Cryoconite holes

From Andrew Fountain

One project this season was to examine the meltwater chemistry of the surface waters on the glaciers. Previous studies examined the chemistry of stream waters that flow on the valley floor from the glaciers to the lakes. This is important for understanding the nutrients in the streams that feed the mosses and microbal mats in the streams, and for the nutrient supply to the lakes. We were uncomfortable with the assumption that the waters coming from the glaciers were nearly pure water (electrical conductance of 3-5 microsiemens), characteristic of distilled water. Sand is transported on to the glaciers during windy periods (primarily in winter) and the lack of sufficient meltwater may not flush the surface ice clean, thus the meltwater that does drain off the glacier is not as pure as we expect, as from temperate glaciers.

Under the leadership of Dr. Martyn Tranter from the University of Bristol (UK) we sampled several stream channels on two glaciers. Many of the water courses exhibit a pool and riffle pattern characteristic of alpine streams but with an exaggerated stream profile of large ponds separated by very steep sections. The first two figures show such ponds. Don't let the blue color fool you, the pools are completely ice-covered. In fact, we observe very little water directly open to the atmosphere. Most water flows beneath the ice surface a few 10's of cm down into the glacier.

In addition to measuring the chemistry of the flowing water, we are also measuring the chemistry of isolated water pockets that are frequently found just beneath the ice surface. These water pockets are "cryoconite" holes (cryo meaning cold and conite meaning rock dust). They are formed by rock dust absorbing solar radiation, warming, and melting into the ice. Although such cryoconite holes are common to most glaciers world wide, here in the dry valleys they seem to be perennially ice-covered. A cross-section of such a hole is shown in the following figure.

Cryoconite Hole on Taylor Glacier (Click on image for larger version)

Cryoconite holes appear as clear-ish or dark colored patches on the ice surface as the figure shows. Measuring the chemistry of cryoconite holes is useful for two reasons. First, because the water is isolated from the atmosphere and not moving, yet reside in the same climatic/ice environment, their chemistries may evolve differently. Second, the walls of the melt pools melt back and intersect cryoconite holes. The contents of the holes, both water and sediment, drain into the large melt pools. The attached picture shows Martyn Tranter sampling one of the holes.

Andrew