Lithofacies, chronostratigraphy, minerals and diagenetic silica facies at DSDP Leg 80 Holes

X-ray powder diffraction and optical and scanning-electron microscope analyses of sediment samples taken from four sites drilled in the Goban Spur area of the northeast Atlantic show variable diagenetic silicification of sediments at several stratigraphic horizons. The results are as follows: 1. The silicified sediments are middle Eocene at Site 548, Paleocene to lower Albian at Site 549, upper to lower Paleocene at Site 550, and lower Turanian at Site 551. 2. There are three types of these silicified sediments: nodular type in carbonate-rich host sediments, bedded type in clayey host sediments, and a type transitional between the other two. 3. Silica diagenesis is considered to progress as follows: dissolution of siliceous fossils; precipitation of opal CT in pore spaces and transformation of biogenic silica (opal A) to opal CT, development of opal CT cement; chalcedonic quartz precipitation in pore spaces and replacement of foraminiferal tests by chalcedonic quartz; and finally, transformation of opal CT to quartz, and cementation. But the strong influence of host-sediment types on diagenetic silica fades is recognized. Bedded-type silicified sediments in a clayey environment indicate a lower grade of silica diagenesis. Only very weak chalcedonic quartz formation is recognized, and there is no opal CT cementation, even in Lower Cretaceous bedded-type clayey silicified sediments. 4. The rf(101) spacing of opal CT shows two distinct trends of ordering or decrease with burial depth; one is a rapid change, in the case of nodular silicified sediments, and the other is a more gentle shift, found in bedded silicified sediments. 5. Diagenetic silica facies of the nodular type develop as irregular concentric zones around some nodule nuclei. Also, quartz-chert nodule formation occurs at rather shallower horizons, and is discordant with the trend of decreasing d(101) spacing in opal CT. 6. Silicified sediments at Site 551 are shallower than at the other sites. The diagenetic silica facies suggest the probable erosion of 300 m or more of sediment at this site. 7. The zeolites clinoptilolite and phillipsite were found in the sediment samples recovered on Leg 80. Clinoptilolite occurs from the shallower levels to the deepest horizons of diagenetically silicified zones, suggesting that clinoptilolite formation is related to diagenesis of biogenic silica. Phillipsite at Site 551 (Section 551-5-2) may originate from volcanogenie material.

Geochemistry, physical properties, diatom abundance and datums of ODP Site 186-1150 and Hole 186-1151A

Detection of climate response to orbital forcing during Cenozoic long-term global cooling is a key to understanding the behavior of Earth's icehouse climate. Sedimentary rhythm, which is a rhythmic or cyclic variation in the sequence of sediments and sedimentary rocks, is useful for quantitative reconstruction of Earth's evolution during geological time. In this study, we attempt to (1) identify sources of natural gamma ray (NGR) emissions of core recovered during Ocean Drilling Program (ODP) Leg 186 by analyses of physical properties, major element concentrations, diatom abundances, and total organic carbon contents, (2) integrate whole-core NGR intensity of recovered core with wireline logging NGR measurements in order to construct a continuous sedimentary sequence, and (3) discuss changes in the NGR signal in the time domain. This attempt gives us preliminary information to discuss climate stability in relation to orbital forcing thorough geologic time. NGR values are obtained mainly by indirectly measuring the amount of terrigenous minerals including potassium and related elements in the sediments. NGR intensity is also affected by high porosity, which in these sediments was related to the amount of diatom valves. NGR signals might be a proxy of the intensity of the East Asian monsoon off Sanriku. A continuous sedimentary record was constructed by integration of the whole-core NGR intensity measured in sediments obtained from the drilled holes with that measured directly in the borehole by wireline logging, then using a stratigraphic age model to convert to a time series covering 1.3-9.7 Ma with a short break at ~5 Ma. High sedimentation rate (H) stages were identified in the sequence, related to intervals of low-amplitude precession and eccentricity variations. The transition of the dominant periodicities through the four H stages may correlate to major shifts in the climate system, including the onset of major Northern Hemisphere glaciation, the initial stage of the East Asian monsoon intensification, and the onset of the East Asian monsoon with uplift of the Himalayas and the Tibetan Plateau.

Foraminiferal study of ODP Hole 189-1171

Sequence boundary ages determined in shallow-water sediments obtained from ODP (Ocean Drilling Program) Leg 189 Site 1171 (South Tasman Rise) compare well with other stratigraphic records (New Jersey, United States, and northwestern Europe) and d18O increases from deep-sea records, indicating that significant (>10 m) eustatic changes occurred during the early to middle Eocene (51-42 Ma). Sequence boundaries were identified and dated using lithology, bio- and magnetostratigraphy, water-depth changes, CaCO3 content, and physical properties (e.g., photospectrometry). They are characterized by a sharp bioturbated surface, low CaCO3 content, and an abrupt increase in glauconite above the surface. Foraminiferal biofacies and planktonic/benthic foraminiferal ratios were used to estimate water-depth changes. Ages of six sequence boundaries (50.9, 49.2, 48.5-47.8, 47.1, 44.5, and 42.6 Ma) from Site 1171 correlate well to the timings of d18O increases and sequence boundaries identified from other Eocene studies. The synchronous nature of sequence boundary development from globally distal sites and d18O increases indicates a global control and that glacioeustasy was operating in this supposedly ice-free world. This is supported by previous modeling studies and atmospheric pCO2 estimates showing that the first time pCO2 levels decreased below a threshold that would support the development of an Antarctic ice sheet occurred at ca. 51 Ma. Estimates of sea-level amplitudes range from ~20 m for the early Eocene (51-49 Ma) and ~25 m to ~45 m for the middle Eocene (48-42 Ma) using constraints established for Oligocene d18O records.

Stable carbon isotope ratios and the formation of dolomite in ODP Leg 175 sites

We examine the link between organic matter degradation, anaerobic methane oxidation (AMO), and sulfate depletion and explore how these processes potentially influence dolomitization. We determined rates and depths of AMO and dolomite formation for a variety of organic-rich sites along the west African Margin using data from Ocean Drilling Program (ODP) Leg 175. Rates of AMO are calculated from the diffusive fluxes of CH4 and SO4, and rates of dolomite formation are calculated from the diffusive flux of Mg. We find that the rates of dolomite formation are relatively constant regardless of the depth at which it is forming, indicating that the diffusive fluxes of Mg and Ca are not limiting. Based upon the calculated log IAP values, log K(sp) values for dolomite were found to narrowly range between -16.1 and -16.4. Dolomite formation is controlled in part by competition between AMO and methanogenesis, which controls the speciation of dissolved CO2. AMO increases the concentration of CO3[2-] through sulfate reduction, favoring dolomite formation, while methanogenesis increases the pCO2 of the pore waters, inhibiting dolomite formation. By regulating the pCO2 and alkalinity, methanogenesis and AMO can regulate the formation of dolomite in organic-rich marine sediments. In addition to providing a mechanistic link between AMO and dolomite formation, our findings provide a method by which the stability constant of dolomite can be calculated in modern sediments and allow prediction of regions and depth domains in which dolomite may be forming.