Deep Sea Drilling Project Cores Research Articles

Evidence for deep-water deposition of abyssal Mediterranean evaporites during the Messinian salinity crisis

Scientific drilling of the abyssal evaporites beneath the deepest parts of the Mediterranean basin gave rise to the idea that the Mediterranean sea completely evaporated at the end of the Messinian. Herein, we show, using new organic geochemical data, that those evaporites were deposited beneath a deep-water saline basin, not in a subaerial saltpan, as originally proposed. Abundant fossil organic lipids were extracted from evaporites in Mediterranean Deep Sea Drilling Project cores. The archaeal lipid distribution and new analyses, using the ACE salinity proxy and TEX86 temperature proxy, indicate that surface waters at the time of evaporite deposition had normal marine salinity, ranging from ∼26 to 34 practical salinity units, and temperatures of 25–28 °C. These conditions require a deep-water setting, with a mixed layer with normal marine salinity and an underlying brine layer at gypsum and halite saturation. After correction for isostatic rebound, our results indicate maximum drawdown of ∼2000 m and ∼2900 m relative to modern sea level in the western and eastern Mediterranean basins, respectively. Our results are consistent with previously proposed scenarios for sea level drawdown based on both subaerial and submarine incision and backfilling of the Rhone and Nile rivers, which require Messinian sea level drops of ∼1300 m and ∼200 m, respectively. This study provides new evidence for an old debate and also demonstrates the importance of further scientific drilling and sampling of deeper part of the abyssal Messinian units.

Extreme platinum-group element fractionation and variable Os isotope compositions in Philippine Sea Plate basalts: Tracing mantle source heterogeneity

Platinum-group elements (PGE), rhenium and osmium isotope data are reported for basalts from Deep Sea Drilling Project cores in the Philippine Sea Plate (PSP). Lithophile trace element and isotopic characteristics indicate a range of source components including DMM, EMII and subduction-enriched mantle. MORB-like basalts possess smooth, inclined chondrite-normalised PGE patterns with high palladium-PGE/iridium-PGE ratios, consistent with previously published data for MORB, and with the inferred compatibility of PGE. In contrast, while basalts with EMII-type lithophile element chemistry possess high Pt/Ir ratios, many have much lower Pd/Ir and unusually high Ru/Ir of > 10. Similarly, back-arc samples from the Shikoku and Parece-Vela basins have very high Ru/Ir ratios (> 30) and Pd/Ir as low as 1.1. Such extreme Pd/Ir and Ru/Ir ratios have not been previously reported in mafic volcanic suites and cannot be easily explained by variable degrees of melting, fractional crystallisation or by a shallow-level process such as alteration or degassing. The data appear most consistent with sampling of at least two mantle components with distinct PGE compositions. Peridotites with the required PGE characteristics (i.e. low Pd, but relatively high Ru and Re) have not been documented in oceanic mantle, but have been found in sub-continental mantle lithosphere and are the result of considerable melt depletion and selective metasomatic enrichment (mainly Re). The long-term presence of subduction zones surrounding the Philippine Sea Plate makes this a prime location for metasomatic enrichment of mantle, either through fluid enrichment or infiltration by small melt fractions. The Re–Os isotope data are difficult to interpret with confidence due to low Os concentrations in most samples and the uncertainty in sample age. Data for Site 444A (Shikoku Basin) give an age of 17.7 ± 1.3 Ma (MSWD = 14), consistent with the proposed age of basement at the site and thus provides the first robust radiometric age for these samples. The initial 187Os/ 188Os of 0.1298 ± 0.0069 is consistent with global MORB, and precludes significant metasomatic enrichment of Os by radiogenic slab fluids. Re–Os data for Sites 446A (two suites, Daito Basin) and 450 (Parece-Vela Basin) indicate ages of ∼ 73, 68 and 43 Ma, which are respectively, 30, 17 and > 12 Ma older than previously proposed ages. The alkalic and tholeiitic suites from Site 446A define regression lines with different 187Os/ 188Os initial (0.170 ± 0.033 and 0.112 ± 0.024, respectively) which could perhaps be explained by preferential sampling of interstitial, metasomatic sulphides (with higher time-integrated Re/Os ratios) by smaller percentage alkalic melts. One sample, with lithophile elements indistinguishable from MORB, is Os-rich (146 pg/g) and has an initial 187Os/ 188Os of 0.1594, which is at the upper limit of the accepted OIB range. Given the Os-rich nature of this sample and the lack of evidence for subduction or recycled crust inputs, this osmium isotope ratio likely reflects heterogeneity in the DMM. The dataset as a whole is a striking indication of the possible PGE and Os isotope variability within a region of mantle that has experienced a complex tectonic history.

Modeling Cenozoic sedimentation in the central equatorial Pacific and implications for true polar wander

High productivity along the central Pacific equator has left thick deposits of calcareous and siliceous ooze. West of 102°W, these have deposited on the Pacific plate which has been drifting northwards and therefore the thickest deposits are displaced north of the equator. This study attempts to simulate the sediment distribution using information from Deep Sea Drilling Project (DSDP) cores and models for Pacific plate motion. The DSDP core data are used to account for accumulation rate variations with time, paleolatitude and paleodepth, accounting for variable carbonate dissolution. The model predicts the distribution on 35–50 Ma seafloor relatively well and demonstrates that the sedimentary bulge is distorted by variable dissolution over a regional bathymetry gradient due to the northern edge of the South Pacific Superswell. A westerly deepening of the carbonate compensation depth and sedimentary lysocline evolution are proposed, which improve the model for seafloor 30 Ma and older. The degree of match between model and data provides a test of the amount of northward drift of the Pacific plate and, assuming a model for Pacific hotspots motion, of true polar wander (TPW, motion of the hotspot frame relative to the Earth's spin axis). Assuming the Duncan and Clague [1985] model for Pacific hotspots motion, the sedimentary modeling favors TPW models which have little or no effect on the central Pacific paleoequator over the past 30 Ma. We also explore implications of interhotspot motion and the suggestion that the bend in the Hawaiian volcanic chain represents an abrupt change in Pacific hotspot motion within the last 5 Ma.

Cenozoic history of the Himalayan-Bengal system: Sand composition in the Bengal basin, Bangladesh

Stratigraphic sequences preserved in the Bengal basin provide detrital information that documents a significantly older history of the eastern Himalaya than that available from Ocean Drilling Program and Deep Sea Drilling Project cores recovered from the Bengal fan. The Bengal basin, formed as a result of the Himalayan collision, is located at the juncture of the Indian craton to the west, the Shillong massif and the Himalayan belt to the north, and the Indo-Burman ranges to the east. Modal analyses of Eocene and Oligocene sandstones of the Cherra, Kopili, and Barail Formations document compositions that are dominated by subangular monocrystalline quartz grains with scarce to no feldspar grains, and few lithic fragments (Cherra and Kopili: Qt 99 F 1 L 0 ; Barail: Qt 90 F 3 L 7 ; Qt—total quartz, F—feldspar, L—lithic fragments). These compositions are similar to sands derived from the Indian craton, suggesting that they underwent intense chemical weathering, likely in a source with low relief and considerable transport. Himalayan tectonism during this time was probably significantly more distant from the Bengal basin than at present. Sandstones of the Miocene Surma Group (Bhuban and Boka Bil Formations) are rich in feldspar grains, argillite, and very low-grade metamorphic lithic fragments (Bhuban: Qt 66 F 15 L 19 ; Boka Bil: Qt 57 F 23 L 20 ) relative to older sandstones, suggesting onset of uplift and erosional unroofing in the eastern Himalaya, and initiation of stream systems supplying orogenic detritus to the Bengal basin. Sands of the upper Miocene to Pliocene Tipam Group and the Pliocene–Pleistocene Dupi Tila Sandstone contain abundant argillitic and low- to medium-grade metamorphic lithic fragments and feldspar grains (Tipam: Qt 61 F 19 L 19 ; Dupi Tila: Qt 70 F 13 L 17 ), suggesting continued orogenic unroofing. These younger sands are rich in potassium feldspar (P/F=0.30, 0.20) relative to plagioclase (P)-rich Bhuban and Boka Bil sandstones (P/F=0.66 and 0.48), suggesting a granitic source, probably the Miocene leucogranites of the High Himalayan Crystalline terrane. These results document for the first time contrasts in orogenic history recorded in the Bengal system vs. western Himalayan foreland basins. Sands deposited in the Bengal basin are generally more quartzose and less lithic than those from the western Himalayan foreland basins, and pre-Miocene strata in the Bengal system show little to no evidence of orogenic activity. In part, this probably reflects west to east progression of the Himalayan collision, but it probably also results from sedimentary systems propagating southward, ahead of the advancing mountain belt as it has been consuming the remnant ocean basin trapped between the Indian craton and the Burmese block.

Heat flow in the western abyssal plain of the Gulf of Mexico: Implications for thermal evolution of the old oceanic lithosphere

The seafloor depth of an oceanic basin reflects the average temperature of the lithosphere. Thus the western abyssal plain of the Gulf of Mexico, which has tectonically subsided much (>1 km) deeper than other basins of comparable ages (late Jurassic), should be underlain by an anomalously cold lithosphere. In order to examine this hypothesis, we made suites of high‐accuracy heat flow measurements at 10 sites along a line connecting Deep Sea Drilling Project (DSDP) sites 90 and 91 in the Sigsbee abyssal plain. The new heat flow sites were initially surveyed by 3.5‐kHz echo sounding, 4‐channel seismic reflection, seismic refraction with eight ocean bottom seismometers, and nine piston cores. We occupied a total of 48 heat flow stations along the seismic survey line (3 to 6 at each site), including 28 where we measured in situ thermal conductivities over the practical depth interval (4 m) of the new multioutrigger bow heat flow probe. We determined the heat flow associated with the lithosphere by correcting the values measured at the seafloor (41 to 45 mW/m2) for (1) the thermal effect of the sedimentation and (2) the additional heat from the radioactive elements within the sediments. The sedimentation history, required for the first, was reconstructed at each heat flow site based on ages and thicknesses of the major seismic stratigraphical sequences, age data from the DSDP cores, 3.5‐kHz subbottom reflectors, and correlation of turbidite units found in the piston cores. Radiogenic heat production was measured for 55 sediment samples from four DSDP holes in the gulf, whose age ranged from present to Early Cretaceous (0.83 μW/m3 on the average). This provided the correction for the second. The effects of these two secondary factors approximately cancel one another. The lithospheric heat flow under the abyssal plain thus estimated ranges from 40 to 47 mW/m2. These heat flow values are among the lowest in the Mesozoic ocean basins where highly reliable data (45 to 55 mW/m2) have been reported. Therefore the lithosphere under the gulf seems indeed colder than that under other old ocean basins. However, it is not as cold as expected from the large tectonic subsidence. The inconsistency between the depth and heat flow may imply an anomaly in the regional thermal isostasy.

Ocean Drilling Program: Recent Highlights and Future Expeditions

During the past year, exploration of the deep ocean floor through scientific ocean drilling has yielded important results with respect to evolution of ocean crust and continental margins and paleoceanography. This paper describes the Ocean Drilling Program's (ODP's) scientific and technical achievements during its ninth year of field operations and discusses areas of future study.

Volcanogenic sediment distributions in the Indian Ocean through the Cretaceous and Cenozoic, and their paleoenvironmental implications

Volcanogenic sediments are significant components of the cores that have been drilled over the past two decades by the Deep Sea Drilling Project (DSDP) and the Ocean Drilling Program (ODP). The spatial distribution of these sediment components were used, in connection with paleogeographical reconstructions, to determine the primary dispersal processes (direct air fall input, surface circulation or atmospheric circulation) for volcanic material across the Indian Ocean. The identification of volcanic-rich intervals in cores is based on the recognition of primary and secondary (authigenic) components or, where sediment samples have not been obtained, from analysis of downhole logging results e.g. spectral gamma-ray and whole core magnetic susceptibility. The data generated from DSDP cores by Vallier and Kidd (1977) have been upgraded in this study to fit a new integrated timescale that has been adopted for synthesis of ODP and DSDP drilling data (Kidd et al., 1992). It is now possible to map the distributions of volcanic-rich sediments on paleogeographic reconstructions at 1 Ma intervals. Through a review of its tectonic development the timing of accumulations of volcanic components in the Indian Ocean can be related to rifting of continents, formation of new crust at ridge crests and at hotspots, and to landmass volcanism, linked to subduction along the Indonesian Arc. The Kerguelen and Réunion hotspots were the dominant sources of volcanogenic sediments to the Indian Ocean since the early Cretaceous. The main accumulations of volcanogenic sediments occur within the basal sequences of drillholes, immediately above basement. Controls on the transport of volcanic components were linked to atmospheric, surface and, possibly deeper, water circulation patterns. The opening Indian Ocean experienced a change from westerly surface currents, generated by the high latitude westerly winds, during the Mesozoic to a gyral circulation pattern in the Cenozoic. The northwest Indian Ocean has also experienced seasonal reversal of winds since the development of the monsoon system during the late Miocene to Recent. These changes in the wind and water circulation patterns are reflected in the distribution patterns of volcanogenic sediments in the Indian Ocean. The Kerguelen hotspot led to the development of Kerguelen Plateau which was formed above sea level during the Albian (110 Ma) and remained above sea level for the next 30–40 Ma. Volcaniclastic material from this source was mainly distributed to the northeast. In contrast, the Réunion hotspot has contributed volcanic material mainly to the northwest during much of its evolution. Volcanism related to the Réunion and Kerguelen hotspots has apparently influenced volcanogenic sediment distributions in the developing Indian Ocean basins more than the regional rates of sea floor spreading, which were previously thought to exert a major control (Valluer and Kidd, 1977).

Compressional wave attenuation in oceanic basalts

To understand better the seismic attenuation in the upper volcanic regions of the oceanic crust, compressional wave attenuations of oceanic basalts have been measured as a function of confining pressure using an ultrasonic pulse‐echo spectral ratio technique capable of measuring attenuations to pressures of 500 MPa. Seven basalts, five from Deep Sea Drilling Project cores and two from dredge samples, have wide ranges of densities, porosities, and alterations, making possible an analysis of the parameters influencing basalt attenuation. Attenuation increases with the volume of secondary minerals present and with increasing porosity. Thus vesicularity and compositional changes associated with basalt alteration will produce variations in attenuation. With the application of hydrostatic pressure, cracks close, thereby reducing attenuations. This pressure dependence should be manifested in oceanic layer 2 by decreasing attenuation with depth. An inverse relationship between velocity and attenuation is observed at high hydrostatic pressures. Water saturation increases attenuation at pressures below 200 MPa and enhances the sensitivity of attenuation to pressure, thus making the state of saturation important in the 40 to 100 MPa range generally found in layer 2. These results provide a framework for interpreting marine attenuation data.

Relative frequency of Neogene volcanic events as recorded in coal partings from the Kenai lowland, Alaska: A comparison with deep-sea core data

Tephra layers occur as partings in Neogene coal beds from the Kenai Peninsula lowland, Alaska. The coal is time-equivalent with Deep Sea Drilling Project (DSDP) cores from the Gulf of Alaska and along the Aleutian Islands and appears to preserve a more detailed but less complete record of volcanism. Coal from the lower Beluga Formation has abundant thin (<10 cm) and dispersed partings recording an eruption once every 125-500 yr of peat accumulation, probably coinciding with a volcanic pulse more than 10.5 m.y. ago. This pulse is not well recorded in nearby DSDP cores, possibly because of bioturbation and distance from source vents. Coal beds in the upper Beluga Formation were deposited during a period of reduced volcanic activity 10.5-7.5 m.y. ago and record volcanic events that occurred approximately every 9,000 yr. This is also manifested in a near absence of tephra layers in DSDP cores of equivalent age. A volcanic pulse occurred about 7.5 m.y. ago, concurrent with the deposition of the lower Sterling Formation; however, intervals between volcanic events average 11,000 yr or longer. An absence of tephra layers in the Gulf of Alaska DSDP core for this time interval indicates that volcanic centers were distant. A dramatic change in frequency and magnitude of volcanism occurred about 5 m.y. ago. Tephra layers recur at intervals of 1,700-2,400 yr, and thicknesses of some layers exceed 2 m. This increase in volcanism is also recorded in DSDP cores.

Brief reversed polarity interval during the Cretaceous Normal Polarity Superchron

A striking feature of the geomagnetic time scale is the ∼34 m.y. period of normal polarity field bias known as the Cretaceous Normal Polarity Superchron. New paleomagnetic data from three Deep Sea Drilling Project cores (Sites 317A, 402A, and 463) indicate that a brief interval of reversed polarity occurred during the Superchron. This reversed polarity interval may help refine past motions of the Pacific plate and aid resolution of the statistical structure of the geomagnetic reversal chronology.

Eustatic, Volcanic, and Tectonic Contols on Redeposition of Shallow-Water Carbonate: A Reanalysis of Pacific DSDP Examples: ABSTRACT

On the basis of comparisons between eustatic sea level curves and occurrences of displaced shallow marine carbonate sediment in deep water Pacific DSDP (Deep Sea Drilling Project) cores, several previous workers have proposed that most displaced neritic carbonate is redeposited in the deep sea during eustatic sea level lowstands in a manner similar to siliciclastic sediment. However, reanalysis of these same DSDP data indicates a much more complex set of controls on redeposition including eustatic, volcanic, and tectonic effects. Of the 43 occurrences identified in 20 DSDP sites, eight (including all occurrences of true allodapic calcarenites) were deposited either during eustatic highstands or immediately after cessation of island-building volcanism; establishment of a shallow-water source area probably represents the dominant control on this type of deposition. Twenty-three other occurrences contain volcanic detritus mixed with minor amounts of reworked carbonate. Of these occurrences eight contain volcaniclastic sediment and exhibit no clear correlation with sea level, indicating probable redeposition of shelf carbonate during volcanic events as a result of either gravitational or seismic instability of the shelf. In contrast, the 15 other mixed volcanic/carbonate occurrences represent lowstand deposit and contain epiclastic volcanic detritus, suggesting lowstand erosion of mixed volcanic/carbonate shelf sediments. Five othermore » lowstand occurrences contain only reworked carbonate sediment and probably record either continued outer shelf/slope lowstand carbonate deposition or minor mechanical erosion of shelf carbonate. Six occurrences contain only reworked Inoceramus fragments, which are not necessarily indicative of a shelf source area.« less

Alkenone molecular stratigraphy in an oceanic environment affected by glacial freshwater events

The sedimentary record of a ratio (UK37) of long chain (C37) unsaturated alkenones is a useful indicator of glacial‐interglacial climatic change in the Late Quaternary northern Gulf of Mexico where a planktonic foraminiferal δ18O‐CaCO3 record is complicated by meltwater and/or fluvial events (Williams and Kohl, 1986). Application of a laboratory temperature calibration of the UK37 ratio (Prahl and Wakeham, 1987) to a Pigmy Basin hydraulic piston core record (Deep Sea Drilling Project core 619) suggested that the minimum glacial surface mixed layer (SML) temperature was 8°±1°C colder than the Holocene high SML temperature of 25.6°±0.5°C. This predicted glacial‐interglacial temperature difference was significantly larger than the differences predicted by either the foraminiferal δ18O or foraminiferal assemblage temperature methods (0.8°‐2.0°C). This large difference may be caused by local Prymnesiophyte assemblage changes in response to climatically induced hydrographic changes. Interglacial periods may be dominated by pelagic Prymnesiophyte assemblages, while glacial periods may be dominated by neritic assemblages. A combined mechanism of both climatically varying Prymnesiophyte species assemblage and depth of alkenone biosynthesis may account for the large difference between the temperature methods.

Compactional behavior of fine-grained sediments — examples from Deep Sea Drilling Project cores

Laboratory experiments, statistical analysis of field data and theoretical models are combined in the attempt to derive standard compaction curves for various deep-sea sediments which are in compaction equilibrium with the overburden. A revised and generalized version of Athy's law together with laboratory experiments allows the classification of sediments in terms of the amount of porewater structurally bound to the particles' surfaces. Especially suction pressure experiments are useful in quantifying the forces that bind water on the particle surfaces. The sediments analyzed almost range from systems of »suspended grains« to nearly perfect »colloidal systems«. The methods and equations required to derive the relevant parameters are given for both, downhole porosity/density measurements and laboratory experiments.

Application of the Graphic Correlation method to Pliocene marine sequences

Biostratigraphy — the use of paleontological evidence to establish relative chronologies, forms the cornerstone of many sedimentary geological investigations. Several different approaches to biochronology are available. Traditional interval zones, defined on lowest and/or highest occurrences of selected taxa, are used to place bodies of rock in a relative chronological framework. Fossil datum levels, which are more numerous than zones, are often used as chronohorizons for correlation purposs. The Graphic Correlation method, like interval zonations, synthesizes information from a number of different taxa but does not assume synchrony of any one taxon. A magnetobiostratigraphic model for deep-sea Pliocene sequences has been constructed by graphic correlation of Deep Sea Drilling project cores from the North Atlantic (606), Caribbean Sea (502), South Atlantic (516), Tasman Sea (590), Equatorial Pacific (573) and North Pacific (577). All cores are hydraulic piston cores which contain abundant planktonic foraminifers, calcareous nannofossils and which record many of the magnetic reversals expected in the Pliocene. The model is based on internally consistent paleontologic data gathered by the author. This study demonstrates the advantages of graphic correlation over conventional biostratigraphic procedures. Accurate inter-regional correlations can be made between core sites without resorting to multiple microfossil zonations and without invoking synchrony of fossil events. Important results of this study are: (1) many Pliocene planktonic foraminifer and calcareous nannofossil events are diachronous by more than 0.20 m.y., (2) Globorotalia truncatulinoides first occurs in the Southwest Pacific Ocean, approximately 0.50 m.y. earlier than previously reported, (3) a previously undetected hiatus of short duration (0.38 m.y.) exists just above the Cochiti subchron at DSDP 577A.

Paleontologic/Stratigraphic/Lithologic Expert System for Cenozoic Open-Ocean Sediments: ABSTRACT

COREXPERT is a developing system of Turbo Prolog programs to assist in the accumulation, manipulation, synthesis, and interpretation of information on Cenozoic sediments of the open oceans. The original data base is from Deep Sea Drilling Project cores. Information from individual sequences is retained intact in the data base and is also summarized in a grid of 5/degree/ squares of latitude and longitude, each with its range of values for sediment properties (carbonate content, biogenous silica content, etc). There is a grid for each subepoch of the Cenozoic (Quaternary, late Pliocene, early Pliocene, late Miocene, middle Miocene, etc). When a user enters data on a new sequence, COREXPERT points out any departures from expected lithologies, accumulation rates, fossil assemblages, etc, for the particular locality and age being entered so that the nonconforming observation can be checked. If it is substantiated, the appropriate summary grid can be modified accordingly. Several modules of this system are operational, in preliminary form. One assists in identifying fossils, another deduces age from fossil assemblages, a third adds new information to the growing data base, and a fourth displays sedimentary properties on a world ocean map - as minimum and maximum values for each 5/degree/ squaremore » for two subepochs simultaneously for comparison. The authors are developing this complex of programs so its essential elements will constitute an expert system shell that will be equally applicable to land-based stratigraphy, with higher geographic and stratigraphic resolution and different sets of sediment properties.« less

Stratigraphic Distribution of Late Neogene Species of the Planktonic Foraminifer Streptochilus in the Indo-Pacific

Study of four species of the biserial planktonic foraminifer Streptochilus from Deep Sea Drilling Project cores of the Eauripik Rise, western equatorial Pacific, and Ninetyeast Ridge, Indian Ocean, shows that both the stratigraphic distribution of species and their frequency patterns (though not actual frequencies or abundances) are correlative in the two areas, supporting their use as stratigraphic and paleoecologic index fossils. Their distributional trends are linked to eustatic sea level changes and to changes in the mixing of surface waters; low frequencies and species turnovers occur during regressive phases when strong circulation of oxygenated waters could lead to the subsequent decline of their oxygen-minimum habitat. The species S. subglobigerum, S. latum, S. globigerum, and S. globulosum succeed one another at intervals averaging 2.5 my from late middle Miocene Zone N 15 through Quaternary Zone N23. The new species, Streptochilus subglobigerum, is described for what was formerly thought to be a stratigraphically lower, disjunct part of the range of S. globigerum. These four species most likely belong to a single phylogenetic lineage as evidenced by some transitional morphologies.

Origin of gases and condensates in the Guaymas Basin hydrothermal system (Gulf of California)

The hydrothermal system in Guaymas Basin, Gulf of California, converts immature organic matter in the overlying sedimentary cover to petroleum (methane to asphalt). This petroleum is scavenged by the fluids and thus migrates upward. The more volatile and soluble hydrocarbons (<C 10) brought to the seabed are injected into the ambient seawater by the hydrothermal circulation, whereas the heavy ends (>C 15) solidify in the mineral mounds at the sea floor. Such volatile and dissolved petroliferous material was sampled and analyzed in terms of its composition and boiling ranges to evaluate its sources, genetic processes and temperatures of both formation and migration. Interstitial gases in sediments of Deep Sea Drilling Project cores consisted of microbial methane mixed with thermogenic C 1-C 5 hydrocarbons at increasing subbottom depths. The water and surface sediments and mounds (young hydrothermal systems) all contained large concentrations of C 1-C 10 hydrocarbons (condensates). These are the volatile components missing in most of the mound samples (i.e. old, established systems). One plume sampled from a transform fault region consisted of natural gas (C 1-C 5 hydrocarbons). All gas and volatile samples had C 1 ( C 2- C 5) concentration ratios typical for a thermogenic origin (< 500) and contained varying amounts of aromatic hydrocarbons, primarily benzene and toluene, with minor amounts of ethylbenzene and xylenes. Headspace gases of the ambient waters contained relatively larger percentages of the aromatic volatiles when compared with those of the sediments, which is consistent with the higher aqueous solubilities of aromatics. Thus, the immature sedimentary organic matter (derived mainly from microbial and diatomaceous marine detritus) is easily pyrolyzed to aliphatic and aromatic products at subbottom depths and differentially transported and deposited by the hydrothermal fluids at the seabed. Post-depositional alteration by solubilization (water-washing) leads to continued scavenging of the more volatile and soluble components from the exposed hydrothermal deposits. Additional alteration by biodegradation leads to selective removal of the normal-, iso- and anteiso-alkanes and relative enhancements of the cycloalkanes or naphthenic components.

Diachrony of Late Neogene microfossils in the southwest Pacific Ocean: Application of the graphic correlation method

The assumption of synchrony of first and last occurrences of fossil taxa can be tested using graphic correlation procedures which, by allowing measured stratigraphic sections to be compared on a common depth scale, make it possible to develop a correlation model which integrates information from a number of cores. The strategy of the test presented here is to use a graphic correlation model that is based on data from the Atlantic (Deep Sea Drilling Project (DSDP) sites 502, 516A) and north Pacific (DSDP site 577A) as a basis for determining to what extent fossil datums in the southwest Pacific are synchronous.First and last occurrences of Pliocene calcareous nannofossils and planktonic foraminifers have been compared in five DSDP cores from the southwest Pacific ocean (sites 586, 587, 588, 590A, and 592). All cores were recovered using hydraulic piston coring technology, which assures the best recovery and minimal disturbance. Most of these cores contain abundant, well‐preserved foraminifers and nannofossils, as well as a partial record of many of the expected magnetic polarity reversals in this part of the section. To assure taxonomic consistency, all taxonomic identifications were made by the author.Graphic correlation of this data set suggests that several important biostratigraphic markers are highly diachronous. For example, this study confirms that Globorotalia truncatulinoides first occurs at approximately 2.4 Ma between 20° and 35° south latitude in the southwest Pacific, approximately 0.5 m.y. earlier than it is found elsewhere in the Atlantic and Pacific. Other datums, such as the last occurrence of Discoaster brouweri, are essentially synchronous. These findings suggest that biostratigraphic models based on the assumption of synchrony of first and last occurrences of fossil taxa may be incorrect. Biostratigraphic models created with the Graphic Correlation method offer an opportunity to examine the biogeographic dimensions of origination, migration, and extinction of planktonic taxa.

Barium, equatorial high productivity, and the northward wandering of the Indian continent

High concentrations of biologically precipitated barite characterize the deep‐sea sediments underlying the equatorial high‐productivity belt of the Indian Ocean. By studying the barium distribution in five Deep Sea Drilling Project cores (213–217), drilled in the eastern Indian Ocean north of and below the present high‐productivity zone, it is possible to determine the times when the different coring sites passed beneath the zone. From this information, net northward spreading rates of the Indian plate can be deduced. The results suggest that northward movement was very rapid during the early and middle Paleogene, slow during the mid‐Cenozoic, and then rapid again from the Late Miocene onwards. The Late Miocene revitalization of active seafloor spreading south of the Indian continent coincides with the main phase of the Himalayan uplift.Supplemental tables are availablewith entire article on microfiche. Orderfrom American Geophysical Union, 2000Florida Avenue, N. W., Washington, D. C.,20009. Document P86‐002; $2.00. Paymentmust accompany order.

The [formula omitted] ratio in the Cenozoic Bengal Abyssal Fan sediments and its use as a paleostream energy indicator

The TiO 2 Al 2 O 3 ratio in the Cenozoic Bengal Abyssal Fan sediments displays decisive potential as a paleostream velocity indicator. The reason for this is that, even in the finest fan suspendates, hydrodynamic sorting determines the amounts of heavy TiO 2-rich minerals relative to lighter Al 2O 3-rich clay minerals. In five Deep Sea Drilling Project cores (213–217), which have been recovered in, and south of, the Bay of Bengal, TiO 2 Al 2 O 3 ratios increase linearly with time from the Late Miocene to Recent. The increase reflects fan progradation and intensified bottom current activity on the southern Bengal Fan. These processes are related to the Himalayan elevation by factors such as the maturation of northern India river systems, evolution of orographic monsoon rains and elevation-attributed increases in denudation rates.