Harry H. Roberts

Hydrocarbon-derived carbonate buildups of the northern Gulf of Mexico continental slope: A review of submersible investigations

Hydrocarbon-derived and microbially mediated authigenic carbonates occur over the entire depth range of the northern Gulf of Mexico slope. These carbonates consist of nodules and incipient nodules in surface sediments, hardgrounds and isolated slabs, and moundlike buildups of up to 10–20 m relief above the surrounding seafloor. The authigenic carbonates are characterized byδ13C negative values in the range −18‰ to −55‰ (PDB) suggesting mixing of seawater carbon with13C-depleted carbon sources ranging from crude oil to biogenic methane. Near the shelf edge, carbonates are “diluted” with biogenic material produced by reefs—bioherms developed at low sea level stands. Fossil-poor carbonates over salt diapirs of the upper and middle slope formed in the shallow subsurface and have been exhumed by the combined processes of uplift and physical erosion. Middle and lower slope carbonates are generally rich in fossil shells of chemosynthetic organisms. Mg calcite pelloidal matrix and acicular to botryoidal aragonitic void-filling cements are common petrographic features of these hydrocarbonderived carbonates. At two sites carbonates are mixed with barite.

Impact of the Deepwater Horizon oil spill on a deep-water coral community in the Gulf of Mexico

To assess the potential impact of the Deepwater Horizon oil spill on offshore ecosystems, 11 sites hosting deep-water coral communities were examined 3 to 4 mo after the well was capped. Healthy coral communities were observed at all sites >20 km from the Macondo well, including seven sites previously visited in September 2009, where the corals and communities appeared unchanged. However, at one site 11 km southwest of the Macondo well, coral colonies presented widespread signs of stress, including varying degrees of tissue loss, sclerite enlargement, excess mucous production, bleached commensal ophiuroids, and covering by brown flocculent material (floc). On the basis of these criteria the level of impact to individual colonies was ranked from 0 (least impact) to 4 (greatest impact). Of the 43 corals imaged at that site, 46% exhibited evidence of impact on more than half of the colony, whereas nearly a quarter of all of the corals showed impact to >90% of the colony. Additionally, 53% of these corals’ ophiuroid associates displayed abnormal color and/or attachment posture. Analysis of hopanoid petroleum biomarkers isolated from the floc provides strong evidence that this material contained oil from the Macondo well. The presence of recently damaged and deceased corals beneath the path of a previously documented plume emanating from the Macondo well provides compelling evidence that the oil impacted deep-water ecosystems. Our findings underscore the unprecedented nature of the spill in terms of its magnitude, release at depth, and impact to deep-water ecosystems.

Chemosynthetic bacterial mats at cold hydrocarbon seeps, Gulf of Mexico continental slope

Abstract White and pigmented filamentous bacterial mats dominated by several undescribed species of Beggiatoa were sampled during research submersible dives to cold hydrocarbon seep sites on the upper continental slope off Louisiana (130–550 m). Mats occur at the interface between reducing sediments and the oxygenated water column. They are localized at sea floor features related to seepage of biogenic methane and crude oil, but there is little evidence that the organisms utilize the hydrocarbons directly. Granules of elemental sulfur (S0) are visible within cells of Beggiatoa, and mat material is characterized by high contents of S0 (up to 193,940 ppm). The Beggiatoa biomass is isotopically light ( δ 13 C = −27.9‰ PDB ). Our geochemical data suggest that the Beggiatoa species are part of a complex bacterial assemblage in cold seep sediments. They oxidize H2S derived from the bacterial sulfate reduction that accompanies bacterial hydrocarbon oxidation when O2 is depleted in sediments, and fix isotopically light carbon from CO2 that is the result of bacterial hydrocarbon oxidation. Beggiatoa mats appear to retard loss of hydrocarbons to the water column by physically retaining fluids in sediments, a function that could enhance production by other bacteria of the H2S and CO2 needed by Beggiatoa.

Cold-water stress in Florida Bay and northern Bahamas; a product of winter cold-air outbreaks

ABSTRACT During January 1977 three consecutive cold fronts crossed south Florida and the northern Bahamas which depressed shallow-water temperatures below the lethal limit for most reef corals. Digital thermal infrared data acquired by the NOAA-5 meteorological satellite, in situ water temperatures, and meteorological data were used to study the thermal evolution of Florida Bay and Bahama Bank waters. The third and most important frontal system depressed Florida Bay water below 16° C, a thermal stress threshold for most reef corals, for 8 days. The minimum water temperature recorded in situ was 12.6° C. Satellite data suggest that some Florida Bay coastal waters were at least 1° C cooler than water at this site. Cold-water plumes (detected on satellite imagery) suggest that offshelf or offbank movement of cold, dense water follows bathymetry-controlled routes. Absence of viable shelf reefs opposite tidal passes supports this contention. Coral mortality at Dry Tortugas was up to 91 percent during the 1977 event. Coral and fish kills were also reported from other parts of the Florida Reef Tract and northern Bahamas. Study results show that cold-water stress conditions can exist over vast shallow-water areas and have residence times of several days. These observations suggest that aperiodic chilling processes have a limiting influence on reef community development throughout the Florida Reef Tract and northern Bahamas.

Radiometric dating of submarine hydrocarbon seeps in the Gulf of Mexico

Massive abiotic carbonates and calcareous shells of the chemosynthetic mytilid Bathymodiolus sp. containing a detailed history of hydrocarbon seepage were investigated using radiocarbon and U-series isotopes. Stable carbon isotopes and 87 Sr/ 86 Sr ratios were also determined in order to provide insights on the carbon source and the nature of the hydrocarbon-rich fluids. Samples from five seepage sites on the northern Gulf of Mexico sea floor overlying subsurface salt diapirs and encompassing depths from 125 to >2000 m were selected as representative of the spectrum of active and extinct seeps examined from submersible dives. In general, paired 14 C and 230 Th dates of carbonate buildups consisting of aragonite, high-Mg calcite, and dolomite mineralogies are highly discordant. The cause of the discordance, established on the basis of paired Δ 14 C and δ 13 C values (Δ 14 C = −898‰ to −992‰ δ 13 C = −9.5‰ to −53.3‰, for n = 27), lies with the impairment of the radiocarbon dates resulting from dilution of the 14 C pool with fossil-hydrocarbon–derived carbon. The validity of the ionium dates based on U-rich (2.4–7.6 ppm) samples is demonstrated by the concordance between 234 U/ 238 U and 230 Th/ 234 U evolution in time, and by the ( 234 U/ 238 U) o activity ratios that are generally within the range of sea-water value of 1.14 ± 0.04 (2 sigma). Some dolomite-rich samples are exceptional because their ( 234 U/ 238 U) o ratios are significantly higher (1.22–1.38) than sea water, suggesting deposition from anoxic pore waters where the soluble U reached anomalous 234 U/ 238 U ratios. The formation of the carbonates from sea-water–derived fluids, rather than from formation fluids advecting from deep aquifers, is supported by the 87 Sr/ 86 Sr composition of the samples (mean 0.709145 ± 19 × 10 −6 , n = 14) that compares well with modern nonseep marine carbonates and the ambient Gulf of Mexico sea water (0.709171 ± 8 × 10 −6 ). Calcareous shells, the δ 13 C values of which indicate a carbon source in sea water (δ 13 C = −4.3‰ to −1.1‰), yield valid radiocarbon ages and show fair concordancy between their radiocarbon and ionium dates. Isotope migration attested by the observed U uptake in the fossil shells is likely to affect the accuracy of their ionium dates. Radiometric ages from extinct and senescent seep sites at upper bathyal depths indicate that hydrocarbon seepage occurred there during late Pleistocene time (195–13 ka). Ages derived from nascent seep sites at mid-bathyal and abyssal depths (12.3–0.0 ka) indicate that currently vigorous seepage was initiated at the end of the last deglaciation. These radiometric ages most likely reflect the time of sedimentary loading and associated salt diapirism that activated the fault conduits to the sea floor.

Biologic and geologic responses to physical processes: examples from modern reef systems of the Caribbean-Atlantic region

Abstract Coral reefs and associated depositional environments of the Caribbean-Atlantic region have characteristics that reflect control by physical processes, both oceanic and atmospheric. Wave direction and wave power help determine sites for productive reef development and shape reef morphology as well as community structure. Spur and groove orientations reflect changes in direction of waves as they refract across a reef-dominated shelf. Abrupt topography of reef-dominated shelf margins interacts with tidally modulated flows to create an energetic and productive deep reef environment which is buffered from the modifying effects of forceful wave action. Shallow wave-reef interactions involve dissipative effects of wave breaking, turbulence, and friction, resulting in measured wave energy transformations ranging from 72 to 97% depending on reef configuration and water depth. Dissipative processes produce strong reef-normal surge currents that transport sediment lagoonward, drive backreef lagoon circulation, and influence fluid flow and diagenesis within the reef. The intensity of these processes is modulated at the tidal frequency. Other long period waves (infragravity) are important agents of mass transport of water and fine sediment. Low speed, long duration currents forced by long waves are potentially important for transporting larvae as well as fine sediment out of a given reef-lagoon system. Ocean-scale currents impinging on steep island and continental margin topography may cause reef-limiting upwelling and nutrient loading. The Caribbean Current upwells on the Nicaragua shelf and carbonate platforms of the Nicaraguan Rise. High trophic resources favor algal rather than coral communities and large (20–30 m relief) Halimeda biotherms occupy niches normally reserved for coral reefs. Thermodynamic air-sea interactions (heat, moisture and momentum flux) regulate the physical properties of reef lagoon and bank top waters. In extra-tropical reef settings (e.g. Bermuda, Florida, Bahamas and Arabian Gulf) cold air outbreaks cause precipitous drops in bank water temperatures and significant increases in bank water salinity and suspended sediment load. Water temperatures are routinely forced below the limit for survival of reef corals and many species of calcareous green algae. Associated increases in the density of shallow waters produce a disequilibrium with surface waters of the adjacent ocean favoring shelf transport to deep water sites of reef development and beyond.

Black Phytokarst from Hell, Cayman Islands, British West Indies

Phytokarst is a distinctive landform resulting from a curious type of biologic erosion. Filamentous algae bore their way into limestone to produce black-coated, jagged pinnacles marked by delicate, lacy dissection that lacks any gravitational orientation. Ordinary rainfall-produced karst and littoral karst are characterized by flat-bottomed pans and vertically oriented flutes, thus differing from phytokarst. Algae attack by dissolving calcite preferentially to dolomite.

Water level and currents of tidal and infragravity periods at Tague Reef, St. Croix (USVI)

Abstract A two-week study, at Tague Reef, St. Croix, USVI investigated the magnitude and spatial variation of tides, sea level differences, infragravity waves, and unidirectional cross-reef currents on a modern coral reef. Infragravity oscillations of water level (∼ 27 min period) of 1–2 cm height correlate with a quarter wavelength resonance over the shelf. Particle displacements associated with these waves may be important to the dispersive characteristics of the reef environment. Estimates of cross-reef mass transport per unit width ranged from 0.058 to 0.032 m2s -1. Sea level differences across the reef (1–4 cm) varied at diurnal and infragravity periods with contributions from wave set-up, and a small contribution from cross-shelf wind stress to the observed sea level differences. The quadratic bottom friction coefficient over the reef was estimated at 0.06–0.2, 20–70 times greater than on open shelves, reflecting the reef’s extreme bottom roughness.

Sea floor responses to hydrocarbon seeps, Louisiana continental slope

Observations and samples from research submersible dives confirm that brines, crude oil, fluid mud, and gases are common seep products. Through this mechanism a unique interplay of geochemical, geologic, and biological processes resulting in unusual sea floor features ranging from carbonate-rich nodular sediments to mounds with tens of meters relief. Stable carbon isotopes occluded in the carbonates provide a permanent imprint that links these authigenic carbonates to by-products of microbial breakdown of crude oil and gas. Recent DSV ALVIN dives confirm that hydrocarbon seeps and their accompanying chemosynthetic communities and authigenic carbonate mounds occur over the entire depth range of the slope.

Morphology and sedimentology of Halimeda bioherms from the eastern Java Sea (Indonesia)

Halimeda bioherms, occurring primarily along the western and southern margins of Kalukalukuang Bank in the eastern Java Sea, display a wide variety of thicknesses and shapes. In general, high-frequency forms of the northern bank are replaced by thicker and lower frequency forms along the deeper southern margin. Sidescan sonar data suggest aperiodic reworking of shallow bioherm crests of the northern bank into features suggestive of bedforms. These features are not associated with deeper bioherms of the southern bank. Cores from the bioherms indicate that they consist mostly of disarticulated Halimeda plates set in a lime mud matrix composed largely of Halimeda fragments and foraminifera tests. Carbon-14 dating shows that, with exception of some deep southern bank examples, bioherms are actively accreting. Results of mineralogy and elemental chemistry on piston core PC-12 suggest cyclic variations in Mg-calcite (cement in Halimeda utricles), which may be related to periodic excursions of cold Pacific throughflow water onto the bank. Composition of the Mg-calcite (8.6 mole-%) suggests a temperature of formation of about 22 °C, which is 7 °C below average surface water temperatures. The carbon and oxygen isotope compositions of both aragonite and Mg-calcite phases are remarkably homogeneous, but were inconclusive with regard to the cold-water intrusion hypothesis. However, a lack of reef-building corals below a depth of ∼15 m, abundance of Halimeda bioherms on the western margin of K-Bank, where upwelling is predicted, extensive boring of sedimentary particles by endolithic boring algae, and high nutrient values of water at the thermocline (50–70 m deep) all support the incursion of cold, nutrient-rich water onto the bank. Upwelling and nutrient overloading are suggested as explanations for remarkable algal growth at the expense of reef-building corals.