Paleomagnetic results from lakes Victoria and albert,Uganda

Summary Unfortunately most of the cores taken from Salisbury Channel, Lake Victoria which contained discrete pieces of wood suitable for¹⁴C dating were organic-rich and therefore showed a considerable scattering of the remanent magnetic direction plots resulting in poorly defined oscillations. However, the cores from Pilkington Bay as well as core C5 from Salisbury Channel, Lake Victoria and the cores from Lake Albert show well-defined paleodeclination and paleoinclination logs. Several cores from Pilkington Bay and Lake Albert with some¹⁴C control can be used to verify the isochronous correlation of the paleodeclination and paleoinclination oscillations. Core C5, from Salisbury Channel, Lake Victoria, which did not contain any discrete pieces of plant or shell material has been dated by correlation with other cores which had¹⁴C control. This core which shows the best-defined paleodeclination and paleoinclination logs of this study would appear to have bottomed in sediments in excess of 6000 years BP age and with the work of Maley et al. (1990) from the Cameroons provides a complete record of the Holocene for Central Africa.     

The late Wisconsin flood into the Gulf of Mexico

¹⁸O/¹⁶O and¹⁴C analysis of deep-sea cores from the Gulf of Mexico revealed a 2.4% isotopic anomaly between 12,000 and 11,000 years ago. This value, together with estimates of the oxygen isotopic composition of ice meltwater and rates of mixing within the Gulf of Mexico indicates that the average yearly discharge of the Mississippi River was between 100,000 and 230,000 m³/s (as compared to 57,000 m³/s for the peak flood of February 17, 1937), with probable yearly peak floods twice as large. Corresponding sea-level rise was between 1 and 2 m per century.     

Variability of natural and bomb-produced radionuclide distributions in abyssal red clay sediments

The nature of sedimentation and mixing are examined in abyssal red clay sediments from the North Central Pacific using three types of indicators: ²³⁰Th/²³²Th, organic¹⁴C, and¹³⁷Cs and ^239.240Pu.²³⁰Th/²³²Th analysed revealed that the clay sedimentation rate in three box cores collected within a 50 km radius was less than 1.0 mm/10³ yr. However, analyses of the organic carbon in thin layers of sediment revealed that radiocarbon was present much deeper in the cores (down to 20 cm) than was expected from the ²³⁰Th/²³²Th distribution. In addition, both the stratigraphy and inventory of radiocarbon was significantly different between box cores. The distributions and inventories of¹³⁷Cs and^239.240Pu were similar to that found for radiocarbon, further illustrating the spatial variability of radionuclides in oligotrophic North Pacific red clays. These data suggest that bioturbational processes are important for transporting organic carbon down into the sediment column.     

Tephrostratigraphy of the last 170 ka in sedimentary successions from the Adriatic Sea

In this study are discussed new SEM-EDS analyses performed on glass shards from five cores collected in the Central Adriatic Sea and two cores recovered from the South Adriatic Sea. A total of 26 tephra layers have been characterized and compared with the geochemical features of terrestrial deposits and other tephra archives in the area (South Adriatic Sea and Lago Grande di Monticchio, Vulture volcano). The compositions are compatible with either a Campanian or a Roman provenance. The cores, located on the Central Adriatic inner and outer shelf, recorded tephra referred to explosive events described in the literature: AP3 (sub-Plinian activity of the Somma-Vesuvius, 2710 ± 60 ¹⁴C years BP); Avellino eruption (Somma–Vesuvius, 3548 ± 129 ¹⁴C years BP); Agnano Monte Spina (Phlegrean Fields, 4100 ± 400 years BP); Mercato eruption (Somma–Vesuvius, 8010 ± 35 ¹⁴C years BP; Agnano Pomici Principali eruption (Phlegrean Fields, 10,320 ± 50 ¹⁴C years BP); Neapolitan Yellow Tuff (Phlegrean Fields, 12,100 ± 170 ¹⁴C years BP). Some of these layers were also observed in the South Adriatic core IN68-9 in addition to younger (AP2, sub-Plinian eruption, Somma–Vesuvius, 3225 ± 140 ¹⁴C years BP), and older layers (Pomici di Base eruption, Somma–Vesuvius, 18,300 ± 150 ¹⁴C years BP). Significant is the tephra record of core RF95-7 that, for the first time in the Adriatic Sea, reports the occurrence of tephra layers older than 60 ka: the well known Mediterranean tephra layers X2 (ca. 70 ka), W1 (ca. 140 ka) and V2 (Roman origin, ca. 170 ka) as well as other tephra layers attributed, on the basis of geochemistry and biostratigraphy, to explosive eruptions occurred at Vico (138 ± 2 and 151 ± 3 ka BP) and Ischia (147–140 ka BP).     

230Th,226Ra and222Rn in abyssal sediments

A model that predicts the flux of²²²Rn out of deep-sea sediment is presented. The radon is ultimately generated by²³⁰Th which is stripped from the overlying water into the sediment. Data from many authors are compared with the model predictions. It is shown that the continental contribution of ionium is not significant, and that at low sedimentation rates, biological mixing and erosional processes strongly affect the surface concentration of the ionium. Two cores from areas of slow sediment accumulation, one from a manganese nodule region of the central Pacific and one from the Rio Grande Rise in the Atlantic were analyzed at closely spaced intervals for²³⁰Th,²²⁶Ra, and²¹⁰Pb. The Pacific core displayed evidence of biological mixing down to 12 cm and had a sedimentation rate of only 0.04 cm/kyr. The Atlantic core seemed to be mixed to 8 cm and had a sedimentation rate of 0.07 cm/kyr. Both cores had less total excess²³⁰Th than predicted.Radium sediment profiles are generated from the²³⁰Th model. Adsorbed, dissolved, and solid-phase radium is considered. According to the model, diffusional losses of radium are especially important at low sedimentation rates. Any particulate, or excess radium input is ignored in this model. The model fits the two analyzed cores if the fraction of total radium available for adsorption-desorption is about 0.5–0.7, and ifK, the distribution coefficient, is about 1000.Finally, the flux of radon out of the sediments is derived from the model-generated radium profiles. It is shown that the resulting standing crop of²²²Rn in the overlying water may be considered as an added constraint in budgeting²³⁰Th and²²⁶Ra in deep-sea sediments.     

Penetration of bomb radiocarbon in the tropical Indian Ocean measured by means of accelerator mass spectrometry

Radiocarbon measurements performed on seawater samples by means of accelerator mass spectrometry (AMS) enable to reduce by a factor of 2000 the water sample size needed for the¹⁴C measurements. Therefore no chemical treatment on board the oceanographic vessel is required. Seventy-four AMS¹⁴C determinations on samples collected in the tropical-equatorial Indian Ocean during the second leg of the INDIGO program (1986) are presented and compared with the β-counting results obtained during the same campaign and the GEOSECS program (1978). A pronounced reduction of the equatorial¹⁴C deficit suggests that substantial amounts of bomb-¹⁴C are associated with the westward flowing Pacific water which enters the Indian Ocean via passages through the Indonesia archipelago and/or to meridional mixing with¹⁴C-rich water of the southern subtropical gyre.     

Sedimentary record of anthropogenic metal inputs in the Tagus prodelta (Portugal)

Favourable oceanographic and environmental conditions allow the formation of a fine-grained deposit (Tagus prodelta) located at the mouth of the Tagus River. This fine-grained deposit results from the sink and accumulation of terrestrial and marine-derived materials. Three short sediment cores collected in the Tagus prodelta were investigated through the variability in grain-size, major and trace elements, C_org, N_tot, δ¹³C and ²¹⁰Pb dating to characterise the historical development of trace metal contamination. Historical trends indicated significant anthropogenic enrichments for Hg, Pb, Zn, Cu, Sb and Sn since the 1930s. Hg presents the highest level of anthropogenic enrichment (ca. EF_Hg=20) in cores 3576 (PO287-26-1B) and 3579 (PO287-27-1B) collected closer to the shore. Despite these elements were derived from distinct industrial sources, all of them presented similar temporal trends, which points to the importance of estuarine mixing processes originated by the tidal regime and wind before transfer to the adjoining coastal areas. The cores were characterised by smoothness of down-core variations and river flood events occurred in the last decades were not registered. Furthermore, the improvement of effluents treatment together with the closing of some industrial point sources in the last two decades is not evidenced in recent sediment composition of the Tagus prodelta. Besides sediment reworking in the upper sediment layers, sediment mixing inside the estuary may explain minor abrupt alterations with the depth.     

Using tephrostratigraphy and cryptotephrostratigraphy to re-evaluate and improve the Middle Pleistocene age model for marine sequences in northeast Japan (Chikyu C9001C)

We refine the tephrostratigraphy and cryptotephrostratigraphy of the Middle Pleistocene deep-sea sediments in the C9001C cores and reconstruct the age model of the cores to provide a standard stratigraphy for the NW Pacific Ocean area. In the lower half of the cores (corresponding to marine isotopic stages [MISs] 9–18), we newly identify nine concentrations (spikes) of glass shards (numbered G17–G25 from top to bottom) below sixteen previously reported spikes (numbered G1–G16) in the upper half of the cores (MISs 1–9). We correlated two spikes (both cryptotephras) with known widespread tephras: G18 with Naruohama-IV (MIS 10d; 357 ka) and G19 with Kasamori 5 (MIS 12b–c; 434–458 ka). Although the two cryptotephra spikes cannot be distinguished by the major- and trace-element compositions of glass shards, these correlations are based upon their stratigraphic occurrence within separate glacial periods (MISs 10 and 12) as indicated by the δ¹⁸O stratigraphy of the C9001C sequence. Based on the stratigraphic positions of the Naruohama-IV and Kasamori 5 tephras, we assign the sediment units corresponding to MISs 10, 11, and 12, and revise the previous assignment of core segments corresponding to MISs 7, 8, and 9. The resulting tephra-based age model is consistent with the last occurrence of Proboscia curvirostris, but it is inconsistent with the first occurrence of Emiliania huxleyi. According to the new age model, the sedimentation rate during MISs 18–2 has ranged from 0.21 m/ky (during MIS 17) to 0.74 m/ky (during MISs 8 and 9), however, the sedimentation rate was never as high as the rate of 1.1 m/ky indicated by an earlier age model.     

Geochemical and isotopic study of the multilayer aquifer system in the Moulares-Redayef basin,southern Tunisia / Etude géochimique et isotopique du système aquifère multicouche du bassin de Moulares-Redayef,sud tunisien

Abstract

Major ion geochemistry, and water molecule isotopes (¹⁸O, ²H) and radiogenic carbon (¹⁴C) of dissolved inorganic carbon (DIC) were used to investigate the hydrodynamic functioning of the multilayer aquifer system in the Moulares-Redayef basin, southern Tunisia. The groundwater of different aquifer levels is characterized by sulphate to calcium sulphate water type. The major geochemical processes in the aquifer system are evaporite mineral dissolution and mixing. The isotopic study allows two groundwater types to be identified: an old palaeoclimatic groundwater, marked by low ¹⁴C activity and relatively depleted stable isotope (¹⁸O and ²H) content characterizes the shallowest aquifers of the Plio-Quaternary and Miocene formations; however, a recent groundwater, distinguished by relatively high ¹⁴C activity and slightly enriched ¹⁸O and ²H content, characterizes the deep Upper Cretaceous artesian aquifer. In addition to these two water groups, other groundwaters are identified, indicating a mixing effect.     

Spatiotemporal patterns of carbon sequestration in a large shallow lake,Chaohu Lake: Evidence from multiple-core records

Lake sediment can sequestrate large amounts of carbon and this issue has become a research hotspot. However, most of research on carbon burial in lakes is based on a single (or a few) sediment core records and so may underestimate the variability of carbon burial features within a single lake. In this study, therefore, Chaohu Lake, a typical large shallow lake in the middle and lower reaches of the Yangtze River, was selected to conduct multiple, high resolution sediment core studies to elucidate that variability. Overall 18 sediment cores are analyzed according to paleolimnological proxies (including ²¹⁰Pb/¹³⁷Cs for 3 master cores); sediment accumulation rate, total organic carbon, grain size and loss of ignition is measured or estimated for most cores. The spatiotemporal variations of organic carbon burial rate (OCBR), carbon storage and their driving factors were examined. Results show: 1) There was a clear temporal difference in carbon burial during the past 150 years, with OCBR varying from 1.1 g C/m²/y to 25.6 g C/m²/y (mean 9.8 g C/m²/y). OCBR began to increase after around 1900, a rapid increase followed after 1950s and a downward trend after 1970s. Total carbon burial amount (OCBA) in the lake since the 1850s is 1.11 x 10¹⁰ g. 2) The average OCBR of six sediment cores in the northwest lake area is 13.4 g C/m²/y, significantly higher than that for sediment cores in other areas (9.6 g C/m²/y). 3) TOC, OCBR, OCBA in all 18 cores exhibited similar temporal patterns (i.e. marked increase since 1950s in most of the cores) but with significant differences in several lake locations. 4) During the last 150 years, carbon burial in Chaohu Lake appears to be greatly affected by changes in regional temperature and population size, according to their significant correlations. OCBR also has a significant correlation with the average lake level in the past 50 years, indicating human activity (notably dam building). This has imposed an important impact on OCBR in Chaohu Lake. This multi-sediment core study reveals the spatiotemporal characteristics of carbon burial in the lake and provides an important basis for increasing the accuracy of calculating carbon storage in large shallow lakes.     

Dating of sediments from Lake Zurich (Switzerland) with210Pb and137Cs

²¹⁰Pb- and¹³⁷Cs-measurements, and varve counting have been used to date sediment cores from Lake Zurich (Switzerland). Two cores from different water depths were dated with²¹⁰Pb/²¹⁰Po and revealed sediment accumulation rates of (0.055±0.015) g·cm^?2·y^?1 and (0.09±0.03) g·cm^?2·y^?1, respectively. A comparable rate of (0.07±0.01) g·cm^?2·y^?1 has been obtained from¹³⁷Cs measurements. These rates were confirmed by annual layer (varve)-counts which lead to rates of 0.07 g·cm^?2·y^?1. Constant²¹⁰Pb activities were observed in the top 6 cm of the sediment cores. This constancy is generally explained in the literature by mixing processes caused by bioturbation and by distortion during coring operations of the uppermost water-rich fluffy sediments. However the distinct¹³⁷Cs-maxima and the regular and undisturbed varve lamination of the top sediment observed in the cores of Lake Zurich contradict this assumption. In addition, measurements of⁷Be at the water/sediment interaces proved complete sediment core recovery and mechanically undisturbed sediments. Remobilization processes are assumed to cause the observed constant²¹⁰Pb activities. Remobilization may also be the reason for an incomplete²¹⁰Pb inventory in the sediments which contain only about 50% of the fallout from atmosphere. The results of the²¹⁰Pb dating should therefore be considered with some care. If existent, varve counting represents the easiest and most reliable means for dating lake sediments.     

Periodical mixing of MORB magmas near East Pacific Rise 13°N: Evidence from modeling and zoned plagioclase phenocrysts

Thirty-six basalt samples from near East Pacific Rise 13°N are analyzed for major and trace elements. Different types of zoned plagioclase phenocrysts in basalts are also backscatter imaged, and major element profiles scanned and analyzed for microprobe. Basalts dredged from a restricted area have evolved to different extents (MgO=9.38wt%—6.76wt%). High MgO basalts are modeled for crystalliza-tion to MgO of about 7wt%, and resulted in the Ni contents (≈28 ppm) that are generally lower than that in observed basalts (>60 ppm). It suggests that low MgO basalts may have experienced more intensive magma mixing. High MgO (9.38wt%) basalt is modeled for self-"mixing-crystallization", and the high Ni contents in low MgO basalts can be generated in small scale and periodical self-mixing of new magma (high MgO). "Mixing-crystallization" processes that low MgO magmas experienced accord with recent 226Ra/230Th disequilibria studies for magma residence time, in which low MgO magmas have experi-enced more circles of "mixing-crystallization" in relatively longer residence time. Magma mixing is not homogeneous in magma chamber, however, low MgO magmas are closer to stable composition pro-duced by periodical "mixing-crystallization", which is also an important reason for magma diversity in East Pacific Rise. Zoned plagioclase phenocrysts can be divided into two types: with and without high An# cores, both of which have multiple reversed An# zones, suggesting periodical mixing of their host magmas. Cores of zoned plagioclase in low MgO (7.45wt%) basalt differ significantly with their mantle in An#, but are similar in An# with microlite cores (products of equilibrium crystallization) in high MgO (9.38wt%) basalt, which further shows that plagioclase phenocryst cores in low MgO basalts may have formed in their parental magmas before entering into the magma chamber.     

Chronology of Holocene lacustrine sediments in Co Ngoin, central Tibetan Plateau

Lake sediments, in which abundant, continuous and high resolved signals are preserved, have the potential to recover the history of terrestrial environmental variation. The value of such paleo-environment signals depends firstly on tying them to accurate …     

Hydrochemical Relation Between Shallow Groundwater with Elevated Fluoride and Groundwater in Underlying Carbonates

In the arid to semi-arid district of Chengcheng, Weinan City, in central Shaanxi Province, diminishing groundwater reserves in the shallow Quaternary (QLB) aquifer and elevated fluoride in the similarly shallow Permo-Triassic (PTF) aquifer, have promoted interest in the development of groundwater resources in the deep but poorly understood Cambrian-Ordovician carbonate aquifer system (COC). To investigate the origin of the COC groundwaters and the relationship between the deep and shallower systems, a hydrochemical study was undertaken involving 179 major and minor ion analyses, 39 stable isotope analyses (δD and δ¹⁸O), and 14 carbon isotope analyses (¹⁴C and δ¹³C). PHREEQC 3.0 was used to investigate mixing. Hydrochemical data support the presence of a well-connected regional flow system extending southwards from the more mountainous north. Stable isotope data indicate that the COC groundwaters originate as soil zone infiltration, under a much cooler regime than is found locally today. This is confirmed by ¹⁴C, which indicates the groundwater to be palaeowater recharged during the late Pleistocene (∼10–12 ka B.P.). The presence of nitrate in the COC groundwaters suggests leakage from overlying shallow aquifers currently provides an additional source of COC recharge, with major faults possibly providing the primary pathways for downward vertical flow.     

湖泊底泥疏浚环境效应:Ⅰ.内源磷释放控制作用

通过为期一年的疏浚模拟试验,在试验室培养疏浚与对照柱样研究了底泥疏浚对内源磷释放的控制效果．结果发现,疏浚表层30cm能够有效的消减沉积物中不f司形态磷含量与孔隙水中PO4^3-P含量．在一年的试验周期内,疏浚和对照柱沉积物-水界面的PO4^3-P通量分别为-143．8至14．4与-237．3至3047．6μg／（m^2·d）,疏浚柱沉积物-水界面的磷通量总体上低于未疏浚对照的磷通量,尤其是在温度较高的月份,从2006年3-10月疏浚柱沉积物-水界面磷释放通量显著低于未疏浚对照柱,疏浚沉积物的磷的释放潜力低于未疏浚对照沉积物．研究结果表明,在外源磷得到有效控制的前提下,底泥疏浚是消减研究区内源磷负荷有效的技术手段．     

Effects of biological sediment mixing on the210Pb chronology and trace metal distribution in a Long Island Sound sediment core

An experiment was designed to assess the relative importance of sediment accumulation and bioturbation in determining the vertical distribution of nuclides in estuarine sediments. A diver-collected core, 120 cm long, was raised from central Long Island Sound and analyzed down its length for:²¹⁰Pb and²²⁶Ra;^{239, 240}Pu; and Mn, Zn, Cu, and Pb. Sampling for chemical analysis was guided by X-radiography of the core. Excess²¹⁰Pb (relative to²²⁶Ra) is roughly homogeneous in the top 2–4 cm of the core, then decreases quasi-exponentially to zero at (or above) 15 cm.^{239, 240}Pu and excess Zn, Cu, and Pb, relative to background values at greater depths in the core, are distributed like excess²¹⁰Pb in the top 10–15 cm. The absence of Mn enrichment at the top of the core, in contrast to other cores raised from this station, suggests that 1–3 cm of sediment was lost by erosion at the site of this core sometime prior to sampling. Below 15 cm excess²¹⁰Pb and excess Zn, Cu, and Pb are found only in the bulk sample from 25 to 30 cm and in clearly identifiable burrow fillings dissected from 70 cm and 115 cm depth. Infilling of large burrows, excavated and then abandoned by crustaceans, is therefore a mechanism for transfer of surficial material to depth in these sediments.The bioturbation rate in the top several centimeters at this station has been determined previously using²³⁴Th (24-day half-life). The distribution of^{239, 240}Pu can be used to estimate a bioturbation rate for the underlying layer (to ～10 cm depth); this rate is found to be 1–3% of the maximum mixing rate for the top 2–3 cm. Using these two mixing rates in a composite-layer, mixing + sedimentation model, the distribution of excess²¹⁰Pb in the top 15 cm was used to constrain the sediment accumulation rate, ω. While the apparent rate of sediment accumulation (assuming no mixing below 2–4 cm) is 0.11 cm/yr, the model requires ω < 0.05 cm/yr. Thus in an area of slow sediment accumulation, a low rate of bioturbation below the surficial zone of rapid mixing causes an increase of at least a factor of two in apparent accumulation rate.     

Radioactivity in sediments of the Great Lakes: Post-depositional redistribution by deposit-feeding organisms

At two locations in southern Lake Huron (U.S.A.), twelve 35.5-cm diameter cores of fine-grained sediments were taken for comparison of the vertical distributions of²¹⁰Pb and falllout¹³⁷Cs with the distributions of benthic macroinvertebrates, mainly oligochaete worms (Tubificidae) and the amphipod,Pontoporeia affinis. Locations were selected on the basis of²¹⁰Pb distributions measured a year earlier which indicated contrasting depths of mixing of surface sediments. At one location the activity of²¹⁰Pb is uniform down to about 6 cm and 95% of total invertebrates occur within this zone; at the other location the zone of constant activity is only 3 cm deep but 90% of the invertebrates occur within it. Comparison of published tubificid defecation rates with sediment accumulation rates based on²¹⁰Pb shows that oligochaetes alone can account for mixing in one case while the effects of amphipods may be required in the case of shallower mixing. If mixing is represented as a diffusional process, eddy diffusion coefficients are at least 5.8 and 3.3 cm² yr^?1 at respective locations. In comparison with bioturbation, molecular diffusion is of minor importance in the post-depositional mobility of¹³⁷Cs. The necessity for introducing a diffusion coefficient varying continuously with depth is indicated by characteristics of the distribution of¹³⁷Cs. Biological reworking of near-surface sediments is an important process affecting radioactivity and chemical profiles in profundal deposits of this and probably other Great Lakes.     

Geosecs North Atlantic radiocarbon and tritium results

Radiocarbon data for 11 stations and tritium data for 16 stations in the North Atlantic Ocean from 74°N to 3°N are presented. For radiocarbon, normal errors inΔ¹⁴C are± 4‰, and in tritium,± 0.09TU or± 3%, whichever is larger. There is a remarkable, but not simply linear, correlation between oceanic bomb transients in¹⁴C and³H. The deep convective mixing in the Greenland Sea is reflected in substantial bomb tracer penetration to all depths, with residence time for the deep, cold core water that seems to be 20 to 30 years. The outflow in the bottom layer southward over the sills of the Denmark Strait and Faroe Passage carries significant tritium concentration, at least to 40°N. Complicated, but coherent, profile structures in the subtropical Atlantic suggest effects of large-scale lateral advection. In particular, a pronounced minimum in both¹⁴C and³H might be associated with the Antarctic intermediate water.     

Preliminary Results from Paleomagnetic Records on Lake Sediments from South America

The preliminary results of paleomagnetic and radiocarbon dating of late pleistocene-holocene sediments from two lakes of south-western Argentina (41°S, 71.5°W) are presented. The magnetic susceptibility, intensity and direction of the natural remanent magnetisation were measured. The stability of the natural remanent magnetisation was investigated by alternating field demagnetisation. The magnetic parameters allowed the cores within each lake to be correlated. ¹³ C analysis, total organic content measurements and C ¹⁴ dating were carried out. A model of sedimentation is suggested. Using this model and the correlation, curves of variations of magnetic inclination and declination in time are shown.     

A deep226Ra maximum in the Northeast Pacific

Seven vertical profiles of²²⁶Ra have been measured along an east-west traverse at about 30°N from San Diego to northwest of Hawaii. These profiles show that there is a distinct core of Ra maximum spreading westward as a tongue in the northeast Pacific deep water. This core starts in the east with 21.1 Ra units (1Ra unit= 10^?14g/kg) at 3.9 km depth at about 130°W, and deepens westward to 4.1 km with its Ra reduced to 18.3 units at 150°W. A similar core with some uncertainty due to possible sampling errors extends westward near the bottom at 5.2 km depth from 19.4 Ra units at 150°W to 15.9 units at about 180° longitude. In addition, these profiles appear to be correlated with each other in structure above the cores of Ra maximum. These cores indicate that the Ra input depends locally on the type and composition of sediments and so the flux varies over the ocean bottom. On the basis of a one-dimensional diffusion-decay model, a horizontal diffusion coefficient of 10⁶ cm²/sec has been computed along these cores. Although this value appears to be slightly lower, it is not inconsistent with those derived from other physical methods.