Low-temperature magnetic properties of pelagic carbonates: Oxidation of biogenic magnetite and identification of magnetosome chains

Pelagic marine carbonates provide important records of pastenvironmental change. We carried out detailed low-temperature magneticmeasurements on biogenic magnetite-bearing sediments from the SouthernOcean (Ocean Drilling Program (ODP) Holes 738B, 738C, 689D, and 690C)and on samples containing whole magnetotactic bacteria cells. Wedocument a range of low-temperature magnetic properties, includingreversible humped low-temperature cycling (LTC) curves. Differentdegrees of magnetite oxidation are considered to be responsible for theobserved variable shapes of LTC curves. A dipole spring mechanism inmagnetosome chains is introduced to explain reversible LTC curves. Thisdipole spring mechanism is proposed to result from the uniaxialanisotropy that originates from the chain arrangement of biogenicmagnetite, similar to published results for uniaxial stable singledomain (SD) particles. The dipole spring mechanism reversibly restoresthe remanence during warming in LTC measurements. This supports aprevious idea that remanence of magnetosome chains is completelyreversible during LTC experiments. We suggest that this magneticfingerprint is a diagnostic indicator for intact magnetosome chains,although the presence of isolated uniaxial stable SD particles andmagnetically interacting particles can complicate this test. Magneticmeasurements through the Eocene section of ODP Hole 738B reveal aninterval with distinct magnetic properties that we interpret tooriginate from less oxidized biogenic magnetite and enrichment of abiogenic hard component. Co-occurrence of these two magneticfingerprints during the late Eocene in the Southern Ocean indicates lessoxic conditions, probably due to increased oceanic primary productivityand organic carbon burial