Magnetic grain size, relative paleointensity, and an apparent magnetic excursion from the Rockall Trough (NW Scotland) during the last glacial cycle

Abstract:

Cores MD04-2822 and MD95-2006, from the Rockall Trough, provide high resolution records over marine isotope stage (MIS) 2 and Termination I as a result of sedimentation rates >1 m/kyr and high-quality age control based on AMS radiocarbon ages, the correlation of sea-surface temperature (SST) proxies to Greenland ice-core records, and benthic oxygen isotope data. Component magnetization directions indicate an apparent magnetic excursion in Core MD04-2822 at 16 meters below seafloor (mbsf), corresponding to an age of 26.5 ka, and occupying ~30 cm of core implying an excursion duration of ~350 yrs. The relative paleointensity (RPI) records from the two cores can be matched to each other, and only in part to other RPI records and to lower resolution RPI reference templates. Magnetic concentration and grain-size parameters change together with benthic d¹³C/d¹⁸O and with SST proxies, with coarser magnetic grain size and higher magnetic concentration during glacial stages and cold SST (Dansgaard-Oescheger) intervals. The magnetic parameters are, however, lagged by several kyr relative to benthic d¹⁸O at Terminations I and II. The mean grain size of sortable-silt (10-63 µm) across Termination I in Core MD04-2822 implies changes in bottom-current velocity, although these are decoupled from the progressive fining of (magnetite) grain size across Termination I implied by magnetic grain size parameters. The magnetic parameters, respond to a fine (micron-scale) grain size fraction, and are interpreted as being sensitive to changes in detrital source (bottom-current path) rather than bottom-current strength.