East Asian Monsoon Variability Research Articles

Upper Ocean variations at IODP Hole U1505C in the northern South China Sea and their response to the East Asian Monsoon during the middle Miocene

We used the samples from International Ocean Discovery Program (IODP) Hole U1505C in the northern South China Sea (SCS) to reconstruct the upper ocean profiles and discussed the East Asian Monsoon (EAM) variations during 15.0–12.5 Ma. 15 genera and 41 species were identified, and 7 planktonic foraminifer datums were recognized, then a reliable chronostratigraphic framework was updated. A principal component analysis (PCA) was used to identify the suitable assemblages for reconstructing upper water profiles, based on planktonic foraminifera relative abundance and isotope records. The sea surface temperature (SST), paleo-productivity, and the depth of thermocline (DOT) were reconstructed by the content of warm water species, high productivity species, and the ratio of mixed layer to thermocline species, respectively. Three main phases were identified: (1) during 15.0–14.7 Ma, the SST and paleo-productivity were high, while the thermocline was shallow, indicating a warm and rainy climate influenced by East Asian Summer Monsoon (EASM); (2) during 14.7–13.8 Ma, the SST was lower, the paleo-productivity increased significantly, and the thermocline deepened, indicating an enhanced East Asian Winter Monsoon (EAWM); (3) during 13.8–12.5 Ma, the SST increased slightly, the paleo-productivity deceased, while the DOT showed neglectable change, indicating a stable EAWM and a waning EASM. The results of the Morlet wavelet spectrum revealed that EAM and the upper ocean profiles in the northern SCS were predominantly regulated by the eccentricity cycles, emphasizing the significant impact of low-latitude processes on climate variations.

Robust land surface temperature record for north China over the past 21,000 years.

Numerous proxy reconstructions have provided general insight into late Quaternary East Asian Monsoon variability. However, challenges persist in precisely assessing absolute temperature impacts on proxy variations. Here, we use two independent paleothermometers, based on bacterial membrane lipids and clumped isotopes of snail shells, in the same section of the western Chinese Loess Plateau to establish a robust land surface temperature record spanning the past approximately 21,000 years. Our independent temperature records consistently reveal (i) similar land surface temperatures between the Last Glacial Maximum and late Holocene and (ii) a gradual cooling Holocene, which contrasts with the climate model predictions. We propose that changes in soil moisture availability over the deglaciation modulates the land surface temperature recorded by the proxies. A land surface energy partitioning model confirms this mechanism, suggesting that effects of soil moisture availability should be properly considered when comparing proxy records with climate model outputs.

East Asian monsoon variations in the loess–desert transitional zone (northern China) during the past 14 ka and their comparison with TraCE21K simulation results

Abstract The Holocene is a critical period for understanding the East Asian monsoon system (EAM) over long timescales, but high-precision dating and high-resolution records from the Holocene epoch at monsoonal margins of East Asia are lacking. Here, on the basis of closely spaced radiocarbon and optically stimulated luminescence dating results obtained from a typical loess–paleosol sequence on the northern Chinese Loess Plateau (CLP), we provide an independent age-based, high-resolution depositional record of East Asian summer (EASM) and winter monsoons (EAWM) variations over the past ~14 ka. We find that both the EASM and EAWM simultaneously strengthened sometime during the Holocene optimum (~7–5 ka BP), with greater seasonality, and weakened during the Late Holocene. These findings are counterintuitive to our understanding of the EAM variations based on loess records at suborbital scales during interglacial periods, providing an alternative scenario of the monsoon system evolution. We postulate that high-latitude forcing and surface feedbacks, such as vegetation change, have modulated the EAM variations during the Holocene warmth.

Isotopic and magnetic proxies are good indicators of millennial-scale variability of the East Asian monsoon

Past East Asian Monsoon variability has been reconstructed using oxygen isotopes of cave speleothems as well as proxy indicators from Chinese loess sequences. However, where the speleothem record is dominated by precession cycles, loess magnetic susceptibility primarily shows a glacial-interglacial pattern. Here we generate a ~ 130,000 years high resolution record of plant wax hydrogen isotopes from a loess section on the western Chinese Loess Plateau that can directly be compared with both speleothem oxygen isotopes and with magnetic susceptibility from the same section. We find that variations in our plant wax hydrogen isotope record follow the precessional pattern of the speleothem record as opposed to the glacial-interglacial changes in magnetic susceptibility. We propose that hydrogen isotopes mainly record precipitation during the growing season, whereas magnetic susceptibility represents an annual climate signal, including precipitation and temperature. Our findings imply that summer vs annual climate variability is driven by distinct orbital forcings.

Multidecadal-to-interannual modulations of Late Holocene climate variations reconstructed from oxygen isotope ratios of a 237-year-long lived fossil coral in Kikai Island, southwestern Japan

For this study, to elucidate the East Asian monsoon variations on seasonal, interannual, and decadal timescales during the Late Holocene, we generated a 237-year-long time series of bimonthly resolved oxygen isotope ratios (δ18O) from a 3.5–3.2 ka Porites coral in Kikai Island, southwestern Japan. The coral data clearly show seasonal-to-interdecadal variations reflecting seawater temperature and salinity variations for 237 years, which can be a robust benchmark with respect to subtropical Pacific climate variability during the Late Holocene. Along with modern and fossil coral records published earlier from Kikai Island and Okinawa Island, the time-weighted δ18O averages for overlapped 14C ages demonstrate that the annual and summer values had slightly increased by <0.2‰ for 6–3 ka and that the winter values had increased by ∼0.2‰ for 6–5 ka and decreased by ∼0.2‰ for 5–3 ka. The result indicates that the mean climate state around the island had been almost stable on a centennial-to-millennium timescale throughout the transition from the Middle to Late Holocene, but with several short-term cooling excursions of ∼1 °C at 4–3 ka. On interdecadal-to-interannual timescales, amplitudes of the fossil coral δ18O variations for winter and summer were similar or larger at 3.5–3.2 ka relative to today, revealing significant periodicities at 36–38 years, 20–21 years, 6–9 years, and 3–4 years by spectral analysis at 95% confidence intervals. Furthermore, winter and summer coral δ18O time series of the respective periodic components indicate that interannual variations of SST (and/or seawater δ18O) around the island have changed on interdecadal and decadal timescales, which have been modulated on a multidecadal timescale of approximately 60–90 years/cycle. These results imply that the relationships of the East Asian winter and summer monsoon to the El Niño/Southern Oscillation and Pacific Decadal Oscillation dominant in the North Pacific might have been modulated by or associated with the Atlantic Multidecadal Oscillation dominant in the North Atlantic during the Late Holocene.

Changes in Kuroshio Current dynamics and East Asian monsoon variability during the last 26 kyr

The Kuroshio Current flows northward along the east coast of Taiwan toward the Okinawa Trough and the East China Sea, but its dynamics and trajectory were probably different during the Last Glacial Maximum (LGM) due to the globally lower sea level that could have caused a (debated) deflection of the current along the eastern edge of the Ryukyu Arc.Core MD18–3532 has been recovered in an intra-slope basin of the Ryukyu accretionary prism, currently disconnected from the Kuroshio Current, but would have been on its trajectory in case of a NE deflection. Measurements of clay mineral assemblages and illite crystallinity revealed that Taiwan has been the main sediment source at this site over the last 26 kyr. The significantly higher sedimentation rate from the Last Glacial Maximum to the Bølling–Allerød compared with the period from the Younger Dryas to the Holocene, coupled with very low δ15Nsed during LGM and Heinrich Stadial 1, provide evidence for the transport of sediments and Trichodesmium spp. cyanobacteria by the partially deflected Kuroshio Current toward the eastern edge of the Ryukyu Arc. Combined with δ13Corg, TOC, TN, and XRF analyses, an increase in primary productivity has been observed during LGM and Heinrich Stadial 1. This would have been caused by an enhanced East Asian Winter monsoon winds resulting in the deepening of the mixed layer that would have led to the upwelling of the Kuroshio Current nutrient-enriched subsurface waters to the oligotrophic surface waters, and the supply of dust-borne iron from the Chinese Loess Plateau.

Co-evolution of orbital-to-millennial East Asian monsoon and westerlies variability over the past 150 ka

Understanding the evolution and interaction of the mid-latitude westerlies and East Asian monsoon (EAM) is crucial for comprehending Asian climate dynamics. While abundant evidence from loess, lake and speleothem records in arid Central Asia (ACA) and East Asia has revealed the interplay between the mid-latitude westerlies and the EAM on orbital and millennial scales, most of these records only cover the period since the last glacial maximum. Thus, the trade-off relationship between the mid-latitude westerlies and the EAM on glacial-interglacial to millennial timescale since the last interglacial period remains unclear. In this study, we present 150 ka-long grain-size and organic carbon records from two loess sections in the western Chinese Loess Plateau (CLP) and Yili Basin to assess the wind intensity and moisture variability between East Asia and ACA at glacial-interglacial to millennial timescales. Our results indicate that the East Asian winter monsoon (EAWM) and westerly wind exhibit synchronous glacial-interglacial and precession-scale fluctuations. However, the moisture exhibits consistent variations between East Asia and ACA on glacial-interglacial timescale, whereas inconsistent pattern at precessional timescale. The moisture variations in ACA lagged those in the East Asia by 2 kyr during marine isotope stages (MIS)5. Millennial-scale wind and moisture variability in East Asia and ACA are broadly correlated with cooling events in the North Atlantic. Based on the modeling results of the TraCE-21 ka, we suggest that the wind and moisture variations in East Asia and ACA are controlled by the combined effects of ice sheets, insolation, and meltwater flux from orbital to millennial timescales. Therefore, our study provides new insights into the complex interplay between the EAM and mid-latitude westerlies, which can contribute to a better understanding of Asian climate dynamics.

High-resolution ∼55 ka paleomagnetic record of Biển Hồ maar lake sediment from Vietnam in relation to detailed 14C and 137Cs geochronologies

Vietnam's Central Highlands in Pleiku feature numerous volcanic craters (i.e. maars) formed over 0.2 Ma ago that contain continuous high-resolution lacustrine sediment records extending from the Late Pleistocene through the Holocene. Their evolution and sedimentary proxies are potentially powerful archives for interpretation of East-Asian monsoon variability across Vietnam, as well as for local to regional changes in landcover. The focus of this study is to develop a reliable chronology for the Biển Hồ 25-m-long lake sediment sequence in Vietnam's Central Highlands. We present a combined sediment chronology extending back to 55 ka cal BP that is based on 137Cs surface sediment chronology, paleomagnetic dating, and 47 14C dates distributed evenly throughout the sequence. Based on sedimentary facies analysis and a smooth age-depth model based on 137Cs and 14C data, we infer a long-term stable rate of sedimentation in Biển Hồ lake without unconformities or erosional hiatuses, except for the uppermost section (ca. 100 years) where human activities increase the rate of sedimentation significantly. The Biển Hồ sequence thus provides a complete and uninterrupted record of the lake's depositional history. The present contribution is a geochronologic opening salvo for future contributions from Biển Hồ lake featuring multi-disciplinary evidence for longer-term paleoclimate and environmental changes.

Changes in the mean and extreme temperature in the Yangtze River Basin over the past 120 years

Much attention has been paid to extreme temperature changes since 1961 because of the absence of data in the early 20th century. Based on the sub-daily meteorological records prior to 1950 and modern daily meteorological records from 1951 to 2020, we established a daily maximum/minimum temperature series that was processed for data quality control, data interpolation, data merging, and homogeneity in 10 cities in the Yangtze River Basin from 1901 to 2020. The anomalies of the annual mean temperature showed a significant upward trend, especially in the last 40 years. Most stations observed a slight decrease before 1961, whereas they experienced a significant increase in the last 60 years. The upward trend of the entire time period in Chongqing and Shanghai was significant in the last 120 years, with the increase in Shanghai being higher than that in Chongqing. The extreme temperature indices also showed significant trends as well as clear periodic variability. The extreme high-temperature indices rapidly increased in the 1930s, decreased until the 1980s, and then increased again. The extreme low temperature indices fluctuated with positive and negative anomalous values, but generally increased prior to the 1970s and then decreased over the last 40 years. The changes in the extreme temperature indices in Chongqing and Shanghai were highly consistent, except for frost days. The upward/downward trend in Shanghai was more significant than that in Chongqing over the last 40 years. The analysis also showed that the observed changes in regional extreme events were due to rapid urbanization and multi decadal variability of East Asian monsoon, in addition to regional and global warming.

Greenhouse gases modulate the strength of millennial-scale subtropical rainfall, consistent with future predictions

Abstract. Millennial-scale East Asian monsoon variability is closely associated with natural hazards through long-term variability in flood and drought cycles. Therefore, exploring what drives the millennial-scale variability is of significant importance for future prediction of extreme climates. Here we present a new East Asian summer monsoon (EASM) rainfall reconstruction from the northwest Chinese Loess Plateau (CLP) spanning the past 650 kyr. The magnitude of millennial-scale variability (MMV) in EASM rainfall is linked to ice volume and greenhouse gas (GHG) at the 100 kyr eccentricity band and to GHG and summer insolation at the precession band. At the glacial–interglacial timescale, gradual changes in CO2 during intermediate glaciations lead to increased variability in North Atlantic stratification and Atlantic meridional overturning circulation, propagating abrupt climate changes into East Asia via the westerlies. Within the 100 kyr cycle, precession variability further enhances the response, showing that stronger insolation and increased atmospheric GHG cause increases in the MMV of EASM rainfall. These findings indicate increased extreme precipitation events under future warming scenarios, consistent with model results.

East Asian monsoon precipitation decrease during Plio-Pleistocene transition revealed by changes in the chemical weathering intensity of Red Clay and loess-paleosol

East Asian Monsoon variations during the Plio-Pleistocene transition have attracted much scientific interest, but they are still not well understood. We analysed the trace and major element concentrations of the Red Clay and loess-paleosol sequence of the XSC section, on the southern Chinese Loess Plateau (CLP), over the time interval of 4–2 Ma. From the results of geochemical and mineralogical analysis, we conclude that the Na/K ratio can be used to reflect the weathering intensity of plagioclase. Grain- size effects on the Na/K ratio were minimized based on the analysis of multiple size fractions. The Na/K ratios show similar trends between different grain-size fractions during the Pliocene-Pleistocene transition. Additionally, the Na/K ratio for various Holocene eolian sediments is well correlated with the regional mean annual precipitation (MAP). Based on this observation, we relate the Na/K ratio to East Asian Summer Monsoon (EASM) precipitation and propose that the EASM is the primary control of the post-depositional weathering intensity. The Na/K record of the EASM precipitation, as inferred from the Na/K record of the XSC section, progressively weakened and increased in amplitude between 4 and 2 Ma, indicating that the climate on the monsoonal margin evolved from warm and moist during the Late Pliocene to dry and cold during the Early Pleistocene. Our results support the hypothesis that global cooling was responsible for driving the decrease in EASM-derived rainfall via the migration of the intertropical convergence zone (ITCZ) and the associated monsoon precipitation belt during the Plio-Pleistocene transition.

Changes in the Kuroshio Current Dynamics and the East Asian Monsoon Variability from East Taiwan During the Last 26 Kyr

Changes in the Kuroshio Current Dynamics and the East Asian Monsoon Variability from East Taiwan During the Last 26 Kyr

One million years of seasonal seesaw in East Asian monsoon winds

Glacial-to-interglacial variations in East Asian summer and winter monsoon are widely ascribed to orbital and/or global ice-sheet forcing. However, the relative impact of orbital and millennial-scale factors on Pleistocene variations in East Asian monsoon still remains controversial. To better constrain the differential response of seasonal monsoon winds over the last million years we present paired records of siliciclastic silt grain sizes, pollen, minerals, and geochemical tracers obtained from high-sedimentation rate deposits at ODP Sites 1144 and 1146 in the northern South China Sea. The proxy records show that loess-style eolian dust supply of winter monsoon was dominant and fluvial input reduced during peak glacials over the last 900 kyr, moreover, during Heinrich stadials, while abundant fluvial mud marked interglacial regimes as result of enhanced summer monsoon, then completely overlying the weakened dust input of winter monsoon. Likewise, an excess of fluvial mud suppressed the eolian dust signal during the initial part of most glacial stages during and after the Mid-Pleistocene Transition (MPT), in part possibly induced by long-term groundwater reserves and/or unknown climate forcings linked to the southern Hemisphere. Prior to the MPT, during glacial stages 24–32, prolonged groundwater reserves and/or a more limited extent of northern-Hemisphere ice sheets, or unknown southern Hemisphere forcing may have controlled an ongoing interglacial-style humid monsoon climate in East Asia. In summary, our findings suggest that the sediment record of seasonal East Asian monsoon variability in part may have been more sensitive to secondary factors of groundwater storage, plant cover, as well as to the redistribution of insolation energy amongst various climate subsystems than to direct orbital and/or northern ice-sheet forcing. • Loess-style dust supply was predominant during peak glacials and Heinrich stadials. • Fluvial mud earmarked interglacials and initial parts of glacial stages since the MPT. • Limited NH ice sheets during glacial stages 24–32 led to ongoing humidity over Eastern Asia. • Sediment record may be more sensitive to groundwater storage, plant cover and energy redistribution among climate subsystems.

A ∼12 Myr Miocene Record of East Asian Monsoon Variability From the South China Sea

AbstractThe long‐term evolution of the East Asian Monsoon and the processes controlling its variability under changing climate boundary conditions remain enigmatic. Here, we integrate new and published high‐resolution planktic and benthic foraminiferal isotope data with proxy records for chemical weathering derived from diffuse reflectance spectroscopy at Ocean Drilling Program Site 1146 (South China Sea) to reconstruct the evolution of the summer monsoon between ∼17 and 5 Ma. Our records show that an overall warm and humid tropical climate prevailed over southeastern Asia during the Miocene Climatic Optimum, suggesting northward expansion of the tropical rain belt in response to greenhouse gas forcing. By contrast, monsoon seasonality increased during the middle Miocene Climatic Transition in tandem with Antarctic glacial expansion and global cooling. Substantial weakening of the summer monsoon between ∼12.7 and 10.9 Ma supports that decreased weathering and riverine input of nutrients and alkalinity contributed to carbonate depletion in the deep ocean during the Carbonate Crash. Intensification of monsoonal circulation and strengthening of the biological pump through the late Miocene promoted carbon burial, drawdown of atmospheric CO2, and climate cooling during the Biogenic Bloom. These results underscore the dynamic evolution of the East Asian Monsoon throughout the middle to late Miocene. Variations in local insolation forcing and in Southern Hemisphere ice volume, influencing the latitudinal thermal gradient, evaporation‐moisture budgets, and the strength of the tropical convection, exerted major controls on the development of the monsoon.

East Asian monsoon during the past 10,000 years recorded by grain size of Yangtze River delta

Abstract Paleoenvironmental research is critical for understanding delta evolution processes and managing delta sustainability, particularly for delta experiencing significant recent fluvial sediment discharge. Based on other previously reported optically stimulated luminescence (OSL) data, Holocene environmental changes of the Yangtze River delta in response to climate fluctuations and human activities were reviewed on the basis of grain-size analyses of core YZ07. The results of grain-size and end-member analysis (EMA) provide a detailed history of East Asian monsoon variability and environmental changes since ∼10,000 cal year B.P. The lower median values (Md) and sand content reflect relatively cool and dry climate conditions between 10,000 and 9,570 cal year B.P. During the early Holocene (9,570–7,630 cal year B.P.), the highest Md values and sand contents and the lowest end member 2 (EM2) contents suggest the Holocene transgression. The increased Md values and sand contents indicate that the climate conditions were warm and wet during the mid-Holocene thermal optimum. From 4,690 to 4,150 cal year B.P., the climate was cool and dry, corresponding to the cool event, as indicated by the finer grain size. Subsequently, between 4,150 and 2,850 cal year B.P., the grain size derived from the Md value and sand content increased, which reflect a wet and warm episode. The climate, which shifted from warm and wet to cool and dry between 2,850 and 1,020 cal year B.P., may have caused a reduction in the sand contents and Md values. After 1,020 cal year B.P., the lowest values of Md and Standard deviation (Sd) and the highest contents of EM2 and clay suggest that the Yangtze River delta has been severely affected by anthropogenic activity. The variability of the East Asian monsoon intensity in the Yangtze River delta strongly correlates with other East Asian monsoon paleoclimate records in China. These results are important for investigations into the interactions between regional systems and global change in monsoonal climatic regions and can provide an example of the evolution of a large scale geomorphic feature resulting from river-sea interaction.

Disturbed climate changes preserved in terrigenous sediments associated with anthropogenic activities during the last century in the Taiwan Strait, East Asia

Accelerated human activity has greatly reduced sediment discharge into the global oceans in recent decades; however, the potential influence on climate change signals carried by sediments during source-to-sink transport processes remains poorly understood. To this end, multiple sedimentological analyses, including X-ray fluorescence scanning, grain-size composition, clay mineralogy, and 210Pb dating, of three short sediment cores retrieved from the Taiwan Strait, East Asia were performed. The results show that the terrigenous sediments of these cores spanning the last 150 years are mainly derived from the Yangtze River, with relatively small contributions from the Taiwan, Zhejiang, and Fujian rivers. The grain-size compositions are predominantly silt and clay (>90%), with minor sand, and the clay mineral assemblages consist of primarily illite, smectite, and chlorite, with moderate kaolinite. Both the clay mineral assemblage and grain-size compositions inferred from both end-member (EM) modeling and standard deviation analysis provide indirect indicators to reveal the East Asian monsoon variations, realized mainly through the current transport processes. The decreased smectite/(illite + chlorite) ratios since the 1850s represent a relatively long-term weakened East Asian winter monsoon (EAWM), while the overall decreased mean grain-size of the fine sensitive component and EM3/(EM1 + EM2) ratio during the 1850s–1950s also implies a gradually weakened EAWM associated with the synchronously weakened Zhe-Min Coastal Current. These sedimentary records of the East Asian monsoon variations can be compared with numerous published paleoclimate records. However, the largely increased mean grain-size of the fine sensitive component and EM3/(EM1 + EM2) ratios since the 1950s indicates a false strengthened EAWM signal, which conflicts with instrumental data and the clay mineralogy record. This observation, coupled with the coarsened terrigenous grain-size and reduced terrigenous fluxes (decreased Ti/Ca ratios), is mainly attributed to intense anthropogenic influences associated mainly with dam proliferation. Therefore, disturbed climate change information preserved in the terrigenous sediments can be linked to increased anthropogenic activities, perhaps representing an additional feature during the Anthropocene epoch.

Stepwise coupling between Chinese loess deposition and global temperature since the early Pleistocene tested by a multiple-state model

Chinese loess deposits provide a valuable archive of East Asian monsoon variations and compare well with marine oxygen isotope records. However, post-depositional pedogenesis disturbances during some long and warm interglacials may decrease the time resolution and recording efficiency of loess deposits. In this study, we build a simple multiple-state model that involves deposition and pedogenesis processes to simulate loess-paleosol grain size variations and evaluate the recording efficiency under the impact of pedogenesis disturbance. Meanwhile, the modeling results are compared with the measured grain size time series of Xifeng profile from the center of the Chinese Loess Plateau (CLP). The results show that the recording efficiency of the loess deposits in the central CLP has been stepwise enhanced since the early Pleistocene. High-latitude signals in the Chinese loess record between 2.5 and 1.6 Ma were partly erased by post-depositional pedogenesis and only those from several intense glacial periods remain better preserved. Three finer-grained loess units appear at ~550 ka (marine isotope stage (MIS) 14), ~1045 ka (MIS 30), and ~1500 ka (MIS 52), which were likely caused by low dust accumulation rates, suggesting the moderate growth of ice sheets in the Northern Hemisphere. After the mid-Pleistocene transition (~0.9 Ma), the occurrence of grain size changes related to precessional cycles within the long-term glacial periods did not appear in our modeling results. Such variations may thus represent changes of the dust source areas controlled by the East Asian summer monsoon. However, the suppression of precessional variations in the major paleosol layers indicates that loess deposits at least in the central CLP are a low time-resolution archive for capturing precession-scale paleoclimatic changes during interglacial periods.

Linking Holocene East Asian monsoon variability to solar forcing and ENSO activity: Multi-proxy evidence from a peatland in Northeastern China

High-resolution proxy-based paleoenvironmental records derived from peatlands provide important insights into climate changes over centennial to millennial timescales. In this study, we present a composite climatic index (CCI) for the Hani peatland from northeastern China, based on an innovative combination of pollen-spore, phytolith, and grain size data. We use the CCI to reconstruct variations of the East Asian summer monsoon (EASM) intensity during the Holocene. This is accomplished with complete ensemble empirical mode decomposition (CEEMD), REDFIT, and cross-wavelet coherency analysis to reveal the periodicities (frequencies) of the multi-proxy derived CCI sequences and to assess potential external forcing of the EASM. The results showed that periodicities of ca. 300–350, 475, 600, 1075, and 1875 years were present in the Hani CCI sequence. Those periodicities are consistent with previously published periodicities in East Asia, indicating they are a product of external climate controls over an extensive region, rather than random variations caused by peatland-specific factors. Cross-wavelet coherency analysis between the decomposed CCI components and past solar activity reconstructions suggests that variations of solar irradiation are most likely responsible for the cyclic characteristics at 500-year frequency. We propose a conceptual model to interpret how the sun regulates the monsoon climate via coupling with oceanic and atmospheric circulations. It seems that slight solar irradiation changes can be amplified by coupling with ENSO events, which result in a significant impact on the regional climate in the East Asian monsoon area.

Monsoon variations inferred from high-resolution geochemical records of the Linxia loess/paleosol sequence, western Chinese Loess Plateau

Chinese eolian deposits have been investigated intensively to reconstruct East Asian monsoon changes at tectonic to millennial timescales. However, high-resolution elemental proxies sensitive to rapid monsoon changes remains sparse. Here we present multiple elemental ratios of the Linxia loess sequence generated by scanning and conventional X-ray fluorescence (XRF) to assess East Asian monsoon variability at glacial-interglacial to sub-orbital timescales. The results show that 11 elements (Al, Si, K, Ca, Ti, Mn, Fe, Rb, Sr, Zr and Ba) can be reliably measured by XRF scanning of loess cores, albeit with relatively lower precision and increased noise levels (due to variable water content and matrix effects) when compared to XRF scanning of dry and homogenized powder samples. Seven elements (Ca, Fe, Ti, Mn, Zr, Rb and Sr), measured by XRF scanning of the core face and powder samples show high correlation (R2 > 0.7, n = 100) with those measured by conventional XRF methods, and thus can be considered as robust indicators for chemical weathering and grain-size sorting. We employ element ratios of Ca/Ti and Rb/Sr to reflect precipitation-induced weathering intensity, and the Zr/Rb ratio and mean grain size to infer the sorting effects related to winter-monsoon winds. These element ratios exhibit significant glacial-interglacial fluctuations, similar to records of magnetic susceptibility and mean grain size. Precessional- to millennial-scale monsoon changes are also persistent in our high-resolution elemental proxies, consistent with those in Gulang loess elemental ratios and Hulu/Sanbao speleothem δ18O records. Our results suggest that high resolution XRF scanning elemental ratios of Chinese loess have great potential for evaluating co-evolution of orbital- and millennial-scale monsoon variations.

Joint insolation and ice sheet/CO2 forcing on northern China precipitation during Pliocene warmth

We demonstrate that the precipitation record on the Chinese Loess Plateau during the middle Piacenzian (3.264–3.025 Ma) has strong 20-ka precessional cycles, challenging past knowledge about East Asian monsoon variations at the orbital timescales.