OS11D-01 INVITED 08:00h
PROMESS 1: Past Global Changes Investigated by Drilling Mediterranean Continental Margins
Between June, 24th and July, 22nd, 2004, a team of European scientists embarked from Brindisi (Italy) to Barcelona (Spain) onboard the Russian vessel "Bavenit", operated by the Dutch geotechnical company FUGRO, for a drilling expedition in the Adriatic Sea and the NW Mediterranean Sea. The purpose of this cruise was to collect long sediment sections and in situ measurements from two deltaic margins where the history of global changes during the last ca. 400 kyr is particularly well preserved. In the Adriatic, two boreholes were drilled at site PRAD1 (water depth 184 m), where the objective was to study the record of the last 4 glacial cycles. A pilot hole was first drilled for assessing the risk of shallow gases, a downhole logging was carried out in this borehole. A second site allowed continuous coring to the targeted depth (71m below sea-floor) with excellent recovery (better than 95%). Very preliminary interpretation indicates that seismic sequences previously identified correspond to 100 kyr glacial cycles. Downhole logging and physical properties of cores allow to identify magnetic events, and tephras. Site PRAD2 was devoted to the study of the recent most sediments (last 12,000 yrs) near the coastline, at a water depth of 56m. The targeted depth was 32 m below sea floor, sufficient to obtain a good record for the last ca 12,000 years. All together, six boreholes were drilled at PRAD2, including a pilot hole, one for continuous sediment recovery, and additional holes for in situ geotechnical tests and sampling. One of the objectives of these tests is to determine whether the wavy features shaping the sedimentary sequences are caused by near-bottom currents or result from liquefaction of unstable sediments triggered by earthquakes or storms. Site PRGL1 in the Gulf of Lion is at 298 m water depth, and the targeted depth below sea floor was 300 m, allowing to reach an expected age of about 430 kyr BP. A pilot hole was drilled down to 310 mbsf, and logged. Two geotechnical boreholes were drilled, allowing tests and measurements to a depth of 150 mbsf. Another borehole was drilled for continuous coring to the depth of 300 mbsf. The recovery was excellent (>95%). Preliminary estimations of coccolithophore assemblages provide a general time-frame for this site. Marine isotope stage (MIS) 12 was reached at the bottom of the hole. We have also good estimates of the position of the intervals corresponding to MIS 2-3, MIS 4, MIS 5a-d, and the transition between MIS 8 and 7. This shows that, as in the Adriatic Sea, seismic bounding surfaces are linked to 100 kyr cycles, that modify lithology and sedimentation rates on the upper slope. The presence of coarser sediment at the end of each "forced regression", and the occurrence of some biogenic gas, trapped by the overlying clayey sediments deposited during the ensuing warm period, is likely at the origin of seismic anomalies. Site PRGL2 is at 103 m water depth, an area where glacial shorelines that formed duringthe last ca. 500 kyr are the best preserved. A CPTU borehole was first drilled, followed by a sampling borehole, down to 100 mbsf. As expected, many sandy intervals were encountered, but the overall recovery was however quite good, in the order of 82%. Gamma ray downhole logging was performed in the drill pipe afterward. PROMESS 1 is an European Community funded project of the 5th framework programme (EVR1-2001-41). It belongs to the OMARC cluster of projects. It is a companion project of the joint Euro-US "EUROSTRATAFORM" project. The "PROMESS 1" shipboard party: S. Berne, M. Canals, A. Cattaneo, E. Colmenero, G. Floch, B. Dennielou, J. Frigola, R. Gelfort, J. Gravalosa, D. Ridente, T. Schoolmeester, N. Sultan, G. Tulloch, H.J. Wallrabe-Adams
http://promess1.wdc-mare.org/
OS11D-02 08:15h
Paleosealevels and strata formation in the Gulf of Lion during the Plioquaternary : from field evidence to stratigraphic numerical modeling : what has been done, what will be done.
Observation and analysis of sedimentary strata, identification of sequences within the series enable the reconstruction of paleoenvironments and paleogeographies that describe the spatio-temporal evolution of sediment deposition. Eustasy, tectonic and sediment flux are the three main factors that control variations in accommodation and therefore geometries of sequences. However deciphering between the factors is not an easy task and stratigraphic simulation can be a tool to test different geological scenario and try reproduce numerically the observed geometries in a sedimentary basin. The Gulf of Lion is particularly well suited to undertake such an approach with field observation and numerical modelling. In particular, the Gulf of Lion offers a perfect natural laboratory for sea level calibration of glacial maxima for Plioquaternary climatic cycles because : (a) the Mediterranean Sea has been connected to the global ocean throughout Plio-Quaternary time, so that sea level variations are directly linked to Arctic and Antarctic ice caps fluctuations. (b) The Gulf of Lions is a young margin with subsidence still underway at present that continually creates a large amount of accommodation to be filled by sediments. (c) During the Last Glacial Maximum, mountain glaciers covered a large part of the Alps, and smaller parts of the Pyrenees and the Massif Central. However, neither the drainage basins nor the shelf were covered by ice. Moreover, the Gulf of Lions is situated sufficiently far from former ice sheets for significant glacio-isostasy of the margin to be excluded. (d) The presence of mountain glaciers led to important erosion and scouring providing an important source of sediments to be deposited in the Gulf of Lions. This approach applied for the late Quaternary already showed that the last five sequences recorded on the outer shelf were related to 100,000 years climatic cycles (MIS12 to MIS2). Preliminary results from the recent and successful PROMESS project (see the presentation by S. Berne) confirmed this general chronotratigraphic framework. Further work (G. Jouet) will enable to go a step further for the last 430, 000 years of sedimentation as the detailed chronostratigraphic framework, sedimentation rates, lithologies. will be measured and used as an input parameter in the 3D numerical simulation. On a broader time scale (Plioquaternary), both backstripping of sequences (see the presentation by Steckler) and numerical stratigraphic simulation are planned. This study is part of the national French GDR Marges ("Golfe du Lion") project, the european PROMESS project (S. Berne) and the US-Eurostrataform project (C. Nittrouer/ P. Weaver) which has been and will be a cooperation among CNRS Brest, IFREMER, IFP (Dionisos), INSTAAR (Sedflux), Lamont (Sequence), Minessota University and Utrecht. Participants in alphabetical order : Aslanian, Baztan, Berne, Granjeon, Gorini, Graindorge, Hutton, Jouet, Joseph, Kettner, Olivet, Overeem, Rabineau, Steckler, Storms, Swenson, Syvitsky.
OS11D-03 08:30h
Interactions Between Storms And Dense Water Cascading In The Gulf Of Lion, NW Mediterranean
The Gulf of Lions is a river-dominated shelf of the northwestern Mediterranean that represents an interesting environment to explore variability of seabed erosional and depositional fluxes. Sediment dynamics is very active in fall and winter when rivers are in flood and southeastern storms impinge the coast. These occasional and brief events inject large amounts of fine sediment on the inner-shelf, which are eventually advected on the outer shelf by a complex shelf circulation. Currents are mainly driven by the continental winds, and usually transport suspended sediment cyclonically along the coast. In late winter and early spring, cascading of cold, dense water reinforces the transport of suspended sediment across the western part of the shelf and towards the slope. From a ten-year time-series of current and flux measurements on the slope, it appears that this mechanism controls the intensity of the export of particulate matter to the continental slope though with a strong internannual variability. Further information on this density underflow was recently gathered and permitted to elucidate its impact on sediment dynamics. A comprehensive field experiment was performed during winter 2004 in the framework of the Eurostrataform program. It comprised several hydrological and ADCP surveys, a monitoring of the storms' impact on the sediment dynamics on the inner shelf, as well as of current, hydrology and particulate fluxes in several canyons that dissect the Gulf of Lions shelf. The data sets offer the opportunity to evidence the downward propagation in the bottom boundary layer of cold and turbid plumes in the canyons, especially those at the southwestern end of the Gulf. It also allows to examine the interactions between the resuspension of fine sediment by two storms that occur during that period and the transport of suspended sediment by dense water cascading.
OS11D-04 08:45h
Contemporary Suspended Sediment Fluxes Through the Gulf of Lion Submarine Canyons.
The Gulf of Lion continental margin receives sediment inputs from the River Rhone as well as from some smaller rivers. From the west to the east ends of this margin, the slope is incised by numerous submarine canyons and the shelf-slope suspended sediment transfer through these canyons is not well known. In the framework of the EUROSTRATAFORM Project (EVK3-CT-2002-00079, EU Fifth Framework Programme: Energy, Environment and Sustainable Development), contemporary suspended sediment transport in the head of seven submarine canyon of the Gulf of Lion was studied by deploying moorings with Aanderaa RCM9/11 current meters equipped with turbidimeters 5 m above bottom at 300 m depth from November 2003 to May 2004. In these time period, there were several sediment transport events, but two of them were especially relevant: one in Early December 2003 and another one in late February 2004. The December event was generated by a "wet storm" during which rivers discharges increased up to one order of magnitude and a cyclonic along-shore advection of a cold, brackish and turbid stream developed in the shelf, whereas the February event was generated by a "dry storm" without associated relevant rivers discharges increases and with an advective transport of fine sediment across the shelf due to cascading of cold, dense water. During these events, the maximum suspended sediment transport took place through the Cap de Creus canyon (at the western end of the Gulf of Lion) where sediment fluxes increased up to two orders of magnitude, and also a relevant transport occurred in other western (Lacaze-Duthier) and central canyons (Herault and Petit Rhone) where sediment fluxes increased up to one order of magnitude. However sediment fluxes in the eastern canyons (Planier and Grand Rhone) did not increase during these events. Other minor sediment flux increases in winter were associated to dense water cascading. This study indicates a preferential shelf-slope sediment transfer through the central and western canyons of the Gulf of Lion and especially through the Cap de Creus canyon.
OS11D-05 09:00h
Modern Shelf Sedimentation Along the Apennine Coast, Adriatic Sea
Sediment supply to the Adriatic Sea is dominated by the Po and Apennine Rivers in the northern and southern regions, respectively. The Po River is the largest point source of sediment, delivering 1.5x10$^{7}$ tons annually to the adjacent shelf. Investigations into the pattern of 100-y sediment accumulation using $^{210}$Pb (half-life 22.3 y) have revealed that accumulation rates are greatest in the northern portion of the dispersal system, near the Pila distributary ($\sim$2 cm/y), and decreases southward toward the Apennine coast. In contrast to the Po, sedimentation in the southern region results from a series of small, distributed fluvial sources (i.e., a line source), draining the Apennine mountains. These rivers deliver approximately 2.4x10$^{7}$ t/y of sediment, contributing to the development of a shore-parallel shelf clinoform that has formed throughout the Holocene. Across-shelf sediment accumulation rates on the clinoform are greatest on the foreset ($\sim$1 cm/y), which is characterized by the presence of seafloor crenulations in some areas. Along-shelf accumulation rates show some variability with proximity to major rivers, but a general increase is observed southward along the margin and rates are greatest near the Gargano Peninsula ($\sim$1.5 cm/y). This trend differs from other line-source systems. Distinct depocenters are associated with individual fluvial sources in the Gulf of Alaska, and river inputs coalesce into a central locus of sedimentation in the Gulf of Papua. The differences among these systems lend further insight into the mechanisms controlling deposits emanating from line-source systems. Anthropogenic activities, particularly dam construction following WWII, have significantly impacted sedimentation in this region. Sediment accumulation along the Apennine region has been reduced, evidenced by changes in $^{210}$Pb profiles. However, observations during winter 2002-2003 using the short-lived radioisotopes $^{7}$Be and $^{234}$Th (half-life 53.3 d and 24.1 d, respectively) have indicated that some new sediment is delivered to the shelf during periods of elevated discharge.
OS11D-06 09:15h
Fine Sediment Distribution in the Western Adriatic Coastal Current
A study of water properties and suspended sediments on the Apennine margin was conducted as part of the Po and Apennine Sediment Transport and Accumulation (PASTA) component of the EuroSTRATAFORM project. Repeated hydrographic transects encompassing a Bora wind event during February 2003 documented variability of the cross-shelf extent of the coastal current, significant changes in the width of the front, vertical stratification, and suspended sediment inventory over periods of a few days. During calm conditions, suspended sediment concentrations (SSCs) off the Apennine margin were generally low, 10 mg/l or less in water depths of < 10 m, with the maximum close to shore. During Bora conditions, the coastal current deepens and destratifies as the result of downwelling. The SSC maximum increases and moves seaward to approximately 15 m depth in the vicinity of the sand-mud transition (SMT), bounded on the seaward side by the cross-shelf density front. The maximum in SSC is likely the result of interrelated factors: a) the mix of grain sizes at the SMT are more easily resuspended than coarser sediments closer to shore and more cohesive sediments seaward; b) depth of resuspension due to waves; c) adjustment of the coastal current during Bora events and subsequent relaxation. The latter could be important with respect to moving sediment offshore, as well as governing the location of the sand-mud transition. Fine sediments from the Apennine Rivers initially deposited in shallow water are easily resuspended and transported along shore (most of the flux) within the coastal current. With wave resuspension and downwelling, sediments are distributed throughout the water column extending to greater depths, but are confined by the density front. With relaxation of the Bora event, stratification is re-established and sediments in the vicinity of the front are deposited in deeper water, resulting in net transport offshore. Comparison with realistic simulations from a 3-dimensional numerical model support the key features of the interpretation including change in width and depth of the coastal current, location of the SSC maximum, and a boundary shear stress minimum at the front.
OS11D-07 09:30h
Dispersal of Fluvial Sediment in the Adriatic Sea, Italy
The Adriatic Sea provides a setting for contrasting a range of sediment dispersal processes. The largest single source of sediment, the Po River, delivers an estimated 15 million tons of sediment annually, much during floods that last for several weeks. South of the Po, several rivers drain smaller catchments in the Apennine Mountains, delivering 32 millions tons of sediment annually. Sediment from these Apennine rivers is delivered during floods that tend to last only a few days. Field observations have shown that the fine-grained sediment delivered by the Po River is predominantly packaged as fast-settling flocculates, whereas the Apennine rivers deliver slowly-settling unflocculated material. The distribution of material from the Po and Apennine rivers results from dispersal patterns that differ due to the sediment properties of fluvial material, the timescales of sediment delivery, and along coast variations in wave and current speeds. A three-dimensional numerical sediment transport model is used to evaluate dispersal paths of fine-grained fluvial sediment delivered to the Adriatic Sea, Italy for September, 2002 -- May, 2003. These calculations estimate resuspension, transport, delivery from fluvial sources, and net deposition. Currents are driven by buoyancy forcing and winds. Wave bottom orbital velocities are based on predictions made using the SWAN wave model. Predicted patterns of fluvial deposition are similar to those observed in Holocene accumulation maps. Deposition is thickest near the Po River mouth, and the Po sediment bed extends alongshore towards the south. Sediment is preferentially deposited on the northern side of the Gargano Peninsula, tens of kilometers south of any major fluvial source. On a regional scale, sediment flux occurs preferentially during northeasterly wind events because these both enhance waves and intensify the western Adriatic coastal current. Sediment from the Po River is initially deposited near the river mouth, whereas sediment from Apennine rivers can traverse long distances during floods. Po River sediment, is, however, remobilized by energetic waves soon after delivery and some of the material may reach depocenters offshore of the Gargano Peninsula within a single storm season.
OS11D-08 09:45h
Modeling Deposition by Wave-Supported Gravity Flows on the Po River Subaqueous Delta: From Seasonal Floods to Prograding Clinoforms
A simple model for wave-supported gravity flows is applied to sediment deposition at time scales ranging from a single major flood to steady-state progradation of an equilibrium clinoform. The modeling approach, which limits sediment load via a critical Richardson number, is applicable to fine sediment transport near river mouths wherever wave energy is available to move abundant sediment offshore during floods. Results suggest this phemomena can account for the majority of the fall 2000 flood deposit mapped by EuroSTRATAFORM investigators in the vicinity of the Po subaqueous delta and also for the rate of delta progradation observed off the dominant Pila outlet of the Po over the last 150 years. Model results predict that convergence of down-slope sediment transport by wave-supported gravity flows increases with bed slope but decreases with slope gradient, such that greatest deposition occurs near where steep slopes first stop increasing with distance offshore. Thus on profiles which reach maximum steepness near shore, like those off Tolle-Gnocca-Goro mouths today or off the Pila mouth 150 years ago, gravity-driven deposition occurs in shallower water. Over time, if deposition overwhelms subsidence, the subaqueous delta becomes less steep near shore and steeper offshore, and the locus of deposition moves progressively into deeper water. If the subaqueous delta is prograding across a relatively flat shelf, the form of the prodelta eventually reaches an approximate equilibrium which progrades seaward as a unit. This has occurred off the Pila; but subsidence has likely overwhelmed deposition off the Tolle-Gnocca-Gorro, keeping steepest slopes and deposition in shallower water.