Introduction

In view of the considerable economic and social importance of European coasts, as well as the increasing need to manage and safely exploit the marine environment, it is vital to establish the long-term role and development of prodeltas. The main objective of the EURODELTA Concerted Action is to pool existing geomorphological, geophysical, stratigraphic, sedimentological, and oceanographic data related to prodelta formation during the recent past. The resulting data will be used to determine the future research urgently required to understand how modern Mediterranean and Black Sea prodeltas (Figure 1) have evolved, how vital they are to the long-term stability of coastal regions, and how they could be managed, in a such a way as to sustain economic activities and the natural environment.

Prodelta is defined here as: shelf areas offshore river mouth that are characterised by significant mud accumulation below storm wave base (Figure 2).

Objectives

Prodeltas are large, shallow-marine features that are typical of the coastal margins of the Mediterranean and Black Seas. Much data on the origin and growth of prodeltas already exists, but this has never been properly evaluated and synthesised, so that it is difficult to establish their full importance, and to predict how they might alter in the future. This could be vital for the long-term success of coastal management plans. Because of their location (offshore but adjacent to and influenced by deltaic and coastal systems) prodeltas represent a crucial link in our understanding of the short- and long-term interactions between fluviodeltaic and marine sedimentation processes. Prodeltas are extremely important for the long-term stability of sites of human settlement, economic endeavours and amenities throughout the Mediterranean coastal belt. Modern prodeltas represent the subaqueous counterpart of deltaic systems and are typically made up of fine-grained deposits that have accumulated at varying sedimentation rates (up to 10 cm/yr). Late-Holocene prodeltas are ubiquitous on Mediterranean and Black Sea margins where they accumulated (and prograded seaward) since the attainment of the modern sea-level highstand, about 5,500 calendar years ago. Prodeltas typically reach their maximum thickness offshore major deltas and represent:

• a stratigraphic and historical archive of short-term environmental changes and human impacts affecting the adjacent river catchments, coastlines and shelves
• a preferential sink for river-borne fine-grained deposits that contain high concentrations of pollutants and carbon compounds derived from human activities
• an area of potential sediment instability, deformation and failure, as has been demonstrated recently for the Central Adriatic and the Nice coast (COntinantal slope STAbility project)

Understanding the evolution of, and significance of, prodeltas will require a major, inter-disciplinary and international co-operative effort, since an extensive range of geological and environmental factors (e.g. subsidence, sediment compaction, wave erosion, flood deposition, landslides) will no doubt have played a role in their formation, in addition to the fluviodeltaic and marine sedimentary processes already alluded to.

The immediate aims of the EURODELTA Concerted Action are to define and quantify:

• the impact of known recent depositional events (e.g. exceptional river floods) on prodeltas;
• how water-mass circulation relates to shear stress changes on the sea floor and to the growth of prograding clinoforms which characterise prodeltas;
• the relationships between prodelta growth and short-term (century-scale) changes in river supply brought about by, for example, climate changes or local human impacts;
• the link between catchment-based records on land and the pattern of shelf sedimentation and nutrient flux in prodeltas over the time period concerned;
• how prodeltas have evolved and, in particular, how major phases of delta construction relate to episodes of increased river discharge,
• the sediment budgets during the period of the modern high sea-level stand, to establish temporal and spatial changes in the rates and patterns of sediment accumulation during the last 5,500 years, and, more especially, during the last few hundred years;
• the detailed architecture of prodeltas, and of distinctive units of sediment accumulation.

The construction of forecast models will provide best-estimate scenarios of future shoreline and prodelta growth and stability, taking into account historical trends and the effects of the extensive regulation that now prevails within the Mediterranean watershed.