Increase In Sediment Accumulation Rates Research Articles

Paleomagnetic and fission-track dating of a Late Cenozoic red earth section in the Liupan Shan and associated tectonic implications

The north-trending Liupan Shan (六盘山) is an important tectonic boundary between the Tibetan Plateau and the Ordos platform. The Late Cenozoic red earth deposits of the Liupan Shan record its tectonic history and environmental effects. In this article we report a new Late Cenozoic red earth section from an intermontane basin in the southern part of the Liupan Shan. Lithofacies analysis, paleomagnetic and fission-track chronologies, and paleocurrent analysis have been employed to identify the tectonic uplift events of the Liupan Shan. Based on the age constraints of mammal fossils, the paleomagnetic polarity zones of the Huating (华亭) Section can be approximately correlated with the standard polarity zones that lie between C3An.2n and C5n.1n of the Geomagnetic Polarity Timescale; the bottom age of this section is approximately 10 Ma. Based on this and the previous studies, we infer that a tectonic event commenced in the southern Liupan Shan in this interval between 8.3 and 8.7 Ma, accompanied by a remarkable increase in sediment accumulation rate. Field observations, fission-track dating, determinations of grain-size frequency distributions and the vertebrate fossils found there suggest that the red earth deposits were reworked by water and mainly transported by fluvial-alluvial processes from the adjacent area.

Sediment accumulation rates and turbidite frequency in the eastern Algerian margin. An attempt to examine the triggering mechanisms

From the analysis of seven new sediment piston-cores sampled in 2005 (MARADJA-2 French–Algerian cruise), this study aims to identify for the first time possible late Pleistocene to recent sedimentary instabilities controlled by seismicity off or close to the eastern coast of Algeria. The detailed lithologic study allows us to identify the frequency of the gravity events (turbidites, debrites) and to discuss their geographical sources and triggering mechanisms. Based on a chronostratigraphy of 24 14C AMS datings, sediment accumulation rates in zones extending off Bejaia and Annaba and, in particular, semi-quantitative analysis of the microfossils and lithogenic tracers of the origin of gravity flows was discussed. Two sediment cores, here considered as reference cores, enabled the estimation of palaeoenvironmental parameters that controlled sedimentation: (1) in the prodelta of Soummam Oued, after 2215calyrBP, floods were less frequent and sediment accumulation rates decreased because of a drier climate; (2) in the middle slope to the NE of Annaba, a location shielded from gravity flows, an increased sedimentation rate coincided with the passage of warmer waters leading to maxima of carbonate biogenic fluxes (particularly pteropods). Off Bejaia, two deep sediment cores show a spectacular increase in sediment accumulation rate between 2200 and 1000calyrBP while turbidites become more frequent. According to the eustatic and climatic stability of this interval, an episode of strong slope instability of the slope is suggested. Both sediment cores on the slope of Annaba indicate an increase in gravity flows during the same last thousand years, which is tentatively related to a regional increase of seismicity during this interval. This spatial distribution of gravity events is clearly different to that of the western margin where the sedimentation is less perturbed.

Strike‐slip tectonics and basin inversion in the Western Mediterranean: the Post‐Messinian evolution of the Alboran Sea

AbstractA comprehensive interpretation of single and multichannel seismic reflection profiles integrated with biostratigraphical data and log information from nearby DSDP and ODP wells has been used to constrain the late Messinian to Quaternary basin evolution of the central part of the Alboran Sea Basin. We found that deformation is heterogeneously distributed in space and time and that three major shortening phases have affected the basin as a result of convergence between the Eurasian and African plates. During the Messinian salinity crisis, significant erosion and local subsidence resulted in the formation of small, isolated, basins with shallow marine and lacustrine sedimentation. The first shortening event occurred during the Early Pliocene (ca. 5.33–4.57 Ma) along the Alboran Ridge. This was followed by a major transgression that widened the basin and was accompanied by increased sediment accumulation rates. The second, and main, phase of shortening on the Alboran Ridge took place during the Late Pliocene (ca. 3.28–2.59 Ma) as a result of thrusting and folding which was accompanied by a change in the Eurasian/African plate convergence vector from NW‐SE to WNW‐ESE. This phase also caused uplift of the southern basins and right‐lateral transtension along the WNW‐ENE Yusuf fault zone. Deformation along the Yusuf and Alboran ridges continued during the early Pleistocene (ca. 1.81–1.19 Ma) and appears to continue at the present day together with the active NNE‐SSW trending Al‐Idrisi strike‐slip fault. The Alboran Sea Basin is a region of complex interplay between sediment supply from the surrounding Betic and Rif mountains and tectonics in a zone of transpression between the converging African and European plates. The partitioning of the deformation since the Pliocene, and the resulting subsidence and uplift in the basin was partially controlled by the inherited pre‐Messinian basin geometry.

Holocene oceanographic and glacial changes recorded in Maxwell Bay, West Antarctica

Three Holocene core sections from Maxwell Bay, South Shetland Islands, West Antarctica are investigated, using MS, grain size, TOC, foraminifera, δ18O and δ13C, 14C, and 210Pb, in order to examine Holocene oceanographic, glacial and climatic changes in the region. They come from 190, 221, and 333m water-depths and the bay margin-proximal locations, complementing earlier studies from the deep central part. The three records vary between locations and reflect combined marine and terrigenous influence from the open ocean and surrounding islands, respectively. The longest and best dated composite section comes from the inner part of the bay (core JTC/JPC-17) and extends back to 6.3calkyr BP, close to the termination of the Holocene Climatic Optimum.A combination of dating problems and mixed foraminiferal assemblages prevented reliable interpretation of core JPC-15 from the outer part of the bay. Core JPC-16, collected from the inner bay near the inlet between King George and Nelson Islands, revealed repeated highs in planktonic Neogloboquadrina pachyderma occurrence during last 3000yr. These highs are interpreted as indicating periods of decreased meltwater flow into the bay, and thus greater influence of open-marine surface-waters. Core JTC/JPC-17, located closest to King George Island and the Collins Ice Cap, records increased glacial conditions at ~2000calyr BP. This event is recorded by turbidite deposition at core site JPC-16.Major components of the benthic foraminiferal assemblage indicate decreasing tolerance to near-glacier or near-shore conditions, however, local climate variability appears to have overall little impact on benthic foraminiferal biota. Thus, it is unlikely that, in similar settings, recent warming would be strongly manifest as changes in benthic foraminiferal communities. Stable isotopic analysis is based on Cassidulinoides parkerianus and Globocassidulina biora. Subtle decrease in δ18O at ~3500calyr BP, followed by episodic increases at ~1000calyr BP and in the youngest part of the record, appear to correlate with the period prior to the beginning of the Neoglacial, with Medieval Warm Period, and with post Little Ice Age warming, respectively. The latest event is also marked by a significant increase in sediment accumulation rate in the second half of the 20th century.

Lake sediment cores as indicators of historical metal(loid) accumulation – A case study in Mexico

Abstract To examine and compare historical accumulation of metal(loid)s in Mexican lakes and reservoirs, 210 Pb and 137 Cs dated sediment cores were evaluated: two from the remote Zempoala and Miramar Lagoons and three from Lake Patzcuaro, and the Intermedia and Silva dams that are affected by human activities. Sediment ecotoxicology was assessed using consensus-based sediment quality guidelines for freshwater ecosystems. The +100 a sediment core from the remote Miramar Lagoon had the highest concentrations of Cr and Ni these being higher than the Probable Effect Levels (PELs). Zinc concentrations were also higher in the Miramar Lagoon compared to the other lakes and reservoirs, with concentrations higher than the Threshold Effect Level (TEL). Mercury concentrations from this lagoon were comparable to those for the Intermedia dam that receives water from urban, industrial and agricultural areas. The higher metal concentrations in the core from the Miramar Lagoon suggest that metal concentrations in the rocks of the watershed are high. Another explanation for the higher metal concentrations is the slow sediment accumulation that causes metals to be accumulated over longer time-periods at the sediment–water interface. A decrease in the concentration of As in the Intermedia dam was observed in sediments corresponding to the last decades. This may be due to an increase in sediment accumulation rate or to the reduction in sources of this metalloid in the watershed. In the Miramar Lagoon, an increase was observed in concentrations of As and Cr in more recent sediments, probably related to increased deforestation in the area or the eruption of El Chichonal volcano in 1982. Concentrations of Pb showed a decreasing tendency over the past decades in the Lake Patzcuaro, Miramar and Zempoala Lagoons sediment cores while such behavior was not be observed for the Intermedia dam. This reduction in concentrations of Pb was attributed to the decrease in use of leaded gasoline.

Magnetostratigraphy of the Suerkuli Basin indicates Pliocene (3.2 Ma) activity of the middle Altyn Tagh Fault, northern Tibetan Plateau

The left-lateral strike-slip Altyn Tagh Fault (ATF) forming the northern boundary of the Tibetan Plateau accommodates parts of the overall convergence between the colliding Indian and Eurasian plates. Precise dating of the ATF activity is essential for understanding possible mechanisms of Tibetan Plateau deformation and uplift. Here we report a magnetostratigraphic study of the Suerkuli Basin deposits recording depositional changes during the ATF activity. Field investigations reveal a remarkable and widespread change in depositional environment in the Suerkuli Basin, i.e. a transformation from low-energy lacustrine deposits (grayish-green mud-siltstone and brown mud-siltstone) into high-energy alluviul fan deposits (poorly sorted gray pebble and cobble conglomerates). Detailed magnetostratigraphy of the 390-m-thick Daban section, at the southeastern margin of the Suerkuli Basin (38°43.09′N, 90°58.84′E), shows that this change in depositional facies occurred at ∼3.2 Ma, accompanied by a remarkable increase in sediment accumulation rate. We attribute this depositional change to the Piocene tectonic activity of the middle ATF although the contribution of the Pliocene global climate deterioration cannot be excluded.

Environmental variability in Lake Naivasha, Kenya, over the last two centuries

Lake Naivasha, Kenya, is one of a number of freshwater lakes in the East African Rift System. Since the beginning of the twentieth century, it has experienced greater anthropogenic influence as a result of increasingly intensive farming of coffee, tea, flowers, and other horticultural crops within its catchment. The water-level history of Lake Naivasha over the past 200 years was derived from a combination of instrumental records and sediment data. In this study, we analysed diatoms in a lake sediment core to infer past lacustrine conductivity and total phosphorus concentrations. We also measured total nitrogen and carbon concentrations in the sediments. Core chronology was established by 210Pb dating and covered a ~186-year history of natural (climatic) and human-induced environmental changes. Three stratigraphic zones in the core were identified using diatom assemblages. There was a change from littoral/epiphytic diatoms such as Gomphonema gracile and Cymbella muelleri, which occurred during a prolonged dry period from ca. 1820 to 1896 AD, through a transition period, to the present planktonic Aulacoseira sp. that favors nutrient-rich waters. This marked change in the diatom assemblage was caused by climate change, and later a strong anthropogenic overprint on the lake system. Increases in sediment accumulation rates since 1928, from 0.01 to 0.08 g cm−2 year−1 correlate with an increase in diatom-inferred total phosphorus concentrations since the beginning of the twentieth century. The increase in phosphorus accumulation suggests increasing eutrophication of freshwater Lake Naivasha. This study identified two major periods in the lake’s history: (1) the period from 1820 to 1950 AD, during which the lake was affected mainly by natural climate variations, and (2) the period since 1950, during which the effects of anthropogenic activity overprinted those of natural climate variation.

Holocene trophic state history of a subtropical blackwater lake, South Georgia, USA

Organic-rich sediment from Lake Louise, a dystrophic sinkhole lake in south Georgia, displays variations in C, N, P, C/N, δ13C, δ15N, biogenic silica (BSi) and diatom flora that document changes in trophic state over the past ~9,500 years. The lake initially was oligotrophic and moderately productive, but by the middle Holocene a rising regional water table, driven by eustatic sea level rise, caused expansion of wetlands around the lake and a shift to humic waters. Low rates of sediment accumulation, low C contents, rising C/N, and light δ13C and δ15N indicate this was a time of low productivity, more anoxic bottom waters and extensive recycling of littoral organic matter. These conditions persisted until ~1800 AD when a physical disturbance to the watershed, probably the Great Hurricane of 1780, resulted in a dramatic increase in productivity that has continued to the present day. We attribute this shift, recorded by a >tenfold increase in sediment accumulation rate, higher C, P, and δ15N, and lower BSi, to establishment of an inflow stream that increased nutrient delivery to the lake, raised water level, and expanded the wetland area around the lake. Since ~1930, logging, farming, and highway construction have impacted the lake, further accelerating biological productivity as well as the delivery of terrigenous sediment. Results of this study illustrate the potential of a single, catastrophic event to permanently alter the hydrology and chemistry of a lacustrine system and confirm that dystrophic lakes can be highly productive and therefore promising targets for paleolimnological study.

Neogene uplift of the Tian Shan Mountains observed in the magnetic record of the Jingou River section (northwest China)

The Tian Shan Mountains constitute central Asia's longest and highest mountain range. Understanding their Cenozoic uplift history thus bears on mountain building processes in general, and on how deformation has occurred under the influence of the India‐Asia collision in particular. In order to help decipher the uplift history of the Tian Shan, we collected 970 samples for magnetostratigraphic analysis along a 4571‐m‐thick section at the Jingou River (Xinjiang Province, China). Stepwise alternating field and thermal demagnetization isolate a linear magnetization component that is interpreted as primary. From this component, a magnetostratigraphic column composed of 67 polarity chrons are correlated with the reference geomagnetic polarity timescale between ∼1 Ma and ∼23.6 Ma, with some uncertainty below ∼21 Ma. This correlation places precise temporal control on the Neogene stratigraphy of the southern Junggar Basin and provides evidence for two significant stepwise increases in sediment accumulation rate at ∼16–15 Ma and ∼11–10 Ma. Rock magnetic parameters also undergo important changes at ∼16–15 Ma and ∼11–10 Ma that correlate with changes in sedimentary depositional environments. Together with previous work, we conclude that growth history of the modern Tian Shan Mountains includes two pulses of uplift and erosion at ∼16–15 Ma and ∼11–10 Ma. Middle to upper Tertiary rocks around the Tian Shan record very young (<∼5 Ma) counterclockwise paleomagnetic rotations, on the order of 15° to 20°, which are interpreted as because of strain partitioning with a component of sinistral shear that localized rotations in the piedmont.

Magnetic polarity stratigraphy of the Neogene foreland basin deposits of Nepal

ABSTRACTThe early Miocene Dumri Formation and middle Miocene–Pliocene Siwalik Group were deposited in the Himalayan foreland basin in response to uplift and erosion in the Himalayan fold‐thrust belt. We report magnetostratigraphic data from four sections of these rocks in Nepal. Three of these sections are in the Siwalik Group in the hanging wall of the Main Frontal thrust, and one section is from the Dumri Formation in the hanging wall of the Main Boundary thrust (MBT). Thermal demagnetization experiments demonstrate that laminated siltstones yield palaeomagnetic data useful for tectonic and magnetostratigraphic studies whereas other lithofacies yield data of questionable reliability. Magnetostratigraphic data have been acquired from 297 sites within a 4200‐m‐thick section of Siwalik deposits at Surai Khola. The observed sequence of polarity zones correlates with the geomagnetic polarity time scale (GPTS) from chron C5Ar.1n to chron C2r.2n, spanning the time frame ca. 12.5–2.0 Ma. At Muksar Khola (eastern Nepal), 111 palaeomagnetic sites from a 2600‐m‐thick section of the Siwalik Group define a polarity zonation that correlates with the GPTS from chron C4Ar.2n to chron C2Br.1r, indicating an age range of ca. 10.0–3.5 Ma. At Tinau Khola, 121 sites from a 1824‐m‐thick section of the Siwalik Group are correlated to chrons C5An.1n through C4r.1n, equivalent to the time span ca. 11.8–8.1 Ma. At Swat Khola, 68 sites within a 1200‐m‐thick section of lower Miocene Dumri Formation are correlated with chrons C6n through C5Bn.2n, covering the time span ca. 19.9–15.1 Ma. Together with previous results from Khutia Khola and Bakiya Khola, these data provide the first magnetostratigraphic correlation along nearly the entire NW–SE length of Nepal. The correlation demonstrates that major lithostratigraphic boundaries in the Siwalik Group are highly diachronous, with roughly 2 Myr of variability. In turn, this suggests that the major sedimentological changes commonly inferred to reflect strengthening of the Asian monsoon are not isochronous. Sediment accumulation curves exhibit a 30–50% increase in accumulation rate in four of the five sections of the Siwalik Group, but the timing of this increase ranges systematically from ∼11.1 Ma in western Nepal to ∼5.3 Ma in eastern Nepal. If this increase in sediment accumulation rate is interpreted as a result of more rapid subsidence owing to thrust loading in the Himalaya, then the diachroneity of this increase suggests lateral propagation of a major thrust system, perhaps the MBT, at a rate of ca. 103 mm year−1 across the length of Nepal.

Anthropogenic Impacts on Sedimentary Sources and Processes in a Small Urbanized Subtropical Estuary, Florida

The Loxahatchee River, a 500 km2 watershed on the southeast Florida coast, is a barrier-impounded drowned river valley experiencing rapid urbanization during the past 50 years. The estuary is currently composed of a sandy central basin with bifurcating forks accumulating organic-rich muddy sands. The Northwest Fork drains natural forest, residential, and agricultural catchments and has a much larger bayhead delta than the channelized Southwest Fork, which is only 50 years old and diverts much of the flow from the Northwest Fork. Sediment accumulation rates within muddy sand deposits are about 2–3 mm a−1, commensurate within error of the current rate of local sea-level rise. Previously established Holocene accumulation rates are close to relative sea-level rise, implying that sediment accumulation is in equilibrium with the creation of accommodation space. The main anthropogenic influences have been changes in surface sediment texture corresponding to dredging-induced modification of tidal currents and a localized >threefold increase in sediment accumulation rate supplied by sources local to the estuary. Late Holocene sedimentation has been episodic, and lithofacies vary from bioturbated transgressive sands at depth to peat and laminated fine-grained sediments then back to mottled muddy sands in the uppermost strata. This facies transition is attributed to a change of water column stratification conditions caused by decreased tidal flushing in the estuary possibly brought about by variation in sea level or episodic inlet closure by littoral sand transport. The magnitude of the anthropogenic changes in sedimentation is roughly equal with natural changes in lithofacies formation.

Oceanographic and climatologic controls on the compositions and fluxes of biogenic materials in the water column and sediments of the Cariaco Basin over the Late Holocene

Materials collected by sediment traps over a 3-y period and sedimentary horizons from a gravity core covering the last 6000 y were used to investigate the effects of climate-related processes such as wind-driven upwelling and regional rainfall on the production, export and burial of particulate organic matter in the Cariaco Basin. A variety of chemical analyses, including organic carbon and nitrogen, biogenic opal, calcite, lithogenic contents, stable carbon isotopic ratios of organic matter and the yields of CuO reaction products derived from distinct biochemicals such as amino acids, fatty acids and lignins, were carried out for this purpose. Principal component analyses were used to investigate the trends in this multivariate data set. These analyses reveal marked temporal differences in the composition of the materials sinking through the water column, which were related to distinct oceanographic and climatic forcings. For example, autochthonous fluxes, characterized by elevated contents of organic carbon and opal as well as high yields of amino acid and fatty acid reaction products, displayed peaks during periods of intense wind-driven upwelling. In contrast, allochthonous materials, characterized by elevated lithogenic contents and elevated yields of lignin-derived products, were more important during periods of high rainfall, low wind and enhanced stratification. In addition to the strong seasonal contrasts, there was significant temporal variability at both shorter (monthly) and longer (inter-annual) time scales. Hence, other factors, such as zooplankton grazing and El Niño effects on local climatology, may also be important. Examination of the gravity core record yielded several significant trends. For example, there was a marked increase in sediment accumulation rates from 5000 to ca. 700 y before present with concomitant increases in the concentrations of organic carbon, opal and most biomarkers. These results suggest that the Cariaco Basin experienced a marked increase in primary productivity and particle flux to the underlying sediments since the Holocene Thermal Maximum. Also within the sedimentary record, we observed distinct variations in the relative contributions of autochthonous and allochthonous organic matter. The frequency of these variations is roughly 1500 y and appears to match ice-rafted debris records from the North Atlantic. Such coincidence indicates cold periods within the Holocene, which are related to minima in insolation, may have led to the southern migration of the inter-tropical convergence zone and the enhancement of wind-driven upwelling, primary productivity and autochthonous organic matter flux to the seabed in the Cariaco Basin. Alternatively, during warm periods, the opposite climatic conditions would have increased both the thermal stratification of the water column and average rainfall in the Cariaco Basin, leading to elevated inputs of allochthonous materials.

Recent habitat degradation in karstic Lake Uluabat, western Turkey: A coupled limnological–palaeolimnological approach

The Ramsar site of Lake Uluabat, western Turkey, suffers from eutrophication, urban and industrial pollution and water abstraction, and its water levels are managed artificially. Here we combine monitoring and palaeolimnological techniques to investigate spatial and temporal limnological variability and ecosystem impact, using an ostracod and mollusc survey to strengthen interpretation of the fossil record. A combination of low invertebrate Biological Monitoring Working Party scores (<10) and the dominance of eutrophic diatoms in the modern lake confirms its poor ecological status. Palaeolimnological analysis of recent (last >200 yr) changes in organic and carbonate content, diatoms, stable isotopes, ostracods and molluscs in a lake sediment core (UL20A) indicates a 20th century trend towards increased sediment accumulation rates and eutrophication which was probably initiated by deforestation and agriculture. The most marked ecological shift occurs in the early 1960s, however. A subtle rise in diatom-inferred total phosphorus and an inferred reduction in submerged aquatic macrophyte cover accompanies a major increase in sediment accumulation rate. An associated marked shift in ostracod stable isotope data indicative of reduced seasonality and a change in hydrological input suggests major impact from artificial water management practices, all of which appears to have culminated in the sustained loss of submerged macrophytes since 2000. The study indicates it is vital to take both land-use and water management practices into account in devising restoration strategies. In a wider context, the results have important implications for the conservation of shallow karstic lakes, the functioning of which is still poorly understood.

Cooling-driven climate change at 12–11 Ma: Multiproxy records from a long fluviolacustrine sequence at Guyuan, Ningxia, China

To explore the evolution of climate through the Miocene, especially during the Middle Miocene climate transition, multiproxy continental records were analyzed by susceptibility (SUS), lightness (L⁎), total inorganic carbon (TIC) and total organic carbon (TOC) content from the 2900-m-thick fluviolacustrine sediment sequence at Guyuan, Ningxia, China and compared with the redness (a⁎) and the pollen humidity indices from the same section. The published Miocene records of sedimentation rate, sediment grain size, biota turnover, and palaeoproductivity from other regions were also compiled and compared. Results show that multiproxies of the Sikouzi section have changed substantially since 12–11 Ma ago. SUS, L⁎ and TIC have increased while a⁎, TOC and the pollen humidity index decreased, all implying that the palaeoclimate in the study area has got cooler and drier since that time. This climate change also left imprints in many other regions, probably resulting from global cooling and the development of the East Antarctic Ice Sheet since about 14 Ma. Several positive feedback mechanisms are proposed to have effectively modulated and magnified the Mid-Miocene global cooling, including vegetation change, greenhouse gas (atmospheric CO 2 and water vapor) fluctuations. They led to the strengthing of ocean and atmospheric circulation, significant lowering of sea level and increasing aridification over the globe, especially in middle-high latitudes. Since 12–11 Ma ago, a continuously cooler and drier climate may have caused the substantial intensification of physical weathering and exhumation on the earth's surface, the increased sediment accumulation rates on the land and in the sea, and the inception of productivity increase in the oceans. Nevertheless, when and where these mechanisms operated and how they were tied to each other remain ambiguous, deserving further investigation.

A Holocene molluscan succession from floodplain sediments of the upper Lena River (Lake Baikal region), Siberia

Floodplain sediments of the upper Lena River near Basovo in south-central Siberia have yielded the most detailed Holocene molluscan succession yet reported from the entire eastern Palaearctic. Over 72,500 shells from at least 28 species of terrestrial and 23 species of freshwater mollusc have been recovered, an abundance and diversity far higher than previously reported from the region. The molluscan assemblages are dominated by land snails, especially members of the genus Vallonia, represented by five species including Vallonia tenuilabris and two poorly known species Vallonia kamtschatica and Vallonia cf. chinensis. Other noteworthy species recovered include Gastrocopta theeli, Carychium pessimum, Vertigo extima (southernmost record), Vertigo microsphaera and the first Asian records of three other taxa ( Vertigo geyeri, Vertigo genesii and Vertigo parcedentata). Illustrations are provided for the critical species, since opinions differ about the status of various taxa and the correct names that should be used. The molluscan assemblages show clear successional trends during the early to mid-Holocene, reflecting episodes of dryness/wetness on the floodplain. Drier conditions at ca 6350 14C yr BP coincide with major changes in the archaeological record seen at other sites in the region but it remains unclear whether the two are linked. A prominent charcoal-rich horizon dated to ca 2800 14C yr BP marks a burning event in the catchment, which resulted in a two-fold increase in sediment accumulation rate. Remains of small mammals occurred throughout the sequence including a tooth of Microtus cf. maximowiczii, possibly the first occurrence of Ungar vole west of Lake Baikal. The faunal analyses have been integrated with a detailed pedological study of the sedimentary profile and a chronology was obtained by means of 12 AMS radiocarbon dates. This study provides the first detailed palaeoecological information relating to Holocene molluscan assemblages from the Cis-Baikal region and lays the foundation for future work in the eastern Palaearctic.

The Huqf Supergroup of Oman: Basin development and context for Neoproterozoic glaciation

The Huqf Supergroup of the Sultanate of Oman provides important information on the geological evolution of the Arabian–Persian Gulf region during a protracted period of continental dispersal and reassembly on the periphery of the Gondwanan supercontinent during the Neoproterozoic, and also provides important constraints on the nature of extreme climate swings during this critical period in the evolution of Earth's biosphere. The Huqf Supergroup spans the period ca. 725–540 Ma, and is composed of three groups. The Abu Mahara Group ( ca. 725 to < 645 Ma) hosts two glacial successions separated by an interval of non-glacial, deep to shallow marine sedimentary rocks. The base of the overlying Nafun Group ( ca.< 645–547 Ma) is marked by a transgressive post-glacial carbonate, which initiates an overstepping of basement-cored structural highs and the deposition of an extensive blanket of carbonate and siliciclastic stratigraphy. The Ara Group ( ca. 547–540 Ma), which is known mostly from the subsurface, comprises carbonates, evaporites and organic-rich shales, with interbedded ashes, deposited in a large number of N–S trending troughs and platforms. The three groups of the Huqf Supergroup correspond to three phases of basin development. The Abu Mahara Group was deposited on an eroded crystalline and metasedimentary basement. An early stage of basin formation preserved < 1.5 km of marginal to deeper marine sedimentary rocks, including an older Cryogenian glacial succession infilling erosional palaeovalleys. Renewed tectonic subsidence associated with submarine volcanism allowed the preservation of a > 1 km-thick, cyclical, rift basin-fill of glacial and non-glacial sedimentary rocks representing a younger Cryogenian icehouse epoch. Progressively older source areas were exhumed during the interval ca. 725 to < 645 Ma, with unroofed 800+ Ma granitoid plutons providing the bulk of sediment, supplemented by syn-extension volcanics, and eventually by distant Meso- and Palaeoproterozoic sources. The wide extent of the Nafun Group, basin-wide correlation of major lithostratigraphic units, and its modest thickness (∼ 1 km) suggest a period of thermal contraction following Abu Mahara rifting. The Oman area was probably a region of slightly stretched continental lithosphere (which passed to the NE into a passive continental margin), occupied by a continental margin rim basin during deposition of the Nafun Group. The Nafun Group bears little resemblance to the coeval small basin-fills choked with calc-alkaline volcanic detritus in the Arabian Shield, but the correlation of the Nafun Group with the Jibalah Group indicates that the contiguous continental rim basin extended from Oman across the tectonically deformed eastern fringe of the Arabian–Nubian Shield. Inundation of the Arabian Shield area and incorporation within the Nafun basin was probably facilitated by extensional collapse and tectonic escape. Sources for 600–640 Ma zircons, found in the Nafun Group, can readily be identified in the Arabian–Nubian Shield. Renewed volcanism, compartmentalization of the basin by N–S trending structural highs and troughs, and an increase in sediment accumulation rates, typifies the Ara Group. The Ara Group deposits formed part of an extensive, latitudinal evaporite belt, with a depocentre translated outwards relative to the Nafun basin, suggesting continuing tectonic progradation of the eastern margin of the Arabian–Nubian Shield. Subduction of ocean floor along the former passive continental margin along the periphery of eastern Gondwana is the most likely cause of Ara volcanism and tectonism, in which case the Oman area can be viewed as occupying a retro-arc setting at this stage, between a subducting margin and the East African orogen. Coeval calc-alkaline plutons and rhyolitic to andesitic volcanics are found in the Central Iranian Terrane. Neoproterozoic glaciations are recorded in the rift and passive margin stratigraphy predating the final amalgamation of continental fragments into greater Gondwana. In Oman, glaciation was contemporaneous with the development of an Andean-type orogen in the Arabian–Nubian Shield while passive margin basins continued to subside in northern Iran, India and South China. The elevated topography associated with the Andean-type orogen together with passive margin mega-escarpments may have promoted nucleation of ice caps that sourced marine-terminating valley glaciers and ice streams. However, the triggering of Cryogenian glaciation must ultimately be related to the biogeochemical cycles operating on a planet with a nascent land biota, lack of calcifying plankton and reduced solar luminosity.

Increased sediment accumulation rates and climatic forcing in the central Andes during the late Miocene

New zircon U-Pb ages from six volcanic ashes in the Subandean foothills provide for the first time well-constrained depositional ages of the late Cenozoic strata in this region. The radiometric ages indicate 12.4 ± 0.5 Ma, 7.93 ± 0.26 Ma, and 5.94 ± 0.20 Ma depositional ages for the bases of Yecua, Tariquia, and Guandacay Formations, respectively. We used these zircon ages to determine the accumulation rates of the late Cenozoic foreland sediments in the Subandes. Our results show a fourfold increase (from 130 to 628 m/m.y.) in sediment accumulation rates between ca. 7.9 and 6 Ma during the deposition of Andean-derived coarsening- and thickening-upward sandstone-dominated Tariquia strata. This increased accumulation rate correlates well with monsoon intensification and climate variability in South America, which was accompanied by the development of fluvial megafan paleodrainage networks in the central Andes. This correlation suggests that climate may have been an important factor between the late Miocene and Pliocene.

Changing sedimentation patterns due to historical land-use change in Frenchman’s Bay, Pickering, Canada: evidence from high-resolution textural analysis

This study examines the anthropogenic alteration of sedimentation in Frenchman’s Bay in Lake Ontario, using high-resolution particle size analysis in two 200 cm cores. Lithofacies were determined using the particle size data of both the terrigenous sediment and terrigenous sediment + diatom fractions. Terrigenous particle size data from the centre of the lagoon provided the most representative record of anthropogenic impacts. Three distinctive lithofacies were recognized: (1) a Natural Wetland (NW) lithofacies (106–200 cm) had an average mean particle size of 49.4 μm, a mode of 29.4 μm and an average standard deviation of 119.1 μm; (2) an Agricultural and Deforestation (AD) lithofacies (40–105 cm) had a statistically significant lower average mean (30.8 μm), mode (13.5 μm), and standard deviation (48.5) μm; (3) an Urbanized (U) lithofacies (0–40 cm) showed a continued trend towards smaller particle sizes with an average mean of 21.2 μm, a mode of 9.4 μm, and an average standard deviation of 32.7 μm. The lithofacies correlated with previously identified trends in thecamoebian biofacies and magnetic susceptibility data showing post-colonial lagoon eutrophication and increased overland soil erosion. The up-core trend towards finer and less variable particle sizes is attributed to erosion of fine-grained watershed sediments (glacial Lake Iroquois silts and clay) during land-clearance and modification of natural drainage patterns. The influx of silts and clays into the lagoon is also recorded by increased sediment accumulation rates and a reduction in seasonal sediment variability in the wetland. Based on the 210Pb dates, sedimentation rates increased at 1850 ±56 AD (AD lithofacies) and suggest an exponentially increasing trend in accumulation rates. Increasing sedimentation rates can be attributed to the progressive loss of native vegetation and intensified erosion of Lake Iroquois deposits via stream and hillslope erosion. Ecologically, the increased input of fine-grained sediments into the wetland has resulted in reduced water clarity and has altered the wetland substrate contributing to wetland loss in Frenchman’s Bay.

The influence of salt withdrawal subsidence on palaeosol maturity and cyclic fluvial deposition in the Upper Triassic Chinle Formation: Castle Valley, Utah

AbstractIntegrated fluvial sequence stratigraphic and palaeosol analysis can be used to better reconstruct depositional systems, but these approaches have not been combined to examine halokinetic minibasins. This study characterizes the temporal and spatial patterns of lithofacies and palaeosols in a sequence stratigraphic framework to reconstruct a model of minibasin evolution and identify halokinetic influences on fluvial deposition. This research documents fluvial cycles and stratigraphic hierarchy, palaeosol maturity and apparent sediment accumulation rates in the Chinle Formation within the Big Bend minibasin. This study also uses palaeosols to help identify fluvial aggradational cycle (FAC) sets. The Chinle is divided into two hectometre‐scale (102 m) fluvial sequences, six decametre‐scale (101 m) FAC sets, and variable numbers of metre‐scale FACs depending on proximity to the minibasin. Ten pedotypes representing 225 palaeosol profiles are recognized. The pedotypes include palaeosols similar to modern Entisols, Inceptisols, Aridisols, Vertisols and Alfisols. A maturity index (1–5) is assigned to each pedotype to assess its variability in palaeosol development. Estimated palaeosol development time is used to approximate apparent sediment accumulation rates. Increased subsidence resulted in a greater number and thicker FACs, thicker FAC sets and fluvial sequence sections, and lithofacies associations reflecting more rapid sedimentation along the minibasin axis. Palaeocurrent indicators converge towards the minibasin axis and indicate that it formed and drifted through time. Relative palaeosol maturity is inversely related to stratal thickness, and decreases towards the minibasin where episodic burial by fluvial sediment was more frequent. Metre‐scale FACs are most abundant towards the minibasin axis, and locally have Entisols and Inceptisols developed upon their upper boundaries reflecting increased sediment accumulation rates. Areas outside the minibasin are characterized by fewer FACs that are associated with more mature palaeosols. Palaeosol‐derived apparent sediment accumulation rates are as much as two orders of magnitude greater within the minibasin than in marginal areas. The combined stratigraphic, palaeocurrent and palaeosol evidence is used to develop a model for the evolution of the Big Bend minibasin that illustrates the halokinetic affect on fluvial and landscape processes.

Paleolimnological investigations of anthropogenic environmental change in Lake Tanganyika: IX. Summary of paleorecords of environmental change and catchment deforestation at Lake Tanganyika and impacts on the Lake Tanganyika ecosystem

Paleorecords from multiple indicators of environmental change provide evidence for the interactions between climate, human alteration of watersheds and lake ecosystem processes at Lake Tanganyika, Africa, a lake renowned for its extraordinary biodiversity, endemism and fisheries. This paper synthesizes geochronology, sedimentology, paleoecology, geochemistry and hydrology studies comparing the history of deltaic deposits from watersheds of various sizes and deforestation disturbance levels along the eastern coast of the lake in Tanzania and Burundi. Intersite differences are related to climate change, differences in the histories of forested vs. deforested watersheds, differences related to regional patterns of deforestation, and differences related to interactions of deforestation and climate effects. Climate change is linked to variations in sediment accumulation rates, charcoal accumulation, lake level and water chemistry, especially during the arid-humid fluctuations of the latter part of the Little Ice Age. Differences between forested and deforested watersheds are manifested by major increases in sediment accumulation rates in the latter (outside the range of climatically driven variability and for the last ∼40 years unprecedented in comparison with other records from the lake in the late Holocene), differences in eroded sediment and watershed stream composition, and compositional or diversity trends in lake faunal communities related to sediment inundation. Variability in regional patterns of deforestation is illustrated by the timing of transitions from numerous sedimentologic, paleoecologic and geochemical indicators. These data suggest that extensive watershed deforestation occurred as early as the late-18th to the early-19th centuries in the northern part of the Lake Tanganyika catchment, in the late-19th to early-20th centuries in the northern parts of modern-day Tanzania, and in the mid-20th century in central Tanzania. Rapid increases in sediment and charcoal accumulation rates, palynological and lake faunal changes occurred in the early-1960s. We interpret this to be the result of greatly enhanced flushing of sediments in previously deforested watersheds triggered by extraordinary rainfall in 1961/62. Regional differences in deforestation histories can be understood in light of the very different cultural and demographic histories of the northern and central parts of the lake shoreline. Incursion of slaving and ivory caravans from the Indian Ocean to the central coast of Lake Tanganyika by the early-19th century, with their attendant diseases, reduced human and elephant populations and therefore maintained forest cover in this region through the late-19th to early-20th centuries. In contrast, the northeastern portion of the lakeshore did not experience the effects of the caravan trades and consequently experienced high human population densities and widespread deforestation much earlier. These studies demonstrate the importance of paleolimnological data for making informed risk assessments of the potential effects of watershed deforestation on long-term lake ecosystem response in the Lake Tanganyika catchment. Differences in sediment yield and lake floor distribution of that yield, linked to factors such as watershed size, slope, and sediment retention, must be accounted for in management plans for both human occupation of currently forested watersheds and the development of future underwater reserves.