Drilling on Antarctica’s continental shelf to uncover ice sheet history

We have finished drilling at our first site on the Ross Sea’s continental shelf, where we had overall excellent recovery for the method of drilling (rotary core barrel; RCB). The RCB is designed to cut through all types of sediment. Because we were targeting both hard diamictites (mixture of poorly-sorted clasts in a muddy/sandy matrix) and softer mudstones, the RCB method was the most appropriate for our site. The diamictites were likely deposited underneath or proximal to ice, and the mudstones which contain microfossils of phytoplankton (mainly diatoms in Antarctica) represent deposition in open marine conditions, far away from ice influence. We can use downcore changes in these sediment types to understand past advances and retreats of the West Antarctic Ice Sheet.

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Drilling in Antarctic conditions (Photo by Kim Kenny)

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Example of sediment core recovery from the Ross Sea from the ANDRILL program, and modeled ice sheet configuration as a result of this project (Gasson et al., 2016). When there are open water conditions, phytoplankton like diatoms thrive and are incorporated into the sedimentary record once they die, creating layers of diatomaceous sediment. At this time Antarctica’s ice sheets are retreated (bottom right model). As the ice sheet advances (top right model), rocks, mud, and sand carried by the ice sheet are deposited, creating layers of lithogenic sediment. Alternating layers of these two units through time tells us about the past environment used to infer ice sheet history.

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Some excited scientists: Imogen Browne, Tim van Peer, and Jeanine Ash (Photo by Laura de Santis)

Ice sheet models and paleoclimate data tell us that the Ross Sea is one of the last places on the continent to become glaciated, so drilling here helps us to constrain the biggest advances of the West Antarctic Ice Sheet over the past 20 million years or so. One of our main aims at this site is to constrain ice advance at a regional erosional surface, which we think happened around 16 million years ago during a period of Earth’s history called the Miocene. This surface represents regional advance and grounding of ice across the continental shelf of the Ross Sea, which only happens every few million years or so, and results in a fall in global sea level.

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Co-chief Laura de Santis presenting an image of the sea floor during the middle Miocene (Photo by Jule Müller)

We rely on microfossils among other methods to tell us the age of erosional surfaces. Microfossils can be different types of phytoplankton, zooplankton, or pollen. In Antarctica, the dominant group of phytoplankton are called diatoms, which form discs and chains called ‘frustules’, composed of silica (glass). We know from regional studies of well-dated sedimentary sequences when different taxa of these groups appear and disappear through the geologic record due to evolution and extinction through time. These biological events are often associated with abrupt changes in the marine environment, such as initiation of new currents and warming or cooling of surface waters.

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Diatoms collected from surface waters south of 70S (Photos by Expedition 374 Paleontology team)

During the Miocene Climate Optimum (~17-15 million years ago), climate was warm enough for plants to grow around the edges of the continent. During this time, Antarctica’s ice sheets were generally reduced in extent and global sea level was much lower. However, recent studies from the Ross Sea and the ANDRILL project indicate that both the West and East Antarctic Ice Sheets were dynamic and capable of modulating global sea level, even during warmer climates of the middle Miocene. By studying past ice sheet behavior and response to warm climates like the middle Miocene, we can better model and predict how Antarctica will respond to ongoing anthropogenic warming.

 

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