East Asian summer monsoon precipitation variability since the last deglaciation

The lack of a precisely-dated, unequivocal climate proxy from northern China, where precipitation variability is traditionally considered as an East Asian summer monsoon (EASM) indicator, impedes our understanding of the behaviour and dynamics of the EASM. Here we present a well-dated, pollen-based, ~20-yr-resolution quantitative precipitation reconstruction (derived using a transfer function) from an alpine lake in North China, which provides for the first time a direct record of EASM evolution since 14.7 ka (ka = thousands of years before present, where the “present” is defined as the year AD 1950). Our record reveals a gradually intensifying monsoon from 14.7–7.0 ka, a maximum monsoon (30% higher precipitation than present) from ~7.8–5.3 ka, and a rapid decline since ~3.3 ka. These insolation-driven EASM trends were punctuated by two millennial-scale weakening events which occurred synchronously to the cold Younger Dryas and at ~9.5–8.5 ka, and by two centennial-scale intervals of enhanced (weakened) monsoon during the Medieval Warm Period (Little Ice Age). Our precipitation reconstruction, consistent with temperature changes but quite different from the prevailing view of EASM evolution, points to strong internal feedback processes driving the EASM, and may aid our understanding of future monsoon behaviour under ongoing anthropogenic climate change.


Supplementary Text
Text S1: Pollen source area It has been suggested that the pollen source area for lakes in forest region with a radius smaller than 750 m (radius of the Gonghai Lake is smaller than 400 m) is generally not exceeding 3 km S1,S2 . The modern vegetation around Lake Gonghai is grass and shrub dominated by Artemisia, Chenopodiaceae and Hippophae, while the forest vegetation on Guancen Mountain (potential regional pollen source area, 5-30 km from Gonghai Lake) is mainly Picea and Larix. The pollen assemblages of the top layers of GH09B are mainly composed of herb pollen such as Artemisia and Chenopodiaceae with extremely low (mostly lower than 10%) content of tree pollens, further indicating that the fossil pollens in Gonghai Lake sediments represent vegetation compositions around the lake basin S3 . Given both the dominance of regional zonal vegetation (very few intra-zonal taxa) and the significant altitudinal zonation around the Gonghai Lake, the stratigraphic pollen data can thus be used to reflect the regional vegetation succession and climatic changes.

Text S2: Pollen diagram description
The pollen assemblages are dominated by arboreal pollen types including Pinus, Picea, Betula, Quercus and Ulmus. The herb pollen taxa are dominated by Artemisia, Chenopodiaceae and Gramineae; they exhibit a higher diversity than the tree pollen types. According to CONISS, five major zones can be delimited (Fig. S6): Pollen Zone 1 (14.7-11.1 ka, 9.42-7.46 m) This zone is dominated by Artemisia, Chenopodiaceae and Betula, with higher percentages of dry-land taxa including Hippophae and Ephedra. Two subzones are defined, which may correspond to the Bølling-Allerød interstadial (GH-1a) and the Younger-Dryas event (GH-1b). Subzone GH-1a has higher percentages of Pinus, Picea, Ulmus and Quercus and also has more desert pollen taxa, including Ephedra and Chenopodiaceae, compared to subzone GH-1b. High percentages of hydrophytes in this zone also indicate a shift from a terrestrial to a shallow aquatic environment, with fluvial sediment inputs below 9.42 m and lacustrine sediment above. Subzone GH-1b is characterised by rapidly increasing Artemisia and Betula pollen and decreasing herb pollen values.
Pollen Zone 2 (11.1-7.3 ka, 7.46-6.24 m) There are two subzones in this interval. Subzone GH-2a (11.1-9.6 ka) is characterised by increasing tree pollen frequencies and declining herb pollen frequencies. Ulmus and Ostryopsis exhibit peak values for the entire zone, and in general the pollen assemblages indicate the establishment of forest near the site. In Subzone GH-2b (9.6-7.3 ka), Pinus and Quercus increase and there is a peak in Picea. Ulmus and Betula values are somewhat reduced.
Pollen Zone 3 (7. 3-5.0 ka, 6.24-5.59 m) This zone is characterised by peak Quercus and total tree pollen frequencies. It reflects stable forest vegetation consisting mainly of conifers and mixed broadleaf trees dominated by Quercus.
Pollen Zone 5 (1.6 ka-Present, 3.78-0.00 m) This zone exhibits low tree pollen and the highest herb pollen frequencies, suggesting a significant change from a forest to a forest-grassland environment. There was some forest recovery during the Medieval Warm Period at around 600-800 cal yr BP; however, very low tree pollen frequencies occurred during the LIA. The presence of cereal pollen types (less than 4%) indicates some human impact on the vegetation during this period; however, documentary evidence suggests that intensive deforestation in this region did not occur until ~600 cal yr BP (Ming Dynasty) S4 .

Text S3: Cultural evolution
Neolithic cultural evolution in the Yellow River valley S5,S6 corresponds closely to the precipitation variations reconstructed from the Gonghai Lake pollen record (Fig. S10). Pre-Yangshao cultures were widely scattered with a small number sites during ca. 8.3-7 ka when precipitation was lower than during the subsequent two millennia. The Yangshao culture expanded rapidly both in sites numbers and occupied areas, and became the prevailing cultural system from 7-5 ka when precipitation was highest and stable. During the Post-Yangshao period (5-4 ka), when precipitation began to decrease, it disintegrated into many different cultures.       Figure S7. The spatial distribution of modern surface samples used in this study. The white dots are the 1860 samples remaining after removing those from tropical and arid regions, and those which could potentially be significantly influenced by human activity (i.e. close to roads, farmlands, etc.) according to the original field notes. The red dots are the 509 samples used for development of the calibration function; outliers are indicated by crosses. The blue dot marks the location of Gonghai Lake. It is evident from the satellite image that the red dots are located at the transitional zone between the vegetated region and relatively barren land. The map was generated using ESRI ArcGIS v9.3 S9 . Figure S8. Histogram of the proportion of variance in the GH09B pollen record explained by 999 calibration functions (WAPLS-2) trained with random environmental data. Solid black lines indicate the proportion of variance explained by our T ANN and P ANN transfer functions, from right to left, respectively. The red dotted line indicates the proportion of variance below which 95% of the random data-trained calibration functions could be explained. The black dotted line indicates the proportion of variance explained by the first axis of a PCA of the fossil data. It is showed that the P ANN reconstruction based on the WAPLS-2 model and stratigraphical pollen data from core GH09B explains more of the variation in the fossil data than 99% of reconstructions (p = 0.001) derived from calibration functions trained on random environmental data, whereas the T ANN reconstruction is not statistically significant (p = 0.434).     and ENSO S11 . The shaded area shows a relatively active ENSO during the late Holocene. Table S1. Radiocarbon dates of terrestrial plant-macrofossil samples for core GH09B and GH09C from Gonghai Lake, using for chronology construction.