Geology topics
E.A. Shinn
Publications and source records attributed to E.A. Shinn.
African dust and the demise of Caribbean coral reefs
The vitality of Caribbean coral reefs has undergone a continual state of decline since the late 1970s, a period of time coincidental with large increases in transatlantic dust transport. It is proposed that the hundreds of millions of tons/year of soil dust that have been crossing the Atlantic during the last 25 years could be a significant contributor to coral reef decline and may be affecting other ecosystems. Benchmark events, such as near synchronous Caribbean-wide mortalities of acroporid corals and the urchin Diadema in 1983, and coral bleaching beginning in 1987, correlate with the years of maximum dust flux into the Caribbean. Besides crustal elements, in particular Fe, Si, and aluminosilicate clays, the dust can serve as a substrate for numerous species of viable spores, especially the soil fungus Aspergillus . Aspergillus sydowii , the cause of an ongoing Caribbean-wide seafan disease, has been cultured from Caribbean air samples and used to inoculate sea fans.
Maps showing sedimentary and biological environments, depth to Pleistocene bedrock, and Holocene sediment and reef thickness from Molasses Reef to Elbow Reef, Key Largo, south Florida
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Contrasts within an outlier-reef system: Evidence for differential quaternary evolution, south Florida windward margin, U.S.A.
Closely spaced, high-resolution, seismic-reflection profiles acquired off the upper Florida Keys (i.e., north) reveal a platform-margin reef-and-trough system grossly similar to, yet quite different from, that previously described off the lower Keys (i.e., south). Profiles and maps generated for both areas show that development was controlled by antecedent Pleistocene topography (presence or absence of an upper-slope bedrock terrace), sediment availability, fluctuating sea level, and coral growth rate and distribution. The north terrace is sediment-covered and exhibits linear, buried, low-relief, seismic features of unknown character and origin. The south terrace is essentially sediment-free and supports multiple, massive, high-relief outlier reefs. Uranium disequilibrium series dates on outlier-reef corals indicate a Pleistocene age (~83-84 ka). A massive Pleistocene reef with both aggradational (north) and progradational (south) aspects forms the modern margin escarpment landward of the terrace. Depending upon interpretation (the north margin-escarpment reef may or may not be an outlier reef), the north margin is either more advanced or less advanced than the south margin. During Holocene sea-level rise, Pleistocene bedrock was inundated earlier and faster first to the north (deeper offbank terrace), then to the south (deeper platform surface). Holocene overgrowth is thick (8 m) on the north outer-bank reefs but thin (0.3 m) on the south outlier reefs. Differential evolution resulted from interplay between fluctuating sea level and energy regime established by prevailing east-southeasterly winds and waves along an arcuate (ENE-WSW) platform margin.
Sea-level records at ~80 ka from tectonically stable platforms: Florida and Bermuda
Studies from tectonically active coasts on New Guinea and Barbados have suggested that sea level at ∼ 80 ka was significantly lower than present, whereas data from the Atlantic and Pacific coasts of North America indicate an ∼ 80 ka sea level close to that of the present. We determined ages of corals from a shallow submerged reef off the Florida Keys and an emergent marine deposit on Bermuda. Both localities are on tectonically stable platforms distant from plate boundaries. Uranium-series ages show that corals at both localities grew during the ∼80 ka sea-level highstand, and geologic data show that sea level at that time was no lower than 7–9 m below present (Florida) and may have been 1–2 m above present (Bermuda). The ice-volume discrepancy of the 80 ka sea-level estimates is greater than the volume of the Greenland or West Antarctic ice sheets. Comparison of our ages with high-latitude insolation values indicates that the sea-level stand near the present at ∼80 ka could have been orbitally forced.
Magnitude and timing of episodic sea-level rise during the last deglaciation
A succession of elevated ridge deposits on the south Florida margin was mapped using high-resolution seismic and side-scan sonar imaging in water depths ranging from 50 to 124 m. The ridges are interpreted to be subtidal shoal complexes and paleoshorelines (eolian dune or beach) formed during the last sea-level transgression. Oolitic and skeletal grainstones and mixed skeletal-peloidal-ooid packstones were recovered using a research submersible. All of the grains are of shallow-water or intertidal origin, and both marine and nonmarine cements were identified.
A giant sediment trap in the Florida keys
Aerial photography, high-resolution seismic profiling, coring and jet probing have revealed a large sediment-filled sinkhole in the Key Largo National Marine Sanctuary off Key Largo, Florida. The 600-m-diameter feature straddles coral reef and carbonate-sand facies and contains >55 m of marine lime sand and aragonite mud. Bulk 14C age determinations of mud from a 30- m sediment core indicate infilling rates exceeding 20 m/ka between 3 and 5.6 ka. The total thickness and nature of the sediment near the base of the sinkhole are not known.
Lime-mud layers in high-energy tidal channels: A record of hurricane deposition
During or immediately following the transit of Hurricane Andrew (August 23-24, 1992) across the northern part of the Great Bahama Bank, thin laminated beds of carbonate mud were deposited in high-energy subtidal channels (4 m depth) through the ooid shoals of south Cat Cay and Joulters Cays. During our reconnaissance seven weeks later, we observed lime-mud beds exposed in the troughs of submarine oolite dunes and ripples. The mud layers were underlain and locally covered by ooid sand. The mud beds were lenticular and up to 5 cm thick. Their bases cast the underlying rippled surface. The layers were composed of soft silt- and sand-sized pellets and peloids and in some areas contained freshly preserved Thalassia blades and other organic debris along planes of lamination. The beds had a gelatinous consistency and locally had been penetrated by burrowers and plants. Layers of lime mud had also settled on bioturbated, plant-stabilized flats and in lagoonal settings but were quickly reworked and made unrecognizable by the burrowing of organisms. Thicker, more cohesive (and therefore older) mud beds and angular mud fragments associated with ooids from Joulters Cays have similar characteristics but lack fresh plant fragments. We infer that these older beds were similarly deposited and thus record the passage of previous hurricanes or tropical storms. Storm layers are preserved within channel sediments because migrating ooids prevent attack by the burrowing activity off organisms.
Paleoshorelines, reefs, and a rising sea: South Florida, USA
The porous limestone bedrock, thin sediment cover, and tectonic stability of the Florida Platform during the past 15 ka BP provide an exceptionally suitable setting for reconstruction of paleoshorelines and onshore projection of future shorelines in a rising-sea scenario. Paleoshorelines for 8, 6, 4 and 2 ka BP show that 1) a series of limestone islands formed, then drowned, along the outer platform; 2) a distinct trough, called Hawk Channel, separated the outer islands from shore; 3) the lower Keys flooded earlier and more rapidly than the rest of the Keys; and 4) Florida Bay and tidal passes through the middle Keys into the bay developed within the past 4 ka BP. Projection of future shorelines onto land shows that most land forming the Florida Keys would flood in a rise of 1 to 2 m and that a rise of little more than 5 m would submerge all land. Offshore reefs would die, while nearshore reefs would shift landward as the mainland shoreline migrated northward. -from Authors
Underwater research methods for study of nuclear bomb craters, Enewetak, Marshall Islands
Three craters, created by the explosion of nuclear fusion devices, were mapped, sampled, core drilled and excavated with airlifts at Enewetak Atoll in the Marshall Islands by using scuba and a research submersible. The craters studied were Mike, Oak, and Koa. Tests took place near sea level at the transition between lithified reef flat and unlithified lagoonal sediments, where water depth ranged from 1 to 4 m. Craters produced by the blasts ranged from 30 to 60 m in depth. The purpose of our study was to determine crater diameter and depth immediately after detonation. Observations of submerged roadways and testing structures and upturned crater rims similar to those characteristic of meteor impacts indicate that the initial, or transient, craters were smaller than their present size. At some later time, while the area was too radioactive for direct examination, the sides of the craters slumped owing to dewatering of under lying pulverized rock. Core drilling of crater margins with a diver-operated hydraulic coring device provided additional data. On the seaward margin of the atoll, opposite Mike, a large portion of the atoll rim approximately the size of a city block had slumped into the deep ocean, leaving a clean vertical rock section more than 400m high. An abundance of aggressive grey reef sharks displaying classic territorial behavior prevented use of scuba at the Mike slump site. The two-person submersible R.V. Delta provided protection and allowed observations down to 300 m. During the 6-week period of study, we made more than 300 scuba and 275 submersible dives. Mapping was with side scan sonar and continuous video sweeps supplemented by tape-recorded verbal descriptions made from within the submersible. A mini-ranger navigation system linked to the submersible allowed plotting of bottom features, depth and sediment type with spatial accuracy to within 2 m.
Whitings, a sedimentologic dilemma
Whitings, drifting clouds of water, milky because of suspended carbonate, have been claimed to originate from either the action of bottom-feeding fish or direct precipitation of calcium carbonate. Five cruises during different seasons were made to the Great Bahama Bank to collect data pertinent to the controversy. Measurements of particulate concentrations average 10 mg/liter with a maximum of 20 mg/liter of carbonate sediment suspended in whiting water, compared with an average of 1.5 mg/liter for clear water outside the whitings. The particles are dominantly acicular aragonite, but Mg calcite composes as much as 20 percent of some whitings. Sedimentation rates, measured with fixed and drifting sediment traps, were as great as 34 g/m 2 /hr. Sediment suspended in whitings aggregated into silt- and sand-size fioccules and settled to the bottom of settling tanks within six hours, even on a rocking ship. Sediment in artificial whitings, created by stirring sediment from the bottom with a shrimp trawl, settled to the bottom in about the same time. Natural whitings, on the other hand, were never observed to dissipate. Because sedimentation from whitings occurs at rates sufficient to cause dissipation of the whitings within six hours, we conclude that the natural whitings are continually replenished by direct precipitation. The search for fish in whitings utilized sidescan sonar and fathometer imaging, shrimp trawls, rotenone, remote video, and direct scuba observation. These methods and 25 years of casual observations leading to this study indicate that fish are not involved in the formation of most Bahamian whitings. Several whitings were found over rocky or sandy bottoms where there was no mud available for fish to suspend. The distance of these whitings from areas of muddy bottom precluded their having been made elsewhere by fish. Stable carbon- and oxygen-isotopic analyses and Delta 14 C activity are interpreted to indicate that the suspended sediment in whitings contains some precipitated calcium carbonate and is not merely bottom sediment stirred into suspension. Estimates indicate that the amount of new carbonate produced in whitings on the Great Bahama Bank is substantially higher than that arising from algal production. Consequently, the amount of sediment transported to deep water may be much greater than previously thought.
Artificial reef observations from a manned submersible off southeast Florida
Examination of 16 artificial reef structures in depths ranging from 30-120m indicated that the highest numbers of fish are found around reefs in water shallower than 46m. Fewer fish, especially those with tropical coral reef affinities, <46m was probably caused by a thermocline. Algae and reef community encrusters, abundant on shallower structures, were absent below 46m. Structures that penetrated above the thermocline, such as upright oil rigs and a hopper barge, were also effective reefs. The open structure and high profile of the rigs enhance their use as artificial reefs by providing a range of well-aerated habitats. Greatest diversity and numbers of fish were observed at the Miami sewer outfall. -from Authors
The geology of the Florida Keys
The Florida reefs have evolved over the last 7000 years during rising sea level. An ancient reef existed from sand shoals on which Miami and Key West are now sited before the last glaciation (100 000 to 125 000 years ago). During the glacial period sea level fell to >300 feet below its present level. Since sea level rose again invading the south Florida shelf again, corals have returned. The bulk of the Florida reef tract is composed of carbonate sand, some coming from corals but most from calcified algae. Future scenarios are considered for the Florida reefs assuming sea level to remain stable or to continue to rise. -J.G.Harvey
Giant subtidal stromatolites forming in normal salinity waters
We report here the discovery of giant lithified subtidal columnar stromatolites (>2 m high) growing in 7-8 m of clear oceanic water in current-swept channels between the Exuma Islands on the eastern Bahama Bank. They grow by trapping ooid and pelletal carbonate sand and synsedimentary precipitation of carbonate cement within a field of giant megaripples. The discovery is important to geologists and biologists because similar organo-sedimentary structures built by a combination of cementation and the trapping of sediment by microbes were the dominant fossil types during the Precambrian. Stromatolites are thought to have been responsible for the production of free oxygen and thus the evolution of animal life1,2. Until the discovery of small lithified subtidal columnar stromatolites in the Bahamas3, the only subtidal marine examples known to be living while undergoing lithification were in the hypersaline waters of Hamelin Pool at Shark Bay, Western Australia4-7. Shark Bay stromatolites range from intertidal to the shallow subtidal with the larger columns reaching 1 m in height. The Shark Bay stromatolites have strongly influenced geological interpretation; by analogy, many ancient stromatolites have been considered to have grown in intertidal and/or hypersaline conditions8, although hypersalinity was not a necessity for growth during the Precambrian because grazing metazoan life had not then evolved. ?? 1986 Nature Publishing Group.
Holocene carbonate sedimentary petrology and facies accumulation, Looe Key National Marine Sanctuary, Florida.
Suggests a greater production of coralline sand (rather than slower Halimeda growth) results from a marked acceleration of bioerosion of abundant dead reefs in the area. -from Authors
Mechanical and chemical compaction in fine-grained shallow-water limestones
Artificial compaction of in-situ cores of sediments resulted in: 1) reduction of sediment thickness by 50 percent and more; 2) reduction of initial porosities of 65 to 75 percent to 35 to 45 percent; 3) creation of megascopic textures almost identical to many ancient lime mud- and wackestone; 4) creation of organic, wispy "stylolite-like" layers; 5) chemical compaction, evidenced by thin sections showing quartz grains piercing mollusc shells without causing fractures and SEM evidence of solutional interfitting of 1 to 4-mu m-size aragonitic carbonate grains; 6) obliteration of pellets and birdseye or fenestral voids in those sediments where early cementation was lacking; obliteration of identifiable marine grasses and vertical "root" tube voids; 8) mashing of sediment-filled circular burrows to produce ellipsoidal structures. Significant mechanical compaction resulted from pressures simulating less than 1,000 ft of burial. Increasing loads to more than 10,000 ft did not significantly increase compaction. Chemical compaction was detected only in cores compacted to pressures greater than 10,000 ft of burial. These experiments suggest that chemical compaction would begin at much shallower depths given geologic time. Experiments that caused chemical compaction lend support to the hypothesis that cement required to produce a low-porosity/low-permeability fine-grained limestone is derived internally. Dissolution, ion diffusion, and reprecipitation are the most likely processes for creating significant thicknesses of dense limestones. Continuation of chemical compaction after significant porosity reduction necessitates expulsion of connate fluids, possibly including hydrocarbons.--Modified journal abstract.
Birdseyes, fenestrae, shrinkage pores, and loferites: a reevaluation
Birdseyes, birdseye limestone, fenestrae, fenestral fabric, shrinkage pores, and loferites are considered similar or synonymous when occurring in lime mudstone or syndepositional dolomite, especially in association with mudcracks and stromatolites. Compaction experiments indicate, however, that without early cementation, these vugs can be obliterated, whereas mudcracks and stromatolites are unchanged. Lumping of burrows and root tubes under the general terms birdseyes or fenestrae, such as for example tubular fenestrae, burrow fenestrae, or root-tube fenestrae, is discouraged. They should be called burrows or root tubes because the birdseyes, fenestrae, and shrinkage pores are so intimately associated with tidal flats to most geologists. Submarine cementation of pelletal and oolitic botryoidal grainstone under 5 to 6 m of water on the Bahama Banks has resulted in vugs provocatively similar to many birdseyes and fenestrae generally attributed to peritidal conditions in the literature. Birdseyes and fenestrae in ancient grainstones cannot therefore be reliably identified without consideration of the sedimentary sequence and associated sedimentary structures.--Modified journal abstract.
Effects of offshore oil drilling on Philippine reef corals.
An offshore drilling site in an area of extensive live-coral bottom off NW Palawan Island, Philippines, was examined 15 months after well completion. Porites lutea growth rates showed that little suppression of head coral growth could be attributed to drilling. Diver observation, however, together with analysis of sampling transect photomosaics, revealed 70-90% reduction in foliose, branching, and plate-like corals in an iron-stained area that extended out from the wellheads in a 115 X 85-m ellipse.-from Authors