Terrestrial carbon-isotope stratigraphy: an exploration of the method from Miocene and Jurassic examples

Terrestrial carbon-isotope stratigraphy has proven a promising tool for stratigraphic correlation between the different exchangeable carbon-isotope reservoirs, as well as a powerful approach to reconstructing the evolution of δ13C of atmospheric CO2, which is closely associated with the evolution of palaeoenvironment and palaeoclimate. However, the limited understanding of pitfalls in specific application potentially restricts the method’s utility for stratigraphic correlation and palaeoenvironmental reconstruction. This thesis takes advantage of three case studies at two vital geological intervals which are both characterized by the significant carbon-isotope perturbation in the exchangeable reservoirs, to explore the nature of terrestrial carbon-isotope stratigraphy. Two of the case studies focus on the late Early to Middle Miocene, the period of the so called Monterey Event that is marked by remarkable positive carbon-isotope excursions in benthic and pelagic marine carbonate records. There are few terrestrial carbon-isotope records for the Monterey Event. In the present study, shallow marine sediments were collected from boreholes in the New Jersey margin, USA (IODP, Expedition 313) and North Sea Basin, Denmark. Phytoclasts are concentrated from palynological residues as the basis for a terrestrial carbon-isotope stratigraphy from the two locations. The carbon-isotope curves obtained can be correlated in detail locally, and correlated crudely on a global scale. However, there are no definite positive carbon-isotope excursions observed in the terrestrial isotopic stratigraphic records through the biostratigraphically determined Langhian interval equivalent to the Monterey Event. The reasons for the absence of relatively positive carbon-isotope excursions in terrestrial carbon-isotope stratigraphy might be caused by the reworking deposits of woody phytoclasts from older strata or some other process related to reworking. Another case study centres on the Triassic-Jurassic boundary and Early Jurassic fluvial and lacustrine succession in the Kuqa section, Tarim Basin, NW China. Macrofossil wood samples were collected to generate the terrestrial carbon-isotope stratigraphy. On the basis of the biostratigraphy and potential Stage/Age (sub-) boundaries implied by biological overturns, the terrestrial carbon-isotope stratigraphy in the Kuqa section can be well correlated with both terrestrial and marine carbon-isotope stratigraphic records from UK through the Early Jurassic. For the Triassic-Jurassic boundary, more precise correlation was made globally and an exact the position of Triassic-Jurassic boundary is proposed in the Kuqa section. In light of the biostratigraphy and the carbon-isotope stratigraphy obtained in the present study, an updated age assignment of the lithostratigraphic units is proposed to Age/Stage level in the Early Jurassic across the Northern Tarim Basin. The carbon-isotope stratigraphy thus significantly improves the terrestrial stratigraphic resolution. Terrestrial carbon-isotope stratigraphy is a powerful tool for global stratigraphic correlation and unifies stratigraphic correlation over marine and non-marine strata in cases when potential biasing factors are excluded.