Scientists from Germany and Argentina investigate climate-induced mass changes of the Patagonian Ice Fields

Everyone knows the anecdote about Newton and the apple. Since gravity is an interacting force, the Earth is also attracted by the apple. It is therefore clear that even small changes in mass lead to a change in the gravitational field.

To detect these small changes, a team of German and Argentinean scientists travelled to southern Patagonia, in the Argentinean province of Santa Cruz. Scientists from TU Dresden (Dresden University of Technology, TUD), the Federal Agency for Cartography and Geodesy (BKG) Leipzig, the Universidad Nacional de La Plata (UNLP) and the Argentine-German Geodetic Observatory (AGGO) completed the five-week measurement campaign at the beginning of March 2022 and then began the more than 2,700 km return journey from El Calafate to La Plata.

The Patagonian Ice Fields are situated in southern Patagonia, in the border region between Argentina and Chile. They form the largest continuous ice mass in the southern hemisphere outside Antarctica. These ice fields are subject to a considerable mass loss, which is largely due to climate change, but also to natural climate variability and ongoing dynamic adjustment processes since the last glacial maximum. What proportions can actually be attributed to these various causes is one of the important open questions that is currently under investigation and to which this project will make a contribution. These mass changes reach a magnitude of up to one fifth of the Antarctic ice mass loss and cause both a change in the gravity field and deformations of the Earth's crust. The latter manifest themselves in a crustal uplift of up to 4 cm per year, determined by previous measurements of the TUD scientists. However, the detection of these resulting changes is made more difficult by a complicated tectonic situation: About 150 km northwest of the Northern Patagonian Ice Field, three tectonic plates meet. Where the divergent plate boundary between the Antarctic and Nazca Plates subducts under the South American Plate, a so-called slab window opens, leading to extraordinary mass redistributions in the Earth's interior that have so far been poorly understood.

© Andreas Richter (UNLP)

As the most important parameter, the absolute gravity value was determined at eight measuring points, from the Atlantic coast in Puerto San Julian to the eastern edge of the southern Patagonian ice field with Glaciar Perito Moreno as the best-known glacier. An absolute gravimeter of the type Micro-g LaCoste FG5 was used for this purpose, with the help of which it is possible to determine the gravity acceleration with an extremely high accuracy of almost one billionth of the gravity. These measurements are supplemented by relative gravity measurements with a Scintrex CG5 gravimeter to determine local gravity gradients and connections to securing points or points of the Argentine gravity network (RAGA) as well as by precise GPS measurements to determine crustal

© Andreas Richter (UNLP)

deformation. A special technique was realized by water level measurements using GPS reflectometry at the large Patagonian lakes and the Atlantic coast: The indirectly received signals of the GPS satellites reflected on the water surface are used in the analysis to determine the height of the water level and, thus, the gravitational effect of changes in the level of the respective body of water.

© Andreas Richter (UNLP)

"In order to detect the gravity changes due to mass loss, at least one more measurement campaign is necessary after this already second one," says Thorben Döhne, PhD student at the Chair of Geodetic Earth System Research, responsible primarily for the relative gravity measurements. "However, the weather conditions, especially with the fierce, persistent winds, and the not always easy logistical circumstances, e.g. transporting people and material over unpaved roads and fords, pose special challenges to such a measurement campaign," reports Andreas Richter, formerly at TU Dresden and now working at UNLP. By relating the measured absolute gravity changes and the crustal deformations determined by GPS, the researchers hope to gain new insights into the structure of the Earth's interior and the nature of the deformation processes observed. "It will also help to link the measurements made at the Earth's surface, i.e. ground-based measurements, with satellite geodetic measurements of the mass balance and height change of the Patagonian Ice Fields," adds Döhne.

The research project "Gravimetric Determination of the Response of the Solid Earth to ice mass changes in Southern Patagonia" is jointly led by Mirko Scheinert (TU Dresden) and Axel Rülke (BKG Leipzig) and funded by a research grant of the German Research Foundation (DFG).

© Mirko Scheinert (TU Dresden)

Acknowledgement
The participating scientists would like to express their gratitude for the support provided by the Universidad Nacional de La Plata, Facultad de Ciencias Astronómicas y Geofísicas (Dean: Prof. Raúl Perdomo), the Argentine-German Geodetic Observatory (Scientific Director: Prof. Claudio Brunini), the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) and the Instituto Geográfico Nacional (Director: Sergio Cimbaro).

Links

• Chair of Geodetic Earth System Research, TU Dresden:
  https://tu-dresden.de/bu/umwelt/geo/ipg/gef?set_language=en
• Federal Agency for Cartography and Geodesy (BKG): https://www.bkg.bund.de/EN/Home/home.html
• Universidad Nacional de La Plata (UNLP), Facultad de Ciencias Astronómicas y Geofísicas: http://www.fcaglp.unlp.edu.ar/
• Argentine-German Geodetic Observatory (AGGO) – CONICET: https://www.conicet.gov.ar/the-future-of-geodetic-data-measurement-in-argentina/
• Instituto Geográfico Nacional (IGN): https://www.ign.gob.ar/

© B. D. Gutknecht

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