PRELIMINARY FINDINGS FOR CENTURY-SCALE (DEVRIES AND GLEISSBERG) SOLAR FORCING ON LATE CRETACEOUS CLIMATE

The Cenomanian-Turonian Eagle Ford Fm consists of a succession of calcite-rich mudstones (marls and limestones) and over 300 volcanic bentonite layers. Astronomical analyses on >150 m intervals have demonstrated that the limestone and marl cycles reflect climatic forcing driven by solar insolation resulting from integrated Milankovitch periodicities. In particular, periodic solar-terrestrial orbital variations including obliquity (37-50ka) and precessional (19-23ka) forcing on summer insolation and its impact on seasonality may have been responsible for the observed lithologic and environmental variations. Furthermore, preliminary analyses of three 0.3-0.5 m thick precession cycles (limestone-marl couplets) have identified periodicities in similar range to the DeVries (200 years) and Gleissberg (83 years) solar cycles, with 99% F-test significance and passing red noise test. These millimeter-scale laminations therefore may reflect century-scale depositional processes.

The exact nature of these century-scale variations in solar forcing on individual lamina is uncertain. To better understand the depositional effects of solar and possible volcanic forcing on these sediments, numerous high resolution analyses on a “continuous” 35 cm long thin section from the same precession cycle as the astronomic analyses have been undertaken. These analyses include millimeter-scale sedimentologic descriptions, micropalaeontologic assemblage reconstructions for individual lamina, combined with high resolution (250µm) X-Ray fluorescence (XRF) and total organic carbon (TOC) measurements. This research may contribute to a better understand of the role and impact of natural climate forcing mechanisms in greenhouse paleoclimates, and improve confidence in present-day simulations and future projections of solar and volcanic influence on century-scale change in the anthropogenically-driven climate of future centuries.