Greigite-bearing nodules in the last glacial Black Sea sediments: rapid post-depositional formation and carbon-preservation implications

Summary During the last glacial period, the Black Sea developed oxic bottom waters that generally preserved detrital titano-/magnetite, yet greigite (Fe₃S₄) occurs sporadically within nodules in these otherwise oxic sediments. To clarify their origin and significance, we combined rock magnetism, X-ray diffraction, and scanning electron microscopy with energy-dispersive X-ray spectroscopy on nodules from core MSM33-55-1 and screened fifteen nearby cores using the ratio of saturation isothermal remanent magnetization to low-field magnetic susceptibility (SIRM/κLF). SIRM/κLF thresholds were applied to classify samples as greigite-bearing and greigite-free, then the greigite-bearing sample proportion was calculated in 0.3-kyr bins from 70 to 20 ka. The nodules have S-rich interiors (elemental sulphur plus greigite) coated by Fe-hydroxide rims (goethite), indicating formation in localized sulphidic microenvironments embedded in otherwise oxic sediment. Palaeomagnetic comparisons indicate that greigite-bearing samples track greigite-free (detrital titano-/magnetite) records for relative palaeointensity (RPI) and inclination (zero lag for inclination; ∼0.5 kyr lag for RPI), which implies that greigite formed shortly after deposition and acquired an early-diagenetic chemical remanent magnetization near-synchronous with a detrital post-depositional remanent magnetization. Greigite-bearing sample proportion increases during interstadials with modest but significant positive correlations with total organic carbon (TOC) content (and TOC/κLF) and a negative correlation with κLF, consistent with enhanced organic-matter supply relative to detrital input promoting sulphidic microenvironments. Together, these results demonstrate that greigite-bearing nodules can be incorporated into palaeomagnetic reconstructions when carefully screened, and that they serve as markers of micro-scale sulphidisation coupled to conditions that favour organic-carbon retention in glacial Black Sea sediments.