Recent Sedimentation Rates Research Articles

Influences of Holocene sea level, regional tectonics, and fluvial, gravity and slope currents induced sedimentation on the regional geomorphology of the continental slope off northwestern India

Regional geomorphology derived from echosounding and bathymetric data (lines of a total of 12,000 km), depth to basement, regional deep seated tectonic features, surficial sediment distribution and Holocene sedimentation rates of the continental margin off northwestern India are presented. Results indicate that the shelf edge over most of the region (shelf break between 80 and 154 m water depth) is covered with relict carbonate facies and partially buried bioherms of Holocene age, particularly off Bombay and Saurashtra regions. Significant changes in the depth and orientation of the shelf edge adjacent to Saurashtra and the Bombay High areas in relict carbonate facies are recorded. At locations, shelf edge depth is much more (up to 150 m) than the Early Holocene eustatic sea-level still stand in this region (about 100 m). The upper boundary of the slope also varies considerably in depth: about 1450 m in the vicinity of the Indus Fan having high sedimentation and 2900 m further southwards. The gradient of the slope off the Kachchh is steepest and some regional notches are observed at the upper slope. The gentler slope off Saurashtra and the Bombay High area has large benches at depths of 180–230 m (width 2–10 km) and 650–780 m, well below the Holocene sea level. The Bombay high area has slope breaks between 400 and 600 m, whereas off Saurashtra steep breaks in the slope occur between 560 and 960 m depth. Further southwards, at the foot of the slope, elevations and depressions are present. Variations in the geomorphology of the slope off the northwestern continental margin of India are attributed to: (1) Late Pleistocene-Holocene sea levels, (2) regional tectonics, (3) fluvial sedimentation, and (4) gravity and slope currents induced sediment dynamics. The locations under influences of slumping, turbidity and tectonic movements, with altered in situ sequence of sedimentations, are delineated.

Chemical indicators of sediment chronology

The determination of recent sedimentation rates has traditionally used radiochemical dating techniques but, because of limited laboratory facilities and the costs of these techniques, alternative methods are desirable. Information on sedimentation rates and sediment history can be obtained from anthropogenic chemical components whose presence in sediments, especially near urban areas, can be used as markers to assess their chronology. More than one marker, with a known date of introduction, should be used. In this study, organosilicons first used in the early 1950s and tributyltin used in marine antifouling paints from 1972 were examined in sediments from Lake Macquarie, NSW, to supplement the known input of heavy metals from the lead-zinc smelter that has been operating at the northern end of the lake since 1897. The most reliable data were obtained at sites where all three markers were present, and use of these data allowed independent determination of the year of initial operation of a sewage treatment works.

Holocene sedimentation patterns and bedforms in the wave-current-dominated nearshore waters of eastern Mersin Bay (eastern Mediterranean)

The distribution and bedforms of Holocene sediments in relatively high energy, wave-dominated nearshore waters of the eastern Mersin Bay (eastern Mediterranean) were investigated by high resolution seismic profiles, sidescan sonar records and surface sediment samples. High-resolution seismic reflection profiles show that the sedimentary column consists of two main sequences separated by a mid-reflector (R), showing irregular and erosional surface features, which is interpreted as the Late Pleistocene/Holocene boundary. The upper sequence which is seen as parallel to slightly divergent reflection configurations in a sheet-like to wedge-shaped geometry, approximates to Holocene deposits reaching up to 10 m in thickness about 2 km seaward of the shoreline, with a slight tendency to increase offshore. Seismic data indicate that the unconformity between the late glacial subaerial surface and the overlying post-glacial transgressive deposits occurs at depths of 1–10 m below the seafloor. Holocene sedimentation rates are estimated to be 10–100 cm per 1000 yr. Typical ridge-furrow systems cutting into beachrocks are apparently products of mechanical erosion at very shallow water depths. The lower seismic sequence underlying the reflector R may represent pre-Holocene deposits and is characterized by complex stratified reflection patterns, commonly showing hummocky to mounded and chaotic cut-and-fill facies. Sidescan sonar records indicate prominent features of these nearshore areas such as gravel waves between sand patches and beachrocks: The former are thought to have been produced as a result of the interaction of waves and currents on the sediment surface with an apparently westerly migration direction for sand, whereas the latter were formed during the lowstand of sea level in the Late Holocene. Muddy sand is the dominant sediment type in the nearshore waters of Mersin Bay (in depths less than 15 m) with local sand and gravel patches and beachrock outcrops.

Recent sedimentation rates of Poyang Lake

由于受到长江和入湖河流双重影响,鄱阳湖沉积可分为南部三角洲前缘、中部开阔湖和北部入江水道三个区域。按各区特点,采用了C.J.C.和C.R.S.不同计算模式,分别得出其沉积速率为0.52、0.16和0.34cm/a。湖区不同沉积地貌单元沉积速率的差异,受物源和水动力条件的影响:水下天然堤、漫滩、堤外洼地及开阔湖沉积速率分别为0.80、0.36、0.13和0.17cm/a。此外,三角洲的迅速伸长,河道分汊或改道,流域水土流失及大型水利工程等天然和人为活动也影响到沉积速率的时空变化。;The process of sediment accumulation in Poyang Lake is complex due to spatial and temporal changes in environmental conditions controlled both by inlet rivers and the Changjiang River.The whole lake can be divided into three different parts according to the characteristics of sedimentation processes,i.e.southern, middle and northern parts,the average rates of which are 0.52cm/a,0.16cm/a and 0.34cm/a,respectively.Moreover,the sedimentation rates in different sedimentary geomorphic units are varied with changes of water dynamics and material sources, g.0.80cm/a for levee,0.36cm/a for flood plain,0.13cm/a for small secondary depression and 0.17cm/a for open area.

Uranium in the oceans: Where it goes and why

Uranium is removed from the oceans by diffusion across the sediment-water interface of organic-rich sediments. This pathway is the largest single sink in the global budget of this element. Dissolved uranium is drawn into suboxic sediments along a concentration gradient established by the precipitation of an insoluble phase which forms when U(VI) is reduced to U(IV). This transformation occurs relatively late in the diagenetic sequence, after the microbially mediated dissolution of manganese and iron oxides, and may be induced by the onset of sulfate reduction. Metallo-organics play an important role in the diagenetic behavior of this element as some uranium is released into solution when labile organics are consumed at the sediment-water interface. In contrast, the diagenesis of authigenic Fe- and Mn-oxides exerts negligible influence on the uranium diagenetic cycle. Variations in the uranium concentration of sediment with time are controlled directly by the uranium content of the source material settling from the water column, and indirectly, by the organic content of this material and the sedimentation rate. Since diffusion from seawater influences dramatically the short-term burial rate of uranium, downcore distributions of dissolved and solid uranium can provide an estimate of recent sedimentation rates in rapidly accumulating sediments.

Innovative methods for quantifying poorly known parameters necessary for numerical basin modeling.

Deterministic models, in which mathematical equations are used to predict the occurrence of various phenomena in the subsurface, are widely used today in basin modeling. However, these models suffer from a number of weaknesses, including incorrect or incomplete equations, incorrect constants in the equations, and errors in input variables. Several alternatives to present deterministic basin models are illustrated here by showing their application in describing permeability.Self similarity, the basis of fractal theory, states that many patterns observed on one scale are also present on other scales. Measured permeability data from hand samples of reservoir rocks must be scaled to adequately describe effective formation permeabilities for carrier beds. Self similarity may also be valuable in evaluating the contribution of fracture permeability to fluid flow. A self-similarity approach may be useful even where self similarity is not strictly valid.A new method for predicting the occurrence of overpressuring and the volume of fluid expelled from both normally pressured and overpressured rocks is based on acquisition of empirical data on the maximum sedimentation rates at which normal pressuring can be maintained under well-constrained conditions. The parameters required to use this method (original and present porosity, viscosity of water, compressibility, present thickness, and recent sedimentation rate) can be obtained more accurately from empirical data than the quantities required for calculation of fluid flow from fine-grained rocks using Darcy's law.Tensional fractures can play a dominant role controlling fluid flow in rocks with low matrix permeabilities. The occurrence and intensity of tensile fracturing can be predicted from stress-strain considerations. It may also prove possible to estimate the degree of fracturing by applying self-similarity concepts to fault patterns observed from seismic data, or from empirical data on the occurrence of overpressuring.Great differences exist between human time scales, which govern reservoir engineering, and geologic time scales, which govern fluid movement in basins. Differences that are of acute importance to a reservoir engineer are often of no practical importance for basin modeling. In basin modeling it is most important to specify permeabilities of seals and of fine-grained rocks as they expell water and hydrocarbons. It is also important to determine the minimum permeabilities capable of permitting long-distance lateral fluid flow. Obtaining reliable data for such rocks should be a main objective of research on basin modeling.

Use of organochlorine contaminants to measure sedimentation rates in estuaries: a case study from the Manukau Harbour

Abstract Organochlorine contaminants are widely distributed in sediments of the Manukau Harbour. The first appearance of DDT in cores is a potential stratigraphic marker because it can have appeared only since extensive applications of DDT on New Zealand pastures for grass grub control began in c. 1950. The technique permits calculation of average annual sedimentation rates, but precision is limited by bioturbation. A nett sedimentation rate of about 5 mm yr-1 was determined from measurements of DDT for muddy tidal flats in Drury Creek. This compared well with a rate of 5 mm yr-1 based upon a pollen dating technique. The past widespread use of DDT may enable comparisons to be made of very recent sedimentation rates in other New Zealand estuaries. Other contaminants identified in the Manukau Harbour sediments include dieldrin, lindane, PCB's and chlordane.

Carbonate mud sedimentation on a temperate shelf: Bass Basin, southeastern Australia

Bass Basin (66,000 km 2) is unusual as a carbonate-dominated basin because calcitic carbonate muds are accumulating in relatively shallow (70–85 m) water depths in the central part of the basin (20,000 km 2). The carbonate muds are produced by the primary accumulation and disintegration of nannoplankton, as well as through the biodegradation of skeletal carbonate grains accumulating on the sea floor. The muds are transported to the south of the basin to the end of tidal current transport paths, where they accumulate in the lowest-energy environment available in enclosed Bass Basin, although they are still subject to periodic reworking by storm energy. The basin margins consist of coarser, partially palimpsest, carbonate sands, whereas the central muds overlie a Late Pleistocene disequilibrium surface. In cores these muds are up to 1 m thick and thin away from the centre towards the margins. Bioturbation is important in modifying the textural character of the muds by increasing the grain size as faecal pellets, and therefore creating a sea floor not in equilibrium with modern hydrodynamic conditions. Radiocarbon dates for an early post-glacial marine transgression embayment facies in the basin gave ages of 10,290 ± 250 to 11,660 ± 300 years B.P. An age of 8700 ± 710 years B.P. was obtained for the base of the modern strait facies. These ages and facies thicknesses were used to establish Holocene sedimentation rates of < 12 cm/1000 y for the basin centre, falling to < 6 cm/1000 y towards the margins.

Periodicity of Mesoscale Phanerozoic Sedimentary Cycles and the Role of Milankovitch Orbital Modulation

Data on more than 200 mesoscale sedimentary cycles indicate that Phanerozoic cycle periods are randomly distributed with respect to the four major Milankovitch orbital parameters, except Late Mississippian through Late Pennsylvanian cycles which show positive clustering about the 413,000 yr eccentricity period. Cycle periods, calculated from average thicknesses and accumulation rates, are strongly dependent on the former (correlation coefficient = 0.802), but relatively independent of the latter (correlation coefficient = -0.360). Hence, calculated period is largely a function of average cycle thickness, and Milankovitch-range periods will be preferentially calculated for cycles with average thicknesses between 1 m and 20 m, regardless of actual causes of cyclicity. For individual cyclic sequences, arguments for Milankovitch control based solely on average period are unfounded. Comparison of short-term Holocene sedimentation rates and long-term Phanerozoic accumulation rates for cyclic peritidal and deltaic facies suggests that average cycle deposition occurs in about 1/30 of the time represented by average cycle period. Thus, long, unconstrained intervals of non-deposition predominate in most cratonic and continent-margin cyclic sequences.

Seismic Stratigraphy in the South Cretan Fault Valley System: A Comparison with the Upper Quaternary Gravitative Sedimentation of the Region: ABSTRACT

A synthesis of the sedimentation processes as deduced from the seismic stratigraphy and the deduced facies associations suggests the following. Small fans developed along the northern major fault line of the SCFVS and contains sediment fed directly from Crete through a series of small canyons, most of which trend perpendicular to the coast. However, the main east-west-trending valley transects the mid- and lower fan sectors and contains several intravalley basinal areas, converging toward the main Messara basin. Thus much of the suprafan sediment is reworked and longitudinally transported into the deeper basins. In these deeper intrabasinal and main basinal areas the thickness of the post-Messinian sediments generally exceeds 800 m and in places exceeds 1,500 m. Toward the south the SCFVS receives additional sediment from the Ptolemy Mountains and the Gavdos rise. Cores recovered along the SCFVS contain a remarkable association of sedimentary sequences which are interpreted as deposits transported largely by gravity-induced mass-flow processes. Calculated Holocene sedimentation rates in these cores range from 6 to more than 40 cm/1,000 years, while late Pleistocene rates were greater than 9 cm/1,000 yr. Good agreement exists between the sedimentation rates calculated from cores and observed on the seismic profiles of the Pliocene-Quaternarymore » sediments. This is a strong indication that the depositional mechanisms affecting the nature of the upper Quaternary sediments could be representative of the upper Cenozoic column.« less

Application of 14C-dating to sedimentary geology and climatology: sea-level and climate change during the holocene

AMS 14C dating of small sized fossil shells, wood chips and sedimentary humic matter has been applied to the investigation of paleoclimatic and sea-level changes in the Holocene, using three estuarine and brackish lake sediment cores, drilled in Kawasaki city and Lake Hamanako, Japan. Precise and detailed ages at differing depths clarified large transitions of Holocene sedimentation rates, of two orders of magnitude. The vertical variation in δ 13C and C/N results for sedimentary organic matter, combined with AMS 14C ages, established continuous climatic and sea-level fluctuation patterns through time and indicated the existence of neoglaciation coincident with the marine regression at 7500 to 7000 yr BP.

Recent sediment accumulation in the Skagerrak, Northeastern North Sea

Recent sedimentation and sediment accumulation rates calculated from downcore 210Pb excess activity profiles in 16 boxcores from the Skagerrak show great variations. Highest sedimentation rates, in excess of 0.40 cm·y −1, are found in the northeastern Skagerrak. Intermediate values (∼0.20 cm·y −1) occur both along the southern and northern slopes of the basin. The central basin is characterized by values of ∼0.10 cm·y −1. These sedimentation rates evidence that 28.4 million tons (dry weight) of sediment accumulate annually in the Skagerrak, of which some 59% (16.7 million tons) are finer than 63 μm. The organic carbon content of the bottom sediments is 2.5% of dry weight in the deepest part of the basin where the finest sediment settles, around 2% in the northern and eastern sectors and on average around 1.5% in the southern part of the study area. Downcore deviations from the logarithmic decrease of 210Pb with depth could generally be attributed to biological disturbance causing reworking of the sediment, as shown by ETS measurements and X-radiography. The Skagerrak is evidently the major depository of fine-grained sediments in the North Sea. The role of sea-floor erosion as an additional source of sediment has most probably been highly underestimated in mass budget studies so far.

Sedimentation Rates and Sediment Core Profiles of 238U and 232Th Decay Chain Radionuclides in a Lake Affected by Uranium Mining and Milling

Radionuclide concentration profiles in sediment cores from three deep basins of Quirke Lake, Ontario, showed enrichment in the surficial layers related to the period of uranium mining in the watershed, based on sedimentation rates determined from a milling-related pH decline indicated by fossil diatoms. Recent sedimentation rates are 96 ± 7 to 185 ± 14 g∙m−2∙yr−1. 210Pb profiles could not be used to determine sedimentation rates due to loadings of 226Ra and 210Ph from mining and milling activities. Profiles of 238U and 232Th decay chain radionuclides showed an enrichment of 1–3 orders of magnitude in surficial sediments relative to background in deeper sediments. Radionudide levels in surficial sediments exceeded those reported in lake sediments in uncontaminated systems and, for some radionuclides, approached or equalled levels in sediments contaminated with uranium and radium refining residues. Radionudide activity ratios in the surficial layers of the Quirke Lake cores and in downstream sediments demonstrated the relative mobilities of the elements in the watershed. Net sediment loading rates for radionuclides in the three basins were 130–230 mg∙m−2∙yr−1 for U, 340–4000 Bq∙m−2∙yr−1 for other members of the 238U decay chain, and 210–530 Bq∙m−2∙yr−1 for 232Th and 228Th.

Sediment incorporation in island-arc magmas: Inferences from 10Be

The radioisotope 10Be has been used as a tracer to evaluate subduction and recycling of sediments in island arcs. As a cosmogenic isotope strongly enriched in oceanic sediments, it is especially suitable for monitoring sediment subduction. We report here 10Be results for 106 arc volcanic rocks and 33 basalts from mid-ocean ridges, oceanic islands, continental rifts, and continental flood basalt provinces. The 33 basalts from non-arc environments all contain < 1 × 10 6 atom/gram (g −1) and form a reference or “control group” against which the arc data may be compared. Lavas from the Sunda, Mariana and Halmahera arcs, each with 12 samples, are indistinguishable from the control group. Nine samples from Japan range from 0.1 × 10 6 to 13.5 × 10 6 g −1 of 10Be. Lavas from the Aleutians and Central America (excluding Costa Rica) consistently have 10Be contents greater than those of the control group, extending to 15.3 × 10 6 g −1 for the Aleutians and 24 × 10 6 g −1 for Central America. Both the Japan and Central American arcs show a geographic variation in 10Be abundances; the Aleutian arc does not. Conceptually, four models may be used to explain the incorporation of 10Be in island arcs. Model 1 is the limiting case calculation presented in Brown et al. (1982). Physically, it corresponds to an assumption that all 10Be in the uppermost sediment layers is carried to depth, the 10Be is mechanically decoupled from the sediment pile and is added to the arc magma source region. This model results in the highest calculated 10Be contents in arc lavas. In the more physically reasonable Model 2, the 10Be-rich upper sediments are assumed to mix with the deeper, 10Be-poor, sediments and the sediments are subsequently incorporated into the source region. Model 3 assumes 10Be to be incorporated from sedimentary layers encountered during magma ascent. In Model 4 only sediments contained within grabens in the downgoing slab may be subducted. As yet insufficient data exist to permit conclusive evaluation of these models, but correlations between 10Be contents in arc volcanic rocks and recent sedimentation rates and sediment thickness, measured outboard of the trench, suggest that Model 2 may be the most important mechanism for 10Be incorporation in many island arcs.

Uranium and thorium series isotopes in recent sediments of the Venezuela Basin, Caribbean Sea

Cores from three sedimentary provinces of the Venezuela Basin were analyzed for uranium and thorium series isotopes. Carbonate free 232Th activity was nearly constant with depth and location. Uranium activities, at two sites, correspond to activities found in Amazon River particulates, the major sediment source for the Venezuela Basin. A third site had higher uranium concentrations, possibly due to the incorporation of uraniumrich shelf sediments or in-situ diagenetic processes. 230Th activities ranged from 1.5 to 5 dpm cm −3. These activities suggest recent sedimentation rates of 7.2, 4.6 and 2.9 cm ky −1 at the turbidite, hemipelagic, and pelagic carbonate sites, respectively. These rates are in good agreement with those based on radiocarbon dating and on carbonate stratigraphy. Concurrent variations in the sedimentation rates at the pelagic carbonate and hemipelagic sites are shown to be due primarily to changes in the rate of CaCO 3 delivery or preservation. A large 226Ra maximum was observed at the redox boundaries of the hemipelagic and turbidite cores, associated with a zone rich in iron and manganese. 226Ra diffusion from the pelagic carbonate sediments, calculated from 226Ra deficits in near-surface sediments, is estimated to be 27 dpm m −2 yr −1. No 226Ra deficiency was measured for the turbidite and hemipelagic sediments, indicating 226Ra migration to the metal-rich layers below, or scavenging of dissolved 226Ra by manganese in the mixed layer. 226Ra transport into the deep basin on biogenic particles was estimated to be similar in magnitude to the flux from the pelagic carbonate site. Abyssal fluxes of 226Ra and particle fluxes of 226Ra from surface waters both appear to be an order of magnitude smaller than the total flux required to balance deep-water removal and radioactive decay in the deep water of the basin.

The210Pb and137Cs profiles in sediment cores from Bay of Quinte, Lake Ontario

210Pb,226Ra and137Cs profiles have been measured for sediment cores from three locations in Bay of Quinte, Lake Ontario, Canada. Compaction of the sediment with depth of burial is taken into account in calculating recent sedimentation rates and age profiles at these locations from the excess210Pb profiles. The rates are 1.803+0.592 cm·y−1 (0.254±0.083 g·cm−2·y−1), 0.101±0.017 cm·y−1 (0.036+0.006 g·cm−2·y−1), and 0.795±0.218 cm·y−1 (0.076±0.021 g·cm−2·y−1). The corresponding fluxes of excess210Pb to sediment/water interface are determined to be 1.652, 0.253, and 1.123 pCi·cm−2·y−1, respectively. The anomalies observed in the radionuclide concentration profiles are attributed to physical mixing. It is postulated that the significantly higher flux of210Pb observed at one of the locations is due to inputs from Trent River which joins the Bay near this location.

Recent sedimentation rates and 210Pb fluxes in Georgian Bay and Lake Huron

The 210Pb and 137Cs profiles are determined at five locations in Georgian Bay and Lake Huron. Compaction-corrected recent sedimentation rates are derived for all locations using the 210Pb data. The rates range from 0.13 ± 0.03 mm y −1 (2.2 ± 0.4 mg cm −2 y −1) to 0.78 ± 0.06 mm y −1 (14.5 ± 1.1 mg cm −2 y −1). Except for one location, the rates show marginal to good agreement with those estimated from palynological measurements at four of the five study locations. 137Cs profiles at study locations suggest that the top portions of the sediment cores were satisfactorily retrieved during coring and sectioning operations. However, considerable downward migration of 13Cs is noted at three locations. The fluxes of excess 210Pb vary between 0.06 and 0.27 pCi cm −2 y −1. The average depositional flux is found to be considerably lower than the flux of atmospheric 210Pb. It is postulated that the deposition of atmospheric 210Pb in Lake Huron and Georgian Bay sediments is limited by the supply of scavenging particulates.

Modelling of 210Pb behaviour in the catchment and sediment of Lake Tali Karng, Victoria, and estimation of recent sedimentation rates.

The technique of 210Pb dating of sediments over time scales of up to 120 years is described and illustrated by a study of Lake Tali Karng. Results from the study of a sediment core from the deepest part of the lake shows that the flux of unsupported 210Pb to the sediment is about 145 mBq cm-2 year-1. Over the last 100 years, about 75 cm of sediment has accumulated at an average rate of 150-200 mg cm-2 year-1. Calculations using two models to convert 210Pb activity-depth profiles to age-depth profiles indicate that the sedimentation rate has increased slightly over the last 100 years. It is estimated that about 14% of the 210Pb falling in the catchment reaches the lake sediment, but depending on the assumptions made the possible range is 7-60%.

Continental freeboard, sedimentation rates and growth of continental crust

To understand how the Earth has evolved requires a model for the timing and nature of the formation of the continental crust. Of the two contrasting models, the first and most popular is that the continental crust of the Earth has grown throughout geological time. Models of linear or exponential increasing growth with time1,2 are not currently accepted, and it is now generally held that much of the crust was in place by 2,500 Myr with a particularly active period of crustal growth and differentiation between about 3,200 and 2,500 Myr (refs 3–13). The second model argues that the present mass of the continental crust formed very early in Earth history (>4,000 Myr) and has subsequently been recycled through the mantle in a steady-state fashion such that the mass of the crust has not changed during most of Earth history14–16. Resolving these conflicting views is not simple because interpretation of the isotopic evidence is non-unique. The resolution of this debate lies in the interpretation of the record of continental freeboard17–19 and in the plausibility of large scale crustal recycling through the mantle via sediment subduction. We point out here that although the record of continental freeboard is consistent with either limited growth or no-growth models, recent sedimentation rates appear insufficient to support no-growth models. Note that evidence against a no-growth model is not necessarily evidence against minor sediment subduction.

Holocene sedimentation rates on the hellenic outer ridge: A comparison by 14C and 230Th excess methods

An examination of the most recent Eastern Mediterranean sapropel (S 1 or A)is made by 14C and 230Th excess techniques, to test the observation (Mangini and Dominik, 1979) that sapropels in this area often represent periods of slower than average sedimentation. Two cores from the Hellenic Outer Ridge with long clear-cut sapropel sections were analysed. Together the 230Th excess and 14C data are equivocal and suggest a complex sedimentation process. The 14C ages call for high sedimentation rates in the sapropel sections, while the estimated 230Th excess values in the sapropel sections are relatively high, and would normally be taken to indicate a slower sedimentation rate than in the overlying carbonate ooze. The 14C ages are more direct and ought to be valid for the core sites. From consideration of a model based on water-column supply of 230Th excess, however, it is suggested that the sapropel in the local area as a whole probably accumulated at a rate similar to, or slightly higher than, the carbonate ooze subsequently deposited.