Redox-sensitive element mobility in mid-Miocene clays from Unit II of Hole M0027A, the New Jersey Shallow Shelf (IODP Expedition 313)

The geochemistry of an unusual seven metre interval of distinctive color-banded clays of Serravallian age has been analysed. This interval is from Hole M0027A, the most landward of the three holes cored during IODP Expedition 313. The clays lie immediately above a key surface (the m4.1 reflector), with the available age constraints indicating deposition of the clays during a time of global cooling and major sea level change. The behaviour of major and trace elements from this interval will help to elucidate the paleoenvironmental conditions and nature of shelf sedimentation during this time. Conventional XRF analysis reveals enrichments in the redox-sensitive trace elements As, Co, Sc, V, Cr, the LREEs La, Ce and Nd, and also Ba and Cs throughout this interval. The enrichment of Co, Sc and V is particularly notable, being much higher than both average New Jersey clay and North Atlantic Average Shale Composition (NASC). Examination of cut cores from Unit II reveals that the enriched zone consists of a sequence of cm- to metre-scale packages of banded clays, alternating with thinner, paler homogeneous clays. The cm-scale color-bands are interpreted to represent diagenetic features due to the mobilisation and relocation of redox-sensitive elements in the sediments during, or shortly after, deposition. Manganese and iron peaks likely reflect previous locations of the manganese redox boundary in the sediment - i.e. they represent relict or paleo-redox fronts. The dark bands are very organic rich, show little bioturbation and contain pyrite or possibly authigenic greigite. They also consistently correspond to peaks in magnetic susceptibility. The homogeneous clays are extensively mottled due to bioturbation, contain little organic matter and generally correspond to lows in magnetic susceptibility. The dark bands are organic-rich clays that are cyclically intercalated with organic-poor lighter colored sediments. The degradation and oxidation of organic matter likely contributes to the mobilisation of redox-sensitive elements. The enriched zone is encountered only in Hole M0027A. Holes M0028A and M0029A show no evidence of this enrichment, suggesting that conditions were localised to this part of the shelf. The observations presented here are consistent with a restricted marine environment, with limited water circulation, decreased bottom water ventilation and sulfidic conditions. A low energy environment is inferred, in which pulses of oxygen-rich river-supplied organic material contribute manganese and iron to the water column, driving redox fronts through the sediment. Increased precipitation and runoff during the mid-Miocene global cooling event, when the climate was cooler and possibly wetter, may have contributed more river-derived sediment to the shelf, and thus played a key role in the formation of this distinctive interval. Our data will be interpreted in light of ongoing research by Expedition 313 Scientists relating to the sedimentology, depositional environments and stratigraphic framework of the New Jersey Shallow Shelf.