ABSTRACT In this study, we provide an ~47.0‐kyr mercury (Hg) deposition record derived from a lacustrine sedimentary sequence in Dahu swamp in the eastern Nanling Mountains, China. The results indicate that organic matter (OM) input played a role in Hg accumulation, whereas the contribution of inorganic weathering detritus had a negligible impact. On the other hand, evolution of the plant community and wildfire events were also important factors affecting Hg accumulation. Compared to arboreal plants, lower (herbs and ferns) plants had stronger absorption and enrichment ability for Hg thus favoring enhanced Hg accumulation in the sediments. We therefore infer that Hg deposition in Dahu sediments was greatly controlled by changes in climatic conditions. Relatively dry and cold climatic conditions, which resulted in reduced arboreal plant communities and increased lower plants, would favor an increased Hg accumulation rate; furthermore, dry conditions may cause more wildfire events thus leading to enhanced release of Hg into the environment and increased contribution of Hg to the sediments, whereas relatively wet and warm conditions would result in the opposite situation. The record of Hg deposition demonstrates a synchronous orbital‐scale variation trend with the intensity of the East Asian summer monsoon. Period analysis reveals several millennial‐ and centennial‐scale cycles similar to solar insolation, together with several millennial events of increased Hg concentration and accumulation rate, which coincided with the corresponding Heinrich events, suggesting joint impacts of solar irradiation and the action of melting water as well as changes in zonal atmospheric circulation on Hg accumulation in Dahu sediments.
ABSTRACT Deciphering the source(s) of sediment deposited in Hugunagyan Lake in South China using magnetic properties will advance our understanding of the evolution of East Asian monsoon systems. However, the source of magnetic minerals in the surface sediments of this lake is unknown, still needing further exploration. Due to simple hydrological systems, the Hugunagyan Lake provides a unique record for investigating the magnetic property of sediments and surface processes. In this study, based on the magnetic properties of 43 surface sediment, 12 surface soil, and 7 volcanic rock samples from the catchment, we demonstrate that the magnetic minerals in sediments were dominated by fine super paramagnetic magnetic particles, with coexistence of pseudo‐single‐domain states minerals. There are no signatures showing the presence of biogenic and authigenic magnetite. The magnetic properties of surface sediments display significant differences between the potential sources of aeolian dust from the loess in North China and modern surface soils in Central Asia, suggesting that the magnetic minerals mainly originate from the weathered materials in the catchment, and there are no further signals of winter monsoon‐driven aeolian dust input.
Understanding the responses of vegetation composition to climate, fire regimes and human disturbance is crucial to provide valuable insights to protect current and future ecosystems. However, relevant studies were poorly performed on the heavily forested montane areas in subtropic East Asia. In this study, we present pollen and charcoal records from a 95-cm long peat core in the subalpine Daping swamp in the western Nanling Mountains in subtropic China, to discuss the possible interactions among vegetation, fire, climatic change, and human activities in the late-Holocene. Our results suggest that the vegetation in the study area was composed of deciduous–evergreen mixed forests with few fire events during 3140–630 cal yr BP, and its changes were mainly controlled by climate with rare human impacts reflected by low concentrations of charcoals, low proportions of Poaceae pollen, and high AP/NAP ratios. After 630 cal yr BP, pollen data indicate an obvious shift from dense forests to more open landscape. The sharp increase of charcoal concentrations, the extremely low AP/NAP and increased Poaceae percentages suggest that this vegetation shift is not only impacted by the dry climate conditions, but also by the intensified deforestation due to enhanced human activities such as the slash-and-burn cultivation, etc. This study reveals a vital transitional timing from the natural to the superimposed anthropogenic forcing of vegetation composition and fire regimes at ~630 cal yr BP. Regional comparison of charcoal records indicates that the onset timing of intensified human disturbance in the inland montane areas is much later than that in the coastal areas in subtropic China. We infer the obstacle impacts of the montane terrain and the different responses of the dissemination of agriculture and enhanced population migration have played a crucial role in this asynchronous spreading pattern.
The western Guangdong Province (WGDP) in China, influenced by both the East Asian and the Indian summer monsoon, is a pivotal region for exploring the Asian summer monsoon (ASM) evolution. So far, there still lacks detail studies on late Holocene climatic/environmental changes here. In this study, multiple magnetic parameters were measured on the lake sediments of Hedong section in this area. Results indicated a dominance of pseudo-single-domain detrital (titano-)magnetite or maghemite mixed with paramagnetic and anti-ferromagnetic minerals in sediments, and the impacts of authigenic greigite, bacterial activity, post-depositional dissolution, and dilution effects were trivial. Changes in rainfall-induced surface erosion intensity were determined as a role in sediment magnetic properties. Intensified surface erosion deduced by increased rainfall favored enhanced concentrations and more coarse-grained magnetic minerals in sediments, and vice versa. Thus, the magnetic parameters can be used to reflect varying ASM strength in the past ~4300 years. They revealed four relatively humid (strengthened ASM) (i.e., 4300–4000 cal. yr BP, 3500–3130 cal. yr BP, 2640–1950 cal. yr BP, and 1250–850 cal. yr BP), and three dry periods (weakened ASM) (i.e., 4000–3500 cal. yr BP, 3130–2640 cal. yr BP, and 1950–1250 cal. yr BP) which coincided with the Bond events 3, 2, and 1 respectively. In combination with the three significant cycles (i.e., 1250-, 75-, and 65-year) revealed by spectral analysis on SIRM record, we inferred both the external forcing (e.g., solar activity) and internal hemispheric-scale atmospheric processes (e.g., ENSO, North Atlantic Oscillation) have played an integral role in late Holocene climate changes in the study region.
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