Radiolarian biostratigraphy in the central Indian Ocean

Identifiable radiolarians of stratigraphic importance were recovered at eight of the sites drilled on Leg 115. The assemblages range in age from Holocene to middle Eocene (Dictyoprora mongolfieri Zone, about 48 Ma). Faunal preservation is particularly good in two stratigraphic intervals: the Holocene through upper Miocene (0-9 Ma), and the lowermost Oligocene to middle Eocene (35-48 Ma). Fluctuating rates of silica accumulation at these drill sites during the Cenozoic reflect changing tectonic and paleoceanographic conditions. In particular, the gradual closure of the Indonesian and Tethyan seaways and the northward migration of the Indian subcontinent severely restricted zonal circulation and silica accumulation in tropical latitudes during the late Oligocene through middle Miocene. By the late Miocene the Indian subcontinent had moved sufficiently north of the equator to allow trans-Indian zonal circulation patterns to become reestablished, and biosiliceous sedimentation resumed. The composition of the radiolarian assemblages in the tropical Indian Ocean is closely comparable with that of the 'stratotype' sequences in the equatorial Pacific. However, there are some notable exceptions in Indian Ocean assemblages: (1) the scarcity of the genera Pterocanium and Spongaster in the Neogene; (2) the absence of the stratigraphically important Podocyrtis lineage, P. diamesa -> P. phyxis -> P. ampla, in the middle Eocene; and (3) the scarcity of taxa of the genus Dorcadospyris, with the exception of D. ateuchus. The succession of radiolarian events was tabulated for those stratigraphic intervals where the assemblages were well preserved. We identified 55 events in the middle Eocene to earliest Oligocene, and 31 events in the late Miocene to Holocene. The succession of events is closely comparable with that of the tropical Pacific. However, there are exceptions that appear to be real, rather than artifacts of sample preservation, mixing, and core disturbance.

Methane concentration in sediment cores during METEOR cruise M92

We studied the concurrence of methanogenesis and sulfate reduction in surface sediments (0-25 cm below sea floor, cmbsf) at six stations (70, 145, 253, 407, 770 and 1024 m) along the Peruvian margin (12° S). This oceanographic region is characterized by high carbon export to the seafloor, creating an extensive oxygen minimum zone (OMZ) on the shelf, both factors that could favor surface methanogenesis. Sediments sampled along the depth transect traversed areas of anoxic and oxic conditions in the bottom-near water. Net methane production (batch incubations) and sulfate reduction (35S-sulfate radiotracer incubation) were determined in the upper 0-25 cmbsf of multicorer cores from all stations, while deep hydrogenotrophic methanogenesis (> 30 cmbsf, 14C-bicarbonate radiotracer incubation) was determined in two gravity cores at selected sites (78 and 407 m). Furthermore, stimulation (methanol addition) and inhibition (molybdate addition) experiments were carried out to investigate the relationship between sulfate reduction and methanogenesis. Highest rates of methanogenesis and sulfate reduction in the surface sediments, integrated over 0-25 cmbsf, were observed on the shelf (70-253 m, 0.06-0.1 and 0.5-4.7 mmol m-2 d-1, respectively), while lowest rates were discovered at the deepest site (1024 m, 0.03 and 0.2 mmol m-2 d-1, respectively). The addition of methanol resulted in significantly higher surface methanogenesis activity, suggesting that the process was mostly based on non-competitive substrates, i.e., substrates not used by sulfate reducers. In the deeper sediment horizons, where competition was probably relieved due to the decline of sulfate, the usage of competitive substrates was confirmed by the detection of hydrogenotrophic activity in the sulfate-depleted zone at the shallow shelf station (70 m). Surface methanogenesis appeared to be correlated to the availability of labile organic matter (C / N ratio) and organic carbon degradation (DIC production), both of which support the supply of methanogenic substrates. A negative correlation of methanogenesis rates with dissolved oxygen in the bottom-near water was not obvious, however, anoxic conditions within the OMZ might be advantageous for methanogenic organisms at the sediment-water interface. Our results revealed a high relevance of surface methanogenesis on the shelf, where the ratio between surface to deep (below sulfate penetration) methanogenic activity ranged between 0.13 and 105. In addition, methane concentration profiles indicate a partial release of surface methane into the water column as well as a partial consumption of methane by anaerobic methane oxidation (AOM) in the surface sediment. The present study suggests that surface methanogenesis might play a greater role in benthic methane budgeting than previously thought, especially for fueling AOM above the sulfate-methane transition zone.

Geochemistry and stratigraphy on sediment cores from Leg198 and Leg 208

The first complete cyclic sedimentary successions for the early Paleogene from drilling multiple holes have been retrieved during two ODP expeditions: Leg 198 (Shatsky Rise, NW Pacific Ocean) and Leg 208 (Walvis Ridge, SE Atlantic Ocean). These new records allow us to construct a comprehensive astronomically calibrated stratigraphic framework with an unprecedented accuracy for both the Atlantic and the Pacific Oceans covering the entire Paleocene epoch based on the identification of the stable long-eccentricity cycle (405-kyr). High resolution X-ray fluorescence (XRF) core scanner and non-destructive core logging data from Sites 1209 through1211 (Leg 198) and Sites 1262, 1267 (Leg 208) are the basis for such a robust chronostratigraphy. Former investigated marine (ODP Sites 1001 and 1051) and land-based (e.g., Zumaia) sections have been integrated as well. The high-fidelity chronology is the prerequisite for deciphering mechanisms in relation to prominent transient climatic events as well as completely new insights into Greenhouse climate variability in the early Paleogene. We demonstrate that the Paleocene epoch covers 24 long eccentricity cycles. We also show that no definite absolute age datums for the K/Pg boundary or the Paleocene - Eocene Thermal Maximum (PETM) can be provided by now, because of still existing uncertainties in orbital solutions and radiometric dating. However, we provide two options for tuning of the Paleocene which are only offset by 405-kyr. Our orbitally calibrated integrated Leg 208 magnetostratigraphy is used to revise the Geomagnetic Polarity Time Scale (GPTS) for Chron C29 to C25. We established a high-resolution calcareous nannofossil biostratigraphy for the South Atlantic which allows a much more detailed relative scaling of stages with biozones. The re-evaluation of the South Atlantic spreading rate model features higher frequent oscillations in spreading rates for magnetochron C28r, C27n, and C26n.

Chemical composition of basalts from the Atlantic Ocean

Basalt recovered beneath Jurassic sediments in the western Atlantic at Deep Sea Drilling Project sites 100 and 105 of leg 11 has petrographic features characteristic of water-quenched basalt extruded along modern ocean ridges. Site 100 basalt appears to represent two or three massive cooling units, and an extrusive emplacement is probable. Site 105 basalt is less altered and appears to be a compositionally homogeneous pillow lava sequence related to a single eruptive episode. Although the leg 11 basalts are much more closely related in time to the Triassic lavas and intrusives of eastern continental North America, their geochemical features are closely comparable to those of modern Mid-Atlantic Ridge basalts unrelated to postulated "mantle plume" activity. Projection of leg 11 sites back along accepted spreading "flow lines" to their presumed points of origin shows that these origins are also outside the influence of modern" plume" activity. Thus, these oldest Atlantic seafloor basalts provide no information on the time of initiation of these "plumes". The Triassic continental diabases show north to south compositional variations in Rb, Ba, La, and Sr which lie within the range of " plume "-related basalt on the Mid-Atlantic Ridge (20° - 40° N) This suggests that these diabases had mantle sources similar in composition to those beneath the present Mid-Atlantic Ridge. "Plumes" related to deep mantle sources may have contributed to the LIL-element enrichment in the Triassic diabase and may also have been instrumental in initiating the rifting of the North Atlantic. Systematically high values for K and Sr87/Sr86 in the Triassic diabases may reflect superimposed effects of crustal contamination in the Triassic magmas.

Geochemical studies of surface sediment samples off North-West Africa

One hundred and twenty-eight surface-sediment samples collected off North-West Africa were studied geochemically to detect the expressions of different meridional climate regimes and zonal productivity gradients in the surface sediments. This geochemical multi-tracer approach, coupled with additional information on the bulk carbonate and TOC contents makes it possible to characterise the sedimentological regime in detail. Typical terrigenous elements like Al, K and Fe mirror the importance of the humid (fluvial) influence in the north of the study area and the dominance of aeolian input in the south. Furthermore, the distributions of Ti and Fe in the surface sediments serve as tracers for the supply of eolian volcanic material from the Canary Islands. The spatial variability of the TOC contents in the surface sediments closely follows the ocean surface productivity patterns, whereas the CaCO3 contents are mainly controlled by dilution with terrigenous matter. The potential productivity proxy Ba is not a reliable tracer for productivity in this region, since it is mainly supplied by terrigenous input (coupled with aluminosilicates).

Radiocarbon ages and 230Th data of five sediment cores from the Björn and Gardar Drift, North Atlantic

In locations of rapid sediment accumulation receiving substantial amounts of laterally transported material the timescales of transport and accurate quantification of the transported material are at the focus of intense research. Here we present radiocarbon data obtained on co-occurring planktic foraminifera, marine haptophyte biomarkers (alkenones) and total organic carbon (TOC) coupled with excess Thorium-230 (230Thxs) measurements on four sediment cores retrieved in 1649-2879 m water depth from two such high accumulation drift deposits in the Northeast Atlantic, Björn and Gardar Drifts. While 230Thxs inventories imply strong sediment focussing, no age offsets are observed between planktic foraminifera and alkenones, suggesting that redistribution of sediments is rapid and occurs soon after formation of marine organic matter, or that transported material contains negligible amounts of alkenones. An isotopic mass balance calculation based on radiocarbon concentrations of co-occurring sediment components leads us to estimate that transported sediment components contain up to 12% of fossil organic matter that is free of or very poor in alkenones, but nevertheless appears to consist of a mixture of fresh and eroded fossil material. Considering all available constraints to characterize transported material, our results show that although focussing factors calculated from bulk sediment 230Thxs inventories may allow useful approximations of bulk redeposition, they do not provide a unique estimate of the amount of each laterally transported sediment component. Furthermore, our findings provide evidence that the occurrence of lateral sediment redistribution alone does not always hinder the use of multiple proxies but that individual sediment fractions are affected to variable extents by sediment focussing.