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In this paper, grain‐size records of 128 surface samples and six sediment cores retrieved from freshwater marshes across the Sanjiang Plain were employed to reconstruct the marshes' hydrological variation during the Holocene and to discuss its links to the East Asian summer monsoon (EASM) variation. The grain‐size hydrological significance for the marshes' deposits was first interpreted by quantitative analyses of 128 modern samples. The results indicate that the sand fraction exhibits a higher level significance to modern water‐level conditions than other grain‐size parameters. Thus, the sand accumulation rates of the six cores were calculated and employed to reconstruct the Holocene hydrological regime of the marshes on the plain. Based on the cluster analysis, it can be identified that three higher water‐level stages occurred during 6.2–5.6 thousand yr BP, 5.6–4.5 thousand yr BP, and 2.8–2.4 thousand yr BP, whereas four lower water‐level stages occurred during ~8.0–6.2 thousand yr BP, 4.5–2.8 thousand yr BP, 2.4–0.7 thousand yr BP, and 0.7–0 thousand yr BP, respectively. Considering the prevalent monsoon climate on the modern Sanjiang Plain, it is reasonable to believe that the local water‐level fluctuations were mainly regulated by the EASM variation rather than local factors including flooding, waterway modification, depression infilling, and human activities. The higher water‐level stages were linked to a stronger EASM and vice versa. This study provides essential background datasets for understanding the potential controls on the marsh development and degradation on the Sanjiang Plain. [ABSTRACT FROM AUTHOR]

A comprehensive and integrative view of East Asian monsoon evolution during the Holocene is still under debate, and additional high-resolution proxy records from climatically sensitive locations are requisite to solve this complex issue. In this paper, we present three well-dated mud/peat cores from a paleo-pingo depression in the Sanjiang Plain, a climatically sensitive region to monsoon variation, to reveal the paleoenvironmental history of the wetland and discuss the regional impacts from monsoon evolution. A paleolake developed in the study area before 5.5 ka BP, and a peatland initiated thereafter consequent upon the gradual shrinking of the paleolake. This transition lasted until 4.5 ka BP, when the paleolake changed entirely to a wetland. Considering the prevalent monsoon climate in the Sanjiang Plain, we suggest that the lake-to-wetland transition from 5.5 to 4.5 ka BP indicates a rapid decline of the East Asian summer monsoon in addition to autogenic basin infilling processes. Such a remarkable monsoon weakening event has been documented across northern China, and we associate this with ocean–atmosphere interactions throughout low-latitude regions. [ABSTRACT FROM AUTHOR]

Understanding the response of wetlands to climate changes could provide useful insights toward predicting the wetlands future in a warmer world. Here, we present a well-dated peat/mud profile with multiple proxies to reconstruct the historic development of Honghe wetland and discuss its response to the East Asian monsoon variations during the Holocene. The results show that the Honghe wetland developed as a shallow-water lake in the mid-early Holocene during a time of high precipitation from the East Asian monsoon. At 4600 years BP, peat layers appeared with sharp increases in the accumulation rate, grain-size Md and arboreal plants, marking a decline in water levels with the relative dry climate. Moreover, the transition corresponds well to the mid-Holocne monsoon weakening event and we suggest the decrease of the monsoon associated precipitation plays a critical role in driving the lake-peatland transition in Honghe. Facing the intensified summer monsoon in future, we suggest some practical measures should be taken to protect the present wetlands from changing back to lakes. [ABSTRACT FROM AUTHOR]

Compared to the well-studied the grain-size depositional process in hydraulic and aeolian sediments, the process in wetlands is still poorly understood. In this study, the grain-size components of a well-dated mud/peat profile (SJ2) in the Sanjiang Plain were partitioned using a lognormal distribution function. The results suggest that the lacustrine deposits contain four distinct unimodal grain-size distributions, C-1 through C-4, with their modal sizes ranging 0.7–1.6, 2.4–8.7, 15.7–27.8, and 380.2–529.6 μm respectively. Only the C-1 and C-3 were identified in the peat layers. A grain-size depositional modal was introduced to explain the environmental meanings of the four components. C-3, as a nearshore suspension component, was the most sensitive component to the variation of local hydraulic conditions. Based on the variation of C-3 percentages of the profile SJ2, the local hydraulic conditions were tentatively reconstructed, and the results showed a close relation with the monsoon variations during the Holocene. Accordingly, we suggest the relative coarse component of the polymodal peat deposits in monsoonal regions can be used as a sensitive indicator for the hydraulic conditions and the monsoon circulations in the geological past. [ABSTRACT FROM AUTHOR]

Understanding the historical flooding regime and its driving mechanisms provide useful insights into prediction of disasters in future. In this study, the paleo-flooding activities along the upper Amur River Delta were reconstructed based on the grain-size dataset of three sedimentary cores along a 120-km transect from its alluvial valley. Our results indicate that the coarse sand fraction in marshes surrounding the upper Amur River Delta is closely linked to historical flooding activities. Six "busy" flooding periods were identified at 4.4–4.0 ka BP, 3.6–3.2 ka BP, 1.7–1.4 ka BP, 1.1–0.9 ka BP, 0.7–0.5 ka BP, and 0.2–0 ka BP, respectively. These flooding intervals correspond well to other sedimentary flooding records from the East Asia Summer Monsoon (EASM) domain, and are in-line with the strengthened EASM stages throughout the Late-Holocene. Thus, our dataset suggests that the EASM circulation, especially its associated precipitation, has played a critical role in the evolution of flooding regime during the Late-Holocene epoch in the Amur River Delta. The last 0.2 ka BP is an exception as the coarse sand accumulation rate increased significantly while the EASM precipitation shows no equivalent shift. The increased coarse sand signals were likely attributed to floods induced by human interference instead of the long-term climate influence. [Display omitted] • The coarse sand fraction in marshes surrounding the Amur River is closely linked to historical flooding activities. • Six "busy" flooding periods were identified at 4.4-4.0, 3.6-3.2, 1.7-1.4, 1.1-0.9, 0.7-0.5, and 0.2-0 ka BP. • The Historical flooding activities in our study region were primarily controlled by EASM associated precipitation. • The flooding-rich period over the last 200 years was ascribed to human disturbance. [ABSTRACT FROM AUTHOR]

Schematic figure showing climate driving mechanisms on wetland hydrological and vegetational evolution. The relatively strong East Asian summer monsoon (EASM) with high precipitation tends to generate a higher water-level condition with more hygrophilous herbaceous plants while blocking the deposition of coarse grain-size fractions (a), and vise versa for weak EASM stages (b). [Display omitted] • The late-Holocene hydrological and vegetational regimes were reconstructed. • An ecological lake-to-wetland transition occurred at ~4.5 ka BP. • The wetlands evolution was largely controlled by monsoonal precipitation. The responding mechanism of wetland ecosystem to climate change is currently unclear due to a lack of long-term records incorporating multiple ecological factors. This paper presents a well-dated and multi-proxy cored record from a natural freshwater wetland developed in a paleo-pingo-related depression on the Sanjiang Plain. The study aims to reconstruct the wetland water-level and vegetational regimes during the past ~6.0 ka BP and discuss the wetland evolution response to the East Asian summer monsoon (EASM) circulation. The results show that a shallow-water lake with hygrophilous herbs developed in the depression with a strong EASM during ~6.0–4.5 ka BP. The paleolake was succeeded by a wetland with a lowered mean water level and an increase in tree cover controlled by the weakened EASM during the past 4.5 ka BP. Such an ecological lake-to-wetland transition corresponds well to the sharp decline of the EASM around 4.5 ka BP which has been well documented in various geological records across the EASM domain. During the wetland developing stage over the past 4.5 ka BP, the water-level and vegetational changes in the studied depression were also closely related to the EASM variations. The strengthened EASM usually generated higher water levels with more hygrophilous herbs around 4.0 ka BP, 3.5 ka BP, 3.0 ka BP, 2.0 ka BP, and 1.3 ka BP. Considering the prevalent EASM climate serving as the predominant water supply for the wetlands on the modern Sanjiang Plain, we suggest the EASM circulation has played a critical role in driving the wetland ecological evolution during the mid-to-late Holocene Epoch. The past 0.4 ka BP is an exception as the local Trees/Herbs ratio visibly increased with the declined water level, while the monsoonal precipitation shows no equivalent shift. The wetland evolution at this stage was attributed to human interference. [ABSTRACT FROM AUTHOR]

An integrated view of wetland's evolution is currently poorly understood due to a lack of knowledge on long-term interactions of multiple ecological factors. Here, we present a cored palynological record covering the Holocene Epoch from a depressional wetland on Heixiazi Island (China). With the aid of principal component analysis and cluster analysis of the palynological data, the historical vegetation regime has been well reconstructed for wetlands on the island. With further assistance from the published data on local hydrology and regional East Asian summer monsoon (EASM) variations, the interactions of vegetation, hydrology, and climate in the island's wetlands have been thoroughly analyzed with correlation analysis of the three factors. The results indicate that a strong EASM generally led to a high water level in the wetlands from increased monsoonal precipitation, causing an increase in arboreal vegetation and a decrease in herbs. Alternatively, a weak EASM generally led to low water levels due to decreased monsoonal precipitation, causing a decrease in arboreal vegetation and an increase in herbs. The local vegetation regime during the early Holocene was marked by an increase in tree/herb ratio due to rising water levels under the influence of an increasingly strengthened EASM. Subsequently, a general decline of the tree/herb ratio occurred from a gradual decrease in water level during the mid and late Holocene when the EASM gradually retreated. The wettest stage marked by the highest water level and tree/herb ratio occurred 8.0–4.6 ka BP with the strongest EASM. The driest stage occurred during the last 0.4 ka BP, which was attributed to both the weakest EASM influence and anthropogenic disturbance. Our study provides an integrated view of the wetlands' ecological dynamics incorporating multi-factor interactions, which further sheds light on the EASM driving mechanisms on wetlands evolution during the Holocene Epoch. Schematic figure showing the historical interactions of vegetation, hydrology, and climate in a typical wetland on Heixiazi Island, NE China. The relatively strong East Asian summer monsoon (EASM) with high atmospheric precipitation tends to generate a higher water level with less herbaceous plants (a), while a weaker EASM stage with less precipitation leads to a lower water level and more herbaceous plants (b). The light blue and gray arrows indicate the separate pollen contributions of arboreal trees in elevated areas and herbs within the depressional wetland, respectively. Unlabelled Image • A palynological methodology aided in reconstructing vegetation history. • Holocene climatological and hydrological regimes were determined. • Holocene interactions of vegetation, hydrology, and climate were presented. • Wetland evolution was largely controlled by monsoonal precipitation. [ABSTRACT FROM AUTHOR]

Peatlands contain around one third of the global soil carbon (C) and play an important role in the C cycle. In particular, the response of the productivity-decay balance to climate variability is critical for understanding both the past and future global C cycle. Most studies of peatland C dynamics have been carried out on boreal and subarctic peatlands, where climate models predict a greater increase in temperature compared to the global average. Less is known about peatlands at lower latitudes, yet there are significant peatland C stocks in these regions that may be more vulnerable to future climate change because they are closer to the climatic limit of peatland distribution. Northeast China is China's largest wetland region, with extensive peatlands in mountain regions and across the plains. Here, we used core data from 134 peatland sites to quantify the C accumulation rate over different timescales and estimate C storage across northeast China. The results show that the Holocene long-term apparent rate of C accumulation (LORCA) ranged from 5.74 to 129.31 g C m −2 yr −1 , with a mean rate of 33.66 g C m −2 yr −1 . The total wetland area and C storage within this region is 82,870 km 2 and 4.34 Gt C, and about 80% of the C is contained in mountain peatlands. We find that total C accumulated over the last 2000 years is linearly related to photosynthetically active radiation over the growing season, supporting the hypothesis that rates of net primary productivity (NPP) are more important than decomposition rates in determining long-term C accumulation. Although peatlands in northeast China are close to the southern limit of major peatland extent, our data suggest that future warming will lead to greater future C accumulation, as long as moisture balance or cloudiness do not become limiting factors. [ABSTRACT FROM AUTHOR]