HR: 15:10h
AN: OS22C-07    [PDF]
TI: Stagnation of the Eastern Tropical Pacific During Deglaciation?: Porewater Oxygen Concentration Shifts in the Gulf of Tehuantepec
AU: * Hendy, I L
EM: ihendy@umich.edu
AF: University of Michigan, 2534 C.C. Little Building, 425 E. University Ave,.University of Michigan, Ann Arbor, MI 48109-1063
AU: Thunell, R C
EM: Thunell@geol.sc.edu
AF: University of South Carolina, Department of Geological Sciences, 700 Sumter Street,University of South Carolina, Columbia, SC 29208
AU: Pedersen, T F
EM: tfp@uvic.ca
AF: University of Victoria, School of Earth and Ocean Sciences, P.O. Box 3055 STN CSC, University of Victoria, Victoria, BC V8W 3P6
AB: The oxygen minimum zone (OMZ; 400 m water depth) is a significant oceanographic feature along the North American Margin. However, during the last glacial, large changes in porewater oxygen concentrations have been documented suggesting the waxing and waning of the OMZ in close concert with rapid climate change events. It has been suggested that changes in California Undercurrent flow along the North American Margin and/or increased ventilation of intermediate waters played a role in OMZ ventilation off California. However little is known about ventilation at the source of the California Undercurrent (intermediate water in the Eastern Tropical Pacific). A depth transect of cores ($15.7\deg$N; $95.3\deg$ E; 570 to 750 m water depth) through the OMZ collected in the Gulf of Tehuantepec has provided a unique opportunity to examine regional intermediate water mass ventilation and local productivity from the present to the last glacial maximum (24 ka). The resolution of the cores (sedimentation rates of 40 to 100 cmkyr$^{-1}$) is such that evidence of OMZ responses to rapid climate timescales should have been recorded through the transition from glacial to interglacial. The cores show surprisingly similar sedimentation rates, trace metal (Mo, Re, U, Cd and Ag) and organic carbon concentrations and $\delta^{15}$N values suggesting that the water column between 570 and 750 m was relatively homogenous. A significant increase in trace metal concentration occurs during the deglacial interval coincident with increases in % organic carbon and $\delta^{15}$N, decreases in bulk sediment Si and Al contents and distinct sediment laminations. Most significantly this time interval contains the lowest sedimentation rate in the cores and yields an organic carbon flux that was constant through the deglaciation. The invariant organic carbon flux implies that the increase in trace metal concentrations seen during deglaciation cannot be the result of an increase in productivity. Further, an increase in organic carbon flux during the Holocene is not matched by an enrichment in trace metal concentration. These observations together show that porewater oxygen concentrations are not primarily controlled by productivity in this area. The low deglacial sedimentation rate suggests that less detrital clay was delivered to the site during deglaciation. Disappearance of detrital clay may be due to a number of factors including flooding of the shelf during sea level rise, winnowing of bottom sediments by contour hugging bottom currents and reduced rainfall in the region. What, then, produced the observed trace-element enrichments? The most probable explanation is that very low oxygen contents in regional bottom waters coupled with ample time for downward diffusion of trace metals allowed the elements to accumulate in the deposits, most likely as authigenic sulphides. These results imply an intensification of the oxygen minimum in the Gulf of Tehuantepec region during the deglaciation. This cannot be attributed to increased export production (and thus subsurface oxidant demand) but instead requires a primary physical control.
DE: 3025 Marine seismics (0935)
DE: 4267 Paleoceanography
DE: 4851 Oxidation/reduction reactions
DE: 4863 Sedimentation
DE: 4870 Stable isotopes
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting