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1.
An extensive 234Th data set was collected at two sites in the North Pacific: ALOHA, an oligotrophic site near Hawaii, and K2, a mesotrophic HNLC site in the NW Pacific as part of the VERTIGO (VERtical Transport In the Global Ocean) study. Total 234Th:238U activity ratios near 1.0 indicated low particle fluxes at ALOHA, while 234Th:238U ～0.6 in the euphotic zone at K2 indicated higher particle export. However, spatial variability was large at both sites—even greater than seasonal variability as reported in prior studies. This variability in space and time confounds the use of single profiles of 234Th for sediment trap calibration purposes. At K2, there was a decrease in export flux and increase in 234Th activities over time associated with the declining phase of a summer diatom bloom, which required the use of non-steady state models for flux predictions. This variability in space and time confounds the use of single profiles of 234Th for sediment trap calibration purposes. High vertical resolution profiles show narrow layers (20–30 m) of excess 234Th below the deep chlorophyll maximum at K2 associated with particle remineralization resulting in a decrease in flux at depth that may be missed with standard sampling for 234Th and/or with sediment traps. Also, the application of 234Th as POC flux tracer relies on accurate sampling of particulate POC/234Th ratios and here the ratio is similar on sinking particles and mid-sized particles collected by in-situ filtration (>10–50 μm at ALOHA and >5–350 μm at K2). To further address variability in particle fluxes at K2, a simple model of the drawdown of 234Th and nutrients is used to demonstrate that while coupled during export, their ratios in the water column will vary with time and depth after export. Overall these 234Th data provide a detailed view into particle flux and remineralization in the North Pacific over time and space scales that are varying over days to weeks, and 10's–100's km at a resolution that is difficult to obtain with other methods.  相似文献   

2.
Measurements of 234Th/238U disequilibria and particle size-fractionated (1, 10, 20, 53, 70, 100 μm) organic C and 234Th were made to constrain estimates of the export flux of particulate organic C (POC) from the surface waters of the Ligurian, Tyrrhenian and Aegean Seas in March–June 2004. POC exported from the surface waters (75–100 m depth) averaged 9.2 mmol m−2 d−1 in the Ligurian and Tyrrhenian Seas (2.3±0.5–14.9±3.0 mmol m−2 d−1) and 0.9 mmol m−2 d−1 in the Aegean Sea. These results are comparable to previous measurements of 234Th-derived and sediment-trap POC fluxes from the upper 200 m in the Mediterranean Sea. Depth variations in the POC/234Th ratio suggest two possible controls. First, decreasing POC/234Th ratios with depth were attributed to preferential remineralization of organic C. Second, the occurrence of maxima or minima in the POC/234Th ratio near the DCM suggests influence by phytoplankton dynamics. To assess the accuracy of these data, the empirical 234Th-method was evaluated by quantifying the extent to which the 234Th-based estimate of POC flux, PPOC, deviates from the true flux, FPOC, defined as the p-ratio (p-ratio=PPOC/FPOC=STh/SPOC, where S=particle sinking rate). Estimates of the p-ratio made using Stokes’ Law and the particle size distributions of organic C and 234Th yield values ranging from 0.93–1.45. The proximity of the p-ratio to unity implies that differences in the sinking rates of POC- and 234Th-carrying particles did not bias 234Th-normalized POC fluxes by more than a factor of two.  相似文献   

3.
234Th was used to quantify sinking fluxes and residence times of particles in surface waters of the north-western Mediterranean Sea. Measurements of dissolved and particulate 234Th were made at the DYFAMED station (43°25′N–7°51′E, JGOFS-France program). Sampling covered 1 year on four cruises in 1994 (February 9, April 29, June 3, October 1) and focused on a transition period in mid-spring with six repeated profiles collected during May 1995. 234Th was nearly in equilibrium with its parent 238U most of the year, except in spring. The intensive sampling in May shows a rapid evolution throughout the month from a moderate 234Th deficit to near-equilibrium values. The time-series of 234Th were treated with steady-state and non-steady-state models. 234Th particulate fluxes clearly indicate large variability in export, with the highest values observed in spring. Particle residence times in the upper 40 m range from <10 to >250 days, and could increase by a factor of 10 within 2 weeks. POC fluxes from the upper 40 m and export ratios (ThE: ratio of 234Th-derived POC export to primary production), derived from the 234Th/238U disequilibrium in the water column and POC/234Th ratio on trapped material, decrease from about 9.5 mmol C m−2 d−1 and >22% in early May to less than 5 mmol C m−2 d−1 and 15% after mid-May. The 234Th-derived information is in agreement with the annual variations in Mediterranean Sea productivity.  相似文献   

4.
In this study at the Bermuda Atlantic Time-series Study (BATS) site we demonstrate that the polonium–lead disequilibrium system may perform better as a tracer of organic carbon export under low-flux conditions (in this case, <2.5 mmol C m?2 d?1) than under bloom conditions in an oligotrophic setting. With very few exceptions, the POC flux predictions calculated from the water-column 210Po deficit were within a factor of 2 of the POC flux caught in surface-tethered sediment traps. However, we found higher correlation between size-fractionated particulate 210Po activity and POC concentration in November 2006 (r=0.93) than in January (r=0.79) and during the spring bloom in March 2007 (r=0.80). We suggest that this is due to the ability of polonium to distinguish between bulk mass flux and organic carbon export under oligotrophic and lithogenic-driven flux regimes. Further, we found that the POC/Po ratio on particles was largely independent of size class between 10 and 100 μm (P=0.13) during each season, supporting the notion that export in this oligotrophic system is driven by sinking aggregates of smaller cells and not by large, individual cells.  相似文献   

5.
Direct measurements of new production and carbon export in the subtropical North Atlantic Ocean appear to be too low when compared to geochemical-based estimates. It has been hypothesized that episodic inputs of new nutrients into surface water via the passage of mesoscale eddies or winter storms may resolve at least some of this discrepancy. Here, we investigated particulate organic carbon (POC), particulate organic nitrogen (PON), and biogenic silica (BSiO2) export using a combination of water column 234Th:238U disequilibria and free-floating sediment traps during and immediately following two weather systems encountered in February and March 2004. While these storms resulted in a 2–4-fold increase in mixed layer NO3 inventories, total chlorophyll a and an increase in diatom biomass, the systems were dominated by generally low 234Th:238U disequilibria, suggesting limited particle export. Several 234Th models were tested, with only those including non-steady state and vertical upwelling processes able to describe the observed 234Th activities. Although upwelling velocities were not measured directly in this study, the 234Th model suggests reasonable rates of 2.2–3.7 m d?1.Given the uncertainties associated with 234Th derived particle export rates and sediment traps, both were used to provide a range in sinking particle fluxes from the upper ocean during the study. 234Th particle fluxes were determined applying the more commonly used steady state, one-dimensional model with element/234Th ratios measured in sediment traps. Export fluxes at 200 m ranged from 1.91±0.20 to 4.92±1.22 mmol C m?2 d?1, 0.25±0.08 to 0.54±0.09 mmol N m?2 d?1, and 0.22±0.04 to 0.50±0.06 mmol Si m?2 d?1. POC export efficiencies (Primary Production/Export) were not significantly different from the annual average or from time periods without storms, although absolute POC fluxes were elevated by 1–11%. This increase was not sufficient, however, to resolve the discrepancy between our observations and geochemical-based estimates of particle export. Comparison of PON export rates with simultaneous measurements of NO3? uptake derived new production rates suggest that only a fraction, <35%, of new production was exported as particles to deep waters during these events. Measured bSiO2 export rates were more than a factor of two higher (p<0.01) than the annual average, with storm events contributing as much as 50% of annual bSiO2 export in the Sargasso Sea. Furthermore it appears that 65–95% (average 86±14%) of the total POC export measured in this study was due to diatoms.Combined these results suggest that winter storms do not significantly increase POC and PON export to depth. Rather, these storms may play a role in the export of bSiO2 to deep waters. Given the slower remineralization rates of bSiO2 relative to POC and PON, this transport may, over time, slowly decrease water column silicate inventories, and further drive the Sargasso Sea towards increasing silica limitation. These storm events may further affect the quality of the POC and PON exported, given the large association of this material with diatoms during these periods.  相似文献   

6.
The vertical sinking flux of particulate Al, Fe, Pb, and Ba from the upper 250 m of the Labrador Sea has been estimated from measurements of 234Th/238U disequilibrium and the respective metal/234Th ratios in >53 μm size particles. 234Th-derived particulate metal fluxes include in situ scavenged metals, labile lithogenic metals, and metals derived from external input (e.g., atmospheric supply). In contrast to the POC/234Th ratio, particle size-fractionated (0.4–10 μm, 10–53 μm, and >53 μm) Al/234Th, Fe/234Th and Pb/234Th, and Ba/234Th ratios generally increase with depth and exhibit no systematic change with particle diameter. Sinking fluxes of particulate Al (2.47–22.3 μmol m−2 d−1), Fe (2.69–16.3 μmol m−2 d−1), Pb (2.85–70 nmol m−2 d−1), and Ba (0.13–2.1 μmol m−2 d−1) at 50 m (base of the euphotic zone) and 100 m (base of the mixed layer) are largely within the range of previous sediment trap results from other ocean basins. Estimates of the upper ocean residence time of Al (0.07–0.28 yr) and Pb (0.8–2.9 yr) are short compared to previously reported values. The settling rate of >53 μm particles calculated from the 234Th data ranges from 14 to 38 m d−1.  相似文献   

7.
To gain new insights into the variability of particulate organic carbon (POC) fluxes and to better understand the factors controlling the POC/234Th ratios in suspended and sinking particulate matter, we investigated the relationships between POC/234Th ratios and biochemical composition (uronic acids, URA; total carbohydrates, TCHO; acid polysaccharides, APS; and POC) of suspended and sinking matter from the Gulf of Mexico in 2005 and 2006. Our data show that URA/POC in sediment traps (STs), APS/POC in the suspended particles, and turnover times of particulate 234Th in the water column and those of bacteria in STs inside eddies usually increased with depth, whereas particulate POC/234Th (10–50 μm) and the sediment-trap parameters (POC flux, POC/234Th ratio, bacterial biomass, and bacterial production) decreased with depth. However, this trend was not the case for most biological parameters (e.g., phytoplankton and bacterial biomass) or for the other parameters at the edges of eddies or at coastal-upwelling sites.In general, the following relationships were observed: 1) 234Th/POC ratios in STs were correlated with APS flux, and these ratios in the 10–50 μm suspended particles also correlated with URA/POC ratios; 2) neither URA fluxes nor URA/POC ratios were significantly related to bacterial biomass; 3) the sum of two uronic acids (G2, glucuronic, and galacturonic acid, which composed most of the URA pool) was positively related to bacterial biomass; and 4) the POC/234Th ratios in intermediate-sized particles (10–50 μm) were close to those in sinking particles but much lower than those in > 50 μm particles. The results indicate that acid polysaccharides, though a minor fraction (~ 1%) of the organic carbon, act more likely as proxy compound classes that might contain the more refractory 234Th-binding biopolymer, rather than acting as the original 234Th “scavenger” compound. Moreover, these acid polysaccharides, which might first be produced by phytoplankton and then modified by bacteria, also influence the on-and-off “piggy-back” processes of organic matter and 234Th, thus causing additional variability of the POC/234Th in particles of different sizes.  相似文献   

8.
Mass, carbon, and nitrogen fluxes and carbon and nitrogen compositions were determined for particulate samples from plankton net tows, shallow floating sediment traps, intermediate and deep moored sediment traps, and sediment cores collected along 140°W in the central equatorial Pacific Ocean during the US JGOFS EqPac program. Mass, particulate organic carbon (POC), and particulate inorganic carbon (PIC) fluxes measured by the floating sediment traps during the Survey I (El Niño) and Survey II (non-El Niño) cruises follow essentially the same pattern as primary production: high near the equator and decreasing poleward. POC fluxes caught in free-floating traps were compared with alternative estimates of export fluxes, including 234Th models, new production, and other sediment trap studies, resulting in widely differing estimates. Applying 234Th corrections to the trap-based fluxes yielded more consistent results relative to primary production and new production. Despite factors of five differences in measured fluxes between different trap types, POC : 234Th ratios of trap material were generally within a factor of two and provided a robust means of converting modeled 234Th export fluxes to POC export fluxes. All measured fluxes decrease with depth. Trap compositional data suggest that mineral “ballasting” may be a prerequisite for POC settling. POC remineralization is most pronounced in the epipelagic zone and at the sediment–water interface, with two orders of magnitude loss at each level. Despite seawater supersaturation with respect to calcium carbonate in the upper ocean, 80% of PIC is dissolved in the epipelagic zone. Given the time-scale differences of processes throughout the water column, the contrasting environments, and the fact that only 0.01% of primary production is buried, sedimentary organic carbon accumulation rates along the transect are remarkably well correlated to primary production in the overlying surface waters. POC to particulate total nitrogen (PTN) ratios for all samples are close to Redfield values, indicating that POC and PTN are non-selectively remineralized. This constancy is somewhat surprising given conventional wisdom and previous equatorial Pacific results suggesting that particulate nitrogen is lost preferentially to organic carbon.  相似文献   

9.
《Marine Chemistry》2007,103(1-2):185-196
Large-volume sampling of 234Th and drifting sediment trap deployments were conducted as part of the 2004 Western Arctic Shelf–Basin Interactions (SBI) spring (May 15–June 23) and summer (July 17–August 26) process cruises in the Chukchi Sea. Measurements of 234Th and particulate organic carbon (POC) export fluxes were obtained at five stations during the spring cruise and four stations during the summer cruise along Barrow Canyon (BC) and along a parallel shelf-to-basin transect from East Hanna Shoal (EHS) to the Canada Basin. 234Th and POC fluxes obtained with in situ pumps and drifting sediment traps agreed to within a factor of 2 for 70% of the measurements. POC export fluxes measured with in situ pumps at 50 m along BC were similar in spring and summer (average = 14.0 ± 8.0 mmol C m− 2 day− 1 and 16.5 ± 6.5 mmol C m− 2 day− 1, respectively), but increased from spring to summer at the EHS transect (average = 1.9 ± 1.1 mmol C m− 2 day− 1 and 19.5 ± 3.3 mmol C m− 2 day− 1, respectively). POC fluxes measured with sediment traps at 50 m along BC were also similar in both seasons (31.3 ± 9.3 mmol C m− 2 day− 1 and 29.1 ± 14.2 mmol C m− 2 day− 1, respectively), but were approximately twice as high as POC fluxes measured with in situ pumps. Sediment trap POC fluxes measured along the EHS transect also increased from spring to summer (3.0 ± 1.9 mmol C m− 2 day− 1 and 13.0 ± 6.4 mmol C m− 2 day− 1, respectively), and these fluxes were similar to the POC fluxes obtained with in situ pumps. Discrepancies in POC export fluxes measured using in situ pumps and sediment traps may be reasonably explained by differences in the estimated POC/234Th ratios that arise from differences between the techniques, such as time-scale of measurement and size and composition of the collected particles. Despite this variability, in situ pump and sediment trap-derived POC fluxes were only significantly different at a highly productive station in BC during the spring.  相似文献   

10.
《Marine Chemistry》2002,80(1):11-26
Profiles of particulate and dissolved 234Th (t1/2=24.1 days) in seawater and particulate 234Th collected in drifting traps were analyzed in the Barents Sea at five stations during the ALV3 cruise (from June 28 to July 12, 1999) along a transect from 78°15′N–34°09′E to 73°49′N–31°43′E. 234Th/238U disequilibrium was observed at all locations. 234Th data measured in suspended and trapped particles were used to calibrate the catchment efficiency of the sediment traps. Model-derived 234Th fluxes were similar to 234Th fluxes measured in sediment traps based on a steady-state 234Th model. This suggests that the sediment traps were not subject to large trapping efficiency problems (collection efficiency ranges from 70% to 100% for four traps). The export flux of particulate organic carbon (POC) can be calculated from the model-derived export flux of 234Th and the POC/234Th ratio. POC/234Th ratios measured in suspended and trapped particles were very different (52.0±9.9 and 5.3±2.2 μmol dpm−1, respectively). The agreement between calculated and measured POC fluxes when the POC/234Th ratio of trapped particles was used confirms that the POC/234Th ratio in trap particles is representative of sinking particles. Large discrepancies were observed between calculated and measured POC fluxes when the POC/234Th ratio of suspended particles was used. In the Barents Sea, vertical POC fluxes are higher than POC fluxes estimated in the central Arctic Ocean and the Beaufort Sea and lower than those calculated in the Northeast Water Polynya and the Chukchi Sea. We suggest that the latter fluxes may have been strongly overestimated, because they were based on high POC/234Th ratios measured on suspended particles. It seems that POC fluxes cannot be reliably derived from thorium budgets without measuring the POC/234Th ratio of sediment trap material or of large filtered particles.  相似文献   

11.
234Th (T1/2=24.1 d) and 210Po (T1/2=138.4 d) are particle reactive radioisotopes that are used as tracers for particle cycling in the upper ocean. Particulate organic carbon (POC) export has frequently been estimated using 234Th/238U disequilibrium. Recent evidence suggests that 210Po/210Pb disequilibrium may be used as an additional tool to examine particle export, given the direct biological uptake of 210Po into cellular material. Differences in these two radioisotope pairs with regard to their half-lives, particle reactivity and scavenging affinity in seawater should provide complementary information to be obtained on the processes occurring in the water column. Here, we review eight different studies that have simultaneously used both approaches to estimate POC export fluxes from the surface ocean. Our aim is to provide a complete “dataset” of all the existing POC flux data derived from the coupled use of both 234Th and 210Po and to evaluate the advantages and limitations of each tracer pair. Our analysis suggests that the simultaneous use of both radiotracers provides more useful comparative data than can be derived from the use of a single tracer alone. The difference in half-lives of 234Th and 210Po enables the study of export production rates over different time scales. In addition, their different biogeochemical behaviour and preferred affinity for specific types of particles leads to the conclusion that 234Th is a better tracer of total mass flux, whereas 210Po tracks POC export more specifically. The synthesis presented here is also intended to provide a basis for planning future sampling strategies and promoting further work in this field to help reveal the more specific application of each tracer under specific water column biogeochemistries.  相似文献   

12.
Time-series measurements of 234Th activities and particulate organic carbon (POC) concentrations were made at time-series stations (K1, K2, K3, and KNOT) in the northwestern North Pacific from October 2002 to August 2004. Seasonal changes in POC export fluxes from the surface layer (∼100 m) were estimated using 234Th as a tracer. POC fluxes varied seasonally from approximately 0 to 180 mg C m−2 d−1 and were higher in spring–summer than in autumn–winter. The export ratio (e-ratio) ranged from 6% to 55% and was also higher in spring–summer. Annual POC fluxes were estimated to be 31 g C m−2 y−1 in the subarctic region (station K2) and 23 g C m−2 y−1 in the region between the subarctic and subtropical gyres (station K3). POC fluxes and e-ratios in the northwestern North Pacific were much higher than those in most other oceans. The annual POC flux corresponded to 69% of annual new production estimated from the seasonal difference of the nutrient in the Western Subarctic Gyre (45 g C m−2 y−1). These results indicate that much of the organic carbon assimilated in the surface layer of the northwestern North Pacific is transferred to the deep ocean in particulate form. Our conclusions support previous reports that diatoms play an important role in the biological pump.  相似文献   

13.
Depth profiles of total 234Th (dissolved+particulate) were collected at Station ALOHA (22°45N, 158°00W) in the North Pacific Subtropical Gyre during 9 cruises from April 1999 to March 2000. Samples were collected and processed by a new 2 L technique that enables more detailed depth resolution then previous 234Th studies. Significant zones of particle export (234Th deficiency) and particle remineralization (234Th excess) were measured both temporally and with depth. 234Th derived particulate carbon (PC) and nitrogen (PN) fluxes were determined with steady-state and non-steady-state models and PC/234Th and PN/234Th ratios measured with both in situ pumps and free-drifting particle interceptor traps deployed at 150 m. 234Th based export estimates of 4.0±2.3 mmol C m−2 d−1 and 0.53±0.19 mmol N m−2 d−1, were approximately 60% higher than those measured in PIT style sediment traps from the same time period, 2.4±0.2 mmol C m−2 d−1 and 0.32±0.08 mmol N m−2 d−1. Most of this difference is attributable to two large export events that occurred during October and December 1999, when traps undercollected for 234Th by a factor of 2 to 4. 234Th export (ThE) ratios based on 234Th derived PC flux/14C based primary production ranged from 4% to 22% (average=8.8%). Our results confirm the recent estimates of C export by Emerson et al. (Nature 389 (1997) 951) and Sonnerup et al. (Deep-Sea Research I 46 (1999) 777) and indicate that C export from the oligotrophic ocean must be considered when discussing C sequestration in global climate change.  相似文献   

14.
Dynamics of the natural radioactive particle tracer 234Th (half-life: 24.1 days) within the abyssal water column up to 1000 m above bottom and within surface sediments of the northeast Atlantic (Porcupine Abyssal Plain; depth: ≈4845 m) were investigated. Distributions of transmissometer voltages and potential temperature indicated a subdivision of the near-bottom water column into a benthic mixed layer (BML; thickness: ≈10–65 m) and the layer above the BML up to the upper boundary of the bottom nepheloid layer (BNL; thickness: ≈1000 m). Comparison of 234Th fluxes (dpm m−2d−1) in sediment traps, vertical fluxes derived from 234Th/238U-disequilibrium in the near-bottom water column and excess 234Th inventories in the surface sediment provided evidence for lateral advection of 234Th-depleted water and a ‘missing sink’ for 234Th. A simple one-dimensional steady-state box-model approach was applied to investigate 234Th dynamics. Estimated residence times suggest the BML and the resuspension zone of the surface sediment to be highly dynamic systems with respect to particle cycling and sorptive reactions on a time scale of weeks. Model results indicate that, through the chemical forcing of changing particle concentration, a thickening BML results in (1) increasing residence times of particulate 234Th in the BML with respect to the net fluxes across the upper boundary of the BML and into the surface sediment; (2) declining adsorption rate constants in the BML; (3) increasing desorptive fluxes in the BML resulting in (4) enhanced 234Th decay in the BML; (5) decreasing net fluxes of particulate 234Th from the BML to the upper BNL above the BML and to the sediment. Potential consequences for carbon cycling in the water column of the deep ocean are discussed.  相似文献   

15.
Export fluxes of particulate organic carbon (POC) were estimated from the 234Th/238U disequilibrium in the Ulleung Basin1 (UB) of the East/Japan Sea1 (EJS) over four seasons. The fluxes were calculated by multiplying the average POC/234Th ratio of sinking particles larger than 0.7 μm at 100- and 200-m water depths to 234Th fluxes by the integrated 234Th/238U disequilibrium from the surface to 100-m water depth. In spring, the 234Th profiles changed dramatically with sampling time, and hence a non-steady-state 234Th model was used to estimate the 234Th fluxes. The 234Th flux estimated from the non-steady-state model was an order of magnitude higher than that estimated from the steady-state model. The 234Th fluxes estimated using the steady-state model showed distinct seasonal variation, with high values in summer and winter and low values in autumn. In spring, the phytoplankton biomass had the highest value, and primary production was higher than in summer and autumn, but the 234Th fluxes were moderate. However, these values might have been significantly underestimated, as the 234Th fluxes were estimated using the steady-state model. The POC export fluxes estimated in autumn were about four times lower than those in other seasons when they were rather similar. The annually averaged POC flux was estimated to be 161 ± 76 mgC m−2 day−1, which was somewhat lower than that in highly productive coastal areas, and higher than that in oligotrophic regions. The export/primary production (ThE) ratios ranged from 7.0 to 56.1%, with higher values in spring and summer and lower values in autumn and winter. In summer, a high ThE ratio of 48.4 ± 7.0% was measured. This may be attributed to the mass diatom sinking event following nitrate depletion. In the UB1, the annually averaged ThE ratio was estimated to be 34.4 ± 12.9%, much higher than that in oligotrophic oceans. The high ThE ratio may have contributed to the high organic carbon accumulation in the UB1.  相似文献   

16.
Total 230Th and 228Th vertical distributions were measured in the eastern Indian Ocean. Based on disequilibria of 234U–230Th and 228Ra–228Th, we estimated the particle flux from the surface to 950 m. The estimated particle flux ranged from 190±60 (150 m in the Southern Ocean) to 8.4±2.5 mg/m2/day (950 m in the West Australian Basin). The particle fluxes correlate well with estimates of nitrate supply by vertical mixing to the euphotic zone, suggesting local control by new production. Flux attenuation at mesopelagic depths appears to be related to biogenic carbonate and silica remineralization based on comparison to alkalinity and dissolved silicate distributions, with possible additional control by lithogenic particle inputs. The particle flux estimates obtained by combining the 234U–230Th and 228Ra–228Th disequilibria offer advantages over those from 228Ra–228Th disequilibrium alone, in that they can account for some aspects of vertical mixing and thus are applicable to a wider range of oceanographic settings.  相似文献   

17.
Sinking particles were collected every 4 h with drifting sediment traps deployed at 200 m depth in May 1995 in a 1-D vertical system during the DYNAPROC observations in the northwestern Mediterranean sea. POC, proteins, glucosamine and lipid classes were used as indicators of the intensity and quality of the particle flux. The roles of day/night cycle and wind on the particle flux were examined. The transient regime of production from late spring bloom to pre-oligotrophy determined the flux intensity and quality. POC fluxes decreased from, on average, 34 to 11 mg m−2 d−1, representing 6–14% of the primary production under late spring bloom conditions to 1–2% under pre-oligotrophic conditions. Total protein and chloroplast lipid fluxes correlated with POC and reflected the input of algal biomass into the traps. As the season proceeded, changes in the biochemical composition of the exported material were observed. The C/N ratio rose from 7.8 to 12. Increases of serine (10–28% of total proteins), total lipids (7–9 to 14–28% of POC) and reserve lipids (1–5 to 5–22% of total lipids) were noticeable, whereas total protein content in POC decreased (20–27 to 18–7%). N-acetyl glucosamine, a tracer of fecal pellet flux, showed that zooplankton grazing was a major vector of downward export during the decaying bloom. Against this background pattern, episodic events specifically increased the flux, modifying the quality and the settling velocity of particles. Day/night signals in biotracers (POC, N-acetyl glucosamine, protein and chloroplast lipids) showed that zooplankton migrations were responsible for sedimentation of fresh material through fast sinking particles (V=170–180 m d−1) at night. Periodic signatures of re-processed material (high lipolysis and bacterial biomass indices) suggested that other zooplankton fecal pellets or small aggregates, probably of lower settling velocities (V<170 m d−1), contributed to the flux during calm periods. At the beginning of the experiment, during the development of a prymnesiophyte bloom in the upper layers, the sterol signal with no periodicity enabled us to estimate high particle settling velocities (⩾600 m d−1) likely related to large aggregate formation. A wind event increased biotracer fluxes (POC, protein, chloroplast lipids). The rapid transmission of surface signals through extremely fast sinking particles could be a general feature of particle fluxes in marine areas unaffected by horizontal advection.  相似文献   

18.
Key aspects of deep-ocean fluid dynamics such as basin-scale (residual) and tidal flow are believed to have changed over glacial/interglacial cycles, with potential relevance for climatic change. To constrain the mechanistic links, magnitudes and temporal succession of events analyses of sedimentary paleo-records are of great importance. Efforts have been underway for some time to reconstruct residual-flow patterns from sedimentary records. Attempts to reconstruct tidal flow characteristics from deep-sea sediment deposits, however, are at a very early stage and first require a better understanding of the reflection of modern tides in sediment dynamics. In this context internal (baroclinic) tides, which are formed by the surface (barotropic) tide interacting with seafloor obstacles, are believed to play a particularly important role. Here we compare two modern deep-sea environments with respect to the effect of tides on sediment dynamics. Both environments are influenced by kilometre-scale topographic features but with vastly different tidal forcing: (1) two sites in the Northeast Atlantic (NEA) being surrounded by, or located downstream of, fields of short seamounts (maximum barotropic tidal current velocities ～5 cm s?1); and (2) a site next to the Anaximenes seamount in the Eastern Mediterranean (EMed) (maximum barotropic tidal current velocities ～0.5 cm s?1). With respect to other key fluid-dynamical parameters both environments are very similar. Signals of sedimentary particle dynamics, as influenced by processes taking place in the bottom boundary layer, were traced by the vertical water-column distribution of radioactive disequilibria (daughter/parent activity ratios≠1) between the naturally occurring, short-lived (half-life: 24.1 d) particulate-matter tracer 234Th relative to its very long-lived and non-particle-reactive parent nuclide 238U. Activity ratios of 234Th/238U<1 in water samples collected near the seafloor indicate fast 234Th scavenging onto particles followed by fast settling of these particles from the sampled parcel of water and, therefore, imply active sediment resuspension and dynamics on time scales of up to several weeks. In the Northeast Atlantic study region tides (in particular internal tides) are very likely to locally push total current velocities near the seafloor across the critical current velocity threshold for sediment erosion or resuspension whereas in the Eastern Mediterranean the tides are much too weak for this to happen. This difference in tidal forcing is reflected in a difference of the frequency of the occurrence of radioactive disequilibria <1 between total 234Th and 238U: In the near-bottom water column of the Northeast Atlantic region 59% of samples had detectable 234Th/238U disequilibria whereas at the Eastern Mediterranean site this fraction was only 7% (including a few disequilibria >1). The results of this study, therefore, add to the evidence suggesting that tides in the deep sea of the open oceans are more important for sediment dynamics than previously thought. It is hypothesised that (a) tide/seamount interactions in the deep open ocean control the local distribution of erosivity proxies (e.g., distributions of sediment grain sizes, heavy minerals and particle-reactive radionuclides) in sedimentary deposits and (b) the aforementioned topographically controlled sedimentary imprints of (internal) tides are useful in the reconstruction of past changes of tidal forcing in the deep sea.  相似文献   

19.
The natural isotope 234Th is used in a small-scale survey of particle transport and exchange processes at the sediment–water interface in the Benguela upwelling area. Results from water and suspended particulate matter (SPM) samples from the uppermost and lowermost water column as well as the underlying sediment of three stations are compared. The stations are situated in different sedimentological environments at 1200–1350 m water depth at the continental slope off Namibia. Highly differing extent and particle content of the bottom nepheloid layer (BNL) are determined from transmissometer data. Three models are presented, all explaining the 234Th depletion of the BNL and 234Th excess of the surface sediment that were observed. While the first model is based solely on local resuspension of surface sediment particles, the second evaluates the influence of vertical particle settling from the surface waters on the 234Th budget in the BNL. The third model explains 234Th depletion in the BNL by sedimentation of particles that were suspended in the BNL during long-range transport. Particle inventory of the BNL is highest at a depocenter of organic matter at 25.5°S, where strong deposition is presently taking place and lateral particle transport is suggested to predominate sediment accumulation. This is supported by the high settling flux of particles out of the BNL into the sediments of the depocenter, exceeding the vertical particle flux into sediment traps at intermediate depth in the same area by up to an order of magnitude. High particle residence/removal times in the BNL above the depocenter in the range of 5–9 weeks support this interpretation. Comparison with carbon mineralization rates that are known from the area reveals that, notwithstanding the large fraction of advected particles, organic carbon flux into the surface sediment is remineralized to a large extent. The deployment of a bottom water sampler served as an in situ resuspension experiment and provided the first data of 234Th activity on in situ resuspended particles. We found a mean specific activity of 86 disintegrations per minute (dpm) g−1 (39–339 dpm g−1), intermediate between the high values for suspended particles (in situ pump: 580–760 dpm g−1; CTD rosette: 870–1560 dpm g−1) and the low values measured at the sediment surface (26–37 dpm g−1). This represents essential information for the modeling of 234Th exchange processes at the sediment–water interface.  相似文献   

20.
The deficit of 234Th relative to its radioactive parent 238U in the surface ocean can yield reliable estimates of vertical Particulate Organic Carbon (POC) fluxes to deeper waters, but only when coupled with an accurate ratio of POC concentration to activity of 234Th on sinking matter. Assuming a simple partitioning of suspended phytoplankton mass between single cells and flocs, we calculate the ratio of the POC flux estimated from 234Th deficit to the actual POC flux (p ratio, Smith, J.N., Moran, S.B., Speicher, E.A., in press. The p-ratio: a new diagnostic for evaluating the accuracy of upper ocean particulate organic carbon export fluxes estimated from 234Th/238U disequilibrium. Deep-Sea Research I.). The p ratios are calculated under the assumption that particle surface area is correlated with 234Th activity and particle volume is correlated with POC concentration. The value of the p ratio depends on the relative contributions of single cells and flocs to the vertical flux. When large single cells make up a significant fraction of the vertical flux, p ratios are less than one, meaning POC fluxes estimated from 234Th deficits underestimate actual POC fluxes. When large single cells are abundant but do not sink fast enough to contribute to vertical POC flux, p ratios are greater than one (up to 3 × overestimate). Factor analysis of the model indicates that altering the extent of flocculation in suspension and changing the density and maximum size of phytoplankton cells have the greatest effects on the p ratio. Failure to measure the properties of flocs when characterizing the ratio of POC to thorium on sinking matter potentially leads to large overestimation of the POC flux (over 20 ×). Failure to characterize the POC to thorium ratio of large particles, by, for example, destruction of phytoplankton cells in pumps, can lead to underestimation of POC flux. Estimates of POC flux should be most reliable in highly flocculated suspensions populated by small cells and rapidly sinking flocs. These conditions are often associated with intense phytoplankton blooms.  相似文献   

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