Net Sediment Accumulation Research Articles

Budgets for fine-grained sediment in the Danish Wadden Sea

The northern and central part of the Danish Wadden Sea consists of three tidal areas belonging to the tidal inlets Grådyb, Knudedyb and Juvre Dyb. The sizes of the inter-tidal areas are 130 km 2, 143 km 2 and 124 km 2, respectively. The tide is semi-diurnal with a tidal range of about 1.5 m corresponding to tidal prisms in the three tidal areas of 138 · 10 6 m 3, 175 · 10 6 m 3 and 155 · 10 6 m 3, respectively. Sediment budgets are constructed for all three tidal areas on the basis of determination of the net accumulation of fine-grained sediment by means of 210Pb-based sedimentation rates. The input from the North Sea is estimated as the residual amount of the total accumulation after subtraction of all other sources (fluvial, primary production, atmospheric deposition, and estuarine coastal erosion). The total annual amount of deposited fine-grained material in the three tidal areas is 173.3 · 10 3 t y − 1 which corresponds to an average accumulation rate of 1.9 kg m − 2 y − 1 , or 3.5 mm y − 1 . The largest contribution to the accumulation is input from the North Sea which adds up to 111.0 · 10 3 t y − 1 , or 64% of the deposition. Accumulation varies considerably between the three areas. The northernmost tidal area (Grådyb) receives 50% while the others receive only 21% and 29%, respectively. The import variations are suggested to be a consequence of three factors: (i) an assumed increasing availability of fine-grained sediment from south to north on the shelf adjacent to the Danish Wadden Sea; (ii) a smaller trapping competence in the open tidal area of Knudedyb compared to the two other areas, equipped with large closed embayments; (iii) a larger combined salt-marsh area in the Grådyb tidal area compared to the two other areas, where large reclaimed salt-marsh areas are protected by dikes. Comparing the present results of salt-marsh accretion with similar results from the mid-20th century suggests that raised dynamics during the period after 1980 has increased salt-marsh accretion by up to about 50%.

Quantity, composition, and source of sediment collected in sediment traps along the fringing coral reef off Molokai, Hawaii

Sediment traps were used to evaluate the frequency, cause, and relative intensity of sediment mobility/resuspension along the fringing coral reef off southern Molokai (February 2000–May 2002). Two storms with high rainfall, floods, and exceptionally high waves resulted in sediment collection rates > 1000 times higher than during non-storm periods, primarily because of sediment resuspension by waves. Based on quantity and composition of trapped sediment, floods recharged the reef flat with land-derived sediment, but had a low potential for burying coral on the fore reef when accompanied by high waves. The trapped sediments have low concentrations of anthropogenic metals. The magnetic properties of trapped sediment may provide information about the sources of land-derived sediment reaching the fore reef. The high trapping rate and low sediment cover indicate that coral surfaces on the fore reef are exposed to transient resuspended sediment, and that the traps do not measure net sediment accumulation on the reef surface.

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.

Evidence of fine‐grained sediment transport and deposition in Sheboygan River, Wisconsin, based on sediment core chemical tracer profiles

Fine‐grained sediment transport and deposition is observed in nine dated sediment cores obtained from the Sheboygan River. The sediment cores were dated by 137Cs and 210Pb methods. Sediment dating was supported by a polychlorinated biphenyl (PCB) analysis and U.S. Army Corps of Engineers (USACE) hydrographic surveys from 1976 to 2002. Plots of 137Cs activity versus depth show distinct peaks related to peak Sheboygan River discharges in the years 1966, 1975, 1979, and 1989. These peaks result from the scouring of sediments during storm discharges and the increased deposition of the fine‐grained fraction of particles, including colloidal clay particles (associated with 137Cs and, to some extent, PCBs), in place of the scoured sediments. Core SR1a, upstream of the Pennsylvania Avenue Bridge, and cores SR5‐SR8, farther downstream, show continuous sedimentation (1.2–11.8 cm/yr) since the late 1950s, whereas net sediment accumulation virtually ceased after 1988 at the intermediate sites of cores SR1–SR4. The maximum PCB concentration (161 ppm) occurs in 1969 in core SR4.

Fluvial sediment budget of a Mediterranean river: the lower Tordera (Catalan Coastal Ranges, NE Spain)

The lower Tordera River is a representative case of water and sediment dynamics and uses in a large river in the Western Mediterranean region. Along the study reach, the fluvial regime of the river changes, shifting from a perennial water and sediment circulation in the upper parts to an ephemeral flow pattern in the lower sections. The fluvial sediment budget of the lower Tordera was estimated based on field measurements of suspended and bedload and a survey of cross-sections. Field data were obtained between January 1997 and June 1999, a period that was dryer than the average. Most sediment is transported as bedload (more than 3/4 of the total load), both at the entrance section of the river reach and at the outlet section, close to the Mediterranean Sea. A total of 156,700 tonnes of sediment (62,680 tonnes/year) entered the system, mostly during discharges below bankfull, while 107,000 tonnes (42,800 tonnes/year) were exported to the sea, almost exclusively during floods. The net accumulation of sediment was 14,600 tonnes per year (as bedload), especially during the dry years of 1998 and 1999, when small floods circulated through the upper sections but did not reach the lowermost part of the river. As a consequence, a mean aggradation of the riverbed of around 6.8 mm/year has occurred, a figure that has been corroborated after the analysis of a series of cross-sections. Residence time of sediment in the perennial reach, calculated at a long-term, ranges from 6 years for sediments in the active base flow channel to 164 years for sediment placed above bankfull level, which is only entrained during high flows. Residence time of sediment in the ephemeral reach, calculated for the period 1997–1999, is only slightly higher than that of the perennial part of the river. This fact can be related to the current adjustment of the river system to historical mining. The riverbed is slowly adjusting to the new conditions, and important sedimentation is taking place in many upstream sections, while erosion still occurs in the lowermost reaches, thus causing the relatively low residence time of sediment.

Impact of storms on mixed carbonate and siliciclastic shelves: insights from combined diffusive and fluid‐flow transport stratigraphic forward model

AbstractA quantitative stratigraphic model of mixed carbonate/siliciclastic continental shelves is presented to investigate the relationships between depositional processes and stratigraphic responses at long‐term, large spatial scales. A diffusion model is combined with a fluid‐flow approach to simulate both long‐term factors, i.e. the processes controlling large‐scale architecture, and short‐term processes, i.e. sediment redistribution by storms. Any net sediment accumulation is the result of the succession of a storm and a fair‐weather period. Sediments are mobilized by waves and advected by low‐frequency currents during storm events. Sediments are then reworked and redistributed downslope by diffusive processes during fair‐weather period. The results are successful in capturing several major characteristics of both modern and ancient depositional systems (geometry, differential preservation, net accumulation rates). The study highlights the importance of waves and unidirectional currents. Depositional geometry and shelf morphology depend on the balance between available sediment supply (generated in situ or detrital) and the transport energy, which is related to the style of sediment transport (diffusive or advective), and to the magnitude and frequency of storms.

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.

FACIES DISTRIBUTION OF FUSULINOID GENERA IN THE MYACHKOVIAN (UPPER CARBONIFEROUS, UPPER MOSCOVIAN) OF THE REGION SOUTH OF MOSCOW

The Moscovian Stage of Central European Russia is represented by a cyclic shallow-marine carbonate succession deposited in a gentle ramp setting part of a vast epeiric sea. Fusulinoids occur in all the marine lithofacies studied including most extreme intertidal grainstones. The ecological tolerance of fusulinoids decreases in the following order: Fusiella , Schubertella , Ozawainella , Fusulinella , Fusulina , Reitlingerina and Hemifusulina , Neostaffella , and Parastaffelloides . The scarcity of Taitzehoella , Beedeina , Eostaffella , Kamaina , and Praeobsoletes did not allow the reliable estimation of their facies distribution and tolerance. Three main zones of fusulinoid distribution are revealed. The predominantly autochthonous assemblages of these zones probably correspond to original fusulinoid biofacies. Biofacies 1 contains the most tolerant genera Fusiella and Schubertella that lived in the most severe conditions of the restricted peritidal zone. The optimal shallow normal-marine settings of Biofacies 2 maintained the highest diversity of fusulinoids ( Fusiella , Schubertella , Ozawainella , Fusulinella , Fusulina , Reitlingerina , Hemifusulina , Neostaffella , Parastaffelloides , Taitzehoella , Beedeina , Eostaffella , Kamaina , and Praeobsoletes ). Most abundant were Fusulinella , Fusulina , and Schubertella . The habitats of staffellids ( Reitlingerina and Parastaffelloides ) were shifted from a wide normal-marine facies spectrum in Podolskian time to less favourable settings of shoals and fine small-foraminiferal sands. The apparent peaks of some genera in the shoal lithofacies of coarse skeletal-peloidal grainstones must be increased several times as a consequence of much slower net sediment accumulation, but the ratio of different fusulinoid genera in this lithofacies is probably retained. Biofacies 3 contains distal tempestites and skeletal mudstones representing the deepest (considerably more than 20 m) depositional environments. The fusulinoid assemblage is less diverse than in Biofacies 2. In the Upper Gubastovo tempestites, Biofacies 3 is highly dominated by Hemifusulina bocki Moeller. Fusiella , Schubertella , Ozawainella , Taitzehoella , Fusulinella , and Fusulina occur in lesser quantities. Some of these infrequent fusulinoids may be allochthonous, and the original diversity might have been even lower. Hemifusulina in the Upper Gubastovo tempestites tends to form monospecific ( H. bocki Moell.) concentrations exhibiting the signs of sudden storm burial.

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.

Suspended Sediment Transport Dynamics in the Sub-tropical Micro-tidal Richmond River Estuary, Australia

Suspended sediment transport and sedimentation dynamics in the sub-tropical Richmond River estuary were investigated between July 1994 and June 1996. During low flow months the estuary received net sediment inputs from the continental shelf; during high flow the estuary exported sediment to the continental shelf depending on the magnitude of floods. Flow-weighted sampling along with a one-dimensional unsteady flow hydrodynamic model of the estuary estimated that about fifty percent of the flood-borne suspended sediment was transported through the estuary to the continental shelf during two minor floods (with a 2-year return period), whereas, one hundred percent of the flood-borne sediment was transported to the continental shelf during a moderate flood (with a 5-year return period). Marker horizons confirmed a net accumulation of sediment in the Richmond River estuary at the salt-fresh water interface during all post flood recovery stages.

Sediment dynamics in the fluvial lakes of the St. Lawrence River: accumulation rates and characterization of the mixed sediment layer

Permanent sedimentation (210Pb and 137Cs), sediment mixed layer thicknesses, and mixing coefficients (7Be) were measured in the St. Lawrence River in order to evaluate the importance of sediment retention in the particulate matter budget and to characterize the system's resilience to changing contaminant loads. Net sediment accumulation (1 to &gt; 18 kg·m-2·year-1) is observed at most sites deeper than 4.5 m located outside the main channels. Annual sediment retention in the lakes ranges from 1.5% (Lake St. Pierre) to 17% (Lake St. Francis) of their total load of suspended solids. 7Be profiles indicate that the average mixed layer thickness, mixed layer mass, and mixing coefficient are 3.3 ± 0.2 cm, 17.8 ± 1.7 kg·m-2, and 14.9 ± 2.8 cm2·year-1, respectively. The average depth of the long-term (approximately 5 years) mixed layer determined from the 137Cs : anthropogenic Pb ratio is 5.1 ± 0.4 cm, corresponding to 30.6 ± 4.6 kg·m-2. Because the mixing coefficient in superficial sediments is relatively high, and because annual particulate matter loading to the river is comparable with its mixed sediment inventory, the system is expected to have a rather short memory of past conditions and to recover rapidly (2-5 years) following a decrease in contaminant loading.

Late Quaternary vegetation dynamics and hydroseral development in a shrub swamp in southern Ontario, Canada

Using a multiple-core paleoecological approach, we studied the development of a small kettle-hole wetland (informal name Spiraea wetland) in southern Ontario. The sedimentary record begins at around 11 700 BP, when Picea and herbs characteristic of a disturbed environment were the principal components of the upland vegetation. These were replaced by ca. 9500 BP by Pinus dominance, which persisted until at least 6300 BP. Mixed deciduous forest communities then developed and were only disturbed by Euro-Canadian settlement in the last few hundred years. Initially, the basin contained an open lake, which was progressively colonized by a range of aquatic plants, becoming a shallow open water wetland community by 9500 BP. A marsh community spread rapidly from the edge of the basin, and a mat of aquatic mosses filled the central area of open water before marsh vegetation became established there. Between 6300 and 1500 BP, there is a marked decrease in sedimentation rates, and the paleoecological data imply that the vegetation communities at the coring points varied between marsh, shrub swamp, and conditions where no net sediment accumulation occurred. Sedimentation rates increased in the upper part of the core, as the modern tall-shrub swamp developed.

Terrigenous sedimentation and coral reef growth: a conceptual framework

The role of terrigenous (and non-framework carbonate) sedimentation has not been prominent in models of coral reef growth and evolution. We derive and discuss a semi-quantitative model which relates coral reef growth to sedimentation. The model is independent of coral biology and is based upon the relative net rates of framework and non-framework sediment accumulation and/or removal. The model might enable some forecasting of long-term responses to changes in sedimentation regime and other environmental factors. The occurrence of turbid-zone reefs is successfully explained in regions of very high turbidity but with little or no net sediment accumulation. Potential future use of the model may include aiding prediction of the effects of other geological and oceanographic factors on the growth or demise of coralline communities.

Modern accumulation rates and a sediment budget for the Eel shelf: a flood-dominated depositional environment

The northern California continental margin is periodically impacted by geologically significant storms, which have a marked influence on terrigenous sediment supply, flood deposition, and long-term accumulation of fine-grained sediment on the Eel shelf. Accumulation of Eel River muds on the adjacent shelf was investigated using 210Pb and 137Cs geochronologies, in order to understand the fate of sediment discharged by the Eel River and to relate patterns of net sediment accumulation (100-yr time scale) to sediment dynamics. 210Pb data demonstrate that modern accumulation of river mud occurs from the 50-m isobath seaward. Across-shelf accumulation rates decrease from maximum mid-shelf values of 0.6–1.7 g cm −2 yr −1 to values of 0.2–0.4 g cm −2 yr −1 at the shelf break, with a spatially weighted mean of 0.5 g cm −2 yr −1 (0.4 cm/yr) for the entire shelf. 210Pb xs sediment-depth profiles from the region of highest accumulation rate are characterized by subsurface intervals of low and uniform activity, which are produced by flood deposition. In some cores, particular 210Pb xs activity intervals may be associated with major Eel River floods of 1955, 1964, and 1974. It is postulated that, because of the coincidence of high-river-flow events and southerly winds during cyclonic winter storms, net northward transport allows for preferential deposition of fine-grained sediment north of the river mouth. Over the past ∼100 years, fluvial sediment input combined with marine dispersal processes have produced a mid-shelf depocenter, evident by both the spatial distribution of 210Pb accumulation rates and by clay-rich flood layers partially preserved in shelf deposits. A fine-grained sediment budget for the dispersal system, based on hydrological data and 210Pb geochronologies, demonstrates that a maximum of ∼20% (3×10 9 kg/yr) of the mean annual supply of fluvial mud (14×10 9 kg/yr) is trapped on the shelf. The results of this study demonstrate that: (1) short-term sedimentation processes associated with floods can influence sediment accumulation on longer time scales; and (2) a major fraction of fine-grained sediment supplied to tectonically active margins by flood-prone mountainous rivers bypasses narrow continental shelves.

Mobile deltaic and continental shelf muds as suboxic, fluidized bed reactors

The oft-cited general correlation between net sediment accumulation and preservation of organic matter, while revealing in many ways, can be a misleading indicator of general elemental cycling processes and controls on storage of biogenic material at the continental-ocean boundary. Deltaic environments are characterized by the highest rates of net sedimentation and are the single most important class of depocenters on Earth. Available data indicate that sedimentary organic C (C org ) of both terrestrial and marine origin is efficiently decomposed in deltaic areas, with decomposition percentages reaching ≥70% and ≥90%, respectively, the latter percentage (marine) being quite comparable to deep-sea, low sedimentation environments. Despite high primary productivity associated with most deltas and evidence of substantial deposition of fresh planktonic debris, patterns of SO 4 = reduction indicate that the reactivity of organic material being buried is low, and that a larger proportion of C org is often degraded compared to other marine deposits of similar net accumulation rate. As indicated by properties of the surficial Amazon delta and downdrift coastal region of northeast South America (∼1600-km extent), the primary reasons for efficient remineralization are related to intense and massive physical reworking of sediment associated with estuarine fronts , upwelling, tidal oscillation, and wind-driven waves. Fluid muds and mobile surface material cause the seafloor and continental boundary to act as a massive, suboxic, fluidized bed reactor dominated in some cases by bacterial rather than macrofaunal biomass. Reoxidation, repetitive redox successions, metabolite exchange, and continual mixing-in of fresh planktonic debris with refractory terrestrial components, result in an efficient decomposition system largely decoupled from net accumulation. Similar processes occur on smaller scales in most estuarine-shelf systems, but appear to be most dramatically expressed off the major rivers forming deltas.

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.

A fine-grained sediment budget for the Sylt-Rømø tidal basin

A budget for net accumulation of fine-grained sediment (<63 μm) has been set up for the Sylt-Romo tidal basin. Net accumulation within the basin was computed from210Pb core dating and mapping of the intertidal and supratidal surface sediments. It was found that a yearly mean value of 58·103 tons of sediment was deposited in the tidal basin. The largest sediment source for the net input of fine-grained sediment is the North Sea contributing about 64% to the net budget; the fluvial input and primary production contribute 14% and 15%, respectively. Local salt marsh erosion accounts for about 5% of the budget and atmospheric deposition for only 2%. The total amount of sediment deposited in the investigated area was low compared with earlier investigations in the Wadden Sea. This is explained partly by the intensive diking of the natural salt marshes fringing the area in the past, and partly by the exposed conditions of most of the intertidal flats. An index describing the trapping efficiency of the water exchanged between the North Sea and the Sylt-Romo tidal area is defined as the ratio between yearly net sediment input from the North Sea and yearly exchanged water volume between the tidal basin and the sea. This index shows that in the Sylt-Romo tidal basin, fine-grained suspended sediment “filters” out of the exchanged sea water at a rate that is 12 times lower than in the Gr»dyb tidal basin. It is concluded that the net deposition of fine-grained sediment in a tidal basin is mainly a function of physiographical and hydrodynamical parameters and to a lesser degree of sediment availability

Sediment accumulation of nutrients (N, P) in the eastern Gulf of Finland (Baltic Sea)

Dry weight (DW), ignition loss (IL) and concentrations of total nitrogen (TN) and total phosphorus (TP) of the sediment surface layer (0 to 10 cm, 1 cm slices) were analyzed from 20 sites in the eastern Gulf of Finland. The distance of the sampling sites from the mouth of the River Neva explained the nutrient concentrations of the sediments well, while the effect of water depth was negligible. The increase of TN and the decrease of TP along the transect from the river mouth towards the open Gulf were caused by the diminishing share of allochthonous material supplied from the River Neva. The mean TN concentration of the different accumulation areas was about 40 % higher in the sediment surface than in the deeper layer (9 to 10 cm). The corresponding difference for TP varied from 53 to 56 %. The results suggest considerable netflux of nutrients from sediment to water. The net sediment accumulation of nutrients were estimated as 6.0 g m−2 a−1 of N and 1.7 g m−2 a−1 of P corresponding 22 000 t a− of N and 6 100 t a−1 of P for the whole eastern Gulf.

Stratigraphy and paleoenvironments of rudist biostromes in the Upper Cretaceous (Turonian-upper Santonian) limestones of southern Istria, Croatia

Abstract Rudists are well preserved in the Upper Cretaceous limestones of southern Istria, where they flourished within different shallow-water environments during the Turonian to upper Santonian. In order to determine paleoenvironments, a vertical succession of the rich rudist assemblages has been investigated and described within sequence stratigraphy framework. The Turonian to upper Santonian sediments of southern Istria, in the sense of sequence stratigraphy, comprise one complete third-order sequence and the beginning of the transgressive systems track (TST) of the next sequence. Lower Turonian pelagic limestones were deposited on a foundered carbonate platform, and represent the beginning of the TST. Limestones of the late TST-lowstand systems track (LST) were deposited on the inner shelf, and the early TST of the next sequence was deposited on the outer shelf. All the rudists which appeared after the early Turonian disappearance and partial extinction were elevators. The genera Durania and Praeradiolites thrived in bouquets, which may coalesce to form thickets in inner shelf environments of the late TST-LST. Their growth depended completely ( Durania ) or partly ( Praeradiolites ) on net sediment accumulation. The environment in which Durania and Praeradiolites thrived was subtidal and deep enough to be out of reach of emersion. Within the same depositional environment, these genera were later replaced vertically by Radiolites . Individuals with slim, cylindrical shells, and wide growth lamellae, have full contact within the thickets, in which their growth depended little, if at all, on net sedimentation rate. Also, Radiolites occupied the shallower parts of the subtidal zone with a commissure rim rising above the sediment surface. Renewed deepening in the late Santonian resulted in the outer shelf environment of the next TST. Gorjanovicia , together with rare Vaccinites , flourished after the Radiolites . Gorjanovicia differed from Radiolites only in possessing more pronounced vertical ribs. In their slightly deeper subtidal environment with pelagic influxes, Gorjanovicia thrived in bouquets. Loosely packed Gorjanovicia gave Vaccinites an opportunity to grow among them. The environmental setting of the southern Istria rudist biostromes and their correlation with other shallow-water deposits in the Mediterranean region suggests that a sequence stratigraphy approach can be applied to correlate rudist biostratigraphy on a regional, or even inter-regional, scale.