List of contributors Preface Acknowledgements 1. Introduction K. J. Hsu and H. J. Weissert 2. Dissolution effects on size distribution of recent planktonic foraminiferal species, South Atlantic Ocean B. Malmgren 3. Middle and Late Quaternary carbonate production and dissolution and paleoceanography of the eastern Angola Basin, South Atlantic Ocean J. H. F. Jansen 4. Late Quaternary upwelling history off southwest Africa (DSDP Leg 75, HPC 532) L. Diester-Haass 5. History of the Walvis Ridge. A precis of the results of DSDP Leg 74 T. C. Moore, P. D. Rabinowitz, P. E. Borella, N. J. Shackleton and A. Boersma 6. Cyclic variations in calcium carbonate and organic carbon in Miocene to Holocene sediments, Walvis Ridge, South Atlantic Ocean W. Dean and J. Gardner 7. Pliocene oceanography and climate: an isotope record from the southwestern Angola Basin H. J. Weissert and H. Oberhansli 8. Paleoceanographic expressions of the Messinian salinity crisis J. A. McKenzie and H. Oberhansli 9. The Paleogene oxygen and carbon isotope history of Sites 522, 523, and 524 from the Central South Atlantic H. Oberhansli and M. Toumarkine 10. History of calcite dissolution of the South Atlantic Ocean K. J. Hsu and R. Wright 11. Cenozoic carbon-isotope record in South Atlantic sediments K. J. Hsu, J. A. McKenzie and H. J. Weissert 12. Hiatuses in Mesozoic and Cenozoic sediments of the Zaire-Congo continental shelf, slope, and deep-sea fan J. H. F. Jansen 13. Synthesis of lake Cretaceous, Tertiary, and Quaternary stable isotope records of the South Atlantic based on Leg 72 DSDP core material D. F. Williams, R. C. Thunell, D. A. Hodell and C. Vergnaud-Grazzini 14. Abyssal teleconnections II. Initiation of Antarctic Bottom Water flow in the southwestern Atlantic D. A. Johnson 15. Cenozoic evolution of polar water masses, southwest Atlantic Ocean S. W. Wise, A. M. Gombos and J. P. Muza 16. Evidence from the Ostracoda of major events in the South Atlantic and world-wide over the past 80 million years R. H. Benson, R. E. Chapman and L. T. Deck.
Gibbs et al . question our reconstruction of surface- and deepwater acidification around Oceanic Anoxic Event 1a. We answer their criticisms to better substantiate our arguments and original conclusions. Contrary to their suggestion, preservation cannot explain the nannofossil changes we documented, which trace perturbations in the photic zone, including a substantial increase in partial pressure of CO 2 ( p CO 2 ) and an inferred decreased pH as derived from geochemical proxies.
ABSTRACT Paleoclimatic general circulation models suggest the existence of a monsoonal climate during the Permo-Triassic over wide parts of the megacontinent Pangea and its adjacent oceans. This paper discusses how Ladinian-Carnian sedimentary successions outcropping in the Southern Alps record the signature of this climate. Sedimentological associations of tepees capped by terra-rossa paleokarst, braided fluvial sediments capped by caliche soils, and evaporite beds alternating with clay-rich delta deposits all indicate that net precipitation values changed substantially over short geological time scales. Early diagenetic features including episodes of dissolution in stratigraphies of meteoric calcite cements, corrosion and hematitization of siliciclastic detritus prior to deposition, early eu edral tectosilicate cementation, and dolomitization from evaporation-concentrated seawater record frequently changing paleohydrological conditions. Sedimentological and early diagenetic data recording highly variable, seemingly conflicting paleoclimate information can best be attributed to fluctuations in net precipitation intensities controlled by monsoonal climate, ranging from seasons to 106 yr. d18O values in early meteoric cements (-5.2 to -6.4) reflect the presence of strongly depleted meteoric waters, which are not compatible with the Southern Alps paleolatitude or paleotopography in the Triassic and can be directly related to precipitation intensities associated with a monsoonal climate. Strong precipitation could have also resulted in decreased surface-water salinities and depleted d18O in surface waters. d18O time series from marine rocks and early meteoric cements indicate a trend from Middle Triassic values, generally deplet d with respect to the expected marine signature, to less depleted Late Triassic values. This is interpreted to represent an unusually wet episode (high net precipitation), transitionally grading in the Late Triassic into a relatively add period (or low net precipitation). Despite the fact that several factors controlled the intensity of monsoonal precipitation, the effects of its variations through time are evident at different time scales in the Middle and Upper Triassic record of the Southern Alps.
A stable-isotope stratigraphy was established for planktonic and benthic foraminifers from upper Miocene-lower Pliocene pelagic sediments from the Mid-Atlantic Ridge.A correlation of stable-isotope and biostratigraphic data with magnetostratigraphic age revealed the following: (1) the late Miocene carbon-isotope shift in the South Atlantic bottom waters was minute compared with the shift reported for other deep-sea locations (Haq et al., 1980), (2) a significant cooling or continental ice-volume increase occurred between 5.7 and 5.2 Ma, and (3) a period of warming or ice-volume decrease followed, with the rate of warming increasing beginning at 4.5 Ma and reaching a climax at 4.3 Ma.The timing of these paleoceanographic events is correlated with the onset and termination of the Messinian salinity crisis in the Mediterranean Sea.
Abstract Tectono-sedimentary breccias, known as ophicalcites, overlie serpentinised peridotites at a Jurassic ocean–continent transition along the Penninic-Austroalpine transition in the Eastern Alps of Switzerland. Deformation of the exhumed mantle rocks and breccia formation occurred under decreasing temperatures and along low-angle detachment faults exposing the mantle rocks at the sea floor and was coupled with hydrothermal activity and carbonation of the serpentinites at shallow depth and/or at the sea floor. Carbon isotopes in the ophicalcites persistently show marine values; however, the interpretation of oxygen-isotope values remained controversial: are they related to Jurassic hydrothermal activity or do they reflect Alpine metamorphic overprint? Here we discuss recent interpretations that relate oxygen isotope values measured in ophicalcites exclusively to Jurassic hydrothermal activity; to this end we use data that we earlier obtained along a north–south profile across Graubünden (eastern Switzerland). We revisited the sites of controversial interpretation along a north–south profile in eastern Switzerland. Along this profile, oxygen isotope values in ophicalcites and overlying pelagic sediments, up to 25 my younger than the ophicalcites, show identical values and become systematically lower with increasing Alpine metamorphism; they strongly deviate from values in ophicalcites and pelagic sediments measured along the Mid-Atlantic Ridge or ancient Atlantic ocean-continent transitions as e.g. in the Iberia–Newfoundland transect. The oxygen-isotope values measured in Alpine ophicarbonates thus reflect isotopic resetting during the Alpine orogeny, related to fluid-rock interaction during regional metamorphism. Hydrothermal processes that accompanied the formation of ophicalcites are not disputed; however, they cannot be traced by oxygen isotope geochemistry.
