Keywords: Oceanic Anoxic Event, Pliensbachian-Toarcian, carbon isotope excursion, Arctic 30 climate, sea level changes 31 2 The Toarcian Oceanic Anoxic Event (T-OAE) (ca. 182 mya, Early Jurassic) represents one of 32 the best-recognized examples of greenhouse warming, decreased seawater oxygenation and 33 mass extinction. The leading hypothesis to explain these changes is the massive injection of 34 thermogenic or gas hydrate-derived 13 C-depleted carbon into the atmosphere, resulting in a >3 35 per mil negative carbon isotope excursion (CIE), accelerated nutrient input and dissolved 36 oxygen consumption in the oceans. Nevertheless, the lack of a precisely dated record of the T-37 OAE outside low latitudes has led to considerable debate about both its temporal and spatial 38 extent and hence concerning its underlying causes. Here we present new isotopic and 39 lithological data from three precisely dated N Siberian sections, which demonstrate that mass 40 extinction and onset of strong oxygen-deficiency occurred near synchronously in polar and 41 most tropical sites and were intimately linked to the onset of a marked 6‰ negative CIE 42 recorded by bulk organic carbon. Rock Eval pyrolysis data from Siberia and comparisons 43 with low latitudes show that the CIE cannot be explained by the extent of stratification of the 44 studied basins or changes in organic matter sourcing and suggest that the negative CIE 45 reflects rapid 13 C-depleted carbon injection to all exchangeable reservoirs. Sedimentological 46 and palynological indicators show that the injection coincided with a change from cold 47 (abundant glendonites and exotic boulder-sized clasts) to exceptionally warm conditions 48 (dominance of the thermophyllic pollen genus Classopollis) in the Arctic, which likely 49 triggered a rapid, possibly partly glacioeustatic sea-level rise. Comparisons with low latitude 50 records reveal that warm climate conditions and poor marine oxygenation persisted in 51 continental margins at least 600 ky after the CIE, features that can be attributed to protracted 52 and massive volcanic carbon dioxide degassing. Our data reveal that the T-OAE profoundly 53 affected Arctic climate and oceanography and suggest that the CIE was a consequence of 54 global and massive 13 C-depleted carbon injection. 55 56 57
We show the present state of the set of parallel zonal scales for the Siberian Jurassic, based on various fossil groups, and the principles of their construction. We discuss the significance of Siberian biostratigraphic scales for the Boreal zonal standard of the Jurassic units. The stratotype region for this standard must have a typical Boreal (Arctic rather than mixed) fauna. A possible candidate is Siberia (and the Arctic biochorema), which is located in the center of the Panboreal Superrealm, where the set of interrelated scales for various fossil groups is the most complete. The set of parallel zonal scales for the Siberian Jurassic is efficient for the subdivision and correlation of Jurassic units in various Arctic regions (Barents Sea shelf, northeastern Russia, Arctic Alaska, Arctic Canada).
Recent integrated studies of Mesozoic reference sections of the Anabar area (northern Middle Siberia, Laptev Sea coast) and the reinterpretation of all the previous data on a modern stratigraphic basis permit considerable improvement of the bio-and lithostratigraphic division and facies zoning of Jurassic and Cretaceous sediments in the region. Analysis of abundant paleontological data allows the development or considerable improvement of zonal scales for ammonites, belemnites, bivalves, foraminifers, ostracods, dinocysts, and terrestrial palynomorphs from several Jurassic and Cretaceous intervals. All the zonal scales have been calibrated against one another and against regional ammonite scale. Reference levels of different scales useful for interregional correlation have been defined and substantiated based on the analysis of lateral distribution of fossils in different regions of the Northern Hemisphere. It provides the possibilities to propose and consider parallel zonal scales within the Boreal zonal standard for the Jurassic and Cretaceous periods. A combination of these scales forms an integrated biostratigraphic basis for a detailed division of Boreal-type sediments regardless of the place of their formation and for the comparison with the international stratigraphic standard as far as a possible use of a set of reference levels for correlation.
The Late Jurassic evolution of Boreal and Arctic basins is reflected in the widespread deposition of organic-rich black shales (source rocks). In this connection, the priority should be placed on the development and refinement of zonal schemes for the Upper Jurassic of the Laptev Sea coast based on ammonites, foraminifers, ostracods, dinocysts, and spores and pollen from reference sections as the basis for stratigraphic, paleogeographic, and facies studies. The Upper Jurassic and Lower Cretaceous reference section of interest is located on the left side of the Anabar Bay of the Laptev Sea (Nordvik Peninsula, Urdyuk-Khaya Cape). An uninterrupted and continuous section from Upper Oxfordian to Lower Valanginian is exposed in coastal cliffs and consists mainly of silty clay deposits with abundant macro- and microfossils. A reliable biostratigraphic subdivision of the Upper Jurassic interval of this section was taken as the basis for the assessment of the correlation potential of different fossil groups and subsequent interregional correlations, facies analysis, and detailed paleogeographic reconstructions of the study area. The analysis of variations in the composition of macrobenthic communities and microphytoplankton and terrestrial palynomorph assemblages and the biofacies analysis allowed the reconstruction of the evolution of marine paleoenvironmental settings in the western part of the Anabar–Lena sea and in the terrestrial settings in the adjacent land area of Siberia.
The Oxfordian–Lower Hauterivian section of the Nordvik Peninsula (northern Central Siberia) is a reference for developing zonal scales for various fossil groups and improving the Boreal zonal standard. In the middle 1950s–late 1980s, it was studied extensively by geologists, stratigraphers, lithologists, and experts on various fossil groups. These studies yielded rich fossil and microfossil collections and a set of parallel zonal scales for various faunal groups. Recently, a new detailed ammonite zonation of the Oxfordian and Kimmeridgian units of this section has been proposed. These results contradict the previous biostratigraphic data on ammonites, foraminifers, and palynomorphs. In the present paper, all the biostratigraphic data on the Oxfordian and Kimmeridgian units of the Nordvik Peninsula (Cape Urdyuk-Khaya) and northern Central Siberia undergo a comprehensive analysis and comparison with those on the Boreal Realm. The ammonite-constrained stratigraphic position of the lower Upper Jurassic in the Cape Urdyuk-Khaya section is interpreted as Upper Oxfordian or Middle Oxfordian. In our view, this difference in the understanding is due to the misidentification of some Oxfordian ammonite forms. The zones based on other fossil groups (foraminifers, dinocysts) which were distinguished in the Upper Oxfordian and Kimmeridgian sections of the Nordvik Peninsula are well traceable circumarctically. Their stratigraphic position in various regions of the Northern Hemisphere is constrained by ammonites and bivalves. However, if we use the last alternative ammonite zonation of this section part, hardly explicable contradictions will appear in interregional foraminiferal and dinocyst correlations.
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