HR: 08:35h
AN: OS51D-03    [Abstracts]
TI: The use of 210Pb/Pb as a Tracer of Chemosynthetic and Photosynthetic Organic Carbon in Hydrothermal Vent Particles
AU: * Kadko, D
EM: dkadko@rsmas.miami.edu
AF: University of Miami, RSMAS/MAC 4600 Rickenbacker Causeway, Miami, FL 33149 United States
AB: Jack Dymond pioneered the use of sediment traps to understand the chemistry and flux of material emanating from submarine hydrothermal vents. For example, in one paper Roth and Dymond (1989) used the ratio of C$_org$/C$_CaCO3$ between traps to determine the fraction of chemosynthetically derived organic carbon (OC) in collected material. Knowledge of the relative proportion of chemosynthetic and photosynthetic OC is critical to mass balance calculations of hydrothermal OC inputs/exports and can also be determined from the 210Pb/Pb ratio measured in the sediment traps and suspended particles. The 210Pb/Pb ratio of particles in the ridge environment is diagnostic of the source and path of the particles. At the Endeavour Ridge, particles emanating directly from vents have a ratio close to that of the vent fluids and basalt ($\sim$0.5 dpm/$\mu$g) from where they are derived, and subsequently scavenge additional 210Pb as they disperse. It is therefore reasonable to assume that particles with a 210Pb/Pb ratio of 0.5 are "fresh" and any OC associated with them must be chemosynthetic in origin. This ratio is much smaller than that of typical (non-vent) seawater ($>$3 dpm/$\mu$g) and thus particles in the upper water column (or distant from vents) will have a high 210Pb/Pb ratio and contain OC predominantly from photosynthetic production. The 210Pb/Pb ratio of Endeavour particles increase markedly from $<$0.70 at a vent orifice, to 1.22 within the buoyant plume ($\sim$20 ma vent), to 1.4 downstream (~2 km) within the neutrally-buoyant plume at 2100 m depth. These plume particles are distinguished from those above the plume (at 1700 m with 210Pb/Pb = 3.2 dpm/$\mu$g) by lower 210Pb/Pb ratios but much higher 210Pb content. The high Pb content attests to a hydrothermal component of the plume particles. Therefore, the 210Pb/Pb ratio can be used to define two conservative endmembers for a particle population: those derived from the vents will have a ratio of 0.5 (with 100% chemosynthetic OC) and those derived from the surface ocean will have a ratio of 3.2 (from the 1700m trap, with 100% photosynthetic OC). With these endmembers, a linear relationship between the % chemosynthetic OC and 210Pb/Pb is: % OCchemo = -37 x (210Pb/Pb) + 118.45 The flux of OCchemo can be compared to the downward flux of photosynthetic OC from the surface ocean by another method utilizing 210Pb as a proxy of OC flux in the Pacific which was introduced in Moore and Dymond, (1988).
DE: 4806 Carbon cycling
DE: 4808 Chemical tracers
DE: 4825 Geochemistry
DE: 4832 Hydrothermal systems
DE: 4860 Radioactivity and radioisotopes
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting