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Middle Eocene to early Oligocene calcareous nannofossil biostratigraphy at IODP Site U1333 (equatorial Pacific)
Full text linkWe present a biostratigraphic and biochronologic study of calcareous nannofossils of middle Eocene - early Oligocene age recovered during IODP Expedition 320, at Hole U1333C in the equatorial Pacific Ocean. The study succession encompasses nannofossil Zones NP16–NP21 (equivalent to CP13–CP16) and Chrons C20r–C12r (middle Eocene-early Oligocene). The distribution patterns of calcareous nannofossil taxa are studied by means of relative abundance and semiquantitative counts with the final aim to test the reliability of biohorizons used in the Paleogene standard biozonations (Martini 1971; Okada and Bukry 1980) and check alternative bioevents included in a more recent mid-latitudes biostratigraphic scheme (Fornaciari et al. 2010). Calibration ages are estimated based on the ranges of the biozones relative to a detailed magnetostratigraphy constructed for the site. Of particular biostratigraphic significance, our study shows that the Top of Sphenolithus furcatolithoides, the Base of common and continuous occurrence (Bc) of Dictyococcites bisectus and the total range of Sphenolithus obtusus can be used to better constrain the middle Eocene interval. The studied sediments cover the crucial time period that
followed maximum Cenozoic warmth and led up to the initial major glaciation on Antarctica, including two important climatic events, the Middle Eocene Climatic Optimum (MECO), a transient episode of global warming during a long-term cooling trend, and the Oi-1 event. The peculiar regime in sedimentation observed in the equatorial Pacific, which roughly consists of alternating phases of Carbonate Accumulation Events (CAE) and crashes in carbonate content, are correlated with increases and decreases in calcareous nannofossil abundances. A more detailed comparison indicates that the MECO corresponds to an interval with very low carbonate in between CAE3 and CAE4. This event is correlative with the Top of S. furcatolithoides, the Bc of D. bisectus and a prominent increase in the relative abundance of heavy calcified nannofossils (e.g., discoasters)
A new age model for the late Paleocene at ODP Site 1263, Walvis Ridge: new stable isotope and calcareous nannofossil data
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Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Calcareous nannofossils across the Eocene-Oligocene transition: Preservation signals and biostratigraphic remarks from ODP Site 1209 (NW Pacific, Shatsky Rise) and IODP Hole U1411B (NW Atlantic Ocean, Newfoundland Ridge)
Full text linkThis work provides a detailed biostratigraphic correlation through the Eocene-Oligocene Transition (EOT), based on an integrated stratigraphic approach and the study of calcareous nannofossils, between two disparate sites, one in the NW Atlantic (IODP Hole U1411B) and one in the NW Pacific (ODP Site 1209). The precise site-to-site correlation provided by these data allows for a comparison of carbonate preservation across the EOT including identification of the main post-depositional processes that impact the calcareous nannofossil ooze at Site 1209. The main aim of this work is to understand the extent to which the bulk δ18O and δ13C records and their sources (mainly calcareous nannofossils) are altered by diagenesis. Our detailed SEM study highlights some differences before, during and after the EOT, suggesting local diagenetic dynamics. At Site 1209, a distinctive change, both in nannofossil assemblage composition and preservation state, is observed from the pre-EOT phase to the Late Eocene Event (LEE), with a shift in the dominant process from dissolution to recrystallisation. Surprisingly, despite the overall poor preservation, only the interval between 141 and 142.4 (adj. rmcd) was compromised in term of isotopic values and assemblage diversity and abundance. This interval, recorded in the upper Eocene, was characterized by severe dissolution, concomitant with deposition of secondary calcite on solution-resistant forms. Diagenetic processes have strongly biased the δ18O isotopic signal, resulting in a positive oxygen isotope anomaly through the upper Eocene that is difficult to reconcile with other published trends. For the remaining time intervals, diagenesis seems not to have altered the bulk δ18O profile, which closely resembles that of other sites across the world, and is particularly consistent with other data from the Pacific Ocean. In summary, the impact of diagenesis on nannofossil preservation even if clearly visible both in SEM and optical microscope observations does not always cause a pervasive alteration of the primary isotopic signal and can instead provide important clues on local depositional dynamics
Planktic foraminiferal response to the Early Eocene Climatic Optimum (EECO, ca 53-49Ma): biostratigraphy and quantitative abundances from Hole 762C (Exmouth Plateau, Indian Ocean)
No full textThe EECO (ca 53-49 Ma) is a crucial interval of time to explore the marine biota response to past global warming as it records the Earth temperatures and pCO2 peak of the entire Cenozoic Era. The unicellular planktic foraminifera are a major group of open-marine calcifiers to investigate in this view as a group extremely sensitive to paleoenvironmental changes and largely utilized in biostratigraphy. Recent studies highlight that the EECO significantly impacted the abundance and diversity of the symbiont-bearing genus Morozovella. This genus, close to the carbon isotope excursion (CIE) known as J event, markedly and permanently decreased in abundance and diversity in the Atlantic and Pacific Oceans whereas abundance and diversity of genus Acarinina concomitantly increased. In addition, Morozovella species switched their coiling direction (the ability to add chambers in clock or counter clock-wise) from dominantly dextral to dominantly sinistral within 200-400 kyrs after the CIE K/X event, whereas Acarinina maintains both below and within the EECO rather proportional dextral and sinistral coiling direction. The detailed record from Atlantic and Pacific Oceans also underlines diachroneities among planktic foraminiferal biohorizons. We decided to explore planktic foraminiferal biostratigraphy, quantitative abundance and coiling direction from Site 762 C (Exmouth Plateau). This site, though affected by some core breaks, records several CIEs below and within the EECO. The study of this site is essential to outline a global perspective of planktic foraminiferal response to the EECO due to its far southern high latitude location of northwest margin of Australia. Our dataset provides new biostratigraphic data suitable for a required Eocene zonal scheme revision and new insights on the resilience of planktic foraminifera from southern Indian Ocean, essential for a more comprehensive understanding of past global warming events in light of the current climate changes
High-resolution nannofossil biochronology of middle Paleocene to early Eocene at ODP Site 1262: Implications for calcareous nannoplankton evolution
No full textOver the last several decades debates on the ‘tempo and mode’ of evolution have centered on the question whether morphological
evolution preferentially occurs gradually or punctuated, i.e., with long periods of stasis alternating with short periods of rapid
morphological change and generation of new species. Another major debate is focused on the question whether long-term evolution is
driven by, or at least strongly influenced by changes in the environment, or by interaction with other life forms. Microfossils offer a
unique opportunity to obtain the large datasets as well as the precision in dating of subsequent samples to study both these questions.We
present high-resolution analyses of selected calcareous nannofossils fromthe deep-sea section recovered at ODP Site 1262 (Leg 208) in
the South-eastern Atlantic. The studied section encompasses nannofossil Zones NP4–NP12 (equivalent to CP3–CP10) and Chrons
C27r–C24n.We document more than 70 biohorizons occurring over an about 10 Myr time interval, (∼62.5 Ma to ∼52.5 Ma), and
discuss their reliability and reproducibilitywith respect to previous data, thus providing an improved biostratigraphic framework, which
we relate to magnetostratigraphic information, and present for two possible options of a new Paleocene stratigraphic framework based
on cyclostratigraphy. This new framework enabled us to tentatively reconstruct steps in the evolution of early Paleogene calcareous
nannoplankton through documentation of transitional morphotypes between genera and/or species and of the phylogenetic relations
between the genera Fasciculithus, Heliolithus, Discoasteroides and Discoaster, as well as between Rhomboaster and Tribrachiatus.
The exceptional record provided by the continuous, composite sequence recovered atWalvis Ridge allows us to describe the mode of
evolution among calcareous nannoplankton: new genera and/or new species usually originated through branching of lineages via
gradual, but relatively rapid, morphological transitions, as documented by the presence of intermediate forms between the end-member
ancestral and descendant forms.
Significant modifications in the calcareous nannofossil assemblages are often “related” to significant changes in environmental
conditions, but the appearance of structural innovations and radiations within a single genus also occurred during “stable” environmental conditions. These lines of evidence suggest that nannoplankton evolution is not always directly triggered by stressed
environmental conditions but could be also driven by endogenous biotic control
Unravelling changes in the productivity regime during the Late Miocene-Early Pliocene Biogenic Bloom: Insights from the western equatorial Pacific (IODP Site U1488)
Full text linkInteractions between early Paleogene calcareous nannoplankton evolution and changes in environmental conditions: evidence from ODP Site 1262
No full textA benthic foraminifera perspective of the Late Miocene-Early Pliocene Biogenic Bloom at ODP Site 1085 (Southeast Atlantic Ocean)
Full text linkThe Late Miocene-Early Pliocene Biogenic Bloom (ca. 9.0–3.5 Ma) was a phase of high marine biological productivity documented globally at multiple ocean sites, related to an increase in nutrient input and/or a significant reorganization of nutrients in the oceans. Here, we studied the Biogenic Bloom at Ocean Drilling Program (ODP) Site 1085 in the Southeast Atlantic Ocean, additionally providing an updated age model based on calcareous nannofossil biostratigraphy. During the event, we identified four intervals characterised by distinct benthic foraminiferal assemblages, suggesting changes in paleoenvironmental/paleoceanographic conditions. The Biogenic Bloom extends from 8.1 to 3.0 Ma at Site 1085, as detected by different proxies such as linear sedimentation rates, carbonate mass accumulation rates, benthic foraminiferal indices and assemblage data. The inferred paleoenvironmental changes allowed us to differentiate four intervals within the Biogenic Bloom. From 8.1 to 5.2 Ma and from 3.8 to 3.0 Ma, the high benthic foraminiferal accumulation rates (BFARs) and the abundance of phytodetritus-exploiting taxa point to highly seasonal phytoplankton blooms. Between 5.2 and 4.8 Ma, we document short-term fluctuations between well‐oxygenated conditions with transient input of phytodetritus and phases of low oxygen eutrophic conditions. Between 4.8 and 3.8 Ma, a decrease in opportunistic species and an increase in eutrophic taxa likely suggest a switch to a higher food supply to the seafloor. Our data shows that the onset of the Biogenic Bloom was synchronous with other global well-dated records and its end appears to align with other Atlantic records. Lastly, our findings support the hypothesis that the Biogenic Bloom was not a single productivity event, but a complex event made up of several short-lived, high-productivity regimes with different driving forces
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