SOURCES, FATE AND DISTRIBUTION OF ORGANIC MATTER ONTHE WESTERN ADRIATIC CONTINENTAL SHELF, ITALY

TOMMASO TESI1,*, STEFANO MISEROCCHI1, LEONARDO LANGONE1,LAURITA BONI2 and FRANCA GUERRINI2

1ISMAR-CNR, Sede di Bologna – Geologia Marina, via P. Gobetti 101 40129 Bologna, Italy2Laboratorio di Biologia e Fisiologia Algale, Sc. Ambientali, Università di Bologna,

via Sant’Alberto, 163 48100 Ravenna, Italy(*author for correspondence, e-mail: tommaso.tesi@bo.ismar.cnr.it; phone: +39-051-6398863;

fax: +39-051-6398940)

Abstract. In the framework of the EUROSTRATAFORM projects, a multidisciplinary research wasfocused on processes that involve transport and deposition of riverine material in the Adriatic Sea.The aim of our contribution was to increase a more complete understanding of organic matterdeposition on the Adriatic shelf, also taking into account the role of Apennine rivers beyond the Poinfluence. In order to characterize origin, fate and variability of sedimentary organic carbon weutilized elemental and stable carbon isotope data in surficial sediments along shallow cross-shelftransects on the western Adriatic shelf.

Keywords: organic matter, Adriatic Sea, Po and Apennine rivers, surface sediment, stable isotopes

1. Introduction

Burial of terrestrial and marine organic matter (OM) in marine sediments is animportant reservoir representing the predominant long-term sink in the globalbiogeochemical cycle of organic carbon (OC) (Berner, 1982, 1989). Deltas andcontinental shelves receive large inputs from both autochthonous and allochtho-nous sources and account for ∼90% OC burial in the ocean (Berner, 1982, 1989;Hedges & Keil, 1995). However, these same massive storage systems are oftensites of efficient remineralization (Aller & Blair, 2004). Therefore despite theirgeochemical implications, the mechanisms controlling OM burial are not yetentirely understood (Gordon, Goñi, Roberts, Kineke, & Allison, 2001).

2. Background

The Adriatic epicontinental shelf exhibits an extensive and continuous mud-dominated regressive wedge formed after the attainment of the modern sea levelhigh-stand under the combined influence of fluvial supply and marine processes(Cattaneo, Correggiari, Langone, & Trincardi, 2003; Correggiari, Trincardi,Langone, & Roveri, 2001). An important feature of the Adriatic basin is that itsclastic sources are mainly located along the western side. The Po (Figure 1),

Water, Air, and Soil Pollution: Focus (2006) 6: 593–603DOI: 10.1007/s11267-006-9044-3 © Springer Science + Business Media B.V. 2006

which is the main river, is characterized by two annual floods associated withincreased rainfall in autumn and snowmelt in spring. Along the eastern Italiancoast an array of Apennine rivers (Figure 1) is characterized by high sedimentyield and typical torrential regime (Cattaneo et al., 2003). The circulation ischaracterized by cyclonic movement driven by buoyancy effects and windpatterns (Malanotte Rizzoli & Bergamasco, 1983). During the winter the cycloniccurrents steer Italian river plumes southward (Poulain, 2001). Fluvial sediments,initially deposited on the prodeltas, are resuspended and efficiently transportedalong the shelf following the general current circulation.

3. Materials and Methods

Sampling sites were located along shore-normal transects across the inner westernAdriatic shelf. Surface sediments were collected at 5–74 m water depths utilizinga large diameter box corer. Box-cores were subsampled and only the uppermost

Figure 1. Map of study area in the Adriatic Sea. Each box core station is indicated by a blackcrosshair. Rivers and thickness of the Late Holocene mud-wedge (modified from Correggiari et al.,2001) are showed.

594 T. TESI ET AL.

sediment slices (0–1 cm) are used in this study. Surficial sediments were collectedduring four cruises over one year (April and October 2002; February and May2003). All samples were refrigerated during transport back to the laboratory wherethey were oven-dried (55°C). A portion of the dried sediment was ground tocarry out elemental and isotopic analyses. In order to investigate the relativecontribution of the terrestrial and marine sources, we also collected surfacesediment in the lower part of the Po and Apennine rivers during July 2004. Aportion of the dried riverine sediment was ground to pass a mesh (250 μm) sievebefore elemental and isotopic analyses. Furthermore, two experiments with acommon diatom (Skeletonema costatum) were carried out to define the isotopicfractionation during the photosynthesis. Algae were maintained in 18 dilute batchcultures (2.5 l): nine bottles contained oligotrophic water from the Adriatic Seaand the remaining nine bottles contained the same volume with 50% of eutrophicwater from the Po River and 50% from the Adriatic Sea. The bottles were filledand closed without headspace. The Po and Adriatic water was filtered, with amethod designed to protect the isotopic composition of the dissolved CO2.Therefore the algae were grown in estuarine and marine conditions at differentsalinity, CO2 isotopic composition and nutrient concentrations. The algae wereinoculated at low concentration (∼100 cells ml−1). After two days, we filteredthree bottles of both marine and estuarine water on precombusted (4 h, 500°C)GF/F glass fiber filters for elemental and isotopic analyses. The operation wasrepeated after 24 and 48 h. OC and total nitrogen (TN) contents were measured insilver capsules using a FISONS NA2000 Element Analyzer after removal of thecarbonate fraction by dissolution in 1.5 N HCl. The average standard deviation ofeach measurement, calculated by replicate analyses of the same sample, was±0.07% for OC and ±0.009% for TN. Stable isotopic analyses of OC were carriedout on the same samples by using a FINNIGAN Delta Plus mass spectrometer,that was directly coupled to the FISONS NA2000 EA. The IAEA standardNBS19 was used as calibration material for carbon. Uncertainties were lower than±0.2‰ as determined from routine replicate measurements of the IAEA referencesample CH-7.

4. Results and Discussion

The OC contents (Figure 2) in the Po prodelta area showed a wide range from0.41% to 2.25%. South of 44.6°N, the OC content along shelf decreaseddrastically showing a narrower range between 0.50% and 0.98%. Mean valuesand standard deviations of OC for each cruise did not show significant differences(Table I), implying low seasonal variability that allowed combination of allelemental data in a single contour map (Figure 3a). In the Po prodelta, the highprimary productivity (up to 10-fold the production calculated in southern portion,Giordani, Helder, Koning, Miserocchi, Danovaro & Malaguti, 2002) and the

595SOURCES, FATE AND DISTRIBUTION OF ORGANIC MATTER

considerable amount of OC delivered by the Po River (Pettine, Patrolecco,Camuso, & Crescenzio, 1998), directly affect the surface sediments off the Pomouth (Figure 3b). OC contents higher than 1% compose a concentrateddepocenter developing around the delta, probably as a result of high and rapidsediment accumulation enhanced by large rapidly sinking flocs (Fox, Hill,Milligan, & Boldrin, 2004). Furthermore, four isolated smaller depocenters, justin front of each distributary, underline the key role of the river load on surfacesediments. Going southward, the region between 44 and 44.5°N seems to be stillunder the influence of the Po River. Beyond this region, on the shelf, OC valuesincrease offshore. This trend can be explained by material hydraulically sorted ingrain size because of water dynamics, in accordance with the classic modernsedimentation pattern on continental shelves: coastal sands, mud and relict sandfarther offshore (Brambati, Ciabatti, Fanzutti, Marabini, & Marocco, 1983). Incontrast, in the Po region the rapid sinking of flocs prevents grain-size sorting.The deposition of fine sediment, which generally has elevated levels of associatedOM relative to their coarse counterparts (Hedges & Keil, 1995), deeply influencesthe OC distribution in surface sediments. Indeed, using the map of grain-sizesurficial distribution of Brambati et al. (1983), it was possible to cluster samplesin three broad classes characterized by different ranges of mud and OC contents.The positive correlation shown in Figure 3c confirms the preferential associationof the organic material with the finest sediment fraction. Geochemical–isotopicinvestigations in the Adriatic have historically employed a two-end-membermixing model between marine phytoplankton and isotopically depleted vascularplants to quantify the terrigenous component of OM in marine sediments(Faganeli, Malei, Pezdič, & Malačič, 1988; Martinotti, Camusso, Guzzi,Patrolecco, & Pettine, 1997). Based on atomic ratio TN and OC (N:C) and

Figure 2. Organic carbon content of surface sediments along the western Adriatic shelf. Each cruiseis indicated by different symbols.

596 T. TESI ET AL.

TABLEI

Sum

marystatisticsof

OC

contentandisotop

iccompo

sitio

n,andC:N

atom

icratio

from

superficial

sediments

collected

inthewestern

Adriatic

shelf

Date

OC(%

)δ1

3OC(‰

)C:N

Num

ber

Mean

Min

Max

Mean

Min

Max

Mean

Min

Max

April20

020.83

±0.24

0.19

1.52

−24.05

±0.59

−25.54

−23.08

8.68

±0.87

7.08

10.61

87Octob

er20

020.83

±0.31

0.14

1.64

−23.92

±1.01

−26.09

−22.71

8.30

±0.69

5.64

9.65

62February

2003

0.92

±0.41

0.41

2.25

−24.89

±0.84

−26.38

−23.63

9.23

±1.04

7.96

12.52

35May

2003

0.90

±0.30

0.37

1.62

−24.83

±1.02

−26.85

−23.10

9.12

±0.79

7.53

11.02

92Average

0.86

±0.30

−24.39

±0.97

8.80

±0.89

276

597SOURCES, FATE AND DISTRIBUTION OF ORGANIC MATTER

δ13OC values, Boldrin, Langone, Miserocchi, Turchetto, and Acri (2005) andMiserocchi, Langone & Tesi (in press) have identified the marine–estuarine algaeas an additional important source of OC. In addition, in the Adriatic basin, ancientsedimentary OC represents a significant component of the terrestrial fractionsupplied by many rivers (Ogrinc, Fontolan, Faganeli, & Covelli, 2005). Therefore,interpretation of the isotopic data and the reliability of a mixing-model depend onthe choice of the end-members, which are usually defined according to sampledistribution. To this target, surface sediment samples were collected also in therivers and two experiments were carried out with different starting isotopiccomposition of dissolved CO2 in cultures of S. costatum. Utilizing the data set ofPOC from the Po prodelta (Boldrin et al., 2005) and western Adriatic shelf(unpublished data), it has been possible to obtain a plot pointing out keyindications of the origin and fate of OC (Figure 4); it was preferred to plot the N:Cratio to the C:N ratio because the first parameter behaves linearly in a mixingmodel (Goñi, Teixeir, & Perkley, 2003), even if the classical C:N ratio is betterknown. In the first place, the compositional variability among the rivers is reducedcompared to algae; the same species of diatom, grown in identical nutrient and

Figure 3. (a) OC distribution in surficial sediments along the western Adriatic shelf; (b) the Poprodelta is shown in detail; (c) relationship between contents of OC and mud.

598 T. TESI ET AL.

isotopic conditions, showed a wider variability in both δ13OC and N:C values.The distribution of filtered suspended material, closer to algal values, shows asimilar scattered distribution whereas the Adriatic surface sediments are moreconsistent. Marine–estuarine algae can exhibit a rather broad range of stable

Figure 5. Relationship between OC and TN for surficial sediments for the Po prodelta andApennine shelf area. The figure includes the dashed and solid linear trend lines for the Po prodeltaand the Adriatic shelf, respectively.

Figure 4. Plot of the carbon stable isotopic composition and nitrogen:carbon atomic ratio. The opensymbols indicate three different sources: culture of marine and estuarine diatoms and riverinesediments. Enclosed circles indicate suspended particulate from the column and enclosed squaresindicate surficial sediments, dark gray for the Po prodelta and gray for the western Adriatic shelf.

599SOURCES, FATE AND DISTRIBUTION OF ORGANIC MATTER

carbon isotopic and N:C ratios that are related to the concentrations of nutrientsand sources of the DIC (Fry & Sherr, 1984; Schneider, Schlitzer, Fischer, &Nöthig, 2003). Many authors explain the differences from the classical Redfieldratio in two ways: deviations during particle production and changes duringsubsequent remineralization processes. During particle production, variations inthe elemental compositions may be related to ambient CO2 concentrations and/ornutrient availability. Changes of elemental ratios during OM degradation aredescribed as preferential remineralization of N with respect to C, or as existenceof micro organisms, that could be responsible for the presence of nitrogen-richbiochemicals (i.e., proteins) (Goñi et al., 2003; Schneider et al., 2003). Thevariability of isotopic composition in the particulate matter depends on thespecies, growth rate and the shape of the algae, but the main factor is the isotopiccomposition of DIC (Fry & Sherr, 1984; Popp, Laws, Bidigare, Dore, Hanson, &Wakeham, 1998). For this reason the algae, which reside in environments wherethe isotopic DIC signature is variable, exhibit a wide range (from −22 to −28‰,Goñi et al., 2003). A strong, linear relationship was detected between the OC andTN contents in all surficial sediments (r2 = 0.94 and 0.90 in the Apennine shelfand Po prodelta, respectively; Figure 5). The x-intercept of these regressions areclose to zero, which indicates that the majority of nitrogen in these sediments isassociated with OM (Goñi et al., 2003). The lower slope gradient (lower N:Cvalues or higher C:N values) in the Po prodelta samples might indicate the major

Figure 6. The whisker-plots show the inner OC isotopic variability for different data set. The lowervalue (−27.1‰, 10th percentile) and the higher value (−20.4‰, 90th percentile) indicate theterrestrial and marine end-member, respectively.

600 T. TESI ET AL.

presence of woody debris (N:C < 0.071, C:N > 14; Goñi et al., 2003) compared tothe Apennine sediments. Sand-sized plant fragments, rich in OC, hydraulicallybehave like very fine-sand (Leithold & Hope, 1999), and this might explain thefour depocenters in front the main mouths of the Po. Indeed, the terrestrial end-member is generally considered as a single value, although in some cases thiscommon simplification leads to underestimate the contribution of terrigenous OM,which in coastal sediments is complicated by the heterogeneous composition ofthe terrigenous end-member itself (Gordon & Goñi, 2003). Finally, the globalburial flux of OC within modern marine sediments is estimated at 0.1–0.2 Gtyear−1 (Hedges & Keil, 1995) while the marine plankton photosynthesizes about50 Gt year−1 of OC. Based on this mass balance, the fresh marine particulate isefficiently recycled at least to 99.5%, throughout the ocean water column and thewater/sediment interface (Hedges, Keil, & Benner, 1997; Mayer, 1994). Anefficient algal degradation, in our case, could easily explain the decouplingbetween suspended POC and OC in surficial sediments, which are clearly moreterrestrial (Figure 3). The ‘refractory’ organic material delivered by the rivers,being partially degraded, especially in the soil fraction, is more efficientlyaccumulated and preserved on the surficial sediment than ‘labile’ algalcompounds. Considering both the influence of estuarine algae limited to theprodelta Po area and this highly efficient algal degradation, we propose for therest of the Adriatic a two end-member mixing model between ‘heavier’ marinephytoplankton (−20.4‰, 90th percentile) and isotopically depleted vascular plant(−27.1‰, 10th percentile) (Figure 6).

5. Conclusions

In the Po prodelta, the content and distribution patterns of OC are principallyrelated to the high in situ primary production and the high discharge of the Po,whereas the dispersion of fine material drives the surficial OC distribution on thewestern Adriatic continental shelf. Comparison between isotopic signature fromsedimentary organic carbon and from filtered suspended material indicates anefficient degradation of the algal particulate matter. The ‘lighter’ terrigenous OC,especially in the Po prodelta, is preferentially accumulated and preserved on thesurficial sediment relative to the ‘labile’ algal material.

Acknowledgments

This work was supported by the ONR–EUROSTRATAFORM project, grant no.N00014-03-01-0154 and by EU–EUROSTRATAFORM project, contract EVK3-CT-2002-00079. The authors would like to thank P. Giordani and G. Rovatti. Thisis contribution no. 1505 of ISMAR-CNR, Sede di Bologna – Geologia Marina.

601SOURCES, FATE AND DISTRIBUTION OF ORGANIC MATTER

References

Aller, R. C., & Blair, N. E. (2004). Early diagenetic remineralization of sedimentary organic C in theGulf of Papua deltaic complex (Papua New Guinea): Net loss of terrestrial C and diageneticfractionation of C isotopes. Geochimica et Cosmochimica Acta, 68, 1815–1825.

Berner, R. A. (1982). Burial of organic carbon and pyrite sulfur in the modern ocean: Its

geochemical and environmental significance. American Journal of Science, 282, 451–473.Berner, R. A. (1989). Biogeochemical cycles of carbon and sulfur and their effect on

atmospheric oxygen over phanerozoic time. Palaeogeography, Palaeoclimatology, Palaeoe-cology, 75, 97–122.

Boldrin, A., Langone, L., Miserocchi S., Turchetto M., & Acri A. (2005). Po River plume onthe Adriatic continental shelf: Observations on dispersion and sedimentation dynamics ofdissolved and suspended matter during different river discharge rates. Marine Geology,222–223, 135–158.

Brambati, A., Ciabatti, M., Fanzutti, G. P., Marabini, F., & Marocco, R. (1983). A newsedimentological textural map of the northern and central Adriatic Sea. Bollettino diOceanografia Teorica ed Applicata, I, 267–271.

Cattaneo, A., Correggiari, A., Langone, L., & Trincardi, F. (2003). The late-HoloceneGargano subaqueous delta, Adriatic shelf: Sediment pathways and supply fluctuations.Marine Geology, 193/1–2, 61–91.

Correggiari, A., Trincardi, F., Langone, L., & Roveri, M. (2001). Styles of failure in lateHolocene highstand prodelta wedges on the Adriatic shelf. Journal of Sedimentary Research,Part B, 71, 218–236.

Faganeli, J., Malei, J., Pezdic, J., & Malacic, V. (1988). C:N:P ratios and stable C isotopicratios as indicators of sources of organic matter in the Gulf of Trieste (northern Adriatic).Oceanologica Acta, 11, 377–382.

Fox, J. M., Hill, P. S., Milligan T. G., & Boldrin, A. (2004). Flocculation and sedimentationon the Po River delta. Marine Geology, 203, 95–107.

Fry, B., & Sherr, E. B. (1984). δ13C measurements as indicators of carbon flow in marine andfreshwater ecosystems. Marine Science, 27, 13–47.

Giordani, P., Helder, W., Koning, E., Miserocchi, S., Danovaro, R., & Malaguti A. (2002).Gradients of pelagic–benthic coupling and carbon budgets in the Adriatic and northernIonian Sea. Journal of Marine Systems, 33–34, 365–387.

Goni, M. A., Teixeir, M. J., & Perkley D. W. (2003). Sources and distribution of organicmatter in a river-dominated estuary (Winyah Bay, SC, USA). Estuarine, Coastal and ShelfScience, 57, 1023–1048.

Gordon, E. S., & Goni, M. A. (2003). Sources and distribution of terrigenous organic matterdelivered by the Atchafalaya River to sediments in the northern Gulf of Mexico.Geochimica et Cosmochimica Acta, 67, 2359–2375.

Gordon E. S., Goni M. A., Roberts Q. N., Kineke G. C., & Allison M. A. (2001). Organicmatter distribution and accumulation on the inner Louisiana shelf west of the AtchafalayaRiver. Continental Shelf Research, 21, 1691–1721.

Hedges, J. I., & Keil, R. G. (1995). Sedimentary organic matter preservation: An assessment

and speculative synthesis. Marine Chemistry, 49, 81–115.Hedges, J. I., Keil, R. G., & Benner, R. (1997). What happens to terrestrial organic matter in

the ocean? Organic Geochemistry, 27, 195–212.Leithold, E. L., & Hope, R. S. (1999). Deposition and modification of a flood layer on the

northern California shelf: Lessons from and about the fate of terrestrial particulate organiccarbon. Marine Geology, 154, 183–195.

602 T. TESI ET AL.

Malanotte Rizzoli, P., & Bergamasco, A. (1983). The dynamics of the coastal region of the

northern Adriatic Sea. Journal of Physical Oceanography, 13, 1105–1130.Martinotti, W., Camusso, M., Guzzi, L., Patrolecco, L., & Pettine, M. (1997). C, N and their

stable isotopes in suspended and sedimented matter from the Po estuary (Italy). Water, Airand Soil Pollution, 99, 325–332.

Mayer, P. A. (1994). Preservation of elemental and isotopic source identification ofsedimentary organic matter. Chemical Geology, 144, 289–302.

Miserocchi, S., Langone, L., & Tesi, T. (in press). Content and isotopic composition of

organic carbon within a flood layer in the Po River prodelta (Adriatic Sea). ContinentalShelf Research.

Ogrinc, N., Fontolan, G., Faganeli, J., Covelli S. (2005). Carbon and nitrogen isotope

compositions of organic matter in coastal marine sediments (the Gulf of Trieste, NAdriatic Sea): Indicators of sources and preservation. Marine Chemistry, 95, 163–181.

Pettine, M., Patrolecco, L., Camuso, M., & Crescenzio, S. (1998). Transport of carbon and

nitrogen to the northern Adriatic Sea by the Po River. Estuarine, Coastal and Shelf Science,46, 127–142.

Popp, B. N., Laws E. A., Bidigare R. R., Dore J. E., Hanson K. L., & Wakeham S. G. (1998).Effect of phytoplankton cell geometry on carbon isotopic fractionation. Geochimica etCosmochimica Acta, 62, 69–77.

Poulain, P.-M. (2001). Adriatic Sea surface circulation as derived from drifter data between1990 and 1999. Journal of Marine Systems, 29, 3–32.

Schneider, B., Schlitzer, B., Fischer, G., & Nothig E. (2003). Depth-dependent elementalcompositions of particulate organic matter (POM) in the ocean. Global BiogeochemicalCycles, 17-2, 1–16.

603SOURCES, FATE AND DISTRIBUTION OF ORGANIC MATTER