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Compositional Changes of Sedimentary Organic Matter from a 100-year Flood Deposit: Insights into Event-Driven Processes in the Coastal Ocean

Periodic Report Summary 2 - SOMFLOOD (Compositional changes of sedimentary organic matter from a 100-year flood deposit: insights into event-driven processes in the coastal ocean)

The main goal of the second period was to assess the effect of the Po delta architecture (Italy) on the composition of the material supplied to the Northern Adriatic. Our effort focused on the Po river and its five tributary channels. Recently, event-dominated rivers like the Po have received renewed consideration as they account for roughly 50 % of the sediment supply to the ocean. Therefore, it has been suggested in several studies that about half of the land-ocean exchange of particulate organic carbon is driven by these event-dominated systems. As it is not feasible to individually study each watershed, several land-ocean studies have provided general conceptual models to discriminate the major geological and hydrological variables affecting the character of organic carbon supplied to the ocean. Recent studies indicated that river discharge exerts first-order control on the concentration and character of organic carbon in event dominated watersheds. According to these studies, mobilization of distinct pools of organic carbon occurs at different discharges and the transport effectiveness of each pool is the product of flow frequency and transport magnitude (i.e. effective discharge). As part of our project, we proposed the internal architecture of prodeltaic systems as an additional variable affecting the land-ocean exchange of particulate organic carbon because it has been shown that their morphology influences the transfer of sediment to the coastal ocean.

Our main working hypotheses were:
1) The composition of particulate and dissolved material along distributary channels changes as the river discharge increases;
2) Each distributary channel exhibits its own distinctive temporal trend with increasing water discharge.

Contrarily to our working hypotheses, the material collected along the distributary channels was relatively homogenous indicating that the morphology of the prodelta does not affect the composition of the organic material. In addition, the organic composition did not exhibit significant changes with time indicating that the source of the particles is relatively similar and independent of the river discharge, although our time-series analysis highlighted significant spotty resuspension events along distributary channels. However, our dataset was acquired when the discharge range was relatively limited (from 1 500 to 4 000 m3 s-1). As the Po river can experience floods exceeding 8 000 m3 s-1, further work is needed to understand the role of large floods events in the transfer of material from land to the coastal system.