Geochemistry of marin ash layers and epiclastic rocks from the Kerguelen Plateau

Epiclastic volcanogenic rocks recovered from the Kerguelen Plateau during Ocean Drilling Program Legs 119 and 120 comprise (pre-)Cenomanian(?) claystones (52 m thick, Site 750); a Turonian(?) basaltic pebble conglomerate (1.2 m thick, Site 748; Danian mass flows (45 m thick, Site 747); and volcanogenic debris flows of Quaternary age at Site 736 (clastic apron of Kerguelen Island). Pyroclastic rocks comprise numerous Oligocene to Quaternary marine ash layers. The epiclastic sediments with transitional mid-ocean-ridge basalt (T-MORB) origin indicate weathering (Site 750) and erosion (Site 747) of Early Cretaceous T-MORB from a then-emergent Kerguelen Plateau, connected to Late Cretaceous tectonic events. The basal pebble conglomerate of Site 748 has an oceanic-island basalt (OIB) composition and denotes erosion and reworking of seamount to oceanic-island-type volcanic sources. The vitric- to crystal-rich marine ash layers are a few centimeters thick, have rather uniform grain sizes around 60 ± 40 µm, and are a result of Plinian eruptions. Crystal-poor silicic vitric ashes may also represent co-ignimbrite ashes. The ash layers have bimodal, basaltic, and silicic compositions with a few intermediate shards. The basaltic ashes are evolved high-titanium T-MORB; a few grains in a silicic pumice lapilli layer have a low-titanium basaltic composition. The silicic ashes comprise trachytic and rhyolitic glass shards belonging to a high-K series, except for a few low-K glasses admixed to a basaltic ash layer. Feldspar and clinopyroxene compositions fit the glass chemistry: high-Ti tholeiite-basaltic glasses have Plagioclase of An40-80 and pigeonite to augite clinopyroxene compositions. Silicic ashes have K-rich anorthoclase and minor Plagioclase around An20 and ferriaugitic to hedenbergitic clinopyroxene compositions. The line of magmatic evolution for the glass shards is not compatible with simple two-end member (high-Ti T-MORB and high-K rhyolite) mixing, but favors successive Ca-Mg-Fe pyroxene, Ti magnetite, and apatite fractionation, and K-rich alkali feldspar fractionation in trachytic magmas to yield rhyolitic compositions. Plagioclase fractionation occurs throughout. This qualitative model is in basic accordance with the observed mineral assemblage. However, as the time span for explosive volcanism spans >30 m.y., this basic model cannot comply with fractional crystallization in a single magma reservoir. The ash layers resulted from highly explosive eruptions on Kerguelen and, with less probability, Heard islands since the Oligocene. The explosive history starts with widespread Oligocene basaltic ash layers that indicate sea-level or subaerial volcanism on the Northern Kerguelen Plateau. After a hiatus of 24 m.y.(?), explosive magmatic activity was vigorously renewed in the late Miocene with more silicic eruptions. A peak in explosive activity is inferred for the Pliocene-Pleistocene. The composition and evolution of Kerguelen Plateau ash layers resemble those from other hotspot-induced, oceanic-island realms such as Iceland and Jan Mayen in the North Atlantic, and the Canary Islands archipelago in the Central Atlantic.

Calcareous nannoplankton from the Southwest Pacific Ocean

The positions of all cores recovered during Leg 90 in the southwest Pacific are shown within the standard calcareous nannoplankton zonation. The stratigraphic and regional occurrences and preservation of Paleogene calcareous nannoplankton found at Sites 588, 592, and 593 are discussed, and fossil lists are given for selected samples. Data on the Eocene/Oligocene boundary found in Holes 592 and 593 and on the Oligocene/Miocene boundary in Hole 588C are presented. Regional unconformities are noted in Hole 588C, where the upper Eocene to middle Oligocene interval (Zones NP17 to NP23) is missing, and in Hole 592, in which the middle Oligocene to lowest Miocene interval (Zones NP23 to NN1) is not represented.

Jurassic to Cretaceous calcareous nannofossils from the Argo Abyssal Plain

A summary of calcareous nannofossil biostratigraphy performed for Late Jurassic (Kimmeridgian) to Early Cretaceous (Hauterivian) cores of Site 765 (Cores 123-765C-58R to -55R) and Site 261 (Cores 27-261-33 to -27), Argo Abyssal Plain, off northwestern Australia is presented. Precise age determinations were limited by variable preservation and the exclusion of a number of marker species due to provincialism. However, the presence of species, such as, Stephanolithion bigotii bigotii, Watznaueria manivitae, Tubodiscus verenae, and Cruciellipsis cuvillieri results in a reasonably good degree of biostratigraphic control. Biogeographic interpretation of the nannofossil data suggests that the Argo Basin occupied a position transitional between the Tethyan and Austral nannofloral realms. A cooler water regime is suggested by the absence of thermophyllic Tethyan forms, such as Nannoconus, and the presence of taxa that display bipolar distribution, such as Crucibiscutum salebrosum. Two new species, Zeugrhabdotus cooperi and Cyclagelosphaera argoensis, and one new combination, Haqius ellipticus are described.

Paleomagnetic of lower Cretaceous sediments from the Argo and Gascoyne Abyssal Plain

Lower Cretaceous sediments were sampled for magnetostratigraphy at three sites. ODP Site 765 and DSDP Site 261, in the Argo Abyssal Plain, consist primarily of brownish-red to gray claystone having hematite and magnetite carriers of characteristic magnetization. ODP Site 766, in the Gascoyne Abyssal Plain, consists mainly of dark greenish-gray volcaniclastic turbidites with magnetite as the carrier of characteristic magnetization. Progressive thermal demagnetization (Sites 765 and 261) or alternating field demagnetization (Site 766) yielded well-defined polarity zones and a set of reliable paleolatitudes. Magnetic polarity chrons were assigned to polarity zones using biostratigraphic correlations. Late Aptian chron M"-1"r, a brief reversed-polarity chron younger than MOr, is a narrow, 40-cm feature delimited in Hole 765C. Early Aptian reversed-polarity chron MOr is also present in Hole 765C. Polarity chrons Mir through M3r were observed in the Barremian of all three sites. Valanginian and Hauterivian polarity chrons can be tentatively assigned to polarity zones only in Hole 766A. The paleolatitude of this region remained at 35° to 37°S from the Berriasian through the Aptian. During this interval, there was approximately 16° of clockwise rotation, with the oriented sample suites of Site 765 displaying a Berriasian declination of 307° to an Aptian declination of 323°. These results are consistent with the interpolated Early Cretaceous apparent polar wander for Australia, but indicate that this region was approximately 5? farther north than predicted.

Paleomagneitc of early Cretaceous sediments from the western Central Atlantic

Drilling at Sites 534 and 603 of the Deep Sea Drilling Project recovered thick sections of Berriasian through Aptian white limestones to dark gray marls, interbedded with claystone and clastic turbidites. Progressive thermal demagnetization removed a normal-polarity overprint carried by goethite and/or pyrrhotite. The resulting characteristic magnetization is carried predominantly by magnetite. Directions and reliability of characteristic magnetization of each sample were computed by using least squares line-fits of magnetization vectors. The corrected true mean inclinations of the sites suggest that the western North Atlantic underwent approximately 6° of steady southward motion between the Berriasian and Aptian stages. The patterns of magnetic polarity of the two sites, when plotted on stratigraphic columns of the pelagic sediments without turbidite beds, display a fairly consistent magnetostratigraphy through most of the Hauterivian-Barremian interval, using dinoflagellate and nannofossil events and facies changes in pelagic sediment as controls on the correlations. The composite magnetostratigraphy appears to include most of the features of the M-sequence block model of magnetic anomalies from Ml to Ml ON (Barremian-Hauterivian) and from M16 to M23 (Berriasian-Tithonian). The Valanginian magnetostratigraphy of the sites does not exhibit reversed polarity intervals corresponding to Ml 1 to M13 of the M-sequence model; this may be the result of poor magnetization, of a major unrecognized hiatus in the early to middle Valanginian in the western North Atlantic, or of an error in the standard block model. Based on these tentative polarity-zone correlations, the Hauterivian/Barremian boundary occurs in or near the reversed-polarity Chron M7 or M5, depending upon whether the dinoflagellate or nannofossil zonation, respectively, is used; the Valanginian/Hauterivian boundary, as defined by the dinoflagellate zonation, is near reversed-polarity Chron M10N.

Geochemistry of plagioclase from the Mid-Atlantic Ridge

We have studied the chemical zoning of plagioclase phenocrysts from the slow-spreading Mid-Atlantic Ridge and the intermediate-spreading rate Costa Rica Rift to obtain the time scales of magmatic processes beneath these ridges. The anorthite content, Mg, and Sr in plagioclase phenocrysts from the Mid-Atlantic Ridge can be interpreted as recording initial crystallisation from a primitive magma (~11 wt% MgO) in an open system. This was followed by crystal accumulation in a mush zone and later entrainment of crystals into the erupted magma. The initial magma crystallised plagioclase more anorthitic than those in equilibrium with any erupted basalt. Evidence that the crystals accumulated in a mush zone comes from both: (1) plagioclase rims that were in equilibrium with a Sr-poor melt requiring extreme differentiation; and (2) different crystals found in the same thin section having different histories. Diffusion modelling shows that crystal residence times in the mush were <140 years, whereas the interval between mush disaggregation and eruption was ?1.5 years. Zoning of anorthite content and Mg in plagioclase phenocrysts from the Costa Rica Rift show that they partially or completely equilibrated with a MgO-rich melt (>11 wt%). Partial equilibration in some crystals can be modelled as starting <1 year prior to eruption but for others longer times are required for complete equilibration. This variety of times is most readily explained if the mixing occurred in a mush zone. None of the plagioclase phenocrysts from the Costa Rica Rift that we studied have Mg contents in equilibrium with their host basalt even at their rims, requiring mixing into a much more evolved magma within days of eruption. In combination these observations suggest that at both intermediate- and slow-spreading ridges: (1) the chemical environment to which crystals are exposed changes on annual to decadal time scales; (2) plagioclase crystals record the existence of melts unlike those erupted; and (3) disaggregation of crystal mush zones appears to precede eruption, providing an efficient mechanism by which evolved interstitial melt can be mixed into erupted basalts.

Documentation of sediment cores from the Great Meteor Seamount, North Atlantic

Sedimentological and biostratigraphic investigations of 15 cores (total length: 88 m) from the vicinity of Great Meteor seamount (about 30° N, 28° W) showed that the calcareous ooze are asymmetrically distributed around the seamount and vertically differentiated into two intervals. East and west of the seampunt, the upper "A"-interval is characterized by yellowish-brown sediment colors and bioturbation; ash layers and diatoms are restricted to the eastern cores. On both seamount flanks, the sediment of the lower "B"-interval are white and very rich in CaCO3 with a major fine silt (2-16 µ) mode (mainly coccoliths). Lamination, manganese micronodules, Tertiary foraminifera and discoasters, and small limestone and basalt fragments are typical of the "B"-interval of the eastern cores only. The sediments contain abundant displaced material which was reworked from the upper parts of the seamount. The sedimentation around the seamount is strongly influenced by the kind of displaced material and the intensity of its differentiated dispersal: the sedimentation rates are generally higher on the east than on the west flank /e.g. in "B": 0.9 cm/1000 y in the W; 3.1 cm/1000 y in the E), and lower for the "A" than for the "B"-interval. The lamination is explained by the combination of increased sedimentation rates with a strong input of material poor in organic carbon producing a hostile environment for benthic life. The CaCO3 content of the core is highly influenced by the proportion of displaced bigenous carbonate material (mainly coccoliths). The genuine in-situ conditions of the dissolution facies are only reflected by the minimum CaCO3 values of the cores (CCD = about 5,500 m; first bend in dissolution curve = 4,000 m; ACD = about 3,400 m). The preservation of the total foraminiferal association depends on the proportions of in-situ versus displaced specimens. In greater water depths (stronger dissolution), for example, the preservation can be improved by the admixture of relatively well preserved displaced foraminifera. Carbonate cementation and the formation of manganese micronodules are restricted to microenvironments with locally increased organic carbon contents (e.g. pellets; foraminifera). The ash layers consist of redeposited, silicic volcanic glass of trachytic composition and Mio-Pliocene age; possibly, they can be derived from the upper part of the seamount. Siliceous organisms, especially diatoms, are frequent close to the ash layers and probably also redeposited. Their preservation was favoured by the increase of the SiO2 content in the pore water caused by the silicic volcanic glass. The cores were biostraftsraphically subdivided with the aid of planktonic foraminifera and partly alsococcoliths. In most cases, the biostratigraphically determined cold- and warm sections could be correlated from core to core. Almost all cores do not penetrate the Late Pleistocene. All Tertiary fossils are reworked. In general, the warm/cold boundary W2/C2 corresponds with the lithostratigraphic A/B boundray. Benthonic foraminifera indicate the original site deposition of the displaced material (summit plateau or flanks of the seamount). The asymmetric distribution of the sediments around the seamount east and west of the NE-directed antarctic bottom current (AABW) is explained by the distortion of the streamlines by the Coriolis force; by this process the current velocity is increased west of the seamount and decreased east of it. The different proportion of displaced material within the "A" and "B" interval is explained by changes of the intensity of the oceanic circulation. At the time of "B" the flow of the AABW around the seamount was stronger than during "A"; this can be inferred from the presence of characteristic benthonic foraminifera. The increased oceanic circulation implies an enhanced differentiation of the current velocities, and by that, also of the sedimentation rates, and intensifies the winnowed sediment material was transported downslope by turbid layers into the deep-sea, incorporated into the current system of the AABW, and asymmetrically deposited around the seamount.

Oligocene planktonic foraminifera from the Ontong Java Plateau

Ocean Drilling Program Hole 803D (Leg 130) from the western tropical Pacific (Ontong Java Plateau) and Hole 628A (Leg 101) from the western subtropical North Atlantic (Little Bahama Bank) contain rich assemblages of planktonic foraminifers. The uppermost Eocene-basal Miocene section of Hole 803D is apparently complete, whereas the Oligocene section of Hole 628A contains three unconformities based on planktonic foraminiferal evidence. Anomalous ranges are recorded for Chiloguembelina cubensis and Globigerinoides primordius. C. cubensis is found to range throughout the upper Oligocene of both sites, and G. primordius first occurs near the base of upper Oligocene Zone P22 in Hole 628A. Paleomagnetic stratigraphy provides constraints on the last occurrence (LO) of Subbotina angiporoides, the first occurrence (FO) of Globigerina angulisuturalis, the FO of Globigerinoides primordius, the FO of Paragloborotalia pseudokugleri, and the LO of Chiloguembelina cubensis. In general, taxon ranges, total diversity, and the composition of the planktonic foraminiferal assemblages from Holes 628A and 803D are similar. Differences in the composition of planktonic foraminiferal assemblages between the two sites are interpreted to be primarily the result of enhanced dissolution at Site 803 (e.g., paucity of Globigerina angulisuturalis and absence of G. ciperoensis). However, the greater abundances of Subbotina angiporoides in subtropical Hole 628A and Paragloborotalia opima in tropical Hole 803D are probably related to oceanographic differences between the two low-latitude sites. Comparison between the low and southern high latitudes illustrates some similarities in the composition of Oligocene planktonic foraminiferal assemblages as well as some important differences. Species such as Pseudohastigerina spp., Turborotalia increbescens, "Turborotalia" ampliapertura, Paragloborotalia opima, P. pseudokugleri, P. semivera/mayeri, Globigerinella obesa, Globigerina angulisuturalis, G. gortanii, G. ouachitaensis, G. sellii, G. tapuriensis, G. tripartita, G. pseudovenezuelana, Subbotina? eocaena and S.? yeguaensis are absent or have rare occurrences in the subantarctic Oligocene assemblages. Biogeographic gradients, although not as pronounced as during the late Neogene, were nonetheless significant during the Oligocene.

Age models and sea-surface paleotemperature reconstruction of sediment cores from the eastern equatorial Atlantic

Based on the faunal record of planktonic foraminifers in three long gravity sediment cores from the eastern equatorial Atlantic, the sea-surface temperature history ove the last 750,000 years was studied at a resolution of 3,000 to 10,000 years. Detailed oxygen-isotope and paleomagnetic stratigraphy helped to identify the following major faunal events: Globorotaloides hexagonus and Globorotalia tumida flexuosa became extinct in the eastern tropical Atlantic at the isotope stage 4/5 boundary, now dated at 68,000 years B.P. The persistent occurrence of the pink variety of Globigerinoides ruber started during the late stage 12 at 410,000 years B.P. CARTUNE-age. This datum may provide an easily detectible faunal stratigraphic marker for the mid-Brunhes Chron. The updated scheme of the Ericson zones helped the recognition of a hiatus at the northwestern slope of the Sierra Leone Basin covering oxygen-isotope stages 10 to 12. Classifying the planktonic foraminifer counts into six faunal assemblages, according to the factor analysis derived model of Pflaumann (1985), the tropical and the tropical-upwelling communities account for 57 % at Site 16415, and 86 % at Site 13519, respectively of the variance of the faunal record. A largely continuous paleotemperature record for both winter and summer seasons was obtained from the top of the Sierra Leone Rise with the winter temperatures ranging between 20 and 25 °C, and the summer ones between 24 and 30 °C. The record of cores from greater water depths is frequently interrupted by samples with no-analogue faunal communities and/or poor preservation. Based on the seasonality signal, during cold periods the termal equator shifted to a geographically mnore asymmetrical northern position. Dissolution altering the faunal communities becomes stronger with greater water depth, the estimated mean minimum loss of specimens increases from 70 % to 80 % between 2,860 and 3,850 water depth although some species will be more susceptible than others. Enhanced dissolution occured during stage 4 but also during cold phases in the warm stage 7 and 9. Correlations between the Foraminiferal Dissolution Index and the estimated sea-surface temperatures are significant. Foraminiferal flux rates, negatively correlated to the flux rates of organic carbon and of diatoms, may be a result of enhanced dissolution during cold stages, destroying still more of the faunal signal than indicated by the calculated minimum loss. The fluctuations of the oxygen-isotope curves and the hibernal sea-surfave temperatures are fairly coherent. During warm oxygen-isotope stages the temperature maxima lag often by 5 to 15 ka behind the respective sotope minima. During cold stages, sea-surface temperature changes are partly out of phase and contain additional fluctuations.

Ostracoda fauna of surface sediments from the Persian Gulf

During the "Meteor"-Expedition to the Persian Gulf in March-May 1965, approximately 300 samples were collected. Most of them have been already studied by various authors in sedimentological as well as micropaleontological respects. 49 samples were selected for ostracode studies. These samples are arranged to form a long-axis section ("Laengsprofil"), and 4 shorter cross-profiles, perpendicular to the long-axis profile in the Persian Gulf and Gulf of Oman. 52 species of ostracodes in this area were specifically determined; 39 of them are described under open nomenclature. 13 species are already known from surrounding sea areas: 2 species from the Red Sea; 2 species from the east coast of Africa; 1 species from the Mediterranean Sea; and others from the Indian and Pacific Oceans. 12 species show close relationships to species from the Indopacific Ocean. The ostracode species found in the area can be grouped after the method of BRAUN-BLANQUT into 2 bioassociations. Association 1 with the following 4 characteristic species : Cytherella cf. pulchra, Loxoconcha sp. A, Neomonoceratina sp. A, Alocopocythere reticulata. Association 2 with 1 characteristic species: Ruggieria (Ruggieria) sp. B. The association 1 is widespread in the entire studied area of the Persian Gulf, where it is considered to characterize the shallow water region down to 200 m. The association 2 is restricted to the deeper water below 200 m of the inner part of the Oman Gulf. Only a few species known from the shallow water association of the Persian Gulf are present. Within the two ostracode associations mentioned above 4 zones from the total studied area could be related to the water depth. The zones A-D are characterized more or less readily by the relative abundance of certain species: Zone A : 7-30 m depth, on substrates of poorly coarse-grained clayey marl; Zone B: 30-94 m depth, on substrates of richly coarse-grained calcareous marl; Zone C: 94-1961208 m depth, on substrates of richly coarse-grained calcareous marl; Zone D: 196/208-500 m depth, on substrates of calcareous clay, poor in benthos. The regional and bathymetric distribution of the ostracode fauna in the area studied was compared in relation to 10 environmental factors: water depth, temperature, salinity, water density, O2-concentration, phosphate-silica contents, pH-values, stratification of the water body, water currents and type of sediments. The major environmental factors which appear to control the ostracode distribution are water depth (as a complex factor), O2-concentration and the type of sediment. At 3 stations (GIK01058, GIK01074 and GIK01204) species of the shallow water association were found together with a few bathyal species. These stations are situated at the outer Biaban shelf, in an area where the bottom water of the Persian Gulf flows down the slope towards the Oman Gulf. Several samples of the Zone B in the major part of the Persian Gulf show also a high species diversity containing a high percentage of subfossil ostracode carapaces. It is probable that the recent biocoenosis has been mixed with a late quarternary thanatocoenosis.

Eocene to Quaternary planktonic foraminifers from the Northwest Pacific

Eocene through Quaternary planktonic foraminifers were identified in cores recovered during Leg 126. Turbidites and volcanic ash beds are intercalated with hemipelagic sediments. Preservation of foraminifers is variable, ranging from excellent to poor and appears to have been affected by fluctuations in the carbonate compensation depth (CCD), depth of burial, changes in bottom water temperature, current velocity, sediment accumulation rates and seafloor topography. Preservation of foraminifers in Quaternary sediments is generally good, however, species abundance varies by a factor of I05-106 and reflects dilution by volcanogenic as well as terrigenous constituents and cannot be used for paleoceanographic reconstructions. In pre-Quaternary deposits planktonic foraminiferal tests frequently exhibit dissolution effects; biostratigraphic zonation and placement of zonal boundaries is difficult owing to hiatuses, dissolution facies, extraneously deposited sediments, and discontinuous coring. The Eocene foraminiferal faunas include specimens of the Globorotalia cerroazulensis plexus, markers of Zone P16 as well as Globigerina senni and Globigerinatheka spp., which became extinct before the end of the Eocene. Six hiatuses and/or dissolution periods, probably reflecting global cooling events and/or changes in oceanic circulation patterns were recorded at Site 792. Recrystallized, poorly preserved, possibly reworked Eocene species (Globigerina senni and Globigerapsis sp.) were recorded in sediments at Site 793.

Oligocene and Neogene radiolarians from the Labrador Sea

Two sites in the Labrador Sea and one site in Baffin Bay were drilled during Leg 105. Radiolarians were recovered at all three sites, although at Site 645 (Baffin Bay), radiolarians were present in useful numbers only in the mudline sample. Radiolarians of late Neogene age were recovered at Site 646 south of Greenland, while early Oligocene and early Miocene radiolarians were recovered from the Labrador Sea at Site 647. In Site 646, radiolarian and other coarse-fraction abundances vary dramatically from sample to sample and may reflect deep-water depositional processes as well as changes in surface-water conditions. Site 647 siliceous microfossils reach their peak abundance and preservation in Core 105-647A-25R and decline gradually upward into the lower Miocene (Cores 105-647A-13R and -14R). Siliceous microfossil abundances in counts of the > 38-µm Carbonate-free coarse fraction from the siliceous interval are correlated to each other, but not to the abundance of nonbiogenic coarse-fraction components. Radiolarian abundances in specimens per gram (but not diatom abundances) are correlated to bulk opal concentration and to the organic carbon content of the sediment. The abundance of radiolarians and other siliceous microfossils within the lower Oligocene to lower Miocene is interpreted as reflecting changes in surface-water productivity. With only a few exceptions, no stratigraphic indicator species were seen in samples from either Site 646 or Site 647. The absence of both tropical/subtropical and Norwegian-Greenland Sea stratigraphic forms is due to the dominance of subarctic North Atlantic taxa in Leg 105 assemblages. The early Oligocene and early Miocene assemblages recovered at Site 647 are of particular interest, as very little material of these ages has previously been recovered from the subarctic North Atlantic region, and virtually no descriptive work has been conducted on the more endemic components of the radiolarian assemblages from these time intervals. Thus, this report concentrates on providing, at least in part, the first comprehensive documentation of early Oligocene and early Miocene radiolarians from the subarctic North Atlantic, with emphasis on basic descriptions, measurements, and photographic documentation. However, synonymic work and formal designation of new species names has been deferred until additional material from other regions can be examined. The sole exception is the emendation of Theocalyptra tetracantha Bjorklund and Kellogg 1972 to Cycladophora tetracantha n. comb.

Organic chemistry of sediments from the Tyrrhenian Sea

Thirty sediment samples from Tortonian to Pleistocene age of five ODP locations (Holes 650A, 651A, and 652A, and Sites 654 and 655) in the Marsili Basin, Vavilov Basin, and Sardinia Margin (Tyrrhenian Sea) were studied by organic geochemical methods including total organic carbon determination, Rock-Eval pyrolysis, bitumen extraction, pyrolysis-gas chromatography, and organic petrography. Six organic facies, including open ocean anoxia with variable terrestrial input, oxic open ocean, oxic tidal flat, mildly oxic lagoon, and anoxic lacustrine algal-bacterial mat environments, have been recognized in these sediments. The sediments below 500 m in Sardinia Margin are mature for significant hydrocarbon generation. Possible mature source-rock (Type I and IIB/III kerogen) and migrated bitumen occur in the deeper part of the section in Vavilov Basin and Sardinia Margin sediments. Sporadic sapropel formation observed in the studied Pliocene-Pleistocene sediment section is probably controlled by organic productivity due to nutrient supply by the rivers and terrestrial input associated with open ocean anoxia or anoxia caused by the material balance between rate of organic matter supplied by turbidites and organic matter consumption. Pliocene and Pleistocene sapropels are mostly immature and lie within Type II-III (precisely as IIA-IIB and IIB source rocks) kerogen maturation path.

Cenozoic calcareous nannofossils from the Lesser Antilles Forearc Region

During Leg 110 of the Ocean Drilling Program, sediment was recovered from six sites in the vicinity of the Lesser Antilles Forearc. Hole 671B, drilled near the toe of the Barbados deformation front, was the first-ever penetration of the decollement between the underthrusting Atlantic Plate and the off scraped Barbados accretionary prism. Stratigraphic repetitions in sequence associated with tectonic movement along the decollement zone, first observed on DSDP Leg 78A, were further documented at four ODP Leg 110 sites. A significant biostratigraphic inversion is present at Site 671 at 128 mbsf in which upper Miocene sediments rest atop lower Pleistocene strata. Smaller repetitions in sequence are recorded at Sites 671, 673, 674, and 676. Leg 110 sediments range from middle Eocene to early Pleistocene in age. Pliocene/Pleistocene assemblages are generally well preserved; however, Miocene assemblages have undergone extensive dissolution at all Leg 110 sites. Paleogene sediments are sometimes recrystallized and the nannofossils contained within exhibit a range in preservation from poor to good.

Organic geochemistry of Upper Albian sediments from the Kirchrode I borehole

In the framework of a multidisciplinary research program, an organic geochemical study was carried out on a drill core which comprises a 245 m thick sequence of light-colored, Upper Albian marlstones that were deposited in the central part of the Lower Saxony basin (northern Germany). For part of the Upper Albian sequence, high-resolution measurements of carbonate contents reveal cycles which can be related to earth orbital forcing. Based on these data, sediment accumulation rates were calculated to be in the order of 15 g/m**2/yr. These high accumulation rates contrast with very low organic carbon contents and an extremely poor preservation of the autochthonous organic matter. Most of the sedimentary organic matter is of terrigenous origin and mainly derived from the erosion of older sedimentary rocks. Organic petrography reveals only a very small fraction of marine organic particles. Carbon/sulphur ratios, pristane/phytane ratios as well as the predominance of resedimented organic particles over autochthonous organic particles suggest that aerobic degradation processes rather than anaerobic processes (sulphate reduction) were responsible for the degradation of the organic matter. Furthermore, the scarcity of terrigenous organic particles (vitrinite) indicates that there was little vegetation on nearby land areas. To explain these analytical results, a depositional model was developed which could explain the scarcity of organic matter in the Upper Albian sediments. This model is based on downwelling of oxygen-rich, saline waters of Tethyan origin, which reduces the nutrient content of surface waters and thus primary bioproductivity while degradation of primary organic matter in the water column is enhanced at the same time. These conditions contrast to those which existed in Barremian and early Aptian times in this basin, when limited water exchange with adjacent oceans caused oxygen deficiency and the deposition of numerous organic carbon-rich black shales. The thick, organic matter-poor Upper Albian sequence of northern Germany also contrasts with comparatively thin, time-equivalent, deep-sea black shales from Italy. This discrepancy indicates that local and regional oceanographic factors (at least in this case) have a greater influence on organic matter deposition than global events.