Submerged Cultural Resources Study:
USS Arizona and Pearl Harbor National Historic Landmark
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USS ARIZONA MEMORIAL Submerged Cultural Resources Study: USS Arizona and Pearl Harbor National Historic Landmark |
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Chapter IV: Biofouling And Corrosion Study Sedimentation and Condition of Horizontal Surfaces Results Descriptions of sediment, biota and underlying surfaces observed at the horizontal stations are presented in Table 4.4. Four distinct zones of differing sediment types were delineated: Table 4.4. Horizontal stations, USS ARIZONA, descriptions of sediment, biota and underlying surfaces. Station 1 Station 2 Station 3 Station 4 Station 5 Station 6 Station 7 Station 8 Station 9 Station 10 Station 11 Station 12 Station 13 Station 14 Station 15 Station 16 Station 17 Station 18 Station 19 Station 20 Station 21 Station 22 Station 23 Station 24 Station 25 Station 26 Station 27 Station 28 Station 29 Station 30 Station 31 Station 32 Station 33 Station 34 Station 35 Station 36 Station 37 Station 38 Station 39 Station 40 Station 41 Station 42 Station 43 Station 44 Station 45 Station 46 Station 47 Station 48 Station 49 Station 50 Station 51 Station 52 Station 53 - No station Station 54 Station 55 Station 56
Zone 1 Aft area, about 40 percent of ship length (Stations 1 through 22). This area lies in an average depth of 8 to 10 feet. Sediment is generally 1 to 6 inches deep and is characteristically about 50 percent sand, 20 percent rubble, and 10 percent mud/silt. The sediment is held together in a spongy mat by pervading growth of colonial feather-duster worms (Branchiomma cingulata) and sponges. Large feather-duster worms (Sabellastarte sanctijosephi) inhabit the sediment in densities of about 1 to 5 per square meter. Encrusting and erect sponges are common on the sediment surface in area coverage of 1 to 10 percent. Most substrate underlying Zone 1 sediment is teak wood. Teak is smooth and relatively dense under the thickest sediment layer. Finger-sized burrows, largely created by burrowing mollusks (primarily Martesia striata and possible Teredinid species), are evident in teak that is exposed or covered by thinner sediment cap. The color of teak in some areas is normal, whereas in other areas it has been blackened, possibly stained by exposure to sulfide compounds formed in anoxic environments under thicker sediment, Gelatinous veneers of fish eggs (clear to purplish in color) are commonly found laid on teak and smooth metal areas that are exposed in Zone 1. Egg nests are often situated in areas that appear to have been naturally exposed by water motion, but more than 50 percent of the nest depressions have apparently been created or enhanced by fanning actions of egg-laying fish. These egg nests are likely built by the Maomao (Sargeant Major fish, Abudefduf abdominalis) that schools abundantly on the wreck. An estimated 100+ nests of 1-to-3-foot diameter were present in Zone 1 at the time of the survey. Teak surfaces exposed in fish-egg nests exhibit considerably more mollusk-burrow damage than surfaces covered by sediment. Therefore, nesting activity should be considered as a potential chronic, deleterious process that should be monitored in future studies. Such monitoring should examine the density and seasonality of nesting activity and should compare the decomposition state of wood areas in and near nests to the control sites continuously covered by moderate sediment thickness. Metal surfaces were encountered under sediment at only two stations in Zone 1, and that material was largely irregular texture corrosion products.
Zone 2 Midships, about 25 percent of ship length (Stations 23 through 38). Depths in this area range from about 4 to 10 feet. Sediments are characterized by high rubble content (50 to 90 percent) with lesser quantities of sand and mud/silt. Coins were very common in the sediment immediately beneath the visitor viewing area of the Arizona Memorial. Colonial tube worms and burrowing sponges are present, although in much lesser abundance than in the finer sediments of Zone 1. Erect and encrusting sponges are present at some stations. Alpheid snapping shrimps and commensal gobies are common in burrow complexes in the sediment. The higher content of coarse material in these sediments is probably due to the presence of superstructure that provides abundant vertical substrate conducive to growth of vermetid worms and oysters. Those organisms provide shell material that comprises the bulk of the rubble. Also, the surge tends to wash finer sediments out of this shallow zone or works them into concentrated horizons in lower sediment layers. The occurrence and thickness of mud/silt horizons under the rubble varied considerably from station to station, and where present often made viewing of the underlying ship fabric difficult or impractical because of persistent turbidity. Surfaces encountered under sediment were usually irregular corroded metal of red dish-brown or blackish coloration. In several areas a black "tarry" hydrocarbon layer 3/4 to 2 inches thick existed over grey metal. The tarry layer is probably made up of heavier fractions of fuel oil that has settled into the lower sediment horizons over the 45-year interval since the ship's sinking. That layer appears to provide some corrosion protection to the deck steel, because most oxides observed under the layer were darker (black and grey) forms resembling "stable" oxides (magnetite and hematite). Deck surface under sediment at Station 23 was found to be porcelain tile in good condition. That area is reported to have been the ship's galley. Zone 3 Area surrounding gun turrets No. 1 and 2, and about 25 percent of ship length (stations 39 through 50). Depths in this area are about 20 to 25 feet. Sediments are predominantly (75 to 95 percent) mud and silts with some admixed rubble and sand. These fine sediments are generally 2 to 3+ feet in thickness and contain abundant burrows of alpheid snapping shrimp and portunid crabs. Colonial feather-duster worms and sponges occur on the sediment surface in some areas but are generally rare. Water motion in this zone is gentle and turbidity is high, indicative of an area of high sedimentation. Some of the turbidity and influx of suspended fine sediment to this area is probably caused by propeller wash from shuttle boats going to and from the Arizona Memorial dock. Fabric underneath a 1- to 2-inch layer of "Branchiomma mat" on the top of gun No. 1 was found to be shiny steel armor plate covered by a thin black oxide coat. Surfaces under sediments at all other stations in this zone were not examined because of the extreme thickness of sediment and the turbid conditions. Zone 4 Bow deck, 12 percent of ship length (Stations 51 through 56). Depths range from 4 to 10 feet. Sediments in this zone are similar to those found in Zones 1 and 2 (as described above) and ranged from 1 to 17 inches in thickness. Fabric under one station was found to be wood (probably teak), and corroded metal was observed under sediment at the other two stations where examination holes were dug. Water motion conditions are similar to Zone 2.
http://www.nps.gov/usar/scrs/scrs4d.htm Last Updated: 27-Apr-2001 |
