(Table 1) Chemical composition and bulk densities of dolerites from ODP Hole 504B lower dikes

Chemical interactions between seawater and the oceanic crust have been widely investigated during recent years. However, most of these studies concern the uppermost volcanic part of the crust. The contribution of the underlying sheeted dike complex to the global budget of the oceans is inferred solely from some ophiolite studies and from the 500-m high-level dike section of DSDP/ODP 504B which was drilled in 1981. Hole 504B is the only place where a continuous and long (1260 m) section in the sheeted dike complex has been cored, and it is now regarded as a reference section for the upper oceanic crust. Many petrological and chemical data from these dolerites are available, including the relative proportions of veins, extensively altered adjacent rocks, and less altered 'host-rocks'. For these three reasons, considering the entire dike section penetrated by Hole 504B is a unique chance to study chemical fluxes related to hydrothermal alteration of this part of the oceanic crust. The calculation of any chemical flux implies knowledge of the chemical composition of the fresh precursor (protolith). Previously, mean compositions of glasses (=P1a) or basalts from the Hole 504B volcanics have been used as protoliths. In this paper, we calculate and discuss the use of various protoliths based on dolerites from Hole 504B. We show that the most adequate and realistic protolith is the mean of individual protoliths that we calculated from the acquisition, by automatic mode, of about 1000 microprobe analyses in each thin-section of dolerite from the Hole 504B lower dikes. Consequently, PFm is further used to calculate chemical fluxes in the dike section of Hole 504B. The chemical compositions of the host-rocks adjacent to alteration halos tend to converge to that of PFm with depth, except for Fe2O3t and TiO2. Because the volume percent of alteration halos increases with depth, the total fluxes related to these halos increase with depth. This explains why the mean flux (host-rocks+halos+veins) of the upper dikes is roughly similar to the mean flux of the lower dikes. During the alteration of the entire Hole 504B dike section, the dolerites gained relatively large quantities of Fe2O3t (+4.0 g/100 cm**3) and released much SiO2 (-6.8 g/100 cm**3), CaO (-5.8 g/100 cm**3), and TiO2 (1.6 g/100 cm**3), and minor Al2O3 (-0.7 g/100 cm**3) and MgO (-0.7 g/100 cm**3). We show the importance of the choice of the protolith in the calculation of chemical budget, particularly for elements showing low flux values. In Hole 504B, the Mg uptake by the volcanics during low temperature alteration added to the Mg release by the dikes gives a net flux of -0.07x10**14 g/year. We propose that part of the Mg uptake by the oceanic crust, which is necessary to compensate the rivers input (-1.33x10**14 g/year), occurs in the underlying gabbros and/or in sections which are altered such as Trinity and Troodos ophiolites. Compared with ophiolites, fluxes calculated for elements other than Mg for the entire crust are generally similar (in tendency, if not in absolute value) to that we obtained from Hole 504B.

Geochemistry and stable isotope record of the hydtrothermally altered lower sheeted dike complex in ODP Hole 140-504B

Drilling during Legs 137 and 140 of the Ocean Drilling Program deepened Hole 504B, the only hole to penetrate through the volcanic section and into the underlying hydrothermally altered sheeted dike complex, by 438.1 m to a total depth of 2000.4 meters below seafloor. This paper presents the secondary mineralogy, bulk-rock sulfur contents, and stable isotopic (O, S) compositions, plus oxygen isotopic compositions of secondary minerals from the lower sheeted dike complex drilled during Legs 137 and 140. Various evidence indicates higher temperatures of hydrothermal alteration in the lower dikes than in the upper dikes, including: the local presence of secondary clinopyroxene in the lower dikes; secondary anorthite and hornblende in the lower dikes vs. mainly actinolite and albite-oligoclase in the upper dikes; generally increasing Al and Ti contents of amphibole downward in the dike section; and greater 18O depletions of the lower dikes (d18O = 3.6-5.0 per mil) compared with the upper dikes. Early high-temperature alteration stages (T = 350°-500°C) resulted in 18O depletions and losses of metals (Cu, Zn) and sulfur from the rocks. Local incorporation of reduced seawater sulfate led to elevated d34S values of sulfide in the rocks (up to 2.5 per mil). Quartz + epidote formed in crosscutting veins at temperatures of 310°-320°C from more evolved fluids (d18O = 1 per mil). Late-stage lower-temperature (~250°C) reactions producing albite, prehnite, and zeolites in the rocks caused slight 18O enrichments, but these were insufficient to offset the 18O depletions caused by earlier higher-temperature reactions. Addition of anhydrite to the rocks during seawater recharge led to increased S contents of rocks that had previously lost S during axial hydrothermal alteration, and to further increases in d34S values of total S in the rocks (up to 12 per mil). Despite the evidence for seawater recharge to near the base of the sheeted dike complex, the paucity of late zeolites in the lower dikes suggests that late-stage, off-axis circulation was mainly restricted to the volcanics and shallowest dikes, or to localized high-permeability zones (faults) at depth.

(Table 1) Distribution of Lower Cretaceous calcareous nannofossils at DSDP Hole 93-603B

Upper Berriasian to lower Aptian calcareous nannofossil assemblages have been studied from a siliciclastic deep-sea fan complex and a subjacent limestone sequence drilled beneath the lower continental rise in the western North American Basin, 270 miles (435 km) off Cape Hatteras, North Carolina (USA). Sharp lithologic facies changes and reworking by turbidites complicate the biostratigraphic interpretation, but provide an excellent opportunity to better distinguish "nearshore" from open-ocean nannofossil species, and to investigate the introduction of neritic taxa into the deep-see environment, a phenomenon that appears to have been widespread within the circum-North Atlantic during Neocomian times. Well-preserved assemblages in dark, carbonaceous claystones were probably displaced from the oxygen minimum zone along the upper slope or outer shelf. Neritic, continental margin species prevalent in this facies include the holococcolith Zebrashapka vanhintei n. gen., n. sp., Lithraphidites alatus magnus n. spp., Pickelhaube furtiva n. gen., and a host of nannoconids and micrantholiths. A qualitative evaluation of widely used guide fossils suggests that the triad of proposed markers for the base of Roth's Zone NC3 make their first appearances in the following (ascending) order: Diadorhombus rectus, TUbodiscus verenae, Calcicalathina oblongata. Of these, we chose the nominative species for the zone, T. verenae, to mark its base and to approximate the Berriasian/Valangian boundary. Cyclagelosphaera deflandrei is strongly affected by diagenesis and is therefore not a reliable index species for the base of Zone NC4 near the Valanginian/Hauterivian boundary (the last occurrence of T. verenae is also not suitable there). In addition, Lithraphidites bollii, a form apparently confined to the low latitudes of the Tethyan region, was absent at the more temperate Site 603 and not available as a subzonal marker for the upper Hautervian-lower Barremian (mid-NC4 and mid-NC5, respectively). Cruciellipsis cuvillieri, however, provides a reliable datum just below the Hauterivian/Barremian boundary (base of NC5), despite the potential for reworking in this section. Nannoconids tend to be reworked in this section, and do not provide trustworthy forms to mark the Barremian/Aptian boundary (base of NC6). Hayesites irregularis n. comb, probably does provide a useful first appearance datum within the lower Aptian, if it is not confused with a more birefringent and globular form, Rucinolithus terebrodentarius n. sp. Rhagodiscus angustus is mimicked by a similar form (Zeughrabdotusl pseudoangustus n. sp.), which apparently ranges down to the Hauterivian, thus Lithastrinus floralis provides a more useful first appearance datum for the base of the middle-upper Aptian Rhagodiscus angustus Zone (NC7). Aside from the new taxa mentioned above, the following are also described: Cretarhabdusl delicatus n. sp. and Cyclagelosphaera jiangii n. sp.

(Figure 2) Foraminifera in lower to middle Albian sediments of DSDP Hole 80-549

The Early Cretaceous series of Site 549 have been analyzed from both an ecological and a chronostratigraphical viewpoint. Lithology and microfaunal assemblages allow the division of these deposits into two parts. The lower part extends from the lower Barremian to the basal upper Barremian and possibly down into the upper Hauterivian. Three foraminiferal associations suggest an orderly evolution from a low-energy, littoral environment, in which continental influences dominated, to progressively more open marine conditions. A dolosparite bed (which has not yet been dated) separates these synrift deposits from lower to basal middle Albian postrift sediments that developed in a pelagic bathyal environment on the middle to nearby lower continental slope.

(Figure 2) Distribution of planktonic foraminifera in the upper Paleocene of DSDP Hole 93-605

Good faunal preservation in the upper part of the Planorotatites pseudomenardii Zone at Deep Sea Drilling Project Site 605, northwestern Atlantic, allows a biometric analysis of the upper Paleocene planktonic foraminiferal species Planorotatites pseudomenardii (Belli), a keeled species that probably developed from a middle Paleocene unkeeled Planorotalites form. Multivariate analysis shows a consistent separation of all Planorotatites specimens into two groups, which are differentiated by the presence or absence of a complete keel; other variables are only of minor importance. The keeled group coincides with P. pseudomenardii. We recognize only one unkeeled species, Planorotalites chapmani (Parr), with Planorotalites ehrenbergi (Bolli), Planorotalites imitata (Subbotina), Planorotalites planoconica (Subbotina), Planorotalites troelseni (Loeblich and Tappan), and Planorotalites hausbergensis (Gohrbrandt) as junior synonyms. P. chapmani ranges from the middle Paleocene to at least the top of the upper Paleocene. The morphology of P. pseudomenardii does not change significantly, and although the frequency of Planorotalites is variable, the proportion of P. pseudomenardii to all Planorotalites varies only slightly around 65% in the upper two-thirds of its range at Site 605. However, in the top 1.5 m of its range the proportion of P. pseudomenardii decreases; in the same section, all Planorotalites specimens show a reduction in the size of their tests, suggesting that a temporary change in environmental conditions led to the exit of P. pseudomenardii\ in Magnetozone C24R at Site 605-apparently higher than expected from current standard zonations. Unkeeled Planorotalites, in contrast to R. pseudomenardii, persisted and regained normal size. The entry of P. pseudomenardii at Site 605 cannot be described in the same detail because of low frequencies of Planorotalites specimens and an erratic distribution of P. pseudomenardii in the lower part of its range. Many of the washed residues of the samples from these sediments are dominated by radiolarians, and the poorly preserved foraminiferal faunas may have abundant benthics, indicating carbonate dissolution. The initially low frequencies of P pseudomenardii relative to the unkeeled Planorotalites show a strong negative correlation with the total amount of radiolarians per sample and could be the result of preferential preservation, as well as of the same environmental conditions that caused the abundance of radiolarians.

Geochemical and stable isotope composition of Middle Campanian marl-limestone rhythmites of the Lehrte West Syncline near Hannover (Lower Saxony, northern Germany)

A cyclic marl-limestone succession of Middle-Late Campanian age has been investigated with respect to a Milankovitch-controlled origin of geochemical data. In general, the major element geochemistry of the marl-limestone rhythmites can be explained by a simple two-component mixing model with the end-members calcium carbonate and 'average shale'-like material. Carbonate content varies from 55 to 90%. Non-carbonate components are clay minerals (illite, smectite) and biogenic silica from sponge spicules, as well as authigenically formed zeolites (strontian heulandite) and quartz. The redox potential suggests oxidizing conditions throughout the section. Trace element and stable isotopic data as well as SEM investigations show that the carbonate mud is mostly composed of low-magnesium calcitic tests of planktic coccolithophorids and calcareous dinoflagellate cysts (calcispheres). Diagenetic overprint results in a decrease of 2% d18O and an increase in Mn of up to 250 ppm. However, the sediment seems to preserve most of its high Sr content compared to the primary low-magnesium calcite of co-occurring belemnite rostra. The periodicity of geochemical cycles is dominated by 413 ka and weak signals between 51 and 22.5 ka, attributable to orbital forcing. Accumulation rates within these cycles vary between 40 and 50 m/Ma. The resulting cyclic sedimentary sequence is the product of (a) changes in primary production of low-magnesium calcitic biogenic material in surface waters within the long eccentricity and the precession, demonstrated by the CaCO3 content and the Mg/Al, Mn/Al and Sr/Al ratios, and (b) fluctuations in climate and continental weathering, which changed the quality of supplied clay minerals (the illite/smectite ratio), demonstrated by the K/Al ratio. High carbonate productivity correlates with smectite-favouring weathering (semi-arid conditions, conspicuously dry and moist seasonal changes in warmer climates). Ti as the proxy indicator for the detrital terrigenous influx, as well as Rb, Si, Zr and Na, shows only low frequency signals, indicating nearly constant rates of supply throughout the more or less pure pelagic carbonate deposition of the long-lasting third-order Middle-Upper Campanian sedimentary cycle.