Estimation Of Sediment Accumulation Rates Research Articles

Carbonate uranium isotopes across Cretaceous OAE 2 in southern Mexico: New constraints on the global spread of marine anoxia and organic carbon burial

Oceanic anoxic events (OAEs) represent discrete intervals of decreased marine oxygen concentrations often associated with volcanism, enhanced organic carbon burial coupled with positive δ13C excursions, and significant biotic turnover. Cretaceous OAE 2 (ca. 94 Mya) is especially notable for globally-distributed changes in calcareous invertebrate and plankton populations. While the presence of organic-rich facies is consistent with locally anoxic environments in many cases, determining the global extent of anoxia is more problematic. To address this issue, we investigate uranium isotope (δ238U) compositions of upper Cretaceous (Cenomanian-Turonian) open marine platform carbonates from southern Mexico as a proxy for global seawater redox conditions. These data are complementary to previous δ238U studies across OAE 2 in both black shales and pelagic carbonates, which have yielded variable results that reflect both global redox and local depositional processes. In Morelos Formation carbonates, a significant and well-defined negative δ238U excursion down to a nadir of −0.6‰ is recorded over an ∼40 m interval. This is consistent with the expansion of marine anoxia, pointing to an areal extent of anoxic seawater of about 1–10% of the global seafloor (or ∼5 to 50 times the modern value). Importantly, based on biostratigraphically-controlled estimates of sediment accumulation rates, the δ238U anomaly precedes the δ13C excursion by a median of ∼45 to 51 kyr (95th percentile confidence interval, CI) or ∼105 to 120 kyr (95th percentile CI) depending on how the onset of the δ238U anomaly is estimated. These results, along with previously reported thallium isotope and trace metal data, suggest that anoxic expansion preceded carbon cycle perturbation. This observation further increases estimates of the duration of OAE 2, implying that widespread ocean anoxia may have lasted >900 kyr.

Cyclostratigraphy of the Lower Triassic Kangan Formation in the Salman gas field, eastern Persian Gulf, Iran

Cyclostratigraphy plays a pivotal role in detecting and reporting Milankovitch cycles in sedimentary successions which allows for creating an independent astronomical time scale and estimates of sediment accumulation rates. In this study, cyclostratigraphic analysis of petrophysical logs from Lower Triassic Kangan Formation was performed in two wells of the Salman gas field, eastern part of the Persian Gulf, Iran. The results of power spectral analysis in the two studied wells reveal the main spectral peaks at wavelengths of long-eccentricity, short-eccentricity, obliquity, and precession cycles. Based on the orbital tuning using the long-eccentricity cycles, a floating astronomical time scale was established, according to which the depositional duration of Kangan Formation was estimated to be 1.6 ± 0.1 Myr. As well as, the duration of the Induan Stage of the Early Triassic Epoch in this study is estimated to be at least 1.6 Myr. Also, by integrating cyclostratigraphic analysis, results of the statistical methods of sedimentation rate evaluation, i.e., evolutionary correlation coefficient (COCO), it was estimated that the sedimentation rates of Kangan Formation in the two studied wells of Salman gas field increased from ~8 cm/kyr in the bottom to about 12–18 cm/kyr in the upper part, which were linked with the delayed life recovery in Early Triassic. We also observed an enhanced sedimentary noise (ESN) interval in eCOCO maps of the K1 member (upper part of Kangan Formation) at two studied wells, which were linked with the global sea-level fall in the late Griesbachian substage of the Early Triassic. The results of this study show that cyclostratigraphy has made an effective contribution towards analyzing sea-level change and estimating sediment accumulation rates and depositional duration of the Kangan Formation.

Retraction: Estimation of sediment accumulation rates using naturally occuring 210Pb models and heavy metal measurements in Gülbahçe Hydrothermal Vent Zone

In this study, sediment chronology was determined by using Lead-210 (210Pb) dating models in Gulbahce Bay. Sediment cores were obtained via a gravity core sampler from hydrothermal area and also from reference point and surficial distribution of Polonium-210 (210Po) was investigated in the study area. The average surficial 210Po concentration was relatively higher in hydrothermal area than the reference point. Sedimentation rates in Core reference point (RN) displayed an increase till the 1940’s and a decrease afterwards, sedimentation rates in any core from hydrothermal area did not exhibit regular continuous increase or decrease. The 210Pb fluxes measured from core inventories are close to each other but they are a bit higher than the average mediterranean and mean atmospheric 210Pb flux values. This situation indicates that there are outer contribution to the study area apart from atmospheric precipitation of 210Pb concentration. Average enrichment factors indicating that the sediments have not been co...

Estimation of sediment accumulation rates using naturally occuring 210Pb models and heavy metal measurements in Gülbahçe Hydrothermal Vent Zone

Estimation of sediment accumulation rates using naturally occuring ²¹⁰Pb models and heavy metal measurements in Gülbahçe Hydrothermal Vent Zone

Settling fluxes and sediment accumulation rates by the combined use of sediment traps and sediment cores in Tema Harbour (Ghana)

Settling fluxes and sediment accumulation rates in coastal Tema Harbour (Ghana) were investigated by the combined analyses of results in sediment traps and sediment cores. Sediment traps were deployed at 5 stations within the Tema Harbour at two sampling depths and were retrieved every two weeks till the end of 12weeks to estimate the Settling Fluxes (SFs). Four sediment cores from the harbour were analysed for their radioactivity (7Be, 234Th, 210Pb, 212Pb, 226Ra, 40K and 137Cs) profiles to quantify Sediment Accumulation Rates (SARs). The sediment cores exhibited variable bulk density profiles, indicating highly dynamic and non-steady sedimentation conditions. 7Be-derived gross-estimates of very recent SARs using the constant flux-constant sedimentation (CF-CS) model were in the range of 2.5–9.0g·cm−2·y−1. These values were much lower than the estimated average SFs (15.2–53.8g·cm−2·y−1), indicating sediment resuspension plays an important role. On a decadal time scale, conventional 210Pb sediment dating models did not allow any estimation of SARs in the Tema Harbour. Thus, the 210Pb-based TERESA model was applied to depict a reliable scenario for sedimentation with time-averaged SARs in the range of 1.4–3.0g·cm−2·y−1 and fluxes of matter contributed by the marine inflow and local sources. Sediment accretion rates of 1.7–3cm·y−1 were also inferred, which may pose a moderate problem of sustainability for the Tema Harbour. This study reveals how the geochemical behaviour of different radionuclides with Gamma spectrometry in the marine environment can be used to obtain reliable information on the complex dynamics of Suspended Particulate Matter (SPM), even in a very disturbed and anthropic environment as a coastal harbour area where (1) conventional 210Pb-based dating methods fail and (2) the use of sediment traps and 234Th and 7Be profiles in sediment cores show serious constraints.

A suspended sediment budget for the agricultural Can Revull catchment (Mallorca, Spain)

Extensive rainfed herbaceous crops are one of the most representative agricultural elements of Mediterranean region, which should be one of the major factors affecting erosion processes. Although land use is commonly seen as resulting in increased sediment yields, the implementation of soil and water conservation practices can have the reverse effect. A sediment budget has been established in Can Revull, a small agricultural catchment (1.03 km2) on the island of Mallorca, by using 137Cs measurements, sediment source fi ngerprinting and more traditional monitoring techniques to quantify the individual components of the budget. A large proportion of the material mobilized from cultivated fi elds without conservation practices (gross erosion of 775 t yr 1, 1,270 t km 2 yr 1) was subsequently deposited either within the fi eld of origin (112 t yr 1, 180 t km 2 yr 1) or at intermediate locations between the source fi eld and the channel network (fi eld-to-channel conveyance loss was 454 t yr 1, 744 t km 2 yr 1). The estimates of sediment accumulation rates on the fl oodplain in the lower reaches of the catchment indicate that the mean sedimentation rate was 0.14 g cm-2 yr-1. This value was extrapolated to the total area of the fl oodplain to estimate a total annual conveyance loss or storage of 445 t yr 1. Monitoring at the catchment outlet over the study period (2004–2008) indicated a mean annual suspended sediment yield of 7 t km-2 yr-1. The sum of the estimates of sediment yield and fl oodplain storage (452 t yr-1) was taken to represent the total annual input of suspended sediment to the channel system. This value was subsequently apportioned using the information provided by the fi ngerprinting investigation, to estimate the mass of sediment reaching the channel network from cultivated fi elds and from eroding channel banks. Thus the annual contribution from channel banks was estimated to be 242 t yr-1. In the case of the contributions from cultivated fi elds, the estimates obtained were, as expected, signifi cantly less than the values of net soil loss from these zones provided by the 137Cs measurements due to conveyance losses associated to fi eld-to-channel conveyance loss. The overall sediment delivery ratios (< 1 %) indicate that approximately 99 % of the sediment mobilized by erosion within the Can Revull catchment is subsequently deposited before reaching the monitoring station. As such, the low sediment outputs from the study catchment should be seen as refl ecting the importance of conveyance losses and storage rather than a lack of sediment mobilization from the catchment surface, although part of the catchment headwaters was modifi ed historically by means of terraces and transverse walls to prevent erosion.

Estimation of sediment accumulation rates using naturally occuring 210Pb models in Gülbahçe Bay, Aegean Sea, Turkey

The sediment chronology was determined using Lead-210 (210Pb) dating method in Gülbahçe Bay. Sediment cores were obtained using gravity core sampler from hydrothermal area and also from reference point and surfical distribution of Polonium-210 (210Po) was investigated in the study area. The average of surfical concentration of 210Po was found as 175.25 ± 3.80 Bq kg−1 in the hydrothermal area which was higher than the reference point value of 133.20 ± 3.30 Bq kg−1. Sedimentation rates in Core reference point (RN) displayed an increase till the year 1942 and a decrease afterwards, sedimentation rates in any core from hydrothermal area have not exhibited any continuous increase or decrease. The 210Pb fluxes measured from core inventories are close to each other. Average enrichment factors for Cu, Cd, Sr, Zn, Cr vary between 20.35 and 12.18 indicating that the sediments have been contaminated with these metals in the reference point. Maximum enrichment factor values for Cr, Cu, Zn, Ba, Sr, Ni, Co vary between 22.91 and 11.42, implying that the sediments in hydrothermal area have been contaminated with these metals.

Bottom-current control on sedimentation in the western Bellingshausen Sea, West Antarctica

A set of single- and multi-channel seismic reflection profiles provide insights into the younger Cenozoic sedimentation history of the continental rise in the western Bellingshausen Sea, west and north of Peter I Island. This area has been strongly influenced by glacially controlled sediment supply from the continental shelf, interacting with a westward-flowing bottom current. From south to north, the seismic data show changes in the symmetry and structure of a prominent sediment depocentre. Its southernmost sector provides evidence of sediment drift whereas northwards the data show a large channel-levee complex, with a western levee oriented in the opposite direction to that of the drift in the south. This pattern indicates the northward-decreasing influence of a westward-flowing bottom contour current in the study area. Topographic data suggest the morphologic ridges at Peter I Island to be the main features responsible for variable bottom-current influence, these acting as barrier to the bottom current and entrained sedimentary material. West of Peter I Island, the east-orientated Coriolis force remains effective in deflecting the suspended load of the turbidity currents towards the west, thereby promoting growth of the western channel levee. Calculated sediment accumulation rates based on seismic data reveal Depocentre C to consist of younger Cenozoic material supplied by glacial transport and modified by contour currents in the western Bellingshausen Sea. These findings demonstrate that the shape, structure and distribution of sediment mounds and estimates of sediment accumulation rates can be associated to the influence of bottom currents and their long-term evolution in response to tectonic movements, ice-sheet dynamics and deep-water formation.

LATE TO MIDDLE JURASSIC SOURCE FACIES AND QUALITY VARIATIONS, SOUTH VIKING GRABEN, NORTH SEA

Source facies and quality of the Late to Middle Jurassic source rock system in the South Viking Graben between 58°N and 60°15'N are highly variable both regionally and stratigraphically. In order to assess the degree of variability and to create a model of source rock quality and potential, isochore maps of the syn‐ and post‐rift sections of the Upper Jurassic Draupne Formation and underlying Heather Formation were generated from seismic and well data, and maturity‐corrected Rock‐Eval data were used to generate quantitative maps of oil and gas potential. The thin post‐rift section at the top of the Draupne Formation is a rich oil‐prone source, while the up to 1,600 m thick syn‐rift section contains a mixture of Type III and Type II material with substantial amounts of gas‐prone and inert organic matter. The Heather Formation, which reaches modelled thicknesses of up to 930 m, is a lean source and is generally gas‐prone.Detailed analyses and interpretations of biomarker and isotopic characteristics support this upward increase in oil‐prone Type II material. The analytical parameters include increasing relative amounts of C27 regular steranes; decreasing ratios of C30 moretane relative to C30 hopane; and an increasing predominance of short chain n‐alkanes and progressively lighter isotopic values for saturate and aromatic fractions of source rock extracts. In addition, increasing amounts of 17α(H),21β(H)‐28,30‐bisnorhopane and decreasing amounts of C34 homohopanes relative to C35 homohopanes, as well as decreasing Pr/Ph ratios, suggest a general decrease in oxygenation upwards. Maps of average Pr/Ph ratios for the syn‐ and post‐rift Draupne Formation and for the Heather Formation are consistent with permanent water column stratification and gradual ascent of the O2:H2S interface from the Callovian to the Ryazanian.Interpretation of oil and gas potential maps, molecular parameters and estimates of sediment accumulation rates in combination suggest that the source facies of the upper, post‐rift Draupne Formation is controlled by widespread anoxia, reduced siliciclastic dilution and reduced input of gas‐prone organic and inert material; by contrast, the potential of the lower, syn‐rift Draupne Formation is strongly controlled by dilution by gas‐prone and inert organic matter resulting from mass flows and also by varying degrees of oxygenation. The oil and gas potential of the Heather Formation is mainly controlled by the degree of oxygenation and siliciclastic dilution.

Alluvial Terraces and Paleosols as Indicators of Early Oligocene Climate Change (John Day Formation, Oregon)

ABSTRACT Flights of paleoterraces exposed in the Painted Hills area of central Oregon record periods of alluvial cutting and filling that correspond with early Oligocene climate changes. The terraced interval is situated chronologically between two global climate steps: the Eocene-Oligocene boundary at 34 Ma and the mid-Oligocene climatic transition at 30 Ma. The paleoterraces, and associated paleosols, are in the middle and upper Big Basin Members of the John Day Formation and are bracketed by both 40Ar/39Ar age determinations and Oligocene floral and faunal fossil assemblages. The paleoterraces are defined by truncation surfaces that cut underlying alluvial strata, are nlapped by paleosols, and are mantled locally by colluvium-influenced, iron-rich paleosols. One well-exposed truncation surface has a stepped geometry with nearly horizontal sections (treads) separated by steep sections (risers) with relief of 4-5 m. The truncation surfaces represent episodes of incision of the overbank alluvium, and the overlying disconformable paleosols (mantling the surfaces) represent periods of landscape stabilization following incision. The strata as a whole can be grouped into three categories according to the degree of development of paleosols in the sequences. These paleosol sequences can be interpreted in terms of climatic-geomorphic-fluvial response. (1) Rapidly deposited alluvium with weakly developed paleosols is interpreted to represent deposition during periods of rapid climate change. During these periods, the vegetation-landscape system was in disequilibrium, thereby causing accelerated soil erosion from the hinterland landscape, which was accompanied by floodplain aggradation. (2) Slowly deposited alluvium with well-developed, clayey paleosols may indicate climatic optimum conditions when vegetation both suppressed soil erosion and enhanced sediment storage in hinterland soils. (3) Most of the alluvial st ata falls into a third category, which consists of alternating moderately and weakly developed paleosols. This alluvium is interpreted as fluvial deposition responding to a geomorphic system in dynamic equilibrium. The largest terrace surface cuts into rapidly deposited alluvium with weak paleosol development and may represent a complex-response incision feature associated with a preceding period of rapid alluviation. A chronology for the alluvial cut-and-fill episodes is constructed based on stratigraphic relationships, time estimates of paleosol development, and 40Ar/39Ar age determinations. Time estimates from paleosols are made both by comparison of paleosols to modern soils from dated chronosequences and directly from 40Ar/39Ar age determinations of tuffs interbedded with complete sequences. These methods allow estimation of sediment accumulation rates and of the timing and duration of aggradation and degradation episodes. In the Painted Hills area, the Eocene-Oligocene boundary is marked by a truncation surface overlain by rapidly deposited alluvium. This alluviation episode is estimated to have begun at about 33.2 Ma. A 10 m stack of red beds (well developed, clay-rich, and oxidized paleosols) cap the before-mentioned rapidly deposited strata and mark the top of the middle Big Basin Member. These paleosols are estimated to span the interval from 32.0 to 32.6 Ma, which corresponds to an early Oligocene warming trend that immediately followed the Eocene-Oligocene (Oi1) cooling event, both recorded in marine isotopic data. Thus, these red beds may represent an early Oligocene climatic optimum in the Pacific Northwest.

Land-sea correlations and evolution of the Cambridge Fiord marine basin during the last deglaciation of Northern Baffin island

Cambridge Fiord functions as a major outlet trough whenever Laurentide ice approaches the eastern coast of Baffin Island. During the last glacial maximum (late Foxe glaciation), an outlet glacier occupied most of the fiord, possibly terminating as far seaward as Buchan Gulf. Glacier retreat began prior to 8800 yr B.P. and was punctuated by the formation of two onshore moraines with upper age limits of 7900 and 7200 yr B.P. Correlative offshore moraines (recognized on seismic reflection profiles) formed prominent submarine sills which partitioned the fiord into three basins. These formed effective sediment traps that restricted the seaward migration of debris flows and turbidites, thus facilitating the accumulation of thick (> 100 m) stratified glacialmarine sediments in the inner fiord. Correlations of onshore and offshore moraines allow time lines to be placed within an offshore seismic stratigraphic section that is inaccessible to piston coring. Resultant estimates of sediment accumulation rates give a mean value of 2.34 × 10 6 m 3/yer for deposits trapped in the inner fiord basins. These thick sequences of sediments do not represent extended Foxe/Wisconsin time periods, but, rather, were rapidly deposited in the inner fiord during the last deglaciation.

Stable isotopic characterization of organic carbon accumulation on the Amazon continental shelf

The stable carbon isotopic ratio ( 13C: 12C) of organic matter in the water column and sediments has been used to examine the distribution and accumulation of organic carbon on the Amazon continental shelf. Near the river mouth, in 0 × 10 −3 salinity surface waters, particulate organic carbon (POC) is isotopically light (−27.3 per mil). Isotopic values of POC in continental shelf waters north and east of the river mouth range from −19.5 to −25.7 per mil. The isotopic variations in water column POC can be related to productivity, turbidity, and water density. The isotopic character of water column POC is controlled, therefore, by the dynamic mixing and northwestward migration of riverine and marine shelf waters. Terrestrial organic carbon dominates the isotopic signal in surface sediments (upper 10 cm) from the river mouth to areas 400 km to the northwest. Only on the outermost and northern parts of the shelf is marine organic carbon dominant in surface sediments. A sharp boundary between terrestrial and marine isotopic values is present in continental shelf sediments at the outer edge of the prograding mud delta. This boundary is associated with an abrupt decrease in sediment accumulation rate and a change in stratigraphic setting from topset and foreset regions to bottomset regions. The average TOC content of Amazon continental shelf sediments is 0.66 ± 0.20%. Based on the average TOC of Amazon shelf sediments and current estimates of sediment accumulation rates, approximately 4.5 × 10 12 g-TOC y −1 is accumulating in Amazon shelf sediments. Using δ 13C-POC values at salinities of 0 and 35 × 10 −3 as end-members to isotopically resolve the contribution of terrestrial and marine carbon sources, approximately 3.1 × 10 12 g-TOC y −1 or 69% of the organic carbon accumulating in Amazon shelf surface sediments is from terrestrial sources. This represents 6% of current estimates for Amazon riverine carbon discharge.

A total dissolution method for determination of the α-emitting isotopes of uranium and thorium in deep-sea sediments

A method is described for the determination of specific activities of the natural α-emitting isotopes of uranium and thorium in deep-sea marine sediments. Such information is needed primarily for estimation of sediment accumulation rates. The method is based on total dissolution of sample after fusion with potassium fluoride (to dissolve silicates) and with sodium/potassium pyrosulphate (to dissolve other refractory minerals and convert to the sulphate system). Uranium and thorium fractions are purified for electroplating by anion exchange, with elements naturally present used as carriers. Final determinations are made by α-spectrometry by means of isotopic tracers added at the outset of processing. Experience gained with different types of deep-sea sediment is discussed.

Comparative 210Pb, 137Cs, and pollen geochronologies of sediments from Lakes Ontario and Erie

The distribution of 210Pb, 137Cs, and Ambrosia (ragweed) pollen in two sediment cores from Lake Ontario and in three cores from Lake Erie provides independent estimates of sediment accumulation rates. Geochronology with 210Pb is based on radioactive decay of the isotope following burial in sediments. The method can reveal with precision changes in sedimentation occurring over the past 100 yr or so. Geochronologies with 137Cs and Ambrosia are based on the occurrence of a horizon corresponding, respectively, to the onset of nuclear testing 25 yr ago and to regional forest clearance in the middle 1800s. These methods provide estimates of long-term average sediment accumulation rates. In all but one core, the distributions of 137Cs and 210Pb indicate no physical mixing of near-surface sediments. In two cores, including one from central Lake Erie collected by diver, all three estimates of sedimentation rates are in excellent agreement. In two other cores, rates based on 210Pb are significantly higher than those inferred from Ambrosia pollen profiles. Lower average rates appear to result from occasional massive losses of sediments. Such events, apparent in the distribution of 210Pb but not in pollen records, correlate with the occurrence of major storm surges on the lakes during this century. In one core from western Lake Erie, exponential distributions of both 210Pb and Ambrosia appear to be artifacts which may result from extensive biological or physical reworking of sediments in shallow water (11 m). Previous indications of increased sedimentation in Lake Erie since about 1935 based on Castanea (chestnut) pollen data are not substantiated.