Forschungs- & Entwicklungsinformationsdienst der Gemeinschaft - CORDIS

Periodic Report Summary 1 - NOGOS (Noble gases in ocean sediments as proxies for fluid transport at subduction zones and past climate conditions)

Project objectives
In order to widen and further stimulate the use of noble-gas geochemistry in the sediment pore water the proposed research aims to systematically apply the state-of-the-art of noble-gas analysis to ocean sediments.
On the one hand the researcher will study the fluid transport in forearc regions off shore Japan. A first sampling region is located close to the epicentre of the devastating Tohoku-Oki earthquake occurred in 2011 and the second sampling region close to the Nankai Trough which is expected to be hit by an earthquake in the future. The He isotopes should allow for the first time studying the release dynamics of deep fluids in these tectonically active regions and its relation to seismic activity. Moreover, the investigation of the noble-gas signature in the pore water will set a first data basis to interpret noble-gas anomalies in regions where major earthquakes occurred or are expected and will be a further step towards the use noble gases in terrestrial fluids as indicator of seismic activity.
Although recently noble gases in pore water of lacustrine sediments allow the determination of lake level and salinity fluctuations, and the reconstruction of past environmental conditions and past eutrophic states in lakes, assessment of noble gases in ocean sediments is not yet fully developed by present experimental techniques - in particular if considering the degassing affecting ICDP/IODP deep-drilling sediments. Based on the experience gained in the field of oceanography during this fellowship, the researcher's internationally acknowledged experimental skills will allow to overcome the restrictions that still limit the application of noble gases in deep sediments from the oceans (and from the continents) by developing a new in situ sampling technique. This technique, conceived, produced, and tested in collaboration with other international partners might also allow climate reconstruction using noble gases dissolved in the pore water of deep-drilling ocean sediment cores.

Work performed during the first phase of the project
After settling at the Atmosphere and Ocean Research Institute (AORI) of The University of Tokyo, the researcher adapted the state-of-the-art methods to determine noble-gas concentrations in the pore water of ocean sediments to the laboratory of the Japanese host, Marine Analytical Chemistry group led by Prof. Dr. Sano. The implementation of the new methods set the analytical basis for the work intended offshore Japan.
As the host research group was able to characterize the geochemical changes in the He isotope signature produced in 2011 by the Tohoku-Oki earthquake based on long-term observations in the water column (Sano et al., 2014), the project shifted its focus towards the region of the Nankai Trough. This allowed comprehensive collection of water and sediment samples using the remotely operated vehicle (ROV) HyperDolphin during RV Natsushima cruises NT13-08 and NT14-07. All acquired samples have been successfully analyzed to determine noble-gas concentrations at AORI.

Main results
The measurements conducted in sediment samples collected at cold seep sites at the splay fault offshore Kumano underline the emission of crustal fluids enriched in 4He (i.e., the estimated share of crustal He is about 95%). This seems to be somehow in contrast with the emission of 3He-rich fluids observed after the Tohoku-Oki earthquake in 2011 (Sano et al., 2014). The present data basis is obviously too limited to enable any assessment of causality, but it seems that fundamentally different processes/structures are involved in the observed fluid release.
Enrichment in 3He is observed in the pore water of shallower stations suggesting that the underlying splay fault does not provide a significant share of fluids. Mantle He is present in proportions of approximately 30-50% which is slightly larger than the 20-30% observed in the subduction system offshore New Zealand (Tomonaga et al., 2013).
The noble-gas measurements allow for the first time the estimation of terrigenic He fluxes in the investigated region. It turns out that He fluxes at the splay fault are extremely variable within distances of a few tens of meters. The inferred fluxes are the highest ever determined along the subduction zone offshore Japan.
The very similar He isotope signature found in all water samples is hardly sufficient to constrain the local/regional fluid emission in the investigated area. Therefore, the measurement of (noble) gases at the sediment/water interface and in the sediment pore water seems to be the most promising approach to understand fluid emission from solid earth at specific locations.

Expected final results and potential impact and use
The results of the first phase of the project pave the road towards the identification and characterization of future changes in noble-gas geochemistry at the sea floor produced in relation to major seismic events. The work carried out offshore Japan allows characterization of different transport processes and geochemical origins of emitted terrigenic fluids that can be interpreted within the specific geologic context. The new insights produced by the project will help to develop future strategies to monitor fluid emissions in the same region (e.g., by targeting the high He flux spots).

- Sano, Y., Hara, T., Takahata, N., Kawagucci, S., Honda, M., Nishio, Y., Tanikawa, W., Hasegawa, A., & Hattori, K. (2014). Helium anomalies suggest a fluid pathway from mantle to trench during the 2011 Tohoku-Oki earthquake. Nat. Commun., 5.
- Tomonaga, Y., Brennwald, M. S., & Kipfer, R. (2013). Using helium and other noble gases in ocean sediments to characterize active methane seepage off the coast of New Zealand. Mar. Geol., 344, 34–40.


Laura Klein, (Financial Officer)
Tel.: +41 58 765 5388


Life Sciences
Datensatznummer: 184234 / Zuletzt geändert am: 2016-06-07
Informationsquelle: SESAM