Age and isotopic and chemical compositions of volcanic rocks from the Japan and Okhotsk Seas

This paper reports results of petrographic and geochemical studies of Miocene-Pleistocene volcanic rocks that accompanied formation of deep-water basins of the Sea of Japan and Sea of Okhotsk. Geochemical types of these rocks, their geodynamic settings, and their derivation from different magmatic sources were determined. Marginal-sea basaltoids from the Sea of Japan are derivatives of fluid-enriched mantle (EMI), while volcanics from the Kuril basin were generated from mantle enriched in continental crust matter (EMU). In spite of different conditions of their genesis, they have some common geochemical features, in particular, their calc-alkaline signatures. These traces of influence of the sialic crust on magma generation confirm development of the basins of both these seas on the continental basement.

Pollen record and calculated holocene vegetation and climate dynamics from Sihailongwan Maar Lake, NE China

High-resolution palynological analysis on annually laminated sediments of Sihailongwan Maar Lake (SHL) provides new insights into the Holocene vegetation and climate dynamics of NE China. The robust chronology of the presented record is based on varve counting and AMS radiocarbon dates from terrestrial plant macro-remains. In addition to the qualitative interpretation of the pollen data, we provide quantitative reconstructions of vegetation and climate based on the method of biomization and weighted averaging partial least squares regression (WA-PLS) technique, respectively. Power spectra were computed to investigate the frequency domain distribution of proxy signals and potential natural periodicities. Pollen assemblages, pollen-derived biome scores and climate variables as well as the cyclicity pattern indicate that NE China experienced significant changes in temperature and moisture conditions during the Holocene. Within the earliest phase of the Holocene, a large-scale reorganization of vegetation occurred, reflecting the reconstructed shift towards higher temperatures and precipitation values and the initial Holocene strengthening and northward expansion of the East Asian summer monsoon (EASM). Afterwards, summer temperatures remain at a high level, whereas the reconstructed precipitation shows an increasing trend until approximately 4000 cal. yr BP. Since 3500 cal. yr BP, temperature and precipitation values decline, indicating moderate cooling and weakening of the EASM. A distinct periodicity of 550-600 years and evidence of a Mid-Holocene transition from a temperature-triggered to a predominantly moisture-triggered climate regime are derived from the power spectra analysis. The results obtained from SHL are largely consistent with other palaeoenvironmental records from NE China, substantiating the regional nature of the reconstructed vegetation and climate patterns. However, the reconstructed climate changes contrast with the moisture evolution recorded in S China and the mid-latitude (semi-)arid regions of N China. Whereas a clear insolation-related trend of monsoon intensity over the Holocene is lacking from the SHL record, variations in the coupled atmosphere-Pacific Ocean system can largely explain the reconstructed changes in NE China.

The magnesium isotope composition of diagenetic dolomites from ODP Hole 112-685A and 201-1230A of the Peru Margin

The magnesium isotope composition of diagenetic dolomites and their adjacent pore fluids were studied in a 250 m thick sedimentary section drilled into the Peru Margin during Ocean Drilling Program (ODP) Leg 201 (Site 1230) and Leg 112 (Site 685). Previous studies revealed the presence of two types of dolomite: type I dolomite forms at ~ 6 m below seafloor (mbsf) due to an increase in alkalinity associated with anaerobic methane oxidation, and type II dolomite forms at focused sites below ~ 230 mbsf due to episodic inflow of deep-sourced fluids into an intense methanogenesis zone. The pore fluid delta 26Mg composition becomes progressively enriched in 26Mg with depth from values similar to seawater (i.e. -0.8 per mil, relative to DSM3 Mg reference material) in the top few meters below seafloor (mbsf) to 0.8 ± 0.2 per mil within the sediments located below 100 mbsf. Type I dolomites have a delta 26Mg of -3.5 per mil, and exhibit apparent dolomite-pore fluid fractionation factors of about -2.6 per mil consistent with previous studies of dolomite precipitation from seawater. In contrast, type II dolomites have delta 26Mg values ranging from -2.5 to -3.0 per mil and are up to -3.6 per mil lighter than the modern pore fluid Mg isotope composition. The enrichment of pore fluids in 26Mg and depletion in total Mg concentration below ~ 200 mbsf is likely the result of Mg isotope fractionation during dolomite formation, The 26Mg enrichment of pore fluids in the upper ~ 200 mbsf of the sediment sequence can be attributed to desorption of Mg from clay mineral surfaces. The obtained results indicate that Mg isotopes recorded in the diagenetic carbonate record can distinguish near surface versus deep formed dolomite demonstrating their usefulness as a paleo-diagenetic proxy.

Geochemistry and minerals at DSDP Hole 76-534A

Leg 76 sampled 31.5 m of basaltic basement at Deep Sea Drilling Project Hole 534A in the Blake-Bahama Basin. The basalts represent a short section of mineralogically uniform, sparsely plagioclase-phyric pillow flows, composed mainly of plagioclase, augitic clinopyroxene, iron-titanium oxides with variable amounts of alteration products (smectite ± carbonate ± quartz). Their major element chemistry is typical of mid-ocean ridge tholeiites and has normative compositions of olivine tholeiites. Mg/(Mg + Fe**2+) ratios range from 0.58 to 0.60, which suggests that these basalts are evolved compared to primitive mantle melts.

Neodymium isotope composition of planktonic foraminifera from ODP Hole 121-758A

This study presents neodymium isotope and elemental data for cleaned planktonic foraminifera from ODP site 758 in the southernmost reaches of the Bay of Bengal in the north-east Indian Ocean. Cleaning experiments using oxidative-reductive techniques suggest that diagenetic Fe-Mn oxyhydroxide coatings can be effectively removed, and that the measured Nd isotope composition reflects the composition of seawater from which the foraminiferal calcium carbonate was precipitated. Modern core-top Pulleniatina obliquiloculata and Globorotalia menardii give epsilon-Nd values of 310.12 +/- 0.16 and 310.28 +/- 0.16, respectively, indistinguishable from recent direct measurements of surface seawater in this area. A high-resolution Nd isotope record obtained from G. menardii for the past 150 kyr shows systematic variations (Delta epsilon-Nd = 3) on glacial-interglacial timescales. The timing of those variations shows a remarkable correspondence with the global oxygen isotope record, which suggests a process controlling the Nd isotope composition that responds in phase with global climate cycles. Palaeoclimate reconstruction indicates that during the last glacial maximum changes in monsoon circulation resulted in a reduction in rainfall over the Indian subcontinent, and a decrease in the flux of river water delivered to the Bay of Bengal. Thus, changes in the riverine input of Nd, a change in either flux or composition, most likely caused the isotope variations, although changes in dust source or local ocean circulation may have also played a role. These results clearly establish a link between climate change and variations in radiogenic isotopes in the oceans, and illustrate the potential of Nd isotopes in foraminifera for highresolution palaeoceanographic reconstruction.

Chemical and boron isotopic compositions of volcanics from ODP Sites 135-834 to 135-840

The influence of fluid flux on petrogenesis in the Tonga-Kermadec Arc was investigated using ion microprobe measurements of B/Be and boron isotope ratios (11B/10B) to document the source and relative volumes of the fluids released from the subducting oceanic plate. We analyzed young lavas from eight different islands along the Tonga-Kermadec Arc, as well as glass shards in volcanic sediments from Ocean Drilling Program (ODP) Site 840, which record the variations in the chemistry of Tonga magmatism since 7 Ma. B/Be is variable (5.8-122), in young Tonga-Kermadec Arc lavas. In contrast, glass shards from around 3 to 4 Ma old volcanic sediments at Site 840 have the highest B/Be values yet reported for arc lavas (18-607). These values are too high to be related simply to a sediment influence on petrogenesis. Together with very high d11B values (-11.6 to +37.5) for the same shards and lavas these data indicate that most of the B is derived from fluid escaped from the subducting altered Pacific oceanic crust, rather than from sediment. High d11B values also reflect large degrees of isotopic fractionation in this cold fast subduction zone. Lower d11B values noted in the Kermadec Arc (17 to -4.4) are related to the influence of sediment eroded from New Zealand and slower convergence. High fluid flux (B/Be) is synchronous in Tonga and the Marianas at 3 to 4 Ma and may be related to acceleration of the Pacific Plate just prior to this time. The timing of maximum B/Be at 3 to 4 Ma correlates with maximum light rare earth (LREE) and high field strength element depletion. This suggests maximum degrees of partial melting at this time. Although thinning of the arc lithosphere during rifting to form the Lau Basin is expected to influence the arc geochemistry, variable aqueous fluid flux from the subducting plate alone appears capable of explaining boron and other trace element systematics in the Tonga-Kermadec Arc with no indication of slab melting.

Sr, C and O isotope ratios of Neogene sediments from DSDP Hole 38-341, Voring Plateau

The Neogene sediments from DSDP site 341 on the Voring Plateau, Norwegian Sea, contain a thin glauconitic pellet-bearing subunit, which separates underlying pelagic clays from overlying glacial-marine sediments. Oxygen isotope measurements of benthic foraminifera show a delta18O shift of + 1? during deposition of this subunit, probably a combined effect of a drop in bottom water temperature and a rise in seawater delta18O. The chronology of this sedimentological and O isotope transition is, however, poorly constrained by fossil evidence. Rb-Sr dating of glauconitic pellets indicates that the lower part of the glauconitic subunit was deposited 11.6 +/- 0.2 Ma ago. Further geochronological evidence, derived from the Sr and C isotopic compositions of foraminifera compared with known seawater-time variations, indicates that the lower pelagic clays are early to middle Miocene, deposited at a mean rate of ~15 m/Ma. The glauconitic subunit contains part of the middle Miocene and probably all of the late Miocene in a condensed sequence with a very low mean depositional rate (~0.2 m/Ma). The overlying glacial marine sediments are probably Pliocene, with a high mean rate of deposition, ~45 m/Ma. This is the first application of C, O and Sr isotopic stratigraphy combined with Rb-Sr dating of glauconitic minerals, and it illustrates the applications of this integrated approach in geochronology.

Paleoceanography of sediment cores SO202-27-6 and MD01-2416

The glacial-to-Holocene evolution of subarctic Pacific surface water stratification and silicic acid (Si) dynamics is investigated based on new combined diatom oxygen (d18Odiat) and silicon (d30Sidiat) isotope records, along with new biogenic opal, subsurface foraminiferal d18O, alkenone-based sea surface temperature, sea ice, diatom, and core logging data from the NE Pacific. Our results suggest that d18Odiat values are primarily influenced by changes in freshwater discharge from the Cordilleran Ice Sheet (CIS), while corresponding d30Sidiat are primarily influenced by changes in Si supply to surface waters. Our data indicate enhanced glacial to mid Heinrich Stadial 1 (HS1) NE Pacific surface water stratification, generally limiting the Si supply to surface waters. However, we suggest that an increase in Si supply during early HS1, when surface waters were still stratified, is linked to increased North Pacific Intermediate Water formation. The coincidence between fresh surface waters during HS1 and enhanced ice-rafted debris sedimentation in the North Atlantic indicates a close link between CIS and Laurentide Ice Sheet dynamics and a dominant atmospheric control on CIS deglaciation. The Bølling/Allerød (B/A) is characterized by destratification in the subarctic Pacific and an increased supply of saline, Si-rich waters to surface waters. This change toward increased convection occurred prior to the Bølling warming and is likely triggered by a switch to sea ice-free conditions during late HS1. Our results furthermore indicate a decreased efficiency of the biological pump during late HS1 and the B/A (possibly also the Younger Dryas), suggesting that the subarctic Pacific has then been a source region of atmospheric CO2.

Uranium concentrations and isotope ratios Bahamas seawater and ODP holes pore-water

The geometry, timing, and rate of fluid-flow through carbonate margins and platforms is not well constrained. In this study, we use U concentrations and isotope ratios measured on small volumes of pore-water from Bahamas slope sediment, coupled with existing chlorinity data, to place constraints on the fluid-flow in this region and, by implication, other carbonate platforms. These data also allow an assessment of the behaviour of U isotopes in an unusually well constrained water-rock system. We report pore-water U concentrations which are controlled by dissolution of high-U organic material at shallow depths in the sediment and by reduction of U to its insoluble 4+ state at greater depths. The dominant process influencing pore-water (234U/238U) is alpha recoil. In Holocene sediments, the increase of pore-water (234U/238U) due to recoil provides an estimate of the horizontal flow rate of 11 cm/year, but with considerable uncertainty. At depths in the sediment where conditions are reducing, features in the U concentration and (234U/238U) profiles are offset from one another which constrains the effective diffusivity for U in these sediments to be c. 1-2 * 10**-8 cm**2/s. At depths between the Holocene and these reducing sediments, pore-water (234U/238U) values are unusually low due to a recent increase in the dissolution rate of grain surfaces. This suggests a strengthening of fluid flow, probably due to the flooding of the banks at the last deglaciation and the re-initiation of thermally-driven venting of fluid on the bank top and accompanying recharge on the slopes. Interpretation of existing chlorinity data, in the light of this change in flow rate, constrain the recent horizontal flow rate to be 10.6 ( 3.4) cm/year. Estimates of flow rate from (234U/238U) and Cl[-] are therefore in agreement and suggest flow rates close to those predicted by thermally-driven models of fluid flow. This agreement supports the idea that flow within the Bahamas Banks is mostly thermally driven and suggests that flow rates on the order of 10 cm/year are typical for carbonate platforms where such flow occurs.

Geochemistry of Central American Volcanic Arc basement samples

The Central American Volcanic Arc (CAVA) has been the subject of intensive research over the past few years, leading to a variety of distinct models for the origin of CAVA lavas with various source components. We present a new model for the NW Central American Volcanic Arc based on a comprehensive new geochemical data set (major and trace element and Sr-Nd-Pb-Hf-O isotope ratios) of mafic volcanic front (VF), behind the volcanic front (BVF) and back-arc (BA) lava and tephra samples from NW Nicaragua, Honduras, El Salvador and Guatemala. Additionally we present data on subducting Cocos Plate sediments (from DSDP Leg 67 Sites 495 and 499) and igneous oceanic crust (from DSDP Leg 67 Site 495), and Guatemalan (Chortis Block) granitic and metamorphic continental basement. We observe systematic variations in trace element and isotopic compositions both along and across the arc. The data require at least three different endmembers for the volcanism in NW Central America. (1) The NW Nicaragua VF lavas require an endmember with very high Ba/(La, Th) and U/Th, relatively radiogenic Sr, Nd and Hf but unradiogenic Pb and low d18O, reflecting a largely serpentinite-derived fluid/hydrous melt flux from the subducting slab into a depleted N-MORB type of mantle wedge. (2) The Guatemala VF and BVF mafic lavas require an enriched endmember with low Ba/(La, Th), U/Th, high d18O and radiogenic Sr and Pb but unradiogenic Nd and Hf isotope ratios. Correlations of Hf with both Nd and Pb isotopic compositions are not consistent with this endmember being subducted sediments. Granitic samples from the Chiquimula Plutonic Complex in Guatemala have the appropriate isotopic composition to serve as this endmember, but the large amounts of assimilation required to explain the isotope data are not consistent with the basaltic compositions of the volcanic rocks. In addition, mixing regressions on Nd vs. Hf and the Sr and O isotope plots do not go through the data. Therefore, we propose that this endmember could represent pyroxenites in the lithosphere (mantle and possibly lower crust), derived from parental magmas for the plutonic rocks. (3) The Honduras and Caribbean BA lavas define an isotopically depleted endmember (with unradiogenic Sr but radiogenic Nd, Hf and Pb isotope ratios), having OIB-like major and trace element compositions (e.g. low Ba/(La, Th) and U/Th, high La/Yb). This endmember is possibly derived from melting of young, recycled oceanic crust in the asthenosphere upwelling in the back-arc. Mixing between these three endmember types of magmas can explain the observed systematic geochemical variations along and across the NW Central American Arc.

Late Miocene to Pleistocene paleoceanographic records from the Feni and Gardar Drifts: Pliocene reduction in abyssal flow

Magnetic fabric analyses from two North Atlantic drift deposits provide proxies for determining relative variations in the strength of abyssal flow over the last 10 my. The data show a cessation of current-controlled sedimentation at the shallower Feni Drift (2417 m) at the time of onset of Northern Hemisphere glaciation (2.6 Ma). Drift formation ended nearly 2 my earlier (4.2 Ma) at the deeper Gardar Drift (3220 m), implying stepwise reduction in deep-water flow. Relatively light delta18O values at the deeper Gardar Drift indicate a warmer, thus also more salty, water mass site prior to 6 Ma. We interpret this as representing Mediterranean Sea water, which flowed north at depths greater than that of the Feni Drift Site. The supply of Mediterranean Water to the North Atlantic was shut off as the Gibraltar Straits closed, causing the Messinian salinity crisis, and never returned to that position in the water column after the Mediterranean opened again.

Os isotope ratios of Late Eocene sediments from the tropical Pacific

High resolution records (ca. 100 kyr) of Os isotope composition (187Os/188Os) in bulk sediments from two tropical Pacific sites (ODP Sites 1218 and 1219) capture the complete Late Eocene 187Os/188Os excursion and confirm that the Late Eocene 187Os/ 188Os minimum, earlier reported by Ravizza and Peucker-Ehrenbrink (2003, doi:10.1016/S0012-821X(03)00137-7), is a global feature. Using the astronomically tuned age models available for these sites, it is suggested that the Late Eocene 187Os/188Os minimum can be placed at 34.5 +/- 0.1 Ma in the marine records. In addition, two other distinct features of the 187Os/188Os excursion that are correlatable among sections are proposed as chemostratigraphic markers which can serve as age control points with a precision of ca. +/-0.1 Myr. We propose a speculative hypothesis that higher cosmic dust flux in the Late Eocene may have contributed to global cooling and Early Oligocene glaciation (Oi-1) by supplying bio-essential trace elements to the oceans and thereby resulting in higher ocean productivity, enhanced burial of organic carbon and draw down of atmospheric CO2. To determine if the hypothesis that enhanced cosmic dust flux in the Late Eocene was a cause for the 187Os/188Os excursion can be tested by using the paired bulk sediment and leachate Os isotope composition; 187Os/188Os were also measured in sediment leachates. Results of analyses of leachates are inconsistent between the south Atlantic and the Pacific sites, and therefore do not yield a robust test of this hypothesis. Comparison of 187Os/188Os records with high resolution benthic foraminiferal delta18O records across the Eocene-Oligocene transition suggests that 187Os flux to the oceans decreased during cooling and ice growth leading to the Oi-1 glaciation, whereas subsequent decay of ice-sheets and deglacial weathering drove seawater 187Os/188Os to higher values. Although the precise timing and magnitude of these changes in weathering fluxes and their effects on the marine 187Os/188Os records are obscured by recovery from the Late Eocene 187Os/188Os excursion, evidence of the global influence of glaciation on supply of Os to the ocean is robust as it has now been documented in both Pacific and Atlantic records.

Mineral composition and 40Ar-39Ar data of samples from Garnet Point, Aurora Peak, Correll Nunatak, Penguin Point and Horn Bluffs, East Antarctica

George V Land (Antarctica) includes the boundary between Late Archean-Paleoproterozoic metamorphic terrains of the East Antarctic craton and the intrusive and metasedimentary rocks of the Early Paleozoic Ross-Delamerian Orogen. This therefore represents a key region for understanding the tectono-metamorphic evolution of the East Antarctic Craton and the Ross Orogen and for defining their structural relationship in East Antarctica, with potential implications for Gondwana reconstructions. In the East Antarctic Craton the outcrops closest to the Ross orogenic belt form the Mertz Shear Zone, a prominent ductile shear zone up to 5 km wide. Its deformation fabric includes a series of progressive, overprinting shear structures developed under different metamorphic conditions: from an early medium-P granulite-facies metamorphism, through amphibolite-facies to late greenschist-facies conditions. 40Ar-39Ar laserprobe data on biotite in mylonitic rocks from the Mertz Shear Zone indicate that the minimum age for ductile deformation under greenschist-facies conditions is 1502 ± 9 Ma and reveal no evidence of reactivation processes linked to the Ross Orogeny. 40Ar-39Ar laserprobe data on amphibole, although plagued by excess argon, suggest the presence of a ~1.7 Ga old phase of regional-scale retrogression under amphibolite-facies conditions. Results support the correlation between the East Antarctic Craton in the Mertz Glacier area and the Sleaford Complex of the Gawler Craton in southern Australia, and suggest that the Mertz Shear Zone may be considered a correlative of the Kalinjala Shear Zone. An erratic immature metasandstone collected east of Ninnis Glacier (~180 km east of the Mertz Glacier) and petrographically similar to metasedimentary rocks enclosed as xenoliths in Cambro-Ordovician granites cropping out along the western side of Ninnis Glacier, yielded detrital white-mica 40Ar-39Ar ages from ~530 to 640 Ma and a minimum age of 518 ± 5 Ma. This pattern compares remarkably well with those previously obtained for the Kanmantoo Group from the Adelaide Rift Complex of southern Australia, thereby suggesting that the segment of the Ross Orogen exposed east of the Mertz Glacier may represent a continuation of the eastern part of the Delamerian Orogen.

Age determination, carbon geochemistry and palynology of section AC003

Sedimentary records from California's Northern Channel Islands and the adjacent Santa Barbara Basin (SBB) indicate intense regional biomass burning (wildfire) at the Ållerød-Younger Dryas boundary (~13.0-12.9 ka) (All age ranges in this paper are expressed in thousands of calendar years before present [ka]. Radiocarbon ages will be identified and clearly marked "14C years".). Multiproxy records in SBB Ocean Drilling Project (ODP) Site 893 indicate that these wildfires coincided with the onset of regional cooling and an abrupt vegetational shift from closed montane forest to more open habitats. Abrupt ecosystem disruption is evident on the Northern Channel Islands at the Ållerød-Younger Dryas boundary with the onset of biomass burning and resulting mass sediment wasting of the landscape. These wildfires coincide with the extinction of Mammuthus exilis [pygmy mammoth]. The earliest evidence for human presence on these islands at 13.1-12.9 ka (~11,000-10,900 14C years) is followed by an apparent 600-800 year gap in the archaeological record, which is followed by indications of a larger-scale colonization after 12.2 ka. Although a number of processes could have contributed to a post 18 ka decline in M. exilis populations (e.g., reduction of habitat due to sea-level rise and human exploitation of limited insular populations), we argue that the ultimate demise of M. exilis was more likely a result of continental scale ecosystem disruption that registered across North America at the onset of the Younger Dryas cooling episode, contemporaneous with the extinction of other megafaunal taxa. Evidence for ecosystem disruption at 13-12.9 ka on these offshore islands is consistent with the Younger Dryas boundary cosmic impact hypothesis [Firestone et al., 2007, doi:10.1073/pnas.0706977104].

40Ar-39Ar dating and isotopic composition of groundmass and felspar samples from core AND-2A

The AND-2A drillcore (Antarctic Drilling Program-ANDRILL) was successfully completed in late 2007 on the Antarctic continental margin (Southern McMurdo Sound, Ross Sea) with the aim of tracking ice proximal to shallow marine environmental fluctuations and to document the 20-Ma evolution of the Erebus Volcanic Province. Lava clasts and tephra layers from the AND-2A drillcore were investigated from a petrographic and stratigraphic point of view and analyzed by the 40Ar-39Ar laser technique in order to constrain the age model of the core and to gain information on the style and nature of sediment deposition in the Victoria Land Basin since Early Miocene. Ten out of 17 samples yielded statistically robust 40Ar-39Ar ages, indicating that the AND-2A drillcore recovered <230 m of Middle Miocene (~128-358 m below sea floor, ~11.5-16.0 Ma) and >780 m of Early Miocene (~358-1093 m below sea floor, ~16.0-20.1 Ma). Results also highlight a nearly continuous stratigraphic record from at least 358 m below sea floor down hole, characterized by a mean sedimentation rate of ~19 cm/ka, possible oscillations of no more than a few hundreds of ka and a break within ~17.5-18.1 Ma. Comparison with available data from volcanic deposits on land, suggests that volcanic rocks within the AND-2A core were supplied from the south, possibly with source areas closer to the drill site for the upper core levels, and from 358 m below sea floor down hole, with the 'proto-Mount Morning' as the main source.