Under scrutiny: Greenland’s deep underwater mud-pump

Greenland’s huge glaciers pump sediment far into biodiverse and biologically rich fjords. We tracked this sediment and developed a simple formula to estimate the sediment quantity and its distribution from any marine-terminating glacier, based on the amount of melt occuring above the glacier.
Published in Earth & Environment
Under scrutiny: Greenland’s deep underwater mud-pump
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When we think about climate change, the image that often comes to mind is that of the Greenland ice sheet, covered with thousands of bright blue spots of meltwater lakes. It's an unsettling scene, akin to a spreading plague that eventually leads to steadily rising sea level encroaching upon distant coastlines. Alongside these conspicuous changes, there's a hidden, but important, step that should not go unnoticed when studying the effects of climate change: the impressive jets of sediment shooting up from beneath the deep glaciers that flow into Greenland’s fjords.

As meltwater generated at the surface of the Greenland ice sheet percolates downwards through crevasses, moulins, and channels, it eventually encounters a landscape deep beneath the ice—a hidden terrain of mountains, valleys, plains and riverbeds over which the meltwater travels purposefully towards the coasts. We know that these pulses of melt water jet out, deep below the surface in Greenland’s fjords, and mixes with deep sea water, that is rich in macronutrients. Through upwelling the nutrients are delivered to the photic zone with great benefits for the distal marine life.

But we know far less about the sediment encountered from the ancient under-ice landscape and dragged along with this water. Firstly, how much sediment comes? And how nutritious is it? The availability of the sediment for uptake in living organisms is under scrutiny, and many researchers advocate that it contains significant amounts of micronutrients, such as trace metals, that help sustain ecosystems in and far outside the fjords. On the other hand, sediment-rich waters allow less light penetration and therefore may be detrimental to photosynthesis near the glacier’s edge, especially given that the amount of incoming sunlight is limited in polar regions, so more sediment might be quite harmful in some places. Altogether, our understanding of the connection between glacial sediment plumes and marine life is not yet in place.

Turning the binocular from the fjords towards the neighbouring ice sheet, there are unanswered questions here too: How much sediment lies beneath the ice sheet? And what role does sediment play in influencing the speed of glacier flow? It is really important to accurately understand the properties of the substrate beneath the ice when using computer models to simulate the future of the Greenland Ice Sheet, small changes can completely alter how the ice sheet flows. Finally, how much sediment is emerging from beneath the ice sheet margins, and is it sufficient to create barriers that can protect the ice from incoming warm waters and buffer the ice sheet from further melting?

Answering these many and complex questions is made challenging by the lack of observations of sediment flux in fjords with marine-terminating glaciers. Not surprisingly, measuring the sediment flux at the deep margins of glaciers in iceberg-infested fjords is difficult and dangerous.

Our study provides observations of the annual sediment discharge from multiple marine-terminating glaciers and its distribution within fjords. Interestingly, our data collection approach turned out to be serendipitous; it wasn’t what we initially intended. Over several field seasons we collected sediment cores from a number of glaciers in Greenland. This work was made possible thanks to the support and knowledge of Greenlandic captains and crews that safely navigated the fjords and allowed for the sediment core retrieval. Our original goal was to analyze these cores to reconstruct historical changes in ocean and glacier variability for specific marine-terminating glaciers around Greenland. In essence, the dated sediment cores serve as diaries for the individual glaciers, reaching thousands of years back in time. Naturally, we also compared the diaries of the various glaciers, to see if their memories of a long-gone past were the same. But after years of data collection and subsequent laboratory analysis, we realized that the amount of data we, and other researchers, had gathered also allowed for a solid comparison among the various glaciers of the amount of sediment deposition over recent decades. This comparison would be analogous to comparing the length of their diary books to see if all glaciers are equally ‘productive writers’. In doing so, we discovered that there is a direct correlation with the amount of meltwater exiting the nearby glacier. To say it poetically, the more it ‘rains inside them’ the more pages the glaciers will fill in their diaries. While this connection with surface melt is not surprising, what intrigued us was that the sediment flux and its distribution within the fjord could be described by a relatively simple formula that applies to all different glaciers we sampled. This simple and first-order formula enables the calculation of annual sediment flux at point in the fjord, using surface melt data from nearby marine-terminating outlet glaciers.

 

Our observations provide valuable information to help understand the effects of glacial sediment discharge into the ocean. An important conclusion is that as the melting of the Greenland Ice Sheet increases into the future we should expect higher fluxes of sediment into the fjords.  While this may pose potential harm, it may also present new opportunities - the key is to stay ahead of the game and proactively navigate the evolving changes.

 

 Photo credit: Rolige bræ by Laurence Dyke

 

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