A large carbon pool in lake sediments over the arid/semiarid region, NW China

Carbon burial in lake sediments is an important component of the global carbon cycle. However, little is known about the magnitude of carbon sequestered in lake sediments over the arid/semiarid region of China(ASAC).In this study, we estimate both organic and inorganic carbon burial since *AD 1800 based on nine lakes in ASAC,and discuss the most plausible factors controlling carbon burial. Our estimates show that the annual organic carbon burial rate(OCBR) ranges from 5.3 to 129.8 g cm-2year-1(weighted mean of 49.9 g cm-2year-1), leading to a standing stock of 1.1–24.0 kg cm-2(weighted mean of 8.6 kg cm-2)and a regional sum of *108 Tg organic carbon sequestered since *AD 1800. The annual inorganic carbon burial rate(ICBR) ranges from 11.4 to 124.0 g cm-2year-1(weighted mean of 48.3 g cm-2year-1), which is slightly lower than OCBR. The inorganic carbon standing stock ranges from2.4 to 26.0 kg cm-2(weighted mean of 8.1 kg cm-2),resulting in a sum of *101 Tg regional inorganic carbon burial since *AD 1800, which is slightly lower than the organic carbon sequestration. OCBR in ASAC shows a continuously increasing trend since *AD 1950, which is possibly due to the high autochthonous and allochthonous primary production and subsequently high sedimentation rate in the lakes. This increasing carbon burial is possibly related to both climatic changes and enhanced anthropogenic activities, such as land use change, deforestation, and eutrophication in the lake. Furthermore, OCBR and ICBR are expected to continuously increase under the scenario of increasing precipitation and runoff and enhanced anthropogenic activities.The results of this research show that the buried carbon in lake sediments of the ASAC region constitutes a significant and large carbon pool, which should be considered and integrated into the global carbon cycle.

Ice-volume and insolation influences on hydroclimate changes in central eastern China during the antepenultimate glacial period

The rainfall changes in East Asian summer monsoon(EASM)regions on the orbital timescale remain controversial due to the lack of reliable rainfall records.Here,we present new multiproxy records(δ^(18)O,δ^(13)C,Sr/Ca and Mg/Ca)of a230Thdated stalagmite from Hulu Cave in central eastern China.Multiproxy records reconstruct a regional hydroclimate history from 340 to 261 kyr BP(thousand years before present),approximately covering the antepenultimate glacial period.Theδ^(18)O record is dominated by the precessional cycles,suggesting that EASM responds to changes in Northern Hemisphere summer insolation(NHSI)on the orbital timescale.Significant correlations amongst theδ^(13)C,Sr/Ca and Mg/Ca suggest that a common factor,i.e.,the local hydrological cycle,controlled their variability,and their leading principal component can be used as a proxy linked to regional rainfall.This composite record bears a good similarity to those from the Chinese Loess Plateau,showing a gradually decreasing rainfall during the antepenultimate glacial period,consistent with changes in global ice volume.Superimposed on the long-term trend,three relative wetter intervals were responding to the higher NHSI periods,suggesting that EASM rainfall variability was induced by integrated effects of global ice volume and NHSI.The increased ice sheets and lower NHSI resulted in an increased meridional temperature gradient and southward shift of the westerlies,which shortened the duration of Meiyu and midsummer rainfall.The differences between the rainfall record and the stalagmiteδ^(18)O record indicate that the latter represents the overall EASM intensity linked to monsoon circulation,but does not directly reflect the rainfall changes at the cave sites.

Megadrought and cultural exchange along the proto-silk road

Arid Central Asia(ACA), with its diverse landscapes of high mountains, oases, and deserts, hosted the central routes of the Silk Roads that linked trade centers from East Asia to the eastern Mediterranean.Ecological pockets and ecoclines in ACA are largely determined by local precipitation. However, little research has gone into the effects of hydroclimatic changes on trans-Eurasian cultural exchange. Here,we reconstruct precipitation changes in ACA, covering the mid-late Holocene with a U-Th dated, ~3 a resolution, multi-proxy time series of replicated stalagmites from the southeastern Fergana Valley,Kyrgyzstan. Our data reveal a 640-a megadrought between 5820 and 5180 a BP, which likely impacted cultural development in ACA and impeded the expansion of cultural traits along oasis routes. Instead,it may have diverted the earliest transcontinental exchange along the Eurasian steppe during the 5 th millennium BP. With gradually increasing precipitation after the megadrought, settlement of peoples in the oases and river valleys may have facilitated the opening of the oasis routes, ‘‘prehistoric Silk Roads", of trans-Eurasian exchange. By the 4 th millennium BP, this process may have reshaped cultures across the two continents, laying the foundation for the organized Silk Roads.

Late Holocene hydroclimatic variations and possible forcing mechanisms over the eastern Central Asia

Hydroclimatic variations over the eastern Central Asia are highly sensitive to changes in hemispheric-scale atmospheric circulation systems. To fully understand the long-term variability and relationship between hydroclimate and atmospheric circulation system, we present a high-resolution lascustrine record of late Holocene hydroclimate from Lake Sayram, Central Tianshan Mountains, China, based on the total organic carbon, total nitrogen, and carbonate contents, carbon/nitrogen ratios, and grain size. Our results reveal four periods of substantially increased precipitation at the interval of 4000–3780, 3590–3210, 2800–2160, and 890–280 cal yr BP, and one period of slightly increased precipitation from 1700–1370 cal yr BP. These wetter periods broadly coincide with those identified in other records from the mid-latitude Westerlies-dominated eastern Central Asia, including the northern Tibetan Plateau. As such, a similar hydroclimatic pattern existed over this entire region during the late Holocene. Based on a close similarity of our record with reconstruction of North Atlantic Oscillation indices and solar irradiance, we propose that decreased solar irradiance and southern migration of the entire circum-North Atlantic circulation system, particularly the main pathway of the mid-latitude Westerlies, significantly influenced hydroclimate in eastern Central Asia during the late Holocene. Finally, the inferred precipitation at Lake Sayram has increased markedly over the past 100 years, although this potential future changes in hydroclimate in Central Asia need for further investigation.

Biogenic silica contents of Lake Qinghai sediments and its environmental significance

Changes in the levels ofbiogenic silica （BSi%） in lake sediments have been widely used in order to study lake productivity and palaeoclimatic changes. However, the provenance of biogenic silica （BSi） needs to be investigated for each lake, especially for large lakes, as does the relationship between levels of BSi and relevant environmental factors. In this study, we measured the percentage of BSi contained in lake sediments, river sediments, and surface soils within the Lake Qinghai catchment, and compared the quantities and shapes of diatoms and phytoliths before and after the extraction processes. The results suggest that BSi in lake sediments is primarily derived from endogenous diatoms; therefore, BSi levels can be used to reflect the changes in primary productivity within the lake. Further comparisons showed that on long-term timescales, the variations in BSi% are generally consistent with those in total organic carbon （TOC） and grain size, reflecting the dominant impacts of precipitation on primary productivity in Lake Qinghai. On short-term timescales, however, the relationship between BSi% and TOC and that between BSi% and grain size are not clear or stable. For example, BSi% sometimes covaried with grain size, but it was sometimes out of phase with or even inversely related to grain size. We speculate that both climate and environmental processes, such as the dilution effect, influence short-term BSi% and its related environmental significance. As a result, BSi% should be used selectively as an indicator of climatic changes on different time scales.

The selection of a primary marker for the Anthropocene

Since Homo sapiens appeared,the planet has experienced continuing environmental changes[1,2].In 2000,Crutzen and Stoermer[3]systematically analysed the global changes caused by human activity over the previous 300 years and suggested that the Holocene epoch ended in the 1750s and a new geological age,the Anthropocene,began.Afterwards,the Anthropocene drew a lot of attention from both scientific community and the general public.The recognition and placement of the Anthropocene/Holocene boundary has been a subject of heated debates over the past two decades.At the 35th International Geological Congress(2016;IGC),the Anthropocene Working Group(AWG)recommended the Anthropocene to be formalised with an epoch rank,with the 1950s as its lower boundary[4].

Dust storms in northern China during the last 500 years

The history and mechanisms of dust storms in northern China remain unclear owing to the paucity of reliable longterm,high-resolution geological records.In this study,we reconstructed the dust storm history of the last~500 years in northern China,based on sedimentary coarse fraction(>63μm)of a well-dated core from Lake Daihai,Inner Mongolia.The highresolution data reveal three intervals of frequent dust storms:AD 1520–1580,AD 1610–1720,and AD 1870–2000.The dust storm events in the Lake Daihai area were broadly synchronous with those inferred from other historical or geological records and generally occurred during cold intervals.Changes in the intensity of Siberian High and the westerlies modulated by temperature variations are the likely major factors controlling dust storm dynamics.An interesting feature is that although the intensities of dust storms have been systematically increased during the recent warming period,a clear decreasing trend within this period is evident.The recent increase in average dust storm intensity may be ascribed to an increase in particle supply resulting from a rapid increase in human activity,whereas the weakening trend was likely caused by decreases in average wind speed resulting from the recent global warming.

A multiple-proxy stalagmite record reveals historical deforestation in central Shandong, northern China

Evaluating anthropogenic impacts on regional vegetation changes during historical time is not only important for a better understanding of the Anthropocene but also valuable in improving the vegetation-climate models.In this study,we analyzed stable isotopes(δ^18O,δ^13C)and trace elements(Mg/Ca,Sr/Ca)of a stalagmite from Huangchao Cave in central Shandong,northern China.230Th and AMS14C dating results indicate the stalagmite deposited during 174BC and AD1810,with a hiatus between AD638 and 1102.Broad similarities of theδ^18O and trace elements in the stalagmite suggest they are reliable precipitation indexes.Theδ^13C of the stalagmite,a proxy of vegetation change,was generally consistent with local precipitation and temperature variations on a centennial-scale before the 15th century.It typically varied from–9.6‰to–6.3‰,indicating climate controlled C3 type vegetation during this period.However,a persistent and marked increasing trend in theδ^13C record was observed since the 15th century,resulting inδ^13C values from–7.7‰to–1.6‰in the next four centuries.This unprecedentedδ^13C change caused by vegetation deterioration cannot be explained by climate change but is fairly consistent with the dramatically increasing population and farmland in Shandong.We suggest that the increasing deforestation and reclamation in central Shandong began to affect vegetation in the mountain region of central Shandong since the 15th century and severely destroyed or even cleared the forest during the 16th–18th century.

Variations in monsoon precipitation over southwest China during the last 1500 years and possible driving forces

Understanding hydroclimatic patterns and their possible driving mechanisms during distinct climate periods over the last 1500 years—such as the Medieval Warm Period(MWP),the Little Ice Age(LIA),and the Current Warm Period—is crucial for predicting future changes to monsoon precipitation in southwest China under global warming scenarios.In this study,based on ^(210)Pb and ^(137)Cs dating of surface sediments and AMS ^(14)C dating of terrestrial plant residues,we establish a robust age model that covers the last~1500 years(AD 439–2012)at Lake Yihai in southwest China.We use analyses of multiple geochemical proxy indices,including loss on ignition at 550℃,total organic carbon,total nitrogen,C/N ratios,and stable carbon isotopes of organic matter to reconstruct changes in summer monsoon precipitation at Lake Yihai during the last~1500 years.The results show that,over southwest China,warm and dry climate conditions prevailed during the MWP(AD 1000–1400)and the past 200 years,whereas conditions during the LIA(AD 1400–1800)were cold and wet.This is consistent with evidence from other geological records over southwest China,such as stalagmite and lake sediment data.Similar hydroclimatic patterns have occurred over the last 1500 years in adjacent tropical/subtropical monsoon regions where the climate is similarly dominated by the Indian summer monsoon(e.g.,South China,the South China Sea,Southeast Asia,Northeast India).We suggest that the meridional migration of the mean position of the Inter-tropical Convergence Zone,and El Niño/Southern Oscillation conditions which are linked to tropical Pacific sea surface temperature,are responsible for centennial-scale hydroclimatic patterns over southwest China and adjacent areas during the last 1500 years.