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1.
We have measured inorganic nitrogen (IN) content and the isotope ratio of IN (δ15NIN) in a sediment core covering the last 145 kyr in the western subarctic Pacific (WSAP). IN content was generally high during glacial periods and shows positive correlations with both eolian dust content and the ratio of organic carbon (C) to organic nitrogen (ON) (C/ON) found in our previous studies. This means that IN was transported from continental areas to the WSAP together with eolian dust and that the IN was not contaminated by volcanic materials, because the eolian dust content was reconstructed using metal components to remove contaminating volcanic materials. Therefore, IN content in the WSAP sediments, the clay fraction of which is not greatly affected by drift deposits seen at the other sites in this region, may potentially be an effective proxy for eolian dust, without the need to consider contamination by volcanic materials. δ15NIN was generally low during glacial periods and shows negative correlations with IN, eolian dust, and C/ON. The possible causes of the observed variations in δ15NIN are as follows: (1) authigenic fixation of NH4 + in water-column and pore water of sea-floor sediments to clay minerals; (2) contamination of measured IN by highly resistant organic matter; or (3) variations in the continental source region of the eolian dust supplied to the WSAP and climatically induced changes in δ15N of soil organic matter there. The last mechanism shows the potential for δ15NIN to be used as a proxy for climate change on land, and is consistent with other published explanations of the spatial distribution of δ15NIN in modern sea-floor sediments.  相似文献   

2.
Fixed nitrogen is a key nutrient involved in regulating global marine productivity and hence the global oceanic carbon cycle. Oceanic nitrogen (N2) fixation is estimated to supply 8×1012 moles N y?1 to the ocean, approximately equal to current riverine and the atmospheric inputs of fixed N, and between 50 and 100% of current estimates of oceanic denitrification. However, the spatial and temporal variability of N2 fixation remains uncertain, mostly because of the normal low resolution sampling for diazotroph distribution and fixation rates. It is well established that N2 fixation, mediated by the enzyme nitrogenase, is a source of hydrogen (H2), but the extent to which it leads to supersaturation of H2 in oceanic waters is unresolved. Here, we present simultaneous measurements of upper ocean dissolved H2 concentration (nmol L?1), and rates of N2 fixation (μmol N m?3 d?1), determined using 15N2 tracer techniques (at 7 or 15 m), on a transect from Fiji to Hawaii. We find a significant correlation (r=0.98) between dissolved H2 and rates of N2 fixation, with the greatest supersaturation of H2 and highest rates of N2 fixation being observed in the subtropical gyres at the southern (～18°S) and northern (18°N) reaches of the transect. The lowest H2 saturation and N2 fixation were observed in the equatorial region between 8°S and 14°N. We propose that an empirical relationship between H2 supersaturations and N2 fixation measurements could be used to guide sampling for 15N fixation measurements or to aid the spatial interpolation of such measurements.  相似文献   

3.
Nitrogen isotope compositions of particulate organic matter and nitrate were analyzed for seawater sampled at five stations at the Alaskan Gyre, Western Subarctic Gyre and East China Sea, focusing on the samples from the surface to 5000 m water to characterize the nitrogen cycling in the subarctic North Pacific Ocean and its marginal sea. The 15N of particulate organic matter showed little agreement with a conceptual closed model that interprets isotopic variation as being caused by isotope discrimination on nitrate utilization. The 15N and 13C of particulate organic matter varied with the water depth. A correlation between isotope compositions and C/N elemental ratio was found generally at all stations, although some irregular data were also found in deep layers. We developed a hypothetical nitrogen balance model based on N2 fixation and denitrification in seawater and attempted to apply it to distinguish nutrient cycling using both 15N-NO3 – and N* variation in seawater. This model was applied to the observed data set of 15N-NO3 – and N* in the North Pacific water and estimated the 15N-NO3 – of primordial nitrate in the North Pacific deep water as 4.8. The North Pacific intermediate water for all stations showed similar 15N-NO3 – and N* values of 6 and –3 µmol/kg, respectively, suggesting a similar nitrogen biogeochemistry. In the East China Sea, analysis showed evidence of water exchange with the North Pacific intermediate water but a significant influence of nitrogen from the river runoff was found in depths shallower than 400 m.  相似文献   

4.
Surface sediment from the coastal bays of Gwangyang and Masan in South Korea were analyzed for their contents and isotopic values of organic carbon and total nitrogen. The sources and diagenetic alteration of organic matter were also assessed. Total organic carbon varied from 0.22% to 3.48% (average = 1.40%, n = 75), and C/N ratios varied from 2.4 to 15.2 (average = 8.79, n = 75). δ13Corg ranged from −19.92‰ to −25.86‰ (average = −21.21‰, n = 75), and δ15NTN ranged from 8.57‰ to 3.93‰ (average = 6.49‰, n = 75). Total organic carbon in both areas was associated with grain-size, with higher contents in finer grained sediment. The high carbon content observed in Masan Bay sediment correlated with its higher C/N ratio. δ13Corg and δ15NTN varied widely, attributable to various influences such as the input of terrestrial organic matter and diagenetic alteration. The depleted δ13Corg and higher δ15NTN observed in the sediment of Gwangyang Bay reflected terrestrial supply, implying that biogeochemical processes, i.e. bacterial degradation, were more active in Masan Bay sediment, which showed less depleted δ13Corg and higher δ15NTN than Gwangyang Bay sediment. δ15NTN was the more useful indicator of biogeochemical processes in the highly anoxic sediment. These results indicate that the δ13Corg and δ15NTN of sedimentary organic matter in coastal bays can indicate the source and degree of diagenetic alteration of sedimentary organic matter.  相似文献   

5.
生物固氮作用是一个重要的海洋新氮来源,在海洋生物地球化学循环中扮演着不可替代的角色。基于稳定同位素（15N2）示踪吸收法,是目前直接测定海洋生物固氮速率最有效的手段。其中,高效、洁净地将15N2引入海水培养体系,并准确定量培养体系底物的同位素示踪水平,是同位素示踪吸收法准确获取固氮速率的关键。本研究针对15N2同位素示踪剂引入这一关键环节进行了探讨,确认改进气泡法是将15N2引入海水培养体系的首选操作。在对培养体系造成的较小扰动的情况下,可将培养体系氮气底物的15N原子丰度提升至10%以上,相对于另一种导入同位素示踪剂的手段——预溶解海水法,改进气泡法将培养瓶中氮气底物的15N原子丰度提升了近200%。此外,改进气泡法还具有最小化痕量金属沾污、操作简便等优点。将改进气泡法结合与稳定同位素比值质谱测定结合,是准确测定水体生物固氮速率的推荐方法。  相似文献   

6.
Concentration and stable isotopic compositions (δ 18O) of dissolved O2 were measured in seawater samples collected from the Philippine Sea in June 2006. The in-situ O2 consumption rate and the isotopic fractionation factor (α r ) during dissolved O2 consumption were obtained from field observations by applying a vertical one-dimensional advection diffusion model to the deep water mass of about 1000–4000 m. The average O2 consumption rate and α r were, respectively, 0.11 ± 0.07 μmol kg−1yr−1 and 0.990 ± 0.001. These estimated values agree well with values from earlier estimations of Pacific deep water. The in-situ O2 consumption rates are two or more times higher north of 20°N, although the value of α r was not significantly different between the north and south. Its levels varied rapidly in the water mass of less about 2000 m depth. These results suggest that organic matter from the continent imparts a meaningful contribution to the upper water in the northern part of the area; it might produce the strong O2 minimum that is evident in the water mass from about 1000–2000 m in the northern part of the Philippine Sea.  相似文献   

7.
Temporal changes in nitrogen isotopic composition (δ15N) of the NO3 − pool in the water column below the pycnocline in Ise Bay, Japan were investigated to evaluate the effect of nitrification on the change in the δ15N in the water column. The δ15N of NO3 − in the lower layers varied from −8.5‰ in May to +8.4‰ in July in response to the development of seasonal hypoxia and conversion from NH4 + to NO3 −. The significantly 15N-depleted NO3 − in May most likely arose from nitrification in the water column. The calculated apparent isotopic discrimination for water column nitrification (ɛnit = δ15Nsubstrate − δ15Nproduct) was 24.5‰, which lies within the range of previous laboratory-based estimates. Though prominent deficits of NO3 − from hypoxic bottom waters due to denitrification were revealed in July, the isotopic discrimination of denitrification in the sediments was low (ɛdenit = ∼1‰). δ15NNO3 in the hypoxic lower layer mainly reflects the isotopic effect of water column nitrification, given that water column nitrification is not directly linked with sedimentary denitrification and the effect of sedimentary denitrification on the change in δ15NNO3 is relatively small.  相似文献   

8.
Most marginal seas in the North Pacific are fed by nutrients supported mainly by upwelling and many are undersaturated with respect to atmospheric CO2 in the surface water mainly as a result of the biological pump and winter cooling. These seas absorb CO2 at an average rate of 1.1 ± 0.3 mol C m−2yr−1 but release N2/N2O at an average rate of 0.07 ± 0.03 mol N m−2yr−1. Most of primary production, however, is regenerated on the shelves, and only less than 15% is transported to the open oceans as dissolved and particulate organic carbon (POC) with a small amount of POC deposited in the sediments. It is estimated that seawater in the marginal seas in the North Pacific alone may have taken up 1.6 ± 0.3 Gt (1015 g) of excess carbon, including 0.21 ± 0.05 Gt for the Bering Sea, 0.18 ± 0.08 Gt for the Okhotsk Sea; 0.31 ± 0.05 Gt for the Japan/East Sea; 0.07 ± 0.02 Gt for the East China and Yellow Seas; 0.80 ± 0.15 Gt for the South China Sea; and 0.015 ± 0.005 Gt for the Gulf of California. More importantly, high latitude marginal seas such as the Bering and Okhotsk Seas may act as conveyer belts in exporting 0.1 ± 0.08 Gt C anthropogenic, excess CO2 into the North Pacific Intermediate Water per year. The upward migration of calcite and aragonite saturation horizons due to the penetration of excess CO2 may also make the shelf deposits on the Bering and Okhotsk Seas more susceptible to dissolution, which would then neutralize excess CO2 in the near future. Further, because most nutrients come from upwelling, increased water consumption on land and damming of major rivers may reduce freshwater output and the buoyancy effect on the shelves. As a result, upwelling, nutrient input and biological productivity may all be reduced in the future. As a final note, the Japan/East Sea has started to show responses to global warming. Warmer surface layer has reduced upwelling of nutrient-rich subsurface water, resulting in a decline of spring phytoplankton biomass. Less bottom water formation because of less winter cooling may lead to the disappearance of the bottom water as early as 2040. Or else, an anoxic condition may form as early as 2200 AD. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

9.
Nitrous oxide (N2O) is a trace gas that is increasing in the atmosphere. It contributes to the greenhouse effect and influences the global ozone distribution. Recent reports suggest that regions such as the Arabian Sea may be significant sources of atmospheric N2O.In the ocean, N2O is formed as a by-product of nitrification and as an intermediary of denitrification. In the latter process, N2O can be further reduced to N2. These processes, which operate on different source pools and have different magnitudes of isotopic fractionation, make separate contributions to the 15N and18O isotopic composition of N2O. In the case of nitrification in oxic waters, the isotopic composition of N2O appears to depend mainly on the 15N/14N ratio of NH+4 and the 18O/16O ratio of O2 and H2O. In suboxic waters, denitrification causes progressive 15N and 18O enrichment of N2O as a function of degree of depletion of nitrate and dissolved oxygen. Thus the isotopic signature of N2O should be a useful tool for studying the sources and sinks for N2O in the ocean and its impact on the atmosphere.We have made observations of N2O concentrations and of the dual stable isotopic composition of N2O in the eastern tropical North Pacific (ETNP) and the Arabian Sea. The stable isotopic composition of N2O was determined by a new method that required only 80–100 nmol of N2O per sample analysis. Our observations include determinations across the oxic/suboxic boundaries that occur in the water columns of the ETNP and Arabian Sea. In these suboxic waters, the values of δ15N and δ18O increased linearly with one another and with decreasing N2O concentrations, presumably reflecting the effects of denitrification. Our results suggest that the ocean could be an important source of isotopically enriched N2O to the atmosphere.  相似文献   

10.
A method for the determination of the δ15N of nitrate in seawater described by Cline and Kaplan (1975) has been modified for application to low-level nitrate samples. We have minimized the reagent blank problem by replacing the Devarda's alloy with an aluminum reagent, and have also established a procedure that yields quantitative (93 ± 2%) extraction of nitrogen even at low nitrate levels. Though the amounts and the δ15N of the blank N varied from one reagent set to another, with these modifications, an overall N blank was reduced to approximately 0.80 ± 0.33 μmole N having an estimated δ15N value of −1.8‰. After blank and yield corrections, the measured isotopic composition of nitrate differed by approximately 0.1‰ from the actual value while the precision was within ±0.2‰ at the 1.25 μM level. The modified procedure was applied to seawater samples collected from the equatorial Pacific in order to compare the N blanks in field samples with those derived from laboratory experiments. The results support the suitability of the modified approach for isotopic analysis of oceanic nitrate in shallow water. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

11.
In this study we used two stable isotopes, δ13C and δ18O, for water mass classification in the coastal region off eastern Hokkaido. δ13C* values, which were corrected for the biological effect, and δ 18O values up to 300 m depth suggested that the isotopic character of the onshore and offshore water in the southern Okhotsk Sea, the Nemuro Strait and the western North Pacific could be explained by the mixing of three source waters: the Oyashio water (OYW), Soya Warm Current water (SWCW) and East Sakhalin Current water (ESCW). In summer, δ 13C*-δ 18O plots indicated mixing between SWCW from the southern Okhotsk Sea and OYW in the Pacific coast of southeastern Hokkaido, while temperature-salinity plots of the onshore water showed minimal difference from the offshore OYW. In winter, on the other hand, the mixed water of ESCW and OYW (or SWCW) appeared in the Pacific coastal region, distributed as cold, low salinity onshore water. Finally, we estimated mixing ratios of OYW, SWCW and ESCW in the coastal region of western North Pacific using their mean values of δ 13C* and δ 18O as endmembers. These results suggest seasonal and yearly changes of water mass combination en route from the southern Okhotsk Sea to the western North Pacific.  相似文献   

12.
We have developed an ecosystem model including two nitrogen isotopes (14N and 15N), and validated this model using an actual data set. A study of nitrogen isotopic ratios (δ15N) using a marine ecosystem model is thought to be most helpful in quantitatively understanding the marine nitrogen cycle. Moreover, the model study may indicate a new potential of δ15N as a tracer. This model has six compartments: phytoplankton, zooplankton, particulate organic nitrogen, dissolved organic nitrogen, nitrate and ammonium in a two-box model, and has biological processes with/without isotopic fractionation. We have applied this model to the Sea of Okhotsk and successfully reproduced the δ15N of nitrate measured in seawater and the seasonal variations in δ15N of sinking particles obtained from sediment trap experiments. Simulated δ15N of phytoplankton are determined by δ 15N of nitrate and ammonium, and the nitrogen f-ratio, defined as the ratio of nitrate assimilation by phytoplankton to total nitrogenous nutrient assimilation. Detailed considerations of biological processes in the spring and autumn blooms have demonstrated that there is a significant difference between simulated δ15N values of phytoplankton, which assimilates only nitrate, and only ammonium, respectively. We suggest that observations of δ 15N values of phytoplankton, nitrate and ammonium in the spring and autumn blooms may indicate the ratios of nutrient selectivity by phytoplankton. In winter, most of the simulated biogeochemical fluxes decrease rapidly, but nitrification flux decreases much more slowly than the other biogeochemical fluxes. Therefore, simulated δ15N values and concentrations of ammonium reflect almost only nitrification. We suggest that the nitrification rate can be parameterized with observations of δ15N of ammonium in winter and a sensitive study varying the parameter of nitrification rate.  相似文献   

13.
The circulation of intermediate and deep waters in the Philippine Sea west of the Izu-Ogasawara-Mariana-Yap Ridge is estimated with use of an inverse model applied to the World Ocean Circulation Experiment (WOCE) Hydrographic Program data set. Above 1500 m depth, the subtropical gyre is dominant, but the circulation is split in small cells below the thermocline, causing multiple zonal inflows of intermediate waters toward the western boundary. The inflows along 20°N and 26°N carry the North Pacific Intermediate Water (NPIW) of 11 × 109 kg s−1 in total, at the density range of 26.5σθ–36.7σ2 (approximately 500–1500 m depths), 8 × 109 kg s−1 of the NPIW circulate within the subtropical gyre, whereas the rest is conveyed to the tropics and the South China Sea. The inflow south of 15°N carries the Tropical Salinity Minimum water of 35 × 109 kg s−1, nearly half of which return to the east through a narrow undercurrent at 15–17°N, and the rest is transported into the lower part of the North Equatorial Countercurrent. Below 1500 m depth, the deep circulation regime is anti-cyclonic. At the density range of 36.7σ2, – 45.845σ4 (approximately 1500–3500 m depths), deep waters of 17 × 109 kg s−1 flow northward, and three quarters of them return to the east at 16–24°N. The remainder flows further north of 24°N, then turns eastward out of the Philippine Sea, together with a small amount of subarctic-origin North Pacific Deep Water (NPDW) which enters the Philippine Sea through the gap between the Izu Ridge and Ogasawara Ridge. The full-depth structure and transportation of the Kuroshio in total and net are also examined. It is suggested that low potential vorticity of the Subtropical Mode Water is useful for distinguishing the net Kuroshio flow from recirculation flows. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

14.
We studied the seasonal, diel, and vertical distribution of phytoplankton N2 fixation to understand the relative contributions of unicellular and filamentous nitrogen fixers (diazotrophs) to N2 fixation and nitrogen recycling in the northern South China Sea (SCS) and the neighboring upstream Kuroshio. N2-fixation rates were measured by the 15N2 tracer technique (addition by bubble) on unicellular (<10 or 20 µm) and the filamentous diazotrophs (>10 or 20 µm, mostly Trichodesmium and Richelia) fractionated by 10- or 20-µm mesh sizes. The mean depth-integrated total (unicellular+filamentous) N2-fixation rates in the SCS (51.7±6.2 µmol N m−2 d−1) averaged 1/3 of that in the Kuroshio (142.7±29.6 µmol N m−2 d−1), with higher rates in the winter than in other seasons in the SCS and the opposite seasonal pattern in the Kuroshio. Unicellular diazotrophs contributed 65% of the total N2 fixation in the SCS, which were negatively correlated with surface temperature and, as for total N2 fixation, were higher in the winter when Trichodesmium spp. were scarce. In comparison, the unicellular diazotrophs contributed 50% of total N2 fixation in the Kuroshio, and their contributions were not significantly correlated with surface temperature. In both the SCS and the Kuroshio, the unicellular N2 fixation was more important during the night than during the day, and in the deep euphotic layer than in the surface layer, even in the daytime. Our results show that the unicellular diazotrophs were important N2 fixers and contributed significantly to N2 fixation in the tropical marginal seas, more so in the SCS than the Kuroshio.  相似文献   

15.
Concentrations of particulate organic nitrogen (PN), dissolved inorganic nitrogen (DIN), and their nitrogen isotope ratios (δ 15N) in the Kiso-Sansen Rivers were determined from monthly observations over the course of a year to assess variations in the form and sources of riverine nitrogen discharged into Ise Bay. The δ 15N values of NO3 − observed in the Kiso-Sansen Rivers showed a logarithmic decreasing trend from 8 to 0‰, which varied with the river discharge, indicating mixing between point sources with high δ 15N and non-point sources with low δ 15N. The influence of isotope fractionation of in situ biogeochemical processes (mainly DIN assimilation by phytoplankton) on δ 15N of NO3 − was negligible, because sufficient concentrations of NH4 + for phytoplankton demand would inhibit the assimilation of NO3 −. A simple relationship between river discharge and δ 15N of NO3 − showed that the fraction of total NO3 − flux arising from point sources increased from 4.0–6.3% (1.1–1.8 tN day−1) during higher discharge (>600 m3 s−1) to 30.2–48.3% (2.6–4.1 tN day−1) during lower discharge (<300 m3 s−1). Riverine NO3 − discharge from the Kiso-Sansen Rivers can explain 75% of the variations in surface NO3 − at the head of Ise Bay over the year.  相似文献   

16.
To understand the processes transporting nitrate to the surface layer of the western and central equatorial Pacific, we measured the nitrogen isotopic ratio of nitrate (δ 15NO 3 − ), which is a very useful tracer of the source of nitrate, above 200 m depth in this region in December 1999. δ 15NO 3 − is higher (about 13.0‰) in the surface water than in the subsurface water (where it is about 6.5‰) due to isotopic fractionation during nitrate uptake by phytoplankton. The δ 15NO 3 − value has a roughly linear relationship with the natural logarithm of nitrate concentration (ln[NO 3 − ]). However, for values above 150 m depth, the intercept of this linear relationship varies with position from east to west. On the other hand, the data at 200 m depth at all observation stations are concentrated around a single point (ln[NO 3 − ] = 2.5 and δ 15NO 3 − = 6.5‰) and do not fit the linear relationships for the shallower values. To examine the meaning of the observed distributions of δ 15NO 3 − and nitrate concentration we developed a box model including nitrogen and nitrogen isotopic cycles. By reproducing the observed relationship between δ 15NO 3 − and nitrate concentration using this model we found that most nitrate is transported horizontally from the eastern equatorial Pacific. We also conducted case studies and investigated the effects of differences in pathways of nitrate transport on the distributions of δ 15NO 3 − and nitrate concentration. From these studies we concluded that the observed linear relationships between δ 15NO 3 − and ln[NO 3 − ], having a common slope around 6‰ but different intercepts at each station, are evidence of the significant horizontal transport of nitrate to the surface water in this area.  相似文献   

17.
New N inputs via biological N2-fixation play a critical role in supporting oceanic primary production and influencing global biogeochemical cycles. Numerous studies have documented significant N2-fixation rates in the North Atlantic, but relatively little is known of the pathways and fluxes of new N through planktonic food webs. Here, we report the natural abundance of 15N in, and contribution of diazotroph N (ND) inputs to, suspended particles and mesozooplankton collected along two transects in the subtropical North Atlantic Ocean (STNA). Samples were collected in April-May 2000 along the two main transects to evaluate spatial trends of 15N within the Western Atlantic and across the basin. We found that N2-fixation is a key component in supporting both primary and secondary productions throughout the STNA. ND contribution was generally higher for suspended particles than for mesozooplankton, and we observed a high ND contribution to suspended particles over large spatial scales in the western and central STNA. Mesozooplankton ND incorporation suggests that diazotroph production supports oceanic food webs over longer timescales (e.g., weeks to months) than that of particle turnover (e.g., days). Larger mesozooplankton (1000-4000 ??m) generally incorporated more ND than smaller mesozooplankton (250-1000 ??m), and thus may exert a stronger influence on an ND movement within the water column. Spatial and vertical patterns of variation in mesozooplankton ??15N also suggest either broad geographical differences in an ND contribution to secondary production, or temporal variations in ND incorporation via mesozooplankton communities.  相似文献   

18.
The paper presents the records of several paleoproductivity proxies (PP) (biogenic opal and Ba (Si_bio, Ba_bio), organic carbon (C_org) and carbonate Ca_carb, chlorin and benthic foraminifera abundance (BFA)) in sediments of the Okhotsk Sea for the last 80 kyr with a resolution of ∼100–300 years. The sediment age model was based on the AMS 14C data, records of benthic foraminifera δ18O, paleointensity of the Earth’s geomagnetic field and magnetic susceptibility. PP values demonstrate series of severe prolonged productivity drops in the Okhotsk Sea followed by a sharp increase during the last glaciation. On the basis of quantitative estimations of the paleoproductivity in the Okhotsk Sea during the cold MIS 2 and warm Holocene (Gorbarenko and Goldberg, 2005), it is suggested that the millennium scale relationship in productivity-climate changes of this basin was similar: an increase in the sea’s productivity was related with regional climate warming and vice-versa. The PP records of productivity/climate oscillations in the Okhotsk Sea during MIS 2–4 occurred contemporaneously with the North Atlantic cold Heinrich events (HE) and Greenland Dansgaard-Oyeshger interstadial (DOI). Observed successions of prolonged climate cooling events followed by rapid, abrupt warming are similar to climate and environmental oscillations in the N. Atlantic and Greenland, that confirms the millennium-scale climate changes synchronicities in the Northern Hemisphere including the far NW Pacific, the hydrology and climate conditions of which are close to those of the Okhotsk Sea. Synchronism of the N. Hemisphere severe cooling (including the Okhotsk Sea) with the Antarctic warming suggests that mechanisms of the “seesaw” effect (Blunier and Brook, 2001) in the low latitude heat redistribution between high latitudes of both hemispheres were more complicated than direct NADW formation forcing and encompasses the global atmospheric reorganization. Within the PP used a closer connection in the Okhotsk Sea millennium oscillations was observed for the C_org, BFA and chlorin; Ba_bio increases more closely to DOI. Si_bio variability does not show any evident correlation with productivity changes.  相似文献   

19.
We used the nitrogen isotope ratio of algae, suspended particles and nitrate in the water column to track spatial variations in the marine nitrogen cycle in the Mediterranean Sea. Surface PON (5–74 m) was more depleted in 15N in the eastern basin (−0.3±0.5‰) than in the western basin (+2.4±1.4‰), suggesting that nitrogen supplied by biological N2 fixation may be an important source of new nitrogen in the eastern basin, where preformed nitrate from the Atlantic Ocean could have been depleted during its transit eastward. The δ15N of nitrate in the deep Mediterranean (∼3‰ in the western-most Mediterranean and decreasing toward the east) is significantly lower than nitrate at similar depths from the North Atlantic (4.8–5‰), also suggesting an important role for N2 fixation. The eastward decrease in the δ15N of surface PON is greater than the eastward decrease in the δ15N of the subsurface nitrate, implying that the amount of N2 fixation in the eastern Mediterranean is great enough to cause a major divergence in the δ15N of phytoplankton biomass from the δ15N of the nitrate upwelled from below. Variations in productivity associated with frontal processes, including shoaling of the nitracline, did not lead to detectable variations in the δ15N of PON. This indicates that no differential fertilization or productivity gradient occurred in the Almerian/Oran area. Our results are consistent with a lack of gradient in chlorophyll-a (chl-a) and nitrate concentration in the Alboran Sea. 15N enrichment in particles below 500 m depth was detected in the Alboran Sea with respect to surface PON, reaching an average value of +7.4±0.7‰. The δ15N in sinking particles caught at 100 m depth (4.9–5.6‰) was intermediate between suspended surface and suspended deep particles. We found a consistent difference in the isotopic composition of nitrogen in PON compared with that of chlorophyll (Δδ15N[PON-chlorin]=+6.4±1.4‰) in the surface, similar to the offset reported earlier in cultures for cellular N and chl-a. This indicates that δ15N of phytoplankton biomass was retained in surface PON, and that alteration of the isotopic signal of PON at depth was due to heterotrophic activity.  相似文献   

20.
Spatial distribution of the carbon and nitrogen content and their isotopic enrichment in suspended matter and sediments were measured in the Godavari estuary to identify the sources and transformation mechanism of organic matter. Significant variability in isotopic distribution was found over the entire length of the Godavari estuary, suggesting multiple sources of organic matter. The mean isotopic ratios (δ13Csed −25.1 ± 0.9, δ13Csus −24.9 ± 1, δ15Nsed 8.0 ± 2 and δ15Nsus 6.5 ± 0.9‰) and elemental concentrations (Csed 0.45 ± 0.2%, Csus 0.9 ± 0.7%, Nsed 0.07 ± 0.05% and Nsus 0.16 ± 0.1%) support a predominantly terrigenous source. Significant enrichment in the isotopic ratios of δ13C from the upper to lower estuary in both suspended (−27.5 and −24.3‰, respectively) and sedimentary (−26.2 and −24.9‰, respectively) phases indicates a decrease in the influence of terrigeneous material toward the mouth of the estuary. A significant positive relationship exists between the δ13C of suspended and sediment, which indicates that these two organic carbon pools are likely coupled in the form of a significant exchange between the two phases. A positive relationship exists between chlorophyll a and suspended organic matter, which may mean that a significant source of organic carbon is the in situ produced phytoplankton. But, applying a simple mixing model to our isotopes, data yielded about 46% as the contribution of the terrestrial source to suspended matter, which may support the excessive heterotrophic activity in the Godavari estuary reported earlier.  相似文献   

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