ENVR 36 |
| Stratigraphic analysis of alkenone unsaturation patterns (UK'37) in marine sediment cores provides a tool for past sea surface temperature (SST) reconstruction. Nonetheless, such reconstructions bear uncertainty given unknowns about when during the year this phytoplankton signal is exported from surface waters, where in the euphotic zone the export originates and how such oceanographic details vary regionally. A prior long-term sediment trap study showed alkenone export from Gulf of California (GoCAL) surface waters is greatest in summer and, the exported UK'37 values, if interpreted using an accepted global calibration, yield significantly ‘colder' estimates than actual measured SST. To explain this observation, we analyzed in two different summers UK'37 with depth in suspended particulate material (SPM) from the GoCAL euphotic zone and in settling particle flux collected by one-day sediment trap deployments at each site. We found the UK'37 - T relationship was remarkably linear for SPM collected in waters ≤25°C, paralleling but significantly offset from the global calibration line. SPM collected in waters >25°C displayed UK'37 values of ~1, the maximum possible. UK'37 values measured in the settling particle flux were consistently lower than those measured in SPM from the overlying nitrate-deficient, surface-mixed layer. Assuming the linear UK'37 - T relationship found applicable to SPM collected in waters ≤25°C depicts the actual temperature calibration for this proxy in the summertime GoCAL, our sediment trap results imply alkenones are exported from a euphotic zone depth where waters are ~23.5°C. Comparison with hydrographic profiles established during our fieldwork showed such water temperature occurs at a depth depicting the top of the nitricline. Consequently, we conclude alkenone export in the summertime GoCAL is a subsurface, biological process sustained by diffusive nitrate availability from underlying waters and UK'37 values exported in summer provide a record of thermocline temperatures rather than SST. |
|
Symposium Honoring Dr. Walter Giger
8:30 AM-12:15 PM, Monday, August 20, 2007 Boston Park Plaza -- Stanbro Rm, Oral
Division of Environmental Chemistry |