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

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