Tephra layers are common in lake sediments and although they have often been used as chronological controls, few studies have investigated the impacts of past tephra depositions on lake ecosystems (Tephropalaeoecology). For the first time we systematically assess how different types of tephra layers vary in their ecological impact on the same lacustrine system. We use a diatom-based tephropalaeoecological approach to infer the impacts of five tephra deposits on Lake Sihailongwan, a well-studied volcanic lake in Northeast China, over the past 30,000 years. The five tephra layers (including two micro-tephras) have varying thicknesses and were deposited in time periods with different climatic conditions. Changes in diatom communities and chrysophyte-cyst concentrations between pre- and post-tephra samples were used to infer changes in lake conditions and highlight the importance of lake background conditions in mediating the impact of tephra. While the two micro-tephra layers did not cause observable changes, the three thicker tephras induced pronounced changes in lake conditions and thus diatom communities. The two thick tephras deposited in more eutrophic and warmer lake conditions caused larger responses from diatoms. We argue that water-column phosphorus decreased due to reduced sediment–water-phosphorus loading as thick tephra layers formed an impermeable layer at the lake bottom. This is supported by a decrease in total diatom concentration and a decline in high phosphorus-requiring taxa such as Discostella stelligeroides and Stephanodiscus minutulus as well as modern limnological observations which showed that groundwater influxes from the lake bottom are the main source of nutrients to the lake. By contrast, the thick tephra deposited in more oligotrophic and colder lake conditions caused less conspicuous changes. When the lake was already low in phosphorus, diatoms did not respond to a further decline in phosphorus but rather responded to the minor increase in silica from the dissolution of tephra particles in the water column. This was inferred from the slight increases in overall diatom concentration and opportunistic taxa such as Pantocsekiella comensis f. minima. Diatom analysis of the post-tephra sediments above the three thick tephras showed that the aquatic ecosystem did not completely recover, indicating the long-lasting effects of these thick tephras and shifts to new lake-ecosystem equilibria.

The rigorous assessment of volcanic hazards relies on setting contemporary monitoring observations within an accurate, longer-term geological context. Revealing that geological context requires the detailed fieldwork, mapping and laboratory analysis of the erupted materials. However, many of the world’s most dangerous volcanic systems are located on or near coasts (e.g., the Phlegraean Fields and Vesuvius in Italy), islands (e.g., the volcanic archipelagos of the Pacific, south-east Asia, and Eastern Caribbean), or underwater (e.g., the recently erupting Hunga Tonga–Hunga Ha’apai volcano), meaning that much of their erupted material is deposited on the sea bed. The only way to sample this material directly is with seafloor sediment cores. This perspectives paper outlines how marine sediment cores are a vital yet underused resource for assessing volcanic hazards by: (1) outlining the spatio-temporal scope of the marine volcanic record and its main deposit types, (2) providing existing examples where marine sediments have contributed to volcanic hazard assessments; (3) highlighting the Sunda Arc, Indonesia as an example location where marine sediment cores are yet to contribute to hazard assessments, and (4) proposing that marine sediment cores can contribute to our understanding of very large eruptions that have a global impact. Overall, this perspectives paper aims to promote the utility of marine sediment cores in future volcanic hazard assessments, while also providing some basic information to assist researchers who are considering integrating marine sediment cores into their volcanological research.

As the world’s strongest zonal surface winds, changes in the Southern Hemisphere Westerlies (SHW) can profoundly affect atmospheric carbon dioxide (CO ₂ ) concentrations, ocean-cryosphere domains and precipitation patterns in the Southern Hemisphere mid- to high-latitudes. The strengthening and poleward migration of the SHW in the last few decades points to an increase in Southern Ocean ventilation and CO ₂ outgassing with significant implications for future global temperatures and Southern Hemisphere mid-latitude precipitation variability. A more in-depth and longer-term understanding of past SHW behaviour is required to improve projections of future SHW variability. Macquarie Island (54°S), located within the modern core belt of the SHW, is an ideal location because the influx of westerly wind-blown sea spray creates a strong conductivity gradient in lakes from west to east across the island. We used the known positive correlation between increasing wind-blown sea spray driven water conductivity and halophilic and halophobic diatoms preserved in lake sediments to reconstruct past changes in SHW influence on Macquarie Island over the last 2500 years. Low values in the accumulation rate of the dominant low-conductivity Aulacoseira principissa and Psammothidum taxa at ~2500–2300 cal. yr BP and after ~800 cal. yr BP suggest relatively stronger SHW over Macquarie Island at those times. Contrastingly, an increase in Psammothidum taxa at ~2300–800 cal. yr BP signals weaker SHW as a consequence of lower sea spray input. This is supported by the coeval, anti-phased changes observed in the relative abundance of high-conductivity species such as Platessa oblongella , Planothidium renei , Pinnularia sp. and Fragilaria capucina . This study provides a detailed and valuable record of SHW variability for the last few millennia in the Pacific sector of the Southern Ocean.

The Southern Hemisphere Westerlies (SHW) are the world’s strongest zonal surface winds, and they profoundly influence ocean-atmospheric CO2 exchange, southern mid-latitude precipitation patterns and ocean-cryosphere dynamics. Stronger and more poleward-shifted SHW over the last few decades have been linked to anthropogenic warming and increased Southern Ocean ventilation and CO2 outgassing. A more in-depth understanding of past natural SHW variability is required to investigate the SHW response to future, anthropogenically impacted climate change scenarios (e.g., IPCC2023). Macquarie Island (54°30’S, 158°57’E) is located in the Southern Ocean within the SHW core belt, providing an ideal location for reconstructing past changes in the SHW. A strong and decreasing west-east conductivity gradient exists in lakes across the island due to the input of westerly wind-blown sea spray. Moreover, since diatom species present in surface sediments from these lakes are strongly determined by conductivity, we reconstructed variations in the SHW over the last 3000 years using a sediment record from Lake Tiobunga on the west coast. Decreases in the sediment accumulation rate and the dominant, low-conductivity Psammothidium taxa imply that the SHW over Macquarie Island were relatively stronger between ~3000–2300 cal BP and in the last thousand years. Conversely, an increase in Psammothidium taxa implies weaker SHW ~2300–1000 cal BP. Superimposed on these longer-term trends are centennial-scale fluctuations and a dramatic increase in diatom production after 1900 CE, which we associate with the well-documented invasive rabbit infestation. Our results provide a record and improve the understanding of the complex SHW atmospheric system for the last few millennia in the Pacific sector of the Southern Ocean.

Tephra layers are common in lake sediments and although they have often been used as chronological controls, few studies have investigated the impacts of past tephra depositions on lake ecosystems (Tephropalaeoecology). For the first time we systematically assess how different types of tephra layers vary in their ecological impact on the same lacustrine system. We use a diatom-based tephropalaeoecological approach to infer the impacts of five tephra deposits on Lake Sihailongwan, a well-studied volcanic lake in Northeast China, over the past 30,000 years. The five tephra layers (including two micro-tephras) have varying thicknesses and were deposited in time periods with different climatic conditions. Changes in diatom communities and chrysophyte cyst concentrations between pre- and post-tephra samples were used to infer changes in lake conditions and highlight the importance of lake background conditions in mediating the impact of tephra. While the two micro-tephra layers did not cause observable changes, the three thicker tephras induced pronounced changes in lake conditions and thus diatom communities. The two thick tephras deposited in more eutrophic and warmer lake conditions caused larger responses from diatoms. We argue that water column phosphorous decreased due to reduced sediment-water phosphorous loading as thick tephra layers formed an impermeable layer at the lake bottom. This is supported by a decrease in total diatom concentration and a decline in high phosphorous-requiring taxa such as Discostella stelligeroides and Stephanodiscus minutulus as well as modern limnological observations which showed that groundwater influxes from the lake bottom are the main source of nutrients to the lake. By contrast, the thick tephra deposited in more oligotrophic and colder lake conditions caused less conspicuous changes. When the lake was already low in phosphorous, diatoms did not respond to a further decline in phosphorous but rather responded to the minor increase in silica from the dissolution of tephra particles in the water column. This was inferred from the slight increases in overall diatom concentration and opportunistic taxa such as Cyclotella comensis fo. minima. Diatom analysis of the post-tephra sediments above the three thick tephras showed that the aquatic ecosystem did not completely recover, indicating the long-lasting effects of these thick tephras and shifts to new lake ecosystem equilibria.