Oxygen and carbon isotope stratigraphy of ODP Holes 107-653A and 107-654A

Stable isotope analysis of two species (or groups of species) of planktonic foraminifers: Globigerinoides ruber (or G. obliquus and G. obliquus extremus) and Globigerina bulloides (or G. falconensis and G. obesa) from ODP Hole 653A and Site 654 in the Tyrrhenian basin, records the Pliocene-Pleistocene glacial history of the Northern Hemisphere. The overall increase in mean d18O values through the interval 4.6-0.08 Ma is 1.7 per mil for G. bulloides and 1.5 per mil for G. ruber. The time interval 3.1-2.5 Ma corresponds to an important phase of 18O enrichment for planktonic foraminifers. In this interval, glacial d18O values of both species G. bulloides and G. ruber increase by about l per mil, this increase being more progressive for G. ruber than for G. bulloides. The increase of interglacial d18O values is higher for G. bulloides (1.5 per mil) than for the Gruber group (1 per mil). These data suggest a more pronounced seasonal stratification of the water masses during interglacial phases. Large positive d18O fluctuations of increasing magnitude are also recorded at 2.25 and 2.15 Ma by G bulloides and appear to be diachronous with those of Site 606 in the Atlantic Ocean. Other events of increasing d18O values are recorded between 1.55 and 1.3 Ma, at 0.9 Ma, 0.8 Ma, and near 0.34 Ma. In the early Pliocene the d18O variability recorded by the planktonic species G. bulloides was higher in the Mediterranean than in the Atlantic at the same latitude. This suggests that important cyclic variations in the water budget of the Mediterranean occurred since that time. Step increases in the d18O variability are synchronous with those of the open ocean at 0.9 and 0.34 Ma. The higher variability as well as the higher amplitude of the peaks of 18O enrichment may be partly accounted for by increase of dryness over the Mediterranean area. In particular the high amplitude d18O fluctuations recorded between 3.1 and 2.1 Ma are correlated with the onset of a marked seasonal contrast and a summer dryness, revealed by pollen analyses. Strong fluctuations towards d13C values higher than modern ones are recorded by the G. ruber group species before 1.7 Ma and suggest a high production of phytoplankton. When such episodes of high primary production are correlated with episodes of decreasing 13C content of G. bulloides, they are interpreted as the consequence of a higher stratification of the upper water masses resulting itself from a marked seasonality. Such episodes occur between 4.6 and 4.05 Ma, 3.9 and 3.6 Ma, and 3.25 and 2.66 Ma. The interval 2.66-1.65 Ma corresponds to a weakening of the stratification of the upper water layers. This may be related to episodes of cooling and increasing dryness induced by the Northern Hemisphere Glaciations. The Pleistocene may have been a less productive period. The transition from highly productive to less productive surface waters also coincides with a new step increase in dryness and cooling, between 1.5 and 1.3 Ma. The comparison of the 13C records of G ruber and G. bulloides in fact suggests that a high vertical convection became a dominant feature after 2.6 Ma. Increases in the nutrient input and the stratification of the upper water masses may be suspected, however, during short episodes near 0.86 Ma (isotopic stage 25), 0.57-0.59 Ma (isotopic stage 16), 0.49 Ma (isotopic stage 13), 0.4-0.43 Ma (isotopic stage 11), and 0.22 and 0.26 Ma (part of isotopic stage 7 and transition 7/8). In fact, changes in the C02 balance within the different water masses of the Tyrrhenian basin as well as in the local primary production did not follow the general patterns of the open ocean.

Oxygen, carbon and strontium isotope data for DSDP Hole 35-323

A downhole decrease in 18O, Mg(2+) and K+, an increase in Ca(2+) and a low 87Sr/86Sr ratio of 0.7067 in the pore fluids of DSDP site 323 were caused principally by the alteration of volcanic material. These chemical and isotopic patterns were produced by the alteration, in order of decreasing importance of: a 60-m thick basal layer of volcanic ash; the underlying basalts; and igneous components in the 640-m thick upper sequence composed largely of terrigenous material. A significant portion of the alteration of the ash in the basal sequence must have occurred before the deposition of the upper sediments, perhaps under the influence of advecting solutions. The rest of the alteration occurred during the deposition of the thick upper sediments. Mass balance considerations and the low d18O values of most of the alteration products suggest that much of the later alteration occurred progressively over the last 13 Myr. The principal alteration products were smectite, potassium feldspar, clinoptilolite and calcite.

Nannofossil and stable isotope record across the Paleocene/Eocene Thermal Maximum in ODP Hole 183-1135A

The Paleocene/Eocene Thermal Maximum (PETM, ca. 55 Ma) is an abrupt, profound perturbation of climate and the carbon cycle associated with a massive injection of isotopically light carbon into the ocean-atmosphere system. As such, it provides an analogue for understanding the interplay between phytoplankton and climate under modern anthropogenic global-warming conditions. However, the accompanying enhanced dissolution poses uncertainty on the reconstruction of the affected ecology and productivity. We present a high-resolution record of bulk isotopes and nannofossil absolute abundance from Ocean Drilling Program (ODP) Site 1135 on the Kerguelen Plateau, Southern Indian Ocean to quantitatively constrain for the first time the influence of dissolution on paleoecological reconstruction. Our bulk-carbonate isotope record closely resembles that of the classic PETM site at ODP Site 690 on the opposite side of the Antarctic continent, and its correlation with those from ODP Sites 690, 1262 and 1263 records allows recognition of 14 precessional cycles upsection from the onset of the carbon isotopic excursion (CIE). This, together with a full range of common Discoasteraraneus and an abundance crossover between Fasciculithus and Zygrhablithusbijugatus, indicates the presence of the PETM at Site 1135, a poorly known record with calcareous fossils throughout the interval. The strong correlation between the absolute abundances of Chiasmolithus and coccolith assemblages reveals a dominant paleoecological signal in the poorly preserved fossil assemblages, while the influence of dissolution is only strong during the CIE. This suggests that r-selected taxa can preserve faithful ecological information even in the severely-altered assemblages studied here, and therefore provide a strong case for the application of nannofossils to paleoecological studies in better-preserved PETM sections. The inferred nannoplankton productivity drops abruptly at the CIE onset, but rapidly increases after the CIE peak, both of which may be driven by nutrient availability related to ocean stratification and vertical mixing due to changed sea-surface temperatures.

Secondary mineral assemblages in a volcanic sequence of ODP Hole 104-642E

Mineralogical and geochemical analyses of alteration products from upper and lower volcanic series recovered during ODP Leg 104 reveal variations both in composition and order of crystallization of clay minerals vesicles and voids filling and replacing glass. These results provide information about successive alteration stages of rocks and interlayered volcaniclastic sediments. The first stage, related to initial basalt-seawater interaction, is characterized by development of Fe-smectites, especially Fe-rich saponite. A second stage of intermittently superimposed subaerial weathering is marked by iron-oxides-halloysite-kaolinite formation. The third episode, interpreted as hydrothermal on the basis of O-isotopic data, is defined by postburial coprecipitation of Fe-poor, Mg-rich saponite and celadonite. A distinct final and pervasive hydrothermal stage, occurring mainly in the lower series and dominated by Al-smectites-zeolites assemblage, indicates changes toward a more reducing alteration environment.

Pliocene stable oxygen and carbon isotope record of benthic foraminifera from ODP Site 138-846 in the eastern equatorial Pacific

Oxygen and carbon isotope ratios were measured in benthic foraminifers from the entire Pliocene and latest Miocene sections of Site 846, a 180-m section, at a sampling interval of 10 cm. This provides a temporal resolution of about 2500 yr. The documented continuity of the record is excellent. Using the time scale that was developed on the basis of orbital tuning of GRAPE density records, we observed a fairly constant phase relationship between delta18O and variations in the obliquity of Earth's rotational axis. A new numbering scheme for Pliocene isotope stages is proposed. This high-resolution delta18O record clarifies several interesting aspects of late Neogene climatic evolution, including a "glacial" event that may have caused the final Messinian desiccation of the Mediterranean Sea; one or more "interglacial" events that might have caused refilling of the Mediterranean; a well-resolved couplet of glacial events at about the age of the Sidujfall Subchron; interglacial extremes in the early part of the Gauss that could have resulted from either significant deglaciation on Antarctica or from warming of deep water; and a gradual ramp of increasingly extreme "glacial" events, starting at about the Kaena Subchron and culminating with delta18O stage 100 in the earliest Matuyama.

Isotope composition of sulfur, oxygen and carbon in Black Sea sediments

Inversion of isotopic composition in the SO4(2-)-H2S system is shown to be universal in Neoeuxine sediments and an explanation of its occurrence is proposed. Change in isotopic composition of sulfate sulfur in Black Sea waters over last 10-15 thousand years is reconstructed. Periods of alteration between aerobic and anaerobic situations are identified, the beginning of entry of Mediterranean waters into the basin is dated, presence of authigenic carbonates in sediments of the sea is established and amounts are determined. Methane generation from carbon dioxide is shown to have been replaced by its generation from acetate in the paleo-Black Sea period.

Oxygen isotopic and species composition of planktonic foraminiferas in sediment cores from the North Atlantic

Paleotemperature curves were drawn from oxygen-isotope ratios in CaCO3 of planktonic foraminiferal tests and by the micropaleontological method using quantitative relationships of their species. Two series of curves yield similar results. These data confirm that isotope composition of oxygen reflects primarily temperature, and not isotope composition in ocean water. Temperature of the upper layer of ocean water increased from north to south both during the last two glaciations and in the interglacials. All three sediment cores collected from different latitudes show approximately the same amplitudes of fluctuation of mean annual temperature during times of their accumulation, as determined independently by different methods; these amplitudes are estimated as 5-7°C.