Northern Coast Of Gulf Research Articles

Arkansas crustal xenoliths: Implications for basement rocks of the northern Gulf Coast, USA

Crustal xenoliths recovered from the diamondiferous Prairie Creek lamproite province in southwestern Arkansas directly constrain the nature, age, and origin of Laurentian basement and indirectly provide a means to test models for development of the southeastern margin of Laurentia and the Ouachita trough. The majority of the crustal xenoliths are near-surface sedimentary rocks, and the rest are mainly amphibolite and both fresh and altered granitic rocks. K-Ar dates of four amphibolite samples average ca. 1.41 Ga and indicate an association with the Laurentian craton. Xenolith compositions and ages, in conjunction with published geophysical and drill data, can be used to test two tectonic models for the development of the southeastern margin of Laurentia and the Ouachita system. One model suggests that the Ouachita trough is a failed rift basin essentially contained within the North American craton. The second model suggests that the Ouachita trough is a rifted continental margin combined with late Paleozoic accretion of an island arc terrane. Application of these data to the two models of Ouachita development suggests that the Ouachita trough is extended and thinned continental lithosphere associated with a Paleozoic rift system that developed separately from the Appalachian continental margin.

Hurricane frequency and landfall distribution for coastal wetlands of the Gulf coast, USA

The regularity and severity of tropical storms are major determinants controlling ecosystem structure and succession for coastal ecosystems. Hurricane landfall rates vary greatly with high and low frequency for given coastal stretches of the southeastern United States. Site-specific meteorological data of hurricane wind speeds and direction, however, are only available for select populated cities of relatively sparse distribution and inland from the coast. A spatial simulation model of hurricane circulation, HURASIM, was applied to reconstruct chronologies of hurricane wind speeds and vectors for northern Gulf coast locations derived from historical tracking data of North Atlantic tropical storms dating back to 1851. Contrasts of storm frequencies showed that tropical storm incidence is nearly double for Florida coastal ecosystems than the westernmost stretches of Texas coastline. Finer-scale spatial simulations for the north-central Gulf coast exhibited sub-regional differences in storm strength and frequency with coastal position and latitude. The overall pattern of storm incidence in the Gulf basin indicates that the disturbance regime of coastal areas varies greatly along the coast, inland from the coast, and temporally over the period of record. Field and modeling studies of coastal ecosystems will benefit from this retrospective analysis of hurricane incidence and intensity both on a local or regional basis.

Holocene Climate and Climate Variability of the Northern Gulf of Mexico and Adjacent Northern Gulf Coast: A Review

Marine records from the northern Gulf of Mexico indicate that significant multidecadal- and century-scale variability was common during the Holocene. Mean annual sea-surface temperature (SST) during the last 1,400 years may have varied by 3°C, and excursions to cold SST coincide with reductions in solar output. Broad trends in Holocene terrestrial climate and environmental change along the eastern portion of the northern Gulf Coast are evident from existing pollen records, but the high-frequency details of climate variability are not well known. Continuous and well-dated records of climate change and climate variability in the western portion of the northern Gulf Coast are essentially lacking.

Hurricane effects on seagrasses along Alabama's Gulf Coast

In November 2004, we evaluated the effect of Hurricane Ivan on seagrass meadows in Alabama by surveying all coastal locations known to support seagrass prior to Hurricane Ivan's landfall in September 2004. We found that 82% of the sites containing seagrass in 2002 still supported seagrass, and that, as in 2002, the most abundant species was Halodule wrightii (shoalgrass). We also found more sites containing Ruppia maritima (widgeongrass) than previously recorded. We confirmed the existence of Thalassia testudinum (turtlegrass) in Little Lagoon, Alabama, whose first record in the state had been noted in 2002. A resurvey of the western half of Alabama's coastal waters in October 2005 after Hurricane Katrina found no loss of seagrass, with all sites that supported seagrass in 2004 still containing seagrass in 2005. There was no major loss of Alabama's seagrass resources due to Hurricanes Ivan or Katrina, even though both category 3 hurricanes severely affected the northern Gulf Coast.

Atmospheric trajectories and spring bird migration across the Gulf of Mexico

We examined the relationship between the longitude of peak arrival of trans-Gulf migrants on the northern coast of the Gulf of Mexico in spring and wind trajectories over the Gulf at three different altitudes (500, 1,500, and 2,500 m above ground level). We used data from 10 WSR-88D radars (weather surveillance radar-1988-Doppler) from Brownsville, Texas, to Key West, Florida, to record the time and longitude of peak arrival on the northern Gulf coast for four spring migrations (2001–2004). We used the National Oceanic Atmospheric Administration Air Resources Laboratory HYSPLIT transport and dispersion model at the READY Web site to generate backward, 24-h atmospheric trajectories based on archived atmospheric data for each trans-Gulf flight. The trajectories began at the geographic location where radar indicated the greatest concentrations of arriving migrants. Although the longitude of peak arrival varied, peak densities of most trans-Gulf migrants arrived on the northern coast near longitude 95°W. Regression analyses showed that the relationship between the longitude of peak trans-Gulf arrival and the direction of atmospheric trajectory was significant but weak at the 500-m level, where few migrants occurred, and was insignificant for the 1,500- and 2,500-m altitudes, where migrant densities were greater. We conclude that winds aloft over the Gulf have little influence on the longitude of peak trans-Gulf arrival on the northern coast of the Gulf of Mexico, and we speculate that the arrival pattern may reflect the trans-Gulf migration pathways that evolved during the Last Glacial Maximum.

Evidence of regional subsidence and associated interior wetland loss induced by hydrocarbon production, Gulf Coast region, USA

Analysis of remote images, elevation surveys, stratigraphic cross-sections, and hydrocarbon production data demonstrates that extensive areas of wetland loss in the northern Gulf Coast region of the United States were associated with large-volume fluid production from mature petroleum fields. Interior wetland losses at many sites in coastal Louisiana and Texas are attributed largely to accelerated land subsidence and fault reactivation induced by decreased reservoir pressures as a result of rapid or prolonged extraction of gas, oil, and associated brines. Evidence that moderately-deep hydrocarbon production has induced land-surface subsidence and reactivated faults that intersect the surface include: (1) close temporal and spatial correlation of fluid production with surficial changes including rapid subsidence of wetland sediments near producing fields, (2) measurable offsets of shallow strata across the zones of wetland loss, (3) large reductions in subsurface pressures where subsidence rates are high, (4) coincidence of orientation and direction of displacement between surface fault traces and faults that bound the reservoirs, and (5) accelerated subsidence rates near producing fields compared to subsidence rates in surrounding areas or compared to geological rates of subsidence. Based on historical trends, subsidence rates in the Gulf Coast region near producing fields most likely will decrease in the future because most petroleum fields are nearly depleted. Alternatively, continued extraction of conventional energy resources as well as potential production of alternative energy resources (geopressured-geothermal fluids) in the Gulf Coast region could increase subsidence and land losses and also contribute to inundation of areas of higher elevation.

The centric diatom genus Cyclotella, (Stephanodiscaceae: Bacillariophyta) from Florida Bay, USA, with special reference to Cyclotella choctawhatcheeana and Cyclotella desikacharyi, a new marine species related to the Cyclotella striata complex†

A.K.S.K. Prasad and J.A. Nienow. 2006. The centric diatom genus Cyclotella, (Stephanodiscaceae: Bacillariophyta) from Florida Bay, USA, with special reference to Cyclotella choctawhatcheeana and Cyclotella desikacharyi, a new marine species related to the Cyclotella striata complex. Phycologia 45: 127–140. DOI: 10.2216/05-13.1A new marine species of the centric diatoms of the genus Cyclotella, Cyclotella desikacharyi A.K.S.K. Prasad collected from several localities in Florida Bay, USA, where it is a dominant part of the planktonic assemblage, is described by means of light and scanning electron microscopy. Cyclotella desikacharyi is characterized by the presence of dense striae (14–19 in 10 µm) in the marginal zone and small granules all through the surface of the valve, including the central area, fultoportulae on the valve mantle located on every second or third interstria, each surrounded by three satellite pores, a pronounced tangential undulation, 3–17 valve-face fultoportulae in an arc on the uplift of the undulation and numerous discoid plastids. The new species is compared with Cyclotella striata complex; C. desikacharyi closely resembles C. striata in valve dimensions and in the structure and arrangement of alveolate striae, and in the presence of three satellite pores surrounding each fultoportula on the valve mantle; it differs from it in the stria density and the presence of siliceous granules on the entire valve surface. The new species is compared with the other closely related marine members of the genus, Cyclotella stylorum and Cyclotella litoralis. Cyclotella choctawhatcheeana Prasad, originally described from the northern Gulf Coast of Florida, is also recorded in several localities in Florida Bay and its global distribution is discussed. The evolution of euryhalinity in the genus Cyclotella is also discussed in brief.

Warm Season Lightning Distributions over the Northern Gulf of Mexico Coast and Their Relation to Synoptic-Scale and Mesoscale Environments

Abstract Cloud-to-ground lightning data from the National Lightning Detection Network are used to create a warm season (May–September) lightning climatology for the northern Gulf of Mexico coast for the 14-yr period 1989–2002. Each day is placed into one of five flow regimes based on the orientation of the low-level flow with respect to the coastline. This determination is made using the vector mean 1000–700-hPa wind data at Lake Charles and Slidell, Louisiana. Flash densities are calculated for daily, hourly, and nocturnal periods. Spatial patterns of composite 24-h and nocturnal flash density indicate that lightning decreases in an east-to-west direction over the region. Flash densities for the 24-h period are greatest over land near the coast, with relative maxima located near Houston, Texas; Lake Charles, Baton Rouge, and New Orleans, Louisiana; Biloxi, Mississippi; and Mobile, Alabama. Flash densities during the nocturnal period are greatest over the coastal waters. Lightning across the northern Gulf coast is closely related to the prevailing low-level synoptic flow, which controls the sea breeze, the dominant forcing mechanism during the warm season. Southwest flow, the most unstable and humid of the five regimes, exhibits the most flashes. In this case, sea-breeze-induced convection is located slightly inland from the coast. Northeast flow, the driest and most stable of the regimes, exhibits the least amount of lightning. The large-scale flow restricts the sea breeze to near the coastline. Geographic features and local mesoscale circulations are found to affect lightning across the region. Geographic features include lakes, bays, marshes, swamps, and coastline orientations. Thermal circulations associated with these features interact with the main sea breeze to produce complex lightning patterns over the area.

Fossil Sirenia of the West Atlantic and Caribbean region. VII. Pleistocene Trichechus manatus linnaeus, 1758

ABSTRACT All known Pleistocene fossils of manatees from North America are conspecific with the living West Indian manatee, Trichechus manatus. However, those of late Pleistocene (late Rancholabrean) age differ from all modern manatees, and are here assigned to a new subspecies, T. m. bakerorum. In contrast, early Pleistocene manatees from the late early Irvingtonian Leisey Shell Pit Local Fauna, Florida, do not clearly differ from modern West Indian manatees and are here referred to T. manatus, subspecies indeterminate. I hypothesize that warm intervals during the Quaternary allowed manatees from the Caribbean (represented today by the Antillean subspecies, T. m. manatus) to disperse northward into the United States. Ecological barriers (cool winters on the northern Gulf of Mexico coast, deep water and strong currents in the Straits of Florida) then impeded genetic contact with Caribbean populations and permitted evolution of endemic North American forms (T. m. bakerorum and the living Florida manatee, T. m. latirostris). Conditions much warmer than today might have eliminated the northern Gulf Coast barrier and allowed gene flow from the Caribbean to swamp such endemic gene pools. Return of glacial conditions reversed these northward range extensions and brought about periodic extinction of manatees in North America. This mechanism might explain the morphological uniqueness of the late Rancholabrean manatees, as well as the fact that early Pleistocene manatees from Florida are more similar to modern T. m. manatus than the late Pleistocene ones are. This model also helps explain phylogeographic patterns detected in mitochondrial DNA data from living manatees.

Santonian-Maastrichtian planktonic foraminiferal and ostracode biostratigraphy of the northern Gulf Coastal Plain, USA

The Santonian-Maastrichtian deposits of the northern Gulf Coastal Plain of the United States (Mississippi-Alabama) are richly fossiliferous and structurally uncomplicated, offering an excellent setting in which to study calcareous microfossil biostratigraphy. Eight planktonic foraminiferal zones and seven ostracode zones are recognized in these strata, based on the stratigraphic occurrence of 45 species of planktonic foraminifera and 112 taxa of ostracodes. The planktonic foraminiferal zonation is based on standard global biozones. These include the Dicarinella asymetrica Taxon Range Zone, the Globotruncanita elevata Interval Zone, the Globotruncana ventricosa Interval Zone, the Globotruncanita calcarata Taxon Range Zone, the Globotruncanella havanensis Interval Zone, the Globotruncana aegyptiaca Interval Zone, the Gansserina gansseri Interval Zone, and the Contusotruncana contusa-Racemiguembelina fructicosa Interval Zone. Three new ostracode zones are defined, and the upper boundary of two previously defined zones are amended. These zones are the Veenia quadrialira Interval Zone (amended), the Acuminobrachycythere acuminata Interval Zone (new), the Brachycythere pyriforma Interval Zone (new), the Ascetoleberis plummeri Taxon Range Zone (amended), the Curfsina communis Interval Zone (new), the Escharacytheridea pinochii Interval Zone, and the Platycosta lixula Interval Zone. The planktonic foraminiferal biostratigraphy indicates that the age of these deposits range from early Santonian to early Maastrichtian. The Santonian-Campanian boundary is recognized on the basis of the highest occurrence surface of Dicarinella asymetrica, and demonstrates that the contact between the Tombigbee Sand Member of the Eutaw Formation and the Mooreville Chalk is diachronous. The Campanian-Maastrichtian boundary cannot be defined by calcareous microfossils due to the lack of a proxy for the newly established definition. The high-resolution biostratigraphy of the ostracodes, which are indigenous to the North American Coastal Plain, will enable subsequent studies of Late Cretaceous paleobiogeography and Gulf of Mexico-Caribbean tectonic development to be conducted.

Water mass stability reconstructions from greenhouse (Eocene) to icehouse (Oligocene) for the northern Gulf Coast continental shelf (USA)

Shallow water mass characteristics such as temperature and density profile play a critical role in the climate system. We have developed a new method by which to reconstruct the ancient shallow water mass stability on the continental shelf using oxygen isotope variation within mollusc shells and fish otoliths and applied the method to an important interval in Earth history, the most recent transition from Greenhouse (Eocene) to Icehouse (Oligocene) climate modes. We define the slope of summer temperature (density) versus the seasonal range in temperature (density) as an indicator of water mass stability. In addition, extrapolation of the regression to zero seasonality is a proxy for temperature at the bottom of the seasonal thermocline (TBST). During the greenhouse world (the early Eocene and middle Eocene) the water mass plot shows an unstable water mass, agreeing with previous planktonic foraminiferal studies showing that temperature gradients at this time were much smaller than at present. During the middle to late Eocene transition, a substantial increase in water mass stability occurred. Significant cooling (∼5°C) of the TBST at this transition indicates that the greater cooling of deeper water relative to surface water caused the increase in water mass stability. The changes in water column structure at this transition were the most likely cause of a major extinction of planktonic foraminifera from warm to cold water taxa. The late Eocene T‐ΔT profile is very similar to modern profiles, suggesting that shallow water mass structure became similar to that of the modern Gulf Coastal shelf by the late Eocene. At the Eocene/Oligocene (E/O) boundary, no major change in water mass structure is identified. This agrees with the observation that no major extinction of planktonic foraminifera is found at the E/O boundary.

A new depositional model for the buried 4000 yr BP New Orleans barrier: implications for sea-level fluctuations and onshore transport from a nearshore shelf source

The Holocene New Orleans Barrier Complex, now buried by the St. Bernard delta of the Mississippi River, provides an excellent example of barrier deposition fed by a nearshore sediment source. This reworking and onshore transport was initiated by a sudden change in the shelf equilibrium profile caused by a sea-level fall about 4100 yr BP. Here we present a new model of barrier formation which does not invoke an Shepard-type Holocene sea-level curve nor the supply of sediment from a longshore source. The Holocene New Orleans Barrier Complex consists of fine-grained, locally cross-bedded, quartz sand that contains Ophiomorpha nodosa burrows and disarticulated mollusks, primarily marine, buried beneath up to 4 m of silty mud of the St. Bernard Lobe. This barrier island and shoal deposit overlies interbedded, fine-grained sand and mud containing marine mollusks, some articulated, that is interpreted to be a nearshore shelf deposit. Its deposition took place between 5500 and 4200 yr BP ( 14C), based on individual dates on seven articulated and seven disarticulated shells. The barrier formation is effectively limited to a several-hundred-year window approximately 4000 yr BP by the 3800 yr BP Rangia sp. shells from the immediately overlying St. Bernard Lobe delta-plain deposits and the buried 3900–3500 yr BP Linsley archeological site, situated on a more gulfward distributary levee. In this paper we present a new depositional model on the New Orleans Barrier. The barrier complex contains an abundance of large mollusk shells that have been reworked to the extent that 14 individual shells yield a 2500-year range, 6000–3500 yr BP. An older, nearby source is required. The current model of a spit/shoal complex migrating westward from an eroding eastern Pleistocene headland probably cannot account for the deposition of large reworked shells given the effects of abrasion and selective size sorting over approximately 50 km of longshore transport. Furthermore, this model demands transport rates of millions of cubic meters per year for the present northern Gulf coast which are at least an order of magnitude higher than its highest known rates. We postulate a nearby shell and sand source that is subjacent and offshore rather than adjacent and littoral. We propose it to be the underlying nearshore shelf deposit that could be mobilized by a brief fall of sea level and carried landward. The barrier complex and the uppermost nearshore shelf deposit have markedly different net deposition rates. The upper 25 cm of the nearshore shelf deposit were deposited over 800 years, based on ages of articulated marine pelecypods. A 20-km-long, 3-km-wide and 4-m-thick segment of the barrier complex was deposited in less than 300 years. A 10-cm-thick lag pavement of bryozoan- and oyster-encrusted mollusk shells that comprises the nearshore shelf deposit beneath the northern edge of the barrier complex is evidence for an essentially zero net deposition rate. The current interpretation of a conformable, progradational relationship between these two units is rejected in favor of a disconformable contact. The hiatus across this disconformity must be less than the several-hundred-year duration of barrier complex deposition. The positioning of the shallower-water barrier complex disconformably over the deeper-water nearshore shelf deposits indicates a sea-level fall.

Compositional analysis of glazed earthenwares from eighteenth-century sites on the northern Gulf Coast

Compositional classification of colonial-era ceramics using neutron activation analysis has heretofore focused primarily on majolicas from Spain and Mexico. In order to expand the chemical database to include 18th-century French ceramics, 186 sherds from Old Mobile and nearby sites on the north-central Gulf Coast have been analyzed, including faience blanche, faience brune, Mexican majolicas, and several types of coarse earthenwares. Quantitative analysis of 23 elements provides a basis for distinguishing French and Spanish-colonial earthenwares, as well as suggesting some preliminary chemical groupings of French faience.

French colonial archaeology at old mobile: An introduction

Twelve years of excavation and analysis of the Old Mobile archaeological site (1MB94) are revealing the physical and social dimensions of the capital of French colonial Louisiana between 1702 and 1711. The specialized studies included in this thematic issue document the interplay of French colonists, enslaved and free Native Americans, and Hispanic trade contacts as a new colonial society developed on the northern Gulf Coast at the turn of the 18th century.

An experimental approach to altering mating tactics in male horseshoe crabs (Limulus polyphemus)

Alternative reproductive tactics are often correlated with phenotype, density, environment, or social context. Male horseshoe crabs (Limulus polyphemus) have two mating tactics that are associated with phenotype. Males in good condition arrive at the nesting beach and spawn while attached to females, whereas those in poorer condition come ashore unattached and crowd around the nesting couples as satellites, fertilizing eggs through sperm competition. The correlation between mating tactic and phenotype may be due to males choosing tactics based on condition, or it may be that males that have not found a female choose to come ashore as satellites. To distinguish between these two possibilities, I conducted an experiment on male horseshoe crabs in the field at Seahorse Key on the northern Gulf coast of Florida. I prevented males from attaching to females by placing small plastic bags over the claws they use to attach. The results showed that males in poor condition came ashore as satellites, whereas males in good condition remained at sea. This means that mating tactics are cued by information about the male's condition and not about whether he found a female. The evolution of phenotype-correlated mating tactics can be represented by a model in which the fitness of each tactic changes with condition and fitness curves cross. I hypothesize that male horseshoe crabs in good condition have higher fitness when attached and that males in poorer condition to better when unattached.

3-D seismic detection of undrilled prospective areas in a mature province, South Marsh Island, Gulf of Mexico

This paper focuses on the most effective approach to defining additional resources using 3-D seismic as the sole data source. Although this example is from a densely drilled province, such a seismic-based approach is often used in frontier areas with sparse well coverage, in step-out regions around known production, can be used for a “quick look” evaluation of existing properties, and for real-time interpretation of newly acquired data. This approach—integrating multiple seismic attributes and interpretation methods—refined the structural framework and detected undrilled prospective areas in a mature province in the Gulf of Mexico. Coherency technologies refined the fault interpretations. Surface mapping identified structural highs, and stratal-surface techniques were used to evaluate seismic stratigraphic facies. The study area covered approximately 350 square miles of coastal waters just south of Marsh Island, Louisiana (Figure 1). Data consisted of two merged 3-D surveys, Outer Continental Shelf (OCS) 310 and State Lands (SL) 340 that cover six productive offshore fields: Starfak, Tiger Shoal, Mound Point, Amber Complex, Lighthouse Point, and North Lighthouse Point. Figure 1. Tiger Shoal area in Vermilion Block 50 offshore Louisiana. Sur-rounding fields and the outline of the two major 3-D seismic surveys are indicated. The area is in the Oligocene-Miocene detachment province of the northern Gulf Coast continental margin—a region generally characterized by large-displacement, dominantly down-to-the-basin, listric growth faults that sole on a regional detachment zone above the Oligocene section. Regional deformation is a product of salt mobilization. The faults originate in the autochthonous Jurassic Louann Salt or in the detachment zone represented by a salt weld that formerly contained a thick, allochthonous salt body. A characteristic feature of this province is the thickness (typically exceeding three miles) of deltaic and shelf sediments above the detachment zone. This remarkable stacking of deltaic/shelf sandstone reservoirs helps make this province one of the world's …

Sequence Stratigraphy of Lower Cretaceous (Barremian-Albian) Carbonate Platforms of Northeastern Mexico: Regional and Global Correlations

ABSTRACT The Lower Cretaceous Cupido and Coahuila platforms of northeastern Mexico form part of the extensive carbonate platform system that surrounded the ancestral Gulf of Mexico. A sequence-stratigraphic model for these Barremian to Albian platforms was constructed from regional correlations of vertical cycle stacking patterns constrained by newly acquired biostratigraphic data and C and Sr isotope stratigraphy. The Cupido shelf lagoon (Barremian-Aptian) is composed of up to 150 peritidal cycles, which stack into high-frequency sequences that are correlated across the shelf. Cupido high-frequency sequences build into a lower partial composite sequence (Cu1), an overlying full composite sequence (Cu2), and the transgressive base of a third composite sequence (Cu-Co3) related to backstepping of the Cupido platform. The highstand part of Cu-Co3 is recorded in evaporite and carbonate facies of the Coahuila platform (Aptian-Albian). More than 80 meter-scale evaporite-to-carbonate cycles characterize the restricted interior of the Coahuila platform and stack into two full composite sequences (Co4, Co5). The Coahuila platform was drowned in the latest Albian or earliest Cenomanian, terminating shallow-water carbonate sedimentation in the region. Regional correlations of the depositional sequences in northeast Mexico with coeval platforms in Texas and the northern Gulf coast indicate that the lower two composite sequences of the Cupido shelf correlate with the lower part of the Sligo-Hosston platform in Texas. The transgressive systems tract of the third composite sequence of the Cupido-La Pena is recorded in Texas by the upper Sligo and Pearsall formations. The highstand systems tract and the disconformable top of the third composite sequence (Coahuila platform) is likely coeval with the upper Glen Rose platform in Texas. The fourth and fifth composite sequences are correlative with the Fredericksburg platform and the Washita-Devils River platforms of Texas, respectively. We conservatively correlated our sequence-stratigraphic framework with composite global sequences and eustatic curves derived from several other basins. Given the available time resolution, results are variable, but sequence boundaries at the top of Cu-Co3 and Co5 appear to have clear interbasinal equivalents.

Molecular population structure of the marine benthic copepod Microarthridion littorale along the southeastern and Gulf coasts of the USA

Relationships among populations (southeast Atlantic and northern Gulf coast, USA) of a ubiquitous, estuarine, harpacticoid copepod (Microarthridion littorale Poppe) were estimated using sequence data from two loci: one mitochondrial [cytochrome b (Cyt b)] and one nuclear [first internal transcribed spacer of ribosomal DNA (ITS-1)]. Copepods were collected from seven estuaries in 1997/98. Allelic phylogenies based on both genes were generally concordant, and suggested that M. littorale populations are structured over large geographic scales (hundreds of kilometers). Three well-supported groups were found in both gene trees comprising clades of alleles sampled from South Carolina, Florida, and Louisiana. Alleles from the Savannah, Georgia sample formed a monophyletic group using the Cyt b data, but this clade was not distinguishable with comparable ITS-1 data. A single specimen from Louisiana was classified in different clades depending on the locus assayed.

Influence of water temperature and salinity on Vibrio vulnificus in Northern Gulf and Atlantic Coast oysters (Crassostrea virginica).

This study investigated the temperature and salinity parameters associated with waters and oysters linked to food-borne Vibrio vulnificus infections. V. vulnificus was enumerated in oysters collected at three northern Gulf Coast sites and two Atlantic Coast sites from July 1994 through September 1995. Two of these sites, Black Bay, La., and Apalachicola Bay, Fla., are the source of the majority of the oysters implicated in V. vulnificus cases. Oysters in all Gulf Coast sites exhibited a similar seasonal distribution of V. vulnificus: a consistently large number (median concentration, 2,300 organisms [most probable number] per g of oyster meat) from May through October followed by a gradual reduction during November and December to < or = 10 per g, where it remained from January through mid-March, and a sharp increase in late March and April to summer levels. V. vulnificus was undetectable (< 3 per g) in oysters from the North and South Carolina sites for most of the year. An exception occurred when a late-summer flood caused a drop in salinity in the North Carolina estuary, apparently causing V. vulnificus numbers to increase briefly to Gulf Coast levels. At Gulf Coast sites, V. vulnificus numbers increased with water temperatures up to 26 degrees C and were constant at higher temperatures. High V. vulnificus levels (> 10(3) per g) were typically found in oysters from intermediate salinities (5 to 25 ppt). Smaller V. vulnificus numbers (< 10(2) per g) were found at salinities above 28 ppt, typical of Atlantic Coast sites. On 11 occasions oysters were sampled at times and locations near the source of oysters implicated in 13 V. vulnificus cases; the V. vulnificus levels and environmental parameters associated with these samples were consistent with those of other study samples collected from the Gulf Coast from April through November. These findings suggest that the hazard of V. vulnificus infection is not limited to brief periods of unusual abundance of V. vulnificus in Gulf Coast oysters or to environmental conditions that are unusual to Gulf Coast estuaries.

Depositional environments of Late Eocene lignite-bearing strata, east-central Texas

Late Eocene lignites in the Brazos River Valley area occur in parasequence-scale cycles containing mostly marine sediments. Lignite is present at the base of the marine interval and forms the base of the cycle in a majority of cases. Less frequently, lignite is underlain by a thin interval of nonmarine deposits that form the base of the cycle. In both situations, the lignite is part of a thin transgressive interval. Cycles are separated by an exposure surface, usually marked by plant rooting and/or development of a paleosol in the top of the underlying sediments. Peat accumulated on top of these exposure surfaces and lignite is preserved where marine flooding allowed ocean waters to cover the peat deposits with fine grained sediments, producing high-ash lignite horizons of great lateral extent but variable thickness and quality. This strandplain model of lignite occurrence provides a better understanding of Late Eocene lignites in Texas than the deltaic or fluvial-deltaic models previously used. The strandplain model of lignite occurrence is appropriate for most of the Yegua-Jackson lignites (Middle-Late Eocene) of Texas and should be considered when evaluating depositional conditions of Paleogene lignites in the northern Gulf Coast region. A re-evaluation of Late Eocene stratigraphy of the Brazos Valley area accompanies the documentation of depositional environments.