New research published this week in the leading international journal PNAS has revealed novel insights into the complicated interplay of factors affecting global climate and the carbon cycle.
The study, based on geochemical data from ancient mudstone deposits, involved an international team of scientists from the University of Oxford, including emeritus Professor Hugh Jenkyns and led by former graduate student Marisa Storm. The group included colleagues from Trinity College, Dublin, the University of Exeter and the British Geological Survey (BGS).
The group re-examined almost 2 km of core from the Mochras Farm Borehole, drilled on the coast of west Wales over the period 1967 to 1969. The borehole material, archived at the BGS in Keyworth, Nottinghamshire, samples a Lower Jurassic section of marine calcareous mudstone (the thickest known from northern Europe) containing abundant ammonites. The variations in species of these ancient marine fossils, reflected in their shape, pattern of coiling and ornament, have long-enabled geologists (biostratigraphers) to date the rocks to a specific ‘zone’: a sub-million year interval of geological time. Hence, accurate characterization of reference sections is crucial in the development and refinement of the geological timescale.
The team performed new carbon-isotope analyses on the marine organic matter contained in these calcareous mudstones, representing over ~27 million years of geological time. The carbon-isotope record in these rocks (the ratio between the two stable isotopes of carbon, 12C and 13C), revealed variations on timescales of ~405 kyr that match periodic changes in the shape of the Earth’s orbit around the Sun (specifically the long eccentricity cycle).
Orbitally controlled cyclicity in sedimentary strata has been recognized since the late eighteen-hundreds. Changes in the Earth’s orbit impact the amount and latitudinal distribution of energy received by the Earth from the sun, which in turn influences climatic and environmental processes, on local, regional and global scales. Such astronomical signals can be preserved in the sedimentary rock record (the study of such cycles is called cyclostratigraphy).
The discovery of a faithfully recorded eccentricity-based signature in the Mochras Farm Core is significant for the rare detail and information it provides. The cyclicity can be tracked over a period of at least 18 million years, coinciding with the interval between the Triassic–Jurassic mass extinction and the so-called Toarcian Oceanic Anoxic Event, which recorded extreme global change in the carbon cycle. The results of this study reveal that orbital forcing was a significant player in the behaviour of the ancient global carbon cycle, although the exact nature of this relationship remains to be explored.
This new study has also resulted in an astronomically calibrated time scale for some of the ammonite-defined stages of the Lower Jurassic. Such studies are vital for continuing our understanding of the timing and relationships of events that have occurred throughout Earth history.
Research by the team studying the Lower Jurassic is on-going, with plans to drill a new site located in the Cheshire Basin (Shropshire, England), to obtain material whose age equivalent in the Mochras Farm Borehole is degraded or missing. This new borehole will recover approximately 850 m of primarily uppermost Triassic to Lower Jurassic strata, including the Triassic–Jurassic boundary. This new section should be ideal for integrating multiple datasets including biostratigraphic, cyclostratigraphic, chemical and magnetic analyses which, combined with data just published from the old Mochras core, will become the international standard for 25 million years of Earth history.
Details in Storm et al., Orbital pacing and secular evolution of the Early Jurassic carbon cycle, PNAS, doi: 10.1073/pnas.1912094117