Responses of the upriver valley sediment to Holocene environmental changes in the Paju area of Korea

Multidisciplinary paleoproxy data from five sedimentary cores (UJ-03, 06, 07, 10, and 12) recovered from the broad, flat upriver valley in the Paju area of Korea are presented. Twelve AMS radiocarbon dates from cores UJ-03 and 12 provided a high-resolution Holocene record for the sedimentary sequence, ranging from about 8000 YBP to the present. From 6400 to 4400 14C YBP (7100–5000 cal. YBP), the sediments were dominantly poorly sorted, medium to coarse sands containing angular to subangular pebbles, suggesting that the materials were locally derived. The increased abundance of coarse-grained sediment was attributed to intensified rainfall during the mid-Holocene, despite the dense regional vegetational cover. This period probably corresponded to the Holocene Climate Optimum in the Korean Peninsula. Total organic carbon (TOC) values showed a marked increase from 4400 to 2100 14C YBP (5000–2200 cal. YBP), which coincided with an observed decrease in particle size (i.e., a decrease in sand content). These changes indicated the onset of paludification on the silty or sandy valley bottom. Contemporaneously, relative abundances of Pinus and herbaceous pollen increased, and the East Asian summer monsoon became weakened. Several intermittent depositional layers were observed, ranging from 2100 14C YBP (2200 cal. YBP) to the present, which contained a high concentration of rootlets, sand, or clay particles. These types of recurring sedimentation events are attributable to climatic shifts and/or human impacts, such as timber harvest and land clearing. The upriver valley catchment was probably sensitive to flash floods due to poorly vegetated slopes, accelerating rates of erosion as a consequence of land use change. This research indicates that the response of a valley system such as that in Paju to climate- or human-induced environmental changes can vary greatly on local and regional scales.