Sedimentology, foraminiferal assemblage, and isotope record of the Agulhas Bank region

The Agulhas Bank region, south of Africa, is an oceanographically important and complex area. The leakage of warm saline Indian Ocean water into the South Atlantic around the southern tip of Africa is a crucial factor in the global thermohaline circulation. Foraminiferal assemblage, stable isotope and sedimentological data from the top 10 m of core MD962080, recovered from the western Agulhas Bank Slope, are used to indicate changes in water mass circulation in the southeastern South Atlantic for the last 450 kyr. Sedimentological and planktonic foraminiferal data give clear signals of cold water intrusions. The benthic stable isotope record provides the stratigraphic framework and indicates that the last four climatic cycles are represented (i.e. down to marine isotope stage (MIS) 12). The planktonic foraminiferal assemblages bear a clear transitional to subantarctic character with Globorotalia inflata and Neogloboquadrina pachyderma (dextral) being the dominant taxa. Input of cold, subantarctic waters into the region by means of leakage through the Subtropical Convergence, as part of Agulhas ring shedding, and a general cooling of surface waters is suggested by increased occurrence of the subantarctic assemblage during glacial periods. Variable input of Indian Ocean waters via the Agulhas Current is indicated by the presence of tropical/subtropical planktonic foraminiferal species Globoquadrina dutertrei, Globigerinoides ruber (alba) and Globorotalia menardii with maximum leakage occurring at glacial terminations. The continuous presence of G. menardii throughout the core suggests that the exchange of water from the South Indian Ocean to the South Atlantic Ocean was never entirely obstructed in the last 450 kyr. The benthic carbon isotope record and sediment textural data reflect a change in bottom water masses over the core location from North Atlantic Deep Water to Upper Southern Component Water. Planktonic foraminiferal assemblages and sediment composition indicate a profound change in surface water conditions over the core site approximately 200-250 kyr BP, during MIS 7, from mixed subantarctic and transitional water masses to overall warmer surface water conditions.

Mineralogy and chemical composition of ODP Sites 129-800 and 129-801

Sites 800 and 801 in the Pigafetta Basin allow the sedimentary history over the oldest remaining Pacific oceanic crust to be established. Six major deposition stages and events are defined by the main lithologic units from both sites. Mineralogical and chemical investigations were run on a large set of samples from these units. The data enable the evolution of the sediments and their depositional environments to be characterized in relation to the paleolatitudinal motion of the sites. The upper part of the basaltic crust at Site 801 displays a complex hydrothermal and alteration evolution expressed particularly by an ochre siliceous deposit comparable to that found in the Cyprus ophiolite. The oldest sedimentary cover at Site 801 was formed during the Callovian-Bathonian (stage 1) with red basal siliceous and metalliferous sediments similar to those found in supraophiolite sequences, and formed near an active ridge axis in an open ocean. Biosiliceous sedimentation prevailed throughout the Oxfordian to Campanian, with rare incursions of calcareous input during the middle Cretaceous (stages 2, 4, and 5). The biosiliceous sedimentation was drastically interrupted during the Aptian-Albian by thick volcaniclastic turbidite deposits (stage 3). The volcanogenic phases are pervasively altered and the successive secondary mineral parageneses (with smectites, celadonite, clinoptilolite, phillipsite, analcime, calcite, and quartz) define a "mineral stratigraphy" within these deposits. From this mineral stratigraphy, a similar lithologic layer is defined at the top of the Site 800 turbidite unit and the bottom of the Site 801 turbidite unit. Then, the two sites appear to have been located at the same distal distance from a volcanic source (hotspot). They crossed this locality, at about 10°S, at different times (latest Aptian for Site 800, middle Albian for Site 801). The Cretaceous siliceous sedimentation stopped during the late Campanian and was followed by deposition of Cenozoic pelagic red clay (stage 6). This deep-sea facies, which formed below the carbonate compensation depth, contains variable zeolite authigenesis in relation to the age of deposition, and records the global middle Cenozoic hiatus events. At the surface, the red clay from this part of the Pacific shows a greater detrital component than its equivalents from the central Pacific deep basins.

Planktic foraminiferal assemblage turnover at the Cretaceous-Tertiary boundary

Three uppermost Cretaceous through basal Paleocene stratigraphic sequences are examined for planktic foraminiferal assemblage stability and temporal succession patterns. These sequences are at mid-latitude South Atlantic DSDP Site 528, then-equatorial Pacific DSDP Site 577 and the Tethyan shelf Ben Gurion section of the Negev, Israel. In order to better estimate biogeographic patterns and habitat preferences, the results of these analyses are compared to previous Cretaceous biogeographic studies and to previous analyses of Cretaceous-Tertiary (K/T) boundary shelf and epicontinental sections. Results indicate that immediately following the K/T boundary, the examined epicontinental and open-ocean sites were exploited primarily by previously epicontinental planktic foraminiferal assemblages. This pattern of K/T boundary assemblage dominance suggests the geologically instantaneous break-down of Late Cretaceous epicontinental and open-ocean biogeographic provincialization. This shift in open-ocean foraminiferal assemblages is not consistent with models of nonselective K/T boundary extinctions, but is consistent with models of extinction resistence and offshore expansion of nearshore taxa. The re-establishment of stable biogeographic differences between open-ocean and epicontinental planktic foraminiferal assemblages occurs by the basal Parvularugoglobigerina eugubina Zone. At open-ocean sites 528 and 577 and the outershelf Ben Gurion section, P0 and P. eugubina Zone faunal records are marked by a pronounced alternation between Paleocene biserial- and non-biserial-dominated assemblages, This alternation appears strongly damped at shelf and epicontinental sections previously examined. The first appearance and peak magnitude of abundant earliest Paleocene trochospiral forms (Parvularugoglobigerina, Eoglobigerina, Morozovella, Globoconusa) also vary from site to site and may depend closely on levels of primary carbonate productivity.

Mineralogy and major element oxide composition of sediments from ODP Hole 172-1063D

Dansgaard-Oeschger (D-O) cycles in sediment at Site 1063 are characterized by distinct fluctuations in physical properties. Stadials are marked by low bulk density and interstadials by high bulk density. Compressional (P-)wave velocity is in phase with bulk density over some but not all depth intervals. Four of the D-O cycles straddling the oxygen isotope Stage 4/5 boundary have been studied in detail to understand the origin of the physical properties changes. Sediment on the Bermuda Rise is comprised of three main components: calcite, aluminosilicate minerals, and biogenic silica. Calcite concentrations vary from 1% to 43% of bulk sediment and are highest during interstadials. Aluminosilicate concentrations vary from 52% to 92% of bulk sediment and are highest during stadials. The major element ratios Al2O3/TiO2 and K2O/Al2O3 show increases across bulk density cycles, suggesting a change in the composition of aluminosilicates. This interpretation is supported by mineralogical analyses, which show a subtle change in clay composition. Biogenic silica concentrations vary from 0% to 23% of bulk sediment and are also highest during stadials. However, the abundance of silica varies significantly from one D-O cycle to another. Silt and fine sand abundance also increase during the first of the four stadials. This coarsening of sediment coincides with the increase in biogenic silica. The low grain density and high porosity associated with biogenic silica result in intervals of low bulk-sediment density. The abundance of biogenic silica closely matches P-wave velocity, suggesting that silica imparts a greater rigidity to the sediment.

Petrology and geochemistry of basalts from the Vavilov Basin

During ODP Leg 107, two holes were drilled in the basement of Vavilov Basin, a central oceanic area of the Tyrrhenian sea. Hole 655B is located near the Gortani ridge in off-axis position at the western rim of the basin; Hole 651A is located on a basement swell at the axis of the basin. This paper deals with mineral chemistry, major and trace element geochemistry, and petrogenesis of the basalts recovered in the two holes. The mineralogy of the basalts is broadly homogeneous, but all of them have suffered important seawater alteration. Their major-element compositions are similar to both normal-mid-ocean-ridge-basalts (N-MORB) and back-arc-basalts (BAB) except for Na2O contents (BAB-like), and K2O which is somewhat enriched in upper unit of Hole 651 A. Their affinity with N-MORB and BAB is confirmed by using immobile trace elements such as Zr, Y, and Nb. However, basalts from the two sites present contrasting geochemical characteristics on spidergrams using incompatible elements. Hole 655B basalts are homogeneous enriched tholeiites, similar to those from DSDP Hole 373 (located on the opposite side of the basin near the eastern rim), and show affinities with enriched MORB (E-MORB). At Hole 651 A, the two basalt units are chemically distinct. One sample recovered in lower unit is rather similar to those from Hole 655B, but basalts from upper unit display calc-alkaline characteristic evidenced by the increase of light-ion-lithophile-element (LILE)/high-field-strength-element (HFSE) ratio, and appearance of a negative Nb-anomaly, making them comparable with orogenic lavas from the adjacent Eolian arc. The observed chemical compositions of the basalts are consistent with a derivation of the magmas from a N-MORB type source progressively contaminated by LILE-enriched fluids released from dehydration of the bordering subducted plate. Implications for evolution of the Tyrrhenian basin are tentatively proposed taking into consideration geochemical and chronological relationships between basalts from Leg 107 Holes 655B and 651 A, together with data from Leg 42 Site 373 and Vavilov Seamount. These data illustrate back-arc spreading in ensialic basin closely associated with the maturation of the adjacent subduction, followed by the growth of late off-axis central volcano, whereas the active subduction retreats southeastward.

Petrography and geochemistry of lithic clasts in sediments obtained from ODP Leg 180 sites

This report includes the petrographic description and reviews the distribution of lithic clasts in sediments drilled during Leg 180 in the Woodlark Basin (southwest Pacific). The lithic clasts include (1) metamorphic rocks; (2) granites; (3) serpentinites, gabbros, dolerites, and basalts likely derived from the Papuan ophiolite belt; (4) rare alkaline volcanites reworked in middle Miocene sediments; (5) medium- to high-K calc-alkaline island arc volcanites, in part as reworked clasts, and explosive products deposited by fallout or reworked by turbiditic currents; and (6) rare sedimentary fragments. At the footwall sites the clast assemblage evidences the association of dolerites and evolved gabbroic rocks; the serpentinite likely pertaining to the same ophiolitic complex are likely derived from onland outcrops and transported by means of turbidity currents. On the whole, extensional tectonics active at least since the middle Pliocene can be inferred. The calc-alkaline volcanism is in continuity with the arc-related products from the Papua Peninsula and D'Entrecasteaux Islands and with the latest volcanics of the Miocene Trobrian arc. However, the medium- to high-K and shoshonitic products do not display a significant temporal evolution within the stratigraphic setting. Lava clasts, volcanogenic grains, and glass shards are associated with turbidity currents, whereas in the Pliocene of northern margin the increasing frequency of tephra (glass shards and vesicular silicic fragments) suggests more explosive activity and increasing contribution to the sediments from aerial fallout materials. Evidence of localized alkalic volcanism of presumable early to middle Miocene age is a new finding. It could represent a rift phase earlier than or coeval to the first opening of the Woodlark Basin or, less probably, could derive from depositional trajectories diverted from an adjacent basin.

Geochemistry of volcanivlastic sediments of ODP Sites 134-832 and 134-833

Volcaniclastic sediments of North Aoba Basin (Vanuatu) recovered during Ocean Drilling Program (ODP) Leg 134 show a mineralogical and chemical overprint of low grade hydrothermal alteration superimposed on the primary magmatic source compositions. The purpose of this study was to identify authigenic mineral phases incorporated in the volcaniclastic sediments, to distinguish authigenic chemical and mineralogical signals from the original volcaniclastic mineralogical and chemical compositions, and to determine the mechanism of authigenic minerals formation. Mineralogical, micro-chemical and bulk chemical analyses were utilized to identify and characterize authigenic phases and determine the original unaltered ash compositions. 117 volcaniclastic sediment samples from North Aoba Basin Sites 832 and 833 were analyzed. Primary volcaniclastic materials accumulated in North Aoba Basin can be divided into three types. The older basin-filling sequences show three different magmatic trends: high K, calc-alkaline, and low K series. The most recent accumulations are rhyodacitic composition and can be attributed to Santa Maria or Aoba volcanic emissions. Original depositional porosity of volcaniclastic sediments is an important factor in influencing distribution of authigenic phases. Finer-grained units are less altered and retain a bulk mineralogical and chemical composition close to the original pyroclastic rock composition. Coarser grained units (microbreccia and sandstones) are the major hosts of authigenic minerals. At both sites, authigenic minerals (including zeolites, clay minerals, Mg-carbonates, and quartz) exhibit complex zonation with depth that crosses original ash depositional boundaries and stratigraphic limits. The zeolite minerals phillipsite and analcime are ubiquitous throughout the altered intervals. At Site 832, the first zeolite minerals (phillipsite) occur in Pleistocene deposits as shallow as 146 meters below seafloor (mbsf). At Site 833 the first zeolite minerals (analcime) occur in Pleistocene deposits as shallow as 224 mbsf. The assemblage phillipsite + analcime + chabazite appears at 635 mbsf (Site 832) and at 376 mbsf (Site 833). Phillipsite + analcime + chabazite + thomsonite + heulandite are observed between 443 and 732 mbsf at Site 833. Thomsonite is no longer observed below 732 mbsf at Site 833. Heulandite is present to the base of the sections cored. The zeolite assemblages are associated with authigenic clay minerals (nontronite and saponite), calcite, and quartz. Chlorite is noticeable at Site 832 as deep as 851 mbsf. Zeolite zones are present but are less well defined at Site 832. Dolomite and rare magnesite are present below 940 m at Site 832. The coarse-grained authigenic mineral host intervals exhibit geochemical signatures that can be attributed to low grade hydrothermal alteration. The altered intervals show evidence of K2O, CaO, and rare earth elements mobilization. When compared to fine-grained, unaltered units, and to Santa Maria Island volcanics rocks, the altered zones are relatively depleted in rare earth elements, with light rare earth elements-heavy rare earth elements fractionation. Drilling at Site 833 penetrated a sill complex below 840 m. No sill was encountered at Site 832. Complex zonation of zeolite facies, authigenic smectites, carbonates and quartz, and associated geochemical signatures are present at both sites. The mineralogical and chemical alteration overprint is most pronounced in the deeper sections at Site 832. Based on mineralogical and chemical evidence at two locations less than 50 km apart, there is vertical and lateral variation in alteration of the volcaniclastic sediments of North Aoba Basin. The alteration observed may be activated by sill intrusion and associated expulsion of heated fluids into intervals of greater porosity. Such spatial variation in alteration could be attributed to the evolution of the basin axis associated with subduction processes along the New Hebrides Trench.

Geochemistry and trace elements at DSDP Hole 78-543A

The upper part of the basaltic substratum of the Atlantic abyssal plain, approaching subduction beneath the Barbados Ridge and thus presumably beneath the Lesser Antilles island arc, is made of typical LREE-depleted oceanic tholeiites. Mineralogical (microprobe) and geochemical (X-ray fluorescence, neutron activation analyses) data are given for 12 samples from the bottom of Hole 543A, which is 3.5 km seaward of the deformation front of the Barbados Ridge complex. These basalts are overlain by a Quaternary to Maestrichtian-Campanian sedimentary sequence. Most of the basalts are relatively fresh (in spite of the alteration of olivine and development of some celadonite, clays, and chlorite in their groundmass), and their mineralogical and geochemical compositions are similar to those of LREE-depleted recent basalts from the Mid-Atlantic Ridge. The most altered samples occur at the top of the basaltic sequence, and show trends of enrichment in alkali metals typical of altered oceanic tholeiites.

Results of bulk sediment X-ray diffraction analysis and quantification of mineral phases based on the RockJock and on the QUAX quantitative analysis

We have performed quantitative X-ray diffraction (qXRD) analysis of 157 grab or core-top samples from the western Nordic Seas between (WNS) ~57°-75°N and 5° to 45° W. The RockJock Vs6 analysis includes non-clay (20) and clay (10) mineral species in the <2 mm size fraction that sum to 100 weight %. The data matrix was reduced to 9 and 6 variables respectively by excluding minerals with low weight% and by grouping into larger groups, such as the alkali and plagioclase feldspars. Because of its potential dual origins calcite was placed outside of the sum. We initially hypothesized that a combination of regional bedrock outcrops and transport associated with drift-ice, meltwater plumes, and bottom currents would result in 6 clusters defined by "similar" mineral compositions. The hypothesis was tested by use of a fuzzy k-mean clustering algorithm and key minerals were identified by step-wise Discriminant Function Analysis. Key minerals in defining the clusters include quartz, pyroxene, muscovite, and amphibole. With 5 clusters, 87.5% of the observations are correctly classified. The geographic distributions of the five k-mean clusters compares reasonably well with the original hypothesis. The close spatial relationship between bedrock geology and discrete cluster membership stresses the importance of this variable at both the WNS-scale and at a more local scale in NE Greenland.

Chemistry of altered basalts from ODP Hole 123-765D

Volcanic basement recovered at Hole 765D is characterized by nonpervasive, oxidative alteration, typical of seafloor weathering. Chilled margins and the mesostasis of the lavas are variably altered to assemblages of celadonite, Fe-oxyhydroxides, zeolites, and calcite with trace saponite. Plagioclase is partially altered to Ca-Na zeolites and/or albite. Well-developed alteration halos parallel fracture surfaces and extend several centimeters into the surrounding rock. These clay-rich halos are enriched in K2O and Fe2O3 relative to the adjacent clay-poor rock. The halos and adjacent rock are characterized by d18O values 2 per mil-3 per mil higher than those of fresh MORB. The "freshness" of the samples and the scarcity of saponite suggest that the duration of seawater circulation was short-lived. Albitization of plagioclase indicates that the volcanic rocks were altered initially at low temperatures and were subsequently reheated off-axis in a closed environment. Reheating did not result in significant modification of the bulk composition of the crust.

Mineral chemistry of basalts from DSDP Holes 65-483 and 65-483B

During DSDP Leg 65, a series of holes was drilled into the oceanic basement across the mouth of the Gulf of California to study the composition of the crust and the nature of its construction at a young spreading center. In Holes 483 and 483B, two of the deepest basement holes drilled on this leg, the basement is characterized by an upper sequence of interlayered massive basalts and sediments underlain by a lower sequence of interlayered pillow and massive basalts. Electron microprobe analyses were performed on pyroxene, plagioclase, olivine, spinel, and glass from 14 representative samples of 10 of the 16 major lithologic units. These analyses along with petrographic results can be used to interpret the detailed crystallization history of the basalts. We believe from the results of this study that the basalts were formed by at least a three-stage cooling process, followed by eruption and formation of quench phases. Our data do not support magma mixing.

Composition and alteration of volcanic rocks of ODP Hole 119-738C

During ODP Leg 119 one basement hole was drilled at Site 738, on the Southern Kerguelen Plateau. The 38.2 m of basement rocks drilled comprises three basaltic aa-lava flows with basal and top breccias, overlain by Turanian marine carbonates. Site 738 basalts probably erupted near a fracture zone, and were emplaced during the plateau-forming stage of Kerguelen Plateau evolution under quiet, subaerial to shallow water conditions. The basalts are T-MORB, chemically resembling Mesozoic continental flood basalts of the southern hemisphere. Two slightly different magma batches are distinguished by Fe, Ti, Al, Zr, and REE concentrations. Prior to eruption, the magmas had undergone significant olivine and some clinopyroxene fractionation. Incompatible and immobile trace element concentrations and ratios point to a veined upper mantle source, where a refractory mineral assemblage retains Nb, Ta, and the HREE. The basaltic melts derived from this regionally veined, enriched upper mantle have high LREE, and especially Ba and Th concentrations and bear the DUPAL isotopic signature gained from deep- seated, recycled, old oceanic(?) crust. A saponite-celadonite secondary mineral assemblage confines the alteration temperature to <170°C. Alteration is accompanied by net gains of H2O, CO2, K2O, and Rb, higher oxidation, minor Na2O, SiO2 gains, and losses of V and CaO. Released Ca, together with Ca from seawater, precipitated as calcite in veins and vesicles, plumbed the circulation system and terminated the rock/open seawater interaction.