Orbital forcing of Devonian Lacustrine Ecosystems

Devonian lacustrine strata of the Caithness Flagstone Group of the Orcadian Basin in North Scotland are famous for fossil fishes, lithological rhythms, and a stunning record of terrestrialization. While the marked sedimentary cyclicity has recently been attributed to climatic variation with Milankovitch cyclical control, there has been no quantitative analysis of the cyclicity in outcrop or attempts to examine the stable isotope geochemistry of the sequence in the context of the Milankovitch climate hierarchy. We measured bulk carbon and nitrogen stable isotopes, molecule-specific carbon isotopes of plant biomarkers, and proxies of water depth sensitive sedimentary facies (depth ranks) from eight sections of the Caithness to explore the quantitative aspects of this cyclicity and the way it affected lacustrine ecosystems. We sampled long stratigraphic sections as well as multiple shorter sections that cross geography (along the Achanarras fish bed horizon) to reveal both long-term environmental changes and higher frequency variations. Sedimentary facies and bulk isotopes show distinctive hierarchical Milankovitch frequency relationships (i.e., a strong signal of precession and the eccentricity-driven modulators of precession and obliquity.) Orbitally paced changes in lake depth modulate ecosystem dynamics with deep-water intervals tending to preserve relatively more 13C-depleted labile organic matter derived mostly from phytoplankton while shallower lake episodes preserve the emergent vascular plant material. Given the great thickness of the Orcadian basin sequence (>3 km) and the apparent well-developed Milankovitch periodicities, the Caithness Flagstone and contiguous units could, with the acquisition of long core records, provide a basis for an astronomically calibrated timescale for the Devonian, calibrating chaotic drift in solar system behavior, and a natural laboratory for understanding ecosystem dynamics through a major part of the terrestrialization process.