Marine Geology, 79 (1988) 159-170 159 Elsevier Science Publishers B.V., Amsterdam - - Pr inted in The Nether lands

RECENT DEPOSITS AND SUSPENDED SEDIMENTS OFF THE PO DELLA PILA (PO RIVER, MAIN MOUTH), ITALY

A. BOLDRIN 1, G. BORTOLUZZI 2, F. FRASCARI 2, S. GUERZONI 2 and S. RABITTI 1

l lstituto di Biologia del Mare, C.N.R., Riva 7 Martiri 1364/A, 30122 Venice (Italy) 21stituto per la Geologia Marina, C.N.R., Via Zamboni 65, 40127 Bologna (Italy)

(Received March 11, 1987; revised and accepted September 10, 1987)

Abstract

Boldrin, A., Bortoluzzi, G., Frascari , F., Guerzoni, S. and Rabitti , S., 1988. Recent deposits and suspended sediments off the Po della Pila (Po River, main mouth), Italy. Mar. Geol., 79: 159-170.

Present sedimentary conditions in the prodeltaic marine area immediately in front of the Po della Pila (Po River main mouth) are character ized by a maximum thickness of Holocene muds of 10-12 m at a depth of 20-25 m and by two different sedimentary cycles, with the more recent lens-shaped uni t (2.5-4.0 m thick) probably deposited in the last 300 yrs. The nearshore prodelta area is characterized by rapidly accumulat ing deposits (2-4 cm/yr) enriched in smectite, gas, organic mater ial (1 2~o) and trace metals.

The water system presented well marked vert ical strat if ication during the summer of 1985, revealing three dist inct water bodies: (1) r iverine and diluted waters (average depth of 3 m), (2) middle depth waters (10-20 m) and (3) bottom waters.

Temperature, oxygen and part iculate organic carbon decrease with depth whilst total suspended matter, yellow substances and total part iculate carbon show maxima at the surface and in the bottom layers, and minima in the intermediate waters, where the organic fract ion comprises 42% of the total.

Introduct ion

The Po is the largest river in Italy (mean liquid discharge=1500 ma/s; mean solid dis- charge 14 million tonnes/yr). It drains the most industrialized Ital ian regions and affects the particle and pollutant budgets of the entire Adriatic Sea. Its delta is therefore a "filter" and serves as a sensitive barometer for pollu- tion problems related to trace metals and nutrients.

The mean flow in the Northern Adriatic is mainly due to thermohaline factors (Franco et al., 1982) and can be generally attributed to an all year round cyclonic system which is faster moving during the winter season. During the summer, the water column is characterized by horizontal stratification and local oceanograph-

ic conditions may prevail. In this season, freshwater from the Po River is injected into the surface layer and spreads into the basin, causing dilution.

Most sand is deposited all year round in the nearshore (delta front) at depths of less than 10 m. Some of the fine sediments settle in the prodelta in the immediate proximity of the mouths at water depths of 15-25 m. During flood periods, most of the suspended matter is trans- ported over longer distances. Major floods occur during the winter when the current circulation pushes freshwater discharges onto the Italian coast (Malanotte-Rizzoli and Bergamasco, 1983). Accordingly, a long-distance southerly trans- port pattern is established and the materials from the Po River can be traced in a mud belt several hundred kilometers southwards

0025-3227/88/$03.50 © 1988 Elsevier Science Publishers B.V.

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along the I tal ian coast. These deposits have a maximum thickness (25 m) at Ravenna (Colan- toni et al., 1979).

Morphology, sedimentology, hydrodynamics , mineralogy and sediment pol lut ion of the Po River delta have been discussed by several authors, among them Nelson (1970), Frascar i et al. (1979), Colantoni et al. (1979), Tomadin (1979), Gandolfi et al. (1982), Dal Cin (1983), Donazzolo et al. (1984) and Guerzoni et al. (1984).

The area invest igated (Fig.l) is direct ly in front of the main out le t of the r iver (Po della Pila). This branch of the Po is responsible for about one hal f of the present total liquid dis- charge and debouches into a zone of accret ion. Al though recent data (Dal Cin, 1983) demon- s t ra te the historical reduct ion of part icle dis- charge by the Po, the removal of r iver channel sands for cons t ruc t ion mater ia ls and the result- ing general re t rea t of the del ta shorelines have led to cont inued deposit ion in the marine areas close to the principal distributaries.

Our goal is to add new da ta to tha t on the r ecen t Q u a t e r n a r y deve lopment in this del ta ic a rea and to describe the present deposi t ional processes, focusing on the te rmina l par t of the r iver and on the a rea of f ine-grained sediments deposi ted in f ront of it. The d is t r ibut ion of the suspended load and the b iogeochemical pro- cesses in the wate r co lumn was also s tudied in detail. The mud deposits at shal low depths and the s t ruc tu re of a mul t i l ayer system immedi- a te ly offshore from the f reshwate r seawate r mixing zone, dur ing a typical summer s i tua t ion are emphasized.

Mater ia l s and m e t h o d s

Bot tom and suspended sediments were sam- pled in Ju ly 1985, dur ing a low discharge period of the r iver (700m3/s). Bot tom sedi- ments were col lected using a box corer and core samples were recovered with a modified gravi ty corer (Busat t i et al., 1980). UNIBOOM profiles were shot (see Fig. l ) while the da ta

4.5 ° 0 0 '

PO d~llm

" \ \ . \ • . . " % . . .

" , , d , ' , .. , ,

. . . , , , ,, 7 Fig.1. Study area with location of sampling stations. The dashed lines indicate the bathymetry (m), the dotted lines, the surface water salinity and the solid lines the UNIBOOM profiles.

161

obtained were reduced by the method of Boldrin et al. (1984) to calculate the thickness of the reflectors in meters. Hydrological parameters were measured using a n ME Kompact Multi- sonde System. Salinity data were calculated with the Practical Salinity Scale (Lewis, 1980). Water transmittance was measured with the Marine Adviser Transmissometer, Model C-2, with an optical beam of 1 m. Total suspended matter (TSM) was determined by the gravime- tric method after filtration on Whatman GF/C filters and subsequent ashing (Strickland and Parsons, 1972). Grain size spectra of the total particulate carbon (range 2-32 gm) were deter- mined by electronic counting (Sheldon and Parsons, 1967) using a Coulter Counter, Model TA II, interfaced with a personal computer (Boldrin et al., 1986). The dissolved organic matter was analyzed (after filtration and stabili- zation) with the fluorimetric method, using excitation and emission wavelengths of 337 and 432 #m respectively (Russo, 1984). Clay mineral analyses were performed by X-ray diffraction on the < 2 #m fractions (Tomadin, 1979). Extrac- tions of the sediments for metals were made with HNO 3 for Co, Fe, Mn, Ni and Zn and with 1 N HC1 for Cu and Pb, using procedures documented in detail elsewhere (Guerzoni et al., 1987). Humic substances were isolated from the sediments using alkaline extraction, follow- ing the procedures described by Poutanen and Morris (1983), excluding the fractionation step. The carbon and nitrogen content (total and organic) was determinated on freeze-dried samples using the CHN analyzer, Carlo Erba model 1106, following the methods of Froelich (1980) and Sharp (1974) for bottom and sus- pended sediments respectively.

R esu l t s

Postglacial evolution of the area

During the Wiirm regression, the North Adriatic Sea was a fluviolacustrine plain. The sea would have reached the study area about 6000 yrs B.P., with the postglacial transgres-

sion. The coastline matched the present-day 25 m isobath, and sand dune ridges (Brambati and Venzo, 1967) have been found to the north, where relict sands outcrop and indicate a stillstand in the transgression. The marine transgression reached its maximum probably at about 2000-3000 yrs B.P. The Po River delta prograded at mean rates of 5m/yr until 1600 A.D. and at 70 m/yr for the last 400 yrs. However, retreating phenomena over the last 30 yrs have been documented also (Dal Cin, 1983).

Subbottom and UNIBOOM profiles show a mud prism overlying reworked sands (see reflector 1 in Fig.3), which in turn cover the eroded surface due to subaerial exposure. The total thickness of the muddy sediments reaches a maximum (10-12 m) in the profiles running at depths of 20-25 m in front of the main mouth. The prism thins seaward and is only a few centimeters thick over the relict sands 8 km offshore (Fig.2). The thickness of the Holocene muds is the same as that in other Mediter- ranean river mouths, even taking into ac- count their much lower mean solid discharge (in the Rh6ne River, the discharge is 3 million tonnes/yr (Aloisi and Monaco, 1975)) and is similar to the thickness in the Yukon River which shows 14 m of marine Holocene sedi- ments, with an annual average discharge of 80-90 million tonnes/yr (Nelson, 1982).

Two different sedimentary cycles may be defined by a marked change in acoustic properties (reflector 2, Fig.3). The lower se- quence displays subparallel internal reflectors, thus indicating continuous mud deposition. Deposition of the sequence probably occurred in an offshore environment before and during the maximum marine transgression. The upper, more evidently lens-shaped unit, probably re- sults from the deposition of a progradational sequence related to the seaward outbuilding of the river delta. It may be pointed out that a larger supply of materials was made available after 1600A.D., due to three main factors affecting the drainage basin: (1) deforestation, (2) the construction of barriers to protect the towns of Ferrara and Venezia from floods and

162

J L . . . . . I I I

Fig.2. Thickness (m) of the Holocene muds off the Po River delta (solid lines). Dashed lines indicate the bathymetry (m).

A B

A, B,

SE , 5 0 0 m m N W

Fig.3. UNIBOOM profiles AB and A'B' showing two marked reflectors: 1= top of the reworked sands; 2= base of the lens- shaped unit. See text for fur ther explanation, and Fig.2 for location of the profiles.

(3) climatic changes. In our profiles, the thickness of the upper sequence is larger (4 m) in front of and south of the northern dis- t r ibutary (Busa di Tramontana) but the thick- ness is only 2.5 m in front of Busa Dritta, in an area that at present has the highest sedimenta- tion rate. This fact is in agreement with documented diversions of the main mouth which occurred in the last 300 yrs (Ciabatti, 1966).

Abrupt disappearance of deeper beds, as shown by acoustic absorption of the seismic signal, was found at depths of 20-25 m (Fig.3) in the landward part of the submerged delta. This anomaly is thought to be caused by a rapid accumulation of organic matter-rich sediments which prevents the escape of gas produced by decomposition in the lower strata (the "table phenomenon" as also reported by Got et al., 1985 for the Rh6ne and Ebro delta areas).

Present sedimentary processes in the prodeltaic area

The Holocene mud belts and the present prodeltaic deposits related to the river dis- charges onto the large continental shelves of the Mediterranean Sea show sedimentological affinities (Got et al., 1985). In particular, the study area shows sediment patterns that are

163

similar to the ones described by Aloisi and Monaco (1975) for the Gulf of Lion.

In the Po River zone, surface samples provide the following data:

The distribution pat tern of clay minerals ranging from the fluvial zone to the offshore area confirm the data of Tomadin (1979). Illite and smectite are the prevailing minerals in the < 2 #m fraction (Table 1) and comprise 70-85% of the total. Smectite richer sediments (17-19% of the total) are found landwards of the 25 m isobath where materials of essentially Po River origin settle. There are no significant differ- ences between riverine and marine sediments regarding the chlorite, kaolinite and serpen- tine content. This area also receives sediment derived from the Northern Adriatic rivers (Bortoluzzi et al., 1986a).

High organic matter contents and high hydration coefficients characterize these muds. Total organic carbon (TOC) values in all samples range from 0.8 to 2.1%. The maximum was found in the river samples; thereafter the values decreased towards the sea. Total humic substances (THS) comprise 40% of the TOC inside the river and decrease to approximately 20% at stations 6, 7 and 8 (Fig.4). This is probably clue to the high sedimentation rates and the correlated reduction in degradation processes. Redox (Eh) data show low to nega- tive values everywhere. The minimum value

TABLE 1

E s s e n t i a l d a t a for sur face samples from the Po R ive r and the offshore area. H20 - - h y d r a t i o n coefficient;

TOC - - t o t a l o rga n i c carbon; SM - - smect i te ; I - - i l l i te ; K - - k a o l i n i t e ; CL - - ch lor i te ; S - - se rpen t ine . H y d r a t i o n coefficient, TOC and c lay c o n t e n t s as p e r c e n t a g e s

Sample no Profi le dep th pH E h H20 TOC SM I K CL S (m) (my)

01 (r iver) 5.5 7.54 - 100 60 2.1 18 49 10 16 7

02 (r iver) 3.5 6.98 - 40 61 1.5 17 51 10 15 7 03 (sea) 5.5 6.42 - 40 51 1.2 17 50 11 15 7 04 (sea) 8.0 7.66 - 6 55 1.3 15 56 9 14 6 05 (sea) 12.0 6.98 - 10 52 0.8 18 51 9 14 8 06 (sea) 15.0 7.25 + 35 52 1.5 18 50 9 15 7 07 (sea) 20.0 7.33 + 2 59 1.9 19 52 10 14 6 08 (sea) 23.5 7.36 + 10 54 1.6 19 53 10 14 4 09 (sea) 30.0 7.68 ÷ 28 49 1.3 13 55 11 17 4

164

0

10

o~ w

w

2O

3O

_%~o- []

[]

Q

Q O 1 2 mg /L 0

2

_.0 0 U 1 O

O S t a t . 1 2 3 - 4 5 6 7 8 9

Fig.4. Particulate organic carbon (Poe) (mg/1) of suspended sediments (squares) and yellow substance distribution (arbitrary fluorescence unit (AFU) contours) off the Po River delta. The histograms show total organic carbon (TOC) (%) and total humic substances (THS) (%) content of bottom sediments at the same stations.

found within the mouth stresses a prevailing "ear ly" deposition of r iverine materials during low discharge periods.

S e d i m e n t a t i o n rates a n d core profi les

A 2 m core was collected in 1980 within the previously described area, close to station 7, where the accumulation velocity is on average more significant than the amount of periodical resuspension and transport due to waves and currents.

This core was analyzed for 21°pb, trace metals and other parameters, in order to study the concentrat ion mechanisms of pollutants and to measure the sedimentation rate. The lat ter was presumed to be very high due to the superposition of flocculation in addition to simple gravitat ional processes (Guerzoni et al., 1985).

Under the assumption made in the previous

paragraph, the average sedimentation rate of the sedimentary unit overlying reflector 2 (Fig.3) was approximately 1 cm/yr. Since it is a regressive sequence, the velocity was probably lower in the older sediments (distal deposits), becoming higher towards the top of the core, where the more recent deposits, rich in flocs, were found. The core comprised a predomi- nantly muddy sediment (more than 98% silt and clay fractions).

Total 21°pb activities were found to be relatively high (2.5-5.8 dpm/g) when compared with estimated 21°pb activities supported by decay of 226Ra in similar environments (1.0-1.5 dpm/g) (Donazzolo et al., 1982; Deconick et al., 1983). Accordingly, the apparent mean sedi- mentat ion rate was in the order of 2-4 cm/yr. These results agree with data reported by Albertazzi et al. (1986), who found substantial amounts of 13 ~Cs at depths of 50-90 cm in cores collected in front of the main distributaries.

165

Trace metals

Trace metal concentrations of total Cu, Cr, Hg and Pb in the upper sediment layers have been previously studied (Guerzoni et al., 1985). The concentrations appeared to be higher than the average values found for surficial fine- grained sediments from the surrounding area (Frascari et al., 1986).

To more effectively characterize the prodel- taic area and to check the possible use of this area as a "sensitive barometer" for pollution problems, more detailed analyses on organic matter, together with other trace metal analy- ses and selective leaching extraction for Cu and Pb were carried out.

The organic matter distribution shows that the THS depends on the total organic matter (TOM) content. The maximum values (1.8%) were found in the subsurface (depths of 19-22cm) with values decreasing to 0.5~/o lower in the core. Low values of the "degree of humification" (THS/TOM) were found in the core (Fig.5) and the data partially contrast with the findings of Poutanen (1985), who observed a slight increase of the value as a function of depth. The homogeneity of the data in our core is probably due to the very high sedimentation rate which reduces the degrada- tion processes (Guerzoni and Rovatti, 1987).

Other data on Zn, Ni, Fe and Mn confirm the

high surficial values, with the exception of Co. The maxima for the different metals are in different core layers, but high values are found down to a depth of 56 cm in the core. Further- more, trace metal concentrations at the bottom of the core did not reach values as low as those which appear as reasonable estimates for background concentrations in this area, confirming that the age of the lowest layer is not older than the preindustrial period (Table 2).

To distinguish the anthropogenic from the lithogenic content, a selective leaching extrac- tion (Chester and Voutsinou, 1981) was also applied to the samples and the data showed that Cu and Pb variations are mainly due to the hydrochloric leachate (carbonates, Fe and easily organic-bound fractions), whilst the residual part is almost constant downwards through the core, with average values of 15 and 20 ppm for Cu and Pb respectively (Bortoluzzi et al., 1986b).

Suspended sediments

A well marked vertical stratification as a result of different interacting processes that began during the spring season characterized the hydrological structure of the Northern Adriatic basin in July 1985. CTD measure- ments revealed three different bodies: (1) a

E v

-1"- ~- 50 o_ w a

W n- O

150

T H S / " TOM 015

0

4" M

c/N

0.30 0.45

I

f % I

I I

I I

,i I 1 6 9

v

o

50

THS ( m g l g ) 2 0

150 --

TOM(rag /g) 12

4

THS X TOM

' /

,iy 1 I

24

Fig.5. Total organic mat ter (TOM) (TOC x 1.8), THS, C/N and "degree of humificat ion" (THS/TOM) versus depth in the core.

166

T A B L E 2

Ver t ica l profiles of TOC and t he a n t h r o p o g e n i c (1), l i thogenic (2) and to ta l (3) c o n t e n t of Cu and Pb. To ta l a m o u n t s (ppm) of Zn, Ni, Co, M n and Fe are s h o w n (mg/kg, dry weight) . A v e r a g e and b a c k g r o u n d va lue s for su r face del ta ic f ine-gra ined samples a re ind ica ted by (a) and (b) r espec t ive ly (from Guerzon i et al., 1984)

Dep th TOC Cu Cu Cu Pb Pb Pb Zn Ni Co M n Fe (cm) (%) (1) (2) (3) (1) (2) (3) (ppm) (%)

0 3 0.8 30 20 50 54 21 75 168 84 17 546 2.5 9 12 . . . . . 192 80 16 656 2.5

19-22 1.8 60 26 86 84 22 106 232 60 14 573 2.2 30-33 0.9 33 15 48 64 19 83 173 53 13 454 2.0 40-43 . . . . . . 234 77 15 513 2.5 56-58 0.8 44 16 60 77 14 91 133 67 15 589 2.1 70-73 . . . . 169 51 13 494 1.9

147-150 0.5 20 12 32 26 12 36 89 62 14 455 2.1

(a ) - - -- -- 4 7 -- -- 64 181 49 13 464 2 .0

(b) . . . . . 70 35 12 ?

Stat . 1 2 5 6 7 8 g . . . . . . . . . . . . .

N [ N . ~ 10

STATION I ~ X

hl - - •

20 0 "~gm

30

l o -

o b o t t o m

10-

0 i , J

2 4 S 16 32 /Jm 0 20 mg 1-1

Fig.6. Dens i ty (at) and to ta l s u s p e n d e d m a t t e r (TSM) (mg/1) off the ma in m o u t h of the Po River. The 21 and 24 a t c o n t o u r s s epa ra t e th ree d i f ferent wa te r bodies (see tex t for explana t ion) . Par t i c le size spec t r a a t the su r f ace (0.5 m deep), a t 19 m deep and a t t he sea bo t t om are s h o w n for s t a t i on 7.

surface layer of riverine diluted waters, (2) middle depth waters and (3) bottom waters. The characteristics of these water bodies are shown in Table 3 and Fig.6. Figure 6 shows the TSM distribution while Fig.7 shows grain size spec- tra of suspended sediments collected off the Po della Pila.

In riverine waters, the TSM presented the

highest values (mean concentra t ion= 18.3 mg/1) with 70% of the inorganic fraction and 1.9mg/1 of the particulate organic carbon (POC). The grain size spectra were strongly marked by two modal classes (7 and 18/tm) corresponding to two phytoplankton popula- tions of centric diatoms (Socal, pers. commun., 1985).

TABLE 3

Mean values and standard deviations of oceanographic characteristics in front of the Po River delta during July 1985. n=number of samples, TSM - - total suspended matter; TPC - - total particulate carbon; POC - - particulate organic carbon; AFU - - arbitrary fluorescence units

167

Parameter Unit Riverine and diluted Middle depth Bottom waters waters (n = 8) waters (n = 14) (n = 5)

Temperature °C 25.96 (1.1) 23.72 (2.4) 18.31 (2.9) Salinity %o 18.13 (14.0) 33.16 (1.1) 36.08 (1.1) Density at 10.43 (10.7) 22.34 (1.4) 26.01 (1.6) Oxygen % 112.18 (17.8) 108.23 (9.1) 73.64 (8.4) Yellow substances AFU 10.18 (5.6) 2.99 (1.9) 3.83 (2.0) TSM ng/dm 3 11.73 (8.7) 2.29 (0.5) 5.83 (2.0) Organic fraction % 35.34 (11.3) 41.58 (8.4) 22.98 (5.0) TPC ng/dm 3 1.47 (0.7) 0.36 (0.1) 0.42 (0.1) POC ng/dm 3 1.32 (0.7) 0.25 (0.1) 0.24 (0.1)

Stat. 1 2 5 6 7 ~ 8 9

. . . .

10

nr W

W

20'

30

1 "

mm m3 . . . . 0.5

0 1Km 0 i , i , I i

2 4 8 16 32 ~m

Fig.7. Gra in size (range 2-32 p_m) spect ra (v/v) of suspended sediments collected off the Po della Pila.

The s i t u a t i o n obse rved a t s t a t i on 2 is r a t h e r pecul iar : The pa r t i c l e size d i s t r ibu t ion of suspended m a t t e r n e a r the b o t t o m showed an inc rease in the coarse f r ac t i on w h e n c o m p a r e d wi th su r face samples . At the same t ime, the h ighes t m e a s u r e d c o n c e n t r a t i o n (28 mg/l , 81% of the i no rgan ic f rac t ion) was found. Such a d i s t r ibu t ion of suspended m a t t e r m u s t be r e l a t ed to f loccu la t ion processes and the t rap- p ing of suspended sed imen t o c c u r r i n g in th is zone, where p rev ious a u t h o r s (Nelson, 1970)

r epor t ed a t u rb id i t y m a x i m u m a t the b o u n d a r y be tween the f r e s h w a t e r and the sa l t wedge.

The offshore ex tens ion of the r i ve r in jec t ion l aye r is r ap id ly r educed due to the low r ive r d i scha rge bu t i t is stil l r ecogn izab le a t the surface , wi th sa l in i ty lower t h a n 30% and t r a n s m i t t a n c e lower t h a n 10%. In th is layer , f avo rab l e condi t ions for b io logica l p roduc t i on are present , as ind ica ted by h igh va lues of dissolved oxygen s a t u r a t i o n pe rcen tages (about 146%), TSM organ ic f r ac t ion and h igh

168

POC values. Particle size distribution is domi- nated by phytoplankton populations (3 modal classes at 4.5, 11 and 28.5 #m).

Below the thermocline, the TSM concentra- tion decreases regularly, with a minimum value of lower than 2 mg/1, but the particle distribution spectra remain substantially the same, indicating that the surface layer is the source of suspended solids. A progressive increase of the inorganic fraction of TSM was observed in the water column, increasing to values of greater than 75% towards the bottom. Conversely, the POC values decreased from 75 to 55% of the TPC in the bottom layer.

The middle depth water showed maximum transmittance values (30%), diminishing bot- tomward to less than 10%.

The suspended matter is also produced at the surface, and apart from being highly biologi- cally utilized, is transferred to the bottom layer as particles of relatively large dimensions (skeletal fragments, fecal pellets, etc.). These particles disintegrate and contribute to the bottom turbidity layer. During the transfer from the surface to the bottom, decaying processes slowly modify the particle size distri- bution and the chemical composition of the particulate matter, and the size differences become evident near the bottom.

In the bottom water layer, TSM values are higher (more than 4 mg/1) and the size spectra are different: Phytoplankton peaks disap- peared and the percentage of finer, inorganic particles (<6 pm in size) strongly increased. Dissolved oxygen saturation values decreased to less than 75%, stressing the prevailing respiration processes with the associated or- ganic matter mineralization.

Near the sea floor at the front of the delta (stations 6 and 7), the values of important parameters are increased relative to adjacent areas. Thus, the bottom water in that area shows a high density (> 26 at), low transmit- tance ( < 10%), high TSM values (> 9 mg/1) and low dissolved oxygen concentration ( < 3.5 cm3/1, i.e., <70% saturation). Particle size spectra revealed a noticeable increase in finer compo- nents (< 5 #m). The fine particles are believed

to be from direct river discharge, and are normally trapped between the flocculation area and the offshore area (stations 6 and 7). Alternating processes of sedimentation and resuspension (the latter demonstrated by the similarity between the particle spectra of suspended sediment 1 m above the bottom and the uppermost veneer of bottom sediments), induced by hydrodynamical conditions (tide flow and bottom currents) also periodically contribute to the dome structure by means of lateral advection from the deeper and denser water layer.

The offshore limit of this area, with sus- pended particle segregation of direct riverine influence, may also be drawn using the distri- bution of yellow substances. Around station 7 (water depth, 20 m), there is a break, from values higher than 5 to values lower than 3 arbitrary fluorescence units (AFU). (Similar values were found by Russo (1984)). The described distribution of dissolved organic matter agrees with the highest values of organic matter found in the bottom sediments at the same station (Fig.4).

Conclus ions

Geological considerations linked with chem- ical and mineralogical studies on riverine and marine bottom sediments give evidence of the formation of a nearshore prodeltaic area char- acterized by (1) a high sedimentation rate (2-4 cm/yr), (2) smectite-rich sediments, (3) high levels of organic matter (1-2%) and trace metals and (4) gas charging.

In the studied summer situation, the bottom water layer, between 15 and 20 m, has a dome structure with different compositional charac- teristics and interactions with bottom sedi- ments. The highest values of organic matter and trace metals are present in this area.

Such a picture may be the result of diverse processes as follows: (1) biophysicochemical differentiation of suspended matter in the water column, (2) hydrodynamic resuspension of the fine portion of bottom sediments, or (3) transfer by gravity and/or hydrodynamics of

suspended matter aggregates and flocs from the flocculation area (in the river) to the bottom layer.

The high sedimentation rate of the area, together with high levels of organic matter gives rise to the decrease in degradation processes, and probably to the gas charging of sediments.

The high seasonal variabili ty of hydrody- namical and oceanographic conditions never- theless demand a winter survey in order to better define the sea-r iver interactions.

Acknowledgements

This paper is contribution number 657 of the Insti tuto per la Geologia Marina del Consiglio Nazionale delle Ricerche, Bologna, Italy. Thanks are due to the captain, officers, crew and Colleagues onboard the R.V. Bannock during cruise PD 85. Clay mineral analyses were carried out by W. Landuzzi and chemical analyses on suspended matter by F. Cioce. The figures were drawn by G. Zini. Constructive criticisms by F. Trincardi and M. Marani are gratefully acknowledged.

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