Cretaceous calcareous nannofossils from Exmouth and Wombat Plateau, Northwest Australia

Three sites drilled during Leg 122, Site 761 on the Wombat Plateau and Sites 762 and 763 on the Exmouth Plateau, provide a composite Cretaceous section ranging in age from Berriasian to Maestrichtian. Together, these sites contain an apparently complete, expanded Aptian-Maestrichtian record. Consistently occurring and moderately well-preserved nannofossil assemblages allow reasonably high biostratigraphic resolution. Our data indicate that traditional middle and Upper Cretaceous nannofossil biozonations are not entirely applicable in this region. In this investigation, we compare in detail the relative ranges of key Cretaceous nannofossil markers in the eastern Indian Ocean and in sections from Europe and North Africa. We have determined which previously used events are applicable, and which additional markers have biostratigraphic utility in this region. Significant differences in Campanian-Maestrichtian assemblages exist between the more northern Site 761 and Sites 762 and 763. Such differences are surprising, considering that these sites are only separated by 3° of latitude. We interpret them as marking a strong thermal gradient over the Exmouth Plateau region. Other results include the recovery of an expanded Albian-Cenomanian sequence containing a mixture of Austral and Tethyan floras, which will enable correlation of biozonations established for these two realms; the recovery of two condensed but apparently complete Cenomanian-Turonian boundary sections; correlation of Upper Cretaceous calcareous nannofossil biostratigraphy with magneto- and foraminifer stratigraphy; and correlation of portions of the Barrow Group equivalents to the Berriasian and Valanginian stages.

Chemical and isotope compositions of rocks dredged from the Shatsky Rise

Magmatic rocks of the Shatsky Rise form two groups replacing one another in time. The earlier ferrotholeiites enriched in potassium compose large massifs. Trachybasalts form seamounts and neotectonic ridges. Composition of volcanites indicates that two sources of magmatism took part in their formation: a depleted source characteristic of basalts of mid-ocean ridges and a ''plume'' source participating in formation of oceanic plateaus.

Age determination of ODP Leg 120 samples

Conventional K-Ar and 40Ar/39Ar analyses on whole-rock samples are reported for basaltic samples retrieved on the Central and Southern Kerguelen plateaus during Ocean Drilling Program Leg 120. Sites 747, 749, and 750 recovered basalts from the plateau basement, whereas Site 748 drilled a lava flow interbedded with sediments of probable Albian age. The freshest core basalts from the basement yielded dates falling in the 110-100 m.y. interval. Sample 120-749C-15R-3 (26-31 cm) gave conventional K-Ar, total fusion, and plateau 40Ar/39Ar ages that are closely concordant: 111.5 ± 3.2 m.y., 109.9 ± 1.2 m.y., and 109.6 ± 0.7 m.y., respectively. Sample 120-750B-15R-5 (54-60 cm), when taking into account the analytical uncertainties, yields conventional K-Ar and 40Ar/39Ar plateau ages that can be considered similar: 101.2 ± 7.5 and 118.2 ± 5 m.y., respectively. Inspection of the 39Ar/40Ar vs. 36Ar/40Ar diagram does not reveal the occurrence of an initial argon component of radiogenic composition in the two samples. Accordingly, our results suggest that the formation of the basement of the Central Kerguelen Plateau was closed at 110 m.y.. Furthermore, these results are in agreement with a K-Ar age of 114 ± 1 m.y. mentioned in the literature for a basalt dredged in the 77°E Graben. The still scant amount of data indicates that the outpourings of the Central Kerguelen Plateau correspond rather well with widespread continental magmatism in Gondwanaland that is believed to mark the incipient opening of the eastern Indian Ocean. This implies a huge head for the mantle plume at the source of these liquids. Nevertheless, on land and at sea the exact duration of magmatism remains unknown. Therefore, a catastrophic pattern similar to that currently invoked for the Deccan Traps at the end of the Cretaceous, though possible, is not yet required by present geochronologic data.

Paleogene planktonic foraminifera from the southern Kerguelen Plateau

An essentially complete Paleogene record was recovered on the Central and Southern Kerguelen plateaus (55°-59°S) in a calcareous biofacies. Recovery deteriorated in the middle Eocene and down to the upper Paleocene because of the presence of interbedded cherts and chalks. The stratigraphic distribution of about 70 taxa of planktonic foraminifers recovered at Sites 747-749 is reported in this paper. Faunas exhibited fairly high diversity (approximately 20-25 species) in the early Eocene, followed by a gradual reduction in diversity in the middle Eocene. A brief incursion of tropical keeled morozovellids occurred near the Paleocene/Eocene boundary, similar to that recorded on the Maud Rise (ODP Sites 689 and 690). The high-latitude Paleogene zonal scheme developed for ODP Leg 113 sites has been adopted (with minor modifications) for the lower Eocene-Oligocene part of the Kerguelen Plateau record. A representative Oligocene (polarity chronozones 7-13) and late Eocene-late middle Eocene (questionably polarity chronozones 16-18) magnetostratigraphic record has allowed the calibration of several biostratigraphic datum levels to the standard Global Polarity Time Scale (GPTS) and established their essential synchrony between low and high latitudes.

Reconstructed ice-sheet limit in the Kara and Barents Sea area at about 50,000 years ago (Markhida Line)

Glacial landforms in northern Russia, from the Timan Ridge in the west to the east of the Urals, have been mapped by aerial photographs and satellite images supported by field observations. An east-west trending belt of fresh hummock-and-lake glaciokarst landscapes has been traced to the north of 67°N. The southern boundary of these landscapes is called the Markhida Line, which is interpreted as a nearly synchronous limit of the last ice sheet that affected this region. The hummocky landscapes are subdivided into three types according to the stage of postglacial modification: Markhida, Harbei and Halmer. The Halmer landscape on the Uralian piedmont in the east is the freshest, whereas the westernmost Markhida landscape is more eroded. The west- east gradient in morphology is considered to be a result of the time-transgressive melting of stagnant glacier ice and of the underlying permafrost. The pattern of ice-pushed ridges and other directional features reflects a dominant ice flow direction from the Kara Sea shelf. Traces of ice movement from the central Barents Sea are only discernible in the Pechora River left bank area west of 50°E. In the Polar Urals the horseshoe-shaped end moraines at altitudes of up to 560 m a.s.l. reflect ice movement up-valley from the Kara Ice Sheet, indicating the absence of a contemporaneous ice dome in the mountains. The Markhida moraines, superimposed onto the Eemian strata, represent the maximum ice sheet extent in the western part of the Pechora Basin during the Weichselian. The Markhida Line truncates the huge arcs of the Laya-Adzva and Rogovaya ice-pushed ridges protruding to the south. The latter moraines therefore reflect an older ice advance, probably also of Weichselian age. Still farther south, fluvially dissected morainic plateaus without lakes are of pre-Eemian age, because they plunge northwards under marine Eemian sediments. Shorelines of the large ice-dammed Lake Komi, identified between 90 and 110 m a.s.l. in the areas south of the Markhida Line, are radiocarbon dated to be older than 45 ka. The shorelines, incised into the Laya-Adzva moraines, morphologically interfinger with the Markhida moraines, indicating that the last ice advance onto the Russian mainland reached the Markhida Line during the Middle or Early Weichselian, before 45 ka ago.

Sedimentation in the vicinity of Leg 92 Drill Sites: Studies of site survey cores

Surveys of the areas surrounding the sites drilled on the Leg 92 19°S transect showed that sedimentation at all except the oldest site is dominated by calcium carbonate deposition. The sediments in the area of the oldest site, west of the Austral Fracture Zone, are being deposited beneath the calcium carbonate compensation depth and are dominated by terrigenous and metal-rich hydrogenous and hydrothermal sediments. The noncarbonate sediments in all of the areas east of the Austral Fracture Zone are dominated by hydrothermal sediment similar in composition to that presently being deposited at the East Pacific Rise. Although no biogenic microfossils were present in smear slides of the sediment, geochemical partitioning suggests that a remnant signal of siliceous biogenic deposition may be preserved, especially in gravity core (GC) 8, which was collected from a high heat flow zone near Site 600. The siliceous sediment may also result from the deposition of amorphous hydrothermal silica from the higher concentrations of pore water SiO2 characteristic of the upwelling waters. Sedimentation on the broad plateaus that characterize each area is quite uniform and suggests that sites on these plateaus will be broadly representative of pelagic sedimentation in the area.

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.

Phytomass and soil carbon storage of different land cover classes in the Usa river basin

This study describes detailed partitioning of phytomass carbon (C) and soil organic carbon (SOC) for four study areas in discontinuous permafrost terrain, Northeast European Russia. The mean aboveground phytomass C storage is 0.7 kg C/m**2. Estimated landscape SOC storage in the four areas varies between 34.5 and 47.0 kg C/m**2 with LCC (land cover classification) upscaling and 32.5-49.0 kg C/m**2 with soil map upscaling. A nested upscaling approach using a Landsat thematic mapper land cover classification for the surrounding region provides estimates within 5 ± 5% of the local high-resolution estimates. Permafrost peat plateaus hold the majority of total and frozen SOC, especially in the more southern study areas. Burying of SOC through cryoturbation of O- or A-horizons contributes between 1% and 16% (mean 5%) of total landscape SOC. The effect of active layer deepening and thermokarst expansion on SOC remobilization is modeled for one of the four areas. The active layer thickness dynamics from 1980 to 2099 is modeled using a transient spatially distributed permafrost model and lateral expansion of peat plateau thermokarst lakes is simulated using geographic information system analyses. Active layer deepening is expected to increase the proportion of SOC affected by seasonal thawing from 29% to 58%. A lateral expansion of 30 m would increase the amount of SOC stored in thermokarst lakes/fens from 2% to 22% of all SOC. By the end of this century, active layer deepening will likely affect more SOC than thermokarst expansion, but the SOC stores vulnerable to thermokarst are less decomposed.

Radiocarbon dating of peat profiles from the subarctic ecoclimatic region, west-central Canada

Long-term vegetation succession and permafrost dynamics in subarctic peat plateaus of west-central Canada have been studied through detailed plant macrofossil analysis and extensive AMS radiocarbon dating of two peat profiles. Peatland inception at these sites occurred around 5800-5100 yr BP (6600-5900 cal. BP) as a result of paludification of upland forests. At the northern peat plateau site, located in the continuous permafrost zone, palaeobotanical evidence suggests that permafrost was already present under the forested upland prior to peatland development. Paludification was initiated by permafrost collapse, but re-aggradation of permafrost occurred soon after peatland inception. At the southern site, located in the discontinuous permafrost zone, the aggradation of permafrost occurred soon after peatland inception. In the peat plateaus, permafrost conditions have remained very stable until present. Sphagnum fuscum-dominated stages have alternated with more xerophytic communities characterized by ericaceous shrubs. Local peat fires have occurred, but most of these did not cause degradation of the permafrost. Starting from 2800-1100 yr BP (2900-1000 cal. BP) consistently dry surface conditions have prevailed, possibly related to continued frost heave or nearby polygon crack formation.

Composition of marine phosphorites and phosphate-bearing sediments

The book deals with behavior of phosphorus and its concentration in oceanic phosphorites. The major stages of marine geochemical cycle of phosphorus including its supply to sedimentary basins, precipitation from sea water, distribution and speciation in bottom sediments, diagenetic redistribution, and relation to other elements are under consideration. Formation of recent phosphorites as a culmination of phosphate accumulation in marine and oceanic sediments is examined. Distribution, structure, mineral and chemical compositions of major phosphorite deposits of various age on continental margins, as well as on submarine plateaus, uplifts and seamounts and some islands are described. A summary of trace element abundances in oceanic phosphorites is presented. Problems of phosphorite origin are discussed.

Neogene planktonic foraminifera from the southern Kerguelen Plateau

With the exception of a brief (2 m.y.) late Miocene-early Pliocene hiatus, an essentially complete Neogene record was recovered on the Kerguelen Plateau in a calcareous biofacies. The stratigraphic distribution of about 30 taxa of Neogene planktonic foraminifers recovered at Sites 747, 748,and 751 (Central and Southern Kerguelen plateaus; approximately 54°-58°S) is recorded. Faunas are characterized by low diversity and high dominance and exhibit a gradual decline in species numbers (reflecting a concomitant increase in biosiliceous forms, particularly diatoms) from about 10 in the early Miocene to 5-8 in the middle Miocene, 3-4 in the late Miocene, to essentially a lone (Neogloboquadrina pachyderma) form in the Pliocene-Pleistocene. A provisional sevenfold biostratigraphic zonation has been formulated that, together with the recovery of a representative Neogene magnetostratigraphic record, may ultimately lead to a correlation with low-latitude magnetobiostratigraphies. The initial appearance of Neogloboquadrina pachyderma is associated with magnetic polarity Chron (MPC) 4 (~7 Ma) and MPC 4A (>8 Ma) at Sites 747 and 751, respectively.