The contemporary distribution of benthic foraminifera in the Pearl River estuary, southeast China, and their use in reconstructing mid‐ to late‐Holocene fluvial flux

Abstract

This thesis investigates the contemporary distribution of benthic foraminifera in the Pearl River estuary, southeast China, and establishes a new proxy record of estuarine salinity and fluvial flux over the mid‐ to late‐Holocene. Seventy‐six samples from the contemporary estuary yielded 118 benthic taxa. Ammonia beccarii dominates a large proportion of the estuary, with contributions from a number of other calcareous taxa, including Haynesina sp., Elphidium spp., Quinqueloculina spp. and Rotalinoides annectens, and agglutinated taxa, for example Haplophragmoides sp. and Ammobaculites formosensis. Species distribution maps, cluster analysis and unconstrained ordination suggest a link between foraminiferal assemblages and the magnitude of fluvial influence. Constrained ordination links the faunal data to recognised environmental gradients and highlights salinity as the most important factor in controlling species distributions. Transfer function development enables quantitative estimates of palaeosalinity to be obtained from fossil faunal data.

A radiocarbon dated mid‐ to late‐Holocene sediment core from the same estuary yielded 102 benthic taxa, 83 % of which were also found in contemporary samples. Ammonia beccarii and Quinqueloculina akneriana dominate fossil assemblages. In light of relative stability in mid‐ to late‐Holocene sea level along the South China coast (Zong, 2004), the long‐term increase in taxa indicative of high salinity environments is primarily interpreted in terms of declining fluvial flux. The trends in quantitative palaeosalinity reconstructions are in agreement with qualitative interpretations, however absolute salinity values are subject to large error terms. The inferred decline in fluvial flux is in accord with bulk organic carbon isotope and diatom data from the same core (Zong et al., submitted a; Yu, 2009) and with records of weakening precipitation intensity derived from terrestrial proxies (e.g. An et al., 2000; Y. Wang et al., 2005; Hu et al., 2008). Millennial‐scale variability in the East Asian monsoon, linked to precession and obliquity cycles of solar insolation, is the hypothesised cause of these trends. A number of abrupt low discharge events, postulated to reflect northern hemisphere teleconnections, punctuate the Pearl River estuary record.