We aim at exploring the biogeochemical processes in seafloor and subseafloor environments with multiple approaches, including molecular characterization, isotopic analysis, and labeling experiments. The major focus for the first five years (2013-2017) is sedimentary dissolved organic matter (DOM).
DOM is a strange mixture that seems to have contradictory properties. Most prokaryotes, a major group of inhabitants in marine environemtns, are osmotrophic with their metabolic waste also in the dissolved state. Therefore, it might be reasonale to assume that DOM is the carbon pool most reflective to microbial activities. However, previous study of seawater and pore-water water DOM pointed out that marine DOM has a refractory nature. This observation led to the proposition of "microbial carbon pump", a hypothesis stating that refractory DOM is a product of mocrobial activity and a mechanism of carbon sequestration at the millennial time scale. Reconcilation of such a dichtomy relies on a better understanding of the composition of alteration mechanisms of this carbon pool.
How to analyze?
DOM can be analyzed by spectroscopic, mass spectrometric, and compound-specific approaches. We will perform molecular and isotopic analysis of DOM with the latter two approaches.
Other supplementary work?
Other supplementary work, such as analysis biogeochemical paramters or microbial composition, is essential for understanding DOM composition and alteration within the broader context of early diagenesis. The study of the relationship between DOM production and anaerobic methane oxidaition is an excellent example how DOM study can be linked to biogeochemistry.
We are currently investigating the following materials in order to answer process-specific questions:
- Hydeothermal field: The ultimate fate of most sedimentary organic matter is thermal alteration by Earth's internal heat. Hydrothermal environments are few exceptions in today's ocean where we can observe such processes near the seafloor. Our research material is from the Guaymas Basin, Gulf of California. The Guaymas Basin is one of the few hydrothermal systems covered with thick organic-rich sediment; hydrothermal petroleum production has been reported. Our goal is to clarify the effects of heat on sedimentary dissolved organic matter in such a dynamic system. Our future research target includes the shallow hydrothermal field of the Turtle Mountain Island, offshore northeastern Taiwan.
- Cold seeps: Marine cold seeps are best known for the presence of methane oxidation acitvities, which has been proposed to be a mechanism that contributes radiocarbon-dead DOM to seawater. To assess the rate and mechanism of methane oxidation to DOM, we propose to investigate the DOM and other major carbon pools in the cold seep field off the coast of southwestern Taiwan (northern South China Sea) under the framework of "National Science and Technology Program - Energy" (in collaboration with Prof. Chin-Chang Hung).
- Estuary: Estuaries are hot spots where diverse sedimentary biogeochemical processes are taking place. We choose one of the largest estuarine system in East Asia - the Yangtze River Estuary - for our study of the relationship between DOM and nitrogen cycle (in collaboration with Prof. Huaiyang Zhou and Hang Gao; funded via the Open Fund Project of the State Kay Laboratory of Marine Geology, Tongji University).
- Subseafloor coalbed: There are extensive coalbed systems along the northwestern coast of the Pacific Ocean, including those offshore western Taiwan. Once deeply buried and heated, coal may release large amount of DOM, which nurishes the microbial community in the surrounding rock and potentially promotes methane production. Dr. Lin has participated in the IODP Expedition 337 in 2012 and retrieved pristine coal and formation fluid samples from a deep (~2 km below seafloor) coalbed off the coast of Shimokita, Japan. These precious materials will provide insights into the relationship between coal diagenesis and methanogenesis.