Reconstruction of extreme hydrological events based on environmental magnetic properties of Holocene pyrite-bearing layers from the East River Delta Plain,southern China

Abstract

The frequent occurrence of extreme hydrological events (EHEs) poses a serious threat to human living environment. Magnetic profiles of deltaic sediments contain high-resolution EHE records, and magnetic minerals could be finally altered to pyrite by reductive diagenesis. Till now, the preservation of EHEs in the pyrite-bearing layers that formed through diagenesis is poorly known. In this study, systematic magnetic tests were conducted on Holocene deposits from core S23 (3.00–18.00 m, spanning the past 7.60 ka) in the East River Delta, southern China. Pyrite was widely discovered within the core, while magnetite, hematite, and greigite were the main magnetic contributors. An abrupt increase in magnetic mineral content along with the occurrence of coarse magnetic grains were observed in the intervals of ~0.70–0.60, 0.87–0.78, 1.07–0.94, 1.18–1.12, 1.40–1.33, 1.58–1.45, 1.92–1.82, 2.09–2.02, 2.62–2.48, 3.44–3.36, 3.90–3.62, 6.14–6.00, 6.74–6.67, and 7.42–7.36 cal. ka BP. These changes, consistent with the EHE sediment records in the nearby peat, indicate the regional EHE deposits. We propose a model in which EHEs promoted the quickly input of coarse, clastic materials (including magnetic minerals), which inhibit the sulfidation progress of greigite to pyrite, and enable the preservation of greigite. The occurrence of EHEs was mainly controlled by the El Niño-Southern Oscillation activity, while their frequency between 2.10–0.50 cal. ka was partly caused by the intensification of the anthropogenic activities in the basin. The continuous weakening of the Asian Summer Monsoon during 7.60–0.50 cal. ka BP led to a long-term increase in the frequency of EHEs in South China. This study provides a new approach for reconstructing ancient EHEs in South China deltas.