A DEFINITION OF DOMINANT DISCHARGE

ABSTRACT

The concept of dominant (stream) discharge in geomorphology has not been firmly defined. By defining it, on rational grounds, as that discharge at which, over a long period of time, most sediment has been transported, a definite value can be computed for a stream for which there are available duration-curve values of daily discharge and a sediment-rating curve. This has been done for 5 eastern and 4 western rivers. The dominant discharge, when expressed in terms of percentage duration, is remarkably consistent, and is found to be a lower discharge than has generally been considered as dominant.     

Vegetation and climate history in the Westeifel Volcanic Field (Germany) during the past 11 000 years based on annually laminated lacustrine maar sediments

Lakes Holzmaar and Meerfelder Maar are located in the Westeifel Volcanic Field less than 10 km apart. Both maar lakes are well known for their annually laminated (varved) sediments covering the past 15 000 years. Here, we focus on reconstructing the history of Holocene vegetation, human impact and climate using high-resolution pollen data. Detailed correlation between the two records using palynologically defined tie-points provides for the first time a test of the precision of the individual varve chronologies. The high-resolution pollen records of both Holzmaar and Meerfelder Maar show continuous natural successions of vegetation during the early and mid-Holocene controlled by the development of soil, climate, immigration and competitive expansion of thermophilous tree species. From 6500 varve yr BP onwards, human impact became an increasingly more important factor. Given the high chronological precision of both records, regional similarities, but also local differences of anthropogenically influenced vegetation changes, can be recognized. The reconstructed July temperature between 8500 and 5000 varve yr BP is around 1 °C higher than today, most likely in response to higher summer insolation related to orbital forcing. High variability of reconstructed July and January temperatures as well as annual precipitation around 5000 varve yr BP is a prominent climatic signal. This is consistent with other records in several areas of Europe which also identify this period as climatically highly variable.     

ANIE: A mathematical algorithm for automated indexing of planar deformation features in quartz grains

Abstract– Planar deformation features (PDFs) in quartz, one of the most commonly used diagnostic indicators of shock metamorphism, are planes of amorphous material that follow crystallographic orientations, and can thus be distinguished from non‐shock‐induced fractures in quartz. The process of indexing data for PDFs from universal‐stage measurements has traditionally been performed using a manual graphical method, a time‐consuming process in which errors can easily be introduced. A mathematical method and computer algorithm, which we call the Automated Numerical Index Executor (ANIE) program for indexing PDFs, was produced, and is presented here. The ANIE program is more accurate and faster than the manual graphical determination of Miller–Bravais indices, as it allows control of the exact error used in the calculation and removal of human error from the process.     

Testate amoebae as a proxy for reconstructing Holocene water table dynamics in southern Patagonian peat bogs

Testate amoebae are abundant and diverse in Sphagnum peat bogs and have been used extensively as indicators of past water table depths. Although these unicellular protists are widely dispersed with globally similar hydrological preferences, regional variations in communities demand region‐specific transfer functions. Here we present the first transfer function for southern Patagonian bogs, based on 154 surface samples obtained from transects in five bogs sampled in 2012 and 2013. Significant variance was explained by pH, electrical conductivity and, in particular, water table depth. Transfer functions for water table were constructed using weighted averaging and evaluated by cross‐validation and independent test sets. The optimal transfer function has predictive ability, but relatively high prediction errors given the wide range in sampled water tables. The use of independent test sets, as well as cross‐validation, allows a more rigorous assessment of model performance than most previous studies. For a subset of locations we compare surface and subsurface samples to demonstrate significant differences in community composition, possibly due to vertical zonation. Our results provide the first quantification of hydrological optima and tolerances for several rare species, which may include Southern Hemisphere endemics and pave the way for palaeohydrological reconstructions in southern Patagonian bogs.     

Controls on the alluviation of oxbow lakes by bed‐material load along the Sacramento River,California

Differences in the nature and quantity of sediment filling oxbow lakes have significant implications for the evolution of meandering rivers and the development of floodplains, influencing rates of meander migration and the valley width over which migration takes place. In an effort to identify the controls on the alluviation of oxbow lakes by coarse bed material, this study examined the sedimentary records stored within oxbow lakes of the Sacramento River of California, USA, and found that the volume of gravel in storage correlated negatively with the diversion angle separating flow between the river channel and the entrance into each lake. A method was devised for estimating the original channel bathymetry of the studied lakes and for modelling the hydraulic and sediment‐transport effects of the diversion angle within channels recently abandoned by meander cut‐off. The diversion angle determines the width of a flow separation within the abandoned‐channel entrance, reducing the discharge diverted from the river channel and thus limiting the ability of the abandoned channel to transport bed material. Aggradation rates are faster within entrances to abandoned channels with high diversion angles, resulting in the rapid isolation of lakes that store only a small volume of coarse‐grained sediment. Aggradation rates are slower within channel entrances where diversion angles are low, resulting in the slow transitioning of such channels into oxbow lakes with a larger and more extensive accumulation of coarse‐grained sediment. These findings compare well with observations in other natural settings and the mechanism which is described for the control of the diversion may explain why some oxbow lakes remain as open‐water environments for centuries, whereas others are filled completely within decades of cut‐off.     

Latitudinally different responses of Tethyan shoal‐water carbonate systems to the Early Aptian oceanic anoxic event (OAE 1a)

Ocean‐wide anoxic events represent intensively investigated anomalies in the global carbon cycle. Most previous research has focussed on hemipelagic and pelagic settings and on the relationship between black‐shale deposition and carbon‐isotope excursions. The study of ocean‐wide anoxic events and coeval shallow‐water settings is now increasingly seen as an interesting complementary approach, but one that is not without problems. Whereas platform drowning characterizes the Early Aptian of the northern Tethyan margin, Lithocodium–Bacinella‐rich facies and ongoing shoal‐water sedimentation at the southern Tethyan margin (Oman) bears important information on potential causes of carbon‐cycle perturbations. The present paper seeks to test the supra‐regional relevance of the Oman data by investigating coeval central Tethyan limestones. Three Lower Aptian shoal‐water sections in Istria (Croatia), deposited on the isolated Adriatic Carbonate Platform, are investigated applying chemostratigraphy (carbon and strontium) and detailed sedimentological analysis. The focus is on peritidal to lagoonal facies characterized by mass occurrences of Lithocodium–Bacinella, an enigmatic microencruster community. Lithocodium–Bacinella facies occurs predominantly in layers ranging from one to several centimetres in stratigraphic thickness, with several layers merging to metre‐thick packages. Growth fabrics within the layers include oncoidal morphotypes, lumps, interconnected patches and columns, layers and rare nodular to massive bindstone facies. These growth patterns show a remarkable regional extent and consistency over study sites distributed several kilometres apart. This widespread distribution suggests that specific Lithocodium–Bacinella morphotypes might serve as regional stratigraphic markers. The high‐resolution carbon‐isotope chemostratigraphy presented here is based on pristine rudist shells and matrix micrite samples and calibrated against strontium‐isotope data obtained from screened rudist low‐Mg calcite. The chemostratigraphic data are consistent with existing biostratigraphic data and place the studied strata at the onset of Early Aptian oceanic anoxic event 1a. Moreover, results indicate the near‐coeval nature of Lithocodium–Bacinella bloom facies in Istria and Oman. The outcomes of this study point to latitudinally different responses of Tethyan shoal‐water carbonate systems (platform drowning versus Lithocodium–Bacinella blooms) to the ocean‐wide anoxic event 1a.     

Sequential and climatic framework of the growth and demise of a carbonate platform: implications for the peritidal cycles (Late Jurassic,North‐eastern France)

The Middle Oxfordian of the eastern Paris Basin constitutes a remarkable example of the growth and demise of a carbonate platform. Fischer plots, sedimentary and diagenetic features allow the identification of four depositional cycles (S5 to S8) in the Transversarium Zone; they are inserted in a lower frequency cycle of increased/decreased accommodation space (SoIII). The long‐term period of accommodation creation occurred during the older S5 and S6 cycles, the maximum accommodation zone being located in the lower part of the S6 cycle. This high accommodation period was tectonically controlled and was coeval with local distensive activity of a Hercynian fault. A major minimum accommodation zone exists during the S8 cycle. At that time, the platform was isolated and presented both a windward and a leeward margin. The growth of the platform was favoured by a warm and arid climate, oligotrophic conditions and reduced siliciclastic input during a highstand in relative sea‐level. These palaeoenvironmental features favoured the proliferation of phototrophic organisms producing carbonate material. The death of the platform was generated by a reduction in the carbonate production surface during a lowstand in relative sea‐level and by the appearance of mesotrophic conditions induced by the increase in siliciclastic inputs at the beginning of a period with a cooler and more humid climate. In the eastern Paris Basin, during the Middle Oxfordian, the parasequences are ordered and present ‘greenhouse’ characteristics. In contrast, at the beginning of the S8 cycle, the randomness in the thickness of contiguous parasequences increased. Decreased carbonate production during the lowstand caused by a transition from photozoan to heterozoan benthic communities certainly favoured this randomness and the appearance of catch‐down parasequences.     

Molecular identification of the extinct mountain goat, Oreamnos harringtoni (Bovidae)

Campos, P. F., Willerslev, E., Mead, J. I., Hofreiter, M. & Gilbert, M. T. P. 2009: Molecular identification of the extinct mountain goat, Oreamnos harringtoni (Bovidae). Boreas , 10.1111/j.1502-3885.2009.00111.x. ISSN 0300-9483.
Harrington's mountain goat ( Oreamnos harringtoni ), an extinct North American herbivore, is one of the least known mammals of the Pleistocene. Fossil specimens are predominantly known from dry cave localities throughout the arid American west – the Grand Canyon, Colorado Plateau, Nevada and Mexico. Morphological analysis of the recovered fossils suggests a close phylogenetic relationship between Harrington's mountain goat and the extant mountain goats from the American northwest ( Oreamnos americanus ). However, the degree of genetic similarity between the two species, and their overall placement within the Caprinae, is not clear. In this study, we recovered and sequenced the first DNA fragments from O. harringtoni in order to investigate these relationships. Genetic analysis further supports the morphological hypothesis that O. harringtoni and O. americanus are two distinct species.     

Vertically resolved dust optical properties during SAMUM: Tinfou compared to Ouarzazate

Vertical profiles of dust key optical properties are presented from measurements during the Saharan Mineral Dust Experiment (SAMUM) by Raman and depolarization lidar at two ground-based sites and by airborne high spectral resolution lidar. One of the sites, Tinfou, is located close to the border of the Sahara in Southern Morocco and was the main in situ site during SAMUM. The other site was Ouarzazate airport, the main lidar site. From the lidar measurements the spatial distribution of the dust between Tinfou and Ouarzazate was derived for 1 d. The retrieved profiles of backscatter and extinction coefficients and particle depolarization ratios show comparable dust optical properties, a similar vertical structure of the dust layer, and a height of about 4 km asl at both sites. The airborne cross-section of the extinction coefficient at the two sites confirms the low variability in dust properties. Although the general picture of the dust layer was similar, the lidar measurements reveal a higher dust load closer to the dust source. Nevertheless, the observed intensive optical properties were the same. These results indicate that the lidar measurements at two sites close to the dust source are both representative for the SAMUM dust conditions.