Integrating carbon isotope excursions into automated stratigraphic correlation: an example from the Silurian of Baltica

 

Authors: Sadler PM

Published in: Bulletin of Geosciences, volume 87, issue 4; pages: 681 - 694; Received 31 July 2011; Accepted in revised form 10 January 2012; Online 16 May 2012

Keywords: biostratigraphy, chemostratigraphy, bentonite, graphic correlation, constrained optimization, Silurian, Baltica,

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Abstract

To correlate the burgeoning volume of biostratigraphic information available from the Silurian rocks of Baltica, it is advantageous to use numerical algorithms. Graphical correlation and related numerical methods routinely incorporate taxon range-ends and bentonites. Bentonites must be matched exactly. Local taxon ranges match or under-represent true ranges and may be stretched to fit hypotheses of correlation. Carbon isotope excursions are better handled as conservative uncertainty intervals – a new data class with properties and freedoms that are the opposite of taxon ranges. Uncertainty intervals match or over-represent truly correlative segments and inflexion points in the local series of isotopic ratios; they may be shrunk to fit correlations between sections. By these means, three fundamentally different kinds of stratigraphic information may be combined in a mutually supportive fashion without over-stating the precision of any of them. When incorporated as uncertainty intervals, Silurian isotopic excursions improve automated construction of inter-regional, ordinal time-lines of taxon first- and last-appearances at both the coarsest and finest resolution. The known succession of a few named excursions helps to stack relatively brief stratigraphic sections into their correct order in longer time-lines. The limits and details of individual excursions support some of the highest-resolution segments of optimized time-lines of individual taxon originations and extinctions. At intermediate scales, local taxon ranges are the unrivalled information source. For the best results, detailed taxon ranges, bentonite analyses and stable isotope time series need to be developed from the same rocks as often as possible.