Determination Of Sediment Accumulation Rates Research Articles

Sediment accumulation and mixing in the Penobscot River and estuary, Maine

Mercury (Hg) was discharged in the late 1960s into the Penobscot River by the Holtra-Chem chlor-alkali production facility, which was in operation from 1967 to 2000. To assess the transport and distribution of total Hg, and recovery of the river and estuary system from Hg pollution, physical and radiochemical data were assembled from sediment cores collected from 58 of 72 coring stations sampled in 2009. These stations were located throughout the lower Penobscot River, and included four principal study regions, the Penobscot River (PBR), Mendall Marsh (MM), the Orland River (OR), and the Penobscot estuary (ES). To provide the geochronology required to evaluate sedimentary total Hg profiles, 58 of 72 sediment cores were dated using the atmospheric radionuclide tracers 137Cs, 210Pb, and 239,240Pu. Sediment cores were assessed for depths of mixing, and for the determination of sediment accumulation rates using both geochemical (total Hg) and radiochemical data. At most stations, evidence for significant vertical mixing, derived from profiles of 7Be (where possible) and porosity, was restricted to the upper ~1–3cm. Thus, historic profiles of both total Hg and radionuclides were only minimally distorted, allowing a reconstruction of their depositional history. The pulse input tracers 137Cs and 239,240Pu used to assess sediment accumulation rates agreed well, while the steady state tracer 210Pb exhibited weaker agreement, likely due to irregular lateral sediment inputs.

Application of Plutonium Isotopes to the Sediment Geochronology of Coarse-Grained Sediments from Englebright Lake, California (USA)

The determination of sediment accumulation rates in environments with temporal variations in texture is challenging using traditional radioisotope methods, largely due to low activities associated with coarse sediments. This study used Englebright Lake, an impoundment in northern California, as a model system to examine the application of plutonium isotopes in lacustrine environments where the interlayering of coarse and fine sediments complicates the geochronology. Inductively coupled plasma mass spectrometry was used to quantify plutonium isotopes and low limits of detection allowed for the measurement of plutonium in sand, clay, and silt fractions. Although measurable levels of plutonium were found in sand fractions, over 75 % of the total plutonium activity was found in fine-grain-size fractions (<63 μm). Correlations between cesium-137 and plutonium activities in fine-grained sediments (r = 0.81–0.98, p < 0.005) suggest that plutonium isotopes may be substituted for cesium isotopes in coarse-grained sediments where cesium is typically below detectable levels. Sediment accumulation rates calculated from grain-size normalized plutonium activity profiles ranged from 6 to 145 cm year−1 in Englebright Lake and identified a sediment depocenter at the delta front upstream of Englebright Dam. Progradation of the delta front reflected changes in sediment supply from the watershed in response to flood events, whereas average annual accumulation responded to human impacts. This study extends the application of plutonium isotopes for sediment geochronology to aquatic environments dominated by coarse sediments and provides new information that contributes to a better understanding of the processes influencing sediment deposition in Englebright Lake.

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.

A COMPARISON OF THREE METHODS FOR MEASURING RECENT RATES OF SEDIMENT ACCUMULATION1

ABSTRACT: The determination of sediment accumulation rates is important in understanding how these materials are affecting lakes and reservoirs ecosystems. In this study three methods were used to estimate sediment accumulation rates in the impounded backwater lakes behind Lock and Dam Nos. 8 and 9 on the upper Mississippi River. The three methods were: 1) a “spud” survey, 2) a survey of bottom contours, and 3) the use of fallout cesium‐137. The field use of these three methods of determining sediment accumulation and the potential errors and merits involved in each method are discussed. The results from the field study in backwater areas along the upper Mississippi River showed the survey of bottom contour method gave the lowest rate of sediment deposition and the 137Cs method gave the highest rates. Sediment accumulation rates from 0 to 7.8 cm per year were measured in the study area. All three methods are useful and have unique characteristics for determining rates and patterns of sediment accumulation. Thus the choice of a method to be used in a sediment survey is dependent on the type of information needed and the time available.

Magnetic polarity dating of tectonic events at passive continental margins

Palaeomagnetic studies of continental margin sediments drilled during IPOD Legs 47a (NW African margin), 47b (NW Portugal margin) and 48 (NE Bay of Biscay and SW Rockall margins) have provided magnetic polarity records for the late Neogene, early Palaeogene and Cretaceous periods. The general pattern of geomagnetic field polarity reversals during late Neogene times is now well established, and the absolute age of major polarity transitions during the past 5 Ma or so has been determined from palaeomagnetic studies of radiometrically dated igneous rocks (see, for example, McDougall et al. 1977). Consequently, a comparison of the late Neogene polarity sequences identified in the sediments of NE Biscay and the NW African margins, with this ‘standard’ polarity reversal time scale, allows the precise determination of sediment accumulation rates, and the times at which significant changes in these rates occurred. These comparisons have led to the recognition of a short but significant hiatus in early Pleistocene times in both areas. Such information has importance in evaluating the recent geological evolution of these margins. For earlier geological periods the correlation between the magnetic polarity time scale and the geological time scale is less well established but studies of the type described here can still provide important information on the timing of certain geological events recorded in the magnetic anomaly patterns of the oceanic lithosphere. For example, in the early Palaeogene sediments cored at Sites 403-405, off the SW Rockall continental margin, a sequence of magnetic reversals has been identified, which shows a good correlation with marine magnetic anomalies 22-24. Since anomaly 24 is the oldest recognizable anomaly in the Atlantic, and lies immediately adjacent to the continental margins, the biostratigraphic age of this anomaly, determined from the nannofossil and dinocyst zonal determinations at Site 404, provides important information on the date of initial rifting of Rockall (together with the rest of NW Europe) from Greenland. The Site 404 results indicate that this important tectonic event occurred in early Eocene times, at about 52 Ma B.P., rather than at 60 Ma B.P. as was originally proposed by Heirtzler et al .(1968). A further example of the potential value of this type of study is provided by the Cretaceous sediments cored at Sites 397, 398, 400A and 402A. A long section of predominantly normal polarity sediments at the latter three sites appears to correlate with the long Cretaceous interval of dominantly normal polarity identified in marine magnetic anomaly patterns. The combination of palaeomagnetic and biostratigraphic studies allows useful constraints to be placed on the maximum duration of this interval, and on the age of short reversals within and below it. This information has direct relevance to the interpretation of Mesozoic marine magnetic anomaly patterns in terms of the history of seafloor spreading and evolution of continental margins during the early stages of opening of the South Atlantic in Cretaceous times.