Clim. Past, 12, 21612179, 2016 www.clim-past.net/12/2161/2016/ doi:10.5194/cp-12-2161-2016 Author(s) 2016. CC Attribution 3.0 License.
Anne-Sophie Fanget1, Maria-Angela Bassetti1, Christophe Fontanier2, Alina Tudryn3, and Serge Bern1,2
1Universit de Perpignan Via Domitia, Centre de Formation et de Recherche sur les Environnements Mditerranens (CEFREM), UMR 5110-CNRS, 66860, Perpignan, France
2IFREMER, Laboratoire Environnements Sdimentaires, BP70, 29280 Plouzan, France
3GEOPS UMR 8148, Univ. Paris-Sud, CNRS, Universit Paris-Saclay, Rue du Belvdre, Bt. 504-509, 91405 Orsay, France
Correspondence to: Anne-Sophie Fanget ([email protected])
Received: 29 May 2016 Published in Clim. Past Discuss.: 27 June 2016
Revised: 31 October 2016 Accepted: 7 November 2016 Published: 6 December 2016
Abstract. A 7.38 m long sediment core was collected from the eastern section of the Rhne prodelta (NW Mediterranean) at 67 m water depth. A multi-proxy study (including sedimentary facies, benthic foraminifera, ostracods, and clay mineralogy) provides a multi-decadal to century-scale record of climate and sea-level changes during the Holocene. The early Holocene is marked by alternative silt and clay layers interpreted as distal tempestites deposited in a context of rising sea level. This interval contains shallow infra-littoral benthic meiofauna (e.g., Pontocythere elongata, Elphidium spp., Quinqueloculina lata) and formed between ca. 20 and 50 m water depth. The middle Holocene (ca. 8.3 to 4.5 ka cal. BP) is characterized, at the core site, by a period of sediment starvation (accumulation rate of ca. 0.01 cm yr1) resulting from the maximum landward shift of the shoreline and the Rhne outlet(s). From a sequence stratigraphic point of view, this condensed section, about 35 cm thick, can be identied on seismic proles as a maximum ooding surface that marks the transition between delta retrogradation and delta progradation. The transition between the early Holocene deposits and the middle Holocene condensed section is marked by a gradual change in all proxy records. Following the stabilization of sea level at a global scale, the late Holocene is marked by the establishment of prodeltaic conditions at the core site, as shown by the lithofacies and by the presence of benthic meiofauna typical of the modern Rhne prodelta (e.g., Valvulineria bradyana, Cassidulina carinata, Bulimina marginata). Several periods of increased uvial discharge are also emphasized by the presence of species com-
Sedimentary archives of climate and sea-level changes during the Holocene in the Rhne prodelta (NW Mediterranean Sea)
monly found in brackish and shallow-water environments (e.g., Leptocythere spp.). Some of these periods correspond to the multi-decadal to centennial late Holocene humid periods recognized in Europe (i.e., the 2.8 ka event and the Little Ice Age). Two other periods of increased runoffs at ca. 1.3 and 1.1 ka cal. BP are recognized, which are likely to reect periods of regional climate deterioration that are observed in the Rhne watershed. Conversely, the Migration Period Cooling (ca. 1.4 ka cal. BP) and the Medieval Climate Anomaly (ca. AD 9501250) correspond locally to periods of increased dryness.
1 Introduction
Deltas comprise a subaerial delta plain, where river processes dominate; a coarser-grained delta front, where river and basinal processes interact; and a muddy submarine prodelta dominated by oceanic processes (Bhattacharya and Giosan, 2003; Galloway, 1975; Postma, 1995). Most of the worlds deltas were initiated during the early Holocene, between ca. 9.5 and 6 ka cal. BP, owing to a deceleration of global sea-level rise (Stanley and Warne, 1994). They constitute key element of the continental margin system as they represent the rst sink of sediments delivered by rivers (Trincardi et al., 2004).
Over the last decennia, numerous investigations have documented the landsea evolution of these systems including the Amazon delta (Nittrouer et al., 1986), the Mekong delta (Ta et al., 2002; Xue et al., 2010), the Yellow River delta
Published by Copernicus Publications on behalf of the European Geosciences Union.
2162 A.-S. Fanget et al.: Sedimentary archives of climate and sea-level changes
(J. P. Liu et al., 2004; Liu et al., 2007), and the delta of the Po River (Amorosi et al., 2008; Cattaneo et al., 2003). The Rhne delta is one of the most important of the Mediterranean Sea and has also been widely investigated combining seismic, sedimentological, and micropaleontological approaches (e.g., Boyer et al., 2005; Fanget et al., 2013a, b, 2014; Gensous et al., 1993; Labaune et al., 2005).
In this paper, we study the evolution of the Rhne prodelta in terms of sedimentary environments, during the last 10.5 kyr, as marked by changes in lithofacies and benthic meiofaunal assemblages (i.e., foraminifera and ostracods) and in relation to the Holocene sea-level rise and climate changes. This study shows that (1) major phases of sea-level rise and delta evolution can be clearly identied based on several independent proxy records, and that (2) changes in uvial discharge inferred, in particular, from ostracod assemblages in the upper part of the core are linked to the last major periods of rapid climate changes of the Holocene (Mayewski et al., 2004; Wanner et al., 2014).
2 Regional geological and climatic setting
2.1 Geological evolution of the Rhne subaqueous delta
In the Gulf of Lion (NW Mediterranean), the Rhne delta (Fig. 1) occupies a deep valley incised during the Messinian and inlled with ca. 2 km of Plio-Quaternary sediments (Lo et al., 2003), mainly delivered by the Rhne River (Alosi et al., 1977). For the last ca. 500 kyr, borehole data demonstrated that shelf deposits are primarily made up of forced-regressed sequences formed in response to 100 kyr glacio-eustatic cycles (Bassetti et al., 2008; Frigola et al., 2012; Sierro et al., 2009). These authors also demonstrated that higher frequency cycles, as well as suborbital climate changes, were nicely recorded within paleo-prodeltaic sedimentary archives. Following the Last Glacial Maximum (LGM, ca. 21 ka cal. BP; Mix et al., 2001), rapid sea-level rise led to the retrogradation of Rhne delta, and formation of a wedge of transgressive (backstepping) deposits thickening landward. The most prominent feature is an elongated paleo-deltaic complex, named the Early Rhne Deltaic Complex (Bern et al., 2007), which formed during the Younger Dryas and the Preboreal (Fig. 1). After ca. 7 ka, stabilization of sea level allowed the progradation of a series of regressive deltaic lobes (Fanget et al., 2014), corresponding to the overall eastward migration of the Rhne distributaries. In total, these transgressive and regressive deposits form the Rhne subaqueous delta that reaches, along the modern delta front, up to 50 m in thickness and pinches out at a present water depth of ca. 90 m (Gensous and Tesson, 1997).
The early Holocene deposits (called seismic unit U500), which rest on a wave ravinement surface (called D500), formed a transgressive parasequence (Labaune et al., 2005) made of tempestite deposits (Fanget et al., 2014). They are separated from middle and late Holocene deposits by a con-
densed section which corresponds on seismic prole to a maximum ooding surface (MFS, called D600). The age of the MFS varies along-strike between ca. 8 and 3 ka cal. BP, and reects, at a given site, the duration of condensation and/or erosion (Fanget et al., 2014). After the stabilization of global sea level (ca. 7 ka cal. BP), the middle and late Holocene Rhne outlets shifted progressively eastward, under natural and/or anthropic inuence. As a result, several deltaic lobes are formed (Fig. 1) (Arnaud-Fassetta, 1998;LHomer et al., 1981; Provansal et al., 2003; Rey et al., 2005;Vella et al., 2005, 2008). Saint Ferrol, which is related to the Rhne de Saint Ferrol Channel, is the rst and largest paleo-deltaic lobe. It started to prograde around 7 ka cal. BP (LHomer et al., 1981). The Ulmet lobe, located to the east and linked to the Rhne dUlmet Channel formed simultaneously to the Saint Ferrol lobe. To the west of the Saint Ferrol lobe, the Peccais lobe, related to the Rhne de Peccais Channel, appeared to be posterior to the erosion of the St Ferrol lobe (Rey et al., 2005; Vella et al., 2005). During the Little Ice Age, the Bras de Fer lobe, linked to the Rhne de Bras de Fer Channel, formed between AD 1587 and 1711 (Arnaud-Fassetta, 1998). Until AD 1650, the Rhne de Bras de Fer Channel is considered as synchronous to the Rhne du Grand Passon Channel (Arnaud-Fassetta, 1998). The Rhne de Bras de Fer Channel shifted to the east up to the present-day position of the Grand Rhne River after several severe oods that occurred in AD 17091711. The progradation of these lobes is primarily inuenced by changes in sediment uxes and thus by climate (Arnaud-Fassetta, 2002;Bruneton et al., 2001; Provansal et al., 2003).
2.2 Holocene climate and its regional characteristics
High uctuations in rainfall and low-amplitude temperature variations are observed during the Holocene (Davis et al., 2003; Mayewski et al., 2004; Sepp et al., 2009; Wanner et al., 2008, 2011). Examination of globally distributed paleoclimate records led to identication of six multi-decadal to century-scale cooling events interrupting periods of relatively stable and warmer climate (Mayewski et al., 2004;Wanner et al., 2008, 2011, 2014). These periods are known as rapid climate changes (RCCs; Mayewski et al., 2004) or cold relapses (CRs; Bassetti et al., 2016; Jalali et al., 2016;Wanner et al., 2014). The 8.2 ka cal. BP cold relapse (CR0, Table 1) took place at the beginning of the Holocene (Barber et al., 1999), a period of progressive warming that induced ice cap melting and freshwater outbursts to the oceans from North American glacial lakes.
During the warm middle Holocene, several signicant episodes were identied at 6.4, 5.3, and 4.2 ka cal. BP (CR1, CR2, and CR3, respectively; Table 1) (Walker et al., 2012;Wanner et al., 2014). The 4.2 ka event, which may have played a role in the collapse of various civilizations (Magny et al., 2013), is characterized by increased drought in North America, Asia, and the southern Mediterranean region.
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A.-S. Fanget et al.: Sedimentary archives of climate and sea-level changes 2163
Figure 1. Offshore and onshore morphology of the Rhne deltaic system. This map is based on the compilation of Bern et al. (2007) and Vella et al. (2008). The successive shifting of the Rhne distributaries under natural and/or anthropic inuence during the middle and late Holocene led to the formation of several deltaic lobes. Steps in the present-day morphology correspond to the position of the paleo-delta fronts that can be linked to known paleo-distributaries of the Rhne: (1) early Saint Ferrol, (2) Peccais, (3) Bras de Fer, (4) Pgoulier, and(5) modern Roustan distributary. The map of relict morpho-sedimentary units in the Rhne delta plain (paleo-shorelines, beach ridges and onshore deltaic lobes) is based on LHomer et al. (1981), Arnaud-Fassetta (1998), Vella (1999), and Provansal et al. (2003). Thick line and black dot correspond respectively to chirp seismic prole (Fig. 2) and sediment core RHKS-55 presented in this study.
Table 1. Chronology of Holocene cold relapses (CRs) based on existing literature.
Event Time slice References (kyr)
CR0 8.2 Barber et al. (1999)
CR1 6.46.2 Wanner et al. (2011)CR2 5.35.0 Magny and Haas (2004);
Roberts et al. (2011)
CR3 4.23.9 Walker et al. (2012)CR4 2.83.1 Chambers et al. (2007);
Swindles et al. (2007)
CR5 1.451.65 Wanner et al. (2011)CR6 0.650.15 Wanner et al. (2011)
During the cooler late Holocene, the 2.8 ka cal. BP cold period (CR4, Table 1) might be at the origin of the decline of the Late Bronze Age civilization (Do Carmo and Sanguinetti, 1999; Weiss, 1982), and CR5 (Table 1) matches the Migra-
tion Period that occurred around 1.4 ka cal. BP (Wanner et al., 2014). The late Holocene cooling trend reached the highest development during the Little Ice Age (LIA, CR6 in Table 1) between the 13th and 19th centuries (Frezza and Carboni, 2009).
In the Rhne catchment area, these CRs (or at least CR0 and CR6) are marked by increased river runoff. CR0, or the so-called 8.2 ka event (Alley et al., 1997), is indeed marked by high lake levels generated by more intense rainfall (Magny and Begeot, 2004; Magny et al., 2003). The LIA (CR6), which is well documented in the Rhne watershed, is marked by a period of signicant Alpine glacier advance (Goehring et al., 2011; Ivy-Ochs et al., 2009), high lake levels (Magny et al., 2010), and an increase in Rhne River oods (Pichard, 1995; Pichard and Roucaute, 2014).
3 Material and methods
The present paper is based on a multi-proxy study of a7.38 m long piston core (RHS-KS55) collected in front
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of the paleo-Rhne de Bras de Fer and Grand Passon channels at 67 m water depth (latitude = 43 14.35[prime] N;
longitude = 4 40.96[prime] E) during the RHOSOS oceanographic
cruise (R/V Le Surot, September 2008).
The detailed architecture of the Rhne subaqueous delta was determined from high-resolution seismic data (2000 5200 Hz Chirp system), together with core data constrained by 14C dates (Fanget et al., 2014). Radiometric dates were measured with accelerator mass spectrometer (AMS) 14C
on benthic foraminifera or shells at Poznan Radiocarbon Laboratory (PRL, Poland) and at Laboratoire de Mesure
14C (LMC14) at Commissariat lEnergie Atomique (CEA, France). Nevertheless, because of the low quantity of biogenic carbonates in the proximal part of the Rhne prodelta, we experienced difculties in dating core RHS-KS55 and observed three age inversions from 336 to 122 cm. Based on seismic and lithofacies correlations at the regional scale (Fanget et al., 2014), we excluded these three 14C dates for core RHS-KS55. We also excluded the three older 14C dates
(unit U500, base of the core) since measured ages are considered older than true ages (due to incorporation of reworked benthic foraminifera; for more information, see Sects. 4.1 and 5.1). As a result, eight robust 14C dates were used to create the age model for the last ca. 9 kyr. Ages are 13C-
normalized conventional 14C years and are corrected for an assumed airsea reservoir effect of 400 years. Calendar ages were calculated using the program Clam (version 2.2, Blaauw, 2010) and the Marine13 calibration curve (Reimer et al., 2013). The age model was based on the linear interpolation between the dated levels using basic (non-Bayesian) agedepth modeling software (Blaauw, 2010).
Core RHS-KS55 was split lengthwise, imaged, and visually described for identifying sedimentological facies. Core RHS-KS55 was then subsampled for benthic microfaunal analyses (i.e., ostracods and foraminifera) and clay miner-alogy. Three-centimeter-thick slides were collected using a 10 cm sampling step through the core, except from the bottom of the core to ca. 500 cm, where thick slides and sampling step were slightly modied. A total of 79 samples were sieved through a 63 m mesh screen and the residues were dried and dry-sieved using a 125 and 150 m mesh screens.Ostracods and benthic foraminifera were hand-sorted from the > 125 and > 150 m fractions, respectively, and stored in Plummer slides. To illustrate the diversity of benthic meio-fauna, total abundance (values normalized for a 100 cm3 sample volume), species richness (S), Shannon index (H), and evenness index (E) (Hayek and Buzas, 1997; Shannon, 1948) were calculated, as described in Murray (2006), for each level. To highlight vertical patterns in benthic meiofaunal communities, hierarchical clustering was also performed on the 79 samples and on the major species (i.e., occur-ring with more than 5 % in at least one sample) by mean of the PAST software (version 2.09, 2011; Hammer et al., 2001). Cluster analyses were based on the arcsine values of the square of root pi, where pi is the relative abundance
(%) of the species i divided by 100. A tree diagram was constructed according to the Wards method based on the squared Euclidean distances.
The fraction less than 63 m was used to perform clay mineral analyses. X-ray diffraction (XRD) on oriented mounts of carbonate-free clay-sized (< 2 m) particles was conducted to identify clay minerals with the PANalytical diffractometer, following the routine of the GEOPS Laboratory (Paris Sud University, France) (Z. Liu et al., 2004; Liu et al., 2008).Three XRD runs were carried out, following air drying, ethyleneglycol solvation over 24 h, and heating at 490 C over 2 h. The position of the (001) series of basal reections on the three XRD diagrams was used to identify clay minerals. Semi-quantitative estimates of peak areas of the basal reections for the main clay mineral assemblages of illite (10 ), smectite (including mixed layers) (1517 ), and kaolinite / chlorite (7 ) were performed on the glycolated curve using the MacDiff software (Petschick, 2000). Relative proportions of kaolinite and chlorite were determined using the ratio 3.57 / 3.54 of the peak areas.
4 Results
4.1 Seismic stratigraphic framework, age model, and sedimentation rates
Seismic discontinuities and seismic units described in the following section are based on Fanget et al. (2014). Several deglacial and Holocene seismic units bounded by well-marked discontinuities are identied at the core site (Fig. 2).Seismic data show that core RHS-KS55 goes through surface D500 and represents an expanded record of seismic units U500, U600, and U610. Seismic unit U620a is missing in the studied core (Fig. 2).
The age of seismic unit U500, which corresponds to the early Holocene transgressive parasequence (Fanget et al., 2014), is poorly constrained in core RHS-KS55 (Figs. 3 and4). Considering the age of the underlying deposits of seismic unit U400 in this area (i.e., paleo-deltaic complex of the Rhne (ERDC), ca. 10.5 ka cal. BP; Bern et al., 2007), we assume that the three 14C dates obtained at 732, 672, and 512 cm (i.e., 12.7, 13.3, and 12.2 ka cal. BP, respectively) are biased (Fig. 4) because of reworking occurring in shallow-water environments. This interpretation is supported by the nature of sediments that composed this unit.Indeed, at the regional scale, seismic unit U500 is made of tempestite deposits (Facies 1, Sect. 4.2.) which mainly contain infra-littoral (i.e., upper shoreface) benthic foraminifera (e.g., Elphidium crispum; see Table 2 and Fanget et al., 2014). These infra-littoral benthic foraminifera are likely reworked by high-energy hydrodynamic processes that regularly winnowed the seaoor. The three older ages obtained within seismic unit U500 are thus probably the result of reworking during the transgression of an underlying Younger Dryas/Preboreal delta front. It is likely that infra-littoral ben-
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Figure 2. Dip (NS) chirp seismic prole across the Grand Passon and Bras de Fer subaqueous delta (position in Fig. 1). Surface D500 corresponds to the ooding surface (which coincides here with a wave ravinement surface) that separates the Younger Dryas (seismic unit U400) deposits from the Preboreal deposits (seismic unit U500). The downlap surface D600 is the maximum ooding surface formed during the turnaround between retrogradation and progradation. D610 and D620 are erosional surfaces that mimic ooding surfaces and separate the middle and late Holocene sedimentary wedges (seismic units U600, U610, and U620a) formed in response to the successive shifts of Rhne Channel. Twtt: two-way travel time.
progrades on D600, and is related to the marine component of the St Ferrol and Ulmet lobes (Fanget et al., 2014).
14C dates indicate that seismic unit U600 was deposited between ca. 4.5 and 0.9 ka cal. BP (Fig. 3). Sedimentation rates through this interval oscillated between 0.03 and 0.4 cm yr1 (Fig. 4). Highest sedimentation rates are recorded along the uppermost part of this unit (i.e., between 300 and 110 cm, Fig. 4). Finally, seismic unit U610, corresponding to the activity of the Grand Passon and Bras de Fer channels, was formed between ca. 900 and 280 yr cal. BP (Fig. 3) (Fanget et al., 2014). As seismic unit U620a is missing within core RHS-KS55, we estimate that the top of the core has an age of ca. 280 yr cal. BP. Sedimentation rate through seismic unit U610 is estimated at ca. 0.11 cm yr1 (Fig. 4).
4.2 Sedimentary features
Based on lithological description (including lithofacies, sedimentary structures, bioturbation, and color) of core RHSKS55 (Fanget et al., 2014), three main sedimentary facies(i.e., Facies 1, 2, and 3 (including 3a and b)) are identied and summarized as follows:
Facies 1: from 738 (core bottom) to 460 cm, core RHSKS55 consists of numerous silt or very ne sand laminae (in the sense of Campbell, 1967) interlaminated with grayish and beige silty clay with spacing of millimeters to several centimeters (Fig. 4). Within these very thin laminae (millimeters to a few centimeters thick), which are characterized by erosional basal contacts, no sedimentary structures can be identied.
Facies 2: from 460 to 430 cm, a peculiar interval consisting of heterolithic content in a grayish silty clay ma-
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Figure 3. Age model for the upper part (i.e., from 472 to 0 cm; middle and late Holocene) of core RHS-KS55 based on linear interpolation using the Clam 2.2 software (Blaauw, 2010).
thic foraminifera picked for these three 14C dates are reworked from older deposits (for more details, see Sect. 5.1).
Only the uppermost part of this unit is condently dated between ca. 9.2 and 8.3 ka cal. BP (Table 2). The upper boundary of seismic unit U500, called D600 and interpreted as a MFS (Fanget et al., 2014), corresponds to a condensed section formed between ca. 8.3 and 4.5 ka cal. BP in the studied core (Fig. 3). It is characterized by a very low sedimentation rate of ca. 0.01 cm yr1 (Fig. 4). Seismic unit U600
exhibits higher (from ca. 5 to 25 %) and erratic values along the uppermost 460 cm of the core. Chlorite content reaches ca. 20 % in the lower part of the core (from the bottom to 350 cm) and drops near 0 % between 350 and 120 cm (Fig. 5).Chlorite content increases to ca. 20 % along the uppermost 120 cm of the core. Kaolinite content is very low throughout core RHS-KS55 with values close to 0 % from the bottom of the core to 360 cm, and ranging from ca. 7 to 10 % between 360 cm and the top of the core (Fig. 5).
4.4 Ostracod fauna
4.4.1 Ostracod density and diversity indices
The number of ostracods per sample varies greatly from 68 to 12 821 ind./100 cm3 in core RHS-KS55 (Fig. 6). From the base of the core to 482 cm, density ranges from 68 to 1266 ind./100 cm3. The lowest values are observed from 642 to 632 cm, whereas four peaks of 1141, 1243, 1006, and 1266 ind./100 cm3 are identied at 702, 622, 571, and 541 cm, respectively. From 471 to 352 cm, the number of counted ostracods increases strongly with values reaching up to 12 821 ind./100 cm3 and 10 539 ind./100 cm3 at 432 and 412 cm, respectively. The number of ostracods per sample drops to 222 ind./100 cm3 at 332 cm and ranges from 305 to 1182 ind./100 cm3 between 322 and 162 cm. Density decreases between 152 and 82 cm and then increases progressively along the uppermost 82 cm of the core.
Species richness (S) oscillates between 7 and 31 species per sample through the core and approximately follows the
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Table 2. Summary of 14C dates. Absolute dates were obtained with accelerator mass spectrometer (AMS) 14C dating of well-preserved shells and benthic foraminifera at Laboratoire de Mesure 14C (LMC14) at Commissariat lEnergie Atomique (CEA, Saclay) and at Poznan Radiocarbon Laboratory (PRL). The ages reported herein are delta 13C-normalized conventional 14C years, corrected for an assumed air-sea reservoir effect of 400 years. Calendar ages were calculated using Clam software (Blaauw, 2010) and the Marine13 calibration curve (Reimer et al., 2013).
Depth Material Weight Sample 14C conventional Calibrated age Mean calibrated (cm) (mg) number age (yr BP) (yr cal. BP) age (yr cal. BP)
9093 Benthic foraminifera 9.5 SacA 27201 1335 [notdef] 30 790944 867
120123 Benthic foraminifera 9.9 SacA 27202 1840 [notdef] 30 13001477 1389
150153 Benthic foraminifera 11 SacA 23204 2080 [notdef] 35 15511761 1656
200203 Benthic foraminifera 10.6 SacA 23205 1655 [notdef] 30 11541282 1218
300303 Benthic foraminifera+Turritella sp. 10.9 SacA 23206 1900 [notdef] 30 13621527 1445
335336 Benthic foraminifera +Turritella sp. 10 SacA 23207 1705 [notdef] 30 11851318 1252 + mixed bivalves
350353 Benthic foraminifera 11.3 SacA 27203 2760 [notdef] 35 23512619 2485
417420 Turritella sp. 896 Poz-35061 4335 [notdef] 35 43754574 4475
430433 Benthic foraminifera 10.5 SacA 27204 6190 [notdef] 40 65136735 6624
440443 Nucula sp. 11.2 SacA 27205 7830 [notdef] 40 81928374 8283
470473 Benthic foraminifera 10.2 SacA 23208 8565 [notdef] 35 90759333 9204
510513 Elphidium crispum 10.1 SacA 23209 10790 [notdef] 40 1205812467 12 263
670673 Benthic foraminifera 13 SacA 23210 11855 [notdef] 45 1321513433 13 324
730733 Elphidium crispum 10.6 SacA 23211 11280 [notdef] 40 1264412870 12 757 Mean calibrated ages with an asterisk were excluded from the age model since they are considered biased (see text for more details).
trix is observed (Fig. 4). Turritella sp., as well as several bivalves (e.g., Acanthocardia echinata, Arca tetragona, Nucula sp.) and bryozoans, is identied.
Facies 3a: from 430 to 320 cm, sediment consists of beige silty clay without visible sedimentary structures.Diffuse veneers of yellowish lighter levels and spots of darker deposits (richer in hydrotroilite), clearly obliterated by intense bioturbation, are observed (Fig. 4). Scattered bryozoans debris, Turritella sp., and bivalves are encountered in this interval.
Facies 3b: from 320 to 0 cm, sediment consists of gray-ish and beige silty clay and contains abundant hydrotroilites and bioturbation.
4.3 Clay mineralogy
Clay mineral assemblages are dominantly composed of illite, with values ranging from ca. 55 to 85 % (Fig. 5). Illite content exhibits high and quite constant values (ca. 70 %) from the bottom of the core to 465 cm. From 455 to 360 cm, illite content decreases to ca. 60 %. A strong increase is observed at 340 cm, with values reaching ca. 80 %. Illite content remains high between 340 and 110 cm. At 110 cm, illite content drops to ca. 55 % and is generally lower from 100 to 30 cm. Along the uppermost 30 cm of the core, a progressive increase in illite content is identied. Smectite content is inversely correlated to illite content throughout core RHS-KS55 (Fig. 5).It has very low values (ca. 2.5 %) from 738 to 465 cm and
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is between 0.2 and 0.7, follows approximately the same trend as H (Fig. 4). The lowest values of E are observed from 738 to 492 cm, whereas the maximal values are observed from 482 to 422 cm. E is relatively constant along the uppermost 412 cm of the core, with values oscillating around 0.5.
4.4.2 Cluster analysis
R-mode cluster analysis allows us to identify six ostracod clusters plus one single species when a cut-off level of 1.4 is applied (Figs. 7 and 8).
Cluster A is made of Semicytherura incongruens, Pontocythere elongata, and Semicytherura sp. It has a maximal contribution from 738 to 512 cm, with erratic values ranging from ca. 5 to 25%. It decreases strongly from 512 to 442 cm and disappears completely along the uppermost 432 cm of the core.
Cluster B is composed of Propontocypris pirifera, Cytherissa sp., Eucythere sp., Aurila sp., and Cytheridea neapolitana. It shows a low contribution through the core, with values generally < 10 %. It increases only between 482 and 442 cm, where a maximal value of 25 % is reached.
Cluster C is constituted by Cytherella sp., Cytheropteron alatum, Cytheropteron monoceros, and Carinocythereis carinata. It exhibits a low contribution (< 10 %) from 738 to 492 cm. It increases strongly between 492 and 372 cm, to reach up to ca. 40 % of the ostracod fauna at 432 cm. Cluster C decreases progressively, and has a minimal contribution along the uppermost 362 cm of the core.
The single species corresponds to Leptocythere spp. This species dominates ostracod fauna from 738 to 492 cm, with values ranging from ca. 41 to 73 %. Along the uppermost 492 cm of the core, Leptocythere spp. oscillates between low (< 5 %) and high (ca. 3040 %) values. The lowest contributions of this species are recorded from 472 to 432, 332 to 302, 232 to 182, and 122 to 82 cm.
Cluster D is made of Cytheropteron rotundatum and Krithe spp. (juvenile Krithe and K. pernoides). From 738 to 362 cm, it has a low contribution (< 5 %). It increases strongly along the uppermost 352 cm of the core, with values ranging from ca. 30 to 60 %, and reaches a peak of ca. 77 % at 92 and 82 cm.
Cluster E is composed of Argilloecia spp., Loxoconcha laevis, and Paradoxostoma sp. It exhibits a moderate contribution through the core, with values oscillating generally between ca. 5 and 15 %. However, four peaks of ca. 37, 45, 35, and 27 % are recorded at 492, 342332, 112, and 72 cm, respectively.
Cluster F is constituted by Sagmatocythere sp., Bosquetina dentata, and Pterigocythereis jonesii. It shows a minimal contribution (< 5 %) from 738 to 492 cm. It increases between 482 and 162 cm, with values ranging from ca. 20 to 35 %, except between 352 and 332 cm, where a decrease (< 15 %) is observed. From 162 to 72 cm, Cluster F exhibits
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Figure 4. Sediment features and sedimentation rates of core RHSKS55. Correlations with seismic units and discontinuities and with the Holocene chronology are shown on the left side of the gure. Black rectangles represent the most signicant periods of climate deterioration (known as cold relapses (CRs); Wanner et al., 2014) during the Holocene (e.g., the 8.2 ka event (CR0) and the Little Ice Age (CR6)). AMS 14C dates are mentioned to the right; italic values correspond to age inversions. 14C dates are summarized in Table 2.
same trend as density (Fig. 6). Lowest values of S are recorded when ostracods abundances are minimal, i.e., at 642 and 632 cm and between 152 and 82 cm. S is maximal within the interval between 482 and 442 cm. The Shannon index (H) varies between 1.2 and 3.0 through the core (Fig. 6). H increases progressively from the base of the core to 492 cm. H reaches maximal values (ca. 3.0) between 482 and 442 cm and decreases progressively from 433 to 82 cm. Along the uppermost 82 cm of the core, H gradually increases (from ca. 1.4 to 2.2). The evenness index (E), which
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Figure 5. Distribution of the main clay minerals in core RHS-KS55. Correlations with seismic units and discontinuities are shown on the left side of the gure. The corresponding positions of seismic units and discontinuities (U500, U600, U610, D600, and D610) are also reported on the plots (gray bands and dashed lines). Black rectangles on the left side of the gure represent the most signicant periods of climate deterioration (i.e., cold relapses (CRs)) during the Holocene.
erratic values oscillating between ca. 2 and 20 %. Along the uppermost 72 cm of the core, it increases slightly.
4.5 Benthic foraminiferal fauna
4.5.1 Benthic foraminiferal density and diversity indices
The number of benthic foraminifera per sample varies greatly from 404 to 74 642 ind./100 cm3 through the core (Fig. 6). From 738 to 482 cm, density shows erratic values oscillating between 404 and 7130 ind./100 cm3 and increases progressively. The number of counted specimens increases strongly between 472 and 382 cm, with values ranging from 12 386 to 74 642 ind./100 cm3. From 382 to 352 cm, density decreases rapidly and drops to 2995 ind./100 cm3. Along the uppermost 342 cm of the core, benthic foraminiferal densities remain quite constant and below 3000 ind./100 cm3.
Species richness (S) oscillates between 13 and 49 species per sample through the core (Fig. 6). The lowest values of S (from 13 to 26) are recorded from 738 to 632 cm. S increases from 622 to 492 cm, except between 542 and 532 cm, where a decreased is observed. The highest values of S are encountered from 482 to 212 cm, with values oscillating between 34
and 49 species per sample. S slightly decreases along the uppermost 202 cm of the core and remains quite constant. The Shannon index (H) varies between 1.2 and 3.2 through the core RHS-KS55 (Fig. 6). From 738 to 642 cm, H decreases progressively from 2.3 to 1.2. Erratic values, oscillating between 1.6 and 2.7, are observed from 632 to 492 cm. Even if H slightly decreased between 452 and 402 cm, the highest values are recorded between 482 and 352 cm. From 342 to 132 cm, H decreases progressively, whereas it increases slightly along the uppermost 122 cm of the core. The evenness index (E) exhibits relatively low values through the core, ranging from 0.2 to 0.5, and follows exactly the same trend as H (Fig. 6).
4.5.2 Cluster analysis
R-mode cluster analysis allows us to distinguish four benthic foraminiferal clusters when a cut-off level of 2.4 is applied (Figs. 7 and 9).
Cluster 1 is composed of Elphidium spp. (including E. advenum, E. crispum, E. decipiens, E. granosum, E. incertum, E. macellum, and E. margaritaceum), Nonionella turgida, and Quinqueloculina lata. From 738 to 492 cm,
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Figure 6. Ecological indices (total abundance per 100 cm3, number of species (S), Shannon (H), and evenness (E) index) describing benthic foraminiferal and ostracod populations. Correlations with seismic units and discontinuities are shown on the left side of the gure. The corresponding positions of seismic units and discontinuities (U500, U600, U610, D600, and D610) are also reported on the plots (gray bands and dashed lines). Black rectangles on the left side of the gure represent the most signicant periods of climate deterioration (i.e., cold relapses (CRs)) during the Holocene.
Figure 7. R-mode cluster analyses of the 16 major (more than 5 % in at least one sample) benthic foraminiferal species, and of the 21 major ostracod species according to Wards method, based on standardized percentages pi of these species.
Cluster 1 dominates strongly with abundances oscillating between ca. 58 and 91 %. The contribution of Cluster 1 drops to ca. 15 % at 472 cm. It has a minimal contribution along the uppermost 472 cm of the core, and especially between 342 and 62 cm, where it exhibits values < 10 %.
Cluster 2 is constituted by Cassidulina carinata, Bulimina marginata, and Valvulineria bradyana. It is characterized by a low contribution (less than 10 %) from 738 to 492 cm. It increases progressively from ca. 15 to 72 % between 482 and 202 cm and remains quite constant along the uppermost
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Figure 8. Cumulative percentages of taxa composing the dened ostracod clusters along core RHSKS55. Correlations with seismic units and discontinuities are shown on the left side of the gure. The corresponding positions of seismic units and discontinuities (U500, U600, U610, D600, and D610) are also reported on the plots (gray bands and dashed lines). Holocene cold relapses (CRs) are also indicated on the y axis by the black rectangles (left side of the gure). Periods of increased uvial discharge are highlighted by peaks in Leptocythere (hatched rectangles). These periods of intensied runoffs correlate to the 2.8 ka event (i.e., CR4) and the LIA (i.e., CR6). Two periods of regional climate deterioration are also observed at 1.3 and 1.1 ka cal. BP. Conversely, the Migration Period Cooling (i.e., CR5) and the Medieval Climate Anomaly (MCA) correspond to periods of increased dryness.
192 cm of the core, with values oscillating between ca. 50 and 72 %.
Cluster 3 is made of Haynesina depressula, Ammonia beccarii, and Eggerella scabra. It has a low contribution (< 15 %) through the core RHS-KS55. From 738 to 492 cm, it exhibits relatively erratic values, and three peaks of ca. 12, 5, and 6 % are observed at 699, 632, and 532 cm, respectively. Cluster 3 has a minimal contribution (< 1 %) between 482 and 342 cm and increases slightly from 332 to 162 cm. From 162 to 82 cm, it increases progressively to reach a value of ca. 9 %. Cluster 3 decreases again between 82 and 32 cm and increases slightly along the uppermost 32 cm of the core.
Cluster 4 is constituted by Pseudoeponides falsobeccarii, Textularia agglutinans, Hyalinea balthica, Melonis barleeanus, Bulimina aculeata, Sigmoilopsis schlumbergeri, and Cibicides lobatulus. It shows a low contribution from 738 to 492 cm, with erratic values ranging from ca. 5 to 22 %. It increases strongly between 482 and 402 cm, where values of ca. 55 % are reached. From 402 to 292 cm, Cluster 4 decreases progressively. It remains quite constant along the uppermost 292 cm of the core, with values oscillating between ca. 13 and 25 %.
5 Discussion
5.1 Benthic meiofauna reworking processes in subaqueous deltaic environment
In subaqueous deltaic environments, reworking processes appear to be common within transgressive deposits (Cattaneo and Steel, 2003). In the Rhne subaqueous delta, transgressive deposits consist of tempestite deposits (seismic unit U500), which are the result of regular occurrence of high-energy hydrodynamic processes (including combined storm and ood events; Fanget et al., 2014). These processes regularly winnowed the seaoor and generate erosion, reworking, and transport of sediments. Thus, it is likely that benthic calcareous meiofauna are reworked from older deposits into modern deposits having the same faunal assemblages (Cearreta and Murray, 2000). These reworked benthic meiofauna cannot be considered as in situ, but it appears impossible to distinguish them from the unreworked modern tests and cara-paces. This will directly affect AMS dating with measured ages older than true ages, as observed within the transgressive seismic unit U500. Such phenomena have been observed in Denmark (Heier-Nielsen et al., 1995) and Spain (Cearreta
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and Murray, 2000), and these highlight the difculty to obtain reliable AMS dates from high-energy transgressive deposits.
Within the most recent prograding units of the highstand systems tract (4.5 to 0.3 ka cal. BP in the present study), we also observe the regular occurrence of reworking and transport of benthic meiofauna. Reworking processes are regularly encountered in shallow-water environments (e.g., Frenzel and Boomer, 2005; Loureiro et al., 2009; Fanget et al., 2013a). Conversely to the transgressive systems tract, reworked benthic meiofauna are easier to identify since they originate from shallow-water environments and deposit into deeper settings. Reworking processes in AMS dating are thus considered as less important and problematic in the high-stand systems tract. It is likely than these allochthonous benthic meiofauna are transported and redeposited further offshore within the river plume during periods of increased river discharge (Fanget et al., 2013a). Thus, it can be relevant to use allochthonous meiofauna as biomarkers for a better understanding of transport and reworking processes (Cronin, 1983; van Harten, 1986; Zhou and Zhao, 1999;Fanget et al., 2013a; Angue Mintoo et al., 2015) and to study paleo-hydrology. The distribution pattern of reworked benthic meiofauna through highstand deposits is likely to reect hydrological uctuations in the past (see Sect. 5.3).
5.2 Record of Holocene sea-level rise and Rhne delta evolution
Seismic stratigraphy, sedimentological (including clay minerals) and benthic meiofauna data described in the previous section (Sect. 4) allow the subdivision of the studied core into three main intervals. These intervals fairly match the tripartite subdivision of the Holocene (Walker et al., 2012; Wanner et al., 2014) and might be linked to the Holocene sea-level history as well as the Rhne deltaic system evolution.
5.2.1 Interval 1 (ca. 10.58.3 ka cal. BP)
This interval encompasses most of the early Holocene. The age of the bottom of the core up to ca. 460 cm cannot be dated precisely because it corresponds to a transgressive parasequence (seismic unit U500, Fig. 2) that formed in a context of shallow-marine environment. Based on the age of the underlying deposits (i.e., the ERDC, seismic unit U400) and on
14C dates, this interval was deposited between ca. 10.5 and8.3 ka cal. BP (i.e., the early Holocene; Walker et al., 2012).
Tempestite (storm-induced) deposits, which are commonly found in lower to middle shoreface environments during periods of storm decelerating ows (Myrow, 1992; Myrow and Southard, 1996; Prez-Lpez and Prez-Valera, 2012), characterize this interval (Fig. 4). The intercalation of ne clay and silt layers suggests that these deposits are distal tempestites (i.e., turbidite-like deposited below the storm wave base; Myrow, 1992; Prez-Lpez and Prez-Valera, 2012).This facies is interpreted to correspond to an hydrodynamic
regime resulting from the combination of E-SE storm waves and ood events (i.e., wet storms of Guilln et al., 2006), which regularly winnow the seaoor (Fanget et al., 2014).Tempestite deposits mainly contain foraminifera belonging to Cluster 1 (Elphidium spp., N. turgida, Q. lata) and ostracods belonging to Cluster A (S. incongruens, P. elongata, Semicytherura sp.), as well as to the genus Leptocythere (Figs. 8 and 9). Benthic foraminiferal species, likeN. turgida, are typical of shallow prodeltaic environment enriched in organic matter of continental origin (e.g., Barmawidjaja et al., 1992; De Rijk et al., 2000; Diz and Francs, 2008; Van der Zwaan and Jorissen, 1991). Elphidium spp. and Q. lata are commonly reported in sandy silty substrates subject to strong hydrodynamic processes (e.g., Donnici and Serandrei Barbero, 2002; Jorissen, 1988; Rossi and Vaiani, 2008; Sgarrella and Moncharmont Zei, 1993). Similar observations are described in the modern Rhne subaqueous delta (Goineau et al., 2011, 2015; Mojtahid et al., 2009). Similarly, ostracods content of Cluster A are represented by littoral to sublittoral/phytal marine forms (e.g., Bonaduce et al., 1975;Cabral et al., 2006; Carbonel, 1980; Peypouquet and Nachite, 1984; Zabi et al., 2012). The genus Leptocythere is commonly found in brackish and shallow-water environments, and many Leptocythere are known to be euryhaline species (e.g., Anadon et al., 2002; Boomer and Eisenhauer, 2002;Carbonel, 1973, 1980; Frenzel and Boomer, 2005; Gliozzi et al., 2005; Van Morkhoven, 1963).
According to paleoenvironmental reconstruction based on benthic meiofauna from core RHS-KS55, the early Holocene is characterized, in the Rhne subaqueous delta, by high-energy hydrodynamic processes and signicant organic matter input of continental origin typical of shallow infra-littoral setting. This interpretation is in agreement with the occurrence of tempestite deposits, as well as the global estimates of sea-level rise during the early Holocene (e.g., Bard et al., 1996; Fairbanks, 1989; Smith et al., 2011). Based on the sea-level curve of Stanford et al. (2011), the base of the core (estimated at ca. 10.5 ka cal. BP) corresponds to a sea level of ca. 50 m below its present-day position. Due to the location of core RHS-KS55 at a water depth of 67 m and its length of 7.38 m, a paleo-water depth of ca. 24 m can be estimated at the base of the core (subsidence and compaction being considered as negligible). At the top of the tempestite facies(i.e., at ca. 460 cm), which is dated at ca. 9.2 ka cal. BP, a paleo-water depth of ca. 52 m is estimated. Thus, the resulting rate of sea-level rise within this interval (738460 cm) is ca. 20 mm yr1. This value matches the one found by Stanford et al. (2011) for the early Holocene. Thus, we consider that tempestite deposits, preserved within this transgressive interval (seismic unit U500), are formed at water depth ranging from ca. 20 to 50 m. In the Rhne subaqueous delta, we consider the tempestite facies as a relatively good paleobathymetric marker and we have been able to correlate it over a large prodelta area (see cores RHS-KS40, RHS-KS22, and RHS-KS39 in Fanget et al., 2014).
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Figure 9. Cumulative percentages of taxa composing the dened benthic foraminiferal clusters along core RHS-KS55. Correlations with seismic units and discontinuities are shown on the left side of the gure. The corresponding positions of seismic units and discontinuities (U500, U600, U610, D600, and D610) are also reported on the plots (gray bands and dashed lines). Holocene cold relapses (CRs) are also indicated on the y axis by the black rectangles (left side of the gure).
5.2.2 Interval 2 (ca. 8.34.5 ka cal. BP)
The interval between 460 and 430 cm corresponds to a period ranging from ca. 8.3 to 4.5 ka cal. BP (i.e., the middle Holocene, Fig. 3), when the Rhne outlet(s) was situated 10 to 30 km landward from the modern shoreline. Considering the resolution of our seismic data (in the order of ca. 0.5 m), it corresponds to the position of the MFS (surface D600, Fig. 2) that marks the transition between retrogradation and progradation. Very low sediment accumulation (ca. 0.01 cm yr1,
Fig. 4), abundant shell concentration, and very rich microfossil content (up to 13 000 ostracods/100 cm3 and 75 000 foraminifera/100 cm3, Fig. 6) indicate sediment starvation within this condensed section which separates transgressive (below) from regressive (above) deposits. This condensed section consists of a silty clay matrix incorporating coarse-grained sediments with reworked shoreface material and shell hash. Clay mineralogy assemblages indicate a clear change in clay mineralogy proportions with a signicant decrease in illite content (from 80 to 60 %) and a sharp increase in smectite content (from 0 to 20 %; Fig. 5).
Benthic foraminifera belonging to Cluster 4 (P. falsobeccarii, T. agglutinans, H. balthica, M. barleeanus, B. aculeata,
S. schlumbergeri, C. lobatulus), and ostracods belonging to Cluster B (P. pirifera, Cytherissa sp., Eucythere sp., Aurila sp., and C. neapolitana), Cluster C (Cytherella sp., C. alatum, C. monoceros, and C. carinata), and Cluster F (Sagmatocythere sp., B. dentata, and P. jonesii) are dominant within this interval (Figs. 8 and 9). Except for C. lobatulus, which is preferentially found in a high-energy shallow-water setting (e.g., Bartels-Jnsdttir et al., 2006; Javaux and Scott, 2003; Milker et al., 2011; Murray, 2006), foraminifera assemblage is mainly composed of species thriving under a stable environment characterized by marine-derived organic matter input and well-oxygenated sediments (e.g., De Rijk et al., 2000; Debenay and Redois, 1997; Fontanier et al., 2008; Goineau, 2011; Goineau et al., 2011, 2015; Mendes et al., 2004; Mojtahid et al., 2009). Clusters B and F are mainly composed of shallow infra-littoral ostracods (e.g., Bonaduce et al., 1975; Frenzel and Boomer, 2005; Guernet et al., 2003; Ruiz et al., 1997; Zabi et al., 2012), whereas Cluster C is primarily made of circa-littoral and epi-bathyal species (e.g., Bonaduce et al., 1975; El Hmaidi et al., 2010; Yamaguchi and Norris, 2012).
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species can be tolerant to moderate river inuence (Fanget et al., 2013b). At the core site, strong decreases in Cluster 1 and Cluster B (shallow infra-littoral species), as well as increases in Cluster 2 and Cluster D, reveal the establishment of prodeltaic conditions since 4.5 ka cal. BP (Figs. 8 and 9).More precisely, they correspond to the progradation of the St Ferrol and Ulmet lobes. A similar pattern is identied on boreholes in the Rhne delta plain, where the onset of prodeltaic sedimentation is marked by the dominance of V. bradyana around 4 ka cal. BP (Amorosi et al., 2013).
Within Interval 3, we note also the presence of benthic foraminifera belonging to Cluster 3 (H. depressula, A. beccarii, E. scabra), and ostracods belonging to the genus Leptocythere and to Cluster E and F (Figs. 8 and 9). The vertical pattern of these ostracods in this interval will be discussed in further details in the next section (5.3). Foraminifera constituting Cluster 3 are typical of very shallow-water environments, and E. scabra is notably known to be adapted to thrive in organic-matter-enriched and hypoxic sediments (Diz and Francs, 2008; Donnici and Serandrei Barbero, 2002; Jorissen, 1987; Mendes et al., 2004). This assemblage increases in the uppermost 300 cm of the core, in concomitance with increased hydrotroilite content. Authigenic minerals generated by sulfate reduction (hydrotroilite) can be related both to high sedimentation rate (as observed in the core), leading to reducing conditions, and to high organic matter supply.These observations suggest increased river inuence that can be linked to the progressive progradation of Rhne delta and to the beginning of the Bras de Fer and Grand Passon channels activity, located in front of the studied core.
5.3 Record of Holocene cold events (CRs)
Ostracods belonging to Clusters E and F, and especially to the genus Leptocythere, show well-marked peaks within high-stand prodeltaic deposits (Fig. 8). As previously described, the genus Leptocythere is widely distributed in brackish and shallow marine water environments (Anadon et al., 2002;Boomer and Eisenhauer, 2002; Carbonel, 1973, 1980; Frenzel and Boomer, 2005; Gliozzi et al., 2005; Van Morkhoven, 1963). In the Po delta, the occurrence of Leptocythere sp. is notably related to local increase in uvial inuence (Rossi, 2009), and in the Rhne delta, few valves of Leptocythere are encountered in restricted environmental areas characterized by estuarine conditions (Amorosi et al., 2013). Thus, the distribution pattern of Leptocythere through the highstand deposits would reect hydrological uctuations. During the Holocene, high uctuations in precipitations are recorded, notably during the CRs (Mayewski et al., 2004; Wanner et al., 2014). In Europe, these CRs (or at least CR0, i.e., the8.2 ka event, and CR6, i.e., the LIA) are characterized by intensied rainfalls (Arnaud et al., 2012; Magny et al., 2010;Magny and Begeot, 2004).
Two intervals of increased occurrence of Leptocythere (and therefore increased rainfall) are identied between
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The middle Holocene condensed section is very well identied thanks to benthic microfossils indicating mixed assemblages belonging to diverse environments, from infra-littoral to epi-bathyal settings. Shallow-water species highlight incorporation of the previous shoreface and delta mouth sediments that were left in situ during the transgressive submersion. Circa-littoral and epi-bathyal species indicate an abrupt increase in water depth (peak of transgression) and mark the time of maximum landward shift of the shoreline.
5.2.3 Interval 3 (ca. 4.50.3 ka cal. BP)
The most recent interval (from 430 cm to the top of core) corresponds to seismic units U600 and U610 (Fig. 2), which formed during the late Holocene a series of regressive deltaic lobes that make up the highstand systems tract in the sequence stratigraphic terminology. They consist of ne-grained prodeltaic deposits and are related to the activity of the St Ferrol and Ulmet distributaries (seismic unit U600), as well as to the synchronous, and thus successive, activity of the Grand Passon and Bras de Fer channels (seismic unit U610) (Fanget et al., 2014). At the core site, clay minerals are dominated by illite, as elsewhere in the Rhne prodelta (Chamley, 1971). Indeed, the Rhne River, receiving its detrital material primarily from the Alps, is particularly rich in illite, associated with some chlorite (Chamley, 1971) that tends to be trapped in sandy sediments during deposition (Chamley, 1971; Giresse et al., 2004). Both minerals represent the relative contribution of physical weathering to sedimentation, since they are resistant to degradation and transport (Chamley, 1971). Relative contents of illite are changing simultaneously with changes in sedimentary facies and activity of different distributaries (Fig. 5). Smectite content is low as a whole but higher than within underlying intervals when sea level was lower. The onset of seaward progradation of the Rhne deltaic lobes corresponds, by denition, to the age of deposits situated immediately above the condensed section. This age is ca. 4.5 ka cal. BP according to the age model.It corresponds to a marked increase in smectite content. It also corresponds to a marked increase in benthic foraminifera belonging to Cluster 2 (C. carinata, B. marginata, and V. bradyana) and ostracods belonging to cluster D (C. rotun-datum and Krithe spp.) (Figs. 8 and 9). Foraminifera assemblage is constituted by typical species living in the distal part of the Rhne prodelta, with ne-grained sediments enriched in both terrestrial and marine organic matter (Goineau et al., 2011, 2015; Kruit, 1955; Mojtahid et al., 2009). They are also reported as opportunistic species which respond quickly to fresh phytodetritus input by higher reproduction (De Rijk et al., 2000; Fontanier et al., 2003; Goineau et al., 2011;Jorissen, 1987). Ostracods content of Cluster D is known as common assemblage of circa-littoral to epi-bathyal environments (Bonaduce et al., 1975; Coles et al., 1994; Cronin et al., 1999; Didi et al., 2002; Yamaguchi and Norris, 2012).In the Rhne subaqueous delta, we hypothesize that these
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ca. 400 and 350, and ca. 70 and 0 cm (Fig. 8). They correspond to ages between ca. 4.0 and 2.2 and between 0.6 and0.2 ka cal. BP, respectively. These intervals are close to CR4 and CR6 (i.e., the LIA) that are dated between ca. 3.1 and 2.8 and ca. 0.65 and 0.15 ka cal. BP, respectively (Wanner et al., 2014). The hypothesis of increased rainfall and river runoff, in the Rhne watershed, during the cooler late Holocene is supported, at least for the LIA, by the observed advance of the Rhne Glacier (Goehring et al., 2011), the high level in Lake Bourget (France) (Arnaud et al., 2012), higher soil erosion in the French Pre-Alps (Simonneau et al., 2013), increased detritism in the Rhne delta plain (Bruneton et al., 2001; Provansal et al., 2003), and increased Rhne River oods (Pichard, 1995). In contrast, CR5, the so-called Migration Period Cooling, is not characterized by any increase in Leptocythere, suggesting drier conditions in the Rhne watershed compared to CR4 and CR6.
The signature of CR0 (the 8.2 ka event), CR1, CR2, and CR3 is difcult to discriminate since the events are incorporated within a condensed section with very low accumulation rate (Fig. 8).
On the other hand, we also notice that two other periods of increased Leptocythere are recorded between ca. 300 and 230 cm and between ca. 180 and 120 cm, i.e., between ca. 1.4 and 1.3 ka cal. BP and ca. 1.2 and 1.0 ka cal. BP (Fig. 8).These periods are not related to global events (CR-like) but might correlate to periods of regional climate deterioration as attested by a high level of Lake Bourget (Arnaud et al., 2012), as well as by periods of increased detritism in the Rhne delta plain (Provansal et al., 2003).
Conversely, a strong decrease in Leptocythere is observed from 120 to 80 cm (Fig. 8). This suggests drier conditions during this interval, which corresponds to the MCA (ca. AD 9501250). In the Northern Hemisphere, the MCA is generally described as a warm period characterized by intense dryness. In the Mediterranean, several studies have highlighted drier conditions during this event (e.g., Wassenburg et al., 2013; Martinez-Ruiz et al., 2015; Bassetti et al., 2016). The same signature is also observed in the Alps and the Rhne watershed, with periods of low lake level and low ood frequency, respectively (e.g., Magny, 2004; Wilhelm et al., 2016). Thus, the hypothesis of increased drought at the studied site during the MCA ts well with regional and local observations.
Within the late Holocene interval, the distribution pattern of Cluster E is slightly offset of the single species Leptocythere (Fig. 8). This cluster is constituted by the shallow infra-littoral Paradoxostoma and Loxoconcha species (Bonaduce et al., 1975; El Hmaidi et al., 2010) and by the epibathyal Argilloecia species. Argilloecia sp., in the Rhne subaqueous delta, appears to be tolerant to uvial inuence and potentially responds to organic matter supply (Fanget et al., 2013b). Nevertheless, an increase in Cluster E is not recorded within the periods characterized by higher river supply such as the CR4 and CR6 (i.e., the LIA) but slightly
after these periods. This possibly indicates that Leptocythere is a better competitor during periods of increased detritism and uvial discharge.
6 Conclusion
Our study shows that some environmental and sea-level changes during the Holocene can be clearly depicted from sedimentological and benthic meiofauna proxies.
During the early Holocene (11.7 to 78 ka cal. BP), sea-level rise led to the deposition of tempestite sediments that contain shallow infra-littoral benthic meiofauna. These deposits are thought to be formed between ca. 20 and 50 m water depth, and we believe that this feature can be used as a good regional scale paleobathymetric marker.
The middle Holocene (78 to 45 ka cal. BP) corresponds to a phase of very low sedimentation at the core site, resulting in the formation of a condensed section (i.e., the MFS in sequence stratigraphic terminology) reecting the further landward position of the shoreline and Rhne outlet(s). This condensed section contains reworked shoreface material within a ne-grained matrix. It displays mixed faunal assemblages, ranging from infra-littoral to epi-bathyal environments, which are the result of erosion processes that occurred during the period of transgressive submersion and, thus, mark the peak of transgression and the subsequent sediment starvation.
Following the transgressive maximum, the late Holocene (45 ka cal. BP to 19th century AD) sediment deposits are inuenced by a combination of allocyclic and autocyclic factors. The progressive shoreline progradation and prodeltaic lobes switching is characterized by changes in clay mineral-ogy content, by the setting-up of benthic meiofauna adapted to thrive in the distal part of the Rhne River inuence (i.e., distal St Ferrol and Ulmet lobes), and by the presence of very shallow-water species (i.e., proximal Grand Passon and Bras de Fer lobes).
Within the late Holocene deposits, ostracod assemblages emphasize uctuations in the Rhne River hydrological activity. In particular, the occurrence of the ostracod genus Leptocythere highlights periods of increased uvial discharge.These periods of intensied runoffs can be attributed to the2.8 ka event (CR4) and the Little Ice Age (CR6) that are known to be at the origin of regional climate deterioration in western Europe, as well as periods of regional climate deterioration at ca. 1.3 and 1.1 ka cal. BP. In contrast, the signatures of the early and middle Holocene cold relapses are difcult to explore in the Rhne subaqueous delta since they correspond respectively to (a) a phase of rapid sea-level rise at the origin of shoreline reworking and deposition of tempestite and (b) a period of very low sedimentation at the core site resulting in a condensed interval with low temporal resolution.
Finally, our study demonstrates that prodeltas may provide interesting expanded archives of climate changes at
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A.-S. Fanget et al.: Sedimentary archives of climate and sea-level changes 2175
the land/sea interface, with accumulation rates reaching0.4 m yr1. On the other hand, such resolution can be achieved at one single site for only short time intervals, since depot centers migrate rapidly as a consequence of sea-level changes, high sediment uxes, and lateral shifting of delta lobes. This highlights the need for acquiring series of long cores/boreholes, parallel and orthogonal to deltaic systems.
7 Data availability
For access to the proxy data, the respective authors should be contacted personally.
Acknowledgements. Core RHS-KS55 was collected during the RHOSOS cruise (2008) on board R/V Le Surot. We thank the captain and crew of this cruise together with the Genavir technical staff and the scientic parties. Special thanks are due to Bernard Dennielou (IFREMER, Brest) for his commitment at sea and during the processing of the data in the laboratory. We thank the Laboratoire de Mesure du Carbone 14, UMS 2572, ARTEMIS in Saclay for 14C measurements by SMA in the frame of the National Service to CEA, CNRS, IRD, IRSN, and Ministre de la Culture et de la Communication. This work was partly supported by the CNRS-INSU Mistrals-Paleomex. We are grateful to Bertil Hebert (CEFREM, University of Perpignan), Gilbert Floch, Anglique Roubi, and Mickal Rovere (IFREMER, Brest) for their technical support. We would like to thank A. Amorosi and L. Goisan for their comments and suggestions that helped to improve the manuscript.S. Luening is also thanked for commenting on the manuscript during the open discussion.
Edited by: N. Combourieu Nebout Reviewed by: A. Amorosi and L. Giosan
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Abstract
A 7.38m long sediment core was collected from the eastern section of the Rhône prodelta (NW Mediterranean) at 67m water depth. A multi-proxy study (including sedimentary facies, benthic foraminifera, ostracods, and clay mineralogy) provides a multi-decadal to century-scale record of climate and sea-level changes during the Holocene. The early Holocene is marked by alternative silt and clay layers interpreted as distal tempestites deposited in a context of rising sea level. This interval contains shallow infra-littoral benthic meiofauna (e.g., Pontocythere elongata, Elphidium spp., Quinqueloculina lata) and formed between ca. 20 and 50m water depth. The middle Holocene (ca. 8.3 to 4.5kacal. BP) is characterized, at the core site, by a period of sediment starvation (accumulation rate of ca. 0.01cmyr<sup>-1</sup>) resulting from the maximum landward shift of the shoreline and the Rhône outlet(s). From a sequence stratigraphic point of view, this condensed section, about 35cm thick, can be identified on seismic profiles as a maximum flooding surface that marks the transition between delta retrogradation and delta progradation. The transition between the early Holocene deposits and the middle Holocene condensed section is marked by a gradual change in all proxy records. Following the stabilization of sea level at a global scale, the late Holocene is marked by the establishment of prodeltaic conditions at the core site, as shown by the lithofacies and by the presence of benthic meiofauna typical of the modern Rhône prodelta (e.g., Valvulineria bradyana, Cassidulina carinata, Bulimina marginata). Several periods of increased fluvial discharge are also emphasized by the presence of species commonly found in brackish and shallow-water environments (e.g., Leptocythere spp.). Some of these periods correspond to the multi-decadal to centennial late Holocene humid periods recognized in Europe (i.e., the 2.8ka event and the Little Ice Age). Two other periods of increased runoffs at ca. 1.3 and 1.1kacal.BP are recognized, which are likely to reflect periods of regional climate deterioration that are observed in the Rhône watershed. Conversely, the Migration Period Cooling (ca. 1.4kacal.BP) and the Medieval Climate Anomaly (ca. AD950-1250) correspond locally to periods of increased dryness.
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