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南海北部颗粒有机碳输出通量、季节变化及其调控过程 【作者】陈蔚芳 【导师】戴民汉

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南海北部颗粒有机碳输出通量、季节变化及其调控过程
`p:NkRfC|2I0On the Export Fluxes, Seasonality and Controls of Particulate Organic Carbon in the Northern South China Sea

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【作者基本信息】厦门大学,环境科学,2008年,博士水利论文8D!dZdq8`

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【中文摘要】 基于234Th/238U不平衡法,本论文主要研究南海北部颗粒有机碳(POC)输出通量及其季节变化。南海为陆架边缘海,物理-生物地球化学过程的时空变化剧烈,需要高时空分辨率的234Th分布来估算POC输出通量,本研究采用改进的小体积二氧化锰测定234Th的方法,所需的海水体积由传统方法的20 L缩减至2—4 L,前处理流程亦大大简化,籍此我们进行了南海北部春、夏、秋、冬四个季节(2004—2005年)高空间分辨率的234Th调查,并参考了南海南部春季的结果。本论文亦分析了南海北部四个季节(2000—2004年)的POC分布特征。本研究首先评估234Th示踪颗粒输出通量的方法,这包括不同采样方法(CTD瓶采与大体积泵采)对POC和颗粒态234Th结果的影响、POC/234Th随颗粒物粒径变化的主要原因。比较CTD瓶采与大体积泵采得到的POC,发现过滤过程中溶解有机碳(DOC)的吸附会引起瓶采POC高于泵采结果,溶解态234Th的吸附同样会造成瓶采颗粒态234Th高于泵采结果,而通过增大采样体积可减少吸附效应。大体积泵收集的分粒径颗...更多粒物样品(1—10μm,10—53μm,>53μm)的结果显示,POC/234Th随粒径的变化出现多种变化趋势,并无规律可循。比较三个粒径的POC/234Th和POC/228Th比值,发现POC的优先矿化和234Th衰变的共同作用是引起POC/234Th比值随粒径出现多变趋势的主要原因。南海北部234Th垂向分布的季节变化明显。冬季234Th在表层出现明显亏损,其活度随深度增加而增加,在真光层底部与238U趋近平衡;其它三个季节则多为层化结构,即表层与真光层底部234Th与238U处于或者接近平衡,次表层出现不平衡程度极大值,表现出寡营养盐海区的特征。水平方向上,234Th活度从陆架向海盆出现增加趋势,不平衡程度减弱,即颗粒物清除程度随离岸距离增加而减小。通过分析2000—2004年南海北部陆架区的POC分布,发现POC浓度随离岸距离增加而减少,在近岸,POC受到珠江冲淡水的影响。在依据盐度判定的冲淡水影响范围(离岸50 km)内,POC浓度的季节变化与珠江径流量季节变化一致,即夏季>春季、秋季>冬季。南海POC的垂直分布呈现表层高,底层低的一般规律,但陆架区部分站位出现底部浓度剧增的现象,主要与底部再悬浮有关。而在陆坡区,POC浓度在200 m与400—500 m附近出现明显增加,可能与南海颗粒物的沿陆坡搬运或水平运移有关。本研究采用了三维模型估算234Th输出通量。结果表明垂直输送过程对234Th通量的影响可以忽略不计;在大部分站位,水平输送过程的影响仅占234Th通量的10%以内,但在一些特殊物理水文条件下,如锋面,水平过程的影响可达30—40%。因此在陆架边缘海应用234Th/238U不平衡法估算POC输出通量,进行高时空分辨率的234Th采样是非常有必要的。需要说明的是,由于船时及调查条件的限制,本论文采用悬浮颗粒物上的POC/234Th比值计算POC输出通量,相当于POC输出通量的上限值。在2004—2005年四个季节调查期间,南海北部陆架区POC输出通量季节变化不明显,变化范围为8.0—11.3 mmol C m-2 d-1。陆坡及海盆区POC输出通量则季节变化明显,冬季表现为高输出特征,输出通量平均为9.0±6.4 mmol C m-2 d-1,其余三个季节POC输出通量在3—5 mmol Cm-2 d-1之间变化;POC输出通量的季节变化与由季风驱动的初级生产力季节变化一致,同时也与浮游植物群落结构季节变化(冬季以微型浮游植物为主,其余季节以微微型浮游植物为主)相对应,三者的耦合表明季风是驱动南海北部POC输出通量季节变化的主要原因,即在冬季混合作用加强使得下层营养盐进入上层海洋,促进初级生产力增加和浮游植物群落结构改变,进而增加POC输出通量。南海北部的ThE(234Th法得到的POC输出通量/初级生产力比值)在4—68%之间变化,大部分落在10—50%之间,平均为24%,其生物泵效率高于寡营养盐海区(<10%)。简化的箱式模型计算结果显示,春季南海北部DOC向下扩散通量平均为0.91 mmolC m-2d-1,占POC输出通量的~30%,如果仅考虑POC输出通量,南海生物泵效率可能被低估。综合分析南海与其它不同纬度陆架边缘海的生物泵效率(ThE),发现ThE均出现显著季节变化,高输出通量季节的ThE是其他季节的2—3倍。若ThE的季节变化为陆架边缘海基本特征,假设高输出通量季节的ThE是其他季节的3倍,则全球陆架边缘海的POC输出通量约占到全球海洋输出通量的~30%。因此,在全球输出通量估算中不能忽略陆架边缘海的贡献,而且进行陆架边缘海的输出通量研究时必须考虑其季节性变化,若仅以单一季节的POC输出通量外推全年的输出通量,可能低估陆架边缘海的贡献。  还原

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#R8C0]0r_-fR:n0【英文摘要】 Continental margins play a disproportionally important role in the global ocean carbon cycling and the flux of biologically produced organic carbon from the upper ocean implies the net sequestration of carbon dioxide(CO2) and the recycling of organic matter.However our understanding of the spatial and temporal variability of particulate organic carbon(POC) flux in the extremely dynamic marginal seas remains limited.In this study,we attempt to elucidate the spatial and seasonal variation of POC export based on the high resolution 234Th sampling in the northern South China Sea(SCS).234Th were measured with a modified small-volume MnO2 eoprecipitation method.Samples were collected in four cruises conducted in the winter(Feb.2004),the summer(Jul.2004),the fall(Sept.2004) and the spring(May. 2005).In this study we examined the effect of different sampling methods on POC and particulate 234Th(PTh) data,which were collected by small-volume samplers(1-2 L) and large-volume(100-600 L...更多) pumping methods.POC and PTh derived from small-volume samplers were frequently observable to be higher than from large-volume pumping.The absorption of dissolved organic carbon(DOC) and dissolved 234Th onto the filters appeared to be a primary factor causing the discrepancy.We also examined POC/234Th and POC/228Th ratios in three particle size classes (1-10μm,10-53μm,>53μm).The various trends in the POC/234Th ratio versus particle size were observed,which may be explained by a combination of 234Th decay and the preferential remineralization of POC relative to Th during particle aggregation.During the survey,the deficit of 234Th generally decreased with the distance offshore in the northern SCS.Depth profiles of 234Th may be categorized into three types with seasonal variations.In the winter,the deficit of 234Th appeared to be substantial in the surface and decreased with depth.In other seasons,234Th basically showed a stratified structure,i.e.,the subsurface maxima of 234Th deficit.Such a stratified structure is characteristic of oligotrophic ocean.At some locations on the shelf,234Th activities were lower than 238U throughout the water column and the enhanced deficit existed at the base of the eupotic zone,which were mainly caused by the resuspension of the bottom sediments.Besides the distribution of 234Th,POC distributions on the shelf and the slope were also investigated from 2000 to 2004.POC concentration decreased with the distance offshore,which indicates the influence of the Pearl River.POC concentration in the summer was higher than in the spring and the autumn and was lowest in the winter,which is consistent with the seasonal variation of the discharge of the Pearl River.POC concentration generally showed a decrease with depth.At some locations on the slope,POC peaked at 200 m and 400-500 m,which suggests the horizontal particle-transport or the particle transport along the slope in the northern SCS.Using a three-dimensional steady state model,234Th flux was estimated.It is shown that horizontal and vertical transport account for less than 10%of the overall 234Th balance for most of stations.However,the contribution of physical transport to the 234Th flux was up to 30-40%at physically dynamic regimes such as at the salinity fronts.This strongly suggests that high resolution 234Th sampling is necessary to correctly resolve the variability of POC fluxes in the marginal sea. Combined with the measurements of POC/234Th ratio on suspended particles,234Th fluxes were converted to POC export rates and regarded as the upper limit of POC flux in the northern SCS.In the northern SCS,POC export on the shelf varied from 8.0 to 11.3 mmol C m-2 d-1 with no apparent seasonal variations.In contrast,the average POC export in the regime of the slope and the basin changed from a low of 3.6±2.2 mmol C m-2 d-1 in the spring to a high of 9.0±6.4 mmol C m-2 d-1 in the winter,with a medium value in the summer(4.7 mmol C m-2 d-1) and the autumn(4.8±4.8 mmol C m-2 d-1).The seasonal pattern of POC export matched the pattern of primary production and the community structure of the phytoplankton,the enhancement of which in winter may be related to the nutrient supplies from the wind-induced mixing in the winter.ThE ratio(234Th-derived export production/Primary production ratio) in the northern SCS varied in the range of 4-68%,with an average of 24%,which was higher than the tyical value in the oligotrophic ocean(<10%).Using a simple box model,we estimated that the diffusive flux of dissolved organic carbon(DOC) was 0.91 mmolC m-2d-1 and equaled to~30%of POC export in the northern SCS in the spring,the efficiency of biological pump in the northern SCS would thus be underestimated without the consideration of the downward flux of DOC.Our updated synthesis of ThE in some marginal seas indicated the seasonal variation of ThE,which showed were two or three times of difference between seasons. Assuming that this seasonal signature is of characteristics of the marginal sea,it is estimated that POC export of the marginal sea might contribute up to 30%of the global export flux.  还原

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d;v`-X$_]Y4?0【中文关键词】 234Th/238U不平衡法; POC输出通量; 季节变化; 陆架边缘海
8e[bs;j0【英文关键词】 234Th/238U; POC Export Flux; Seasonal Variations; Marginal Sea水利论文2u@NYj"C

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