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Authigenic greigite may form at any time within a sediment during diagenesis. Its formation pathway, timing of formation, and geological preservation potential are key to resolving the fidelity of (paleo‐)magnetic signals in greigite‐bearing sediments. In the cored sequence of the International Ocean Discovery Program Expedition 362 (Sumatra Subduction Margin), multiple organic‐rich mudstone horizons have high magnetic susceptibilities. The high‐susceptibility horizons occur immediately below the most bioturbated intervals at the top of muddy turbidite beds. Combined mineral magnetic, microscopic, and chemical analyses on both thin sections and magnetic mineral extracts of sediments from a typical interval (∼1,103.80–1,108.80 m below seafloor) reveal the presence of coarse‐grained greigite aggregates (particles up to 50–75 μm in size). The greigite formed under nonsteady state conditions caused by the successive turbidites. Organic matter, iron (oxy)(hydr)oxides, Fe²⁺, and sulfides and/or sulfate were enriched in these intensively bioturbated horizons. This facilitated greigite formation and preservation within a closed diagenetic system created by the ensuing turbidite pulse, where pyritization was arrested due to insufficient sulfate supply relative to Fe (oxy)(hydr)oxide. This may represent a novel greigite formation pathway under conditions modulated by turbidites and bioturbation. Paleomagnetic analyses indicate that the early diagenetic greigite preserves primary (quasi‐)syn‐sedimentary magnetic records. The extremely high greigite content (0.06–1.30 wt% with an average of 0.50 wt% estimated from their saturation magnetization) implies that the bioturbated turbiditic deposits are an important sink for iron and sulfur. Mineral magnetic methods, thus, may offer a window to better understand the marine Fe–S–C cycle.

Recognition of coring‐induced disturbance, which is essential for magnetic fabric and paleomagnetic studies of poorly lithified sediments, is generally not straightforward. Here, we report on anisotropy of magnetic susceptibility (AMS) and paleomagnetic data of the sediments from Holes U1480E and U1480H, IODP Expedition 362, west of the Sumatra subduction zone. AMS is characterized by steep minimum principal axes (K_min) in undisturbed sediments. However, a considerable portion of the recovered sediments are affected by significant coring‐induced disturbance. In these cases, we observed three AMS patterns: (1) AMS principal axes are randomly distributed for sediments with mingling and distortion of beds, (2)K_minaxes of sediments with upward‐arching beds are deflected out of the splitting face of the working half, and (3) suck‐in sediments are characterized by verticalK_maxaxes. These deformation‐dependent AMS patterns can be attributed to the realignment of mineral particles caused by the coring process and subsequent sampling procedures. Besides a low‐coercivity, vertical, drilling‐induced overprint, we observed a high‐coercivity component that is likely a composite of the primary magnetization with a demagnetization‐resistant portion of the drilling overprint. After accounting for the disturbed intervals, several polarity transitions can be identified in the undisturbed sediments which correlate well with the Pleistocene geomagnetic polarity timescale. These observations demonstrate that great caution is required when attributing geological significance to AMS and paleomagnetic data obtained from soft sediment cores, which are highly susceptible to coring‐induced disturbance. In addition, AMS measurements provide a potential tool for identifying core deformation for further paleomagnetic studies.

Gyro‐remanent magnetization (GRM) is a frequently occurring yet unwanted remanence contamination for certain samples during alternating field (AF) demagnetization of the natural remanent magnetization. The origin and detailed properties of GRM have not yet been fully understood. In this study, systematic rock magnetic analyses were conducted on marine greigite‐bearing samples of Hole U1433A drilled by the IODP Expedition 349 from the South China Sea. Results show that GRM is mostly acquired above ~55 mT AF demagnetization and can be effectively removed by heating to ~400°C during thermal demagnetization but a secondary tail could remain until ~585°C. In addition, no apparent GRM was observed during the AF demagnetization for the 400°C thermally treated samples. These results strongly suggest that GRM is dominantly carried by single domain (SD) greigite but with minor contributions from SD magnetite. Thus, thermal treatment alone or the hybrid demagnetization (i.e., thermal demagnetization at ~400°C first then systematical AF demagnetization) can efficiently avoid the GRM acquisition and be beneficial for relative paleointensity estimation for greigite‐bearing samples. Besides, GRM carried by greigite has a low thermal stability. Our results also show AF demagnetization spectra of anhysteretic remanent magnetization (ARM) could be strongly distorted by GRM effects due to both have a preference of SD particles. Thus, the median destructive field of ARM is improper to be used as a coercivity proxy for greigite‐bearing samples. Instead, the biplot analysis of AF demagnetization of natural remanent magnetization and ARM can be used to evaluate the relative content of greigite.

Macrostructures preserved in deformed rocks are essential for the understanding of their evolution, especially when the deformation is weak and hard to discriminate in regional scale or purely through geophysical data. In order to resolve the inconsistency between NS trending fracture zones and NE oriented spreading fabrics of the South China Sea during the latest spreading stage, we analyzed macrostructures identifiable from the basalt and consolidated sediment samples of the Integrated Ocean Drilling Program (IODP) Sites U1431 and U1433. These two sites are close to the East and Southwest relict spreading ridges and provide critical information on the latest spreading stages. The structures in the basalt of both sites suggest two dominant orientations of NS and NE. At U1431, sediments show mainly WNW trending slickensides, different from that of basalt. At U1433, no structures were found in postspreading sediment. Thus, NE and NS trending structures in basalt are most possibly formed by seafloor spreading. Crosscutting relationship suggests that NE trending structures formed first, followed by NS and finally WNW trending structures. These observations are consistent with geophysical features. Magnetic anomalies and ocean bottom seismometer velocity suggest that the latest relict ridge of the East Subbasin coincides with the EW trending seamount chain. Located between the relict ridges of East and Southwest Subbasins, NS trending Zhongnan‐Liyue Fracture Zone had acted as the latest transform fault. Based on the above evidences, we proposed that the South China Sea may have experienced a short period of NS oriented spreading after earlier SE spreading. These results resolve the previous inconsistencies.