Nutrient profiles in the pore water of a deltaic lagoon: Methodological considerations and evaluation of benthic fluxes

Estuarine, Coastal and Shelf Science (1991) 33.361-382

Nutrient Profiles in the Pore Water a Deltaic Lagoon: Methodological Considerations and Evaluation of Benthic Fluxes

Monique Giuseppe Gabriella

Viel”, Andrea Barbantib, Leonardo Buffoni”, Diego Paltrinierib and Rossob

of

Langoneb,

“ENEA, C.R.E.A., CP316,19100 La Spezia, Italy, and bIstitutoper Marina CNR, Via Zamboni 65,40127 Bologna, Italy

la Geologia

Received 2OJuly 1990 and in revised form 25 March 1991

Keywords: nutrients; pore waters; dialysers; early diagenesis; benthic fluxes; kinetic models; PO Delta Several aspects of the early diagenesis of major nutrients (silica, ammonia, phosphorus) and iron, based on sediment and porewater studies, were examined in Sacca di Scardovari (PO Delta, Italy). Radionuclide measurements (““Pb, “7Cs, ’ YZs) were carried out in order to calculate modern sediment accumulation and mixing rates. The pore water obtained from two different sampling techniques is also compared. One method involves direct sampling through the use of dialysis samplers; the second one obtains samples through the centrifugation of sediment cores. The two methods compare well for all the elements considered, except for Fe values in the topmost centimetres of the sediment and for NH, in the lowest 15 cm. These differences can be explained by alterations of porewater composition during the manipulation of core samples and by spatial heterogeneity. The distributions obtained for the different ionic species are typical of anoxic sedimentary environments, rich in organic matter. Evaluation of silica, ammonia and phosphorus fluxes at the water-sediment interface is conducted by applying a one-dimensional model: the sediment is considered as a two-layer system and a steady-state diagenesis is assumed. The model takes into account molecular diffusion and irrigation in a cumulative coefficient D, or distinguishes irrigation effects in a separate term, 1. The two schemes furnish different flux values, and explanations for these differences are proposed.

Introduction Numerous papers have shown that the exchange at the water-sediment interface in lagoons and coastal marine environments plays a fundamental role in the biochemical cycles of several elements (for example Aller, 1980a,b; McCaffrey et al., 1980; Rosenfeld, 1981; Elderfield et al., 1981; Klump &Martens, 1981; Berelson et al., 1987; Bender et al., 0272-7714/91/100361+22

$03.00/O

@ 1991 Academic

Press Limited

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1989; Hammond et al., 1985; Holdren & Armstrong, 1986; van Eck & Smits, 1986). The regeneration of nutrients and other chemical speciesin sedimentsis controlled by physical, chemical and biological factors and is particularly related to the degradation of organic matter and to the physical-chemical conditions of the sediment. The evaluation of nutrient fluxes from the sediments to the water has been conducted by different authors through various approaches:in situ measurementsutilizing benthic chambers (Hammond et al., 1985; Frascari et al., 1988; Callender & Hammond, 1982; Klump & Martens, 1981); evaluations based on nutrient concentrations in the pore waters and on the sediment characteristics at various depths (Klump & Martens, 1981; Aller, 1980a). Both approaches, though, display limitations. The latter one, chosen for our experiments, requires great care during the extraction of pore water from the sediment, taking into account the sensitive nature of dissolved elements to temperature and atmospheric conditions (Bray et aE., 1973; Carignan, 1984). The integrated study of solid fractions and pore waters renders the inclusion of the recycling processesas part of the geochemical characteristics and the definition of the diagenetic processesof the areapossible. Solute flux evaluation basedon the concentration profiles of the elements in pore waters is greatly influenced by the model used for the interpretation of the concentration gradients and by the spatial detail that defines the gradients, particularly near the sediment-water interface. The releaseof nutrients from sedimentsrepresents an important factor in determining the trophic level of an aquatic system. The problem of eutrophication in the coastal areas of the northern Adriatic Seahasbeen discussedin detail by many authors (AA.VV., 1978; Marchetti, 1984; Bortoluzzi et al., 1984a,6). They generally agree in that the principal causeof eutrophication is related to the nutrient inputs from the PORiver, but the relative importance of nutrient regeneration in the sedimentsis unknown. In this paper we present the first results concerning the geochemical characteristics of sediments from Sacca di Scardovari, a deltaic lagoon of the PORiver. These results are basedon the interpretation of porewater profiles of nutrients and iron, and on the application of a mathematical model for the evaluation of fluxes. The result of a methodological test is also reported consisting of the comparison between two porewater sampling techniques: centrifugation of core slicesin inert atmosphere (Holdren et al., 1977; Elderfield et al., 1981; Hammond et al., 1985) and in situ sampling by dialysers (Hesslein, 1976; Mayer, 1976; Carignan, 1984). The comparability between the two methods hasbeen tested by Carignan et al. (1985) in lacustrine sediments. They found good comparability for Fe, Mn and other metals. Fernex et al. (1986), working in coastal areasof the Mediterranean Sea, seem to agree with that conclusion, but only show profiles without an exhaustive discussion of their results. Study

area

Saccadi Scardovari (Figure 1) is a 30-km2 subtidal lagoon; it is distinguishable from the surrounding areasby its morphological stability that has persisted for at least a century (Bondesan & Simeoni, 1983). At present it hasno direct freshwater input. The lagoon has a mean water depth of 2 m, and two areas with different characteristics can be distinguished (Ceccherelli et al., 1985): in the northern one, which is higher in salinity (19-239~~),tide currents are relatively weak; in contrast, the central-southern part of the lagoon hasgreater exchange with coastalwaters, which are diluted during most of the year

Nutrient profiles in pore water

Figure

1. Study

363

area.

by PO River discharges. The lagoon is relatively well protected from marine wave action by a belt of sandridges. Sediments are mainly fine-grained silty clays in the inner part and become coarser near the inlet (Ceccherelli et al., 1985). Benthic activity is very intense: the macrobenthos communities are dominated by polychaetes (Capitella capitata, Polydora ciliata) and molluscs (Cerastoderma glaucum, Hydrobia sp.) (Ceccherelli et al., 1985). During recent years the biocenosisof the PODelta lagoonshas been enriched by the presenceof the bivalve Scapharca inequivalvis, which is able to produce burrows lo-12 cm deep (Poluzzi & Taviani, 1984;V.U. Ceccherelli, pers. comm.). Based on the existing biocenosis, it can be stated that particle reworking due to organisms is limited to the upper few centimetres of sediment, whereas irrigation phenomena could affect thicker sediment sections. Sample collection The dialysers were constructed following the model described by Carignan et al. (1985). They are composed of plexiglass, containing two columns of cells, I cm deep, each with a volume of 3.5 cm’, spaced 1 cm apart along a total length of 30 cm. The cells were filled with Milli-Q water and covered with an inert polysulphone membrane (Gelman HT450) with a pore size of 0.45 urn (Carignan, 1984; Carignan et al., 1985). Before their insertion in the sediment, the dialysers were deoxygenated by a 24-h immersion in distilled water into which nitrogen gaswas bubbled. In October 1986, the dialysers were put in place by a diver at points A and B, about 100 m apart (Figure 1). At each point two dialysers, 0.5 to 1.0 m apart and with three to

364

M. Vie1 et al.

four cells emerging above the sea-bottom, were positioned. A third dialyser was also positioned at B. They were recovered after 23 days, a time deemed sufficient to reach equilibrium acrossthe membrane, both in coastalmarine sediments (Klump & Martens, 1981; Gaillard et al., 1986;van Eck & Smits, 1986)and in lake sediments(Carignan, 1984). The porous membraneswere intact at the time of recovery. Pore waters were sampled from the dialyser within 5 to 10 min after recovery, using syringes. The samples were split into aliquots of l-3ml suitable for each analysis. Aliquots for nutrients were immediately stored in vacuum vials to avoid oxidation; for Fe and Ca concentrated HCl wasadded to pH 1-2; pH and Eh were measuredwith calibrated microelectrodes on a separate aliquot. The corresponding levels of dialysers B, and B, were mixed (Table I), to have a greater quantity of water for the analysis. The third dialyser at B functioned as a control for the evaluation of the effects of atmospheric exposure on porewater composition during the sampling operations. A column of cells was sampledimmediately after recovery, while the other one was sampled 15 to 20 min after recovery. Two cores, each about 30 cm long, were obtained with a water-sediment corer (6 cm diameter; Busatti et al., 1988) at each site during the recovery of the dialysers, using great care to limit the disturbance of the interface. The cores were immediately stored at 4 “C. Two hours after collection they were extruded in a glove box under a nitrogen atmosphere and sliced into sections 2 cm thick. The E,, and pH values of the sections were measured with microelectrodes and a small portion of each section was weighed and dried at 60 C for porosity determination. Each slice was placed in a polypropylene (150 cc) test-tube and centrifuged at 5000 rpm for 20 min. The resulting supernatant (7-10 cc) was filtered in an inert atmosphere through Millipore HGA 0.45 urn filters and divided into two parts, one of which was acidified with HCl. Chemical

analysis

Within 24 h after recovery, reactive P-PO,, N-NH,, N-NO,, N-NO, and reactive Si-SiO, were determined from the unacidified samplesutilizing a Technicon Autoanalyser II. The sampleswere diluted I:100 before the analysis. Ammonia was determined using Berthelot’s reaction, with readings at 630 nm (detection limit: 0.2 PM) (Grasshoff et al., 1983). Nitrite was measured using an adaptation of the denitrogenation method with readings at 550 nm; nitrate wasevaluated by subtraction from nitrites after reduction by a mixture of copper and cadmium (Rand et al., 1976). Unfortunately the initial sample dilution (1: 100) resulted in the concentrations being close to the sensitivity of the test, which is respectively 0.4 l.rM and 0.1 PM; thus the values obtained for NO, and NO, were not reliable. DRP concentration was determined by the phosphomolybdenum blue method (Murphy & Riley, 1962) with readings at 880 nm (detection limit 0.08 PM); silica concentration wasmeasuredusing the silicomolybdate reduction method (Grasshoff et al., 1983) with readings at 815 nm (detection limit 1 PM). Total Fe and Ca were measuredin the acidified samplesusing flame AAS. The main composition of the sediment was determined through energy dispersive Xray fluorescence, using pastilles of freeze-dried and finely milled sediment. The grain-size analyseswere conducted through wet-sieving of the fraction > 63 urn and analysis of the fine fraction with an X-ray sedigraph. The organic carbon and total nitrogen content were determined through a CHN Elemental Analyser after the removal of carbonateswith HCl (Froelich, 1980).

7

0

-3 -4 -5 -6 -7 -8 -9 -10 -11 -12 -13 14 -15 - 16 -17 -18 -19 -20 -21 -22 23 -24 25 -26 -27

-1

f4 f3 f2 fl

Depth fcm)

7.99 7.92 7.96 7.85 7.58 7.79 7.78 7.77 7.69 7.68 7.64 7.63 7.63 7.70 7.63 7.65 7.65 7.63 7.63 764 7.65 7.65 7.68 7.64 7.67 7.64 7.65 7.65 7.68 7.68

PH

8 -7 ~ 11 12 -- 11 ~ 10 -8 ~ 10 - 11 12 10 10 7

-I

1272.86 1272.86 1249.29 1237.86 1284.29 1278.57 1240.71 1272.86 1284.29 1243.57 1237.86 1191.07 1232.14

63.21 93.93 60.71 93.93 84.29 97.14 108.21 119.29 213.21 521.07 858.93 870.00 870.00 1197.14 1243.57 1240.71

Dialyser

WC

Si

1. Chemical

Et’

t217 t217 t 155 +72 f56 +29 + 18 +9 +1 -3 ~ 16

(mv)

'I'ABLE

48 48 48 96

1020 1080 1185 1080 1080 1130 1110 995 1130 1080 1530 1080 1067

580 370 250 370 720 800 830 880 960 1010 1050

A,

(PM)

NH,

parameters

PO,

92.0 94.2 104.5 91.3 89.6 90.6 90.6 87.8 96.2 96.5 94.8 70.5 78.8

5.8 5.8 2.7 8.6 28.1 54.5 73.2 26.7 84.7 92.7 92.7

1.0 1.0 1.0 4.6

(PM)

9.0 9.0 6.1 54.6 33.1 21.8 7.5 7.5 6.1 6.1 6.1 6.1 7.5 6.1 6.1 6.1 6.1 4.7 7.5 6.1 7.5 7.5 6.1 6.1 4.7 7.5 7.5 7.5 6.1 7.5

ipM)

Fe

of overlying

PH

8.06 7.99 7.84 7.64 7.75 7.66 7.61 7.60 7.59 7.60 7.56 7.63 7.56 7.58 7.55 7.56 7.58 7.56 7.66 7.61 7.61 7.59 7.58 7.58 7.59 7.60 7.57 7.59 7.61 7.58

water

and

+214 +231 +231 f215 + 180 + 107 +75 f49 f31 f22 + 14 + 11 +5 +10 f4 0 0 -3 -2 -4 -5 -2 -2 -3 -3 +1 fl fl 0 0

Eh (mV)

porewater

61.79 61.79 91.07 178.21 265.71 358.57 408.93 696.43 876.79 917.50 1080.71 1124.29 1185.36 917.50 1173.57 1173.57 1160.71 1138.93 1127.14 1156.43 1127.14 1040~00 1156.43 1173.57 1165.00 1160.71 1162.14 1138.93 1160.71 1098.21

Dialyser

(PM)

Si

samples

70 20 45 90 220 270 280 600 590 630 730 830 890 805 995 1460 1020 1040 1240 1080 1020 1010 1090 1040 1130 1080 1155 1110 1200 1090

AZ

WV

NH,

obtained

0.2 2.0 4.8 5.1 4.1 12.8 20.8 43.7 44.4 67.0 76.0 90.6 92.7 98.6 98.3 73.2 97.9 97.6 92.0 94.1 101.4 109.7 95.5 90.6 81.2 91.3 94.1 91.3 95.8 96.9

(PM)

PO,

6.4 7.7 53.7 28.1 11.5 5.2 3.8 5.2 6.4 7.7 5.2 5.2 5.2 5.2 6.4 6.4 6.4 5.2 5.2 6.4 6.4 6.4 6.4 6.4 6.4 5.2 5.2 7.7 7.7 6.4

@Ml

Fe

by dialysers

7.91 8.08 7.94 7.75 7.61 7.62 7.63 7.65 7.66 7.57 7.57 7.62 7.59 7.59 7.56 7.56 7.53 7.55 7.53 7.59 7.56 7.63 7.69 7.50 7.50 7.51 7.55 7.48 7.51 7.48

PH

Eh

+216 +227 +218 f219 + 176 + 162 + 167 + 171 + 123 +45 +26 + 12 +5 - 10 -4 -8 - 16 - 16 - 19 - 12 -23 - 23 -22 -26 ~ 27 -27 - 28 ~ 30 -- 28 ~ 30

(mv)

1316.07 1216.43 1233.21 1266.43 1415.89 1515.53 1382.68 1399.29 1482.50 1432.50 1440.90 1490.72

65.36 89.29 93.93 112.86 179.29 217.14 212.50 241.07 316.79 763.57 926.43 1128.93 1240.71 1517.86 1142.86 1402.14 1354.64

Dialyser

Si WC

14 28 70 57 130 120 140 230 350 500 690 860 765 880 790 800 980 810 850 950 790 1150 1085 1080 1135 1125 1140 1185 1128 1203

Elm2

(;%j

1.6 1.6 1.6 1.6 8.0 10.7 10.7 14.5 18.3 18.3 44.7 80.3 80.3 89.6 93.9 80.3 81.3 97.0 94.9 97.0 96.0 96.0 94.9 95.4 95.9 93.8 94.9 93.9 95.9 94.9

(2)

7.5 9.0 8.2 7.5 31.5 46.6 51.6 20.8 6.1 7.0 6.8 7.5 6.8 6.8 6.8 6.8 7.5 7.5 6.8 6.8 6.8 5.4 6.8 6.8 5.4 5.4 6.8 6.1 8.2 6.1

(?)

366

M. VieZ et al.

TABLE 2. centrifugation

-% PH

(mV)

Level (cm)

o-2 2-4 4-6 6-8 8-10 lo-12 12-14 14-16 16-18 18-20 20-22 22-24 24-26 26-28 28-30

Chemical

Si WV

NH, (PM)

parameters

of

PO, WV

Fe @MI

5.1 16.4 33.0 60.6 79.1 83.4 92.9 96.0 93.9 82.3 84.4 84.4 78.0 80.3 72.0

6.4 11.6 7.7 5.2 7.7 7.7 6.4 6.4 6.4 6.4 6.4 5.2 5.2 5.2 7.7

porewater

samples

through

core

Core A, 7.88 7.73 7.53 7.47 7.45 7.44 7.44 7.43 7.35 7.44 7.34 7.37 7.95 7.47 7.33

97 -76 - 131 - 176 - 205 - 209 -219 -219 - 230 ~ 239 - 232 -240 -238 -226 -247

155.00 326.43 592.50 888.21 1058.93 1124.64 1242.86 1278.21 1248.93 1078.21 1349.29 1189.64 1166.07 1260.71 120750

110 210 330 570 750 935 1150 1220 1275 1060 1390 1200 1370 1350 1540

7.66 7.64 7.48 7.45 7.37 7.37 7.39 7.46 7.63 7.35 7.32 7.55 7.33 7.32 7.30

179 25 -4 -26 -50 -51 -48 -65 -74 ~ 209 -219 -219 -229 -229 -236

Core B, O-2 211 4-6 6-8 8-10 lo-12 12-14 14-16 16-18 18-20 20-22 22-24 24-26 2&28 28-30

obtained

7.87 7.57 7.60 7.52 7.45 7.43 7.46 7.35 7.33 7.32 7.46 7.28 7.26 7.24 7.23

56 - 129 - 176 -200 -302 -310 - 270 -204 ~ 177 - 146 - 152 -146 -146 -134 ~ 133

900.00 533.21 864.29 1053.57 1083.21 1231.07 1266.43 1278.21 1225.00 1207.50 1231.07 1213.21 1189.64 1160.00 1189.64

1280 370 640 765 950 1260 1390 1300 1275 1350 1400 1370 1370 1520 1530

300.36 391.43 719.64 976.79 1053.57 1225.00 1254.64 1260.71

425 240 440 790 980 1290 1330 1420

5.1 10.7 16.4 47.7 78.9 94.9 112.0 107.7

1272.50 1260.71 1266.43 1260.71 1278.21 1242.86

1560 1350 1470 1400 1400 1530

103.4 105.6 102.4 97.0 90.8 94.9

6.4 6.4 6.4 7.7 6.4 9.0 6.4 7.7 7.7 6.4 10.4 11.6 7.7 7.7 7.7

Core B, 47.7 29.9 53.7 76.1 84.4 92.9 99.2 97.0 94.9 92.9 97.0 88.6 82.3 76.1 76.5

7.7 7.7 7.7 9.0 11.6 7.7 7.7 10.4 9.0 7.7 9.0 9.0 10.4 9.0 9.0

7.65 7.40 7.34 7.43 7.36 7.35 7.37 7.34 7.21 7.14 7.31 7.31 7.14 7.10 7.03

115 53 -58 -81 - 96 ~ 88 -73 - 98 - 121 - 125 ~ 128 - 130 - 134 - 135 -133

225.71 367.86 705.00 870.36 1059.64 1224.29 1201.43 1242.86 1254.64 1248.93 1254.64 1278.21 1260.71 1201.43 1189.64

280 240 450 620 960 1090 1180 1190 1330 1390 1390 1500 1500 1530 1570

10.7 20.2 49.6 63.7 85.8 94.9 92.9 94.9 94.9 94.9 92.9 88.6 84.4 78.0 74.1

7.7 12.2 7.7 6.4 6.4 6.4 6.4 7.7 6.4 3.8 6.4 7.7 7.7 7.7

Activity-depth profiles of 137Cs, 134Cs and ‘lOPb of core A, were used in order to calculate modern sediment accumulation and mixing rate. Activities of 134Cs and 137Cs were determined by direct gamma counting using an intrinsic germanium detector coupled with a multichannel analyser. Alpha counting of ““PO was used for “‘Pb determinations by assuming secular equilibrium between the two isotopes. Polonium-210 was extracted from the sediment and plated following a modification of the method outlined by Wan et al. (1987). Results

and discussion

Comparison between sampling methods and data reliability Tables 1 and 2 show the full data-set, whereas Figure 2 illustrates only station A, B being analogous. From the porewater profiles in Figure 2, it can be seen that the two methods are comparable for all the nutrients. This is true for both the concentration gradients and for

Nutrient profiles in pore

water

367

the absolute quantities measured, although ammonia values are about 30% higher in the lower 10-15 cm of the cores. We would exclude lossof ammonia due to oxidation because we do not have evidence of this in elementsparticularly sensitive to oxidation, such asPO, and Fe. Furthermore, we do not find in the dialysers the increasein NO, that the oxidation of about 300 uM of ammonia would give. Therefore, we are prone to consider this discrepancy asdue to spatial heterogeneity. The better vertical definition obtained using the dialysers explains in part the difference in gradient variation under the interface. Comparability between Eh and pH profiles is limited. The general course of the profiles with depth appears comparable, while the absolute values for the cores are decidedly lower: they differ by 0.2 for pH and by 150-200 mV for E,,. The measurement of these parameters was conducted in air; the data seemto indicate that even in air exposure of a few minutes during the sampling can produce partial CO, degassingand oxidation. Also the Ca profiles compare acceptably well [Figure 2(c)]: the pore waters extracted from cores show Ca contents that are always higher by 5-6?/,, probably because of a dilution effect causedby the distilled water contained in the dialysis cells. It is more difficult to compare the Fe profiles [Figure 2(b)]. Concentrations are similar below the first 5-6 cm. Above this depth significant differences can be noted. The variation may be explained in two ways: (a) the presence,in the glove-box, of small quantities of oxygen (Carignan et al., 1985) sufficient to oxidize the Fe’+ ion, particularly sensitive in concentrations greater than 20 PM (R. Cranston, pers. comm.); (b) oxidation due to interface disturbance during the transport of the cores to the laboratory. Several factors can influence and causelocal variation in the composition of pore water [heterogeneous grain size, microenvironment formation due to the presenceof living or dead organisms and aquatic plant roots (Lord & Church, 1983), bioturbation and irrigation (Hammond et al., 1985: Ray & Aller, 1985)], and difficulties in samplemanipulation (Gaillard et al., 1989). For this reason it is important to assessthe reliability of the data, and whether they are useful in defining the general geochemicalcharacteristics of the investigated environment and evaluating benthic fluxes. The comparison between the results obtained from the four dialysers and cores pinpoints local variations; these variations, nevertheless, do not hinder geochemical interpretation of the basic processesrelative to the nutrient speciesthat occur in the area, since the trends, rather than the values of the single measurementsthemselves, are consistent. Carignan (1984) recommended that sampling from the dialysers must be carried out within 5 min of their exposure to the atmosphere. Although slightly longer, the comparison with core profiles shows that our sampling time did not permit a substantial sample alteration. We tested the effect of an exposure time of 15-20 min and noted somedrops in the concentrations of ammonia, DRP and silica (Figure 3). This could be due to partial oxidation and precipitation (for ammonia and phosphate) and/or to spatial heterogeneity. Characteristics

of some early diageneticprocesses

in the studied environment

Through observation of the coresbefore cutting, we identified a thin brown oxidized layer in the top 1 mm of the sediment, while we did not observe any evident sign of bioturbation. The grain-size composition (Table 3) shows a slight decreasein the percentage of silt upward, indicating a progressive decreasein time of the mean energy inside the lagoon. The surface layer of these sediments (upper 2 cm) has a high organic content (organic C = 3-4’<,,). The percentage of major oxides are homogeneously distributed along the examined core (Table 3).

368

M. Vie1 et al.

Activity-depth p.rofilesof “‘Pb and 137Cs of core A, (Figures 4 and 5) were usedin order to calculate modern sediment accumulation and mixing rates. The accumulation rate, from excess210Pbprofile below the mixed layer, was tested by the 137Cs profile, applying the formalism reported by Nittrouer et al. (1984) and DeMaster et al. (1985) on the depth of the 1963137Cs peak. We computed a sedimentation rate of 0.23 cm year-’ and a dry mass accumulation rate of 0.11 g cm 2year - ’ with a mixed surfrcial layer of 2-4 cm and no deep mixing.

(0) Cores

5 0

0

-5

-5

-10

-10

5

-15

-15

-20

-20 -25

-25 -30

. Ii

7.3 7.5

7.7

79

8.1

8.3

-30

t.

I

7.3 7.5

1

77

79

8.1 8

PH

5

5 0

0 0

. .

.

0 -5

. .

-10

.

. .

.

-15 -20

Stlvza (FM) Figure 2. (a) Comparison ofpH, E, and silica concentrations in pore waters from cores and dialysers: dialyser and core A, (3); dialyser and core A, (0). (b) Comparison of ammonia, phosphate and iron concentrations in pore waters from cores and dialysers: dialyser and core A, (0); dialyser and core AZ (0). (c) Comparison of calcium concentrations in pore waters from core (0) and dialyser (0) at station A.

Nutrient

profiles

360

in pore water

(b) Diolysers

Cores 5 0

B

-25

Ammonia (

I

80 . 0 . 0. 0 0 0 0

.

.

0

20

40

-10-15

60

-

-25

-

80

(cl

-25 6 Co

7 (mM)

-

-20

0 Fe (MM)

5

o

.

0 . 0 .

-25

( FM) 51

20

40

60

80

370

M. Vie1 et al.

5 0

Y

5?

a*.

(0

-2.

.

1

P

~~

.x -5 r 0

-10

5 & 0

-15

l

l .

0. -0 .t*

-5-

1.

l.

(bl

0 z

.

” .CI l .

-10 l.

-15

. . .

2 .

.’

- .

-

.l

.

.

.

.

-20

l.

-20

-

-25

-

:

-25

l

.. .

.

-30

0 -5 2 2 5 E 0

-10 -15 -20 -25

Figure 3. Nutrient concentrations (PM) in dialyser pore waters sampled immediately after dialyser retrieval (0) and 15 min later (0). (a) A mmonia, (b) phosphate, (c) silica.

Profile of 134Cs (Figure 5) wasused to evaluate a sediment mixing coefficient. The 134Cs is a result of fallout from the Chernobyl accident (April 1986). Core A, wassampledabout 6 months later (November 1986) and in this period we can estimate that only about 1 mm of sediment should have settled. Thus, it may be assumedthat 134Cs at greater depths than the water-sediment interface is due to physical reworking of particles by organisms,waves and currents. For a pulse input, the activity A of 134Cs may be approximated by (Crank, 1975) A = A, exp (- z2/4Dat)

(1)

where A, is the activity at .z= 0 (water-sediment interface) at t = 0 (time of the Chernobyl accident) and Da is the diffusion coefficient due to bioturbation. In our case,134Cs profile consistsof only two points becausethe elapsedtime from the accident is only 6 months and the activity is averaged on slicesof 2 cm (thickness of core sampling). However, equation (1) fitted to 134Cs profile gives D, = 1.8 cm’ year-’ = 5.8 E - 8 cmzsP1. The porewater profiles obtained for the different nutrients are generally similar to the ones in past studies on coastal sediments rich in organic matter (Sholkovitz, 1973; McCaffrey et al., 1980; Aller, 1980~; Elderfield et al., 1981; Nembrini et al., 1982; Fernex

Nutrient profiles in pore water

TABLE 3. Sediment characteristics: nitrogen; grain size

371

main geochemical

composition;

organic

carbon

and

Level (cm) O-2 4-6 8-10 12-14 18-20 26-28

3.49 3.44 3.51 3.62 3.24 4.18

14.57 15.56 14.88 15.03 15.38 15.27

54.61 54.18 52.80 53.47 51.48 52.85

0.73 1.23 1.18 1.41 1.25 0.80

3.95 4.15 3.67 3.54 3.23 3.31

11.16 10.49 13.00 14.02 14.16 14.15

Level (cm)

N tot 0
C org 0 II

C/N atoms

&2 2-4 C8 lo-12 18-20 28-30

0.590 0.306 0.240 0.134 0,142 0.147

3.570 2.294 1592 1.428 1.517 1.034

7.05 8.75 7.73 12.43 12.47 8.20

Level (cm)

Sand 00

Silt l’cl

Clay
G2 4-6 12-14 18-20 2628

0.0 0.0 0.0 0.0 0.0

25.0 26.0 35.0 35.0 47.0

75.0 74.0 65.0 65.0 53.0

et al., 1986; Gaillard et al.,

1.02 0.95 0.78 0.73 0.87 1.08

9.01 9.03 8.75 7.76 9.01 7.58

1.17 0.77 1.11 0.60 0.63 0.72

1986); and the trends are principally determined by the products of its decomposition. This decomposition causesthe releaseof phosphate and ammonia (van Eck & Smits, 1986), whereas silica is principally the product of the dissolution of diatom skeletons (McCaffrey et al., 1980; Aller, 1980~). The vertical gradients observed in the profiles are controlled by the production rate in combination with other processessuch asadsorption on particulates (Berner, 1974; Rosenfeld, 1979; Krom & Berner, 1980), consumption by living organisms (Aller, 1980a; Lord & Church, 1983) and differences in transport rates of the different ionic species(Klump & Martens, 1981). In our case,the strong gradient observed from 4 to 10 cm (Figure 2) and the low gradient from 0 to 4 cm presumably reflect the combined effects of molecular diffusion, particle mixing (caused by bioturbation, waves, current, tides) (Klump & Martens, 1981), irrigation and limited scavenging by Fe-Mn hydroxide in the first centimetre. Iron showsan evident spike in the first 34 cm of sediment [Figure 2 (b)]; this characteristic can be explained by the possible existence of a transition layer between the sulphide and oxide zonespossessingintermediate conditions that allow Fe*+ to be present in solution (Aller, 1980b; Balzer, 1982; Nembrini et al., 1982; van Eck & Smits, 1986) and/or taking into account the complexing capacity of dissolved organic matter with iron (Singer, 1977; Krom & Sholkovitz, 1978; Elderfield, 1981). Unfortunately we do not have data for the alkalinity, sulphide and sulphate, which would facilitate a better definition of the mineral phasesin equilibrium with these dissolved species.Nevertheless we can hypothesize, on

372

M. Vie2 et al.

Activity 04

(dpm

g-‘)

I.0

5

15

20 Figure

4. Total

(0)

and excess (0)

*‘Vb

activity-depth

profile

of core A,.

the basisof these considerations and the data of several authors, that hydroxides control the presence of dissolved Fe near the interface, whereas, deeper, the role is played by monosulphide (Aller, 1980b;Berner, 1981; Elderfield et al., 1981; Salomons & Forstner, 1984). Analysis of depth distribution the water-sediment interface

of chemical species and evaluation

offluxes at

Besidesa qualitative description of the processestaking place during the early diagenesis of the sediments, a more quantitative study can be conducted, using kinetic models (Berner, 1980). From the analysis and interpretation of the experimental distributions of the different chemical species(Figure 2), and from the characteristics of the sediment at the interface, it is possible to estimate diffusive fluxes of silica, ammonia and phosphorus towards the overlying water. The Si, NH, and PO, profiles show nearly constant concentrations in the first 4-5 cm which suggestthat mixing (bioturbation, wave and current) and/or irrigation hasoccurred; so in this layer molecular diffusion may be only a secondary factor. This is supported by the trend of the artificial and natural radionuclides. The models proposed for silica by Vanderborght et al. (1977) and for ammonia and phosphorus by Berner (1980), that take into account disturbances at the water-sediment interface in coastal areas,seemadequate to describe the present case.

Nutrient

profiles

in pore water

Actiwty 0 I

20 .1

.

373

( Bq. kg-’ ) 1

40 !

1

60 I 0

0 0

20; Figure

5. Activity-depth

profile

of ‘Ts

(0)

and “‘Cs

(0)

of core A,.

The models are based on several assumptions: (1) a one-dimensional depth model is adopted with the sediment-water interface as origin; (2) steady-state diagenesis is assumed; (3) to take into account the disturbances at the sediment-water interface in coastal areas, the sediment is considered a two-layer system, in which each layer is characterized by a constant mass transfer coefficient; (4) compaction and porosity gradients are taken into account setting @ = 082 in the upper layer and @ = 0.75 in the lower layer. The physical reworking of particles may be quantified by a diffusion coefficient Da; the low value of& (Da about 10 E-8 cm2 s-l, estimated as described in the previous section) is not sufficient to explain the trend of our depth profiles. Therefore, irrigation must be considered as the main vertical dispersion process in the pore waters. This process may be mathematically described, in the mass balance equation, either by a diffusion term with a suitable diffusion coefficient D, (Berner, 1980) or by a linear term Z (C-C,) proportional to the difference of concentration between the pore water at depth and the supernatant water (Hammond et al., 1985). In the mathematical model both terms are alternatively included; this allows us to compare the results obtained by the two schemes. The diffusion coefficient D, and the irrigation coefficient Z of the first (disturbed) layer are unknown and are considered as control parameters; their values are estimated by fitting the computed distribution to the experimental data. The diffusion coefficient D,.of the second (undisturbed) layer is the molecular diffusion coefficient D, of the species corrected by the assessed tortuosity @ of the sediment D, = D, = DO/@; o2 is set equal to the porosity @ times the ‘ formation resistivity factor ’ (McDuff & Ellis, 1979). For the nearshore sediment, Ullman and Aller (1982) proposed for 82 a value of W2, when the porosity is >0.7 (average porosity of the studied sediment=0.8). The molecular diffusion

M. VieZet al.

374

coefficient assumedfor silica is that proposed by Wollast and Garrels (1971): D,(Si)= 8.6 E- 6 cm2s-l. The molecular diffusion coefficients for NH, and PO, were taken from Li and Gregory (1974) and modified for temperature of the bottom water (m 12 “C) following the relation proposed by Krom and Berner (1981) and the interaction with cations present in solution (Klump & Martens, 1981): D,(NH,)= 14.2 E-6cm2 sl, D,(PO,)=7.4 E-6 cm2s-l. The two-layer system is shown in the following scheme: z=o

Layer 1

D, ‘Ds

andI=O

disturbed sediment

or

E G I2

D,=D,

andI>

(mixed or irrigated) Z=Z*

2 Layer 2

undisturbed sediment

D,=D,

(molecular diffusion) For the assumption described above, the steady-state mass balance equation for a dissolved component in the pore water is written as(Berner, 1980) $D(z);-wg-I(C-C,)+R=O

(2)

where C(z) is the concentration of the dissolved component in PM at depth z, the diffusion coefficient is given by D(z) = D, for the upper (disturbed) layer (O,z*) and D(z)= D, (z > z*) for the lower layer, the parameter w is the sedimentation rate, which in our caseis givenby7.29E-9cms-‘. The terms in equation (2) represent respectively diffusion, advection, irrigation and diagenetic reactions specific for each ionic species. The following boundary conditions are imposed to solve equation (2): (a) at z = 0, the concentration is equal to the concentration in the overlying water. In our case,we take for C,, the concentration measuredin the first dialyser cell (1 cm) outside the sediment, considering it asrepresentative of the ‘ diffusive boundary layer ’ (Boudreau & Guinasso, 1982; Klump & Martens, 1981; Santschi et al., 1983), whose thickness is unknown; (b) at z+ m, the concentration tends toward a finite value (C,%).In our case,this value is reached at about 1O-l 5 cm; (c) at the interface between the upper and lower layer (z = .a*), we assumecontinuity of both concentration and flux c(z* -) = c(z* +) J(z*)

= - Q1 D,

Nutrient

profiles

375

in pore water

The diagenetic reactions affecting the components Si, NH, and PO, are described by the following: (a) the net rate of dissolved silica production results from competition between dissolution of highly reactive opal (diatoms) and reprecipitation processes.It can be shown (Vanderborght et al., 1977) that the global rate of reaction can be simply described by the kinetic reaction R = k, CC,, - C>

(3)

where k, is an apparent kinetic constant, and C,, is the equilibrium concentration, reached at depth; (b) in the upper layer we have neglected the oxidation of NH, to nitrate becauseof the anoxic conditions of the sediment (Aller, 1980~). Therefore, only the processof decomposition of organic nitrogen to ammonia is considered. Biological consumption of ammonia is not considered, as is the case in most diagenetic models. Our opinion is that such a processcan be important only in environments like salt marshes,where macrophytes are abundant (Lord & Church, 1983), but is negligible in most coastal and marine systems. The model assumesthat decomposition occurs via first-order kinetics, with a constant decomposition rate k,. Thus, the concentration N(z) of non-diffusible, metabolizable organic nitrogen is given by (Berner, 1980) N(z) = No W

- (k,lw)zl

(4)

where N, is the concentration at the water-sediment interface. The model (Berner, 1980) considers NH, adsorption as a simple linear isotherm. Furthermore, precipitation of authigenic minerals can be ignored because it generally doesnot occur. Thus the contribution R is given by R=

F 4, N(z) 1+K

(5)

where F=p, (I-@)/@ (p, is the mean density of total sediment solids, p,=25 g cm-‘) is the concentration conversion factor and I( is the equilibrium adsorption constant (K= 1.6; Rosenfeld, 1979; Berner, 1980); (c) the assumptionsfor PO, are the sameused for NH,. In our case,we do not consider precipitation of phosphate to form authigenic minerals becausethere is no evidence of this processin our profiles [Figure 2(b)]. Strong adsorption of phosphate by ferric oxides and hydroxides near the interface is also not considered, even if linear adsorption is taken into account through K (K= 1.8; Krom & Berner, 1980,198l). We have to emphasizethat the assumptionsand simplifications made in modelling phosphate lead to lessreliable results. The fluxesJ, of dissolved speciesat the sediment-water interface may be evaluated by a modification of Fick’s first law (6) J, is expressed in mmoles m-2 day-‘. The diffusion coefficient D, = Da+ D, + D,, + D, (expressed in cm2s-i) integrates the action of the bioturbation (Ds), irrigation (DI), wave

376

M. Vie1 et al.

and current stirring (D,,) and molecular diffusion of the species(D,) when I is assumed equal to zero. The term Z (C, - C,), where C, is the averaged concentration in the upper layer, represents the averaged flux due to irrigation when D, is assumedequal to D,. In many studies, irrigation, bioturbation, wave and current stirring are ignored: D, is set equal to D,, the molecular diffusion coefficient, and Z is set equal to zero. This is more justifiable when dealing with deep-sea sediment, but, in the caseof biologically active near-shore sediment, this approximation can lead to an underestimation of the fluxes at the interface. The calculations were undertaken on the dialyser samples because the values are time-averaged and also because of the much better definition of the profiles at the water-sediment interface. A brief description of how the computation wasperformed follows. The values used for parameters D,, w, K, @and p, have been reported in the foregoing text. The depths z* of the interface between the two layers are estimated from the trend of the experimental profiles; we obtained z* = 5 cm for all the SiO, and NH, profiles, while we have different values of .a*, ranging between 4 and 6 cm, for PO,. The experimental evidence disagreeswith the expectation of the samez* for all speciesin the samedialyser; this can be explained by the slightly different reactivity of phosphate to irrigation processes.Moreover, it must be outlined that small variations in z* (within 1 cm) do not significantly affect the output of the model. The values C, and C,, are deduced directly from the experimental profiles. The only unknown parameters, whose values must be assigned,are the decomposition rates k, in equation (3), k, in equations (4) and (5) and k, in equations similar to equations (4) and (5) not illustrated, and the diffusion and/or irrigation coefficients D, and I respectively. These parameters are estimated by fitting the computed solutions of equation (2), together with the given boundary conditions, to the experimental profiles. As pointed out by Vanderborght et al. (1977) for dissolved silica and experienced by us for the three species,the distribution in the upper part of layer 2 is very sensitive to the value of k, which allows us to select its value. The rangesof values obtained for the three speciesare k,= 1.47-2.05 E-6 s-l; k,= 1.46 E-9 sc’; and k,=2.92-4.37 E-9 sc’. The values of parameters D, and I for the upper layer are obtained by one of the following two schemes:(i) assumeI=0 and estimate the best value of D,; (ii) assume D, = D, and estimate the best value of I. Some numerical results for different values of D, [scheme (i)] and different values of Z [scheme(ii)] are shown in Figures 6 and 7 respectively for the speciesSiO, (dialyser A,). These examples give an idea of the responseof the model to variations in D, and I. A comparison between the best results obtained from schemes(i) and (ii) is shown in Figure 8 for the speciesNH, (dialyser A,). The results of the computations performed on dialyser data are shown in Table 4; the values obtained for coefficients D, and Z and the evaluated fluxes at the two interfaces (water-sediment and layer l-layer 2) are also reported. Furthermore, the quantities E, and E, are given; they are the squareroot of the sumof squareerrors in the first and second layer respectively. These values give a measureof the error and help in the interpretation of the best fit. In Table 4 the results for NH, relative to the dialyser A, are missing becauseit is not possibleto explain, by meansof the mathematical model used, the peak of the profile in the upper layer.

Nutrient

profiles

in pore

water

377

pM.IO

pM.IO

25

pM.10 0

50

/ 30

Figure

6. Examples

of fitting

.

I=0

25

D,;25.

I

100

1O-4

’

with various

D,: silica in dialyser

A,.

The schemes (i) and (ii) are equivalent for silica. In fact, the computed depth distributions are in both cases a good approximation of the experimental ones and the values of the fluxes are comparable. Scheme (ii) shows that about 909,) of the total flux is due to the irrigation component. For NH, and PO, scheme (ii) produces fluxes which are about one-half of the fluxes obtained by scheme (i). For these two species it seems difficult to choose the most reliable scheme from the results of best fitting. The two schemes produce a different value of total fluxes when the diffusion (J,& and irrigation (Jir,) component are comparable. We have verified that scheme (ii) cannot be applied when both the kinetic constant k and the irrigation coefficient I are high, as for the profiles of PO, for dialysers A and BieZ. In this case, at the interface between layers 1 and 2, the concentration must have a very high gradient; to satisfy this boundary condition the term Z(C-C,), in contrast to the diffusion term, produces a sharp maximum and minimum in the first layer. Due to the limited time-span of our experiment it is important to carry out more studies on the sediment-water interface in this lagoon. A better understanding of its biological cycles is also needed, for the validation of the data and the model. Benthic chamber measurements coupled with the modelling of porewater profiles can help to verify the reliability of scheme (i) and scheme (ii). Nevertheless, experiments conducted recently in other lagoons of the PO Delta (Frascari et al., 1988; Barbanti et al., 1991), using benthic chambers, give fluxes that are comparable with the ones obtained from our calculations.

378

M. Vie1 et al.

25

i0

0

5

.

.

. . 25 -

.

I=10.10‘5 0, = D*

. .

30

Figure 7. Examples of fitting with various I: silica in dialyser A,.

pM.10

25

-

I=0 0,=3.10-5

.

.

. .

25

30

Figure 8. Comparison of the two schemes (see text) fitting ammonia data of dialyser 1 Conclusions Some aspects of early diagenesis of nutrients based on sediments were investigated in the Sacca di Scardovari environment.

and porewater

studies

Nutrient

profiles

in pore water

379

TABLE 4. Gradients and fluxes for silica, ammonia and phosphorus: D, = cumulative diffusion coefficient for layer 1 (cm* s ‘); D, = molecular diffusion coefficient for layer 2 (cm* ss’); I = irrigation coefficient (s I); 3,,,= diffusive fluxes (mM m-z day I); J,,, = irrigation fluxes (mM mm * day- ‘); 3,, = total fluxes (mM m * day ‘); E,-E, = best fit evaluation in the two layers Dialyser Scheme

D, I Layer J,,, 1 3,,< 3 tot Si

NH,

E,

PO,

Q Layer 3,,, 2 E?

ii

Scheme

4.84 E-06 5.00 E-05 0.4 7.8 8.2 20

1-O E-04 0 -

4.84 E-06 2.6 392

4-84 E-06 2.3 487

i

A, Scheme

Dialsyer ii

Scheme

i

B,-, Scheme

4.84 E-06 8.00 E-06 1.1 6.7 7.8 135

3.0 E-04 0 11.3 72

4.84 E-06 2.00 E-05 0.9 9.7 10.6 40

4-84 E-06 2.0 328

4.84 E-06 1.6 386

4.84 E-06 3.0 550

4.84 E-06 2.4 472

3.0 E-05 0

7.99 E-06 0.80 E-06 0.54 0.48 1.02 84

4.0 E-05 0 2.1 105

7.99 E-06 1 ,oo E-06 0.50 0.53 1.03 79

7.99 E-06 1.0 652

7.99 E-06 0.98 623

7.99 E-i)6 1.1 542

7.99 E-06 1.03 501

7.5 E-04 0 1.9 5

2.0 E-04 0

4.16 E-06 1 .OO E-05 0.03 0.08 0.11 1

7.0 E-04 0

4.16 E-06 0.17 70

4.16 E-06 0.13 50

4.16 E-06 0.11 56

4.16 E-06 0.17 35

8.2 53

1.9 62

D? Layer Jr,,, 2 E,

J-wr 1

Scheme

Dialyser

7.5 E-04 0 8.5 28

Layer rdlff * 3,,, 3 ml E,

D, I J,,, 3,,* 3 IO, El

i

A,

0.28 2

ii

4.0 6

The distributions obtained for the different ionic species are typical of anoxic sedimentary environments, rich in organic matter. The distributions show strong gradients for ammonia, silica and phosphate between the interface and the deeper zone. The distribution of soluble iron suggests the existence of a transit layer between an oxidizing layer close to the interface and a reducing zone where Fe is depleted in pore waters, probably controlled by precipitation of sulphide. The two tested sampling techniques (in situ sampling by dialysers and laboratory centrifugation of core slices) of pore water showed good comparability for the main nutrients except for NH, in the deepest part of the sediment, probably a result of spatial heterogeneity. Iron showed divergence near the water-sediment interface, probably due to alteration of core samples during transport and extraction. The two methods give results that are chemically comparable, but they have, in our opinion, a different meaning. In fact, pore water extracted by centrifuge furnishes an

M. Viel et al.

380

instantaneous datum, while that extracted from dialysers furnishes data that are averaged in time. It has also been confirmed that porewater subsampling from dialysers must be done immediately to prevent alteration of the dissolved concentrations. An evaluation of ammonia, silica and phosphate fluxes at the water-sediment interface has been conducted. A two-layer model is used to interpret the porewater profiles of nutrients. In the upper layer, the flux at the water-sediment interface is controlled by mechanisms such as molecular diffusion, irrigation and physical mixing; at the second interface, the flux is determined only by molecular diffusion. Considerations on 134Cs distribution in the sediment allow us to assumethat particle bioturbation, in this case,has only a secondary influence on the fluxes, and irrigation becomesthe prevalent factor. The contribution of irrigation to fluxes can be either included in a cumulative diffusion coefficient, or evaluated separately. The two schemescan give quite different values of total fluxes: for silica, where irrigation is the dominant factor in diffusion processes,the computed curves obtained by application of the two schemesare very comparable and the total fluxes are similar. In the case of phosphorus and ammonia, the contribution of molecular diffusion becomeshigher and the total fluxes obtained by application of the two schemesare significantly different. Unfortunately, from our experimental data it wasnot possibleto evaluate which schemedescribesthe real phenomenon. Acknowledgements Our thanks are due to Dr F. Frascari, Dr C. Peroni, Professor R. C. Aller and Professor D. E. Hammond for their critical review and improvements to the manuscript. We are grateful to L. Casoni for the drawings. Contribution no. 796 of Istituto per la Geologia Marina, CNR, Bologna, Italy. References AA.VV. 1978 11 problema dell’eurrofizzazione delle acque costiere dell’Emilia-Romagna: situazioni e prospettive. Dipartimento Ambiente Territorio Trasporti Regione Em&z-Romagna, Studi e Documentazioni 14, 182 pp. Aller, R. C. 1980a Diagenetic processes near the sediment-water interface of Long Island Sound. I. Decomposition and nutrient element geochemistry (S, N, I’). Advances in Geophysics 22,237-350. Aller, R. C. 19806 Diagenetic processes near the sediment-water interface of Long Island Sound. II. Fe and Mn. Advances in Geophysics22,351-415. Balzer, W. 1982 On the distribution of iron and manganese at the sediment-water interface: thermodynamic versus kinetic control. Geochimica et Cosmochimica Acta 46, 1153-l 161. Barbanti, A., Ceccherelli, V. U., Frascari, F., Rosso, G. & Reggiani, G. 1991 Nutrient regeneration processes in bottom sediments in a PO delta lagoon (Italy) and the role of bioturbation in determining the fluxes at the sediment-water interface (in press). Bender, M., Jahnke, R. &Weiss, R. 1989 Organic carbon oxidation and benthic nitrogen and silica dynamics in San Clemente Basin, a continental borderland site. Geochimica et Cosmochimica Acta 53,685-697. Berelson, W. M., Hammond, D. E. &Johnson, K. S. 1987 Benthic fluxes and the cyclingolbiogenic silica and carbon in two southern California borderland basins. Geochimica et Cosmochimica Acta 51,1345-1363. Berner, R. A. 1974 Kinetic models for the early diagenesis of nitrogen, sulfur, phosforus and silicon in anoxic marine sediments. In The Sea (Goldberg, E. D., ed.). Wiley, New York. Vol. 5,427-450. Berner, R. A. 1980 Early Diagenesis. A Theoretical Approach. Princeton University Press, N.J., 241 pp. Berner, R. A. 1981 A new geochemical classification of sedimentary environments. Journal of Sedimentar-v Petrolog-v

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ion diffusion,

sulfate

dell’Emilia-Romagna. e Documentaziom

35,

382

Mayer,

M.

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