Rock Magnetic-Based Cyclic Expression in Late Visean Ramp Carbonates and an Astrochronology for the Late Asbian From Northwest England

The late Asbian appears to mark the initial, well-documented, onset of far-field glacio-eustatic changes in equatorial Mississippian strata. This work unravels the nature of cyclicity in upper Asbian shallow marine carbonates, using a combination of petrographic study, rock magnetic proxies and astrochronological testing on samples from the Trowbarrow section, NW England. Rock magnetic data express the content of two types of siliciclastic sources; a marine-delivered magnetite-dominated source, and an eolian-delivered, hematite-dominated source. The eolian-sourced material generally peaked during regressive and low-stand parts of the carbonate rhythms. Astrochronologic testing methods based around the average spectral misfit and TimeOpt methodology show the magnetite abundance proxies are principally carrying the astronomically forced signal. Two likely sedimentation rate models are derived from the five better magnetic proxies using evolutive methods. In addition, a set of three likely major hiatus levels are inferred in the sedimentation rate models, based on testing possible major hiatus scenarios with TimeOpt methods, using eccentricity modulation. From these, using the three best proxies, an average astrochronologic duration for the Trowbarrow section suggests a late Asbian duration of 1976 +/- 86 kyr (1 sigma), and a basal late Asbian age of 334.48 +/- 0.35 Ma (2 sigma). Coupled atmosphere-ocean models for the late Paleozoic, suggest that lows in short eccentricity correspond to glacials, when inferred delivery of siliciclastic sediment to the carbonate ramp is generally at a maximum. The glacial and lower sea-level intervals also coincide with maximum delivery of eolian siliciclastics, likely linked to increased aridity and less vegetation cover on adjacent and distal parts of Laurentia. Around 332-333 million years ago changes in sea-level driven by changes in polar ice volume were an important control on the sedimentation patterns through time in carbonates from low paleolatitudes. Understanding the pacing of these changes has implications for timescales and paleoceanographic processes. These cyclical changes in carbonate lithology are related to changes in magnetic properties within the late Asbian (late Visean) section at Trowbarrow in NW England. The magnetic changes principally express differences in the small content of silica-based clastics, but also reflect changing Fe-oxide mineralogy between times of hematite-rich and magnetite-rich clastic input. The changes are responding to differences in eolian delivery and marine dispersal of the clastics. Eolian delivery of clastics was at its peak near sea-level lowstands, when nearby terrestrial systems were the most arid and supplying the most dust. Statistical assessments show that the magnetite abundance signal principally expresses changes in astronomical eccentricity. Primarily using the expected eccentricity pacing, two age models are constructed, which includes three levels of detectable hiatus in the section. Using these two age models and the three best magnetic proxies, the average late Asbian duration is 1976 +/- 86 kyr, with the base of the late Asbian at 334.48 +/- 0.35 Ma. Rock magnetic properties show the included siliciclastics are from hematite-rich eolian dust and marine-dispersed sources The magnetite mineral abundance proxies principally carry the primary eccentricity-driven astronomical signal An astrochronology for the late Asbian indicates its duration is 1976 +/- 86 kyr