Acid sulphate soil-landscape relationships in an undrained, tide-dominated estuarine floodplain, Eastern Australia

Shallow stratigraphic sequences associated with the genesis of acid sulphate soils were examined in a tide-dominated estuarine floodplain, eastern Australia. The results show that the accumulation of pyrite is more significant in the site, compared to the results from the previous studies in a wave-dominated and a fluvial-dominated estuarine plain. This is in agreement with the suggestion of Pons and Van Breemen (1982) that strong tidal action favours the enrichment of pyrite in the sediments. c sediments are covered by up to 1.5 metres of flood overbank sediments in the site. Alternating flood-dominated regimes (FDR) and drought-dominated regimes (DDR) are responsible for intensified overbank sedimentation. During transitional periods from DDR to FDR, channel bed aggradation and increasing flood frequency largely accelerate overbank deposition. The overlying overbank sediments have overburdened and compacted the low-bulk density pyritic sediments and resulted in insignificant increase in surface elevations. This enables the accretion of overbank sediments to a considerable thickness. -induced acidification has occurred in this undrained estuarine floodplain. Rainfall analysis suggests that this process was initiated by lowering of the watertable down to the pyritic layers during prolonged drought episodes in the past. Re-pyritization of oxidation products appears limited during high rainfall periods following these prolonged drought episodes. The acidified pyritic layer may act as a storage sink of acid sulphate materials, which can be transferred upwards by capillary action and acidify the non-pyritic topsoils.