Last millennia sedimentary record on a micro-tidal, low-accumulation prodelta (Têt NW Mediterranean)

Abstract

Statistical sequential analysis was performed on a series of sediment cores collected from the Têt prodelta in the Gulf of Lions, northwestern Mediterranean Sea, between October 2003 and October 2004. Seabed changes during that period were correlated to hydrodynamic conditions (waves and currents) and river discharge. Low sediment supply prevents full preservation of new sediment strata on this low-accumulation prodelta located on a microtidal, storm-dominated inner shelf. Severe meteorological events caused a rapid succession of erosion and deposition phases. For example, the December 2003 flood and storm produced a flood layer deposit that persisted for 2 months with only slight transformations due to early diagenesis and/or bioturbation, until a new storm event eroded this layer. A typical sedimentary sequence was observed for the secular deposits composed of a 10-cm-thick sandy layer overlaying siltyclayey layers. These characteristic features were used to analyse the last millennia sedimentary record of the Têt prodelta. The low preservation of freshly deposited sediments and variable sedimentation rates during the last millennia period yield a sedimentary sequence formed by the outcropping of muddy prodeltaic units intersected by heterogeneous siltysandy units similar to those formed under present day hydrodynamic conditions. No flood layer was found related to catastrophic flooding of the last century in the sedimentary record. The Little Ice Age (∼ 1550–1850 AD) probably favoured the formation of a well-developed muddy prodelta in the mouth of the Têt River. Later on, the decrease of sediment supply by rivers due to climate change and/or human activities (damming, irrigation), and the increase of the number of high-energy storms reaching the coast, induced a coarsening of the top sediment layer on this prodelta. This modern change of the substrate is also observed in the composition of benthic biota found in the substrate.

Introduction

Small rivers contribute a significant amount of the total sediment load to the global marine systems (Milliman and Syvitski, 1992, Mulder and Syvitski, 1995) and most of the sediment entering shallow coastal areas is entrapped in deltas. Prodeltas are the subaqueous part of aerial deltas located in water depth below the storm wave base action (Coleman and Wright, 1975). Prodeltas correspond to preferential depositional areas of riverborne and organic material. They are composed of fine grained deposits (clays and silts) (Aloïsi and Monaco, 1975), and are commonly found off river mouths at depths range from 20 to 50 m, where wave and current energy decrease and flocculation by purely physico-chemical processes may cause nearshore mud accumulation on the shelf (Drake, 1976). Nevertheless, it has been demonstrated that waves and currents energy remain occasionally strong enough to induce a part of resuspension and transport prodeltaic material seawards (Smith and Hopkins, 1972). For these reasons, prodeltas do not constitute static reservoir of fine sediment, but are temporal deposit areas of clays and silts located in the sediment transport pathway between the turbid nearshore zone and the middle and outer shelf mud deposits.

Whatever the amount of sediment input from the nearby river, the prodelta records a trace of hydrological and biological changes, notably meteorological events such as floods and storms (Nittrouer and Wright, 1994). It is now well understood that in prodeltas of flood-prone rivers, well-preserved sedimentary strata are rather easy to identify, e.g. offshore of the Eel River, northern California (Sommerfield and Nittrouer, 1999), the Mississippi–Atchafalaya deltaic system (Allison et al., 2005); the Rhône River in the Gulf of Lions (Beaudouin et al., 2005), or the Po River (Trincardi et al., 2004). Conversely, sedimentary features of small flood-prone prodeltas are presumed to be more complex, due to slow accumulation rates and the low preservation potential of deposits. To date, only a limited number of studies round the world have addressed the modern to last millennia (century to millennial time-scale) sedimentary record in such small prodeltaic environments (Ingram et al., 1996), which are common in the Mediterranean. However, none of these studies addressed the sedimentary record itself, nor estimated the shelf mass accumulation rates over longer timescales. Are the conditions in small prodeltaic environments favourable to the formation of new sediment strata? Is there an identifiable sedimentary record and what is the structure of the record in small prodeltas from the last century to the last millennia period?

As part of the Eurostrataform project, which investigated the transfer of sediment from sources (catchment areas) to sinks (deep basins), this study aims at better understanding the formation and evolution of new sediment strata on continental shelves under the influence of meteorological events (floods, storms). Starting with the relationship between hydrodynamic forcing and resulting sedimentological and geochemical characteristics, an attempt is made to define the last millennia record on a prodelta located on a microtidal continental shelf influenced by small rivers and dominated by storms.