Mark R. Boardman

Observations of a Nassau grouper, Epinephelus striatus, Spawning Aggregation Site in Little Cayman, Cayman Islands, Including Multi-Species Spawning Information

Mass spawning aggregations of Caribbean grouper species are a conservation priority because of declines due to over-fishing. Previous studies have documented five historical aggregation sites in the Cayman Islands. Today, three of these sites are inactive or commercially extinct. In January 2002, the Reef Environmental Education Foundation led an expedition to Little Cayman Island to document a recently re-discovered spawning aggregation of Nassau grouper, Epinephelus striatus. A team of divers estimated the abundance, color phase composition, and courtship and spawning behavior of the aggregating grouper. The color phase composition of the aggregation shifted both during the course of each evening and throughout the 10-day project. Divers documented atypical coloration and courtship behavior in 10 additional fish species, of which five were seen spawning. Artisanal fishing occurred daily on the aggregation. The Cayman Islands Department of the Environment collected landings data and sampled catches to obtain length and sex ratios. The Cayman fishing fleet, while small, had a significant impact on the aggregation with a harvest of almost 2 000 Nassau grouper during the 10-day project. The study site supports the largest known Nassau grouper aggregation in the Cayman Islands. The relatively large size of fish and the high proportion of males indicate that this site supports a relatively healthy aggregation compared to other Nassau grouper aggregation sites throughout the Caribbean.

Sources of Periplatform Carbonates: Northwest Providence Channel, Bahamas

ABSTRACT Sources and quantities of periplatform carbonate sediments have been determined in Northwest Providence Channel, Bahamas, by an end-member method employing geochemical, textural, petrographic, and SEM data. The Holocene sediment over most of this open seaway, 200 to 2,000 m deep, between Great Bahama Bank and Little Bahama Bank, is a drape of largely fine grained lime mud 50 cm thick. Shallow platform sources contribute 75-90 percent; the remainder is planktonic foraminifera, pteropods, and coccoliths. Mud (< 62 µm) from each bank and the planktonic source are each geochemically distinct and can be quantified. The two shallow platform sources dominate sedimentation in the deep basin. Trend-surface maps of mud derived from each bank reveal no windward-leeward asymmetry in cont ast to reported sand-transport patterns. This indicates that the fines are swept off both platforms by storm and tidal exchange. Concentric distribution of fines relative to source indicates no offset by currents, suggesting that fecal pelleting must rapidly remove fine sediment from the water column. Canyon valleys and intercanyon highs have equal thicknesses of Holocene sediment, which indicates that pelagic processes of deposition presently dominate over gravity-flow processes. Confirmation of this depositional process is that the deposition rate increased from 2 cm/1,000 years to 10 cm/1,000 years when postglacial sea level flooded the bank tops. Thus 80 percent of the present sedimentation rate results from bank-top contribution. Furthermore, the calculated mass of mud traceable to Little Bahama Bank agrees excellently with the independent estimate of overproduction there. In cores, bank sediments resulting from the most recent sea-level highstand sharply overlie planktonic sediments, writing a sea-level history in alternating stable and unstable mineral suites.

Banktop responses to Quaternary fluctuations in sea level recorded in periplatform sediments

Periplatform sediment from a 12-m core recovered from Northwest Providence Channel, Bahamas, contains a geochemical and paleontological record of sea-surface temperatures, ice volumes, and the response of banktops to highstands of sea level. A comparison of these data suggests that fluctuations of carbonate mineralogy in periplatform sediments result from fluctuations of sea level and from patterns of banktop sedimentation. Highstands of sea level that flood carbonate platforms are recorded in periplatfonn sediments as abrupt increases of exported Sr-rich aragonite derived from banktop orgiuiisms superposed on a background of Sr-poor aragonite (pteropods) and calcite (foraminifera and coccoliths) derived from planktonic sources. The pulses of banktop sediment coincide with increases of water temperature determined from foraminiferal assemblages and with decreases in ratios of oxygen isotopes, indicating decreased ice volume and rising sea level. Following these abrupt changes is a more gradual decline in bank-derived sediment, although paleotemperatures from foraminiferal assemblages and oxygen isotopic data clearly show that warm conditions and a highstand of sea level persist. We suggest that this decrease of offbank transport is part of autocyclic sedimentation patterns of shallow-water carbonate environments. Offbank transport is restricted as reefs, sand shoals, and islands reach sea level. Also, green algal production may decline because progradation of tidal flats decreases lagoon area, and the hydrologic and ecologic conditions change as lagoons are filled.

Delineation of shallow stratigraphy using ground penetrating radar

Abstract Ground penetrating radar surveys were conducted at four sites to investigate the shallow stratigraphy and to determine the applicability and performance of the radar technique. A basic analog-recording radar system with a single 80 MHz antenna was used for all of the surveys. Good stratigraphic control existed at all of the sites so that the effectiveness of the radar could be evaluated. The four sites were distinctly different in composition and extent. At an upland farm site in the glaciated region of southwestern Ohio a clay-rich soil covers shallow bedrock. Some soil horizons were identifiable on the radar profiles, but only to a depth of 1.4 m. At a sand and gravel quarry in southwestern Ohio the depositional patterns of the unsaturated deposits were clearly imaged to a depth greater than 4 m. At a hydraulic fill dam in western Ohio the changes in the internal composition of the earthen fill were observed on the radar records to depths of 4 m. On San Salvador in the Bahamas radar profiles over partially consolidated carbonate sand revealed an extensive series of buried beach ridges to depths of 4 m. These data were useful in understanding the depositional history of the area, which could not be determined from surface and pit sampling alone. These studies show that ground penetrating radar is an important tool for studying shallow stratigraphy where the ground conductivity is low enough to permit radar reflections from depths of interest.

Cement Origin of Supratidal Dolomite, Andros Island, Bahamas

ABSTRACT Dolomite in supratidal carbonates of northwest Andros Island, Bahamas, can be seen, by means of scanning electron microscope (SEM), to be cement in primary porosity and not replacement of the carbonate sediments, as one widely held view suggests. These microcrystalline, calcian dolomite cements commonly occur in lithified lime mud crusts, but they also occur in unconsolidated sediments, mainly as intragranular precipitates. The voids in which the dolomite occurs vary in size from minute wall pores in rotaliid forams and endolithic borings to major intra- and interparticle pores. In the larger pores, dolomite commonly forms an isopachous fringe and may be followed by either aragonite or high-magnesium calcite (HMC) cement. Silt-sized bioclasts and pellets within crusts are often lithif ed by dolomite cement, which may totally fill much of the interparticle porosity. Within unpelleted lime mud, the interparticle porosity may also be filled by dolomite, which engulfs the needle mud particles. The needle mud inclusions or molds visible on polished, etched sections of this dolomite-filled mud are equivalent in abundance, size, and shape to the mud particles in non-dolomitic, unlithified muds embedded in plastic. That equivalence indicates that dolomite replacement of mud has not occurred. Framboidal pyrite present in a few samples appears to have formed prior to or contemporaneously with dolomite cement. The presence of pyrite and the depleted 13C values (-3.2 to -3.9 per mil) of dolomite-cemented crusts sug est that sulfate reduction may have had a role in the formation of Andros Island dolomite cement. Other Holocene and older dolomites from similar settings should be reexamined by means of SEM to determine if they formed partly or wholly as a cement rather than by replacement.

Effect of Hurricane Hugo on molluscan skeletal distributions,Salt River Bay, St. Croix, U.S. Virgin Islands

Just prior to the passage of Hurricane Hugo over St. Croix, U.S. Virgin Islands, 35 molluscan skeletal samples were collected at 30 m intervals along a sampling transect in Salt River Bay, on the north-central coast. Three months after the hurricane, the transect was resampled to permit direct assessment of storm effects on skeletal distributions. Results indicate that spatial zonation of molluscan accumulations, associated with environmental transitions along the transect, was maintained in the wake of the hurricane. However, limited transport was diagnosed by comparing the compositions of prestorm and poststorm samples from the deepest, mud-rich subenvironment on the transect. In aggregate, the species richness of samples from the southern half of this zone increased from 16 to 40, and the abundance of species that were not among the characteristic molluscs of this subenvironment increased from 11% to 26%. These storm effects could probably not have been recognized, and attributed directly to Hugo, had there been no prestorm samples with which to compare directly the poststorm samples.

New microtextural criterion for differentiation of compaction and early cementation in fine-grained limestones

Mechanisms of porosity reduction in fine-grained limestones (micrites), the most abundant type of limestone, have been difficult to evaluate because of the fine crystal sizes. Scanning electron microscopy reveals common minute voids termed microfenestrae ({approximately} 1.5-40{mu}m diameter) in Holocene lime muds from all marine carbonate environments studied. Experimental compaction of lime muds greatly reduces abundance of microfenestrae at pressures less than 10 bar. Thus, the abundance of cement-filled microfenestrae in micrites appears to be a widely applicable criterion for recognition of the extent of cementation prior to significant compaction in any given micrite. Microfenestrae in most Phanerozoic micrites studied are about as common as in Holocene lime muds, suggesting that early cementation is a more important mechanism in micrite porosity reduction than recent views have suggested.

Origin and Accumulation of Lime Mud in Ooid Tidal Channels, Bahamas

ABSTRACT Mud layers have been found within the ooid sands of Joulters Cays and Lee Stocking Island, Bahamas. A 1.3-m vibracore extracted from 4 m water depth in the Lee Stocking tidal channel contains a lower unit of dark brown muddy sand (skeletal rich) overlain by two layers of ooid sands intercalated with two layers of creamy, white mud. The two upper mud layers are aragonite-rich, sometimes pelleted, and contain very few skeletal grains. The contacts between the ooid sands and mud layers are commonly sharp, but some contacts show evidence of burrowing and mud clast formation. A mud layer with similar textural, mineralogic and petrographic characteristics was recovered from a 16-cm core from a tidal channel on Joulters Cays, Bahamas. This mud layer was also enclosed within ooid sands. Mud in the Lee Stocking ooid tidal channel is apparently of two origins. Mud in the lower unit of the core contains a more equal distribution of aragonite and calcite (50% each) and abundant skeletal grains of normal marine origin, indicating that the lower unit is lagoonal. Mud that is interlayered with ooid sands in the same core is dominated by aragonite (80%) and contains little sand-sized material, suggesting that it is not a typical lagoon mud. SEM examination also confirms that this mud is quite different from the lagoonal mud found at the base of the core. C-14 dating of the mud from the lagoonal unit shows that this sediment was deposited in water depths of 2 to 3 m approximately 5000 years ago when sea level was 3 m lower than present. C-14 dating coupled with knowledge of Holocene sea level indicates that both the ooid sands and the mud layers were also deposited as subtidal sediments. The mechanism by which the mud layers are accumulated with ooid sands is problematic. Suspension by storms and transport to tidal channels is one possibility; however, evaluation of the data suggests an alternative explanation. The post-lagoon history of deposition of the tidal channel may include nearly continuous restriction during which mud layers were deposited. The restriction was possibly caused by the formation and maintenance of ooid sand barri rs. Ooid sand deposition and burial of these layers may have accompanied barrier destruction.

Drinking water quality in the Mount Kasigau region of Kenya: a source to point-of-use assessment

Drinking water quality was investigated in seven rural villages surrounding Mount Kasigau in southeastern Kenya, where water is piped from unprotected dammed streams and springs in the Kasigau cloud forest down to taps, kiosks, and tanks in the villages. Analyses were conducted for nutrients, trace metals, and pathogen indicators in water from community taps, water stored in homes, and collection points along the pipelines up to catchment dams on the mountain. Water was relatively free from nutrient and trace-metal contamination; however, all samples were contaminated with total coliforms and nearly all were contaminated with Escherichia coli. There was no discernable pattern in the extent of contamination from the catchment dams to the villages. In each of three villages chosen for further study, six residents were selected for a more in-depth investigation. Water quality was generally worse in water stored in those homes compared to water collected at the village taps. The quality of drinking water in homes where treatment was applied was no better than in homes with no water treatment. The Kasigau villages, as many other areas in the developing world, need inexpensive and effective water treatment, as well as an assessment of the effectiveness of sanitary and hygienic practices.

Trends of Sedimentary Microfabrics of Ooid Tidal Channels and Deltas

Tidal channels within ooid sand shoals are involved in the longshore transport of ooids, the offbank and onbank transport of ooids and perhaps the generation of ooids. Channels are oriented perpendicular to the axis of highest ooid concentration and are linked to the transport system by their ebb-tidal deltas.