Description: 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.

Description: Published

Description: 102965

Description: 1A. Geomagnetismo e Paleomagnetismo

Description: JCR Journal

Description: The Global Stratotype Section and Point (GSSP) approach to define stage boundaries leaves the unit or body of the stage undefined. At the same time, previous arguments against the use of unit-stratotypes have been invalidated for the younger Cenozoic part of the geological record through the revolutionary advance in integrated high-resolution stratigraphy and astronomical dating. Combined, these provide unprecedented age control and ensure continuity of sedimentary successions, at least within the time scales of the calibrated astronomical-forced climate oscillations, and offer the possibility to introduce amended unit-stratotypes for global stages. Here we propose that such unit-stratotypes should comprise the entire stage in an astronomically age calibrated deep-marine succession, preferably but not necessarily containing the GSSP. Furthermore, cycles used for the tuning can be formally defined as chronozones, i.e. chronostratigraphic units of either unspecified rank or of a smaller scale than the stage, and independent of the standard hierarchy in global chronostratigraphy. In this way, the standard Geological Time Scale and Global Chronostratigraphic Scale can be brought in line with the progress in integrated high-resolution stratigraphy and astronomical dating. However, the more fundamental formal definition of unit-stratotypes does not depend on the formalization of astrochonozones, and both issues should be separately considered and voted upon.

Description: Published

Description: 19-39

Description: 1A. Geomagnetismo e Paleomagnetismo

Description: JCR Journal

Description: The mid-Cretaceous period was one of the warmest intervals of the past 140 million years, driven by atmospheric carbon dioxide levels of around 1,000 parts per million by volume. In the near absence of proximal geological records from south of the Antarctic Circle, it is disputed whether polar ice could exist under such environmental conditions. Here we use a sedimentary sequence recovered from the West Antarctic shelf—the southernmost Cretaceous record reported so far—and show that a temperate lowland rainforest environment existed at a palaeolatitude of about 82° S during the Turonian–Santonian age (92 to 83 million years ago). This record contains an intact 3-metre-long network of in situ fossil roots embedded in a mudstone matrix containing diverse pollen and spores. A climate model simulation shows that the reconstructed temperate climate at this high latitude requires a combination of both atmospheric carbon dioxide concentrations of 1,120–1,680 parts per million by volume and a vegetated land surface without major Antarctic glaciation, highlighting the important cooling effect exerted by ice albedo under high levels of atmospheric carbon dioxide.