Evidence of Ectasian/Stenian MesoProterozoic River Morphology

Deeply channelled Precambrian rivers: Remote sensing and outcrop evidence from the 1.2 Ga Stoer Group of NW Scotland

Authors:

Ielpi et al

Abstract:

The current paradigm on Precambrian fluvial sedimentology assumes that pre-vegetation environments did not allow for the establishment of deep, stable channels. However, few studies have documented a continuum of fluvial-depositional architectures along km-scale transects where clusters of channel bodies can be observed in their entirety. The Stoer Group consists of a 1.2 Ga rift-basin fill, its type area occurring at Stoer Peninsula, NW Scotland. Ground observations on classic coastal sections are integrated with remote sensing on a restored transect 7 km wide, representing 400 m of stratigraphy. The transect spans a set of palaeovalleys carved on high-relief gneissic basement. Remote sensing and ground-based sedimentology unveil a set of depositional domains, including: (i) perennial channels filled with downstream-lateral accretionary bars; (ii) poorly drained muddy floodbasins; (iii) well-drained sandy floodbasins containing splays and distributary channels, at times meandering; (iv) gravelly piedmonts composed of talus cones and fluvial-fan deposits; and (v) aeolian fields comprising dunes, ponded interdunes, and sandsheets. These depositional domains reveal an original geomorphic complexity higher than previously recognized for Precambrian terrestrial environments.

The occurrence of clustered channel bodies alongside or within both muddy and sandy depositional domains indicates that stable fluvial channels developed independently from the cohesive nature of the substrate. Their development was possibly aided by drainage focusing along valley thalwegs. Three types of fluvial channels have been identified: laterally extensive, multistorey channels akin to mobile, weakly sinuous rivers; vertically stacked, multistorey channels reminiscent of confined, weakly sinuous rivers; and laterally extensive, multilateral channels, indicative of highly mobile, moderately sinuous rivers. Eighty-four preserved channel fills display width-to-thickness ratios fully overlapping with those of post-vegetation and modern braided rivers, disproving the commonly held notion that pre-vegetation rivers could only generate sheet-like sandbodies. This study provides new insights on Precambrian fluvial styles, and underscores the potential of remote-sensing methods to analyze very large exposures of fluvial rocks.

How the Mississippi River Drained During the Late Pleistocene

The ancestral Mississippi drainage archived in the late Wisconsin Mississippi deep-sea fan

Authors:

Fildani et al

Abstract:

The response of continental-scale drainage systems to short-term (i.e., millennial-scale) climate change is unknown but has wide implications for understanding climate feedbacks and terrestrial-marine fluxes. The late Wisconsin Mississippi River to deep-sea fan of North America was one of Earth's largest sediment-routing networks during the most recent glacio-eustatic cycle. To understand late Pleistocene sediment production and dispersal related to the partly glaciated, ancestral Mississippi system, we sampled late Wisconsin deep-sea fan channel-fill and lobe deposits for detrital zircon U-Pb and (U-Th)/He double-dating analyses, from Deep Sea Drilling Project (Leg 96) cores and U.S. Geological Survey piston cores. Our results suggest a late Pleistocene glacial Mississippi system that forced a larger transfer of sediment from Cordilleran magmatic provinces and the Canadian Shield when compared to the modern drainage. This indicates a potentially more expansive and/or erosive ancestral Mississippi catchment, and the efficient dispersal of terrigenous sediment, nutrients, and solutes into the deep-sea via high-discharge meltwater and glacial-lake outbursts during ice retreat.

Greenhouse gas Emissions From Freshwater HIGHER Than Thought

Do not underestimate the babbling brook. When it comes to greenhouse gases, these bucolic water bodies have the potential to create a lot of hot air.

According to a new analysis in the journal Ecological Monographs, by researchers at the University of Wisconsin-Madison and colleagues, the world's rivers and streams pump about 10 times more methane into our atmosphere than scientists estimated in previous studies. The new study also found that human activity seems to drive which streams are the biggest contributors.

"Scientists know that inland waters, like lakes and reservoirs, are big sources of methane," says Emily Stanley, a professor at the UW-Madison Center for Limnology and lead author of the paper. Yet accurately measuring emissions of methane from these sources has remained a challenge.

Like carbon dioxide, methane is a greenhouse gas that traps heat at the Earth's surface. It is less prevalent than carbon dioxide in the atmosphere but also more potent: A molecule of methane results in more warming than a molecule of carbon dioxide. Understanding how much methane is emitted into the atmosphere from all sources helps scientists account for the full global greenhouse gas budget, and take measures to mitigate its impact.

Rivers and streams haven't received much attention in accounting for that budget, Stanley says, because they don't take up much surface area on a global scale and, with respect to methane, didn't seem to be all that gassy. But over the years, measurements taken by Stanley and her lab members seemed to indicate these sources may produce more methane than scientists had previously known.

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Early Permian Paleopolar, River-Dominated, Shallow Marine Delta's Ichnology From Antarctica

Ichnology of a Paleopolar, River-Dominated, Shallow Marine Deltaic Succession in the Mackellar Sea: The Mackellar Formation (Lower Permian), Central Transantarctic Mountains, Antarctica

Authors:

Jackson et al

Abstract:

The Lower Permian Mackellar Formation in the Beardmore Glacier Area of the Central Transantarctic Mountains, Antarctica, was deposited between 80 and 85°S paleolatitude. Previous studies suggest that Mackellar Formation strata were deposited in either a large glacial lake or inland sea. Our study identified 30 ichnogenera from Turnabout Ridge and Buckley Island of which: 1) none are exclusively freshwater forms; 2) 20 ichnogenera are found in freshwater, brackish, and marine settings; and 3) 10 ichnogenera are exclusively found in marine settings. Combining ichnologic evidence with sedimentologic observations suggests that the Mackellar Formation was deposited in a river-dominated delta in a fully marine to brackish-marine setting. Trace-fossil associations are grouped into the Arenicolites–Phycodes, Lingulichnus, Arenicolites–Planolites, Kouphichnium, Phycodes–Teichichnus–Arenicolites, and Planolites–Teichichnus–Phycodes ichnocoenoses of a mixed Skolithos–Cruziana ichnofacies. These associations are characteristic of epi- and endobenthic worms arthropods, and mollusks, with the vast majority of the traces being diminutive in diameter and length, and exhibit shallow (≤ 10 mm) penetration depths. These ichnocoenoses occur in paleoenvironments that include the prodelta, distal to proximal delta front including mouth bars, and subaqueous terminal distributary channels. The diminutive morphology, shallow penetration depth, low bed ichnodiversity, high overall ichnodiversity, and sedimentologic characteristics are indicative of benthic organisms in a marine deltaic environment with short-lived communities composed of small-bodied organisms stressed by high freshwater input and high sedimentation rates.

The Dinosaur Graveyards at Las Águilas, Mexico Were a Campanian Cretaceous River Delta Environment

Age and depositional environment of the “dinosaur graveyard” at Las Águilas, southern Coahuila, NE Mexico

Authors:

Vogt et al

Abstract:

Here we provide a detailed description of the upper Campanian sediment succession at Las Águilas, southern Coahuila, northeastern Mexico, including the first absolute age dating for this interval, paleoenvironmental reconstructions and taphonomic observations on the abundant dinosaur remains at the locality. Stratigraphic investigations of the dinosaur-bearing succession at the Las Águilas vertebrate fossil area near Porvenir de Jalpa reveal a diverse vertebrate assemblage, including dinosaurs, crocodilians and turtles. New findings in adjacent sites include eusuchian crocodylomorphs, four different kinds of turtles, dromaeosaurids, lambeosaurines, pterosaurs and elasmosaurid plesiosaurs. Strontium isotope measurements on fossil oyster shells provide an absolute age of 73 ± 1 Ma for the lower part of the Las Águilas section. The locality is thus of late Campanian age. The vertebrate, invertebrate and plant materials as well as the sediment structures observed in a 50 m thick predominantly siliciclastic succession of the Cerro del Pueblo Formation suggest deposition in an extensive delta plain environment. The facies succession indicates a short-termed cyclicity of limnic, brackish and shallow marine environments during the late Campanian–early Maastrichtian Cerro del Pueblo Formation with numerous layers containing dinosaur fossil remains.

OFF Paris Seine Floating Hotel Rendering

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An Early Permian Paleopolar, Postglacial, River-dominated Delta From Antarctica

An Early Permian, paleopolar, postglacial, river-dominated deltaic succession in the Mackellar–Fairchild formations at Turnabout Ridge, Central Transantarctic Mountains, Antarctica

Authors:

Flaig et al

Abstract:

Turnabout Ridge, a remote outcrop belt in the Beardmore Glacier Region of the Central Transantarctic Mountains, comprises subglacial–glaciomarine deposits of the Pagoda Formation (Fm) overlain by a postglacial deltaic succession of the Mackellar–Fairchild formations (fms). Four depositional environments were identified in the Mackellar–Fairchild fms: 1) prodelta to distal-delta-front deposits that record the initial filling of a newly formed intracratonic basin; 2) an interval dominated by subaqueous terminal distributary channels and levees that signal the advancement of a delta into the region; 3) mouth bars and associated subaqueous terminal distributaries representing the proximal delta front; and 4) an overlying sand-dominated braidplain.

A companion ichnologic study identified 30 ichnogenera in the Mackellar Fm that combine to form six ichnocoenoses. Twelve traces are known only from marine settings, 18 are found across marine, brackish, and freshwater systems, and none are known solely from freshwater systems. The ichnology refines paleoenvironmental and paleosalinity interpretations for the Mackellar Fm, and indicates a predominantly brackish water paleoenvironment.

Subaqueous terminal distributary channels record the highest energies, suspended sediment concentrations, and sediment loads, and are the conduits that delivered sediment to the delta front and prodelta. Abundant traction deposits (hyperpycnites) displaying only minor wave or tide modification suggest that the delta is best classified as river-flood dominated. The succession exhibits similarities to the Panther Tongue and Ferron Notom deltas of the Cretaceous Western Interior Seaway, USA, and a Late Ordovician proglacial delta from the Murzuq Basin in Libya.

The combined ichnologic and sedimentologic evidence suggests that sediment-laden glacial outburst floods produced freshets that recurrently prepped a shallow-marine basin, reducing salinities and allowing for abundant channelization and hyperpycnite deposition along the delta front. Turnabout Ridge likely occupied a proximal and axial position relative to an Antarctic freshwater- and sedimentation-stressed, river-dominated marine deltaic system along the shoreline of an epeiric seaway during the Early Permian.

Heterolithic Meandering-channel Deposits From Neoproterozoic Scotland may Give Insight to Martian Riverbeds

Heterolithic meandering-channel deposits from the Neoproterozoic of NW Scotland: Implications for palaeogeographic reconstructions of Precambrian sedimentary environments

Authors:

Santos et al

Abstract:

Pre-Silurian fine-grained meandering river deposits are apparently rare in the rock record. Most modern-day river dynamics are influenced by vegetation through bank stabilization, fine-grained sediment production and retention, and runoff control, leading to the development of highly sinuous, single-channel systems. In contrast, pre-vegetation river dynamics are poorly understood, in part because there are no modern-day analogues for completely non-vegetated meandering river systems, particularly under humid climates. Some models attribute the paucity of fine-grained sediments described from studies of pre-vegetation fluvial deposits to lower rates of chemical weathering in the absence of land plants. The architecture of precambrian, fine-grained meandering stream deposits is here described for the first time.

The Allt na Béiste Member at the base of the Applecross Formation of the Torridon Group (Neoproterozoic, NW Scotland) is characterized by heterolithic deposits, preserving a varied suite of fluvial forms including inclined heterolithic strata, lateral-accretion sets, up to 8 m-thick muddy floodplain deposits with preserved crevasse-splays. Successions of laterally-accreting strata are interbedded with metre-scale channel-fill sandstones, in a succession up to 190 m-thick related to the early stages of the Applecross Formation fluvial system, as large-scale rivers buried the Lewisian palaeovalleys over which fluvial and lacustrine sediments of the Diabaig Formation had been deposited. The lacustrine fine-grained sediments, coupled with relatively low gradients of the shallow lacustrine environments and denuded, highly weathered Lewisian basement, apparently provided considerable amounts of fine-grained sediment which added sufficient cohesion to this fluvial system to induce the adoption of laterally-migrating, meandering channel planforms at the expense of multi-thread braided channels.

Our data suggest that, given appropriate conditions, pre-vegetation meandering channel planforms were indeed able to develop, without the buffering effects of land plants. The paucity of fine-grained sediments in the pre-vegetation rock record may be more a consequence of preservation-related issues than the actual paucity of such sediments. These results provide novel insights into the characteristics of Earth's landscape prior to the Silurian, and provide potential analogues for meandering channels interpreted from satellite imagery of Mars.

Some Pebbles in the Gale Crater on Mars may Have Been Transported for tens of Miles

While new evidence suggests that Mars may harbor a tiny amount of liquid water, it exists today as a largely cold and arid planet. Three billion years ago, however, the situation may have been much different.

In 2012 the Mars Curiosity rover beamed images back to Earth containing some of the most concrete evidence that water once flowed in abundance on the planet. Small, remarkably round and smooth pebbles suggested that an ancient riverbed had once carried these rocks and abraded them as they traveled.

To Douglas Jerolmack, a geophysicist at the University of Pennsylvania, and his collaborator Gábor Domokos, a mathematician at Budapest University of Technology and Economics, Curiosity's findings raised a fundamental geological question: Can we use shape alone to interpret the transport history of river pebbles -- on Mars, Earth or any planet?

"Thousands of years ago, Aristotle pondered the question of pebbles on the beach and how they become rounded," Jerolmack said. "But until recently, descriptions of pebble shape have been qualitative, and we lacked a basic understanding of the rounding process."

Now that has changed. In a new report in Nature Communications, Jerolmack, Domokos and colleagues report the first-ever method to quantitatively estimate the transport distance of river pebbles from their shape alone. The researchers' estimate that the Martian pebbles traveled roughly 30 miles from their source, providing additional evidence for the idea that Mars once had an extensive river system, conditions that could support life.

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Upper Maastrichtian Cretaceous Riparian Forest was Surprisingly Modern

A riparian plant community from the upper Maastrichtian of the Pyrenees (Catalonia, NE Spain)

Authors:

Marmi et al

Abstract:

Angiosperms began to colonise riparian habitats very soon in their evolutionary history, probably already in the Aptian, but it is still poorly known when flowering plants finally dominated entirely these kind of communities as they do in the present. A new fossil plant locality (Molí del Baró-1) from the upper Maastrichtian of the Southern Pyrenees is described in which meandering river facies represent one of the first riparian communities formed only by angiosperms. The fossil assemblage consists of abundant leaves, seeds, logs and sporomorphs. Angiosperms remains dominate in all these cases and the leaf sample is mostly composed of a new eudicot willow-like species, Saliciphyllum gaetei sp. nov., the palm Sabalites longirhachis and an helophytic monocot. Pollen remains suggest that the later belonged to Typhaceae. Most of these plant remains were parautochthonous and deposited in a pond formed in the accretional part of a meander loop. The locality of Molí del Baró-1 represents an unique plant fossil assemblage in the uppermost Cretaceous of southern Europe. It clearly differs from those reported in other Maastrichtian localities of the Pyrenees (Fumanya and South Isona) and from the Campanian-Maastrichtian of Austria and Romania. In addition, it reflects a surprisingly modern physiognomy for a Late Cretaceous riverine plant assemblage that was built up with willow-like plants, palms and reeds.

Alluvial response to the Paleocene–Eocene Thermal Maximum

Alluvial response to the Paleocene–Eocene Thermal Maximum climatic event, Polecat Bench, Wyoming (U.S.A.)

Authors:

Kraus et al

Abstract:

The stratigraphic interval spanning the Paleocene–Eocene Thermal Maximum in the northern Bighorn Basin, Wyoming shows changes in the alluvial record that can be tied to a high resolution climate record. The complexity and stratigraphic spacing of paleosols change through the study section. Comparison to the climate record, reconstructed from paleosols, indicates the changes correspond to Paleocene–Eocene Thermal Maximum climatic fluctuations. In particular, the middle of the section contains thick, welded paleosols and thin avulsion deposits that link to times of well-drained floodplains and lower mean annual precipitation. Stratigraphic intervals below and above the main part of the Paleocene–Eocene Thermal Maximum interval correspond to times of less well drained floodplains and higher mean annual precipitation. These strata contain thinner paleosols and thick avulsion deposits.

Differences in paleosol complexity and spacing suggest that sediment flux to the depositional site varied in response to precipitation fluctuations associated with the Paleocene–Eocene Thermal Maximum. Welded paleosols and thin avulsion deposits indicate reduced floodplain accretion during deposition of the middle of the Paleocene–Eocene Thermal Maximum. Intervals with widely spaced paleosols indicate more rapid accretion. We hypothesize that drier episodes associated with warming caused reduced vegetation in source areas and promoted erosion and increased sediment yield. Because precipitation was reduced, much of that sediment was stored in upstream reaches of the fluvial system rather than moving to the depositional basin. Welded paleosols formed because of diminished sediment supply to the basin. With a return to wetter conditions during the recovery phase of the Paleocene–Eocene Thermal Maximum, upstream water flux increased, stored sediment moved to the basin, and vertically spaced, thinner paleosols developed. The results demonstrate how vertical sections of alluvial paleosols can provide information on how climate fluctuated through time and how the fluvial system responded to climate change.

Ecological & Environmental Transition in Eocene Paleogene Germany

FROM OPEN ESTUARY TO MEANDERING RIVER IN A GREENHOUSE WORLD: AN ECOLOGICAL CASE STUDY FROM THE MIDDLE EOCENE OF HELMSTEDT, NORTHERN GERMANY

Authors:

Riegel et al

Abstract:

Tropical climates reached their northernmost expansion during the early Paleogene greenhouse phase, supporting a paratropical biota as far north as the southern shore of the ancient North Sea. There, relative sea-level fluctuations led to the formation of transgressive-regressive sequences, exposed in open-cast lignite mines in the area of Helmstedt (Lower Saxony, Germany), which record environmental changes at the marine-terrestrial interface. We studied an example from the middle Eocene of the Helmstedt Mine from which we reconstruct ecological successions from open estuary to meandering river environments, which represent vegetation at the Paleogene climatic optimum. In the absence of vertebrate and shelly fossils, environmental interpretations are made exclusively on plant matter in the sediment. Vegetational reconstructions are based mainly on quantitative palynological analyses, supported by paleobotanical and organic-petrographical evidence and confirmed by Principal Component Analysis. Diverse dinocyst assemblages in conjunction with Ophiomorpha-type bioturbation indicate open estuarine conditions. These are succeeded by mangrove, represented by pollen of Rhizophora, Avicennia, Nypa and the form species Psilodiporites iszkaszentgoergyi, and marsh environments, indicated by pollen of Restionaceae, Sparganiaceae/Typhaceae and fern spores. Subsequent lowland mire forests are characterized by a dominance of either the Tricolporopollenites cingulum group (Fagaceae) or triporate pollen of the Triporopollenites robustus/rhenanus complex (Myricaceae/Betulaceae). Within this landscape coexists a meandering river system, represented by channel lag and point-bar deposits of the active channel and by the clastic to lignitic sedimentary fill of an abandoned channel.

Evidence of Calymmian MesoProterozoic Tidal/River Delta System From Brazil

Mesoproterozoic delta systems of the Açuruá Formation, Chapada Diamantina, Brazil

Authors:

Magalhães

Abstract:

Facies analysis, distinct depositional paleoflow directions and grain size ranges suggest that different fluvial systems fed coeval distinct, independent river-dominated and tide-dominated braid delta systems in the Mesoproterozoic Açuruá Formation. The river-dominated delta system is characterized by: (a) vertical stacking of parasequences up to 30 m which are characterized by coarsening- and thickening-upward trends; (b) gradationally based vertical succession of prodelta, delta front, distributary channels, and floodplain; (c) grain size of sandstone from delta front and distributary channels varies respectively from very fine to medium and medium to coarse; (d) lacking of wave or tide reworking suggests delta development in a relatively low energy marine basin; (e) paleocurrent readings indicate delta front sandstone bodies shifting laterally towards NE to SE directions. The tide-dominated delta system is characterized by: (a) vertical stacking of parasequences up to 25 m which are characterized by coarsening- and thickening-upward trends; (b) gradationally based vertical succession of prodelta, delta front and tide-influenced distributary channels; (c) grain size of sandstone from delta front and tide-influenced distributary channels varies respectively from very fine to coarse and fine to granular; (d) abundance of tide-influenced sedimentary structures suggest tidal dominated delta development; (e) consistent bidirectional paleocurrents with northwards ebb-tide direction and no lobe shifting. The studied braid delta systems belong to a highstand system tract and as such, they exhibit classic parasequence sets with progradational stacking pattern. The facies association variability and overall characteristics of these Mesoproteozoic delta systems are compatible with delta systems of all ages. However, contrary to typically Phanerozoic deltas – which are characterized by progradational clinoforms – the coalescence of the studied braid delta facies associations promoted fairly sheet-like geometry limited by marine flooding surface that more likely resulted from the response of deposition on shallow and wide epeiric sea.

Modeling River Deltas on Titan and Earth

Dynamical modelling of river deltas on Titan and earth

Authors:

Witek et al

Abstract:

The surface of Titan hosts a unique Earth-like environment with lakes and rivers, and active 'hydrologic' cycle of methane. We investigate sediment transport in Titanian rivers and deposition in Titanian lakes with particular attention to formation of river deltas. The obtained results are compared with analogous terrestrial processes. The numerical model based on Navier-Stokes equations for depth-integrated two dimensional turbulent flow and additional equations for bed-load and suspended-load sediment transport was used in our research. It is found that transport of icy grains in Titanian rivers is more effective than silicate grains of the same size in terrestrial rivers for the same assumed total discharge. This effect is explained theoretically using dimensionless form of equations or comparing forces acting on the grains. Our calculations confirm previous results (Burr et al., 2006). We calculate also models with organic sediments of different densities, namely 1500 and 800 kg m−3. We found substantial differences between materials of varying densities on Titan, but they are less pronounced than differences between Titan and Earth.

Mid-Cretaceous to Paleocene Paleogene North American Drainage Reorganization

Mid-Cretaceous to Paleocene North American drainage reorganization from detrital zircons

Authors:

Blum et al

Abstract:

Detrital zircons (DZ) from fluvial sandstones of the Western Canada Sedimentary Basin and the U.S. Gulf of Mexico (GoM) passive margin indicate mid-Cretaceous through Paleocene continental-scale drainage reorganization. DZ populations from the Early Cretaceous Mannville Group of Alberta represent a continental-scale system that routed sediment from the Appalachian Mountains and the eastern three-quarters of North America to the Boreal Sea. In contrast, DZ populations from the GoM coastal plain show that only the southern United States and Appalachian-Ouachita orogen contributed sediment to the GoM through the Late Cretaceous, whereas by the Paleocene, southern North America, from the Western Cordillera to the Appalachian Mountains, had been routed to the GoM. This continental-scale drainage reorganization reflects the culmination of an ∼300 m.y. trajectory that began with Paleozoic Appalachian assembly, and broad east to west sediment routing, followed by assembly of the Mesozoic Western Cordillera, which resulted in west-derived rivers in the United States draining to the GoM in Texas, or to an ancestral Mississippi River in the Mississippi embayment, setting up the template for sediment routing that persists today.

Shift in Snow From Climate Change Will Impact Rivers

New research has shown for the first time that the amount of water flowing through rivers in snow-affected regions depends significantly on how much of the precipitation falls as snowfall. This means in a warming climate, if less of the precipitation falls as snow, rivers will discharge less water than they currently do.

The study by PhD student Wouter Berghuijs and Dr Ross Woods, Senior Lecturer in Water and Environmental Engineering in the Department of Civil Engineering at the University of Bristol together with a colleague from Delft University of Technology is published online in Nature Climate Change.

The researchers, using historical data from several hundred river basins located across the United States, investigated the effect of snow on the amount of water that rivers discharge.

How river flow is generated in snowy areas is poorly understood due to the difficulty in getting appropriate measurements. Previous studies have mostly focused on the role of snowfall for the within-year distribution of streamflow - how much water is there in the river during a particular period of the year - and assumed that there was no important effect of snow on the average streamflow. This study is the first to focus on the role of snow for how much water is on average available in rivers.

With data from 420 catchments located throughout the United States the researchers show that snowiness is an important factor for the average river discharge.

Global warming is very likely to reduce the amount of snow significantly in snow-affected catchments, even if temperatures rise only two degrees Celsius. The new research suggests that the amount of water in rivers will be reduced as a result of the decrease in snow.

The authors of the study said: "With more than one-sixth of the Earth's population depending on meltwater for their water supply, and ecosystems that can be sensitive to streamflow alterations, the socio-economic consequences of a reduction in streamflow can be substantial.

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Accurate are Rivers as Gauges of Chemical Weathering of the Continents?

How accurate are rivers as gauges of chemical denudation of the Earth surface?

Authors:

Bouchez et al

Abstract:

Examination of the behavior of oxygen and hydrogen during weathering reactions shows that river dissolved load, although widely used, is an imperfect tracer of chemical denudation. At the current state of knowledge, none of the metrics for river total dissolved loads (such as the silicate-derived total dissolved solids, TDSsil = Ca2+ + Mg2+ + Na+ + K+ + SiO2, converted or not to equivalent oxides) account, in a mechanistic manner, for the transfer of oxygen and hydrogen between the solid and fluid phase during weathering reactions. We assess that chemical denudation derived from TDSsil will significantly overestimate the true chemical denudation for weathering of Ca-feldspar to kaolinite, whereas weathering of water-rich sedimentary rocks will be characterized by an underestimation of chemical denudation by TDSsil. For a handful of field sites, we estimate that the bias is lower than ±10%. The sign and extent of the bias depends on the nature of bedrock and on weathering conditions. Altogether, our analysis questions the broadly accepted concept of chemical denudation rate.

Evidence of a River Delta From Statherian PaleoProterozoic Australia

Riverine mixing and fluvial iron formation: A new type of Precambrian biochemical sediment

Authors:

Pufahl et al

Abstract:

Precambrian iron formations are biochemical sediments that record ocean chemistry and circulation on the early Earth. The appearance of large, economically important continental margin iron formation reflects the creation of extensive continental shelves and oxygenation of the ocean-atmosphere system near the end of the Archean. Exhalative iron formation contains a record of hydrothermal vent chemistry through time. We introduce here fluvial iron formation, a new type of Fe-rich microbial-biochemical sediment that formed by mixing river discharge and seawater in coastal environments. The Paleoproterozoic Chiall Formation (ca. 1.8 Ga), Earaheedy Basin, Western Australia, contains laminated and granular hematitic iron formation in delta channel deposits. Where mixing occurred in adjacent peritidal settings, laminated iron formation and hematitic oncoids formed. Because fluvial iron formations precipitated at the interface between terrestrial and marine realms, the locus of known Fe precipitation processes is shifted landward into paleoestuarine settings and reflects Fe derived from both terrestrial weathering and coastal upwelling, providing a new window into ocean-atmosphere evolution.

The Nile Started Because of Eocene Neogene Uplift in East Africa

Eocene initiation of Nile drainage due to East African uplift

Authors:

Charlie J. Underwood, Chris King, and Etienne Steurbaut

Abstract:

The Late Eocene and Early Oligocene sedimentary succession in the Fayum, Egypt, records the progressive development of northerly flowing Nile-type African drainage. New biostratigraphic dating of these units allows the calibration of the paleomagnetic record, the combination of dating methods enabling a detailed chronology of events to be studied. Between about 38 and 35 Ma there was a dramatic change in sedimentary regime and vast quantities of clastic material were transported into the area, smothering the underlying carbonate platform and initiating a stepwise progradation of clastic units. The sudden change in sediment availability coincides with the beginning of uplift and volcanic activity in the Turkana region of East Africa, cutting off preexisting easterly drainage from the middle of the continent. The Fayum succession therefore records the initiation of northerly drainage of central and eastern Africa, and the origins of the modern Nile watershed. The development of the current route of the Nile, with the incision of the current Nile Valley, was slightly later and related to mid Oligocene uplift of the Red Sea margins and Messinian base level fall.

A Carbon Cycle Twist: Amazon River Exhales Almost All Carbon Absorbed by Rain Forest

The Amazon rain forest, popularly known as the lungs of the planet, inhales carbon dioxide as it exudes oxygen. Plants use carbon dioxide from the air to grow parts that eventually fall to the ground to decompose or get washed away by the region's plentiful rainfall.

Until recently people believed much of the rain forest's carbon floated down the Amazon River and ended up deep in the ocean. University of Washington research showed a decade ago that rivers exhale huge amounts of carbon dioxide – though left open the question of how that was possible, since bark and stems were thought to be too tough for river bacteria to digest.

A study published this week in Nature Geoscience resolves the conundrum, proving that woody plant matter is almost completely digested by bacteria living in the Amazon River, and that this tough stuff plays a major part in fueling the river's breath.

The finding has implications for global carbon models, and for the ecology of the Amazon and the world's other rivers.

"People thought this was one of the components that just got dumped into the ocean," said first author Nick Ward, a UW doctoral student in oceanography. "We've found that terrestrial carbon is respired and basically turned into carbon dioxide as it travels down the river."

Tough lignin, which helps form the main part of woody tissue, is the second most common component of terrestrial plants. Scientists believed that much of it got buried on the seafloor to stay there for centuries or millennia. The new paper shows river bacteria break it down within two weeks, and that just 5 percent of the Amazon rainforest's carbon ever reaches the ocean.

"Rivers were once thought of as passive pipes," said co-author Jeffrey Richey, a UW professor of oceanography. "This shows they're more like metabolic hotspots."

When previous research showed how much carbon dioxide was outgassing from rivers, scientists knew it didn't add up. They speculated there might be some unknown, short-lived carbon source that freshwater bacteria could turn into carbon dioxide.

"The fact that lignin is proving to be this metabolically active is a big surprise," Richey said. "It's a mechanism for the rivers' role in the global carbon cycle – it's the food for the river breath."

The Amazon alone discharges about one-fifth of the world's freshwater and plays a large role in global processes, but it also serves as a test bed for natural river ecosystems.

Richey and his collaborators have studied the Amazon River for more than three decades. Earlier research took place more than 500 miles upstream. This time the U.S. and Brazilian team sought to understand the connection between the river and ocean, which meant working at the mouth of the world's largest river – a treacherous study site.

"There's a reason that no one's really studied in this area," Ward said. "Pulling it off has been quite a challenge. It's a humongous, sloppy piece of water."

The team used flat-bottomed boats to traverse the three river mouths, each so wide that you cannot see land, in water so rich with sediment that it looks like chocolate milk. Tides raise the ocean by 30 feet, reversing the flow of freshwater at the river mouth, and winds blow at up to 35 mph.

Under these conditions, Ward collected river water samples in all four seasons. He compared the original samples with ones left to sit for up to a week at river temperatures. Back at the UW, he used newly developed techniques to scan the samples for some 100 compounds, covering 95 percent of all plant-based lignin. Previous techniques could identify only 1 percent of the plant-based carbon in the water.

Based on the results, the authors estimate that about 45 percent of the Amazon's lignin breaks down in soils, 55 percent breaks down in the river system, and 5 percent reaches the ocean, where it may break down or sink to the ocean floor.