We quantify sea surface temperature (SST) in the past Southern Ocean using the diatom Fragilariopsis kerguelensis that displays variable population with SST.  We explore the use of this relatively new proxy by applying it to sediment assemblages from the Sabrina Coast and Amundsen Sea. We find that Amundsen Sea and Sabrina Coast F. kerguelensis populations are different from each other.  An understanding of F. kerguelensis dynamics may help us generate an SST proxy to apply to ancient sediments.

Ocean alkalinity enhancement (OAE) is a promising method of atmospheric carbon removal; however, its ecological impacts remain largely unknown. We assessed the effects of simulated silicate- and calcium-based mineral OAE on diatom silicification. We found that increased silicate concentrations from silicate-based OAE increased diatom silicification. In contrast, the enhancement of alkalinity had no effect on community silicification and minimal effects on the silicification of different genera.

Shellfish aquaculture operates in nearshore areas where data on ocean acidification parameters are limited. We show daily and seasonal variability in pH and saturation states of calcium carbonate at nearshore aquaculture sites in British Columbia, Canada, and determine the contributing drivers of this variability. We find that nearshore locations have greater variability than open waters and that the uptake of carbon by phytoplankton is the major driver of pH and saturation state variability.

The diatom-bound nitrogen isotope proxy is used to study how efficiently diatoms in the Southern Ocean help to remove CO₂ from the atmosphere, but may be biased by different diatom species. We examine a specific type of diatom, Chaetoceros resting spores (CRS), commonly preserved in Southern Ocean sediments. We find that CRS record surprisingly low δ¹⁵N_DB values compared to other diatoms, yet changes in their relative abundance over time does not significantly bias previously published records.

Sea ice is important to stabilize the ice sheet in Antarctica. To understand how the global climate and sea ice were related in the past we looked at ancient molecules (IPSO25) from sea-ice algae and other species whose dead cells accumulated on the ocean floor over time. With chemical analyses we could reconstruct the history of sea ice and ocean temperatures of the past 14 000 years. We found out that sea ice became less as the ocean warmed, and more phytoplankton grew towards today's level.

This study uses diatom assemblages, biogenic silica and Si/Al data over the last 140 kyr from core TAN1302-44 (64°54' S, 144°32' E) to define glacial-to-interglacial paleoenvironments near Antarctica with respect to sea ice duration and ocean circulation. It has found that the sea ice season increased gradually during the last glacial, reaching a maximum before decreasing at the end of MIS 2. Following this, Circumpolar Deep Water increased relative to other times prior to ice sheet retreat.

Salt marshes are thought to be important carbon sinks because of their ability to store carbon in their soils. We provide the first estimates of how much blue carbon is stored in salt marshes on the Pacific coast of Canada. We find that the carbon stored in the marshes is low compared to other marshes around the world, likely because of their young age. Still, the high marshes take up carbon at rates faster than the global average, making them potentially important carbon sinks in the future.

Ocean alkalinity enhancement has the capacity to remove vast quantities of carbon from the atmosphere, but its effect on marine ecosystems is largely unknown. We assessed the effect of increased alkalinity on a coastal phytoplankton community when seawater was equilibrated and not equilibrated with atmospheric CO₂. We found that the phytoplankton community was moderately affected by increased alkalinity and equilibration with atmospheric CO₂ had little influence on this effect.

Algae preserved in seafloor sediments have allowed us to reconstruct how Antarctic sea ice has varied between cold and warm time periods in the last 130 000 years. The patterns and timings of sea-ice increase and decrease vary between different parts of the Southern Ocean. Sea ice is most sensitive to changing climate at the external edges of Southern Ocean gyres (large areas of rotating ocean currents).

Despite its importance in the global climate, our knowledge of Antarctic sea-ice changes throughout the last glacial–interglacial cycle is extremely limited. As part of the Cycles of Sea Ice Dynamics in the Earth system (C-SIDE) Working Group, we review marine- and ice-core-based sea-ice proxies to provide insights into their applicability and limitations. By compiling published records, we provide information on Antarctic sea-ice dynamics over the past 130 000 years.

Stable isotope ratios of foraminiferal shells from deep-sea sediments preserve key information on the variability of ocean circulation and ice volume. We present the first global atlas of harmonized raw downcore oxygen and carbon isotope ratios of various planktonic and benthic foraminiferal species. The atlas is a foundation for the analyses of the history of Earth system components, for finding future coring sites, and for teaching marine stratigraphy and paleoceanography.

Climate models are used to predict future climate changes and as such, it is important to assess their performance in simulating past climate changes. We analyze seasonal sea-ice cover over the Southern Ocean simulated from numerical PMIP3, PMIP4 and LOVECLIM simulations during the Last Glacial Maximum (LGM). Comparing these simulations to proxy data, we provide improved estimates of LGM seasonal sea-ice cover. Our estimate of summer sea-ice extent is 20 %–30 % larger than previous estimates.

How much of the West Antarctic Ice Sheet will melt and how quickly it will happen when average global temperatures exceed 2 °C is currently unknown. Given the far-reaching and international consequences of Antarctica’s future contribution to global sea level rise, the SWAIS 2C Project was developed in order to better forecast the size and timing of future changes.

Algae preserved in marine sediments have allowed us to reconstruct how much winter sea ice was present around Antarctica during a past time period (130 000 years ago) when the climate was warmer than today. The patterns of sea-ice increase and decrease vary between different parts of the Southern Ocean. The Pacific sector has a largely stable sea-ice extent, whereas the amount of sea ice in the Atlantic sector is much more variable with bigger decreases and increases than other regions.

Radiolarians found in marine sediments are used to reconstruct past Southern Ocean environments. This requires a comprehensive modern dataset. The Southern Ocean Radiolarian (SO-RAD) dataset includes radiolarian counts from sites in the Southern Ocean. It can be used for palaeoceanographic reconstructions or to study modern species diversity and abundance. We describe the data collection and include recommendations for users unfamiliar with procedures typically used by the radiolarian community.

We explore the potential for the use of carbon isotopes of algal fatty acid as a new proxy for past primary productivity in Antarctic coastal zones. Coastal polynyas are hotspots of primary productivity and are known to draw down CO₂ from the atmosphere. Reconstructions of past productivity changes could provide a baseline for the role of these areas as sinks for atmospheric CO₂.

Climate models struggle to simulate a LGM ocean circulation in agreement with paleotracer data. Using a set of simulations, we test the impact of boundary conditions and other modelling choices. Model–data comparisons of sea-surface temperatures and sea-ice cover support an overall cold Southern Ocean, with implications on the AMOC strength. Changes in implemented boundary conditions are not sufficient to simulate a shallower AMOC; other mechanisms to better represent convection are required.

We present a multi-proxy record of Holocene glacial meltwater input, sediment transport, and sea-ice variability off East Antarctica. Our record shows that a rapid Antarctic sea-ice increase during the mid-Holocene (~ 4.5 ka) occurred against a backdrop of increasing glacial meltwater input and gradual climate warming. We suggest that mid-Holocene ice shelf cavity expansion led to cooling of surface waters and sea-ice growth, which slowed basal ice shelf melting.