Research 研究 

海洋近100年來的流動與循環，可以利用在1950年代於太平洋赤道區域進行的核子試爆所產生的大量人造核種（如鈾-236與碳-14），隨著海流傳送到其他海域的時間與豐度變化來追蹤。本研究團隊在上一期的深耕計畫補助下，發展了一套於臺大分析超低豐度鈾-236的方法。應用在重建南灣珊瑚岩芯中的鈾-236豐度變動，加上配合碳-14豐度變化來探討赤道太平洋海流的傳送速度與黑潮水侵入北呂宋海峽的幅度與頻率。接續過去的研究成果，我們將會重建從1950年來蘭嶼、菲律賓西部海域以及位於北赤道洋流上的三根珊瑚岩心中化學示蹤劑的記錄，探討北赤道洋流與黑潮的淨流速與變動情形。

The natural 236U/238U in the ocean is very low. Since the 1940s, thermonuclear bomb tests, spent nuclear fuel, and nuclear power plant accidents injected large amounts of 236U into the ocean. The so-called "anthropogenic 236U" input to the surface ocean produced high 236U/238U ratios, making 236U/238U a sensitive tracer for ocean circulation and environmental safeguard monitoring. However, the 236U abundance is less than only one-billion-time smaller than the 238U. To detect the two isotopes simultaneously is challenging and only became possible about ten years ago using an accelerator mass spectrometer (AMS) with power up to several million volts. The AMS requires a large sample size, long analytical time, and high overall cost, limiting its popularity. 

As the corresponding author and first author, my team and I accomplished an impossible scientific mission to establish a novel technique using a piece of commonly available instrumentation, a multi-collector high-resolution inductively coupled plasma mass spectrometer (MC-ICPMS). As a result, we can determine atomic 236U/238U ratios in samples with only femtograms of 236U.  The major interference, the abundance sensitivity of 238U tail at 236 atomic mass unit, is reduced from 10-6 to 10-10 with the retarding potential quadrupole lens. We also reduced and corrected for other interferences, including the polyatomic interferences from hydride, nitride, lead, and plutonium. In addition, we carefully evaluate the non-linear detector behavior. The breakthrough in measuring the difficult-to-get uranium isotope ratio (236U/238U) of an extremely small sample size for ocean water columns and coral core samples is expected to significantly progress modern oceanography and a new direction in ocean circulation research. The new method is also applicable to safeguard purposes for nuclear fuel/waste products.

Taiwan's beautiful yet degrading coral reef ecosystems received much more attention in their biodiversity than in their water chemistry. An urgent need is to investigate the abiotic forcing, including the chemical compositions of the reef water, influencing the reef system to guide environmental management. Thus, I collaborated with coral ecologist Dr. Vianney Denis, advising his student to conduct the chemical analyses to combine with the community structures research for Taiwan's northeast coastal ecosystem (Hsiao et al., 2021). My lab has also made efforts to understand dissolved organic carbon and nitrogen biogeochemistry. Our research suggests that the organic components are more suitable than the inorganic nutrients in reflecting the changes in water quality (Lin et al. nearly ready for submission; Yen et al. in prep ).

Submarine hydrothermal systems are unique settings that support living organisms via chemical energy. However, accessing the deep-sea vents is challenging, especially the hydrothermal fluids flowing deeply in the subbasement below the thick marine sediment. Therefore, collaborators and I modified sampling systems (Lin et al., 2020a) to collect high-quality samples for dissolved organic matter biogeochemical research (Lin et al., 2019a) and for primordial helium-3 degassing research (Lin et al., 2020b). Although shallow-hydrothermal vents are relatively easier to sample, it is difficult to process the hydrothermal fluid samples with complex matrixes for further analysis. Thus, we have modified an existing method to comprehensively survey the Kueishantao shallow hydrothermal vent H2S. In addition, we use the H2S dynamics to constrain the amount of sulfide energy available for living organisms in this system (Lin et al. in prep). 

Marine dissolved organic carbon (DOC) is the largest fixed carbon pool in the ocean and is highly depleted in radiocarbon. Thus, DOC is inferred to be largely refractory to removal processes that operate less than millennial timescales. However, a growing number of reports have shown that a large fraction of marine DOC can be effectively removed during circulation through submarine hydrothermal systems. However, what is not clear is whether the DOC that remains in hydrothermal fluids is remnant non-reactive DOC from recharged seawater or DOC that has been largely produced in the subsurface.

Being the first and the corresponding author, our team provided the first, and a detailed discussion emphasized the importance of aromatic compounds observed by the nuclear magnetic resonance (NMR), which may contribute to the depleted radiocarbon of the DOC. The main conclusion is that most DOC in ridge flank hydrothermal fluids results from the selective removal of seawater DOC, leaving behind isotopically depleted and non-reactive fractions. We included the Pacific warm ridge flank data, making the findings global when taken with the Atlantic data reported by Walter et al. 2018. Furthermore, we identified that isotopically enriched seawater DOC is removed in the hydrothermal flow paths, leaving isotopically depleted seawater DOC in basaltic basement fluids. Finally, we provided the first quantitative data demonstrating the net addition of aromatic compounds to crustal fluids, either through transformation and/or synthesis of aromatic compounds within the warm (65 °C) ridge-flank basaltic basement. Thus, chemolithoautotrophic or methanotrophic production in the subsurface is not a primary contributor to DOC within this system's deep crustal hydrothermal fluids.

Lin, H.-T.*, Hsieh, C. C., Repeta, D. J., Rappé, M. S. (2020) Sampling of basement fluids via Circulation Obviation Retrofit Kits (CORKs) for dissolved gases, fluid fixation at the seafloor, and the characterization of organic carbon. MethodsX, 7, 101033 (2-yr IF: 1.84).

The exploration of the chemoautotrophic hydrothermal ecosystems in the deep ocean requires precise sampling on the seafloor. Efficient usage of the human-occupied deep submersible vehicles (HOV) or the remotely operated vehicles (ROV) guarantees fruitful research cruises. It was an impossible challenge to collect hydrothermal fluids of high integrity up to tens of liters. The deep-sea sampling technique can be applied to extend our understanding of the deep-sea hydrothermal systems.

Being the leading role of the deep-sea sampling team and the first and corresponding author, I provide details of an HOV or ROV-assisted hydrothermal fluid sampling system. It featured a mobile pumping system (MPS) and included a fluid trap from which a gastight sampler can withdraw fluids. We also applied the MPS to demonstrate the value of fixing samples at the seafloor to determine redox-sensitive dissolved iron concentrations and speciation measurements. To make the best use of the deep-sea landers, we describe a miniature and mobile version of a named GeoMICROBE (MGM) sled, which permits rapid turn-over and is relatively easy for preparation and operation. MGM is capable of collecting fluid samples, filtering suspended particles, and extracting organic compounds. We validate this approach by demonstrating the seafloor extraction of hydrophobic organics from a large volume (247L) of hydrothermal fluids.