Tidal currents,bed sediments,and bedforms at the South Branch and the South Channel of the Changjiang (Yangtze) estuary,China: Implications for the ripple-dune transition

Measurements were made of tidal currents, bed sediment particle sizes, and bedform dimensions at the South Branch and the South Channel of the Changjiang estuary, China, during the dry season in 1997 and the flood season in 1998. The near bottom current speed and direction were measured by a mechanical current meter for 10 h in 1997. The near surface current speed and direction were measured by a Current Meter of Endeco/YSI Inc. 174 SSM for 14 h in 1997 and 1998. Nine bed sediment samples were taken and their particle sizes were analyzed with sieves and siphons. The bedforms were nautically detected by an echo sounder and a side scan sonar. Results show that the ebb tides had larger near-bottom and near-surface current speeds and longer durations than the flood tides, in which the former occurred during the flood season in 1998 and not in the dry season in 1997. The bed sediments were composed of coarse silts and very fine sands during the dry season but of fine sands during the flood season. Bedforms were dominated by ebb tidal currents, the height∶length ratios of dunes and lee face angles were low, and heights and lengths were larger during the flood season in 1998. The ebb and flood tidal currents, bed sediment sizes, and dune morphology were largely controlled by the seasonal runoff variations. A new tentative boundary might be proposed for natural dunes in very fine sand with the availability of additional field data in the future.     

Sediment transport and bedforms in a carbonate tidal inlet; Lee Stocking Island, Exumas, Bahamas

An active oolitic sand wave was monitored for a period of 37 days in order to address the relationship between the direction and strength of tidal currents and the resultant geometry, and amount and direction of migration of bedforms in carbonate sands. The study area is situated in a tidal channel near Lee Stocking Island (Exumas, Bahamas) containing an estimated 5.5 to 6 × 10⁵ m³ of mobile oolitic sand. Tidal ranges within the inlet are microtidal and the maximum current velocity at the studied site is 0.6 m s^?1. At least 300–400 m³ of mostly oolitic sand are formed within, or brought into, the channel area every year. The tidal inlet is subdivided into an ocean-orientated segment, in which sand waves are shaped by both flood and ebb tides, and a platform-orientated segment, where sand waves are mainly shaped by flood tides. The studied sand wave lies on the platformward flood-tide dominated segment in a water depth of 3.5.4.5 m. During the 37 days of observation, the oolitic and bioclastic sand wave migrated 4 m in the direction of the dominant flood current. The increments of migration were directly related to the strength of the tide. During each tidal cycle, bedforms formed depending on the strength of the tidal current, tidal range and their location on the sand wave. During flood tides, a steep lee and a gentle stoss side formed and current ripples and small dunes developed on the crest of the sand wave, while the trough developed only ripples. The average lee slope of the sand wave is 24.2°, and therefore steeper than typical siliciclastic sand waves. During ebb tides, portions of the crest are eroded creating a convex upward ebb stoss side, covered with climbing cuspate and linguoid ripples and composite dunes. The area between the ebb-lee side and the trough is covered with fan systems, sinuous ripples and dunes. The migration of all bedforms deviated to a variable degree from the main current direction, reflecting complex flow patterns in the tidal inlet. Small bedforms displayed the largest deviation, migrating at an angle of up to 90° and more to the dominant current direction during spring tides.     

Sedimentological characteristics of small rivers with loessic headwaters in the Chaco,South America

Some sedimentological characteristics of fluvial environments located in the northeastern region of the Chaco–Pampa plain were analysed in order to determine the possible influence of the loessic headwaters of local basins. Concentrations of suspended sediments, as well as grain size of the river beds, were compared. Unexpectedly, it was found that the suspended load has an inverse relationship with the hydrometric levels, with the exception of highly saline streams (conductivity values between 4300 and 11,000 μS cm⁻¹). The mean concentration of suspended sediments was 63.1 and 206.3 mg l⁻¹ in flood and low water periods, respectively. The grain size of bed sediments was relatively homogeneous along the whole hydrological cycle. Sandy–silty bottoms predominate in channels, followed by silty–sandy and clayey–silty bottom types. Sediment transport mainly occurs in accelerating flow suspension, overcoming the critical erosion velocity. During flood periods, some parameters exceeded by 70% the values recorded during the low water phase.     

Dynamics of coarse particulate matter in the turbidity maximum zone of the Gironde Estuary

There is a lack of studies devoted to coarse particulate matter (CPM) in estuaries, although this fraction can disturb activities that filter large volumes of water, such as industrial or fishery activities. In the macrotidal and highly-turbid Gironde Estuary, a monthly sampling of CPM was performed in 2011 and 2013 at two stations in the Turbidity Maximum Zone (TMZ) to understand its seasonal, tidal and hydrological dynamics. Regardless of the season and station, low quantities of CPM (few g m⁻³) were observed in comparison with suspended particulate matter (several 10³ g m⁻³). The highest concentrations were consistently recorded in bottom waters and at the upstream station. Whereas there is no clear link between the CPM present in the column water and spring or neap tides, an increase in the CPM size has been identified at the two stations after a flood event, fact potentially critical regarding filtering functioning of estuarine activities.     

Dissolved and particulate zinc and nickel in the Yangtze River (China): Distribution,sources and fluxes

Dissolved and particulate Zn and Ni concentrations were determined at 76 locations along the Yangtze River basin from the headwaters to the estuary during flood and dry seasons. Spatial and temporal variations of Zn and Ni were investigated and six major source zones were identified. The Three Gorges Dam (TGD) blocked most of the suspended loads and extremely low concentration of Zn and Ni were observed downstream of the dam. Dissolved (ranging from 0.062 to 8.0 μg L⁻¹) and particulate (ranging from 12 to 110 mg kg⁻¹) Ni showed similar levels of concentrations during flood and dry seasons, whereas dissolved (ranging from 0.43 to 49 μg L⁻¹) and particulate (ranging from 54 to 1100 mg kg⁻¹) Zn were slightly and much lower in the flood season than dry season, respectively. This was attributed to the increased water discharge during the flood season causing a dilution effect and sediment resuspension. In the flood season, average concentrations of Zn and Ni were higher in the main channel than in tributaries, due to soil erosion and mining activities providing the dominant inputs. The situation was opposite in the dry season, attributed to the contribution of municipal sewage, industrial activities, and waste disposal. During the flood season, dissolved Zn and Ni concentrations were negatively correlated with pH. Water and suspended particulate matter (SPM) from the upper reaches, middle reaches, and lower reaches of the Yangtze River were characterized by their Zn and Ni concentrations. The Panzhihua, Nanling and Tongling mining areas were considered as the most important source zones of particulate Zn and Ni. The Chongqing region, Wuhan region and the Yangtze River Delta provided most of the dissolved Zn and Ni inputs into the river. Annual net flux of Zn (10–72 × 10⁵ kg a⁻¹) and Ni (5.0–19 × 10⁵ kg a⁻¹) in each source zone were estimated according to their respective influent and effluent fluxes. Contributions of the source zones to Zn and Ni transport decreased from the upper reaches to the lower reaches.     

Late Quaternary linear dune accumulation and chronostratigraphy of the southwestern Kalahari: implications for aeolian palaeoclimatic reconstructions and predictions of future dynamics

The linear dunes of the Kalahari, now largely inactive, have long been identified as having potential palaeoenvironmental significance. The application of optically stimulated luminescence (OSL) dating to these dunes in the 1990s provided the first chronology of aeolian accumulation in this region, though field methodologies and time-consuming multiple-aliquot laboratory protocols limited both the depth of sampling in dune bodies and the total number of samples dated.In order to permit a more thorough investigation of the potential of these dunes to preserve long chronological records, this intensive study presents 71 OSL ages from the linear dunes of the southwestern Kalahari at Witpan, South Africa, sampled with coring equipment at regular and frequent intervals down to bedrock.The earliest sand accumulation recorded at Witpan is at 104 ka, and in spatially discrete locations, other evidence of dune activity is recorded at 77–76, 57–52 and 35–27 ka. Although an inherently discontinuous archive, the linear dunes of the southwestern Kalahari have the potential to record multiple phases of dune construction. Following the Last Glacial Maximum there is near continuous evidence of dune-building, with a peak of accumulation recorded from 15 to 9 ka at five individual sites. This latter period is generally recognised from other proxy evidence as being unusually arid in this region, and such periods of dune activity are likely to be related to intensification of the continental anticyclone. During the Holocene, accumulation has continued at most sites sampled, albeit at a lesser intensity. This may imply that these dunes are presently not far from thresholds of activation.     

Holocene dune formation at Ash Meadows National Wildlife Area,Nevada, USA

Small isolated dune fields in the northern Mojave Desert are important centers of biodiversity and archaeological occupation sites. Currently dunes at Ash Meadows, Nevada, are stabilized by vegetation and are experiencing erosion of their upwind margins, indicating a negative sediment budget. New OSL ages from dunes at Ash Meadows indicate continuous eolian accumulation from 1.5 to 0.8 ka, with further accumulation around 0.2 ka. Prior studies (e.g., Mehringer and Warren, 1976) indicate periods of dune accumulation prior to 3.3 ka; 1.9–1 ka; and after 0.9 ka. These periods of eolian accumulation are largely synchronous with those identified elsewhere in the Mojave Desert. The composition of the Ash Meadows dunes indicates their derivation from regional fluvial sources, most likely during periods when axial washes were active as a result of enhanced winter precipitation.     

Cross-strata and bedforms of probable transitional dune to upper-stage plane-bed origin from a Late Precambrian fluvial sandstone, northern Norway.

A Late Precambrian fluvial sandstone sequence in northern Norway is dominated by large-scale cross-sets that show either lenticular or tabular geometries in the streamwise sections. The lenticular sets interdigitate and in places show nearly symmetrical formsets. The tabular sets are in places solitary, but are mainly grouped in cosets. In both cross-set types, the cross-strata range from concave-up to sigmoidal in shape, with the latter variety comprising subhorizontal to gently inclined topset strata (with parting lineation) that merge uninterruptedly downflow into the steeper (10–2°) foresets. Within the cross-sets the geometry and dip azimuths of the foresets are conspicuously consistent, although the concave-up and sigmoidal strata commonly alternate downcurrent. The cross-strata characteristics suggest flood stage deposition from relatively high velocity steady currents heavily laden with suspended sand. Both cross-set types are interpreted as representing bedforms generated by flow in the dune to upper-stage plane-bed transition. The lenticular cross-sets probably represent periodic dunes, but it is far less clear whether the long bedforms represented by the tabular sets should be classified as dunes, or rather as solitary to quasi-periodic bars.     

Evolution and mechanics of a Miocene tidal sandwave

A remarkable exposure of Miocene marine molasse in western Switzerland records the evolution of a tidal sandwave over a period of approximately 2 1/2 months. The sandwave is composed of tidal ‘bundles’ in which a sandwave reactivation stage and full vortex stage can be recognized for the dominant flow (ebb tide) and a rippled flood apron overlain by high water drape for the reversed flow. Bundle thicknesses vary systematically through neap–spring cycles, with a periodicity of 27 demonstrating the semi-diurnal lunar control of sedimentation. Waves were an additional component, especially when superimposed on flood tides, producing near-symmetrical combined-flow ripple marks in the flood apron. Tidal current velocities are estimated using critical shear velocities for entrainment, the ripple-dune transition and the dune-plane bed transition. Using appropriate estimates of roughness lengths and a logarithmic velocity law, maximum tidal speeds at 1 m above the bed were approximately 0·6 m sec^?1 for ebbs and up to 0·5 m sec^?1 for floods. The enhancement by waves of bed shear stress (τ_wc/τ of approximately 2 for 1 m high waves) under flood currents implies flood tidal velocities closer to 0·2–0·3 m sec^?1. Peak instantaneous bedload sediment transport rates using a modified Bagnold equation are nearly 5 times greater under ebb tides than floods. The average net sediment transport rate at springs (0·04 kg m^?1 sec^?1) is over 10 times greater than at neaps (0·002 kg m^?1 sec^?1). Comparison with transport rates in modern tidal environments suggests that the marine molasse of Switzerland was deposited under spatially confined and relatively swift tidal flows not dissimilar to those of the present Dutch tidal estuaries.     

Bedform topography and basal conditions beneath a fast-flowing West Antarctic ice stream

A grid of seismic reflection lines has been used to image basal topography and infer basal conditions and flow processes beneath ～140 km² of Rutford Ice Stream, West Antarctica. The subglacial topography in this region consists of two troughs flanking a central high and the bed is composed of water-saturated sediments. The two troughs are filled with deforming sediment, whereas the bed in the central region appears to undergo a transition from largely deforming conditions upstream to basal sliding downstream. The deforming bed is very flat along flow, but undulates across flow. Sliding areas show rougher bed topography. Cross-stream bed topography is characterised by streamlined mounds of deforming sediment aligned in the ice flow direction. These bedforms occur superimposed on the bed in regions of both basal sliding and sediment deformation. In places, they form finger-like mounds of material, which extend into the sliding region further downstream. Mean bedform height is 22 m, mean width is 267 m, and many of them extend for at least 1–2 km along flow. We interpret most of these bedforms as drumlins and one as a mega-scale glacial lineation. The juxtaposition of different basal conditions is consistent with models proposed from terrestrial studies in which the glacier bed is a mosaic of stable and deforming bed areas, variable both spatially and temporally. Any theory of subglacial sediment rheology must also be able to account for our conclusion that, at any given time, pervasive deformation extends at least a few metres into the bed and can persist over a considerable area (many km²). Bedform geometry and basal conditions concur with interpretations of former ice streams, with evidence for increasing elongation ratio with distance downstream. However, those studies also identified bedrock cropping out at the ice-bed interface, for which there is no evidence on Rutford Ice Stream.     

Megaripple dynamics and sediment transport in a mesotidal mangrove creek: implications for palaeoflow reconstructions

An experiment was conducted to study megaripple morpho dynamics on a sandy intertidal shoal in a mesotidal mangrove creek (Gordon Creek, Townsville, Australia). Tidal current velocity and depth were recorded with S4 current meters over a period of 35 tides. The tidal megaripples were 0.06–0.2 m in height and 1–2 m in wavelength, and their movement was monitored by (1) electromagnetic bed-elevation probes (which automatically recorded bed level every 2 min at three positions along the survey transect) and (2) daily surveying for 8 days around spring tidals. The tidal currents in Gordon Creek are ebb-dominated, with maximum depth-mean current velocities for the flood and ebb tides of 0.62 and 0.98 m s^?1 respectively. Significant bedload transport occurs only during spring tides, and only on the larger of the unequal semi-diurnal tides. Bedload transport is overwhelmingly in the ebb direction. Megaripple migration rates reach 5.6 m per tide in the ebb direction and up to 0.1 m min^?1 within individual tides. Within-tide ‘bedform transport rates’are up to 0.29 kg m^?1 s^?1. The results suggest that for reconstruction of palaeoflows from deposits of preserved fine- to medium-grained sandy tidal megaripples, it is valid to use a depth-averaged velocity of 0.5–0.6 m s^?1 as the migration threshold. Velocity thresholds associated with partial or complete reversal of megaripple asymmetry are invalid.     

Hoanib River flood deposits of Namib Desert interdunes asanalogues for thin permeability barrier mudstone layers inaeolianite reservoirs

ABSTRACT The ephemeral braided Hoanib River of NW Namibia flows for a few days a year, and only high discharges enable the river to pass through interdunal depressions within the northern Namib Desert dune field to the Atlantic. The dune field comprises mainly large transverse dunes resulting from predominant SSW winds. River flood deposits between aeolian dunes are analogous to mudstone layers conformably interbedded with ancient aeolianite dune foresets. Deep floods pond laterally to considerable depths (metres to >10 m) in adjacent interdunes, depositing mud layers (1–50 cm) to considerable heights on avalanche and stoss faces of bounding dunes. Fairly passive flooding only disturbs aeolian stratification minimally. Floodwater clay infiltrates and settles as an impermeable seal, with a flood pond on top, perched, above regional groundwater. Flood ponds evaporate slowly for long periods (>3 years). Early emergence desiccates higher parts of a mud layer. Subsequent floods can refill a predecessor pond, benefiting from the existing impervious seal. Potential preservation of such mud layers is lower on the stoss face, but high on the avalanche face after burial by subsequent dune reactivation and migration. The leeward (right) Hoanib bank, a dune stoss face, is river and wind eroded to exhume fossil interdune pond mud layers of an earlier Hoanib channel. The highly inclined layers are interbedded with dune avalanche foresets and represent the edges of two successive fossil ponds exposed in plan. Ancient flood pond mudstones occur in the Permian–Triassic hydrocarbon reservoir, the Sherwood Sandstone Group of the Cheshire Basin (Kinnerton Formation) and Irish Sea Basin and were previously used erroneously to argue against the aeolian origin of cross‐bed sets. Hoanib studies show that primary river interaction with a dune field might preserve only localized erosional omission surfaces in ancient aeolianites, with little sandy barform preservation, prone to aeolian reworking. Around the main fluvial channel locus, however, flood pond mudstone layers should form a predictable halo, within which fluid permeability will decrease.     

Flow and sediment transport over large subaqueous dunes: Fraser River, Canada

Large symmetric and asymmetric dunes occur in the Fraser River, Canada. Symmetric dunes have stoss and lee sides of similar length, stoss and lee slope angles <8°, and rounded crests. Asymmetric dunes have superimposed small dunes on stoss sides, sharp crests, stoss sides longer than lee sides, stoss side slopes <3° and straight lee side slopes up to 19°. There is no evidence for lee side flow separation, although intermittent separated flow is possible, especially over asymmetric dunes. Dune symmetry and crest rounding of symmetric dunes are associated with high sediment transport rates. High near-bed velocity and bed load transport near dune crests result in crest rounding. Long, low-angle lee sides are produced by deposition of suspended sediment in dune troughs. Asymmetric dunes appear to be transitional features between large symmetric dunes and smaller dunes adjusted to lower flow velocity and sediment transport conditions. Small dunes on stoss sides reduce near-bed flow velocity and bed load transport, causing a sharper dune crest. Reduced deposition of suspended sediment in troughs results in a short, steep lee slope. Dunes in the Fraser River fall into upper plane bed or antidune stability fields on flume-based bedform phase diagrams. These diagrams are probably not applicable to large dunes in deep natural flows and care must be taken in modelling procedures that use phase diagram relations to predict bed configuration in such flows.     

Morphology,sedimentology and palaeohydraulic significance of large gravel dunes,Altai Mountains,Siberia

Coarse-gravel bedforms which resulted from Pleistocene glacial outburst floods are identified as subaqueous dunes. Comparison of the morphology of these ‘fossil’ structures with modern dunes shows that the form of two-dimensional (2-D) transverse dunes and 3-D cuspate and lunate dunes developed in coarse gravels is comparable with sand-dune morphology within lesser-scale geophysical flows. The similarity of the steepest gravel dunes with equilibrium dunes in sand indicates that grain size is not a major factor in constraining primary duneform. Internal structure indicates that flow over 2-D dunes was relatively uniform but over 3-D bedforms flow was locally variable. Flow separation and complex streaming of flow occurred over the steepest 3-D dunes. Cross-beds are thin and few approach the angle of repose; consequently most dunes did not migrate primarily by avalanching but by stoss-entrained gravel transported over the crests rolling-down and depositing on the lee slopes. Lee-side sediments are often finer than the stoss-slope sediments, which indicates the lee formed when flood power was waning. Some dunes were slightly planed-down during falling stage because lee-side cross-beds tend to be steeper than the angle of the preserved lee slope. However, silt-rich caps indicate that any height reduction was contemporary with the final deposition of foresets. Post-flood modification has been negligible although the modern topography is subdued by loess deposits within the dune troughs.     

Groundwater fluxes into a submerged sinkhole area,Central Italy,using radon and water chemistry

The groundwater contribution into Green Lake and Black Lake (Vescovo Lakes Group), two cover collapse sinkholes in Pontina Plain (Central Italy), was estimated using water chemistry and a ²²²Rn budget. These data can constrain the interactions between sinkholes and deep seated fluid circulation, with a special focus on the possibility of the bedrock karst aquifer feeding the lake. The Rn budget accounted for all quantifiable surface and subsurface input and output fluxes including the flux across the sediment–water interface. The total value of groundwater discharge into Green Lake and Black Lake (∼540 ± 160 L s⁻¹) obtained from the Rn budget is lower than, but comparable with historical data on the springs group discharge estimated in the same period of the year (800 ± 90 L s⁻¹). Besides being an indirect test for the reliability of the Rn-budget “tool”, it confirms that both Green and Black Lake are effectively springs and not simply “water filled” sinkholes. New data on the water chemistry and the groundwater fluxes into the sinkhole area of Vescovo Lakes allows the assessment of the mechanism responsible for sinkhole formation in Pontina Plain and suggests the necessity of monitoring the changes of physical and chemical parameters of groundwater below the plain in order to mitigate the associated risk.     

Soil-geochemical factors controlling the distribution and oral bioaccessibility of nickel,vanadium and chromium in soil

Geogenic nickel (Ni), vanadium (V) and chromium (Cr) are present at elevated levels in soils in Northern Ireland. Whilst Ni, V and Cr total soil concentrations share common geological origins, their respective levels of oral bioaccessibility are influenced by different soil-geochemical factors. Oral bioaccessibility extractions were carried out on 145 soil samples overlying 9 different bedrock types to measure the bioaccessible portions of Ni, V and Cr. Principal component analysis identified two components (PC1 and PC2) accounting for 69% of variance across 13 variables from the Northern Ireland Tellus Survey geochemical data. PC1 was associated with underlying basalt bedrock, higher bioaccessible Cr concentrations and lower Ni bioaccessibility. PC2 was associated with regional variance in soil chemistry and hosted factors accounting for higher Ni and V bioaccessibility. Eight percent of total V was solubilised by gastric extraction on average across the study area. High median proportions of bioaccessible Ni were observed in soils overlying sedimentary rock types. Whilst Cr bioaccessible fractions were low (max = 5.4%), the highest measured bioaccessible Cr concentration reached 10.0 mg kg⁻¹, explained by factors linked to PC1 including high total Cr concentrations in soils overlying basalt bedrock.     

The OSL chronology of eolian sand deposition in a perched dune field along the northwestern shore of Lower Michigan

Extensive coastal dunes occur in the Great Lakes region of North America, including northwestern Michigan where some are perched on high (~ 100 m) bluffs. This study focuses on such a system at Arcadia Dunes and is the first to systematically generate optical ages from stratigraphic sections containing buried soils. Dune growth began ca. 4.5 ka during the Nipissing high lake stand and continued episodically thereafter, with periods of increased sand supply at ca. 3.5 ka and ca. 1.7 ka. The most volumetrically dominant phase of dune growth began ca. 1.0 ka and continued intermittently for about 500 years. It may have begun due to the combined effects of a high lake phase, potential changes in lake hydrodynamics with final isostatic separation of Lake Superior from Lakes Michigan and Huron, and increased drought and hydrologic variability associated with the Medieval Warm Period. Thus, this latest eolian phase likely reflects multiple processes associated with Great Lakes water level and climate variability that may also explain older eolian depositional events. Comparison of Arcadia ages and calendar corrected ¹⁴C ages from previous studies indicate broad chronological agreement between events at all sites, although it appears that dune growth began later at Arcadia.     

Mercury and methylmercury in riparian soil,sediments, mine-waste calcines,and moss from abandoned Hg mines in east Guizhou province,southwestern China

Concentrations of total Hg and methylmercury (MMHg) in riparian soil, mine-waste calcine, sediment, and moss samples collected from abandoned Hg mines in Wanshan district, Guizhou province, China, were measured to show regional dispersion of Hg-contamination. High total Hg and MMHg concentrations obtained in riparian soils from mined areas, ranged from 5.1 to 790 mg kg⁻¹ and 0.13 to 15 ng g⁻¹, respectively. However, total Hg and MMHg concentrations in the soils collected from control sites were significantly lower varying from 0.1 to 1.2 mg kg⁻¹ and 0.10 to 1.6 ng g⁻¹, respectively. Total Hg and MMHg concentrations in sediments varied from 90 to 930 mg kg⁻¹ and 3.0 to 20 ng g⁻¹, respectively. Total Hg concentrations in mine-waste calcines were highly elevated ranging from 5.7 to 4400 mg kg⁻¹, but MMHg concentrations were generally low ranging from 0.17 to 1.1 ng g⁻¹. Similar to the high Hg concentrations in soil and sediments, moss samples collected from rocks ranged from 1.0 to 95 mg kg⁻¹ in total Hg and from 0.21 to 20 ng g⁻¹ in MMHg. Elevated Hg concentrations in mosses suggest that atmospheric deposition might be an important pathway of Hg to the local terrestrial system. Moreover, the spatial distribution patterns of Hg contamination in the local environment suggest derivation from historic Hg mining sites in the Wanshan area.     

Phosphorus and iron cycling in deep saprolite,Luquillo Mountains,Puerto Rico

Rapid weathering and erosion rates in mountainous tropical watersheds lead to highly variable soil and saprolite thicknesses which in turn impact nutrient fluxes and biological populations. In the Luquillo Mountains of Puerto Rico, a 5-m thick saprolite contains high microorganism densities at the surface and at depth overlying bedrock. We test the hypotheses that the organisms at depth are limited by the availability of two nutrients, P and Fe. Many tropical soils are P-limited, rather than N-limited, and dissolution of apatite is the dominant source of P. We document patterns of apatite weathering and of bioavailable Fe derived from the weathering of primary minerals hornblende and biotite in cores augered to 7.5 m on a ridgetop as compared to spheroidally weathering bedrock sampled in a nearby roadcut.Iron isotopic compositions of 0.5 N HCl extracts of soil and saprolite range from about δ⁵⁶Fe = 0 to ? 0.1‰ throughout the saprolite except at the surface and at 5 m depth where δ⁵⁶Fe = ? 0.26 to ? 0.64‰. The enrichment of light isotopes in HCl-extractable Fe in the soil and at the saprolite–bedrock interface is consistent with active Fe cycling and consistent with the locations of high cell densities and Fe(II)-oxidizing bacteria, identified previously. To evaluate the potential P-limitation of Fe-cycling bacteria in the profile, solid-state concentrations of P were measured as a function of depth in the soil, saprolite, and weathering bedrock. Weathering apatite crystals were examined in thin sections and an apatite dissolution rate of 6.8 × 10^? 14 mol m^? 2 s^? 1 was calculated. While surface communities depend on recycled nutrients and atmospheric inputs, deep communities survive primarily on nutrients released by the weathering bedrock and thus are tightly coupled to processes related to saprolite formation including mineral weathering. While low available P may limit microbial activity within the middle saprolite, fluxes of P from apatite weathering should be sufficient to support robust growth of microorganisms in the deep saprolite.     

Feeding ecology of o-group sea bass,Dicentrarchus labrax, in salt marshes of Mont Saint Michel Bay (France)

0-group sea bass,Dicentrarchus labrax, colonize intertidal marsh creeks of Mont Saint Michel Bay, France, on spring tides (e.g., 43% of the tides) during flood and return to coastal waters during ebb. Most arrived with empty stomachs (33%), and feed actively during their short stay in the creeks (from 1 to 2 h) where they consumed on average a minimum of 8% of their body weight. During flood tide, diet was dominated by mysids,Neomysis integer, which feed on marsh detritus. During ebb, when young sea bass left tidal marsh creeks, the majority had full stomachs (more than 98%) and diet was dominated by the most abundant marsh (including vegetated tidal flats and associated marsh creeks) resident amphipod,Orchestia gammarellus. Temporal and tidal effects on diet composition were shown to be insignificant. Foraging in vegetated flats occurs very rarely since they are only flooded by about 5% of the tides. It was shown that primary and secondary production of intertidal salt marshes play a fundamental role in the feeding of 0-group sea bass. This suggests that the well known nursery function of estuarine systems, which is usually restricted to subtidal and intertidal flats, ought to be extended to the supratidal, vegetated marshes and mainly to intertidal marsh creeks.