Abstract Marine carbonate carbon isotopes (δ 13 C), over orbital to multi‐million‐year timescales, provide critical insight into the carbon cycle connecting Earth's atmosphere, lithosphere, hydrosphere and biosphere. However, the influence of astronomical forcing on deep time carbon cycle dynamics remains poorly constrained. Here, we present a ∼8 Myr‐long astrochronology based on a carbonate δ 13 C record from a Late Jurassic succession in the Lower Saxony Basin, Germany, northwestern Tethys. Astronomical tuning and spectral analyses reveal dominant 405‐kyr cycles of the orbital eccentricity amplitude modulations, and 173‐kyr and 1.2 Myr cycles of the obliquity amplitude modulations. The new astrochronology precisely dates one of the Late Jurassic carbon isotope excursions (Middle Oxfordian Event) from 159.0 to 157.2 Ma. Notably, the subordinate, low‐amplitude positive δ 13 C excursions within the Event correlate with 405‐kyr orbital eccentricity minima and 173‐kyr obliquity maxima, whereas the negative shifts correspond to eccentricity maxima and obliquity minima. Our results demonstrate a binary carbon cycle response to orbital eccentricity and obliquity extremes in the northwestern Tethyan region. Low eccentricity likely maintains permanently wet seasons and high obliquity enhances monsoon‐induced precipitation. Both mechanisms accelerate hydrological dynamics and subsequently intensify continental weathering and 12 C‐enriched organic carbon burial in marine sediments, leading to carbonate positive δ 13 C excursions. Conversely, high eccentricity generates strong dry‐wet seasonality and low obliquity weakens precipitation, both of which decelerate organic carbon burial and ultimately cause negative δ 13 C shifts. These findings advance our understanding of how astronomical forcing modulates the Late Jurassic marine carbon cycle in the northwestern Tethys.
Zhang et al. (Mon,) studied this question.