UTIG RESEARCH PROJECTS ARCHIVE

Ingo Pecher, University of Texas
Peter Clift, Woods Hole Oceanographic Institution
Carolyn Ruppel, Georgia Institute of Technology

National Science Foundation (10/1/99-9/30/01)

SUMMARY

This is a proposal to use seismic, stratigraphic, and numerical modeling techniques to examine the dynamics of the gas hydrate system on the convergent Peruvian margin. With its high fluid flux, high organic productivity, and well-characterized vertical tectonism, the margin provides an important endmember for the study of bottom simulating reflector (BSR) evolution and destruction and the relationship between the free gas and gas hydrate zones. The availability of a probable gas hydrate-free reference site in the vicinity provides an important baseline for measurements that constrain the seismic properties of gas hydrate-bearing sediments. The submission of a new Peruvian margin drilling proposal in October 1998, coupled with a two-leg R/V Sonne cruise scheduled for 2000, makes the timing ripe for project. We here propose to conduct a "piggyback" ocean bottom seismometer study as part of the R/V Sonne cruise, to use existing and new seismic reflection data from the upper-slope Lima Basin to closely constrain subsidence, sedimentation, and erosion rates, and to test the tectonic BSR suppression hypothesis through state-of-the-art numerical modeling of the dynamics of the free gas and gas hydrate system in the Lima Basin.

A major goal of this project is to test hypothesis of the tectonic suppression of BSRs:

Tectonic suppression of BSRs

Bottom simulating reflectors (BSRs) appear to be mostly caused by free gas beneath the base of the gas hydrate stability zone (BHSZ). Strong BSRs are present on the Peruvian lower slope at 12 degree S, the southern transect of ODP Leg 112. In Lima Basin further upslope no BSRs have been observed along this transect although a high organic carbon content suggests the presence of gas hydrates. The lower slope undergoes moderate uplift and strong sedimentation, whereas Lima Basin is subsiding rapidly with moderate sedimentation rates. Sedimentation causes temperature at a given point in the sediment column to increase if assuming that the thermal gradient stays constant. The BHSZ moves upward, gas hydrates dissociate to free gas. Subsidence leads to an increase in pressure and a shift of gas hydrate stability toward higher temperatures. The BHSZ moves downward, free gas at the BHSZ is absorbed into the gas hydrate zone. We quantified both effects for expected sedimentation and subsidence rates in Lima Basin. The pressure effect from subsidence is stronger than the temperature effect from sedimentation. This explains the lack of BSRs in this part of Lima Basin (Pecher et al., 1996b; von Huene and Pecher, 1999). This hypothesis based on data acquired during a R/V SONNE cruise in March and April of 2000 (GEOPEKO), a joint project of GEOMAR Kiel, the GeoForschungsZentrum Potsdam, the U. of Hamburg (all Germany) and UTIG.

Related sites:

• GeoForschungsZentrum Potsdam
• GEOMAR

References:
Pecher, I.A., von Huene, R., Ranero, C., Kukowski, N., Minshull, T.A., and Singh, S.C., 1996. Formation mechanisms of free gas beneath the hydrate stability zone at convergent margins - geophysical evidence from bottom simulating reflectors at the Peruvian and Pacific Costa Rican margin. International conference on natural gas hydrates, 593-600.
Von Huene, R. and Pecher, I.A. 1999. Neotectonics and the origins of BSRs along the Peru margin. Earth Planet. Sci. Lett., 166, 47-55.