Onset of Antarctic ice sheet vulnerability to ocean conditions

Oceanographic observations show that the circumpolar deep water (CDW) intrudes at several locations of the continental shelf break around Antarctica and threatens the outlet glaciers and ice shelves. A recent analysis of the last decades of Antarctic observations have, indeed, concluded that many glaciers and ice shelves experienced a thinning (Paolo et al., 2016) and a retreat of their grounding line over the last decade (Konrad et al., 2018). Current location of those CDW intrusions are controlled, among other factors (e.g., tides and wind system), by the depth and morphology of the continental shelf, by the steepness and curvature of the continental slope and by the outflow of cold bottom waters (Nitsche et al., 2017). The magnitude of bottom water outflow and CDW inflow is also controlled by the capability of the continental shelf basins to store the bottom waters. Determining how much and where the ocean heat exchanges occur is crucial to determine which sectors of the Antarctic ice sheet (AIS) are most at risk of retreat/collapse and how fast this will happen in the next decades to centuries (Lenton et al., 2008).

The morphology of the pan-Antarctic continental shelf largely evolved since the Eocene-Oligocene transition, presumed onset of the Antarctic glaciations (~34 Ma). The wax and waning of the AIS eroded large amount of sediments that were released at the continental shelf edge, thus contributing to its progradation until the late Miocene-early Pliocene (10 - 5 Ma, De Santis et al., 1999). Stabilisation of the continental shelf occurred during the Pliocene and Plio/Pleistocene glaciations and led to the actual landward-deepened continental shelf extent. Thus, Miocene is a key period in the AIS climatic history. Sediment core records and seismic stratigraphy show that the AIS dynamics evolved from highly dynamical and largely wet-based ice sheet until the Mid Miocene characterised by high atmospheric CO2 concentration (> 450 ppm) to less dynamical and mostly cod-based and expanded ice sheet by the late Miocene (10-5 Ma), characterised by lower atmospheric CO2 concentrations (< 250 ppm).

Atmospheric CO2 (and inherent climatic feedbacks) is not the only driver of AIS dynamical changes through time. Recent oceanic simulations using reconstructed mid-Miocene bathymetry of the Weddell Sea, characterised by a smaller and shallower continental shelf, show that the location of the Weddell Sea gyre is shifted southward and eastward from its current location, which modifies the heat transport from open ocean to the Antarctic continental shelf edge (Huang et al., 2017). Thus, the location and strength of the CDW inflow and bottom waters outflow across the continental shelf edge evolved along with the morphological changes of the continental shelf edge. On another hand, conceptual ice-sheet simulations show that the gradual deepening of the Antarctic bathymetry in the Ross Sea amplifies the West AIS sensitivity to atmospheric and oceanic conditions (Bart et al., 2016). It is likely that the impact of the inflow/outflow oceanic mechanism on the AIS advances and retreats over the expanding continental shelf has strengthened and became a primary factor of the AIS variability through the Plio/Pleistocene, at least in the main marine-based sectors of the AIS.

The Weddell Sea and Ross Sea are both strategic areas to investigate the response of the AIS to climate changes. Both the East and West AIS, indeed, flow and converge to sustain the Ronnie-Filchner ice shelves and Ross Ice Shelf, respectively (Fig 1). In the present proposal, we focus on the Ross Sea ocean circulation and its impact on the AIS dynamics. Different water masses interact on the Ross Sea continental shelf and slope (Orsi et al., 2009, Rosso et al., 2011). Here, we mainly focus on the interactions between impinging CDW (in its Along Slope Current configuration), Ross Sea Bottom Water (RSBW) and Ice Shelf Water (ISW), west of Iselin Bank and their evolution through time, as revealed by the sedimentary processes of this area. Ice Shelf Water (ISW) forms as a result of sub-shelf melting occurring beneath the Ross Ice Shelf. ISW is generally supercool (< -1.9°C) and fresher than the RSBW and flows along the eastern flank of the Glomar Challenger Basin (Fig.1). Along its pathway on the continental shelf, the ISW maintain its characteristics (Bergamasco et al., 2002) but it partly mixes with the RSBW and both water masses interact with the ASC at the shelf edge, at the mouth of the Pennell Trough near the head of the Hillary Canyon (Fig. 1), sinking downslope and locally forming the AABW. In the Ross Sea, inflow of CDW also occurs at the mouth of the Joides Basin and Cap Adare (Budillon et al., 2016).

Understanding the inception of this oceanic inflow/outflow mechanism and its impact on the AIS are some of the objectives that the recent International Discovery Ocean Program (IODP) expedition 374 in Ross Sea proposed to investigate (McKay et al., 2018). In the present proposal, we want to test whether or not this feedback established during the Miocene, as a result of morphological and circulation constraints, and strengthened along with the gradual global climate cooling and expansion of the AIS and its continental shelf during the Plio/Pleistocene glaciations.

[1] Analyze the impact of the continental shelves and slope morphology evolution through time on the heat and salt exchange at key locations of the Ross Sea continental shelf edge (Fig. 1) and under different climatic conditions (from Miocene to the last deglaciation).

[2] Investigate the influence of the geometry of the Ross Sea basins (Little America, Whale Deep, Glomar Challenger, Pennell Trough, Joides basin, Draglyski basin) on the formation and retention of bottom waters and the impact of this retention on the overflow through the shelf break (Fig. 1).

[3] Characterise the water masses evolution (dynamics and physical parameters) through time and compare this evolution with the present-day state of Ross Sea waters.

[4] Analyse the impact of ocean waters variability and paleo-bathymetry on the advance and retreat of the Antarctic ice sheet in the different time periods of interest in this project (Miocene, Pliocene, LGM and last deglaciation).

Achieved results:

Due to the pandemic, the project surffered significant delayed in its agenda and manny results are coing out now, as the project is now closed

Objective [1]: During the project, the numerical oceanic model MIT GCM has been implemented onto the Ross Sea domain to achieve this objective. The MITgcm model allows oceanic circulation within the cavity and as such, was thus perfect to investigate the impact of bathymetry on the the heat and salt exchange under different climatic conditions. Numerical simulations over the last deglaciations, carried out each 1000 years show that the changes in slope morphology affect the CDW intrusion on the continental shelf. This will be the object of a publication within the next months (2024).

Objective [2] & [3]: Numerical simulations using the MITgcm allowed us to simulate the entire last deglaciation, accounting for glacio-isostatic adjustment and therefore change sin the bathymetry. Results are currently being analysed.

Objective [4]: A software decidated to 3D backtripping of paleo-bathymetry has been developed and published - PALEOSTRIP. The software is open source and thus accessible to anyone. Thanks to this software, we are now producing a set of paleo-bathymetry for the Miocene in the Ross Sea to bettwer understand the origine of the ice-sheet ocean interactions. In addition, a post-doc co-supervised with University of Trieste, investigated the evolution of the rift in the Ross sea during the Cenozoic. The idea was to improve the PALEOSTRIP software, in particular the thermal subsidence component, function of the lithosphere/astenosphere evolution through time. The study led to the develop of a numerical model that we used to generate an ensemble of tectonical simulations, accouting the the thermal evolution of the area. Results are currently being written in a paper to be submitted soon.

What could not be achieved:

• - Sediment core analysis: after the pandemic, administrative issues between OGS and the external laboraories (one in Poland and one in Germany) could not be solved in time to carry out the analysis.

• Numerical simulations of the oceanic circulation during the Miocene: due to the pandemic, the implementation of the numerical model have been delayed, as such, we were able to carry out many simulations of the last deglaciations, but we did not had the time to focus on the Miocene. This will be the objecct of further future investigations.

From the deliverables initially listed, here are the status:

D1: Simulated Antarctic ice sheet volume and extent variations in response to bathymetry: DONE

D2: Simulations of continental shelf and cavity oceanic circulation: DONE

D3: Ross Sea oceanic circulation maps for Miocene, Pliocene and last deglaciation periods based on both sediments, oceanographic data as well as modeling results: PARTIALLY DONE - only for the last deglaciation

D4: Age model for PNRA gravity cores and box cores.: NOT ACHIEVED DUE TO: PANDEMIC DELAYS,  TECHNICAL/ADMINISTRATIVE ISSUES WITH DATING LABS.

D5: Scientific publications in international journals: DONE

OUTREACH:

TV, Radio, Giornali:

2022:

17 Dicembre: Luoghi di Scienza, Rai scuola

13 Dicembre: Corriere della Sera, https://www.corriere.it/buone-notizie/22_dicembre_15/missione-florence-signora-ghiacci-l-antartide-rischio-b08b7fe0-78e7-11ed-826e-c8ce3b8cd17b.shtml

21 Novembre: Radio Capodistria, Zona 30 con Barbara Urizzi

7-14 Novembre: vari talks divulgativi alla COP27, Sharm el Sheik.

2 Novembre: RADAR, Radio Rai 3 con Simona Regina

2 Novembre: intervista su “La Stampa” – “SOS per i ghiacci del polo Sud, ventre molle di un pianeta sempre piu bollente”.

25 ottobre: Radio Capodistria, Zona 30 con Barbara Urizzi

11 Ottobre: Puntata Tele 4, Trieste Science+Fiction, con Massimiliano Maltoni: https://youtu.be/fIHJTjU-oYs

3 ottobre: TGR Friuli-Venezia Giulia, Buongiorno Regione

28 Settembre: Linea Verde, RAI 1

24 Agosto: articolo nel piccolo sull’innalzamento del livello del mare.
 

2020:

11 dicembre: TV Capodistria – intervista sull’innalzamento del livello del mare.

9 ottobre: Pubblicazione del libro divulgativo “Life of the Antarctic ice” – in collaborazione con il Norwegian Polar Institute.

Attività nelle scuole e per il pubblico:
 

2022: 

2 Dicembre: conferenza per le scuole – a cura di Where is the Science – “Cambiamenti climatici e livello del mare”, Municipio di Aquileia.

5 Novembre: conferenza pubblica, Mondi Futuri, Trieste Science+Fiction

4 Ottobre: ADAPT YOUR FUTURE, Università di Trieste, seminario a 300 liceali internazionali.

6-8 Ottobre: Barcola SeaSummit – Barcolana Jobs, laboratori per le scuole

24 Settembre: NEXT, Festival della Scienza di Trieste

16 Settembre: Terra in trasformazione – Gruppo 78, Conferenza pubblica al magazzino 26

8 Settembre: Scuola di biologia marina – “Cambiamenti climatici” – per una classe del liceo Galilei, sede OGS dei Filtri

15 Luglio: Il Mare si fa in 7, conferenza pubblica “Operazione Antartide: al polo Sub con la Laura Bassi”, biblioteca comunale di Grado.

28 Giugno: Evento pubblico con il Principe Alberto di Monaco, Polar oceans: Engines to global ocean, United Nation Ocean Decade conference, Lisbon.

29 Maggio: Conferenza pubblica al Yatch Club Adriaco di Trieste

Playlist Youtube sul sito di OGS di 25 filmati che ho girato e montato io durante la 37 campagna in Antartide a bordo della Laura Bassi. Immagini ripresi in onda tante volte nel corso delle varie interviste etc…

Da Gennaio ad Aprile: “Operazione Antartide” – serie di seminari al liceo Scientifico Statate G. Galileo di Trieste - nell’ambito della campagna in antartide.

2021:

Ottobre a dicembre: “Operazione Antartide” – serie di seminari al liceo Scientifico Statate G. Galileo di Trieste - nell’ambito della campagna in antartide

11 settembre: Science and Law – a cura dell’UIP – “cambiamenti climatici: cosa puo fare la legge”, museo Revoltella, Trieste.

2 Settembre: Scuola di biologia marina – “Cambiamenti climatici” – per una classe del liceo Galilei, sede OGS dei Filtr

18 Giugno: confernza pubblica, Mare Dire Fare – “Monitorare l’Antartide è importante per i Triestini?”, BIOMA, Riserva di Miramare, Trieste.
 

2020: 

28 Novembre: National geographics Festival – “Il clima di ieri, oggi e domani, un viaggio nel tempo” – webminar.

3 settembre: Radio RAI 3 – Podcast per ESOF – a cura di Simona Regina, “il Clima che cambia”.

13 Maggio: “Webminar per gli studenti a cura degli enti di ricerca” – attività istituzionale con OGS durante la pandemia – webminar sui cambiamenti climatici.

23 Marzo: Science Web Festival - conferenza in streaming sui cambiamenti climatici

Febbraio: seminario per le classi elementari della scuola di Tricesimo “Calote polari e livello del mare”

The following publications were produced during the life time of the project and some are still in progress and will be published in the next months:

[1] Colleoni, F., T. Naish, R. DeConto, L. De Santis, and P. L. Whitehouse (2022), The uncertain future of Antarctica’s melting ice, Eos, 103,

[2] Tim R. Naish, Bella Duncan, Richard Levy, Robert M. McKay, Carlota Escutia, Laura De Santis, Florence Colleoni, Edward G.W. Gasson, Robert M. DeConto, Gary Wilson, Chapter 8 - Antarctic Ice Sheet dynamics during the Late Oligocene and Early Miocene: climatic conundrums revisited, Editor(s): Fabio Florindo, Martin Siegert, Laura De Santis, Tim Naish, Antarctic Climate Evolution (Second Edition), Elsevier, 2022, Pages 363-387, ISBN 9780128191095.

[3] Florence Colleoni, Laura De Santis, Tim R. Naish, Robert M. DeConto, Carlota Escutia, Paolo Stocchi, Gabriele Uenzelmann-Neben, Katharina Hochmuth, Claus-Dieter Hillenbrand, Tina van de Flierdt, Lara F. Pérez, German Leitchenkov, Francesca Sangiorgi, Stewart Jamieson, Michael J. Bentley, David J. Wilson, Chapter 12 - Past Antarctic ice sheet dynamics (PAIS) and implications for future sea-level change, Editor(s): Fabio Florindo, Martin Siegert, Laura De Santis, Tim Naish, Antarctic Climate Evolution (Second Edition), Elsevier, 2022, Pages 689-768, ISBN 9780128191095, 

[4] R. Conte, M. Rebesco, L. De Santis, F. Colleoni, M. Bensi, A. Bergamasco, V. Kovacevic, J. Gales, F. Zgur, D. Accettella, L. De Steur, L. Ursella, R. McKay, S. Kim, R.G. Lucchi, (2021), Bottom current control on sediment deposition between the Iselin Bank and the Hillary Canyon (Antarctica) since the late Miocene: An integrated seismic-oceanographic approach, Deep Sea Research Part I: Oceanographic Research Papers, Volume 176, 

[5] Di Roberto, A., Scateni, B., Di Vincenzo, G., Petrelli, M., Fisauli, G., Barker, S. J., Del Carlo, P., Colleoni, F., Kulhanek, D. K., McKay, R., De Santis, L., The IODP Expedition 374 Scientific Party (2021), Tephrochronology and Provenance of an Early Pleistocene (Calabrian) Tephra From IODP Expedition 374 Site U1524, Ross Sea (Antarctica), Geochemistry, Geophysics, Geosystems, 22(8), 1525-2027,

[6] Marschalek, J. W. and Zurli, L. and Talarico, F. and van de Flierdt, T. and Vermeesch, P. and Carter, A. and Beny, F. and Bout-Roumazeilles, V. and Sangiorgi, F. and Hemming, S. R. and Pérez, L. F. and Colleoni, F., et al., (2021), A large West Antarctic Ice Sheet explains early Neogene sea-level amplitude, Nature, 600 (7889), 450-455.

[7] Colleoni, F., De Santis, L., Pochini, E., Forlin, E., Geletti, R., Brancatelli, G., Tesauro Magdala, Busetti Martina, Braitenberg, C. (2021). PALEOSTRIPv1. 0–a user-friendly 3D backtracking software to reconstruct paleo-bathymetries. Geoscientific Model Development (2021), 14 (8), 5285-5305