8ο Πανελλήνιο Συμποσιο Ωκεανογραφίας & Αλιείας 843 BIOGENIC SILICA STORAGE IN THE DANUBE DELTA Massimo Presti 1, Panagiotis Michalopoulos, Eleni Kamperi, Christos Anagnostou Hellenic Center for Marine Research Athinon-Souniou Av., km 46.7, 19013 Anavyssos (Mavro Lithari), GREECE 1 e-mail address: massimo@ath.hcmr.gr ABSTRACT The Danube River has a significant influence on the sedimentation of the western Black Sea area. We estimated the long-term reactive silica storage capacity of Danube delta-front and prodelta areas (~7300 km 2 ) by using 210 Pb sedimentation rate measurements and operational leaches for the determination of biogenic silica. A two-step method for reactive silica determinations developed for deltaic environments was used in order to account for the formation of authigenic aluminosilicate coatings on diatom frustules. Average total reactive silica concentration from Danube subaqueous delta range from 130 µmol g -1 in the delta front area to 641 µmol g -1 in the offshore, prodelta area. The total storage of reactive Si is ~8.4*10 9 mol Si yr -1 (~57% of the average total riverine DSi + freshwater BSi inputs). The sediment-fed delta-front area (~1300 km 2 ) stores about 47% of the total buried Si. Approximately 39% of total reactive silica stored in the delta-front is affected by early diagenetic reactions. συσσωρευση βιογενουσ πυριτιου στο δελτα του ποταμου δουναβη περιληψη Ο ποταμός Δούναβης είναι η σημαντικότερη πηγή ιζήματων και διαλυμένων θρεπτικών αλάτων της Δυτικής Μαύρης Θάλασσας. Στην εργασία αυτή εξετάζεται η μακροχρόνια αποθήκευση δραστικού πυριτίου στα ιζήματα του υποθαλάσσιου Δέλτα του Δούναβη που περιλαμβάνει το κύριο τμήμα του δέλτα καθώς και την περιοχή του προδέλτα (7300m 2 ) μετά τον υπολογισμό του ρυθμού απόθεσης ιζημάτων με μετρήσεις 210 Pb και μέτρηση βιογενούς πυριτίου με εργαστηριακές μεθόδους. Στα πλαίσια αυτής της έρευνας χρησιμοποιήθηκε μέθοδος μέτρησης του δραστικού πυριτίου που έχει αναπτυχθεί για δελταϊκά περιβάλλοντα, αποτελείται από δύο στάδια ανάλυσης και προσμετρά τη επίδραση που έχει ο σχηματισμός διαγενετικών αργυλοπυριτικών φάσεων πάνω σε σωματίδια διατόμων μετά των απόθεση των σωματιδίων αυτών σε δελταϊκά ιζήματα. Η μέση τιμή ολικού δραστικού πυριτίου στο υποθαλάσσιο δελταϊκό σύστημα κυμαίνεται από 130 μmol g -1 στο κυρίως δέλτα μέχρι 641 μmol g -1 σε περιοχές του προδέλτα. Η συνολική κατακράτηση δραστικού πυριτίου στο δελταϊκό σύστημα είναι ~8.4*10 9 mol Si yr -1 (περίπου το 57% του συνολικού πυριτίου, διαλυμένου και βιογενούς σωματιδιακού πυριτίου που εκχύει ο ποταμός Δούναβης). Στην περιοχή κύριας απόθεσης ιζημάτων του δέλτα (1300 Km 2 ) αποθηκεύεται το 47% του δραστικού βιογενούς πυριτίου που κατακρατάται συνολικά στο υποθαλάσσιο δέλτα του Δούναβη. Το 39% του δραστικού πυριτίου που βρίσκεται στην περιοχή κύριας απόθεσης ιζημάτων συμμετέχει σε αντιδράσεις σχηματισμού διαγενετικών αργιλοπυριτικών φάσεων. Keywords: Danube; western Black Sea; Biogenic Silica; authigenic minerals.
844 8ο Πανελλήνιο Συμποσιο Ωκεανογραφίας & Αλιείας INTRODUCTION The water and nutrient discharge of the Danube River, the second largest in Europe, significantly affects the western Black Sea ecosystem. The concept that in tropical deltaic systems reactions occur that convert biogenic silica into newlyformed aluminosilicate phases (Michalopoulos and Aller, 1995; Michalopoulos et al., 2000), which result in increased capacity of these systems for silica storage, has been discussed recently (Michalopoulos and Aller, 2004). In this study of the the Danube subacqueous delta we estimate for the first time in a temperate system the storage of biogenic silica, including the influence of early diagenetic aluminosilicate formation. STUDY AREA The Danube river delivers on average ~210*10 9 m 3 water yr -1 and 40χ51*10 6 tons sediment yr -1 contributing 55% of the freshwater discharge to the Black Sea (Friedrich et al., 2002; Panin and Jipa, 2002). Over 90% of the waters cross the delta through the three main Danube branches. The main nutrient sources (N, P, Si) for the continental shelf production are the river and benthic fluxes (Ragueneau et al., 2002). The construction the Iron Gates dams (1972 and 1984) decreased the transport of sediments and dissolved silica (DSi), thus accelerating costal erosion (Humborg et al., 1997). However, N and P discharges have increased continuously since the early 60s leading to eutrophication the western Black Sea. The DSi concentration of the Danube outflow in the Black Sea is on average 50-75 µm, whereas the concentration of biogenic silica (BSi), delivered by the river mainly as diatom-derived detrital matter, is 5-18 µmol/l (Ragueneau et al., 2002). The DSi annually delivered by the river amounts to 2.8-3.2 Fig. 1. Study area and location of sampling sites. Delta front and prodelta morphological and depositional units are taken from Panin and Jipa (2002).
8ο Πανελλήνιο Συμποσιο Ωκεανογραφίας & Αλιείας 845 Fig. 2. Relative pools of reactive Si in cored deposits from the delta front to the prodelta sites. *10 5 t/yr (Humborg et al., 1997; Friedrich et al., 2002). The sediment-fed submersed part of the Danube delta can be divided into 2 depositional systems (Fig. 1; Panin & Jipa, 2002), the delta front (~1300 km 2 ) and the prodelta (~6000 km 2 ). Highest nutrient concentrations in bottom waters are found in these areas (Friedrich et al., 2002). METHODS Sediment cores were collected across the delta front, prodelta and continental shelf during the September 2002 research cruise of the R/V AEGAEON, in the frame of the EU danubs project (Fig. 1). Sediment accumulation rates were calculated using the 210 Pb method. 210 Pb was measured via its granddaughter 210 Po, after total dissolution of the dry sediment sample and total alpha counting of the deposited 210 Po on silver discs. Sediment subsamples (0.5-2 cm intervals) were packed into polypropylene bottles. Porewaters were separated using centrifugation and filtration and analysed for their Si content. The resulting sediment plug was frozen and stored. The biogenic silica content has been determined on three sediment cores (MT1, NT1 and NT3) located on the delta front and two (CT4 and ST4) in the prodelta areas (Fig. 1) by using a two-step operational leaching method, which includes acid and alkaline leaches (Michalopoulos & Aller, 2004). The first step of the protocol is an acid-leach, which allowes to target authigenic silicates formed by conversion of opaline silica during early phases of diagenesis, without affecting well-crystallized clays, and exposes fresh silica surfaces for the subsequent alkaline leach. For comparison, we used as well the standard operational leach protocol, which measures fresh biogenic silica in a predominantly detrital matrix (DeMaster et al., 1983). The operational pools are defined as: Si-HCl (mild acid-leachable), Si-Alk (mild alkaline-leachable following acid pretreatment), ΣSi (total reactive Si = Si-HCl + Si-Alk) and BSi (mild alkalineleachable, no acid pretreatment). Microscopic
846 8ο Πανελλήνιο Συμποσιο Ωκεανογραφίας & Αλιείας Fig. 3. A) SEM micrograph of a diatom frustule buried in the delta front station MT1, (depth 1-2cm). The frustule is covered by authigenic aluminosilicate material precipitated during early diagenesis. B) SEM backscatter image of a buried diatom frustule at prodelta station ST4 (depth 15-17cm). The frustule is relatively fresh and shows signs of alteration (K, Mg, Al enrichments data not shown here) in the bright areas of the image. examination of diatoms and particles of interest was performed on sediment samples selected on the basis of leaching results. Samples were thawed, rinsed briefly with distilled-deionized water, centrifuged and the supernatant decanted. The centrifuged sediment plug was resuspended in DI water and wet sieved through a 38-µm sieve. Diatoms in the fine fraction (38-5 µm) were separated from <5 µm particles through several repeated settling and centrifuging steps. Storage, microscopic observation and handpicking of the separated diatoms were done while the specimens were immersed in DI water. Observations of particles were carried out with a Philips XL scanning electron microscope (SEM). An EDAX micro-analyser was also used for qualitative EDS and semiquantitative chemical analyses. RESULTS Sediment accumulation rates, estimated by using the 210 Pb method, range from 2.1±0.48 cm y -1 for station MT1, directly in front of the S. Gheorge branch, to 0.088±0.015 cm y -1 for station ST4, in the prodelta area (Fig. 1). The latter value is consistent with previous estimates for the same zone (0.11 cm y -1 ; Wijsman et al., 2002). An increase in the magnitude of downcore averaged Si content can be observed from station NT1 to station ST4, with differences in the relative importance of the operational Si pools (Fig. 2). While the Si-HCl pool does not increase significantly, the average Si-Alk (and thus total reactive silica, ΣSi) is far higher at the distal site (ST4, 641 µmol g -1 ) compared to the river mouth sites (e.g. NT1, 130 µmol g - 1 ). Similarly to the Si-Alk pool, BSi increases progressively away from the sites proximal to the river branches. The difference between the Si-Alk and BSi pools instead decreases along the same pattern. The concentration of the acidleachable Si pool does not show significant down-core variations at any site, whereas a net increase of ΣSi and BSi contents characterises the uppermost 5-10 cm of cores ST4 and CT4 (ΣSi up to 7 wt.% and 6.1 wt.% respectively, data not shown here). Relatively low terminal pore-water concentrations of Si(OH) 4 (~600 µm) are found down-core in the delta front sites, whereas at stations ST4 and CT4 downcore values increase up to 1000 µm (profiles not shown). The higher pore-water Si(OH) 4 concentration found at the prodelta sites is consistent with less terrigenous content and higher BSi
8ο Πανελλήνιο Συμποσιο Ωκεανογραφίας & Αλιείας 847 concentrations, resulting in less interaction of BSi with reactive detrital aluminosilicate components in the sediments. DISCUSSION We have used the 210Pb accumulation rate and the average down-core concentrations of total reactive Si in the Danube delta sites, in order to estimate net Si storage in the delta front and prodelta areas (~7300 km2). The ΣSi is assumed to represent the total biogenic SiO2, whilst the Si-HCl pool the labile authigenic coatings. Previous studies on the Amazon (Michalopoulos &Aller, 2004) and the Mississippi deltas (data not shown here) showed that even with the 2-step modified operational leach, converted frustules are not completely dissolved. Therefore, we can assume that our estimated net Si storage is still a conservative measure. The total accumulation of reactive Si is ~8.4*109 mol Si yr-1 (~57% of the average total riverine DSi + freshwater BSi inputs). Although the delta-front is much smaller than the prodelta (1300 km2 vs. 6000 km2), this sediment-fed area stores about 47% of the total buried Si. By comparing the two-step alkaline leach results with the one-step alkaline measurements we estimate that approximately 39% of total reactive silica stored in the deltafront is affected by early diagenetic reactions (i.e. precipitation of authigenic aluminosilicate coatings on diatom frustules). In the prodelta area, where the reactive/biogenic silica fraction is higher but there is less detrital material delivered, post-burial transformation reactions affect about 27% of the total stored Si. We suggest that the availability of reactive detrital material in the suspended riverine terrigenous debris deposited in the different parts of the subaqueous delta is the factor limiting formation of authigenic coatings in Danube delta sediments. The decreased importance of the Si-HCl pool and the decreased difference in the Si-Alk and BSi operational pools from the nearshore to the distal prodelta sites is consistent with fewer authigenic aluminosilicate phases covering frustules. Microscopic observations of selected diatoms material revealed that heavily coated frustules are found at the near-shore areas, whilst the surface of diatom frustules buried in prodelta sites is relatively fresh (Fig. 3). Acknowledgements The data presented here were collected as a part of the EU-funded project danubs: Nutrient management in the Danube basin and its impact on the Black Sea (contract evk1-ct-2000-00051). M.P. acknowledges the support of the EU-funded Si-WEBS network. We wish to thank G. Kampouri for her help in the laboratory. REFERENCES DeMaster, D.J., Knapp, G.B. and Nittrouer, C.A., 1983. Biological uptake and accumulation of silica on the Amazon continental shelf. Geochimica et Cosmochimica Acta, 47(10): 1713-1723. Friedrich, J. et al., 2002. Benthic Nutrient Cycling and Diagenetic Pathways in the Northwestern Black Sea. Estuarine, Coastal and Shelf Science, 54(3): 369-383. Humborg, C., Ittekkot, V., Coclasu, A. and Bodungen, B.V., 1997. Effect of Danube River dam on Black Sea biogeochemistry and ecosystem structure. Nature, 386(6623): 385-388. Michalopoulos, P. and Aller, R.C., 1995. Rapid clay mineral formation in Amazon delta sediments: reserve weathering and oceanic elemental cycles. Science, 270(5236): 614-617. Michalopoulos, P. and Aller, R.C., 2004. Early diagenesis of biogenic silica in the Amazon delta: alteration, authigenic clay formation, and storage. Geochimica et Cosmochimica Acta, 68(5): 1061-1085. Michalopoulos, P., Aller, R.C. and Reeder, R.J., 2000. Conversion of diatoms to clays during
848 8ο Πανελλήνιο Συμποσιο Ωκεανογραφίας & Αλιείας early diagenesis in tropical, continental shelf muds. Geology, 28(12): 1095-1098. Panin, N. and Jipa, D., 2002. Danube River Sediment Input and its Interaction with the North-western Black Sea. Estuarine, Coastal and Shelf Science, 54(3): 551-562. Ragueneau, O. et al., 2002. Biogeochemical Transformations of Inorganic Nutrients in the Mixing Zone between the Danube River and the North-western Black Sea. Estuarine, Coastal and Shelf Science, 54(3): 321-336. Wijsman, J.W.M., Herman, P.M.J., Middelburg, J.J. and Soetaert, K., 2002. A Model for Early Diagenetic Processes in Sediments of the Continental Shelf of the Black Sea. Estuarine, Coastal and Shelf Science, 54(3): 403-421.