Net Sediment Accumulation Rates Research Articles

A novel subsurface sediment plate method for quantifying sediment accumulation and erosion in seagrass meadows

Sediment dynamics in seagrass meadows are key determinants of carbon sequestration and storage, surface elevation, and resilience and recovery from disturbance. However, current methods for measuring sediment accumulation are limited. For example, 210Pb dating, the most popular tool for quantifying sediment accretion rates over decadal timescales, relies on assumptions often at odds with seagrass meadows. Here, we have developed a novel subsurface sediment plate method to detect changes in sediment accumulation and erosion in real time that: 1) is affordable and simple to implement, 2) can quantify short-term (weeks to months) sediment dynamics of accumulation and erosion, 3) is non-destructive and minimizes impacts to surface-level processes, and 4) can quantify long-term (years) net sediment accumulation rates. We deployed subsurface sediment plates at two sites within a 20 km2 seagrass meadow in the Virginia Coast Reserve Long-Term Ecological Research site, USA. Here, we discuss spatial and temporal trends in sediment dynamics over a 25-month period, the sediment accretion rates estimated using the subsurface sediment plate method compared to previous estimates based on 210Pb dating, the precision of the method, and our recommendations for implementing the method for measuring surface sediment dynamics in other seagrass settings. We recommend the application of this method for quantifying short- and long-term changes in seagrass surface sediments across various spatial scales to improve our understanding of disturbance, recovery, restoration, carbon cycling, sediment budgets, and the response of seagrasses to rising sea levels.

Sedimentation as a Support Ecosystem Service in Different Ecological Types of Mangroves

Mangrove vegetation is strongly dependent on the climate, the physicochemical variables of the sediment, and the hydrological dynamics. These drivers regulate the distribution of different mangrove ecotypes and their ecosystem services, so the net sediment accumulation rates in different mangrove ecotypes in Celestun Lagoon, a karstic zone in the NW Yucatan Peninsula, SE Mexico, were estimated. The measurements considering mangrove ecotypes and their spatial variability concerning the lagoon's salinity gradient (inner, middle, and outer lagoon zones) in three climate seasons (dry, rain, and “nortes”) were realized. We registered the structural variables of the forest, interstitial water physicochemical characteristics, and sediment variables that could influence the net sediment deposition. Fringe mangroves are exposed to low hydrodynamism and show the highest sedimentation rate (3.37 ± 0.49 kg m−2 year−1) compared to basin (1.68 ± 0.22 kg m−2 year−1), dwarf (1.27 ± 0.27 kg m−2 year−1), and “peten” (0.52 ± 0.12 kg m−2 year−1) mangroves. The highest sedimentation rate was recorded in the rainy season (0.24 ± 0.08 kg m−2 month−1), while spatially, the highest value was registered in the outer zone (0.44 ± 0.09 kg m−2 month−1). If the extension of each mangrove ecotype is considered, dwarf mangroves have the highest annual sediment accumulation (1,465 t year−1 in 14,706 ha). The structural, physicochemical, and sediment variables of the sites by mangrove ecotype show that dwarf mangroves represent a distinct group from those formed by fringe, basin, and peten mangroves. However, the sedimentation is high in fringe mangroves at the front of the lagoon and diminishes inland where peten mangroves exist. The differences are given by tree density, but salinity, as a proxy variable of the freshwater influence, significantly influences the sedimentation rate. These results indicate that mangroves in karstic environments can have critical roles in confronting climate change, considering water and sediment flows are the basis of sediment accumulation. According to their hydrogeomorphological drivers, conserving, managing, and restoring the mosaic of mangrove ecotypes improves ecosystem services, including mitigation and adaptation to climate change.

Light Limitation and Depth-Variable Sedimentation Drives Vertical Reef Compression on Turbid Coral Reefs

Turbid coral reefs experience high suspended sediment loads and low-light conditions that vertically compress the maximum depth of reef growth. Although vertical reef compression is hypothesized to further decrease available coral habitat as environmental conditions on reefs change, its causative processes have not been fully quantified. Here, we present a high-resolution time series of environmental parameters known to influence coral depth distribution (light, turbidity, sedimentation, currents) within reef crest (2–3 m) and reef slope (7 m) habitats on two turbid reefs in Singapore. Light levels on reef crests were low [mean daily light integral (DLI): 13.9 ± 5.6 and 6.4 ± 3.0 mol photons m–2 day–1 at Kusu and Hantu, respectively], and light differences between reefs were driven by a 2-fold increase in turbidity at Hantu (typically 10–50 mg l–1), despite its similar distance offshore. Light attenuation was rapid (KdPAR: 0.49–0.57 m–1) resulting in a shallow euphotic depth of <11 m, and daily fluctuations of up to 8 m. Remote sensing indicates a regional west-to-east gradient in light availability and turbidity across southern Singapore attributed to spatial variability in suspended sediment, chlorophyll-a and colored dissolved organic matter. Net sediment accumulation rates were ∼5% of gross rates on reefs (9.8–22.9 mg cm–2 day–1) due to the resuspension of sediment by tidal currents, which contribute to the ecological stability of reef crest coral communities. Lower current velocities on the reef slope deposit ∼4 kg m2 more silt annually, and result in high soft-sediment benthic cover. Our findings confirm that vertical reef compression is driven from the bottom-up, as the photic zone contracts and fine silt accumulates at depth, reducing available habitat for coral growth. Assuming no further declines in water quality, future sea level rise could decrease the depth distribution of these turbid reefs by a further 8–12%. This highlights the vulnerability of deeper coral communities on turbid reefs to the combined effects of both local anthropogenic inputs and climate-related impacts.

Large‐scale connectivity of fluvio‐deltaic stratigraphy: Inferences from simulated accommodation‐to‐supply cycles and automated extraction of chronosomes

AbstractMultiscale simulation of fluvio‐deltaic stratigraphy was used to quantify the elements of the geometry and architectural arrangement of sub‐seismic‐scale fluvial‐to‐shelf sedimentary segments. We conducted numerical experiments of fluvio‐deltaic system evolution by simulating the accommodation‐to‐sediment‐supply (A/S) cycles of varying wavelength and amplitude with the objective to produce synthetic 3‐D stratigraphic records. Post‐processing routines were developed in order to investigate delta lobe architecture in relation to channel‐network evolution throughout A/S cycles, estimate net sediment accumulation rates in 3‐D space, and extract chronostratigraphically constrained lithosomes (or chronosomes) to quantify large‐scale connectivity, that is, the spatial distribution of high net‐to‐gross lithologies. Chronosomes formed under the conditions of channel‐belt aggradation are separated by laterally continuous abandonment surfaces associated with major avulsions and delta‐lobe switches. Chronosomes corresponding to periods in which sea level drops below the inherited shelf break, that is, the youngest portions of the late falling stage systems tract (FSST), form in the virtual absence of major avulsions, owing to the incision in their upstream parts, and thus display purely degradational architecture. Detailed investigation of chronosomes within the late FSST showed that their spatial continuity may be disrupted by higher‐frequency A/S cycles to produce “stranded” sand‐rich bodies encased in shales. Chronosomes formed during early and late falling stage (FSST) demonstrate the highest large‐scale connectivity in their proximal and distal areas, respectively. Lower‐amplitude base level changes, representative of greenhouse periods during which the shelf break is not exposed, increase the magnitude of delta‐lobe switching and favour the development of system‐wide abandonment surfaces, whose expression in real‐world stratigraphy is likely to reflect the intertwined effects of high‐frequency allogenic forcing and differential subsidence.

Abrupt onset of carbonate deposition in Lake Kivu during the 1960s: response to recent environmental changes

This study interprets the recent history of Lake Kivu, a tropical lake in the East African Rift Valley. The current gross sedimentation was characterized from a moored sediment trap array deployed over 2 years. The past net sedimentation was investigated with three short cores from two different basins. Diatom assemblages from cores were interpreted as reflecting changes in mixing depth, surface salinity and nutrient availability. The contemporary sediment trap data indicate seasonal variability, governed by diatom blooms during the annual mixing in the dry season, similar to Lakes Malawi and Tanganyika. The ratio of settling fluxes to net sediment accumulation rates implies mineralization rates of 80–90% at the sediment-water interface. The sediment cores revealed an abrupt change ~40 years ago, when carbonate precipitation started. Since the 1960s, deep-water methane concentrations, nutrient fluxes and soil mineral inputs have increased considerably and diatom assemblages have altered. These modifications probably resulted from a combination of three factors, commonly altering lake systems: the introduction of a non-native fish species, eutrophication, and hydrological changes inducing greater upwelling. Both the fish introduction and increased rainfall occurred at the time when the onset of carbonate precipitation was observed, whereas catchment population growth accompanied by intensified land use increased the flux of soil minerals already since the early twentieth century due to more intense erosion.

Mangrove forest sedimentation and its reference to sea level rise, Cananeia, Brazil

The stability of a mangrove ecosystem in Cananeia, Brazil, is assessed based on investigations of the site-specific temporal rise in relative sea level during the past 50 years, 100-year sediment accumulation rates (SAR) and sources of organic matter (OM). Addressing this, three sediment cores were collected in a transect, intertidal mud flat, mangrove margin and well into the forest. The net SAR, as estimated by the age-depth relationships of 210 Pb and 137 Cs, is between 2.5 and 3.9 mm year -1 . These rates are comparable to the estimates based on the Pb and Zn contaminant markers corresponding to mining initiation in the region in 1918. Further, the SARs are lower than the rate of regional relative sea level rise (4 mm year -1 )a s indicated by the past 50-year tide gauge record, but the rate is higher than the eustatic sea level rise (1.7 ± 0.3 mm year -1 ). The stratigraphies of TOC/TN, d 13 C(OC), OP and d 15 N indicate site-specific mangal vegetal litter,

Short‐lived radionuclides (7Be and 210Pb) as tracers of particle dynamics in a river system in southeast Michigan

Riverine water samples (dissolved and particulate), surficial bottom sediments, and trapped sediments were collected mostly monthly from three stations along the Clinton River, in southeastern Michigan, over a period of ~1 yr and analyzed for 7Be and 210Pb to elucidate the types and rates of processes affecting particle dynamics in a riverine system. Using a simple irreversible scavenging box model approach, sources and sinks for dissolved and particulate 7Be and 210Pb were quantified to estimate their residence times in the dissolved and particulate phases. Resuspension rates of surficial bottom sediments calculated from the mass balance of particulate 7Be varied from 0.50 to 1.34 (geometric mean: 0.83 ± 0.34) g cm−2 yr−1, while corresponding values varied from 0.16 to 1.48 (GM: 0.38 ± 0.38) g cm−2 yr−1 using particulate 210Pb. Based on the 210Pb mass balance, it appears that only ∼2% of 210Pb was derived from direct atmospheric deposition, while ∼98% was derived from resuspension of bottom sediments. Additionally, there was a large discrepancy between mass flux collected in the trap (GM: 8.9 g cm−2 yr−1) compared to net sediment accumulation rates (GM: 0.88 ± 0.38 g cm−2 yr−1), which was attributed to sediment resuspension; this may provide insight into the frequency of particle‐recycling events. Furthermore, desorption of particle‐reactive species during resuspension events could result in the mobility of contaminants to farther distances from the contaminated site. Therefore, this study has direct relevance to the uptake of particle‐reactive species in a riverine system and thereby the water quality of rivers.

Changes in material fluxes from the Changjiang River and their implications on the adjoining continental shelf ecosystem

This study aims to examine the changing patterns of Changjiang material fluxes, which are influenced by anthropogenic activities, and the resultant modifications to the coastal and shelf oceanographic conditions, and to propose future research about the effect of these changes on the estuarine and shelf ecosystem. Within the catchment basin of the Changjiang River, the construction of more than 48,000 dams has caused significant sediment discharge reduction, together with modifications to the timing of seasonal freshwater discharge. In the future, the mean freshwater discharge will decrease following the completion of the water-diverting project for water supply to northern China. At the same time, the riverine nutrient loadings (N and P) have increased due to the extensive use of chemical fertilizers and the large discharge of industrial wastewater and domestic sewage. These changes are modifying the oceanographic conditions of the estuarine and shelf waters. The flushing time for the river water becomes longer in wet seasons but shorter in dry seasons. An increase in salinity can be expected after the completion of the water-diverting project. Nutrient concentrations will be enhanced in the shelf waters. In contrast to the decrease in the suspended sediment concentration of the river water, field measurements have not shown well-defined patterns of changes within the estuary; nevertheless, net sediment accumulation and carbon burial rates would be reduced in the deltaic areas because of the reduced sediment discharge. Finally, increase in the nutrient input appears to enhance the primary production in the East China Sea region, which, in turn, may enhance the fishery catch.

Sedimentary and microbial record of the Middle/Late Ordovician phosphogenetic episode in the northern Holy Cross Mountains, Poland

Numerous phosphorite beds and phosphatic nodules occur in the upper Middle and lower Upper Ordovician carbonate-shale succession of the Bukowiany Formation outcropping in the northern Holy Cross Mountains, central Poland. The vertical stacking pattern of this succession indicates that phosphatic and accompanying strata reflect a conformable sedimentary succession accumulated during a rise of relative sea level. The base of the Bukowiany Formation is marked by a conspicuous phosphorite horizon revealing a low net sediment accumulation rate reflecting a switch into a mesotrophic ecological system. This horizon was produced by reworking and redeposition of pristine phosphate sediment (e.g. by currents activity) during the late Darriwilian transgression. The overlying sedimentary record appears to reflect nucleation of the phosphate phase in the sediment–water interface and its subsequent burial by the accompanying sediment. The phosphatized tiny stromatolites and nodules preserved within the Bukowiany Formation indicate that benthic microbial communities played an important role in redistribution and concentration of phosphate during deposition of this succession.

Condensation and phosphatization of the Middle and Upper Ordovician limestones on the Malopolska Block (Poland): Response to paleoceanographic conditions

The growth fault activity in the late Middle Ordovician resulted in the establishment of a horst-like submarine paleohigh in the northern Malopolska Block. This topographic submarine swell was the site of carbonate deposition characterized by a low overall net sediment accumulation rate. The sedimentary and ecological conditions on this paleohigh were controlled by a paleocurrent that flowed along the northern margin of the Malopolska Block, however seasonal intense storms associated with the mid-latitude location of this “terrane” cannot be excluded. The paleocurrent induced (seasonal?) upwelling of nutrient-rich waters from deeper parts of the current system that shifted onto the paleohigh and caused phosphatization of the sediment. The winnowing and reworking of sediment, together with slow carbonate production by a temperate-water biota and relatively long periods of omission, resulted in low accumulation rates and sediment condensation. The cessation of the paleocurrent and associated phosphatization coincides with changes in the oceanic circulation system due to the Late Ordovician collision of Avalonia with Baltica and gradual closure of the Tornquist Sea.

Environmental Change and Atmospheric Contamination on Svalbard: Sediment Chronology

Sediment cores collected from eight lakes along the western coast of Svalbard as part of a project investigating atmospheric pollution and environmental change in Arctic regions were dated radiometrically using natural (210Pb) and artificial (137Cs and 241Am) fallout radionuclides. At four sites the sedimentation rates were relatively uniform, and in consequence the 210Pb dates were relatively unambiguous. At the remaining sites there were irregularities in the 210Pb activity versus depth profiles, indicating significant variations in the net sediment accumulation rate during the past 120 years. At these sites, there were significant differences between 210Pb dates calculated using the two standard simple dating models, constant rate of supply (CRS) and constant initial concentration (CIC). In most cases, stratigraphic dates based on the 137Cs and 241Am records supported use of the CRS model, though at one site (Ossian Sarsfjellet) the CIC model appeared more appropriate. The irregularities in the 210Pb records were mainly caused by episodes of accelerated sedimentation due, for example, to inwash or slump events, though at some sites there appears to have been a systematic increase in sediment accumulation rates in recent decades. Sediment accumulation rates were generally lower at the northern sites, and higher at the more southerly locations. Mean sediment accumulation rates varied by an order of magnitude, ranging from 0.002 to 0.050 g cm−2 y−1 (0.02–0.10 cm y−1). 210Pb fluxes measured from the core inventories were mostly in the range 34–80 Bq m−2 y−1 typical of Arctic sites. Much higher values, recorded at two sites (Birgervatnet and Daltjørna), may be due to significant inputs from the catchments during spring thaw.

Early diagenetic remineralization of sedimentary organic C in the Gulf of Papua deltaic complex (Papua New Guinea): Net loss of terrestrial C and diagenetic fractionation of C isotopes

Oceania supplies ∼40% of the global riverine flux of organic carbon, approximately half of which is injected onto broad continental shelves and processed in shallow deltaic systems. The Gulf of Papua, on the south coast of the large island of New Guinea, is one such deltaic clinoform complex. It receives ∼4 Mt yr −1 particulate terrestrial organic carbon with initial particle C org loading ∼0.7 mg m −2. C org loading is reduced to ∼0.3 mg m −2 in the topset-upper foreset zones of the delta despite additional inputs of mangrove and planktonic detritus, and high net sediment accumulation rates of 1–4 cm yr −1. Carbon isotopic analyses (δ 13C, Δ 14C) of ΣCO 2 and C org demonstrate rapid (<100 yr) remineralization of both terrestrial (δ 13C <−28.6) and marine C org (δ 13C ∼−20.5) ranging in average age from modern (bomb) (Δ 14C ∼60) to ∼1000 yr (Δ 14C ∼−140). Efficient and rapid remineralization in the topset-upper foreset zone is promoted by frequent physical reworking, bioturbation, exposure, and reoxidation of deposits. The seafloor in these regions, particularly <20 m, apparently functions as a periodically mixed, suboxic batch reactor dominated by microbial biomass. Although terrestrial sources can be the primary metabolic substrates at inshore sites, relatively young marine C org often preferentially dominates pore water ΣCO 2 relative to bulk C org in the upper foreset. Thus a small quantity of young, rapidly recycled marine organic material is often superimposed on a generally older, less reactive terrestrial background. Whereas the pore water ΣCO 2 reflects both rapidly cycled marine and terrestrial sources, terrestrial material dominates the slower overall net loss of C org from particles in the topset-upper foreset zone (i.e. recycled marine C org leaves little residue). Preferential utilization of C org subpools and diagenetic fractionation of C isotopes supports the reactive continuum model as a conceptual basis for net decomposition kinetics. Early diagenetic fractionation of C isotopes relative to the bulk sedimentary C org composition can produce changes in 14C activity independent of radioactive decay. In the Gulf of Papua topset-upper foreset, Δ 14C of pore water ΣCO 2 averaged ∼ 300‰ greater than C org sediment between ∼1–3 m depth in deposits. Diagenetic fractionation and decomposition aging of sedimentary C org compromises simple application of 14C for determination of sediment accumulation rates in diagenetically reactive deposits.

Stratigraphic Significance of Ooidal Ironstones from the Cretaceous Western Interior Seaway: The Peace River Formation, Alberta, Canada, and the Castlegate Sandstone, Utah, U.S.A.

Ooidal ironstones form prominent units, and an appreciation of their sedimentological and stratigraphic significance is important for the interpretation of sedimentary successions. Ooidal ironstones from the Albian Paddy Member of the Peace River Formation, Alberta, Canada, and the Campanian Castlegate Sandstone of the Mesaverde Group in Colorado have been analyzed. The ooidal ironstones are associated with a major sequence boundary and subsequent transgressive surface. Both are interpreted to have been deposited under conditions of low net sediment accumulation in well-oxygenated bottom-water conditions, with episodic storm events reworking the sediments. The Paddy Member ironstone is interpreted to have been deposited in an estuarine environment, and the Castlegate ironstone is interpreted to have been deposited in a shallow-marine environment. Both ironstones are dominated by berthierine ooids and grain-rimming and pore-filling siderite, with later ferroan dolomite and calcite in the Castlegate ooidal ironstone. Petrographic, geochemical, and isotopic evidence, coupled with thermodynamic considerations, indicates that the ooidal ironstone mineralogy formed during suboxic diagenesis. The conditions required for suboxic diagenesis were extensive sediment reworking and slow net sediment accumulation rates. We propose that these conditions arose as a result of marine transgression during the initial relative sea-level rise following the development of a sequence boundary. Ooidal ironstone formation ceased once sedimentation rates increased and transgression deepened water depths considerably. These results illustrate the association between ooidal ironstones and major stratal surfaces in sedimentary successions, and highlights the complexity that early diagenesis at such surfaces can take.

Spatial and temporal variations in early diagenetic organic matter oxidation pathways in Lower Jurassic mudstones of eastern England

The Lower Liassic succession of the East Midlands Platform, eastern England, comprises shallow marine mudstones and oolitic ironstones. In these sediments, temporal and spatial variations in the relative importance of early diagenetic anaerobic organic matter oxidation pathways are recorded. The ironstones are stratigraphically condensed and were deposited upon the Market Weighton Swell (MWS), a topographic high in the basin, in shallow water and high energy conditions at times of low relative sea-level. At times of higher sea-level stratigraphically condensed mudstones were deposited upon the MWS. Away from the MWS stratigraphically expanded mudstones were deposited throughout the Lower Liassic. The ironstones contain berthierine (ooids and pore-filling cement) and siderite (pore-filling cement), with rare pyrite replacing berthierine and siderite. Mudstones deposited away from the MWS contain pyrite as the sole early diagenetic iron mineral. Contrastingly, mudstones deposited upon the MWS contain both early diagenetic pyrite and Mn-rich siderite. The presence of berthierine and siderite in the ironstones indicates that sulphide was absent from early diagenetic pore-waters and that Fe(III) reduction dominated anaerobic organic matter oxidation. In contrast, in mudstones deposited away from the MWS the presence of pyrite indicates that sulphate reduction was a predominant pathway for anaerobic organic matter oxidation. In mudstones deposited upon the MWS the presence of both pyrite and Mn-rich siderite suggests that both sulphate reduction and Fe(III) reduction were important during early diagenesis. It is proposed that this spatial and temporal variation in relative importance of early diagenetic anaerobic organic matter oxidation reactions was a result of variation in physical sedimentary parameters (sediment accumulation rate and energy of environment). At times of low net sediment accumulation rates, in shallow water, high energy environments, suboxic diagenesis (Fe(III) and Mn(IV) reduction) dominated, giving rise to the characteristic berthierine-siderite ironstone assemblage. At times of higher sedimentation accumulation rates in deeper, lower energy environments, sulphate reduction dominated, leading to pyrite formation. At intermediate conditions, Fe(III), Mn(IV) and sulphate reduction were all important, leading to precipitation of both pyrite and Mn-rich siderite.

Stability and facies association of early diagenetic mineral assemblages; an example from a Jurassic ironstone-mudstone succession, U.K.

ABSTRACT Sedimentological and petrographic data from two Lower Liassic ooidal ironstones (the Frodingham and Pecten Ironstones) and associated mudstones from eastern England have been collected. Within these ironstones early diagenetic berthierine, siderite, and minor glauconite have a limited lateral extent, associated with an intrabasinal high (the Market Weighton Swell). This mineral assemblage passes laterally and vertically into mudstones and silty mudstones within which pyrite is the dominant early diagenetic iron mineral. The ironstones are stratigraphically condensed, having experienced low net sediment accumulation rates and shallow, high-energy conditions. Thermodynamic considerations indicate that berthierine and siderite should be stable in anoxic environments where sulfide activity is negligible. Pyrite is rare within the ironstones, suggesting that sulfate reduction was a negligible process during ironstone formation. Where pyrite is present it postdates berthierine and siderite, indicating that sulfate reduction may have been a later process in some beds. We propose that berthierine and siderite were precipitated during suboxic diagenesis, where sulfate reduction was inhibited and iron reduction was the dominant process of organic-matter oxidation. Suboxic diagenesis resulted from the extended periods of physical and biological sediment reworking during ironstone deposition, which favored the oxidation of iron minerals and aerobic egradation and loss of organic matter. Berthierine formed during early suboxic diagenesis at the expense of detrital clays and iron oxides. During later suboxic diagenesis, pore-water bicarbonate activities increased, leading to siderite precipitation. Early glauconite also formed in places, and it seems likely that it formed close to the oxic/suboxic interface. The predominance of pyrite in the mudstones indicates that sulfate reduction was the major process of organic-matter oxidation during early diagenesis.

Spatial variability in sediment composition and evidence for resuspension in a large, deep lake

Spatial variability in the composition of sediments in Pyramid Lake, Nevada, was assessed in June 1992 by collecting surficial samples at 32 sites. For those sites, organic fractions were highest in the centre of the lake and low near the single inflow to the system. The spatial patterns for organic C, N, and P in lake sediments indicated that small organic-rich particles are preferentially deposited in the centre of the lake. This sorting of particles by water depth was supported by a corresponding decrease in the particle size distribution of sediments with station depth and by limited sediment-trap data. Further, a comparison of sediment-trap particle flux rates with the net sediment accumulation rate for the centre of the lake based on 210Pb decay indicated that the resuspension of bottom sediments in Pyramid Lake (zmax = 102 m) must occur throughout the lake.

Floodplains and paleosols in the Wyoming Eocene sequence: implications for the taphonomy and paleoecology of faunas

The Willwood Fm. of the southern Bighorn Basin of Wyoming, U.S.A., comprises 700 m of lower Eocene alluvial molasse, nearly all of which contains relict pedogenic features. These rocks are grouped into pedofacies–alluvial sediment prisms, thick with immature paleosols proximal to streams and thinner with mature paleosols distally. Pedofacies are bounded by either trunk-stream channel or crevasse-splay deposits, which represent time-stratigraphic markers. The floodplain widths of the Willwood rivers varied from 15 to 20+ km. Paleosols occur throughout the Willwood Formation and the most mature paleosols required about 60 Ka to form whereas the least mature, required 0.5 to 1.0 Ka. Paleosol thicknesses vary from about 0.3–8.0 m and are directly related to net sediment accumulation rate (NSAR) and profile maturity. Pedofacies also reflect NSAR controls; pedofacies are continuously superposed, 15–35-m-thick, and represent time intervals of 30–60 Ka.In the earliest Eocene, paleosol maturity rose sharply, and NSAR plummeted (Fort Union Fm./Willwood Fm. contact), after which maturity gradually declined (and NSAR rose) throughout the early Eocene. This decline was punctuated by two episodes of severe decline, each corresponding with major increases in NSAR, increased tectonism, and episodes of faunal turnover (“Biohorizons” A and B). Above the biohorizons, species earlier tied to particular paleosol maturities were replaced by closely related though more generalized species with no marked paleosol preferences. Time-stratigraphic reconstruction of the Willwood Fm. shows that “Biohorizons” B and C record the same faunal event; B the extinctions, and C the immigrations.The 1,300 Willwood fossil vertebrate localities, which are distributed throughout the entire formation, occur in the surface horizons of cumulative alluvial paleosols. All fossil accumulations in paleosols are attritional and formed during pedogenesis. The most complete remains occur in immature paleosols, whereas the most abundant remains are found in mature paleosols. Within the large-scale Willwood ecologic setting, studies of discrete (m's to tens of m's thick) stratigraphic intervals suggest that the paleontology and sedimentology of these intervals can be significantly influenced by lateral differences in paleosol hydromorphy (soil wetness) and maturity (lateral position of a fossil-bearing paleosol with respect to an ancient river channel). These smaller-scale controls on fossil occurrences are important for distinguishing between real and apparent changes in faunal compositions over time and emphasize the value of three-dimensional stratigraphic analysis for interpreting paleontologic events.Supported by National Geographic Society grant 3985-89.

Accumulation rates and 137Cs distribution in sediments off the Po River delta and the Emilia-Romagna coast (northwestern Adriatic Sea, Italy)

Sediment accumulation rate studies were conducted using 210Pb and 137Cs with the aim of understanding recent and present sedimentation patterns on the Italian Adriatic continental shelf. Since there is no simple model governing the delivery of radionuclides to coastal sediments a variety of approaches were used to estimate dry mass accumulation rates. At many sites there was little divergence between the various estimates, and the mean value is taken to be a reliable estimate of the net sediment accumulation rate. At other sites greater variation was observed. Reasons for these variations can include both secular changes in accumulation rates, and interruptions to the process of sedimentation arising from, for example, sediment mixing, scouring and focussing. At these sites best estimates of accumulation rates are given on the basis of an assessment of the most appropriate dating technique. Highest mass accumulation rates are found near the Po river (up to 1.80 ± 0.29g cm −2 year −1), reflecting the large supply of sedimentary materials from this source. South of the delta accumulation rates were much lower, ranging from 0.22 ± 0.06 to 0.39 ± 0.06g cm −2 year −1. At a number of sites the 137Cs activity versus depth profile had a well defined peak (resulting from weapon fallout) which served to identify the depth corresponding to 1963, providing a valuable check on the 210Pb derived accumulation rates. At other sites this peak was too broad or confused to be a valuable reference. The distribution of 210Pb and 137Cs reflect their different geochemical behaviour and sources, and provide information on the process of sediment accumulation.

Paleomagnetic Isochrons, Unsteadiness, and Uniformity of Sedimentation in Miocene Intermontane Basin Sediments at Salla, Eastern Andean Cordillera, Bolivia

Fine-scaled stratigraphic data are combined with chronologic information provided by paleomagnetic stratigraphy to document and analyze chronostratigraphic variability of the Salla Beds, a sequence of distal floodplain and lacustrine intermontane basin sediments within the eastern Andean cordillera of Bolivia. Unsteadiness in sediment accumulation is identified using a conceptual model of episodic accumulation in which simulated accumulation histories are constructed using estimated rates for various fluvial facies and taking into account gaps in deposition and possible effects of erosion. Variability in the Salla Beds is compared to that in sequence of Miocene fluvial sediments in northern Pakistan to evaluate controls on spatial and temporal stratigraphic variability. Sedimentation in the Salla sequence was generally steady over short time scales, but over longer intervals infrequent carbonates, associated with unusually long hiatuses, served to reduce the net sediment accumulation rate. In the Chinji sequence of Pakistan, unsteady accumulation evolved into a pattern of steadier sediment accumulation as recurring cycles of unsteadiness were superimposed on the overall sequence. The controls on sediment accumulation over time in these two sequences may be similarly related to the presence and extent of atypical and infrequent events of deposition, non-deposition, or erosion that exert a significant effect on net sediment accumulation. The discrepancy of short-term episodic and long-term net stratigraphic accumulation rates in both these sequences suggests that the decrease in net sediment accumulation rate with increasing time reflects the superimposed effects of different levels of local geomorphic and extrabasinal controls.

Factors affecting sulfur incorporation into lake sediments: paleoecological implications

This paper discusses the use of S as a paleolimnological tracer of limnetic sulfate concentration. A positive relationship (p<0.05) was found between limnetic sulfate and sediment S concentrations for the Great Lakes, English Lakes, and lakes from the Adirondack and Northern New England regions. There is a positive correlation (p<0.05) between C and S concentration in sediment across all regions studied. The importance of C in affecting S content in sediment was also examined by a series of cores taken at different water depths in Big Moose Lake (Adirondacks). There was a strong relationship between C and S among cores with sediment from deeper water having higher C and S concentrations (r 2=0.99). Sulfur from the shallower cores had greater concentrations of chromium-reducible S (pyrite), while cores from deeper waters had a greater proportion of organic S fractions including C-bonded S and ester sulfates. For assessing historical changes in S accumulation in sediments, enrichment factors were calculated for the PIRLA lakes. Pre-1900 net sediment accumulation rates of S were very similar across all regions. Sulfur enrichment was greatest in Adirondack sediment which had total post-1900 S accumulation of 1.1 to 7.4 times pre-1900 S accumulation. Sediment from Northern New England (NNE) generally had lower S concentration than Adirondack sediments and S enrichment factors ranged from 1.2 to 2.1. Sediment from the Northern Great Lakes States region had similar S concentration and distribution with depth to NNE sediment. In two Northern Florida lakes, sediment showed little variation in S concentration with depth, but in two other lakes from the same region, there was higher S concentration in deeper layers. Lakes which had the greatest enrichment factors also exhibited the most marked changes in C:S ratios. Ratios of C:N showed little variation (10.6 to 26.1) among the PIRLA lakes. A first order model indicated slow decomposition within these organic rich sediments. Elemental concentrations and ratios of sediment from a variety of lakes and reservoirs were complied. Maximum and minimum elemental ratios for all the data were 28 to 8.1 for C:N, 0.81 to 0.11 for C:H, and 675 to 12.5 for C:S, respectively. For the C:S ratios in all regions except the Great Lakes, the maximum ratio was less than 231. Both the maximum and minimum amount of N and H concentration of organic matter is related to biotic processes. The minimum concentration of S is regulated not only by nutrient demands but also by non-assimilatory processes. Sulfur incorporation into sediments is a function of a complex of factors, but limnetic sulfate concentration and organic matter content play a major role in regulating the S content of sediment. Further quantification of S incorporation pathways will aid in the paleolimnological interpretation of sediment S profiles. Such information is also important in assessing how S sediment pools will respond to decreases in limnetic sulfate concentration which may occur with decreases in inputs from acidic deposition.