Abstract:The biostratigraphy of Sinemurian to lower Toarcian calcareous nannofossils has been investigated in the Sancerre-Couy core (Paris Basin), which contains a mixed assemblage of species with affinities to the northern and southern areas of the peritethyan realm, thus allowing for the use and calibration of the Mediterranean Province (Italy/S France) and NW Europe (UK) biozonation schemes. This study is based on semi-quantitative analyses of the calcareous nannofossil assemblage performed on 145 samples and the recorded bioevents are calibrated to the NW European Ammonite Zonation and to a new organic carbon isotope curve based on 385 data points. The main bioevents, i.e. the first occurrences of Parhabdolithus liasicus, Crepidolithus pliensbachensis, Crepidolithus crassus, Mitrolithus lenticularis, Similiscutum cruciulus sensu lato, Lotharingius hauffii, Crepidolithus cavus and Lotharingius sigillatus as well as the last occurrence of Parhabdolithus robustus, have been identified. However, we show that a large number of standard biostratigraphic markers show inconsistent occurrences at the base and top of their range, possibly accounting for some of the significant discrepancies observed between the different domains. In addition to the nine main bioevents used for the biozonation of the core, we document an additional 50 distinct bioevents, evaluate their reliability and discuss their potential significance by comparison to previous studies. A total of five significant negative organic carbon isotope excursions are identified and defined in the Paris Basin including the well-documented Sinemurian-Pliensbachian boundary event. One positive excursion is further defined in the Pliensbachian interval. Our calibration of high-resolution calcareous nannofossil biostratigraphy to ammonite biostratigraphy and organic carbon isotopes represents a new stratigraphic reference for the Lower Jurassic series.

Abstract. Global paleo-climate reconstructions are largely based on observations from the Northern Hemisphere despite increasing recognition of the importance of the Southern Hemisphere mid-latitudes for understanding the drivers of the global climate system. Unfortunately, the required complete and high-resolution terrestrial records from the Southern Hemisphere mid-latitudes are few. However, the maar lakes in the Auckland Volcanic Field (AVF), New Zealand, are crucial in this regard as they form outstanding depositional basins due to their small surface-to-depth ratio, restricted catchment, and absence of ice cover since their formation, hence ensuring continuous sedimentation with anoxic bottom water. Significantly, the estimated age of the AVF of ca. 250 ka may allow development of a continuous sediment record spanning the last two glacial cycles. The Orakei maar lake sediment sequence examined in this study spans the Last Glacial Cycle (ca. 126 to ca. 9.5 ka cal BP) from the phreatomagmatic eruption to the crater rim breach due to post-glacial sea-level rise. Two overlapping cores of >100 m sediment were retrieved and combined to develop a complete composite stratigraphy that is presently undergoing a wide range of multi-proxy analyses.

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Itrax micro X‐ray fluorescence (μ‐XRF) core scanning is a non‐destructive, rapid approach to measuring elemental concentrations and their variability in sediment cores. As such, it records elemental signatures of tephra layers, which serve as correlation tie points and chronological markers for these sedimentary archives of past climatic changes. The traditional tephra identification approach using electron microprobe‐based geochemical fingerprinting of glass shards is a slow and invasive process, whilst μ‐XRF scanning of rhyolite tephra in sediment cores from Auckland (New Zealand) could provide a faster, non‐invasive approach to aid the recognition of tephra layers. This study highlights the potential and pitfalls in this novel approach: changes in most scanning parameters, and the use of two different Itrax core scanners, still led to similar chemical characterizations of the tephra layers. Changes in other scanning parameters have a biasing influence on the chemical characterization of the tephra, which would lead to misidentification of unknown layers. We demonstrate that μ‐XRF core scanning provides a faster and non‐invasive approach to correlation of sediment sequences using chemically distinct, visually pure tephra layers if a strict scanning protocol is followed. Nevertheless, an extensive database of μ‐XRF‐scanned rhyolite tephra is required for recognition of unknown tephra units using this approach.