Orbital- to sub-orbital-scale cyclicity in seismic reflections and sediment character in early Pleistocene mudstone, Santa Barbara, California.

摘要

High-resolution seismic reflection records from the Santa Barbara basin suggest that much of the early Pleistocene hemipelagic sedimentary sequence records climate variability on orbital to sub-orbital scales, much like strata of the last glacial cycle studied at ODP Site 893. This thesis develops and tests a new method to extract lithologic cyclicity from high-resolution marine seismic reflection data (towed chirp) collected on the R/V Melville in 2008 that penetrate 10's of meters below seafloor into a ∼1 km-long sequence of south-dipping seismic reflections. Spectral analysis of these data reveals orbital-scale cyclicity in Pleistocene sediments that shifts to higher frequencies at the location of an unconformity. This analysis suggests that acoustic impedance and physical properties of sediment are controlled by climatically-driven oscillations in lithologic composition and fabric during deposition. Furthermore, shifts in spectral character permit identification of unconformities and changes in sedimentation rate prior to physical sampling by core.;Cyclostratigraphic analysis of sedimentary sequences usually requires measurement of geochemical proxies on sediment material recovered from coring or drilling efforts which can be expensive and time-consuming. Seismic reflection data are a remotely sensed record of acoustic impedance contrasts in sediments which vary with sediment density and velocity changes imparted by organic matter fluctuations which are controlled by climate oscillations. With sufficient resolution, this method could allow remote detection of sedimentary cycles imparted by climate forces without retrieving sediments. Paleoclimatologically significant, orbital-scale cycles have been detected in marine seismic reflection data from the outer California Continental Borderland basins (Janik et al., 2004), Mediterranean outflow contourites (Llave et al., 2006), and the Cape Basin off southwest Africa (Weigelt and Uenzelmann-Neben, 2007). In Santa Barbara basin, where sediment character is documented to be sensitive to climatic variation at a sub-millennial scale, strata older than 1 Ma have been uplifted to the surface. These Pleistocene-age sediments dip to the south at ∼30° allowing for short core recovery and acquisition of high-resolution seismic reflection data at a constant shallow depth. Due to consistently high sedimentation rates in the basin (0.1-1 m/kyr), high-resolution seismic data such as the towed chirp seismic reflection data acquired on the 2008 R/V Melville Cruise allows detection of cycles as fine as ∼4 kyrs.;Mid-way through the seismic section, an abrupt shift in spectral character illuminates the location of an otherwise unnoticeable unconformity, and the magnitude of the shift to lower frequencies suggests a sedimentation rate increase of ∼0.06 m/kyr. This method can be used to help with coring expeditions, to identify uninterrupted sedimentary successions for cyclic analysis, and to locate discontinuities in the sedimentary record.