Historical analyses of coastal marine sediments reveal land-based impacts on the benthos

We document from a sediment core collected at 23 m water depth offshore of Separation Point on the northern coast of Aotearoa New Zealand’s South Island, 3000 years of natural and human-induced change to undisturbed offshore sediments that are protected from contact fishing methods. Multivariate analysis of sediment characteristics and sources of primary productivity successfully differentiated the pre-human (< 1500 AD), Māori, and European periods (> 1850). Pre-human sediments were characterised by elevated proportions of shell gravels, reduced carbonate content and low sediment accumulation rates (SAR) (0.19–0.22 mm yr−1). Māori period sediments were identified by presence of bracken-fern spores and higher levels of pelagic diatom than benthic production, higher proportions of silt, and a four-fold increase in SAR (0.86 mm yr−1). European-influenced sediments containing introduced pine pollen were deposited at ca. 11 times pre-human rates (2.5 mm yr−1) and contained the greatest proportions of clay and organic material. Two pre-human sediment samples that contained elevated pelagic diatom remains were misclassified by our analysis as Māori period samples and overlapped in time with known earthquake and palaeotsunami events, suggesting both disturbances have long-term effects on the ecology of diatoms. Changes to Māori period sediments were consistent with the use of fire, coastal land clearance for dwellings and modifications of soils for horticulture. These effects were later amplified by widespread land clearance for the development of pastoral farming and roading by Europeans.