Abstract High topography is the manifestation of the balance between deep and surficial erosional processes. Hence, reconstructions of paleotopography are critical for disentangling records of orogenesis and climate. Here we used a new approach by combining detrital zircon U‐Pb geochronology and tetraether‐based paleothermometry to characterize the Neogene paleotopography of Northern Tibetan Plateau. Detrital zircon U‐Pb data reveal that the Qilian Shan has been uplifted, providing sediments to bounding basins since circa 15.8 Ma. The paleothermometry studies show warm temperatures for paleosols (<12.4–9.5 Ma and 3.7–2.0 Ma) and low temperatures for lacustrine facies (12.4 Ma and 9.5–3.7 Ma). We interpret the different temperatures to reflect the in situ production of tetraethers under warm temperatures within the basin (paleosols) versus terrestrial inputs from high and cold drainage to the paleolake (lacustrine facies). The study supports a topography with significant relief in the Northern Tibetan Plateau since 12.4 Ma.
Abstract The Qaidam Basin marks a crucial boundary between the Westerlies and the Asian summer monsoons. Previous studies in the Qaidam Basin have advanced our knowledge of the paleoclimate over glacial to interglacial cycles. However, our understanding of the paleoclimatic sensitivity of the Qaidam Basin to the relative strength of these two climatic driving forces remains limited due to the lack of regional paleoclimatic reconstructions. The Qaidam Basin is proposed as a regional and global eolian dust source during the glacial periods, during which a cold, dry climate is associated with the equatorward shift of the jet stream. On the contrary, paleoshoreline records suggest that a highstand lake stage prevailed in late Marine Isotope Stage 3 (MIS 3) and lasted until 15 ka. To address this conundrum, we have applied an integrated approach to reconstructing the regional paleoclimatic history by combining compound-specific isotope analysis, lake temperature reconstruction, and numerical modeling. Our results show varying paleoclimate associated with the dynamic climate boundary since 45 ka: (1) a wet climate during late MIS 3, when the Asian summer monsoons are strengthened under high summer insolation and penetrate further into Central Asia; (2) a general cold, dry but wetter than at present climate in the Last Glacial Maximum (LGM), when the Asian summer monsoons retreat and the Westerlies become dominant; and (3) three short periods of extreme aridity corresponding to the Younger Dryas and Heinrich 2 and 4 events, when the normal moisture transport via the Westerlies and Asian summer monsoons is interrupted. The numerical modeling supports an increase in the effective precipitation during the LGM due to reduced evaporation under low summer insolation. These results suggest that the Westerlies and Asian summer monsoons alternately controlled the climate in the Qaidam Basin in response to precessional forcing during the late Pleistocene.
Abstract Studies reveal that the sea-surface temperature (SST) of the Northern Hemisphere decreased at a smaller amplitude than that of the Southern Hemisphere during the Eocene–Oligocene transition (EOT). This interhemispheric temperature asymmetry has been associated with intensified Atlantic Meridional Overturning Circulation (AMOC) that may have driven enhanced precipitation and weathering in low latitudes and the subsequent drawdown of atmospheric carbon dioxide. However, no quantitative constraints on paleo-precipitation have been reported in low latitudes to characterize the AMOC effect across the EOT. Here, we present the results of high-resolution (ca. 6 k.y. per sample) isotopic and biomarker records from the Gulf of Mexico. Reconstructed precipitation using leaf wax carbon isotopes shows an increase of 44% across the EOT (34.1–33.6 Ma), which is accompanied by a secular increase in SST of ~2 °C during the latest Eocene. We attribute the enhanced precipitation in the Gulf of Mexico to the northward shift of the Intertropical Convergence Zone that was driven by an enlarged polar-tropic temperature gradient in the Southern Hemisphere and an invigorated AMOC. Our findings link changes in meridional temperature gradient and large-scale oceanic circulation to the lowlatitude terrestrial hydroclimate and provide paleohydrological evidence that supports CO2-weathering feedback during the EOT “greenhouse” to “icehouse” transition.
Compound-specific stable isotope analysis and biomarker-based paleothermometry have been increasingly applied to for paleoclimate and paleoecology studies. The first project aims to reconstruct the paleoclimate in the Qaidam Basin during the last glacial period. Qaidam Basin in the Northern Tibetan Plateau is a critical eolian factory located at a unique geographic location that links the Westerlies and the Asian summer monsoons. However, how the interactions of two climatic systems influence the paleohydrology that in turn impacts the eolian production in the Qaidam Basin has been seldom explored. Our compound-specific hydrogen isotope and UK 37 temperature data found that the Westerlies and Asian summer monsoons alternately controlled the Qaidam Basin’s climate in response to precessional forcing during the late Pleistocene. The second project studies the hydrological change in the Gulf of Mexico (GoM) region during the “greenhouse” to “icehouse” transition near the Eocene-Oligocene boundary (33.9 Ma). The geological evidence and climate model proposes that cooling in the Northern Hemisphere was delayed and weaker than in the Southern Hemisphere during the climate transition, which intensifies the Atlantic Meridional Overturning Circulation, increased precipitation in low-latitudes, and subsequent drawdown of atmospheric carbon dioxide during weathering processes. However, there are no quantitative constraints on variations in low latitude precipitation during this interval. Our paleoclimate data from the GoM through the climate transition interval (33.45 – 34.13 Ma) show increased precipitation up to 50%, supporting the CO2- weathering feedback hypothesis and highlighting the low-high latitude climate and atmospheric-oceanic connectivities. The third project focuses on the post middle Miocene paleoclimate and ecological change in the northern Tibetan Plateau (TP). The relative importance of high topography on the TP and the global cooling in regional climate and ecology in Central Asia has been a longstanding debate. Our carbon isotope records from Hexi Corridor, northern TP (spanning from ca. 16 Ma to 2 Ma) show that (1) global cooling plays a primary role in the decline in C4 plant contribution in northern Tibetan Plateau since the ~14.8 Ma, and (2) the disparate regional climate patterns in northern Tibetan Plateau since ~12 Ma are owing to the uplift of plateau.
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