Source of the climate signal recorded by magnetic susceptibility variations in Indian Ocean sediments.

The origins of magnetic susceptibility variations in deep-sea sediments from the Indian Ocean (Ocean Drilling Program) (ODP Leg 117) have been examined to identify the specific connections between climate shifts and sediment magnetic properties. In a previous study, the magnetic susceptibility variations in ODP Hole 722B were identified as an outstanding proxy paleoceanographic record, despite evidence of postdepositional loss of magnetic minerals through reductive diagenesis. This paradox is addressed by assessing the contributions of ferrimagnetic detrital iron oxides, bacterial magnetosomes, and paramagnetic detrital Fe silicate minerals to the magnetic susceptibility signal. In addition to detailed magnetic analyses, mineralogical, morphological, and grain size data have been obtained from representative magnetic extracts. For Hole 722B, we find that magnetic responses to climate change result from (1) fluctuations in the volume magnetic susceptibility, which is primarily controlled by carbonate dilution; (2) a ferrimagnetic signal, which is restricted to the upper 7 meters below sea floor (mbsf) and which largely reflects source area aridity; and (3) a paramagnetic susceptibility record below 7 mbsf, which is coincident in frequency with variations in lithogenic grain size.

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