Thinking outside the zone: High-resolution quantitative diatom biochronology for the Antarctic Neogene

Developing a coherent Neogene history of the Antarctic ice sheet and Southern Ocean requires a high-resolution biostratigraphic age model, one that is applicable to the array of existing onshore and offshore paleoenvironmental records. This study integrates comprehensive diatom biostratigraphy magnetostratigraphy, and tephrostratigraphy from 32 Neogene sections around the Southern Ocean and Antarctic continental margin. Framing their correlation as a Constrained Optimization (CONOP) permits the use of computer implemented approaches that resemble multidimensional versions of graphic correlation. The method establishes the most parsimonious sequence of events subject to an expert's selection of suitable input data, determination of geological feasibility (constraints) and the most appropriate measure of fit to the data (optimization). Quantitative output includes assessments of record quality and confidence intervals on all age assignments. Thorough review and testing of CONOP model settings and input datasets established the solution's sensitivity to these factors and guided the selective removal of unreliable data to produce a robust, precise, and reproducible biochronology. Two complementary models, based on different assumptions about reworking, yield independent estimates of average local ranges and total regional ranges of fossil taxa in the southern high latitudes. The resulting composite sequences include range data of 116 diatom taxa, as well as 52 paleomagnetic reversals and two radiometrically dated ashes, which enable age calibration to within an average of +/- 0.08 m.y. for first and last appearances as old as 18 Ma. Deviations of new model ages from previously published estimates are generally small, and reflect the different sample sizes available for traditional versus new quantitative biostratigraphic calibrations. Our results confirm the reliability and regional isochroneity of most commonly utilized diatom zonal marker events, identify many new potentially useful events, and provide up to an order of magnitude greater temporal resolution than traditional zonations. The anticipated expansion of this new Southern Ocean chronostratigraphic framework to new localities and other microfossil groups will pave the way to further age refinement and broader application of this approach in the future. (C) 2007 Elsevier B.V. All rights reserved.