Calcareous nannofossil biostratigraphy of the central East Pacific Rise

During Leg 92 of the Deep Sea Drilling Project, sediments containing calcareous nannofossils of latest Oligocene to Holocene age were recovered from 14 holes at six sites (597 to 602) along the East Pacific Rise. The combined sections yield a virtually complete record for the region, with a compressed upper Miocene to Pleistocene interval. The nannofossil content of 14 U.S.N.S. Eltanin piston cores from the study area were also examined in order to supplement data generated during Leg 92. Two taxonomically new combinations are presented: Sphenolithus umbellus and Pontosphaera segmenta. Assemblages of calcareous nannofossils juxtaposed in reversed stratigraphic order within the upper Miocene provide strong evidence for downslope transport of sediments along the East Pacific Rise during the Messinian. Narrow bands of dark metalliferous sediment of coccolith Zone CN8b alternate with normal light-colored, in situ, pelagic sequences of Zone CN9b. This may indicate more vigorous bottom current activity between 5.40 and 6.70 Ma.

Representative analyses of pre-tectonic metabasic rocks and metapelitic gneiss from Sverdrupfjella, East Antarctica

Extensive high-grade polydeformed metamorphic provinces surrounding Archaean cratonic nuclei in the East Antarctic Shield record two tectono-thermal episodes in late Mesoproterozoic and late Neoproterozoic-Cambrian times. In Western Dronning Maud Land, the high-grade Mesoproterozoic Maud Belt is juxtaposed against the Archaean Grunehogna Province and has traditionally been interpreted as a Grenvillian mobile belt that was thermally overprinted during the Early Palaeozoic. Integration of new U-Pb sensitive high-resolution ion microprobe and conventional single zircon and monazite age data, and Ar-Ar data on hornblende and biotite, with thermobarometric calculations on rocks from the H.U. Sverdrupfjella, northern Maud Belt, resulted in a more complex P-T-t evolution than previously assumed. A c. 540?Ma monazite, hosted by an upper ampibolite-facies mineral assemblage defining a regionally dominant top-to-NW shear fabric, provides strong evidence for the penetrative deformation in the area being of Pan-African age and not of Grenvillian age as previously reported. Relics of an eclogite-facies garnet-omphacite assemblage within strain-protected mafic boudins indicate that the peak metamorphic conditions recorded by most rocks in the area (T = 687-758°C, P = 9·4-11·3?kbar) were attained subsequent to decompression from P > 12·9?kbar. By analogy with limited U-Pb single zircon age data and on circumstantial textural grounds, this earlier eclogite-facies metamorphism is ascribed to subduction and accretion around 565?Ma. Post-peak metamorphic K-metasomatism under amphibolite-facies conditions is ascribed to the intrusion of post-orogenic granite at c. 480?Ma. The recognition of extensive Pan-African tectonism in the Maud Belt casts doubts on previous Rodinia reconstructions, in which this belt takes a pivotal position between East Antarctica, the Kalahari Craton and Laurentia. Evidence of late Mesoproterozoic high-grade metamorphism during the formation of the Maud Belt exists in the form of c. 1035?Ma zircon overgrowths that are probably related to relics of granulite-facies metamorphism recorded from other parts of the Maud Belt. The polymetamorphic rocks are largely derived from a c. 1140?Ma volcanic arc and 1072 ± 10?Ma granite.

Mineral chemistry of basalts at DSDP Leg 55 Holes

Microprobe mineral compositions of olivine, plagioclase, clinopyroxene, chrome spinel, ilmenite, and titanomagnetite are presented for 7 samples from 4 flows of hawaiite and one flow of tholeiitic basalt from Hole 430A at Ojin Seamount, 4 samples from 3 flows of alkalic basalt from Hole 432A at Nintoku Seamount, and 29 samples from 2 flows of alkalic basalt and 24 flows of tholeiitic basalt from Holes 433A, 433B, and 433C at Suiko Seamount. The four hawaiite flows from Hole 430A on Ojin Seamount have nearly identical mineralogy. The plagioclase phenocrysts and calculated equilibrium olivine appear to have crystallized at about 1175°C; the groundmass plagioclase crystallized from about 1135° to 1010°C; and the Fe-Ti oxides equilibrated at temperatures from 1000°C to 720°C under oxygen fugacities of 10**-11 to 10**-17. The single tholeiitic flow contains glomerocrysts of plagioclase (An80 to An65) and clinopyroxene (Wo43En46Fsn to Wo42En45Fs13). The plagioclase phenocrysts give calculated temperatures as high as 1400°C, indicating that they were not equilibrated with a magma having the bulk rock composition. The plagioclase groundmass crystallized at 1120° to 1070°C, and the Fe-Ti oxides equilibrated at 1070° to 930°C under oxygen fugacities of 10**-10 to 10**-12. Using mineral compositions of Hawaiian basalts as a guide, we infer that the hawaiite flows were erupted during the post-caldera alkalic eruptive stage and the tholeiite was erupted during the shield-building or caldera collapse stage. The three alkalic basalt flows from Hole 432A on Nintoku Seamount have similar mineralogy, although Flow Units 1 and 2 contain much more abundant plagioclase phenocrysts. The groundmass plagioclase crystallized at temperatures between 1175° and 1000°C. The olivine and plagioclase phenocrysts do not appear to be in equilibrium with the enclosing magmas. The mineral compositions suggest that these samples are intermediate between alkalic basalt and hawaiite; they probably erupted during the post-caldera alkalic stage of eruption. The two analyzed alkalic basalt flows are the two youngest flows recovered at Holes 433A, 433B, and 433C. Flow Unit 1 contains abundant sector-zoned clinopyroxene, and Flow Unit 2 contains rare kink-banded olivine xenocrysts. The plagioclase phenocrysts yield calculated temperatures of 1440° to 1250°C, indicating that they are probably not cognate. Calculated-equilibrium olivine indicates crystallization of olivine at about 1170°C. The Fe-Ti oxides equilibrated at temperatures of 1140° to 870°C under oxygen fugacities of 10**-9 to 10**-14. The groundmass plagioclase crystallized at temperatures of 1178° to 1035 °C. The mineral compositions indicate that these alkalic basalts erupted during the post-caldera alkalic eruptive stage. The 24 analyzed tholeiitic basalts are subdivided on the basis of phenocryst abundances into olivine tholeiites, plagioclase tholeiites, and tholeiites. The crystallization sequence appears to have been chrome spinel, olivine, plagioclase, and clinopyroxene as phenocryst phases, followed by and overlapping with groundmass crystallization of plagioclase (1180° to 920°C), clinopyroxene, and Fe-Ti oxides (1140° to 670°C). At least three flows contain pigeonite. The mineral compositions indicate that all the samples from Flow Unit 4 downward are tholeiitic basalts, although Flow Unit 64 has mineral compositions transitional to those in alkalic basalts.

Strontium and Neodymium isotope composition of sediments from the North Atlantic

Sr and Nd isotopic compositions have been measured on the lithic fraction of last climatic cycle sediments from the North Atlantic (~40°N/~60°N), in order to identify the origins of the particles. From the reconstruction of their transport pathways, we deduce the mechanisms that explain their distributions. The main source regions are the Canadian shield (mostly the area of Baffin Bay and western Greenland), the Scandinavian shield, the European region (British Isles and Bay of Biscay), and Iceland. We observe a significant glacial/interglacial contrast, characterized by a dominant Icelandic input via near-bottom transport by North Atlantic Deep Water (NADW) during the interglacials and a largely continent-derived contribution of surface-transported, ice-rafted detritus (IRD) during the glacial period. During the last glacial period, the Heinrich events (abrupt, massive discharges of IRD) originated not only from the Laurentide ice sheet as heretofore envisioned but also from other sources. Three other major North Atlantic ice sheets (Fennoscandian, British Isles, and Icelandic) probably surged simultaneously, discharging ice and IRD into the North Atlantic. As opposed to theories implying a unique, Laurentide origin [Gwiazda et al., 1996 doi:10.1029/95PA03135] driven by an internal mechanism [MacAyeal, 1993 doi:10.1029/93PA02200], we confirm that the Icelandic and the Fennoscandian ice sheets also surged as recently proposed by other authors, and we here also distinguish a possible detrital contribution from the British Isles ice sheet. This pan-North Atlantic phenomenon thus requires a common regional, external forcing.

Lead isotopes of feldspar in North Atlantic Ocean sediments

The provenance of ice-rafted debris (IRD) deposited in the North Atlantic before, during, and after Heinrich event 2 has been determined through measuring the lead isotopic composition of single feldspar grains and multiple-grain composites from the larger than 150-µm size fraction, from cores from the eastern and western North Atlantic and from the Labrador Sea. Single-grain analyses are used to identify the specific continental sources of the IRD, whereas composite samples are used to assess the relative IRD contributions from different sources. All single grains from Heinrich layer 2 (H 2) as well as H 2 composites plot along a correlation line on a 207Pb/204Pb versus 206Pb/204Pb diagram characteristic of the Churchill province of the Canadian shield. This is yet another strong piece of evidence that this Heinrich event was dominated by a massive iceberg discharge of the Laurentide ice sheet lobe located over Hudson Bay. In contrast, single grains from the ambient glacial sediment (above and below H 2) have multiple sources: many of them also lie along the correlation line with H 2 grains, but many others have Pb signatures consistent with derivation from the Grenville province and the Appalachian range in North America and possibly from Scandinavia and Greenland. Composites from the ambient sediment generally lie well to the right of the H 2 reference line in agreement with the results of the single-grain analyses. The evidence provided by lead isotopes regarding the dominant role played by the Hudson Bay lobe of the Laurentide ice sheet in the development of the Heinrich events lends support to the binge/purge model advanced by MacAyeal [1993a, b] that invokes trapping of geothermal heat by the base of the icecap and subsequent basal melting as the mechanism that triggered the Heinrich events.

Geochemistry and radiogenic isotopes of ODP Hole 157-953C lavas

The Canary Island primitive basaltic magmas are thought to be derived from an HIMU-type upwelling mantle containing isotopically depleted (NMORB)-type component having interacted with an enriched (EM)-type component, the origin of which is still a subject of debate. We studied the relationships between Ni, Mn and Ca concentrations in olivine phenocrysts (85.6-90.0 mol.% Fo, 1,722-3,915 ppm Ni, 1,085-1,552 ppm Mn, 1,222-3,002 ppm Ca) from the most primitive subaerial and ODP Leg 157 high-silica (picritic to olivine basaltic) lavas with their bulk rock Sr-Nd-Pb isotope compositions (87Sr/86Sr = 0.70315-0.70331, 143Nd/144Nd = 0.51288-0.51292, 206Pb/204Pb = 19.55-19.93, 207Pb/204Pb = 15.60-15.63, 208Pb/204Pb = 39.31-39.69). Our data point toward the presence of both a peridotitic and a pyroxenitic component in the magma source. Using the model (Sobolev et al., 2007, Science Vol 316) in which the reaction of Si-rich melts originated during partial melting of eclogite (a high pressure product of subducted oceanic crust) with ambient peridotitic mantle forms olivine-free reaction pyroxenite, we obtain an end member composition for peridotite with 87Sr/86Sr = 0.70337, 143Nd/144Nd = 0.51291, 206Pb/204Pb = 19.36, 207Pb/204Pb = 15.61 and 208Pb/204Pb = 39.07 (EM-type end member), and pyroxenite with 87Sr/86Sr = 0.70309, 143Nd/144Nd = 0.51289, 206Pb/204Pb = 20.03, 207Pb/204Pb = 15.62 and 208Pb/204Pb = 39.84 (HIMU-type end member). Mixing of melts from these end members in proportions ranging from 70% peridotite and 30% pyroxenite to 28% peridotite and 72% pyroxenite derived melt fractions can generate the compositions of the most primitive Gran Canaria shield stage lavas. Combining our results with those from the low-silica rocks from the western Canary Islands (Gurenko et al., 2009, doi:10.1016/j.epsl.2008.11.013), at least four distinct components are required. We propose that they are (1) HIMU-type pyroxenitic component (representing recycled ocean crust of intermediate age) from the plume center, (2) HIMU-type peridotitic component (ancient recycled ocean crust stirred into the ambient mantle) from the plume margin, (3) depleted, MORB-type pyroxenitic component (young recycled oceanic crust) in the upper mantle entrained by the plume, and (4) EM-type peridotitic component from the asthenosphere or lithosphere above the plume center.

Chemical composition of glass inclusions from ODP Holes 157-953C and 157-956B

We report S concentrations and relative proportions of (SO4)2- and S2- in OL- and CPX-hosted glass inclusions and in host glassy lapilli from Miocene basaltic hyaloclastites drilled north and south of Gran Canaria during ODP Leg 157. Compositions of glass inclusions and lapilli resemble those of subaerial Miocene shield basalts on Gran Canaria and comprise mafic to more evolved tholeiitic to alkali basalt and basanite (10.3-3.7 wt.% MgO, 44.5-56.9 wt.% SiO2). Glass inclusions fall into three groups based on their S concentrations: a high-sulfur group (1050 to 5810 ppm S), an intermediate-sulfur group (510 to 1740 ppm S), and a low-sulfur group (<500 ppm S). The most S-rich inclusions have the highest and nearly constant proportion of sulfur dissolved as sulfate determined by electron microprobe measurements of SKa peak shift. Their average S6+/S_total value is 0.75+/-0.09, unusually high for ocean island basalt magmas. The low-sulfur group inclusions have low S6+/S_total ratios (0.08+/-0.05), whereas intermediate sulfur group inclusions show a wide range of S6+/S_total (0.05-0.83). Glassy lapilli and their crystal-hosted glass inclusions with S concentrations of 50 to 1140 ppm S have very similar S6+/S_total ratios of 0.36+/-0.06 implying that sulfur degassing does not affect the proportion of (SO4)2- and S2- in the magma. The oxygen fugacities estimated from S6+/S_total ratios and from Fe3+/Fe2+ ratios in spinel inclusions range from NNO-1.1 to NNO+1.8. The origin of S-rich magmas is unclear. We discuss (1) partial melting of a mantle source at relatively oxidized fO2 conditions, and (2) magma contamination by seawater either directly or through magma interaction with seawater-altered Jurassic oceanic crust. The intermediate sulfur group inclusions represent undegassed or slightly degassed magmas similar to submarine OIB glasses, whereas the low-sulfur group inclusions are likely to have formed from magmas significantly degassed in near-surface reservoirs. Mixing of these degassed magmas with stored volatile-rich ones or volatile-rich magma replenishing the chamber filled by partially degassed magmas may produce hybrid melts with strongly varying S concentrations and S6+/S_total ratios.

Age determination of Paleogene sediments from the Prysz Bay

Age dating of Paleogene diamictites from ODP Site 739 in Prydz Bay with marine microfossils (diatoms and calcareous nannofossils) suggests the build-up of a major East Antarctic ice shield in latest Eocene to earliest Oligocene time, about 35-38 m.y. ago. Strontium isotopic analyses of small mollusk remains found within these diamictites, however, yield younger ages ranging from 29 to 23 Ma (i.e., latest early Oligocene to earliest Miocene). These age discrepancies could be caused by repeated glacial reworking of microfossils, macrofossils, and sediment clasts through the late Oligocene or, alternatively, by ion exchange in the still aragonitic mollusk shells.

Pollen and non-pollen palynomorph records of the Holocene part of the composite sediment core TMD from lake Tso Moriri (NW India) and of modern surface samples from the Tso Moriri region

The high-altitude lake Tso Moriri (32°55'46'' N, 78°19'24'' E; 4522 m a.s.l.) is situated at the margin of the ISM and westerly influences in the Trans-Himalayan region of Ladakh. Human settlements are rare and domestic and wild animals are concentrating at the alpine meadows. A set of modern surface samples and fossil pollen from deep-water TMD core was evaluated with a focus on indicator types revealing human impact, grazing activities and lake system development during the last ca. 12 cal ka BP. Furthermore, the non-pollen palynomorph (NPP) record, comprising remains of limnic algae and invertebrates as well as fungal spores and charred plant tissue fragments, were examined in order to attest palaeolimnic phases and human impact, respectively. Changes in the early and middle Holocene limnic environment are mainly influenced by regional climatic conditions and glacier-fed meltwater flow in the catchment area. The NPP record indicates low lake productivity with high influx of freshwater between ca. 11.5 and 4.5 cal ka BP which is in agreement with the regional monsoon dynamics and published climate reconstructions. Geomorphologic observations suggest that during this period of enhanced precipitation the lake had a regular outflow and contributed large amounts of water to the Sutlej River, the lower reaches of which were integral part of the Indus Civilization area. The inferred minimum fresh water input and maximum lake productivity between ca. 4.5-1.8 cal ka BP coincides with the reconstruction of greatest aridity and glaciation in the Korzong valley resulting in significantly reduced or even ceased outflow. We suggest that lowered lake levels and river discharge on a larger regional scale may have caused irrigation problems and harvest losses in the Indus valley and lowlands occupied by sedentary agricultural communities. This scenario, in turn, supports the theory that, Mature Harappan urbanism (ca. 4.5-3.9 cal ka BP) emerged in order to facilitate storage, protection, administration, and redistribution of crop yields and secondly, the eventual collapse of the Harappan Culture (ca. 3.5-3 cal ka BP) was promoted by prolonged aridity. There is no clear evidence for human impact around Tso Moriri prior to ca. 3.7 cal ka BP, with a more distinct record since ca. 2.7 cal ka BP. This suggests that the sedimentary record from Tso Moriri primarily archives the regional climate history.

Geochemistry of metasedimentary units of the Cambro-Ordovician Ross Orogen in North Victoria Land and Oates Land, Antarctica

Metasediments in the three early Palaeozoic Ross orogenic terranes in northern Victoria Land and Oates Land (Antarctica) are geochemically classified as immature litharenites to wackes and moderately mature shales. Highly mature lithotypes with Chemical Index of Weathering values of >=95 are typically absent. Geochemical and Rb-Sr and Sm-Nd isotope results indicate that the turbiditic metasediments of the Cambro-Ordovician Robertson Bay Group in the eastern Robertson Bay Terrane represent a very homogeneous series lacking significant compositional variations. Major variations are only found in chemical parameters which reflect differences in degree of chemical weathering of their protoliths and in mechanical sorting of the detritus. Geochemical data, 87Sr/ 86Sr t=490 Ma ratios of 0.7120 - 0.7174, epsilonNd, t=490 Ma values of -7.6 to -10.3 and single-stage Nd-model ages of 1.7 - 1.9 Ga are indicative of an origin from a chemically evolved crustal source of on average late Palaeoproterozoic formation age. There is no evidence for significant sedimentary infill from primitive "ophiolitic" sources. Metasediments of the Middle Cambrian Molar Formation (Bowers Terrane) are compositionally strongly heterogeneous. Their major and trace element data and Sm-Nd isotope data (epsilonNd, t=500 Ma values of -14.3 to -1.2 and single-stage Nd-model ages of 1.7 - 2.1 Ga) can be explained by mixing of sedimentary input from an evolved crustal source of at least early Palaeoproterozoic formation age and from a primitive basaltic source. The chemical heterogeneity of metasediments from the Wilson Terrane is largely inherited from compositional variations of their precursor rocks as indicated by the Ni vs TiO2 diagram. Single-stage Nd-model ages of 1.6 -2.2 Ga for samples from more western inboard areas of the Wilson Terrane (epsilonNd, t=510 Ma -7.0 to -14.3) indicate a relatively high proportion of material derived from a crustal source with on average early Palaeoproterozoic formation age. Metasedimentary series in an eastern, more outboard position (epsilonNd, t=510 Ma -5.4 to -10.0; single-stage Nd model ages 1.4 - 1.9) on the contrary document stronger influence of a more primitive source with younger formation ages. The chemical and isotopic characteristics of metasediments from the Bowers and Wilson terranes can be explained by variable contributions from two contrasting sources: a cratonic continental crust similar to the Antarctic Shield exposed in Georg V Land and Terre Adélie some hundred kilometers west of the study area and a primitive basaltic source probably represented by the Cambrian island-arc of the Bowers Terrane. While the data for metasediments of the Robertson Bay Terrane are also compatible with an origin from an Antarctic-Shield-type source, there is no direct evidence from their geochemistry or isotope geochemistry for an island-arc component in these series.