(Table 2) Mineralogy and carbon, oxygen, and hydrogen isotope ratios for secondary mineral samples from igneous rocks from ODP Hole 128-794D

The basaltic rocks of Hole 794D drilled during Leg 128 are strongly altered. Microprobe analyses and XRD spectra on small quantities of matter extracted from thin sections show that primary minerals and glassy zones of the groundmass are totally or partially replaced by clay minerals with chlorite/saponite mixed-layer composition whatever the rock sample considered. This mixed-layer was also identified in veins and vesicles where it crystallizes in spheroidal aggregates. The largest veins and vesicles are filled by a zoned deposit: the chlorite/saponite mixed-layer always occupies the central part and is rimmed by pure saponite. Calcite crystallizes in secondary fractures which crosscut the clayey veins and vesicles. Chemographic analysis based on the M+-4Si-3R2+ projection shows that the chemical composition of the saponite component in the mixed-layer is identical to that of the free saponite. This indicates that the clay mineral crystallization was controlled by the chemical composition of the alteration fluids. From petrographic evidence, it is suggested that both chlorite/saponite mixed-layer and free saponite belong to the same hydrothermal event and are produced by a temperature decrease. This is supported by the stable isotopic data. The isotopic data show very little variation: d18O saponite ranges from 13.1 per mil to 13.5 per mil, and dD saponite from -73.6 per mil to -70.0 per mil. d18O calcite varies from +19.7 per mil to +21.9 per mil vs SMOW and d13C from -3.2 per mil to +0.4 per mil vs. PDB. These values are consistent with seawater alteration of the basalt. The formation of saponite took place at 150°-180°C and the formation of calcite at about 65°C.

A low-latitude Upper Pleistocene oxygen isotope stack

Below oxygen isotope stage 16, the orbitally derived time-scale developed by Shackleton et al. (1990) from ODP site 677 in the equatorial Pacific differs significantly from previous ones (e.g. Kominz and Pisias, 1979 doi:10.1126/science.204.4389.171; Morley and Hays, 1981 doi:10.1016/0012-821X(81)90034-0, Imbrie et al. 1984), yielding estimated ages for the last Earth magnetic reversals that are 5-7% older than the K/Ar values (Mankinen and Dalrymple, 1979 doi:10.1029/JB084iB02p00615; Berggren et al., 1985; Harland and Armstrong, 1989) but are in good agreement with recent Ar/Ar dating (Baksi et al., 1991; 1992 doi:10.1126/science.256.5055.356; Spell and McDougall, 1992 doi:10.1029/92GL01125). These results suggest that in the lower Brunhes and upper Matuyama chronozones most deep-sea climatic records retrieved so far apparently missed or misinterpreted several oscillations predicted by the astronomical theory of climate. To test this hypothesis, we studied a high-resolution oxygen isotope record from giant piston core MD900963 (Maldives area, tropical Indian Ocean) in which precession-related oscillations in delta18O are particularly well expressed, owing to the superimposition of a local salinity signal on the global ice volume signal (Rostek et al., 1993 doi:10.1038/364319a0). Three additional precession-related cycles are observed in oxygen isotope stages 17 and 18 of core MD900963, compared to the SPECMAP composite curves (Imbrie et al., 1984; Prell et al., 1986 doi:10.1029/PA001i002p00137), and stage 21 clearly presents three precession oscillations, as predicted by Shackleton et al. (1990). The precession peaks found in the delta18O record from core MD900963 are in excellent agreement with climatic oscillations predicted by the astronomical theory of climate. Our delta18O record therefore permits the development of an accurate astronomical time-scale. Based on our age model, the Brunhes-Matuyama reversal is dated at 775 +/- 10 ka, in good agreement with the age estimate of 780 ka obtained by Shackleton et al. (1990) and recent radiochronological Ar/Ar datings on lavas (Baksi et al., 1991; 1992; Spell and McDougall, 1992). We developed a new low-latitude, Upper Pleistocene delta18O reference record by stacking and tuning the delta18O records from core MD900963 and site 677 to orbital forcing functions.

(Table 1) Oxygen and carbon isotope ratios of benthic foraminifera of DSDP Holes 12-119 and 48-401

We present here oxygen and carbon isotopic records of Eocene to Oligocene benthic foraminifera from two Bay of Biscay Deep Sea Drilling Project (DSDP) sites (119 and 401). d18O of benthic foraminifera increases 1.9 per mil from a middle Eocene minimum (Zones P10-P11) to an earliest Oligocene maximum (Zone NP21). Approximately 1.4 per mil of the increase in benthic foraminiferal d18O occurs during the late Eocene to earliest Oligocene (Zones P15/16-NP21). Previous results from other North Atlantic DSDP sites (400A and 398) have significantly lower d18O values of benthic foraminifera, some by as much as 2 per mil (Vergnaud-Grazzini et al., 1978; 1989, doi:10.2973/dsdp.proc.48.119.1979; Vergnaud-Grazzini, 1979, doi:10.2973/dsdp.proc.47-2.117.1979 ). We believe that these differences result from diagenetic alteration of the sediments in the deeper-buried Sites 400A and 398.

Stable carbon and oxygen isotope records of Cibicidoides wuellerstorfi in DSDP and ODP sites in the Atlantic and Pacific Ocean

Carbon isotopic records from benthic foraminifera are used to map patterns of deep ocean circulation between 3 and 2 million years ago, the interval when significant northern hemisphere glaciation began. The delta18O and delta13C data from four Atlantic sites (552, 607, 610, and 704) and one Pacific site (677) show that global cooling over this interval was associated with increased suppression of North Atlantic Deep Water (NADW) formation. However, the relative strength of NADW production was always greater than is observed during late Pleistocene glaciations when extreme decreases in NADW are observed in the deep North Atlantic. Our data indicate that an increase in the equator-to-pole temperature gradient associated with the onset of northern hemisphere glaciation did not intensify deepwater production in the North Atlantic but rather the opposite occurred. This is not unexpected as it is the "warm high-salinity" characteristic, rather than the "low temperature", of thermocline waters that is critical to the deepwater formation process in this region today.

Stable oxygen isotopes of calcite veins in ODP Site 146-892

In situ secondary ionization mass spectrometry (SIMS) analyses of oxygen isotopes in authigenic calcite veins were obtained from an active thrust fault system drilled at Ocean Drilling Program (ODP) Site 892 (44°40.4'N, 125°07.1'W) along the Cascadia subduction margin. The average d18OPDB value of all samples is -9.9 per mil and the values are the lowest of any measured in active accretionary prisms. Ranges in individual veins can be as much as 19.6 per mil. There is an isotopic stratigraphy related to the structural stratigraphy. Mean isotope values in the hanging wall, thrust, and footwall are -14.4 per mil, -9.5 per mil, and -5.2 per mil, respectively. Several veins and crosscutting vein sequences show a general trend from lower to higher d18O values over time. Isotopic and textural data indicate several veins formed by a crack-seal mechanism and growth into open fractures. The best explanation for the strong 18O depletions is periodic rapid flow from 2-3 km deeper in the prism. Relatively narrow isotopic ranges for most veins suggest that fluids were derived from a similar source depth for each episode of fluid pulse and calcite crystallization. Structural and mass balance considerations are consistent with a record preserved in the veins of ten to hundreds of thousands of years. The fluid pulses may relate to periodic large earthquake events such as those recognized in the paleoseismicity records from the Cascadia margin.

Element contents, sulfur and oxygen isotopes, and molecular biomarkers of phosphatic crust samples from the Chimbote Platform of the shelf off Peru

Authigenic phosphatic laminites enclosed in phosphorite crusts from the shelf off Peru (10°01' S and 10°24' S) consist of carbonate fluorapatite layers, which contain abundant sulfide minerals including pyrite (FeS2) and sphalerite (ZnS). Low d34Spyrite values (average -28.8 per mill) agree with bacterial sulfate reduction and subsequent pyrite formation. Stable sulfur isotopic compositions of sulfate bound in carbonate fluorapatite are lower than that of sulfate from ambient sea water, suggesting bacterial reoxidation of sulfide by sulfide-oxidizing bacteria. The release of phosphorus and subsequent formation of the autochthonous phosphatic laminites are apparently caused by the activity of sulfate-reducing bacteria and associated sulfide-oxidizing bacteria. Following an extraction-phosphorite dissolution-extraction procedure, molecular fossils of sulfate-reducing bacteria (mono-O-alkyl glycerol ethers, di-O-alkyl glycerol ethers, as well as the short-chain branched fatty acids i/ai-C15:0, i/ai-C17:0 and 10MeC16:0) are found to be among the most abundant compounds. The fact that these molecular fossils of sulfate-reducing bacteria are distinctly more abundant after dissolution of the phosphatic laminite reveals that the lipids are tightly bound to the mineral lattice of carbonate fluorapatite. Moreover, compared with the autochthonous laminite, molecular fossils of sulfate-reducing bacteria are: (1) significantly less abundant and (2) not as tightly bound to the mineral lattice in the other, allochthonous facies of the Peruvian crusts consisting of phosphatic coated grains. These observations confirm the importance of sulfate-reducing bacteria in the formation of the phosphatic laminite. Model calculations highlight that organic matter degradation by sulfate-reducing bacteria has the potential to liberate sufficient phosphorus for phosphogenesis.

Isotopic composition of foraminifera from sediments of DSDP Legs 3, 6-9, 17

Oxygen isotopic compositions of the tests of planktonic foraminifera from several Deep Sea Drilling Project sites provide a general picture of low-latitude marine temperatures from Maastrichtian time to the present. Bottom temperatures determined from the isotopic compositions of benthonic foraminifera are interpreted as being indicative of high-latitude surface temperatures. Prior to the beginning of middle Miocene time, high- and low-latitude temperatures changed in parallel fashion. Following an apparently small and short-lived drop in temperature near the Tertiary-Cretaceous boundary, temperatures remained warm and relatively constant through Paleocene and early and middle Eocene time; bottom temperatures then were on the order of 12°C. A sharp temperature drop in late Eocene time was followed by a more gradual lowering of temperature, culminating in a late Oligocene high-latitude temperature minimum of about 4°C. A temperature rise through early Miocene time was followed in middle Miocene time by a sudden divergence of high- and low-latitude temperatures: high-latitude temperatures dropped dramatically, perhaps corresponding to the onset of major glaciation in Antarctica, but low-latitude temperatures remained constant or perhaps increased. This uncoupling of high-and low-latitude temperatures is postulated to be related to the establishment of a circum-Antarctic circulation similar to that of today. A further drop in high-latitude temperatures in late Pliocene time probably signaled the onset of a major increase in polar glaciation, including extensive sea-ice formation. Early Miocene, small-amplitude (1 per mil) sympathetic fluctuations in isotopic compositions of planktonic and benthonic foraminifera have been identified. These have a period of several hundred thousand years. Superimposed upon these are much more rapid and smaller fluctuations (0.2 to 0.5 per mil) with a period of about 80000 to 90000 yr. This is similar to the period observed for Pleistocene isotopic temperature fluctuations. In low latitudes, much smaller vertical temperature gradients seem to have existed during Maastrichtian and Paleogene time than exist at present. The absence of a sharply defined thermocline during early Tertiary time is also suggested.

Stable oxygen and carbon isotope ratios of Pliocene benthic foraminifera in the Atlantic Ocean

Large changes in benthic foraminiferal delta180 and delta13C occurred during the Pliocene (between 3.0 and 2.0 Ma) at Hole 665A. Oxygen isotopic compositions increased to maximum values at 2.4 Ma, correlating with an 18O enrichment observed at Hole 552A and other locations (Shackleton et al., 1984). As at Hole 606 (Keigwin, 1986), however, maximum delta180 values at 2.4 Ma were not as great as at Hole 552A, and enrichments in delta180 also occurred before 2.4 Ma. We believe that the section representing sediments from 2.5 to 2.7 or 2.8 Ma is missing at Hole 552A because of incomplete core recovery. Consequently, the older delta180 increases are not found at Hole 552A. Benthic foraminiferal delta13C values are much lower at Hole 665A than at Hole 552A, approaching the low values observed in the Pliocene Pacific Ocean. This geographic distribution of delta13C suggests that, like late Quaternary glaciations, the equatorial Atlantic Ocean was dominated during the Pliocene by deep water that originated in the Southern Ocean and had chemical characteristics very similar to the Pacific Ocean. Reduced O2 values were probably associated with low delta13C values and contributed to increased preservation of organic carbon during enriched 180 intervals of the Pliocene equatorial Atlantic.

Stable oxygen isotope record of Neogloboquadrina pachyderma of ODP Sites 177-1091 and 177-1094

Ocean Drilling Program (ODP) cores permit us to extend the study of millennial-scale climate variability beyond the time period that is generally accessible for piston cores (i.e., the last glacial cycle). ODP Leg 177 provided for the first time continuous high sedimentation rate cores along a north-south transect from 41°to 53°S across the main subdivisions of the Southern Ocean (Shipboard Scientific Party, 1999, doi:10.2973/odp.proc.ir.177.101.1999). The main purpose of this drilling was to investigate the Pleistocene and Holocene paleoceanographic history of this region, documented in the sedimentary records. ODP Sites 1094, 1093, 1091, and 1089 accumulated throughout the Pleistocene at rates >10 cm/k.y. and are the most detailed Pleistocene climatic records ever retrieved from the Southern Ocean. These sections provide a unique opportunity to fill an important gap in the knowledge of the paleoclimatic evolution of the high southern latitude regions. The composite sections at each site were generated shipboard using magnetic susceptibility, gamma ray attenuation (GRA) density, and reflectance data to correlate the drill holes and splice together an optimal (complete and undisturbed) record of the sedimentary sequence at each site. A preliminary magnetic polarity stratigraphy was generated on the 'archive' halves of the core sections from each hole, using the shipboard pass-through magnetometer after demagnetization at a single peak alternating field (Shipboard Scientific Party, 1999). During July 1998, we sampled core sections spanning the mid-Pleistocene interval (0.65-1.2 Ma) from Sites 1094, 1093, and 1091 at the ODP Bremen Core Repository and have since then analyzed the stable isotopic ratios of foraminifers in samples from Sites 1094 and 1091. Our goals for these studies are to establish detailed chronology for the mid-Pleistocene Southern Ocean records from Leg 177 using high-resolution stable isotope analyses, and furthermore, to trace the evolution of millennial-scale variability in proxy records from older glacial and interglacial periods characterized by higher-frequency variation. Here, we report on our stratigraphic results to date and describe the laboratory methods employed for sample preparation and stable isotope analysis. Furthermore, we provide tab-delimited text files of the age models.

Stable carbon and oxygen isotope ratios of foraminifera in surface sediments of the South China Sea

The relationship between planktonic and benthic foraminiferal stable-isotope values and oceanographic conditions and factors controlling isotopic variations are discussed on the basis of oxygen and carbon isotopic analyses of 192 modern surface and Last Glacial Maximum (LGM) samples from the South China Sea (SCS). The harmonic variation of benthic delta18O in surface sediments with water depth and temperature implies that the temperature is the main factor influencing benthic delta18O variations. Planktonic delta18O fluctuates with sea surface temperature (SST) and salinity (SSS). The N-S temperature gradient results in planktonic delta18O decreasing from the northeast to the south. Cool, saline waters driven by the winter monsoon are interpreted to have been responsible for the high delta18O values in the northeast SCS. The East Asian monsoons not only bring nutrients into the South China Sea and maintain high nutrient concentration levels at the southwestern and northeastern ends, which cause depleted delta13C both in planktonic (surface) and benthic (bottom) samples but also reduce planktonic/benthic delta18O differences. The distribution of delta18O and delta13C in the surface and LGM samples are strikingly similar, indicating that the impact of SST and SSS has been maintained, and nutrient inputs, mainly from the northeastern and southwestern ends, have been controlled by monsoons since the LGM. Comparisons of the modern and LGM delta18O indicate a difference of about 3.6 °C in bottom-water temperature and a large surface-to-bottom temperature gradient during the LGM as compared to today.

Carbonate mineralogical and isotopic analyses of TV-guided grab samples and TV-guided box corer samples from the Shumagin area

Methane seepage leads to Mg-calcite and aragonite precipitation at a depth of 4,850 m on the Aleutian accretionary margin. Stromatolitic and oncoid growth structures imply encrustation of microorganisms (microbial mats) in the host sediment with a unique growth direction downward into the sediment, forming crust-shaped lithologies. Biomarker investigations of the residue after carbonate dissolution show strong enrichments in crocetane and archaeol, which contain extremely low d13C values. This indicates the presence of methane-consuming archaea, and d13C values of -42 to -51 per mill PDB indicate that methane is the carbon source for the carbonate crusts. Thus, it appears that stromatolitic encrustations of methanotrophic anaerobic archaea probably occurs in a consortium with sulphate-reducing bacteria and that carbonate precipitation proceeds downward into the sediment, where ascending cold fluids provide a methane source. Strontium and oxygen isotope analyses as well as 14C ages of the carbonates suggest that the fluids come from deep within the sediment and that carbonate precipitation began about 3,000 years ago.

Sedimentology and geochemistry of Oxygen Isotope Stage 3 and Holocene sediments from Laguna Potrok Aike, Patagonia

Seismic reflection studies in the maar lake Laguna Potrok Aike (51°58? S, 70°23? W) revealed an erosional unconformity associated with a sub-aquatic lake-level terrace at a water depth of 30m. Radiocarbon-dated, multi-proxy sediment studies of a piston core from this location indicate that the sediment below this discontinuity has an age of 45kyr BP (Oxygen Isotope Stage 3), and was deposited during an interval of high lake level. In comparison to the Holocene section, geochemical indicators of this older part of the record either point towards a different sediment source or to a different transport mechanism for Oxygen Isotope Stage 3 sediments. Holocene sedimentation started again before 6790cal. yr BP, providing a sediment record of hydrological variability until the present. Geochemical and isotopic data indicate a fluctuating lake level until 5310cal. yr BP. During the late Holocene the lake level shows a receding tendency. Nevertheless, the lake level did not drop below the 30m terrace to create another unconformity. The geochemical characterization of volcanic ashes reveals evidence for previously unknown explosive activity of the Reclús and Mt. Burney volcanoes during Oxygen Isotope Stage 3.

Strontium isotopic data for calcareous ooze, chalk, and pore waters from DSDP Hole 590B

A detailed study of strontium isotope variations in Neogene marine carbonate sediments from Deep Sea Drilling Project Site 590B, using techniques that allow the 87Sr/86Sr ratio to be determined to better than +/-0.00001, gives a high-resolution record of the Sr isotopic evolution of seawater. The data show that the rate of change of the marine 87Sr/86Sr ratio has varied significantly even on time scales as short as 1 m.y. Periods of particularly rapid growth appear to follow major marine regressions and probably reflect an increase in the delivery of radiogenic Sr from the continents coupled with a decreased submarine carbonate dissolution rate (greater carbonate compensation depth). Periods of relatively slowly changing 87Sr/86Sr follow major marine transgressions. On the basis of correlations with the marine oxygen isotope record and the times of major continental glacier growth, it is inferred that the effects of sea-level variations are modified by climatic factors that affect the intensity of continental weathering and runoff. The effects of sea-floor generation rate variations are not discernible for the Neogene. The maximum attainable stratigraphic resolution using Sr isotopes is between 0.1 and 2 m.y. for this time period.