Oxygen microprofiles and porewater chemistry in sediments of the Skagerrak

Sediment porewater oxygen profiles were measured with micro and needle electrodes in sediment cores of 27 stations in the Skagerrak (northeastern North Sea). Oxygen penetration depth ranged from 3 to 20 mm depth. Fluxes estimated from the oxygen gradients varied from 3 to 18 mmol m**-2 d**-1. Oxygen penetration and flux depend on water depth, but possibly more on the hydrological conditions, related to the import of fresh organic matter by primary production in the water column. Oxygen fluxes were not related to the total organic carbon (TOC) content of the sediments. Stations in the eastern part of the Skagerrak showed high burial rates of TOC. At 6 stations porewater chemistry of Fe, Mn and NO3- was strongly associated with the oxygen distribution. The average relative contribution of terminal electron acceptors to carbon mineralisation was estimated at 85% for O2, 0.5% for Mn, 4.5% for [NO3]3-, 1% for Fe and 9% for [SO4]2-. At one station the occurrence of exceptionally high solid manganese oxyhydroxides was probably related to an active internal manganese cycle.

Stable oxygen isotope record of planktonic foraminifera from late Quaternary sediments of the equatorial Pacific

Planktonic foraminiferal test fragmentation in three cores along a depth transect from the western equatorial Pacific (ERDC-93P, 1619 m; RC17-177, 2600 m; V28-238, 3120 m [Thompson, 1976]) were examined for the last 500 kyr at sample intervals from 2.5 to 5 kyr to study the fluctuations of dissolution in the western equatorial Pacific. The age models were constructed by correlating the delta18O records with the SPECMAP stack [Imbrie et al., 1984]. Results showed that intermediate and deep waters experienced the same patterns of dissolution through climatic cycles. Fragmentation varied with a greater amplitude, and the carbonate ion concentration changed less, in the deep than in the intermediate water. Dissolution has significant variance distributions and coherencies with delta18O over the 100, 41, and 23 kyr periods of orbital variations; dissolution maxima lag ice volume minima by 6 to 20 kyr. The dissolution variability was consistent with recent geochemical models which seek to explain the reduction of atmospheric CO2 concentration at the last glacial maximum [Broecker, 1982; Boyle, 1988].

Stable oxygen isotope record of foraminifera from the Ontong Java Plateau

We provide a reconstruction of atmospheric CO2 from deep-sea sediments, for the past 625000 years (Milankovitch chron). Our database consists of a Milankovitch template of sea-level variation in combination with a unique data set for the deep-sea record for Ontong Java plateau in the western equatorial Pacific. We redate the Vostok ice-core data of Barnola et al. (1987, doi:10.1038/329408a0). To make the reconstructions we employ multiple regression between deep-sea data, on one hand, and ice-core CO2 data in Antarctica, on the other. The patterns of correlation suggest that the main factors controlling atmospheric CO2 can be described as a combination of sea-level state and sea-level change. For best results squared values of state and change are used. The square-of-sea-level rule agrees with the concept that shelf processes are important modulators of atmospheric CO2 (e.g., budgets of shelf organic carbon and shelf carbonate, nitrate reduction). The square-of-change rule implies that, on short timescales, any major disturbance of the system results in a temporary rise in atmospheric CO2.

Radiogenic neodymium, strontium and lead isotopes of authigenic and detrital sedimentary phases and stable oxygen and carbon isotopes of benthic foraminifera from gravity cores SO213-59-2 and SO213-60-1 (central South Pacific).

The South Pacific is a sensitive location for the variability of the global oceanic thermohaline circulation given that deep waters from the Atlantic Ocean, the Southern Ocean, and the Pacific Basin are exchanged. Here we reconstruct the deep water circulation of the central South Pacific for the last two glacial cycles (from 240,000 years ago to the Holocene) based on radiogenic neodymium (Nd) and lead (Pb) isotope records complemented by benthic stable carbon data obtained from two sediment cores located on the flanks of the East Pacific Rise. The records show small but consistent glacial/interglacial changes in all three isotopic systems with interglacial average values of -5.8 and 18.757 for epsilon Nd and 206Pb/204Pb, respectively, whereas glacial averages are -5.3 and 18.744. Comparison of this variability of Circumpolar Deep Water (CDW) to previously published records along the pathway of the global thermohaline circulation is consistent with reduced admixture of North Atlantic Deep Water to CDW during cold stages. The absolute values and amplitudes of the benthic delta13C variations are essentially indistinguishable from other records of the Southern Hemisphere and confirm that the low central South Pacific sedimentation rates did not result in a significant reduction of the amplitude of any of the measured proxies. In addition, the combined detrital Nd and strontium (87Sr/86Sr) isotope signatures imply that Australian and New Zealand dust has remained the principal contributor of lithogenic material to the central South Pacific.

Stable oxygen isotope record of benthic foraminifera from the tropical Atlantic

Benthic foraminiferal oxygen isotope ratios from two sediment cores recovered at 426 and 1299 m water depth in the eastern and western tropical Atlantic show that a slowdown of the thermohaline circulation (THC) during Heinrich event H1 and the Younger Dryas was accompanied by rapid and intense warming of intermediate depth waters. Millennial-scale covariations of low paleosalinities in the subpolar North Atlantic with decreased benthic oxygen isotope ratios in the eastern tropical Atlantic throughout the past 10,000 years suggest that THC weakening might be related to middepth warming during the Holocene period as well. Climate model experiments simulating a strong reduction of the THC in the Atlantic Ocean under present-day and glacial conditions reveal that the increase of temperature in the middepth tropical and South Atlantic is a common feature for both climatic states, caused by a reduced ventilation of cold intermediate and deep waters in conjunction with downward mixing of heat from the thermocline. From the similarity of the paleoclimatic records with the model simulations, we infer that the characteristic pattern of temperature change in the Atlantic Ocean related to weakened thermohaline circulation can serve as an indicator of present-day and future THC slowdown.

(Table 1) Sulfur and oxygen isotopes from two anhydrite samples recovered from DSDP Hole 70-504B

The first anhydrite reported from oceanic basalts occurs in altered basalts drilled during DSDP Leg 70 from Hole 504B. Anhydrite has been identified in several samples, two of which were studied in detail. Anhydrite in Sample 504B-40-3 (130-135 cm), which was acquired at 310 meters sub-basement, occurs in a dolerite at the center of a vug rimmed by saponite and calcite. Red iron-hydroxide-rich alteration halos occur from 0 to 310 meters sub-basement; primary sulfides in these halos are oxidized, and the rocks have lost large amounts of sulfur. The anhydrite in this sample has a d34S value of 18.5 per mil, and it is interpreted to have formed from a fluid containing a mixture of seawater sulfate (20.9 per mil) and basaltic sulfur (0 per mil) released during the oxidation of primary sulfides. Anhydrite in Sample 504B-48-3 (14-18 cm), which was found at 376 meters sub-basement, occurs intergrown with gyrolite at the center of a 1-cm-wide vein that is rimmed by saponite and quartz. At sub-basement depths below 310 meters to the bottom of the Leg 70 section (562 m sub-basement), the rocks exhibit the effects of anoxic alteration with common secondary pyrite. Anhydrite in Sample 504B-48-3 (14-18 cm) has a d34S value of 36.7 per mil, and it is interpreted to have formed from seawater-derived fluids enriched in 34S through sulfate reduction. Temperatures of alteration calculated from oxygen isotope data range from 60 to 100°C. Sulfate reduction may have occurred in situ, or elsewhere at higher temperature, possibly deeper in the crust. The secondary mineral paragenetic sequence indicates a progressive decrease in Mg and increase in Ca in the circulating fluids. This eventually led to anhydrite formation late in the alteration process.

(Table 1) Peak intensities and stable carbon and oxygen isotope ratios of various diagenetic carbonates of ODP Site 128-799

Abundant and various diagenetic carbonates were recovered from a 1084-m-thick, Quaternary to lower Miocene section at ODP Site 799 in the Japan Sea. Petrographic, XRD, SEM, EDS-chemical, and isotopic analyses revealed wide variations in occurrence and textural relations and complex mineralogy and chemistry. Diagenetic carbonates include calcite, calcium-rich rhodochrosite, iron- and manganese-rich magnesite, iron- and manganese-rich dolomite and ankerite, and iron- and manganeserich lansfordite (hydrous Mg-carbonate). Rhodochrosite commonly occurs as small, solid nodules and semi-indurated, thin layers in bioturbated, mottled sediments of Units I and II (late Miocene to Quaternary). Lansfordite occurs as unindurated nodules and layers in Unit II (late Miocene and Pliocene), whereas magnesite forms indurated beds a few centimeters thick in slightly bioturbated-to-faintly laminated sediments of Unit III (middle and late Miocene). Some rhodochrosite nodules have dark-colored, pyritic cores, and some pyrite-rhodochrosite nodules are overgrown by and included within magnesite beds. Dolomite and ankerite tend to form thick beds (>10 cm) in bedded to laminated sediments of Units III, IV, and V (early to late Miocene). Calcite occurs sporadically throughout the Site 799 sediments. The d18O values of carbonates and the interstitial waters, and the measured geothermal gradient indicate that almost all of the Site 799 carbonates are not in isotopic equilibrium with the ambient waters, but were precipitated in the past when the sediments were at shallower depths. Depths of precipitation obtained from the d18O of carbonates span from 310 to 510 mbsf for magnesite and from 60 to 580 mbsf for dolomite-ankerite. Rhodochrosite and calcite are estimated to have formed within sediments at depths shallower than 80 mbsf. Diagenetic history in the Site 799 sediments have been determined primarily by the environment of deposition; in particular, by the oxidation-reduction state of the bottom waters and the alkalinity level of the interstitial waters. Under the well-oxygenated bottom-water conditions in the late Miocene and Pliocene, manganese initially accumulated on the seafloor as hydrogenous oxides and subsequently was mobilized and reprecipitated as rhodochrosite within the shallow sulfate-reduction, sub-oxic zone. Precipitation of lansfordite occurred in the near-surface sediments with abundant organic carbon and an extremely high alkalinity during the latest Miocene and Pliocene. The lansfordite was transformed to magnesite upon burial in the depth interval 310 to 510 mbsf. Dolomite first precipitated at shallow depths in Mn-poor, anoxic, moderately biocalcareous sediments of early to late Miocene. With increasing temperature and depth, the dolomite recrystallized and reequilibrated with ambient waters at depths below about 400 mbsf.

Alkenone stable carbon isotopes, carbonate stable carbon and oxygen isotopes, planktic foraminifera boron isotopes and atmospheric CO2 calculations and box model results from Ras il-Pellegrin section, Malta

The development of a permanent, stable ice sheet in East Antarctica happened during the middle Miocene, about 14 million years (Myr) ago. The middle Miocene therefore represents one of the distinct phases of rapid change in the transition from the "greenhouse" of the early Eocene to the "icehouse" of the present day. Carbonate carbon isotope records of the period immediately following the main stage of ice sheet development reveal a major perturbation in the carbon system, represented by the positive d13C excursion known as carbon maximum 6 ("M6"), which has traditionally been interpreted as reflecting increased burial of organic matter and atmospheric pCO2 drawdown. More recently, it has been suggested that the d13C excursion records a negative feedback resulting from the reduction of silicate weathering and an increase in atmospheric pCO2. Here we present high-resolution multi-proxy (alkenone carbon and foraminiferal boron isotope) records of atmospheric carbon dioxide and sea surface temperature across CM6. Similar to previously published records spanning this interval, our records document a world of generally low (~300 ppm) atmospheric pCO2 at a time generally accepted to be much warmer than today. Crucially, they also reveal a pCO2 decrease with associated cooling, which demonstrates that the carbon burial hypothesis for CM6 is feasible and could have acted as a positive feedback on global cooling.