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1. Strontium and carbon isotopic evidence for decoupling of pCO2 from continental weathering at the apex of the late Paleozoic glaciation

   https://doi.org/10.1130/G40093.1  

   Chen, Jitao; Montañez, Isabel P.; Qi, Yuping; Shen, Shuzhong; Wang, Xiangdong (March 2018, Geology)

   Earth’s penultimate icehouse (ca. 340–285 Ma) was a time of low atmospheric pCO2 and high pO2, formation of the supercontinent Pangaea, dynamic glaciation in the Southern Hemisphere, and radiation of the oldest tropical rainforests. Although it has been long appreciated that these major tectonic, climatic, and biotic events left their signature on seawater 87Sr/86Sr through their influence on Sr fluxes to the ocean, the temporal resolution and precision of the late Paleozoic seawater 87Sr/86Sr record remain relatively low. Here we present a high-temporal-resolution and high-fidelity record of Carboniferous– early Permian seawater 87Sr/86Sr based on conodont bioapatite from an open-water carbonate slope succession in south China. The new data define a rate of long-term rise in 87Sr/86Sr (0.000035/m.y.) from ca. 334–318 Ma comparable to that of the middle to late Cenozoic. The onset of the rapid decline in 87Sr/86Sr (0.000043/m.y.), following a prolonged plateau (318–303 Ma), is constrained to ca. 303 Ma. A major decoupling of 87Sr/86Sr and pCO2 during 303–297 Ma, coincident with the Paleozoic peak in pO2, widespread low-latitude aridification, and demise of the pan-tropical wetland forests, suggests a major shift in the dominant influence on pCO2 from continental weathering and organic carbon sequestration (as coals) on land to organic carbon burial in the ocean. 

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2. Atmospheric CO ₂ over the Past 66 Million Years from Marine Archives

   https://doi.org/10.1146/annurev-earth-082420-063026  

   Rae, James W.B.; Zhang, Yi Ge; Liu, Xiaoqing; Foster, Gavin L.; Stoll, Heather M.; Whiteford, Ross D.M. (May 2021, Annual Review of Earth and Planetary Sciences)

   null (Ed.)

   Throughout Earth's history, CO 2 is thought to have exerted a fundamental control on environmental change. Here we review and revise CO 2 reconstructions from boron isotopes in carbonates and carbon isotopes in organic matter over the Cenozoic—the past 66 million years. We find close coupling between CO 2 and climate throughout the Cenozoic, with peak CO 2 levels of ∼1,500 ppm in the Eocene greenhouse, decreasing to ∼500 ppm in the Miocene, and falling further into the ice age world of the Plio–Pleistocene. Around two-thirds of Cenozoic CO 2 drawdown is explained by an increase in the ratio of ocean alkalinity to dissolved inorganic carbon, likely linked to a change in the balance of weathering to outgassing, with the remaining one-third due to changing ocean temperature and major ion composition. Earth system climate sensitivity is explored and may vary between different time intervals. The Cenozoic CO 2 record highlights the truly geological scale of anthropogenic CO 2 change: Current CO 2 levels were last seen around 3 million years ago, and major cuts in emissions are required to prevent a return to the CO 2 levels of the Miocene or Eocene in the coming century. ▪  CO 2 reconstructions over the past 66 Myr from boron isotopes and alkenones are reviewed and re-evaluated. ▪  CO 2 estimates from the different proxies show close agreement, yielding a consistent picture of the evolution of the ocean-atmosphere CO 2 system over the Cenozoic. ▪  CO 2 and climate are coupled throughout the past 66 Myr, providing broad constraints on Earth system climate sensitivity. ▪  Twenty-first-century carbon emissions have the potential to return CO 2 to levels not seen since the much warmer climates of Earth's distant past. 

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3. Episodic Late Cretaceous to Neogene crustal thickness variation in southern Tibet

   https://doi.org/10.1111/ter.12689  

   Sundell, Kurt E.; Laskowski, Andrew K.; Howlett, Caden; Kapp, Paul; Ducea, Mihai; Chapman, James B.; Ding, Lin (October 2023, Terra Nova)

   Abstract Recent advancements in quantitatively estimating the thickness of Earth's crust in the geologic past provide an opportunity to test hypotheses explaining the tectonic evolution of southern Tibet. Outstanding debate on southern Tibet's Cenozoic geological evolution is complicated by poorly understood Mesozoic tectonics. We present new U‐Pb geochronology and trace element chemistry of detrital zircon from modern rivers draining the Gangdese Mountains in southern Tibet. Results are similar to recently published quantitative estimates of crustal thickness derived from intermediate‐composition whole rock records and show ~30 km of crustal thinning from 90 to 70 Ma followed by thickening to near‐modern values from 70 to 40 Ma. These results extend evidence of Late Cretaceous north–south extension along strike to the west by ~200 km, and support a tectonic model in which an east–west striking back‐arc basin formed along Eurasia's southern margin during slab rollback, prior to terminal collision of India with Eurasia. 

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4. Small mammals in a large tree: examining the timing of the early therian radiation using a novel phylogenetic metatree; Journal of Vertebrate Paleontology, Program and Abstracts, 2023

   Grossnickle, D; Hellert, S; Ely, R; Slater, G; Lloyd, G; Angielczyk, K_D; Bormet, A; Wilson_Mantilla, G (October 2023, Society of Vertebrate Paleontology)

   Mammals achieved considerably greater ecological diversity in the Cenozoic compared to the Mesozoic. However, there remains uncertainty about the origins of this rise in diversity, such as whether it was triggered by novel ecological opportunities following the Cretaceous-Paleogene (K-Pg) mass extinction event 66 Ma. To test hypotheses on the timing of the mammalian radiation, studies commonly rely on analyses of fossil-only datasets, which often ignore the influence of phylogeny, or analyses of extant- only datasets, which struggle to recreate macroevolutionary patterns in deep time. Thus, an integrative approach is needed that incorporates paleontological data, neontological data, and phylogenetic comparative methods. To this end, we generated a time-calibrated meta-phylogeny(‘metatree’) comprising over 3000 species of trechnotherians (therians and close relatives) from the Mesozoic and Cenozoic, based on 115 published character matrices and a molecular phylogeny of extant mammals. To quantify ecomorphological patterns, we collected jaw lengths (as a proxy for size) and jaw measurements that correlate with diet for 430 extinct and extant species. We then fit a suite of evolutionary models to the data to test various hypotheses on the i) timing of the start of the therian radiation and ii) potential shift in mode of evolution. Although results are sensitive to time-calibration methods and phylogenetic uncertainty, they generally show evidence for a shift in mode of evolution prior to the K-Pg boundary. For both the jaw correlates of diet and body size analyses, results suggest a shift from a constrained (Ornstein-Uhlenbeck) or stochastic (Brownian motion) mode of evolution to an ‘early burst’ mode of evolution, with the best-fitting models being those that model this shift occurring between 76 and 66 Ma. These results suggest that the ecological diversification of therians began in the latest Cretaceous, prior to the K-Pg extinction event. However, our results are also congruent with the hypothesis that the therian radiation was a multiple-step process that involved bursts in diversification at multiple points in time, such as following both the Cretaceous Terrestrial Revolution (ca. 80 Ma) and the K-Pg extinction event. Further, we emphasize the importance of continued efforts to collect early mammal fossils and resolve relevant stratigraphic information, which will allow for more robust tests on the timing of the mammalian radiation. 

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5. Regional exhumation of the Laramide Province

   https://doi.org/10.1130/B37625.1  

   Jepson, Gilby; Carrapa, Barbara; Reeher, Lauren J; DeCelles, Peter G; Afonso, Walter D; Howlett, Caden J; Caylor, Emilia A; Sherpa, Tshering ZL; Wang, Jordan W; Constenius, Kurt N (February 2025, Geological Society of America Bulletin)

   Western North America is the archetypical Cordilleran orogenic system that preserves a Mesozoic to Cenozoic record of oceanic Farallon plate subduction-related processes. After prolonged Late Jurassic through mid-Cretaceous normal-angle Farallon plate subduction that produced the western North American batholith belt and retroarc fold-thrust belt, a period of low-angle, flat-slab subduction during Late Cretaceous−Paleogene time caused upper plate deformation to migrate eastward in the form of the Laramide basement-involved uplifts, which partitioned the original regional foreland basin. Major questions persist about the mechanism and timing of flat-slab subduction, the trajectory of the flat-slab, inter-plate coupling mechanism(s), and the upper-plate deformational response to such processes. Critical for testing various flat-slab hypotheses are the timing, rate, and distribution of exhumation experienced by the Laramide uplifts as recorded by low-temperature thermochronology. In this contribution, we address the timing of regional exhumation of the Laramide uplifts by combining apatite fission-track (AFT) and (U-Th-Sm)/He (AHe) data from 29 new samples with 564 previously published AFT, AHe, and zircon (U-Th)/He ages from Laramide structures in Arizona, Utah, Wyoming, Colorado, Montana, and South Dakota, USA. We integrate our results with existing geological constraints and with new regional cross sections to reconstruct the spatial and temporal history of exhumation driven by Laramide deformation from the mid-Cretaceous to Paleogene. Our analysis suggests a two-stage exhumation of the Laramide province, with an early phase of localized exhumation occurring at ca. 100−80 Ma in Wyoming and Montana, followed by a more regional period of exhumation at ca. 70−50 Ma. Generally, the onset of enhanced exhumation occurs earlier in the northern Laramide province (ca. 90 Ma) and later in the southern Laramide province (ca. 80 Ma). Thermal history models of selected samples along regional cross sections through Utah−Arizona−New Mexico and Wyoming−South Dakota show that exhumation occurred contemporaneously with deformation, implying that Laramide basement block exhumation is coupled with regional deformation. These results have implications for testing proposed migration pathway models of Farallon flat-slab and for how upper-plate deformation is expressed in flat-slab subduction zones in general. 

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