234Th Fluxes Research Articles

The accretion rate of extraterrestrial 3He based on oceanic 230Th flux and the relation to Os isotope variation over the past 200,000 years in an Indian Ocean core

In the eastern equatorial Indian Ocean, the flux of extraterrestrial 3He, a proxy of interplanetary dust particles (IDPs), has been relatively constant over the past 200 ka. The flux is equal to (1.1±0.4)×10 −12 cm 3 STP cm −2 ka −1, a value obtained using the xs 230Th profiling method. Variations in mass accumulation rates (MARs) derived assuming a constant extraterrestrial 3He flux have a 40-ka periodicity similar to that observed in the δ 18O-derived MARs. This frequency is similar to that of the Earth's obliquity. Measured 187Os/ 188Os ratios are less radiogenic than present-day seawater (0.49–0.98), reflecting the mixing of Os derived from extraterrestrial, terrigenous and hydrogenous sources. When coupled with He data measured on the same samples, Os isotope data yield important information about the terrigenous component supplied to the eastern equatorial Indian Ocean. The amount of Os in the sample derived from the extraterrestrial component can be deduced with the help of the helium systematics. Once corrected for the extraterrestrial component of Os, Os isotope signatures, in conjunction with the 4He concentrations, suggest a supply of terrigenous material from Indonesian ultramafic and Himalayan crustal sources that clearly varies through time.

Sensitivity of 234Th export to physical processes in the central equatorial Pacific

We determined the sensitivity of the calculated sinking flux of 234Th in the central equatorial Pacific to physical processes and scavenging mechanisms by imposing a meridional and vertical advection and diffusion field on a simple dissolved and particulate 234Th cycle. We used the model to estimate the efficiency with which the 234Th deficiency relative to 238U reflected the predicted sinking flux of 234Th on particles and compared our results with 234Th data taken during the JGOFS-EqPac 1992 Survey II Cruise. 234Th deficiencies near the equator were strongly affected by both vertical advection and horizontal diffusion. The model 234Th deficiency at the equator underestimated the model 234Th sinking flux by 144% in neglecting advection and diffusion in the presence of strong upwelling at the equator. The model 234Th deficiency at the equator corrected for advection overestimated the sinking flux of 234Th by 33% in neglecting horizontal diffusion. Analysis of the scavenging mechanism suggests that, during situations of export governed by rapidly sinking particles, 234Th-based estimates of particle export are only half as sensitive to advection compared to situations of export governed by slowly sinking particles. Given that results using the mechanism of slowly sinking particles compare better with the observed 234Th deficiency and calculated meridional 234Th fluxes at the equator than the mechanism of rapidly sinking particles, we consider the mechanism of slowly sinking particle more appropriate for this region. In agreement with previous studies based on observed 234Th gradients, this study supports the incorporation of vertical advection terms in the 234Th balance to estimate particulate carbon export at the equator but suggests that this method may have overestimated the sinking flux at the equator during EqPac Survey II by 0–63% due to the role of horizontal diffusion.

Upper ocean export of particulate organic carbon in the Arabian Sea derived from thorium-234

Thorium-234 is used in the Arabian Sea as a tracer of sinking particle fluxes. Samples were collected from January to August 1995 on four cruises during the Northeast Monsoon, the Spring Intermonsoon and the mid- and late-Southwest Monsoon periods. In this study, 234Th activity distributions are used to quantify the 234Th flux on sinking particles, and the measured ratio of particulate organic carbon (POC) to particulate 234Th is used to convert from 234Th to POC export at 100 m. The calculated POC fluxes range from <1 to >25 mmols C m -2 d -1, and strong seasonal and spatial gradients are observed. The single largest feature is a basinwide export maximum associated with the late-SW Monsoon cruise when POC export rates are 17–28% of the observed primary production rates along the southern sampling line. During all other cruises, this export ratio is <2–10%, with an increase near shore where POC fluxes are generally elevated. Also, during the Spring Intermonsoon, a POC export maximum is observed along the northern sampling line. Both this Spring export feature and late-SW Monsoon flux maximum appear to be associated with a phytoplankton community structure dominated by diatoms. The timing of the late-SW Monsoon flux peak agrees with the observed flux maximum in the deep moored time-series sediment traps (Honjo et al., 1998). This dramatic increase in export between the mid- and late-SW Monsoon also corresponds to measured decreases in the stocks of total organic C in the upper 150 m (Hansell and Peltzer, 1998) and a sharp decline in surface water Al and Fe (Measures and Vink, 1998). These 100 m flux results, plus a series of POC flux profiles, allow for a more complete understanding of the magnitude and timing of sub-euphotic zone export in the Arabian Sea.

230Th in the eastern North Atlantic: the importance of water mass ventilation in the balance of 230Th

The 230Th distribution in the water column on a north-south transect in the eastern North Atlantic was measured using thermal ionization mass spectrometry (TIMS). Water samples at three locations (L1, 33°N 21.6°W; L2, 47.4°N 19.5W; and L3, 54.4°N 21.1°W) were filtered and the dissolved and particulate fractions were analysed for their 230Th and 232Th contents. In the upper water column (< 1000 m water depth), the total 230Th ex distribution at the three locations could be explained by a reversible scavenging model. In the deep water column, however, 230Th ex concentrations decrease from south to north with relatively low and constant concentrations at stations L2 and L3. The major water mass below 2000 m is North East Atlantic Deep Water (NEADW), filling the deep water column at L1. The water column at L2 and L3 is influenced by Labrador Sea Water (LSW) down to 2000 m, whereas at L3, the deep water is strongly influenced by Iceland-Scotland Overflow Water (ISOW). The anomalous distribution of 230Th ex in the deep water column of stations L2 and L3 could be described after a ventilation term was added to the equations of the reversible scavenging model. For L3, a ventilation time (3–12 years) was found comparable to results derived from other tracers. However, the ventilation of the deep water (>2000 m) at L2 suggested by our model (8–25 years) is probably too rapid in comparison with previous findings based on chlorofluorocarbon data. Enhanced scavenging of 230Th would be an alternative explanation for the low concentration in NEADW at L2. With the scavenging-mixing model, we calculated an advective export flux of 230Th of up to 50% of the production in the water column in the eastern North Atlantic. Advective loss of 230Th has to be taken into account when using the 230Th flux into sediments for paleoceanographic studies in ocean basins where deep water residence time is comparable to the 230Th scavenging residence time.

Scavenging of 234Th and 7Be in Lake Constance

Particulate and dissolved concentrations of 234Th and 7Be, along with fluxes of total mass and particulate 234Th and 7Be into sediment traps, were measured in Lake Constance (Bodensee) from March to December 1993. During the same period, atmospheric deposition rates of 7Be and 210Pb were determined. Variations in atmospheric fluxes of 7Be and 210Pb were found to reflect variability in rainfall. The mean atmospheric flux for 7Be was 13.5 dpm cm−2 yr−1 and for 210Pb it was 0.65 dpm cm−2 yr−1. Total 234Th activities in surface water were generally high in winter and low in summer, primarily in response to the seasonality of biological activity. About 50–70% of 234Th was found in particulate form, depending on particle concentration and particle residence time. Scavenging rates for dissolved 234Th ranged from 0.14 to 0.88 d−1, having increased slightly with total particle concentration. 7Be activities in surface water exhibited large variations because of variable atmospheric input and particle flux. During periods of high biological productivity, particulate 7Be in surface water increased from ~20 to ~60% of the total 7Be concentration. The scavenging rates for dissolved 7Be(0.01−0.12 d−1) were generally lower than those for dissolved 234Th because of the lower particle reactivity of 7Be. Although the 7Be flux into sediment traps at 50 m did not differ significantly from the flux at 120 m, the 234Th flux increased with depth because of the further adsorption of water column‐produced 234Th onto sinking particles. The range of residence times for the total radionuclide inventories estimated from an irreversible scavenging model was 33–83 d for 234Th and 40–340 d for 7Be (box depth, 50 m). The residence times of both nuclides were controlled by the particle flux, which, in turn, was determined by biological productivity in the surface water.

Export flux of particulate organic carbon from the central equatorial Pacific determined using a combined drifting trap- 234Th approach

The export flux of particulate organic carbon from the euphotic zone in the central equatorial Pacific was measured using an approach that utilizes 234Th and organic carbon analyses on water column and drifting sediment trap samples. This study was conducted as part of the U.S. Joint Global Ocean Flux Study (U.S. JGOFS) EqPac process study from 12°N to 12°S at 140°W. Samples were collected during the Survey I (February–March 1992) and Survey II (August–September 1992) cruises. The accuracy of drifting sediment traps was evaluated by comparing the measured flux of 234Th with the flux calculated from the deficiency of 234Th relative to 238U in the water column. Calculated 234Th fluxes were corrected for the effects of horizontal and vertical advection. The uncertainties on these 234Th fluxes averaged 39% for Survey I and 20% for Survey II. Comparison of measured and calculated 234Th fluxes revealed evidence for overtrapping, especially in the shallow traps (≤ 100 m). Measured and calculated 234Th fluxes agreed to within 50% for traps at 150–250 m. Good correlation was obtained between measured fluxes of organic carbon and 234Th except for some shallow samples high in organic carbon, suggesting that 234Th was a good tracer for organic carbon. The flux of particulate organic carbon (POC) was calculated as the product of the calculated flux of 234Th times the organic carbon/ 234Th ratio in trap samples. Assuming that the organic carbon/ 234Th ratio in trap samples was representative of sinking particles, we used an average value for the organic carbon/ 234Th ratio for each station. The variability in the station-averaged POC/ 234Th ratio ranged from 10% to 30%. The POC fluxes calculated using our combined 234Th-trap approach ranged from 1 to 6 mmol C m −2 day −1 during Survey I, and from 2 to 30 mmol C m −2 day −1 during Survey II. The average uncertainty for the POC fluxes was ±60%. Primary and new production integrated to the depth of the 0.1 % light level varied by factors of 2–3 for Survey I and Survey II, respectively. The export of particulate organic carbon from the euphotic zone also increased by a factor of 3. The corresponding e-ratios (POC export/primary production) ranged from 0.03 to 0.11 for Survey I, and 0.04 to 0.23 for Survey II. Annual average regional rates (10°N–10°S; 90°W–180°E) of new (0.47 Gt C year −1) and particulate export (0.42 Gt C year −1) production were in good agreement, suggesting that, on an annual basis, significant export of DOC need not be invoked to balance new and export production in this region.

Detailed 230Th, 232Th and 210Pb fluxes recorded by the 1989/90 BOFS sediment trap time-series at 48°N, 20°W

Phytoplankton productivity north of 42°N in the NE Atlantic is dominated by intense blooms in spring and early summer. Detailed analyses of deep-water sediment trap samples over a 17 month period, encompassing two such bloom events, reveal that the removal fluxes of the particle-reactive radionuclides 210Pb and 230Th are generally linearly related to mass flux, as observed elsewhere, up to 120 mg m−2 day−1. Above mass fluxes of 120 mg m−2 day−1 , the 230Th flux is limited close to its water column production rate in a trap at 3100 m, and somewhat in excess of its production rate in a trap at 4465 m (full water column depth 4555 m). The 210 Pb flux is also similarly limited in both traps. These limits are ascribed to the differing behaviour of the fastersettling bloom material, rather than to quantitative scavenging of water column radionuclides. Finite dissolved and particulate activities for both radionuclides are observed in the water column. Comparison of 210Pb23Th activity ratios in surface particulate and trap material indicates that the trap material does not represent surface material which has settled without modification through the water column. Rather the 210Pb230Th activity ratio of the trap material is similar to the activity ratio of particulate material in the integrated water column overlying the shallower trap, but the underlying mechanisms controlling the scavenging limitation remain unclear. Comparison of data from the 3 100 m trap in “clear water” 1455 m above the sea floor, with those from the second only 90 m above, illustrates that re-entrainment of current-resuspended near-bottom material must contribute to the deep trap. As a result, the intercepted 230Th in the deep trap regularly exceeds production in the overlying water column throughout the year. The mean CaCO3 230Th and 232Th fluxes to this deep trap agree to within 10% of their long-term fluxes measured in a core from the trap site, although 210Pb fluxes to the trap are only 70% of the flux recorded by the sediment.

Thorium‐234/uranium‐238 disequilibrium as an indicator of scavenging rates and participate organic carbon fluxes in the Northeast Water Polynya, Greenland

Analyses of particulate and dissolved distributions of the naturally occurring radionuclide 234Th (half‐life=24.1 days) in the water column of the Northeast Water Polynya (Greenland continental shelf) show deficiencies relative to equilibrium with 238U that are related to uptake onto sinking particles. The deficiencies are generally enhanced in the upper 50 m of the water column (Polar Water), a zone of high suspended particle, particulate organic carbon, and pigment concentrations. In July–August 1992, residence times of dissolved Th relative to uptake onto particles ranged from 52 to 293 days and those of particulate Th relative to sinking ranged from 6 to 52 days in the upper 50 m. In contrast, removal was enhanced in July–August 1993, with dissolved and particulate 234Th residence times ranging from 45 to 167 days and 4 to 57 days, respectively. The assumption of steady state used in these calculations is not strictly valid, in that both years showed progressive increases in 234Th removal at stations reoccupied several weeks apart. Spatial variation in 234Th removal from the water column also is evident and may be due, in part, to progressive 234Th removal associated with the inferred anticyclonic circulation within the polynya. Inventories of particulate 234Th in the upper 50 m correlate well with inventories of suspended particulate matter and particulate organic carbon (POC). Using the ratio of POC/234Th on suspended particles and the 234Th fluxes obtained from disequilibrium with respect to 238U to calculate POC fluxes gives values of 12 to 45 (mean = 25±14) mmol C m−2 d−1 in 1992 and 17 to 45 (mean = 35±10) mmol m−2 d−1 in 1993. A direct comparison of the Th‐derived POC fluxes from the 1992 data is possible with those obtained independently based on depletions of dissolved nutrients (29.2±23.5 mmol m−2 d−1 (Wallace et al., this issue)) and dissolved inorganic carbon (14±7.4 mmol m−2 d−1 (Yager et al., this issue)). The good agreement reflects the fact that during the summer, particle cycling and flux in the surface water of the polynya are apparently dominated by biogenic particles. Removal of 234Th from the water column is balanced, within the uncertainties, by inventories of excess 234Th in bottom sediments, with both water column deficiencies and sediment inventories greater in 1993 than 1992. This indicates that on the timescale of several half‐lives of 234Th (∼100 days) the polynya is in balance with respect to 234Th (and probably the particles with which it is associated).

Carbon-cycle imbalances in the Sargasso Sea

THE net exchange of carbon dioxide between the atmosphere and the ocean, and thus the nature of the oceanic carbon sink, is dominated by the seasonal dynamics of carbon cycling in the upper ocean. This cycle represents a balance between abiotic and biotic carbon transport into, and export out of, the ocean's upper layer. Here we report measurements of these processes made over five years in the Sargasso Sea off Bermuda, as part of the US Joint Global Ocean Flux Study (JGOFS). We find that the decrease in carbon stocks from the spring to the autumn in the upper 150 m of the ocean is three times larger than the measured sum of biotic and abiotic fluxes out of this layer. This discrepancy can be explained either by failure to account for horizontal advection of carbon or by inaccuracies in the fluxes of sinking particles as measured using sediment traps. Either the traps miss 80% of the sinking particles, or 70% of the carbon cycling is due to advection (or a combination of both processes is responsible). Sediment-trap measurements of the 234Th flux during this period suggest that most of the discrepancy may be due to inaccuracies in the trap methods, which would require a very general reassessment of existing ideas about particle export and remineralization of carbon in the oceans. If, on the other hand, advection is the main source of the discrepancy, the traditional one-dimensional (vertical) modelling of the oceanic carbon cycle cannot give a full account of carbon dynamics.

Beryllium 10, thorium 230, and protactinium 231 in Galapagos microplate sediments: Implications of hydrothermal activity and paleoproductivity changes during the last 100,000 years

Biogenic particle fluxes from highly productive surface waters, boundary scavenging, and hydrothermal activity are the main factors influencing the deposition of radionuclides in the area of the Galapagos microplate, eastern Equatorial Pacific. In order to evaluate the importance of these three processes throughout the last 100 kyr, concentrations of the radionuclides 10Be, 230Th, and 231Pa, and of Mn and Fe were measured at high resolution in sediment samples from two gravity cores KLH 068 and KLH 093. High biological productivity in the surface waters overlying the investigated area has led to 10Be and 231Pa fluxes exceeding production during at least the last 30 kyr and probably the last 100 kyr. However, during periods of high productivity at the up welling centers off Peru and extension of the equatorial high‐productivity zone, a relative loss of 10Be and 231Pa may have occurred in these sediment cores because of boundary scavenging. The effects of hydrothermal activity were investigated by comparing the 230Thex concentrations to the Mn/Fe ratios and by comparing the fluxes of 230Th and 10Be which exceed production. The results suggest an enhanced hydrothermal influence during isotope stages 4 and 5 and to a lesser extent during isotope stage 1 in core KLH 093. During isotope stages 2 and 3, the hydrothermal supply of Mn was deposited elsewhere, probably because of changes in current regime or deep water oxygenation. A strong increase of the Mn/Fe ratio at the beginning of climatic stage 1 which is not accompanied by an increase of the 230Thex concentration is interpreted to be an effect of Mn remobilization and reprecipitation in the sediment.

Sediment trap experiments in the water column off southwestern Taiwan:234Th fluxes

The activity of234Th (t 1/2=24.1 days) in dissolved, particulate and sediment trap samples was determined in the water column off southwestern Taiwan during 2–4 October, 1993. Vertical234Th fluxes measured by the free-floating sediment traps ranged from 363 to 2290 dpm m−2 d−1 in the upper 450 m. Th-234 fluxes predicted from the irreversible scavenging model concur with those measured by the sediment traps. Comparison of the residence times of particulate234Th and particulate organic carbon showed that their respective values differ by a factor of approximately 2∼3, which suggests organic carbon is preferentially recycled relative to234Th in the euphotic zone.

A three dimensional time‐dependent approach to calibrating sediment trap fluxes

We conducted an experiment to test explicitly the accuracy of upper ocean sediment trap fluxes using the particle‐reactive radionuclide 234Th (t1/2 = 24.1 days). Two independent VERTEX‐style multitrap arrays were used for collection of sinking particles at 95 m and 97 m depths over a four‐day period in May 1992 at the U.S. Joint Global Ocean Flux Study Bermuda Atlantic Time‐series Study (BATS) site. Samples for total 234Th were collected every 8 m between the surface and 96 m and immediately combined for analysis to obtain the vertically integrated activity of 234Th. We collected a total of 27 samples over the four‐day period. The234Th samples were collected daily at each of the two traps and every other day on a 6 × 6 km grid to characterize the entire source region for particles collected in the traps. In situ flow sensors at one trap array indicated low horizontal shear at the trap mouth (5‐10 cm/s) compared to normal values at BATS. The predicted 234Th flux from the watercolumn profiles was not significantly different from zero (−30 ± 140 disintegrations per minute/m2/d). The measured trap 234Th flux at both arrays was significantly higher (290 ± 15 dpm/m2/d). We hypothesize that upper ocean traps at Bermuda may overcollect during low‐flux periods and undercollect during high‐flux periods, thus recording a biased signal of the true particle flux.

Scavenging of 234Th and phosphorus removal from the hydrothermal effluent plume over the North Cleft segment of the Juan de Fuca Ridge

Dissolved and total 234Th (t1/2 = 24.1 days) profiles were measured over the hydrothermal field located within the North Cleft segment of the Juan de Fuca Ridge. Above the hydrothermal effluent plume, which rose approximately 200 m from the seafloor, ≥90% of the 234Th was dissolved, and the total 234Th activity was in equilibrium with its parent uranium. This is typical of deep water for which the residence time of dissolved 234Th with respect to particle uptake is greater than 200 days. Within the effluent plume, however, as little as 30% of the 234Th was in the dissolved state and the residence time of dissolved 234Th with respect to particle uptake was then of the order of only a few weeks. The flux (JTh) of dissolved 234Th to the particle phase was about 300 disintegrations per minute (dpm) cm−2 yr−1. The total 234Th was still approximately in equilibrium with U, indicating that the residence time of the 234Th‐bearing particles with respect to sinking was greater than 100 days. Particulate P, V, and As, while insignificant above the plume, attained concentrations as high as 60, 0.65, and 0.45 nmol/L within the plume. The particulate [P]/[234Th] ratio and the 234Th flux (JTh) are used to calculate a removal flux of P resulting from hydrothermal scavenging of ∼50 μg cm−2 yr−1 or approximately 5.0× 106 g P yr−1 per kilometer of active ridge crest. Similarly, 1.0×105 g km−1 of V and As are removed per year from the water column by hydrothermal scavenging over this region. Extrapolation to a global scale suggests that ridge crest scavenging removes ≤10% of these elements from the ocean.

Heinrich events in the North Atlantic: radiochemical evidence

Heinrich events are a series of apparently synchronous sediment horizons in the North Atlantic with unusually high ratios of lithic fragments to foraminifera in the coarse fraction. They occur more prominently during cold climatic periods and could have been produced either by dramatic decreases in surface water productivity, by brief increases in carbonate dissolution on the seafloor, or by rapid accumulation of ice-rafted debris. The excess 230Th activity profile in core CHN82 31 11PC(42°23′N, 31°48′W) provides evidence in support of the last of these explanations for the two most recent events (H1 and H2). Sediment flux reconstruction based on the constant 230Th flux model indicates that sediment rain rates increased from an average of ca 1.6 g cm −2 ka −1 to at least 19 g cm −2 ka −1 during Heinrich event H1 and 11 g cm −2 ka −1 during Heinrich event H2. These estimates are lower limits, as bioturbation likely smoothed the 230Th profile. Our data also suggest a maximum of 600 y for the duration of H1 and 800 y for H2. Two independent approaches provide similar estimates of sediment mass accumulation (g cm −2) at the site during the two events; ca 10 g cm −2 and ca 9 g cm −2 were deposited from the overlying water column during event H1 and H2. An additional ca 4 g cm −2 were brought to the site by sediment focusing during event H1. These methods provide a means of mapping and quantifying IRD mass accumulation over the North Atlantic for studying the processes that controlled the distribution of the Heinrich deposits.

Application of a generalized scavenging model to time series 234Th and particle data obtained during the JGOFS North Atlantic bloom experiment

Primary production, particle concentration and flux data, obtained as part of the JGOFS North Atlantic Bloom Experiment, have been used to construct a self consistent, time dependent, particle cycle model for the upper water column for the period 25 April–30 May 1989. Daily 234Th concentrations and fluxes are modelled over the course of the bloom, and the results compared with available data. Use of a single pseudo-first-order rate constant k′1 for the adsorption of 234Th onto particulate material in the model gives satisfactory agreement with measured 234Th profiles. Modelled concentrations of 234Th on large, sinking particles—which must be known in order to calculate particles fluxes from 234Th disequilibrium—are predicted well by the model. The results also suggest that particle fluxes determined from sediment traps may be systematically low.

Fluxes of Th isotopes in the Santa Monica Basin, offshore California

Fluxes of 234Th, 232Th, 230Th and 228Th increase with depth in the water column of Santa Monica Basin, following the flux of particulate inorganic matter. Temporal variability in flux is generally low for the shorter lived and predominantly radiogenic 234Th and 228Th, but high for 230Th and the nonradiogenic 232Th. The flux profiles are largely controlled by lateral advection of detrital material resuspended from the basin slope. Most of the shorter lived 234Th and 228Th scavenged from the water column are produced in situ or locally, whereas most of the authigenic 230Th is transported from elsewhere. From the concentration (in dissolved and particulate forms) and flux profiles of these isotopes, the residence time estimate of dissolved Th is 1–2 months in the upper 50 m and 1–2 years in the entire water column. The corresponding residence times for particulate Th are 4 days and ∼ 40 days, respectively. The average rate at which suspended particles work their way downward via aggregation and settling is estimated at 6–10 m/day. In the context of the documented circulation pattern in the region, these results suggest that most pollutants introduced to the Santa Monica Basin will be exported out of it.

Late pleistocene sedimentation: a case study of the central Indian Ocean Basin

Concentration-depth profiles of excess 230Th have been measured in five sediment cores from the central Indian Ocean Basin to determine their accumulation rates and excess 230Th flux to the depositional environment. Sedimentation accumulation rates ranged between 1 and 5 mm ky −1. The excess 230Th inventories in the cores range from 44 to 481 dpm cm −2, considerably lower than that expected from its production in the overlying water column. These low excess 230Th inventories (relative to its production) have been attributed to the effects of winnowing by bottom currents. The absence of 230Th ex in core (SK-176) has been attributed to the lack of sediment deposition in this area caused by the influence of Antarctic Bottom Water (AABW). This study shows that the deposition of biogenic silica and lithogenic components exhibited orders of magnitude variations durining Late Pleistocene. The biogenic flux variation are attributed to productivity changes while the episodic variations in the non-biogenic flux could result from major changes in the influx of terrigenous materials via the Ganges-Brahmaputra.

Particulate fluxes of 230Th, 231Pa, and 10Be in the northeastern Pacific Ocean

Measurements of 230Th, 231Pa, and 10Be in sediment trap samples and Holocene sediments collected along a transect in the northeastern Pacific indicate that the removal of these nuclides from the ocean is greatly influenced by boundary scavenging. More than 35% of the 230Th, at least 70% of the 231Pa, and as much as 85% of the 10Be fluxes collected by sediment traps at a nearshore site (located ~120 km off shore) are supplied by lateral transport of the dissolved nuclides from the open ocean. These findings are consistent with the hypothesis that boundary scavenging is proportional to the oceanic residence times of the nuclides (10–40 y for 230Th, ~ 100 y for 231 Pa, and ~ 1000 y for 10Be). Results of this study indicate that particle flux is a principal factor influencing scavenging of 230Th, 231Pa, and 10Be from the water column. The marine geochemistry of 10Be in ocean-margins is fundamentally different from that in the open ocean. Scavenging of 10Be occurs throughout the water column at the margin area in this study, in contrast to the pattern of 10Be being removed only in the surface waters and then remineralized at depth in the open ocean. Aluminosilicate may be a major phase removing 10Be from the water column in the open ocean but not at ocean-margins. Nuclide accumulation rates in the underlying Holocene sediments are all higher than the corresponding fluxes collected by sediment traps. Several possibilities that could account for this observation include: downslope sediment transport; deployment of the traps during a period when the particle flux was not representative of long-term average conditions; and scavenging of the nuclides from near bottom waters. The high accumulation rate of authigenic Mn in some Holocene sediments, which can be attributed to redox cycling of Mn, may have caused near bottom scavenging of 230Th and 231Pa as well as certain other trace elements (such as Ba, Cu, and Zn).

Boundary Scavenging and Deep‐Sea Sediment Dating: Constraints from Excess 230Th and 231Pa

Apparent sedimentation rates derived from excess 230Th and 231Pa in deep‐sea sediments accumulated during the last 25,000 years in the Pacific Ocean are mostly lower than the average rates determined by other independent (such as 14C and ∂18O) dating methods, which may be a consequence of generally higher accumulation rates of aluminosilicate and carbonate during the last glacial that diluted the excess 230Th and 231Pa activities in glacial sediments. The pattern of enhanced scavenging of nuclides in ocean margin areas (i.e., boundary scavenging) also changed from the glacial to the Holocene. Fluxes of 230Th and 231Pa throughout the ocean varied over time responding to changes in the nature and intensity of boundary scavenging which, in turn, were influenced by changes in particle flux and particle composition. The lower intensity of boundary scavenging of 231Pa during the last glacial may also have contributed to the lower apparent sediment accumulation rates based on excess 231Pa for cores in ocean margin areas.

Temporal variations of 234Th activity in the water column of Dabob Bay: Particle scavenging

Monthly measurements of total, dissolved, and particulate 234Th, along with fluxes of total mass and particulate 234Th, were made throughout 1987 in Dabob Bay, Washington. Vertical distributions of total 234Th showed a large deficiency relative to 278U throughout the water column during all sampling periods.Total 234Th activities in surface waters were generally higher in winter and lower in summer, in response to seasonal biological activity. In spite of this variation, the steady state assumption seems valid for modeling 234Th of the bay because the magnitude of variation is insignificant in the mass balance calculation. The range of residence times of total, dissolved, and particulate 234Th estimated from an irreversible scavenging model are 4–70, &lt; 1–10, and 3–60 d, respectively. The residence time of 234Th is controlled by the suspended particles whose distribution is determined by biological production in the surface layer and resuspension of bottom sediments.Vertical fluxes of 214Th measured by sediment traps also showed wide seasonal variation and generally increased monotonically from 200–1,000 dpm m‒2 d‒1 at 20 m to 2,000–4,000 at 100 m. The flux of 234Th predicted with the steady state scavenging model tends to be about twice the directly measured 234Th flux. This difference may be due either to low trap efficiency for 234Th‐containing particles or to boundary scavenging at the sides of the basin. The residence times of particulate 234Th and total suspended matter agree well, and both are influenced by biological processes.