History of Mid- and Late Holocene Palaeofloods in the Yangtze Coastal Lowlands, East China: Evaluation of Non-Pollen Palynomorph Evidence, Review and Synthesis

James B Innes,Yongqiang Zong

doi:10.3390/quat4030021

Abstract

The surface of the lowland deltaic plain around Taihu (Lake Tai), south of the Yangtze river mouth in eastern China, lies near sea level and until recent drainage and development by human societies was mostly covered by wetlands of various types. It was created by regular overbank flooding, mainly from the Yangtze, and the deposition of mostly mineral sediments over the several millennia since sea level regained its current altitude in the early mid-Holocene and progradation of the Yangtze delta began. Fluvial activity has therefore been the dominant influence on sedimentation in the Taihu lowlands, and in the lower Yangtze valley generally, and has determined the character of the mainly inorganic sediment sequences that have accumulated there, with autochthonous deposition of organic sediments within the local wetland plant communities playing a minor role. The presence of both clastic flood horizons and peat layers within the deposits of the Taihu plain attests to great variability in the magnitude of fluvial input from the Yangtze, with repeated extreme floods occurring at some periods, but with periods when the growth of peat layers shows low water tables, little exogenic sediment input and so little fluvial influence. We have examined the published evidence for these different depositional environments in the lower Yangtze and the Taihu plain during the Holocene, comparing the flood history with the middle and upper reaches of the Yangtze catchment. Discrete phases of high or low flooding influence are recognised, and these correspond with large-scale Holocene climate history. Intensified human land use in recent millennia has complicated this relationship, amplifying the flooding signal. Our palynological research shows that algal microfossil type and abundance is a useful proxy for changing water depth and quality in the aquatic environments of the Holocene Taihu wetlands, and can recognise flooding events that are not registered in the floodplain lithological sequences.

Highlights

It can be no surprise that the coastal lowlands around Lake Tai (Taihu), to the west of Shanghai in eastern China, have been greatly influenced by fluvial action in the midand Late Holocene, as their surface altitude is very low, mostly between 2 and 5 m above sea level, and they lie between the Yangtze (Changjiang) river to the north, one of the largest rivers in the world, and the major Qiantang river to the south, which empties into
Much of the lowland plain between the Yangtze River and Hangzhou Bay was created by fluvial activity due to progradation of the Yangtze delta from the early mid-Holocene onwards, and rests upon terrestrial sediments of pre-Holocene age, some of which are fluvial
Subject to marine influence, in the early to mid-Holocene and more recently, the varying input through time of fluvial water and suspended sediment discharge from the Yangtze has been dominant in governing the history of depositional regimes in this coastal plain since the mid-Holocene [23,209]

Summary

Introduction

It can be no surprise that the coastal lowlands around Lake Tai (Taihu), to the west of Shanghai in eastern China, have been greatly influenced by fluvial action in the midand Late Holocene, as their surface altitude is very low, mostly between 2 and 5 m above sea level, and they lie between the Yangtze (Changjiang) river to the north, one of the largest rivers in the world, and the major Qiantang river to the south, which empties intoHangzhou Bay (Figure 1). The Taihu lowland, and nearby coastal lowlands in the area such as the Ningshao plain south of Hangzhou Bay, is an alluvial floodplain that, since its creation about 7000 years ago and its subsequent progradation [6], has always experienced high groundwater tables and surface waterlogging, including many shallow lakes [7,8], and so naturally has supported wetland vegetation of various types [9,10,11] These wetlands have been much altered and reduced by agricultural development, eutrophication, drainage and urbanisation over the last two millennia but in recent times [7,8,12,13,14,15,16], and the plain’s hydrology is likely to be even more affected in future because of recent major damming of the middle Yangtze [17,18], which will result in greatly reduced water discharge in the river’s lower reaches, with reduced fluvial sediment input in the Taihu area [19,20,21,22]. Of these, flooding from the Yangtze has been the most important, certainly since postglacial sea-level readjustment was completed in the late mid-Holocene [28,29,30,31,32,33]

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