New insights into dating the sediment sequence within a landslide-dammed reservoir on the Chinese Loess Plateau

Abstract

The deposition couplets within the check dams and landslide-dammed reservoirs on the Chinese Loess Plateau (CLP) document the processes and histories of watershed soil erosion, transportation and deposition, related to floods, eco-environment and land use changes, and human activities. Previously, dating the couplets was dominantly dependent on multiple intercomparisons among specific sediment yields of visible couplets, 137Cs activities distributions and erosive rainfall events by meteorological records. However, inter-annual division of the deposition couplets and dating historical sequence beyond meteorological records in the landslide-dammed reservoir are little known. Based on high-resolution XRF core scanning on a 22.75 m sediment sequence in Jingbian (JB) landslide-dammed reservoir on the central hilly-gully area of the CLP, and cross checking of multiple dating methods, this study tried to propose a new method to build the accurate chronology sequence based on inter-annual division by annual freeze-thaw layer and to further date the sediment without rainfall records. The results showed that a total of 126 deposition couplets and 78 annual freeze-thaw layers were identified in the JB sequence. Multimethod cross-dating, including 137Cs activities, annual freeze-thaw layers, couplet specific sediment yield and modern rainfall records, was the most accurate method of dating the JB sediment sequences since 1960s with detail meteorological records. Furthermore, the correspondence between annual freeze-thaw layers and the historical grades of flood index from literature was the valid method to date the JB historical sequences without rainfall records. Consequently, the JB sequence was deposited during the period between 1855 and 2014. High-resolution dating of the JB sequence provides the chronology for recovering natural and anthropogenic information on the central hilly-gully area of CLP since 1850s. The proposed methods will also shed new light on the accurate dating of the sediment sequences within other check dams and landslide-dammed reservoirs on the central and northern CLP.

Asynchronized erosion effects due to climate and human activities on the central Chinese Loess Plateau during the Anthropocene and its implications for future soil and water management

“…The couplet specific sediment yield (CSSY) and annual specific sediment yield (ASSY) were calculated based on the methods of Wang et al (2019) and Wei et al (2018), which were the common ways to estimate the sediment yield within the check dams and landslide‐dammed reservoirs on the CLP. Briefly, the CSSY and ASSY in the JB and HS catchments during A.D. 1850–2000 were determined from Equations and , respectively: where S 1 and S 2 are the top and bottom surface areas of each couplet (m 2 ), respectively; H is the thickness of each couplet (m); δ is the bulk density (1.4 t/m 3 as the mean density of representative sediment samples from check dams and landslide‐dammed reservoirs on the CLP; Zhang et al, 2009); A is the catchment area of the reservoir (m 2 ); TE is the trapping efficiency of the landslide dam (assuming 100% TE due to intact dams since the formation of both reservoirs); n is the number of the couplet within an individual year.…”

“…The chronologies of the JB and HS sequences were built by a multi‐proxy combination of 137 Cs activities, records of recent rainfall and historical flood grade indexes, annual freeze–thaw layers that were identified by high concentrations and correlations of Ca and Fe (Wang et al, 2016b). Details of the methods used for sampling, couplets division, and dating for the JB and HS sequences were reported by Wang et al (2019, 2021). As a result, the JB sequence consisted of 126 couplets in 78 individual years over A.D. 1855–2014, which indicated that the JB sequence was deposited by 126 hydrological events at 78 individual years during this period.…”

“…On the northcentral CLP, the JB landslide‐dammed reservoir is located at the headwater area of the Wuding River in the loess hilly–gully region (Figure 1b). This reservoir was created in 1855 following a natural landslide on the left side of the gully, with a total drainage area of 2.87 km 2 (Wang et al, 2019). The JB reservoir is near the 400 mm equipluve and the Great Wall of the Ming Dynasty and belongs to the typical agro‐pastoral ecotone area.…”

“…Many of these reservoirs provided the templates of current check dams that were constructed to control soil and water (Wang et al, 2019). The sedimentary sequences in these reservoirs are composed of deposition couplets (i.e.…”

“…t/km 2 ) for a 2.87 km 2 catchment area (Table2), while the ASSY ranged from 0.09 Â 10 4 to 23.40 Â 10 4 t/(km 2 a) [mean of 1.17 Â 10 4 t/(km 2 a)] according to the interannual division of the couplets (Table2;Wang et al, 2019). The highest and lowest ASSY in the JB catchment occurred in A.D. 1860 and 1898, respectively.…”