About This Item

Share This Item

The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 5. (May)

First Page: 937

Last Page: 938

Title: Interstitial Water Chemistry of DSDP Holes on Guatemala Transect (Leg 67) Related to Occurrence of Methane Clathrates (Gas Hydrates): ABSTRACT

Author(s): Reinhard Hesse, W. E. Harrison

Article Type: Meeting abstract


High gas-concentrations, and methane/ethane ratios approaching the DSDP-safety limit, caused premature termination of drilling on Sites 496 to 498 (2,064 to 5,497-m water depth) on the Pacific continental slope off Guatemala. Methane concentrations in organic-matter-rich Quaternary to Miocene middle and lower trench slope sediments cause the formation of clathrates that were recovered for the first time in volcanic ash from two drill holes. The potential clathrate-bearing zone (below 80 to 100 m subbottom depth) is associated with a spectacular decrease in interstitial water chlorinity, which drops below half the chlorinity of seawater at the bottom of the holes (near 400 m subbottom). It is proposed that the crystallization of gas hydrates, which like ice exclude

End_Page 937------------------------------

salts from the crystal lattice, causes upward ejection (relative to the subsiding solids) of dissolved salts. This is supported by positive 18O-isotopes (^dgr18Osmow up to +1.96 at bottom of hole 497), which make freshening of the pore waters by influx of meteoric water unlikely. If our hypothesis is correct, lowered chlorinities in sections of subtropical continental margins (high organic matter content!) might thus serve as an indicator for the occurrence of clathrates. Organic matter oxidation is associated with strong sulfate depletion (within one or a few meters from the sea floor), a pronounced increase in alkalinity (maximum between 50 and 250 m subbottom) and ammonia as well as phosphate (maxima between 100 and 200 m). Calcium and strontium rem in uniformly low throughout the holes, Mg decreases markedly with decreasing chlorinity. The fact that chlorinity does not drop to zero in the clathrate zone suggests that only a portion of the pore water is tied up in clathrate formation.

End_of_Article - Last_Page 938------------

Copyright 1997 American Association of Petroleum Geologists