Delayed Earth system recovery following the end-Permian mass extinction is often attributed to severe ocean anoxia. However, the extent and duration of Early Triassic anoxia remains poorly constrained. Here we use paired records of uranium concentrations ([U]) and 238 U/ 235 U isotopic compositions (δ 238 U) of Upper Permian−Upper Triassic marine limestones from China and Turkey to quantify variations in global seafloor redox conditions. We observe abrupt decreases in [U] and δ 238 U across the end-Permian extinction horizon, from ∼3 ppm and −0.15‰ to ∼0.3 ppm and −0.77‰, followed by a gradual return to preextinction values over the subsequent 5 million years. These trends imply a factor of 100 increase in the extent of seafloor anoxia and suggest the presence of a shallow oxygen minimum zone (OMZ) that inhibited the recovery of benthic animal diversity and marine ecosystem function. We hypothesize that in the Early Triassic oceans-characterized by prolonged shallow anoxia that may have impinged onto continental shelvesglobal biogeochemical cycles and marine ecosystem structure became more sensitive to variation in the position of the OMZ. Under this hypothesis, the Middle Triassic decline in bottom water anoxia, stabilization of biogeochemical cycles, and diversification of marine animals together reflect the development of a deeper and less extensive OMZ, which regulated Earth system recovery following the end-Permian catastrophe.paleoredox | uranium isotopes | biogeochemical cycling | carbon isotopes | Early TriassicT he end-Permian mass extinction-the most severe biotic crisis in the history of animal life-was followed by 5 million years of reduced biodiversity (1, 2), limited ecosystem complexity (3), and large perturbations in global biogeochemical cycling (4, 5). Ocean anoxia has long been invoked both as a cause of the extinction (6-8) and as a barrier to rediversification (9). Numerous lines of evidence demonstrate widespread anoxic conditions around the time of the end-Permian mass extinction (e.g., refs. 6 and 10-12). In contrast, the prevalence of anoxia during the 5-to 10-millionyear recovery interval remains poorly constrained (13,14).Reconstructing paleoredox conditions is challenging because some indicators of anoxia characterize only the local conditions of the overlying water column, whereas other indicators may be influenced by confounding factors, such as weathering rates on land. Here, we use paired measurements of [U] and δ 238 U in marine carbonate rocks to differentiate changes in weathering of U from variations in global marine redox conditions. Microbially mediated reduction of U(VI) to U(IV) under anoxic conditions at the sediment−water interface results in a substantial decrease in uranium solubility and a measureable change in 238 U/ 235 U (15-18). Because 238 U is preferentially reduced and immobilized relative to 235 U, the δ 238 U value of seawater U(VI) decreases as the areal extent of bottom water anoxia increases (Fig. S1). Consequently, a global increase in the extent of anoxi...
Gugulipid is an extract of the guggul tree, Commiphora mukul, that is used to treat hyperlipidemia in humans. The lipid-lowering activity is found in the stereoisomers and plant sterols Z-guggulsterone and E-guggulsterone. The molecular basis for the lipidlowering action of guggulsterone has been suggested to be antagonism of the farnesoid X receptor, a member of the nuclear receptor superfamily of ligand-activated transcription factors. To determine whether guggulsterone has the ability to function as an agonist of other nuclear receptor family members, we screened a panel of these proteins for their ability to transactivate reporter genes. Here, we show that guggulsterones activate the estrogen receptor ␣ isoform, progesterone receptor, and pregnane X receptor. Concentration-response analysis using reporter gene assays indicate that guggulsterones activate these three receptors with EC 50 values in the low micromolar range. Furthermore, we show that guggulsterone-mediated activation of the pregnane X receptor induces the expression of CYP3A genes in both rodent and human hepatocytes. Protein interaction assays indicate that guggulsterones interact directly with pregnane X receptor, thereby modulating interaction with protein cofactors. We introduce a novel method to screen herbal remedies for their ability to activate pregnane X receptor. Pregnane X receptor activation is known to cause herb-drug interactions, and our data suggest that gugulipid therapy should be used cautiously in patients taking prescription medications that are metabolized by CYP3A family members. Moreover, our data suggest the need for additional studies of guggulsterones agonist activity against estrogen receptor ␣ isoform and the progesterone receptor.
The Nanpanjiang Basin occurs in a key position for resolving controversies of basin tectonics and patterns of plate assembly at the junction between south China and Southeast Asian plates. Paleocurrent measurements indicate that siliciclastic turbidites in the basin were sourced by the Precambrian Jiangnan uplift to the northeast, the Precambrian Yunkai uplift to the southeast and the Triassic Songma suture to the south. Detrital zircon geochronology reveals Archean (2500 Ma), Paleoproterozoic (1800–1900 Ma), Neoproterozoic (900–1000 Ma) and Paleozoic (420–460 Ma) ages consistent with derivation from the Jiangnan and Yunkai uplifts. A large Permian‐Triassic peak of 250 Ma is present in the southern basin and attenuates northward suggesting derivation from an arc developed along the Songma suture. Sandstone QFL compositions average 65/12/23% and plot in the recycled orogen field except for a few samples in the southern basin that fall in the dissected arc field. The compositions are consistent with derivation from Precambrian basement that includes orogenic complexes. In the southern basin, Middle Triassic turbidites contain greater lithics and feldspars and Lower Triassic turbidites have volcaniclastic composition consistent with derivation from a southerly arc. Our preferred interpretation is evolution from remnant basin to a large peripheral foreland with southward subduction and convergence with Indochina along the Songma suture. The previously proposed Dian‐Qiong zone is not a suture as its map location places it within carbonate platforms bounded by identical stratigraphy. The Nan‐Uttaradit zone is too distant to have provided voluminous siliciclastic flux to the basin. The Nanpanjiang Basin provides an example of the evolution of an exceptionally large foreland with far‐field rejuvenation of Precambrian uplifts and carbonate platforms that were significantly influenced by siliciclastic flux. The timing and pattern of turbidite basin fill impacted platform evolution by enabling margin progradation in areas proximal to siliciclastic sources, whereas platforms distant from sources were driven to aggradation and extreme relief with large‐scale gravitational sector collapse.
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