The sea-ice cover around Antarctica has experienced a slight expansion in area over the past decades1, 2. This small overall increase is the sum of much larger opposing trends in different sectors that have been proposed to result from changes in atmospheric temperature or wind stress3, 4, 5, precipitation6, 7, ocean temperature8, and atmosphere or ocean feedbacks9, 10. However, climate models have failed to reproduce the overall increase in sea ice11. Here we present a data set of satellite-tracked sea-ice motion for the period of 1992–2010 that reveals large and statistically significant trends in Antarctic ice drift, which, in most sectors, can be linked to local winds. We quantify dynamic and thermodynamic processes in the internal ice pack and show that wind-driven changes in ice advection are the dominant driver of ice-concentration trends around much of West Antarctica, whereas wind-driven thermodynamic changes dominate elsewhere. The ice-drift trends also imply large changes in the surface stress that drives the Antarctic ocean gyres, and in the fluxes of heat and salt responsible for the production of Antarctic bottom and intermediate waters.

Wind-driven trends in Antarctic sea ice drift

Earth’s climate has undergone different intervals of gradual change as well as abrupt shifts between climate states. Here we aim to characterize the corresponding changes in climate response to astronomical forcing in the icehouse portion of the Cenozoic, from the latest Eocene to the present. As a tool, we use a 35-m.y.-long δ 18 O benthic record compiled from different high-resolution benthic isotope records spliced together (what we refer to as a megasplice). We analyze the climate response to astronomical forcing during four 800-k.y.-long time windows. During the mid-Miocene Climatic Optimum (ca. 15.5 Ma), global climate variability was mainly dependent on Southern Hemisphere summer insolation, amplified by a dynamic Antarctic ice sheet; 2.5 m.y. later, relatively warm global climate states occurred during maxima in both Southern Hemisphere and Northern Hemisphere summer insolation. At that point, the Antarctic ice sheet grew too big to pulse on the beat of precession, and the Southern Hemisphere lost its overwhelming influence on the global climate state. Likewise, we juxtapose response regimes of the Miocene (ca. 19 Ma) and Oligocene (ca. 25.5 Ma) warming periods. Despite the similarity in δ 18 O benthic values and variability, we find different responses to precession forcing. While Miocene warmth occurs during summer insolation maxima in both hemispheres, Oligocene global warmth is consistently triggered when Earth reaches perihelion in the Northern Hemisphere summer. This pattern is in accordance with previously published paleoclimate modeling results, and suggests an amplifying role for Northern Hemisphere sea ice.

Alternating Southern and Northern Hemisphere climate response to astronomical forcing during the past 35 m.y.

Understanding how the Antarctic ice sheet will respond to global warming relies on knowledge of how it has behaved in the past. The use of numerical models, the only means to quantitatively predict the future, is hindered by limitations to topographic data both now and in the past, and in knowledge of how subsurface oceanic, glaciological and hydrological processes interact. Incorporating the variety and interplay of such processes, operating at multiple spatio-temporal scales, is critical to modeling the Antarctic’s system evolution and requires direct observations in challenging locations. As these processes do not observe disciplinary boundaries neither should our future research.

/pdf/spatio-temporal-variability-of-processes-across-antarctic-2vav4rf1yi.pdf

Spatio-temporal variability of processes across Antarctic ice-bed–ocean interfaces

45 The Pliocene Epoch (~5.3-2.6 million years ago, Ma) was characterized by a warmer than 46 present climate with smaller Northern Hemisphere ice sheets, and offers an example of a climate 47 system in long-term equilibrium with current or predicted near-future atmospheric CO2 48 concentrations (pCO2). A long-term trend of ice-sheet expansion led to more pronounced glacial 49 (cold) stages by the end of the Pliocene (~2.6 Ma), known as the “intensification of Northern 50 Hemisphere Glaciation” (iNHG). We assessed the spatial and temporal variability of ocean 51 temperatures and ice-volume indicators through the late Pliocene and early Pleistocene (from 3.3 52 to 2.4 Ma) to determine the character of this climate transition. We identified asynchronous shifts 53 in long-term means and the pacing and amplitude of shorter-term climate variability, between 54 regions and between climate proxies. Early changes in Antarctic glaciation and Southern 55 Hemisphere ocean properties occurred even during the mid-Piacenzian warm period (~3.26456 3.025 Ma) which has been used as an analogue for future warming. Increased climate variability 57 subsequently developed alongside signatures of larger Northern Hemisphere ice sheets (iNHG). 58 Yet, some regions of the ocean felt no impact of iNHG, particularly in lower latitudes. Our 59 analysis has demonstrated the complex, non-uniform and globally asynchronous nature of 60 climate changes associated with the iNHG. Shifting ocean gateways and ocean circulation 61 changes may have pre-conditioned the later evolution of ice sheets with falling atmospheric 62 pCO2. Further development of high-resolution, multi-proxy reconstructions of climate is required 63 so that the full potential of the rich and detailed geological records can be realized. 64 65 Plain Language Summary 66 Warm climates of the geological past provide windows into future environmental responses to 67 elevated atmospheric CO2 concentrations, and past climate transitions identify important or 68 sensitive regions and processes. We assessed the patterns of average ocean temperatures and 69 indicators of ice sheet size over hundreds of thousands of years, and compared to shorter-term 70 variability (tens of thousands of years) during a recent transition from late Pliocene warmth 71 (when CO2 was similar to present) to the onset of the large and repeated advances of northern 72 hemisphere ice sheets referred to as the “ice ages”. We show that different regions of the climate 73 system changed at different times, with some changing before the ice sheets expanded. The 74 development of larger ice sheets in the northern hemisphere then impacted ocean temperatures 75 and circulation, but there were many regions where no impacts were felt. Our analysis highlights 76 regional differences in the timing and amplitudes of change within a globally-significant climate 77 transition as well as in response to the current atmospheric CO2 concentrations in our climate 78 system. 79 80

Climate Evolution Through the Onset and Intensification of Northern Hemisphere Glaciation

The Miocene to Pliocene (Neogene) occurred between 23.04 and 2.58 million years ago and includes intervals of peak global warmth where Earth’s average surface temperature was up to 8℃ warmer than present. Major cooling steps also occurred, across which Antarctica’s ice sheets advanced to the continental shelf for the first time and sea ice expanded across the Southern Ocean. Knowledge of Antarctic environmental change and ice sheet variability through this dynamic period in Earth history has advanced over the past 15 years. Major field and ship-based efforts to obtain new geological information have been completed and significant advances in numerical modelling approaches have occurred. Integration of ice proximal data and coupled climate-ice sheet model outputs with high-resolution reconstructions of ice volume and temperature variability from deep sea δ18O records now offer detailed insight into thresholds and tipping points in Earth’s climate system. Here we review paleoenvironmental data through key episodes in the evolution of Neogene climate to include the Miocene Climatic Optimum (MCO), Middle Miocene Climate Transition (MMCT), Tortonian Thermal Maximum (TTM), Late Miocene Cooling (LMC), and Pliocene Warm Period (PWP). This review shows that Antarctica’s climate and ice sheets remained dynamic throughout the Neogene. Given the analogous nature of warm episodes in the Miocene and Pliocene to future projections, the environmental reconstructions presented in this chapter offer a stark warning about the potential future of the AIS if warming continues at its current rate. If average global surface warming above pre-industrial values exceeds 2℃, a threshold will be crossed and AIS instabilities would likely be irreversible on multi-century timescales.

Antarctic environmental change and ice sheet evolution through the Miocene to Pliocene – a perspective from the Ross Sea and George V to Wilkes Land Coasts

We investigate the stability of the East Antarctic Ice Sheet (EAIS) on the Wilkes Land continental margin, Antarctica, utilizing a high-resolution record of ice-rafted debris (IRD) mass accumulation rates (MAR) from Integrated Ocean Drilling Program Site U1359. The relationship between orbital variations in the IRD record and climate drivers was evaluated to capture changes in the dynamics of a marine-based ice sheet in response to early Pliocene warming. Three IRD MAR excursions were observed and confirmed via scanning electron microscope microtextural analysis of sand grains. Time series analysis of the IRD MAR reveals obliquity-paced expansions of the ice sheet to the outer shelf prior to ~4.6 Ma. A decline in the obliquity and a transition into a dominant precession response of IRD MAR occur at ~4.6 Ma along with a decline in the amplitude of IRD MAR maxima to low background levels between ~4.0 and ~3.5 Ma. We speculate that as sea surface temperatures began to peak above 3°C during the early Pliocene climatic optimum, the ice shelves thinned, leading to a greater susceptibility to precession-forced summer insolation and the onset of persistent retreat of a marine-based portion of the EAIS.

Threshold behavior of a marine‐based sector of the East Antarctic Ice Sheet in response to early Pliocene ocean warming

On high‐latitude continental margins sediment is supplied from land to the deep sea through a variety of processes, including iceberg and sea‐ice rafting, and bottom current transport. The accurate reconstruction of sediment fluxes from these sources through time is important in palaeoclimate reconstructions. The goal of this study was to assess a shift in the intensity of glacial processes, iceberg and sea‐ice rafting during the Pliocene through an investigation of coarse sediment deposited at the AND‐2A site in the Ross Sea and at International Ocean Discovery Program Site U1359 on the Antarctic continental rise. Terrigenous particle‐size distributions and suites of quartz grain microtextures in the sand fraction of the deep‐sea sediments were compared to those from Antarctic glaciomarine diamictites as a baseline for proximal glacial sediment in its source area. Using images acquired through Scanning Electron Microscopy, and following a quantitative approach, fewer immature and potentially glacially transported grains were found in Pliocene deep‐sea sand fractions than in ice‐contact sediments. Specifically, in the lower Pliocene interval silt and fine sand percentages are elevated, and microtextures in at least half of the sand fraction are inconsistent with a primary glacial origin. Larger numbers of chemically altered and abraded grains in the deep‐sea sand fraction, along with microtextures that are diagnostic of periglacial environments, suggest a role for eolian sediment transport. These results highlight the anomalous nature of high‐latitude sediment fluxes during prolonged periods of ice retreat. Furthermore, the identification of a significant offshore sediment flux during Antarctic deglaciation has implications for estimated nutrient supply to the Southern Ocean and the potential for high‐latitude climate feedbacks under warmer climate states.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/dep2.157

Quartz grain microtextures illuminate Pliocene periglacial sand fluxes on the Antarctic continental margin

In the Southern Ocean, unconstrained Westerlies allow for intense mixing between deep waters and the atmosphere. How this system interacts with Antarctic ice sheets and the global ocean circulation is poorly understood due to a paucity of data. The poor abundance and preservation of foraminiferal carbonate in ice-proximal sediments is a major challenge in high-latitude paleoceanography. A new approach is to examine a sediment geochemical record of changing paleoproductivity and sediment redox environment that can be tied to changes in water mass properties. This study focuses on the paleoceanography of the George V Land margin between ~4.7 and 4.3 Ma. This interval at the onset of the early Pliocene Climatic Optimum was characterized by the highest global sea surface temperatures and the lowest sea ice concentrations in East Antarctica in the past 5 million years. At IODP Site U1359, an abrupt increase in Mn/Al ratios ~4.6 Ma indicates an episode of oxic bottom conditions resulting from enhanced wind-driven downwelling of Antarctic surface water. Above, extremely high concentrations of sedimentary barite (Ba excess >40,000 ppm) point to biogenic barite deposition, preservation, and concentration through enhanced upwelling of nutrient-rich Circumpolar Deep Water (CDW). Incursion of CDW onto the continental shelf affected ice discharge and resulted in a stable but reduced ice-sheet configuration over several glacial cycles. The geochemical results along with previous work on Site U1359 for the first time link paleoceanography and cryospheric change based on data from the same high-latitude site.

Oceanic circulation changes during early Pliocene marine ice-sheet instability in Wilkes Land, East Antarctica

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Threshold behavior of a marine‐based sector of the East Antarctic Ice Sheet in response to early Pliocene ocean warming

Quartz grain microtextures illuminate Pliocene periglacial sand fluxes on the Antarctic continental margin

Oceanic circulation changes during early Pliocene marine ice-sheet instability in Wilkes Land, East Antarctica