Gaojun Li

Natural and anthropogenic sources of East Asian dust

Nd-Sr isotopic signatures of loess, modern dust, and their potential source materials are systematically investigated to discriminate the natural and anthropogenic sources of eastern Asian dust. Chinese loess is an eolian deposit that has accumulated over the past few million years, and is characterized by a negative Nd-Sr isotopic correlation that implies binary sources for the natural background of eastern Asian dust. Loess in northeastern China shows the highest eNd(0) value, while loess in western China has the lowest eNd(0). Considering the Nd-Sr isotopic signatures of the potential sources, we propose that the high eNd(0) end member is derived from the arid lands around the northern boundary of China while the low eNd(0) end member originates from the deserts on the northern margin of the Tibetan Plateau. Chinese Loess Plateau deposits are dominated by northern margin Tibetan Plateau dust, suggesting a strong link between the thick eolian deposit in this region and the evolution of the Tibetan Plateau. The long-range transported spring dust in both Nanjing (south China) and Japan has an Nd-Sr isotopic composition similar to that of loess. However, spring dust in Beijing (north China) has a much lower eNd(0) value, indicating an additional anthropogenic contribution of low eNd(0) material, possibly from the sandy lands in the adjacent north and west.

Sulphide oxidation and carbonate dissolution as a source of CO 2 over geological timescales

The observed stability of Earth’s climate over millions of years is thought to depend on the rate of carbon dioxide (CO2) release from the solid Earth being balanced by the rate of CO2 consumption by silicate weathering. During the Cenozoic era, spanning approximately the past 66 million years, the concurrent increases in the marine isotopic ratios of strontium, osmium and lithium suggest that extensive uplift of mountain ranges may have stimulated CO2 consumption by silicate weathering, but reconstructions of sea-floor spreading do not indicate a corresponding increase in CO2 inputs from volcanic degassing. The resulting imbalance would have depleted the atmosphere of all CO2 within a few million years. As a result, reconciling Cenozoic isotopic records with the need for mass balance in the long-term carbon cycle has been a major and unresolved challenge in geochemistry and Earth history. Here we show that enhanced sulphide oxidation coupled to carbonate dissolution can provide a transient source of CO2 to Earth’s atmosphere that is relevant over geological timescales. Like drawdown by means of silicate weathering, this source is probably enhanced by tectonic uplift, and so may have contributed to the relative stability of the partial pressure of atmospheric CO2 during the Cenozoic. A variety of other hypotheses have been put forward to explain the ‘Cenozoic isotope-weathering paradox’, and the evolution of the carbon cycle probably depended on multiple processes. However, an important role for sulphide oxidation coupled to carbonate dissolution is consistent with records of radiogenic isotopes, atmospheric CO2 partial pressure and the evolution of the Cenozoic sulphur cycle, and could be accounted for by geologically reasonable changes in the global dioxygen cycle, suggesting that this CO2 source should be considered a potentially important but as yet generally unrecognized component of the long-term carbon cycle.

Increasing Nd isotopic ratio of Asian dust indicates progressive uplift of the north Tibetan Plateau since the middle Miocene

The time and rate of Tibetan uplift is critical in understanding both the geodynamics of plateau growth and the influence of mountain building on climate change. Models predict that the uplift of the north Tibetan Plateau (NTP) occurred later than uplift of the south and central Tibetan Plateau; uplift of the NTP is believed to have initiated the aridity in the interior of Asia, and to have critically affected the evolution of Asian monsoon and, ultimately, global climate. The history of NTP uplift has remained uncertain, however; scenarios range from Early Miocene to the past million years. This work provides new evidence by relating the Nd isotopic evolution of Asian dust to NTP uplift. The sources of the Asian dust are located in the arid lands between the NTP and the Central Asia orogen. Mass balance calculations indicate that the decreasing e Nd values, from −8.2 to −10.4 for Pacific dust since 15 Ma, reflects increasing relative contribution of NTP materials (40% to 70%) to the dust source. As paleoelevation in the Central Asia orogen has largely remained constant, the increasing relative NTP detritus contribution is interpreted to reflect the progressive surface uplift of the NTP at a rate of at least 0.12 mm yr −1 since 15 Ma.

Spatial and glacial‐interglacial variations in provenance of the Chinese Loess Plateau

The Chinese Loess Plateau (CLP) covers an extensive area over 440,000 km2 and provides an unprecedented terrestrial record of Neogene climate. However, it is still unclear whether the provenance of these loess deposits is uniform or contains spatial and temporal differences. Here this is addressed by comparing detrital-zircon age spectra of typical loess and paleosol samples from three distant sites located at the western, middle, and southeastern parts of the CLP. Our results reveal that the zircon age spectra not only change between loess and paleosol layers but also vary from the western to the eastern CLP, at least during the last glacial cycle. The discrepancies of the zircon age spectra among different sites suggest that the loess provenance of CLP is heterogeneous and spatially variable, although it has been suggested that the mineralogical, elemental and isotopic compositions of loess deposits on CLP are highly homogenous spatially and in glacial-interglacial cycles.

High regional climate sensitivity over continental China constrained by glacial-recent changes in temperature and the hydrological cycle

The East Asian monsoon is one of Earth’s most significant climatic phenomena, and numerous paleoclimate archives have revealed that it exhibits variations on orbital and suborbital time scales. Quantitative constraints on the climate changes associated with these past variations are limited, yet are needed to constrain sensitivity of the region to changes in greenhouse gas levels. Here, we show central China is a region that experienced a much larger temperature change since the Last Glacial Maximum than typically simulated by climate models. We applied clumped isotope thermometry to carbonates from the central Chinese Loess Plateau to reconstruct temperature and water isotope shifts from the Last Glacial Maximum to present. We find a summertime temperature change of 6–7 °C that is reproduced by climate model simulations presented here. Proxy data reveal evidence for a shift to lighter isotopic composition of meteoric waters in glacial times, which is also captured by our model. Analysis of model outputs suggests that glacial cooling over continental China is significantly amplified by the influence of stationary waves, which, in turn, are enhanced by continental ice sheets. These results not only support high regional climate sensitivity in Central China but highlight the fundamental role of planetary-scale atmospheric dynamics in the sensitivity of regional climates to continental glaciation, changing greenhouse gas levels, and insolation.

Evolution of the Cenozoic carbon cycle: The roles of tectonics and CO2 fertilization

Cenozoic carbon fluxes associated with rock weathering, sediment burial, and volcanic degassing are calculated from the mass balance equations coupling marine isotopic records of carbon (both organic and inorganic), strontium, and osmium. The result is confirmed by the good match between modeled carbonate sedimentation rates and carbonate sedimentation rates previously integrated from ocean basins worldwide. The coevolution between weathering and burial of carbonate suggests that marine carbonate accumulation was regulated mainly by the recycling of carbonate rocks, which mediated the bicarbonate ion concentration of the oceans. A reduction in CO2 effusion to the atmosphere caused by reduced volcanic degassing from 52 to 15 Ma and tectonically enhanced organic rock exhumation since 15 Ma is also observed. These changes in CO2 effusion are balanced by concomitant changes in CO2 sequestration by silicate weathering and organic carbon burial. Importantly, we demonstrate a clear decoupling of modeled silicate weathering rates from global climate over the last ∼15 Ma. This observation is inconsistent with temperature-mediated mineral dissolution acting as the key mechanism facilitating the CO2 silicate weathering feedback process. However, we instead observe a clear coupling (positive correlation) between modeled silicate weathering, organic carbon burial, and atmospheric CO2 concentration. We suggest that CO2 fertilization effects on terrestrial biomass productivity and plant weathering could have represented a major negative feedback process helping to balance atmospheric CO2, at least during the Cenozoic ice house periods.

Shifting material source of Chinese loess since ~2.7 Ma reflected by Sr isotopic composition

Deciphering the sources of eolian dust on the Chinese Loess Plateau (CLP) is fundamental to reconstruct paleo-wind patterns and paleo-environmental changes. Existing datasets show contradictory source evolutions of eolian dust on the CLP, both on orbital and tectonic timescales. Here, the silicate Sr and Nd isotopic compositions of a restricted grain size fraction (28–45 μm) were measured to trace the source evolution of the CLP since ~2.7 Ma. Our results revealed an unchanged source on orbital timescales but a gradual source shift from the Qilian Mountains to the Gobi Altay Mountains during the past 2.7 Ma. Both tectonic uplift and climate change may have played important roles for this shift. The later uplift of the Gobi Altay Mountains relative to the Qilian Mountains since 5 ± 3 Ma might be responsible for the increasing contribution of Gobi materials to the source deserts in Alxa arid lands. Enhanced winter monsoon may also facilitate transportation of Gobi materials from the Alxa arid lands to the CLP. The shifting source of Asian dust was also reflected in north Pacific sediments. The finding of this shifting source calls for caution when interpreting the long-term climate changes based on the source-sensitive proxies of the eolian deposits.

Uranium comminution age responds to erosion rate semi-quantitatively

Here we present (234U/238U) data from river sediments collected on the Tibetan Plateau. The (234U/238U) ratios of a specific grain size fraction show good correlation with erosion rates, which were determined by in-situ–produced cosmogenic nuclides. This correlation has previously been observed in a wide range of geomorphic settings, suggesting that (234U/238U) ratios of fluvial sediments have great potential to quantify erosion rates.