Modern Sedimentation Rates Research Articles

FIRST DATA OF MODERN SEDIMENTATION RATE IN THE SOUTHERN TATAR STRAIT OF THE SEA OF JAPAN

The study of natural archives, such as bottom sediments of lakes and seas, becomes promising for paleoreconstructions due to the conditions of rapid changes of the environment, anthropogenic impact, and insufficient data of instrumental measurements. Based on a radioisotope dating using the non-equilibrium 210Pb, the modern sedimentation rates for the last 150 years in the southern part of the Tatar Strait have been established. According to the two models (CIC and CRS), the sedimentation rate was 0.20–0.22 сm/year.

First Data on Modern Sedimentation Rates in the Southern Part of the Tatar Strait of the Sea of Japan

First Data on Modern Sedimentation Rates in the Southern Part of the Tatar Strait of the Sea of Japan

Multidecadal delay between deglaciation and formation of a proglacial lake sediment record

Basal ages obtained from proglacial lake sediments are often used to constrain deglaciation histories, based on the assumption that fine-grained stratigraphic records start forming immediately after glacier retreat. Here, we test this assumption by studying the onset of sedimentation in Calluqueo Lake, Chilean Patagonia, which progressively deglaciated between the 1940s and the end of the 20th century. Although the glacier-proximal basin has been ice-free for at least three decades, it does not yet contain a fine-grained stratigraphic record, despite the modern sedimentation rate of ∼3 cm yr−1. By comparison, the distal basin contains a fine-grained stratigraphic record starting in 1997 ± 2 CE, i.e., 20–50 years after it was deglaciated. Based on these results, we show that several decades are required for ∼1 m of fine-grained sediments to accumulate between the coarse till material and sufficiently smoothen the uneven lake floor to start forming a fine-grained stratigraphic record. Although the exact timing depends on lake floor morphology and sediment accumulation rates, our results suggest that proglacial lake sediment records lack the first decades of sedimentation. This delay is mostly negligible when using radiocarbon ages from basal sediments to date glacier retreat since it falls within the range of radiocarbon uncertainties, though its importance increases throughout the Holocene. It is however significant for chronologies entirely based on varve-counting. Therefore, our results support the use of basal ages to establish deglaciation chronologies but they call for attention when using them to reconstruct Holocene ice retreat, especially with varve chronologies.

Grain-Size Distribution of Surface Sediments in the Bohai Sea and the Northern Yellow Sea: Sediment Supply and Hydrodynamics

The grain-size distribution of surface sediments in the Bohai Sea (BS) and the northern Yellow Sea (NYS), and its relationship with sediment supply and hydrodynamic environment were investigated based on grain-size compositions of surface sediments and modern sedimentation rates. The results showed that the surface sediments in the BS and the NYS were primarily composed of silty sand and clayey silt with a dominant size of silt. In addition, the Yellow River delivered high amount of water and sediments to the BS, and they are dominated in surface sediments (mainly silt) in the Bohai Bay, the Yellow River mouth, the center of the BS, and the north coast of Shandong Peninsula. The coarse-grained sediments were mainly deposited at the river mouth due to the estuarine filtration and physical sorting. Meanwhile, there was a significant relationship among the modern sedimentation rate, the surface sediment grain size distribution and sediment transport pattern. The areas with coarser surface sediments generally corresponded low sedimentation rates because of strong erosion; whereas the sedimentation rate was relatively high at the place that the surface sediments were fine-grained. Furthermore, the grain-size trend analysis showed that the areas with fine-grained surface sediments such as the mud area in the central BS and the upper Liaodong Bay were the convergent centers of surface sediments, except for the Bohai Bay and the subaqueous Yellow River Delta where offshore sediment transport was evident.

Sediment accumulation and budget in the Bohai Sea, Yellow Sea and East China Sea

Sediment accumulation and budget in the continental margins provides abundant information of source-sink processes from the land to the sea, including weathering, human activities and sedimentary environment. Here we show the distribution of mud areas, modern sedimentation rates and sediment budget in the Bohai Sea, Yellow Sea and East China Sea. Using grain size data of >18,000 surface sediment samples and 210Pb data from 413 sites, we identify five areas with sediments mainly composed of fine-grained fractions (mean grain size >6 φ) and find a relatively high sedimentation rates of >1.5mm/yr in the mud areas. Near the Yellow and Yangtze River deltas sedimentation reaches >95.0mm/yr. Approximately 1185.4×106tons of fine-grained sediment accumulate annually in the mud areas of the east China seas. Atmospheric deposition contributes <2% (18.37×106 tons/yr), while the riverine sediment inputs account for >75% (917×106tons/yr). The remainder comes from all other sources including coastal erosion and resuspension of bottom sediments. In addition, ~45% of the fluvial sediment supply deposits on the subaerial delta, ~40–50% is trapped on the subaqueous delta and shelf, and the remainder <5% escapes the shelf edge. The results will be a strong foundation for understanding of the transport, deposit and preservation of sediment and other relevant material (e.g. carbon and nutrient etc.) of terrestrial materials in the sea.

Hiatuses and condensation: an estimation of time lost on a shallow carbonate platform

AbstractOn shallow carbonate platforms, the sedimentary record is highly fragmentary because low accommodation commonly leads to non‐deposition, erosion, reworking and condensation. Consequently, it is difficult to quantify the time that is actually recorded and to estimate sedimentation rates. The Berriasian platform in the Swiss and French Jura Mountains offers an opportunity to address this problem. Facies evolution through time displays deepening‐shallowing trends of different orders, resulting in a hierarchical stacking of sequences. The sequence‐ and cyclostratigraphic analysis relates these sequences to orbital cycles with durations of 20, 100, and 400 kyr. Many surfaces in the record of shallow‐water carbonates can be interpreted as hiatuses due to non‐deposition in the supratidal realm, reactivation of shoals or scouring by currents. Strongly bioturbated intervals and hardgrounds commonly result from sediment starvation. These same processes and products can be observed on modern carbonate platforms, and the duration of the hiatuses or condensed intervals can be inferred (a few hours to a few hundreds of years). Amalgamations and narrow stacking of such surfaces create sequence‐boundary zones that can be followed over the entire platform. Time lost or condensed in these zones may correspond to hundreds of thousands of years. Time distribution in the studied sections thus is highly irregular: short periods of sedimentation alternated with long periods of non‐deposition, erosion, reworking, and condensation. Furthermore, due to substrate morphology and laterally variable depositional environments, the depositional record of a given time interval varied significantly across the platform. In one example, it can be estimated that – applying modern sedimentation rates – a sedimentary record spanning 800 kyr could have been deposited within 44 kyr if there were no hiatuses or condensations. It becomes clear that, when estimating carbonate production and sediment accumulation rates in ancient records, the shortest possible time spans must be used to minimize the effect of time lost in hiatal surfaces and condensed intervals.

Variability of the 14 C reservoir effects in Lake Tangra Yumco, Central Tibet (China), determined from recent sedimentation rates and dating of plant fossils

Abstract Sediments from lakes provide one of the most important archives for past environmental changes on the Tibetan Plateau. The recent sedimentation rate of modern lakes is widely used as an independent method to calibrate 14 C-derived chronologies because 14 C values are often affected by a reservoir effect. Terrestrial plant residues in lake sediments are believed to be the ideal material for 14 C dating because they normally provide the true ages of the sediments. In this study, we present the spatial and temporal variations in modern sedimentation rates over the past ∼150 years and evaluate the reservoir effects of 14 C ages determined from bulk sediments and plant residues from Lake Tangra Yumco on the central Tibetan Plateau. The results show that ages determined from plant residues are systematically younger than those of the bulk sediments. However, the reservoir effects associated with the bulk sediments are much more constant than those of the plant residues, highlighting the complicated composition of these macro-remains and the fact that they might not be the best dating materials in Tangra Yumco, especially in southern part. A similar reservoir effect of ∼2200 years is observed in the southern and northern parts of Tangra Yumco, based on the dating of modern surface sediments and aquatic plants. This study demonstrates the complexity of the reservoir effect in a closed lake on the Tibetan Plateau, and careful consideration must be paid to the use of different approaches to date different materials in order to establish a reliable chronology.

The Emptiness of the Stratigraphic Record: A Preliminary Evaluation of Missing Time In the Mesaverde Group, Book Cliffs, Utah, U.S.A

Abstract Observations of modern sedimentation rates in nonmarine and shallow-marine clastic environments indicate that deposits formed in comparable settings in the ancient record could accumulate in a fraction of the time that would appear to be available according to conventional chronostratigraphic dating methods. Typically as little as 10% of elapsed time is represented by sediment, the remainder by breaks in sedimentation, many of which are inconspicuous. An examination of the chronostratigraphy and sedimentary history of the Mesaverde Group of the Book Cliffs, Utah, U.S.A., confirms this pattern. Shoreface, deltaic, and nonmarine successions are considered to be particularly fragmentary. Ravinement surfaces may each account for up to 105 years of missing time. In the sedimentary record of shallow-marine and nonmarine deposits stratigraphic continuity is not to be expected, and calculation of time-related issues, such as mass-balance transport rates and the trajectories of shoreline transgression and regression, must take this into account.

Radionuclide dating of recent sediment and the validation of pollen-environment reconstruction in a small watershed reservoir in southeastern China

A 76cm sediment core was collected from the Huangdong Reservoir (southeastern China). The radionuclides 137Cs and 210Pb and sediment grain size analysis were applied to establish the chronology and modern sedimentation rates. The chronology established by the 137Cs time markers was consistent with the Constant Flux Constant Sedimentation (CFCS) model and the known time of dam construction (1960AD) recognized from the curve of sediment grain size at the depth of 54cm. The results show that the sedimentation rate of the Huangdong Reservoir is relatively high, implying that soil erosion and water loss were strong. The sedimentation rate before the construction of the reservoir was approximately 1.5cm/a, which belongs to the fluvial flooding deposit. The rate decreased to approximately 1.0cm/a during the reservoir deposition (since 1960s). Pollen and micro-charcoal analysis were performed to test the reliability of pollen-based environment reconstruction, including vegetation cover, deforestation, planting and fire disasters. The results of pollen and micro-charcoal indicated that the herbs (primarily Gramineae), ferns (such as Dicranopteris) and several trees (mainly Pinus) varied following chorography or historical records. We discovered that the most important indicators for local forest-cover changes are pollen concentration and the AP/NAP value (ratio of arboreal pollen to non-arboreal pollen). Human impact on forests is manifested by a sharp decrease in pollen percentage of trees and absolute pollen concentration. Forest fire events were recorded by the peaks of charcoal concentration and the C/P value (ratio of charcoal concentration to pollen concentration). For example, the massive deforestation and intensive fires during the Great Leap Forward movement and the Cultural Revolution are reflected by pollen and micro-charcoal spectra. This work testifies that radionuclide use as a dating method is applicable in southern China, and the 137Cs method has demonstrated its reliability in the small reservoir setting in spite of low absolute values. The environmental proxies, such as grain size, pollen, and charcoal, are sufficiently sensitive to reveal local ecosystem changes caused by natural disasters or human activity.

Investigation of water depth,water quality and modern sedimentation rate in Mapam Yumco and La'ang Co,Tibet

基于2009年和2010年分别在玛旁雍错和拉昂错开展的2次野外湖泊调查,本文简要报道这2个藏南内流水系湖泊的水深分布、湖水理化特征及湖泊现代沉积速率.测深结果显示玛旁雍错湖盆形状规则,深水区坡度平缓,最大水深为72.6 m;拉昂错北部为较浅的河流冲积扇,通过狭长的水道与开阔的南部湖区相连,南部湖区中心平坦,实测最深处49.03 m;根据水深数据计算的玛旁雍错和拉昂错水量分别为1.462×10¹⁰m³和5.711×10⁹m³.玛旁雍错湖水剖面水质在垂直方向上呈现明显的分层结构,温跃层介于25~35 m之间,温度梯度为0.17℃/m,水体pH、电导率和溶解氧在温跃层具有同步变化特征;拉昂错湖水温跃层与玛旁雍错相比不明显,5~15 m间温度梯度为0.16℃/m,湖水pH与溶解氧呈反相变化,这可能与水生植物呼吸作用有关.2个湖泊湖水的主要离子组成特征除Ca²⁺外基本相同,拉昂错主要离子浓度约为玛旁雍错的3倍,而Ca²⁺则约为玛旁雍错的1/2;利用²¹⁰Pb和¹³⁷Cs方法对玛旁雍错和拉昂错的短岩芯进行了测试,结果表明2个湖泊近代沉积速率分别约为0.31 mm/a和0.65 mm/a.;In this paper,we reported water depth,water quality and modern sedimentation rates of Mapam Yumco and La'ang Co, southern Tibet,based on field investigation conducted in 2009 and 2010,respectively.Bathymetric survey showed a rather regular shape of Mapam Yumco with a maximum depth of 72.6 m and flat in the middle part of the lake.In La'ang Co,there are shallow fluvial fans in the north part and open areas in the south,which are connected by a narrow channel.The deepest part of La'ang Co is 49.03 m which lies in southern basin.The water volumes of the two lakes are 1.462×10¹⁰ m³ and 5.711×10⁹ m³,respectively, according to the calculation based on water depth data.For the physicochemical profile feature of Mapam Yumco,a thermocline existed between 25-35 m with a temperature gradient of 0.17℃/m.Conductivity,pH and dissolved oxygen showed synchronous abrupt changes in the thermocline.In La'ang Co,temperature decreased abruptly between 5-15 m with a gradient of 0.16℃/m.Meanwhile,pH changed to weak acid condition from surface to bottom,while the dissolved oxygen changed oppositely which may caused by respiration of aquatic plants.In Mapam Yumco and La'ang Co,major ionic composition showed similar features except Ca²⁺,major ions in La'ang Co were approximately three times of Mapam Yumco in concentration,while Ca²⁺ was only half of Mapam Yumco.210 Pb and ¹³⁷Cs method was used to determine the modern sedimentation rates of both lakes,the results showed that it was 0.31 mm/a and 0.65 mm/a in Mapam Yumco and La'ang Co,respectively.

Glacimarine sedimentary processes and depositional environments in an embayment fed by West Greenland ice streams

The extent and behaviour of the central West Greenland sector of the Greenland Ice Sheet (GIS) during the Last Glacial Maximum (LGM) and the following deglaciation remains poorly constrained. To investigate deglacial marine environments and sedimentary processes in an area fed by several LGM ice streams we now report findings from a 3.5kHz sub-bottom profiler survey acquired in and around Disko Bay in 2009. Sediment thicknesses and volumes in three glaciated areas, Disko Bay, southern Vaigat and Egedesminde Dyb, are fully constrained for the first time. Acoustic facies types and distributions were also mapped. The largest sediment thicknesses (256m) and widest variation in acoustic character was observed in Disko Bay adjacent to the mouth of Jakobshavns Isfjord. Significant sediment thicknesses were also observed in isolated basins in the Egedesminde Dyb trough and in a trough in southern Vaigat Strait (up to 110m). Given low modern sedimentation rates, these thick sediment packages are inferred to record the retreat of glaciers from the middle shelf through Disko Bay and into both the Jakobshavns and Torssukatak fjord systems in central West Greenland. Our results highlight similarities in deglacial sedimentary processes across the study area, when the margins of retreating glaciers supplied large volumes of sediment to the marine environment from sediment-laden meltwater plumes and melting icebergs. We estimate the sediment flux for the palaeo-Jakobshavn ice stream while the ice margin was stable at a fjord-entrance sill to be 1.6–3.6×107m3a−1. Assuming a flux at the upper end of our range for the Torssukatak fjord system requires the grounded ice margin to be stable for about 450years during retreat. The sediment fluxes we estimate are within the range of previously calculated fluxes for other Arctic and Antarctic ice streams and suggest that large volumes of meltwater and sediment were available at the base of the outlet glaciers during deglaciation, even during periods of quasi-stability. Retreat of the ice margins into the fjord systems is characterised by a rapid transition to ice-distal sedimentation by rainout of iceberg-rafted detritus (IRD) and hemipelagic material; accumulation rates drop to ~2mma−1. This study demonstrates the variability in glacimarine processes and environments on high-latitude continental margins and highlights the importance of local factors, such as bedrock morphology, as well as glacio-dynamic factors, on sedimentation.

Estimation of modern sedimentation rate in Zun-Torei Lake (East Trans-Baikal Region) by 137Cs

On the basis of bottom sediment core dating performed by the technogenic isotope 137Cs, the rate of sedimentation in periodically drying Zun-Torei Lake (water area is up to 300 km2) is estimated at 2.88 mm per year over the last 59 years. This value is in agreement with the data of palinological determinations (3.44 mm per year). High sedimentation rates are attributed to intensive evaporite sedimentation and probable eolian addition of terrigenous material.

Regional heterogeneity and the effects of land use and climate on 20 lakes in the big woods region of Minnesota

Few studies have assessed the relative importance of landscape, land use history, climate, and regional heterogeneity on lake ecosystem processes, despite the likelihood that interactions among these factors must be important for controlling lake dynamics. We used 14 sediment measures from 20 lake cores in a climatically sensitive region of the prairie-forest border in southern Minnesota to (1) assess relationships between modern lake productivity (Carlson’s Trophic State Index [TSI]), modern land-use, catchment, and lake morphometry, and (2) contrast regional responses to climatic transitions from the ‘Medieval Climatic Anomaly’(1000–1350) to the ‘Little Ice Age’ (1350–1800) to ‘Modern’(~1980–1996 AD). TSI was significantly positively correlated with modern sedimentation rate, and accumulation rates of organic matter (OM), biogenic silica (BSi), and total phosphorus (TP). TSI was not significantly correlated with “modern” land cover, catchment, or lake morphometry characteristics, but total organic N(N) was negatively correlated with percent cultivation in the catchment area and negatively correlated with δ15N of bulk organic matter in sediment. Regional, among-lake heterogeneity was high over the past 1,000 years, but Little Ice Age (LIA) cooling appeared to result in an approximately 20% decline in OM, BSi and TP accumulation, while warming and cultural eutrophication of the past 150 years corresponded to a 200–400% increase in accumulation rates as well as an 80% increase in carbonates and a small but significant 10% drop in C/N ratios consistent with greater in-lake productivity. Our results indicate that climate does have regional effects on lake ecosystems but that among-lake variability is high, reflecting the importance of local factors and suggesting a need for (1) more explicit definition of what ‘regional’ means, (2) a focus on degree as well as direction of change, and (3) estimating accumulation rates based on multiple lakes and multiple cores within lakes.

Spatial pattern of modern sedimentation rate of Qinghai Lake and a preliminary estimate of the sediment flux

We investigated the spatiotemporal distribution of 137Cs radioactivity in surface sediment of Qinghai Lake, and developed the spatial pattern of sedimentation rate. The 137Cs inventory is higher near the estuary/bank area, but lower at the central lake. On the other hand, the average 137Cs activity is lower near the estuary/bank area, but higher at the central lake. The mass accumulation rate (MAR) and the fluxes and contents of the terrestrial detritus (e.g. SiO2, Fe2O3, and Ti) are higher near the estuary/bank area, but lower at the central lake. The chemical/biogenic deposits (e.g. the autogenic carbonates) take up higher fractions at the central lake. These suggest that the spatial pattern of recent sedimentation rate is dominated by the deposition of terrestrial detritus. The average MAR (0.0337g·cm−2·a−1) was estimated based on those of different cores of Qinghai Lake and was verified by a Ca mass-balance method. The fluxes of total lake sediments, terrestrial detritus, and direct atmospheric deposits were also estimated.

Modern sediments and sediment accumulation rates on the narrow shelf off central Vietnam, South China Sea

The narrow shelf along the coast of central Vietnam is seasonally supplied by large amounts of sediment from the adjacent mountainous hinterland following monsoonal precipitation. This study examines the fate of these sediments, and their accumulation rates along two transects across the shelf, based on analyses of radionuclides (Pb-210, Cs-137), sediment texture and structure, as well as carbonate content. The inner shelf is covered by sands, and probably serves as bypass zone for fine sediments transported offshore. Sediment characteristics suggest that the transport to the mid and outer shelf is related to flood events. Averaged over the last century, the Pb-210-based mud mass accumulation rates on the mid and outer shelf vary between 0.25 g cm (-2) and 0.56 g cm (-2) year (-1) (corresponding to linear sediment accumulation rates of 0.20-0.47 cm year (-1)). Along with high excess Pb-210 inventories, these high accumulation rates suggest a significant sediment depocentre on the mid shelf. The Pb-210-derived sediment accumulation rates were found to be several times higher than C-14-derived rates previously reported for the Holocene, at the same location on the outer shelf. This is probably due to the incompleteness of the Holocene record, and an overestimation of the modern rate. Another explanation would be increased erosion within the rivers' drainage basins, due to 20th century deforestation. This hypothesis is supported by the difference between recent (less sand, more lithic grains in the sand fraction) and older sediments. In terms of modern sedimentation processes and rates, the central Vietnam shelf, although being a part of a narrow passive continental margin, is similar to active flood-dominated continental margins.

Floodplain Chronology and Vertical Sedimentation Rates Along the Red River, Southern Manitoba*

This paper reports the accelerator mass spectrometry radiocarbon ages and geomorphic context of one bone and 38 charcoal samples collected from 12 floodplain exposures along the Red River, southern Manitoba. The stratigraphically oldest dates range from 3760 to 6710 cal BP and indicate that floodplain deposits of at least mid-Holocene age are preserved along the valley bottom. The average rate of lateral channel migration at the meander adjacent to section Ju30-1-98 is up to 0.08 m a-1 over the past 3850 years. In sections Se04-1-97, Ju30-1-98, Ju23-1-98 and Ju21-2-98, the rates of vertical sedimentation above and below 1 to 3 m depth, range from 0.3 to 0.8 mm a-1 and 1.7 to 3.7 mm a-1, respectively. The decease in sedimentation rates is interpreted to represent a shift from oblique accretion/proximal overbank sedimentation to distal overbank sedimentation. In sections Ju28-1-98, Au05-1-00, Au03-1-98 and Ju21-1-98, which are late Holocene in age, the average rates of vertical sedimentation range from 1.8 to 3.9 mm a-1. A toy plastic pail and two radiocarbon ages from section Au05-1-00 reveal an increase in modern sedimentation rates from 1.6 to 14.3 mm a-1. This increase may have been caused by greater fluvial erosion from the large-scale introduction of European agricultural practices in the 19th century.

Dating of Sediment Layers and Sediment Accumulation Studies along the Western Continental Margin of India: A Review

Modern surface sediments and organic matter/peat in sediment cores along the western continental margin of India have been dated by excess 210Pb and conventional or accelerator mass spectrometry 14C techniques. We have established an extensive database of these radiometric dates and related parameters, and have calculated sedimentation rates. Modern sedimentation rates are higher (1.8-19.0 mm/yr) on the continental margin of northwestern India and decline (0.01-2.6 mm/yr) toward the southwest. This trend is attributed to differences in load-carrying capacity of the rivers and lithology of the adjacent hinterland. Sedimentation rates decrease away from the coast, except for higher sedimentation rates at few places on the mid-slope compared to inner slope regions. The data reveal lateral as well as temporal variations in accumulation rates, which are explained in terms of environment of deposition and paleomonsoon conditions. The onset of intensive monsoonal conditions during the Holocene is reflected in very high sedimentation rates on the inner shelf. Peat and carbonized wood layers in the inner shelf cores are of Early Holocene age and indicate marine transgression. Age inversions in the cores of continental shelf and slope regions are attributed to either sediment slumping or reworking and redeposition of foraminifera shells. This study provides additional information to the worldwide database for continental margin sedimentation, and is useful for understanding Late Quaternary sedimentation records, sea level changes, the Holocene marine transgression event, the role of Last Glacial Maxima (LGM), and climatic changes in the input of terrigenous sediments to continental margins.

Historical pollution trends in coastal environments of India.

Seventeen sediment cores were collected from different coastal ecosystems of Tamil Nadu, India that include coastal lagoon (Pulicat), polluted rivers in Chennai (Adyar and Cooum), Coral reef (Gulf of Mannar) and a perennial river (Tamiraparani). Radiometric dating has been used to determine the modern sedimentation rates in these ecosystems. The Pulicat Lake and the polluted rivers (Adyar and Cooum) yield an average sediment accumulation rate of 12.34 and 7.85 mm yr(-1), respectively. In the Gulf of Mannar coral reef, the sedimentation rate averages 17.37 mm yr(-1), while the rate in Tamiraparani River is 11.00 mm yr(-1). In the Tamiraparani River basin, the deposition rates were an order of magnitude higher when compared to the erosion rates, which may be due to bank erosion and the intense human activity. In general high rates of sedimentation observed in the coastal ecosystems not only reflect the capacity of the coastal regions as sinks for trace metals but also denote increased input of pollutants into the coastal environments in the recent past. The deposition rates of heavy metals--Fe, Mn, Zn, Cu, Cr and Ni in the depth profiles have been computed using sedimentation rates and their distribution is discussed. It can be seen that the mean deposition rates of all the measured elements in the Tamil Nadu coastal ecosystems are high compared with rates determined for the sediments of the deltaic regions of India and the Bay of Bengal.

Investigation of a modern glacimarine sedimentary environment in the fjord Kuannersuit Sulluat, Disko, West Greenland

A 56 cm long sediment core from Kuannersuit Sulluat, a fjord on the island Disko, West Greenland is used to infer modern sedimentation rates and describe the sedimentary environment. The fjord abuts a catchment area dominated by a recently surging glacier (1995–1999). Analyses of the core included magnetic susceptibility, x-radiographs and grain size analysis. From these analyses it is concluded that deposition takes place both through sediment settling from the water column and by gravity flows. Modern sedimentation rates in the fjord appear to be more than 10 times higher than average annual rates (3–5 mm/a) during the Holocene. This is due to changes in melt-water discharge and shorter distance to the sediment source caused by the glacier surge. Episodic gravity flows may result in a deposition of 12 cm sediment and thereby strongly influence long-term average sedimentation rates.

Measuring Modern Sedimentation Rates in Caddo Lake (LA,TX) using 137Cs Depth Profiles

ABSTRACT Modern sedimentation rates were measured by using gamma ray spectroscopy to measure the depth profile of 137Cs activity in sample cores from Caddo Lake. Seven cores were taken from various environments including a fresh water delta, back bays, and midlake channels, and also near inlet bayous and near the dam. The sedimentation rates vary from 0.56 ± 0.03 cm/yr (0.22 ± 0.01 in/yr) in a stream channel, 0.22 ± 0.03 cm/yr (0.09 ± 0.01 in/yr) in the freshwater delta region to unobservable amounts in the back bay and near the dam. Future work will involve more measurements of cores and modeling the data.