Long-Term Development and Current Status of the Barcelona Continental Shelf: a Source-To-Sink Approach

ARTICLE IN PRESS

Continental Shelf Research 27 (2007) 1779–1800 www.elsevier.com/locate/csr

Long-term development and current status of the Barcelona continental shelf: A source-to-sink approach

C. Liquetea, M. Canalsa,Ã, G. Lastrasa, D. Amblasa, R. Urgelesa, B. De Mola, M. De Batistb, J.E. Hughes-Clarkec

aGRC Geocie`ncies Marines, Dept. d’Estratigraﬁa, Paleontologia i Geocie`ncies Marines, Facultat de Geologia, Universitat de Barcelona, E-08028 Barcelona, Spain bRenard Centre of Marine Geology, Universiteit Gent, Krijgslaan 281 S8, B-9000 Gent, Belgium cOcean Mapping Group, Department of Geodesy and Geomatics Engineering, University of New Brunswick, Fredericton, New Brunswick, Canada E3B 5A3

Received 18 July 2006; received in revised form 16 January 2007; accepted 28 February 2007 Available online 19 March 2007

Abstract

The Barcelona continental shelf, off the city of Barcelona (NE Spain), is a relatively narrow canyon-bounded shelf in the northwestern Mediterranean Sea. Three medium-size rivers (Tordera, Beso´s and Llobregat) and several ephemeral rivulets flow into this margin. Two main domains have been recognized in the Barcelona shelf: (i) a modern, river-influenced area, and (ii) a relict, sediment depleted area, both affected by a variety of human impacts. A detailed geomorphologic study based on multibeam bathymetry and backscatter data, high resolution seismic profiles, and surface sediment samples allowed mapping and interpreting the main distinctive seafloor features on the Barcelona shelf. Modern sedimentary features reveal that the Llobregat River is the main sediment source of the Barcelona prodeltaic shelf. High-discharge fluvial events result in the formation of suspended sediment plumes and sediment waves on the shelf floor. Relict (late Pleistocene–Holocene) sedimentary features reflect that an important shift occurred in the seashore direction between MIS 4 and MIS 2, and that recent neotectonic reactivation has created a set of seafloor faults. The Barcelona inner and middle shelf is severely impacted by anthropogenic activities such as the enlargement works of the Port of Barcelona, sewage pipes, dredging, anchoring and trawling. r 2007 Elsevier Ltd. All rights reserved.

Keywords: Shelf sedimentary environments; Source-to-sink; Prodelta; Multibeam; River basins; Seismic reﬂection; Neotectonics; Sediment waves; Late Quaternary sea level change; Human impacts; Northwestern Mediterranean Sea; Barcelona continental shelf; Spain

1. Introduction the continental margins. Source-to-sink studies analyze the complex interplay of processes within Most of the processes that shape land–ocean entire sediment-dispersal systems at different tem- systems leave their imprint in the sediment record of poral and spatial scales in order to understand the evolution of continental margins and its associated ÃCorresponding author. Tel.: +34 93 402 13 60; morphology (Driscoll and Nittrouer, 1999). The fax: +34 93 402 13 40. ﬁnal aim of a source-to-sink analysis is to develop a E-mail address: [email protected] (M. Canals). quantitative approach to a given margin dispersal

0278-4343/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.csr.2007.02.007 ARTICLE IN PRESS 1780 C. Liquete et al. / Continental Shelf Research 27 (2007) 1779–1800 system, so that its response to natural or human with modern and relict sedimentary features, and perturbations can be predictable. with anthropogenic impacts. The first requirement of a source-to-sink study is to compile a high-quality landscape and seascape 2. General setting data set so that sediment erosion, transport and accumulation can be analyzed as a whole. High The Catalan margin extends along 300 km from resolution swath bathymetry represented a funda- the Ebro Delta to the Cap of Creus peninsula mental breakthrough for source-to-sink studies. The (Fig. 1). It forms the western border of the northern first swath bathymetry map from a continental Valencia Trough, a 400 300 km mid-Miocene margin in the western Mediterranean Sea was the extensional basin in the western Mediterranean eastern Gulf of Lions multibeam map published by Sea. The Catalan margin is underlain by a SW–NE Orsolini et al. (1981–1982). During the following oriented horst-and-graben structure (Maillard et al., years, researchers from France, Italy and Spain 1992; Roca et al., 1999). Following the Miocene added new contributions (e.g. Berneét al., 2004; extension, the Messinian closure of the strait Canals et al., 2004b, Marani et al., 2004)toa between north Africa and the south of Iberia led general swath mapping effort that led to the recent to an event that had a dramatic influence on the multibeam bathymetry compilation of the entire shaping of Mediterranean margins: the salinity western Mediterranean Sea (Medimap Group, crisis and drying up of the basin between 5.96 and 2005). However, continental shelves were mostly 5.33 million years ago (Duggen et al., 2003). During left out of such a mapping effort mainly because of the initial stage of the Messinian event, the large fall two reasons: (i) the time consuming character of in water level caused extreme erosion and incision swath mapping in shallow areas due to much that led to the development of large canyon systems. smaller swath widths and (ii) the broadness of some The physiography and the seascape of the modern of the continental shelves in the region, such as the Catalan margin are, therefore, strongly determined Gulf of Lions (70 km) and the Ebro (50 km) by the Messinian crisis (Amblas et al., 2004). The shelves. Still today only relatively small areas of the development of the modern terrigenous shelf and continental shelves in the western Mediterranean margin started shortly after the Pliocene transgres- have been swath mapped and the results published. sion, being ultimately reshaped by Quaternary In this paper we present the results achieved so far glacioeustatic sea level changes (Bartrina et al., on the Barcelona continental shelf where a compre- 1992). hensive data set made of multibeam data, high The largest submarine canyons in the Catalan resolution seismic reflection profiles, sediment sam- margin, which clearly have Messinian ancestors, ples and geographic and hydrological data is now segment the margin in different sections. The available. First, the sediment sources and the main continental shelf is narrowest (3 km) where deeply distinctive geomorphological features are presented, incised by submarine canyon heads, and reaches its and then the origin of these features and the modern maximum width off the Ebro Delta (50 km). The seafloor zonation is discussed, regarding aspects Barcelona margin and shelf, off the city of such as fluvial sedimentation, neotectonics, sea level Barcelona, occupy the central stretch of the section changes, and human impacts. bounded by the Foix Canyon to the south and the This paper aims at providing an accurate Blanes Canyon to the north, from 41180Nto morphological description of the Barcelona con- 411320N(Fig. 1). The Barcelona continental shelf tinental shelf in the northwestern Mediterranean is 6–20 km wide with the shelf break at 110–120 m Sea following a source-to-sink perspective that links depth. Its average slope is 0.61 although it reaches fluvial inputs and physical oceanography processes up to 211 in the slope break off the Llobregat River to the development of the shelf morphosedimentary mouth and at the edge of some prominent sandy features. Mostly qualitative sedimentary analyses deposits observed to the north of the Beso´s River. have been developed at this stage due to the lack of The Barcelona shelf consists of an inner, middle and homogeneous quantitative data onshore and off- outer shelf separated by the 30–40 and the 80 m shore. The time fork considered in our study is the isobaths, respectively (ITGE, 1989). late Pleistocene–Holocene, when global sea level The Barcelona continental shelf is mainly fed by oscillations had a profound impact on continental sediment inputs from the Llobregat and Beso´s rivers shelf development. Therefore, our analysis deals (Fig. 1), mostly draining Neogene-Quaternary and ARTICLE IN PRESS C. Liquete et al. / Continental Shelf Research 27 (2007) 1779–1800 1781

1°30'E 2°0'E 2°30'E

8°W 4°W 0° 4°E 8°E 2504m Highest peak in each river basin N FRANCE Pyrenees Dams 46° (letters after Table 1) CC N Cities mentioned in ED 42° the text VT SPAIN Barcelona continental N

shelf 38° C ALGERIA D Northern Current Littoral drift 1 Tordera River 2 Sant Cebrià Rivulet Faults 3 Argentona Rivulet Canyon systems 4 Besós River 5 Llobregat River Shelf break E 6 Sant Perede Ribes Rivulet

Montseny

1684m 1304m A B else|csr|1585 764m Coastal642m Ranges 1 alan Cat 2

Blanes F 3 -50 Masnou -200 C. -100 4

646m Barcelona Areny 5 s C. Besós C

-1000 6

Morrá Berenguer

Foix C. s C.

a C. Profiles in Fig.4 4550000 4600000 4650000

41°0'N400000 41°30'N 450000 42°0'N

Fig. 1. Physiographic and morphological setting of the study area. Blue lines and numbers illustrate the river courses, and bold white lines mark the watersheds limits. The yellow area illustrates the Barcelona continental shelf. VT: Valencia Trough. CC: Cap of Creus. ED: Ebro Delta. Faults after Bartrina et al. (1992). Map projection is UTM Zone 31N Datum WGS 1984. Both UTM and geographic coordinates are shown for ease of reference. ARTICLE IN PRESS 1782 C. Liquete et al. / Continental Shelf Research 27 (2007) 1779–1800

Paleogene calcareous rocks, respectively, and the the study area (Calafat, 1986), while the major Tordera River eroding a predominantly granitic storm wave-face is about 30 m depth (Sorribas et al., massif (SGC, 1992). The Llobregat river basin is the 1993). most regulated in the study area (Table 1). Other ephemeral rivulets or ‘‘rieras’’ draining granitic 3. Methodology and data set rocks and a soft Plio-Quaternary cover punctually add to sediment inputs, essentially during violent Swath bathymetry and associated backscatter data eastern storms with heavy rains that mostly occur in were acquired onboard the R/V Hespe´rides and the late summer and early fall, and are locally known as 12 m fishing inspection boat Arraix during the ‘‘llevantades’’ (Martıń Vide, 1982). The energetic MARINADA (2002) and PRODELTA-1 (2004) wave regime associated to ‘‘llevantades’’ reinforces surveys using multibeam echosounders Simrad EM- the southwards littoral drift, which becomes then 1002S and EM-3000D, respectively (Fig. 2). The able to transport large volumes of sand that may be EM-1002S system operates at a frequency of 95 kHz lost from the coastal sediment cell either escaping to with a maximum ping rate of 10 Hz and uses 111 deeper sections on the shelf or getting trapped by beams. The swath width is up to 7.4 times the water submarine canyon heads. depth when working shallower than 200 m depth, The dominant oceanographic current in the and 1500 m wide on deeper areas. The EM-3000D Gulf of Lions and the Catalan margin is the system operates at a frequency of 300 kHz with a Northern Current, also referred to as Liguro- maximum ping rate of 40 Hz and uses 254 beams. Provenc-al Current, which flows south-westwards The swath width is up to 10 times the water depth or over the continental slope reaching speeds of 200 m from 1 to 150 m water depth. Raw data were 50 cm s1 (Flexas et al., 2002) and occasionally logged using Simrad’s system and later processed intruding and meandering over the shelf, especially using the SwathEd software from the Ocean between December and May (Flexas et al., 2002; Mapping Group of the University of New Bruns- Arnau et al., 2004). The predominant current wick. This processing included editing of naviga- acting near the coast is the south-westwards littoral tion, both automatic filtering and manual beam drift. Ports and man made coastal protection editing, refraction correction and backscatter equal- works severely interfere the drift current leading ization. The total swath mapped area in this work to anthropogenically generated sheltered deposi- covers about 625 km2 (Fig. 2). tional areas and erosional coastal stretches In total, 337 km of high-resolution seismic reflec- (DGPT, 2000). The losses of littoral sediments tion profiles were acquired in July 2004 jointly with are evidenced by the increasing sand volumes the Renard Centre of Marine Geology from the required for beach nourishment. For instance, University of Ghent (Belgium) on board the fishing 150 000 m3 yr1 were required for the 4 km long inspection boat Arraix (Fig. 2). The seismic source ‘‘Olympic Zone’’ beaches in Barcelona during 1999 was a SIG 300 J Sparker at a sampling frequency of and 2000, while the estimated littoral transport 6 kHz. About 200–2000 Hz band-pass filtering and for the same period was of some tens of thousands relative position corrections preceded digital regis- of m3 yr1 (DGPT, 2000). The fair-weather tration onboard. Post-processing included several wave-face has been established at 20 m depth in band-pass filters and centered automatic gain

Table 1 Main characteristics of the dams built along the Tordera, Beso´s and Llobregat river basins

River system Letter in Fig. 1 Reservoir Year of construction Maximum capacity (hm3) Dam height (m) Surface (ha)

Tordera A Santa Fe 1933 1 23 6 Beso´s B Vallforners 1989 2 62 16 Llobregat C Sant Ponc- 1957 24 60 139 D La Baells 1976 115 102 367 E Sant Martı´de Tous 1997 1 32 15 F La Llosa del Cavall 1999 80 122 300

Letters allow identifying the location of each dam on Fig. 1. Dams labeled B, D, E and F are also referred in Fig. 4. ARTICLE IN PRESS C. Liquete et al. / Continental Shelf Research 27 (2007) 1779–1800 1783

2°00'E 2°10'E 2°20'E 2°30'E

llosp38

llosp40

Besós River

llosp45

LLB LLA LL2 LLD LL5 Llobregat LL11 River LL6 LL7 LLC -200 Shelf LL4 edge LL8 LL9 LL1 0

Swath mapped area during PRODELTA-1 survey Swath mapped area during MARINADA survey Seismic reflection profiles Seismic lines in Figure 6 (llosp 38, 40 and 45) Shelf edge Sediment sample 4560000 4570000 4580000 4590000 41°10'N 41°20'N 41°30'N 410000 420000 430000 440000 450000 460000

Fig. 2. Marine data set used in this study, including multibeam coverage, location of sparker seismic reflection profiles, and position of surface sediment samples. Map projection is UTM 31N WGS84. control. The Sparker data have an approximate assembling with a dispersant agent and sieving of resolution of 10–15 m horizontally and 0.4–1.0 m the 41 mm fraction. Grain size measurements of vertically. A triple-line artifact, likely linked to the o1 mm fraction were made with a LS100 operational conditions, was occasionally observed Coulter Counter. Compositional visual analyses overlying the strongest reflectors, namely the sea- were carried out with binocular lens. Organic matter floor reflector. This reverberation is up to 5 ms thick content was determined by measuring the weight (two-way travel time, TWTT), thus yielding an loss after burning sediment samples at 560 1Cina uncertainty of up to four vertical meters. muffle furnace. Sediment samples were collected by means of a Geographic and hydrological data from relevant 45 cm long multicoring system in September 2004 river systems have been gathered from the Spanish on board the R/V Garcıá del Cid totaling 13 ‘‘Instituto Geogra´fico Nacional’’ digital topography sampling sites (Fig. 2). Subsamples were subse- series, ‘‘Centro de Estudios y Experimentacioń quently described and analyzed for grain size, de Obras Pu´blicas’’ (CEDEX) and the compila- composition and organic matter content. Only the tion on Spanish Mediterranean watersheds by topmost centimeter of each sampling site has been Canals et al. (2004a). This data set was inclusive taken into account for this study. Preparation for of watershed divides, morphometry, location of grain size analysis included two hydrogen peroxide gauging stations and liquid discharge time series. attacks for organic matter elimination, grain dis- Fig. 3 shows the longitudinal profiles of the ARTICLE IN PRESS 1784 C. Liquete et al. / Continental Shelf Research 27 (2007) 1779–1800

Offshore Coastline 2000 Tordera 5.0º 1000 Blanes Canyon 0 head -1000 25 km 2000 Besós

Elevation (m) 1000 4.6º

-1000 25 km 2000 5.7º Llobregat 1000

-1000 25 km

Fig. 3. Topographic proﬁles along the three main river systems of the study area, from headwaters to the continental margin segments they open to. The slope of the uppermost river courses is indicated. See location on Fig. 1.

Table 2 Main morphologic and hydrological characteristics of the ﬂuvial systems shown on Fig. 1

No. in River system Basin River River Delta Distance Mean Mean Mean Annual Fig. 1 area length slope area RM–SB water runoff sediment solid (km2) (km) (1) (km2) (km) discharge (l km2 s1) ﬂux (g s1) discharge (m3 s1) (103 T)

1 Tordera 878.8 59.3 1.0 4.2 5.0 7.7 8.8 66.1 2 2 Sant Cebria` 34.5 11.9 2.1 — 15.8 — — — — 3 Argentona 74.3 20.1 1.4 — 18.9 — — — — 4 Beso´s 1028.9 51.6 0.6 8.3 13.9 6.8 6.6 474.7 15 5 Llobregat 5045.1 162.7 0.5 80.0 6.6 16.7 3.3 2153.9 68 6 Sant Pere de Ribes 233.3 22.2 1.0 — 11.2 — — — —

Mean water discharge and runoff values are calculated after 33 years long water discharge time series for the Llobregat and Beso´s rivers and 9 years long for the Tordera River. The small rivulets are non-monitored. Mean sediment ﬂux and solid discharge values are derived from punctual suspended sediment measurements along 7 years. Gauging stations are located less than 15 km upstream from the river mouth. RM–SB: from river mouth to shelf break.

Llobregat (163 km long), Beso´s (52 km) and Tor- estimated by combination of total suspended sedi- dera (59 km) rivers, including the respective adja- ment measurements (in mg l1) and the correspond- cent continental margin. The selected hydrological ing daily water discharge values (in m3 s1)(Table 2). data series are 33 years long for the Llobregat and Water and solid discharge data were taken from the Beso´s rivers and 9 years long for the Tordera River, nearest gauging station to each river mouth that is including temporary gaps of variable duration. The less than 15 km far from the seashore. Information small rivulets in Fig. 1 are non-monitored. Total related to damming history (Table 1) was extracted suspended sediment values of the Tordera, Beso´s from the ‘‘Age` ncia Catalana de l’Aigua’’ database and Llobregat river systems from the period (ACA, 2000) and the Spanish ‘‘Ministerio de Medio 1995–2002 have been gathered from ‘‘Age` ncia Ambiente’’ data collection (MOPU, 1988; MOPT- Catalana de l’Aigua’’. Sediment ﬂux (in g s1) was MA, 1994). ARTICLE IN PRESS C. Liquete et al. / Continental Shelf Research 27 (2007) 1779–1800 1785

4. Results The largest average sediment flux to the sea is supplied by the Llobregat River, and the least by the 4.1. Sediment sources Tordera system. Cumulative sediment deposition at the river mouth as reflected by delta areas is far The hydrological data show that the largest more significant in the Llobregat system (Table 2). fluvial input both in terms of liquid and solid The relatively high slope of the Tordera River and discharge is the one from the Llobregat River, nearby ‘‘rieras’’ (rivulets), especially in the catch- although the top runoff value corresponds to the ment areas (Fig. 3 and Table 2), and their strong Tordera River (Fig. 4 and Table 2). The Tordera seasonal regime favor large short-lived discharge and Beso´s rivers show similar mean water discharge events that are barely reflected by statistical values, but this is largely because of the July 1999 averages or simply remain unrecorded. This may extraordinary flood event in the Tordera Basin. lead to underestimate the significance of these Under normal conditions, the Beso´s system has fluvial systems as sediment vectors to the Barcelona considerably larger discharge values than the continental shelf. Tordera. Water discharge regimes of the Tordera, Damming in the Llobregat, Beso´s and Tordera Beso´s and Llobregat river systems are relatively river basins is essentially limited to upper courses irregular, particularly in the Tordera River, though (Fig. 1), with the main dams (La Baells, La Llosa the overall pattern is similar (Fig. 4A). Maximum del Cavall and Sant Ponc-) located in the Llobregat water discharge usually occurs between November watershed. However, the influence of these dams on and January for the Beso´s and Tordera rivers, and the Llobregat River final water discharge is not in spring for the Llobregat River following snow evident (Fig. 4A). Water extraction for agriculture, melting in the Pyrenees catchment area (Fig. 4B). urban and industrial use is known to be especially

120 D B E F Tordera Besós 3-1

) 100 467 ms

-1 Llobregat s 3 80

40 Discharge (m 20

0 10-67 10-68 10-69 10-70 10-71 10-72 10-73 10-74 10-75 10-76 10-77 10-78 10-79 10-80 10-81 10-82 10-83 10-84 10-85 10-86 10-87 10-88 10-89 10-90 10-91 10-92 10-93 10-94 10-95 10-96 10-97 10-98 10-99 10-00 10-01 Month - Year

) Tordera

-1 20

s Besós 3 15 Llobregat

5 Discharge (m 0 OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP Month

Fig. 4. (A) Monthly water discharge time series from the Tordera (dark gray line), Beso´s (light gray line), and Llobregat (black line) rivers. The Tordera maximum discharge peak (out of scale) occurred in July 1999. Letters point to dam constructions (see location on Fig. 1 and information on Table 1). (B) Mean monthly discharge histograms for the Tordera, Beso´s and Llobregat rivers. ARTICLE IN PRESS 1786 C. Liquete et al. / Continental Shelf Research 27 (2007) 1779–1800 high along the Llobregat low course, where several relevant distinctive seafloor features. This identifica- pumping stations detract more than 60 hm3 of fresh tion is based on the high resolution bathymetric and water per year (ACA, 2005). In addition, the backscatter data, the seismic reflection profiles, and Tordera, Beso´s and Llobregat rivers undergo the sedimentological analyses. The location of some anthropogenic water extractions of less than of the following seafloor features is shown in Fig. 5. 5hm3 yr1 in numerous sites along their upper courses (ACA, 2005). It is to be noted that, related 4.2.1. Abrupt slopes to the enlargement of the Port of Barcelona, the 1–5 m high abrupt slopes are observed in the lowermost course of the Llobregat River was northern part of the Barcelona shelf. They appear as diverted in September 2004 shifting the river mouth straight steps on the seafloor bathymetry and as 2.5 km southwards. vertical slopes on the seismic data set (Fig. 6A and B). Their orientation is rather constant, ranging 4.2. Distinctive seafloor features from N050-N075 with descending angles offshore and southwards, except by one feature nearly N090 The detailed examination of the Barcelona con- oriented. Apart from the 5 m high feature located tinental shelf led to the identification of its most off the Beso´s River at around 55 m depth, the rest of

2°00'E 2°10'E 2°20'E 2°30'E

Masnou B 4 2 A 1 1 D 2 1 1 2 2 3

3 1 Barcelona Prodelta limit 3 5 3

2 3 3

C 6

Examples of distinct sea floor features: 1 - Abrupt slopes 2 - Narrow ridges 3 - Prograding sediment bodies 5 4 - Large submarine step 5 - Low back scatter areas (see Fig.7) 2 6 - Wavy prodelta front Shelf break 4560000 4570000 4580000 4590000 41°10'N 41°20'N 41°30'N 410000 420000 430000 440000 450000 460000

Fig. 5. Swath bathymetry shaded relief map of the Barcelona continental shelf at 10 m resolution. The onland orthophotomap shows the high density population of the area. White lines correspond to river courses. Red boxes and capital letters point to the location of images on Fig. 10. Numbers mark some examples of the distinctive seaﬂoor features discussed in the text. ARTICLE IN PRESS C. Liquete et al. / Continental Shelf Research 27 (2007) 1779–1800 1787

1 km 2 km 0

Large submarine step Line llosp38 NNW-SSE 20 Multiple Abrupt slopes

40 TWTT (ms)

1 km 2 km 50 NNW-SSE Sharp ridge Line llosp40

70 Abrupt slopes

90 TWTT (ms)

110

130

1 km 2 km 70 NNW-SSE Line llosp45 Prograding sediment body 90

110 TWTT (ms)

130

10 m 150

Fig. 6. Sparker seismic reflection profiles illustrating the internal structure of several seafloor features discussed in the main text. Vertical axes show the Two-Way Travel Time in milliseconds, corresponding approximately to the metric bar of the bottom right corner of line llosp45. Vertical exaggeration is 17. See location on Fig. 2. the slopes are 1–3 m high. The length of these these features, while samples LL-A and LL-D features ranges from 150 to 8.5 km. were taken in close proximity (less than 100 m from one of the ridges) (Fig. 2). The multicore system 4.2.2. Narrow ridges could not penetrate more than 5 cm in any of the Discontinuous narrow ridges appear to the north three sites. The grain size and composition analyses of the study area. They are sharp concave-up of the three samples showed relatively coarse structures from tens of centimeters up to 2 m material with a high percentage of bioclasts high observed in the bathymetric and seismic data 41 mm diameter (13–41%) (Table 3). The mean (Fig. 6B). The ridges are W–E to NNE–SSW grain size of sample LL-B was 0.34 mm, while those oriented and their length ranges from 150 m to of LL-A and LL-D were 0.46 and 0.64 mm, 10 km. Sample LL-B was collected from one of respectively. ARTICLE IN PRESS 1788 C. Liquete et al. / Continental Shelf Research 27 (2007) 1779–1800

Table 3 scatter values (Fig. 7). The internal structure of the Water depth and grain size composition of the surface sediment sediment prism forming the main step consists of samples located on Fig. 2 7–121 tilted subparallel strata that get steeper Sample Depth (m) Clay % Silt % Sand % 41mm% seawards. This sediment body is bounded by an upper erosional truncation and a set of downlaping LL-A 72 9.4 20.6 43.3 26.7 basal reﬂectors (Fig. 6A). LL-B 71 13.4 30.7 42.5 13.4 LL-C 132 6.7 23.4 69.1 0.9 LL-D 69 2.5 6.2 50.6 40.7 4.2.4. Featureless seaﬂoor with low backscatter LL-2 69 12.4 53.7 29.4 4.5 Two roughly elliptical patches showing the lowest LL-4 149 7.4 23.3 68.9 0.4 backscatter values are observed S–SW of the Beso´s LL-5 96 18.0 62.9 14.9 4.2 and Llobregat river mouths (Figs. 5 and 7). Their LL-6 57 12.6 52.0 34.9 0.7 main axes are parallel to the coast. The northern LL-7 30 13.7 51.3 35.0 0.0 LL-8 66 22.7 73.6 3.7 0.0 patch is located on the inner-middle shelf, between LL-9 85 35.9 63.5 0.0 0.5 25 and 50 m depth, extending from the Beso´s River LL-10 71 18.7 71.7 4.8 4.9 mouth to the Port of Barcelona. It is 6.5 km long LL-11 92 6.1 19.6 68.2 6.1 and 3 km wide, and it is detached from the coastline

Measurements of the o1 mm fraction were made with a LS100 by 1 km. The larger southern patch is placed mostly Coulter Counter. Grain size is expressed as weight percentage. on the middle-outer shelf, between 25 and 85 m depth. It is 13 km long and 4 km wide, and the average distance from its shallowest boundary to the shoreline is 2 km. 4.2.3. Prograding sediment bodies Low backscatter values are typically linked to fine The Barcelona shelf comprises several outcrop- grain sizes. Surface samples LL-9 and LL-10 were ping and subcropping prograding sediment bodies collected from the southern patch (Fig. 7). Both located around 45, 50, 65, 75 and 105 m depth. They samples are made up of relatively fine material, are E–W to ENE–SWS oriented and have a clear clayey silts and silts with very fine sands (Table 3) seafloor expression. They show relatively high showing a mean grain size of 0.013 and 0.069 mm, backscatter values, most probably indicative of respectively. Their composition, inclusive of abun- relatively coarse material. These sediment bodies dant mica, very fine-grained quartz and some plant can reach 5 km in length and 4 m in height, and their debris, reveals a strong fluvial influence. seafloor slope ranges between 0.61 and 4.01. Internally, they show high-angle cross stratification 4.2.5. Wavy prodelta front with well-defined body base and top (Fig. 6C). The Llobregat prodelta front off the river mouth Another expression of these sedimentary features comprises an elongated area of around 25 km2 is a large, ENE–WSW oriented, thick sediment (2.2 12 km) located between 45 and 95 m depth body limited towards deeper water by a steep slope, characterized by sediment undulations parallel to here called submarine step. It appears on the the bathymetric contours (Fig. 5). The slope in this northern inner shelf near Masnou and extends wavy seafloor ranges from 0.31 to 31, and back- along 11 km deepening northwards. Its height is scatter values are relatively high. The prodelta up to 10 m and its slope up to 211.At41129.50N, sediment waves have a wavelength of 60–100 m around 20 m depth, this large single step splits into and are 30–80 cm high (Urgeles et al., 2006). Surface three smaller steps (Fig. 5). The deepest of these samples LL-6 and LL-8 were retrieved from the smaller steps is known to continue eastwards Llobregat prodelta front (Fig. 7). They contain beyond the study area forming a 20 km long, 30 m poorly sorted sandy mud (Table 3), mainly of high, W–E oriented submarine step. It extends from continental origin, with average grain sizes of 0.076 the northern end of the study area to the Blanes and 0.019 mm, respectively. Their organic matter Canyon, crossing the middle and outer shelf down content is relatively high, exceeding 10% in weight. to 110 m depth (ITGE, 1989; Dıáz and Maldonado, 1990; Liquete et al., 2004b). Sandy seafloor has been 4.2.6. The Port of Barcelona area reported from this large ENE–WSW sediment body The ongoing works for the enlargement of the (ITGE, 1989; Llamas, 2003), which is in agreement Port of Barcelona translate into dredging, loading with the observed high and homogeneous back- and infilling impacts that affect more than 5 km of ARTICLE IN PRESS C. Liquete et al. / Continental Shelf Research 27 (2007) 1779–1800 1789

2°00'E 2°10'E 2°20'E 2°30'E 4590000

Besós River

D B A 2 11 Llobregat 5 River 6 C 7 4 -200 Shelf 8 edge 9 10

-500 4560000 4570000 4580000 41°10'N 41°20'N 41°30'N 410000 420000 430000 440000 450000 460000

Fig. 7. Backscatter map from the PRODELTA-1 survey at 5 m resolution. High backscatter values (lighter tones) represent high seafloor reflectivity, usually corresponding to coarse sediments. Low backscatter values (darker tones) generally correspond to fine soft sediments, and are concentrated south-westwards from the Beso´s and Llobregat river mouths as discussed in the text. Black dots point to the sediment sampling stations (see also Table 3). Map projection is UTM 31N WGS84.

shoreline and 2 km offshore, as deep as 25 m depth (Fig. 8), with the strongest impacts on its north- (Figs. 5 and 8). The two main trenches to be filled eastern part between 40 and 70 m depth. with concrete blocks are 200 m wide and 2.1 and 3.8 km long, respectively. The new port breakwaters 4.2.7. Dredging marks embrace a future infill area of around 7.4 km2. The bathymetric data show several dredge marks Independently of the current official anchoring and trenches resulting from dredging operations. areas off the Port of Barcelona, two zones The largest of these dredging trenches found in the characterized by a rough surface and high and study area is about 400 100 m and is located irregular backscatter values have been interpreted as around 41115.50 2130E. Dredging for beach nour- the ones most affected by anchoring (Fig. 8). Single ishment has been common during the last decades marks found on these areas reach up to 70 cm deep. from Masnou northwards usually between 10 and The first of these zones spreads from the old jetty 40 m depth, and between 2180E and 2130E from 10 to down to 30 m depth, between 411200Nand411220N. 25 m depth (Fig. 8). These activities are assumed to The second one corresponds to the rather large have a significant impact over the benthic commu- official anchoring area of the Barcelona harbor nities and over the water column turbidity. ARTICLE IN PRESS 1790 C. Liquete et al. / Continental Shelf Research 27 (2007) 1779–1800

2°00'E 2°10'E 2°20'E 2°30'E

Besós River Sewage pipes Summer minimum trawling depth (-65m)

Shelf edge

Llobregat River Natural Llobregat low course -200

Sea grass meadows protected areas Winter minimum Trawl marks concentrations trawling depth (-50m) Areas dredged for beach nourishment Artificial reefs Port of Barcelona anchoring area Observed anchoring impact 4560000 4570000 4580000 4590000 Port of Barcelona enlargement area 41°10'N 41°20'N 41°30'N 410000 420000 430000 440000 450000 460000

Fig. 8. Main human impacts on the Barcelona continental shelf. The Llobregat River lowermost course was diverted to the south in September 2004. Map projection is UTM 31N WGS84.

4.2.8. Sewage pipes (DARP, 1997a). Trawling is banned shallower than Two 3-km-long submarine sewer pipes placed 50 m depth during winter and shallower than 65 m near the Llobregat and Beso´s river mouths (Fig. 8) during summer (Fig. 8). Most trawling marks on exit from the ‘‘Baix Llobregat’’ and ‘‘El Beso´s’’ the Barcelona shelf consist of straight furrows less water treatment plants that treat around 0.4 and than 1 m deep and up to a few kilometers long. 0.5 hm3 per day, respectively, for a total population These marks concentrate in three areas: 411230N of nearly 5 million (EMMA, 2003). Nearshore the 21160E between 50 and 70 m depth, 411160N217.50E pipes are placed within up to 2 m deep narrow between 50 and 60 m, and 41113.50N214.50E trenches, while seawards the tubes emerge progres- between 65 and 85 m. Trawling activities have sively till lying on the seaﬂoor where they are strong effects on benthic communities, sediment covered by a sediment pile of a few tens of fabric and resuspension, and seaﬂoor microtopo- centimeters. Leakages from the Beso´s pipe resulted graphy (e.g. Palanques et al., 2001; Liquete et al., in several mound-shaped features along its path. 2004a).

4.2.9. Trawl marks 4.2.10. Artificial reefs Fishing activities on the Barcelona margin are Typical artificial reefs are concrete and steel decreasing but are still quite intense. Trawlers blocks placed on the seafloor to offer shelter and represent around 17% of the Barcelona fishing fleet hard substrata to plant and animal species in and ARTICLE IN PRESS C. Liquete et al. / Continental Shelf Research 27 (2007) 1779–1800 1791 around the reef. Their form and size depend on 5.1. Shelf floor zonation whether they have production or protection pur- poses. The production reef blocks weight around From a morphosedimentary viewpoint, the multi- 4 T and are 3 m in height, while the protection beam bathymetry and backscatter maps of the blocks can be nearly double weighted and half sized Barcelona shelf allow identifying two main do- (DARP, 1997b). The production reefs are intended mains: (i) a relict, sediment depleted shelf floor, and to ease the development of new or existing commu- (ii) a modern, river-influenced shelf floor (Fig. 9). A nities. The protection reefs form barriers to defend variety of anthropogenic impacts are overimposed weak benthic communities from trawling gears. with different intensities on these two domains. Off Barcelona, a reef complex of nearly 100 The relict shelf, markedly depleted of modern production and protection artificial structures was sediment accumulation, covers the southern end of installed around 411230N2112.50E at 15–30 m depth, the study area and most of the northern sector covering about 0.5 km2. The production blocks are (Fig. 9). It comprises the widest part of the grouped in compact clusters while the protection Barcelona shelf, reaching up to 20 km in width. Its blocks are distributed following a sinuous line. A relict character is evidenced by a relatively rough second artificial reef area locates north of the Beso´s topography and a number of paleorelieves and River. It was created in 1992 and it is known as sandy deposits that are described in greater detail in ‘‘El Masnou’’. Some of the individual elements of the following section. The relict shelf is also distin- that reef have been identified in our dataset. guishable by high to medium backscatter intensities corresponding to coarse or cemented materials barely covered by fine grained modern sediments. 4.2.11. Troughs and highs alignments The river-influenced shelf is directly affected by On the inner shelf north of the Beso´s River, two modern fluvial inputs. The main sediment body is troughs and highs alignments have been observed. the Llobregat and Beso´s joint prodelta that appears The highs have elongated irregular forms that on the seismic data as a seaward thinning mud translate into abrupt bathymetric and backscatter wedge up to 54 m thick and 35 km long (Liquete et changes. The highs are up to 1 m high and a few tens al., 2006). Off the Llobregat River mouth the to 200 m long. The deeper alignment starts off the prodelta practically reaches the shelf edge at about Beso´s River mouth and follows the 20 m contour 6.5 km, while off the Beso´s River mouth it reaches along 18 km. It continues northwards along the edge the 50 m isobath at about 2.5 km from the shoreline. of the large submarine step described within the The prodeltaic wedge extends to the southwest due section 4.2.3. The shallower alignment initiates at to the littoral drift and dominant currents in the 411280N on the top of the same large step. It roughly follows the 12 m contour northwards along 7 km. 2°00'E 2°20'E The highs and the area in between the two alignments show relatively high backscatter, which could be indicative of coarse sediments but also of other textural characteristics such as the presence of sedimentary structures or bioturbation.

5. Discussion 41°20’E This section attempts to: (1) identify the different Shelf edge Barcelona shelf sedimentary environments; (2) Main shelf domains: interpret the main sedimentary features and their River-influenced domai n origin linked to fluvial or submarine processes; Relict domain (3) analyze the recent evolution of the shelf from Human impacted domain observable features related to sea level changes; and Fig. 9. Sketch showing the general zonation of the Barcelona (4) emphasize the importance of the current human shelf. It comprises a modern, river-influenced shelf domain; a impacts on the continental shelf off Barcelona and relict, sediment depleted shelf domain; and a severely anthro- their future increasing effects or mitigation. pogenically impacted area. Map projection is UTM 31N WGS84. ARTICLE IN PRESS 1792 C. Liquete et al. / Continental Shelf Research 27 (2007) 1779–1800 study area (see the section 2). North of the Beso´s 11B) has been ascribed to the modern fluvial River mouth, modern sedimentation is confined to a dominated shelf as, according to the available narrow alongshore band (Fig. 9) characterized by information, it is actively involved in the present- modern littoral sandy prisms. day littoral processes. It occurs at water depths The anthropogenic overprint is particularly ap- shallower than the wave-face depth of major storms parent along a 13 km long littoral fringe that (see the General Setting section). Similar submerged extends between the Beso´s and Llobregat river sandy bodies interpreted as coastal paleo-spits have mouths down to 50 m depth (Fig. 9). Dredging and been described at 5–10 m depth off the Tordera infilling for the extension of the Port of Barcelona River mouth (Serra et al., 2003). However, the (still ongoing), sewage pipes placement and releases, origin of the submarine step that appears in the and numerous coastline structures are the main northernmost part of the study area is likely related anthropogenic impacts in the area. to the larger and deeper sediment prism that extends 20 km north-eastwards beyond the Barcelona shelf 5.2. Origin of the sediment features as far as the Blanes Canyon (Calafat, 1986; ITGE, 1989; Dıáz and Maldonado, 1990; Liquete et al., 5.2.1. On the river-influenced shelf 2004b). The dimensions, morphology and sandy The modern river-influenced continental shelf is composition of this submarine step suggest that it characterized by a smooth seafloor and relatively was formed during an ancient lower-than-today sea low backscatter values. This domain reaches its level stage. main development in the adjacent Llobregat and Beso´s prodeltas, which represent the principal 5.2.1.2. Featureless seafloor with low backscatter. Holocene sediment accumulation on the Barcelona Recent sediment accumulation over the modern continental shelf. Water and solid discharge values river-influenced shelf results in a relatively smooth evidence that the Llobregat River is the main seabed, except for the Llobregat wavy prodelta modern sediment supplier to the Barcelona shelf, front and the human impacted areas. The two low despite the slightly decreasing trend of its mean backscatter patches identified in this study are water and sediment discharge. In the Llobregat indicative of a fine sediment cover that overlies delta/prodelta environment, accumulation rates part of the Llobregat and Beso´sprodeltas.Both reached 12–25 mm yr1 between 4.5 and 0.4 ka BP patches appear close to the Llobregat and Beso´s (Ga` mez et al., 2005) and 0.7–1.5 mm yr1 during the river mouths, respectively, and extend south- past century (Sańchez-Cabeza et al., 1999). The westwards. The location and composition of the prodeltas cover an area of about 190 km2 and their low backscatter patches point to a fluvial origin. location southwards of the parent river mouths The larger size and distance to the coast of the is determined by the south-westward dominant southern patch is attributed to the greater input of coastal current. the Llobregat River related to the Beso´sRiver, The Tordera river, with a catchment area and a which is 2.5 times greater in water discharge and mean water discharge similar to the Beso´s ones, 4.5 times greater in solid discharge. It is suggested does not show a recognizable typical prodelta. This that the low backscatter patches represent the is attributable to the lesser suspended sediment accumulation of the finest fluvial material that concentration of the Tordera, linked to the pre- settled from short-lived sediment plumes follow- dominantly granitic character of its basin, and to its ing strong discharge events (Arnau et al., 2004) minor base water regime (i.e. when excluding and spread south-westwards under the direct maximum discharge events). The Tordera system influence of coastal currents. shows a marked torrential character typical of the small rivulets in the study area. It is also likely that 5.2.1.3. Wavy prodelta front. Sediment-wave de- suspended sediment inputs by the Tordera River posits are relatively common on the seabed but evolve into hyperpycnal flows (Mulder and Syvitski, mostly in deep water environments where they have 1995; Mulder et al., 2003) that are trapped by the been linked to turbidity currents or destabilization nearby Blanes submarine canyon head. processes (Alonso et al., 1995; Lee et al., 2002). The sediment undulations that cover the Llobregat 5.2.1.1. Large sediment prisms. The large submar- prodelta front (Fig. 10C) are relatively small sized ine step to the north of Masnou (Figs. 5, 10B and and consist of fluvial material. Shallow-water ARTICLE IN PRESS C. Liquete et al. / Continental Shelf Research 27 (2007) 1779–1800 1793

Fig. 10. 5 m (A) and 1 m (B, C, D) resolution zooms of bathymetric data showing specific features mentioned in the text. (A) Crossing between older N090 beachrocks and younger N030 beachrocks, both used as paleo-coastal indicators. (B) Troughs and highs alignments that could be interpreted either as biogenic constructions (sea grass meadows) or as dredging marks. (C) Trench and sewer pipe exiting from the ‘‘Baix Llobregat’’ water treatment plant. At depths 440 m the pipe lies directly on the sea bed. Large scale seafloor trenching for the enlargement of the Port of Barcelona can be observed in the upper part of the image. The wavy character of the Llobregat prodelta front is shown on the lower-right corner of the image. (D) Seafloor expression of tectonic and morphosedimentary features of the relict Barcelona shelf. BR: Beachrocks. F: Faults. SB: Prograding sediment bodies. See location on Fig. 5. Map projection is UTM 31N WGS84. undulations, often found in prodelta settings, have by the Llobregat River (Urgeles et al., 2006). been associated to sediment deformation, or fluid However, a genetic relationship with deformation expulsion or bottom currents (Trincardi and Nor- processes of the gas-charged, soft prodeltaic sedi- mark, 1988; Bornhold and Prior, 1990; Chiocci et ments cannot be totally ruled out. al., 1996; Lee et al., 2002; Cattaneo et al., 2004). Offshore decreasing wavelengths, internal parallel 5.2.2. On the relict shelf reflectors and the lack of headwall scarps, suggest Most of the study area has been classified as a that the Llobregat prodelta front sediment waves modern sediment-starved shelf where relict mor- are the result of bottom currents possibly associated phological features crop out. The relict shelf owes to hyperpycnal flows during peak discharge events its character to the limited fluvial inputs and to the ARTICLE IN PRESS 1794 C. Liquete et al. / Continental Shelf Research 27 (2007) 1779–1800

2°20'E 2°30'E 2°20'E 2°30'E

Large submarine step

Besos River

Shelf 4580000 4590000 edge 4580000 4590000 41°20'N 41°30'N 41°20'N 41°30'N Outer edge of sediment bodies Outcropping faults N090 paleo-coastline indicators Beachrocks N030 paleo-coastline indicators -200

440000 450000 460000 440000 450000 460000

Fig. 11. (A) Location of distinctive seafloor features identified on the northern, relict Barcelona shelf. (B) Distribution of the two sets of beachrocks and prograding sediment bodies used as paleo-coastline indicators. arrows represent the hypothesized direction of the shoreline landwards migration during the formation of N090 features (arrow ‘‘a’’, post-MIS 4) and N030 features (arrow ‘‘b’’, post-MIS 2). The inner shelf large submarine step to the north has been interpreted as active and, therefore, is not attributed to any of the paleo- coastal sets. Map projection is UTM 31N WGS84. focusing of sedimentation at the inner shelf and Maillard and Mauffret (1999) and Roca et al. prodeltas. The southwards dominated littoral drift (1999), among others, confirmed the tectonic and Northern Current (see the General Setting reactivation of the Catalan margin during the section) and the physiography of the margin favor Quaternary. However, the tectonic regime respon- such a situation. An unquantified part of the silt and sible for this reactivation is disputed. Some authors clay drifting on the continental shelf is suggested to (e.g. Dıáz del Rıó et al., 1986; Maillard and be trapped by canyon heads (Granata et al., 1999; Mauffret, 1999) propose an ongoing distensive Sańchez-Cabeza et al., 1999; Puig et al., 2000). The regime, while others (e.g. Goula et al., 1999) link interpretation of the morphosedimentary features the Catalan margin reactivation to compressional on the relict shelf domain brings to light new stresses. These contrasting views leave open for the evidences about the sedimentary evolution of the submarine faults identified in this work to be either Barcelona shelf during the late Pleistocene period. normal or strike-slip structures. The Barcelona region holds a moderate seismic 5.2.2.1. Abrupt slopes. The straight pronounced activity. Masana (1996) found morphological evi- slopes of the relict shelf have been correlated in dences of younger than 100 ka BP earthquakes with the available crossing seismic profiles with gentle a magnitude 46.3 in the southern Catalan Coastal discontinuities of the seabed and sub-surface Ranges. Also in the Catalan Coastal Ranges, Perea reflectors (Fig. 6A and B), taking into account et al. (2003) recognized two paleoseismic events that these gentle discontinuities can represent between 125 and 34 ka BP and a third one younger several meters high breaks (see the section 3). than 13.5 ka BP, and suggested that earthquakes of The seismic representation of the abrupt slopes magnitude 7 or above could have occurred in the and their characteristic bathymetric expression lead region. Two pre-instrumental epicenters dated on us to interpret them as a possible set of submarine 1427 and 1784 have been identified within a 150 km faults with a rather constant NE–SW orientation distance from Barcelona (NEIC, 2004). Four earth- (Figs. 10D and 11A). quakes ranging between 4.5 and 5.5 magnitude, and ARTICLE IN PRESS C. Liquete et al. / Continental Shelf Research 27 (2007) 1779–1800 1795 tens of them between 3.5 and 4.5 magnitude, have setting and morphology of the prograding sediment been instrumentally detected in the same area since bodies on the Barcelona shelf fit with those of relict 1980 (NEIC, 2004). These seismic events may coastal ridges resulting from transgressive or support the possibility to find active faults on the regressive histories. The prograding sediment bodies shelf. have been interpreted as relict wave- or swash-built Therefore, the abrupt slopes identified in the beach ridges (Figs. 10D and 11A). Therefore, such study area can be attributed to the seafloor sediment bodies of likely sandy nature mark the expression of a local fault set linked to the position of ancient shorelines that are significantly reactivation of the deep Neogene extensional oblique to the present-day shoreline, but semi- structures (horst-and-graben) associated to tectonic parallel to the depth contours of the shelf. deformation and/or to local seismicity. 5.3. Morphosedimentary evidences of sea level 5.2.2.2. Narrow ridges. The sharp ridges on the change relict domain are ascribed to cemented beachrocks (Figs. 10A and D, and 11A), most likely of marine We consider that the beachrocks and prograding vadose origin. Sediment samples LL-B, LL-A and sand bodies on the relict Barcelona shelf formed LL-D support this interpretation as they consist of during relatively short ancient sea level stillstands. bioclastic to siliciclastic-bioclastic, rounded to well- Sea level changes during the late Quaternary are rounded, sands and gravels that are typical of well-documented around the western Mediterra- beachrock formations (Dalongeville and Sanlaville, nean Sea (e.g. Alessio et al., 1996; Lambeck and 1984; Bernier and Dalongeville, 1996; Neumeier, Bard, 2000; Lambeck et al., 2002; Antonioli et al., 1998). Similar structures, also interpreted as bea- 2004). In contrast, there are much less published chrocks, have been found on the inner shelf between seafloor evidences on late Pleistocene stillstands. Argentona and Sant Cebria` rivulets (Fig. 1)(Serra, Collina-Girard (2002) identified up to seven relict 2002). The form of the identified beachrocks (sharp, shorelines between 11 and 100 m depth in the NW irregular, rather symmetrical ridges) suggests that Mediterranean Basin, along the coasts of southern the not-cemented surrounding sediments must have France, Corsica and the Italian Isle of Elba. been eroded while the considerable size of the According to their orientation, the paleo-coastal beachrocks likely favored their preservation. indicators of the relict Barcelona shelf belong to two Beachrocks are useful tools to identify ancient main sets: (i) a N090 oriented set inclusive of coastlines. Their formation implies carbonate diag- beachrocks and prograding sediment bodies and (ii) enesis aided by biotic and abiotic factors in a stable a N030 oriented set made of well-defined beach- sedimentary environment. Amongst promoting fac- rocks (Figs. 10A and 11B). Landwards of 55 m tors the following have been cited: evaporation; depth, both sets tend to deflect towards a N060 rising temperatures; CO2-degassing of interstitial orientation subparallel to the present-day coastline. water pushed into the sediment by waves and tides; (i) The main N090 paleo-coastal indicators lowering of PCO2 by algal photosynthesis; microbial appear at about 104 m (prograding body), 70–77 m activity; and decreasing pH (Tucker and Wright, (beachrock and prograding body), and 48–73 m 1990; Neumeier, 1998). The warm and confined (large branched beachrock) below present sea level Mediterranean Sea is a favorable place for beach (mbpsl). These N090 morphosedimentary elements cementation (e.g. Magaritz et al., 1979; Holail and cut the dominant present-day bathymetric trend, Rashed, 1992; Bernier and Dalongeville, 1996). The which suggests that the landward migration(s) of usefulness of beachrocks to reconstruct the evolu- the coastline during their formation followed a S-N tion of the shoreline in the study area is discussed direction, i.e. shifted 451 related to the present-day farther below. shoreline (Fig. 11B). (ii) The N030 paleo-coastal indicators are pre- 5.2.2.3. Prograding sediment bodies. Inclined served at 69–76, 54–55, 63–69, and 48–53 mbpsl. (o101) cross-strata and parallel/sub-parallel lami- They likely formed as a result of a SE-NW nae are usually indicative of intertidal or shallow advancing shoreline pushed by one or more events subtidal depositional facies, while higher angle of sea level rise (Fig. 11B). stratification should correspond to eolian deposits. In a first view, the N030 paleo-coastal indica- According to the criteria by Otvos (2000), the tors seem to correspond to a coastal regression ARTICLE IN PRESS 1796 C. Liquete et al. / Continental Shelf Research 27 (2007) 1779–1800 semi-parallel to the depth contours, while the N090 This interpretation of the paleo-coastal indicators prograding sediment bodies could match to sandy implies that between MIS 4 and MIS 2 the bars in dynamic equilibrium with those N030 topographic/bathymetric trend of the Barcelona beaches. However, the high angle between the two continental shelf experienced significant directional sets of paleo-coastal indicators, together with the shift from S-N to SE-NW and so did the main intersection of N030 and N090 lithified beaches in direction of the subsequent marine transgressions. the northeastern part of the study area (Fig. 10A), We hypothesize that the causes for such directional make us suggest that both sets were developed shift are (a) sedimentation changes involving during different sea level cycles under different local depocenter migration due to modifications in conditions. sediment point sources and oceanographic condi- Late Quaternary sea level curves (Fig. 12)vary tions; (b) the regional neotectonic activity stressing along a pattern that is governed by ca. 100 ka high- the main NE-SW transfer faults, which have an amplitude (more than 100 m) glacio-eustatic cycles intermediate orientation between the two sets of modulated by higher frequency cyclicities (Imbrie paleo-shoreline indicators; (c) a combination of et al., 1984). Orbital forcing, particularly Earth’s tectonic events and entry points and depocenters eccentricity, obliquity and precession, have been migration. assumed as the triggers for these climatic and eustatic cycles (e.g. Imbrie et al., 1984; Shackelton, 2000). 5.4. Importance of anthropogenic impacts Higher frequency climatic cycles such as Heinrich events or Dansgaard–Oeschger fluctuations influence Man made actions leave their imprint over the sedimentological patterns from river basins to the shallow seafloor, as can be observed in the seafloor (Bard et al., 2006). The two last glacio- Barcelona shelf. Multibeam data are the perfect eustatic lowstands, reaching 80–110 and tool to control submarine human impacts and to 110–120 mbpsl, respectively, occurred during Marine monitor their possible natural mitigation. The Isotope Stages (MIS) 4 and 2 (Fig. 12). The rapid sea following section discusses the relative importance level rises following these lowstands favored the and probable consequences of the main anthropo- flooding and preservation of morphosedimentary genic actions on the Barcelona continental shelf. features along the former shorelines. Therefore we The Port of Barcelona area. The Port of suggest that the paleo-coastal indicators found on the Barcelona enlargement works have a strong impact relict Barcelona continental shelf were formed during over a large part of the Barcelona inner shelf the transgressions that followed MIS 4 and MIS 2. (Fig. 10C) and the Llobregat prodelta depocenter. Sediment sampling and dating is required to confirm As the continental shelf is narrowest off the our interpretation. This would require rock coring enlargement area, the new harbor area will comprise drilling of the hard beachrocks and the subcropping up to 35% of the Llobregat prodelta width and up and buried prograding sediment bodies. to 29% of the entire continental shelf (Fig. 8).

Waelbroeck et al., 2002 0 SPECMAP curve Siddall et al., 2003

Relative sea level (mbpsl) 120 1 2 MIS3 4 MIS5 MIS6 0 50 100 150 200 Age (ka BP)

Fig. 12. Relative sea level curves estimated from d18O records. Vertical scale is in meters with respect to present sea level (zero value). The Waelbroeck et al. (2002) curve corresponds to a global model; the SPECMAP curve (Imbrie et al., 1984; Imbrie et al., 1990) derives from the Atlantic Ocean; and the Siddall et al. (2003) curve originates from the Red Sea. MIS: Marine Isotope Stages. ARTICLE IN PRESS C. Liquete et al. / Continental Shelf Research 27 (2007) 1779–1800 1797

Major alterations are expected on coastal circula- close or even trespassing the shallowest legal tion and sediment-dispersal patterns. Furthermore, boundary for this kind of activity, at short distance the diversion of the Llobregat lowermost course and from artificial reefs and over the buried sewer pipes. mouth will likely cause the starvation of the wavy Artificial reefs. From the observations made, prodelta front and the establishment of a new protection artificial reefs seem locally efficient in sediment balance in the area. preventing direct impacts on the benthic ecosystem, Massive anchor ploughing leads to seafloor such as those from trawling, dredging or structure erosion and sediment resuspension. The ongoing placement, while their impact on sediment transport changes around the Port of Barcelona, involving the is negligible. When deployed on soft bottoms they shifting of anchoring areas, provide an excellent tend to sink and get partially buried, therefore opportunity to monitor the recovery of abandoned potentially loosing part of its efficiency. anchoring sites (like the one that extends off the old Troughs and highs alignments. The troughs and breakwater down to 30 m depth) and the progres- highs alignments on the northern inner shelf sive disturbance of newly established anchoring sites (Fig. 10B) of the study area extend along several (like the southern relatively small anchoring area kilometers. Their origin is a matter of debate. The illustrated in Fig. 8). first hypothesis is that they result from beach Dredging impacts. Dredging for beach nourish- nourishment dredging mostly performed during ment in the study area is mostly performed at short the 1980s. However, their large size, sharp bound- distance from the shoreline, at depths shallower aries and location over sea grass meadows protected than the major storm wave-face. Therefore, as areas (Fig. 8) points to a biogenic origin, likely dredging is performed at water depths undergoing phanerogam upbuilts. The endemic Mediterranean active sediment transport, the marks left on the Posidonia oceanica has been reported very close to seafloor tend to vanish in a matter of months or a the northern limit of the study area down to 30 m few years. The impact of dredging on the benthic depth (Manzanera and Cardell, 2002; Llamas, ecosystem is punctually and locally remarkable. 2003), while another phanerogam, Cymodocea More than two years are required for dredged sandy nodosa, has been identified within the study area bottoms to restructure to their initial ecological around 15 m depth (Castan˜eda, 1994). However, the conditions, as observed on the inner shelf to the upbuilding potential of C. nodosa is negligible north of the study area (Sardaét al., 2000). Whether compared to P. oceanica. Sea grass meadows do or not dredging for beach nourishment will continue not spread southwards likely because of human in the future is essentially a matter of political pressure, too turbid waters related to the Beso´s and decision. However, the impact of coastal infrastruc- Llobregat discharge, and the dominance of soft, tures, as illustrated by the enlargement of the Port prodelta bottoms. of Barcelona, will impose a strong need for infilling and coastline stabilization for which large volumes 6. Conclusions of sediment will be required. Sewage pipes. The two long sewer pipes coming The seafloor and subseafloor of the Barcelona out from Barcelona efficiently dispose the sewage continental shelf records its late Quaternary history water (Fig. 10C). However, the sedimentary signs of and the imprint of the factors controlling this leak away sites, long after the opening of a mud sediment-dispersal system. Two main morphosedi- treatment plant, indicate that either the system is mentary domains have been recognized: (i) a not efficient enough or some old sewer sediment modern, river-influenced area, and (ii) a relict, accumulations have not been dispersed after years. sediment depleted area. Both domains are affected Trawling impacts. Trawl gears cause significant by a variety of anthropogenic impacts. reworking and resuspension of the uppermost The main sediment source of the Barcelona shelf sediment cover, increasing the suspended sediment is the Llobregat river system. However, the con- concentration in the water column and disturbing tinental shelf reaches its minimum width in front of the seafloor ecosystem. Trawling over the muddy the Llobregat River mouth as most of the sediment Llobregat prodelta causes significant turbidity inputs are deviated southward by the dominant increases lasting several days (Palanques et al., coastal circulation. The modern, river-influenced 2001). Trawling marks may remain on the seafloor shelf includes the Llobregat and Beso´s adjacent for years. Trawl marks have been observed very prodeltas, which represent the main Holocene ARTICLE IN PRESS 1798 C. Liquete et al. / Continental Shelf Research 27 (2007) 1779–1800 depocenter in the study area. The wavy nature of program (ref. 2005 SGR-00152). GRCGM also the Llobregat prodelta front may result either from acknowledges the support received from Landmark hyperpycnal and bottom currents or from soft Graphics Corporation via the Landmark University sediment destabilization. The finest fluvial material Grant Program, and from SMT Inc. via the is preferentially accumulated along two patches educational User License for Kingdom Suite inter- shifted south-westwards of the parent river mouth pretation software. as a result of the dominant littoral circulation. A large part of the study area is sediment- depleted where relict morphosedimentary features, References such a set of NE-SW faults, crop out. A number of paleo-coastal indicators consisting of beach ridges ACA, 2000. Embassaments. Web page of the ‘‘Age` ncia Catalana and beachrocks are linked to short late Pleistoce- de l’Aigua,’’ /http://mediambient.gencat.net/aca/ca//actua ne–Holocene stillstands. The paleo-coastal indica- cions/embassaments/inici.jspS. Last revision in 2006. tors point to a 451 shift of the seashore direction ACA, 2005. Caracteritzacio´de masses d’aigua i ana` lisi del risc d’incompliment dels objetius de la Directiva Marc de l’Aigua between MIS 4 and MIS 2. This directional shift is (2000/60/CE) a Catalunya (conques intra i intercomunita` ries). tentatively attributed to tectonic activity along the Age` ncia Catalana de l’Aigua, Generalitat de Catalunya, NE-SW faulting system that involved modifications 860pp. of entry points and depocenter locations. Alessio, M., Allegri, L., Antonioli, F., Belluomini, G., Improta, Anthropogenic impacts are widespread on the S., Manfra, L., Preite, M., 1996. La curva di risalita del Mare Tirreno negli ultimi 43 ka ricavata da datazioni su speleotemi Barcelona inner and middle shelf, especially along sommersi e dati archeologici. Memorie Descrittive del the Beso´s-Llobregat coastal stretch. Coastal infra- Servizio Geologico Nazionale 52, 235–256. structures already have a major impact on sediment Alonso, B., Canals, C., Palanques, A., Rehault, J.-P., 1995. A dispersal patterns, that will likely increase in the Deep-sea Channel in the Northwestern Mediterranean Sea: near future. 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