How Do Scientists Date Historical Glacial Retreats?

 

Have you ever wondered how scientists date historical glacial retreats?

Glacial scientists often seek to understand when and how fast glaciers receded (retreated). This information is critical for understanding paleoclimate trends,  models, and understanding glacier dynamics.

Unfortunately, the answer requires dating old glacial sediments but those glacial sediments are typically difficult to date.  Most methods require that we use indirect methods of dating subglacial sediments.  Most often we accomplish this by dating organic remains above glacial sediments (basal ages in lake cores showing life starting as glaciers retreat) and below glacial sediments (trees overridden by advancing glaciers). Techniques to date organic material are only useful over limited periods of time (i.e., radiocarbon is only relevant going back ~40,000 years).

Radiocarbon dating measures the decay of Carbon-14 contained inside samples of organic matter (such as small fossils in lake cores). This organic matter must have been buried and preserved since it died. Scientists also have to make sure that the sample they select is not contaminated with older radiocarbon (e.g., coal, etc).

To date the recession of a glacier, scientists use radiocarbon dating of organic material to construct the age-depth models for lake core sediments. Modern techniques allow us to collect very small fossils from cores which greatly reduces the errors associated with older radiocarbon dating which required “bulk” material sizes. 

Basal lake core dating was used in the Matanuska area to reconstruct the timing of Matanuska and Knik Glacier retreats since the last glacial maximum (~21,000 years ago).

This digital elevation model (DEM) shows terrain relief over along the Matanuska and Knik Glacier advance / retreat profiles. The terminal moraine (Elmendorf Moraine) created during the Last Glacial Maximum (LGM) advance is very obvious and shows that the twinned lobe (terminus) of the Matanuska and Knik Glaciers extended from Anchorage to Willow. The blue shapes are lakes distributed along the flow pathways of the two glaciers. Twenty-four lakes and bogs in this region were drilled and dated in order to reconstruct the retreat history. 

Lake coring is conducted in the winter when lakes are frozen thereby allowing drill rigs to be hauled onto the ice. The drill core is lowered to the bottom of the lake where sediment starts and then advanced through all the lake sediments until the dense hard glacial sediment (till) contact is reached. The desired organic material will be sampled from the contact between the glacial till and the lake sediments - this organic material radiocarbon date indicates the minimum age ice retreated from this area.

Lake core samples are extruded from the coring device, wrapped carefully in plastic, stored in shipping tubes, and shipped to the laboratory for analysis. This process is hard to execute in the winter because it is nearly impossible to wear gloves while handling the fragile core sediment material thus your wet hands are exposed to temperatures well below freezing.

Lake sediment cores show very significant differences among the different lake basins of the MatSu area. The lighter banded core (left) is from Reed Lake and contains a lot of marl (carbonate material) as well as tephra layers and shows clear laminations. The darker core (right) is from Loon Lake and is dominated by organic material accumulated in this basin. The differences in sediments are tied to the local geology and ecology where the basin is located.

Once the cores are in the laboratory, we sample each layer of sediment in the lake core and process samples to measure the organic content, carbonate, silicate, and magnetic properties (see the example data graph provided below). These tiny sediment samples are processed in extremely hot furnaces to measure Loss on Ignition (how much organic material is contained in the sediments), and run through various measurement systems to quantify other properties.

The data graph above shows the organic content, carbonate, silicate, and magnetic properties of the basal core sample from Reed Lake.

Macrofossils are carefully extracted from the basal contacts of each lake core (the transition between the glacial contact and the start of the organic material) under a powerful microscope.  All of the fossils pictured above are collected from lake cores along the Matanuska and Knik Glacier last glacial maximum flow profiles. Only terrestrial macrofossils are used for radiocarbon dating. Terrestrial macrofossils pictured above include (a) through (j) which are samples of wood, except sample (e) which is an example of charcoal that is never used for such dating. Other interesting non-terrestrial (aquatic) macrofossils: samples k) through n) are gastropods from the Fish Lake death assemblages, o) ostracode, r) beetle fragment, s) bryozoan statoblast, u) & v) oospores, w) chironomid head capsule, y) calcite chara encrustation.

 

The result of the radiocarbon dating of lake cores provides us with the chronology of Matanuska and Knik Glacier retreat. The image above shows the extent of the twinned Matanuska and Knik Glaciers during the Last Glacial Maximum contrasted against their much smaller modern extents.  It should be noted that during the LGM, ice covered almost all of southern Alaska but this image only shows the mapped extent of the Matanuska and Knik Glaciers. The rest of the adjacent region was also ice covered. 

Join us on a tour to learn all about the interesting discoveries made during this study which define the LGM Matanuska and Knik Glacial retreat!