Bioturbation-assisted Polarity Reversal in Pleistocene Marine Sediments; a Case of Chemical Remagnetisation Rather Than Physical Grain Re-alignment

High-fidelity paleomagnetic records retained in marine sediments are generally attributed to pDRM. This process requires a zone below the water-sediment interphase where a gradual lock-in of detrital magnetic particles takes place. Within this lock-in zone, bioturbation probably represents an important factor controlling this lock-in process. There are, however, few direct observations (if any!) of how bioturbation actually affects grain re-alignment. A Reversed-Normal polarity reversal record in Pleistocene marine sediments from the island of Rhodes, Greece (E-Mediterranean), has been studied in detail by retrieving 240 sub-samples (6.4cc) from a sediment-block (12x12x20cm) covering a previously determined polarity reversal. Demagnetisation (af and thermal) reveals an abrupt polarity reversal, coinciding with large variations in NRM, ARM, SIRM-intensities and susceptibility. X-ray graphs of sediment slabs obtained prior to sub-sampling show a dramatic increase in burrows at the polarity boundary. This is not reflected by the almost identical, bimodal grain-size distributions (sedigraph; 1 to 64m) throughout the block. Ore microscopy of magnetic extracts shows a stronger degree of low-temperature alterations in the N-polarity, bioturbated section. This is reflected by thermomagnetic curves run in air and Argon. The N-polarity zone also carries magnetic minerals with significantly higher coercivities (MDFSIRM, MDFARM, Hcr,). Magnetic fabric derived from AMS is completely overprinted by an tectonic overprint suggesting NNE-SSW extension. The significant changes in magnetic mineral composition occurring at the R-N level suggests that bioturbation in the present case acts as a proxy for changes in nutrition/oxygen conditions at the sediment/water inter-phase.