Sangheon Yi

Vegetation and climate changes in the Changjiang (Yangtze River) Delta, China, during the past 13,000 years inferred from pollen records

Abstract A high-resolution pollen record from 2 boreholes from the Changjiang (Yangtze River) delta shows a series of well-defined changes in vegetation and climate over the last 13,000 yr . In latest Pleistocene to early Holocene time (12,900– 10,300 cal yr BP), pollen dominated by the xerophytic herb Artemisia and common conifers indicate coniferous forest and grasslands and a cool, dry climates, possibly represented by the Younger Dryas. From 10,300 to 9000 cal yr BP, a warm, wet climate fostered the development of mixed broadleaved evergreen–deciduous forests to grow on the grasslands and surrounding hills and uplands. From 9000 to 7600 cal yr BP, the dominance of conifers and the reduction in broadleaved evergreen trees pollen suggest cool, dry conditions. The mid-Holocene “hypsithermal” (7600– 4800 cal yr BP) is represented by a large expansion of subtropical evergreen–deciduous broadleaved forests. Between 4800 and 1300 cal yr BP, which corresponds to a Neoglacial period elsewhere, conifers are dominant, along with common herbs, whereas the area with broadleaved evergreen trees shrink under the cool, dry conditions. The first occurrence of Fagopyrum pollen at 4500 cal yr BP suggests human influence. The significant change in arboreal and herbaceous pollen after 1300 cal yr BP reflects widespread intensive human influence.

Cretaceous and Cenozoic non-marine deposits of the Northern South Yellow Sea Basin, offshore western Korea: palynostratigraphy and palaeoenvironments

Abstract Palynological analyses of two wells (Haema-1 and Kachi-1) located in two sub-basins of the Northern South Yellow Sea Basin have been carried out in order to establish a palynostratigraphic breakdown of the sedimentary succession and to determine environments of deposition. Seven assemblage zones and two assemblage subzones have been erected on the basis of frequency variations in, and occurrences of, biostratigraphically significant palynomorphs as follows: Classopollis–Ephedripites Assemblage Zone (AZ): Barremian–Albian; Alisporites–Aquilapollenites–Penetetrapites AZ, which is subdivided into an Alisporites–Rugubivesiculites Assemblage Subzone: Cenomanian–Lower Maastrichtian, and an Aquilapollenites–Penetetrapites Assemblage Subzone: Upper Maastrichtian; Momipites–Coryluspollenites AZ: Paleocene; Caryapollenites–Inaperturopollenites AZ: Lower–Middle Eocene; Quercoidites–Pinuspollenites AZ: Upper Eocene; Liquidambarpollenites–Fupingopollenites–Magnastriatites AZ: Lower–Middle Miocene; Graminidites–Persicarioipollis AZ: Pliocene. The depositional environments represented by the well sections are considered to have been generally fluvio–lacustrine, and the climate to have varied between semi-arid and wet, and subtropical and warm temperate, except during the Late Eocene and Pliocene when a cool-temperate climate prevailed. Six stages in the development of the sub-basins are recognised. These are: (1) initial stage of rift or pull-apart basin-formation during the Late Jurassic?–Cretaceous; (2) subsidence from the Paleocene to Middle Eocene; (3) alternation of uplift and subsidence in the Late Eocene; (4) synrift inversion and erosion through the Oligocene; (5) uplift during the Early Miocene; and (6) widespread subsidence from the Middle Miocene onwards apart from during the Early Pliocene when the region was subjected to uplift once more.

Mid-latitude interhemispheric hydrologic seesaw over the past 550,000 years

An interhemispheric hydrologic seesaw—in which latitudinal migrations of the Intertropical Convergence Zone (ITCZ) produce simultaneous wetting (increased precipitation) in one hemisphere and drying in the other—has been discovered in some tropical and subtropical regions. For instance, Chinese and Brazilian subtropical speleothem (cave formations such as stalactites and stalagmites) records show opposite trends in time series of oxygen isotopes (a proxy for precipitation variability) at millennial to orbital timescales, suggesting that hydrologic cycles were antiphased in the northerly versus southerly subtropics. This tropical to subtropical hydrologic phenomenon is likely to be an initial and important climatic response to orbital forcing. The impacts of such an interhemispheric hydrologic seesaw on higher-latitude regions and the global climate system, however, are unknown. Here we show that the antiphasing seen in the tropical records is also present in both hemispheres of the mid-latitude western Pacific Ocean. Our results are based on a new 550,000-year record of the growth frequency of speleothems from the Korean peninsula, which we compare to Southern Hemisphere equivalents. The Korean data are discontinuous and derived from 24 separate speleothems, but still allow the identification of periods of peak speleothem growth and, thus, precipitation. The clear hemispheric antiphasing indicates that the sphere of influence of the interhemispheric hydrologic seesaw over the past 550,000 years extended at least to the mid-latitudes, such as northeast Asia, and that orbital-timescale ITCZ shifts can have serious effects on temperate climate systems. Furthermore, our result implies that insolation-driven ITCZ dynamics may provoke water vapour and vegetation feedbacks in northern mid-latitude regions and could have regulated global climate conditions throughout the late Quaternary ice age cycles.

Pollen analysis at Paju Unjeong, South Korea: Implications of land-use changes since the late Neolithic:

Age-controlled pollen and microcharcoal records from Holocene sediments in Paju Unjeong, Korea, reflect the response of vegetation dynamics to climate changes and human activity. The pollen spectrum shows clear differences between the natural vegetation stage and the land-use vegetation stage. The land-use vegetation stage is further divided into dry-field cultivation and paddy-field cultivation stages. The natural vegetation stage (c. 8420–4700 cal. BP, early Neolithic) indicates the presence of dense woodlands consisting of various deciduous broadleaved trees on well-drained hills to uplands during the mid-Holocene period. Early Neolithic foraging peoples occupied the area and collected wild plants and fruits, such as acorns. The land-use stage (c. 4700 cal. BP, late Neolithic to Modern) reflects forest transition and, finally, Gramineae-dominated open landscape. During the dry-field cultivation stage (c. 4700–2000 cal. BP, late Neolithic to Bronze Age), inhabitants may have used fire as the principal means to open lowland vegetation for occupation, wood reclamation, and artifact making. During the paddy-field cultivation stage (c. 2000 cal. BP, Iron Age to Modern), inhabitants’ activities on adjacent open lowland increased. The local area became an extensive open landscape on which the Baekje people (c. ad 0–100, first century in Korean chronology) utilized constant fires to manage woodlands at limited levels for more open habitat. Their descendents, the Joseon people (c. ad 1600, seventeenth century in Korean chronology), utilized strategic management of pine trees as a secondary method to obtain wood material for building houses.

Palynological study on vegetation and climatic change in the subaqueous Changjiang (Yangtze River) delta, China, during the past about 1600 years

The well-defined pollen record in massive marine clay deposits from the subaqueous Changjiang (Yangtze River) delta reveals changes in vegetation and inferred climate during the last about 1600 years. Climatic periods inferred from the pollen record include (1) a basal cool/dry period (AD 385–910), (2) a relatively warm/wet conditions comparable to Medieval Warm Period (AD 910-1085) with a strengthen summer monsoon, (3) a relatively cool and wet conditions possibly corresponding to Little Ice Age (LIA; AD 1085–1815) with a weaken summer monsoon, and finally (4) the present warm period, since AD 1815. The pollen aridity index based on variations in humidity suggests that three subperiods within the LIA can be identified: wet LIA-1 (AD 1085-1170), dry LIA-2 (AD 1170–1330), and wet LIA-3 (AD 1330–1815).

The Link between ENSO-like Forcing and Hydroclimate Variability of Coastal East Asia during the Last Millennium

Inconsistent reconstructions of East Asian hydroclimate for the last millennium significantly limit our understanding of the mechanisms behind climate variability during the medieval climate anomaly (MCA) and little ice age (LIA) in the region. In this study, we present new high-resolution multiproxy records (diatom, δ13C, C/N, TS) from the Mulyoungari swamp, Jeju Island, South Korea. Our results indicate that El Niño southern oscillation-like variations caused the dry MCA/wet LIA pattern in the study area. Recent paleo-ENSO studies generally support the hypothesis that the MCA was characterized by more persistent El Niño-like conditions. During El Niño events, the genesis of typhoons affecting coastal East Asia tends to diminish because of warm anomalies of eastern tropical Pacific (ETP) SSTs and downward motions over the western tropical Pacific. Therefore, coastal East Asia likely experienced a decline in typhoon-related precipitation during the MCA, in contrast to monsoon-dominated northern China. Our results additionally imply that SST anomalies in the ETP need to be carefully checked to better understand current hydroclimate variability in coastal East Asia, one of the most populated areas on earth.

Palaeohydrological and Palaeoenvironmental Fluctuations of the Historic Eurimji Lake

Lake Eurimji is located in the northern part of Jaecheon, Chungbug Province, in South Korea. It is well known for its scenic beauty as a nationally registered landscape site (Yang GS, Jungwon Munhwa 14:1–38, 2010). Since the late Bronze Age, its water has been used for agricultural irrigation. The lake was initially formed at the mouth of a fanglomerate where Jurassic granites blocked the main course of a stream channel at about 305 m above sea level (m.a.s.l.). Natural levees grew laterally near the main outlets of the fan-valley mouth up to about 307 m.a.s.l. The lake accumulated organic sandy mud on the sand-and-gravel streambeds extending toward the southern valley mouth near Yongdu Mt., the ages of which were dated as early as ca. 3,000 calibration years Before Present (cal-yr BP). The early evidence of lake deposits is supported by radiocarbon ages for organic muds below the lowermost artificial layers in Lake Eurimji, which vertically constitute the bottom of Eurimji bank or dyke at about 306.5 m.a.s.l. The height is about 15 m. The present study reconstructs the paleo-precipitation of Lake Eurimji by applying an age mode to core sediments in the ER3-1 borehole using the mean grain size of sediments trapped at the bottom of the lake. The results show that lake sediments accumulated at an extremely high rate, ca. 3.49 mm/year during the Early Three Kingdoms Period, around 2,120–1,340 cal-yr BP. The average grain size of the trapped sediments is proportional to the total amount of precipitation during the elapsed time of each monitoring session. Both the paleoclimatic cyclicity and historical documents indicate that droughts in the lake catchment areas prevailed at 80 AD, 120 AD, 160 AD, 310 AD, 350 AD, 460–500 AD, and 620 AD, while relatively wet conditions occurred at approximately 90 AD, 150 AD, 200 AD, 240 AD, 320 AD, 360 AD, 435 AD, and 530 AD. These drought records suggest a multi-decadal cyclicity, particularly during the Early Three Kingdoms Period. This may imply that multi-decadal to century scale fluctuations of winter and summer monsoons controlled the sedimentary regime of Lake Eurimji catchment areas during this period in South Korea.

Pollen record of early- to mid-Holocene vegetation and climate dynamics on the eastern coast of the Yellow Sea, South Korea:

To understand the early- to mid-Holocene vegetation and climate dynamics on the eastern coast of the Yellow Sea, we obtained a sedimentary core with high-resolution accelerator mass spectrometry (AMS) carbon 14 (14C) data from the Gunsan coast in South Korea. The palynological analysis demonstrated that the riverine wetland meadow from 12.1 to 9.8 cal. kyr BP changed to temperate deciduous broad-leaved forest in 9.8–2.8 cal. kyr BP. In addition, the cold climate from 12.1 to 9.8 cal. kyr BP became warmer from 8.5 to 7.3 cal. kyr BP. This was followed by another relatively cold period from 7.3 to 2.8 cal. kyr BP. The temperature change was mainly in response to solar factors. However, there are two relatively humid periods from 12.1 to 9.8 and 8.5 to 7.3 cal. kyr BP, which arose for different reasons. The earlier humid period resulted from strong westerlies and a rapidly rising sea level. The later humid period was produced mainly by the strong East Asian summer monsoon (EASM) and may also be linked to La Niña–like activity. The cold ‘Younger Dryas’ event from 12.0 to 11.4 cal. kyr BP recorded in this study may have been produced by a North Atlantic meltwater pulse. This would have reduced temperatures that were already low because of weak insolation, and the strong winter monsoons would have increased the precipitation.

The 8.2 ka cooling event in coastal East Asia: High-resolution pollen evidence from southwestern Korea

In this study, we present a high-resolution multi-proxy record (pollen, magnetic susceptibility, and mean grain size) from Bigeum Island, South Korea, which mainly showed early Holocene paleoenvironmental change. Bigeum pollen records indicated that early Holocene climate variations in coastal East Asia were principally controlled by the Atlantic meridional overturning circulation. Most importantly, the 8.2 ka cooling event was clearly recognized for the first time in coastal East Asia, where few high-resolution proxy data, such as ice core and stalagmite δ18O records, are available. The insular vegetation in the study site was extremely susceptible to even short-term climate changes, such as the 8.2 ka cooling event, which allowed a detailed climate reconstruction from pollen data. Early Holocene climate teleconnections between coastal East Asia and other regions were identified through regional comparisons of Greenland, China, Brazil, Spain, Madagascar, and Korea. Coastal East Asia is one of world’s most populated regions and will be particularly vulnerable to future climate change. Accurate and detailed paleoclimate proxy data, such as the Bigeum pollen record, will therefore be increasingly important in this region.

1000-Year Quasi-Periodicity of Weak Monsoon Events in Temperate Northeast Asia since the Mid-Holocene

The Holocene variability in the East Asian summer monsoon (EASM) based on speleothem δ18O records has inconsistencies in timing, duration, and expression of millennial-scale events among nearby regions, and even within the same cave. Here, we present another stalagmite δ18O record with multi-decadal time resolution from the temperate Korean Peninsula (KP) for the last 5500 years in order to compare with Holocene millennial-scale EASM events from Southeast Asia. Based on our new stalagmite δ18O record, millennial-scale events since the mid-Holocene were successfully identified in the KP, representing a noticeable cyclic pattern with a periodicity of around 1000 years. We propose that the Holocene millennial-scale events are common hydroclimatic phenomena at least in the East Asian monsoon system. Meanwhile, the shorter periodicity of millennial-scale events than that of the North Atlantic region is likely to decouple the EASM system from the North Atlantic climate system. This observation suggests that weak EASM and North Atlantic Bond events may have been induced independently by direct solar activity (and then possible feedback) and ocean–ice sheet dynamics, respectively, rather than simple propagation from the North Atlantic to the EASM regions.