Changes in calcareous nannofossil assemblages across the Paleocene/Eocene transition from the paleo-equatorial Pacific Ocean

Quantitative analyses of selected calcareous nannofossils in deep-sea sections recovered from the paleo-equatorial Pacific (ODP Leg 199) provide new information about biostratigraphy, biochronology and the evolutionary history of calcareous nannofossils across the Paleocene/Eocene transition interval. The sediment cores from ODP Leg 199 represent the first continuous Paleocene/Eocene boundary sections ever to be sampled in the central equatorial Pacific Ocean. Calcareous nannofossil assemblages are studied to document the distribution of biostratigraphically useful taxa such as Ericsonia, Discoaster, Fasciculithus, Rhomboaster and Tribrachiatus. Focus is given to the evolution of the Rhomboaster-Tribrachiatus lineage in the lower Eocene interval at Site 1215, and on the stratigraphic relationship of these taxa relative to species in the genus Fasciculithus. Critical intervals of North Atlantic DSDP Site 550 have also been re-examined. The Tribrachiatus digitalis morphotype was described at Site 550 from an interval affected by down-hole contamination, partly originating from within the Tribrachiatus orthostylus range. The T. digitalis morphotype represents an evolutionary transitional form between T. contortus and T. orthostylus, entering the stratigraphic record within the range of the former species and disappearing within the lower part of the range of the latter species. The subzonal subdivision of Zone NP10 hence collapses. Lithological and colour variability reflecting orbital cyclicity occur in the lower Eocene of Site 1215, permitting a relative astronomical age calibration of the Tribrachiatus taxa. The distinct Rhomboaster spp.-Discoaster araneus association also occurs in the paleo-equatorial Pacific Ocean, together with a marked decrease in diversity of Fasciculithus spp. Site 1220 reveals a short peak abundance of Thoracosphaera spp. just above the P/E boundary interval, which probably reflects a stressed surface water environment.

Rocks and minerals from the Strakhov Fracture Zone, 4°N Mid-Atlantic Ridge

Geological-geophysical data obtained during Cruises 7, 11, and 12 of R/V Akademic Nikolay Strakhov (1989-1991) within the international project EQUARIDGE in the Strakhov Fracture Zone region (4°N) are presented. The trough of the fracture is interpreted as an open extension joint, a graben produced by stretching along the axis of the Mid-Atlantic Ridge. Bedrock studies showed that typical mid-ocean tholeiitic basalts occur within the narrow (60 nm wide) axial rift zone, whereas igneous rocks not typical for the ocean were found on the eastern and western flank plateaus. This allows to suppose that a reworked relict continental-type basement of pre-Upper Jurassic age possibly exists beneath the flank plateaus, within the segment under discussion. The above data correspond to the hypothesis of E. Bonatti about nonspreading nature of the basement of Mid-Atlantic Ridge within the equatorial segment and the Strakhov Fracture Zone.

Stable isotope and Magnesium/Calcium ratios of 4 profiles from the equatorial Pacific/Caribbean

This thesis examines the closure history of the Central American Seaway (CAS) and its effect on changes in ocean circulation and climate during the time interval from ~6 - 2.5 Ma. It was accomplished within the DFG Research Unit "Impact of Gateways on Ocean Circulation, Climate and Evolution" at the University of Kiel. Proxy records from Ocean Drilling Program (ODP) Sites 999 and 1000 (Caribbean), and from ODP Sites 1237, 1239 and 1241 (low-latitude east Pacific) are developed and examined. In addition, previously established proxy data from Atlantic Sites 925/926 (Ceara Rise) and 1006 (western Great Bahama Bank) and from two east Pacific sites (851, 1236) are included for interpretations. The main objectives of this study are (1) to acquire a consistent stratigraphic framework for all sites, (2) to reconstruct Pliocene changes in Caribbean and tropical east Pacific upper ocean water masses (i.e. temperature, salinity, thermocline depth), and (3) to identify potential underlying forcing mechanisms.

Middle and late Miocene calcareous nannofossil biostratigraphy from equatorial Indian and Pacific Oceans

Selected calcareous nannofossils were investigated by means of quantitative methods in middle and upper Miocene sediments from the tropical Indian Ocean (ODP Leg 115) and equatorial Pacific Ocean (DSDP Leg 85, ODP Legs 130 and 138). Our goal was to test the reliability of the classic biohorizons used in the standard zonations of Martini (1971) and Bukry (1973) and, possibly, to improve biostratigraphic resolution in the Miocene. In a time interval of about 8 m.y., from the last occurrence (LO) of S. heteromorphus (~13.6 Ma) to the LO of D. quinqueramus (~5.5 Ma), a total 37 events were investigated, using both the conventional and some additional markers proposed in the literature. At least 17 of these events proved to be distinct biostratigraphic correlation lines between the two considered areas. This integrated biostratigraphic framework increases the biostratigraphic resolution in the middle-upper Miocene interval (of the order of about 0.5 m.y). All the investigated events were tied to the geomagnetic polarity time scale (GPTS) and compared to biomagnetostratigraphy from mid-latitude North Atlantic Site 94-608 (Olafsson, 1991; Gartner, 1992), thus obtaining further information about the biostratigraphic and biochronologic reliability of the investigated events and a significant improvement of the available nannofossil biomagnetostratigraphic model for the middle and late Miocene.

Magnetic observation of ODP Site 152-919 in the Irminger Basin, North Atlantic

Component natural remanent magnetizations derived from u-channel and 1-qcm discrete samples from ODP Site 919 (Irminger Basin) indicate the existence of four intervals of negative inclinations in the upper Brunhes Chronozone. According to the age model based on planktic oxygen isotope data, these "excursional" intervals occur in sediments deposited during the following time intervals: 32-34 ka, 39-41 ka, 180-188 ka and 205-225 ka. These time intervals correspond to polarity excursions detected elsewhere, known as Mono Lake, Laschamp, Iceland Basin and Pringle Falls. The isotope-based age model is supported by the normalized remanence (paleointensity) record that can be correlated to other calibrated paleointensity records for the 0-500 ka interval, such as that from ODP Site 983. For the intervals associated with the Mono Lake and Laschamp excursions, virtual geomagnetic poles (VGPs) reach equatorial latitudes and mid-southerly latitudes, respectively. For intervals associated with the Iceland Basin and Pringle Falls excursions, repeated excursions of VGPs to high southerly latitudes indicate rapid directional swings rather than a single short-lived polarity reversal. The directional instability associated with polarity excursions is not often recorded, probably due to smoothing of the sedimentary record by the process of detrital remanence (DRM) acquisition.

Planktonic foraminifera distribution during the mid-Piacenzian warm period in the North Atlantic

The mid-Piacenzian (MP) warm period (3.264-3.025 Ma) has been identified as the most recent time in geologic history during which mean global surface temperatures were considerably warmer than today for a sustained period. This interval has therefore been proposed as a potential (albeit imperfect) analog for future climate change and as such, has received much scientific attention over the past two decades. Central to this research effort is the Pliocene Research, Interpretation, and Synoptic Mapping (PRISM) project, an iterative paleoenvironmental reconstruction of the MP focused on increasing our understanding of warm-period climate forcings, dynamics, and feedbacks by providing three-dimensional data sets for general circulation models. A mainstay of the PRISM project has been the development of a global sea surface temperature (SST) data set based primarily upon quantitative analyses of planktic foraminifer assemblages, supplemented with geochemical SST estimates wherever possible. In order to improve spatial coverage of the PRISM faunal data set in the low and mid-latitude North Atlantic, this study provides a description of the MP planktic foraminifer assemblage from five Ocean Drilling Program sites (951, 958, 1006, 1062, and 1063) in the subtropical gyre, a region critical to Atlantic Ocean circulation and tropical heat advection. Assemblages from each core provide evidence for a temperature- and circulation-driven 5-10° northward displacement of MP faunal provinces, as well as regional shifts in planktic foraminifer populations linked to species ecology and interactions. General biogeographic trends also indicate that, relative to modern conditions, gyre circulation was stronger (particularly the Gulf Stream, North Atlantic Current, and North Equatorial Current) and meridionally broader. A comparison of mid-Piacenzian and modern North Atlantic planktic foraminifer assemblages suggests that low latitude western boundary currents were less than 1 °C warmer while eastern boundary currents were ~1-2 °C warmer, supporting the hypothesis of enhanced northward heat advection along western boundary currents and warming of high latitude Northeast Atlantic source regions for the Canary Current. These findings are consistent with a model of reduced meridional SST gradients, with little-to-no low latitude warming, and more vigorous ocean circulation. Results therefore support the theory that enhanced meridional overturn circulation and associated northward heat advection made an important contribution, in conjunction with elevated atmospheric CO2 concentrations, to the 2-3 °C global surface temperature increase (relative to today) and strong polar amplification of SST warmth during the MP warm period.

Concentration of total organic carbon in waters of the Atlantic Ocean

In pursuance of previous studies water samples were taken in the Atlantic and Mediterranean during the 12th, 14th and 15th cruises of RV Mikhail Lomonosov in 1962-1964 to determine total and particulate organic carbon and permanganate oxidizability. Preliminary processing of the water samples was carried out in the normal manner in the on-board laboratory immediately after they had been taken: destruction of bicarbonates and carbonates by precise addition of acid (by alkalinity) and evaporation to dryness at 50-60°C. It is quite probable that the corresponding volatile fraction of organic matter is lost under these conditions. In discussion it was demonstrated that it may now be assumed that the carbon of the volatile fraction averages approximately 15% of total carbon, i.e., 15% of the sum of organic carbon of the volatile and nonvolatile fractions. Oxidizability was determined in all samples in the on-board laboratory.

Sedimentology of ODP sites in the Cape Basin, southeast Atlantic Ocean

Middle/late Miocene to early Pliocene sedimentary sequences along the continental margin of southwest Africa have changes that correspond to the carbonate crash (12-9 Ma) and biogenic bloom events (~7-4 Ma) described in the equatorial Pacific by Farrell et al. (1995, doi:10.2973/odp.proc.sr.138.143.1995). To explore the origins of these changes, we analyzed the carbon and coarse fraction contents of sediments from ODP Sites 1085, 1086, and 1087 at a time resolution of 5 to 30 kyr. Several major drops in CaCO3 concentration between 12 and 9 Ma are caused by dilution from major increases in clastic input from the Oranje River during global sea level regressions. Abundant pyrite crystals and good preservation of fish debris reflect low oxygenation of bottom/pore waters. Regional productivity was enhanced during the time equivalent to the carbonate crash period. Higher benthic/planktic foraminiferal ratios indicate that CaCO3 dissolution at Site 1085 peaked between 9 to 7 Ma, which was after the global carbonate crash. This period of enhanced dissolution suggests that Site 1085 was located within a low-oxygen water mass that dissolved CaCO3 more easily than North Atlantic Deep Water, which began to bathe this site at 7 Ma. At 7 to 6 Ma, the onset of the biogenic bloom, increases and variations in total organic carbon and benthic foraminiferal accumulation rates show that paleoproductivity increased significantly above values observed during the carbonate crash period and fluctuated widely. We attribute the late Miocene paleoproductivity increase off southwest Africa to ocean-wide increases in nutrient supply and delivery.