Glacial to interglacial mineral magnetic and palaeoceanographic changes at Chatham Rise, SW Pacific Ocean

Core CHAT 1K from 3556 m depth east of New Zealand shows marked changes in magnetic susceptibility, correlating very well with δ18O of foraminifera over the last 150 ka. Susceptibility is high during interglacial stages 1 and 5, and substages 5a to 5e are well marked. The susceptibility is shown to be enhanced during interglacials and is neither an artefact of glacial carbonate dilution nor due to an interglacial increase of terrigenous material. Mineral magnetic property measurements (ARM, SIRM, S ratio) also show the same well-defined glacial–interglacial variability, interpreted as an interglacial increase of single domain magnetite grains less than 0.1 μm in size. Examination by TEM of magnetic extracts reveals abundant cubic to octahedral grains of 0.02–0.2 μm size, often arranged in long chains characteristic of bacterial magnetite. This material is much more abundant in interglacial than glacial samples. It is suggested that during interglacials, with lower productivity (and organic carbon input to the bed), the oxic zone in the sediments which can be colonised by magnetotactic bacteria is thicker and therefore is colonised for longer by magnetite producers. This yields a climate-controlled stratigraphy which leads that of infaunal benthic foraminifera by no more than a few centimetres.