Stable Isotopic Values Of Benthic Foraminifers As Possible Indicators Of Episodic Methane Seep Events In Gas Hydrate Geo-system

As a dynamic system, gas hydrate geo-system destabilization can be triggered by several factors (sea-level fluctuations, tectonics, and intermediate water temperature changes) and form episodic methane seep events playing a significant role in global climate change, global carbon budget and atmospheric methane content. The extreme environments developed by methane seep with a relative high methane and hydrogen sulfide flux, low oxygen content and high alkalinity in ambient pore water provide a valuable opportunity to study its influence on biological community and possibly the beginnings of life during the early stage of Earth history. However, several key issues such as the intensity, duration, episodicity, and trigger mechanisms of methane release are particularly poorly understood.Benthic foraminifers are critical to paleoceanograpic and paleoenvironmental reconstruct-tions relying on its high abundance, more sensitive to climate changes. Studies of Benthic foraminifers from modern methane seep environments have been conducted primarily in the South China Sea, Peruvian margin, Gulf of Mexico, California margin, Hydrate Ridge, Cascadia margin, Barents Sea, Adriatic Sea, northeastern Arabian Sea, eastern New Zealand, Papua Gulf, and offshore Japan. These studies have shown that benthic foraminifers may be capable of recording the negative carbon isotopic compositions of methane seep influenced dissolved inorganic carbon in pore water. Gas hydrate destabilization triggered by warmer intermediate water, lowered sea-level, and tectonics can release methane into the ambient and pore water which provides abundant sulfate for anaerobic oxidation of methane. Summarized analysis of previous studies have demonstrated carbon isotopic values of benthic foraminifers at methane seep environments vary from 0‰to-6‰PDB, with a few analyses giving values as low as-15.58%o PDB, whereas, species at normal sea background and little or no methane seep environments exhibitδ13C values from 0‰to-1.75‰PDB. These typical carbon isotopic differences can be used to decipher the methane seep events in gas hydrate geo-system. Also, the negative carbon isotopic values provide much more information about the intensity of methane release. High resolution analysis of benthicδ13C values gives an insight into the duration and episodicity of methane release during the evolution of gas hydrate.Here, we report carbon and oxygen isotopic values of two species of shallow infaunal benthic foraminifers (Uvigerina peregrina and Bulimina mexicana) from all five sites drilled during Integrated Ocean Drilling Program (IODP) Expedition 311, Cascadia Margin. Our main conclusions are:1.Observations by light microscope and SEM demonstrate that the foraminifer shell pores are open with well-defined edges and no evidence of authigenic carbonate precipitation or diagenetic alteration.2.Carbon and oxygen isotopic values of these two species exhibit wide ranges. The negative carbon isotopic values and positive shifts of oxygen isotopes of both species possibly indicate that the benthic foraminifers calcified during periods of methane release which influence the pore water DIC via AOM and release the H218O.3.Carbon and oxygen isotopic values of these two species show episodic fluctuations versus depth, which implies methane seep stages at all five sites are highly episodic.4.The methane release stages recognized at the five sites couple well with the eustatic sea-level fluctuations during the past 1.0 Ma, which demonstrates sea-level fluctuations play significant roles in destabilization of gas hydrate geo-system.5.Comparative analysis of the five sites demonstrates that methane seeps occur widely during the evolution of gas hydrate in accretionary margin. Gas hydrate geo-system in accretionary wedge is supposed to undergo different evolution stages from hydrate formation to hydrate dissociation, methane consumption, and hydrate loss, which leaves imprint on the benthic foraminifers shells.The results of this investigation are unique in showing the history of episodic methane release from gas hydrates as submarine seeps over an extended period of time (last 2 million years) of the Quaternary. No other study has provided a time series record of seep activity over such an extended interval of time. Although the investigation was conducted at relatively low chronologic (stratigraphic) resolution, it has demonstrated general trends in methane instability on this part of the ocean margin. Such history of episodic hydrate instability has not been demonstrated before.