Sedimentary Processes in the Theta Gap, NE Atlantic

Deep marine gaps are topographical choke-points where constricted bottom currents may interact with down-slope gravitational flows originating from surrounding topographic highs. Such gaps can provide unique insight into processes understudied in abyssal settings: pure contouritic systems and mixed (turbidite-contourite) systems. Here, we present a detailed study of the Theta Gap, a deep marine gap that connects the Biscay Abyssal Plain and the Iberian Abyssal Plain. Previous studies identified significant erosion and deposition of turbidites but only focussed on the area between the Finisterre and A Coruña Seamounts. The processes occurring between the Galicia Bank and the Finisterre Seamount are less defined. Our aim was to compare these two areas within the Theta Gap to determine if the south Theta Gap contributes to the exchange of Lower Deep Water between the abyssal plains and how this affects sediment deposition. To achieve this aim, we used sub-bottom profiles along with a detailed analysis of four sedimentary cores, to study the sedimentary facies across the two areas. Hydrological data was used to assess to what degree Lower Deep Water and associated bottom currents are contributing to sediment distribution in the gap. We found clear differences in turbidite deposition between the north and south Theta Gap. The same event often has thinner and coarser deposits in the north compared to the south. In the south, the turbidite sequences are thicker, more complete and include discontinuous silt laminae and mud/silt couplets. The distribution of iostherms and isopycnals in the Theta Gap suggest that Lower Deep Water flows north-eastwards through the south Theta Gap before recirculating and flowing south-westwards through the north Theta Gap. We hypothesise, the discrepancy in turbidite distribution is greatly controlled by these bottom currents which foster deposition in the south Theta Gap and, at times, prevent deposition in the north Theta Gap. Such bottom current – sediment interactions are understudied in abyssal settings and deep marine gaps, such as the Theta Gap, can provide unique insight into these processes of sediment transport and reworking.