Christina R. Riesselman

The Wilkes Subglacial Basin (WSB), the largest marine-based sector of the East Antarctic Ice Sheet (EAIS), contains 3-4 m sea level equivalent ice volume and is considered highly vulnerable to instability processes under future warming scenarios ^1–3 . Loss of marine-based ice in WSB has been implied during Pliocene (5.3-2.6 Ma) interglacials when global temperatures were 2–3°C warmer and atmospheric pCO₂ exceeded ~400 ppm ⁴ . Ice retreat in the WSB has also been suggested within the last 400 kyr under more modest warming, although the magnitude of ice loss relative to the Pliocene remains uncertain ^5–8 . Here, we present a continuous 6-million-year-long record of iceberg-rafted debris and geochemical provenance tracers sourced from the WSB, providing evidence for shifts in iceberg calving and subglacial erosion related to ice sheet dynamics. Climate and ice sheet modelling experiments reveal that, while the pacing of marine-based erosion and iceberg calving in WSB was consistently modulated in a near-linear manner by orbital forcing, expansive grounding-line retreat required crossing a critical atmospheric warming threshold when pCO₂ exceeded 400 ppm. Our results demonstrate non-linear behaviour in the WSB, whereby once a warming threshold is breached, accelerated basal ice flow, dynamic thinning and flotation of the ice margin at its present-day bathymetric high occurs - after which ice loss within the largest EAIS marine catchment becomes irreversible on multi-millennial timescales.

Fjords are globally significant sedimentary archives that record the relationship between glacial, tectonic, and marine processes. Fiordland, on the southwest coast of New Zealand, hosts one of the most extensive temperate fjord provinces in the Southern Hemisphere, yet its postglacial evolution has remained poorly constrained. This study integrates high‐resolution seismic reflection data with existing geomorphic mapping across 15 fjords to reconstruct the regional deglaciation and sedimentary history following the Last Glacial Maximum. The resulting seismostratigraphic framework provides the first region‐wide chronology of marine inundation and sediment accumulation across Fiordland. Two principal depositional phases are identified: a basin‐confined unit representing pre‐marine infill deposited during and immediately following early glacier retreat, and an overlying marine unit that drapes the fjord floors following marine inundation. Uplift‐corrected sill depths combined with global and regional sea‐level curves reveal that marine inundation progressed asynchronously across Fiordland, with first inundation occurring as early as ∼16.9 ka and final fjords becoming marine by >8.7 ka, coincident with rapid sea‐level rise during Meltwater Pulses 1A and 1B. Post‐inundation sedimentation reflects a transition from high‐energy glacial deposition to low‐energy marine accumulation controlled by catchment size, fjord geometry, and tectonic setting. This reconstruction highlights Fiordland's role as efficient sediment and carbon traps, while preserving records of postglacial environmental change in temperate fjord systems. The integrated approach presented here offers a template for evaluating how tectonic, climatic, and eustatic processes collectively shape fjord evolution.

The Southern Ocean is a key driver of global climate variability, yet its dynamics during the Mid‐Pleistocene Transition (MPT; ∼1.25–0.7 Ma) remain poorly understood, especially in the South Pacific sector. The Polar Frontal Zone (PFZ) underwent significant latitudinal shifts during this critical period of climate reorganization. Here, we present new diatom records from four sediment cores along a north‐south transect (54°S–60°S) in the central Pacific Southern Ocean, aimed at tracing spatial and temporal changes in the positions of the SAF and the PF across the MPT. We use the relative abundance of the diatom species Thalassiothrix antarctica as a proxy for frontal movement, along with diatom‐based transfer functions, to reconstruct winter sea‐ice (WSI) extent and summer sea surface temperatures (SSSTs). Results indicate a gradual northward displacement of the PFZ beginning at around 1.4 Ma, followed by a return to near‐modern position at 700 ka. These frontal changes were driven by significant sea‐ice expansion and SSST cooling, contributing to temperate interglacials during the MPT. Additionally, frontal migrations in the Pacific Southern Ocean might be topographically steered by the regional bathymetry, that is, the East Pacific Rise.

Fiordland, Aotearoa/New Zealand (AoNZ) is host to 14 named temperate fjords, however the magnitude and timescales of oceanographic variability in the deep basins remain poorly understood due to limited oceanographic sampling in these basins over the past five decades. Fjords have been identified as globally significant carbon burial sites and oceanographic processes and properties are important components of the sequestration process. Deep‐water renewal (DWR) and vertical mixing are key processes in governing deep‐water conditions. In this study, we present a near‐monthly, 2 yr, in situ timeseries of deep‐water properties in Patea/Doubtful Sound, Fiordland, AoNZ, and determine the roles of DWR and vertical mixing in controlling deep‐water properties and timescales of their variation. In conjunction with pre‐ and post‐winter CTD casts collected in four other AoNZ fjords, these data provide the first direct observations of DWR in the southwest fjords of AoNZ. Using two years of near‐monthly repeat CTD survey data (2023–2025), we estimate vertical diffusivities at two sites using a budget method and use these to hindcast properties in the fjord between January 2012 and April 2025 using a 1‐D model. Hindcast output suggests renewal occurs every winter in both entrance and fjord‐head basins. Comparisons between hindcast output and coastal water properties reveal that annual DWR in AoNZ results from deepening and cooling of the winter mixed layer, contrasting with other fjord systems where DWR is often caused by upwelling, providing a novel renewal preconditioning mechanism.

The Ross Ice Shelf buttresses ice draining from both East and West Antarctica and its collapse could accelerate the loss of inland ice sheets, rapidly raising sea level. Documenting the location, timing and rate of past glacial retreat can help reveal processes driving rapid mass loss, informing projections of ice sheet responses to a warming climate. Here, we present a record of mid-Holocene ice retreat from the southwestern Ross Sea using facies succession and paired ramped pyrolysis oxidation ¹⁴ C/ ²¹⁰ Pb chronology. This record shows rapid ice shelf retreat from 6.9-5.4 cal kyr BP, coeval with thinning of adjacent outlet glaciers. Our findings reconcile earlier discrepancies in terrestrial and marine reconstructions, and indicate that synchronous grounding line retreat from west of Ross Island to the Siple Coast at ~7-6.2 cal kyr BP was likely driven by warm-water incursions, a process active in parts of Antarctica today.

Sea ice in the Ross Sea plays a critical role in the formation of dense water masses, ice sheet stability, and air‐sea gas exchange, and also supports unique ecosystems. However, its seasonal and spatial variability makes it challenging to include in model simulations. To address this, new sea ice records that extend beyond the satellite era and include periods of climate change are essential. This new sediment record from Moubray Bay, northwest Ross Sea, reconstructs environmental conditions between ∼11,300 and ∼10,900 cal yr BP—a time of rapid retreat of marine‐based ice sheets and coastal glaciers in the region. The diatom assemblage is dominated by three taxa: Fragilariopsis curta , Corethron pennatum , and Chaetoceros resting spores. Variations in their relative abundances reveal changes in wind strength, water column structure, and sea ice concentration and duration. Between ∼11,300 and ∼11,200 cal yr BP, environmental conditions are characterized by a stabilized water column due to fresh meltwater influx, and weaker winds, which resulted in shorter sea ice duration and reduced winter sea ice concentration. This continued after ∼11,200 cal yr BP but stronger winds linked to deepening of Amundsen Sea Low‐like circulation triggered short‐term water column stratification. Sea ice concentration and duration increased after ∼11,100 cal yr BP driven by cooling of the sea surface by stronger southerly winds. Concurrent changes in early Holocene marine and terrestrial climate records from the Ross Sea indicate a shift in atmospheric circulation during the early Holocene.

The Mid‐Pleistocene Transition (MPT) marks a major shift in the pacing of Earth's climate changes from 40 to 100 thousand years. The Polar Frontal Zone (PFZ), which controls upwelling and productivity in the Southern Ocean, migrates latitudinally during major climate transitions. Data about past PFZ behavior in the Pacific Sector of the Southern Ocean are sparse. Here, we present new diatom and geochemical records from International Ocean Drilling Program (IODP) Site U1539, located just south of the modern Subantarctic Front (SAF) in the Pacific Sector of the Southern Ocean. At Site U1539, a long‐term shift in the diatom assemblage across the MPT indicates a northward shift of the PFZ around 770 ka. The early‐MPT assemblage is dominated by Thalassiothrix antarctica mats interpreted to reflect the position of the SAF. This assemblage transitions to a more diverse assemblage post‐MPT. The post‐MPT assemblage reveals cyclic increases in the relative abundance of Eucampia antarctica during glacial periods, suggesting potential sensitivity to dust‐derived nutrients. Throughout the 1.4 Myr record, summer sea surface temperature (SSST) and biogenic opal content vary with the global benthic oxygen isotope stack, indicating sensitivity to orbitally‐paced glacial‐interglacial change. South Pacific glacial SSSTs gradually cool across the MPT, with increased siliceous productivity and cooler SSSTs during glacials and reduced siliceous productivity and warmer SSSTs during interglacials. Reduced absolute diatom abundance during Marine Isotope Stages 21–15 (∼860–560 ka) indicates persistent sea ice during lukewarm interglacials, suggesting that decreased ventilation preconditioned the Southern Ocean to store more carbon in the late Pleistocene.

We present a mid‐to‐late Holocene record of relative paleosecular variation from the Ross Sea region of Antarctica. The 6,700‐year‐long record of inclination, declination, and relative paleointensity from a marine sediment core collected near Cape Adare is independently dated using a combination of ramped pyrolysis oxidation and carbonate radiocarbon dates. Agreement between the large‐scale features of the relative paleointensity record and the virtual axial geomagnetic dipole moment suggests that changes in the record are dominated by the dipole component of the Earth's geomagnetic field. Correspondence between the record and a non‐independently dated reconstruction from the Antarctic Peninsula indicates regionally coherent changes in the geomagnetic field intensity in the southern high latitudes during the mid‐to‐late Holocene. The prominent features of the record serve as stratigraphic markers for hard‐to‐date Antarctic sedimentary records and a constraint on Holocene geomagnetic field behavior when incorporated into the next generation of geomagnetic field models.

Three new pennate diatom taxa, Fragilariopsis clava sp. nov. Duke; Fragilariopsis armandae sp. nov. Frazer, Duke et Riesselman; and Rouxia raggattensis sp. nov. Duke et Riesselman, are described and named from Pliocene-Pleistocene sediments collected from the continental rise adjacent to the Wilkes Land coast of East Antarctica. The stratigraphic occurrence of F. clava and F. armandae at IODP Site U1361 are well-constrained to Marine Isotope Stages G9-G7 (2.76–2.74 Ma) and 101–97 (2.58–2.47 Ma), respectively. The short stratigraphic ranges of F. clava and F. armandae are potentially useful biostratigraphic markers for constraining the age of late Pliocene to Early Pleistocene Antarctic sediments. Rouxia raggattensis is observed in the oldest sample examined at Site U1361 from ∼4.05 Ma and is more common between 3.0–2.15 Ma. The rise in abundance of R. raggattensis corresponds to a large turnover in diatom species between 3 and 2 Ma associated with Antarctic cooling, suggesting that sea surface conditions were favorable for R. raggattensis during this dynamic time. Clavate Fragilariopsis species diversified between 2.9–2.7 Ma, but some species quickly went extinct between 2.7–2.5 Ma, possibly because they were marginalized by the cooler climate conditions.