Article The sediment budget and dynamics of a delta-canyon-lobe system over the Anthropocene timescale: The Rhone River Delta, Lake Geneva (Switzerland/France) A. SILVA, Tiago, et al. Abstract Deltas are important coastal sediment accumulation zones in both marine and lacustrine settings. However, currents derived from tides, waves, or rivers can transfer that sediment into distal, deep environments, connecting terrestrial and deep marine depozones. The sediment transfer system of the Rhone River in Lake Geneva is composed of a sublacustrine delta, a deeply incised canyon and a distal lobe, which resembles, at a smaller scale, deep-sea fan systems fed by high discharge rivers. From the comparison of two bathymetric datasets, collected in 1891 and 2014, a sediment budget was calculated for eastern Lake Geneva, based on which sediment distribution patterns were defined. During the past 125 years, sediment deposition occurred mostly in three high sedimentation rate areas: the proximal delta front, the canyon-levée system and the distal lobe. Mean sedimentation rates in these areas vary from 0.0246 m yr-1 (distal lobe) to 0.0737 m yr-1 (delta front). Although the delta front–levées–distal lobe complex only comprises 17.0% of the analysed area, it stored 74.9% of the total deposited sediment. Results show that [...] Reference A. SILVA, Tiago, et al. The sediment budget and dynamics of a delta-canyon-lobe system over the Anthropocene timescale: The Rhone River Delta, Lake Geneva (Switzerland/France). Sedimentology, 2019, vol. 66, p. 838-858 DOI : 10.1111/sed.12519 Available at: http://archive-ouverte.unige.ch/unige:107241 Disclaimer: layout of this document may differ from the published version. 1 / 1 Authors post-print, Sedimentology, Vol. XX, XX-XX, 2018 DOI:10.1111/sed.12519; published online: 05.07.2018 The sediment budget and dynamics of a delta- canyon-lobe system over the Anthropocene timescale: the Rhone River Delta, Lake Geneva (Switzerland/France) Tiago A. Silva1,2,*, Stéphanie Girardclos2,3, Laura Stutenbecker4,‡, Marteen Bakker5, Anna Costa6, Fritz Schlunegger4, Stuart N. Lane5, Peter Molnar6, Jean-Luc Loizeau1,2 Abstract Deltas are important coastal sediment accumulation zones in both marine and lacustrine settings. However, currents derived from tides, waves, or rivers can transfer that sediment into distal, deep environments, connecting terrestrial and deep marine depozones. The sediment transfer system of the Rhone River in Lake Geneva is composed of a sublacustrine delta, a deeply incised canyon and a distal lobe, which resembles, at a smaller scale, deep-sea fan systems fed by high discharge rivers. From the comparison of two bathymetric datasets, collected in 1891 and 2014, a sediment budget was calculated for eastern Lake Geneva, based on which sediment distribution patterns were defined. During the past 125 years, sediment deposition occurred mostly in three high sedimentation rate areas: the proximal delta front, the canyon-levée system and the distal lobe. Mean sedimentation rates in these areas vary from 0.0246 m yr-1 (distal lobe) to 0.0737 m yr-1 (delta front). Although the delta front-levées-distal lobe complex only comprises 17.0% of the analysed area, it stored 74.9% of the total deposited sediment. Results show that 52.5% of the total sediment stored in this complex was transported toward distal locations through the sublacustrine canyon. Namely, the canyon-levée complex stored 15.9% of the total sediment, while 36.6% was deposited in the distal lobe. The results thus show that in deltaic systems where density currents can occur regularly, a significant proportion of riverine sediment input may be transferred to the canyon-lobe systems leading to important distal sediment accumulation zones. Keywords Lake Geneva; Rhone River; sublacustrine delta; sedimentation rates; sediment fluxes; delta front; canyon-levée; distal lobe. 1Department F.-A. Forel for Environmental and Aquatic Sciences, University of Geneva, Geneva, 1205, Switzerland 2Institute for Environmental Sciences, University of Geneva, Geneva ,1205, Switzerland 3Departement of Earth Sciences, University of Geneva, Geneva, 1205, Switzerland; 4Institute for Geological Sciences, University of Bern, Bern, 3012, Switzerland 5Institute of Earth Surface Dynamics, University of Lausanne, Lausanne, 1015, Switzerland 6Institute of Environmental Engineering, ETH Zürich, Zurich, 8093, Switzerland ‡ present adress: Institute of Applied Geosciences, Technische Universität Darmstadt, Darmstadt, 64287, Germany *Corresponding author: [email protected] 1. Introduction by the supply of terrestrial material through underflows that transport and distribute clastic sediments. In marine settings, Sediment accumulation in deltas is a function of sediment underwater deltaic systems fed by high discharge rivers (e.g. supply by rivers. In these environments, the texture, geochem- the Congo and the Yellow Rivers) are also often associated istry (Vogel et al. 2010) and biological characteristics of their with shelf canyons that route terrestrial sediments into deep- deposits reflects changes produced by climatic variability and, sea fan systems, placing these fans among the world’s most where relevant, anthropogenic activities in the associated wa- important sediment depocentres (Talling, 2014). Deltas in tershed (e.g. Anselmetti et al. 2007, Syvitski and Kettner relatively deep lake settings have similar deep sublacustrine 2011, Loizeau et al. 2012,). canyons, such as those first described by Forel (1885; 1888) When underwater deltas are characterized by low gradient for Lake Geneva (Girardclos et al. 2012). delta plains, highly stable channels and high sediment sus- Although canyons are recognized as important sediment rout- pended loads, a delta-fed thalweg and lobe system can develop ing features and distal fans as important sediment depocentres, (e.g. Postma, 1990). These underwater deltas are governed The sediment budget and dynamics of a delta- canyon-lobe system over the Anthropocene timescale: the Rhone River Delta, Lake Geneva (Switzerland/France) — 2/22 the quantification of sediment supply and the partitioning of annual discharge of the Rhone is 183 m3 s-1 and the sedi- this flux along a delta-canyon-distal lobe system has not been ment flux varies between less than 0.5 up to almost 5 Mt yr-1 produced at decadal to centennial timescales. Few studies (Loizeau and Dominik, 2000). Most of the sediment input have compared bathymetric datasets from surveys that are from the Rhone River consists of clays to medium-sized silts separated by almost 100 years (e.g. Hickin, 1989; Sabatier et (Kremer et al., 2015). The silty-sandy fraction of this input al., 2006) but were unable to provide that much detail due to accumulates near the river mouth, or at deeper levels on the the small differences and/or the limited quality of the datasets. canyon levées and in the deep lobe area due to underflows In addition, these studies covered areas too small to quantify (Corella et al., 2014, 2016). the partitioning of the supplied sediment between the delta front, the canyon/levée and the distal fan at the same timescale. Thus, there remains a need for a precise budgeting of sedi- ment deposition in similar source to sink systems, which is the scope of this work. This study provides one of the first complete quantitative centennial-scale analyses of a sediment budget in a delta- canyon-distal lobe system. Broadly speaking, this timescale corresponds to the Anthropocene and the Industrial Revolu- tion (Crutzen, 2002). The spatial resolution (50 x 50 m) allows these budgets to be accurately resolved and also to obtain es- timates of associated uncertainties. To this end, we used the oldest complete bathymetric map of Lake Geneva dating from 1891 (data collected in 1886 to 1889) and a recent multi-beam bathymetric map (data collected from December 2012 to De- cember 2013; Sesa, 2014) to determine the thickness of the Figure 1. Location of Lake Geneva in western Switzerland sediment deposits. This was used as basis to calculate a sedi- with its main tributaries and cities. Isobaths between 10 m mentary budget over the last 125 years, while assessing which and 308 m have an equidistance of 50 m. In the eastern zone parts of the system where responsible for sediment storage. of the Grand-Lac, they point to the Rhone River sublacustrine delta and its active canyon. 2. Study area Lake Geneva has a surface area of 580.1 km2 and hosts the largest volumetric freshwater reservoir in Western Europe (89 2.1 Lake Geneva’s lakebed morphology and sedi- km3). The lake is located between the Jura mountain range ment dynamics to the north and the Alps to the south (Fig. 1). The basin un- The oldest complete bathymetric dataset of Lake Geneva (Fig. derlying the lake was formed through glacial sculpting during 2), hereafter called the historical bathymetry dataset, was col- the Pleistocene (Dupuy et al., 2014). lected by Swiss and French engineers (Gosset, Hörnlimann, The lake is divided into two sub-basins: i) the deep basin and Delebecque) between 1886 and 1889 (Fig. S1 in sup- of the “Grand-Lac” to the east between Bouveret and Nyon; plementary material) and subsequently published by Forel and ii) the “Petit-Lac” in the west between Nyon and Geneva. (1892a) and Delebecque (1898). The Grand-Lac, which comprises 86% of the total area of Forel (1885) reported the occurrence of the Rhone River main the lake, is characterized by a large and flat central bottom underwater canyon, which extended for 6 km into the lake, to area at depths greater than 300 m. The Grand-Lac reaches a a water depth of 200 to 230 m, and was between 500 and 800 maximum depth of 309.7 m and has a mean depth of 172 m m wide and 50 to 10 m deep. (CIPEL, 2015). Forel (1888) compared the large underwater canyons in In the eastern part of the lake, between Bouveret and the Lake Geneva and Lake Constance with similar canyons lo- Evian-Lausanne line (Fig.