Tracking Tectonics and Ice
By Alessa J. Geiger & Max Zundel |
With the Polarstern route change at the very beginning of our expedition, exiting the Strait of Magellan via the Atlantic and not through the Pacific, our pre-selected land study areas seemed far out of reach. Max Zundel and I (Alessa Geiger) had teamed up as one of the land geology groups due to our positive overlap in possible sample areas. Whilst Max’ focus is on understanding the tectonic history of the Scotia Sea and surrounding areas my work deals with reconstructing glacier dynamics in the Chilean West Fjords during the last glacial cycle. Both of us collect rocks – but we utilize different techniques to answer our research questions (see more below). After several days of waiting to either get close enough to our field sites, winds abating, wave heights decreasing and clouds opening up the site of view for our helicopter pilots, we were finally on our first land mission to Isla Londonderry, located approximately 60 km south west of Cordillera Darwin, Chile.
As we fly over the wide expanse of the open ocean with the faint silhouette of distant land ahead, we are once again struck by Patagonia’s natural beauty and feel privileged in being allowed to work here! Approaching Isla Londonderry the helicopter thoroughly hovers over land to identify a solid landing spot where we can be dropped off. Shortly after, we touch down, jump out of our orange helicopter survival suits and into our field gear, make our first satellite call to Polarstern (which will be repeated hourly), wave the helicopter good bye and are off to find our first samples. We waded through some dense vegetation in the first couple of meters above sea level before entering characteristic mountain terrain.
Looking south-east across Bahia Cook, clear signs of former glacier activity are visible in the form of beautiful glacially modified bedrock. On Isla Londonderry bullet shaped erratic boulders sit on bedrock at least up to 300 m a.s.l. (Fig.3). We decided to sample at 100-meter increments and collected five samples for cosmogenic nuclide analysis and four bedrock samples for thermochronology work. About six hours after the helicopter had dropped us off, it was time to return as the weather window was bound to close and Polarstern was driving out of reach for the helicopters. We hope to return to Isla Londonderry during our Polarstern home track to conclude our sampling efforts here.
In order to reconstruct the timing of glacier extent and retreat in the Chilean West Fjords, I look out for rounded erratic boulders (rocks which are of a different geology to the underlying bedrock), which were deposited by glacier ice. Using rare cosmogenic isotopes, which accumulate in the erratic since deposition, I can determine when the glacier left that erratic at said position. This technique is known as terrestrial in-situ produced cosmogenic nuclide analysis and has been used to date a variety of geological processes. In southern South America this technique has been successfully utilized to date glacier extent during the last glacial cycle along the eastern margin of the Andean Cordillera. In the case of the Chilean West Fjords, frontal moraines marking glacier extent are at a sub-marine position; hence dating glacier extent is difficult. For that reason, my work aims to build a vertical chronology of past ice thickness change at western most fjord sites in southern Chile. Using the vertical chronology I can model probable glacier extent. Ultimately the dataset will shed light on a/synchronicities of glacier response between the eastern and western sector of the Patagonian Ice Sheet during the last glacial and associated links with palaeo-climatic changes.
Max’ task is to get rock samples from bedrock. Preferentially, he looks out for coarse-grained rocks, e.g. granites, gneisses, sandstones etc., however anything which contains the mineral apatite is suitable. Back home, these samples will be crushed and processed until only apatite-minerals are left. The spontaneous radioactive decay of uranium isotopes naturally creates tiny cracks within the apatite’s crystal lattice. These are then counted and measured through a microscope in order to constrain a fission track age giving name to the method: Apatite fission track analysis. The fission-track ages together with other measured parameters yield important information about the cooling paths of the samples in the upper crust. The cooling paths in turn are used to identify vertical tectonic movements, e.g. faults, which are often related to motion of tectonic plates. Such plate tectonic movements play an important role in the opening of the Drake Passage, which resulted from the break-up of two major continents, South America and Western Antarctica since the late Cretaceous (c. 80 million years ago). Crustal fragments, which have experienced this dramatic continental break-up are presently at the fringes of the Drake Passage, e.g. Tierra del Fuego and the Chilean West Fjords on the South American side and also the Elephant Island Group and South Shetland Islands on the Antarctic side. Max’ project therefore aims to reconstruct the individual tectonic histories of these fragments to understand how and when the Drake Passage opened.
