Reconstruction of the East Asian monsoon variability since the mid-Holocene from the Pearl River estuary, southern China

Abstract

The principal aim of this thesis is to reconstruct East Asian Monsoon (EAM) variability during the mid-Holocene, developing a relatively new proxy of bulk organic carbon isotopic signature (δ13C). C/N ratios and trace elements are also employed to conduct a multi-proxy case study from the Pearl River estuary, southern China. Sources of sediments within an estuary include river-derived terrestrial/freshwater input, in situ brackish-water suspended sediment and tide-derived marine input. This study assumes the three proxies can help differentiate sources of sediments relating to monsoon-driven freshwater flux and help reconstruct monsoonal precipitation history during the mid-Holocene when the sea level was relatively stable.

To achieve the aims of this thesis a range of modern samples were collected from terrestrial areas, including plants and soil samples, through to estuarine areas, including seasonal estuarine suspended organic matter (SOM) and surface sediment. Results suggest that bulk organic δ13C and C/N ratios can successfully identify sources of the organic component of the estuarine sediment, and thus can be used to infer relative changes in monsoon-driven freshwater flux to the estuary. For example, more negative δ13C values reflect a greater level of contribution of freshwater organic carbon, i.e. stronger monsoonal freshwater discharge. Results also show that a combination of selected metals, such as the terrigenous metals (Fe, Mn, Co and As), can be useful for indicating sediment sources and sedimentary environment.

Analysis of an estuarine core (UV1) shows that freshwater discharge from the Pearl River catchment gradually declined from 6400 to 2000 cal. years BP, suggesting a gradual weakening of summer monsoon precipitation, responding to the weakening insolation controlled by the orbital-driven precession cycle. Superimposed on this are wet/dry intervals, ranging from centennial- to millennial- scale, driven by solar activity. Changes in ENSO and high-latitude cooling events might be responsible for dry/wet events at centennial- to decadal- scale, identified during the mid-Holocene. This study also suggests that the coupling of thermal and moist conditions of the EAM might only have become stable after 4500 cal. yr BP. A sudden shift in the geochemical signature indicates agricultural activity in the Pearl River delta intensified from 2000 cal. yr BP.