Sm-Nd isotopic investigation for rocks of Prince Charles Mountains, East Antarctica

The Prince Charles Mountains have been subject to extensive geological and geophysical investigations by former Soviet, Russian and Australian scientists from the early 1970s. In this paper we summarise, and review available geological and isotopic data, and report results of new isotopic studies (Sm-Nd, Pb-Pb, and U-Pb SHRIMP analyses); field geological data obtained during the PCMEGA 2002/2003 are utilised. The structure of the region is described in terms of four tectonic terranes. Those include Archaean Ruker, Palaeoproterozoic Lambert, Mesoproterozoic Fisher, and Meso- to Neoproterozoic Beaver Terranes. Pan-African activities (granite emplacement and probably tectonics) in the Lambert Terrane are reported. We present a summary of the composition of these terranes, discuss their origin and relationships. We also outline the most striking geological features, and problems, and try to draw attention to those rocks and regional geological features which are important in understanding the composition and evolution of the PCM and might suggest targets for further investigations.

Preliminary results and documentation of sediment cores CRP2 and CRP-2A from the Ross Sea off Cape Roberts, Antarctica

The site for CRP-2, 14 km east of Cape Roberts (77.006°S; 163.719°E), was selected to overlap the early Miocene strata cored in nearby CRP-1, and to sample deeper into the east-dipping strata near the western margin ofe he Victoria Land Basin to investigate Palaeogene climatic and tectonic history. CRP-2 was cored from 5 to 57 mbsf (metres below the sea floor) (core recovery 91 %), with a deviation resulting in CRP-2A being cored at the same site. CRP-2A reached down to 624mbsf (recovery 95%), and to strata with an age of c. 33-35 Ma. Drilling took place from 16 October to 25 November 1998, on 2.0-2.2 m of sea ice and through 178 m of water. Core fractures and other physical properties, such as sonic velocity, density and magnetic susceptibility, were measured throughout the core. Down-hole logs for these and other properties were run from 63 to 167 mbsf and subsequently from 200 to 623 mbsf, although density and velocity data could be obtained only to 440 mbsf because of hole collapse. Sonic velocity averages c. 2.0 km S-1 for the upper part of the hole, but there is an sharp increase to c. 3.0 km s-1 and also a slight angular unconformity, at 306 mbsf, corresponding most likely to the early/late Oligocene boundary (c. 28-30 Ma). Velocity then increases irregularly to around 3.6 km s-1 at the bottom of the hole, which is estimated to lie 120 m above the V4/V5 boundary. The higher velocities below 306 mbsf probably reflect more extensive carbonate and common pyrite cementation, in patches, nodules, bedding-parallel masses and as vein infills. Dip of the strata also increases down-hole from 3° in the upper 300 in to over 10° at the bottom. Temperature gradient is 21° k-1. Over 2 000 fractures were logged through the hole. Borehole televiewer imagery was obtained for the interval from 200 to 440 mbsf to orient the fractures for stress field analysis. Lithostratigraphical descriptions on a scale of 1:20 are presented for the full length of the core, along with core box images, as a 200 page supplement to this issue. The hole initially passed through a layer of muddy gravel to 5.5 mbsf (Lithological Sub-Unit or LSU 1.1), and then into a Quaternary diatom-bearing clast-rich diamicton to 21 mbsf (LSU 2. l), with an interval of alternating compact diamicton and loose sand, and containing a rich Pliocene foraminiferal fauna, to 27 mbsf (LSU 2.2). The unit beneath this (LSU 3.1) has similar physical properties (sonic velocity, porosity, magnetic susceptibility) and includes diamictites of similar character to those of LSU 2.1 and 2.2, but an early Miocene (c. 19 Ma) diatom assemblage at 28 mbsf (top of LSU 3.1) shows that this sub-unit is part of the older section. The strata beneath 27 mbsf, primary target for the project, extend from early Miocene to perhaps latest Eocene age, and are largely cyclic glacimarine nearshore to offshore sediments. They are described as 41 lithological sub-units and interpreted in terms of 12 recurrent lithofacies. These are 1) mudstone, 2) inter-stratified mudstone and sandstone, 3) muddy very fine to coarse sandstone, 4) well-sorted stratified fine sandstone, 5) moderately to well-sorted, medium-grained sandstone, 6) stratified diamictite, 7) massive diamictite, 8) rhythmically inter-stratified sandstone and mudstone, 9) clast-supported conglomerate, 10) matrix-supported conglomerate, 11) mudstone breccia and 12) volcaniclastic sediment. Sequence stratigraphical analysis has identified 22 unconformity-bounded depositional sequences in pre- Pliocene strata. They typically comprise a four-part architecture involving, in ascending order, 1) a sharp-based coarse-grained unit (Facies 6,7,9 or 10), 2) a fining-upward succession of sandstones (Facies 3 and 4), 3) a mudstone interval (Facies l), in some cases coarsening upward to muddy sandstones (Facies 3), and 4) a sharp-based sandstone dominated succession (mainly Facies 4). The cyclicity recorded by the strata is interpreted in terms of a glacier ice margin retreating and advancing from land to the west, and of rises and falls in sea level. Analysis of sequence periodicity awaits afirmer chronology. However, apreliminary spectral analysis of magnetic susceptibility for a deepwater mudstone within one of the sequences (from 339 to 347 mbsf) reveals ratios between hierarchical levels that are similar to those of the three Milankovitch orbital forcing periodicities. The strata contain a wide range of fossils, the most abundant being marine diatoms. These commonly form up to 5% of the sediment, though in places the core is barren (notably between 300 and 412 mbsf). Fifty samples out of 250 reviewed were studied in detail. The assemblages define ten biostratigraphical zones, some of them based on local or as yet undescribed forms. The assemblages are neritic, and largely planktonic, suggesting that the sea floor was mostly below the photic zone throughout deposition of the corcd sequence. Calcareous nannofossils, representing incursions of ocean surface waters, are much less common (72 out of 183 samples examined) and restricted to mudstone intervals a few tens of metres thick, but are important for dating. Foraminifera are also sparse (73 out of 135 samples) and represented only by calcareous benthic species. Changing assemblages indicate a shift from inshore environments in the early Oligocenc to outer shelf in the late Oligocenc, returning to inshore in the early Miocene. Marine palynomorplis yielded large numbers of well-preserved forms from most of the 116 samples examined. The new in situ assemblagc found last year in CRP-1 is extended down into the late Oligocene and a further new assemblage is found in the early Oligoccnc. Many taxa are new, and cannot us yet contribute to an improved understanding of chronology or ecology. Marine invertebrate macrofossils, mostly molluscs and serpulid tubes, are scattered throughout the core. Preservation is good in mudstones but poor in other lithologies. Climate on land is reflected in the content of terrestrial palynomorphs, which are extremely scarce down to c. 300 mbsf. Some forms are reworked, and others represent a low growing sparse tundra with at least one species of Nothofagus. Beneath this level, a significantly greater diversity and abundance suggests a milder climate and a low diversity woody vegetation in the early Oligocene, but still far short of the richness found in known Eocene strata of the region. Sedimentary facies in the oldest strata also suggest a milder climate in the oldest strata cored, with indications of substantial glacial melt-water discharges, but are typical of a coldcr climate in late Oligocene and early Miocene times. Clast analyses from diamictites reveal weak to random fabrics, suggesting either lack of ice-contact deposition or post-depositional modification, but periods when ice grounded at the drill site are inferred from thin zones of in-situ brecciated rock and soft-sediment folding. These are more common above c. 300 mbsf, perhaps reflecting more extensive glacial advances during deposition of those strata. Erosion of the adjacent Transantarctic Mountains through Jurassic basalt and dolerite-intruded Beacon strata into basement rocks beneath is recorded by petrographical studies of clast and sand grain assemblages. Core below 310 mbsf contains a dominance of fine-grained Jurassic dolerite and basalt fragments along with Beacon-derived coal debris and rounded quartz grains, whereas the strata above this level have a much higher proportion of basement derived granitoids, implying that the large areas of the adjacent mountains had been eroded to basement by the end of the early Oligocene. There is little indication of rift-related volcanism below 310 mbsf. Above this, however, basaltic and trachytic tephras are common, especially from 280 to 200 mbsf, from 150 to 46 mbsf, and in Pliocene LSU 2.2 from 21 to 27 mbsf. The largest volcanic eruptions generated layers of coarse (up to 1 cm) trachytic pumice lapilli between 97 and 114 mbsf. The thickest of these (1.2 m at 112 mbsf) may have produced an eruptive column extending tens of km into the stratosphere. A source within a few tens of km of the drill site is considered most likely. Present age estimates for the pre-Pliocene sequence are based mainly on biostratigraphy (using mainly marine diatoms and to a lesser extent calcareous nannofossils), with the age of the tephra from 112 to 114 mbsf (21.44k0.05 Ma from 84 crystals by Ar-Ar) as a key reference point. Although there are varied and well-preserved microfossil assemblages through most of the sequence (notably of diatoms and marine palynomorphs), they comprise largely taxa either known only locally or as yet undescribed. In addition, sequence stratigraphical analysis and features in the core itself indicate numerous disconformities. The present estimate from diatom assemblages is that the interval from 27 to 130 mbsf is early Miocene in age (c. 19 to 23.5 Ma), consistent with the Ar-Ar age from 112 to 114 mbsf. Diatom assemblages also indicate that the late Oligocene epoch extends from c. 130 to 307 mbsf, which is supported by late Oligocene nannofossils from 130 to 185 mbsf. Strata from 307 to 412 mbsf have no age-diagnostic assemblages, but below this early Oligocene diatoms and nannofossils have been recovered. A nannoflora at the bottom of the hole is consistent with an earliest Oligocene or latest Eocene age. Magnetostratigraphical studies based on about 1000 samples, 700 of which have so far undergone demagnetisation treatment, have provided a polarity stratigraphy of 12 pre-Pliocene magnetozones. Samples above 270 mbsf are of consistently high quality. Below this, magnetic behaviour is more variable. A preliminary age-depth plot using the Magnetic Polarity Time Scale (MPTS) and constrained by biostratigraphical data suggests that episodes of relatively rapid sedimentation took place at CRP-2 during Oligocene times (c. 100 m/My), but that more than half of the record was lost in a few major and many minor disconformities. Age estimates from Sr isotopes in shell debris and further tephra dating are expected to lead to a better comparison with the MPTS. CRP-2/2A has recorded a history of subsidence of the Victoria Land Basin margin that is similar to that found in CIROS-170 km to the south, reflecting stability in both basin and the adjacent mountains in late Cenozoic times, but with slow net accumulation in the middle Cenozoic. The climatic indicators from both drill holes show a similar correspondence, indicating polar conditions for the Quaternary but with sub-polar conditions in the early Miocene-late Oligocene and indications of warmer conditions still in the early Oligocene. Correlation between the CRP-2A core and seismic records shows that seismic units V3 and V4, both widespread in the Victoria Land Basin, represent a period of fluctuating ice margins and glacimarine sedimentation. The next drill hole, CRP-3, is expected to core deep into V5 and extend this record of climate and tectonics still further back in time.

Major ion concentrations and chlorine isotope composition of water-soluble chloride and structurally bound chloride in DSDP/ODP serpentinized ultramafic rocks

Eight DSDP/ODP cores were analyzed for major ion concentrations and d37Cl values of water-soluble chloride (d37Clwsc) and structurally bound chloride (d37Clsbc) in serpentinized ultramafic rocks. This diverse set of cores spans a wide range in age, temperature of serpentinization, tectonic setting, and geographic location of drilled serpentinized oceanic crust. Three of the cores were sampled at closely spaced intervals to investigate downhole variation in Cl concentration and chlorine isotope composition. The average total Cl content of all 86 samples is 0.26±0.16 wt.% (0.19±0.10 wt.% as water-soluble Cl (Xwsc) and 0.09±0.09 wt.% as structurally bound Cl (Xsbc)). Structurally bound Cl concentration nearly doubles with depth in all cores; there is no consistent trend in water-soluble Cl content among the cores. Chlorine isotope fractionation between the structurally bound Cl**- site and the water-soluble Cl**- site varies from -1.08? to +1.16?, averaging to +0.21?. Samples with negative fractionations may be related to reequilibration of the water-soluble chloride with seawater post-serpentinite formation. Six of the cores have positive bulk d37Cl values (+0.05? to +0.36?); the other two cores (173-1068A (Leg-Hole) and 84-570) have negative bulk d37Cl values (-1.26? and -0.54?). The cores with negative d37Cl values also have variable Cl**-/SO4**2- ratios, in contrast to all other cores. The isotopically positive cores (153-920D and 147-895E) show no isotopic variation with depth; the isotopically negative core (173-1068A) decreases by ~1? with depth for both the water-soluble and structurally bound Cl fractions. Non-zero bulk d37Cl values indicate Cl in serpentinites was incorporated during original hydration and is not an artifact of seawater infiltration during drilling. Cores with positive d37Cl values are most likely explained by open system fractionation during hydrothermal alteration, with preferential incorporation of 37Cl from seawater into the serpentinite and loss of residual light Cl back to the ocean. Fluid / rock ratios were probably low as evidenced by the presence of water-soluble salts. The two isotopically negative cores are characterized by a thick overlying sedimentary package that was in place prior to serpentinization. We believe the low d37Cl values of these cores are a result of hydration of ultramafic rock by infiltrating aqueous pore fluids from the overlying sediments. The resulting serpentinites inherit the characteristic negative d37Cl values of the pore waters. Chlorine stable isotopes can be used to identify the source of the serpentinizing fluid and ultimately discern chemical and tectonic processes involved in serpentinization.

Element concentrations and isotopic composition of tholeiite and whole rocks samples from the east Mariana Basin

Studies of seafloor magnetic anomaly patterns suggest the presence of Jurassic oceanic crust in a large area in the western Pacific that includes the East Mariana, Nauru and Pigafetta Basins. Sampling of the igneous crust in this area by the Deep Sea Drilling Program (DSDP) and the Ocean Drilling Program (ODP) allows direct evaluation of the age and petrogenesis of this crust. ODP Leg 129 drilled a 51 m sequence of basalt pillows and massive flows in the central East Mariana Basin. 40Ar/39Ar ages determined in this study for two Leg 129 basalts average 114.6 +/- 3.2 Ma. This age is in agreement with the Albian-late Aptian paleontologic age of the overlying sediments, but is distinctively younger than the Jurassic age predicted by magnetic anomaly patterns in the basin. Compositionally, the East Mariana Basin basalts are uniformly low-K tholeiites that are depleted in highly incompatible elements compared to moderately incompatible ones, which is typical of mid-ocean ridge basalts (MORB) erupted near hotspots. The Sr, Nd and Pb isotopic compositions of the tholeiites (87Sr/86Sr init = 0.70360-0.70374; 143Nd/144Nd init = 0.512769-0.512790; 206Pb/204Pb meas = 18.355-18.386) also overlap with some Indian Ocean Ridge MORB, although they are distinct from the isotopic compositions of Jurassic basalts drilled in the Pigafetta Basin, the oldest Pacific MORB. The isotopic compositions of the East Mariana Basin tholeiites are also similar to those of intraplate basalts, and in particular, to the isotopic signature of basalts from the nearby Ontong Java and Manihiki Plateaus. The East Mariana Basin tholeiites also share many petrologic and isotopic characteristics with the oceanic basement drilled in the Nauru Basin at DSDP Site 462. In addition, the new 110.8 +/- 1.0 Ma 40Ar/39Ar age for two flows from the bottom of Site 462 in the Nauru Basin is indistinguishable from the age of the East Mariana Basin flows. Thus, while magnetic anomaly patterns predict that the igneous basement in the Nauru and East Mariana Basins is Jurassic in age, the geochemical and chronological results discussed here suggest that the basement formed during a Cretaceous rifting event within the Jurassic crust. This magmatic and tectonic event was created by the widespread volcanism responsible for the genesis of the large oceanic plateaus of the western Pacific.

Minerals and geochemistry at DSDP Hole 62-466

Alkali-basalt clasts in Upper Cretaceous sediments from Site 466 on southern Hess Rise suggest that parts of Hess Rise were constructed by off-ridge volcanic activity. Apparently, tectonic adjustments at Hess Rise occurred during the Late Cretaceous (Campanian-Maastrichtian), when parts of the original volcanic pedestal were uplifted and provided source rocks for the clasts. Synchronous volcanism may have occurred. Causes for the Late Cretaceous tectonic adjustments (and volcanism?) are not known, but they may be related to intraplate movement along the Mendocino Fracture Zone.

Mineralogy, geochemistry and selected chemical formulas of sediments from ODP Holes 161-986B and 161-977A

Upper Miocene to Pleistocene hemipelagites and resedimented facies recovered at Holes 976B and 977A (Leg 161) in the Alboran Basin consist mainly of biogenic and detrital components, with a minor contribution of neoformed mineral phases. Diagenetic processes have not obliterated the primary deposition signal, and therefore detrital components (quartz, feldspar, detrital dolomite, rock fragments, and clays) provide information about source rocks and provenances. No major bulk or clay mineralogy differences were recognized between resedimented and hemipelagic facies; in fact, similar mineral assemblages in both types of facies suggest common source rocks. However, mineral abundance fluctuations can be related to climate variations and tectonic factors, as the main controls of sediment fill of this basin. A marked increase in smectites in Messinian sediments suggests an extensive development of soils during that time, probably favored by the alternation of wet and dry climate episodes and the relative aridification of the Mediterranean borderlands. A notable increase in detrital components suggests a sea-level fall and/or tectonic uplift during the late Pliocene. The significant increase in detrital dolomite in the uppermost Pliocene deposits suggests the uplift of dolomite-rich rocks as source areas. Mineral components in Pleistocene sediments indicate increasing tectonic stability, and clay-mineral fluctuations during the Pleistocene can be related not only to tectonic events, but also to alternating cooling and warming periods.

Early - Late Miocene palaeoclimate data and palaefloras of 48 samples from Southern China

Southern China, especially Yunnan, has undergone high tectonic activity caused by the uplift of Himalayan Mountains during the Neogene, which led to a fast changing palaeogeography. Previous study shows that Southern China has been influenced by the Asian Monsoon since at least the Early Miocene. However, it is yet not well understood how intense the Miocene monsoon system was. In the present study, 63 fossil floras of 16 localities from Southern China are compiled and evaluated for obtaining available information concerning floristic composition, stratigraphic age, sedimentology, etc. Based on such reliable information, selected mega- and micro-floras have been analysed with the coexistence approach to obtain quantitative palaeoclimate data. Visualization of climate results in maps shows a distinct spatial differentiation in Southern China during the Miocene. Higher seasonalities of temperature and precipitation occur in the north and south parts of Southern China, respectively. During the Miocene, most regions of Southern China and Europe were both warm and humid. Central Eurasia was likely to be an arid center, which gradually spread westward and eastward. Our data provide information about Miocene climate patterns in Southern China and about the evolution of these patterns throughout the Miocene, and is also crucial to unravel and understand the climatic signals of global cooling and tectonic uplift.

Distribution of Cretaceous redeposited benthic foraminifers at DSDP Site 89-585

Cretaceous benthic foraminifers from Site 585 in the East Mariana Basin, western Pacific Ocean, provide an environmental and tectonic history of the Basin and the surrounding seamounts. Age diagnostic species (from a fauna of 155 benthic species identified) range from late Aptian to Maestrichtian in age. Displaced species in sediments derived from the tops and flanks of nearby seamounts were deposited sporadically on the Basin floor well below the carbonate compensation depth (CCD) at abyssal depths of 5000 to 6000 m. These depths, characterized by an indigenous assemblage of benthic foraminifers, recrystallized radiolarians, fish debris, and sponge spicules, existed in the Mariana Basin from late Aptian to the present. Early Albian and older edifice-building volcanism had reached the photic zone with associated shallow-water bank or reef environments. By middle Albian, the dominant source areas subsided to outer-neritic to upper-bathyal depths. Major volcanic activity ceased and fine-grained sediments were deposited by distal turbidites, although intermittent volcanism and the influx of rare neritic material continued until the late Albian. By the Cenomanian to Turonian, upper- to middle-bathyal depths were reached by the dominant source areas, and the sediments recovered from this interval include organic carbon-rich layers. Rare benthic foraminifers from the Coniacian-Santonian interval indicate a continuation of dominantly middle-bathyal source areas. A change in sedimentation during the Campanian-Maestrichtian from older zeolitic claystone to abundant chert in the Campanian, and nannofossil chalk and claystone in the Maestrichtian resulted from migration of the site beneath the equatorial productive zone due to northwestward plate motion. The appearance of rare middle-neritic and upper-bathyal species in the Maestrichtian interval associated with volcanogenic debris gives evidence of the remobilization and downslope transport of pelagic deposits due to thermally induced uplift. Episodic redeposition of shallow-water material during the Aptian-Albian was produced by edifice-building volcanism perhaps combined with eustatic lowering of sea level. The Cenomanian-Turonian pulse coincided with a low global sea-level stand as does the transported material during the Coniacian-Santonian. The Maestrichtian pulse was caused by renewed midplate volcanism that extended over a large area of the central Pacific.

Miocene microflora and palaeoclimatic estimates of the Stetten section (Austria)

Pollen analyses have been proven to possess the possibility to decipher rapid vegetational and climate shifts in Neogene sedimentary records. Herein, a c. 21-kyr-long transgression-regression cycle from the Lower Austrian locality Stetten is analysed in detail to evaluate climatic benchmarks for the early phase of the Middle Miocene Climate Optimum and to estimate the pace of environmental change. Based on the Coexistence Approach, a very clear signal of seasonality can be reconstructed. A warm and wet summer season with c. 204-236 mm precipitation during the wettest month was opposed by a rather dry winter season with precipitation of c. 9-24 mm during the driest month. The mean annual temperature ranged between 15.7 and 20.8 °C, with about 9.6-13.3 °C during the cold season and 24.7-27.9 °C during the warmest month. In contrast, today's climate of this area, with an annual temperature of 9.8 °C and 660 mm rainfall, is characterized by the winter season (mean temperature: -1.4 °C, mean precipitation: 39 mm) and a summer mean temperature of 19.9 °C (mean precipitation: 84 mm). Different modes of environmental shifts shaped the composition of the vegetation. Within few millennia, marshes and salt marshes with abundant Cyperaceae rapidly graded into Taxodiaceae swamps. This quick but gradual process was interrupted by swift marine ingressions which took place on a decadal to centennial scale. The transgression is accompanied by blooms of dinoflagellates and of the green alga Prasinophyta and an increase in Abies and Picea. Afterwards, the retreat of the sea and the progradation of estuarine and wetland settings were a gradual progress again. Despite a clear sedimentological cyclicity, which is related to the 21-kyr precessional forcing, the climate data show little variation. This missing pattern might be due to the buffering of the precessional-related climate signal by the subtropical vegetation. Another explanation could be the method-inherent broad range of climate-parameter estimates that could cover small scale climatic changes.

Grain size distribution and clay mineralogy of upper Miocene sediments of ODP Hole 107-652A

The upper Miocene sedimentary sequence of Site 652, located on the lower continental margin of eastern Sardinia, was cored and logged during Ocean Drilling Program (ODP) Leg 107. Geophysical and geochemical logs from the interval 170-365 m below seafloor (mbsf), as well as various core measurements (CaCO3, grain size, X-ray diffraction), provide a mineralogical-geochemical picture that is interpreted in the framework of the climatic and tectonic evolution of the western Tyrrhenian. The results indicate the presence of short- and long-term mineralogical variations. Short-term variations are represented by calcium-carbonate fluctuations in which the amount of CaCO3 is correlated to the grain size of the sediments; coarser sediments are associated with high carbonate content and abundant detrital material. Long-term variation corresponds to a gross grain-size change in the upper part of the sequence, where predominantly fine-grained sediments may indicate a gradual deepening of the lacustrine basin towards the Pliocene. Regional climatic changes and rift-related tectonism are possible causes of this variability in the sedimentation patterns. The clay association is characterized by chlorite, illite, and smectite as dominant minerals, as well as mixed-layers clays, kaolinite, and palygorskite. Chlorite, mixed-layers clays, and illite increase at the expense of smectite below the pebble zone (335 mbsf). This is indicative of diagenetic processes related to the high geothermal gradient and to the chemistry of the evaporative pore waters, rather than to changes in the depositional environment.

Age estimates of Discoaster datums and corresponding isotope stages (Table 2)

High-resolution records (2 7 kyr) of Upper Pliocene Discoaster abundances obtained from six ODP/DSDP sites are assessed independently using oxygen isotope stratigraphy. Four Atlantic Ocean sites (DSDP Sites 552 and 607, and ODP Sites 659 and 662) comprise a transect from 56°N to 1°S and provide a record of latitudinal variations in Diseoaster biogeography. Low-latitude sites in the Atlantic (ODP Site 662), Pacific (ODP Site 677), and Indian (ODP Site 709) oceans provide additional information about variability in Discoaster abundance patterns within the equatorial region. A common chronology, based on the astronomical time scale developed for ODP Site 677, has been established for all the sites. By integrating oxygen isotope data and Discoaster abundance records at each site we are able to independently evaluate the temporal and spatial distribution of D. brouweri and D. triradiatus in the 500 kyr prior to the extinction of the discoasters near the base of the Olduvai subchron. Major decreases in abundance are evident during some of the more intense late Pliocene glacial events. In particular, glacial isotope stages 82, 96, 98 and 100 are associated with distinct abundance minima. At these times, large-scale changes in surface hydrographic conditions appear to have suppressed Discoaster numbers on a global scale. The increase in abundance of D. triradiatus, which precedes the extinction of the discoasters by around 200 kyr, may also be related to the intensification of environmental pressures that accompanied the build-up of Northern Hemisphere ice sheets during the late Pliocene. In spite of contrasting geographic and oceanographic settings, the various D. brouweri and D. triradiatus records are remarkably similar. This demonstrates that the acme and extinction events are excellent biostratigraphic datums. The simultaneous extinction of D. brouweri and D. triradiatus at 1.95 Ma were synchronous events at both a regional scale within the Atlantic, and on a global scale between the three major oceans. However, the start of the D. triradiatus acme appears to have been diachronous, occurring some 40 kyr earlier in the Atlantic than in the Indo-Pacific, and hence the stratigraphic usefulness of this datum is regional rather than global.

Stable chlorine isotopes in pore waters from DSDP Hole 79-546 and ODP Hole 131-808C

Stable Cl isotope ratios, measured in marine pore waters associated with the Barbados and Nankai subduction zones, extend significantly (to ~-8 per mil) the range of d37Cl values reported for natural waters. These relatively large negative values, together with geologic and chemical evidence from Barbados and Nankai and recent laboratory data showing that hydrous silicate minerals (i.e., those with structural OH sites) are enriched up to 7.5 per mil in 37Cl relative to seawater, strongly suggest that the isotopic composition of Cl in pore waters from subduction zones reflects diagenetic and metamorphic dehydration and transformation reactions. These reactions involve clays and/or other hydrous silicate phases at depth in the fluid source regions. Chlorine therefore cannot be considered geochemically conservative in these systems. The uptake of Cl by hydrous phases provides a mechanism by which Cl can be cycled into the mantle through subduction zones. Thus, stable Cl isotopes should help in determining the extent to which Cl and companion excess volatiles like H2O and CO2 cycle between the crust and mantle.

Paleomagnetic and rock magnetic properties of IODP Expedition 318 cores

The Integrated Ocean Drilling Program Expedition 318 to the Wilkes Land margin of Antarctica recovered a sedimentary succession ranging in age from lower Eocene to the Holocene. Excellent stratigraphic control is key to understanding the timing of paleoceanographic events through critical climate intervals. Drill sites recovered the lower and middle Eocene, nearly the entire Oligocene, the Miocene from about 17 Ma, the entire Pliocene and much of the Pleistocene. The paleomagnetic properties are generally suitable for magnetostratigraphic interpretation, with well-behaved demagnetization diagrams, uniform distribution of declinations, and a clear separation into two inclination modes. Although the sequences were discontinuously recovered with many gaps due to coring, and there are hiatuses from sedimentary and tectonic processes, the magnetostratigraphic patterns are in general readily interpretable. Our interpretations are integrated with the diatom, radiolarian, calcareous nannofossils and dinoflagellate cyst (dinocyst) biostratigraphy. The magnetostratigraphy significantly improves the resolution of the chronostratigraphy, particularly in intervals with poor biostratigraphic control. However, Southern Ocean records with reliable magnetostratigraphies are notably scarce, and the data reported here provide an opportunity for improved calibration of the biostratigraphic records. In particular, we provide a rare magnetostratigraphic calibration for dinocyst biostratigraphy in the Paleogene and a substantially improved diatom calibration for the Pliocene. This paper presents the stratigraphic framework for future paleoceanographic proxy records which are being developed for the Wilkes Land margin cores. It further provides tight constraints on the duration of regional hiatuses inferred from seismic surveys of the region.

Isotopic composition and major and trace element concentrations of mafic rocks obtained from western Dronning Maud Land

On the basis of new bulk major and trace element (including REE) as well as Sm-Nd and Rb-Sr isotope data, used in conjunction with available geochronological data, a post-tectonic mafic igneous province and four groups of pre- to syntectonic amphibolite are distinguished in the polymetamorphic Maud Belt of western Dronning Maud Land, East Antarctica. Protoliths of the Group 1 amphibolites are interpreted as volcanic arc mafic intrusions with Archaean to Palaeoproterozoic Nd model ages and depletion in Nb and Ta. Isotopic and lithogeochemical characteristics of this earliest group of amphibolite indicate that the Maud Belt was once an active continental volcanic arc. The most likely position of this arc, for which a late Mesoproterozoic age (c. 1140 Ma) is indicated by available U-Pb single-zircon age data, was on the southeastern margin of the Kaapvaal-Grunehogna Craton. The protoliths of Group 2 amphibolites are attributed to the 1110 Ma Borgmassivet-Umkondo thermal event on the basis of comparable Nd model ages and trace element distributions. Group 3 amphibolite protoliths are characterized by mid-ocean ridge basalt-type REE patterns and low Th/Yb ratios, and they are related to Neoproterozoic extension. Group 4 amphibolite protoliths are distinguished by high Dy/Yb ratios and are attributed to a phase of syntectonic Pan-African magmatism as indicated by Rb-Sr isotope data.