234Th Fluxes Research Articles

Climatic influences on the growth rates of Mn crusts during the Late Quaternary

The concentration of 230Th in Mn crust VA13-2 (146°W, 9°25′S, 4830 m) from the Pacific Ocean was measured with high depth resolution (0.02 mm) to a depth of 1.4 mm corresponding to an age of ∼226 ka B.P. The mean growth rate estimated from these results is 6.2 ± 0.4 mm/Ma. However, the exponential curve obtained shows two drastic events in the 230Th concentration record with durations of 41 ± 4 ka and 37 ± 6 ka which are interpreted as growth standstills. The decay-corrected 230Th concentration is inversely correlated with those of Mn, Fe and Co indicating that the 230Th flux into the crust has been constant throughout time and that the observed fluctuations in 230Th are due to dilution by the main components. By applying a constant 230Th flux model, high-resolution growth rates were derived from every sample investigated. A detailed comparison of the growth pattern with the SPECMAP time table reveals that the VA13-2 growth rates tend to be higher during interglacial than during glacial periods. In particular, the observed growth standstills are associated with glacial stages 6 and 8. Further investigation of Mn crust 72 DK 9 taken from shallower water (170°35′W, 20°13′N, 1550 m) tend to confirm that there is a link between the growth rates of hydrogenetic crusts and the climate of the Late Quaternary.

Accumulation rates of surface sediments in the Norwegian-Greenland Sea

Assuming a constant flux of 230Th to the sediment, surface, accumulation and sedimentation rates of surface sediments in the Norwegian-Greenland Sea have been calculated. The accumulation rates range from 0.7 to 4.0 g cm −2 ka −1, and the sedimentation rates range from 0.87 to 6.58 cm ka −1. Applying a simple box model it can be shown that the effects of bioturbation on the calculation are of minor importance for the area studied. The input of biological material indicated by the flux of CaCO 3 is the highest (up to 1.2 g cm −2 ka −1) in areas influenced by the Norwegian Current, whereas in the Southern Greenland Sea the flux of CaCO 3 is one order of magnitude lower. The pattern of the terrigenous input calculated by the difference between bulk sediment and CaCO 3 flux reflects the importance of the Norwegian continent, the Greenland continent and the island Jan Mayen as source areas for terrigenous matter delivered to the Norwegian-Greenland Sea.

Depth profiles of 230Th excess, transition metals and mineralogy of ferromanganese crusts of the Central Indian basin and implications for palaeoceanographic influence on crust genesis

Two ferromanganese encrustations of hydrogenetic origin from water depths well below the CCD (carbonate compensation depth ) from a seamount zone in the Central Indian basin were studied for growth rates using U-Th series isotopes and transition-metal fluxes. The 230Th excess inventories and transition-metal fluxes in depth profiles separated by a few cm on one crust (F-380) show insignificant variations. In another crust (SS-663X) this variation is within a factor of 2 as compared to variations by a factor of 5 in the Pacific crusts. The minor variations in growth rates, 230Th and transition-metal fluxes within a single crust as well as two adjacent crusts suggest that they are due to the contact of the accreting crust surface with a dynamic veneer of sediment in space and time. Nearly uniform Mn/Fe ratios (1–1.6), δ-MnO 2 as the main Mn mineral phase and a smooth exponential decay pattern of 230Th excess and 230Th excess/ 232Th activities with depth indicate that these crusts have not recorded any palaeoceanographic events of the past 0.4 Ma. The interpolated age of these crusts is between 10 and 13 Ma which is comparable to the period of increased carbonate dissolution due to enhanced activity of AABW (Antarctic Bottom Water) currents during the middle Miocene. Probably the middle Miocene oceanographic conditions, which were associated with increased carbonate dissolution, might have initiated the formation of crusts in the Central Indian basin.

Depth profiles of 230Th excess, transition metals and mineralogy of ferromanganese crusts of the Central Indian basin and implications for palaeoceanographic influence on crust genesis

Two ferromanganese encrustations of hydrogenetic origin from water depths well below the CCD (carbonate compensation depth) from a seamount zone in the Central Indian basin were studied for growth rates using UTh series isotopes and transition-metal fluxes. The 230Th excess inventories and transition-metal fluxes in depth profiles separated by a few cm on one crust (F- 380) show insignificant variations. In another crust (SS- 663X) this variation is within a factor of 2 as compared to variations by a factor of 5 in the Pacific crusts. The minor variations in growth rates, 230Th and transition-metal fluxes within a single crust as well as two adjacent crusts suggest that they are due to the contact of the accreting crust surface with a dynamic veneer of sediment in space and time. Nearly uniform Mn/Fe ratios (1–1.6), δ-Mn0 2 as the main Mn mineral phase and a smooth exponential decay pattern of 230Th excess and 230Th excess/ 232Th activities with depth indicate that these crusts have not recorded any palaeoceanographic events of the past ∼ 0.4 Ma. The interpolated age of these crusts is between 10 and 13 Ma which is comparable to the period of increased carbonate dissolution due to enhanced activity of AABW (Antarctic Bottom Water) currents during the middle Miocene. Probably the middle Miocene oceanographic conditions, which were associated with increased carbonate dissolution, might have initiated the formation of crusts in the Central Indian basin.

Using 230Th in Marine Sediments to Reconstruct the Late Quaternary History of Sea Level

We propose a method to determine past sea levels based on the record of 230Th accumulating in seafloor sediments. The decay of uranium in seawater produces this nuclide, which then adsorbs rapidly onto particles settling to the seafloor. Since the concentration of uranium is similar at all depths in the water column, the flux of 230Th to the seafloor at any location is proportional to water column height. Consequently, changes in sea level must cause changes in these fluxes. At shallow depths (<1000 m), fluctuations of sea level cause changes in the expected 230Th flux that are large enough to produce measurable changes in the 230Th content of the sediment. We have developed two approaches to determine past sea levels from the content of 230Th in sediments: (1) in a single core, the ratio of the past and present fluxes of 230Th to a site on the seafloor is a measure of the ratio of the past and present depths of the site below sea level and (2) in two or more cores from a region where the vertical input of sediment is similar at all depths in the water column, the concentrations of 230Th in synchronous sediment samples are linearly related to depth and indicate past sea level as the depth where the concentrations extrapolate to zero. The latter approach requires fewer assumptions and allows sea level to be estimated at discrete points in time. The methodology, assumptions, advantages and disadvantages of the first approach is illustrated using preliminary data from a core recovered from the margin of Little Bahama Bank.

234Th/ 238U disequilibria in the Black Sea

Profiles of total suspended matter, dissolved 238U, and dissolved and particulate 234Th were determined at three stations in the Black Sea during the R.V. Knorr cruise in June 1988. Sediment traps were deployed on a free-floating mooring line to measure vertical fluxes of total mass and 234Th at two deep stations. This paper presents the first data set of 234Th/ 238U disequilibria in anoxic environments. Our dissolved 238U results are consistent with literature data. The dissolved 238U deviates from the predicted conservative mixing activities below the depth of oxygen depletion. Dissolved 234Th deviates from secular equilibrium activities in the euphotic and suboxic zones. The profiles of dissolved 234Th across the oxic-anoxic interface are similar to those of phosphate, indicating the importance of Fe-Mn oxyhydroxides in controlling the thorium distributions. Particulate 234Th is enriched in the euphotic zone and in the upper layer of the suboxic zone. Comparison of the profiles of 234Th, Fe and Mn suggests that the distribution of 234Th is mainly controlled by the Mn rather than the Fe redox cycle. In general, BS3-2 (western basin) has higher total mass and 234Th fluxes than BS3-6 (central basin). The fluxes of 234Th measured by sediment traps deployed at depths from 40 to 150 m, range from 200 to 800 dpm m2 day −1, with a maximum at 60 m. The residence times of dissolved and particulate 234Th and total suspended materials are calculated using the irreversible scavenging model and the sediment trap flux data. The results suggest that, under these conditions, Th is a good tracer for the total mass flux.

Abyssal sediment erosion in the Central Indian Basin: Evidence from radiochemical and radiolarian studies

Radiochemical analyses of three spade cores collected around 78°E between 10°S and 12°S from the abyssal depths in the Central Indian Basin yield an average accumulation rate of 2 mm/ka. The ratios of 230Th flux in the sediments to its production rate in the overlying water column ( Fa Fp ) are extremely low (∼0.2). This fact, and the absence of Collosphaera invaginata (first appearance datum, 150–200 ka), an index radiolarian species of Neogene Radiolarian Zone 1, indicate intense erosion and lateral transport of younger sediments from this region. The radiochemical and radiolarian biostratigraphic evidence for nearly 175 ka of erosion of the chronological record of the sediments in this region has been attributed to the effect of turbulent Antarctic Bottom Water entering the Central Indian Basin through the northern saddles of the Ninety East Ridge.

Thorium 230 profiling in deep‐sea sediments: High‐resolution records of flux and dissolution of carbonate in the equatorial Atlantic during the last 24,000 years

Variations in carbonate flux and dissolution, which occurred in the equatorial Atlantic during the last 24,000 years, have been estimated by a new approach that allows the point‐by‐point determination of paleofluxes to the seafloor. An unprecedented time resolution can thus be obtained which allows sequencing of the relatively rapid events occurring during deglaciation. The method is based on observations that the flux of unsupported 230Th into deep‐sea sediments is nearly independent of the total mass flux and is close to the production rate. Thus excess 230Th activity in sediments can be used as a reference against which fluxes of other sedimentary components can be estimated. The study was conducted at two sites (Ceará Rise; western equatorial Atlantic, and Sierra Leone Rise; eastern equatorial Atlantic) in cores raised from three different depths at each site. From measurements of 230Th and CaCO3, changes in carbonate flux with time and depth were obtained. A rapid increase in carbonate production, starting at the onset of deglaciation, was found in both areas. This event may have important implications for the postglacial increase in atmospheric CO2 by increasing the global carbonate carbon to organic carbon rain ratio and decreasing the alkalinity of surface waters (and possibly the North Atlantic Deep Water). Increased carbonate dissolution occurred in the two regions during deglaciation, followed by a minimum during mid‐Holocene and renewed intensification of dissolution in late Holocene. During the last 16,000 years, carbonate dissolution was consistently more pronounced in the western than in the eastern basin, reflecting the influence of Antarctic Bottom Water in the west. This trend was reversed during stage 2, possibly due to the accumulation of metabolic CO2 below the level of the Romanche Fracture Zone in the eastern basin.

Chemistry of uranium, thorium, and radium isotopes in the Ganga-Brahmaputra river system: Weathering processes and fluxes to the Bay of Bengal

The most comprehensive data set on uranium, thorium, and radium isotopes in the Ganga-Brahmaputra, one of the major river systems of the world, is reported here. The dissolved 238U concentration in these river waters ranges between 0.44 and 8.32 μ/1, and it exhibits a positive correlation with major cations (Na + K + Mg + Ca). The 238U ∑Cations ratio in waters is very similar to that measured in the suspended sediments, indicating congruent weathering of uranium and major cations. The regional variations observed in the [ 234U 238U ] activity ratio are consistent with the lithology of the drainage basins. The lowland tributaries (Chambal, Betwa, Ken, and Son), draining through the igneous and metamorphic rocks of the Deccan Traps and the Vindhyan-Bundelkhand Plateau, have [ 234U 238U ] ratio in the range 1.16 to 1.84. This range is significantly higher than the near equilibrium ratio (~1.05) observed in the highland rivers which drain through sedimentary terrains. The dissolved 226Ra concentration ranges between 0.03 and 0.22 dpm/1. The striking feature of the radium isotopes data is the distinct difference in the 228Ra and 226Ra abundances between the highland and lowland rivers. The lowland waters are enriched in 228Ra while the highland waters contain more 226Ra. This difference mainly results from the differences in their weathering regimes. The discharge-weighted mean concentration of dissolved 238U in the Ganga (at Patna) and in the Brahmaputra (at Goalpara) are 1.81 and 0.63 μ/1, respectively. The Ganga-Brahmaputra river system constitutes the major source of dissolved uranium to the Bay of Bengal. These rivers transport annually about 1000 tons of uranium to their estuaries, about 10% of the estimated global supply of dissolved uranium to the oceans via rivers. The transport of uranium by these rivers far exceeds that of the Amazon, although their water discharge is only about 20% of that of the Amazon. The high intensity of weathering of uranium in the Ganga-Brahmapura River system can also be deduced from the [ 232Th 238U ] and [ 230Th 238U ] activity ratios measured in the suspended sediments. 230Th is enriched by about 19% in the suspended sediments relative to its parent 238U. The flux of excess 230Th supplied to the Bay of Bengal via these river sediments is 980 × 10 12 dpm/a, about six times more than its in situ production from seawater in the entire Bay of Bengal.

Response of Th/U in deep Labrador Sea sediments (ODP Site 646 to changes in sedimentation rates and paleoproductivities

Research Article| February 01, 1990 Response of Th/U in deep Labrador Sea sediments (ODP Site 646 to changes in sedimentation rates and paleoproductivities C. Hillaire-Marcel; C. Hillaire-Marcel 1GEOTOP, Université du Québec à Montréal, BP 8888, succursale A, Montréal, Québec H3C 3P8, Canada Search for other works by this author on: GSW Google Scholar A. Aksu; A. Aksu 2Department of Earth Sciences, Memorial University of Newfoundland, St. John's, Newfoundland A1 B 3X5, Canada Search for other works by this author on: GSW Google Scholar C. Causse; C. Causse 3Laboratoire de Géologie du Quaternaire, CNRS Luminy, case 907, 13288 Marseille Cedex 9, France Search for other works by this author on: GSW Google Scholar A.de Vernal; A.de Vernal 1GEOTOP, Université du Québec à Montréal, BP 8888, succursale A, Montréal, Québec H3C 3P8, Canada Search for other works by this author on: GSW Google Scholar B. Ghaleb B. Ghaleb 1GEOTOP, Université du Québec à Montréal, BP 8888, succursale A, Montréal, Québec H3C 3P8, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information C. Hillaire-Marcel 1GEOTOP, Université du Québec à Montréal, BP 8888, succursale A, Montréal, Québec H3C 3P8, Canada A. Aksu 2Department of Earth Sciences, Memorial University of Newfoundland, St. John's, Newfoundland A1 B 3X5, Canada C. Causse 3Laboratoire de Géologie du Quaternaire, CNRS Luminy, case 907, 13288 Marseille Cedex 9, France A.de Vernal 1GEOTOP, Université du Québec à Montréal, BP 8888, succursale A, Montréal, Québec H3C 3P8, Canada B. Ghaleb 1GEOTOP, Université du Québec à Montréal, BP 8888, succursale A, Montréal, Québec H3C 3P8, Canada Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1990) 18 (2): 162–165. https://doi.org/10.1130/0091-7613(1990)018<0162:ROTUID>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation C. Hillaire-Marcel, A. Aksu, C. Causse, A.de Vernal, B. Ghaleb; Response of Th/U in deep Labrador Sea sediments (ODP Site 646 to changes in sedimentation rates and paleoproductivities. Geology 1990;; 18 (2): 162–165. doi: https://doi.org/10.1130/0091-7613(1990)018<0162:ROTUID>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Labrador Sea is a particularly suitable high-latitude basin for investigating U and Th behavior in deep-sea sediments. During the late Quaternary, the cyclic development and decay of huge ice sheets on adjacent land masses resulted in large-amplitude changes in sedimentation rates and organic paleoproductivities. The resulting magnification of U and Th response is well illustrated by high-resolution studies on piston-cored sediments from the Greenland continental rise at Ocean Drilling Program Leg 105 Site 646 spanning isotopic stages 8 to 1. Our results show a clear positive correlation of 238U/232Th ratios with organic paleoproductivity indicators (e.g., dinocyst) due to U uptake in the water column and/or during the early early diagenesis of organic matter responding to carbon fluxes and to their climate forcing. 230Th excesses over 234U exceed the theoretical value of the 230Th rain from the overlying water column, indicating lateral input possibly from the Greenland slope and shelf. Because these horizontal fluxes of 230Th may be partly controlled by physical parameters, 230Th excesses cannot be unequivocally correlated with sedimentation rates and/or productivity as reported elsewhere. In this subarctic basin characterized by low overall organic carbon burial, the 238U/232Th ratio appears to be a sensible geochemical indicator of organic activity and paleoproductivity. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Lateral transport of sediment particles in the Okinawa Trough determined by natural radionuclides.

Settling particles at 640 m, 1390 m, and 1920 m depths in the Okinawa Trough and bottom sediments at six sites in the East China Sea were analyzed for U and Th isotopes, 210Pb, and 210Po. Concentrations of 212Th in the settling particles were higher than those observed in the open ocean and were relatively constant with depth compared with other radiogenic nuclides. Fluxes and concentrations of these radiogenic nuclides increased with depth and the measured fluxes were larger than those calculated from water column inventories. While sinking through the water column of 640-1390 m, the increases in 210Pb and unsupported 228Th fluxes originating intensively in the shelf region were much larger than the unsupported 230Th fluxes produced evenly in sea water. The 210Po/210Pb activity ratio in the settling particles was almost unity and the 228Th/230Th activity ratio of the radiogenic fraction was much smaller than the ratio of the production rates of these nuclides in the shelf water. These results suggest that a large fraction of the deeper settling particles, as well as the sediments in the trough zone originating in the continental shelf zone, are transported laterally and downward via the near bottom layer on a time scale of approximately 16 years on the average.

Nutrient assimilation, export production and 234Th scavenging in the eastern equatorial Pacific

Primary productivity, nitrogen uptake using 15N-labeled NH 4 + and NO 3 −, and the distribution and vertical fluxes of POC, PON, chlorophyll, phaeopigments and 234Th have been analysed at four stations in the eastern equatorial Pacific. Two stations were north and two stations were south of the equatorial front. Most analyses show little variation across the front, which is a boundary between cold, nutrient-rich surface water to the south and warm, nutrient-poor surface water to the north. New production and the f- ratio has been calculated using several approaches, including nitrogen uptake rates, export fluxes of POC and PON and the residence time of 234Th. At three of the four stations these independent estimates agree to within a factor of 2. All estimates of the f- ratio (0.1–0.3) are lower than would be predicted from published fvs NO 3 − concentration or fvs primary production curves. The ammonia concentration may be the controlling factor in this case. The residence time of 234Th is about a factor of three less than the residence time of carbon but agrees well with the residence time of total suspended matter. The residence time of nitrogen is about 50% longer than that of carbon.

High resolution 10Be and 230Th profiles in DSDP site 580

10Be and 230Th profiles were measured at Site 580 at a depth of 20 m ( 230Th and 10Be) and 80 m ( 10Be), corresponding to 360 ka and 1.5 Ma, respectively, with a resolution of approx. 5000 a. The radiometric sediment accumulation rates of 6.2 cm ka ( ± 25%) agree well with the average for the last 730 ka (via paleomagnetic stratigraphy). Age corrected concentrations of 10Be range from 2 to 7 × 10 9 atoms g (average 3.5 ± 1). The variations of the 10Be concentrations can be explained by changes in the sediment supply during different climatic conditions. The maxima and the minima of 10Be follow the fluctuations of excess 230Th in the core section during the last 360 ka. Fluxes of both 10Be and 230Th exceed production and vary remarkably throughout time suggesting enhanced scavenging by bioproductivity. At the Brunhess-Matuyama boundary we observe a maximum of 10Be (6.0 ± 0.3.10 9 atoms g , corresponding to 2.5 σ deviation from the average value). However, the observed large fluctuations of Be-10 throughout the core profile make it difficult to interprete this particular maximum.

Budgets and behaviors of uranium and thorium series isotopes in Santa Monica Basin sediments

Nine natural decay-series isotopes were measured in six box cores collected from a transect across the Santa Monica Basin. The 210Pb-derived sedimentation rate decreases from ~80 mg/cm 2-yr at the slope to ~20 mg/cm 2-yr in the deep central basin. Sediment mixing prevails in sites underlying oxic waters, but is subdued in the anoxic deep basin below the sill depth. Uranium contents in sediments are controlled by levels of authigenic U, which are higher in the more reduced condition in the deep basin. Most of the authigenic U results from precipitation within the sediments. The 232Th- 228Th disequilibrium in sediments indicates that 228Ra is lost from the sediments from a depth of ~ 10 cm upward. Modelling the distribution of excess 228Th and 234Th in the surficial layers of the deep basin sediments results in a mean sediment mixing coefficient of 0.2 cm 2/yr and a sedimentation rate close to that based on 210Pb. There is no evidence of changing sedimentation rate in the central basin during the past century. Fluxes of excess 210Pb, 230Th and 231Pa to the central Santa Monica Basin sediments are much higher than what can be predicted from local supply. Advective input of open ocean waters coupled with enhanced scavenging of these reactive nuclides at the ocean margin is considered to be the primary cause.

The distribution and behaviour of 230Th and 231Pa at an ocean margin, Baja California, Mexico

Uranium, Th and Pa isotopes were measured in sediments collected by box core along a transect normal to the Baja California continental margin. Six cores were analysed, ranging in depositional environment from manganese nodule-bearing, pelagic, red clay to hemipelagic sediments displaying Mn reduction in the upper 5 cm. In the hemipelagic cores, solid-phase Mn peaks due to diagenetic remobilisation occur within the upper layers which are well mixed with respect to unsupported 230Th and 231Pa. The fact that 230Th and 231Pa are not concentrated at the Mn peaks suggests that little mobility of these nuclides occurs within the sediment column. Unsupported 230Th 231Pa activity ratios in the biomixed layer of the sediments range from 10.9 to 6.6, generally decreasing towards the inshore stations. The flux of 230Th and 231Pa across the sediment/ water interface is almost in balance with the theoretical water column supply in the most distal core, but rapidly increases inshore. This evidence from the sediment column confirms that enhanced 230Th and 231Pa removal occurs at ocean margins, and that 231Pa is removed from the water column to the sediments in preference to 230Th.

Fluxes of234Th,210Po and210Pb determined by sediment trap experiments in pelagic oceans

Sediment trap experiments were carried out in two oceans, the eastern Pacific Ocean and the Antarctic Ocean, which have very different biological productivities. The natural radionuclides,234Th,210Po and210Pb were used as tracers of reactive metals. Larger particulate fluxes of these radionuclides were found in the seas where total mass fluxes were larger, although the concentrations of these radionuclides in the settling particles were somewhat smaller. The concentrations of234Th in the settling particles varied widely and irregularly with depth, whereas the concentrations of210Po and210Pb in the settling particles steadily increased with increasing water depth. The ratios of210Po/210Pb in the settling particulates were larger than unity which the ratio of234Th/excess210Po as larger than234Th/210Po in the deep water. These results suggest that, when the particles sink through the water column, these radionuclides are being absorbed by settling particles in the order234Th>210Po>210Pb. The observed particulate fluxes of210Pb are about one eighth of those calculated from the disequilibria between226Ra and210Pb at the stations in the subtropical eastern Pacific, although the observed fluxes are the same as the calculated ones in the northern North Pacific and the Antarctic Ocean. Thus, there must be a horizontal flow carrying these reactive metals from the oligotorophic ocean to the biologically productive ocean where the metals are removed by settling particles even in deep water.

Non-destructive alpha-spectrometry and radiochemical studies of deep-sea manganese nodules

A deep-sea Mn nodule handpicked from top of a 20 cm diameter North Pacific tripod core was directly counted by an α-spectrometer with a silicon surface barrier detector. A distinct 210Po peak was recorded for the nodule upper side in contact with bottom seawater. Based on the spectrum, the 210Po (supported by 210Pb with 22 year half life) is thought to be confined in the top few microns of the nodule. Since the 210Po peak diminishes considerably for the bottom side resting on sediment, nondestructive α-spectrometry is useful for determining the sea floor orientation of nodules. This method was tested for a western North Pacific dredged Mn nodule, and was successful in identifying the orientation (and also surface loss of the nodule by dredging).Radiochemical analysis was also carried out for carefully scraped samples from the top and bottom of the two nodules. The growth rates of the hand-picked nodule based on excess 230Th and 231Pa profiles were 1 to 4 mm/106 y and apparently varied with time. The dredged sample has grown at 2 to 6 mm/ 106 y and the growth has likely been episodic. The radionuclide composition was significantly different between the top and bottom. Evidence was found that the radionuclide fluxes of 230Th and 231Pa into the nodules considerably changed during the growing histories. The 230Th/232Th activity ratios in the nodule tops are comparable with those in bottom seawater suggesting that the top is growing with metals of seawater origin. However it remains unresolved whether the nodule bottoms are growing by utilizing metals in the sediment pore water.

Seasonality in the flux of natural radionuclides and plutonium in the deep Sargasso Sea

A record of radionuclide fluxes at a deep-ocean station near Bermuda (32°05′N, 64°15′W) was obtained from analysis of a 3-year collection of sediment-trap samples. The trap was placed at a depth of 3200 m, 1000 m above the sea floor, and the samples were recovered at 2-month intervals. Concentrations of 238U, 234U, 232Th, 230Th, 228Th, 231Pa, 210Po, and 239, 240Pu were measured in the trapped material. Most of the radionuclide activity was found in the <37- gmgm sieved fraction. All of the radionuclide fluxes showed seasonal variations that were in phase with the variations in total sediment flux, which were shown in earlier work to be closely tied to the annual cycle of primary production in the overlying surface water. The seasonal variations are especially noteworthy for 230Th and 231Pa, considering that most of their production occurs in the water column below the euphotic zone. Evidently the seasonal influence is transmitted downward by the varying particle flux so that radionuclide scavenging rates at depth, as well as at the surface, are affected. It is suggested that this could be brought about by seasonal variations in the flux of marine snow or in the rate of fecal-matter production in the deep-water column. Fluxes of 230Th and 231Pa integrated over the annual cycle yielded a trapping efficiency of 105 ± 17% for the PARFLUX sediment trap used in this investigation.

10Be and Th isotopes in manganese nodules and adjacent sediments: Nodule growth histories and nuclide behavior

The concentration profiles of 9Be, 10Be, 230Th, 232Th, 231Pa (via 227Th) and 238U have been measured in three manganese nodules, one each from the North Pacific (A47-16(4)), the South Pacific (TF-5) and the Indian Ocean (R/V Vitiaz). In addition the 10Be concentration in deep water from the GEOSECS reoccupation station 500 of the North Pacific, and in ☐ cores raised from the manganese nodule field in the North Pacific have been measured. The 10Be concentration in nodule and seawater samples was measured by the accelerator masss spectrometric technique employing the Yale Tandem Van de Graaff accelerator. The concentrations of 10Be, 230Th exc and 231Pa exc and ratios of 10Be/ 9Be and 230Th exc/ 232Th all decrease with depth in the nodules. This decrease, interpreted in terms of nodule growth, yields “average” growth rates of a few millimeters per million years for the nodules. The growth rates of the nodules exhibit temporal variations, both on short time (∼ 50,000 years) and long time (several million years) scales. Of the three nodules studied, only in TF-5 is the short-term average growth rate based on 230Th exc in the top 0–0.5 mm the same as the long-term average rate based on Be isotope data for the 0.5–17 mm interval. For the other two nodules, the recent average growth rates based on 230Th exc data differ significantly from the long-term average growth rates based on Be isotopes. In A47-16(4) the 10Be based rate is less than the 230Th exc rate and in R/V Vitiaz the 10Be based rate is greater than the 230Th exc rate. This observation, coupled with measurable changes in growth rates even during the past few hundred thousand years, suggests, but does not prove, that the discordant growth rates deduced from 230Th exc and 10Be profiles document changes in nodule growth rate with time rather than mixing effects on 230Th exc profiles. The 10Be concentration in the GEOSECS North Pacific deep water is 6100±1200 atoms/g. This value coupled with the average surface 10Be/ 9Be ratio of North Pacific nodules predicts a 9Be concentration within the limits of measured values. The inventory of 10Be and 230Th exc in the nodules is only ∼ 10% of the total, the remaining being in sediments. The 10Be concentrations in the upper portions of two adjacent cores studied are nearly the same, but the deposition fluxes of both 10Be and 230Th based on 230Th dating vary by a factor of two. This difference is attributable to local redistribution of sediment at the time of deposition prior to accumulation.

Fluxes of uranium and thorium series isotopes in the Santa Barbara Basin

Samples from the MANOP Santa Barbara Basin sediment trap intercomparison were analyzed for the isotopes of uranium, thorium, radium, lead, and polonium. All of the traps showed approximately the same compositions and isotopic ratios, indicating that they trapped similar materials. The 234Th flux via falling particles was very close to the flux predicted from the production and scavenging rates of 234Th from the water column. The 210Pb content of the trapped particles and the surface sediments were the same, however, the measured flux of 210Pb was seven times greater than the predicted flux. Predicted and measured fluxes of 228Th and 210Po were similarly out of balance. To explain this apparent inconsistency, we suggest (as others have done) that the Santa Barbara Basin is an area where scavenging from the water column is intensified and where sediments deposited initially on the margins may be physically remobilized on a short time scale. These two effects increase the apparent area from which the basin derives the longer-lived isotopes but does not increase significantly the supply of the short-lived 234Th.