Riverine Carbon and the Sedimentary Record on the Continental Shelves

Transcription

1 Riverine Carbon and the Sedimentary Record on the Continental Shelves Stefano Miserocchi and Leonardo Langone Istituto Scienze Marine, Sede di Bologna-Geologia Marina (formerly Istituto di Geologia Marina) Consiglio Nazionale delle Ricerche Via Gobetti, Bologna, Italy phone: +39 (051) Fax. +39 (051) Award Number: N LONG-TERM GOALS The aim of our contribution (as part of the EuroSTRATAFORM Program) was to improve a more complete understanding of organic matter (OM) transport, deposition and burial in the Gulf of Lions and Adriatic Sea. Continental shelves represent the largest modern repository of particulate organic carbon (OC); nevertheless, some major river systems may be sites where a substantial portion of terrestrial inputs bypass the shelf, and are discharged onto the continental slope and beyond. Characterizing the origin of OM delivered to the ocean and defining the processes that affect its ultimate burial may provide a better understanding of the mechanisms that influence organic carbon preservation in the ocean. OBJECTIVES During this year, our research was mainly focused on the improvement of the characterization of the origin of the organic matter using the lignin content as a tracer of terrestrial organic matter contribution. This has been performed both in the Gulf of Lions and the Adriatic Sea on the same samples on which elemental and stable carbon isotopic analysis have been carried out. Our approach was to use lignin content and isotopic signature of the organic matter in order to: 1) trace the initial distribution of terrigenous material accumulated in the prodelta area and follow its transport along the continental shelf; 2) characterize qualitatively the riverine input defining the amount of soil and woody debris, respectively; APPROACH Terrestrial and marine organic matter burial in marine sediments is an important reservoir, representing the predominant long-term sink in the global biogeochemical cycle of OC. Deltas and continental shelves, considered to be the main repositories for organic carbon in marine sediments, receive large inputs from both autochthonous and allochthonous sources and account for ~90% of the OC burial in the ocean.

2 However, these same massive storage systems are often sites of efficient remineralization: some studies suggest that in highly reactive mobile shelf muds, physical reworking, bioturbation, exposure, and reoxidation inhibit high carbon burial. In the Gulf of Lions and the Adriatic Sea a simple two-end-member mixing model, in which vascular plant debris is diluted by marine phytoplankton detritus, can describe the composition of OM in coastal marine sediments. The isotopic signature and elemental composition of the sedimentary OC are widely used as proxies for decoding origin and fate of OM on various continental margins. Vascular plants of terrestrial origin exhibit many bulk chemical and isotopic proprieties that distinguish them from marine organisms. Lignin is a macromolecule uniquely synthesized by vascular plants ans is a more sensitive biomarker for indirectly quantifying the marine fraction. Lignin has been used in many investigations to identify in detail the terrestrial OM contribution to a wide variety of environments including rivers (Bianchi et al., 2004), estuaries (Goñi et al., 2003), prodeltas and inshore regions (Gordon and Goñi, 2003) as well as offshore regions (Hernes and Benner, 2002). WORK COMPLETED On the basis of elemental and isotopic surficial distribution 120 samples were selected for alkaline CuO oxidations analyses. The yields of individual lignin and non-lignin oxidation products were used to quantify the terrigenous and marine contribution in surficial sediments. Our aim was to build a multi-proxy geochemical tools to increase the understanding of organic matter cycle in Mediterranean margins. Alkaline CuO oxidations were carried out according to the procedure described in Goñi and Montgomery (2000). In order to investigate correlation between sediment dynamics and organic matter we carried out grain size analyses on the same samples. RESULTS In both margins, along the main dispersal system, the lignin contribution showed a drastic drop-off with increasing distance offshore (from 70-80% to 40-60% CuO products). This trend reflects a reduction in direct terrestrial input, with a consequent enhancement of the marine OM contribution (Λ = 0 mg/100 mg OC). Because of the low lignin content in soil (Λ = 1-4 mg/100 mg OC; Hedges et al., 1997) relative to plant fragments (Λ > 6 8 mg/100 mg OC; Hedges et al., 1997), the carbonnormalized lignin-derived is a powerful geochemical tool for identifying diverse terrestrial OM origins. This biomarker can be used to resolve the ambiguity that can result from simply using the isotopic signature, as sometimes isotopic differences between soil and woody material are not statistically significant. The lignin data confirms the variability detected in the prodelta areas concerning the terrestrial sources illustrated using elemental and isotopic values (Fig 1.). Hydrodynamic sorting of the surface mixed layer has been shown to be a fundamental mechanism for the redistribution of riverine-derived organic matter along coastal margins (Leithold and Hope, 1999). The terrestrial composition in the prodelta surficial sediments varied seasonally: physical reworking of flood deposit has an active role in the preferential transport of material which in turn impacted the organic and grain size surface distributions. Riverine material, being heterogeneous with a broad

3 variability in particle size, is not equally removed from the Rhône and Po prodelta areas and this in turn affects OC, N/C and lignin distribution in the surficial sediments. In our hypothesis, the riverine sediment initially deposited in the prodelta areas is impacted by hydraulic sorting that preferentially removes the finest material. Surficial concentrations of plant debris, coupled with coarse-sized particles, reflected lateral sorting in the benthic boundary layer. Wood fragments, rich in organic matter and lignin, hydraulically behave like very fine sand and are retained within the prodelta area. Soil-derived OM adsorbed onto fine fraction, lignin-poor particles, is selectively transported out of the prodelta along the main sediment dispersal system. The composition of OM from surface sediments in different regions of the GoL, are shown in Fig. 2. The plot displays the relationships between δ13coc, lignin and OC compositions of surficial sediments from diverse depositional settings in the GoL. These settings include the Rhône proximal prodelta, where high sediment accumulation rates reach 20 cm y-1, the distal prodelta area, the midshelf mud belt consisting of a fine-grained deposit of Holocene age, and finally the outer-shelf where relict sands are mixed with modern mud (Roussiez et al., 2005). The linear relationship highlighted in the graph identifies a possible mixing of fine soil-derived terrestrial material, isotopically depleted, rich in OC and relatively rich in lignin with marine phytodetritus characterized by low OC, heavier isotopic composition and the absence of lignin. Such a mixture is supported by the elevated coefficient of correlation (r-squared= 0.92). Along the dispersal system, a fraction of the OC adsorbed onto the fine material is diluted by marine OC that reaches the seabed and/or degraded by microbial activity.

4 Lignin content Λ (mg / 100mg OC) [TN:OC] atomic ratio C3-Soil 4 OM 3 2 riverine-estuarine 1 marine phyto. phyto. detritus detritus δ 13 OC C 3 - Vascular plants detritus Po prodelta area riverine-estuarine marine phyto. phyto. detritus detritus C3-Soil OM C3- Vascular plants detritus δ 13 OC Lignin content Λ (mg / 100mg OC) [TN:OC] atomic ratio C 3 -Soil 4 OM riverine-estuarine marine phyto. phyto. detritus detritus δ 13 OC Rhône prodelta area C3- Vascular plants detritus riverine-estuarine marine phyto. phyto. detritus detritus C 3 -Soil OM C 3 - Vascular plants detritus δ 13 OC Figure 1. Graph of the stable isotopic compositions of organic carbon (δ13oc) vs carbonnormalized lignin phenol yields and total nitrogen:organic carbon ratio from Po and Rhône surficial sediments. The compositions of four possible OC sources (C3 vascular plant detritus, C3 soil OM riverine/estuarine phyto detritus and marine phyto detritus) are also plotted in all graphs to illustrate the relative influence. The main difference between a flood and non-flood event is visible in the lignin content. The input of riverine sediment (blue characters flood period Po Dec2000, GoL Apr2005), initially deposited on the prodelta areas, is impacted by hydraulic sorting that preferentially removes the finest material (red charaters non-flood period Po Apr2002, GoL Sept2004).

5 Figure 2. Surficial OC, δ 13COC and Λ distribution for the Gulf of Lions coupled with the sedimentary units characterized by different sedimentological features based on Roussiez et Al The linear regression identifies a mixing between soil-derived OC with phytoplankton detritus. Out the prodelta, the marine contribution increases showing depleted isotopic values and low lignin contents in the offshore regions.

6 IMPACT/APPLICATIONS Studying the sedimentary processes, which affect the distribution and preservation of organic matter on the continental shelves, will provide key insights to understanding the role this environment as sinks for OC with respect to the global carbon cycle. RELATED PROJECTS The alkaline CuO oxidations were carried out in collaboration with Miguel Goñi (COAS-Oregon State University) which has been working on parallel projects focused on the fate of the organic matter in the marine environments We are working with D. Orange and A. Garcia (UCSC) in evaluating the origin of anomalous subsurface shallow gas concentrations in the Rhône prodelta delta and in the Cap de Creus Canyon. REFERENCES Bianchi, T.S., Filley, T, Dria, K., P. Hatcher, Temporal variability in sources of dissolved organic carbon in the lower Mississippi River. Geochimica et Cosmochimica Acta, 68: Goñi M. A. Montgomery S., Alkaline CuO oxidation with a microwave digestion system. Lignin analyses of geochemical samples. Analytical. Chemistry, 72: Goñi, M.A., Teixeir, M.J., Perkley D.W., Sources and distribution of organic matter in a river-dominated estuary (Winyah Bay, SC, USA). Estuarine, Coastal and Shelf Science, 7: Gordon, E.S., Goñi, M.A., Sources and distribution of terrigenous organic matter delivered by the Atchafalaya River to sediments in the northern Gulf of Mexico. Geochimica et Cosmochimica Acta, 67: Hedges, J.I., Keil, R.G., Benner, R., What happens to terrestrial organic matter in the ocean? Organic Geochemistry 27, Hernes P. J.,Benner R., Transport and diagenesis of dissolved and particulate terrigenous organic matter in the North Pacific Ocean. Deep-Sea Research I, 49: Leithold, E. L. Hope, R.S Deposition and modification of a flood layer on the northern California shelf: lessons from and about the fate of terrestrial particulate organic carbon. Marine Geology, 154: Roussiez, V., Aloisi, J. C., Monaco, A. Ludwig W., Early muddy deposits along the Gulf of Lions shoreline: A key for a better understanding of land-to-sea transfer of sediments and associated pollutant fluxes. Marine Geology, :

7 PUBLICATIONS Papers Fabres J., Calafat A., Tesi T., D. Zúñiga D., Canals M., Heussner S., Durrieu de Madron X., Bourrin F., Miserocchi S, Palanques A., Puig P.(in prep). Controlling factors and source of particulate fluxes at the head of Gulf of Lions submarine canyons: Results from a comprehensive sediment trap experiment. Marine Geology García-García A., Orange D., Lorenson T., Radakovitch O., Tesi T., Miserocchi S., Berné S., Friend PL., Nittrouer C., Normand A. (in press). Shallow gas studies off the Rhône prodelta, Gulf of Lions. Marine Geology García-García A., Orange D., Tesi T., Lorenson T., Sansoucy M., Herbert I., Miserocchi S., Dougherty J., Langone L. (in press) Cap de Creus: an active canyon with little shallow gas. Marine Geology García-García A., Tesi T., Orange D., Lorenson T., Miserocchi S., Langone L., Herbert I., Dougherty J. (submitted). Understanding shallow gas occurrences in the Gulf of Lions. Geo Marine Letters Miserocchi S., Langone L., Tesi T. (in press). Concentration and isotopic composition of organic carbon within a flood layer in the Po River prodelta (Adriatic Sea). Continental Shelf Research. Tesi T., Miserocchi S., Langone L. (in press). Sources, fate and distribution of organic matter on the western Adriatic continental shelf, Italy. Water, Air and Soil Pollution Tesi T, Miserocchi S, Goñi MA, Langone L. (submitted). Origin and distribution of organic material in the western Mediterranean Sea, Gulf of Lions, France. Marine chemistry Tesi T, Miserocchi S, Goñi MA, Langone L. (submitted). Organic carbon dynamics along the Western Adriatic shelf in surficial sediments and particulate material. Estuarine coastal and shelf science Abstract Stevens A., Goñi MA., Tesi T., Loewy C., Corbett, D, Walsh J., Mallinson, D. Distribution of Organic matter in Storm Deposits along the Louisiana Shelf Following Hurricanes Katrina and Rita: a Tale of Two Storms, Eos Trans. AGU, 87(36), Ocean Sci. Meet. Suppl., Abstract OS12L-06 Tesi T, Miserocchi S, Goñi MA, Langone L. Comparative organic geochemistries in surficial sediments from the Rhône (France) and Po (Italy) prodelta area., Eos Trans. AGU, 87(36), Ocean Sci. Meet. Suppl., Abstract OS16A-34 Tesi T., Langone L., Miserocchi S. Organic carbon and its isotopic composition in surficial sediments off the Rhône prodelta, along shelf and in the inner part of Cap de Creus canyon, Gulf of Lions (France). EUROSTRATAFORM Final Meeting and PROMESS 2nd Annual Meeting, Tesi T., Miserocchi S. Goñi M. A., Langone L. Development and reworking of a flood deposit on the northern Adriatic continental shelf, Italy: impacts on the supply and cycling of allochthonous organic matter. ASLO Summer Meeting 2006, Victoria, Canada.

8 Tesi T., Goñi M. A., Langone L., Miserocchi S. Sources of terrestrially-derived organic carbon in the prodelta Po river: deposition and reworking of a flood deposit. ECSA 41st International Conference Measuring and managing changes in estuaries and lagoons Venice, October Tesi T, Miserocchi S, Goñi MA, Langone L. Source, Transport and Fate of Terrestrial organic carbon on the Western Mediterranean sea, Gulf of Lions, France. AGU 2006 Fall Meeting