Physico-chemical parameters of sediment core PG1972-1 (09-Tik-03) from a Tundra Lake, Northeastern Siberia

Thermokarst lakes in the Siberian Arctic contain sediment archives that can be used for paleoenvironmental inference. Until now, however, there has been no study from the inner Lena River Delta with a focus on diatoms. The objective of this study was to investigate how the diatom community in a thermokarst lake responded to past limnogeological changes and what specific factors drove variations in the diatom assemblage. We analysed fossil diatom species, organic content, grain-size distribution and elemental composition in a sediment core retrieved in 2009 from a shallow thermokarst lake in the Arga Complex, western Lena River Delta. The core contains a 3,000-year record of sediment accumulation. Shifts in the predominantly benthic and epiphytic diatom species composition parallel changes in sediment characteristics. Paleoenvironmental and limnogeological development, inferred from multiple biological and sedimentological variables, are discussed in the context of four diatom zones, and indicate a strong relation between changes in the diatom assemblage and thermokarst processes. We conclude that limnogeological and thermokarst processes such as lake drainage, rather than direct climate forcing, were the main factors that altered the aquatic ecosystem by influencing, for example, habitat availability, hydrochemistry, and water level.

Chemistry of smectite and volcanic glass of ODP Site 159-959

The Ocean Drilling Program (ODP) Site 959 was drilled in the northern border of the Côte d'Ivoire-Ghana Ridge at a water depth of 2100 m. Pleistocene total thickness does not exceed 20 m. Winnowing processes resulted in a low accumulation rate and notable stratigraphic hiatuses. During the Late Pleistocene, bottom circulation was very active and controlled laminae deposition (contourites) which increased the concentration of glauconitic infillings of foraminifera, and of volcanic glass and blue-green grains more rarely, with one or several subordinate ferromagnesian silicates. Volcanic glass generally was X-ray amorphous and schematically classified as basic to intermediate (44-60% SiO2). Opal-A or opal-CT suggested the beginning of the palagonitisation process, and previous smectitic deposits may have been eroded mechanically. The blue-green grains presented two main types of mineralogic composition: (1) neoformed K, Fe-smectite associated with zeolite (like phillipsite) and unequal amounts of quartz and anorthite; (2) feldspathic grains dominated by albite but including quartz, volcanic glass and smectites as accessory components. They were more or less associated with the volcanic glass. On the basis of their chemical composition, the genetic relationship between the blue-green grains and the volcanic glass seemed to be obvious although some heterogeneous grains seemed to be primary ignimbrite and not the result of glass weathering. The most reasonable origin of these pyroclastic ejecta would be explosive events from the Cameroon Volcanic Ridge, especially from the Sao Thome and Principe Islands and Mount Cameroon area. This is supported both by grain geochemistry and the time of volcanic activity, i.e. Pleistocene. After westward wind transport (some 1200 km) and ash fall-out, the subsequent winnowing by bottom currents controlled the concentration of the volcanic grains previously disseminated inside the hemipelagic sediment. Palagonitisation, and especially phillipsite formation, may result from a relatively rapid reaction during burial diagenesis (<1 m.y.), in deep-sea deposits at relatively low sedimentation rate. However, it cannot be excluded that the weathering had begun widely on the Cameroon Ridge before the explosive event.

Mineralogy and diagenesis of slope sediments at DSDP Leg 84 holes

The distribution and composition of minerals in the silt and clay fraction of the fine-grained slope sediments were examined. Special interest was focused on diagenesis. The results are listed as follows. (1) Smectite, andesitic Plagioclase, quartz, and low-Mg calcite are the main mineral components of the sediment. Authigenic dolomite was observed in the weathering zones of serpentinites, together with aragonite, as well as in clayey silt. (2) The mineralogy and geochemistry of the sediments is analogous to that of the andesitic rocks of Costa Rica and Guatemala. (3) Unstable components like volcanic glass, amphiboles, and pyroxenes show increasing etching with depth. (4) The diagenetic alteration of opal-A skeletons from etching pits and replacement by opal-CT to replacement by chalcedony as a final stage corresponds to the typical opal diagenesis. (5) Clinoptilolite is the stable zeolite mineral according to mineral stability fields; its neoformation is well documented. (6) The early diagenesis of smectites is shown by an increase of crystallinity with depth. Only the smectites in the oldest sediments (Oligocene and early Eocene) contain nonexpanding illite layers.

Major oxides, trace elements and rare earth elements of selected basalt samples at DSDP Hole 83-504B

DSDP Hole 504B is the deepest section drilled into oceanic basement, penetrating through a 571.5-m lava pile and a 209-m transition zone of lavas and dikes into 295 m of a sheeted dike complex. To define the basement composition 194 samples of least altered basalts, representing all lithologic units, were analyzed for their major and 26 trace elements. As is evident from the alteration-sensitive indicators H2O+, CO2, S, K, Mn, Zn, Cu, and the iron oxidation ratio, all rocks recovered are chemically altered to some extent. Downhole variation in these parameters enables us to distinguish five depth-related alteration zones that closely correlate with changes in alteration mineralogy. Alteration in the uppermost basement portion is characterized by pronounced K-uptake, sulfur loss, and iron oxidation and clearly demonstrates low-temperature seawater interaction. A very spectacular type of alteration is confined to the depth range from 910 to 1059 m below seafloor (BSF). Rocks from this basement portion exhibit the lowest iron oxidation, the highest H2O+ contents, and a considerable enrichment in Mn, S, Zn, and Cu. At the top of this zone a stockwork-like sulfide mineralization occurs. The chemical data suggest that this basement portion was at one time within a hydrothermal upflow zone. The steep gradient in alteration chemistry above this zone and the ore precipitation are interpreted as the result of mixing of the upflowing hydrothermal fluids with lower-temperature solutions circulating in the lava pile. Despite the chemical alteration the primary composition and variation of the rocks can be reliably established. All data demonstrate that the pillow lavas and the dikes are remarkably uniform and display almost the same range of variation. A general characteristic of the rocks that classify as olivine tholeiites is their high MgO contents (up to 10.5 wt.%) and their low K abundances (-200 ppm). According to their mg-values, which range from 0.60 to 0.74, most basalts appear to have undergone some high-level crystal fractionation. Despite the overall similarity in composition, there are two major basalt groups that have significantly different abundances and ratios of incompatible elements at similar mg-values. The majority of the basalts from the pillow lava and dike sections are chemically closely related, and most probably represent differentiation products of a common parental magma. They are low in Na2O, TiO2, and P2O5, and very low in the more hygromagmaphile elements. Interdigitated with this basalt group is a very rarely occurring basalt that is higher in Na2O, TiO2, P2O5, much less depleted in hygromagmaphile elements, and similar to normal mid-ocean ridge basalt (MORB). The latter is restricted to Lithologic Units 5 and 36 of the pillow lava section and Lithologic Unit 83 of the dike section. The two basalt groups cannot be related by differentiation processes but have to be regarded as products of two different parental magmas. The compositional uniformity of the majority of the basalts suggests that the magma chamber beneath the Costa Rica Rift reached nearly steady-state conditions. However, the presence of lavas and dikes that crystallized from a different parental magma requires the existence of a separate conduit-magma chamber system for these melts. Occasionally mixing between the two magma types appears to have occurred. The chemical characteristics of the two magma types imply some heterogeneity in the mantle source underlying the Costa Rica Rift. The predominant magma type represents an extremely depleted source, whereas the rare magma type presumably originated from regions of less depleted mantle material (relict or affected by metasomatism).

Major and trace element abundances of igneous rocks from DSDP Site 37-334

REE abundances in gabbros and peridotites from Site 334 of DSDP Leg 37 show that these rocks are cumulates produced by fractional crystallization of a primitive oceanic tholeiite magma. They may be part of a layered oceanic complex. The REE distributions in the residual liquids left after such a fractionation are similar to those of incompatible element-depleted oceanic tholeiites. The REE data indicate that the basalts which overlie the gabbro-peridotite complex, are not genetically related to plutonic rocks.

Chemical composition, characteristics and age of glauconites at DSDP Leg 66 Holes

During the analysis of "glaucony" recovered during Leg 66, off Mexico, we reviewed the data on previously studied glaucony layers in active margin areas. We found the depth of Leg 66 glaucony sediments to be significantly greater than conventionally assumed appropriate to their genesis (100-500 m). Accordingly, we hypothesize their occurrence at unusual depth to be due to (1) transport of shallow sediments and redeposition at greater depths, (2) margin subsidence, or (3) genesis at greater depth than is generally assumed. For the area off Mexico, we reject (1). (2) has already been verified in Japan and is possible as an explanation for the present phenomenon without excluding (3), which we investigate in this chapter.

Geochemistry of basalts fropm DSDP Leg 65 basalts

Samples of basalt collected on Leg 65 near 22°N on the East Pacific Rise all display the depleted light rare-earth pattern of "normal" oceanic crust. Consequently the La/Ta ratio is close to 18, as opposed to the value of 9 associated with the flat or enriched patterns found along parts of the Mid-Atlantic Ridge and the Emperor Seamount chain. The Leg 65 samples are chemically similar to those from the CYAMEX area at 21 °N and to the Leg 54 samples from 9°N, suggesting homogeneity of the upper mantle under the northern part of the East Pacific Rise over a minimum distance of about 1500 km. The geochemistry of the rocks and their field relationships with respect to depth and distance from the axis of the Rise show no pattern of distribution linked to the degree of fractional crystallization and thus cast doubt on any possible model involving large, long-lived magma chambers at the axis of the Rise.