Enrichment of organic carbon in a deep‐water sand‐prone turbidite system: A study from the Eocene Aínsa Basin (Spanish Pyrenees)

Dilute sediment gravity flows transporting fine-grained sediments such as silt or clay in suspension are thought to be the principal carriers of organic carbon into the deep sea. However, observations from submarine fan datasets show that plant debris can also be accumulated in a range of coarser (sandy) facies, which suggests that organic carbon buried in sand-prone turbiditic environments could make a significant contribution to the total amount of sequestered organic carbon in submarine fans. Here, we present insights on the distribution of organic carbon in relation to grain size and facies from slope deposits of the Arro System, Aínsa Basin (Spain). Evaluation of total organic carbon (TOC) content from 82 samples (canyon-fill, channel-overbank, mass-transport intraslope-lobe and slope deposits) shows that there are distinct partitioning patterns of organic carbon between different deposits depending on facies, grain size and depositional sub-environment. The main outcomes of this study are: (1) fine-grained sandstones are relatively enriched in organic carbon (average of 0.78 wt% TOC) compared to silt-prone deposits (average of 0.45 wt% TOC); (2) non-channelised turbiditic and hemipelagic slope deposits (0.38 wt% TOC) are not the main depocentre of organic carbon, which is preferentially stored within the channelised turbidite system; and (3) deposits exhibiting sinusoidal stratification show the highest organic carbon content independent of grain size. Sinusoidal stratification indicates high rates of aggradation by quasi-steady supercritical turbidity flows, allowing for the preservation of organic material in stable antidune deposits with a record of high suspension-load sedimentation. This study shows that the partitioning of organic carbon within slope deposits is controlled by depositional flow regimes and aggradation rates. Therefore, these depositional dynamics need to be considered to understand the variability of carbon sequestration within deep-marine sediments.