TOC, carbonate, opal composition and trace element concentration from ODP Holes 175-1082A and 175-1084A

Distinctive light-dark color cycles in sediment beneath the Benguela Current Upwelling System indicate repetitive alternations in sediment delivery and deposition. Geochemical proxies for paleoproductivity and for depositional conditions were employed to investigate the paleoceanographic processes involved in creating these cycles in three mid-Pleistocene intervals from ODP Sites 1082 and 1084. Concentrations of total organic carbon (TOC) vary between 3.5 and 17.1%. Concentrations of CaCO3 vary inversely to TOC and Al, which suggests that both carbonate dissolution and terrigenous dilution contribute to the light-dark cycles. Opal concentrations are independent of both TOC and CaCO3, therefore eliminating diatom production and lateral transport of shelf material as causes of the light-dark cycles. d13Corg and d15Ntot values do not vary across light-dark sediment intervals, implying that the extent of relative nutrient utilization did not change. The stable d15Ntot values represent a balanced change in nitrate supply and export production and therefore indicate that productivity was elevated during deposition of the TOC-rich layers. Parallel changes in concentrations of indicator trace elements and TOC imply that changes in organic matter delivery influenced geochemical processes on the seafloor by controlling consumption of pore water oxygen. Cu, Ni, and Zn are enriched in the darker sediment as a consequence of greater organic matter delivery. Redox-sensitive metals vary due to loss (Mn and Ba) or enrichment (Mo) under reducing conditions created by TOC oxidation. Organic matter delivery impacts subsequent geochemical changes such as carbonate dissolution, sulfate reduction and the concentration of metals. Thus, export production is considered ultimately responsible for the generation of the color cycles.

Geochemistry of sediments in the central equatorial Pacific Ocean

In order to quantify changes in export production and carbonate dissolution over the past 1 Myr in the central equatorial Pacific Ocean we analyzed Ba, P, Al Ti, and Ca in 1106 samples from five piston cores gathered from 5°S to 4°N at 140°W. We focused on Ba/Ti, Al/Ti, and P/Ti ratios as export proxies and employed areally integrated time slice as well as time series strategies. Carbonate maxima from 0-560 kyr are characterized by 15-30% greater export than carbonate minima. The increases in export fall on glacial delta18O transitions rather than glacial maxima. From 560-800 kyr, overlapping with the mid-Pleistocene transition, there is a very large increase in total export yet no glacial-interglacial variability. The highest latitudes (5°S and 4°N) record minimal absolute export change from glacials to interglacials and yet record the most extreme minima in percent CaCO3, indicating that carbonate records there are dominated by dissolution, whereas near the equator they are more influenced by changes in export.

Chemical and mineral compositions of basalts from the Pacific Ocean, DSDP data

Mineral and chemical alterations of basalts were studied in the upper part of the ocean crust using data of deep-sea drilling from D/S Glomar Challenger in the main structures of the Pacific floor. Extraction of majority of chemical elements (including heavy metals) from basalts results mainly from their interaction with heated sea water. As a result mineralized hydrothermal solutions are formed. On entering the ocean they influence greatly on ocean sedimentation and ore formation.

Geochemistry of Pleistocene volcanic rocks in the Mariana Forearc

Geophysical surveys of the Mariana forearc, in an area equidistant from the Mariana Trench and the active Mariana Island Arc, revealed a 40-m-deep graben about 13 km northwest of Conical Seamount, a serpentine mud volcano. The graben and its bounding horst blocks are part of a fault zone that strikes northwest-southeast beneath Conical Seamount. One horst block was drilled during Leg 125 of the Ocean Drilling Program (Site 781). Three lithologic units were recovered at Site 781: an upper sedimentary unit, a middle basalt unit, and a lower sedimentary unit. The upper unit, between 0 and 72 mbsf, consists of upper Pliocene to Holocene diatomaceous and radiolarian-bearing silty clay that grades down into vitric silty clay and vitric clayey silt. The middle unit is a Pleistocene vesicular, porphyritic basalt, the top of which corresponds to a high-amplitude reflection on the reflection profiles. The lower unit is a middle to upper (and possibly some lower) Pliocene vitric silty clay and vitric clayey silt similar to the lower part of the upper unit. The thickness of the basalt unit can only be estimated to be between 13 and 25 m because of poor core recovery (28% to 55%). The absence of internal flow structures and the presence of an upper glassy chilled zone and a lower, fine-grained margin suggest that the basalt unit is either a single lava flow or a near-surface sill. The basalt consists of plagioclase phenocrysts with subordinate augite and olivine phenocrysts and of plagioclase-augite-olivine glomerocrysts in a groundmass of plagioclase, augite, olivine, and glass. The basalt is an island arc tholeiite enriched in large-ion-lithophile elements relative to high-field-strength elements, similar to the submarine lavas of the southern arc seamounts. In contrast, volcanic rocks from the active volcanoes on Pagan and Agrigan islands, 100 km to the west of the drill site, are calc-alkaline. The basalt layer, the youngest in-situ igneous layer reported from the Izu-Bonin and Mariana forearcs, is enigmatic because of its location more than 100 km from the active volcanic arc. The sediment layers above and below the basalt unit are late Pliocene in age (about 2.5 Ma) and normally magnetized. The basalt has schlierenlike structures, reverse magnetization, and a K-Ar age of 1.68±0.37 Ma. Thus, the basalt layer is probably a sill fed by magma intruded along a fault zone bounding the horst and graben in the forearc. The geochemistry of the basalt is consistent with a magma source similar to that of the active island arc and from a mantle source above the subducting Pacific plate.

Geochemistry of sediments beneath the Tonga-Kermadec arc

The fate of subducted sediment and the extent to which it is dehydrated and/or melted before incorporation into arc lavas has profound implications for the thermo-mechanical nature of the mantle wedge and models for crustal evolution. In order to address these issues, we have undertaken the first measurements of 10Be and light elements in lavas from the Tonga-Kermadec arc and the sediment profile at DSDP site 204 outboard of the trench. The 10Be/9Be ratios in the Tonga lavas are lower than predicted from flux models but can be explained if (a) previously estimated sediment contributions are too high by a factor of 2-10, (b) the top 1-22 m of the incoming sediment is accreted, (c) large amounts of sediment erosion are proposed, or (d) the sediment component takes several Myr longer than the subducting plate to reach the magma source region beneath Tonga. The lavas form negative Th/Be-Li/Be arrays that extend from a depleted mantle source composition to lower Th/Be and Li/Be ratios than that of the bulk sediment. Thus, these arrays are not easily explained by bulk sediment addition and, using partition coefficients derived from experiments on the in-coming sediment, we show that they are also unlikely to result from fluid released during dehydration of the sediment (or altered oceanic crust). However, partial melts of the dehydrated sediment residue formed at ~800 °C during the breakdown of amphibole +/- plagioclase and in the absence of cordierite have significantly lowered Th/Be ratios. The lava arrays can be successfully modelled as 10-15% partial melts of depleted mantle after it has been enriched by the addition of 0.2-2% of these partial melts. Phase relations suggest that this requires that the top of the subducting crust reaches temperatures of ~800 °C by the time it attains ~ 80 km depth which is in excellent agreement with the results of recent numerical models incorporating a temperature-dependent mantle viscosity. Under these conditions the wet basalt solidus is also crossed yet there is no recognisable eclogitic signal in the lavas suggesting that on-going dehydration or strong thermal gradients in the upper part of the subducting plate inhibit partialmelting of the altered oceanic crust.

Major components and major and trace elements from ODP Site 175-1085

We have analyzed 33 Pliocene bulk sediment samples from Ocean Drilling Program Site 1085 in the Cape Basin, located offshore of western Africa in the Angola-Benguela Current system, for 17 major and trace elements, and interpreted their associations and temporal variations in the context of an allied data set of CaCO3, opal, and Corg. We base our interpretations on elemental ratios, accumulation rates, inter-element correlations, and several multi-element statistical techniques. On the basis of qualitative assessment of downhole changes in the distributions of P and Ba, utilized as proxies of export production, we conclude that highs in bulk and biogenic accumulation that occur at 3.2 Ma, 3.0 Ma, 2.4 Ma, and 2.25 Ma were caused by increases in export production as well as terrigenous flux, and record a greater sequestering of organic matter during these time periods. Studies of refractory elements and other indicator proxies (SiO2, Al2O3, TiO2, Fe2O3, MgO, V, Cr, Sr, and Zr) strongly suggest that the terrigenous component of the bulk sediment is composed of two compositional end-members, one being 'basaltic' in composition and the other similar to an 'average shale'. The basaltic end-member comprises approximately 10-15% of the total bulk sediment and its presence is consistent with the local geology of source material in the drainage basin of the nearby Orange River. The increase in bulk accumulation at 2.4 Ma appears to reflect a greater relative increase in basaltic input than the relative increase in shale-type input. Although studies such as this cannot precisely identify the transport mechanisms of the different terrigenous components, these results are most consistent with variations in sea level (and associated changes in shelf geometry and fluvial input) being responsible for the changing depositional conditions along the Angolan Margin during this time period.