Dennis K. Hubbard

Hurricane-Induced Sediment Transport in Open-Shelf Tropical Systems--An Example from St. Croix, U.S. Virgin Islands

Hurricane Hugo passed directly over St. Croix on 17 September 1989. Sustained winds in excess of 110 knots (gusts to 165 knots) and waves 6-7 m in height accompanied the storm. Along the north coast, wave height was lower (ca. 3-4 m) due to the leeward position of the shelf. In the deeper reefs at Cane Bay and Salt River, damage was confined primarily to the soft-bodied benthic community (e.g., sponges, gorgonians); coral damage was much less severe, largely because of the buffering effects of the water column. The greatest change observed after the storm was wholesale flushing of sand from shelf-edge areas. In Salt River submarine canyon, a minimum of 2 million kg of sediment were flushed into deeper water. The transport rate associated with the storm was eleven orders of magnitude above that measured during fair-weather, and the volume of sediment that was removed from the canyon equalled roughly a century of normal sediment accumulation. At Cane Bay, 336,000 kg of sediment were flushed from a single channel, with similar amounts removed from adjacent breaks in the shelf-edge reef. A current meter in Salt River submarine canyon provided information on the timing and intensity of the oceanographic processes related to Hurricane Hugo. As the storm approached, waves piled water against the shoreface and in Salt River Bay. As the storm passed over St. Croix, the change in wind direction, followed by a decrease in wave height, triggered a release of water trapped in the bay and along the adjacent shoreline by waves earlier in the storm. For a period of 4-6 hours, net down canyon currents reaching 2 m/s and oscillatory flows up to 4 m/s occurred along the base of the western canyon wall, removing up to 2 m of sand. Similar events were likely responsible for the wholesale removal of sand in eastern Cane Bay. The paradoxical concurrence of wholesale sediment transport and low-level reef damage is related to the protection from waves but not wind afforded by the north coast of St. Croix, facing away from the direction of storm approach. These observations and measurements provide our first opportunity to relate sediment export in such a high-energy event to the physical processes that were responsible. Calculations based on post-Hugo measurements are in agreement with an earlier sediment budget for Salt River canyon. Sediment export in Cane Bay exceeded the volume similarly predicted. Because such events are probably common on all exposed carbonate shelf margins, storms like Hurricane Hugo are among the most important factors in the cycling of sediment through exposed, open-marine environments both now and in the geologic past. The patterns of reef damage and sediment transport are much more complicated than previously envisioned, and more thoughtful consideration of their variability and the processes responsible is essential to an understanding of the signature that will be left by major storms.

Sedimentation as a control of reef development: St. Croix, U.S.V.I.

Sediment-transport rates in and around Salt River submarine canyon, St. Croix, U.S.V.I., were measured over a 2-year period using bedload traps and Legrangian sediment-tracer experiments. Sodiments generatly move in a westerly direction along the north shore of St. Croix. Consequently, sediment moves into the canyon at a higher rate over the eastern margin (47,000 kg/m-year) than over the west wall (19,000 kg/m-year). As a result, extensive reef growth is limited to the west wall, and the inner portion of the eastern margin is a cobblecovered slope inhabited primarily by gorgonians and a few sediment-tolerant corals. Sedimentation exhibits similar controls on other reefs around the island. At Cane Bay, sedimentation rates are lower than at Salt River, and reef development is accordingly greater. Along the east side of Christiansted canyon and the downdrift margin at Sandy Point, extreme sedimentation has completely buried the reefs. Sediment transport was an order of magnitude higher during storms than during fair weather. Under all conditions measured, the amount of sediment exported from the canyon was considerably below that entering over the canyon walls. It is hypothesized that major hurricanes periodically flush the excess, and keep long-term sediment influx and export in balance.

Utilization of shallow-water seagrass detritus by Carribbean deep-sea macrofauna: δ13C evidence

Abstract Three dives were made using the DSRV Alvin in the deep-sea basin north of St. Croix, Virgin Islands. Detrital seagrasses and macrofaunal distributions at 2455 to 3950 m depth were assessed quantitatively. Counts of the manatee grass Syringodium filiforme (ca. 5 to 100 blades m−2) contrasted sharply with those of the turtle grass Thalassia testudinum (ca. 0.1 to 2.0 blades m−2), reflecting an abundance proportional to previously reported export rates of the same species from Tague Bay, a nearby shallow source lagoon. Of the macrofaunal consumers that could potentially utilize this detrital nutrient source, three species of holothurians (Mesothuria verrilli, Psychropotes semperiana, and Benthodytes linqua) and two species of sea urchins (Hygrosoma petersi and Salencidaris profundi) were collected and/or observed. Gut content analyses revealed that all three holothurians deposit-feed on sediment and at least one species of sea urchin (H. petersi) feeds almost exclusively on Syringodium. Carbon: nitrogen analyses of naturally occurring abyssal Thalassia detritus showed very low nitrogen content (0.21% N) and a high C:N ratio (214.8), thus yielding a loo nutritional value. Fresh Thalassia blades held in a litter bag experiment (by R. D. Turner) at 3950 m changed little in nitrogen content and C:N ratio after four years. A comparison was made of the stable carbon isotope ratios of 13C:12C for abyssal seagrass detritus and other potential carbon sources with those for tissues from the holothurian and urchin consumers. The results indicate that a significant proportion of the nutrition of both groups is derived from detrital seagrasses either by direct consumption (sea urchins) or indirectly by deposit-feeding on sediments enriched by decomposed seagrasses (holothurians).

Where's the reef: The role of framework in the Holocene

Holocene reef models generally emphasize the role of in-place and interlocking framework in the creation of a rigid structure that rises above its surroundings. By extension, a number of ancient biohermal deposits have been disqualified as “true reefs” owing to their lack of recognizable framework. Fifty-four cores from several eastern Caribbean sites (Fig. 1) clearly demonstrate that in-place and interlocking framework is not common in these reefs that are comprised of varying mixtures of recognizable coral (primary framework), loose sediment/rubble and secondary framework made up mostly of coralgal fragments bound together by submarine cementation and biological encrustation. Recovery of primary and secondary framework ranged from 22% (avg.) in branching-coral facies to 33% in intervals dominated by head corals. Accretion rate decreased as expected with water depth. However, the recovery of recognizable coral generally increased with water depth, inversely to presumed coral-growth rates.This pattern reflects a spectrum in the relative importance of coral growth (primary construction), bioerosion, hydromechanical breakdown and the transport of sediment and detritus. The relative importance of each is controlled by the physical-oceanographic conditions at the site of reef development and will dictate both the architecture of the reef and the character of its internal fabric. We do not propose that framework reefs do no exist, as they most assuredly do. However, the fact that so many modern reefs are not dominated by in-place and interlocking framework suggests that its use as the primary determinant of ancient reefs may be unreasonable. We, therefore, propose the abnndonment of framework-based models in favor of those that treat framework generation, physical/biological degradation, sedimentation, and encrustation as equal partners in the development of modern and ancient reefs alike.

Styles of Reef Accretion Along a Steep, Shelf-Edge Reef, St. Croix, U.S. Virgin Islands

ABSTRACT Seven horizontal cores were taken from the reef-dominated margins of Salt River submarine canyon, St. Croix, U.S. Virgin Islands at water depths of 14-30 m. The pattern of east-to-west sediment transport in the area exerts a major control on present-day reef morphology, as well as accretionary styles recorded in the cores. Coral growth is inhibited by sedimentation on much of the eastern (heavily stressed) margin, and slopes are gentle. The regular pattern of alternating reef growth and sedimentary infill in the eastern cores reflects this highly variable environment. On the west wall, away from the source of incoming bedload sediment, coral cover is much greater, and a vertical reef wall forms the canyon margin. Slumping of the steep reef face has caused numerous repetitions of section and, in some instances, accretion rates higher than the growth rates of calcifying organisms occupying the present or past reef surface. This slumping process, along with the highly dissected character of the reef, results in larger sections of reef framework separated by open or sediment-filled vugs and caverns. 14C dates indicate that this complex constructional history has resulted in at least 24 m of lateral accretion during Holocene time. Average lateral-accretion rates in the cores varied from 0.84 to 2.55 m/1,000 yr, with the highest rate occurring in the core from deepest water. Intervals of rapid accretion within the cores were not generally related to patterns of in situ coral growth, but rather with intervals of allochthonous material slumped from the shallower portions of the reef complex. The character of the cores illustrates the potential importance of detrital material and late-stage reworking in the accretion of shelf-edge reefs. The high rates of accretion at depth highlight potential problems with reef models based on net accretion paralleling the abilities of the present-day surface organisms to produce calcium carbonate.

Chapter 10 - Geology and Hydrogeology of St. Croix, Virgin Islands

This chapter describes the geology and hydrogeology of St. Croix, Virgin Islands. St. Croix, the only one of the Virgin Islands that is composed mostly of sedimentary rocks, lies about 150 km southeast of San Juan, Puerto Rico. The island is 40 km long along an east-west axis and tapers to a narrow point on the eastern side. St. Croix contains a carbonate section that reveals a history of uplift and exposure in the late Tertiary. Traditional water use in the Virgin Islands has depended on rainwater catchment and scattered, hand-dug wells. However, the dependence on agriculture in past centuries has diminished with the waning of the sugar industry, and St. Croix now looks up to industries such as oil-refining, alumina-processing, and tourism.. As a water resource, the carbonate section provides a meager supply of groundwater by most hydrologic standards. As with many islands, however, the importance of even a minor resource is made larger by the expense of the alternatives.