, 4 min, 712 words
Tags: alaska how-stuff-works kayak-adventures
Here at KAW, our most popular trip is our Aialik (pronounced eye-Y'ALL-ick) Glacier day trip. On the trip, we paddle right up to the face of Aialik Glacier. Aialik is a tidewater glacier, which means it comes right up to the ocean and drops chunks of ice in it. Today we had the chance to learn more about tidewater glacier dynamics from Luke Rosier, which was lovely. My favorite portion was a discussion of the tidewater glacier cycle, which causes tidewater glaciers to advance and retreat cyclically over centuries.
There are two key components to the tidewater glacier cycle: mass loss due to calving, and the deposit of sediment by subglacial streams.
For most glaciers, mass gain comes from snowfall at the origin of the glacier (in the accumulation zone). In turn, mass loss comes from melting down in the ablation zone, typically closer to the end of the glacier. On top of that, tidewater glaciers are special: their mass loss also comes from calving, when ice falls into the sea. It turns out that the mass loss to calving depends strongly on how deep the water is where it meets the glacier. Deeper water leads to higher calving rates, and shallower slows down that mass loss.
In the right conditions (more on that in a moment), some ice at the bottom of a glacier can melt. That lubricates glacier movement and allows "basal sliding," but it can also drag sediment out from under the glacier, leading to buildup at the terminus.
What are the right conditions? Well, the ice needs to be deep enough to put a lot of pressure on the bottom layer. And it needs to be warm enough that the increased pressure changes the melting point to below the local temperature. That means it needs to be a well-established glacier in a reasonably temperate climate. For example, southern Alaska works for this!
Let's say we start with a calving tidewater glacier. Over time, sediment gets deposited, reducing the water depth at the terminus. That reduces the calving rate, so the glacier can advance over many decades. Eventually, the sediment will build up to sea level. At this point, the glacier loses mass only by melting, not by calving. So it advances until it hits an equilibrium, then pauses for a while. Eventually, the terminus becomes unstable (why??), and the glacier starts falling back. As it does so, it is suddenly exposed to substantial calving again, because it's pushed all that nice sediment that was holding it up forward to its equilibrium position. As a result, its mass balance is off – it's losing way more mass than before, which causes a dramatic and relatively speedy retreat. Eventually it'll find its tidewater equilibrium point again, and the tidewater glacier cycle starts anew.
The interesting consequence of all of this is that some glaciers may advance more with a warming climate. For example, glaciers in Greenland don't exhibit the tidewater glacier cycle yet, because they don't have ingredient two: subglacial meltwater. As the climate warms, however, that may change, and we may see some tidewater glaciers start to advance. This all serves to complicate the standard line of "glaciers are retreating because the climate is warning," but it also just shows how dynamic a system glaciers can be.
If you're interested in more, check out this paper from last year, Sediment transport drives tidewater glacier periodicity, by Brinkerhoff, Truffer, and Aschwanden.
As for me, I want to learn more about:
Guess who had three successful paddle float reentries today! And several successful controlled wet exits, including rolling forward to get out of my cockpit. We've postponed hand of god rescues, keep an eye out in a few days for more news.