Human influence and the changing geomorphology of Mediterranean deltas and coasts over the last 6000 years: From progradation to destruction phase?

摘要

The present geomorphology of the Mediterranean's coasts is largely a product of an intricate long-term relationship between Nature and human societies. A cradle of ancient civilisations, the Mediterranean has seen its shores occupied by Humans since Prehistory, and is, therefore, a particularly pertinent unit of analysis. The morphotectonic context and other forcing agents (e.g., climate) shaped out a highly diversified coastal morphology and generated a sediment-supply regime potentially favourable to the formation of numerous open-coast deltas and bay-head deltas in infilled rias as sea level stabilised during the mid-Holocene. This supply of riverine sediment has also been the key agent in mediating human occupation of the Mediterranean's clastic coasts. Expressions of this relationship have been extensively archived in clastic coastal deposits, including base-level deltaic and estuarine sedimentary sinks, which comprise records to explore the interactions between geosystems and the human environment. The stratigraphic sequences in these coastal sedimentary archives comprise, in many places, a clearly identified anthropogenic signature, notably in ancient harbours, some of which underwent extremely rapid silting up due to massive sediment sourcing generated by new agricultural practices from the Neolithic onwards. Increasing human influence, especially over the last 3000 years, has been, in turn, an important driver of changes in sediment supply, strongly modulating deltaic development. Pulses of sediment supply from catchments rendered vulnerable by human perturbations during the Roman period resulted in a new cycle of inception of many other deltas and in rapid delta growth (e.g. the Ebro, the Po, the Arno and the Ombrone). Another progradation dynamic during the Little Ice Age, at a time of strong rural population growth, river discharge increases, technological developments, and urbanisation, further consolidated delta growth. Understanding the life cycle of these deltas since their initial formation is, in turn, key to unravelling the relative role of natural and anthropogenic forcing agents. Rapid climate changes are deemed to have contributed through both the stripping of landscapes rendered fragile by human activities and active fluvial sediment transport to the coast, but disentangling climate change effects from human impacts in the Mediterranean remains a challenge. The patterns of subsequent deltaic growth and delta morphodynamics reflect adaptations to pulsed sediment supply, river discharge variations, the microtidal, fetch-limited context of the Mediterranean, and direct engineering interventions. The progradation dynamic of the Roman period and Little Ice Age contrasts markedly with the situation of common coastal destabilisation over the last two centuries, particularly well documented for the last 50 years. This period has been characterised by reduced sediment flux to base-level geosystems due to catchment reforestation, retenion within reservoirs, fluvial regulation and dredging, resulting in the erosion of deltas and barrier lagoon and beach dune systems. Large stretches of shoreline and narrow coastal plains have been massively engineered for coastal defence and protection against erosion, but also for the construction of marinas, leisure harbours and artificial beaches, resulting in the emergence of veritable artificial seafronts.