Sediments accretion

Compared to N inputs, the internal loss terms of estuarine N-budgets are not as well documented. In general, internal losses include burial of PN in accreting sediment colunms and denitrification in either the water colunm or sediments (see also Devol, this volume). A few nutrient budgets have estimated N-extraction in the harvest of fish and shellfish biomass and fish migration from estuaries. 

Diatoms have been used to look at the history of changing productivity, and hence nutrient inputs, in lake systems. The use of diatoms as palaeoenvironmental indicators in coastal environments is limited because of the preservation problem (Barker et al., 1994) and the paucity of areas which are continuously accreting sediment. Brown (1994) studied a number of sediment cores from the Wash for preservation of diatom material. The biogenic silica content of the sediments decreases with depth (Figure 5.6) which may indicate progressive corrosion with depth and/or increased input of silica to the sediments with time. Gross changes in the type of diatom present in the sediment may support the latter conclusion but the work is of a preliminary nature. Further examination of siliceous palaeoenvironmental indicators in intertidal sediments will be useful. 

Readman JW, Mantoura RFC, Rhead MM (1987) A record of polycyclic aromatic hydrocarbon (PAH) pollution obtained from accreting sediments of the Tamar Estuary, U.K. evidence for non-equilibrium behaviour of PAH. Sci Total Environ 66 73-94. 

Goodbred, S. L., Jr and Kuehl, S. A. (1998). Floodplain process in the Bengal Basin and the storage of Ganges-Brahmaputra River sediment an accretion study using Cs and Pb geochronology. Sediment. Geol. 121,239-258. 

Building new dams or increasing existing dam water-storage capacities would allow more water storage however, as dams block sediment transport, they could affect the Ebro delta if they further impede sediment accretion. On the other hand, they may act as pollutant traps associated with sediments. If so, removal and disposal of these environmental threats need to be considered. 

Raisbeck et al. have reported on the application of the Grenoble cyclotron for the measurement of 10Be in artifically enriched samples [9]. Later experiments have measured 10Be in melted arctic glacier ice cores [10], marine sediments [32] and ocean surface layers [33]. The Yale group, Turekian et al., [11] have measured the 10Be content in magnesium nodules and demonstrated that these nodules accrete at the rate of approximately 4.5 mm/106 years. 

This mode of formation dominates in red clay sediments and produces rather smooth, spherical nodules with lower Mn/Fe ratios than those produced via the diagenetic mode. These nodules tend to be enriched in Fe, Co, and Pb. Their accretion rates are slow—on the order of millimeters per million years and even less in the southwest Pacific. 

Hydrogenous precipitation Direct precipitation or accumulation from seawater. Nodule surfaces undergoing accretion are not in direct contact with sediment. 1-2 8-Mn02 22% 1 30-50... 

Suboxic Diagenesis Metals remobilized from reducing sediments. Upward diffusive transport through pore waters supplies metals to nodule bottoms. Accretion is episodic, occurring only when the depth of the redox boundary rises close to the sediment-water interface. 100-200 Todorokite/Birnessite (low Cu and Ni content) 48% 20-70 60-200... 

The earth was formed by a process of accretion about 4.6 billion years ago. Initially it was a molten mass lacking the gravitational pull to retain its gases at the prevalent elevated temperatures. And yet, within a mere 700 million years of the planet s birth, as calculated from the isotopic record of sediments, cellular life almost certainly existed. What raw materials were available to bring about this amazing turn of events What were the sources of energy used to drive the necessary reactions Where did the important reactions take place Was it in the atmosphere, in the oceans, on dry land, or all three ... 

Esser and Turekian (1988) estimated an accretion rate of extraterrestrial particles in ocean bottom and in varved glacial lake deposit on the basis of osmium isotope systematics and concluded a maximum accretion rate of between 4.9 x 104 and 5.6 x 104 tons/a. The discrepancy between this estimate and those derived from helium can easily be attributed to the difference in the size of the cosmic dust particles under consideration. Cosmic dusts of greater than a few ten micrometers may not be important in the helium inventory of sediments because the larger grains are likely to lose helium due to atmospheric impact heating (e.g., Brownlee, 1985). Stuart et al. (1999) concluded from studies on Antarctic micrometeorites that 50- to 1 OO-qm micrometeorites may contribute about 5% of the total flux of extraterrestrial 3He to terrestrial sediments. Therefore, the helium-based estimate deals only with these smaller particles. 

Patterson, D. B., Farley, K. A. (1998) Extraterrestrial 3He in seafloor sediments Evidence for correlated lOOkyr periodicity in the accretion rate of interplanetary dusts, orbital parameters, and Quaternary climate. Geochim. Cosmochim. Acta, 62, 3669-82. 

Much of the work to date in estuaries and adjacent marsh/shelf environments using 210Pb has been to determine sediment accumulation and accretion rates (Armentano and Woodwell, 1975 Krishnaswami et al., 1980 Church et al., 1981 Kuehl et al., 1982 Olsen et al., 1985 Paez-Osuna and Mandelli, 1985 McKee et al., 1986 Lynch et al., 1989 Bricker-Urso, 1989 Moore, 1992 Smoak et al., 1996 Dellapenna et al., 1998, 2001 Benoit, 2001 Corbett et al., 2003). Lead-210 is considered to be a reliable method for dating sediments deposited over the last 100 to 110 y (Krishnaswami et al., 1971). In the absence of bioturbation/mixing the activity gradient of excess 210Pb in sediments, which is the net result of accumulation and radioactive decay, can provide information on the sedimentation rate of recent sediments. Unfortunately, in many... 

Figure 7.17 Estimated accretion rates of intertidal salt marshes on the coast of Rhode Island (USA). Despite having well-developed exponential decay curves of excess 210Pb in most marsh sediment cores, indicating a relatively a constant rate of sediment accretion, some variations in the log excess 210Pb indicate that accretion may not have been constant through time. R = correlation coefficient, I = inventories of excess 210Pb (dpm cm-2) (Benninger, 1979 Olsen et al., 1985). (Modified from Bricker-Urso et al., 1989.)...

Lead-210 is produced by radioactive decay of 222Rn and can enter estuaries as a dissolved/complexed ion or in association with particles from the ocean, river, and atmosphere. Much of the work to date in estuaries and adjacent marsh/shelf environments using 210Pb has been to determine sediment accumulation and accretion rates. 

Accretion slow addition to land by deposition of water-borne sediment and/or an increase of land along the shores of a body of water, as by alluvial deposit. 

Bricker-Urso, S., Nixon, S.W., Cochran, J.K., Hirschberg, D.J., and Hunt, C. (1989) Accretion rates and sediment accumulation in Rhode Island salt marshes. Estuaries 12, 300-317. 

Delaune, R.D., Patrick, W.H., and Buresh, R.J. (1978) Sedimentation rates determined by 137Cs dating in a rapidly accreting salt marsh. Nature 275, 532-533. 

Delaune, M.L., Reddy, C.N., and Patrick, W.H. (1981) Accumulation of plant nutrients and heavy metals through sedimentation processes and accretion in a Louisiana salt marsh. Estuaries 4, 328-334. 

Hatton, R.S., Delaune, R.D., and Patrick, W.H. (1983) Sedimentation, accretion, and subsidence in marshes of Barataria Basin, Louisiana. Limnol. Oceanogr. 28, 494-502. 

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