A lacustrine record of the early stage of the Miocene Climatic Optimum in Central Europe from the Most Basin, Ohře (Eger) Graben, Czech Republic.

21 pagesInternational audienceThis study reports on a ~ 150 m thick macrofossil-barren sequence of siliciclastic sediments from a Burdigalian age (Early Miocene) freshwater lake. The lake was located within an incipient rift system of the Most Basin in the Ohře (Eger) Graben, which was part of the European Cenozoic Rift System, and had an original area of ≈ 1000 km2. Sediments from the HK591 core that cover the entire thickness of the lake deposits and some of the adjacent stratigraphic units were analysed by X-ray fluorescence spectroscopy (a proxy for element composition) and magnetic polarity measurement. The element proxies were subjected to frequency analysis, which provided estimated sedimentation rates, and allowed for sediment dating by magnetostratigraphy and orbital tuning of the age model. Based on the resulting age model and the known biostratigraphy, the lake was present between 17.4 and 16.6 Ma, which includes the onset of the Miocene Climatic Optimum in the latest Early Miocene. The identification of orbital forcing (precession, obliquity and short eccentricity cycles) confirms the stability of the sedimentary environment of the perennial lake in an underfilled basin. The dating allowed the sediment record to be interpreted in the context of the current knowledge of the European climate during that period. The stability of the sedimentary environment confirms that precipitation was relatively stable over the period recorded by the sediments

Trans-Atlantic correlation of Late Cretaceous high-frequency sea-level cycles

Previous studies of Cretaceous sedimentary rocks have used multi-proxy correlation methods to suggest eustatic change, modulated by the c. 400 kyr long eccentricity rhythm. Although numerous authors have inferred eustatic changes on shorter timescales, none have demonstrated synchronous sea-level changes in separate basins on different plates, thousands of kilometres apart. Our study integrates basin-scale, three-dimensional sequence architecture, molluscan biostratigraphy, and carbon-isotope chemostratigraphy to demonstrate synchronous sea-level changes in upper Turonian to lower Coniacian shallow-marine clastic successions in the Western Canada Foreland Basin, and the Bohemian Cretaceous Basin. Depositional sequences in both basins are plotted in a common time domain using an astronomically calibrated age model, allowing direct comparison. In both basins, at least seven major transgressive events can be shown to be synchronous within the limits of combined biostratigraphic and chemostratigraphic resolution. ‘Major’ and ‘minor’ sequences of late Turonian age appear to have been paced, respectively, by the long (c. 400 kyr) and short (c. 100 kyr) eccentricity cycles. In contrast, early Coniacian sequences evidence pacing by the c. 38 kyr obliquity rhythm. Stratal architecture suggests that sequences developed in response to eustatic changes of c. 14–20 m at average rates ranging 0.08 to >1.3 m/kyr. At a time of ‘warm greenhouse’ climate, sea-level change of this magnitude and timescale may not be explicable entirely as a result of thermal- and aquifer-eustasy, and hence glacio-eustasy may also have been a contributing factor

The Cyclostratigraphy Intercomparison Project (CIP): consistency, merits and pitfalls

Cyclostratigraphy is an important tool for understanding astronomical climate forcing and reading geological time in sedimentary sequences, provided that an imprint of insolation variations caused by Earth’s orbital eccentricity, obliquity and/or precession is preserved (Milankovitch forcing). Numerous stratigraphic and paleoclimate studies have applied cyclostratigraphy, but the robustness of the methodology and its dependence on the investigator have not been systematically evaluated. We developed the Cyclostratigraphy Intercomparison Project (CIP) to assess the robustness of cyclostratigraphic methods using an experimental design of three artificial cyclostratigraphic case studies with known input parameters. Each case study is designed to address specific challenges that are relevant to cyclostratigraphy. Case 1 represents an offshore research vessel environment, as only a drill-core photo and the approximate position of a late Miocene stage boundary are available for analysis. In Case 2, the Pleistocene proxy record displays clear nonlinear cyclical patterns and the interpretation is complicated by the presence of a hiatus. Case 3 represents a Late Devonian proxy record with a low signal-to-noise ratio with no specific theoretical astronomical solution available for this age. Each case was analyzed by a test group of 17-20 participants, with varying experience levels, methodological preferences and dedicated analysis time. During the CIP 2018 meeting in Brussels, Belgium, the ensuing analyses and discussion demonstrated that most participants did not arrive at a perfect solution, which may be partly explained by the limited amount of time spent on the exercises (∼4.5 hours per case). However, in all three cases, the median solution of all submitted analyses accurately approached the correct result and several participants obtained the exact correct answers. Interestingly, systematically better performances were obtained for cases that represented the data type and stratigraphic age that were closest to the individual participants’ experience. This experiment demonstrates that cyclostratigraphy is a powerful tool for deciphering time in sedimentary successions and, importantly, that it is a trainable skill. Finally, we emphasize the importance of an integrated stratigraphic approach and provide flexible guidelines on what good practices in cyclostratigraphy should include. Our case studies provide valuable insight into current common practices in cyclostratigraphy, their potential merits and pitfalls. Our work does not provide a quantitative measure of reliability and uncertainty of cyclostratigraphy, but rather constitutes a starting point for further discussions on how to move the maturing field of cyclostratigraphy forward

Frequency modulation reveals the phasing of orbital eccentricity during Cretaceous Oceanic Anoxic Event II and the Eocene hyperthermals

Major advances in our understanding of paleoclimate change derive from a precise reconstruction of the periods, amplitudes and phases of the ‘Milankovitch cycles’ of precession, obliquity and eccentricity. While numerous quantitative approaches exist for the identification of these astronomical cycles in stratigraphic data, limitations in radioisotopic dating, and instability of the theoretical astronomical solutions beyond ∼50 Myr ago, can challenge identification of the phase relationships needed to constrain climate response and anchor floating astrochronologies. Here we demonstrate that interference patterns accompanying frequency modulation (FM) of short eccentricity provide a robust basis for identifying the phase of long eccentricity forcing in stratigraphic data. One- and two-dimensional models of sedimentary distortion of the astronomical signal are used to evaluate the veracity of the FM method, and indicate that pristine eccentricity FM can be readily distinguished in paleo-records. Apart from paleoclimatic implications, the FM approach provides a quantitative technique for testing and calibrating theoretical astronomical solutions, and for refining chronologies for the deep past. We present two case studies that use the FM approach to evaluate major carbon-cycle perturbations of the Eocene and Late Cretaceous. Interference patterns in the short-eccentricity band reveal that Eocene hyperthermals ETM2 (‘Elmo’), H2, I1 and ETM3 (X; ∼52–54 Myr ago) were associated with maxima in the 405-kyr cycle of orbital eccentricity. The same eccentricity configuration favored regional anoxic episodes in the Mediterranean during the Middle and Late Cenomanian (∼94.5–97 Myr ago). The initial phase of the global Oceanic Anoxic Event II (OAE II; ∼93.9–94.5 Myr ago) coincides with maximum and falling 405-kyr eccentricity, and the recovery phase occurs during minimum and rising 405-kyr eccentricity. On a Myr scale, the event overlaps with a node in eccentricity amplitudes. Both studies underscore the importance of seasonality in pacing major climatic perturbations during greenhouse times

Coal-bearing fluvial cycles of the late Paleozoic tropics; astronomical control on sediment supply constrained by high-precision radioisotopic ages, Upper Silesian Basin

The late Paleozoic sedimentary record is well known for its mixed marine-terrestrial coal-bearing sedimentary sequences, traditionally called cyclothems, and their hypothetical glacioeustatic sea-level control. In contrast, coeval sedimentary cycles deposited in continental, nonmarine settings have received comparatively little attention. These fluvial and fluvio-lacustrine cycles are common in many late Paleozoic basins across paleoclimatic belts ranging from the tropics to the higher latitudes. Several mechanisms explaining the origins of these sequences have been suggested, including autogenic processes and tectonically driven allocycles. However, progress in understanding the climatic signal recorded in paleosols together with cyclostratigraphic analysis indicate that these terrestrial cycles record climate-driven upstream changes in sediment supply, possibly linked to astronomical ("Milankovitch") forcing. We evaluate this hypothesis by reviewing the existing models for repetitive fluvial facies and the lateral continuity of coal-bearing fluvial cycles in the Upper Silesian Basin (similar to 3 degrees S paleolatitude). In this basin, mixed terrestrial-marine short eccentricity-driven coal-bearing cyclothems of Serpukhovian age are replaced in the Early Pennsylvanian by coal-bearing fluvial allocycles of the Saddle and Lower Such ' a Members (Bashkirian). Facies and cyclic pattern analyses suggest that the terrestrial cycles record alternation of contrasting fluvial styles: a low sinuosity avulsive/laterally migrating sand and gravel dominated braided river and fluvial systems with well-developed floodplains and narrow channels transporting minimal sediment load across widespread peat swamps. Tonsteins in coals indicate that peat swamps were broadly isochronous over a large area. Lateraly persistent coals and fluvial channels indicate clastic deposition not coeval with periods of active peat accretion in extensive and long-lasting swamps. Cyclostratigraphic analysis together with new high-precision radioisotopic U-Pb zircon ages of three tonsteins suggest that major fluvial cycles were linked to the similar to 100-thousand year term of orbital eccentricity. Our conceptual model involves an upstream control on vegetation cover and sediment supply through changes in seasonal rainfall related to interactions of the summer-hemisphere monsoon and the Intertropical Convergence Zone. Furthemore, we develop an integrated time scale for the late Namurian to earliest Westphalian; the Namurian - Westphalian boundary is dated 319.17 +/- 0.41 million years ago (Ma); bases of the Marsdenian and Yeadonian substages are estimated as 320.26 +/- 0.43 Ma and 319.92 +/- 0.43 Ma, respectively.Web of Science228art. no. 10399

Palaeodrainage systems at the basal unconformity of the Bohemian Cretaceous Basin: roles of inherited fault systems and basement lithology during the onset of basin filling

This study presents a synthesis of currently available data on the distribution of Cenomanian-age palaeodrainage systems in the Bohemian Cretaceous Basin, filled by fluvial and estuarine strata, and an interpretation of their relationships to the basement units and fault systems. Much of the progress, compared to previous studies, was made possible by a recent basin-scale evaluation of Cenomanian genetic sequence stratigraphy. Several local palaeodrainage systems developed in the basin, separated by drainage divides of local importance and one major divide – the Holice-Nové Město Palaeohigh – which separated the drainage basins of the Tethyan and Boreal palaeogeographic realms. The locations and directions of palaeovalleys were strongly controlled by the positions of inherited Variscan basement fault zones, whereas the bedrock lithology had the subordinate effect of narrowing or broadening valleys on more vs. less resistant substratum, respectively. The intrabasinal part of the palaeodrainage network followed the slopes toward the Labe (Elbe) System faults and was strongly dominated by the conjugate, NNE-trending, Jizera System faults and fractures. Outlet streams – ultimate trunk streams that drained the basin area – are interpreted to have followed the Lužice Fault Zone toward the Boreal province to the Northwest, and the Železné hory Fault Zone toward the Tethyan province to the Southeast. At both the northwestern and southeastern ends of the Bohemian Cretaceous Basin, shallow-marine or estuarine conditions are proven to have existed during the early Cenomanian. Direct evidence for syn-depositional subsidence during the early to mid-Cenomanian, fluvial to estuarine phase is very rare, and the onset of deposition by fluvial backfilling of the palaeodrainage systems was driven mainly by the long-term rise in global sea level. Subtle surface warping, mostly without detectable discrete faulting, is inferred to have been a response to the onset of the palaeostress regime that later, with further stress accumulation, led to subsidence in fault-bounded depocentres of the Bohemian Cretaceous Basin and uplift of new source areas

The Cyclostratigraphy Intercomparison Project (CIP): consistency, merits and pitfalls

International audienceCyclostratigraphy is an important tool for understanding astronomical climate forcing and reading geological time in sedimentary sequences, provided that an imprint of insolation variations caused by Earth’s orbital eccentricity, obliquity and/or precession is preserved (Milankovitch forcing). Numerous stratigraphic and paleoclimate studies have applied cyclostratigraphy, but the robustness of the methodology and its dependence on the investigator have not been systematically evaluated. We developed the Cyclostratigraphy Intercomparison Project (CIP) to assess the robustness of cyclostratigraphic methods using an experimental design of three artificial cyclostratigraphic case studies with known input parameters. Each case study is designed to address specific challenges that are relevant to cyclostratigraphy. Case 1 represents an offshore research vessel environment, as only a drill-core photo and the approximate position of a late Miocene stage boundary are available for analysis. In Case 2, the Pleistocene proxy record displays clear nonlinear cyclical patterns and the interpretation is complicated by the presence of a hiatus. Case 3 represents a Late Devonian proxy record with a low signal-to-noise ratio with no specific theoretical astronomical solution available for this age. Each case was analyzed by a test group of 17-20 participants, with varying experience levels, methodological preferences and dedicated analysis time. During the CIP 2018 meeting in Brussels, Belgium, the ensuing analyses and discussion demonstrated that most participants did not arrive at a perfect solution, which may be partly explained by the limited amount of time spent on the exercises (∼4.5 hours per case). However, in all three cases, the median solution of all submitted analyses accurately approached the correct result and several participants obtained the exact correct answers. Interestingly, systematically better performances were obtained for cases that represented the data type and stratigraphic age that were closest to the individual participants’ experience. This experiment demonstrates that cyclostratigraphy is a powerful tool for deciphering time in sedimentary successions and, importantly, that it is a trainable skill. Finally, we emphasize the importance of an integrated stratigraphic approach and provide flexible guidelines on what good practices in cyclostratigraphy should include. Our case studies provide valuable insight into current common practices in cyclostratigraphy, their potential merits and pitfalls. Our work does not provide a quantitative measure of reliability and uncertainty of cyclostratigraphy, but rather constitutes a starting point for further discussions on how to move the maturing field of cyclostratigraphy forward

Geochemical and palynological sea-level proxies in hemipelagic sediments: A critical assessment from the Upper Cretaceous of the Czech Republic

Geochemical and palynological records are presented for an expanded Turonian–Coniacian hemipelagic succession in the central Bohemian Cretaceous Basin. A high-resolution stratigraphic framework is provided by biostratigraphy and organic carbon stable-isotope (δ13Corg) chemostratigraphy. A short-term (100 kyr) sea-level curve has been derived from high-resolution transgressive/regressive maxima / shore-proximity data established from basin-wide sediment geometries. The viability of geochemical and palynological parameters as potential sea-level proxies is tested against this independently derived sea-level record. Elemental chemostratigraphy is demonstrated to offer a reliable means of identifying medium- to long-term (0.4–2.4 Myr) sea-level trends. Manganese maxima are associated with periods of high sea level, and troughs with intervals of low sea level. Falling Mn contents accompany regression and rising values transgression. Major transgressive events associated with medium-term sea-level change are marked by sharp increases in Ti/Al ratios, but short-term (100 kyr) sea-level cycles are not consistently identified. Long-term δ13Corg variation and dinoflagellate cyst species richness are positively correlated and show similarities to the sea-level curve. Baseline trends have a cycle duration close to the 2.4 Myr long-eccentricity cycle. Dinocyst species richness closely follows short-term changes in sea level, with marked increases in dinocyst diversity coincident with most short-term flooding events. Periods of rapid sea-level rise caused an influx of a more diverse ‘outer shelf’ assemblage into the study area, together with the addition of shallower water species, some of which may have been transported into the central basin by hypopycnal flows. Changes in the proportion and abundance of peridinioid dinoflagellate cysts (principally Palaeohystrichophora infusorioides) were controlled principally by changing nutrient levels. Proximity proxies derived from geochemical and palynological data are not always consistent with the independent sea-level model. This exemplifies the need to understand all factors influencing elemental geochemical and palynological proxies before making simplistic sea level interpretations