The Antarctic climate system is changing. Geology can help us understand how.

Perhaps the most concerning possible climate scenario is large-scale mass wasting of Antarctica’s ice sheets, the largest reservoirs of ice on Earth. Climate and ice-sheet modelers seek to predict the rate of change in ice volume and how quickly coastal communities will be impacted, but their predictions are dependent on observational data for testing and tuning their models. Because modern subglacial and ice-marginal environments are difficult to access and image, data resolution is spatially and temporally limited. Therefore, geologic observations of past expansions and contractions of ice sheets are more often used for model calibration. My research is primarily focused on utilizing glacial geomorphology and sedimentology to understand ice-proximal processes and rates of change of past marine-based ice sheets.

Geomorphology

Geomorphologic interpretation is one of the first methods I employ to determine paleoenvironments and ice-sheet retreat patterns. Using multibeam swath bathymetry and high-resolution seismic, we can resolve seafloor features as small as <1 m in relief that tell us about past ice behavior. Key features include glacial lineations, which are subglacial features aligned parallel to paleo-ice flow direction, and grounding-zone wedges and moraines, whose crests are oriented perpendicular to ice flow. By mapping the backstep of grounding-zone wedges and moraines, we can track the pattern of retreat.

Other features on the seafloor include iceberg furrows (plowmarks, scours, gouges) and remnants of subglacial channels. In the Ross Sea, widespread, deep iceberg furrows indicate a major collapse of the ancestral Ross Ice Shelf during the late Holocene (Yokoyama et al., 2016, Proceedings of the National Academy of Sciences). Multibeam bathymetry also has revealed an episodically active ancestral subglacial hydrological system that locally restricted the supply of sediments that would otherwise build constructional relief at the grounding line and stabilize the ice (Simkins et al., 2017, Nature Geoscience).

Click on the images below to see enlarged versions.

Use of combined multibeam bathymetry and high resolution seismic (CHIRP) data to select sites for a transect of cores across a grounding-zone wedge (from Prothro et al., 2018)

Multibeam image of a grounding-zone wedge that prograded over moraines and a preexisting subglacial channel (Simkins et al., 2017)

Sedimentology

Multiproxy analysis of cores is crucial to understanding what depositional environments are represented and what events are recorded. I supplement grain-size analyses with microfossil data and geotechnical data such as magnetic susceptibility, gamma density, and x-rays. A common representation of grain-size data is by volume percentage of certain grain size classes (e.g., % sand, % silt, % clay), but much more can be learned by looking at other statistics derived from the shape of the full grain size distribution. Calculations of mean, mode, sorting, and skewness reveal additional information about current conditions and subglacial processes.

For example, a common well-sorted fine silt deposit has been documented in Pine Island Bay, the Ross Sea, and Marguerite Bay (coming soon) in association with meltwater channels identified using multibeam. This deposit is the first direct evidence for subglacial meltwater activity throughout ice-sheet retreat following the Last Glacial Maximum in the Ross Sea, indicated by the superposition of meltwater silt deposits above subglacial deposits and below open marine deposits (Prothro et al., 2018, Marine Geology; Simkins et al., 2017, Nature Geoscience).

Click on the images below to see enlarged versions.

Log of Core KC17 from Cruise NBP1502A, showing multiproxy grain size, geotechnical, and foraminifera data.

 

Comparison of meltwater deposits and till. Better sorting in silty meltwater deposits relative to till and a common 10 micron mode suggest a subglacial origin for the meltwater silt.

Press:

Hidden river once flowed beneath Antarctic Ice (link)

Colossal Antarctic ice-shelf collapse followed last ice age (link)

IFL Science: Colossal Antarctic ice-shelf collapse followed last ice age (link)