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

Abstract

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. These interventions have, collectively and progressively, raised societies to a pervasive and overarching position in the geomorphic stability–instability of the Mediterranean's coasts, a situation that will be exacerbated by pressures from sea-level rise, paving the way for rampant coastal erosion and delta destruction.

Introduction

The subject of the influence of past and present societies on the geomorphology of the coasts of the Mediterranean is very large, and is embedded in fundamental changes in the relationship between natural processes and human activities that have occurred over the course of the Holocene period (van Andel, 1989), marked by the transition from a strongly nature-dominated to an increasingly human-dominated environment (Marsh, 1864, Desjardins, 1876, Sherlock, 1922, Churchill Semple, 1932, Delano-Smith, 1979, Horden and Purcell, 2000, Berger and Guilaine, 2009, Walsh, 2013). The Mediterranean, therefore, stands out as one of the most pertinent geographical zones to look at the nature and trajectories of these interactions. One of the cradles of ancient civilisations, the Mediterranean has seen its shores occupied by societies since Prehistoric times, and the geomorphology of large sectors of the Mediterranean's coasts is a heritage of this relationship between Nature and Humans. Expressions of this relationship are diverse and affect both the bold rocky coasts and low-lying depositional coasts of the Mediterranean. The effects are most clearly evident, however, and increasingly more pervasive, on the latter coasts, especially those formed by river deltas, which have sequestered rich sedimentary archives to probe Holocene change and human impacts.

The aim of this paper is to present a review of the geomorphic human imprint on the coasts of the Mediterranean at different spatial and temporal timescales within the time frame of the Holocene. The evolution of non-deltaic coastal barriers and of many Mediterranean deltas and their adjacent downdrift coastal sectors has been reconstructed using multiproxy analyses of core data, palaeogeographical or geomorphological reconstructions, and coastal archaeology, providing material for the review. This review comes at a time when human impacts are leading to increasing coastal instability, providing a potential framework for exacerbating the effects of climate-induced change and sea-level rise. The overview provided here is both a daunting and necessarily incomplete task, given: (1) the long and complex relationship between the Mediterranean's coasts and societies, (2) the vast range of human interventions, both directly on the coast, and indirectly via modifications of river catchments that supply sediment, and therefore condition, the stability of depositional coasts, (3) the difficulty of disentangling human influence from the effects of commonly rapid changes in climate and vegetation dynamics, (4) the sometimes fragmentary and time-transgressive nature of many large deltaic sediment records, and (5) the overarching geographical and geomorphological diversity of the Mediterranean's clastic coasts. Salient elements of this relationship and the way human societies have profoundly modified the Mediterranean's coasts are highlighted through focus on clastic non-deltaic barriers and river deltas and their associated deposits. These coastal morphosedimentary archives record the temporal progression of human impacts, starting from dispersed coastal strongholds, related mainly to ancient harbours and maritime commerce, through larger-scale indirect impacts on the hinterlands that back the waterfront, and onto the modern era of direct significant anthropogenic transformations of the coast. It must be stated at the outset that direct quantification of the balance between human impacts and natural changes affecting the background environment and the coasts of the Mediterranean is not feasible. However, quantifying river discharge variation and changes to coastal sediment budgets provides an indirect measure of the importance of human activity over the last 200 years.

Following the introduction (Section 1), Section 2 briefly describes the setting and characteristics of the Mediterranean. Section 3 provides an overview of the coasts and deltas of the Mediterranean, including the recently emerged trend of massively engineered and artificial shores. Section 4 deals with ancient harbours and coastal palaeo-engineering works, several of which provide a distinct record of sedimentation reflecting a growing anthropogenic signature. Section 5 briefly describes the morphogenesis of non-deltaic coastal barriers for which identified anthropogenic signatures are lacking. Section 6 concerns an appraisal of the morphogenesis and morphodynamics of Mediterranean deltas hinged on the classic relationship between these deposits and relative sea-level stabilisation, as initially postulated by Stanley and Warne (1994), whereas Section 7 considers Mediterranean river delta inception in terms of the postulate of these deposits being largely man-made constructs generated by land-use changes related to the flourishing of classical civilisations, as initially proposed for deltas in Tuscany, Italy (e.g. Marinelli, 1926 in Pranzini, 1989, Fabbri, 1985, Innocenti and Pranzini, 1993) and in Spain (Vita-Finzi, 1975, Guillén and Palanques, 1997). 8 The impacts of anthropogenic changes in river catchments on coasts and deltas since ca. 1800, 9 Delta morphodynamic changes over time and the human impact focus, respectively, on the massive modifications of river catchments over the last two centuries (Section 8) and direct human interventions on the Mediterranean's coasts that are strongly reflected in the current fragile status of large stretches of the depositional non-deltaic barriers (Section 9). Section 10 provides an overview of delta morphodynamic changes associated with variations in sediment supply and direct and indirect human interventions on river catchments and river mouths. Following these sections, Section 11 discusses the difficulty of disentangling human impacts and changes in the geomorphic functioning of the Mediterranean's coasts from those of rapid climate changes in the course of the Holocene, debouching on a synthesis that warns against a simplistic view of Mediterranean river deltas as human constructs. Section 12 proposes conclusions and perspectives on the future of the Mediterranean's deltas and coasts, against a background of destabilisation by Humans under the overarching framework of sea-level rise and climate change. The review shows a strong imbalance in favour of the literature published since 2000. Much of the post-2000 literature provides more updated and comprehensive coverage of coastal geomorphic changes documented in many pre-2000 publications, especially given the greater attention being paid to deltas in the current circumstances of increasing delta vulnerability (Ericson et al., 2006, Syvitski et al., 2009, Anthony, 2014a). Much of the literature reviewed also reflects the increasing impacts of human activities inventoried over the last 15 years or so.