Erosional Regime Research Articles

Asymmetric growth of the Pyrenees revealed through measurement and modeling of orogenic fluxes

Orogenic asymmetry is ubiquitous in terms of topography, structure and metamorphism, but the relative influences of climate and tectonics in determining such asymmetry remain contentious. This study demonstrates that for the case of the European Pyrenees, orogenic asymmetry is not defined by inherited structural asymmetry, nor by orographic asymmetry of precipitation, but is the simple consequence of the inherent tectonic growth of a doubly-vergent orogen. The Pyrenees record approximately 60 Myr of upper crustal accretion, erosion and sedimentation that developed from Late Cretaceous to early Miocene times. Approximately 165 km of plate convergence was accommodated by four phases of orogenic growth that can be summarized in terms of their generic orogenic development as follows: 1) Early inversion of passive margin extensional faults, initiating the pro-wedge. 2) Growth by a combination of frontal accretion and underplating to the pro-wedge; symmetrical and steady record of erosion and foreland basin sedimentation. 3) Cessation of pro-wedge frontal accretion with propagation of the retro-wedge deformation front. 4) Accelerated growth of pro-wedge antiformal stack induced by underplating; accelerated erosion over antiformal stack and increase in sediment discharge to pro-foreland basin; stabilization of outer pro-wedge by sediment blanketing. Discrete element model experiments are used to explore the natural tendency of the system towards asymmetry. Model output demonstrates that during active frontal accretion to the pro-wedge of an orogen, the maxima in erosional denudation is offset towards the pro-wedge relative to the maximum depths of exhumation. This asymmetry is fundamentally driven by the progressive reduction in advection velocities of material within the orogen from the pro-wedge into the retro-wedge. Hence, for an efficient erosional regime, a dynamic coupling between erosion and structural uplift of the pro-wedge will reduce activity of the retro-wedge and hence drive asymmetry in terms of low temperature thermochronometry, erosion and sedimentation. The models also demonstrate the tendency for pro-wedge growth through frontal accretion to slow. However, the cessation of frontal accretion as recorded by the South Pyrenean Thrust Belt (phases 3 and 4) requires additional forcing; it is proposed that the thick wedge-top sedimentation recorded at this time, increased taper angles and hence stabilized the wedge. Accretionary influx into the orogen at this time was accomplished through underplating. These processes governing asymmetry do not require a spatially variable climate or a laterally heterogeneous underthrust plate.

Erosional and climatic effects on long-term chemical weathering rates in granitic landscapes spanning diverse climate regimes

We used cosmogenic nuclide and geochemical mass balance methods to measure long-term rates of chemical weathering and total denudation in granitic landscapes in diverse climatic regimes. Our 42 study sites encompass widely varying climatic and erosional regimes, with mean annual temperatures ranging from 2 to 25 °C, average precipitation ranging from 22 to 420 cm·year −1, and denudation rates ranging from 23 to 755 t·km −2·year −1. Long-term chemical weathering rates range from 0 to 173 t·km −2 year −1, in several cases exceeding the highest granitic weathering rates on record from previous work. Chemical weathering rates are highest at the sites with rapid denudation rates, consistent with strong coupling between rates of chemical weathering and mineral supply from breakdown of rock. A simple empirical relationship based on temperature, precipitation and long-term denudation rates explains 89–95% of the variation in long-term weathering rates across our network of sites. Our analysis shows that, for a given precipitation and temperature, chemical weathering rates increase proportionally with fresh-material supply rates. We refer to this as “supply-limited” weathering, in which fresh material is chemically depleted to roughly the same degree, regardless of its rate of supply from breakdown of rock. The temperature sensitivity of chemical weathering rates is two to four times smaller than what one would expect from laboratory measurements of activation energies for feldspar weathering and previous inter-comparisons of catchment mass-balance data from the field. Our results suggest that climate change feedbacks between temperature and silicate weathering rates may be weaker than previously thought, at least in actively eroding, unglaciated terrain similar to our study sites. To the extent that chemical weathering rates are supply-limited in mountainous landscapes, factors that regulate rates of mineral supply from erosion, such as tectonic uplift, may lead to significant fluctuations in global climate over the long term.

Uplift, exhumation and precipitation: tectonic and climatic control of Late Cenozoic landscape evolution in the northern Sierras Pampeanas, Argentina

AbstractDeciphering the evolution of mountain belts requires information on the temporal history of both topographic growth and erosion. The exhumation rate of a mountain range undergoing shortening is related to the erodability of the uplifting range as well as the efficiency of erosion, which partly depends on the available precipitation. Young, rapidly deposited sediments have low thermal conductivity and are readily eroded, in contrast to underlying resistant basement rocks that have a higher thermal conductivity. Apatite fission‐track thermochronology can quantify cooling; thermal models constrain the relationship between this cooling and exhumation. By utilizing geological relations for a datum, we can examine the evolution of rock uplift, surface uplift and exhumation. In the northern Sierras Pampeanas of Argentina, a young sedimentary basin that overlay resistant crystalline basement prior to rapid exhumation provides an ideal setting to examine the effect of contrasting thermal and erosional regimes. There, tectonically active reverse‐fault‐bounded blocks partly preserve a basement peneplain at elevations in excess of 4500 m. Prior to exhumation, the two study areas were covered by 1000 and 1600 m of recently deposited sediments; this sequence begins with shallow marine deposits immediately overlying the regional erosion surface. Apatite fission‐track data were obtained from vertical transects in the Calchaquíes and Aconquija ranges. At Cumbres Calchaquíes, erosion leading to the development of the peneplain commenced in the Cretaceous, probably as a result of rift‐shoulder uplift. In contrast, Sierra Aconquija cooled rapidly between 5.5 and 4.5 Myr. At the onset of this rapid exhumation, the sediment was quickly removed, causing fast cooling, but relatively slow rates of surface uplift. Syntectonic conglomerates were produced when faulting exposed resistant bedrock; this change in rock erodability led to enhanced surface uplift rates, but decreased exhumation rates. The creation of an orographic barrier after the range had attained sufficient elevation further decreased exhumation rates and increased surface uplift rates. Differences in the magnitude of exhumation at the two transects are related to both differences in the thickness of the sedimentary basin prior to exhumation and differences in the effective precipitation due to an orographic barrier in the foreland and hence differences in the magnitude of headward erosion.

Geological and geotechnical context of cover collapse and subsidence in mid-continent US clay-mantled karst

This paper presents a synthesis of geologic and geotechnical concepts to present a unified model of conditions controlling the development of cover-collapse sinkholes and associated ground subsidence. Appropriate engineering response to the hazards associated with collapse and subsidence requires a full understanding of the underlying mechanisms that produce such effects. The geotechnical characteristics of the overlying clay mantle and occurrence of the associated cover-collapse features are not random, but rather are directly tied to the underlying water flow routes and their development through time. The clay mantle and underlying epikarst are two components of a single system, each of the components influencing the other. This paper brings together these two aspects in terms of the author's personal experience and observations as a geologist, geotechnical engineer, hydrogeologist, and caver. A summary of the basic model follows. Much of the clay mantle and pinnacled upper surface of the epikarst form while surface drainage still prevails. At this stage, the karst underdrains are insufficiently developed to transport soils, although some subsidence into cutters occurs because of dissolutional rock removal. Soil arches and macropore flow routes associated with cutters have developed by this stage. As competent deep conduits extend into the area by headward linking, the cutters with the most favorable drains are linked to the conduits first and act as attractors for the development of a tributary, laterally integrated drainage system in the epikarst. Once the most efficient cutter drains become competent to transport soils, the depressed top-of-rock and ground surfaces characteristic of dolines develop. A given doline underdrain is likely to have multiple tributary drains from adjacent cutters, which vary in soil transport competence. Soil stiffness in the clay mantle over the limestone varies as a result of the pattern of stresses imposed as the underlying rock surface is lowered by dissolution and later as soil piping locally removes soils. In the absence of karst, these soils would have developed a laterally uniform, stiff to very stiff consistency. Where soil near the soil–bedrock interface is locally removed, however, the weight of the materials overlying this void is transferred to abutment zones on the pinnacles by soil arches. Local soil loading in the abutment areas of these arches would increase at least on the order of 50% in the case of an isolated cavity. In some cases, multiple closely spaced cutters whose soil arches have narrow, laterally constrained abutment zones bearing on the intervening pinnacles may produce substantially higher soil abutment stresses. If the clays in the abutment zones do not fail, they would respond to this increase in stress by consolidating: stiffening and decreasing in volume. The cutters spanned by the soil arches accumulate raveled soils that are "under-consolidated", the soft zones noted between pinnacles by Sowers. A simple integral of stresses analysis makes it obvious, however that no continuous soft zone exists. It is the transfer of load to the pinnacles through the stiffened abutment soils that allows these locally soft areas to exist. Soil stiffness profiles from borings substantiate this pattern. Cover-collapse features develop where soil transport through cutter drains is sufficient to remove the soils from beneath these arched areas. Two types of collapse have been observed: type 1 collapses have an upward-stoping open void whose rubble pile is removed by transport as fast as it is generated, producing a deep, steep-sided final collapse. In some cases, multiple voids in clusters can form with narrow abutments separating them. Large collapses may involve a progressive failure of several members of a cluster, including intervening pillars. Type 2 features are soil-filled voids limited in their rate of upward growth by the rate of soil removal, have little open void space, and migrate to the ground surface as a column of soft soils, finally producing a shallow depression. The type 2 features have geotechnical significance because of their effect on settlement under imposed loads. A single underdrain system may service both types of features, the behavior of particular voids being dependent on the relative efficiencies of their drains. This behavior can also change with time because backfilling of the underdrains with soil or flushing out of the soil filling can occur with changes in hydrologic or erosional regimes.

Modelling late Cenozoic isostatic elevation changes in Storfjorden, NW Barents Sea: an indication of varying erosional regimes

Storfjorden Fan off NW Barents Sea contains ∼116×10 3 km 3 of sediments which are interpreted to have been derived from a drainage area measuring 69×10 3 km 2 during the last ca. 2.3 m.y. Published isopach maps of the Storfjorden deposits have been used in a numerical model to simulate late Cenozoic isostatic elevation changes in the Storfjorden drainage area. The volume estimates in the depositional area for various time intervals together with the corrected density estimates for the eroded sediments indicate that ∼1100 m of sediments have been eroded from the drainage area during the past ca. 2.3 m.y. The results of isostatic modelling show three distinct topographic phases during this time: a subaerial phase (2.3–1.4 Ma), a transition phase (1.4–0.8 Ma) and a submarine phase (0.8 Ma–present). In addition, sediment yield values from the drainage area during various intervals have been calculated. Highest sediment yield values are obtained for the subaerial phase while those for the submerged phase are the lowest. The age estimates are somewhat uncertain, but based on the modelling results together with the estimates of sediment yield, it is suggested that the changes in elevation and sediment yield values can be linked to the documented late Cenozoic paleoclimatic shifts along the western Svalbard–Barents Sea Margin. The doubling of sediment yield in the 1.6–1.4 Ma time from the preceding 2.3–1.6 Ma interval, despite lower topography, is related to major ice expansion under a subaerial scenario. The lowest yield values during the past 0.8 m.y., despite repeated glacial advances, are attributed to submergence of the area below sea level such that periods of erosion were limited to peak glacials with negligible erosion during the intervening interstadials and interglacials.

Three-dimensional flexural modelling of the Ebro Basin�(NE Iberia)

The Ebro Basin, the southern foreland basin of the Pyrenees, has undergone a complex evolution in which, apart from the Pyrenees, the Iberian Range and the Catalan Coastal Ranges have played an important role, both as sediment sources and as basin confining structures. The deflected basement underlying the Ebro Basin dips north, suggesting a lithospheric-scale control on the structure of this basin. This is compatible with the results of subsidence analyses, which show that the study area is not in a local mode of isostatic compensation. In order better to understand the mechanisms that led to the present configuration of the Ebro Basin, and particularly the relevance of the various kinds of (un)loading (e.g. surrounding fold-and-thrust belts, basin topography, subsurface loads), we carried out a 3-D kinematic modelling study that accounts for the flexural state of the lithosphere, subjected to various loads applied at its lateral boundaries, and the sedimentary fill of the basin. We also included the effect of Neogene extensional tectonics along the eastern basin margin, which is related to the opening of the Valencia Trough. We show the suitability of the 3-D lithospheric-scale flexural modelling approach to the study of NE Iberia. Modelling results point to a relatively strong lithosphere in this area, with values of effective elastic thickness ranging from 10 to 35 km in the Ebro Basin, increasing towards the Pyrenees. We also find that the topographic (tectonic) load itself is insufficient to explain the observed basement deflection. Thus an extra subsurface load beneath the Pyrenees, corresponding to the underthrusted Iberian lithosphere, is required. The effect of lithospheric stretching in the Valencia Trough on the Ebro Basin is appreciable only in its eastern part, where the lithosphere was uplifted. This had considerable repercussions on the sedimentary and erosional regime of the Ebro Basin. We have analysed the link between the stretching-related, tectonically uplifted areas and the erosional patterns observed onshore northeast Iberia.

Changes in erosion and ocean circulation recorded in the Hf isotopic compositions of North Atlantic and Indian Ocean ferromanganese crusts

High-resolution Hf isotopic records are presented for hydrogenetic Fe–Mn crusts from the North Atlantic and Indian Oceans. BM1969 from the western North Atlantic has previously been shown to record systematically decreasing Nd isotopic compositions from about 60 to ∼4 Ma, at which time both show a rapid decrease to unradiogenic Nd composition, thought to be related to the increasing influence of NADW or glaciation in the northern hemisphere. During the Oligocene, North Atlantic Hf became progressively less radiogenic until in the mid-Miocene (∼15 Ma) it reached +1. It then shifted gradually back to an ϵ Hf value of +3 at 4 Ma, since when it has decreased rapidly to about −1 at the present day. The observed shifts in the Hf isotopic composition were probably caused by variation in intensity of erosion as glaciation progressed in the northern hemisphere. Ferromanganese crusts SS663 and 109D are from about 5500 m depth in the Indian Ocean and are now separated by ∼2300 km across the Mid-Indian Ridge. They display similar trends in Hf isotopic composition from 20 to 5 Ma, with the more northern crust having a composition that is consistently more radiogenic (by ∼2 ϵ Hf units). Paradoxically, during the last 20 Ma the Hf isotopic compositions of the two crusts have converged despite increased separation and subsidence relative to the ridge. A correlatable negative excursion at ∼5 Ma in the two records may reflect a short-term increase in erosion caused by the activation of the Himalayan main central thrust. Changes to unradiogenic Hf in the central Indian Ocean after 5 Ma may alternatively have been caused by the expanding influence of NADW into the Mid-Indian Basin via circum-Antarctic deep water or a reduction of Pacific flow through the Indonesian gateway. In either case, these results illustrate the utility of the Hf isotope system as a tracer of paleoceanographic changes, capable of responding to subtle changes in erosional regime not readily resolved using other isotope systems.

Dissolved vanadium in rivers: effects of silicate weathering

New measurements of vanadium in 15 small California catchments as well as some tributaries of the Mississippi River are presented here. These measurements complement previously published fluvial vanadium data and allow a re-examination of prior conclusions about the geochemistry of vanadium in rivers. The data suggest a best estimate of the fluvial flux of dissolved vanadium to be 0.52 Gmol year −1, though considerable uncertainty remains in this number. Dissolved vanadium in rivers appears to be derived largely from the weathering of silicate rocks which leads to a general correlation between dissolved V and Si in rivers. However, rock type can modify this basic correlation as can solubility limits on Si. The nature of the erosional regime may play some role in modifying fluvial V/Si ratios, but the effect is weak enough to be obscured by other factors. Fluvial vanadium transport is also potentially affected both by organic matter (through complexation and reduction) and by the effects of oxygen depletion in lakes, reservoirs and fresh water sediments. However, these factors require additional study. The development of methodology to determine vanadium redox speciation in the field would aid the further understanding of the processes affecting the transport of this element through the environment.

Morpho-sedimentary evidence of Holocene coastal changes near the mouth of the Gironde and on the Medoc Peninsula, SW France

The Medoc Peninsula is a triangular area of land between the Atlantic Ocean on the west and the Gironde estuary on the east. The Gironde, orientated SE-NW, is the largest estuary in France. On the Medoc Peninsula Holocene sediments cover a substratum of Tertiary limestones and Plio-Pleistocene fluvial terraces. The Gironde originated as an incised valley during the Weichselian glacio-eustatic fall (100 000–18 000 B.P.) and has acted as a sink for fine sediment throughout the Holocene sea level rise (SLR) (18 000 B.P. onwards). Conversely, the Atlantic littoral zone, characterised by sandy beaches and dunes, has been subjected to erosion throughout the Holocene transgression. The incised valley of the present Gironde was inundated by the sea approximately 10 000 B.P. At this time, the rate of SLR exceeded that of sediment supply, producing a large accommodation space in which transgressive tidal-estuarine muds and sands were deposited. As the rate of SLR decreased around 6000-4000 B.P., sedimentation became more pronounced and the available accommodation space began to decrease significantly. Landward-derived fluvial sediments began to prograde over the tidal muds and sands, and a first generation of salt marshes formed in the lateral valleys. Around 2575-1420 B.P., a sandy chenier ridge formed at the edge of a first generation of salt marshes and wholly or partly separated them from the Gironde. On the eastern side of the ridge a second generation of marshes began to form after 1200 B.P. Reclamation of the first and second generations of marshes occurred during the seventeenth and eighteenth centuries, respectively. On the estuarine side of the eighteenth century dyke the modern intertidal flats and salt marshes began to form after the eighteenth century. Sedimentation within the estuary decreased the accommodation space and led to the increased transport of sediment to the shelf after 2 000 B.P. This process was also aided by climatic and anthropogenic factors. Future evolution of the Gironde estuary is likely to consist of further marsh growth and chenier development. However, future increases in the rate of SLR, and the degree of storminess, may cause a shift to an erosional regime in parts of the lower estuary. According to previous work [25, 66], three different dune fields migrated landwards across the Atlantic littoral zone over the Holocene period. A field of isolated barchan dunes moved landwards over the Plio-Pleistocene fluvial terraces before 5100 B.P. From some time after this, until around 3000 B.P., a field of parabolic dunes was active. Finally, a barchan dune field was active from around 3000-2000 B.P. to the end of the eighteenth century/beginning of the nineteenth century, when the dunes were stabilised by pine plantations. It is tentatively suggested, using these dates and dune morphology, that dune formation was controlled by sand supply governed by the rate of SLR. However, the role of climatic changes such as aridity, storminess, windiness and the associated effects of vegetation cover, is also likely to be important. In the future the Atlantic coast is likely to continue to erode, although the extensive plantation of the aeolian dunes with pine forests is likely to prevent large-scale transgressive activity.

Genesis of some calcretes in the southern Yilgarn Craton, Western Australia: Implications for mineral exploration

Calcrete is extensively distributed in the semi‐arid and arid regions of the southern Yilgarn Craton of Western Australia. Two general types are recognised, groundwater and pedogenic calcretes; a model for the genesis of the latter is proposed, based on detailed studies in two regions (Mt Gibson and Kalgoorlie) in the southern Yilgarn Craton. At Mt Gibson, calcrete is largely restricted to erosional regimes on greenstones and is absent or present in minor amounts in relict and depositional regimes. In granitic terrain, calcrete is absent in relict and erosional regimes, but it is present in minor amounts in depositional regimes. By contrast, it is present in all the geomorphic regimes in the Kalgoorlie region although it is more abundant in erosional regimes on mafic bedrock. In soils over Ca‐poor ultramafic rocks, calcrete abundance is highly variable. Calcium and Mg in calcretes comes from two sources, in situ from weathering and from external sources (such as laterally moving soil water and erosion and...

Sediment-drift development on the continental margin off NW Britain

Abstract The long-term record of sediment-drift development on the continental margin off NW Britain was investigated using seismic-reflection data calibrated with boreholes and short cores. These data indicate that the onset of bottom currents on the NW British margin occurred in the early late Eocene following a major phase of subsidence in the Rockall Trough and Hatton-Rockall Basin. Seismic reflection profiles reveal the downwarped and eroded surface of lower upper Eocene and older strata onlapped by middle to upper Cenozoic sediments. The latter consist predominantly of deep-marine contourites preserved both as mounded, elongate, and broad, sheeted, sediment drifts with associated sediment waves. Seismic-stratigraphical analysis of the drift succession has revealed a broad two-stage depositional history. A vigorous bottom-current regime in the late Eocene to mid-Miocene interval was accompanied by significant lateral migration of sediment by upslope accretion onto the flanks of the basins, including the construction of the bulk of the Feni Ridge. Mid-Miocene tectonism modified the palaeoceanographic and sedimentary regime. In the ensuing mid-Miocene to Holocene interval, drift accumulation persisted in the axial and eastern part of the Rockall Trough whereas a largely erosional regime prevailed on its western margin. The overall locus of drift sedimentation shifted westeards onto the Rockall Plateau.

Genesis of some calcretes in the southern Yilgarn Craton, Western Australia: Implications for mineral exploration

Calcrete is extensively distributed in the semi‐arid and arid regions of the southern Yilgarn Craton of Western Australia. Two general types are recognised, groundwater and pedogenic calcretes; a model for the genesis of the latter is proposed, based on detailed studies in two regions (Mt Gibson and Kalgoorlie) in the southern Yilgarn Craton. At Mt Gibson, calcrete is largely restricted to erosional regimes on greenstones and is absent or present in minor amounts in relict and depositional regimes. In granitic terrain, calcrete is absent in relict and erosional regimes, but it is present in minor amounts in depositional regimes. By contrast, it is present in all the geomorphic regimes in the Kalgoorlie region although it is more abundant in erosional regimes on mafic bedrock. In soils over Ca‐poor ultramafic rocks, calcrete abundance is highly variable. Calcium and Mg in calcretes comes from two sources, in situ from weathering and from external sources (such as laterally moving soil water and erosion and deposition of calcrete fragments). The contribution from these sources varies according to region/area and geomorphic regime. In situ weathering is the dominant source in erosional regimes, whereas external sources have a larger input in depositional regimes. The differences in distribution of calcrete in relict and depositional regimes between the Mt Gibson and Kalgoorlie regions may be attributed to different conditions of weathering and landscape development. Modification of pre‐existing regolith by carbonates is common. Nodular and pisolitic calcrete may result from replacement of clays and of nodular and pisolitic Fe oxides by calcite and dolomite. Calcite and dolomite are the main carbonates species in the soil profiles, dolomite being more abundant on Mg‐rich mafic and ultramafic bedrocks. Dolomite crystals are, in places, coated with fibrous palygorskite. Calcium and Mg are present in both carbonate and non‐carbonate forms, thus Ca/Mg ratios do not directly reflect carbonate chemistry. Two distinct carbonate micromorphologies, euhedral‐rhombohedral and needle, suggest two modes of formation. Needle and micro‐rod calcites probably resulted from replacement of fungi and bacteria. Euhedralrhombohedral crystals of calcite and dolomite suggest inorganic precipitation from solution.

An evaluation of model parameterizations of sediment pathways: a case study for the Tejo estuary

A simple sediment transport model was used to reproduce the evolution of sedimentary features in the Tejo estuary over a 50-yr period. The parameterizations of fundamental processes of sediment dynamics such as erosion, deposition and flocculation were evaluated by conducting sensitivity analysis of model simulations to changes in specific parameters and their interactions. The analysis of the sediment budget for model simulations indicated that deposition and erosion mechanisms are sufficient to describe long-term bathymetric changes in the Tejo. The results suggest that erosion is the controlling mechanism in the classic representations of sediment pathways used in the model, while deposition is highly dependent on the prevailing erosion regime. At low to average concentrations of suspended sediments, dynamic feedbacks between deposition and erosion mechanisms control the sediment regime, but for high concentrations (erosional regime) advection determines the net export or accretion in the estuary. The short-term variability of the sediment budget along the longitudinal axis of the estuary is directly related to flocculation effects, which contribute to a significant increase of the instantaneous rates of removal of suspended sediments in the maximum turbidity zone, especially during peak flood and ebb flows. The role of erosion thresholds was also investigated with the purpose of assessing feedback effects among bottom and water-column processes. This study showed that modeling of bottom consolidation processes needed for a realistic representation of the time-space evolution of bed erosion thresholds is essential to replicate sediment processes consistent with in situ observations.

5508013 Process for the production of sulphur from at least one sour gas containing hydrogen sulphide and a fuel effluent and thermal reactor

Atoll islands are dynamic features that respond to seasonal alterations in wave conditions and sea level. It is unclear how shoreline wave run-up and erosion patterns along these low elevation islands will respond to projected sea-level rise (SLR) and changes in wave climate over the next century, hindering communities' preparation for the future. To elucidate how these processes may respond to climate change, extreme boreal winter and summer wave conditions under future sea-level rise (SLR) and wave climate scenarios were simulated at two atolls, Wake and Midway, using a shallow-water hydrodynamic model. Nearshore wave conditions were used to compute the potential longshore sediment flux along island shorelines via the CERC empirical formula and wave-driven erosion was calculated as the divergence of the longshore drift; run-up and the locations where the run-up exceed the berm elevation were also determined. SLR is projected to predominantly drive future island morphological change and flooding. Seaward shorelines (i.e., ocean fronted shorelines directly facing incident wave energy) were projected to experience greater erosion and flooding with SLR and in hypothetical scenarios where changes to deep water wave directions were altered, as informed by previous climate change forced Pacific wave modeling efforts. These changes caused nearshore waves to become more shore-normal, increasing wave attack along previously protected shorelines. With SLR, leeward shorelines (i.e., an ocean facing shoreline but sheltered from incident wave energy) became more accretive on windward islands and marginally more erosive along leeward islands. These shorelines became more accretionary and subject to more flooding with nearshore waves becoming more shore-normal. Lagoon shorelines demonstrated the greatest SLR-driven increase in erosion and run-up. They exhibited the greatest relative change with increasing wave heights where both erosion and run-up magnitudes increased. Wider reef flat-fronted seaward shorelines became more accretive as all oceanographic forcing parameters increased in magnitude and exhibited large run-up increases following increasing wave heights. Island end shorelines became subject to increased flooding, erosion at Wake, and accretion at Midway with SLR. Under future conditions, windward and leeward islands are projected to become thinner as ocean facing and lagoonal shorelines erode, with leeward islands becoming more elongate. Island shorelines will change dramatically over the next century as SLR and altered wave climates drive new erosional regimes. It is vital to the sustainability of island communities that the relative magnitudes of these effects are addressed when planning for projected future climates.

Multi-element chemistry of some Amazonian waters and soils

The rivers flowing through the Amazon Basin are both physically and chemically heterogeneous. Through detailed geochemical analyses, variability is shown to be primarily controlled by both substrate lithology in the source region and soil geochemistry in the erosional regime. The solute-rich waters of the Rio Solimões reflect the drainage from the Andes Mountains and the fertility of the varzea. In contrast, the solute-deficient waters of the Rio Negro reflect the infertility of the lateritic and podsolitic terrains of the Central Amazon. Rivers flowing through the Precambrian Shield have an intermediate composition. Such observations suggest that it is possible to classify the chemical composition of Amazonian rivers according to the soil types in their catchment regions and vice versa. These findings have profound implications for using water chemistry as an indicator of the agricultural and mineral potential of a region. Results from this study also confirm that precipitation is a significant source of major cations and trace metals to solute-deficient river systems such as the Rio Negro.

Hydrodynamics of salt marsh creek systems: Implications for marsh morphological development and material exchange

AbstractIntegrated ebb‐aligned drainage systems are a feature of tide‐dominated marshes, and are generally regarded as major conduits for material exchange. In north Norfolk, highly unsteady creek flows exhibit well‐developed velocity and stress transients which result from the discontinuous nature of the tidal prism and the interaction of shallow water tidal inputs with hydraulically rough vegetated surfaces. Marsh morphological development is governed by a form‐process feedback, in the sense that the marsh surface acts as a topographic threshold separating the depositional regime of below‐marsh tides from the erosional (ebb‐dominated) regime of over‐marsh tides. Vertical marsh growth results in increasing intermittency of creek sediment transport. Furthermore, velocity transients are associated with large discharges which must be allowed for in material flux computations. Creek flux measurements are not in themselves sufficient to estimate total material budgets, since a large proportion of tidal exchange may take place via the marsh edge. Such studies should focus instead on direct measurement of marsh surface processes. These findings have relevance beyond this back‐barrier setting to marshes of different geometry, occupying a broad range of the tidal energy spectrum.

Geomorphological observations concerning the evolution of the Basin of Athens

Tectonism is an important element in the determination of the present landscape of Greece. It has shaped and is shaping the numerous highlands and depressions and consequently settlement patterns of the country. One such area is the Greater Athens region, a triangular basin, opening to the sea in the southwest and surrounded by four mountain masses. The presence of an old planation surface on the surrounding mountains; of thick lateritic soils in the basin overlain by Miocene and Pliocene marls, marly limestones, sands and conglomerates; Pleistocene fluvio-torrential deposits; alluvial fans; and fluvial and marine terraces, enabled the authors to put together the Tertiary and Quaternary tectonic and geomorphic evolution of the Greater Athens area. The paleogeographic evolution goes back to a probable mountainous topography before the Miocene with near tropical climatic conditions. In the Early Miocene, the breaking up of the highland area and the formation of today's basin begins or has already begun. During the Late Miocene, a transgression of the sea occurs, particularly in the western half of the basin. This is followed by regression, erosion and Late Pliocene transgression. During the Neogene and Quaternary we have continuous fluvio-torrential deposition intercalating with marine sediments. In the Quaternary, the tectonic activity in the area slowly subsides and climate, particularly in the late Pleistocene and Holocene and eustatism, especially along the coastal zone, affect more readily the depositional and erosional regime and geomorphic processes.

Constraints on early events in Martian history as derived from the cratering record

The shapes and densities of crater size‐frequency distribution curves are used to constrain two major events early in Martian history: termination of high obliteration rates and viability of the multiple impact origin of the crustal dichotomy. Distribution curves of fresh craters superposed on uplands, intercrater plains, and ridged plains display shapes and densities indicative of formation prior to the end of heavy bombardment. This observation correlates with other geologic evidence, suggesting a major change in the erosional regime following the last major basin size impact (i.e., Argrye). In addition, the multisloped nature of the curves supports the idea that the downturn in the crater size‐frequency distribution curves reflects the size‐frequency distribution of the impactors rather than being the result of erosion. The crustal dichotomy formed prior to the heavy bombardment intermediate epoch based on distribution curves of knobby terrain; if the dichotomy resulted from a single gigantic impact, this observation places constraints on when this event happened. An alternate theory for dichotomy formation, the multiple‐impact basin idea, is questioned: since distribution curves of large basins as well as heavy bombardment era units are not represented by a −3 differential power law function, this study finds fewer basins missing on Mars compare to the Moon and Mercury than previously reported. The area covered by these missing basins is less than that covered the northern plains.

Characterization of Venera 15/16 Geologic units from Pioneer Venus reflectivity and roughness data

Geologic units have been defined for the surface of Venus from Venera 15/16 image data. A characterization of these geologic units is carried out using information on surface properties derived from Pioneer Venus (PV) reflectivity and rms slope data. The geologic context provided by Venera 15/16 units allows additional, more specific interpretations of surface radar properties to be made. Characterization of Venera units results in the definition of four groups of Venera units: (1) smooth rocky units, 2) rough rocky units, (3) rough high dielectric units, and (4) diffusely scattering units. On the basis of correlations of surface morphology to spatial and statistical distributions in rms slope and reflectivity data, we test models for the origin of the surface properties of some units. We conclude that plains and tectonic units can be contrasted in terms of the average roughness of the surface and that tectonic deformation appears to roughen the surface at 0.5- to 10-m and 5- to 50-cm scales. This tectonic weathering process appears to dominate the erosional regime of Venus. Unlike Earth or Mars, production and transport of soils dominates only a small portion (less than or equal to 5%) of the surface. Some of the Venera units display distinctive spatial and statistical distributions of PV radar data. In particular, apparent low reflectivity in the tesserae appears to be caused by small (5-50 cm) rock fragments on the surface which cause diffuse scattering at Pioneer Venus wavelengths. Analysis of models for the formation of these fragments suggests that they are due to the pervasive deformation undergone by the tesserae. Finally, aspects of this study have been used to extend results of Venera image data analysis southward of 30 deg. N lat, resulting in it prediction of the distribution of tessera. Such results can aid in Magellan investigations.

Interaction of the Western Boundary Undercurrent with the continental margin off Newfoundland

ABSTRACTSedimentation on the Newfoundland rise is strongly influenced by the Western Boundary Undercurrent (WBU). The upper rise (2600‐2800 m) is swept by a rapid (Ū= 8·5 cm sec−1), south‐flowing core of the WBU which has generated a sandy contourite facies characterized by coarse gravelly, sandy sediments; current‐induced bedforms such as scour moats, lineations and lee drifts; ferro‐manganese‐stained gravel clasts; a high proportion of broken foraminiferal tests and a diagnostic benthic foraminiferal assemblage. The overlying nepheloid layer, when compared to adjacent waters, is thickest (800 m), most sediment laden (80 μg 1−1), contains the highest proportion of terrigenous sediment and exhibits the best developed bottom mixed layer (∼ 15 m thick). Comparisons with earlier data from the same area imply the dimensions and sediment load of the nepheloid layer vary with time. Empirical considerations, based on near‐bottom current meter records from Labrador and Newfoundland, suggest the WBU is capable of transporting bedload with threshold friction velocities (u*) of around 0·87‐1·14 cm sec−1 for between 1 and 15% of the time. The prevailing transport direction is southwards along the rise, but this may be punctuated periodically by brief incursions to the north.The erosional regime of the upper rise is bordered by a regime of fine‐grained deposition typified by muddy contourites. Both the lower slope and lower rise are mantled by bioturbated muds, the former zone having terrigenous mud and the latter, biogenic calcareous mud. The accompanying nepheloid layer is thin, biogenic‐rich and devoid of an identifiable mixed layer.