Abstract Marine carbonate carbon isotopes (δ13C), 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 δ13C 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 δ13C 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 12C‐enriched organic carbon burial in marine sediments, leading to carbonate positive δ13C 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 δ13C shifts. These findings advance our understanding of how astronomical forcing modulates the Late Jurassic marine carbon cycle in the northwestern Tethys.
Volcanism in continental rifts, rifted volcanic arcs, and back-arc basins is fundamentally coupled with crustal extension. However, the precise geometry and timing of the fault systems that facilitate magma transport and accommodate extension remain poorly constrained. The Christiana-Santorini-Kolumbo volcanic field lies within the Santorini-Amorgos Tectonic Zone, an actively extending back-arc rift where volcanism transitioned from dominantly effusive and moderately explosive andesitic activity (similar to 570-250 ka) to repeated caldera-forming silicic eruptions after similar to 250 ka. Here we integrate scientific drilling results from IODP Expedition 398 with a dense grid of high-resolution seismic profiles and identify a previously unrecognized rift structure, the Kolumbo Graben, bounded by the NE-SW-striking Kolumbo Fault and hosting Kolumbo Volcano and the Kolumbo Volcanic Chain. Subsidence and fault throw rates across the rift system accelerated between similar to 330 and similar to 160 ka, with the Kolumbo Fault accommodating similar to 220 m of displacement. This phase of accelerated extension preceded the emergence of Kolumbo and the onset of repeated caldera-forming silicic eruptions at Santorini. Volcanic edifices of the Kolumbo Volcanic Chain cluster within the graben interior rather than along the master fault, indicating that distributed intra-graben deformation and locally reduced horizontal stresses provided preferred pathways for magma ascent. Structural continuity further suggest that the Kolumbo Graben extends beneath Santorini. We conclude that accelerated rifting and strain localization exerted first-order control on magma ascent and the transition to highly explosive volcanism over the past similar to 250 kyr, while magma intrusion and associated thermal weakening likely provided a positive feedback that further localized deformation during periods of peak activity.
Information on the past of Earth's magnetic field can be retrieved from magnetic grains in rock samples. Micromagnetic Tomography (MMT) is a recently developed method that uses magnetic surface scans from a Quantum Diamond Microscope (QDM) combined with the position of magnetic grains in rock samples to calculate the magnetic moments of those grains. An important parameter for these calculations is the sample-to-sensor distance during the magnetic measurements. Until recently, sample placement in the QDM was typically done manually. To improve sample placement, we developed an automated sample placement system (JAMES). JAMES eases the sample placement process by enabling the positioning of the sample in the x and y direction, as well as positioning of the sample plan-parallel against the sensor or at a known distance from the sensor with a precision of 1 mu m. We evaluated the importance of a known sample-to-sensor distance for retrieving quantitative magnetic information with MMT. With a known distance, individual grain magnetic moments and summed intensities (summed magnetic moments of individual grains) can be accurately determined. The sample-to-sensor distance is less important for determining directions, when studying isolated grains with dipolar magnetic surface expressions. However, when grains do not meet those conditions, a known sample-to-sensor distance is required to determine accurate directions. JAMES is now a standard element in the MMT procedure, ensuring reliable and repeatable magnetic measurements with the QDM. This greatly improves the precision of the magnetic moment calculations in MMT and advances the extraction of magnetic information from rock samples.
The India-Asia collision continuously loads the Tibetan Plateau (TP), driving its Cenozoic eastward growth. This process has caused extensive crustal deformation and a diverse magmatic response. Here, we report on newly identified 35-6.5 Ma granitoids from Kangding area in southeastern (SE) TP. Based on elemental and isotopic data obtained from this study, including the first silicon isotope study of the Kangding Cenozoic granites, and data from the literature, the samples can be divided into two subgroups. Subgroup 1 (Sg1) has diagnostic geochemical characteristics (lower MgO, epsilon Nd(t), epsilon Hf(t)-zircon, and higher K2O/Na2O, sum of light rare earth elements, Th, Th/U, Rb/Sr, (87Sr/86Sr)i totally) compared with Sg2. Sg1 also shows a strong correlation (R 2 = 0.82) between delta 30Si and Al/Si, while Sg2 does not. The features of Sg1 are best explained by a significant contribution from metasediments. Based on the restricted ages of the Sg1 magmas, we identify a temporary shift in the melt source during the early Miocene (ca. 20-10 Ma). At that time, the source involved the addition of metasedimentary materials, in contrast to the sediment-poor source during the rest of the Cenozoic. Based on previous geophysical and regional deformation observations, the growth of the TP involved a gradual accentuation of imbrication structures. This was followed by the formation of a stress overload zone and the involvement of a crustal sedimentary source. Lastly, stress unloading formed an asymmetric topography across the SE plateau margin.
Rifting is a tectonic process that leads to extensive magmatic activity, continental breakup, and the formation of new oceanic crust. The interplay between rifting and dynamic mantle flow driven by thermal heterogeneity in the mantle along the rift-axis can influence magmatism and deformation beyond the rift zone. We constructed a series of three-dimensional geodynamic models to simulate mantle flow during the syn- and post-rift phases by integrating different initial temperature conditions for the extending rift zone and rift process zone (RPZ). We define the RPZ as the region beyond the rift-tip where strain-rates are elevated relative to the surroundings, enabling potential rift propagation. Our study demonstrated that rift-induced mantle flow, derived from a model with a higher initial temperature (+100K), channelized into the RPZ, reaching up to approximately 900 km beyond the rift-tip. Even after rift termination, the channelized mantle flow continued to transport melt and heat, maintaining a melt flux of approximately 6 & times; 1016 kg/Myr toward the cold and thick lithospheric mantle. Thermal instability induced by the channelized mantle caused localized lithospheric erosion and mantle upwelling. Our study demonstrates that the channelized flow provides new insights into geodynamic evolution, encompassing rift propagation and magma supply to intraplate volcanism. The persistent transport of heat and mass to the RPZ beyond the rift-tip, even after rift termination, can contribute to both accretion and erosion of cold continental roots.
