Fate of terrigenous organic carbon in muddy clinothems on continental shelves revealed by stratal geometries: Insight from the Adriatic sedimentary archive

Continental shelves host 90% of modern Organic Carbon (OC) burial and play a key role in the sequestration of terrigenous OC over geological timescales. The efficiency of OC burial in these systems, however, varies greatly depending on the duration of exposure to oxic-suboxic conditions during sediment transport. In this study, we use observations across a wide range of stratigraphic and sedimentological scales coupled with geochemistry data from muddy shelf deposits along the western Adriatic to investigate the relation between sediment transport and burial of terrigenous (land-derived) fraction of OC (OCTₑᵣᵣ). Our analysis focused on the Little Ice Age (LIA, 1500–1850 CE) interval, which was characterized by wet, cold, and stormy weather conditions, before the time of widespread regulation and damming of rivers. On the Adriatic shelf, LIA deposits are organized as clinothem: strata that dip gently seawards. The LIA clinothem becomes progressively steeper and deeper from north to south. Basin-scale seismic-stratigraphic analysis and biogeochemical data show evidence of elongated stratal units associated with low OCTₑᵣᵣ content in the northern sector of the LIA clinothem, whereas farther south, where clinoforms are steeper, the LIA clinothem exhibits wavy stratal units with limited cross-shelf continuity and high OCTₑᵣᵣ concentrations. Based on these data we infer two contrasting scenarios for OCTₑᵣᵣ deposition during the LIA: 1) protracted sediment redistribution under the influence of coastal currents with efficient OCTₑᵣᵣ degradation prior to final burial in the northern sector; and (2) rapid deposition of OCTₑᵣᵣ-rich event beds as a result of flood-driven hyperpycnal flows with limited dispersion across the shelf in the southern sector. The latter scenario of deposition resulted in scattered hot spots of OCTₑᵣᵣ burial along the apparently homogeneous western Adriatic shelf deposit. Our work documents significant lateral variability of a fine-grained system in which hot spots of OCTₑᵣᵣ can be preserved in scattered prodelta bedsets (<1 km in across-shelf lateral continuity) over a 600 km long shelf. Shelfal clinothems worldwide should not be considered as homogeneous pools of OCTₑᵣᵣ because of the influence of river, storm, and oceanic currents.