Susceptibility measured on 18 sediment cores from METEOR cruise M134/1

• Main Author: von Dobeneck, Tilo
• Format: Article in Journal/Newspaper
• Language: English
• Published: PANGAEA - Data Publisher for Earth & Environmental Science 2020
• Subjects: M134/1; magnetic susceptibility; South Georgia Island; Center for Marine Environmental Sciences MARUM
• Online Access: https://dx.doi.org/10.1594/pangaea.914649; https://doi.pangaea.de/10.1594/PANGAEA.914649

Sediment magnetic properties are determined by the speciation, concentration and grain‐size range of the sedimentary iron mineral assemblage and mirror both, sediment lithology and diagenesis. As geochemical redox reactions involving iron, methane and other types of organic matter were a major issue of Cruise M134/1 to South Georgia Island, magnetic methodology has been routinely applied to all collected sediment series. All opened GC and DAPC cores were investigated for changes in magnetic susceptibility at 2 cm resolution, using a hand‐bench suitable for horizontal full‐ or half‐core logging. Magnetic volume susceptibility κ is the most commonly measured rock magnetic parameter in geoscience and describes how strongly a material is magnetized by a small and alternating external magnetic field (κ = dM/dH with volume magnetization M and magnetizing field H). κ primarily reflects the concentration of lithogenic or authigenic ferrimagnetic iron minerals while paramagnetic minerals such as iron‐bearing silicates have lesser influence except in Fe oxide depleted sediments. Diamagnetic matrix mineral (calcite, quartz, opal) and pore water content (i.e. porosity) all dilute the magnetic mineral fraction and reduce the bulk magnetic volume susceptibility accordingly. On this cruise, magnetic susceptibility was exceptionally measured on a self‐built, hand‐operated roller bench onto which a new BARTINGTON 140 mm loop sensor connected to a MS‐3 control unit was mounted (cycling time set to 5 sec). The sensor's sensitivity extends over an interval (half‐width) of ~8 cm. Consequently, sharp susceptibility changes appear smoothed in the core log and thin, magnetically contrasting layers are not appropriately resolved. Each segment was separately logged at 2 cm intervals with 10 cm extensions to both sides. Start and end data were used to assess and compensate the instrumental temperature drift. A volume correction for core diameter and full or split core data was applied. The systematic decline of the susceptibility signal towards segment boundaries was recovered by summing up the overlapping tail data, thereby (at least theoretically) providing loss‐free data sets. Top and bottom ends of the core were mirror‐corrected. Expansion and bulging of some segment ends due to methane degassing produced signal spikes at segment limits due to numerical overcompensation. These spurious 'segment‐end peaks' were identified by visual inspection and eliminated from the data sets leaving typical data gaps of 4‐8 cm. Some few data gaps represent voids formed by rapid degassing of the core. The possible solution to rescale all segments was considered as too difficult to realize in the shipboard workflow. A second shore‐based precision‐measurement of the archive halves with a spot sensor is advisable to reach publication‐quality data.