The Measurement Of Oxygen Isotope Composition Of Dissolved Inorganic Phosphate In Seawater And Its Application On Tracing Phosphorus Cycle In The Jiulong River Estuary And Taiwan Strait

Phosphorus (P) is an essential element required for organism’s growth. However, the concentrations of dissolved inorganic phosphorus (DIP) in most ocean surface waters are low and may ultimately limit or co-limit (e.g., with N or trace metals) the oceanic primary productivity. Great excess of DIP in the water environment will cause eutrophication, oxygen deficition and eventually cause red tide phenomenon. Furthermore, over long time scales, P can influent primary production, fluxes of buried organic carbon, and the sequestration of atmospheric carbon dioxide ultimately influence global climate.P cannot be used as an isotopic tracer for biogeochemical study due to its solely stable isotope (31P). Therefore, the biogeochemical cycle of P was relatively lack of understanding. However, the orthophosphate (PO43-) is the major inorganic form of P in the ocean, and it is strongly bonded to oxygen (O). The P-O bond in phosphate is resistant to inorganic hydrolysis and do not exchange oxygen with water without biological mediation. Thus, the oxygen isotopic composition of phosphate (δ18Op) can potentially be used to discern phosphate cycling.The first part of this dissertation describes the development of a methodology to extract DIP from natural water samples. In this method, DIP in water samples is concentrated through Mg(OH)2-PO4co-precipitation (MAGIC:magnesium-induced coprecipitation); then the resulting solution is converted to CePO4precipitate to separate DIP from dissolved salts, and through anion resin column treatment to remove the dissolved organic material (DOM). Samples with high content of the DOM need to remove the macromolecules DOM by XAD-2macroporous resin after dissolved of Mg(OH)2-PO4, additional concentrated by freeze-drying are required for low phosphorus samples. Phosphate is converted to silver phosphate ultimately, which in turn pyrolitically decomposed to CO at1350-1380℃, and δ18O is analyzed using thermal conversion-isotope ratio mass spectrometry (TC/EA-IRMS). The inlet sample (Ag3PO4) quantity is limited between0.5mg to0.8mg; and the precision of this measurement is±0.3‰. In the second part of this dissertation,δ18Op is applied to study biogeochemical cycling of phosphate. Results from these studies demonstrate the utility of theδ18OP isotopic tracer. First, the waters of the Jiulong River estuary are best represented by a two end-member mixing system. By comparing the observed values of δ18OP in Jiulong River estuary with end-member mixing values and equlibrium values, we conclude that the P utilization by organisms was slow relative to its input, thus P is not fully recycled. Second, the δ18Op values increase with the concentrations of DIP in surface water of Taiwan Strait. The most possiblly reason is that in the P-limited ocean high proportion of DOP was hydrolysed to support the requirements of primary producer, hence leading to a low value of δ18Op. Although δ18Op values increase with the depth, all of the δ18Op values in deep water are negatively deviated from equilibrium values and close to theoretical equilibrium values with depth, indicating that the utilization of DOP is more active in the shallow water of Taiwan Strait.