Grains size and components distribution in turbidite layesr of ODP Leg 101

Textural and compositional differences were found between gravity-flow sheets in an open-ocean environment on the northern slope of Little Bahama Bank (Site 628, Pliocene turbidite sequence) and in a closed-basin depositional setting (Site 632, Quaternary turbidite sequence). Mud-supported debris-flow sheets were cored at Site 628. Average mean grain size of the turbidite samples was lower, mud content was higher, and sorting was poorer than in comparable samples from Site 632. This reflects the deposition of proximal, low-energy turbidity currents and debris flows on a base-ofslope carbonate apron. No mud-supported debris-flow sheets were deposited in the investigated sediment sequence of Hole 632A. Many larger turbidity currents from around the margins of Exuma Sound may have reached this central basin setting, depositing sediments that had been transported over longer distances. Planktonic components dominate in the grain-sized fraction (500-1000 µm) of turbidite samples from Hole 628A, while platform detritus is rare. We interpreted this as resulting from the erosion and reworking of a large area of open-ocean slope sediments by gravity flows. In contrast, large amounts of benthic and platform components were found in the turbidite samples of Hole 632A. This may be explained by the fact that the slopes of the enclosed Exuma Sound are steep, and turbidity currents bypassed much of these slopes through pronounced channels, delivering more shallow-water detritus to the deep basin. Erosion of slope sediments, a possible source area of planktonic detritus, is assumed to be low. The small slope area in relation to the larger surrounding platform areas and lower production of planktonic components in the enclosed waters of Exuma Sound may also explain the observed low number of planktonic components at Hole 632A. Turbidite material from both open-ocean and enclosed-basin environments was deposited at Site 635.

Dinoflagellate cysts of ODP Leg 101 holes

The Albian-Cenomanian sediments in Holes 627B and 635B contain diverse dinoflagellate-cyst assemblages, which show affinities with coeval assemblages from offshore Morocco and northwest Europe. A total of 34 samples were analyzed from the shallow-water platform sediments and neritic marly chalk of Hole 627B and from the argillaceous chalk and limestone of Hole 635B. Dinoflagellate cysts indicate that the top of the shallow-water platform drilled at Hole 627B must be attributed to the late Albian. Dinocysts also date the drowning of the carbonate platform of the Blake Plateau. This drowning started in the latest Albian (Vraconian) and continued into the Cenomanian. The site area changed from an inner to intermediate or outer(?) neritic environment. The area around Hole 635B from the late Albian appears to have been situated in a deeper environment than the area around Hole 627B during the same period. The new dinoflagellate-cyst species Compositosphaeridiuml bahamaensis n. sp., Maghrebinia breviornata n. sp., and Subtilisphaeral habibi n. sp. are described, and Pervosphaeridium truncatum is emended. Additional taxonomic remarks about other species are included.

Neogene planktonic foraminifera of ODP Leg 101 holes

Leg 101 of the Ocean Drilling Program recovered a large volume of Neogene sediments from sites in the Straits of Florida, Little Bahama Bank, and Exuma Sound. In varying amounts, shallow-water, platform-derived carbonate debris is nearly ubiquitous. Reworked planktonic foraminifers are common, especially in the Pliocene-Pleistocene. At Site 626 in the Straits of Florida, a sequence of Holocene to upper Oligocene sediments was recovered. The greatest Neogene hiatus at this site spans the latest Miocene through Pliocene. Below this, several minor hiatuses are present in a generally conformable sequence. From the Little Bahama Bank transect (Sites 627, 628, and 630), a nearly complete composite Neogene section was sampled. At Site 627, a major unconformity separates lowermost Miocene sediments from middle to upper Eocene sediments. A second major unconformity occurs at Site 628. Here, middle Miocene sediments lie above uppermost Oligocene deposits. Sites 632, 633, and 631 in Exuma Sound all bottomed in a thick, lower Pliocene section. The mid-Pliocene is very thin at Sites 633 and 631, while it is better represented at Site 632. Major unconformities at Sites 627 and 628 appear to correlate with periods of elevated sea level, which suggests that carbonate platform shedding may be greatest during this part of the sea-level cycles. One of the salient features of the Bahamas is the lack of any systematic temporal distribution of hiatuses. Only a brief hiatus in the late Pliocene may be regional. It appears that local platform-shedding events were of equal or greater importance in developing the stratigraphy of the Bahamas than regional or eustatic events.

Strontium isotope dating of ODP Leg 133 sediments

The strontium-isotope dating method, based on the strontium-isotope seawater curve, was used to date stratigraphic events recognized in carbonate sediments drilled during Leg 133 on the Queensland and Marion plateaus. The strontium isotope ages of these events are used to correlate paleoceanographic changes, delineated from oxygen isotope signals, and paleoenvironmental or facies changes recorded in the lithostratigraphy. Results indicate that a strong connection exists between prevailing paleoenvironmental conditions and the developmental style of a carbonate platform. Also, the strontium-isotope ages of discrete dolomite intervals within the sequences were determined, indicating that multiple dolomitization events took place and that a hydrodynamically driven process may be currently active within the modern carbonate platform.

Paleomagnetic of ODP Leg 133 holes

Paleomagnetic and rock-magnetic analyses from discrete samples of carbonate sites on the Queensland Plateau were used to determine magnetic polarity reversal stratigraphy and the nature of magnetization in these sediments. Magnetic polarity zones were correlated with the geomagnetic polarity time scale in the upper portions of cores at Sites 812 through 814, usually back to a late Pliocene age. Loss of reliable directional data was coincidental with a major decrease in magnetic intensity, below which, no stable polarity zones could be identified. The intensity reduction is either an in-situ alteration of magnetic grains, or an input signal representing progressive increase in the magnetic component of Queensland Plateau sediments. Although not conclusive at this point, the geochemical conditions and differing age of intensity reduction support the former hypothesis. Rock-magnetic analysis of carbonate sediments suggests that ultrafine-grained magnetite or maghemite crystals is an important carrier of remanence and may be biogenic in origin. Application of a recently calibrated anhysteretic remanent magnetization test to assess configuration of single-domain crystal within a natural matrix indicates that cementation (ooze-chalk-limestone) may be important in post-depositional changes affecting magnetostatic grain interaction.

Porosity, gas permeability and grain density of ODP Leg 194 sites

We report analyses of porosity and permeability of core samples from Site 1193 in the Northern Marion Platform, Sites 1196 and 1199 in the Southern Marion Platform, and Sites 1194, 1195, 1197, and 1198 from the slopes of these platforms. The samples include 415 horizontal 1-in plugs, 290 vertical 1-in plugs, and 23 whole-core pieces. Porosity and permeability analyses were possible for most, but not all, samples. Grain density measurements were also obtained for the horizontal plugs. Representative photomicrographs are provided of thin sections from 139 of the horizontal plugs and the 23 whole-core pieces.

Benthic foraminifera and stable isotope record of ODP Leg 182 sites

Cores from Sites 1129, 1131, and 1132 (Ocean Drilling Program (ODP) Leg 182) on the uppermost slope at the edge of the continental shelf in the Great Australian Bight reveal the existence of upper Pleistocene bryozoan reef mounds, previously only detected on seismic lines. Benthic foraminiferal oxygen isotope data for the last 450,000 years indicate that bryozoan reef mounds predominantly accumulated during periods of lower sea level and colder climate since stage 8 at Sites 1129 and 1132 and since stage 4 at the deeper Site 1131. During glacials and interstadials (stages 2-8) the combination of lowered sea level, increased upwelling, and absence of the Leeuwin Current probably led to an enhanced carbon flux at the seafloor that favored prolific bryozoan growth and mound formation at Site 1132. At Site 1129, higher temperatures and downwelling appear to have inhibited the full development of bryozoan mounds during stages 2-4. During that time, favorable hydrographic conditions for the growth of bryozoan mounds shifted downslope from Site 1129 to Site 1131. Superimposed on these glacial-interglacial fluctuations is a distinct long-term paleoceanographic change. Prior to stage 8, benthic foraminiferal assemblages indicate low carbon flux to the seafloor, and bryozoan mounds, although present closer inshore, did not accumulate significantly at Sites 1129 and 1132, even during glacials. Our results show that the interplay of sea level change (eustatic and local, linked to platform progradation), glacial-interglacial carbon flux fluctuations (linked to local hydrographic variations), and possibly long-term climatic change strongly influenced the evolution of the Great Australian Bight carbonate margin during the late Pleistocene.

(Table 1) Thermal conductivities of lithified core samples from ODP Leg 166 sites

The geothermal regime of the western margin of the Great Bahama Bank was examined using the bottom hole temperature and thermal conductivity measurements obtained during and after Ocean Drilling Program (ODP) Leg 166. This study focuses on the data from the drilling transect of Sites 1003 through 1007. These data reveal two important observational characteristics. First, temperature vs. cumulative thermal resistance profiles from all the drill sites show significant curvature in the depth range of 40 to 100 mbsf. They tend to be of concave-upward shape. Second, the conductive background heat-flow values for these five drill sites, determined from deep, linear parts of the geothermal profiles, show a systematic variation along the drilling transect. Heat flow is 43-45 mW/m**2 on the seafloor away from the bank and decreases upslope to ~35 mW/m**2. We examine three mechanisms as potential causes for the curved geothermal profiles. They are: (1) a recent increase in sedimentation rate, (2) influx of seawater into shallow sediments, and (3) temporal fluctuation of the bottom water temperature (BWT). Our analysis shows that the first mechanism is negligible. The second mechanism may explain the data from Sites 1004 and 1005. The temperature profile of Site 1006 is most easily explained by the third mechanism. We reconstruct the history of BWT at this site by solving the inverse heat conduction problem. The inversion result indicates gradual warming throughout this century by ~1°C and is agreeable to other hydrographic and climatic data from the western subtropic Atlantic. However, data from Sites 1003 and 1007 do not seem to show such trends. Therefore, none of the three mechanisms tested here explain the observations from all the drill sites. As for the lateral variation of the background heat flow along the drill transect, we believe that much of it is caused by the thermal effect of the topographic variation. We model this effect by obtaining a two-dimensional analytical solution. The model suggests that the background heat flow of this area is ~43 mW/m**2, a value similar to the background heat flow determined for the Gulf of Mexico in the opposite side of the Florida carbonate platform.

(Table T1) Pore-water strontium content, and Sr isotope composition and ages of pore waters and sedimentary carbonates of ODP Leg 171B sites

Strontium concentrations and 87Sr/86Sr values were measured on pore-water and sedimentary carbonate samples from sediments recovered at Sites 1049-1053 on the Blake Spur during Ocean Drilling Program Leg 171B. These sites form a 40-km-long depth transect extending along the crest of the Blake Spur from near the upper edge of the Blake Escarpment (a steep cliff composed of Mesozoic carbonates) westward toward the interior of the Blake-Bahama Platform. Although these sites were selected for paleoceanographic purposes, they also form a hydrologic transect across the upper eastern flank of the Blake-Bahama Platform. Here, we use pore-water strontium concentrations and isotopes as a proxy to define patterns of fluid movement through the flanks of this platform. Pore-water strontium concentration increases with depth at all sites implying that strontium has been added during sediment burial and diagenesis. The isotopic values decrease from seawater-like values in the shallow samples (~0.70913) to values as low as 0.707342 in one of the deepest samples (~625 meters below seafloor). The change in pore-water strontium isotopic values is independent of the strontium isotopic compositions predicted from the host sediment age and measured on bulk carbonate in some samples. In most cases the difference between predicted sediment strontium isotopic composition and measured value is less than ±2 about the mean of the measured strontium value. Both the increase in concentration and the decrease in the strontium isotope values with increasing depth indicate that strontium was expelled from older carbonates. The strontium concentration and isotope profiles vary between sites according to their proximity to the Blake-Bahama Platform edge. Profiles from Site 1049 (nearest the platform edge) show the greatest amount of mixing with modern seawater, whereas the site most distal to the platform edge (Site 1052) shows the most significant influence of older, deeper carbonates on the pore-water strontium isotopic composition.

Beryllium, neodymium and metal composition of ODP Leg 194 sites and survey sites crusts

Be and Nd isotope compositions and metal concentrations (Mn, Fe, Co, Ni, and Cu) of surface and subsurface ferromanganese hardground crusts from Ocean Drilling Program Leg 194 Marion Plateau Sites 1194 and 1196 provide new insights into the crusts' genesis, growth rates, and ages. Metal compositions indicate that the hardgrounds, which have grown on erosional surfaces in water depths of <400 m because of strong bottom currents, are not pure hydrogenetic precipitates. Nevertheless, the ratios between cosmogenic 10Be and stable 9Be in hardgrounds from the present-day seafloor at Site 1196 between 1 x 10**-7 and 1.5 x 10**-7 are within the range of values expected for Pacific seawater, which shows that the hardgrounds recorded the isotope composition of ambient seawater. This is also confirmed by their Nd isotope composition (epsilon Nd between -3 and 0). The 10Be/9Be ratios in the up to 30-mm-thick and partly laminated hardgrounds do not show a decrease with depth, which suggests high growth rates on the present-day seafloor. The subsurface crust at Site 1194 (117 m below the seafloor) grew during a sedimentation hiatus, when bottom currents in the late Miocene prevented sediment accumulation on the carbonate platform during a sea level lowstand. The age of 8.65 ± 0.50 Ma for this crust obtained from 10Be-based dating agrees well with the combined seismostratigraphic and biostratigraphic evidence, which suggests an age for the hiatus between 7.7 and 11.8 Ma.