The PIs propose to use the (U-Th)/He system in apatite to investigate the exhumation history, development of the present topography, and pattern of glacial erosion in the central Antarctic Peninsula. The Antarctic Peninsula has been glaciated since the Eocene and Pleistocene climate cooling is hypothesized to have suppressed, rather than enhanced, glacial erosion. To achieve these goals, the PIs will use a thermochronometric record of when and how the present glacial valley relief formed. A challenge to the proposed research is that, unlike Pleistocene glacial landscapes in temperate areas, the Peninsula is ice-covered and it is not possible to directly sample the bedrock surface. The PIs hope to learn about the timing and process of glacial valley formation through apatite (U-Th)/He and 4He/3He measurements on glacial sediment collected near the grounding lines of major glaciers draining the Peninsula. Learning how the Antarctic Peninsula landscape formed is important to discern how the mechanics of glacial erosion operate on long time scales, and to understand how glaciers mediate the interaction between climate change and orogenic mass balance. This work addresses a fundamental question in Antarctic earth science of how to infer geologic and geomorphic processes active on an ice-covered and inaccessible landscape.
Broader impacts: This proposal will bring new researchers into the Antarctic research community. A proposed collaboration with British Antarctic Survey researchers will build an international collaboration. The outcomes of this project have ancillary importance to other fields and addresses fundamental challenges in Antarctic Earth Science.
The purpose of this project was to develop a method of studying the geologic history of ice-covered regions in Antarctica. This is important because nearly the entire Antarctic continent is covered by ice, so it's not possible to investigate its geologic makeup as we could anywhere else on Earth where rocks and sediments are directly exposed at the surface. This project is a preliminary study of a small region of the Antarctic Peninsula, aimed at determining whether or not it would be possible to use this method at a larger scale throughout the continent. The specific geochemical technique we aim to apply is low-temperature thermochronometry, which is commonly used in other geologic research worldwide to measure rates of past surface erosion and tectonic deformation over geologic time periods spanning millions of years. Normally, this method would simply involve collecting and analyzing surface samples of bedrock from sites spread across the region that we want to study. In Antarctica, that's not possible because the surface is nearly completely covered by glaciers that can be thousands of feet thick. However, these glaciers are actively sliding in many places, so they erode the bedrock beneath them and transport the resulting sediment to the ice margin. Most Antarctic glaciers calve icebergs directly into the ocean, so this sediment is deposited on the seafloor near glacier margins, where it can be collected fairly easily by ship. So the samples we want to collect -- samples of the bedrock surface underneath the Antarctic ice cap -- are, in effect, being collected for us by these glaciers, and we can get them simply by picking up samples of seafloor sediment near the glaciers. In order to take advantage of this idea to learn about the geologic history of ice-covered regions, we had to answer a few questions that no one has considered before, and the aim of this project was to answer some of these questions by analyzing samples of glacial-marine sediment collected from the seafloor near two Antarctic glaciers by past oceanographic research cruise. One important question is whether the process of glacial erosion and transport would destroy the specific minerals that are needed for our geochemical measurements. We discovered that this is not the case; they survive as well as they would in, for example, river transport in temperate areas. So this issue turns out not to be a serious obstacle. Another important question relates to where the glacial sediment comes from: is it derived from a large portion of the upstream area that is covered by ice (good for our purposes) or is it derived only from a few small areas (less useful)? For the sites in this study, we found that glacial sediment is, in fact, derived from a large fraction of the area covered by ice. This means that we can, in principle, get large-scale information about the subglacial geology. Finally, a third focus of the project was to collect some preliminary geochemical measurements of the type that we would aim to make on a larger scale, and ask whether our results agree with some independent information that we already have about the geologic history of this particular region. This part of the project involved putting together a mathematical and physical calculation that is based on some simple assumptions about the geologic history of the region and that predicts the geochemical data we would expect to observe if this geologic history is correct. Of course, in this study this sort of a comparison can't be a complete test, or even a very good test, of the method, because if we already knew everything about the geologic history of Antarctica (which is not the case) there would be no need to develop new methods to study it. So this part of the project could also be described as trying as hard as we can to find evidence that our method doesn't work, or fails in some way. In this case, we couldn't find any such evidence: our observations agree with the sparse existing information we have about the geologic history of the region. To summarize, this project was primarily a feasibility study to determine whether a geochemical method of learning about the geologic history of landscapes could be applied to the ice-covered regions of Antarctica by collecting and analyzing samples of glacial sediment derived from those regions. We found that this approach is very likely to be feasible, which presents a potential new way to learn about the relatively unknown geologic makeup of much of the continent.
