The Matuyama Chronozone at ODP site 982 (Rockall Bank): Evidence for decimeter-scale magnetization lock-in depths

At ODP Site 982, located on the Rockall Bank, component magnetizations define a polarity stratigraphy from the middle part of the Brunhes Chronozone to the top of the Gauss Chronozone (0.3-2.7 Ma). The Cobb Mountain and Reunion subchronozones correlate to marine isotope stages (MIS) 35 and 81, respectively. The Gauss/Matuyama boundary correlates to the base of MIS 103. The slope of the natural remanence (NRM) versus the anhysteretic remanence (ARM) during stepwise alternating field demagnetization is close to linear (r > 0.9) for the 900-1700 ka interval indicating similarity of NRM and ARM coercivity spectra. The normalized remanence (NRM/ARM) can be matched to paleointensity records from ODP Sites 983/984 (Iceland Basin) and from sites in the Pacific Ocean. The age model based delta(18)O stratigraphy at ODP Site 982 indicates sedimentation rates in the 1-4 cm/kyr range. At ODP Site 980/981, located similar to200 km SE of Site 982 on the eastern flank of the Rockall Plateau, sedimentation rates are 2-7 times greater. According to the delta(18)O age models at Site 982 and Site 980/981, the boundaries of the Jaramillo, Cobb Mountain, Olduvai and Reunion subchronozones appear older at Site 982 by 10-15 kyr. We model the lock-in of magnetization using a sigmoidal magnetization lock-in function incorporating a surface mixed layer (base at depth M corresponding to 5% lock-in) and a lock-in depth (L) below M at which 50% lock-in is achieved. The observed age offsets can be simulated by values of DM (difference between values of M at Sites 982 and 980/981) in the 16-23 cm range, and values of L < 10 cm. The low values of L imply that the magnetization is rapidly locked (within 10 cm) after sediment passes through the decimeter scale surface mixed-layer.