Aeolian processes and sources

Silt and sand-sized grains, including microfossils, are well known to be raised by saltation processes and then carried significant distances by wind. Diatom frustules and small clumps of diatomaceous sediment (microclasts) have an extremely high surface-area:mass ratio (Fig. 1), making them significantly more susceptible to wind entrainment from subaerially exposed diatomaceous surfaces than mineral grains of similar size. Wind erosion and aeolian transport of diatoms typically require dry, subaerially exposed source beds—surfaces covered with abundant unconsolidated diatoms—together with appropriate surface wind regimes, including episodic storm events capable of lifting saltated particles to the mid-troposphere42,43.

Figure 1: Scanning electron micrograph of diatomaceous sediment. Electron micrograph of Southern Ocean diatomaceous sediment, presented air-dried without any cleaning or processing, illustrating very high surface area of unconsolidated diatomite. Similar material is likely to have accumulated in the Wilkes and Aurora basins following retreat of ice. Once isostatically emerged and exposed they would be highly susceptible to wind erosion. Scale bar, 50 μm. Full size image

Once aloft, these particles can be carried great distances, as readily seen by the observed distribution of diatoms across the Antarctic ice-sheet surface. Pleistocene and modern long-distance transport and atmospheric precipitation of diatoms on the Antarctic ice sheet and the TAMs have been demonstrated through analysis of ice cores4,44,45, and modern surfaces in the TAMs46. Aeolian diatoms in Antarctica today are strongly dominated by non-marine forms, including those characteristic of Dry Valley lake and stream deposits and some from far-flung terrestrial source beds on other continents4,44,45,46, reflecting a relative paucity of marine sources for modern aeolian dust.

The fact that marine diatoms typically dominate the diatom assemblages extracted from most Sirius tillite surfaces, despite a constant and ongoing rain of non-marine diatoms across these surfaces today, indicates that the accumulation of marine diatoms is not a recent phenomenon, but largely reflects a specific interval or intervals of significant aeolian input with distinct source beds of subaerially exposed Pliocene marine diatomaceous sediments. Diatoms are generally not lifted from ocean surfaces47 and carried in large numbers far from pelagic sources, owing to the weight of enclosed cytoplasm and water, as well as the consequent reduction in their surface area. This is borne out by the relative lack of modern pelagic diatoms reported from ice cores and Sirius deposit surfaces.

The widespread distribution of aeolian marine diatoms in Sirius deposits (Fig. 2) implies that there must have been a significant upwind source of exposed diatomaceous beds. Specific hypothesized provenance of the diatoms was rarely discussed in the papers that made the case for aeolian emplacement of Sirius diatoms, although Stroeven et al.20,21 noted that the East Antarctic coastal margin along the Wilkes Land coast could provide one possible source for some aeolian diatoms. Despite this acknowledgement they explicitly rejected EAIS dynamic responses to Pliocene warmth26. Gersonde et al.48 presented a radically different hypothesis, suggesting that the diatoms might represent ejecta from a Pliocene meteorite impact in the Southern Ocean.

Figure 2: Sirius Group outcrops and Antarctic Ice Sheet configuration during warm Pliocene. Warm Pliocene Antarctic ice-sheet configuration from the simulation in Pollard et al.7, representing intervals of maximum retreat during warm austral summer orbits. Note the ice-sheet retreat across the Wilkes Basin (WB), Aurora Basin (AB) and the Recovery Ice Stream, adjacent to the Shackleton Range (SR). This model result illustrates an intermediate ice-sheet geometry, between the end-member configurations suggested in key papers in the ‘stabilist/dynamicist’ debate. Coastal exposures of previously submarine deposits, following isostatic rebound (Fig. 5), are shown in brown. Diatom-bearing Sirius Group outcrops along the Transantarctic Mountains in the Wisconsin Range (WR), Queen Maud Mountains (QMM), and South Victoria Land (SVL) are indicated in red (following ref. 5). Location of the Eltanin Meteorite impact site48 in the Southern Ocean is indicated. Full size image

Improved models

A new version of an established Antarctic ice-sheet model (PSUICE-3D) (ref. 3), which uses hybrid ice dynamics and allows for the buttressing effects of floating ice shelves and freely migrating grounding lines, now includes physics associated with glaciological processes of melt-water enhanced calving due to hydrofracture and ice cliff failure7. When forced by modest atmospheric and oceanic warming representative of warm Pliocene conditions with 400 p.p.m.v. CO 2 and a warm austral summer orbit7, the model produces a significant retreat of marine-based ice, including most of West Antarctica and the major Wilkes and Aurora basins in East Antarctica, where grounding lines retreat more than 500 km into the interior (Fig. 2), thus providing a physically plausible scenario for EAIS recession, with sea level rise and marine basin development. This result is further enhanced, notably in the Wilkes Basin, by the inclusion of dynamic topography8. Together, these new models further strengthen the case for EAIS retreat during Early and mid-Pliocene warm intervals, though do not indicate retreat as extensive as that implied by Webb et al.10. The recent modelling results are more consistent than earlier work with far-field sea-level49 and climate records50 and with East Antarctic coastal margin geologic records37,38,39,40.

Interior seaways and rebound

The seaways that would have filled the freshly exposed, deglaciated marine basins would have been well-mixed and nutrient-rich, fed by glacier ice-derived iron and other limiting micronutrients51,52, and thus would have hosted rich planktic diatom communities, comparable to those that rapidly accumulated thick diatomaceous oozes in the Ross Embayment during some of the same episodes of Pliocene West Antarctic Ice Sheet retreat2.

In the model, the lagged bedrock rebound in response to the reduced ice load becomes significant several thousand years after ice retreat, resulting in the emergence of large stretches of ice-free coastal plain and numerous islands around the East Antarctic coastline, most notably at the mouths of the Aurora and Wilkes basins, as shown in Fig. 2. Calculated basin depth and land areas are plotted by defined sectors (Fig. 3) in order to evaluate regional changes in ocean depth (Fig. 4) and emerged land area (Fig. 5) over time. According to the model, Aurora and Wilkes basins display more newly exposed land than would have emerged elsewhere around Antarctica, including the Amery Oasis of East Antarctica or around West Antarctica.

Figure 3: Longitudinal sectors defined to distinguish regions of ice-sheet change. Sectors are simply defined by longitude, as follows: MBL/ASE (Marie Byrd Land/Amundsen Sea Embayment) (160° W–60° W), Recovery Basin (60° W–30° E), Aurora Basin (30° E–135° E), Wilkes Basin (135° E–160° E) and Ross Basin (160° E–160° W). These are used for the model ocean and land surface calculations plotted in Figs 4 and 5. Text colours correspond with Figs 4 and 5 graphs. A separate sector was considered focusing on the Lambert Glacier/Prydz Bay area, but the model showed little net emerged coastal area, due to the prevailing steep-walled fjord-like geometry. Full size image

Figure 4: Average ocean depth change over time. Area of ‘new’ open ocean versus time, that is, model cells with no ice and with bedrock depth below sea level that had been land or grounded ice at the start of the run (modern configuration), representing the new oceanic bays exposed by grounded-ice retreat. The early peak and significant subsequent decline in ocean depth in the Aurora Basin (green) and the Wilkes Basin (turquoise) reflect rapid ice-sheet collapse followed by isostatic rebound, creating expansive coastal plain and islands. Coloured lines represent sectors defined in Fig. 3. Full size image

Figure 5: Area of exposed land over time following ice retreat and isostatic rebound. Total area of emerged exposed land (potential regions of aerially exposed diatom deposits, shown as brown patches in Fig. 2) is shown versus time through the model run, subdivided into sectors defined in Fig. 3. At each time, this is the area of model grid cells with no ice cover and with bed elevation that previously experienced deglaciation and isostatic emergence above sea level. Total for each individual sector is shown, as well as the all-Antarctic total (black curve). Note the early increase in land area of the Marie Byrd Land/Amundsen Sea Embayment sector, the late but significant rebound-induced emergence of East Antarctic basins, and the minimal emergence in the Ross Sea sector. Full size image

Before this emergence of ice-free land in the Aurora and Wilkes basins, the overlying ocean would have been mostly ice-free in summer, with air temperatures considerably warmer than today and little sea ice. After several millennia a significant drape of unconsolidated diatomaceous ooze would have accumulated, conformably overlaying glacial deposits. This would create a geological contact comparable to the rapid glacial-interglacial (diamicton to diatomite) transitions described in the ANDRILL-1B drillcore (defined as Motif 2)2.

Following uplift, a subaerially exposed diatom-strewn landscape would have been highly susceptible to wind erosion, especially during summer with reduced snow cover (Fig. 6), thus providing an abundant source for plumes of aeolian diatoms available to be raised aloft by storm-induced saltation. Subsequent transport across the ice-sheet surface would have led to widespread deposition on the Sirius tillite surfaces and elsewhere. Although the model shows some new land exposure in West Antarctica, an East Antarctic source for windblown diatoms is more likely, because the emerged lands around the Aurora and Wilkes sectors include extensive low-lying coastal plain and islands, whereas West Antarctic exposures are characterized by ice retreat from more steeply sloping island and mountain-front coasts, which generate less exposure of emerged coastal plain (Figs 4 and 5).

Figure 6: Pliocene average and summer temperatures. Pliocene calculated average annual (a) and summer (b) temperatures, and Pliocene versus modern annual (c) and summer (d) temperature anomalies. Note the temperatures much higher than modern summer values over Pliocene exposed lands of East Antarctica, indicating the probability of loss of winter snow cover over emerged Pliocene marine deposits. Full size image

The entire process takes several thousand years following climate warming. With grounding line retreat and ice shelf collapse, a marine basin opens, with copious diatom production in open waters during summer with little sea ice. The outer basins shallow with the isostatic response following ice retreat, eventually leading to emergence of uplifted islands and coastal areas. Although the simulation in Fig. 2 shows just one episode of retreat, many such cycles of retreat and re-advance would have occurred, driven by orbital cycles on timescales of 20,000–40,000 years over a period of at least several hundred thousand years in the warm mid-Pliocene around 3 Ma ago53,3,7. The retreat phase of each cycle would have provided an opportunity for diatom production, with subsequent exposure and windblown transport.

Modelling Pliocene winds

To evaluate the hypothesis that marine diatoms associated with Sirius tillites were derived from aeolian transport directly resulting from EAIS partial retreat, the Pliocene wind regime is simulated using the Genesis v3 Global Climate Model with a nested, polar version of the RCM (RegCM3) over Antarctica41. The model simulates three-dimensional winds for a Pliocene-like climate with a retreated Antarctic Ice Sheet and East Antarctic basins with exposed subaerial land as described above. The near-surface winds are mainly katabatic and do not show direct transport from either the Wilkes or Aurora basin surface exposures towards the Sirius locations in the TAMs (Fig. 7a). Surface winds do, however, suggest a potential for some aeolian transport towards the central TAMs from the Recovery Basin sector coast, adjacent to the Shackleton Range (Fig. 2). We consider the Ross Sea sector as a relatively insignificant source of windblown diatoms because there is little isostatic uplift compared with the Aurora and Recovery sectors. Like today, the modelled Pliocene low-level winds on the fringes of the ice sheet are energetic, with summer (December, January and February) average wind speeds exceeding 10 m s−1 in many locations adjacent to the freshly exposed sediments, most notably along the Aurora Basin coastline (Fig. 8). Wind speeds during coastal storm events would have been significantly higher, with low-pressure events enhancing the process of lifting diatoms to the mid-troposphere, where long-distance transport is possible. We suggest that the aeolian process was enhanced during austral summer, because modelled surface temperatures (Fig. 6) indicate complete loss of winter snow cover over emerged land in East Antarctica.

Figure 7: Surface and mid-troposhere winds over Pliocene Antarctic landscape. Modelled wind regimes over the warm Pliocene ice-sheet configuration (Fig. 2) during austral summer, DJF (December, January and February). (a) Near-surface winds are dominated by katabatic processes. Some Pliocene diatoms in the central TAMs may have been carried at relatively low altitudes from coastal exposures of a retreated Recovery Ice Stream near the Shackleton Range in the Weddell Sea sector or along the Ross Sea coast. Coastal storm events over the emerged Wilkes and Aurora basins would have provided the dominant mechanism for lifting a large volume of diatoms and other dust particles off exposed, snow-free surfaces to higher altitudes where they could be carried great distances. (b) Modelled winds at 500 hPa (∼5,000 m). Red arrow indicates likely dominant pathway for aeolian dust from Aurora Basin outcrops towards the TAMs. This configuration suggests that stratospheric rainout from a meteorite impact in the South Pacific Ocean would be towards the Ross Embayment, and thus ejecta would likely be more abundant along Marie Byrd Land and the Whitmore Mountains than the TAMs, though specific ejecta pathways have not investigated. Neither surface nor mid-altitude winds indicate significant pathways from the Ross Sea. Full size image

Figure 8: Average summer wind speed over Antarctic Pliocene landscape. Seasonally averaged summer (DJF) surface (10 m) wind speeds, corresponding to the simulated velocity vectors in Fig. 7a. Note the maximum wind speeds at the ice-sheet margin, adjacent to exposed, emergent marine basins. Antarctic summer months, when the coastal margins are snow-free under warm Pliocene conditions, may provide the best opportunity for lifting diatom tests, though storm events at any time of the year would contribute to significant wind erosion of the deposits. Full size image

In contrast with surface winds, mid-tropospheric winds at 500 hPa, ∼5,000 m altitude, are generally cyclonic over the continental interior and show strongly favourable transport directions towards the TAMs and Sirius outcrop localities, from both Wilkes and Aurora basin marine sediment exposures (Fig. 7b), especially during austral summer.

Although ice-free bedrock patches also emerge around the islands of West Antarctica (Fig. 2), both surface and upper-level winds are unfavourable, blowing away from the TAMs (Fig. 7), which implies they were not a significant source of particulates on Sirius Group deposits. Previous studies39,54 have focused on geological evidence of Pliocene and earlier retreat in the Lambert basin. Our model shows some retreat there (Fig. 2), in basic agreement with ice-erosion modelling of Taylor et al.55 and Jamieson et al.56. However the extent of retreat is much less than in the Wilkes and Aurora basins, and there is significantly less exposed ice-free land available to provide large volumes of aeolian diatoms (Fig. 2).