HR: 16:15h
AN: OS52D-02 INVITED     [PDF]
TI: Carbon Cycling in Gas Hydrate Systems and Consideration of the Time Domain
AU: * Dickens, G R
EM: jerry@rice.edu
AF: Dept. Earth Science, Rice University, Houston, TX 77546 United States
AU: Snyder, G T
EM: gsnyder@rice.edu
AF: Dept. Earth Science, Rice University, Houston, TX 77546 United States
AB: Enormous amounts of microbial CH$_{4}$ reside in gas hydrate, dissolved gas and free gas bubbles in marine sediment. Global carbon cycle models and paleoenvironmental studies habitually neglect this CH$_{4}$, although its distribution clearly depends on carbon fluxes to and from the ocean, and external conditions, especially temperature. As we begin to appropriately connect this large, dynamic seafloor CH$_{4}$ cycle into broad Earth systems science, we are faced with some major conceptual problems, including how amounts, distributions, and fluxes from selected boreholes can be extrapolated globally. Well-reasoned commentary on this issue has led to the prevailing idea of typical gas hydrate systems on passive (low fluid flux) and active (high fluid flux) margins. The Ocean Drilling Program has now targeted three locations explicitly to characterize the amount, distribution and biogeochemical cycling in marine gas hydrate systems: Blake Ridge (passive), Hydrate Ridge (active), and Peru Trench (active). Although much work remains, in situ gas profiles and detailed pore water concentration profiles of many species have now been generated at all three locations. The gas hydrate zone at the Blake Ridge lies beneath a relatively thick interval of SO$_{4}$ reduction, and holds large amounts of CH$_{4}$ fairly well dispersed in pore space surrounded by very fresh waters greatly enriched in I but only moderately enriched in PO$_{4}$ and ammonia. By contrast, the gas hydrate zone at the Peru Trench site lies beneath a relatively thin interval of SO$_{4}$ reduction, and holds relatively low amounts of CH$_{4}$ in moderately fresh waters moderately enriched in I but greatly enriched in PO$_{4}$ and ammonia. Amounts and distribution of gas and pore water species at Hydrate Ridge lie somewhere between, although the most striking generalization is the variability in all components across this area. Our most basic conclusion from this emerging data is that we must move beyond classifying gas hydrate systems on physical differences alone and include the time domain. Blake Ridge represents an old, contracting system (where current CH$_{4}$ outputs exceed inputs) with low, generally unfocused fluid flow. The Peru Trench represents a younger, growing system with moderate, generally unfocused fluid flow. Hydrate Ridge represents a growing system with highly variable fluid flow.
DE: 0300 ATMOSPHERIC COMPOSITION AND STRUCTURE
DE: 1055 Organic geochemistry
DE: 3022 Marine sediments--processes and transport
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting