²³⁸U removal and accumulation
in Concepción bay sediments, Chile

ABSTRACT

Activity profiles of ²³⁴U, ²³⁸U and ²¹⁰Pb, as well as the ²³⁴U/²³⁸U ratio, and ²¹⁰Pb_xs radiochemical parameters and macrobenthic abundance in sediments were measured and calculated at different times during a year (1991), along a transect between the head and the mouth of Concepción Bay, Chile. This bay is a semi-enclosed shallow embayment with a strong seasonality in its hydrographic and biological characteristics due to wind driven upwelling. Upwelling of oxygen-poor and nutrient-rich water occurs and organic detritus reaches the seabed, creating a sub-oxic and anoxic environment during, at least, 57% of the year. ²³⁸U and ²³⁴U activities in the sediments show a gradual increase from 1.57 to 6.07 dpm g^-1 from the surface to 15 cm depth in the sediments. Under suboxic-anoxic conditions, this pattern is consistent with a reduction of U(VI) to U(IV) and its subsequent removal from pore-water. A ²³⁴U/²³⁸U ratio of 1.10 ±0.08 indicates an authigenic origen. The ²³⁸U accumulation rate and inventory show a significant increase from Station 1 (head of the bay) to Station 3 (mouth of the bay) from 348 to 794 dpm cm^-2 kyr^-1 and of 14 to 32 dpm cm^-2, respectively. The organic-rich and highly reducing sediments also show an increase in sulphate-reduction rates, sulfur speciation and bioturbation activities of macrobenthos. These patterns suggest that the extent of U enrichment in these sediments is regulated by bacterial sulphate reduction rate and bioturbation and bioirrigation activities. The estimated ²³⁸U accumulation rates are higher than those reported for other anoxic environments, representing an important U sink in marine sediments. The geochemical behaviour of U seems to be very sensitive to the redox condition and this characteristic could become an important paleoceanographic tool to identify changes in oceanographic conditions during the last millennium.

Key words: U removal and accumulation, bacterial sulphate reduction and bioperturbation, Coastal upwelling, Central Chile.

RESUMEN

Remoción y acumulación de ³²⁸U en los sedimentos de la bahía de Concepción, Chile. Las actividades del ²³⁴U, ²³⁸U y del ²¹⁰Pb, los parámetros radioquímicos como la razón ²³⁴U/²³⁸U y el exceso de ²¹⁰Pb_xs y la abundancia del macrobentos fueron medidas y estimadas en sedimentos marinos a lo largo de una transecta ubicada entre la boca y la cabeza de Bahía Concepción, durante un ciclo anual. Esta bahía es un ambiente semicerrado y somero, sujeto a una fuerte estacionalidad en sus características hidrológicas y biológicas debido a la influencia de la surgencia costera que se desarrolla en dicha región. La surgencia de aguas ricas en nutrientes y pobres en oxígeno da como resultado una alta producción de detritos orgánicos que alcanzan los sedimentos, creando condiciones subóxicas y/o anóxicas durante, al menos, el 57% del año. Los perfiles de ²³⁴U, ²³⁴U y ²³⁸U entre la superficie y los 15 cm de profundidad muestran un incremento gradual de sus actividades en rangos que fluctuaron entre 1.57 y 6.07 dpm g^-1. Bajo condiciones subóxicas y anóxicas, esta distribución es consistente con la reducción del U(VI) a U(IV) y su consecuente remoción del agua intersticial y su acumulación en la fase sólida.La razón ²³⁴U/²³⁸ de 1,10 ± 0,08 indica un origen autigénico. La tasa de acumulación e inventarios de ²³⁸U muestran un aumento significativo desde la Estación 1 (cabeza de la bahía) hacia la Estación 3 (boca de la bahía), con rangos que fluctuaron desde 348 a 794 dpm cm^-2 kyr^-1 y desde 14 a 32 dpm cm^-2, respectivamente. Estos sedimentos, ricos en materia orgánica y altamente reductores, muestran también un incremento similar desde la cabeza hacia la boca de la bahía, en las tasas de sulfato-reducción, especiación de azufre y la actividad bioperturbadora e irrigadora del macrobentos. Estos patrones sugieren que el grado de enriquecimiento de U en estos sedimentos es el resultado de su reducción y remoción del agua intersticial, la cual está regulado por la tasa de sulfato-reducción y la bioperturbación y bio-irrigación. La tasa de acumulación de ²³⁸U es mayor a las encontradas para otros ambientes o cuencas anóxicas e indican una importante área para la pérdida de U en sedimentos marinos. El comportamiento geoquímico del U parece ser muy sensible a las condiciones óxido-reductoras de los sedimentos, representando una herramienta importante para identificar cambios en las condiciones paleo-oceanográficas del último milenio.

Palabras claves: Remoción y acumulación de U, Sulfato-reducción y bioperturbación, Surgencia costera, Chile Central.

INTRODUCTION

Enhanced concentration of U in marine sediments has been well documented in deep sea systems, e.g., siliceous oozes (Bloch, 1980; Cochran and Krishnaswami, 1980), metaliferous sediments (Cochran, 1982), turbidites (Colley and Thomsom, 1992), anoxic basins (Anderson, 1987; Barnes and Cochran, 1991) and coastal sediments (Mckee et al. 1987; Barnes and Cochran, 1991, 1993; Church et al., 1996), both in ancient (Thomsom et al., 1990; Colley and Thomson, 1990, 1992; Klinklammer and Palmer 1991) and recent sedimentary environments.

Under oxic condition and pH greater than 6, U exists as U(VI) and forms the stable (3.3 µg L^-1 or 2.4 dpm L^-1 at salinity of 35) and soluble uranyl-carbonate (UO2(CO₃)₃^4-) species, in which natural occuring ²³⁸U isotope represents 99.27% of the total concentration. Under reducing condition, U(VI) is reduced to much less soluble and particle reactive U(IV) species, which, like Th(IV), can be removed from solution onto particles (Cochran, 1982). In marine surface sediments, the U is mobile only over a short distance, as it is released by the oxidation of organic matter via the dissolved oxygen that penetrates through the sediment surface from seawater. Subsequently, U is immobilized by the reduction to U(IV), leading to substantial U enrichment in reducing zones (Santschi et al., 1988; Barnes and Cochran, 1993). Thus, the main mechanism controlling U accumulation rates in sediments is consistent with a balance between the diagenetic release of U oxide coatings to the pore-water and the reduction and removal of U from pore-water solution to the solid phase (McKee et al., 1987; Santschi et al., 1988; Colley and Thomson, 1990; Barnes and Cochran, 1993).

The microbially mediated degradation of organic material in near-shore sediments controls many important biogeochemical processes. These include organic carbon remineralization of metabolizable fractions and other elements and the regulation of the chemical distributions controlled by redox conditions (Berner, 1980). The oxidation state of U exerts a strong control on its geochemical behaviour. There is a correlation between U removal rates and diagenetic redox conditions associated with organic carbon oxidation processes, e.g., sulphate reduction (Barnes and Cochran, 1990, 1993). In fact, sulphate in coastal marine sediments appears to be the quantitatively most important electron acceptor, accounting for 50-70 % or more of the total sediment respiration (Howarth and Jorgensen, 1984).

There is evidence that bacteria play a role in U reduction, but whether the mechanism of dissolved U removal by bacteria is a passive or active mechanism, or both, has not been clarified yet. The microbial activity can generate HS^- which could act as a reducing agent, but this is only possible at high sulphide concentration (Reynolds and Goldhaber, 1983; Mohagheghi et al., 1985). It is becoming increasingly clear that U reduction in anoxic environments is often the result of a direct enzymatic process by Fe-reducing and sulphate-reducing bacteria, as has been observed both in culture experiments and under natural conditions (Lovley et al., 1991; Lovley and Phillips, 1992; Lovley et al., 1993; McKee and Todd, 1993).

Accordingly, the sediments of Concepción Bay represent an appropriate substrate wherein the distribution of solutes and other processes, related either directly or indirectly to the decomposition of organic matter, shows a natural gradient useful for studying the mechanisms of removal of U from sediments. Concepción bay is the largest, protected and relatively shallow coastal area located in central Chile and, is also surrounded by one of the most industrialised and urbanised areas of Chile. It is characterised by an orientation that enhances the influence of the wind and, therefore, the fertilising effect associated with coastal upwelling of Sub-surface Equatorial Water (SSEW). Thus, the nutrient cycle in Concepción Bay shows a strong complexity due to the combined effects of i- a pronounced O_2, minimum layer and nutrient supplied by the SSEW, which on the continental shelf at 36°S are present at 80-280 m, but during the coastal upwelling it can be elevated to 20 m depth bathing the shelf and bay sediments; ii- high rate of nutrient recycling from sediment and iii- high nutrient load from domestic sewage (Farías et al., 1996).

The ²³⁸U solid phase distribution in the nearshore sediments of Concepción Bay provides an index of the extent to which the U geochemical behaviour is linked to organic carbon oxidation. Thus, a relationship between the accumulation of U in organic-rich sediments and the intensity of the diagenetic redox reactions (e.g., bacterial sulphate-reduction) could be expected to occur in Concepción Bay sediments, where biogeochemical processes in the water column and sediments occur at different rates, both temporal and spatial scales (Ahumada et al., 1983; Arcos and Navarro, 1986; Peterson et al., 1988).

STUDY AREA

Concepción Bay (36°40'S;73°01'W) (Fig. 1) is a semi-enclosed (167.4 km²) and shallow (maximum depth 45 m) embayment with a strong seasonality in hydrographic processes and high biological productivity of the water column due to wind driven upwelling (Ahumada et al., 1983). The hydrographic parameters of the bay are affected by local wind variability producing the following hydrographic regimes (Ahumada et al., 1983): a- wintertime (ca., 43% of the year) is dominated by northerly winds that drive strong and efficient vertical mixing in the water column. Sub-Antarctic Surface Water (SASW) is present within the bay, associated with a high dissolved oxygen content and low nutrient concentrations; b- late spring, summer and early autumn (ca. 57% of the year), when southerly winds dominate, the combined effect of these winds, the orientation of the coast and the Coriolis force give rise to the upwelling of SSEW, characterised by higher salinity, oxygen-poor (< 40 µM) and nutrient-rich (> 25 µM N-NO₃^-) water.

The upwelled water produces an increase in phytoplankton biomass and high levels of primary productivity (Ahumada, 1989; Ahumada, 1991; ). Maximum chlorophyll-a concentration with values of of about 50 mg Chl-a m^-3and primary production of 5.76 g C m^-2 d^-1and up to 9.6 g C m^-2 d^-1, have been reported for the shelf waters, among the highest values in the literature (Fossing et al., 1995). A carbon balance (Bernal et al., 1989) and sediment trap data (Ahumada, 1991; Farías et al., 1994) indicate that ca. 25-50% of the primary production generated during the upwelling season settles onto the bay sediments. Thus, oxygen-poor upwelled water and organic-rich detritus reaching the seabed, create a sub-oxic and anoxic environment during at least 57% of the year.

The sediments of Concepción bay are soft, black reducing mud (mainly silty clay), rich in organic matter. The sediment surface is frequently covered by a filamentous microbial association of the genera Thioploca and Beggiotoa (Gallardo, 1977) and the upper few centimetres of the sediment column is inhabited by an abundant macrobenthic community dominated by polychaete worms, tolerant to low oxygen conditions (Carrasco, 1996). The solutes in pore-water are typical of anoxic sediments that have undergone quick remineralisation of organic matter via sulphate-reduction (Farías and Salamanca, 1990; Ferdelman et al., 1997). These processes in the upper 20 cm of the sediment column are evidenced by a steep positive pore-water concentration gradient in dissolved ammonium concentration. In turn, these gradients support high ammonium fluxes to the overlying waters (up to 10 mmol m^-2 d^-1) during the upwelling season (Farías et al., 1996).

METHODS

SAMPLE COLLECTION

The samples were collected during January, April, August and November of 1991 on board the R.V. Kay-Kay. Rectangular Plexiglass corers, operated by divers (to recover undisturbed samples of 15-20 cm in length), and gravity cores were used to collect sediments from three stations along a transect from Talcahuano Harbour to the mouth of Concepción Bay (Fig. 1). A 212 cm² rectangular core was taken for radiochemical (²³⁸U, ²³⁴U, and ²¹⁰Pb) and sedimentological analyses (percentage of water, bulk density, porosity). The cores were stored at 4°C after collection and then sectioned at 1-2 cm intervals.

In order to characterise the redox conditions from each interval, a gravity core was taken to determine Eh and pH. These measurements were made under a glove bag filled with N_2, by inserting the platinum electrode directly through a sealed hole to avoid air contamination. Macrofauna samples (triplicate) were taken with a 0.1 m² grab. Samples were sieved through a 0.5 mm mesh and biological material was fixed with 10% water-formaline solution and stored in a Rose Bengal ethanol solution for later separation and identification.

ANALYTICAL METHODS

For U isotope determination, ashed sediment samples (4-5 g) were leached in hot 6 N HCl and spiked with a ²³²U/²²⁸Th tracer. The U was separated from Th and other interferences (Ra and Fe) using DOWEX 1x8 anion exchange columns, following a modified procedure of Bhat et al. (1969). The purified U fraction was electro-deposited onto a stainless steel disc from an ammonium sulphate solution of pH 2 (Talvitie, 1972). The ²¹⁰Pb was determined by its granddaughter ²¹⁰Po using the chemical procedures delineated by Flynn (1969). A sample of 1-2 g of dried sediments was digested with successive concentrated acids and then spontaneously deposited onto silver discs. ²⁰⁹Po yield tracer was added to each sample prior to acid digestion.

The ²³⁸U, ²³⁴U and ²¹⁰Pb activities were determined using an alpha-spectrometer Canberra 35 Plus, equipped with four detectors. In order to obtain the excess of ²¹⁰Pb, the measured ²¹⁰Pb activities were then corrected for ²²⁶Ra-supported ²¹⁰Pb activities by subtracting the stable, low-level activities at the base of the cores. In this case, it is assumed that the ²¹⁰Pb is in equilibrium with the ²²⁶Ra activity. This relation has previously been corroborated by Salamanca (1988). The water content, porosity and dry bulk densities of each section were determined by drying at 110°C, assuming a dry weight density of 2.65 g cm^-3. Weight loss on ignition at 475°C, as a measure of Loss Organic Ignition (LOI) was measured in sediment samples after drying to constant weight (Dean, 1974). ²³⁸ U and ²¹⁰Pb_xs inventories from 0 to 15 cm depth were calculated by adding from all horizons the product of isotope activity, internal width and dry bulk sediment density. The sediment apparent accumulation rates were estimated from ²¹⁰Pb distributions by means of a dimensional, steady-state sedimentation model (Cochran, 1982). A multivariate covariance analysis (ANCOVA) between U as the dependent variable, depth as classificatory variable, and stations and sampling periods as co-variables was carried out.

RESULTS AND DISCUSSION

GENERAL SEDIMENT CHARACTERISTICS

Sediments from all stations are grey to black muds ( ca. 80 % clay minerals with mean grain size of 7.05 phi), with high superficial sediment porosity (0.93-0.90 v v^-1)) and high organic content (15-18 % LOI). The sediments reveal reducing characteristics (Eh< -150 mV) from the first few millimetres of depth, except during the winter period (August), when a surface oxidized layer (ca. 5-10 mm) is observed. The pH varies from 7.5 to 7.2. The black to green colour of the sediments can be caused by the presence of common micro-crystalline sulphide (i.e., FeS and FeS₂), which indicate reducing conditions (Ferdelman et al., 1997). The oceanographic and geochemical characteristics at the three stations are presented in table 1.

The silt-clay sediments of Concepción Bay and adjacent shelf may be considered as a record of an organic-rich, anoxic sedimentary environment where the very high superficial carbon oxidation rates (up to 3 mol cm^-3 d^-1) are entirely related to sulphate reduction (Thamdrup and Canfield, 1996). Concepción Bay sediments are only comparable to those measured in highly productive water and organic carbon deposition areas (e.g., enclosed bays, estuaries and salt marshes).

The sulphate reduction rates have been recently measured in a location similar to Stations 2 and 3 of this study and found to range between 50 and 1500 nmol cm^-2 d^-1 at the surface (Thamdrup and Canfield, 1996; Ferdelman et al., 1997; Farías, 1998). The maximum values occurring at Station 3 and these values are consistent with those found in other upwelling and highly productive ecosystems (Canfield, 1989). The ammonium pore-water profiles and their fluxes across the sediment-water interface reflect also an increase in the diagenetic ammoniun production from Station 1 to Station 3 (Farías and Salamanca, 1990; Farías 1998).

Concentrations of pore-water sulphide and speciation of sulphur compounds (e.g., pyrite, acid volatile sulphur 'AVS', elemental S) also exhibit a strong spatial variability in the sediments of Concepción Bay (Ferdelman et al., 1997). Station 1 is one of the most polluted sites in the bay due to the dumping of untreated sewage from fishmeal factories and the sediments have a high sulphide concentration (> 1000 µM) (Rudolph and Ahumada, 1987). The sediments at Station 2 have intermediate to high sulphide concentrations (356-1200 uM), whereas , the lowest values were observed at Station 3 (1-2 µM), despite the fact that high sulphate reduction rates have been reported (Ferdelman et al., 1997). Therefore, the produced sulphide must have been rapidly reoxidized.

The differences in AVS concentration and sulphur speciation in the sediments between the intermediate (Station 2) and outer sites (e.g., Station 3) can be explained by the different physiology of the bacterial mats dominating the sediments. Beggiatoa spp. grow in the highly-reducing sediments of the inner parts of the bay, whereas Thioploca spp. grow in more oxidised sediments with respect to sulphur speciation on shelf locations (Huetel et al., 1996). On shelves, Fe reduction is mainly coupled to oxidation of reduced S and the precipitation of sulphides as pyrite-dominated H₂S scavenging (Thamdrup and Canfield, 1996; Ferdelman et al. 1997). Thioploca is able to oxidise a maximum of 35% of sulphide production (Ferdelmam et al., 1997), revealing a potential to oxidise part of the sulphide with its intracellular nitrate (Fossing et al., 1995).

The macrofauna in Concepción Bay sediments consisted of 26 taxa or species groups during the sampling period, being characterised by large numbers of polychaete deposit feeders. The Spionid Paraprionospio pinatta (Erlers) represented about 75.58-98.57% of the total macrofauna abundance. There are no clear patterns in the temporal abundance patterns over the seasonal cycles, but a spatial trend in the relative abundance along the organic matter gradient was observed. Station 1 was defaunated during the sampling period, while Station 3 had a maximum abundance (ca. 1512 individuals per 0.1 m^-2). The variation of total abundance of the macrobenthic community between stations and sampling periods and the relative contribution of Polychaeta are shown in table 2.

SOLID PHASE RADIOCHEMICAL DATA

The sediments of Concepción bay have a ²³⁸U activity ranging from 1.57 to 6.07 dpm g^-1 with a mean of 3.02±1.12 dpm g^-1. The vertical distributions of ²³⁸U at the different stations are plotted in figure 2. ²³⁸U activity remains relatively constant with depth at Station 1 while Stations 2 and 3 show a significant increase from 2-3 dpm g^-1 at the surface of the sediment to 4-6 dpm g^-1 at 15 cm depth (p< 0.05, ANCOVA analysis, Table 3). The ²³⁸U enrichment with depth increases by a factor of two, particularly in the cores from Station 3. The results indicate that ²³⁸U increases significantly with depth and this pattern is different between sampling stations (p.< 0.05), but no significant differences occur between sampling periods (p.=0.123).

Figure 3 presents the vertical distribution of excess ²¹⁰Pb activity for the three sampling stations. Station 1 presents an almost uniform activity throughout the sediment column. By contrast, at Stations 2 and 3 the distribution is characterised by a break in slope, with a zone of relatively uniform activity from the sediment surface to 5-8 cm depth, due to biological mixing, followed by a decrease of ²¹⁰Pb activity with depth. For an explanation of the ²³⁸U radiochemical profiles, chemical (reduction and removal onto particles) and physical (diffusion and mixing) mechanisms must be considered. The ²³⁴U/²³⁸U ratio in Concepción bay sediments presents an average value of 1.11 ± 0.08, consistent with an authigenic origin (Thomson et al., 1990). The addition of authigenic ²³⁴U increases the ²³⁴U/²³⁸U activity ratios from 0.9-1, characteristic of detrital phases, a value of 1.15, characteristic of seawater (Cochran, 1982). The ²³⁴U and ²³⁸U activities in Concepción Bay sediments show a very strong correlation (r²= 0.92, DF=79), essentially identical in sediments from all depths (Fig. 4). The least squared regression gives a ratio of change in ²³⁴U to change in ²³⁸U of 1.043±0.04 (95% C.L.). The U/Th ratios higher than 2 (Farías, 1992), together with concentrations of Al in the sediment column (Salamanca, 1988), indicate a constant detrital-phase composition in the sediment column. Therefore, the enhancement of U in the sediments suggests authigenic enrichment.

The ²³⁸U profiles in Concepción Bay sediments also seem to be affected by biological mixing (i.e., particle mixing and burrow irrigation by benthic fauna). In fact, these sediments are densely inhabited (except Station 1) by macrobenthic assemblages of deposit feeder Polychaeta (Table 2). Sediment X-radiographs reveal extensive networks of burrows, which indicate an intense circulation of pore-water through the sediments (Farías, 1992). The fluid transfer may be orders of magnitude greater (volumetrically) than particle reworking (Aller and Cochran, 1976). Consequently, benthic organism activities introduce from overlying seawater about 2.45 dpm L^-1 of dissolved ²³⁸U and ²³⁴U (Farías, 1992), penetrating several centimetres down into the reducing sediment column, wherein an intense remineralisation of deposited organic matter coupled with a sulphate reduction from the sediment surface are observed (Thamdrup and Canfield, 1996; Ferdelman et al., 1997; Farías, 1998). Thermodynamic prediction of U reduction indicates that U in pore-water of anoxic sediments that contain dissolved Fe²⁺ and HS^-1 should be gradually reduced and removed from solution (Barnes and Cochran, 1990, 1993).

URANIUM ACCUMULATION RATES

The ²³⁸U inventories fluctuate from 14.22 dpm cm^-2 to 32.21 dpm cm^-2, being considerably higher at Station 3. Also, the excess of ²¹⁰Pb yields average inventories which increase from Station 1 to 3 (Station 1: 8.73±1.48 dpm cm^-2, Station 2: 20.28±3.07 dpm cm^-2 and Station 3: 21.62 + 3.09 dpm cm^-2). The apparent accumulation rates estimated from ²¹⁰Pb_xs vary from 0.17 to 0.28 cm yr^-1. The U accumulation rates can be estimated from the increase in U with depth (average value over 0-20 cm) and dry sediment accumulation rate of 0.15 g cm^-2 yr^-1, derived from the ²¹⁰Pb_xs distribution below the bioturbation zone (Farías, 1992). The estimated U accumulation rates range from 348 to 795 dpm cm^-2 kyr^-1.

The spatial and seasonal variations of total U accumulation rates and ²¹⁰Pb and ²³⁸U inventories are presented in table 4 . A strong spatial increase of ²³⁸U and ²¹⁰Pb_xs inventories and U accumulation rates from Station 1 to Station 3 are observed . Part of this spatial variability could be directly attributed to early diagenetic processes associated with the different intensity and mechanisms of organic carbon remineralization linked directly or indirectly to microbial activity and indirectly to the effect of bioturbation.

The processes of U removal by bacterial mechanisms are not clearly understood yet. The association of U with sulphide sediments (Reynolds and Goldhaber, 1983) and the microbial production of HS^-1 suggest a possible role of this element as a reducing agent. It is becoming increasingly clear that bacteria may enzymatically reduce U(VI); U reduction could be coupled to the oxidation of organic matter with U acting as the electron acceptor (Cochran et al., 1986; Lovley et al., 1991; Lovley and Phillips, 1992; Gorby and Lovley, 1992; Lovley et al., 1993; Barnes and Cochran, 1993; McKee and Todd, 1993).

The U accumulation rates, as well as their inventories, increase gradually from Station 1 to Station 3 (Table 4). By contrast, the pore-water sulphide tend to decrease from Station 1 to Station 3 (Table 1). Therefore, these conspicuous natural gradients in rates and mechanisms of carbon oxidation coupled with sulphate-reduction and sulphur speciation are useful in determining the mechanisms involved in U removal. If the U removal is related to general rates of bacterial oxidation of organic carbon, as defined by the rates of diagenetic redox reactions such as sulphate reduction, a more rapid U removal under conditions of higher sulphate reduction rates is expected. If the U removal is mediated by dissolved sulphide acting as a reducing agent, the most rapid U reduction under high concentrations of sulphide is expected.

Station 1 shows the lowest ²³⁸U and ²¹⁰Pb inventories and total U accumulation rates with respect to the other two stations, but has the highest sulphide concentrations. The observed homogeneity of long-lived radionuclide and geochemical parameters, as well as the absence of biogenic structure, could also be related to physical processes that cause particle mixing during erosion and deposition associated with sediment transport events. This physical mixing as well as the high organic content and sulphide concentrations can be viewed as a local phenomenon given the shallow depth and the proximity of about 1 km to Talcahuano Harbour. By contrast, the radiochemical parameters tend to increase towards the mouth of the bay, but the sulphide concentration shows an inverse trend. Therefore, the rates of U removal have a direct relationship with bacterial sulphate reduction in the studied locations. In addition, the increase in the measured radiochemical and diagenetic parameters between the sampling stations is apparently correlated with macrofaunal abundance and its bioturbation rate. In fact, the macrobenthos stimulates the decomposition reactions and solute fluxes across the sediment-water interface (Aller, 1982).

The U accumulation rates measured in Concepción bay sediments and those found in salt marshes by Church et al. (1996) are higher than those reported for other anoxic basin sediments, ranging from 15 to > 300 dpm cm^-2 ky^-1 (Barnes and Cochran, 1993). The U accumulation in anoxic basin sediments was estimated to account for about 20% of the U input annually by rivers to oceanic U balance (Klinkhammer and Palmer, 1991; Barnesand Cochran, 1990, 1991). Therefore, the inclusion of the U removal in shallow marine coastal sediments (e.g., estuaries, salt marshes, embayments) should increase the importance of marine sediments as U sinks.

Suboxic zones also act as an important sink for certain elements, namely N, Mn, Cr, Cd, and U (Codispoti, 1995; Rosenthal and Boyle, 1995; Rue et al., 1997). In the case of Cr, the Cr(VI) is partially reduced to particle reactive Cr(III), like U(IV) and Th(IV), which can then be scavenged throughout the suboxic area and removed to the sediments (Rue et al., 1997). The geochemical behaviour of U seems also to be very sensitive to the redox conditions (suboxic-anoxic conditions) developed at the 0-15 cm depth of the sediment column, where intense organic carbon remineralisation through sulphate was observed. If this is so, this element could be used as a paleo-oceanographic tool to identify changes in oceanographic conditions (e.g., oxygen level) during the late millennia. Presumably, U(IV) like Cr(III) is scavenged by the rain of particulate organic material through the suboxic waters (e.g., SSEW) and this raises the possibility of using U: organic carbon ratios in sediments as a paleoindicator of suboxic conditions in the overlying water column. This has been poorly reported in the scientific literature, although most recently Rosenthal and Boyle (1995) found glacial enrichments of authigenic Cd and U in sub-Antarctic sediments associated with increased accumulation rates of organic carbon.

ACKNOWLEDGEMENTS

The authors thank C. E. Barnes and C. Morales for critically reading a preliminary version of this paper. The authors also express appreciation to J. Le Roux (Universidad de Chile), S. Ribeiro (Comisión Nacional de Energía Atómica) and J. Boles (University of California, Santa Barbara) for their criticisms which helped to improve the manuscript. Financial assistance was provided by the Dirección de Investigación, Universidad de Concepción, through Grants P.I. No. 95-112.31.1 and 96-112.038-6. This is a contribution to the main FONDAP-Humboldt Programme.

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Manuscript received: December 29, 1999; accepted: September 3, 2000.