Abstract. The timing of the waxing and wining of the East Asian summer monsoon during the Holocene is still under debate. In present study, we present the high-resolution grain-size and LOI records from a well-dated mud/peat profile to reveal the lake-wetland transition in the Sanjiang Plain and discuss its significance to Holocene monsoon evolutions. The results show that the shallow-water lakes have developed in low-lying areas of the plain before 4600 yr BP, corresponding to the Holocene monsoon maximum. Thereafter, the wetlands began to initiate with the extinction of the paleolakes, marking a lake-shrinking stage with the relative dry climate. Considering the prevalent monsoon climate in the Sanjiang Plain, we suggest the lake-wetland transition at 4600 yr BP indicate a sharp decline of the summer monsoon rather than the basin infilling process. Such a remarkable monsoon weakening event has been widely documented in northern China, and we associated it with the ocean–atmosphere interacting processes in low-latitude regions.
Abstract Partitioning evapotranspiration ( ET ) into evaporation ( E ) and transpiration ( T ) in wetlands is important for understanding the hydrological processes in wetlands and the contribution of wetland ET to local and regional water cycling and for designing effective wetland management strategies. Stable water isotopes are useful in the application of ET partitioning through the evaluation of the isotopic compositions of E ( δ E ), T ( δ T ), and ET ( δ ET ) obtained from observation or modelling methods. However, this approach still suffers from potentially large uncertainties in terms of estimating the isotopic endmembers. In this study, we modified the traditional isotope‐based ET partitioning methods to include leaf‐level biological constraints to separately estimate the relative contributions of T from Scirpus triqueter and Phragmites australis and the relative contributions of E from the standing surface water in a semiarid marsh wetland in northeastern China. The results showed that although the δ T values of S . triqueter and P . australis were rather similar, the mean δ T values of the 2 species were different from the values of δ E , making it possible to distinguish the relative contributions of E and T through the use of isotopes. The simulation of leaf water using a non‐steady‐state model indicated obvious deviations in leaf water enrichment ( δ Lb ) from isotopic steady states for both species, especially during early mornings and evenings when relative humidity was highest. The isotopic mass balance showed that E accounted for approximately 60% of ET , and T from S . triqueter and P . australis each contributed approximately 20% to ET ; this implied that the transpiration of one reed was equivalent to that of 5.25 individuals of S . triqueter . Using the estimated ratio of T to ET and the measured leaf transpiration, the total ET was estimated to be approximately 10 mm day −1 . Using the NSS‐ T r method, the estimated ET was higher than the water loss calculated from the water level gauge. This indicated that the river water and surrounding groundwater were the sources of the marsh wetland, with a supply rate of 8.3 mm day −1 .
Studying the peatlands accumulation and carbon (C) storage in monsoonal areas could provide useful insights into the response of C dynamics to climate variation in the geological past. Here, we integrated 40 well-dated peat/lake sediment cores to reveal the peatlands evolution history in the Sanjiang Plain and examine its links to East Asian monsoon variations during the Holocene. The results show that 80% peatlands in the Sanjiang Plain initiated after 4.7 ka (1 ka = 1000 cal yr BP), with the largest initiating frequency around 4.5 ka. The mean C accumulation rate of peatlands in the Sanjiang Plain exhibits a synchronous increase with the peatlands expansion during the Holocene. Such a peatlands expanding and C accumulating pattern corresponds well to the remarkable drying event subsequent to the Holocene monsoon maximum. We suggest that in addition to the locally topographic conditions, Holocene variations of East Asian summer monsoon (especially its associated precipitation) have played a critical role in driving the peatlands initiation and expansion in the Sanjiang Plain.
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