HR: 15:25h
AN: OS52C-07    [PDF]
TI: Migration of the Base of the Hydrate Zone Tracked by Rock Magnetism
AU: * Musgrave, R J
EM: R.Musgrave@latrobe.edu.au
AF: Department of Earth Sciences La Trobe University, Bundoora, Melbourne, VIC 3086 Australia
AU: * Musgrave, R J
EM: R.Musgrave@latrobe.edu.au
AF: School of Geosciences Monash University, Clayton Campus, Melbourne, VIC 3800 Australia
AU: Hollamby, J A
EM: j.hollamby@latrobe.edu.au
AF: Department of Earth Sciences La Trobe University, Bundoora, Melbourne, VIC 3086 Australia
AB: Previous studies have indicated that the presence of gas hydrate produces responses in the authigenic magnetic mineralogy which can be readily distinguished by the methods of rock magnetism. An index based on the Day plot of domain state has proven particularly useful in delineating hydrate occurrence. Recent ODP Legs 201 and 204 have provided the opportunity to test the broad applicability of the rock magnetic proxy for hydrate. Growth of the metastable sulfide mineral greigite in its single-domain size range is stimulated where disseminated hydrate occupies a high proportion of pore spaces. Greigite production apparently results from extracellular reduction of iron derived from iron silicates. Preservation of the metastable greigite, without further reduction to pyrite, seems to be a characteristic feature of the hydrate zone. Disseminated hydrate at Peru Margin Site 1230 from Leg 201 is present in an upper unit of Pleistocene to Holocene diatomaceous mud, but is comparatively rare in the low porosity, consolidated accretionary wedge sediments of Miocene age that occur below a hiatus at 200 meters below seafloor. Bacterial counts mirror this decreased abundance of hydrate below the hiatus: so do the rock magnetic proxies. Leg 204 sites from South Hydrate Ridge similarly show a rock magnetic response to the presence of hydrate, and what appears to be the rock magnetic signature of the base of the hydrate zone; however, this occurs about 30 m below the modern base of hydrate stability, marked by a BSR. Our suggested explanation is a rapid upward migration of the thermally-controlled base of hydrate stability resulting from a warming of bottom-water at the close of the last glacial. This result confirms and extends the Leg 146 results from the Vancouver Island Margin (Site 889), and North Hydrate Ridge (Site 892); water depth appears to have controlled the amount of bottom-water warming experienced by each site, suggesting a control from intermediate water bodies on the shelf.
DE: 1505 Biomagnetism
DE: 1540 Rock and mineral magnetism
DE: 4805 Biogeochemical cycles (1615)
DE: 4820 Gases
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting