HR: 15:25h
AN: GP33E-08    [Abstracts]
TI: At what depth is a post-depositional remanent magnetization locked-in?
AU: Liu, Q
EM: liux0272@yahoo.com
AF: National Oceanography Centre, University of Southampton, European Way, Southampton, SO143ZH, United Kingdom
AU: * Roberts, A P
EM: arob@noc.soton.ac.uk
AF: National Oceanography Centre, University of Southampton, European Way, Southampton, SO143ZH, United Kingdom
AB: Despite the fact that sedimentary magnetizations have been widely analysed in paleomagnetic studies over the last 60 years, considerable debate remains concerning the mechanism(s) by which sediments become magnetized. Bioturbation disturbs detrital magnetic particles after deposition, so accurate recording of the ancient geomagnetic field in bioturbated sediments is widely attributed to acquisition of a post-depositional remanent magnetization (pDRM). A pDRM is acquired when the geomagnetic field exerts a torque on a magnetic particle and aligns it with the field after the final mixing event experienced by the particle. The relationship between the Matuyama-Brunhes (M-B) boundary and oxygen isotope age tie points has been widely used to determine the pDRM lock-in depth in marine sediments. However, such analyses can be badly affected by phase discrepancies among different paleoclimatic proxies and by varying isotopic compositions of seawater in different locations and from the presence of different water masses at different depths at the same location. It is therefore necessary to separately compare benthic and planktonic oxygen isotope records for sites from the same water mass to avoid inadvertently introducing age differences to the analysis. When the global data set is subjected to such a rigorous analysis, few reliable data remain for the M-B boundary. Correlation of two benthic oxygen isotope records from ODP sites 982 and 983 (northeast Atlantic), and two planktonic oxygen isotope records from sites V28-238 and V28-239 (western equatorial Pacific), provides lock-in depth estimations for these two regions of about 23 and 21 cm, respectively. The reliability of the V28 data has been questioned, which leaves a small and poorly constrained global data set. Systematic acquisition of high quality data is needed to resolve this problem. In addition to this empirical analysis, we have analytically evaluated a range of commonly used lock-in functions to better understand complexities associated with pDRM lock-in. This analysis indicates that often-used lock-in functions will not only serve as a filter that smoothes the input geomagnetic signal, but that these functions also complicate recording by downward shifting geomagnetic signals of different wavelengths to different depths, yielding an asymmetric recording. By comparing magnetic records from ODP sites 982 and 983, the downward displacement of the M-B boundary due to the lock-in process is estimated to be about 5 cm. Therefore, we conclude that the total displacement (about 23 cm) of the M-B boundary at these two sites is due to the summation of a thick surficial mixing layer and a 5 cm lock-in depth. Our work provides a practical means of decomposing the downward displacements of magnetic signals due to different processes.
DE: 1522 Paleomagnetic secular variation
DE: 1535 Reversals: process, timescale, magnetostratigraphy
DE: 1540 Rock and mineral magnetism
DE: 1594 Instruments and techniques
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting