Shear experiments from IODP Expedition 322 and 333 samples

The mechanical behavior of the plate boundary fault zone is of paramount importance in subduction zones, because it controls megathrust earthquake nucleation and propagation as well as the structural style of the forearc. In the Nankai area along the NanTroSEIZE (Kumano) drilling transect offshore SW Japan, a heterogeneous sedimentary sequence overlying the oceanic crust enters the subduction zone. In order to predict how variations in lithology, and thus mechanical properties, affect the formation and evolution of the plate boundary fault, we conducted laboratory tests measuring the shear strengths of sediments approaching the trench covering each major lithological sedimentary unit. We observe that shear strength increases nonlinearly with depth, such that the (apparent) coefficient of friction decreases. In combination with a critical taper analysis, the results imply that the plate boundary position is located on the main frontal thrust. Further landward, the plate boundary is expected to step down into progressively lower stratigraphic units, assisted by moderately elevated pore pressures. As seismogenic depths are approached, the décollement may further step down to lower volcaniclastic or pelagic strata but this requires specific overpressure conditions. High-taper angle and elevated strengths in the toe region may be local features restricted to the Kumano transect.

Isotopic composition of Manihiki and Ontong Java basalts

Pb, Nd, and Sr isotopic results for lavas of the Cretaceous Ontong Java and Manihiki oceanic plateaus fall well within the modern-day oceanic island or hot pot field. The data provide no evidence of old continental basements but indicate a major involvement of 'Kerguelen-type' or 'EM-I'-like mantle in the sources of both plateaus, which appear to have probably been formed, at least in part, by hotspots. However, the presently active hotspots that Pacific plate reconstructions suggest might have been possible plateau sources lack Kerguelen-type isotopic compositions. Either these hotspots did not participate in the formation of the two plateaus, or if they did, Kerguelen-type material must have been volumetrically much more important early in their existence. Two hypotheses for the origins of these plateaus which involve hotspot sources are consistent with the sparse available geochemical, geochronological and geophysical data. The first holds that the plateaus formed cataclysmically in association with surfacing plume heads; the second posits a relatively steady but robust hotspot at or near a ridge crest and requires a much longer period of formation. A near-ridge origin appears to be indicated by evidence that most of the Pacific plateaus were built largely on relatively young ocean crust. However, we suggest that a near-ridge origin is also compatible with the plume head concept in that plume heads appear very likely to become associated with spreading axes through their influence on rift propagation, which should be substantially greater than for ordinary hotspots. In either case, the lack of hotspot tracks (seamount chains) attached to the two plateaus would be a consequence of ridge migration or rift propagation in a near-ridge setting.

Physical properties, velocity and attenuation characteristics, and mineralogy of gabbros from ODP Hole 176-735B

Laboratory compressional wave (Vp) and shear wave (Vs) velocities were measured as a function of confining pressure for the gabbros from Hole 735B and compared to results from Leg 118. The upper 500 m of the hole has a Vp mean value of 6895 m/s measured at 200 MPa, and at 500 meters below seafloor (mbsf), Vp measurements show a mean value of 7036 m/s. Vs mean values in the same intervals are 3840 m/s and 3857 m/s, respectively. The mean Vp and Vs values obtained from log data in the upper 600 m are 6520 and 3518 m/s, respectively. These results show a general increase in velocity with depth and the velocity gradients estimate an upper mantle depth of 3.32 km. This value agrees with previous work based on dredged samples and inversion of rare element concentrations in basalts dredged from the conjugate site to the north of the Atlantis Bank. Laboratory measurements show Vp anisotropy ranging between 0.4% and 8.8%, with the majority of the samples having values less than 3.8%. Measurements of velocity anisotropy seem to be associated with zones of high crystal-plastic deformation with predominant preferred mineral orientations of plagioclase, amphiboles, and pyroxenes. These findings are consistent with results on gabbros from the Hess Deep area and suggest that plastic deformation may play an important role in the seismic properties of the lower oceanic crust. In contrast to ophiolite studies, many of the olivine gabbros show a small degree of anisotropy. Log derived Vs anisotropy shows an average of 5.8% for the upper 600 m of Hole 735B and tends to decrease with depth where the overburden pressure and the age of the crustal section suggests closure of cracks and infilling of fractures by alteration minerals. Overall the results indicate that the average shear wave splitting in Hole 735B might be influenced by preferred structural orientations and the average value of shear wave splitting may not be a maximum because structural dips are <90°. The maximum fast-wave orientation values could be influenced by structural features striking slightly oblique to this orientation or by near-field stress concentrations. However, flexural wave dispersion analyses have not been performed to confirm this hypothesis or to indicate to what extent the near-field stresses may be influencing shear wave propagation. Acoustic impedance contrasts calculated from laboratory and logging data were used to generate synthetic seismograms that aid in the interpretation of reflection profiles. Several prominent reflections produced by these calculations suggest that Fe-Ti oxides and shear zones may contribute to the reflective nature of the lower oceanic crust. Laboratory velocity attenuation (Q) measurements from below 500 m have a mean value of 35.1, which is consistent with previous vertical seismic profile (VSP) and laboratory measurements on the upper 500 m.

Age model, raw data and interpolated raw data from proxy records of sediment cores GeoB6007-1 and GeoB6007-2

Here we present a 1200 yr long benthic foraminiferal Mg/Ca based temperature and oxygen isotope record from a ~900 m deep sediment core off northwest Africa to show that atmosphere-ocean interactions in the eastern subpolar gyre are transferred at central water depth into the eastern boundary of the subtropical gyre. Further we link the variability of the NAO (over the past 165 yrs) and solar irradiance (Late Holocene) and their control on subpolar mode water formation to the multidecadal variability observed at mid-depth in the eastern subtropical gyre. Our results show that eastern North Atlantic central waters cooled by up to ~0.8± 0.7 °C and densities decreased by Sigma theta=0.3±0.2 during positive NAO years and during minima in solar irradiance during the Late Holocene. The presented records demonstrate the sensitivity of central water formation to enhanced atmospheric forcing and ice/freshwater fluxes into the eastern subpolar gyre and the importance of central water circulation for cross-gyre climate signal propagation during the Late Holocene.

(Table 1) 137Cs and 90Sr concentrations, salinity, and temperature in waters of the southeastern Baltic Sea on October 16, 2001

An analysis of variations in 137Cs and 90Sr concentrations in Baltic Sea surface waters after the accident at the Chernobyl nuclear power plant was performed. Instability of 137Cs concentrations during the short-term observations was found, when they differed 2- to 3-fold. Concentrations of 90Sr appeared to be more stable; meanwhile, their deviations sometimes exceeded ranges of experimental errors. By variations in the monthly average values of radionuclide concentrations in surface waters of the Baltic Sea in 1989-1995, no trend of water self-purification was observed. Theoretical results obtained confirmed a potential of formation and propagation of patches with increased concentrations of 137Cs in the southeastern Baltic Sea. The most reliable factor that controlled the process of self-purification of Baltic Sea water appeared to be the mean annual value of radionuclide concentration. Pronounced divergences were obtained between the measured and calculated mean annual concentrations of 137Cs and 90Sr in surface waters of the Baltic Sea in 1989-2001. These divergences are explained by potential influence of waters from the Gulf of Bothnia and by other additional supplies of radionuclides to marine environment, which were not included into mathematical models.

Concentrations of dinoflagellate cysts in the surface layer of bottom sediment from the White Sea

Dinoflagellate cysts were studied in 42 samples from surface sediments of the White Sea. Total concentration of dinocysts varies from single cysts to 25000 cyst/g of dry sediments, which reflects biological productivity in White Sea waters and regional particular features of sedimentation processes. The highest concentrations are observed in silts; they are related to the regions of propagation of highly productive Barents Sea waters in the White Sea. Generally, spatial distribution of dinocyst species in the surface sediments corresponds to distribution of the major types of water masses in the White Sea. Cysts of relatively warm-water species (Operculodinium centrocarpum, Spiniferites sp.) of North Atlantic origin that dominate in the sediments indicate an intensive intrusion of Barents Sea water masses to the White Sea along with hydrological dwelling conditions in the White Sea favorable for development of these species during their vegetation period. The cold-water dinocyst assemblage (Islandinium minutum, Polykrikos sp.) is rather strictly confined to inner parts of shallow-water bays, firstly, those adjacent to the Onega and Severnaya Dvina river mouths.

(Table S1) Radiocarbon data of sediment core SO161/5 22SL

At the end of the Last Glacial Maximum (19,000 to 11,000 years ago), atmospheric carbon dioxide concentrations rose while the Delta14C of atmospheric carbon dioxide declined**1, 2. These changes have been attributed to an injection of carbon dioxide with low radiocarbon activity from an oceanic abyssal reservoir that was isolated from the atmosphere for several thousand years before deglaciation**3. The current understanding points to the Southern Ocean as the main area of exchange between these reservoirs4. Intermediate water formed in the Southern Ocean surrounding Antarctica would have then carried the old carbon dioxide signature to the lower-latitude oceans**5, 6. Here we reconstruct the Delta14C signature of Antarctic Intermediate Water off the coast of Chile for the past 20,000 years, using paired 14C ages of benthic and planktonic foraminifera. In contrast to the above scenario, we find that the delta14C signature of the Antarctic Intermediate Water closely matches the modelled surface ocean Delta14C, precluding the influence of an old carbon source. We suggest that if the abyssal ocean is indeed the source of the radiocarbon-depleted carbon dioxide, an alternative path for the mixing and propagation of its carbon dioxide may be required to explain the observed changes in atmospheric carbon dioxide concentration and radiocarbon activity.

(Table 1) Results from XRD analysis as well as sediment type and selected results from ring shear experiments

The location of the seaward tip of a subduction thrust controls material transfer at convergent plate margins, and hence global mass balances. At approximately half of those margins, the material of the subducting plate is completely underthrust so that no accretion or even subduction erosion takes place. Along the remaining margins, material is scraped off the subducting plate and added to the upper plate by frontal accretion. We here examine the physical properties of subducting sediments off Costa Rica and Nankai, type examples for an erosional and an accretionary margin, to investigate which parameters control the level where the frontal thrust cuts into the incoming sediment pile. A series of rotary-shear experiments to measure the frictional strength of the various lithologies entering the two subduction zones were carried out. Results include the following findings: (1) At Costa Rica, clay-rich strata at the top of the incoming succession have the lowest strength (µres = 0.19) while underlying calcareous ooze, chalk and diatomite are strong (up to µres = 0.43; µpeak = 0.56). Hence the entire sediment package is underthrust. (2) Off Japan, clay-rich deposits within the lower Shikoku Basin inventory are weakest (µres = 0.13-0.19) and favour the frontal proto-thrust to migrate into one particular horizon between sandy, competent turbidites below and ash-bearing mud above. (3) Taking in situ data and earlier geotechnical testing into account, it is suggested that mineralogical composition rather than pore-pressure defines the position of the frontal thrust, which locates in the weakest, clay mineral-rich (up to 85 wt.%) materials. (4) Smectite, the dominant clay mineral phase at either margin, shows rate strengthening and stable sliding in the frontal 50 km of the subduction thrust (0.0001-0.1 mm/s, 0.5-25 MPa effective normal stress). (5) Progressive illitization of smectite cannot explain seismogenesis, because illite-rich samples also show velocity strengthening at the conditions tested.

Chemical composition of Late Cenozoic volcanic and volcaniclastic deposits from the western Woodlark Basin area, SW Pacific

Tephra fallout layers and volcaniclastic deposits, derived from volcanic sources around and on the Papuan Peninsula, form a substantial part of the Woodlark Basin marine sedimentary succession. Sampling by the Ocean Drilling Program Leg 180 in the western Woodlark Basin provides the opportunity to document the distribution of the volcanically-derived components as well as to evaluate their chronology, chemistry, and isotope compositions in order to gain information on the volcanic sources and original magmatic systems. Glass shards selected from 57 volcanogenic layers within the sampled Pliocene-Pleistocene sedimentary sequence show predominantly rhyolitic compositions, with subordinate basaltic andesites, basaltic trachy-andesites, andesites, trachy-andesites, dacites, and phonolites. It was possible to correlate only a few of the volcanogenic layers between sites using geochemical and age information apparently because of the formation of strongly compartmentalised sedimentary realms on this actively rifting margin. In many cases it was possible to correlate Leg 180 volcanic components with their eruption source areas based on chemical and isotope compositions. Likely sources for a considerable number of the volcanogenic deposits are Moresby and Dawson Strait volcanoes (D'Entrecasteaux Islands region) for high-K calc-alkaline glasses. The Dawson Strait volcanoes appear to represent the source for five peralkaline tephra layers. One basaltic andesitic volcaniclastic layer shows affinities to basaltic andesites from the Woodlark spreading tip and Cheshire Seamount. For other layers, a clear identification of the sources proved impossible, although their isotope and chemical signatures suggest similarities to south-west Pacific subduction volcanism, e.g. New Britain and Tonga- Kermadec island arcs. Volcanic islands in the Trobriand Arc (for example, Woodlark Island Amphlett Islands and/or Egum Atoll) are probable sources for several volcaniclastic layers with ages between 1.5 to 3 Ma. The Lusancay Islands can be excluded as a source for the volcanogenic layers found during Leg 180. Generally, the volcanogenic layers indicate much calc-alkaline rhyolitic volcanism in eastern Papua since 3.8 Ma. Starting at 135 ka, however, peralkaline tephra layers appear. This geochemical change in source characteristics might reflect the onset of a change in geotectonic regime, from crustal subduction to spreading, affecting the D'Entrecasteaux Islands region. Initial 143Nd/144Nd ratios as low as 0.5121 and 0.5127 for two of the tephra layers are interpreted as indicating that D'Entrecasteaux Islands volcanism younger than 2.9 Ma occasionally interacted with the Late Archean basement, possibly reflecting the mobilisation of the deep continental crust during active rift propagation.

(Table 1) Sample characteristics, test consolidations and calculated elastic parameters of ODP Sites 194-1193 and 194-1196

This study is based on rock mechanical tests of samples from platform carbonate strata to document their petrophysical properties and determine their potential for porosity loss by mechanical compaction. Sixteen core-plug samples, including eleven limestones and five dolostones, from Miocene carbonate platforms on the Marion Plateau, offshore northeast Australia, were tested at vertical effective stress, sigma1', of 0-70 MPa, as lateral strain was kept equal to zero. The samples were deposited as bioclastic facies in platform-top settings having paleo-water depths of <10-90 m. They were variably cemented with low-Mg calcite and five of the samples were dolomitized before burial to present depths of 39-635 m below sea floor with porosities of 8-46%. Ten samples tested under dry conditions had up to 0.22% strain at sigma1' = 50 MPa, whereas six samples tested saturated with brine, under drained conditions, had up to 0.33% strain. The yield strength was reached in five of the plugs. The measured strains show an overall positive correlation with porosity. Vp ranges from 3640 to 5660 m/s and Vs from 1840 to 3530 m/s. Poisson coefficient is 0.20-0.33 and Young's modulus at 30 MPa ranged between 5 and 40 GPa. Water saturated samples had lower shear moduli and slightly higher P- to S-wave velocity ratios. Creep at constant stress was observed only in samples affected by pore collapse, indicating propagation of microcracks. Although deposited as loose carbonate sand and mud, the studied carbonates acquired reef-like petrophysical properties by early calcite and dolomite cementation. The small strains observed experimentally at 50 MPa indicate that little mechanical compaction would occur at deeper burial. However, as these rocks are unlikely to preserve their present high porosities to 4-5 km depth, further porosity loss would proceed mainly by chemical compaction and cementation.

(Table S1) Age determination of Atlantic Ocean sediment cores

Well-dated benthic foraminifer oxygen isotopic records (d18O) from different water depths and locations within the Atlantic Ocean exhibit distinct patterns and significant differences in timing over the last deglaciation. This has two implications: on the one hand, it confirms that benthic d18O cannot be used as a global correlation tool with millennial-scale precision, but on the other hand, the combination of benthic isotopic records with independent dating provides a wealth of information on past circulation changes. Comparing new South Atlantic benthic isotopic data with published benthic isotopic records, we show that (1) circulation changes first affected benthic d18O in the 1000-2200 m range, with marked decreases in benthic d18O taking place at ~17.5 cal. kyr B.P. (ka) due to the southward propagation of brine waters generated in the Nordic Seas during Heinrich Stadial 1 (HS1) cold period; (2) the arrival of d18O-depleted deglacial meltwater took place later at deeper North Atlantic sites; (3) hydrographic changes recorded in North Atlantic cores below 3000 m during HS1 do not correspond to simple alternations between northern- and southern-sourced water but likely reflect instead the incursion of brine-generated deep water of northern as well as southern origin; and (4) South Atlantic waters at ~44°S and ~3800 m depth remained isolated from better-ventilated northern-sourced water masses until after the resumption of North Atlantic Deep Water (NADW) formation at the onset of the Bølling-Allerod, which led to the propagation of NADW into the South Atlantic.

Conventional and 40Ar/39Ar K-Ar ages of volcanic rocks at DSDP Leg 55 Holes

Conventional K-Ar, 40Ar/39Ar total fusion, and 40Ar/39Ar incremental heating data on hawaiite and tholeiitic basalt samples from Ojin (Site 430), alkalic basalt samples from Nintoku (Site 432), and alkalic and tholeiitic basalt samples from Suiko (Site 433) seamounts in the Emperor Seamount chain give the following best ages for these volcanoes: Ojin = 55.2 ± 0.7 m.y., Nintoku = 56.2 ± 0.6 m.y., and Suiko = 64.7 ± 1.1 m.y. These new data bring to 27 the number of dated volcanoes in the Hawaiian-Emperor volcanic chain. The new dates prove that the age progression from Kilauea Volcano on Hawaii (0 m.y.) through the Hawaiian-Emperor bend (- 43 m.y.) to Koko Seamount (48.1 m.y.) in the southernmost Emperor Seamounts continues more than halfway up the Emperor chain to Suiko Seamount. The age versus distance data for the Hawaiian-Emperor chain are consistent with the kinematic hot-spot hypothesis, which predicts that the volcanoes are progressively older west and north away from the active volcanoes of Kilauea and Mauna Loa. The data are consistent with an average volcanic propagation velocity of either 8 cm/year from Suiko to Kilauea or of 6 cm/year from Suiko to Midway followed by a velocity of 9 cm/year from Midway to Kilauea, but it appears that the change in direction that formed the Hawaiian- Emperor bend probably was not accompanied by a major change in velocity.