Storm layer deposition on the Mississippi Atchafalaya subaqueous delta generated by Hurricane Lili in 2002

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1 ARTICLE IN PRESS Continental Shelf Research 25 (2005) Storm layer deposition on the Mississippi Atchafalaya subaqueous delta generated by Hurricane Lili in 2002 Mead A. Allison a,, Alexandru Sheremet b, Miguel A. Gon i c, Gregory W. Stone b a Department of Earth and Environmental Sciences, Tulane University, New Orleans, LA 70118, USA b Coastal Studies Institute and Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, LA 70803, USA c College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR , USA Received 24 May 2005; received in revised form 11 August 2005; accepted 29 August 2005 Abstract The Atchafalaya inner continental shelf, located along the north-central Gulf of Mexico offshore of Louisiana, is an area of rapid mud accumulation associated with the progradation of a subaqueous delta originating from this Mississippi River distributary. In September October 2002, this region was impacted by two tropical cyclones (Tropical Storm Isidore and Hurricane Lili) separated by only 7 days. Water-column and hydrodynamic records from coastal observation platforms (WAVCIS network) are combined with seabed sampling 4 7 days after passage of Lili, to examine the impact of these events on the Atchafalaya inner shelf. Wind speeds at the CSI-3 platform on the delta (located in 4.5 m of water) peaked at 20 m/s during Isidore, and more than 30 m/s during the closer, and stronger, Lili event. Significant wave heights during Lili peaked at more than 2 m at the CSI-3 platform, coincident with a storm surge of about 2 m. Water-column flow structure during both storms was closely tied to the storm surge (coastal setup setdown) cycle despite variations in wind direction with storm passage. Flow was onshore throughout the water column during the waxing phase (1.5 days in Lili, 4 days in Isidore), with a rapid (1 2 h) reversal to offshore flow after storm passage (12 h waning phase). Flow velocities remained above 1 m/s throughout the ADCP-measured water column (465 cm above the bottom) for more than 2 days during the Lili event. Sediment cores reveal the presence of a basal erosional surface, hypothesized to represent seabed deflation from the combined resuspension attributable to both storms, overlain by a silty clay storm deposit 2 19 cm thick. Comparison with 7 Be seabed profiles and X-radiographs taken at two delta stations (5 m water depth) prior to and following the storm suggests erosional deflation of 3 13 and 7 17 cm occurred at these stations. The overlying, physically stratified storm deposit contains radioisotopic inventories ( 7 Be, 234 Th, 137 Cs, 210 Pb) that are consistent with an origin primarily from redeposition of particles resuspended in the waxing phase of the storm. X-radiography and granulometry suggest two-phase re-deposition: an initial, normally graded basal deposit 1 2 cm thick containing sand that likely was deposited from normal settling, and a slightly normally graded, sand-poor unit hypothesized to be deposited from consolidation of a fluid mud (410 g/l), hindered settling suspension later in the waning phase. Macrofaunal burrows in the storm deposit suggest rapid (days) settlement of surviving fauna, likely due to high abundance in the sediments at this time of year when burial rates (from Atchafalaya River sediment supply) and energies sufficient for bottom resuspension are normally low. r 2005 Elsevier Ltd. All rights reserved. Keywords: Hurricane; Storm layer; Mississippi delta; Louisiana; Continental shelf deposition Corresponding author. Tel.: ; fax: address: malliso@tulane.edu (M.A. Allison) /$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi: /j.csr

2 2214 ARTICLE IN PRESS M.A. Allison et al. / Continental Shelf Research 25 (2005) Introduction Tropical cyclones, because of the enormous wave orbital velocities and strong mean flows they create in the benthic boundary layer, may have a disproportionate impact on the transport, deposition and burial of sediments on continental shelves where they are active. Their impact is likely often underestimated, however, due to their unpredictable nature, which makes designed empirical studies of the water column during these storms challenging. Furthermore, the direction that each storm traverses the shelf, and hence, the flow vectors that they impart, are unique. The study of water-column dynamics associated with tropical cyclones has recently been aided by the creation of coastal ocean observational networks (Malone, 1998) of fixed platforms that monitor fluid and flow properties within the water column in addition to meteorological and sea-surface data collected by surface buoy networks. Hydrodynamic modeling also shows great promise (Keen and Slingerland, 1993; Keen and Glenn, 1999; Fan et al., 2004; Keen et al., 2004) for predicting shelf water-column flow structure and possible seabed response for an unlimited range of potential storms of varying magnitude, direction and rate at which they traverse the shelf. Study of the shelf seabed response to tropical cyclones, whether erosional, depositional, or a combination of the two, is complicated in addition to the aforementioned factors by the wide range of sediment types and shelf geometries that are present in areas prone to these storms. Previous investigations of seabed response have involved the examination of a shelf area immediately following a tropical cyclone in an area recently studied for another purpose. This before-and-after approach aids in minimizing another complicating issue definitively identifying the sediment deposit or erosional surface associated with the storm. Here we mention two examples of the most common shelf type in latitudes and regions experiencing tropical cyclones, a sediment-starved (relict) shelf with a thin (o1 m) and discontinuous modern sediment cover. The effect of a series of hurricanes and tropical storms was documented in the Onslow embayment of the North Carolina continental shelf (Riggs et al., 1998) and showed primarily an erosional response, with relatively complete mobilization (bedform migration and resuspension), relocation of the surficial sand sheet and evidence in places for breakup of indurated, horizontally bedded, relict outcrops. Thieler et al. (2001) examined the adjacent barrier island shoreface before and after tropical cyclone passage and found alongshore migration of large (1 km wavelength), shore-oblique sand ridges and associated ripple scour depressions that were observed to be fixed in location for several years before the storm. Another example is the southern Texas continental shelf examined after Hurricane Carla in 1961 by Hayes (1967), who documented extensive sand layers on the innerand mid-shelf extending seaward from several points on the shoreface opposite barrier islands. Several hypotheses have been offered as to the mechanism responsible for the transport of this large volume of sand from the shoreface and adjacent barrier island, including downwelling flows and offshore flow generated by return flow of storm surge waters (Hayes, 1967; Morton, 1981; Snedden et al., 1988). River-dominated continental shelves are another shelf type impacted by tropical cyclones. Because of the enormous sediment contributions from the associated large rivers, these areas are characterized by rapid (cm/yr) rates of sediment burial (Nittrouer and Wright, 1994; McKee et al., 2004). This feature makes these shelves important locations for preserving a sediment record of large storms that can be utilized to identify analogous erosional or depositional characteristics in the rock record. Limited studies have been conducted to date on the effects of tropical cyclones on river-dominated margins. On the Ganges Brahmaputra margin in the Bay of Bengal, Michels et al. (1998), Kudrass et al. (1998) and Kottke et al. (2003) identified a series of acoustically transparent mud layers up to several meters thick in cores and seismic transects on the subaqueous delta and the adjacent head of Swatch of No Ground submarine canyon. Radioisotope geochronologies in the cores were used to tie at least some of these layers to the times of typhoon passage. It was hypothesized that these events (along with earthquakes) can lead to sediment failure and mass flow (debris flows) downslope on the subaqueous delta front and can remobilize thick (several meters) mud layers deposited ephemerally on the topset area of the submarine delta by the river and transport them seaward or into the canyon. Bea et al. (1983) measured seafloor erosion depths of m at the site of a mass flow slide on the Mississippi

3 ARTICLE IN PRESS M.A. Allison et al. / Continental Shelf Research 25 (2005) prodelta ramp caused by wave pumping of the underconsolidated mud seabed during the passage of Hurricane Camille in In 2002, the coast of Louisiana, and the associated river-dominated shelf of the Mississippi Atchafalaya River, was struck by two tropical cyclones within 7 days (September October) of one another (Figs. 1 and 2). Elements of Louisiana State University s Wave-Current-Surge Information System for Coastal Louisiana (WAVCIS) coastal ocean observing system were recording meteorological, sea surface and water column currents over the period of the storms. Beginning 4 days after the second event (Hurricane Lili), a seabed coring cruise was conducted on the inner shelf in the area of storm passage. This area, which includes the Atchafalaya subaqueous delta and adjacent inner shelf offshore of the West Louisiana Chenier Plain, has been the subject of previous sediment transport and accumulation as well as organic geochemistry studies (Allison et al., 2000; Gordon et al., 2001; Gordon and Gon i, 2003; Draut et al., 2004; Neill and Allison, 2005). Combining these data sets provides a unique opportunity to examine fluid and flow characteristics of the storms and the seabed response over a heterogeneous shelf surface of modern muds and relict (sandy) deltaic shoals. The organic geochemistry of the storm deposits are reported in a companion paper by Gon i etal. (in press) TX CSI-3 FIG.3 Isidore (20-26 Sept) 100 km LA 4 ( km/h ) 3 ( km/h ) 25 2 ( km/h ) 1 ( km/h ) TROPICAL STORM(<119) 3 26 MS CSI-5 NOAA AL Yucatan GULF OF MEXICO Lili (1-3 Oct) 2 21 GA Cuba Fig. 1. Map of the storm tracks of Tropical Storm Isidore and Hurricane Lili in the Gulf of Mexico in The location of NOAA buoy and WAVCIS stations CSI-3 and CSI-5 are noted. 20 FL 2. Background 2.1. History of Hurricane Lili and tropical storm Isidore Statistics for both storms reported below are available from the National Weather Service s National Hurricane Center at gov. TS Isidore originated as a tropical wave off the west coast of Africa on September 9, After tracking westward it entered the eastern Caribbean and intensified to a tropical storm on September 18 south of Jamaica. The storm moved slowly to the northwest and intensified to a minimal category 1 hurricane ( km/h) before briefly making landfall on the western tip of Cuba late on the 20th (Fig. 1). The hurricane then moved westward, reaching maximum wind speed of 204 km/h (110 knots) at 1800 GMT on the 21st before moving inland on the northern Yucatan peninsula. The storm reemerged into the Gulf of Mexico as a weak tropical storm after h over land. Isidore never regained hurricane status as it tracked northward across the Gulf (Fig. 2), making landfall with maximum winds of 102 km/h (55 knots) and a minimum pressure of 984 mb just west of Grand Isle, Louisiana at 0600 GMT on September 26. Maximum storm surges of 2.7 m were recorded east of the storm s center along the Louisiana Mississippi border. The center of circulation passed approximately 75 km east of the WAVCIS CSI-5 platform, which is located on the 6 m isobath seaward of Terrebonne Bay (Fig. 1). While TS Isidore was in the Caribbean on September 16, the tropical wave that was to become Hurricane Lili emerged from western Africa. After passing over the Windward Islands into the Caribbean, the storm fluctuated in energy before becoming a hurricane on the 30th near the Cayman Islands. The storm then moved over land with maximum winds of 167 km/hr (90 knots) very close to the same track over western Cuba followed by TS Isidore (Fig. 1). Unlike Isidore, Lili then accelerated to a forward speed 2.8 km/h (15 knots) and followed a northwesterly route across the Gulf of Mexico (Fig. 2 and 3). The storm also strengthened to Category 4 with maximum winds of 232 km/h (125 knots) and a minimum pressure of 940 mb at 0000 GMT on October 3. Several hours earlier (1950 GMT), the storm center passed within 30 km of NOAA buoy moored in 3246 m of water (Fig. 1). Analysis of buoy data by White and Shay (2004) shows that when the storm passed, significant

4 2216 ARTICLE IN PRESS M.A. Allison et al. / Continental Shelf Research 25 (2005) Fig. 2. NOAA visible band images from the GOES-8 satellite of the Gulf of Mexico showing the characteristics of Tropical Storm Isidore at 20:15 GMT on September 25, 2002 (upper panel) and Hurricane Lili at 14:15 GMT on October 2, 2002 (lower panel), prior to landfall on the Louisiana coast (from Limits of the study area on the Atchafalaya inner shelf are shown with a yellow box. wave height peaked at more than 10 m and wave spectral energies exceeded 220 m 2 /Hz. Wave spectral energies were contained largely within the swell part of the spectrum, decaying within hours after passage. In the final 13 h before landfall on October 3, the hurricane rapidly weakened to 148 km/h (80 knots) and minimum pressure of 963 mb at landfall near Intercoastal City, Louisiana (Fig. 3).

5 ARTICLE IN PRESS M.A. Allison et al. / Continental Shelf Research 25 (2005) Chenier Plain Km Freshwater Bayou WH4 WH2 WH MA4 15:00 CSI-3 RADIOISOTOPE STATIONS OTHER CORE STATIONS R/V Longhorn Cruise Track (7-10 Oct) Marsh Island Atchafalaya Bay MI6 9:00 GMT on 3 Oct Atchafalaya River Mouth 10m 20m Fig. 3. Close-up map of the storm track of Hurricane Lili on the Atchafalaya shelf and the coring stations occupied in the period of 4 7 days following the storm. The location of the CSI-3 platform is noted. Maximum storm surges of approximately 3.9 m were recorded on the landward shorelines of Atchafalaya Vermilion Bay. Total Lili rainfall in south-central Louisiana ranged from 10 to 20 cm. The center of circulation passed only about 20 km west of the WAVCIS CSI-3 platform, which is located in 4.5 m of water seaward of Marsh Island (Fig. 3) The Atchafalaya subaqueous delta and adjacent Chenier coast The Atchafalaya River is a distributary of the Mississippi that has been active at least since the 1500s, but began capturing an increasing proportion of the overall water discharge early in the 20th century (Roberts, 1998). In 1963, the construction of a control structure at Old River regulated flow down the Atchafalaya distributary at a maximum of 30% of total Mississippi-Red water discharge to prevent complete capture of the Mississippi flow. During the last century, when relatively large quantities of Mississippi water and sediment have been directed down the Atchafalaya, the river has constructed sandy bayhead deltas within Atchafalaya Bay into which it discharges (Shlemon, 1975; Van Heerden and Roberts, 1980, 1988; Roberts et al., 1980, Roberts, 1988) and has begun to build a subaqueous mud delta on the adjacent continental shelf along a broad front across Atchafalaya Bay and Marsh Island. Recent studies (Allison et al., MI3 5m 2000; Neill and Allison, 2005) have shown that the subaqueous feature is a clinoform up to 3 m thick that extends out to about the 8 m isobath and resembles similar features observed in high-energy deltaic systems like the Amazon (Nittrouer et al., 1986, 1996) and Ganges Brahmaputra (Kuehl et al., 1989, 1997). Peak sediment accumulation rates (2 4 cm/yr) are found on the foreset beds of the clinoform in about 5 7 m water depth (Neill and Allison, 2005). The subaqueous delta is prograding seaward over relict Mississippi deltaic shoals that are capped by a reworked sand facies, and formed by the regression and subsidence of earlier Holocene highstand deltaic lobe deposits (Penland et al., 1988; Penland and Suter, 1989). West of Marsh Island, the subaqueous delta merges into a shoreface mud deposit associated with the West Louisiana Chenier Plain. The nature of these shoreline deposits has been extensively studied over the last 50 years. Relevant to the present study, Morgan et al. (1953) first recognized the linkage between reactivation of mudflat progradation along the eastern Chenier Plain in the 1950s and the increasing supply of fine-grained sediment from the Atchafalaya. The zone of active mudflat growth presently is confined to the area extending about 30 km west of Freshwater Bayou; a tidal water body connected inland with the Gulf Intracoastal Waterway. Accumulation of sediments and shoreline progradation are particularly rapid immediately west of the jetties that protect the shelf extension of the Freshwater Bayou shipping channel (Roberts et al., 1989; Huh et al., 2001; Draut et al., 2005). Numerous studies have described how winter cold fronts and occasional large storms are responsible for shoreward transport of mud, which is deposited at the shoreline and stabilized by marsh grasses (Morgan et al., 1953; Kemp, 1986; Roberts et al., 1987, 1989; Penland and Suter, 1989; Huh et al., 1991, 2001; Stone and Sheremet, 2003). More recently, seismic and radiochemical studies by Draut et al. (2004) have demonstrated that the zone of active shoreline growth also marks an area of the shoreface where there is a transition from convex (clinoform) cross-shore geometry to a relict, concave geometry further west. This latter feature is thought to have been formed by erosional deflation of older chenier plain deposits. Draut and coworkers hypothesize that this transition at about W marks the western limit of modern Atchafalaya accumulation on the inner shelf, and that these modern shoreface deposits serve as the

6 2218 ARTICLE IN PRESS M.A. Allison et al. / Continental Shelf Research 25 (2005) sediment reservoir feeding shoreline growth during resuspension events. 3. Methods 3.1. WAVCIS observations The typical sensor package on WAVCIS platform stations, including CSI-3 and CSI-5, contains above-water meteorological sensors and underwater hydrodynamic sensors. The meteorological sensors provide measurements of wind speed and direction, air temperature, humidity, visibility (all measured at 20 m above mean water level) and barometric pressure (12 m above mean sea level). A Digiquartz pressure transducer is utilized to measure water level changes at the frequency of both waves and tides. Current velocity throughout the water column and directional waves are measured using a bottommounted and upward-looking acoustic Doppler current profiler (ADCP). The instrument is a Rio Grande Workhorse that operates at 1200 khz and was produced by RD Instruments. Water temperature is also measured with an electronic thermometer. All underwater sensors except the ADCP are mounted at 2 m below mean sea level to minimize depth attenuation of the high-frequency wave signal. Meteorological sensors sampled at 1 Hz and then averaged for a 10-min period beginning 10 min before the hour. The hydrodynamic parameters are sampled at 2 Hz for a 17 min burst at the beginning of every hour. Each wave burst recorded by the pressure sensor thus includes 2048 samples. Data are stored on-site on the platform in a Campbell CR23X Data-Logger and on dedicated computers. Data are transmitted back to LSU via satellite cellular phones and after post-processing and quality control protocols are implemented, disseminated at Directional wave spectra are calculated following the procedure described by the US Army Corps of Engineers Field Wave Gaging Program (Earle and McGehee, 1995). The automated directional wave spectra analysis software was developed at the Coastal Studies Institute. Two central functions, the fast-fourier transformation and computation of cross-spectral density, were included from the Wave Data Analysis Standard Spectral Analysis Program (WDASSAP) developed by USACE. Wave parameters derived from this analysis and posted on the Internet include significant wave height, maximum wave height, dominant wave direction, peak wave period, and average wave period. All hourly observations and wave parameters are archived and can be retrieved, queried, and graphically viewed on the aforementioned web page with permission of the program editor. For an in-depth overview of the observing system and data processing the reader is referred to Stone (2001), Stone and Sheremet (2003) and Stone and Zhang (submitted) Seabed studies A cruise was conducted on the Atchafalaya inner shelf in the area that Hurricane Lili made landfall beginning 4 days after the storm. The R/V Longhorn cruise followed a ship track beginning at the Atchafalaya River mouth. A series of 34 coring stations were sampled over the period from 7 to 10 October (Fig. 3 and companion paper by Gon i et al., in press). Cores were collected with an Ocean Instruments multicorer that utilizes 10 cm diameter acrylic tubes and obtains cores of cm length. At 10 of the stations shown in Fig. 3, cores were sealed and returned to a shore-based laboratory, where they were sectioned lengthwise, and a 1.5 cm thick section was removed for X-radiography with a Kramex Model PX-20N unit operating at 15 ma/ 70 kev. Core subsamples were then collected at 1 cm depth intervals, weighed and freeze-dried before reweighing to determine water content (porosity). Cores from the remaining 24 stations were extruded at 1 cm depth intervals onboard ship primarily for organic analysis (see Gon i et al., in press). These cores were only utilized in the present study for depth profiles of sediment water content. Sediment intervals from the 10 X-radiograph stations were analyzed for the particle-reactive radiotracers 234 Th (T 1/2 ¼ 24.5 d), 7 Be (53 d), 137 Cs (30 yr) and 210 Pb (22.3 yr) and were selected using the X-radiograph images of the cores as a guide. Because of their different source functions, this multi-tracer approach yields information about the origin of the particles onto which the radiotracers are adsorbed, as well as sediment geochronology. 234 Th is a naturally occurring 238 U daughter that is present in the seabed due to decay of uranium within the sediments (supported 234 Th) and, in addition, is adsorbed from particle contact with the water column through dissolved uranium ( 234 Th xs ), which is relatively high in marine waters (DeMaster et al., 1986). 7 Be is a natural spallation product of cosmic-ray interaction with atmospheric gases. Fallout takes place through both wet and dry precipitation, with higher fallout

7 ARTICLE IN PRESS M.A. Allison et al. / Continental Shelf Research 25 (2005) correlating with high local precipitation (Olsen et al., 1986; Baskaran, 1995). Riverine particles tend to be relatively enriched in 7 Be because of the focusing effect of the large collection area of the river drainage basin. 137 Cs is an anthropogenic bomb tracer released into the atmosphere with the onset of atmospheric testing of thermonuclear weapons in The fallout and transport pathway to shelf sediments is similar to 7 Be. Unlike Be, however, supply via fallout to the Earth s surface is not only controlled by precipitation factors, but reached its highest level worldwide with the peak in atmospheric testing, and has been declining overall since the 1972 ban on atmospheric weapons testing (Cambray et al., 1982). 210 Pb is a naturally occurring uranium daughter product (via 226 Ra and 222 Ra). Like 234 Th, it is produced in situ (supported Pb) and from dissolved uranium decay in the water column (primarily from marine water). The source function of excess 210 Pb ( 210 Pb xs ) is complicated, however, by an additional source from atmospheric fallout (and focusing in runoff) from the decay of 222 Ra gas. Radioisotope activities were analyzed by gamma spectrometry. Freeze-dried samples were ground to a powder and g were packed in Petri dishes with a 2000 mm 2 surface area, sealed to prevent Ra loss, and counted immediately for h on a Canberra intrinsic germanium detector of 2000 mm 2 planar configuration. Activities were determined using net peak area for gamma photopeaks at 63 kev (total 234 Th), 477 kev ( 7 Be), and kev ( 137 Cs). Detector efficiencies at each energy level were calibrated using a natural sediment standard (IAEA-300 Baltic Sea) and were corrected for self absorption using the method of Cutshall et al. (1983). Samples were then recounted after at least 100 days. This allowed 234 Th activity to be measured after 4 5 half-lives, when 234 Th xs is below detection limits and the remaining Th activity in the sediments is supported. The second count was also utilized to determine total 210 Pb activity (46.5 kev photopeak) after supported levels had ingrown to secular equilibrium. Supported 210 Pb levels, utilized to determine 210 Pb xs (excess ¼ total supported), were obtained using the averaged activity of 226 Ra daughters at 295 and kev ( 214 Pb) and 609 kev ( 214 Bi). Grain size was analyzed for selected samples by weighing out an aliquot of freeze-dried sample, which was then rehydrated and disaggregated ultrasonically, and sieved at 63 mm to separate the sand fraction. The sand fraction was dried and weighed (to determine the sand:mud ratio) and then analyzed at a 0.1 j interval on a 180 cm automated settling column using Femto software. A deflocculant (sodium metaphosphate) was added to the mud fraction which was then analyzed at 0.25j intervals from 63 to 0.25 mm using a Sedigraph 5100 X-ray particle size analyzer. 4. Results 4.1. Sea-surface and water-column properties at CSI-3 and CSI-5 The location of station CSI-3 relative to the wind field and storm surge of the two storms was the major control on the atmospheric and hydrodynamic perturbations observed at the station. TS Isidore was a low intensity storm of large spatial extent; in contrast, Hurricane Lili was a relatively intense but more localized perturbation. The manner in which the two storms impacted the CSI-3 site was also distinct, since Lili was relatively fastmoving but passed virtually on top of the station, while slow-moving TS Isidore made landfall over 200 km east of CSI-3, placing it in the distal part of the wind field and storm surge (Stone and Sheremet, 2003). A summary of the CSI-3 measurements for the two storms is presented in Fig. 4. The wind record at CSI-3 (Atchafalaya shelf) of TS Isidore and Hurricane Lili is shown in Fig. 4a. The TS Isidore event was characterized by a period of relatively high, 10-min-averaged, winds of 20 m/s at its peak. Because of the slow forward motion of TS Isidore, this event lasted for more than four days prior to closest approach (160 km) to the platform, and for only 1.5 days afterward, as the storm accelerated as it moved inland. Northeast winds were present at CSI-3 during most of strengthening phase of TS Isidore, becoming due north at maximum before changing abruptly to the northwest in the weakening phase. Peak wind speed in the stronger and much closer Hurricane Lili reached 30 m/s, but the faster forward motion limited the high wind event to 2 days before and 1.5 days after the storm s closest approach. As in the earlier event, Lili winds were from the northeast during the early storm phase, but, because of the proximity of the landfall point, wind direction shifts were more abrupt (changed to southwest after storm passage). The relative position of the station to the location of landfall for both storms is apparent also in the water level time series (Fig. 4c). Water level at the

8 2220 ARTICLE IN PRESS M.A. Allison et al. / Continental Shelf Research 25 (2005) Fig. 4. Wind, wave and current measurements at CSI-3 and CSI-5 during Tropical Storm Isidore and Hurricane Lili versus time (gray patches mark the approximate time spans of the storms). Directions of wind refer to direction from which it is coming, while current directions refer to direction it is moving: (a) wind measurements at CSI-3 (colors indicate wind direction using the bar adjacent to panel D), (b) significant wave height at CSI-3(red) and CSI-5(blue), (c) water level (in meters) and vertical distribution of horizontal current velocity at CSI-3 (ADCP data), (d) water level (in meters) and vertical distribution of horizontal current direction. The black circles on the water level curves mark the times of measured profiles. The thin vertical lines mark roughly the beginning of the storm decay phase, and clearly separate the setup/setdown currents (NW, purple-blue versus SE, green-yellow). The setup is also significant in Lili s case over 2 m at CSI-3. Gaps in the ADCP data on October 5 and again between October 6 and 11 are due to instrument failure. CSI-3 platform with the approach of TS Isidore showed only a slight increase (10 20 cm) over the diurnal astronomical tide fluctuation that was consistent with the distance of the storm and the offshore wind vector (Fig. 4a). Sea-surface elevation began to decrease after peak significant wave height of about 1 m was reached at CSI-3 (Fig. 4b): this was about 14 h prior to peak wind stress. As the storm moved onshore and began to dissipate, winds were from the northwest, resulting in setdown of approximately 30 cm. Despite the offshore wind vector in the early phases of Hurricane Lili at CSI-3, the storm surge reached almost 2 m above normal, as the platform was located within the disintegrating eyewall at about 1200 GMT on October 3 (Fig. 4c). Setdown began within hours of the relatively synchronous timing of peak winds and maximum significant wave height (2.2 m).

9 ARTICLE IN PRESS M.A. Allison et al. / Continental Shelf Research 25 (2005) Hurricane Lili was characterized by long (14 s) swells at CSI-3, with significant wave height of approximately 0.6 m, approximately 14 h prior to the peak of the storm (Fig. 4b). At the maximum significant wave height, the peak wave period was approximately 10 s. The swell (defined as a frequency less than 0.2 Hz) accounted for about half of the wave energy, creating a swell significant height of approximately 1.1 m. Further east at CSI-5, both storms generated larger significant wave heights than at CSI-3 (Fig. 4b; Sheremet and Stone, 2003; Stone and Sheremet, 2003). Spectral analysis of wave data at CSI-3 and CSI-5 (Fig. 5) reveal wavefield features that are typical for the muddy (CSI-3) and sandy (CSI-5) sedimentary environments at the two stations (see also Sheremet and Stone, 2003). While the local atmospheric forcing is virtually identical at the two stations (Fig. 5a) wave variance evolution differs in a significant way, as illustrated in Fig. 5b,c. For Fig. 5. Wind and wave measurements at CSI-3 and CSI-5 during Tropical Storm Isidore and Hurricane Lili, versus time (gray patches mark the approximate time spans of the storms): (a) wind speed, (b) long wave (swell, fo0.16 Hz) variance, (c) short wave (f40.16 Hz) variance (CSI-3 records plotted in red, CSI-5 records in blue), (d) spectral evolution recorded at CSI-3, (e) spectral evolution at CSI-5. Horizontal lines show the separation between the long and short wave bands.

10 2222 ARTICLE IN PRESS M.A. Allison et al. / Continental Shelf Research 25 (2005) simplicity, the frequency domain is separated here into two bands: low frequency, long waves, with periods larger that 6 s, and short, high-frequency waves, with periods of less than 6 s. The long wave band variance (Fig. 5b) displays strong bottom dissipation typically associated with muddy environments regardless of sea conditions. The short frequency band is characterized by a more rapid response to local forcing and dissipation. This dissipation is responsible for the major difference in maximum significant wave height during Lili (3 m versus 2 m) between the sandy site (CSI-5) and the muddy site (CSI-3) despite the storm s much closer proximity to CSI-3 (Fig. 4b). Short wave variance is comparable at the two sites (Fig. 5c), with the exception of the weakening phase of both storms, when CSI-3 waves decay to about an order of magnitude lower than waves at CSI-5. Observations of fluid muds from suspended sediment sensors (OBS) placed at CSI-3 during Hurricane Claudette in 2003 (Sheremet et al., 2005), support the hypothesis that the strong decay in CSI-3 wave variance at the end of the Lili event is an indication of the formation of a high density suspension layer in the lowest 1 m of the water column. Figs. 5d,e show details of the spectral wave evolution. The evolution of long waves (swell) variance (Fig. 5b) is consistent with friction-like wave dissipation in the muddy environment of CSI-3, with wave energy levels consistently lower at CSI-3 regardless of sea state. Observations of mud dynamics during Hurricane Claudette (Sheremet et al., 2005) show a substantial decrease in wave activity in the waning phase of the storm, coinciding with the formation of a fluid-mud layer due to mud settling. A similar wave-decay effect, which extends throughout the spectrum, is visible at CSI-3 as a deep blue vertical band in Fig. 5d (compare with CSI-5 observations, Fig. 5e), suggests that a similar mechanism could be active during the final stages of Lili (see Section 5). The ADCP at CSI-3 recorded currents down to approximately 65 cm above the bed (Fig. 4c). Shortly after the peak of the Hurricane Lili event, the ADCP failed for a brief period on October 5, and for a longer period between the 6th and 11th. In the period between the storms, tidal currents displayed a regular cycle of peak currents during the flooding phase toward the northwest (onshore) and a weaker ebb flow toward the west (alongshore), reflecting the influence of the westwardflowing Atchafalaya coastal current on the ebb flow vector. During both storm events, currents exceeded 1 m/s to the depth limit (65 cm above the bed) of the sensor for a period of about 12 h prior to closest approach. In the fast-moving Lili event, this is the limit of waxing phase intensification of currents, while stronger than average flows were observed for 3 4 d prior to TS Isidore. In both storms, currents decayed significantly in the 12 h period following closest approach to CSI-3. There is also clear evidence of a coastal setup and setdown currents in the storm records. In both storms, there is a very sharp (1 2 h period) reversal of currents from a predominantly northwest (onshore) vector throughout the water column before closest approach, to an offshore (southeast) vector in the waning phases at CSI-3 (Fig. 4d) Seabed characteristics X-radiographs (Fig. 6) of cores collected during the R/V Longhorn cruise beginning 4 days after Hurricane Lili show a similar surficial sediment pattern at all 10 sites on the Atchafalaya shelf near CSI-3 (Fig. 3). The cores contain a high porosity (75 90%) surface sediment layer of 2 19 cm thickness marked by a basal erosional contact with underlying sediments. The underlying sediments vary in lithology and porosity (55 75%) from station to station. In all but one station where the deposit is more than 3 cm thick, the basal 1 2 cm above the erosional contact appears to fine upward in the X-radiographs (Fig. 6). In one station (MI6) this basal layer is a 1 cm thick sandy lag. The lowdensity surficial layer contains faint, parallel to wavy (mm-to-cm scale) laminations and partial disruption of laminations by soft-bodied macrofauna burrows (Fig. 6), that in some cases extend from the sediment surface past the erosional surface at the base of the deposit. Core MA4 (Fig. 6) isan exception to the relatively massive character of the deposit; the layer at this site contains several 1 3 cm thick sandy interbeds. Grain-size analysis of the surficial sediment layer in the cores show that it is a silty clay with less than 1 2% sand or shell except in the sandy layers in MA4. Table 1 shows a typical detailed breakdown of the sand, silt and clay fraction through the deposit (site WH4). In X-radiograph (Fig. 6), the WH4 layer (16 cm thick) appears faintly laminated above a normally graded basal zone (15 16 cm depth interval). The detailed grain-size data (Table 1) reveal, however, that while the basal depth

11 ARTICLE IN PRESS M.A. Allison et al. / Continental Shelf Research 25 (2005) Fig. 6. X-radiograph negatives (coarser layers are darker) of the basal erosional surface and overlying the Hurricane Lili sediment deposit for 4 of the 5 stations where the deposit was thickest. Due to the enormous difference in X-ray transparency of the high porosity storm deposit with underlying sediments, two different density images are superimposed for each station. The image on the right side is optimized to display the erosional surface and normally graded basal part (1 2 cm thick) of the storm deposit, while the left and overlying image is optimized for the high porosity surface layer. Table 1 Detailed granulometry of the storm layer at core site WH4 Sample % Sand (44j) % Coarse silt (4 6j) % Fine silt (6 8j) % Clay (8 12j) % Fine clay (412j) Mean (D 50 ) in mm WH4 0 1 cm (0.26) WH4 3 4 cm (0.26) WH4 5 6 cm (0.26) WH4 8 9 cm (0.31) WH cm (0.26) WH cm (0.88) interval contains the only significant sand, the entire deposit fines upward, with both coarse silt (16 63 mm) and fine silt (4 16 mm) decreasing, while the % clay (o4 mm) increases. This can also be observed in the fining upward of mean grain size, but is not observable in the median (D 50 ) grain size trend. In addition to the 10 stations where X-radiography and radiochemistry were conducted, cores were collected at 24 other shelf sites (Fig. 3). The surficial sediment layer was identified in these cores using the water-content data at centimeter depth intervals. The base of the surficial layer of 80 90%

12 2224 ARTICLE IN PRESS M.A. Allison et al. / Continental Shelf Research 25 (2005) porosity was marked by a sharp decrease in water content with underlying sediments of at least 10 20% porosity, and up to 40% depending on lithology of the sediments below the contact. The data from all 34 stations were utilized to construct a map of spatial changes in the thickness of the surficial sediment layer shown in Fig Radioisotope trends Fig. 7. Map of the spatial extent and thickness of the Lili storm deposit from X-radiographs and profiles of sediment density. Open circles represent the core locations where X-radiographs were available. Fig. 8 shows representative downcore profiles of activity for the four short-period, particle-reactive radiotracers ( 234 Th xs, 7 Be, 137 Cs, 210 Pb xs )examined at the 10 sites. These examples were selected because they contain 4 of the 5 thickest sediment deposits above the erosional surface and are also shown in X- radiograph (Fig. 6). In all of the 10 sites except MA4 (Fig. 6), activities of all four tracers are relatively homogenous above the erosional surface, although activities decline slightly downcore at some sites (e.g., WH4) consistent with the observed grain size trends. 234 Th xs activity shows the largest errors and downcore variations, due in part to the recount method employed to differentiate supported and Th xs.activities are typically only significantly different (lower) in the coarser zone immediately overlying the erosional Fig. 8. Downcore radioisotopic activities for the four particle-reactive radiotracers ( 234 Th, 7 Be, 137 Cs, 210 Pb) in the upper seabed (including below the storm deposit) for the four stations shown in X-radiograph in Fig. 6. Dashed lines indicate the depth of the erosional surface at the base of the Lili deposit. Profiles indicate that a distinct radiochemical signature (particularly of the short-lived isotopes 7 Be and 234 Th) is present in the storm deposit relative to underlying sediments.

13 ARTICLE IN PRESS M.A. Allison et al. / Continental Shelf Research 25 (2005) contact. At many sites, 234 Th xs and 7 Be activities are below detectability limits in this interval. Core MA4 (Fig. 8) is distinct in that it displays a logarithmic decrease in 234 Th xs and 7 Be activities with depth in the surficial layer. While downcore activities are relatively homogenous, site-to-site variations in radiotracer activity in the surficial layer are significant except for 210 Pb xs, which only varies in mean activity of the surficial layer from dpm/g. In contrast, mean activities (dpm/g) of the other isotopes at each site in the surficial deposit range from 0.12 to 0.40 (Cs), 0.75 to 3.2 (Be), and 0.8 to 6.5 (Th xs ). The Th values are based on only 7 of the 10 sites, since three sites were not counted initially before decay had decreased excess activities below the point at which reliable measurements could be made (41.5 halflives). No 234 Th xs or 7 Be was encountered below the erosional contact at any of the 10 sites. 210 Pb xs and 137 Cs activities also changed sharply in the underlying strata at many sites (Fig. 8). 5. Discussion 5.1. Seabed erosion and deposition caused by the storms The presence of 234 Th xs throughout the surficial sediment layer on the Atchafalaya inner shelf is evidence of its deposition within the last 100 days. Previous coring in the area has not recorded measurable 234 Th xs activities in the seabed (Allison et al., 2000), suggesting this deposit has an atypical origin. Porosities of the layer are higher than has been previously observed in surficial sediments from this area (Allison et al., 2000; Neill and Allison, 2005), also indicative of rapid and recent deposition. The vertically homogenous activity in 9 of 10 core sites of 234 Th xs and 7 Be activity further suggests deposition did not occur steadily, such that decay would resulted in decreased activities with depth: this constrains deposition to having taken place over a period of less than one Th half-life (o24 d). Finally, this faintly laminated, silty clay deposit is remarkably similar in lithology over an area that normally displays a range of surficial sediment facies associated with the Atchafalaya subaqueous delta (interbedded sands and muds inshore to bioturbated muds offshore; seaward limit shown in Fig. 9) and adjacent relict sandy shoals (Neill and Allison, 2005). Although no sampling was done immediately prior to the storms, this evidence (3.9) (1.7) CSI (2.7) 2.3(1.1) 0 0.8(0.3) LAYER THICKNESS >15 (cm) Marsh Island Atchafalaya 1.8 Bay 1.2(0.8) Atchafalaya River Mouth 1.6(1.3) LIMIT OF SUBAQUEOUS DELTA Km Fig. 9. Spatial map of thickness of the Lili deposit with total inventories of 7 Be in the storm layer shown for each of the 10 stations where activities were measured. Downcore-averaged 7 Be/ 234 Th xs ratios for the storm layer are shown in parentheses for the seven stations where 234 Th xs data were collected. See text for an explanation of trends. suggests that the surficial deposit is related to the passage of TS Isidore and Hurricane Lili. The basal erosional surface of the sediment layer is also unique. Although layers with sharp lower boundaries have been observed in Atchafalaya subaqueous delta sediments (Neill and Allison, 2005), they are not traceable over a wide area within the upper 20 cm of the sediment column, nor are they overlain by a uniform lithology. Furthermore, previously detected layers do not typically display characteristics such as incised preexisting macrofaunal burrows and lag deposits of sandy or shelly material (Fig. 6). The presence of a sharp erosional surface, particularly at sites located farther offshore where there is limited modern sediment accumulation (o0.5 cm/yr; Neill and Allison, 2005) and biological destruction of primary sedimentary structures is further evidence for the recent origin of the erosional surface. The X-radiographs (Fig. 6) display some soft-bodied macrofaunal burrowing through the surficial deposit, which in some cases extends into sediments that underlie the erosional surface. We interpret this as burrowing that occurred in the 4 7 day period between the passage of Hurricane Lili and core collection. This rapid settlement, likely of organisms displaced by the storm, is testament to how rapidly this stratal record would be diffused by bioturbation, and hence, its relative youth. In part, this rapid settlement is a

14 2226 ARTICLE IN PRESS M.A. Allison et al. / Continental Shelf Research 25 (2005) function of the fact that the season when tropical cyclones impact south Louisiana (June November, see Stone et al., 1997) coincides with high benthic biological activity, limited burial from new sediment arriving from the Atchafalaya (falling to low discharge) and low wave energy (low potential for disturbance by resuspension). The observation that the erosional surface marks the limit of excess Th and Be penetration at all 10 sites also suggests it is intimately associated with the overlying storm deposit. Our interpretation of this erosional-depositional record on the Atchafalaya shelf is that the erosional surface records the period of peak, combined wavecurrent boundary layer stress (U* cw ), when the inner shelf was being deflated by the storm. We estimate that U* cw at CSI-3, calculated from ADCP-derived bottom shear stress, bottom wave orbital velocities and excursion amplitudes, and using the algorithms of Styles and Glenn (2000), peaked at approximately 12 cm/s (Isidore) and 20 cm/s (Lili). The overlying deposit would then record deposition that occurred in the waning phase of the storm when resuspended sediments were redeposited. However, the fact that there were two events (Isidore and Lili) closely spaced in time that caused a relatively similar benthic boundary layer stress suggests the erosional surface is a combined record of Isidore and Lili erosion, and the overlying deposit was formed in the waning phases of Lili. If there was a storm layer deposited by TS Isidore on the Atchafalaya shelf, it was removed during the period of peak Lili benthic shear stress that reactivated the erosional surface, and may have further deflated the seafloor. Otherwise, two erosional surfaces would have been observed, separated by the remnants of a TS Isidore deposit. A first-order estimate of the depth of seafloor deflation caused by the combined Isidore Lili erosion can be obtained using 7 Be data from two of the coring stations at the 5 m isobath (MI6 and WH6; Fig. 3) that were sampled at different seasons in by Allison et al. (2000) and again in March 2001 (unpub. data). 7 Be penetration varied seasonally at these stations in these previous studies between 4 8 cm (MI6) and 1 7 cm (WH6), with maximum penetration occurring in late spring (April May) when sediment supply from the Atchafalaya remained high but seabed resuspension caused by cold front passage was waning (Allison et al., 2000). In October, Be penetrates about 7 cm at MI6 and 3 cm at WH6. Given that no Be penetration was observed at these stations below the erosional surface in October 2002, these depths provide a minimum estimate of the amount of sediment removed by the storms. Asecondmethodofestimatingdepthofseafloor deflation is possible by matching stratigraphic horizons between X-radiographs taken in March 1999 with the post-storm X-radiographs at MI6 and WH6. Although the erosional surface was not present in the earlier X-radiographs, its location in the physically stratifieddepositsis8cm(mi6)and13cm(wh6) below the seabed surface of March In the 3.5 year interval between cores, additional sediment would be expected to have accumulated at both sites. Longer-term (100 yr) 210 Pb sediment accumulation rates are 2.6 cm/yr at MI6 (Neill and Allison, 2005) and 0.6 cm/yr at WH6 (Allison et al., 2000). Although these values are averaged over decades and so this is not a precise value of additional sedimentation, correcting deflation rates for these estimates yields a storm removal of 17 cm at MI6 and 15 cm at WH6. These estimates of deflation may be expected to vary spatially with changing benthic shear stress (particularly with water depth), and due to the range of substrate lithology and degree of consolidation on the pre-storm Atchafalaya shelf. They do indicate that the volume of sediment eroded locally by the storm is likely the major source of sediment to the overlying Lili storm deposit. This interpretation is supported by radiochemical inventories (see below) and the nature of the organic matter in the deposit (Go ni et al., in press) Origin of the Hurricane Lili sediment layer Because of the divergent sources of 7 Be (high activity on riverine particles) and 234 Th xs (high on marine particles), variations in the 7 Be/ 234 Th xs ratio have been utilized as a measure of changing particle sources (Feng et al., 1999). However, downcore profiles in the Lili sediment layer show a relatively invariant 7 Be/ 234 Th xs ratio, which suggests that (1) observed co-varying changes are a function of the normal grading of the deposit and (2) the source of the sediment particles to the layer was relatively consistent through time. The fact that more 234 Th xs but similar 7 Be activities were observed than was seen previously on the Atchafalaya inner shelf (Allison et al., 2000) indicates a mixture of particle sources, likely containing a greater than usual proportion of