Stable carbon and oxygen isotope ratios of Late Miocene and Early Pliocene foraminifera from the Southwest Pacific

The Late Miocene-Early Pliocene paleoclimatic history has been evaluated for a deep drilled sediment sequence at Deep Sea Drilling Project Site 281 and a shallow water marine sediment sequence at Blind River, New Zealand, both of which lay within the Subantarctic water mass during the Late Miocene. A major, faunally determined, cooling event within the latest Miocene at Site 281 and Blind River coincides with oxygen isotopic changes in benthonic foraminiferal composition at DSDP Site 284 considered by Shackleton and Kennett (1975) to indicate a significant increase in Antarctic ice sheet volume. However, at Site 281 benthonic foraminiferal oxygen isotopic changes do not record such a large increase in Antarctic ice volume. It is possible that the critical interval is within an unsampled section (no recovery) in the latest Miocene. Two benthonic oxygen isotopic events in the Late Miocene (0.5 ? and 1 ? in the light direction) may be useful as time-stratigraphic markers. A permanent, negative, carbon isotopic shift at both Site 281 and Blind River allows precise correlations to be made between the two sections and to other sites in the Pacific region. Close interval sampling below the carbon shift at Site 281 revealed dramatic fluctuations in surface-water temperatures prior to a latest Miocene interval of refrigeration (Kapitean) and a strong pulse of dissolution between 6.6 and 6.2 +/- 0.1 m.y. which may be related to a fundamental geochemical change in the oceans at the time of the carbon shift (6.3-6.2 m.y.). No similar close interval sampling at Blind River was possible because of a lack of outcrop over the critical interval. Paleoclimatic histories from the two sections are very similar. Surface water temperatures and Antarctic ice-cap volume appear to have been relatively stable during the late Middle-early Late Miocene (early-late Tongaporutuan). By 6.4 m.y. cooler conditions prevailed at Site 281. Between 6.3 and 6.2 -+ 0.1 m.y. the carbon isotopic shift occurred followed, within 100,000 yr, by a distinct shallowing of water depths at Blind River. The earliest Pliocene (Opoitian) is marked by increasing surface-water temperatures.

Stable oxygen isotope ratios of marine barite from Cenozoic sediments

We report new data on oxygen isotopes in marine sulfate (delta18O[SO4]), measured in marine barite (BaSO4), over the Cenozoic. The delta18O[SO4] varies by 6x over the Cenozoic, with major peaks 3, 15, 30 and 55 Ma. The delta18O[SO4] does not co-vary with the delta18O[SO4], emphasizing that different processes control the oxygen and sulfur isotopic composition of sulfate. This indicates that temporal changes in the delta18O[SO4] over the Cenozoic must reflect changes in the isotopic fractionation associated with the sulfide reoxidation pathway. This suggests that variations in the aerial extent of different types of organic-rich sediments may have a significant impact on the biogeochemical sulfur cycle and emphasizes that the sulfur cycle is less sensitive to net organic carbon burial than to changes in the conditions of that organic carbon burial. The delta18O[SO4] also does not co-vary with the d18O measured in benthic foraminifera, emphasizing that oxygen isotopes in water and sulfate remain out of equilibrium over the lifetime of sulfate in the ocean. A simple box model was used to explore dynamics of the marine sulfur cycle with respect to both oxygen and sulfur isotopes over the Cenozoic. We interpret variability in the delta18O[SO4] to reflect changes in the aerial distribution of conditions within organic-rich sediments, from periods with more localized, organic-rich sediments, to periods with more diffuse organic carbon burial. While these changes may not impact the net organic carbon burial, they will greatly affect the way that sulfur is processed within organic-rich sediments, impacting the sulfide reoxidation pathway and thus the delta18O[SO4]. Our qualitative interpretation of the record suggests that sulfate concentrations were probably lower earlier in the Cenozoic.

Alkenone variations, total organic carbon and stable oxygen isotope ratios of the western Arabian Sea

We here present records of total organic carbon (TOC) and C37 alkenones, used as indicators for past primary productivity, from the western (WAS) and eastern Arabian Sea (EAS). New data from an open ocean site of the WAS upwelling area are compared with similar records from Ocean Drilling Program (ODP) Site 723 from the continental margin off Oman and MD 900963 from the EAS. These records together with other proxies used to reconstruct upwelling intensity, indicate periods of high productivity in tune with precessional forcing. On the basis of their phase relationship to boreal summer insolation they can be divided into three groups: in the WAS differences between monsoonal proxies (1) and productivity (2) document a combined signal of moderate SW monsoon winds and of strengthened and prolonged NE monsoon winds, whereas in the EAS phasing indicates maximum productivity (3) at times of stronger NE monsoon winds associated with precession-related maxima in ice volume.

Stable carbon and oxygen isotope ratios of foraminifera from Holocene sediments of the North Atlantic

The Denmark Strait Overflow (DSO) today compensates for the northward flowing Norwegian and Irminger branches of the North Atlantic Current that drive the Nordic heat pump. During the Last Glacial Maximum (LGM), ice sheets constricted the Denmark Strait aperture in addition to ice eustatic/isostatic effects which reduced its depth (today ~630 m) by ~130 m. These factors, combined with a reduced north-south density gradient of the water-masses, are expected to have restricted or even reversed the LGM DSO intensity. To better constrain these boundary conditions, we present a first reconstruction of the glacial DSO, using four new and four published epibenthic and planktic stable-isotope records from sites to the north and south of the Denmark Strait. The spatial and temporal distribution of epibenthic delta18O and delta13C maxima reveals a north-south density gradient at intermediate water depths from sigma0 ~28.7 to 28.4/28.1 and suggests that dense and highly ventilated water was convected in the Nordic Seas during the LGM. However, extremely high epibenthic delta13C values on top of the Mid-Atlantic Ridge document a further convection cell of Glacial North Atlantic Intermediate Water to the south of Iceland, which, however, was marked by much lower density (sigma0 ~28.1). The north-south gradient of water density possibly implied that the glacial DSO was directed to the south like today and fed Glacial North Atlantic Deep Water that has underthrusted the Glacial North Atlantic Intermediate Water in the Irminger Basin.

Stable isotope record and trace element ratios of Eocene and Oligocene foraminifers from ODP Site 120-748

Stable carbon and oxygen isotope analyses were conducted on well-preserved planktonic and benthic foraminifers from a continuous middle Eocene to Oligocene sequence at Ocean Drilling Program (ODP) Site 748 on the Kerguelen Plateau. Benthic foraminifer d18O values show a 1.0 per mil increase through the middle and upper Eocene, followed by a rapid 1.2 per mil increase in the lowermost Oligocene (35.5 Ma). Surface-dwelling planktonic foraminifer d18O values increase in the lowermost Oligocene, but only by 0.6 per mil whereas intermediate-depth planktonic foraminifers show an increase of about l.0 per mil. Benthic foraminifer d13C values increase by 0.9 per mil in the lowermost Oligocene at precisely the same time as the large d18O increase, whereas planktonic foraminifer d13C values show little or no change. Site 748 oxygen isotope and paleontological records suggest that southern Indian Ocean surface and intermediate waters underwent significant cooling from the early to late Eocene. The rapid 1.2 per mil oxygen isotope increase recorded by benthic foraminifers just above the Eocene/Oligocene boundary represents the ubiquitous early Oligocene d18O event. The shift here is unique, however, as it coincided with the sudden appearance of ice-rafted debris (IRD), providing the first direct link between Antarctic glacial activity and the earliest Oligocene d18O increase. The d18O increase caused by the ice-volume change in the early Oligocene is constrained by (1) related changes in the planktonic to benthic foraminifer d18O gradient at Site 748 and (2) comparisons of late Eocene and early Oligocene planktonic foraminifer d18Ovalues from various latitudes. Both of these records indicate that 0.3 per mil to 0.4 per mil of the early Oligocene d18O increase was ice-volume related.

Stable carbon and oxygen isotope ratios of carbonates of ODP Site 103-639

A Tithonian sequence of shallow-water limestones, intercalated with siliciclastics and overlain by dolomite, was recovered during drilling at ODP Site 639 on the edge of a tilted fault block. The carbonates were strongly affected by fracturing, dolomitization, dedolomitization, and compaction. The chronology and nature of the fractures, fracture infilling, and diagenesis of the host rock are established and correlated for both the limestone and the dolomite. A first phase of dolomitization affected limestone that was already, at least partially, indurated. In the limestone unit, fractures were filled by calcite and dolomite; most of the dolomite was recrystallized into calcite, except for the upper part. In the dolomitic unit, the first-formed dolomite was progressively recrystallized into saddle dolomite, as fractures were simultaneously activated. The dolomitic textures become less magnesian (the molar ratio mMg/mCa goes from 1.04-0.98 to 0.80), and the d18O (PDB) ranges from -10 per mil to -8 per mil. The varying pores and fissures are either cemented by a calcic saddle dolomite (mMg/mCa ranging from 0.95 to 0.80) or filled with diverse internal sediments of detrital calcic dolomite, consisting of detrital dolomite silt (d18O from -9 per mil to -7 per mil) and laminated yellow filling (with different d18O values that range from -4 per mil to +3 per mil). These internal sediments clearly contain elements of the host rock and fragments of saddle crystals. They are covered by marls with calpionellids of early Valanginian age, which permits dating of most of the diagenetic phases as pre-Valanginian. The dolomitization appears to be related to fracturing resulting from extensional tectonics; it is also partially related to an erosional episode. Two models of dolomitization can be proposed from the petrographic characteristics and isotopic data. Early replacement of aragonite bioclasts by sparite, dissolution linked to dolomitization, and negative d18O values of dolomite suggest a freshwater influence and 'mixing zone' model. On the other hand, the significant presence of saddle dolomite and repeated negative d18O values suggest a temperature effect; because we can dismiss deep burial, hydrothermal formation of dolomite would be the most probable model. For both of these hypotheses, the vadose filling of cavities and fractures by silt suggests emersion, and the different, and even positive, d18O values of the last-formed yellow internal sediment could suggest dolomitization of the top of the sequence under saline to hypersaline conditions. Fracturing resulting in the reopening of porosity and the draining of dolomitizing fluids was linked to extensional tectonics prior to the tilting of the block. These features indicate an earlier beginning to the rifting of the Iberian margin than previously known. Dolomitization, emersion, and erosion correspond to eustatic sea-level lowering at the Berriasian/Valanginian boundary. Diagenesis, rather than sedimentation, seems to mark this global event and to provide a record of the regional tectonic history.