Lacustrine stromatolites were widespread in the Miocene Wudaoliang Group(stromatolites of the Wudaoliang Group), northern Tibetan Plateau;but only at one location nearby the Wudaoliang Town, they occurred intensively ...Lacustrine stromatolites were widespread in the Miocene Wudaoliang Group(stromatolites of the Wudaoliang Group), northern Tibetan Plateau;but only at one location nearby the Wudaoliang Town, they occurred intensively in thick, laterally traceable beds(Wudaoliang stromatolites). Although deposited in lacustrine environment, the lack of fossils in these rocks hampers determining whether the stromatolites formed in freshwater or saline conditions.To address this problem, and in an attempt to identify criteria to distinguish differences of freshwater and saline conditions, we studied the laminae microfabrics, stable carbon and oxygen isotope ratios, rare earth element patterns and biomarkers of the stromatolites. These stromatolites can be divided into fenestral stromatolites and agglutinated stromatolites. The fabric of fenestral stromatolites is formed by microcrystalline carbonate enclosing spar-cemented, angular crystal traces. Essentially, this fabric is interpreted as pseudomorph after former formed evaporite crystals. Faecal pel ets identical to that of the present-day brine shrimp Artemia, lack of other eukaryotic fossils,and stable isotopic signals point to a shal ow, evaporation-dominated hypersaline lake setting. Covariation of carbon and oxygen isotopes indicates hydrological y closed conditions of the Miocene lake on northern Tibetan Plateau.However, if compared to other lacustrine carbonates of the Wudaoliang Group, the high δ13 C values of the investigated Wudaoliang stromatolites reveal an additional photosynthetic effect during the deposition of the stromatolites.Furthermore, although no direct evidence is available from field observations and microfabrics, a positive europium anomaly of Wudaoliang stromatolites indicates that a palaeo-hydrothermal inflow system had existed in the outcrop area.These new results favour a hypersaline lake setting subject to hot spring inflow for the Wudaoliang stromatolites, in contrast to earlier interpretations suggesting a freshwater lake setting(e.g. Yi et al., Jo展开更多
Reconstruction of the paleoaltitude history of the Tibetan Plateau is critical for understanding the linkage between tectonics and its effect on regional and global climate change.Presently,most of the paleoaltitude s...Reconstruction of the paleoaltitude history of the Tibetan Plateau is critical for understanding the linkage between tectonics and its effect on regional and global climate change.Presently,most of the paleoaltitude studies are concentrated on the southern and southeastern part of the Tibetan Plateau,and few studies have been conducted in the central-northern part.In this paper we focused on the Wudaoliang Formation in the Tuotuohe Basin,central to northern Tibetan Plateau,to reconstruct paleoaltitude based on carbonate oxygen isotopes.The carbonate samples are primary or have experienced an early stage of digenesis.Based on the thermodynamic and empirical model results,the paleoaltitude of the Wudaoliang Formation is found to be around 2700–3260 m(average of 2980±280 m)in the early Miocene(~24 Ma).Integrating paleoaltitude results from Wudaoliang Basin and our results,we conclude that crustal shortening and tectonic activity were strong during the late Eocene to late Oligocene-early Miocene and relatively weak during the early Miocene in the central-northern Tibetan Plateau.展开更多
基金funded by the National Natural Science Foundation of China(Grant Nos.41772105 and 41402099)
文摘Lacustrine stromatolites were widespread in the Miocene Wudaoliang Group(stromatolites of the Wudaoliang Group), northern Tibetan Plateau;but only at one location nearby the Wudaoliang Town, they occurred intensively in thick, laterally traceable beds(Wudaoliang stromatolites). Although deposited in lacustrine environment, the lack of fossils in these rocks hampers determining whether the stromatolites formed in freshwater or saline conditions.To address this problem, and in an attempt to identify criteria to distinguish differences of freshwater and saline conditions, we studied the laminae microfabrics, stable carbon and oxygen isotope ratios, rare earth element patterns and biomarkers of the stromatolites. These stromatolites can be divided into fenestral stromatolites and agglutinated stromatolites. The fabric of fenestral stromatolites is formed by microcrystalline carbonate enclosing spar-cemented, angular crystal traces. Essentially, this fabric is interpreted as pseudomorph after former formed evaporite crystals. Faecal pel ets identical to that of the present-day brine shrimp Artemia, lack of other eukaryotic fossils,and stable isotopic signals point to a shal ow, evaporation-dominated hypersaline lake setting. Covariation of carbon and oxygen isotopes indicates hydrological y closed conditions of the Miocene lake on northern Tibetan Plateau.However, if compared to other lacustrine carbonates of the Wudaoliang Group, the high δ13 C values of the investigated Wudaoliang stromatolites reveal an additional photosynthetic effect during the deposition of the stromatolites.Furthermore, although no direct evidence is available from field observations and microfabrics, a positive europium anomaly of Wudaoliang stromatolites indicates that a palaeo-hydrothermal inflow system had existed in the outcrop area.These new results favour a hypersaline lake setting subject to hot spring inflow for the Wudaoliang stromatolites, in contrast to earlier interpretations suggesting a freshwater lake setting(e.g. Yi et al., Jo
基金supported by the“Strategic Priority Research Program”of the Chinese Academy of Sciences(XDB26000000)the National Natural Science Foundation of China(No.42572166,41420104008)+3 种基金Open Fund of the State Key Laboratory of Loess and Quaternary Geology,Institute of Earth Environment,CAS(SKLLQG1629,SKLLQG1708)Breeding programs of State Key Laboratory of Loess and Quaternary Geology(SKLLQGPY1804)the“The Second Tibetan Plateau Scientific Expedition and Research”a part of the“Belt&Road”project of IEECAS。
文摘Reconstruction of the paleoaltitude history of the Tibetan Plateau is critical for understanding the linkage between tectonics and its effect on regional and global climate change.Presently,most of the paleoaltitude studies are concentrated on the southern and southeastern part of the Tibetan Plateau,and few studies have been conducted in the central-northern part.In this paper we focused on the Wudaoliang Formation in the Tuotuohe Basin,central to northern Tibetan Plateau,to reconstruct paleoaltitude based on carbonate oxygen isotopes.The carbonate samples are primary or have experienced an early stage of digenesis.Based on the thermodynamic and empirical model results,the paleoaltitude of the Wudaoliang Formation is found to be around 2700–3260 m(average of 2980±280 m)in the early Miocene(~24 Ma).Integrating paleoaltitude results from Wudaoliang Basin and our results,we conclude that crustal shortening and tectonic activity were strong during the late Eocene to late Oligocene-early Miocene and relatively weak during the early Miocene in the central-northern Tibetan Plateau.
