On depositional processes governing along-strike facies variations of fine-grained deposits: Unlocking the Little Ice Age subaqueous clinothems on the Adriatic shelf

Abstract

Depositional processes recorded by shelf deposits may vary widely along-strike, depending largely on the mode of delivery and deposition of sediments to the basin. In fine-grained systems in particular, depositional processes are difficult to reconstruct with standard facies analysis of sediment cores due to the ostensibly featureless and homogenous appearance of muds. In this study, sedimentological, palaeontological, geochemical and oceanographic data were combined in a detailed characterization of depositional conditions via sedimentary structures, type of organic matter, trace-metal geochemistry and benthic fauna assemblages (foraminifera and ostracods) along the 600 km long shelf delta clinothems of the West Adriatic shelf (Italy). Processes inferred from sedimentary facies and micro-structures were then considered in the context of the modern Adriatic oceanographic regime. Specific attention was given to the Little Ice Age stratigraphic unit (1500–1850 CE), which contains a continuum of genetically related fine-grained strata. The Little Ice Age deposit offers the opportunity to examine a source-to-sink system with the high resolution typical of modernanalogues, at a time interval when Apennine rivers were not yet hydraulically engineered with man-made sediment traps along their trunks. Individual beds within the Little Ice Age muddy prodelta form hectometre to kilometre-wide bedsets that reflect the interplay between energetic meteo-ocean conditions (storm-dominated beds), flood supply (river-dominated beds or hyperpycnites) and along-shelf bottom-current dispersion (drift-dominated beds). The multidisciplinary approach applied at different scales of observations helped in understanding sediment provenance and the relative timing of sediment transport before final burial that strongly promoted organic matter oxygen exposure and the loss of carbon by microbial degradation. Overall, the distinctive depositional processes that acted in concert along the prodelta slope produced a subtle lateral heterogeneity of preserved sedimentary structures, faunal associations and organic matter composition in a laterally-continuous lithostratigraphic unit deposited at centennial scale. These findings have implications on the forcing conditions that ultimately control the location and nature of fine-grained beds in both modern and ancient, mud-dominated depositional systems.