The PASTA Project Investigation of Po and Apennine Sediment Transport and Accumulation

46 The northwestern Adriatic Sea from the Po River mouth to the Gargano Peninsula (Figure 1) is an ideal location to study sedimentary processes that fi ll foreland basins and form epicontinental shelves. These semi-enclosed shelves form in depressions on continental crust; they differ in morphology and origin from pericontinental shelves, which are found along the edges of ocean basins and are largely the result of sea-level rise. A number of epicontinental shelves are extant today (e.g., Yellow Sea, Baltic Sea), and they were more common during the geologic past when sea level was relatively stable. For example, during the Cretaceous Period (~100 million years ago), an epicontinental shelf ran north-south through the middle of the North American continent, received sediments from the Rocky Mountains, and created the oil-producing strata in that region. Mountain building on the Italian Peninsula created the Apennines and Alps, whose weight has depressed the adjacent crust (foreland basin) in the Adriatic Sea. The relief of these ranges continues to stimulate land-surface erosion that produces sediment, fi lls the northern Adriatic, and forms the epicontinental shelf. The sediment is transported by the Po River and a host of smaller Apennine rivers to the south (Figure 1). The Po is one of the dominant drainage basins in Europe, extending eastward across northern Italy—bounded by and receiving sediment from the south fl ank of the Alps and the north fl ank of the Apennines. The river enters the northern Adriatic just south of Venice, and has formed a prominent delta at its mouth. A characteristic of the Adriatic epicontinental shelf is minimal expenditure of energy by tides and ocean swells. Therefore, the wind-driven currents (Figure 2) and waves control sediment dispersal, and transport the Po’s discharge southward. The Apennine mountain range intersects The PASTA Project Investigation of Po and Apennine Sediment Transport and Accumulation


FLUVIAL SEDIMENT SUPPLY
The annual sediment discharge of the

SEDIMENT ACCUMULATION
The fl ood deposit present in early December 2000 revealed sedimentarystructure and radiochemical signatures that allowed its geometry and history to The strong southward transport associated with the WACC creates a conveyor belt of sediment that starts with the input from the Po River and receives the contributions from numerous Apennine rivers as it rolls southward.Are they failure features?
The strong southward transport associated with the WACC creates a conveyor belt of sediment that starts with the input from the Po River and receives the contributions from numerous Apennine rivers as it rolls southward.Some sediment is left along the dispersal system, as indicated by the observed accumulation rates, but much sediment continues to the end of the conveyor belt at the Gargano Peninsula.In this vicinity, far from any signifi cant river supply, the greatest accumulation rates in the Apennine region are observed (>1 cm/yr).
The Po and Apennine sediment supply is thought to have decreased during the late Holocene and especially in the past century.However, the pattern of modern accumulation rates (greatest near Po and Gargano, and continuous accumulation between these end points) is consistent with Holocene sedimentation observed by seismic profi ling (Figure 1) (Correggiari et al., 2001).This consistency suggests that although the mass of sediment in the system may have decreased, the processes of transport and accumulation probably have not changed dramatically.Therefore, the observations described in this paper provide an understanding of the sedimentary processes creating the epicontinental shelf in the northwestern Adriatic Sea.

Figure 1 .
Figure 1.Chart of northern Adriatic Sea, showing bathymetry and thickness of Holocene sediment (i.e., sediment accumulation since sea-level rise fl ooded the shelf surface).Th e thickness was determined by seismic profi ling and is expressed in milliseconds of seismic-wave travel (10 ms ~ 15 m).Th e Po River enters in the northwest corner, and numerous Apennine rivers enter between the Ancona promontory and the Gargano Peninsula (only a small number of rivers are shown).Th e Holocene deposit of sediment extends from the Po mouth to the Gargano Peninsula, and the thickest portions are at the two ends of this sediment dispersal system.Th e regions colored blue-green are foreset beds (steepest portion of the clinoform structure) where a crenulated seafl oor is observed (see Figure 5B for region of red box).Reprinted from Correggiari et al. ( 2001).

Figure 2 .
Figure 2. Th is fi gure shows the mean currents (direction and relative speed) and sediment volumes in transport, as predicted by numerical modeling for the period autumn 2002 to winter 2003.Th e primary driving forces include winds, wave resuspension, and freshwater infl ow.Strong currents and large sediment fl uxes are expected on the western side of the Adriatic and are observed.Figure courtesy of Courtney Harris, College of William and Mary.
).The path of this article starts with fl uvial sediment supply, extends to the physical 1800s), the Tolle River was the dominant distributary in the Po region.It is also important to note that the Venice Republic reconfi gured the Po distributaries around 1600 because of northward progradation into the Venice lagoon.For both natural and artifi cial reasons, the active distributaries carrying Po River sediment have changed; consequently, the locations for the resulting lobes of sedimentation have changed, as well.The net effect of the Po delta is to trap about 15 percent of the sediment provided by the river.The remaining sediment escapes to the ocean.Models of Po River sediment discharge conclude that sediment loads have generally decreased from the early stages of the Holocene (over ten thousand years ago).Since then, there have been natural (Little Ice Age) and manmade (destruction of forests) alterations to the discharge.The biggest impact has been caused by hydraulic structures (e.g., dams, artifi cial levees) created during the past century, especially since World War II, that have accentuated the decrease in sediment discharge.Ironically, the discharge has decreased, but the sediment reaching the Po from its tributaries probably travels more quickly to the Adriatic Sea because artifi cial levees constrain fl ow and prevent sediment from reaching fl ood plains.The short residence time for particles in the river is demonstrated by signifi cant concentrations of the natural radioisotope 7 Be (half life 53 days) on the seabed at the mouth of the Po.This is only possible if many of the particles transit from hill slope to seabed in periods of less than six months.Coastal rivers draining the Apennines from small mountainous drainage basins behave differently from the much larger Po River.Due to the size of the Po River's drainage basin, the weather conditions causing precipitation and river fl ow do not generally correspond to the weather conditions impacting sediment dispersal at its mouth.However, the precipitation that increases fl ow in the Apennine rivers is associated with the same weather systems impacting oceanographic conditions at the river mouth because of the proximity of source and sink.This has a couple of important ramifi cations, including episodic discharge of the Apennine rivers in direct response to precipitation.The fl ood sediment is released into an energetic marine environment (i.e., storm conditions), and together these are the recipe for highconcentration sediment fl ows-as have been observed elsewhere (e.g., Eel River in northern California).If suspendedsediment concentrations exceed 10 g/l (grams per liter), then the material can fl ow downslope as a gravity fl ow driven by its own weight.Numerical modeling

Figure 3 .Figure 4 .
Figure 3. Th is fi gure shows the region near the mouth of the Po River where gravity fl ows were predicted and observed.(A) Surface plume of turbid water that shows the typical southward trajectory of Po discharge.Th is MODIS (Moderate resolution Imaging Spectroradiometer) image shows the region of boundary-layer in C. (B) Nautical chart showing locations of Po distributary channels (Pila, Tolle, Gnocca, and Goro) and locations of boundary-layer instruments on the adjacent seabed.Data from WHOI-12 instruments (red dot) are shown in C and data from University of Washington instruments are shown in Figure 4. (C) Profi les are shown of off shore current velocity very near the seabed during three periods of gravity fl ows.Th e increased velocity (+ seaward) in the lower 10 cm is a signature of gravity fl ows driven by density imparted by wave-suspended sediment.Figure courtesy of Peter Traykovski, Woods Hole Oceanographic Institution.

Figure 5 .
Figure 5. Sedimentary processes and seabed character near the Pescara River.(A) Data collected from two boundary-layer instruments and a mooring (deepest) deployed along a shore-perpendicular transect.With distance seaward, mean southward speed increases and the direction of near-bottom transport veers eastward due to frictional interactions with the seabed.Th e two vectors for the mooring show near-surface (23.7 cm/s) and near-bottom (11.7 cm/s) velocities.B) Seismic profi le showing the crenulated surface of the clinoform foreset beds across the transect near Pescara River, and the locations of instrumentation (a tripod near top of foreset, and a mooring on crenulations).Th e high-stand systems tract (HST) is the sediment accumulated since sea level rose across the maximum fl ooding surface (mfs).Figures courtesy of Pere Puig and Albert Palanques, Institut de Ciències del Mar, Barcelona.Th e seismic profi le is fromCorreggiari et al., 2001.

Figure 6 .
Figure 6.Th ree fi gures showing predicted and observed thicknesses for the deposit resulting from the autumn 2000 fl ood of the Po River.(A) Th e distribution of thicknesses based on predictions of wave-supported gravity fl ows. Figure courtesy of Carl Friedrichs and Malcolm Scully, College of William & Mary.(B) Th e distribution of thicknesses based on observations of sedimentary structures.Figure is courtesy of Robert Wheatcroft, Oregon State University.(C) Th e distribution of thicknesses based on observations of the penetration depth for 7 Be, which represents the latter phase of fl ood discharge.Figure courtesy of Cindy Palinkas and Charles Nittrouer, University of Washington.In all three cases, the thicknesses are greatest near the Pila distributary and become thinner in a southward direction, with a secondary peak in the region of the Tolle, Gnocca, and Goro distributaries.

Table 1 . List of major EuroSTRATAFORM contributors to the research described in this article.
cial publications of Marine Geology andContinental Shelf Research (see Table1