Siliceous phytoplankton in middle Eocene sediments of ODP Hole 207-1260A

The Middle Eocene diatom and silicoflagellate record of ODP Site 1260A (Demerara Rise) is studied quantitatively in order to throw light on the changes that siliceous phytoplankton communities experienced during a Middle Eocene warming event that occurred between 44.0 and 42.0 Ma. Both Pianka's overlap index, calculated per couple of successive samples, and cluster analysis, point to a number of significant turnover events highlighted by changes in the structure of floristic communities. The pre-warming flora, dominated by cosmopolitan species of the diatom genus Triceratium, is replaced during the warming interval by a new and more diverse assemblage, dominated by Paralia sulcata (an indicator of high productivity) and two endemic tropical species of the genus Hemiaulus. The critical warming interval is characterized by a steady increase in biogenic silica and a comparable increase in excess Ba, both reflecting an increase in productivity. In general, it appears that high productivity not only increased the flux of biogenic silica, but also sustained a higher diversity in the siliceous phytoplankton communities. The microflora preserved above the critical interval is once again of low diversity and dominated by various species of the diatom genus Hemiaulus. All assemblages in the studied material are characterized by the total absence of continental and benthic diatoms and the relative abundance of neritic forms, suggesting a transitional depositional environment between the neritic and the oceanic realms.

Composition of palagonitized basalts, their alteration products, and palagonitized basaltic glasses

Detailed petrochemical and geochemical studies of two samples of palagonitized basalts collected from depths 3060 and 4800 m have shown that palagonitization of tholeiitic basalt is accompanied by intensive removal of Ca and Mg and some removal of SiO2 from rocks. Appreciable amount of K is added to rocks in this process. Behavior of Fe, Al, Ti, Cr, and Na is inert. Palagonitization of alkalic basalt is accompanied by loss of SiO2, Ca, and Na from rocks. Contents of K and Mg are not changed. Four stages can be discerned in alteration of basalts under deep-sea conditions: syngenetic and diffusional palagonitization, hydrothermal leaching, and underwater weathering. Crusts of Fe-Mn ores are formed through removal of Fe, Mn, Ni, Co, Sn, and Mo from rocks and sorption of Pb, Hg, Yb, La, Bi, W, and Be from sea water.

(Table 1, page 434-435) Bulk chemical composition of S.W. Pacific island arc Mn-oxide crusts

Hydrothermal Mn-oxide crusts have been removed from the Tonga-Kermadec Ridge, the first such hydrothermal deposits to be reported in the S.W. Pacific island arc. In several respects the deposits are similar to hydrothermal Mn-crusts from oceanic spreading centre settings. They are limited in areal extent, comprise well-crystalline birnessite and generally display extreme fractionation of Mn from Fe. They are strongly depleted in many elements compared to hydrogenous Mn deposits but are comparatively enriched in Li, Zn, Mo and Cd. The Group IA and Group IIA metals show strong intercorrelations and the behaviour of Mg in the purest samples may indicate the extent to which normal seawater has influenced the composition of the deposits. Certain aspects of the deposits are not typical of hydrothermal Mn deposits. In particular at least some of the crusts have developed on a sediment or unconsolidated talus substrate. Some crusts, or specific layers within some crusts, display a chemical composition which suggests a significant input from normal seawater.

Chemistry of interstitial waters of ODP Site 166-718

Preliminary data are presented on dissolved heavy metals in interstitial water samples collected at Site 718 of Ocean Drilling Program Leg 118. The heavy metals at this site are divided into three groups: Group I (B, K, Mn, Ni, Pb, total Si, total P, V) behaves like Mg, which decrease with depth; Group II (Ba, Cu, Sr, Ti) behaves like Ca, which increases with depth; and Group 111 (Cd, Co, Cr, Fe, Na, Mo, Zn) contains metals that are independent of depth. Mg decreases with depth from 50 mM at the seafloor to 21 mM at 900 mbsf. Mn in the sulfate reduction zone (1.0 to 2.8 ppm) is more highly concentrated than in the methane fermentation zone (0.23 to 0.50 ppm), except for Section 116-718-1H-1. A similar behavior is also observed for V and Pb. Ni, B, and K decrease non-uniformly with depth. Ca and Sr increase with depth at the same rates, indicating the dissolution of inorganic calcium carbonate by anaerobic oxidation of organic matter (Sayles, 1981, doi:10.1016/0016-7037(81)90132-0). The distribution of Ba with depth is very similar to those of Ca and Sr. Cu and Ti profiles trend to increase non-uniformly with depth. Fe is constant with depth. The sharp decrease in total silicate concentration at the seafloor probably indicates a decrease in the decomposition of siliceous biological matter (e.g., diatoms) and production of opal. The constant levels of Group 111, except for Na and Fe, may reveal equal sources of supply from surface seawater and the Himalayas over time.

Major, trace element and XRF data for sediments and fish tooth samples, from a shallow marine setting with varying redox conditions (Site U1356, Wilkes Land Margin, E.Antarctica)

Fossil fish teeth from pelagic open ocean settings are considered a robust archive for preserving the neodymium (Nd) isotopic composition of ancient seawater. However, using fossil fish teeth as an archive to reconstruct seawater Nd isotopic compositions in different sedimentary redox environments and in terrigenous-dominated, shallow marine settings is less proven. To address these uncertainties, fish tooth and sediment samples from a middle Eocene section deposited proximal to the East Antarctic margin at Integrated Ocean Drilling Program Site U1356 were analyzed for major and trace element geochemistry, and Nd isotopes. Major and trace element analyses of the sediments reveal changing redox conditions throughout deposition in a shallow marine environment. However, variations in the Nd isotopic composition and rare earth element (REE) patterns of the associated fish teeth do not correspond to redox changes in the sediments. REE patterns in fish teeth at Site U1356 carry a typical mid-REE-enriched signature. However, a consistently positive Ce anomaly marks a deviation from a pure authigenic origin of REEs to the fish tooth. Neodymium isotopic compositions of cleaned and uncleaned fish teeth fall between modern seawater and local sediments and hence could be authigenic in nature, but could also be influenced by sedimentary fluxes. We conclude that the fossil fish tooth Nd isotope proxy is not sensitive to moderate changes in pore water oxygenation. However, combined studies on sediments, pore waters, fish teeth and seawater are needed to fully understand processes driving the reconstructed signature from shallow marine sections in proximity to continental sources. This article is protected by copyright. All rights reserved.