Net Sedimentation Rate Research Articles

Sediment sorting within a relatively wave-exposed and sandy subtidal seagrass (Zostera marina) meadow

Seagrasses impact on sedimentary intertidal and subtidal ecosystems by affecting local hydrodynamics, geomorphology and sediment properties. Their influence on hydrodynamics is to reduce flow velocities in their canopies, and this leads to increased net sedimentation rates and reduction of the grain size. Most investigations of the seagrass-hydrodynamics-sediment feedback system has been carried out over silt and fine-sand beds under tide-dominated conditions, mostly in the intertidal zone. Here, we use sedimentological data from a relatively wave-exposed and subtidal seagrass (Zostera marina) meadow in the Isles of Scilly with a fine-to-medium sand bed and show that the sand within the seagrass meadow is indeed finer than in adjacent, unvegetated regions. However, in contrast to previous studies, this is not due to increased mud/silt content within the seagrass meadow, but an almost 0.1-mm shift in the median sediment size across the sand fraction from 0.25 mm (fine to medium sand) to 0.35 mm (medium sand). The studied seagrass meadow is characterised by a distinct spatial structure comprising of vegetated ‘ridges’ and bare sand ‘valleys’. Even the bare sand valleys within the seagrass meadow are characterised by significantly coarser sand than the adjacent vegetated ridges, providing further confirmation of the efficiency of sediment sorting by wave processes that takes place within seagrass meadows. It is concluded that any numerical modelling involving sediment transport processes associated with seagrass environments must account for variability in the textural characteristics.

Unraveling marine phosphogenesis along the Miocene coast of Peru: Origin and sedimentological significance of the Pisco Formation phosphorites

Phosphorite deposits form through phosphogenesis, a mechanism of diffusion and concentration of interstitial phosphate in sediments leading to Ca-phosphate mineral precipitation in a favorable sedimentary regime at the seafloor. Stretching along the Peruvian coast, the East Pisco Basin is known to host phosphorite beds of Miocene age. In this study, we investigate the petrographic, mineralogical and chemical composition of the Miocene phosphorite layers of the East Pisco Basin, and discuss the phosphogenetic mechanism(s) and sedimentary dynamics that led to the genesis and accumulation of P-rich deposits. Phosphorite samples were collected from layers overlaying stratigraphic unconformities within the Pisco Formation and subsequently analyzed through multiple methods: optical microscopy, XRD, XRF, SEM-EDS, EPMA, ICP-MS and LA-ICP-MS. Our results indicate that these layers consist of phosphatic clasts and nodules along with dolomite clasts, basement boulders, shark teeth, mollusk molds and vertebrate bone fragments. The P-rich clasts are mainly composed of phosphatic intraclasts and small phosphatic nodules. They occur associated with minor amounts of dolomite clasts coated by Fe-oxyhydroxides and laminites. Clasts and nodules are cemented by both early and late diagenetic cements, such as dolomite, calcite, silica, gypsum/anhydrite and baryte. Our observations suggest that the Pisco phosphorite deposits are transgressive lags laying on ravinement surfaces. They formed just below the sediment-water boundary during the early transgression phases, when the porewaters were enriched in phosphorus due to the abundance of organic-rich sediments and the microbial activity, as typical of upwelling settings. These deposits formed in shallow marine conditions, characterized by high biological productivity and low net sedimentation rates, with suboxic conditions at and beneath the sediment-water interface. The concentration of phosphate-coated clasts was then favored by a sedimentary condensation mechanism of dynamic bypassing. Overall, this study provides new insights into the genesis of phosphorite layers in the East Pisco Basin and highlights the importance of understanding the complex interplay between authigenic and sedimentological processes in the formation of these economically and geologically relevant sedimentary rocks.

New data on the Turonian nodular phosphate-bearing bed in the Dobrindol Formation south of Kaspichan, NE Bulgaria

The Turonian nodular phosphate-bearing bed outcropping near Kaspichan consists of early diagenetically mineralized (carbonate-fluorapatite – CFA, glauconite and goethite) limestone lithoclasts (nodules), mineralized macrofauna, and non-mineralized intraclasts, embedded in a glauconite-CFA cement. The characteristics of the nodular bed are consistent with a formation under extremely low net sedimentation rate with nodules demonstrating evidence for reworking. The identified ammonite association assigns late Turonian age of the bed. XRF and LA-ICPMS analyses reveal low P, U, and REE-Y concentrations, with correlations between REE and the other authigenic oxides indicating that glauconite and goethite could be main REE concentrators along with CFA. Fluctuations in the redox conditions are implied by the authigenic mineral association, mineralization stages, presence of diverse fauna and ichnofissils, and applied geochemical proxies.

Application of 210Pbex in elucidating contemporary sedimentary dynamics in the water-level fluctuation zone of the Three Gorges Reservoir, China

Excess 210Pb (210Pbex) has been widely applied as a proxy indicator for establishing the chronology of sediments in lakes where the sedimentation processes comply with the assumptions for using 210Pbex dating models. However, little is known regarding the applicability of 210Pbex to elucidate contemporary seasonal sedimentary dynamics in reservoir water-level fluctuation zones (WLFZs). In the present study, sedimentary core and bulk sediments were collected from the WLFZ of the Three Gorges Reservoir (TGR) in China, and the physical and chemical properties were determined. Results for 210Pbex activity, total organic carbon, and total nitrogen of bulk sediments indicate pronounced elevation-dependent spatial variation (p < 0.05), but 137Cs activity and particle composition show no systematic trends, allowing 210Pbex activity to be used to differentiate seasonal stratigraphy. For the sedimentary core, therefore, sediment layers with lower 210Pbex activity were inferred to represent sedimentation during wet seasons, when the 210Pbex concentration was diluted by a higher suspended sediment load, whereas sediment layers characterised by higher 210Pbex activity were ascribed to sedimentation during dry seasons, when the tranquil water favoured 210Pbex fallout to be absorbed by suspended sediments and then deposited in the WLFZ. Hereupon, temporal variation is observed in annual sedimentary thickness, with relatively low sedimentation rates during the initial stage (2006–2008) of the TGR impoundment, followed by relatively high sedimentation rates from 2009 to 2013. Over the period from 2014 to 2017, sedimentation rates were low because the suspended sediment was intercepted by upstream cascade reservoirs. Subsequently, sedimentation rates were higher during 2018–2019 as a consequence of an increase in extreme rainfall events. The findings herein highlight that 210Pbex-based seasonal stratification discrimination is effective approach to establish high-resolution sedimentary chronosequences and that net sedimentation rates in WLFZ are controlled by inflow suspended load of the TGR.

Authigenic Fe Mineralization in Shallow to Marginal Marine Environments: A Case Study from the Late Paleocene—Early Eocene Cambay Shale Formation

The late Paleocene–early Eocene warm greenhouse conditions, characterized by elevated pCO2 levels in the atmosphere and a dramatic increase in sea surface temperature, prompted abundant authigenic glauconite formation within the shallow marine sediments worldwide by lowering the net sedimentation rate, increasing organic productivity and expanding the oxygen minimum zones to shallow oceans. The early Eocene marginal marine Cambay Shale Formation experienced episodes of marine inundation represented by limestone–green shale alternations. The shales host abundant authigenic light-green, dark-green, and brown pellets. A detailed characterization of the pellets of the Valia and Vastan lignite mines, integrating the sedimentological, petrographical, mineralogical, and mineral geochemical data, suggests two distinct varieties of Fe–silicate formation, viz. glauconite and chamosite. While the glauconitic green pellets are ubiquitous to Valia and Vastan mines, brown chamosite pellets are confined within the basal part of the green shale facies alternating with fossiliferous limestone in the Vastan mine. The glauconites of the Valia mine manifest a ‘nascent’ to ‘slightly evolved’ maturation stage of glauconitization, whereas the glauconites of the Vastan mine represent the ‘evolved’ type. The limestone–green shale alternation in the Valia mine is overlain by a ~4 m-thick spherulitic mudstone facies comprising monomineralic sideritic spherulites, reflecting a pure FeCO3 composition. The glauconites in the Cambay Shale Formation transformed from kaolinite-rich clay pellets under dys-oxic depositional conditions. The increasing anoxicity within the microenvironment, possibly amplified by the rapid oxidation of continent-derived organic matter, facilitated chamosite formation instead of glauconite. The increased freshwater influx into the marginal marine depositional environment resulted in immature, K-poor glauconites of the Valia mine. The formation of siderite spherulites overlying the limestone–green shale alternation relates to the beginning of the regressive phase of sedimentation.

Recent Transport and Deposition of Littoral Sand Across the Inner-Shelf of Central Oregon: Further Constraints on Estimating Depths of Closure (30–34 m) during Future Sea Level Rise in the Pacific Northwest Region, USA

In this article, geologically recent sedimentation rates across a high wave- and wind-energy inner-shelf are used to constrain depths of closure, which are needed to estimate potential offshore displacements of littoral (beach) sand that could result from future sea level rise (SLR). Seven shallow vibracores (1.0&amp;ndash;2.3 m subsurface depth) were analyzed from a single transect across the inner-shelf (19&amp;ndash;72 m water depth) at the South PacWave study site in the high-wave energy (peak wave Hs= 10&amp;ndash;15 m) coast of Central Oregon. The vibracores were 14C dated to establish 1) near-modern mixing depths and 2) net sedimentation rates that equaled or exceeded rates of coeval SLR (10 cm century-1) during very latest-Holocene time (&amp;le;1.0 ka). Sedimentation rates of 17 cm century-1 and 31 cm century-1 for vibracore P1-2A22 in 34 m water depth do exceed coeval SLR, even after accounting for apparent mixing depths of 60&amp;ndash;100 cm. But sedimentation rates in deeper core sites (47&amp;ndash;53 m water depth) do not approach coeval SLR rates. These results support a proposed near future (one century) 30 m depth of closure in the innermost-shelf of northern Oregon. Vibracore sand grain sizes (mean 0.20&amp;plusmn;0.03 mm 1&amp;sigma;, n=18) are similar across the inner-shelf, but heavy-mineral sand tracers confirmed that the latest-Holocene inner-shelf sand accumulations were supplied from seaward transport of littoral sand. The net loss of littoral sand to the inner-shelf sand sink accounts for the narrowing and thinning of beach deposits in northern Oregon during latest-Holocene time. A 1.0 m thickness of littoral sand displaced across the innermost-shelf (5&amp;ndash;30 m water depth) following a 1.0 m SLR, or equivalent increase in offshore accommodation space, would yield a cross-sectional area of 1.5x103 m2. That value is three times larger than the mean cross-sectional area of the modern adjacent beaches (mean 4.8x102 m2) in the South PacWave study area. Following a possible near future SLR of 1.0 m, the popular sandy beaches of northern Oregon could be converted to intertidal gravel/algae covered bedrock platforms.

A Convolution Method to Assess Subgrid‐Scale Interactions Between Flow and Patchy Vegetation in Biogeomorphic Models

AbstractInteractions between water flow and patchy vegetation are governing the functioning of many ecosystems. Yet, numerical models that simulate those interactions explicitly at the submeter patch scale to predict geomorphological and ecological consequences at the landscape scale (order of km2) are still very computationally demanding. Here, we present a novel and efficient convolution technique to incorporate biogeomorphic feedbacks in numerical models across multiple spatial scales (from less than 1 m2 to several km2). This new methodology allows for spatially refining coarse‐resolution hydrodynamic simulations of flow velocities (order of m) around fine‐resolution patchy vegetation patterns (order of 10 cm). Although flow perturbations around each vegetation grid cell are not simulated with the same level of accuracy as with more expensive finer‐resolution models, we show that our approach enables spatial refinement of coarse‐resolution hydrodynamic models by resolving efficiently subgrid‐scale flow velocity patterns within and around vegetation patches (mean error, spatial variability, and spatial correlation improved by, respectively, 13%, 66%, and 49% on average in our test cases). We also provide evidence that our approach can substantially improve the representation of important biogeomorphic processes, such as subgrid‐scale effects on net sedimentation rate and habitable surface area for vegetation (respectively 66% and 39% better on average). Finally, we estimate that replacing a fine‐resolution model by a coarser‐resolution model associated with the convolution method could reduce the computational time of real‐life fluctuating flow simulations by several orders of magnitude. This marks an important step forward toward more computationally efficient multiscale biogeomorphic modeling.

Tidal Flat Depositional Response to Neotectonic Cyclic Uplift and Subsidence (1–2 m) as Superimposed on Latest-Holocene Net Sea Level Rise (1.0 m/ka) in a Large Shallow Mesotidal Wave-Dominated Estuary, Willapa Bay, Washington, USA

The late-Holocene record of tidal flat deposition in the large shallow Willapa Bay estuary (43 km in length), located in the Columbia River Littoral Cell (CRLC) system (160 km length), was investigated with new vibracores (n=30) and gouge cores (n=8), reaching 2–5 m depth subsurface. Reversing up-core trends of muddy sand to peaty mud deposits in marginal tidal flat settings demonstrate episodic submergence events resulting from cyclic tectonic uplift and subsidence (1–2 m) in the Cascadia subduction zone. These short-term reversals are superimposed on longer-term trends of overall sediment coarsening-up, which represent the transgression of higher-energy sandy tidal flats over pre-existing lower-energy tidal flat mud and peaty mud deposits in late-Holocene time. Fining-up trends associated with channel lateral migration and accretionary bank deposition occurred only infrequently in the broad intertidal flats of Willapa Bay. Vibracores and gouge cores were dated by 14C (n=16) and paleo-subsidence event contacts (n=17). Vibracore median probability 14C ages ranged from 0 to 6,992 yr BP and averaged 2,174 yr BP. Dated sample ages and corresponding depths of tidal flat deposits yield net sedimentation rates of 0.9–1.2 m ka-1, depending on the averaging methods used. Net sedimentation rates in the intertidal flat settings (~1.0 m ka-1) are comparable to the rate of net sea level rise (~1.0 m ka-1), as based on dated paleo-tidal marsh deposits in Willapa Bay. Reported modern inputs of river sand (total=1.77x104 m3 yr-1), from the three small rivers that flow into Willapa Bay, fall well short of the estimated increasing accommodation space (1.9x105 m3 yr-1) in the intertidal (MLLW-MHHW) setting (1.9x108 m2 surface area) during the last 3 ka, or 3.0 m of sea level rise. The under-supply of tributary sand permitted the influx of littoral sand (1.1x105 m3 yr-1) into Willapa Bay, as based on the net sedimentation rate (~1.0 m ka-1) and textural composition (average 60 % littoral sand) in analyzed core sections (n=179). The long-term littoral sand sink in Willapa Bay’s intertidal setting (55 % of total estuary area) is estimated to be about 5 % of the Columbia River supply of sand to the CRLC system, and about 30% relative to the littoral sand accumulated in barrier spits and beach plains during late-Holocene time. A 2.0 m rise in future sea level could yield a littoral sand sink of 2.2x108 m3 in the Willapa Bay intertidal setting, resulting in an equivalent shoreline retreat of 600 m along a 50 km distance of the barrier spit and beach plains that are located adjacent to the Willapa Bay tidal inlet. Willapa Bay serves as proxy for potential littoral sand sinks in other shallow mesotidal estuary-barrier-beach systems around the world following future global sea level rise.

Effects of discharge, wind, and tide on sedimentation in a recently restored tidal freshwater wetland

AbstractSediment deposition is one of the key mechanisms to counteract the impact of sea level rise in tidal freshwater wetlands (TFWs). However, information about sediment deposition rates in TFWs is limited, especially for those located in the transition zone between the fluvially dominated and tidally dominated sections of a river delta where sedimentation rates are affected by the combined impact of river discharge, wind, and tides. Using a combined hydrodynamic–morphological model, we examined how hydrometeorological boundary conditions control sedimentation rates and patterns in a TFW located in the Rhine–Meuse estuary in the Netherlands. The modelling results show that net sedimentation rate increases with the magnitude of the river discharge, whereas stronger wind increasingly prevents sedimentation. Sediment trapping efficiency decreases for both increasing river discharge and wind magnitude. The impact of wind storms on the trapping efficiency becomes smaller for higher water discharge. The spatial sedimentation patterns are affected by all controls. Our study illustrates the importance of evaluating both the separate and the joint impact of discharge, wind, and tides when estimating sedimentation rates in a TFW affected by these controls. Such insights are relevant to design measures to reactivate the sedimentation process in these areas.

Persistent or repeated surface habitability on Mars during the late Hesperian ‐ Amazonian

AbstractLarge alluvial fan deposits on Mars record relatively recent habitable surface conditions (≲3.5 Ga, Late Hesperian ‐ Amazonian). We find net sedimentation rate &lt;(4–8) μm/yr in the alluvial fan deposits, using the frequency of craters that are interbedded with alluvial fan deposits as a fluvial‐process chronometer. Considering only the observed interbedded craters sets a lower bound of &gt;20 Myr on the total time interval spanned by alluvial fan aggradation, &gt;103‐fold longer than previous lower limits. A more realistic approach that corrects for craters fully entombed in the fan deposits raises the lower bound to &gt;(100–300) Myr. Several factors not included in our calculations would further increase the lower bound. The lower bound rules out fan formation by a brief climate anomaly. Therefore, during the Late Hesperian ‐ Amazonian on Mars, persistent or repeated processes permitted habitable surface conditions.

Sediment and carbon accumulation rates off the southern coast of Finland

The southern coast of Finland encompasses about one third of the coasts of the Gulf of Finland. It is a mosaic of hundreds if not thousands of islands, peninsulas and bays, which also are reflected in the seabed of the coast. The sea floor is composed of a patchy and fragmented mosaic of mainly quite small basins separated from each other by thresholds of islands, peninsulas or submarine ridges. This affects transport and near bottom currents such that deposition of suspended particles is restricted to certain areas only. Linear sediment accumulation rates along the southern coast of Finland were studied from 28 cores of a sampling campaign in 2000 – 2004 through gamma spectrometry of 137Cs. Sediment accumulation in this environment with such diverse character was found very patchy and net sedimentation rates varying from less than 0.5 cm/a to values of nearly 3 cm/a as well as mass accumulation rates from 0.5 kg/m2/a to 8.8 kg/m2/a were found. The sediment accumulation rates were observed to be higher in shallower water in coastal sheltered or semi-sheltered bays. Total carbon concentrations varied from 1.3 % to 12.6 % and carbon accumulation rates from 20 g/m2/a to 355 g/m2/a such that the highest carbon concentrations were usually found in deeper water with some distance from the coast, while the highest carbon accumulation rates were found in the coastal shallow basins where sediment accumulation was found strongest.

A late Albian ammonite assemblage from the mid- Cretaceous succession at Annopol, Poland

Abstract A previously unrecorded ammonite assemblage, comprising Lepthoplites sp., Callihoplites tetragonus (Seeley, 1865), C. cf. tetragonus, Arrhaphoceras cf. substuderi Spath, 1923, Cantabrigites sp., Stoliczkaiella (Stoliczkaiella) sp., Hamites cf. duplicatus Pictet and Campiche, 1861, H. cf. subvirgulatus Spath, 1941, and H. cf. venetzianus Pictet, 1847, is described from the mid-Cretaceous condensed succession at Annopol, Poland. These specimens are preserved as pale phosphates or sandstone moulds in a bed of reworked phosphatic nodules near the top of the Albian. This assemblage has many species in common with the late late Albian faunas from condensed deposits of England, Switzerland, and France. The presence of Callihoplites tetragonus indicates the lowermost upper upper Albian Mortoniceras fallax Zone. The ammonites studied are the youngest elements in the phosphate bed, which also contains taxa as old as the middle Albian Hoplites dentatus Zone. This bed originated through condensation and reworking of nodules and fossils in a period of low net sedimentation rate, being probably a reflection of a sea-level drop at the boundary between the classic ammonite zones of Mortoniceras inflatum and Stoliczkaiella dispar.

Upland sediment supply and its relation to watershed sediment delivery in the contemporary mid-Atlantic Piedmont (U.S.A.)

We use sediment accumulation in ponds and reservoirs to examine upland sediment sources and sinks in the Piedmont physiographic region of Maryland, USA. In zero-order and first-order watersheds, sediment yield is greatest from suburban land cover, followed by agriculture and forest. The idea that sediment yield is small from mature suburban development appears to not be correct. First-order channel enlargement is an important sediment source, causing sediment yield to increase from zero-order to first-order watersheds. Nonchannel sources provide one-third to two-thirds of the upland sediment load.Long-term sediment accumulation in a reservoir at fifth-order indicates that cumulative sediment load from upland areas is reduced by one-quarter by net valley bottom sedimentation. If upland supply exceeds the load delivered from a watershed, sediment must accumulate along valley bottoms. In our study watershed, net sedimentation rate (sedimentation less erosion) averaged over valley bottom area is 2.6mm/y, a value that is similar to independent direct measurements of sedimentation and erosion in a nearby watershed. Evaluation of the relative contributions to sediment mass balance of upland supply, valley bottom sedimentation and erosion, and watershed delivery indicates that, if valley-bottom rates of sedimentation exceed erosion as indicated by recent studies, then the proportion of watershed sediment delivery derived from stream banks is necessarily small.Although sediment yield estimated from stream gage records is similar in magnitude to that from ponds for watersheds smaller than 20km2, sediment yield from reservoir sedimentation is a factor of five larger than that estimated from gage records for watersheds larger than 140km2. This observation confirms that the different methods provide very different estimates of sediment yield. This possibility is reinforced by a sediment yield of 14Mg/km2/y from a gage immediately above a reservoir with a yield of 142Mg/km2/y based on reservoir accumulation.

Soil greenhouse gas emissions and carbon budgeting in a short‐hydroperiod floodplain wetland

AbstractUnderstanding the controls on floodplain carbon (C) cycling is important for assessing greenhouse gas emissions and the potential for C sequestration in river‐floodplain ecosystems. We hypothesized that greater hydrologic connectivity would increase C inputs to floodplains that would not only stimulate soil C gas emissions but also sequester more C in soils. In an urban Piedmont river (151 km2 watershed) with a floodplain that is dry most of the year, we quantified soil CO2, CH4, and N2O net emissions along gradients of floodplain hydrologic connectivity, identified controls on soil aerobic and anaerobic respiration, and developed a floodplain soil C budget. Sites were chosen along a longitudinal river gradient and across lateral floodplain geomorphic units (levee, backswamp, and toe slope). CO2 emissions decreased downstream in backswamps and toe slopes and were high on the levees. CH4 and N2O fluxes were near zero; however, CH4 emissions were highest in the backswamp. Annual CO2 emissions correlated negatively with soil water‐filled pore space and positively with variables related to drier, coarser soil. Conversely, annual CH4 emissions had the opposite pattern of CO2. Spatial variation in aerobic and anaerobic respiration was thus controlled by oxygen availability but was not related to C inputs from sedimentation or vegetation. The annual mean soil CO2 emission rate was 1091 g C m−2 yr−1, the net sedimentation rate was 111 g C m−2 yr−1, and the vegetation production rate was 240 g C m−2 yr−1, with a soil C balance (loss) of −338 g C m−2 yr−1. This floodplain is losing C likely due to long‐term drying from watershed urbanization.

Water quality effects following establishment of the invasive Dreissena polymorpha (Pallas) in a shallow eutrophic lake: implications for pollution mitigation measures

This study investigates whether ecosystem alteration occurred in a shallow, lowland lake following establishment of the alien zebra mussel, Dreissena polymorpha (Pallas). Measurements of total phosphorus (TP) loads, lake TP concentrations, phytoplankton (chlorophyll a) and transparency levels for the period 1990–2008 were examined to determine the water quality effects of D. polymorpha. The period of time also included the implementation of catchment measures aimed at reducing phosphorus (P) loading to the lake. A range of loading-response models was tested to explore changes in the net sedimentation rate for P. Results show that while high TP loads from the catchment were reduced, TP concentrations in the lake remained high after D. polymorpha invasion. Decoupling of the previous chlorophyll a–TP relationship also occurred. Results from a TP loading-response model that closely simulated observed concentrations in the lake prior to establishment of D. polymorpha indicated that measured TP post establishment was statistically higher than predicted for the same conditions but without the presence of D. polymorpha. Data presented in the paper highlight the need to consider the potential impacts of invasive species in evaluations of the effectiveness of measures aimed at mitigating aquatic pollution.

Accelerated water quality improvement during oligotrophication in peri-alpine lakes.

Monitoring of four eutrophic Swiss lakes undergoing oligotrophication during more than 25 years (i.e., gradually decreasing nutrient loading, productivity, and associated symptoms of eutrophication) revealed that phosphorus (P) net sedimentation rates (the fraction of a lake's total P content that is buried within its sediments each year) and P export rates (the fraction of the lakes' total P content that is exported via the outlet each year) increased as the lakes' P contents decreased. These findings are of scientific as well as practical interest because they imply that, contrary to the hitherto prevailing view, the P concentration of eutrophic lakes will decrease more than proportional to the reduction of their external P load, and faster than predicted by the linear (eutrophic state-based) models.

26Al/10Be dating of an aeolian dust mantle soil in western New South Wales, Australia

Aeolian dust mantle soils are an important element of many landscapes in south-eastern Australia, though the age of these aeolian deposits has not been radiometrically determined. At Fowlers Gap in western New South Wales, surface cobbles of silcrete and quartz overlie a stone-free, aeolian dust mantle soil, which has a thickness of about 1.6m. The clay-rich aeolian dust deposit in turn lies upon a buried silcrete and quartz stone layer. Modelling in-situ cosmogenic 26Al and 10Be concentrations measured in both the surface quartz stones and in the buried quartz layer of rocks, reveals that each has experienced a complex exposure-burial history. Due to the absence of quartz stones or sand at intermediate depths, our cosmogenic 26Al and 10Be modelling was not able to determine a definitive mechanism of stone pavement formation and stone burial. Various scenarios of stone formation, transport, burial and exhumation were tested that constrain the age of the deposit to range from 0.9±0.2Ma to 1.8±0.2Ma, based largely on different assumptions taken for the time-dependency of the net sedimentation rate. This corresponds with the initiation of the Simpson Desert dune fields and the deflation of lakes in central Australia, which probably responded to the shift to longer-wavelength, larger-amplitude Quaternary glacial cycles at around 1Ma. Sensitivity analyses were carried out to identify those parameters which better constrained model outputs. Within model errors, which largely are the result of analytical errors in measured 26Al and 10Be concentrations, all three competing theories of colluvial wash, upward displacement of stones, and cumulic pedogenesis are possible mechanisms for the formation of the surface stone pavement.

A continuous multi-millennial record of surficial bivalve mollusk shells from the São Paulo Bight, Brazilian shelf

To evaluate the potential of using surficial shell accumulations for paleoenvironmental studies, an extensive time series of individually dated specimens of the marine infaunal bivalve mollusk Semele casali was assembled using amino acid racemization (AAR) ratios (n=270) calibrated against radiocarbon ages (n=32). The shells were collected from surface sediments at multiple sites across a sediment-starved shelf in the shallow sub-tropical São Paulo Bight (São Paulo State, Brazil). The resulting 14C-calibrated AAR time series, one of the largest AAR datasets compiled to date, ranges from modern to 10,307calyr BP, is right skewed, and represents a remarkably complete time series: the completeness of the Holocene record is 66% at 250-yr binning resolution and 81% at 500-yr binning resolution. Extensive time-averaging is observed for all sites across the sampled bathymetric range indicating long water depth-invariant survival of carbonate shells at the sediment surface with low net sedimentation rates. Benthic organisms collected from active depositional surfaces can provide multi-millennial time series of biomineral records and serve as a source of geochemical proxy data for reconstructing environmental and climatic trends throughout the Holocene at centennial resolution. Surface sediments can contain time-rich shell accumulations that record the entire Holocene, not just the present.

Establishing relative sea level trends where a coast lacks a long term tide gauge

Vulnerability assessment of coastal areas to projected sea level rise requires incorporation of historic trends in relative sea level change as an exposure factor. Most shorelines of developing countries lack long term tide gauges, such as the Pacific Islands region, which are especially vulnerable to climate change impacts. This study has the objective of demonstrating how long-term relative sea level trends can be derived from proxy records, on the tectonically unstable main island of Fiji. At Tikina Wai on the western coast, while elevations of present mangrove zones of Rhizophora stylosa, Rhizophora samoensis and Bruguiera gymnorrhiza were <1.2 m around mean sea level, sediment cores down to 3 m showed mangrove occurrence meters lower than they can grow today. Pollen analysis identified past locations of these mangrove species zones, and the present day elevations of the species were used to reconstruct past sea levels. Results of this study showed that relative sea-level has been slowly rising for the last several centuries at about 2.1 mm a−1, yet mangrove communities have remained resilient with nearly equivalent net sedimentation rates, though with some zone retreat landwards. With such local subsidence, the Tikina Wai district is more exposed to future sea level rise projections than stable coastal areas elsewhere, with additional exposure in having a micro-tidal range. Adaptation actions identified to address this risk include enhancement of sedimentation under mangrove communities through coastal and catchment planning to remove obstructions to sediment supply, reducing non-climate stresses to increase organic production, and replanting of degraded areas. Such information on relative sea level trends can be used to identify where adaptation resources are best concentrated.

Sedimentary facies of the upper Cambrian (Furongian; Jiangshanian and Sunwaptan) Tunnel City Group, Upper Mississippi Valley: New insight on the old stormy debate

New data from detailed measured sections permit a comprehensive revision of the sedimentary facies of the Furongian (upper Cambrian; Jiangshanian and Sunwaptan stages) Tunnel City Group (Lone Rock Formation and Mazomanie Formation) of Wisconsin and Minnesota. Heterogeneous sandstones, comprising seven lithofacies along a depositional transect from shoreface to transitional-offshore environments, record sedimentation in a storm-dominated, shallow-marine epicontinental sea. The origin of glauconite in the Birkmose Member and Reno Member of the Lone Rock Formation was unclear, but its formation and preserved distribution are linked to inferred depositional energy rather than just net sedimentation rate. Flat-pebble conglomerate, abundant in lower Paleozoic strata, was associated with the formation of a condensed section during cratonic flooding. Hummocky cross-stratification was a valuable tool used to infer depositional settings and relative paleobathymetry, and the model describing formation of this bedform is expanded to address flow types dominant during its genesis, in particular the importance of an early unidirectional component of combined flow. The depositional model developed here for the Lone Rock Formation and Mazomanie Formation is broadly applicable to other strata common to the early Paleozoic that document sedimentation along flooded cratonic interiors or shallow shelves.