Sedimentary facies, clay mineralogy and geochemistry of the Paleogene sediments of ODP Site 105-647

Claystones immediately overlying the early Eocene age ocean-floor basalt, cored at Ocean Drilling Program (ODP) Site 647, underwent hydrothermal and thermal alterations originating from the basalt, which resulted in changes in both the mineralogical and chemical composition of the sediments. Chlorites and higher magnesium and iron concentrations were found in the lowermost sediment sequence. Upcore, changes in the bulk chemical composition of the sediments become smaller, when compensated for variations in the carbonate content originating from biogenic and authigenic components. Chlorite disappears upcore, but still only part of the swelling clay minerals have survived the thermal influence. Thirty meters above the basalt, the clay mineralogy and chemical composition become uniform throughout the Paleogene section. Iron-rich smectites (i.e., nontronitic types), totally dominate the clay mineral assemblage. Biogenic components, responsible for the dominant part of the calcite and cristobalite contents, vary in amount in the upper part, and so do the authigenic carbonate and sulfide contents. Detrital components, such as kaolinite, illite, quartz, and feldspars, make up a very small proportion of the sediment record. The nontronitic smectites are believed to be authigenic, formed by a supply of iron from the continuous formation of ocean-floor basalt in the ridge area that reacted with the detrital and biogenic silicates and alumina silicates.

Petrology and isotope characteristics of basalts from ODP Hole 111-504B

Petrography and isotope geochemical characteristics of H, O, S, Sr, and Nd have been described for basalts recovered from Hole 504B during Leg 111 of the Ocean Drilling Program. The petrographic and chemical features of the recovered basalts are similar to those obtained previously (DSDP Legs 69, 70, and 83); they can be divided into phyric (plagioclase-rich) and aphyric (Plagioclase- and clinopyroxene-rich) basalts and show low abundances of TiO2, Na2O, K2O, and Sr. This indicates that the basalts belong to Group D, comprising the majority of the upper section of the Hole 504B. The diopside-rich nature of the clinopyroxene phenocrysts and Ca-rich nature of the Plagioclase phenocrysts are also consistent with the preceding statement. The Sr and Nd isotope systematics (average 87Sr/86Sr = 0.70267 ± 0.00007 and average 143Nd/144Nd = 0.513157 ± 0.000041) indicate that the magma sources are isotopically heterogeneous, although the analyzed samples represent only the lowermost 200-m section of Hole 504B. The rocks were subjected to moderate hydrothermal alteration throughout the section recovered during Leg 111. Alteration is limited to interstices, microfractures, and grain boundaries of the primary minerals, forming chlorite, actinolite, talc, smectite, quartz, sphene, and pyrite. In harmony with the moderate alteration, the following alteration-sensitive parameters show rather limited ranges of variation: H2O = 1.1 ±0.2 wt%, dD = - 38 per mil ± 4 per mil, d180 = 5.4 per mil ± 0.3 per mil, total S = 562 ± 181 ppm, and d34S = 0.8 per mil ± 0.3 per mil. Based on these data, it was estimated that the hydrothermal fluids had dD and d180 values only slightly higher than those of seawater, the water/rock ratios were as low as 0.02-0.2, and the temperature of alteration was 300°-400°C. Sulfur exists predominantly as pyrite and in minor quantities as chalcopyrite. No primary monosulfide was detected. This and the d34S values of pyrite (d34S = 0.8 per mil) suggest that primary pyrrhotite was almost completely oxidized to pyrite by reaction with hydrothermal fluids containing very little sulfate.

Petrology and carbon and oxygen stable isotopic composition of macrofossils and sediments from the Blake-Bahama Formation, DSDP Hole 93-603B

Strata that record the evolutionary history of the North American continental margin in a region that serves as the basin margin interface between allochthonous sedimentation from the continent and pelagic sedimentation from the oceanic realm were recovered at Deep Sea Drilling Project Site 603, on the lower continental rise. The lowermost unit recovered at this site is composed of upper Berriasian-Aptian interbedded laminated limestone and bioturbated limestone with sandstone to claystone turbidites. This unit can be correlated with the Blake-Bahama Formation in the western North Atlantic. Studies of the laminated and bioturbated limestones were used to determine the depositional environment. Geochemical and petrographic studies suggest that the laminated limestones were deposited from the suspended particulate loads of the nepheloid layer associated with weak bottom-current activity as well as moderate to poorly oxygenated bottom-water conditions. Fragments of macrofossils are also found in the Blake-Bahama Formation drilled at Site 603. Twelve specimens and their host sediment were analyzed for their carbon and oxygen isotopic composition. The macrofossil samples chosen for analysis consist of nine samples of Inoceramus, two ammonite aptychi, and one belemnite sample. Depletion in 18O is observed in recrystallized specimens. The ammonite aptychi have been diagenetically altered and/or exhibit evidence of isotopic fractionation by the organism. Oxygen isotope paleotemperatures obtained from five well-preserved specimens - four of Inoceramus and one of a belemnite - suggest that bottom-water temperatures in the North Atlantic Basin during the Early Cretaceous were very warm, at least 11°C.

Stable isotope record, opal accumulation rate and geochemistry of middle Miocene sediments of IODP Site 321-U1338

During the middle Miocene, Earth's climate transitioned from a relatively warm phase (Miocene climatic optimum) into a colder mode with re-establishment of permanent ice sheets on Antarctica, thus marking a fundamental step in Cenozoic cooling. Carbon sequestration and atmospheric CO2 drawdown through increased terrestrial and/or marine productivity have been proposed as the main drivers of this fundamental transition. We integrate high-resolution (1-3 k.y.) benthic stable isotope data with XRF-scanner derived biogenic silica and carbonate accumulation estimates in an exceptionally well-preserved sedimentary archive, recovered at Integrated Ocean Drilling Program Site U1338, to reconstruct eastern equatorial Pacific productivity variations and to investigate temporal linkages between high- and low-latitude climate change over the interval 16-13 Ma. Our records show that the climatic optimum (16.8-14.7 Ma) was characterized by high amplitude climate variations, marked by intense perturbations of the carbon cycle. Episodes of peak warmth at (southern hemisphere) insolation maxima coincided with transient shoaling of the carbonate compensation depth and enhanced carbonate dissolution in the deep ocean. A switch to obliquity-paced climate variability after 14.7 Ma concurred with a general improvement in carbonate preservation and the onset of stepwise global cooling, culminating with extensive ice growth over Antarctica at ~13.8 Ma. We find that two massive increases in opal accumulation at ~14.0 and ~13.8 Ma occurred just before and during the final and most prominent cooling step, supporting the hypothesis that enhanced siliceous productivity in the eastern equatorial Pacific contributed to CO2 drawdown.

Age models, sedimentology and geochemistry from sediment cores in Drake Passage throughflow

The Drake Passage (DP) is the major geographic constriction for the Antarctic Circumpolar Current (ACC) and exerts a strong control on the exchange of physical, chemical, and biological properties between the Atlantic, Pacific, and Indian Ocean basins. Resolving changes in the flow of circumpolar water masses through this gateway is, therefore, crucial for advancing our understanding of the Southern Ocean's role in global ocean and climate variability. Here, we reconstruct changes in DP throughflow dynamics over the past 65,000 y based on grain size and geochemical properties of sediment records from the southernmost continental margin of South America. Combined with published sediment records from the Scotia Sea, we argue for a considerable total reduction of DP transport and reveal an up to ~40% decrease in flow speed along the northernmost ACC pathway entering the DP during glacial times. Superimposed on this long-term decrease are high-amplitude, millennial-scale variations, which parallel Southern Ocean and Antarctic temperature patterns. The glacial intervals of strong weakening of the ACC entering the DP imply an enhanced export of northern ACC surface and intermediate waters into the South Pacific Gyre and reduced Pacific-Atlantic exchange through the DP ("cold water route"). We conclude that changes in DP throughflow play a critical role for the global meridional overturning circulation and interbasin exchange in the Southern Ocean, most likely regulated by variations in the westerly wind field and changes in Antarctic sea ice extent.