Red or green: Overprinting of the climatic signal in Miocene sediments, South China Sea (<scp>IODP</scp> Expedition 368, Site <scp>U1502</scp>)

Sedimentary beds of alternating red and green colour are commonly interpreted to reflect orbitally-forced cyclic climatic, syn-depositional conditions, although changes caused by post-depositional fluids also documented. Results from IODP Hole U1502A marine sediments in the South China Sea exemplify reducing fluid–rock interactions that locally changed sediment green. Petrographic, rock magnetic paleomagnetic data on cores show colouration originates an early, basin-wide, pervasive diagenetic oxidation event (forming haematite), whereas results subsequent fluid-driven reduction pyrrhotite-magnetite). The dense sulfidic stockwork basaltic basement underlying these was likely source fluids. Drilling deep holes into basin basements can thus provide useful information fluid transfer overlying sedimentary layers.