Cadmium isotopic fractionation in the South China Sea: the roles of biotic and abiotic processes

本研究藉由分析南海之海水、懸浮浮游生物與沉降顆粒的鎘同位素組成，來探討生物與非生物性作用如何控制鎘在海洋中循環之相對貢獻。南海海水水柱中的鎘同位素組成隨著水深的增加而減少。鎘同位素組成（ε114/110CdNIST）在表層80公尺的範圍中維持在+8到+9之間，於100到150公尺的範圍內陡降至+4到+5左右，從水深150到1000公尺間自+5緩慢下降到+3，而在1000到3500公尺的水層中維持在+3左右。我們發現鎘同位素組成在水深150公尺以下之範圍內的變化是保守的，暗示鎘在此範圍的循環主要是由水團的對流與混合作用所控制。在表層海水，我們發現採集自水深30公尺處的沉降顆粒具有與鄰近海水一致的同位素組成，同樣介於+8至+9之間。此同位素組成上的一致性顯示在表層海水中生物作用對鎘同位素分化的總體效應並不顯著。此外，在南海氣膠的沉降是表層海水中鎘的主要來源，海水與沉降顆粒具有一致且較重的鎘同位素組成暗示氣膠也同樣具有較重的組成。這些發現暗示表層海水具有較重的鎘同位素組成未必是受生物的分化作用所致。 本研究之懸浮浮游生物與沉降顆粒的鎘同位素結果更進一步揭示了浮游植物攝取、細菌分解與浮游動物攝食等幾個主要的生物作用是如何控制表層海水中鎘同位素分化的總體效應。我們發現富光層中的浮游生物中的鎘同位素組成介於-9至+7之間，顯著地比表層海水的組成還輕。這暗示了浮游生物偏好從海水攝取較輕的鎘同位素。此外，浮游生物的鎘同位素組成也比水深30公尺處之沉降顆粒還輕。由於浮游生物轉變為沉降顆粒之過程主要受微生物分解作用，與浮游動物打包作用所控制，懸浮與沉降顆粒間之鎘同位素差異暗示這些作用在顆粒轉變的過程中會從顆粒釋放出較輕的鎘同位素。我們也發現沉降顆粒中的鎘同位素組成隨著水深的增加而提升，在水深30公尺處其值介於+8到+13 之間，在水深100公尺處上升到+12至+21，到了160公尺處則上升到+16到+18。此同位素組成隨著水深增加而提升之現象暗示微生物分解作用與浮游動物打包作用在生物顆粒沉降的過程中，同樣從顆粒中分解、釋放出較輕的鎘同位素。這些發現顯示生物攝取、微生物分解與浮游動物打包作用對鎘同位素分化的影響程度是在同一個數量級上。這些作用的綜合效應在調節表層海水中溶解態與顆粒態鎘的同位素組成上扮演著關鍵的腳色。 傳統觀念認為表水生物性沉降顆粒是深層海水的溶解態鎘的主要來源。然而我們發現南海表水的沉降顆粒具有高於+8的鎘同位素組成，與全球大洋深層海水的組成+3並不一致。這個同位素上的差異暗示表水之沉降顆粒可能並非深層海水的溶解態鎘的直接來源。未來研究應探討不同水深之懸浮與沉降顆粒的鎘同位素組成，以解開深層海水中鎘的來源為何，以及表層海水與深層海水內鎘循環之間的交互作用。In this study, we determined the Cd isotopic composition in seawater, suspended plankton, and sinking particles of the South China Sea (SCS) to better understand the relative contributions of biotic and abiotic processes that control Cd cycling in the ocean. The isotopic composition in the water column decreased with depth, with ε114/110CdNIST values ranging from +8 to +9 in the top 80 m, +4 to +5 between 100 and 150 m, decreasing from +5 to +3 at depths from 150 to 1000 m, and remaining at around +3 from 1000 to 3500 m. The isotopic composition in seawater below 150 m varies conservatively, indicating that the Cd cycling is mainly controlled by advection and mixing of water masses. Comparable to the isotopic composition value in surface seawater, the ε114/110CdNIST in the sinking particles collected at 30 m ranged from +8 to +9 suggesting that the net biological isotopic fractionation in the surface water is insignificant. In the SCS, aerosol depositions are known to be the dominant source of the Cd in the surface water. Relatively heavy Cd isotopic composition can be found in the seawater and particles, which suggests that the Cd originating from aerosols is composed of similarly heavy composition. This finding indicates that the heavy Cd isotopic composition in oceanic surface water cannot necessarily be attributed to biological fractionation. The isotopic composition of Cd in suspended and sinking particles further sheds light on how the major biotic activities, including phytoplankton uptake, microbial degradation, and zooplankton grazing, control the net Cd isotopic fractionation in the surface water. The ε114/110CdNIST in the suspended plankton of the euphotic zone ranged from -9 to +7, which is significantly lighter than the ambient seawater. The isotopic difference between plankton and seawater indicates that plankton preferentially assimilate light Cd isotopes from seawater. Additionally, the composition of the plankton was also lighter than the composition of the sinking particles at 30 m. Because microbial degradation and zooplankton grazing are the major processes controlling the transformation from suspended to sinking particles, the isotopic difference between suspended and sinking particles indicates that the processes disseminate relatively light Cd through the transformation. The ε114/110CdNIST in the sinking particles increased with depth, ranging from +8 to +13 at 30 m, from +12 to +21 at 100 m, and from +16 to +18 at 160 m. The increasing isotopic composition with depth indicates that microbial degradation and zooplankton repackaging also disseminate relatively light Cd from biogenic particles during particle sinking. Consequently, the Cd isotopic fractionations via phytoplankton uptake, microbial degradation, and zooplankton repackaging take place at similar magnitudes. The combined effects of these processes play critical roles in regulating the isotopic composition of dissolved and particulate Cd in the surface water. It is traditionally believed that the dissolved Cd in deep waters predominantly originates from the biogenic sinking particles generated in surface waters. However, our study has found that the isotopic composition of Cd in the sinking particles in the SCS surface waters is ≥ +8 ε, which is not comparable to the composition in global deep waters, which is ~ +3 ε. The isotopic difference indicates that the sinking particles exported from surface waters may not serve as the direct source of Cd in deep waters. Future studies should focus on the Cd isotopic composition in suspended and sinking particles at various water depths to determine the source of Cd in deep waters and decipher the interactions of Cd cycling between surface and deep waters