Age-depth relation and red colour reflectance of ODP Leg 177 sites

The Antarctic Polar Front is an important biogeochemical divider in the Southern Ocean. Laminated diatom mat deposits record episodes of massive flux of the diatom Thalassiothrix antarctica beneath the Antarctic Polar Front and provide a marker for tracking the migration of the Front through time. Ocean Drilling Program Sites 1091, 1093 and 1094 are the only deep piston cored record hitherto sampled from the sediments of the circumpolar biogenic opal belt. Mapping of diatom mat deposits between these sites indicates a glacial-interglacial front migration of up to 6 degrees of latitude in the early/mid Pleistocene. The mid-Pleistocene transition marks a stepwise minimum 7° northward migration of the locus of the Polar Front sustained for about 450 kyr until an abrupt southward return to a locus similar to its modern position and further south than any mid-Pleistocene locus. This interval from a "900 ka event" that saw major cooling of the oceans and a d13C minimum through to the 424 ka Mid-Brunhes Event at Termination V is also seemingly characterised by 1) sustained decreased carbonate in the sub-tropical south Atlantic, 2) reduced strength of Antarctic deep meridional circulation, 3) lower interglacial temperatures and lower interglacial atmospheric CO2 levels (by some 30 per mil) than those of the last 400 kyr, evidencing less complete deglaciation. This evidence is consistent with a prolonged period lasting 450 kyr of only partial ventilation of the deep ocean during interglacials and suggests that the mechanisms highlighted by recent hypotheses linking mid-latitude atmospheric conditions to the extent of deep ocean ventilation and carbon sequestration over glacial-interglacial cycles are likely in operation during the longer time scale characteristic of the mid-Pleistocene transition. The cooling that initiated the "900 ka event" may have been driven by minima in insolation amplitude related to eccentricity modulation of precession that also affected low latitude climates as marked by threshold changes in the African monsoon system. The major thresholds in earth system behaviour through the mid-Pleistocene transition were likely governed by an interplay of the 100 kyr and 400 kyr eccentricity modulation of precession.

Carbon and nitrogen isotopic composition of Early Toarcian sediments

Bulk sedimentary nitrogen isotope (d15Ntot) data have been generated from Lower Jurassic black, carbon-rich shales in the British Isles and northern Italy deposited during the early Toarcian oceanic anoxic event. A pronounced positive d15Ntot excursion through the exaratum Subzone of the falciferum Zone (defined by characteristic ammonites in the British Isles) broadly correlates with a relative maximum in weight percent total organic carbon and, in some sections, with a negative d13Corg excursion. Upwelling of a deoxygenated water mass that had undergone partial denitrification is the likely explanation for relative enrichment of d15Ntot, and parallels may be drawn with Quaternary sediments of the Arabian Sea, Gulf of California, and northwest Mexican margin. The development of Early Toarcian suboxic water masses and consequent partial denitrification is attributed to increases in organic productivity. Approximately coincident phenomena include the following: a relative climatic optimum, realignment of major oceanic current systems, and a possible release of methane gas hydrates from continental margin sediments early in the history of the oceanic anoxic event.

Calcareous nannofossil stratigraphy, nannofossil events and carbon isotopic ratios of ODP Leg 183 sites

Cores from Sites 1135, 1136, and 1138 of Ocean Drilling Program Leg 183 to the Kerguelen Plateau (KP) provide the most complete Paleocene and Eocene sections yet recovered from the southern Indian Ocean. These nannofossil-foraminifer oozes and chalks provide an opportunity to study southern high-latitude biostratigraphic and paleoceanographic events, which is the primary subject of this paper. In addition, a stable isotope profile was established across the Cretaceous/Tertiary (K/T) boundary at Site 1138. An apparently complete K/T boundary was recovered at Site 1138 in terms of assemblage succession, isotopic signature, and reworking of older (Cretaceous) nannofossil taxa. There is a significant color change, a negative carbon isotope shift, and nannofossil turnover. The placement of the boundary based on these criteria, however, is not in agreement with the available shipboard paleomagnetic stratigraphy. We await shore-based paleomagnetic study to confirm or deny those preliminary results. The Paleocene nannofossil assemblage is, in general, characteristic of the high latitudes with abundant Chiasmolithus, Prinsius, and Toweius. Placed in context with other Southern Ocean sites, the biogeography of Hornibrookina indicates the presence of some type of water mass boundary over the KP during the earliest Paleocene. This boundary disappeared by the late Paleocene, however, when there was an influx of warm-water discoasters, sphenoliths, and fasciculiths. This not only indicates that during much of the late Paleocene water temperatures were relatively equable, but preliminary floral and stable isotope analyses also indicate that a relatively complete record of the late Paleocene Thermal Maximum event was recovered at Site 1135. It was only at the beginning of the middle Eocene that water temperatures began to decline and the nannofossil assemblage became dominated by cool-water species while discoaster and sphenolith abundances and diversity were dramatically reduced. One new taxonomic combination is proposed, Heliolithus robustus Arney, Ladner, and Wise.

Carbon isotopic composition and dissolved CO2 concentration in pore waters of ODP Leg 174A sites

Microbially mediated redox diagenetic processes in marine sediments are reflected in the amount and carbon isotopic composition of dissolved CO2 and CH4 (Claypool and Kaplan, 1974). Oxidation of organic matter gives rise to dissolved CO2 with about the same 13C/12C ratio as the starting organic matter. Subsequent reduction of CO2 to form CH4 involves a large (~70) kinetic isotopic effect, resulting in significant 13C depletion in the CH4, and 13C enrichment in the residual CO2. Ocean Drilling Program Leg 174A (offshore New Jersey) presented an opportunity to study these processes in shelf and upper slope sediments. Holes 1071A-1071D, 1071F, and 1072A were drilled on the shelf in water depths of 88.0-98.1 m. Hole 1073A was drilled on the slope in 639.4 m of water. Pore-water samples were collected for analysis at all three sites, whereas gas samples could only be obtained from Hole 1073A on the slope.

Measurements of trace metal abundances and isotopes in carbonate sediments, Exuma, Bahamas

In order to validate the use of 238U/235U as a paleoredox proxy in carbonates, we examined the incorporation and early diagenetic evolution of U isotopes in shallow Bahamian carbonate sediments. Our sample set consists of a variety of primary precipitates that represent a range of carbonate producing organisms and components that were important in the past (scleractinian corals, calcareous green and red algae, ooids, and mollusks). In addition, four short push cores were taken in different depositional environments to assess the impact of early diagenesis and pore water chemistry on the U isotopic composition of bulk carbonates. We find that U concentrations are much higher in bulk carbonate sediments (avg. 4.1 ppm) than in primary precipitates (avg. 1.5 ppm). In almost all cases, the lowest bulk sediment U concentrations were as high as or higher than the highest concentrations found in primary precipitates. This is consistent with authigenic accumulation of reduced U(IV) during early diagenesis. The extent of this process appears sensitive to pore water H2S, and thus indirectly to organic matter content. d238/235U values were very close to seawater values in all of the primary precipitates, suggesting that these carbonate components could be used to reconstruct changes in seawater U geochemistry. However, d238/235U of bulk sediments from the push cores was 0.2-0.4 per mil heavier than seawater (and primary precipitates). These results indicate that authigenic accumulation of U under open-system sulfidic pore water conditions commonly found in carbonate sediments strongly affects the bulk U concentrations and 238U/235U ratios. We also report the occurrence of dolomite in a tidal pond core which contains low 234U/238U and 238U/235U ratios and discuss the possibility that the dolomitization process may result in sediments depleted in 238U. From this initial exploration, it is clear that 238U/235U variations in ancient carbonate sediments could be driven by changes in global average seawater, by spatial and temporal variations in the local deposition environment, or subsequent diagenesis. To cope with such effects, proxies for syndepositional pore water redox conditions (e.g., organic matter content, iron speciation, and trace metal distributions) and careful consideration of possible post-deposition alteration will be required to avoid spurious interpretation of 238U/235U data from ancient carbonate sediments.

Stable isotope ratios of foraminifera from the continental shelf off Troms, Northern Norway

A stable oxygen and carbon isotope stratigraphy is established for a Late Weichselian/Holocene glaciomarine/marine seguence in Andfjorden and Malangsdjupet on the continental shelf off Troms, Northern Norway. The stratigraphy demonstrates that the global signals, Termination I B and possibly also I A (upper parts), are present and radiocarbon date to 10.3-9.7 kyr B.P. and >14-13.5 kyr B.P., respectively. A temperature increase of 5°-6°C and possibly a small salinity increase occurred during Term. I. A near-glacial environment between 13 and 14 kyr B.P. was characterized by poorly ventilated bottom waters followed by a meltwater pulse at circa 13 kyr B.P. During the beginning intrusion of Atlantic Water between 13 and 10 kyr B.P., the bottom water was characterized by somewhat fluctuating temperatures and salinities. Temperatures close to those of the present were established around 9.7 kyr B.P. and seem to have been rather stable since.