Taiwan As An Example For "Source To Sink" Processes Of Particulate Organic Matter In High Mountainous River And Surrounding Marginal Seas

The sedimentary "source to sink" system in land-to-ocean boundary is extended from the high mountain,across the continental margin,and eventually dumps into the deep sea.It is the primary locations for the erosion and deposition processes take place,which constitutes the sedimentary cycle of the earth and is also connected to the earth system for a long-term carbon cycle.At geological time scale,the concentration of atmospheric carbon dioxide was regulated mainly by the balance between the oxidation of fossil carbons and the burial of photosynthesis fixed carbon.Therefore,to better constrain the global carbon budget,it is very important to clarify the carbon erosion,transfer and burial processes.The high-standing islands in Western Pacific,occupied less than 3%of the Earth's area,but contribute 17-35%of annual global organic carbon flux to the ocean,thus being an ideal natural laboratory for studying carbon source and sink processes.Therefore,in this thesis,we collected the samples from the suspended sediment in the small mountainous river of Taiwan,the sinking sediments and on-floor sediment of reservoir,soil profiles,the surface shelf sediments,submarine canyon sinking sediment,and a piston core in Okinawa Trough.Stable carbon,nitrogen isotope and 14C of these materials were analyzed to study the erosion,transformation,and bury processes of organic carbon from source to sink perspective.The main conclusions are as follows:(1)By analyzing the 60 vertical profiles of soil carbon content and its isotope compositions,we calculated the soil organic carbon stock,and use the vertical carbon isotope enrichment factor as the proxy for the soil turnover rate.Combined with the topographical and meteorological data,we find that the soil carbon stock in Taiwan is mainly affected by temperature.That is,the higher temperature enhances the degradation rate of organic matter,leading to decline of carbon pool size.On the other hand,we also found that there was a positive relationship between the carbon isotope of the surface soil organic matter and the mean annual rainfall,presumably due to the large amount of nutrients(P and Cu)brought by the rainfall,which led to the increase in primary productivity.And there is a significant positive correlation between mean annual rainfall and carbon stock further support this inference.These findings suggest that,despite the increased rainfall might accelerate the carbon loss via enhance of soil erosion,the brought nutrient alleviate the nutrient limitation and thus promote the primary productivity,leading to an increase in carbon stocks.Thus,we speculate that as the rainfall increases under global warming scenario,Taiwan soil's carbon sink capacity will increase.(2)We investigated the seasonal variation of sinking sediment flux and associated variations of carbon and nitrogen isotope composition by deploying sediment traps in 20m and 70m depth at Feitsui Reservoir(FTR),northeastern Taiwan.During observation periods(Nov.2004-Nov.2005),seven typhoon events were recorded.Allochthonous source material(three soil profiles in upper river reach of FTR)as well as ultimate sediment deposits(five gravity sediment cores)was also collected to evaluate the organic carbon and nitrogen cycling within reservoir from source to sink perspective.We found more than 80%of annual sediment flux was exported during typhoon events,agreeing with previous studies on river side which proposed that episodic events dominate the sediment transfer in Taiwan.The carbon and nitrogen contents and their isotopic composition(?13C20m-POC and ?15N20m-PN)in non-typhoon period displayed evident seasonal variation,which was governed by reservoir intellal biogeochemical processes.In particularly,?13C20m-POC in nontyphoon period was regulated by seasonal variations of temperature,dissolved inorganic carbon pool size,and seasonal community structure alteration;while the?15N20m-PN temporal variation patterns might reflect the phytoplankton uptake multi-nitrogen sources(both ammonium and nitrate).Binary mixing model(?13C versus 1/OC)suggests the organic carbon preserved in FTR was result from mixing of different depth soil,and the reservoir autochthonous summer primary primary signal cannot be found in sediments.Collectively,our results imply that the oxidation of in-situ primary production might be an important route for C02 gas evasion in inland water.(3)A sediment trap was deployed at 610 m deep in the Gaoping submarine canyon at western Pacific during summer 2008,during which a typhoon event(Kalmaegi)was recorded.Basing on cross relations of total organic carbon(TOC),isotopic compositions(?13C,14C)and nitrogen to carbon ratios(N/C)among newly measured and previously reported source materials,i.e.,mountain soils,river suspensions,river sediments,shelf sediments and sediment trap samples,we deciphered particulate organic carbon sources and quantified the content of fossil organic carbon(OCf)and biospheric non-fossil(OCnf)for typhoon and non-typhoon transports.The ages of OCnf components in trap samples during typhoon and non-typhoon periods were similar;both pre-aged and uniformly pre-mixed.During typhoon,canyon was more connected to river and the OCnf in canyon trap was likely sourced from organics buried in ancient landslides.The plant debris cannot be found in trap except in the hyperpycnal layer,suggesting hyperpycnal flow was capable of entrain plant debris while segregation had occurred during non-hyperpycnal periods.Extreme events coupled with canyon system created an efficient way for deep sea burial of freshly produced organic-rich material.During non-typhoon,canyon is more connective to shelf,where wave and tide benefit reworking,thus,allow abiotic and biotic processes to prepare isotopically uniform and aged OCnf for canyon transport.Results shed lights on the ephemeral behavior of organics transport in a canyon system at active margin boundary.(4)We use KOH-KOBr to separate operationally defined total organic matter into oxidizable(labile)and residual fractions for content and isotope measurements.In a sediment core in the Okinawa Trough,significant amounts of carbon and nitrogen existed in the residual phase,in which the C/N ratio was?9 resembling most documented sedimentary bulk C/N ratios in the China marginal seas.Such similarity creates a pseudo-C/N interrupting the application of bulk C/N.The residual carbon,though composition unknown,it displayed a ?13C range(-22.7 to-18.9‰,mean-20.7‰)similar to black carbon(-24.0 to-22.8‰)in East China Sea surface sediments.After removing residual fraction,we found the temporal pattern of ?13CLOC in labile fraction(LOC)was more variable but broadly agreed with the atmospheric pCO2-induced changes in marine endmember ?13C.Thus,we suggested adding pCO2-induced endmember modulation into two-endmember mixing model for paleo-environment reconstruction.Meanwhile,the residual nitrogen revealed an intimate association with illite content suggesting its terrestrial origin.Additionally,?15N in residual fraction likely carried the climate imprint from land.Further studies are required to explore the controlling factors for carbon and nitrogen isotopic speciation and to retrieve the information locked in the residual fraction.