水體中顆粒與沈積物之有機碳、氮及其穩定同位素研究：南海及翠峰湖

Organic carbon and nitrogen in particulate matter and sediments and their isotopic compositions from the South China Sea and the Tsui-Fong Lake

胡漢杰(Han-Chieh Hu)

指導教授 ： 劉康克 ； 高樹基

國立中央大學/地球科學學院/水文與海洋科學研究所/碩士(2007年)

摘要

本研究主要利用碳、氮含量及碳同位素組成，了解海洋及高山湖
泊中有機物之來源，並探討各項環境因子對同位素組成之影響。本研
究參與從2004 年五月到2005 年三月共五個航次，在SEATS 測站
(18.0∘N, 115.6∘E)，收集懸浮性顆粒物質並分析其有機碳及氮含量
和同位素組成，涵蓋了一年的四個季節，分析結果顯示SEATS 測站
200 公尺以淺的懸浮性顆粒有機物之碳同位素值為-25.2‰ ~
-21.3‰。碳和氮的莫爾數比為5.5~11.4，平均值是6.74，非常接近
Redfield ratio，表示在SEATS 測站碳來源以海源為主。
表層的浮游植物碳同位素組成，可以藉由水文、DIC 同位素組成
和化學參數，經由前人發展出之公式來計算。用這種演算法可以模擬
表層海水中顆粒性有機碳的δ13C 數值之季節性變化。在有光層較深之
水中生長速率較低，產生的有機物之碳同位素值會比較輕，但此處的
生產力較低，因此對沈降的通量似乎並不重要。
在2001 年12 月~2002 年11 月於南海北部所設置的沈積物收集
器，提供了沈降的有機顆粒同位素樣本，供本研究分析。分析結果顯
示碳同位素的變化從-25‰~-20.8‰，碳和氮的莫爾數比為5.5~18。利
用混合模式可以顯示沈降的有機物顆粒是由陸源 (-25.5‰，C/N=22)
和海源 (-22.1±1.1‰，C/N=6.63±1)所混合而成的。從南海表層沈積物的碳同位素值，比這所觀測到沈降顆粒之碳同位素值來得重，推測其
原因，不能排除是因為有機物分解之同位素變化所造成，但更可能的
原因是由於索氏效應，以及二氧化碳分壓變高導致海水中水溶性二氧
化碳濃度增加，使得分化效應變大的緣故。
本研究又分析了翠峰湖的岩芯(TFL-1B)，分析項目包括：含水量
及碳、氮、硫含量以及有機碳、氮同位素組成。分析結果顯示翠峰湖
沈積物孔隙率和TOC 含量有良好之線性關係。而沈積物中的有機碳
與硫，有密切之正相關，且其截距接近零，這顯示沈積物中硫可能都
來自有機物，無機的硫很少。
在岩芯大約100 公分深的地方，各種地球化學參數都有很大的改
變。利用岩芯中所發現的定年資料，發現大約2500 年以前，沈積速
率很快，顯示當時在沈積環境上有很大的改變。前人的研究顯示2500
年前當時的溫度可能比現在低大約2~4℃，這說明了台灣高山湖泊的
沈積物碳氮含量可以記錄當時氣候的變化。

關鍵字

氮同位素 ； 顆粒性有機物 ； 翠峰湖 ； 南海 ； 碳同位素

並列摘要

In order to investigate how particulate organic matter (POM) attains
its carbon isotopic composition, POM samples were collected by
filtration from the top 200 m at the SEATS station (18.0 oN, 115.6 oE) on
five cruises between May 2004 and March 2005. The filters containing
the POM samples were analyzed for particulate organic carbon (POC)
and particulate nitrogen (PN). The repeated sampling provided a
time-series of isotope data, which displayed δ13C values ranging
from-25.2‰ to 21.3‰ and C/N ratios ranging from 5.5 to 11.4, with a
weighted mean value of 6.74, very close to the Redfield ratio. This means
that most POC obtained at the SEATS station had a marine origin.
We successful simulated the carbon isotopic composition of
phytoplankton in the surface layer using previously reported isotope
fractionation model and the observed values of local hydrographic,
isotopic and chemical variables. Using the same algorithm, we found that
δ13C values of POC from the subsurface layer were quite low due to low
growth rates. The low growth rate also suggests that the contribution of
the subsurface layer is probably limited.
We also analyzed samples collected from sediment traps deployed in the northern South China Sea from December 2001 to November 2002.
The δ13C values varied from -25.2‰ to 20.8‰ and the C/N ratios ranged
from 5.5 to 18. We demonstrated that the sinking POM originated from
mixing of terrigenous organics (δ13C = -25.5 ‰ and C/N = 22) and
marine organics (δ13C = -22.1±1.1 ‰ and C/N = 6.63±1). Comparing our
observations with published δ13C values of sediments from the South
China Sea, we found that our samples were significantly lighter than
sedimentary organic carbon. We have good reason to believe that the
lower δ13C values of POC in the present day ocean are attributable to the
Suess effect. The increased concentration of aqueous CO2 in the surface
sea water due to the increasing atmospheric CO2 partial pressure and the
lowered δ13C values of the atmospheric CO2 contribute to the lower δ13C
values of POC.
Besides marine samples, we also analyzed a sediment core (TFL-1B)
from the Tsui-Fong Lake. The measurements included the water content,
TOC and TN concentrations, and their isotopic compositions. It was
found that sediment porosity and TOC showed good linear correlation.
TOC and total sulfur also showed very good positive correlation and its
intercept was very close to the origin, which means that most sulfur in the sediments came from organic matter.
At about 100 centimeter below the surface, all chemical parameters
displayed rather drastic changes. Based on 14C dating, we found that the
stratum at 87 cm below surface, the age was about 2500 years before
present. The deposition rate was much faster prior to 2500 years bp.
This indicates that the geochemical changes occurred along with a
drastic change in deposition rate of sediments. The isotopic and
geochemical data support the notion that the climate was colder in the
period 2500 years before present (Jian et al., 2000). The study indicates
that the high altitude lakes in Taiwan could preserve useful records of
the climate change.

並列關鍵字

South China Sea ； Carbon dioxide isotope ； POM

參考文獻

Benner, R., Fogel, M.L., Hodson, R.E., Sprague, E.K., 1987. Depletion of C-13 in Lignin and Its Implications for Stable Carbon Isotope Studies. Nature, Vol 329(Iss 6141): 708-710

Charles, C.D., Hunter, D.E. and Fairbanks, R.G., 1997. Interaction between the ENSO and the Asian monsoon in a coral record of tropical climate. Science, 277, 925-928.

Chen, Y.L.L., 2005. Spatial and seasonal variations of nitrate-based new production and primary production in the South China Sea. Deep-Sea Research Part I-Oceanographic Research Papers, 52(2): 319-340.

Chen, Y.L.L., Chen, H.Y., Karl, D.M., Takahashi, M., 2004. Nitrogen modulates phytoplankton growth in spring in the South China Sea. Continental Shelf Research, 24(4-5): 527-541.

參考資料(中文部分)