Decrease In Mean Grain Size Research Articles

Effect of welding gas mixtures on weld material and bead geometry

Abstract Shielding gas is an essential parameter in welding processes, serves to shield the molten material from the atmosphere and external contaminations while providing a stable electric-arc pathway during the process, thus influencing the quality of the weld bead and several other aspects of the process. Therefore, any welding-based process, such as wire-arc additive manufacturing, could benefit from shielding gas mixture optimisation to selectively improve the processing and possibly the final mechanical properties of the welded material. In this study welding of EN AW 5183 aluminium (single weld bead and wire-arc additive manufacturing build-up) was conducted using several gas mixtures and their influence on final geometry defects, and microstructure measured. The gas mixtures used were a combination of varying amounts of carbon dioxide, nitrogen, and oxygen. These were added to commercially available argon gas. It was observed that: CO2 additions led to an unfavourable bonding defect and increased mean grain size; nitrogen additions caused a decrease in mean grain size, decreased pore count while maintaining a similar pore volume fraction to the reference argon mixture samples; oxygen additions showed slight reduction of mean grain size, decreased pore count and decreased pore volume fraction. These results further showcase that varying shielding gas mixtures influence the final material properties and should be considered as an additional improvement route for conventional welding and additive manufacturing.

Hydrothermal and eco-environmental evolution on the southeastern Chinese Loess Plateau since the last deglaciation: evidence from terrestrial mollusk records

The ongoing global warming is altering hydrothermal patterns and eco-environments. However, it is unclear how they would respond under a natural warming scenario. Terrestrial mollusks in loess-paleosol sequences could serve as natural biological archives to explore this issue. In this study, terrestrial mollusk records from two loess-paleosol profiles on the southeastern Chinese Loess Plateau (CLP) were analyzed. Our results show that the cold-aridiphilous species decreased and the overall biodiversity increased gradually during the last deglaciation (∼19.0–11.5 ka BP), which was accompanied with the shift from cold-dry to warm-humid climate. Moreover, the mollusk communities reacted to Heinrich event 1 (H1) cooling. In the early-middle Holocene (∼11.5–5.5 ka BP), mollusk communities were dominated by the thermo-humidiphilous species, characterized by stable community composition, increased species richness and evenness with warm-humid climate. Thereafter, there was a noticeable decrease in species diversity and increase in the cold-aridiphilous species in the mid-late Holocene (∼5.5–0 ka BP), associated with a gradual shift towards a cold-dry climate. On the whole, it is noteworthy that the count of thermo-humidiphilous species increased remarkably at ∼16 ka BP, which is synchronous with a significant increase in the Northern Hemisphere summer insolation. Interestingly, this change of terrestrial mollusks preceded the formation of the Holocene paleosol at ∼8 ka as indicated by the pronounced increase in magnetic susceptibility and decrease of mean grain size. This suggests that the substantial East Asian summer monsoon (EASM) precipitation lagged the onset of deglacial warming on the southeastern CLP. Furthermore, this delay may be attributed to the negative feedback of North Atlantic meltwater, which caused a decline in the intensity of Atlantic Meridional Overturning Circulation (AMOC) and inhibited the rapid enhancement of EASM precipitation during the last deglaciation. Altogether, this provides crucial geological background evidence for exploring the potential regional hydrothermal and biodiversity responses to global warming in the near future.

Exceptional dielectric and varistor properties of Sr, Zn and Sn co-doped calcium copper titanate ceramics.

Calcium copper titanate (CCTO) powders associated with the chemical formula Ca1-x Sr x Cu3-y Zn y Ti4-z Sn z O12 (where x, y, z varying from 0 to 0.1) were synthesized via a solid-state reaction route. Dense ceramics (>96% of theoretical density) were obtained by sintering these powders comprising micrometer-sized grains at appropriate temperatures. X-ray powder diffraction studies confirmed the formation of monophasic CCTO cubic phase, with no traceable secondary phases present. The lattice parameter 'a' was found to increase on increasing the dopant concentration. The microstructural studies performed on these ceramics confirmed a decrease in mean grain size (18 μm to 5 μm) with the increase in Sr, Zn and Sn doping concentrations as compared to that of undoped CCTO ceramics though they were sintered at the same temperature and duration (1100 °C/15 h). The dielectric studies (dielectric constant (ε') and the dielectric loss (D)) conducted in a wide frequency range (102-107 Hz) demonstrated an increase in ε' and a decrease in D on increasing the doping concentration. Impedance analysis (Nyquist plots) performed on these ceramics revealed a significant increase in grain boundary resistance. The highest value of grain boundary resistance (6.05 × 108 Ω) (in fact this value was 100 times higher than that of pure CCTO) was obtained for the composition corresponding to x = y = z = 0.075 and intriguingly the ceramic pertaining to this composition exhibited enhanced ε' (1.7 × 104) and lower D (0.024) at 1 kHz. Further, these co-doped CCTO ceramics exhibited substantial improvement in breakdown voltages and nonlinear coefficients (α). The temperature independent (30 -210 °C) dielectric response of these samples qualifies them to be suitable dielectric materials for the fabrication of multilayer ceramic chip capacitors.

A new approach to thermo-mechanical treatment of steel T91 by multiple upsetting-extrusion in a ferritic range

The microstructure and mechanical properties of the ferritic-martensitic steel T91 after severe plastic deformation (SPD) and subsequent thermal treatment are investigated. SPD was realized by the method of multiple “upsetting-extrusion” (MUE) in temperature range of stability of ferritic phase (750-575 °C), thermal treatment of SPD specimens was carried out at temperatures 550–730 °C during 1–100 h. It is revealed that decrease of temperature of “upsetting-extrusion” and increase of cycle number to 5 induces the decrease of mean grain size to 145 nm and increase of degree of microstructure homogeneity. At the same time microhardness increases from 1800 MPa to 2850 MPa. Composition, shape, size and density of carbide precipitates in steel after SPD and thermal treatment are determined. Obtained submicron grain ferritic microstructure with high value of microhardness is stable at exposure temperature of 550 °C. It is shown that temperature variation of thermal treatment induces the considerable change in grain size and mechanical characteristics.

Orbital Forcing of Late Miocene–Early Pleistocene Environmental Change in the Zhada Basin, SW Tibetan Plateau

AbstractMechanisms controlling the long‐ and short‐term variability of the Indian Summer Monsoon (ISM) and high‐elevation environmental change have largely been examined using low‐elevation or marine records with less emphasis on high‐elevation non‐marine records. We address this using a high‐resolution, long‐term record from upper Miocene–lower Pleistocene (~9.0–2.2 Ma) fluvio‐lacustrine strata in the Zhada Basin, southwestern Tibetan Plateau. Long‐term changes include the onset of lacustrine deposition, a decrease in mean grain size, and an increase in δ18Ocarb and δ13Ccarb values at ~6.0 Ma in response to basin closure following regional extension. This was followed by a return to palustrine/fluvial deposition, an increase in mean grain size, and a decrease in δ18Ocarb and δ13Ccarb values at ~3.5 Ma in response to tectonically driven long‐term ISM weakening. Spectral analysis reveals that high‐frequency variations in the δ18Ocarb record are dominated by 100 and ~20 kyr cycles from ~6.0–2.2 Ma. Wavelet and spectral analysis of the most densely sampled interval (4.23–3.55 Ma), tuned to the record of daily insolation (21 June at 35°N) confirms and highlights 100 and 20 kyr cycles. The tuned Pliocene δ18Ocarb record is coherent with the record of Northern Hemisphere insolation at precession periods but not at obliquity or eccentricity periods. Additionally, the tuned δ18Ocarb record is anticorrelated to the insolation record, indicating that stronger Northern Hemisphere insolation correlates with a stronger ISM. These results suggest that variations in daily insolation drove late Miocene–early Pleistocene high‐frequency ISM variability and environmental changes in the high‐elevation southwestern Tibetan Plateau.

Investigation of the effect of grain size on liquefaction potential of sands

Due to the permanent damage to structures during earthquakes, soil liquefaction is an important issue in geotechnical earthquake engineering that needs to be investigated. Typical examples of soil liquefaction have been observed in many earthquakes, particularly in Alaska, Niigata (1964), San Fernando (1971), Loma Prieta (1989), Kobe (1995) and Izmit (1999) earthquakes. In this study, liquefaction behavior of uniform sands of different grain sizes was investigated by using the energy-based method. For this purpose, a total of 36 deformation-controlled tests were conducted on water-saturated samples in undrained conditions by using the cyclic simple shear test method and considering the relative density, effective stress and mean grain size parameters that affect the cumulative liquefaction energy. The results showed that as the mean grain size decreases, the liquefaction potential of the sand increases. In addition, with increasing effective stress and relative density, the resistance of sand against liquefaction decreases. Multiple regression analysis was performed on the test results and separate correlations were proposed for the samples with mean grain size of 0.11-0.26 mm and for the ones with 0.45-0.85 mm. The recommended relationships were compared to the ones existing in the literature and compatible results were obtained.

Sediment Bypassing from the New Human-Induced Lobe to the Ancient Lobe of the Turbo Delta (Gulf of Urabá, Southern Caribbean Sea)

Alcantara-Carrio, J.; Caicedo, A.; Hernandez, J.C.; Jaramillo-Velez, A., and Manzolli, R.P., 2019. Sediment bypassing from the new human-induced lobe to the ancient lobe of the Turbo Delta (Gulf of Uraba, Southern Caribbean Sea). Journal of Coastal Research, 35(1), 196–209. Coconut Creek (Florida), ISSN 0749-0208.Artificial river diversions cause intense and unpredictable changes in the geomorphological evolution and sedimentary dynamics of deltas. This study analyzes the effects of the 1958 course diversion of the Turbo River delta, Gulf of Uraba, southern Caribbean Sea. Recent satellite images were analyzed and beach topography was monitored to obtain the shoreline migration and sediment budget; surface sediments were seasonally sampled, and their grain size parameters were determined; and seasonal sediment transport directions were deduced according to grain size trends. It was concluded that the bimodal wave regime controls the seasonal sedimentary patterns associated with southward longshore transport during both the dry and rainy seasons, and the northward longshore transport occurs in the midsummer drought. Beach morphodynamics of Yarumal Point, close to the present-day river mouth, show that this is the most exposed sector to waves, whereas fluvial sediment inputs are evidenced by a decrease in mean grain size and by poorly sorted sediments in the rainy season. A new spit is forming in Yarumal Point, with an intense increase in area and a westward progradation related to high fluvial sediment inputs from the La Nina event of 2010–12 and low erosion from the El Nino events of 2009–10 and 2015–16. In the central sector, longshore drift has formed the Yarumal barrier spit with a southeastward progradation that recently closed the interdistributary El Uno Bay. Consequently, a humaninduced sand barrier–lagoon system resulted with a stable or slightly regressive shoreline. Moreover, the Yarumal barrier spit has generated a new sediment bypass to Barajas Beach in the northern limit of the Las Vacas spit (i.e. the ancient delta lobe). Therefore, there is an unusual accretion of the abandoned delta lobe by sediment supply from the new lobe.

Variation of the mid-latitude westerlies and Indian summer monsoon since the last deglaciation: Evidence from grain-size data from Linggo Co, central Tibet

The mid-latitude westerlies and Indian summer monsoon (ISM) have a significant impact on the water budget and general environment of the Tibetan Plateau (TP), and much work has been conducted in order to characterize variations in these two atmospheric circulation systems on various time scales. However, the nature of the interplay between the Asian summer monsoon and the westerlies during the last deglaciation remains controversial. In this study, a sediment core from Linggo Co, a closed lake in the central TP, was analyzed for grain size and ostracod content. The results suggest that environmental change in the Linggo Co basin during the past 17 ka can be divided into four stages: during Stage 1 (17–11.7 ka) Linggo Co was a shallow lake characterized by high amplitude hydrological fluctuations. Strong winds prevailed in winter, correlative with cold events in the North Atlantic region, and indicating the dominance of the westerlies. During Stage 2 (~11.7 ka), in the early Holocene, Linggo Co expanded rapidly and there was the disappearance of the shallow-water ostracod species Limnocythere inopinata together with a decrease in mean grain-size. These changes indicate that ISM was enhanced and that it was the dominant atmospheric circulation affecting TP rather than the westerlies. During Stage 3 (10–8 ka) Linggo Co experienced several intervals with a negative water budget, which may be attributed to a weakening of the weaken ISM or to the reduced influx of glacial meltwater as a result of the influence of cold climatic events. During Stage 4, after 8 ka, Linggo Co shrank progressively in parallel with a gradual weakening of the ISM and finally a transition to the present environment occurred at ~2.5 ka. A comparison of the record from Linggo Co and other records from the TP reveals that prior to 16.5 ka the mid-latitude westerlies was the dominant atmospheric circulation system controlling the entire TP. From 16.5 to 11.5 ka, the mid-latitude westerlies still controlled the central-northern TP; however, the ISM controlled the southern TP, which had a similar climatic pattern to the present day. During the early-middle Holocene, the ISM was the dominant system affecting the entire TP; however, its influence was considerably reduced during the late Holocene, which was probably accompanied by a strengthening of the westerlies.

Experiment and modeling to evaluate the effects of proppant-pack diagenesis on fracture treatments

Observed reductions in the permeability of propped hydraulic fractures are examined by considering the role of mechanical stresses and the chemistry of pore fluids at elevated temperatures as agents of proppant diagenesis. Stress-enhanced dissolution of proppant increases the density of grain packing and reprecipitation of mineral matter further occludes pores — together these mechanisms additively reduce porosity and permeability. Experiments and analyses are presented which explore the evolution of porosity and permeability in proppant packs subjected to reservoir conditions of stresses to 65 MPa and temperatures to 177 °C. Experiments are completed in two modes: in flow-through reactors absent intergranular stresses to evaluate rates of dissolution and reprecipitation on proppant surfaces, and in uniaxially stressed reactors with stagnant fluids to evaluate the relative role of stress in mediating dissolution and porosity reduction. Lumped parameter models are used to evaluate rates of dissolution and chemical compaction in a range of proppants. Mechanisms include mineral dissolution, transport, and reprecipitation of the resulting products in the particle interstices, resulting in a loss of intergranular porosity. The model uses thermodynamic data derived from the reactor experiments to constrain the projected loss of permeability for the mineralogical composition of available proppants. Evaluated silica dissolution rates vary with temperature but are of the order of 1.1 × 10 −11 to 3.1 × 10 −4 mol/m 2 s in the range of 100 °C to 288 °C. Experimental results show chemical compactions of the order of 10 millistrains over 40 days and these project modeled reductions in porosity of the order of 25% over 1000 days. Rates of porosity loss increase both with an increase in temperature and a decrease in mean grain size of the granular proppant although ultimate porosities at the conclusion of compaction are similar. Rates and magnitudes of ultimate reductions in porosity map directly onto reductions in permeability and for proppants in the range 100 to 600 μm, result in permeability reductions in the range of 25% (163 °C) to 75% (191 °C). Such magnitudes (75%) of permeability loss over short periods (1000 days) are consistent with many field observations, with the mechanistic models providing a rational method of scaling such observations.

Effects of grain size distribution on the creep damage evolution of polycrystalline materials

It is evident that realistic microstructures of polycrystalline materials demonstrate a certain distribution of grain size, which has not been widely studied in most analyses of mechanical properties of materials at high temperatures. In this work, the effects of grain size distribution on the creep damage evolution induced by void growth of polycrystalline materials were investigated by the Voronoi tessellation approach, taking into account the void evolution on the grain boundaries in a grain aggregate cell. The results indicate that with the decrease in mean grain size, the damage variable increases faster. When the mean grain sizes are the same, the more uniform the grain size is, the faster the damage variable increases.

Characterization of Recent Sediments between Abu Dhabi and Dubai Coasts, United Arab Emirates, with Multiple Analytical Techniques

Surface sediments collected from the shoreline area between Abu Dhabi and Dubai, United Arab Emirates (UAE), were analysed to determine their textural and compositional characteristics. The relatively heterogeneous sediments of the UAE coastal areas present a broad spectrum of grain sizes and mineralogical compositions, which have potential as discriminates for depositional environments. The trends in textural characteristics of the sampled sediments include (i) a systematic decrease in mean grain size from Dubai to Abu Dhabi, (ii) a systematic increase in the content of evaporite minerals from Dubai to Abu Dhabi, and (iii) a relative increase in degree of sediment sorting from Dubai to Abu Dhabi. The results also indicate that the sediments of both areas are marine in origin and were delivered to the coastal area during large tidal events. Results of compositional analyses reveal (i) a decrease in abundance of shell fragments from Dubai (>55%) to Abu Dhabi (<9%) and (ii) a complementary increase in percentage of mineral fragments (aragonite, halite, plagioclase, and gypsum, except calcite) in Abu Dhabi sediments compared with Dubai sediments. Geochemically, the coastal sediments from Abu Dhabi have higher concentrations of major and trace elements, except for Pb, than sediments from Dubai. The reported results set the background information of the studied sediments, and could be useful for ongoing development activities.

Paleoenvironmental changes during the last 8400 years in the southern Yellow Sea: Benthic foraminiferal and stable isotopic evidence

We used benthic foraminiferal, stable isotopic and grain-size data from an AMS 14C dated core from the southern Yellow Sea to decipher Holocene paleoenvironmental changes. Large-amplitude changes in δ 18O, δ 13C and benthic foraminiferal assemblage composition indicate that major environmental changes occurred during the last 8400 years in the southern Yellow Sea. We recognize 3 stages of environmental development: 1. A low salinity estuarine environment stage, at 8.4–6.9 cal. kyr BP, 2. A low salinity shallow sea environment stage at 6.9 to about 6–5 cal. kyr BP and 3. A modern marine shelf environment stage after about 6–5 cal. kyr BP The δ 18O and δ 13C values of Ammonia aomoriensis (Asano), combined with benthic foraminiferal assemblage data indicate a low bottom-water salinity (< 15‰) during stage 1, suggesting a large amount of river runoff. Abrupt increases in δ 18O and δ 13C around 6.9 cal. kyr BP were accompanied by a decrease in mean grain-size and in abundance of benthic foraminifera, possibly caused by a North-ward shift of the Yellow River's outlet. During subsequent stage 2 the southern Yellow Sea was still characterized by a low salinity (< 28‰) even though sea-level was high, possibly due to the continued strong influence of river input, and/or limited exchange between the southern Yellow Sea and the open ocean. At about 6–5 cal. kyr BP, an A. aomoriensis dominated benthic foraminiferal assemblage was replaced by an Ammonia ketienziensis (Ishizaki) dominated assemblage, indicating an increase in bottom-water salinity. Grain-size and stable isotope data, suggest that the modern circulation was probably established at that time. We speculate that the combined impacts of reduced river runoff, enhanced East Asia Winter Monsoon, and increased influence of southern, warm ocean currents caused this transition.

Correction to “Development of subaqueous barchanoid-shaped dunes due to lateral grain size variability in a tidal inlet channel of the Danish Wadden Sea”

[1] High-resolution bathymetry at centimeter-scale accuracy acquired with a multibeam echo sounder system revealed the existence of barchanoid-shaped large (i.e., length 10–100 m) to very large (i.e., length >100 m) dunes in the Gradyb tidal inlet channel in the Danish Wadden Sea. The development of these dunes is due to an increase in dune celerity from 12 m/yr in the center of the channel to around 30 m/yr at the sides. This increase in dune celerity can be explained by the fact that dune heights decrease from 3.1 m in the center of the channel to 1.4 m at the sides, as a smaller sediment volume has to be moved per unit time for equal dune celerity. Water depth is uniform across the channel. Likewise, high-resolution acoustic Doppler current profiler measurements across the channel showed a uniform distribution of both ebb and flood flow. Thus no correlation between dune dimensions and water depth or flow velocity was established. Instead, high-accuracy bed sampling along the crests of the dunes showed a decrease in mean grain size from 0.63 mm in the center to 0.36 mm at the sides of the channel. The decrease in dune height is ascribed to this decrease in grain size, given that flow depth and flow velocity are uniform across the channel. The lateral decrease in grain size is suggested to result from sorting effects by secondary currents directed from the center toward the sides of the channel in the trough/lee side region of the barchanoid-shaped dunes.

Development of subaqueous barchanoid‐shaped dunes due to lateral grain size variability in a tidal inlet channel of the Danish Wadden Sea

High‐resolution bathymetry at centimeter‐scale accuracy acquired with a multibeam echo sounder system revealed the existence of barchanoid‐shaped large (i.e., length 10–100 m) to very large (i.e., length &gt;100 m) dunes in the Grådyb tidal inlet channel in the Danish Wadden Sea. The development of these dunes is due to an increase in dune celerity from 12 m/yr in the center of the channel to around 30 m/yr at the sides. This increase in dune celerity can be explained by the fact that dune heights decrease from 3.1 m in the center of the channel to 1.4 m at the sides, as a smaller sediment volume has to be moved per unit time for equal dune celerity. Water depth is uniform across the channel. Likewise, high‐resolution acoustic Doppler current profiler measurements across the channel showed a uniform distribution of both ebb and flood flow. Thus no correlation between dune dimensions and water depth or flow velocity was established. Instead, high‐accuracy bed sampling along the crests of the dunes showed a decrease in mean grain size from 0.63 mm in the center to 0.36 mm at the sides of the channel. The decrease in dune height is ascribed to this decrease in grain size, given that flow depth and flow velocity are uniform across the channel. The lateral decrease in grain size is suggested to result from sorting effects by secondary currents directed from the center toward the sides of the channel in the trough/lee side region of the barchanoid‐shaped dunes.

Deposits of depletive high‐density turbidity currents: a flume analogue of bed geometry, structure and texture

AbstractFlume experiments were performed to study the flow properties and depositional characteristics of high‐density turbidity currents that were depletive and quasi‐steady to waning for periods of several tens of seconds. Such currents may serve as an analogue for rapidly expanding flows at the mouth of submarine channels. The turbidity currents carried up to 35 vol.% of fine‐grained natural sand, very fine sand‐sized glass beads or coarse silt‐sized glass beads. Data analysis focused on: (1) depositional processes related to flow expansion; (2) geometry of sediment bodies generated by the depletive flows; (3) vertical and horizontal sequences of sedimentary structures within the sediment bodies; and (4) spatial trends in grain‐size distribution within the deposits. The experimental turbidity currents formed distinct fan‐shaped sediment bodies within a wide basin. Most fans consisted of a proximal channel‐levee system connected in the downstream direction to a lobe. This basic geometry was independent of flow density, flow velocity, flow volume and sediment type, in spite of the fact that the turbidity currents of relatively high density were different from those of relatively low density in that they exhibited two‐layer flow, with a low‐density turbulent layer moving on top of a dense layer with visibly suppressed large‐scale turbulence. Yet, the geometry of individual morphological elements appeared to relate closely to initial flow conditions and grain size of suspended sediment. Notably, the fans changed from circular to elongate, and lobe and levee thickness increased with increasing grain size and flow velocity. Erosion was confined to the proximal part of the leveed channel. Erosive capacity increased with increasing flow velocity, but appeared to be constant for turbidity currents of different grain size and similar density. Structureless sediment filled the channel during the waning stages of the turbidity currents laden with fine sand. The adjacent levee sands were laminated. The massive character of the channel fills is attributed to rapid settling of suspension load and associated suppression of tractional transport. Sediment bypassing prevailed in fan channels composed of very fine sand and coarse silt, because channel floors remained fully exposed until the end of the experiments. Lobe deposits, formed by the fine sand‐laden, high‐density turbidity currents, contained massive sand in the central part grading to plane parallel‐laminated sand towards the fringes. The depletive flows produced a radial decrease in mean grain size in the lobe deposits of all fans. Vertical trends in grain size comprised inverse‐to‐normal grading in the levees and in the thickest part of the lobes, and normal grading in the channel and fringes of the fine sandy fans. The inverse grading is attributed to a process involving headward‐directed transport of relatively fine‐grained and low‐concentrated fluid at the level of the velocity maximum of the turbidity current. The normal grading is inferred to denote the waning stage of turbidity‐current transport.

Textural and compositional characterization of recent sediments along a paraglacial estuarine coastline, Maine, USA

Textural and mineralogical characteristics of bottom sediments collected from the lower Kennebec River estuary, Maine, and adjacent nearshore region were used to differentiate subtidal sedimentary environments. Five depositional environments, defined on the basis of bottom morphology, bathymetry, and dominant sedimentary processes—estuarine channel, channel margin, outer bar, shoreface, and offshore—possess distinct sedimentological characteristics. Trends in textural characteristics of these environments and those along the adjacent barrier beach include: (1) systematic decrease in mean grain size from the estuarine channel to the offshore facies; (2) similarity between the outer bar and estuarine channel sands; (3) increase in sediment sorting from the channel/margin and offshore to shoreface facies; and (4) fining of the low-tide terrace, beachface, and berm samples away from the bar-welding region. The fine-grained nature of the offshore facies is the result of deposition of suspended sediment delivered to the coast during large flood events, whereas the coarse fraction is contributed through the attrition of carbonate tests of the local macrofauna. Results of compositional analyses reveal: (1) a decrease in shell fragments from the estuarine channel (>50%) to the offshore (<10%); (2) a three-fold increase in mineralogical maturity from channel and outer bar to offshore sands; and (3) increase in mineralogical maturity of the beachface and low-tide terrace samples away from the bar-welding region. The similarity between channel-derived bar sands and those sampled from the bar-welding region of the adjacent barrier beach suggests that the sedimentological signature of subtidal environments is temporarily preserved. The wide range of textural and compositional attributes of sediments in paraglacial settings makes these environments ideal places in which to study the variations in sediment transport mechanisms over relatively short distances. We demonstrate the usefulness of primary physical characteristics of bottom sediments in studying the patterns of fluvial–marine interaction along other physiographically similar coastal settings.

Downstream Fining in a Network of Gravel‐Bedded Rivers

A network‐based routing model is used to determine how spatial variations in sediment supply influence rates of downstream fining in a small watershed in central Pennsylvania. Gravel of a specified size distribution and lithology is supplied to the network at zero‐order basins. Once in transport, the total sample weight and moments of the grain‐size distribution are modified according to empirical functions of transport distance. Contributions from each zero‐order basin are summed to determine the total population of sizes and lithologies at any location. In the study area, resistant quartz sandstone ridges supply sediment an order of magnitude larger than other lithologies. Because these ridges occur primarily in the basin headwaters, spatial variations in supply strongly influence rates of downstream fining. Model calculations indicate that spatial variations in supply alone account for over 80% of the decrease in mean grain size along the main channel.

Benthic invertebrates of a modern carbonate ramp: a preliminary survey

A preliminary survey of benthic invertebrates off central west Florida provides documentation of modern epifaunal communities on a low-gradient carbonate slope. Three large-scale biofacies occur in soft-sediment carbonate environments between 200 and 2,000 m: an Echinoderm biofacies (200–550 m) dominated by a diverse assemblage of echinoderms, gastropods, and decapod crustaceans; a Penaeid shrimp–conical mound biofacies (550–1,200 m) characterized by large bioturbation structures; and a Microbial mat biofacies (1,200–2,000 m) with only rare epifaunal invertebrates. A fourth, hard-substrate biofacies reflects the presence of localized Miocene and Pleistocene hardgrounds in water depths of 200–600 m. This illustrates that hard-substrate biofacies may be laterally correlative with soft-sediment biofacies in a slope setting, thus producing a mosaic of contrasting faunal associations. All four biofacies have low population densities, presumably as a consequence of relatively low surface productivity. All four biofacies also show biogeographic affinity with other faunas at intermediate depths in the Caribbean region. Depth-related faunal transitions on the west Florida slope correlate with substrate and current velocity. Decreasing species diversity and abundance and a biofacies transition from suspension-feeding to deposit-feeding assemblages correlate with increasing depth, a decrease in mean grain size, and an increase in organic content of the sediment. This biofacies model may provide a modern analogue for faunas of ancient low-gradient slopes such as those of Cretaceous “shelf-sea” chalks of northwestern Europe.

Platinum-group minerals in quaternary gold placers in the upper Chindwin area of northern Burma

In the Chindwin Basin in northern Burma, there is a system of five Pleistocene terraces in which gold placers with low concentrations of platinum-group minerals (PGM) occur. Samples were taken from four sites in the Chindwin Basin and one from near an ophiolite occurrence on the northeast side of the Chindwin Basin; they were studied under the microscope, with a scanning electron microscope, and an electron microprobe. The main minerals were Pt-Fe and Os-Ir-Ru alloys, usually in a ratio between 2 and 5. In most cases, the shape of the grains allowed a quick distinction between the two types. Sperrylite, laurite, irarsite, cooperite, tulameenite, and isomertieite occur infrequently as individual mineral grains and sometimes as inclusions in the alloy grains. Braggite, platarsite, hollingworthite, bowieite, keithconnite, cuproiridsite, malanite, stibiopalladinite, geversite, kashinite, several unnamed PGM, and Fe, Ni, and Cu sulfides were observed as inclusions, mainly in the Pt-Fe alloys and also to a lesser extent in the Os-Ir-Ru alloys. Lamellar and myrmekite-like intergrowths, oriented exsolution lamellae, and idiomorphic inclusions of sulfides in the alloys indicate a magmatic origin of the PGM. The origin of the PGM is assumed to be ophiolites in northern Burma. A continual decrease in mean grain size occurred during transport.

Sediment diffusion during overbank flows

ABSTRACTDistinctive overbank sediments deposited since European settlement on the floodplain of the Brandywine Creek, Pennsylvania, are used to calibrate and test a diffusion model of overbank deposition. The predictions of the model can be calibrated to reproduce the topography of the post‐settlement lithosome with an average error of 7%. The model also correctly predicts the decrease in mean grain size away from the channel. The model greatly underestimates the ability of floodwaters to transport sand away from the channel. Apparently, sand is transported across the floodplain by bedload transport and by advective suspended sediment transport as well as by diffusion.If flow duration data for 1912–1981 and the present rating curve for the Brandywine Creek at Chadds Ford, Pennsylvania, are assumed to apply throughout the post‐settlement period, the model may be used to estimate palaeohydraulic characteristics of post‐settlement floods. Calculations indicate that 212 post‐settlement floods covered the floodplain to an average depth of 1.6 m, transported an average excess suspended sediment concentration of 6200 ppm, and deposited an average thickness of 1.4 cm of sediment on levees next to the channel.