of chernozems in the past 5100 years is analyzed. It is shown that the development of chernozems in the center of the East European Plain is characterized by both long-term (interglacial, encompassing the entire Holocene) and short-term (within separate climatic periods of the Holocene) trends. The long-term trend of the evolution of ordinary chernozems developed from the loess-like calcareous loam consists of a continuous increase in the thickness of the soil humus profile and an oscillating increase in the depth of the carbonate accumulations. The Late-Holocene short-term trends of the natural evolution of chernozems are characterized by the high spatial heterogeneity related to the patterns of climatic fluctuations and to the lithological composition of parent materials. In that period, the thickness of genetic horizons in the forest-steppe chernozems was increasing faster than that in the steppe chernozems.     

Soil-landscape chronograms for pedochronological analysis

Pedochronology is defined as the sequence of soil changes over time in a given landscape. Integrated stratigraphical and paleogeomorphic analysis of soil-landscape systems can offer information on landscape history that provides a rational basis for further pedological and soil-geomorphological studies. To connect these aspects of research, a new scheme, called chronogram, is introduced in which geogenic and pedogenic events can be summarized.Soil-landscape chronograms have time on one axis and depict erosion, deposition and soil formation across a given landscape along the other axis. Using open and enclosed hillslope systems as examples, the construction of chronograms and their relations to other summarizing schemes are demonstrated. These schemes include Butler's K-cycle system, Bos and Sevink's site history scheme, the catena, the toposequence, the Nine Unit Land-surface Model, and Huggett's simulation model. Opportunities for further pedochronological analysis, using paleosols, authigenic and allogenic pedorelicts, are identified.     

Dry Weight Deposition in Polluted Onondaga Lake,New York,U.S.A.

Short and long-term trends in dry weight deposition rates are documented for polluted Onondaga Lake, NY, based on analyses of sediment trap collections made below the epilimnion at a single deep-water location in ten years over the 1980–1992 period using cylindrical traps. Additionally, comparisons of dry weight rates obtained with two different diameter (4.0 and 7.6 cm) traps of the same aspect ratio (6), at four different depths below the epilimnion, and from the lake's two basins, in 1993, are presented to establish the representativeness of the long-term data. The near equivalence of fluxes determined with the different size traps and cumulative deposition rates measured over the range of deployment depths, and the minor differences in flux observed for the two basins, supports the representativeness of long-term data in quantifying deposition rates for the pelagic zone of the lake. Short-term variations in downward flux have been common in the lake over the period of monitoring. The summer-average dry weight deposition rate decreased systematically (by about 45%) following the closure (in 1986) of a soda ash facility that caused enhanced rates of calcium carbonate deposition; from an average of 23.2 to 12.8 g m-² d-¹. These dry weight deposition rates are high compared to values reported in the literature for other lakes, and are responsible for the lake's high rate of net sedimentation.     

Comparison of mercury concentrations in modern lake sediments and glacial drift in the Canadian Shield in the region of Ottawa/Kingston to Georgian Bay,Ontario, Canada

An ongoing problem in evaluating the significance of mercury (Hg) in surficial materials is distinguishing sources of natural (spatial) variation of the geological/geochemical environment from sources (airborne, waterborne, etc.) of anthropogenic (temporal) variation. The Geological Survey of Canada (GSC) has carried out a series of sampling programs, including one in the southeastern part of the geologically complex Canadian Shield, in order to link the easily observable lithological variations of bedrock with the chemical composition of overlying glacial deposits and lake sediments. This research aims to provide a base against which observed variations in life systems can be judged as natural or anthropogenic. In the study area, high concentrations of Hg and other trace elements in lake sediment and glacial sediments can be related to glacial dispersal from mineralized bedrock and/or bedrock with high natural background concentrations of these elements.     

Accumulation of pyrite and Fe-rich carbonate and phosphate minerals in a lowland moor area

Pyrite and iron-rich carbonate and phosphate minerals are found in large quantities in an inland moor in western Jutland, Denmark. Sediments with up to 10.4% pyrite (FeS₂) occur well-separated from sediments with iron-rich carbonate and phosphate minerals both reaching up to ∼6–7%. Analysis of the ground water showed that water enters the moor from different sources. Water of high alkalinity (up to 2.05 meq l⁻¹) as well as water rich in sulphate (up to 79 mg l⁻¹), sulphide (up to 1.75 mg l⁻¹) and Fe²⁺ (up to 22.4 mg l⁻¹) were sampled from the upper ground water flowing into the moor. On entering the moor processes within moor sediments control authigenic mineralogy, resulting in segregated mineral precipitation.     

Sedimentological study of aeolian soil parent materials in the Río Sauce Grande basin, Buenos Aires province, Argentina

Sedimentological characteristics of aeolian soil parent materials in the Río Sauce Grande basin, southern Buenos Aires province, were analysed using grain-size distributions, transport mechanisms, dynamics of the sedimentary process and the nature of calcareous layers. Samples were collected from C horizon of 13 profiles distributed over all the piedmont areas and plains. Calcium carbonate in these materials is either inherited from the parent material or was redistributed by pedogenic processes. Their amounts in Ck horizons increase from the upper to the lower basin, and are inversely related to the thickness of soil parent material, to mean annual rainfall and to the calcrete depth.The cumulative grain-size plots show that the sediments were transported by both aeolian saltation and aeolian suspension. Most of them have coarser and more heterogeneous grain size distributions than typical loess of the northern hemisphere, indicating a larger and more fluctuating wind energy than is required for the formation of typical loess. Deposits in the upper and middle basins were deposited in two main aeolian episodes which can be related to the dry periods of the Late Pleistocene and the Late Holocene recorded in southern Buenos Aires province. In some thin profiles of the middle and lower basins, the large clay content of the calcic C horizon was probably inherited from one or more previous pedogenic events. There is a systematic increase of particle size towards the lower basin, which may be related to an increase of energy towards the lower basin or to the influence of littoral dune fields along much of the Atlantic coastal zone.