Land subsidence and sea level rise in the Port Adelaide estuary: Implications for monitoring the greenhouse effect

Abstract

The historic tide gauge records from Port Adelaide and Outer Harbour are one of the more important datasets from the Australian region purporting to show a significant rate of local sea level rise. However, geological evidence including radiocarbon dated palaeosea level indicators, indicates that most of this rise is due to subsidence of the land. The subsidence is significant but localized, and can be largely attributed to human activities associated with port development, reclamation and industrialization. Two principal contributing factors are reclamation of Holocene wetlands and groundwater extraction from deeper Tertiary aquifers. Wetland reclamation has caused surficial soil compaction as a result of artificial lowering of the water table, oxidation of peat and pyrite, and leaching of the substrate by soil acidification. Surficial subsidence at rates of up to 10 mm/year has occurred in reclaimed mangrove woodlands as a result of these processes. Extraction of groundwater from deeper aquifers over the past 50 years has created a potentiometric cone of depression in excess of 20 m beneath the Port Adelaide estuary. Land subsidence estimated at 2.8 mm/year is associated with the central zone of depressed groundwater levels. The tide gauge data from Port Adelaide and Outer Harbour have been used in global sea level rise calculations without adequate local neotectonic correction. Although outside the zone of greatest land subsidence, three‐quarters of the secular rise in mean sea level of 2.5–2.9 mm/year indicated by the tide gauge records can be attributed to land level changes. Hence the local sea level trend is a rise of ∼0.7 mm/year. As many of the world's historic tide gauge sites are expected to be affected by similar anthropogenic effects, it is imperative that local neotectonic corrections be applied to all tide gauge data before drawing any conclusions regarding global or local eustatic sea level change.