Christopher M. Moy

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.

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.

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.

Apart from natural variations in environmental and climatic conditions, human activities (e.g. land-use changes) can have major impacts on freshwater ecosystems. In the context of New Zealand, the arrival of humans approximately 750 years ago has left a permanent mark on landscapes, posing challenges in discerning undisturbed natural conditions for paleoclimatic reconstructions. Yet, remote alpine lakes in the Southwest Pacific can serve as pristine archives of environmental and climate changes, hardly influenced by human activities.This study focuses on sediment cores obtained from two neighbouring catchments of Lake Bright and Lake Laffy in Fiordland National Park, situated in an understudied remote area of New Zealand’s Southern Alps. Employing a multiproxy approach, we utilized bulk C and N stable isotopes, lipid biomarker analysis, and high-resolution X-ray fluorescence and hyperspectral imaging to reconstruct Holocene changes in catchment dynamics and climate. Here, we present a new precipitation record based on the compound-specific composition of hydrogen isotopes of plant wax n-alkanes as well as a branched glycerol dialkyl glycerol tetraethers (brGDGTs) derived temperature reconstruction. Examining a sequence spanning the late Holocene, we discern events of evolving environmental conditions impacting these catchments.Our biomarker paleorecord from Lake Bright revealed a relatively stable temperature range during the last 4 ka, indicating that the changes in the sedimentary record might be dominantly shaped by the local hydroclimate. The low compound δ2H values observed reflect precipitation that has been orographically uplifted over the Southern Alps. Downcore δ2H variations are likely influenced by changes in the isotopic composition of rainfall driven by regional temperature and latitudinal sources of westerly precipitation. In this context, we present these episodes of hydroclimatic variability and derive the ecosystem response to prevalent conditions by correlating the respective changes in lake productivity, water quality and shifts in vegetation composition.Finally, we extend this record through the Holocene to the last local glacial termination about 10 ka ago using the Lake Laffy stratigraphy and engage in a discussion regarding the potential implications of enhanced climate variability on the future integrity of New Zealand's pristine alpine ecosystems.

The Antarctic Circumpolar Current (ACC) represents the world’s largest ocean-current system and affects global ocean circulation, climate and Antarctic ice-sheet stability^1–3. Today, ACC dynamics are controlled by atmospheric forcing, oceanic density gradients and eddy activity⁴. Whereas palaeoceanographic reconstructions exhibit regional heterogeneity in ACC position and strength over Pleistocene glacial–interglacial cycles^5–8, the long-term evolution of the ACC is poorly known. Here we document changes in ACC strength from sediment cores in the Pacific Southern Ocean. We find no linear long-term trend in ACC flow since 5.3 million years ago (Ma), in contrast to global cooling⁹ and increasing global ice volume¹⁰. Instead, we observe a reversal on a million-year timescale, from increasing ACC strength during Pliocene global cooling to a subsequent decrease with further Early Pleistocene cooling. This shift in the ACC regime coincided with a Southern Ocean reconfiguration that altered the sensitivity of the ACC to atmospheric and oceanic forcings^11–13. We find ACC strength changes to be closely linked to 400,000-year eccentricity cycles, probably originating from modulation of precessional changes in the South Pacific jet stream linked to tropical Pacific temperature variability¹⁴. A persistent link between weaker ACC flow, equatorward-shifted opal deposition and reduced atmospheric CO₂ during glacial periods first emerged during the Mid-Pleistocene Transition (MPT). The strongest ACC flow occurred during warmer-than-present intervals of the Plio-Pleistocene, providing evidence of potentially increasing ACC flow with future climate warming.

The modern Gulf of Alaska (GOA) is a Cordilleran Ice Sheet (CIS) region, estimated to be important for nutrient cycling and CO₂ exchange. Little is known of the GOA evolution over the Pliocene and Pleistocene as well as its impact on the CIS development, when other evidence for changing North Pacific circulation has emerged. We analyzed Integrated Ocean Drilling Program Expedition 341 Site U1417 sediments, which extend through the Plio‐Pleistocene transition (4–1.7 Ma), focusing on productivity‐related biomarkers (alkenones, brassicasterol), siliceous microfossils and bulk carbon and nitrogen stable isotopes. Our results show two dominant water column regimes: one characterized by high silica and low organic matter (OM) preservation, containing microorganism remains from a mix of habitats (4–3.7 Ma) and a second characterized by low biogenic silica and increased OM preservation of microorganisms from dominantly open ocean habitats (3.33–3.32 Ma and 2.8–1.66 Ma). An increase of phytoplankton diversity (3.7–3.35 Ma, 3.19–2.82 Ma) characterizes the two transitions of water column conditions, from oxygenated to reductive, that we attribute to a change from ocean mixing to strong stratified conditions with some occasional mixing. The biogeochemical changes in the GOA follow 400 and 100 kyr eccentricity cycles which are also reflected in changes in the CIS. We conclude that the CIS expansion created high nutrient low chlorophyll conditions in the GOA during the Mid Piacenzian Warm Period and the early Pleistocene. In turn, positive feedbacks increased marine productivity export, atmospheric CO₂ drawdown and further CIS expansion.