Pacing of the latest Ordovician and Silurian carbon cycle by a ~4.5 Myr orbital cycle

Abstract

The cyclic eccentricity, precession and obliquity parameters of the Earth's orbit and axial motion have been demonstrated to have a large impact on climate on short (104 to 105 yr) timescales during the Cenozoic and Mesozoic. Amplitude modulation (AM) of these parameters on longer (106 yr) timescales induces “grand orbital cycles” that have also been detected for large parts of the Cenozoic and Mesozoic. However, the response of paleoclimatic variables to these cycles is unknown for the Paleozoic due to past chaotic transitions in the orbital motion of the inner planets. The Late Ordovician to Early Devonian remains an enigmatic period of geological time. During the Hirnantian, extensive Southern Hemisphere glaciation was associated with the second largest mass extinction in Earth's history. The subsequent Silurian was met with several high-amplitude (>4‰) carbon isotope (δ13C) excursions associated with equally erratic climatic, biotic and eustatic fluctuations. Here, I conduct spectral analysis of carbonate δ13C and conodont apatite oxygen isotope (δ18O) compilations for the Katian (~448 Ma) to mid-Lochkovian (~415 Ma). The resulting spectra document a prominent cyclicity of ~4.2 (3.7 to 4.8) Myr in the carbon and climate cycles, similar to dominant periods found in Cenozoic astronomical solutions related to the long-term AM of the 100-kyr eccentricity band, 400-kyr eccentricity band and axial obliquity. Variations in the ~4.5 Myr eccentricity and axial obliquity cycles respectively enhanced organic carbon burial or reservoir stability leading to fluctuations in the Paleozoic δ13C record. This process led to a decline in atmospheric CO2 to threshold levels, after which astronomical forcing triggered the onset of glaciation and extensive global cooling, associated with positive δ18O excursions during the Hirnantian, the early and late Llandovery, the Telychian-Sheinwoodian boundary, the Homerian, the late Ludfordian and the late Pridoli-early Lochkovian.