A. Conrad Neumann

Indicators of methane-derived carbonates and chemosynthetic organic carbon deposits: examples from the Florida Escarpment

Abyssal chemosynthetic communities are supported by bacterial oxidation of reduced chemicals in brines which seep out through sediments at the base of the Florida Escarpment. They are surrounded by carbonate hardgrounds and sediments rich in fresh organic carbon that contain a record of the metabolic pathways and geochemical processes which are active at these sites. The isotopic composition of tissue samples (δ 13 C as low as #7576.40∓), carbonate crusts (δ 13 C as low #7545.19∓) and sedimentary organic matter (δ 13 C as low #7567.87∓) indicate that biogenic methane dissolved in the brines (δ 13 C #7583.3 ± 5.8∓) is a major carbon source for many of the locally synthesized compounds

Rapid sea-level changes at the close of the last interglacial (substage 5e) recorded in Bahamian island geology

Rapid and abrupt relative sea-level changes within the last interglacial (substage 5e) are recorded in the island geology of the tectonically stable Bahamas. From 132 to 118 ka, reef growth reached a maximum elevation near +2 m, as indicated by fossil reef elevation across the platform, whereas bioeroded notches are incised in coastal cliffs as high as +6 m. The end of the interval is characterized by voluminous eolianites exhibiting palm tree and frond impressions. It is inferred that sea level for most of the interval remained near +2 m, restraining reef growth, and that the notch at +6 m represents a rapid and brief excursion just before the close of the substage. The subsequent fall must have been rapid in order to leave the notch profile intact and mobilize windward lagoon ooids into dunes before cementation could anchor them. In order to explain the rapid rise to +6 m, glacial surging is invoked. The subsequent fall, also rapid, may be a consequence of the surge flooding high latitudes and providing enough moisture to initiate reglaciation and drawdown.

Sea level events and Pleistocene coral ages in the northern Bahamas

Abstract Emergent Pleistocene sea level indicators in the northern Bahamas include: a bioerosional notch at +5.3 to 5.9 m; sea caves, notches, and marine terraces at about +4.3 m; and lithified coral rubble and reef deposits between 0 and 3 m. Thorium 230 dates of the fossil corals, which were deposited as these features were being produced, span the age range from 100 to 145 thousand years BP with a majority falling between 115 and 130 thousand years BP. The notch at about +5.6 m is interpreted to be the product of a sea level stand 125 thousand years BP, while the features at +4.3 m are believed to be formed sometime later as sea level fell from the higher position. Part of the age span is inherent in the dating technique and possible sample alteration. Another cause of the spread may be mixing of corals of different ages into a single deposit.

Offbank transport of carbonate sands along open, leeward bank margins: Northern bahamas

Abstract Vigorous offbank transport of carbonate sands occurs along the west-facing, leeward, open margins of Little and Great Bahama Bank. Large, offbank-oriented sand waves, thick sand bodies covering reefs, and shallow-water sands (mostly non-skeletal) obtained from adjacent deep flanks, all demonstrate the existence of this shallow-to-deep pathway of sediment transport. A variety of data suggests that transport occurs during storms and not during normal tidal-current fluctuations. An abundance of composite grains as well as vertical sequences of submarine cemented horizons (seen via seismic profiling within the marginal sand bodies) indicate cyclic periods of relative quiescence followed by moments of intense sediment movement. Current-meter data obtained during six weeks of low wind activity show that critical threshold velocities for sand transport are only barely exceeded during portions of the tidal cycle. These flows coupled with higher-frequency bottom currents (generated by surface gravity waves) winnow the sands but do not provide significant net offbank transport. Offbank sand transport provides material to the shallow and deep bank margins enabling them to expand during sea-level highstands. Each sea-level fluctuation is likely to produce within the shallow margins a vertical sequence of basal reefs, covered by a thick non-skeletal calcarenite, capped by a subaerially exposed, diagenetically altered surface. Shallow-water sands carried off the marginal escarpment help to construct deep flanks and large sediment drifts.

Carbonate Sand Bodies Along Contrasting Shallow Bank Margins Facing Open Seaways in Northern Bahamas

High-resolution seismic profiling, bottom sampling, and aerial and space imagery show that widely contrasting carbonate sand bodies are present along the shallow windward, leeward, and tide-dominated margins of Little and Great Bahama Banks which face open seaways--the Northwest Providence Channel and the northern Straits of Florida. This high diversity in sand bodies and their associated bank margins is due to variations in the level, duration, and magnitude of the physical energy flux across the edges of the platforms. In the Bahamas, the dominant winds, waves, and currents are from the east; hence easterly facing margins are windward, westerly facing margins are leeward. Where strong tidal currents are present (because of adjacent basin shape) the windward and leeward effects are diminished. Thus, bank-margin orientation relative to the climatic energy patterns appears to assert the major controls upon sedimentation. Antecedent topography in the form of islands or subtidal rock ridges creates energy barriers which can control the direction and volume of sediment flux on or off the banks. Along windward margins, energy barriers such as large islands not only block the bankward transport of sands but appear to augment offbank transport by setting up seaward flows. In contrast, along leeward margins, energy barriers prevent the vigorous offbank sand transport that normally occurs when these margins are open and water flow is unrestricted. In addition, sand-body geometry and grain type are controlled by bank-margin orientation. Along windward, open margins, skeletal sands accumulate in the lee of low-relief, mostly relict reefs. These margins are generally sediment-barren, having only a sporadic, thin veneer of sand. Along leeward, open margins a broad (10 × 10 km), thick (12 m), bankward-thinning blanket of sand rich in nonskeletal components (peloids, composite grains) lies on top of rock ridges and reefs. Large, asymmetric sand waves superimposed on this sand body indicate net offbank sand transport. Along the tide-dominated margin, large, broad, oolitic sand lobes have migrated onto adjacent lagoonal muddy sands and indicate net bankward transport. We suggest the following classification of modern, shallow, carbonate bank margins: (1) windward open; (2) windward protected; (3) leeward open; (4) leeward protected; (5) tide dominated. This classification seems to fit many ancient analogs and may be useful in paleoenvironmental reconstructions.

Rapid sea level and climate change at the close of the Last Interglaciation (MIS 5e): evidence from the Bahama Islands

The geology of the Last Interglaciation (sensu stricto, marine isotope substage (MIS) 5e) in the Bahamas records the nature of sea level and climate change. After a period of quasi-stability for most of the interglaciation, during which reefs grew to +2.5 m, sea level rose rapidly at the end of the period, incising notches in older limestone. After brief stillstands at +6 and perhaps +8.5 m, sea level fell with apparent speed to the MIS 5d lowstand and much cooler climatic conditions. It was during this regression from the MIS 5e highstand that the North Atlantic suffered an oceanographic “reorganization” about 118±3 ka ago. During this same interval, massive dune-building greatly enlarged the Bahama Islands. Giant waves reshaped exposed lowlands into chevron-shaped beach ridges, ran up on older coastal ridges, and also broke off and threw megaboulders onto and over 20 m-high cliffs. The oolitic rocks recording these features yield concordant whole-rock amino acid ratios across the archipelago. Whether or not the Last Interglaciation serves as an appropriate analog for our “greenhouse” world, it nonetheless reveals the intricate details of climatic transitions between warm interglaciations and near glacial conditions.

Organic geochemistry and pore water chemistry of sediments from Mangrove Lake, Bermuda

Abstract Mangrove Lake, Bermuda, is a small coastal, brackish-water lake that has accumulated 14 m of banded, gelatinous, sapropelic sediments in less than 10 4 yr. Stratigraphic evidence indicates that Mangrove Lakes sedimentary environment has undergone three major depositional changes (peat, freshwater gel, brackish-water gel) as a result of sea level changes. The deposits were examined geochemically in an effort to delineate sedimentological and diagenetic changes. Gas and pore water studies include measurements of sulfides, ammonia, methane, nitrogen gas, calcium, magnesium, chloride, alkalinity, and pH. Results indicate that sulfate reduction is complete, and some evidence is presented for bacterial denitrification and metal sulfide precipitation. The organic-rich sapropel is predominantly algal in origin, composed mostly of carbohydrates and insoluble macromolecular organic matter called humin with minor amounts of proteins, lipids, and humic acids. Carbohydrates and proteins undergo hydrolysis with depth in the marine sapropel but tend to be preserved in the freshwater sapropel. The humin, which has a predominantly aliphatic structure, increases linearly with depth and composes the greatest fraction of the organic matter. Humic acids are minor components and are more like polysaccharides than typical marine humic acids. Fatty acid distributions reveal that the lipids are of an algal and/or terrestrial plant source. Normal alkanes with a total concentration of 75 ppm exhibit two distribution maxima. One is centered about n -C 22 with no odd/even predominance, suggestive of a degraded algal source. The other is centered at n -C 31 with a distinct odd/even predominance indicative of a vascular plant origin. Stratigraphic changes in the sediment correlate to observed changes in the gas and pore water chemistry and the organic geochemistry.

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.

Continental margin brine seeps: Their geological consequences

Local dissolution observed at the base of the Florida Escarpment appears to result from acid generated by sea-floor oxidation of dissolved sulfide species carried in brines that seep from the base of the escarpment. The extent of seep corrosion depends upon the amount of sulfide that has exited the platform and the efficiency with which the acid attacks the limestones of the escarpment. Calculations suggest that brine seepage influences the shape of these escarpments by undercutting and steepening the carbonate continental margin edges. Other processes competing for the dissolved sulfides are bacterial chemosynthesis and sulfide mineralization, which produces organic-carbon- and sulfide-mineral-rich deposits along the unconformity that separates the buried escarpment from onlapping abyssal sediments.

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.