Charles A. Nittrouer

Rates of sediment accumulation and particle reworking based on radiochemical measurements from continental shelf deposits in the East China Sea

Abstract Radiochemical measurements of 234Th (t1/2 = 24days), 137Cs (bomb-produced), and 210Pb (t1/2 = 22y) have been used to characterize rates of mixing, deposition, and accumulation on 100-day and 100-y time scales in East China Sea sediments. In the inner-shelf mud deposit near the mouth of the Changjiang (Yangtze River), 234Th data indicate deposition rates as rapid as 4.4 cm month−1 on a 100-day time scale. 210Pb data indicate that on a 100-y time scale accumulation rates are an order of magnitude slower (typically 1 to 5 cm y−1) than the short-term deposition rates. Most of the sediment deposited near the mouth of the Changjiang on a 100-day time scale is transported southward along the dispersal system on a 100-y time scale, probably as a result of winter storms and a strong coastal current.210Pb accumulation rates from the inner-shelf mud deposit indicate that approximately 40% of the sediment discharged by the Changjiang can be accounted for in the sediments north of 30°N. The offshore mud deposit in the East China Sea is associated with the Huanghe (Yellow River) dispersal system. The dominant process affecting radionuclide profiles within this deposit is particle mixing (not sediment accumulation). In this area the upper 5 cm of the seabed are intensely reworked (Db = 26cm2 y−1) relative to the zone between 5 and 25 cm (Db = 2cm2 y−1). The maximum accumulation rate in the offshore mud deposit is 0.3 cm y−1. Less than 2% of the sediment discharged by the Huanghe can be accounted for in the offshore mud deposit. The relative intensity of mixing and accumulation is different for the proximal deposits of the Changjiang (where accumulation dominates) relative to the distal deposits of the Huanghe (where mixing dominates). Near the Changjiang mouth radiographs show distinct horizontal stratification, and the value of G (mixing rate/accumulation rate) is near zero. In the off-shore mud deposit radiographs reveal nearly homogeneous sedimentary structure, and the G value is ⩾18.

Sediment accumulation in a modern epicontinental-shelf setting: The Yellow Sea

Sediment accumulation in the Yellow Sea epicontinental-shelf environment is investigated on 100-yr and 1000-yr time scales using 210Pb and 14C geochronologies. The distribution of modern (210Pb) accumulation rates in the Yellow Sea reveals that the loci of modern Huanghe sediment accumulation are the topset (1–2 mm/yr), foreset (4–9) mm/yr), and proximal bottomset deposits (2–4 mm/yr) of the Shandong subaqueous delta (which extends south from the Shandong Peninsula). 210Pb rates in the distal bottomset deposits of the subaqueous delta (in the central and southern Yellow Sea) are generally low (0.3–0.9 mm/yr). A sediment budget demonstrates that between 9–15% of the annual Huanghe discharge is accumulating in the Yellow Sea. About two-thirds of this sediment is accumulating in the topset, foreset and proximal bottomset deposits of the Shandong subaqueous delta, with the remaining third accumulating as widespread distal bottomset deposits. 14C age dates of subaqueous delta sediments indicate that the thick (∼40m) clinoform structure formed predominantly between 6200 and 4060 yrs B.P. (at rates approaching 20 mm/yr). Observations in the Yellow Sea, as well as on the Amazon and Ganges-Brahmaputra shelves demonstrate that subaqueous-deltaic stratigraphy is the general rule where major rivers enter energetic shelves, whether epicontinental or pericontinental. The development of extensive bottomset deposits may be restricted to epicontinental-shelf environments and may be diagnostic of sedimentation in this type of setting.

Nature of sediment accumulation on the Amazon continental shelf

Sediment accumulation on the Brazilian continental shelf near the Amazon River is investigated using radiochemical (e.g.210Pb,14C) techniques to provide a better understanding of this major dispersal system of fine-grained sediment.210Pb profiles from 57 cores collected during 1983 reveal the distribution of modern (100-y time scale) accumulation rates on the Amazon subaqueous delta. Accumulation rates increase from 100 y) sediment in the northwestern portion of the subaqueous delta indicates that this sediment was deposited <1000 y ago. The absence of modern sediment in this area is not understood. A sediment budget for the Amazon shelf indicates that 6.3 ± 2.0 × 108 tons of sediment accumulate annually. Much of the remainder of Amazon River sediment (∼6 × 108tons y−1) probably is transported northwestward beyond the Brazilian shelf and/or is accumulating landward of the shelf as coastal accretion.

Transport of particles across continental shelves

Transport of participate material across continental shelves is well demonstrated by the distributions on the seabed and in the water column of geological, chemical, or biological components, whose sources are found farther landward or farther seaward. This paper addresses passive (incapable of swimming) particles and their transport across (not necessarily off) continental shelves during high stands of sea level. Among the general factors that influence across-shelf transport are shelf geometry, latitudinal constraints, and the timescale of interest. Research studies have investigated the physical mechanisms of transport and have made quantitative estimates of mass flux across continental shelves. Important mechanisms include wind-driven flows, internal waves, wave-orbital flows, infragravity phenomena, buoyant plumes, and surf zone processes. Most particulate transport occurs in the portion of the water column closest to the seabed. Therefore physical processes are effective where and when they influence the bottom boundary layer, causing shear stresses sufficient to erode and transport particulate material. Biological and geological processes at the seabed play important roles within the boundary layer. The coupling of hydrodynamic forces from currents and surface gravity waves has a particularly strong influence on across-shelf transport; during storm events, the combined effect can transport particles tens of kilometers seaward. Several important mechanisms can cause bidirectional (seaward and landward) transport, and estimates of the net flux are difficult to obtain. Also, measurements of across-shelf transport are made difficult by the dominance of along-shelf transport. Geological parameters are often the best indicators of net across-shelf transport integrated over time scales longer than a month. For example, fluvially discharged particles with distinct composition commonly accumulate in the midshelf region. Across-shelf transport of particulate material has important implications for basic and applied oceanographic research (e.g., dispersal of planktonic larvae and particle-reactive pollutants). Continued research is needed to understand the salient mechanisms and to monitor them over a range of timescales.

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. 210Pbxs 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 210Pbxs 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% (3×109 kg/yr) of the mean annual supply of fluvial mud (14×109 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.

The formation of sedimentary strata in an allochthonous shelf environment: The Washington continental shelf

Nittrouer, C.A. and Sternberg, R.W., 1981. The formation of sedimentary strata in an allochthonous shelf environment: the Washington continental shelf. Mar. Geol., 42: 201-232. Models of fine-scale ( 120 m) shelf regions. The inner shelf sand and mid-shelf sandy silt are the predominant accretionary deposits on the shelf. Important sedimento-logical observations obtained by boxcoring the upper 1/2 m of these deposits are: (a) progressive decrease in grain size with distance from the Columbia, (b) downward coarsening within the seabed, (c) loss of distinct sedimentary structure (homogenization of sediment) with distance from the Columbia. Non-dimensional parameters based on sediment transport and benthic biological studies can be used to relate rates of sediment mixing (re-working) to rates of accumulation. These parameters have the potential for quantitative prediction of strata formation, from measurements of active processes. Physical and biological mixing can be assumed to remain relatively constant in an alongshelf direction on the Washington shelf, and accumulation rate is known (from Pb-210 geochronology) to decrease away from the Columbia. Together these factors tend to increase the ratio of mixing to accumulation with distance from the Columbia, and predict the sedimentological observations listed above.

The effect of sediment mixing on Pb-210 accumulation rates for the Washington continental shelf

Abstract Nine cores from the Washington continental shelf were examined by radiochemical techniques in order to evaluate the effect of mixing on the calculation of sediment accumulation rates from Pb-210 profiles. Th-234 profiles indicate mixing coefficients of 140 cm 2 yr −1 for the seabed offshore from the Columbia River and 47 cm 2 yr −1 for the seabed of the Mid-Shelf Silt Deposit (75 km north of the Columbia River). These large mixing coefficients demonstrate that particles can penetrate to the base of the intensely mixed surface layer (∼ 10 cm) within one year after emplacement at the seabed surface. Observed depths of Cs-137 penetration within the seabed are compared with depths predicted from the surface mixed-layer thickness and the Pb-210 accumulation rate. The observed and predicted depths agree well at all but one station. At this station a combination of Th-234, Co-60, Pb-210 and Cs-137 profiles suggests that active mixing occurs below the intensely mixed surface layer. The agreement of Pb-210 and Cs-137 data at the other stations, however, indicates a general absence of deep mixing on the Washington shelf. Thus, the apparent accumulation rates calculated from Pb-210 profiles (below the intensely mixed surface layer) on the Washington shelf generally reflect the true rates of sediment accumulation.

STRATAFORM: overview of its design and synthesis of its results

Abstract The strata formed on continental margins record the history of both marine and terrestrial events occurring on Earth. The STRATAFORM Program was created to develop a better understanding of relationships between these events and margin stratigraphy. In order to obtain the maximum information from strata, they are being examined simultaneously on various temporal and spatial scales, and their formation is being studied where events and accumulation are known to be active. The general objectives of STRATAFORM are: to add geological perspective to studies of sedimentary processes, to constrain stratigraphic interpretations better, and to develop numerical models relating processes and stratigraphy. This research is being undertaken on margins off northern California and New Jersey, which provide unique opportunities for investigation of short-term and long-term stratigraphy, respectively. This volume and paper focus upon studies undertaken off northern California, north of Cape Mendocino. This area is tectonically active, with a coastal mountain range, narrow shelf, and input of sediment primarily from the Eel River. Recent estimates indicate that 1 to 3×10 7 t/yr of mud (silt and clay) are supplied to the continental shelf. Sand accumulates on the inner shelf ( 70%) reaches the continental slope (or deeper). During the previous lowstand of sea level, sediment discharge was ∼25% greater, and probably led to hyperpycnal, debris and other flows transporting sediment down the slope. On the Eel shelf, flood and storm events combine to create normally stratified fine-grained layers with basal sand, and much terrestrial organic debris. These layers are modified through time, but some can be buried and preserved. Subsurface flood layers seem to impact observations of acoustic backscatter on the middle shelf, and subsurface fluids (gas? freshwater?) affect electromagnetic measurements on the inner shelf. Budgets of Eel River mud accumulation for the past century indicate ∼20% trapped on the adjacent shelf and ∼20% on the upper slope (150–600 m). The remaining 60% is dispersed to more distal locations. Over Holocene time scales, Eel River sediment has built a small sandy subaqueous delta, and a subtle bulge of muddy sediment on the middle shelf north of the river. Gullies have been eroded on the open slope during lowstands of sea level, and have accreted upward during highstands. A massive failure has formed the Humboldt slide, due to earthquake activity and unstable substrates (possibly from excess pore pressures and gas). The tectonic setting has impacted margin stratigraphy from small scales (fluid-expulsion pockmarks) to large scales (spatial variation of late Cenozoic sedimentation). The broad dispersal of Eel sediment demonstrates the three-dimensional character of the resulting stratigraphy, including the large-scale clinoform structure developed on the shelf/upper slope. The rollover point (topset/foreset boundary) for this feature is the shelf break (i.e., below sea level), which is important information for stratigraphic interpretations of sea-level change.

Rapid and widespread dispersal of flood sediment on the northern California margin

The dispersal of flood sediment from small river systems is a poorly studied, yet potentially important aspect of active continental-margin sedimentation. In January 1995, during a flood with a 30 yr return period, the Eel River (northern California) delivered an estimated 25 ± 3 × 106 t (metric tons) of fine-grained (<62 µm) sediment to the ocean. The flood formed a distinct layer on the sea bed that was centered on the 70 m isobath, extended for 30 km along shelf and 8 km across shelf, and was as thick as 8.5 cm, but contained only 6 × 106 t of sediment. Thus, 75% of the flood-derived sediment did not form a recognizable deposit, but was instead rapidly and widely dispersed over the continental margin. Stratigraphic models of, and compilations of sediment flux to, active continental margins need to take the dispersive nature of small river systems into account.

The deltaic nature of Amazon shelf sedimentation

Despite the annual discharge of more than a billion tons of sediment by the Amazon River, the sedimentary environment near the river mouth has little subaerial expression and thus does not meet the classic definition of a delta. The river mouth, however, is not an estuary, either. These observations raise a major question as to what type of sedimentary environment the Amazon river mouth represents. Seismic stratigraphy has been examined on the continental shelf at the mouth of the Amazon River using high-frequency (3.5-kHz) seismic records from about 6,000 km of ship track. These records demonstrate three regions. (1) The Amazon River has built a subaqueous feature which stretches for hundreds of kilometres offshore and alongshore from its mouth. The feature is prograding seaward and accreting upward, and it contains fine-scale stratification typical of classic deltas. The feature forming at the mouth of the Amazon is a subaqueous delta; it differs from classic deltas primarily in its lack of subaerial expression. Subaqueous deltas, such as the Amazon, represent the general case of a major river entering an energetic oceanic regime.