Mud-forced turbulence dampening facilitates rapid burial and enhanced preservation of terrestrial organic matter in deep-sea environments

Hybrid event beds (HEBs) are tiered deep-water deposits that can include significant intervals of organic-rich muddy sandstone and sandy mudstone. They are emplaced by decelerating sediment gravity flows in which turbulence becomes extinguished due to increasing cohesion. Whilst several studies have addressed the distribution of organic matter (OM) in turbidites, the extent to which OM is segregated between the component HEB divisions has yet to be quantitatively addressed for clastic systems. Here we document bed scale fractionation of terrestrial OM in HEBs drawn from a range of deep-water sub-environments (basin floor sheets, outer fan fringes and mid-fan lobes) preserved in the Ross Sandstone Formation, western Ireland, a tropical Pennsylvanian deep-water fan complex. A suite of bulk geochemical techniques (TOC, Rock-Eval pyrolysis, δ13C isotopes, and XRF scanning) and petrographic analyses were applied to HEB-dominated core intervals retrieved from four behind-outcrop boreholes. Results shows that muddy sandstones (H3 divisions) of the HEBs have significantly higher OM (average TOC = 1.2 wt% and upto 2.7 wt%) and mud contents than co-genetic cleaner sandstones (H1 divisions; average TOC = 0.2 wt% and upto 0.7 wt%). Muddy caps (H5 divisions) to the event beds have higher mud but relatively low average TOC contents (average TOC = 0.7 wt% and upto 1 wt%) compared to H3 divisions, implying textural fractionation of OM components, greater burn-down linked to slower suspension settling and/or downward propagation of oxidation fronts. HEBs can dominate distal lobe stratigraphy and are thus an important but under-represented sink for terrestrial carbon with enhanced preservation of OM in H3 divisions on account of rapid en-masse deposition, high mud abundances offering enhanced protection, and where thick, emplacement beyond the reach of oxidation fronts descending from the sea floor. The present study has important implications for understanding how carbon is buried and distributed in deep-water successions.