HR: 0800h
AN: H51G-0446    [Abstracts]
TI: Establishing the Temporal Resolution of High-Latitude Paleoclimatic and Paleomagnetic Signals in Bioturbated Gulf of Alaska Continental Margin Sediments
AU: * Rosen, G P
EM: rosengp@ufl.edu
AF: University of Florida Department of Geological Sciences, PO BOX 112120 241 Williamson Hall, Gainesville, FL 32611-2120 United States
AU: Jaeger, J M
EM: jaeger@geology.ufl.edu
AF: University of Florida Department of Geological Sciences, PO BOX 112120 241 Williamson Hall, Gainesville, FL 32611-2120 United States
AU: Stoner, J S
EM: jstoner@coas.oregonstate.edu
AF: College of Oceanic & Atmospheric Sciences Oregon State University, 104 COAS Administration Building, Corvallis, OR 97331-5503 United States
AU: Channell, J E
EM: jetc@geology.ufl.edu
AF: University of Florida Department of Geological Sciences, PO BOX 112120 241 Williamson Hall, Gainesville, FL 32611-2120 United States
AB: Under the right depositional conditions, continental margin strata may preserve valuable records of climatic, tectonic and geochemical changes in the adjacent landscapes. Whereas anoxic basins containing laminated strata are a preferred depositional environment for paleoclimate records, they are geographically limited, thus diminishing their usefulness at examining global landscape changes. Bioturbated margin strata are far more ubiquitous, but under slow sediment accumulation, proxies of decadal-scale climate changes, which may have a large impact on landscape modification, may not be preserved. Additionally, paleosecular variations (PSV) and relative paleointensity of natural remanent magnetization (NRM) in sediments are increasingly being used as global chronometers, but little field data exists from continental margins to examine the use of these tools in rapidly bioturbated strata common to this setting. When utilizing marine sedimentary proxies and strata to interpret paleoclimatic and paleomagnetic signals, respectively, it is necessary to consider the temporal resolution and fidelity of those signals and the conditions under which they are emplaced and preserved. Specifically, to what degree is bioturbation degrading or time-integrating the signal? The degree of degradation is proposed to vary with the transit time (TT) through the biologically mixed surface layer (TT= layer thickness/sediment accumulation rate) and the intensity of bioturbation in this layer, as represented by the biodiffusivity coefficient, D_b, which has been shown to be highly variable (10~100 cm²/yr) on continental margins. Theoretically, weakly mixed strata undergoing rapid accumulation provide the best signal preservation. To quantify preservation potential, samples were collected along the Gulf of Alaska (GoA) margin aboard the R/V Maurice Ewing in 2004 (EW0408). Coring locations included fjord, shelf and fan sites and spanned a range of depositional environments from glacimarine to biologically productive bays. Multicores that preserved the sediment-water interface were x-rayed and subsampled for radioisotopic activity measurement by gamma spectroscopy. Multicores also were subsampled with ODP-style u-channels for measurement of NRM. X-radiographs and ²³⁴Th and ²¹⁰Pb activities were used to establish D_b, sediment accumulation rates and the thickness of the surface mixed layer. D_b values from representative depositional environments are >50 cm²/yr in biogenic-rich fjord sediments, 10-20 cm²/yr in the glacially sourced sediments of the continental shelf and upper slope, and <10 cm²/yr for submarine fan sites based on 100-d ²³⁴Th time scales. The thickness of the annually mixed surface layer covaries with D_b from a high of 15-20 cm to a low of <5 cm. Given the relatively thin mixed layer, low D_b values for shelf and upper slope sites and ²¹⁰Pb-based accumulation rates (1-20 mm/yr), these glacially derived sediments are weakly mixed with signal transit times of only 1-25 years. Estimates of lock-in depth of the NRM appear to also correlate with the intensity and depth of bioturbation. Therefore, GoA environments supplied with more glacially derived sediments have the best potential to record and preserve decadal-scale paleoclimate and paleomagnetic fluctuations with minor degradation attributable to bioturbation.
DE: 3005 Marine magnetics and paleomagnetics (1550)
DE: 4219 Continental shelf and slope processes (3002)
SC: Hydrology [H]
MN: Fall Meeting 2005