Modeling and Monitoring Submerged Prehistoric Sites during Offshore Sand Dredging and Implications for the Study of Early Holocene Coastal Occupation of Southern California

Beach sand dredging projects off the coast of San Diego County in Southern California provide data for improved understanding of the strategraphic setting for early Holocene sediments and the potential for offshore presence of buried archaeological materials. Geophysical data, core sediments, and analysis of recovered fossils allowed models to be developed for six offshore borrow sites within drown river valleys. These site-specific models were tested during dredging operations, and the dredge spoil was monitored for archaeological materials. Two of the borrow sites yielded stone bowls consistent with those found in previous offshore archaeological investigations in this region. These artifacts, however, were determined to come from nearshore and lagoonal sediments, not appropriate for occupation, raising questions about both the function of stone bowls and the process that resulted in their deposition. This project illustrates the potential for commercial development projects to yield information on offshore archaeological resources, as well as the challenges.Track lines plots for sub bottom profiling data are represented by files with names: TracklineDATE.pdf where DATE is either 1999 or 2008. These files use a Lambert Conformal Conic projection with the North American Datum 1983, California Zone 6 in feet. The location of individual seismic lines are designated with a number at the beginning of the line, and distance along the track is designated in meters by the location of the CMP (common-mid-point) stack of the seismic data. Seismic data files are represented by files with names: SITE_LINE#-DATE.pdf where SITE refers to the survey area, LINE# is the line number within that area, DATE is either 1999 or 2008. Files from 1999 are scans from the original graphic recorder output with no additional processing. They show the 3.5 kHz chirp sonar record on top and the Geopulse record on the bottom. The locations of vibracores are shown as annotations (blue triangles with core number). The vertical depth is given at 5 m increments. Files from 2008 were digitally recorded, and are the result of stacking and FK migration using a 24 channel streamer, as described in the methods section. Distance along track is given in m, and depth is given as time (ms) which can be nominally converted to depth using a two-way travel time and assumption of ~1500 m/s (0.75 m/ms), so that major time marks (10 ms) give 7.5 m depth increments. Vibracore Logs are represented by files with names: VibracoreDATE.pdf where DATE is the year of the data collection. These plots give the sediment classification as a function of depth within each core. Locations of grain size measurements are indicated at the appropriate depth. For the 2008 cores, field photographs are included at the end of the files. GrainSize1999.pdf gives grain size distribution plots for samples obtained from cores collected in 1999. Funding provided by: San Diego Association of Governments*Crossref Funder Registry ID: Award Number:San Diego Association of Governments, Beach Replenishment Project 1999 Field Methods: During January 1999, Sea Surveyor, Inc. conducted marine geophysical surveys and vibracore investigations along the San Diego coastline. The geophysical surveys of borrow sites were conducted using the 48' survey boat "WESTWIND". A differential GPS antenna was installed on the mast of the vessel, directly over the transducer well. The subbottom profiling included two acoustic sources: a Datasonics chirp II sonar, operating a frequency of 3-16 kHz, and a Applied Acoustic Engineering Geopulse system operating at 800 -2000 Hz. The Geopulse acoustic source and hydrophones were towed alongside the vessel and 17' behind the GPS navigation antenna. The seismic reflection data from both systems were displayed on an EPC Model 1086 thermal graphic recorder. No additional data processing was conducted on the graphic recorder output. The 165' vessel "AMERICAN PATRIOT" and 20' ALPINE vibratory corer were used to collect sediment core samples at 125 locations in 10 proposed sand borrow sites during 18-24 January 1999. The ALPINE Vibracorer uses 4"-diameter steel barrels for collecting the core samples, and 3.5" diameter cores in clear cellulose acetate butyrate (CAB) liners, which can be split laterally or longitudinally for inspection and geotechnical logging and subsampling. After the vibracore collected a sediment sample, the sediment cores were extracted from the vibracore barrel, and the liner was cut at 5' intervals, capped, taped, and labeled. The collected cores were then shuttled to Oceanside Harbor, where geotechnical personnel split, inspected, described, logged, and subsampled the core samples for laboratory analyses. Subsamples from the sediment cores were transported to MEC Analytical Systems' laboratory in Carlsbad, California for grain-size and chemical analyses. After the sediment cores were split longitudinally, logged, and photographed, the geotechnical personnel collected one or more representative subsamples from the top 1-3 layers of the core. The sediment subsamples were sealed within transparent ziplock bags, labeled, and transported to MEC Analytical Systems' Carlsbad laboratory for grain-size analyses. A total of 235 sediment samples were analyzed for grain-size. In the laboratory, approximately 40 grams of sediment from each subsample was weighed into a coors dish and placed into an oven to dry overnight. Once dry, the sample weight was determined by first weighing the sample in the coors dish, followed by weighing the dish after the sediment sample had been placed in the sieves. The sieves were stacked on top of one another with the sieve having the largest screen mesh diameter above sieves having progressively smaller screen mesh diameters. The sample was shaken for 10-minutes to sieve the sediment, which left the coarsest material on the upper screens and allowed the finer particles to fall through to the bottom. Once shaken, the contents of each sieve were weighed and the results were entered onto a data sheet. The data sheet was then entered into a computer spreadsheet, and the percentage (by weight) of gravels, sands, and silts were calculated. More information on the 1999 field operations can be found at: https://www.sandag.org/index.asp?publicationid=590&fuseaction=publications.detail 2008 Field Methods: The seismic reflection survey was initiated on October 6 and 7, 2008 using Fugro Inc. geophysical systems on the vessel "Julie Ann", a 24-foot aluminum hull vessel. The Starfix Seis navigation system was interfaced to a Trimble 12-channel GPS receiver with an integrated Starfix Differential receiver. The Trimble unit receives ranging information from the same satellites as the Starfix differential reference stations. These corrections are applied to the DGPS receiver's satellite data to produce an accurate (±5 feet) position of the vessel in real-time and a post processed accuracy of less than 2.5 feet. The differentially-corrected position from the Trimble receiver is then passed to the navigation computer. The seismic reflection system used a mechanical "boomer" energy source and a multichannel, Geo-Eel hydrophone array. An Applied Acoustics Engineering AA300J portable seismic energy source was used to power the AA300 boomer plate towed from a catamaran configuration. The boomer plate is an electro- mechanical transducer made of an insulated metal plate and a rubber diaphragm adjacent to a flat wound electrical coil. A short duration high-energy pulse is discharged from the AA300J energy source into the coil and the resulting magnetic field repels the metal plate in the transducer. The plate motion is transferred to the water by the rubber diaphragm, generating a broad-band acoustic pulse that does not have strong cavitations or ringing. The Applied Acoustics Model AA300 sub-bottom tow fish was deployed and towed from the starboard side stern of the vessel. Sufficient tow cable was deployed such that the tow fish was clear of and beneath the vessel's wake. The system was triggered at an 8 to 10 Hz pulse rate and swept frequency range between 2 to 10 kHz. The recorded record length was dependent on water depth. Navigation fix marks were sent to the systems' printer every 100 meters down the survey line. All navigation information and sub-bottom data were time tagged and logged to a hard drive. The reflected acoustic pulse, generated by the boomer source, was received by a multichannel Geo-Eel hydrophone array. The Geo-Eel includes hydrophones enclosed in a silica gel at specified interval. All track lines used a 24-channel (location dependent) hydrophone array. The hydrophone array included 5.1 foot (1.56 meter) group spacing of the channels. The raw data recorded by the hydrophone channels was logged to a Geometrics CNT recording system in a SEG-Y format for later post-processing to an accuracy of within 2.5% of penetration depth. Seismic data processing was conducted by Mark Legg (Legg Geophysical), and the following description of the processing steps applied to the 2008 data was provided by him. "Data processing of the multichannel seismic reflection profiles acquired offshore San Diego County, followed the basic premise of the wavelet processing method used in the petroleum exploration industry. A single-plate boomer source type was used combined with a 24-channel hydrophone streamer configuration, with the objective high resolution at shallow sub-bottom depths (