Hawkings, J. R., Wadham, J. L., Benning, L. G., Hendry, K. R., Tranter, M., Tedstone, A., … & Raiswell, R. (2017). Ice sheets as a missing source of silica to the polar oceans. Nature Communications, 8, 14198.
Glaciers get a lot of attention because they are expansive sheets of ice. They are important to understand because they can impact sea level, circulation, climate, albedo, and they are homes to microbial organisms and large animals (check out some other posts about glaciers here). A new reason they are getting attention is the recently realized importance to the global silica budget. Researchers found that melting glaciers deliver enough silica to the surface ocean that their contribution should not be ignored.
Silica is comprised of silicon and oxygen atoms and is a building block for minerals in the silicate group. Silicate minerals are fundamental components of igneous rocks, the kind of rocks that harden from magma and lava and that make up Earth’s bedrock. In silicates, a silica tetrahedron base combines with a variety of cations like aluminum, potassium, calcium, sodium, magnesium, and iron and arrange in crystalline structures representative of various minerals, like feldspars, amphiboles, pyroxenes, and micas. Mechanical and chemical weathering breaks down the minerals into smaller sizes and alters their chemical compositions as cations are exchanged and released. The ultimate result is that silica is removed from the hard rock and is transported elsewhere as amorphous silica and dissolved silica. Amorphous silica is an inorganic silica gel, produced from the dissolution and precipitation of silicates. Because it is highly soluble in seawater it is dissociated into a biologically available dissolved form called silicic acid.
Dissolved silica is important because it is what diatoms use to make their shells and thus is an essential ingredient for their existence. Diatoms are tiny photosynthetic plants that live in the surface ocean. It is estimated that nearly half of fixed carbon (carbon removed from the atmosphere) is by diatoms. If an abundance of dissolved silica is delivered to the surface ocean then is can enhance diatom production. During photosynthesis CO2 is drawn out of the atmosphere; when the diatoms die, sink the sea floor, and are eventually buried, the CO2 will be stored in Earth’s geology.
Silica cycling is also linked to CO2 cycling in the initial steps of chemical weathering. When CO2 is drawn out of the atmosphere and dissolves in water is creates a weak acid by releasing hydrogen ions. It is these ions that replace the cations in the mineral structure and force a change in the chemical composition. Because silica cycling is closely linked to other biogeochemical cycles it is important to understand the silica budgets of today and in the past.
The current silica budget includes rivers, groundwater, wind-blown dust, hydrothermal activity, and seafloor weathering as the important sources of silica to the ocean. Even though glaciers can cover up to 30% of land surface at glacial maximums and they are known to discharge a lot of nutrients beneficial to biogeochemical cycling, their role in the silica budget is underexplored and underrepresented.
The group of scientists behind this research speculate that glaciers have a greater importance in the silica budget because of the residence time of water that pools beneath ice sheets is long and coupled with intense mechanical and chemical erosion, and because the water flows with enough energy to carry suspended particulate matter, which is thought to be a source of dissolved silica.
To quantify the contribution of glaciers to the global silica budget researchers collected melt water and iceberg samples from sites around the Greenland Ice Sheet (Figure 1)
Melt waters were collected from the Leverett Glacier, an 80km long outlet of the Greenland Ice Sheet that terminates on land (Figure 2). The Leverett Ice sheet is atop Paleoproterozoic (2.5-1.6 billion years old) igneous shield rock. Samples were collected daily during the 2012 melt period of May, June, and July.
Iceberg samples were collected in the Tunulliarfik Fjord in Southern Greenland (Figure 2) in 2013, in the Sermilik Fjord in East Greenland in 2014 (Figure 4).
In the lab researchers used a variety of careful analytical techniques and high-resolution microscopy to measure and characterize amorphous silica, dissolved silica, electrical conductivity, and suspended particulate matter. Suspended particulate matter are particles that cloud the water, like when you disturb a stream bottom and the water gets all cloudy from the fine particles that are entrained into the water column.
The ultimate goals of the research were to determine the amorphous silica component in the suspended particular matter and in ice-rafted debris, investigate the role of salinity of the dissolution of amorphous silica into dissolved silica, calculate potential dissolved silica fluxes from icebergs in the Sermilik Fjord and Tunulliarfik Fjord and melt water from the Leverett Glacier, and expand their results to estimate that the total delivery of silica from the Greenland Ice Sheet to the ocean.
Determining the amorphous silica component in the suspended particular matter and in ice-rafted debris
Concentrations of silica and suspended particulate matter in melt water and iceberg samples are presented in Table 1 in the original article. In melt water, amorphous silica weight percent in suspended particulate matter was .51-1.21% with an average concentration of 392 uM (range: 120-627). Dissolved silica was sparse. The average concentration was 10uM range (.8-41.4uM). Amorphous silica in icebergs was lower than in melt waters. The weight percent of amorphous silica in suspended particulate matter from both the iceberg sites were similar with values in the range of 0.16-~0.47%. The mean concentration of amorphous silica in Sermilik iceberg samples was 47.9 uM and in the Tunulliarfik samples was 50.5 uM. Amorphous silica in the iceberg samples was concentrated in the sediment rich layers of ice. Dissolved silica was sparse in the iceberg samples; concentrations lower than 20uM were measured in sediment rich ice and low concentrations, <1uM, were measured in clean ice.
A decrease through time in dissolved silica in the glacial melt at the terminus may be related to dilution. For instance, at the beginning of melt season the drainage pattern is not fully established or efficient so there is less dilution by melting from the top of the glacier. As dilution increases into the season and drainage patterns are established with time, the dissolved silica content decreases from dilution.
Most of (~95%) the silica exported is amorphous is associated with platy sediment indicative of aluminosilicate source. The presence of amorphous silica is suspected to be associated with large amounts of suspended particulate matter in melt water. In icebergs the lesser amounts of amorphous silica is presumably because the icebergs do not have melt water with abundant suspended material comprised of platy aluminosilicates. Instead they have unsorted layers of sediment.
Role of salinity of the dissolution of amorphous silica into dissolved silica
Much of the amorphous silica initially input changes into dissolved silica when encountered with high salinities. In a transect of silica concentrations at Leverett Glacier from the terminus at the head of the Watson River into the fjord it was found that as salinity increased so did the dissolved silica concentration (Figure 6 original article)
Amorphous silica dissolves into dissolved silica more readily around cations, similar to the kinds found in seawater, like sodium and calcium. Thus in salty water, there typically is less amorphous silica and more dissolved silica.
Calculation of the potential dissolved silica fluxes from icebergs in the Sermilik Fjord and Tunulliarfik Fjord and melt water from the Leverett Glacier
A range of .06-.79 Tmol is the annual input of dissolved silica and amorphous silica from the Greenland Ice Sheet, which is approximately half of what is seen from rivers in the Arctic.
Expanding their results to estimate that the total delivery of silica to the oceans from the Greenland Ice Sheet
Scientists extrapolated their results to estimate that there is a total delivery of .01Tmol of dissolved silica and .16Tmol of amorphous per year to the ocean from Greenland Ice Sheet. In their estimate they neglected the dissolved silica component from icebergs because they were measured at minimal concentrations. They estimate that annually the Greenland Ice Sheet contributes 1.8% of global Si input to the oceans, which represents 3% of the terrestrial input.
The input estimated is the same contribution of hydrothermal activity, atmospheric input and groundwater, and is certainly an important source to pan-arctic ocean waters, maybe as much as 37% of the terrestrial silica load for latitudes higher than 60N. Questions remain about how far out of the Fjord the amorphous and dissolved silica will travel. The contribution to the open ocean is less well understood.
The contribution of silica from melt water and icebergs reported in this study are similar to the hydrothermal, atmospheric, and aeolian sources in the 2014 silica budget. If the glacial contribution is equal to the contribution of sources that are included in the budget, than the glacial contribution surely should be included as well.
Researchers speculate that the presence of amorphous silica in the glacial melt waters and the icebergs is related to two possible processes: Dissolution-Reprecipitation and the Leached surface layer hypothesis. Dissolution-Reprecipitation is when amorphous silica forms as a crust on recently weathering materials. The leached surface layer hypothesis implies that the leaching of weak ions leaves a crust rich in amorphous silica.
Weathering is enhanced by mechanical damage caused as glaciers flow over bedrock. The friction between rocks frozen in the base on the glacier and the bedrock below, results in grinding that can structurally damage the mineral grains, making them easier to chemically weather. Essentially, the more beat up the bedrock becomes, the easier it is harvest silica.
Due to the dissolution of amorphous silica into dissolved silica related to salinity, conservative estimates suggest that within in a year of input, 60-100% amorphous silica will dissolve into dissolved silica.
The fjords have high diatom productivity in the surface water yet the dissolved silica concentration in the fjords is still very high, even when uptake by diatoms and reverse weathering is accounted for. Amorphous silica released from icebergs as they melt may be a source for the excess dissolved silica in the surface ocean.
Ice sheets that melted 15,000 years ago, during the last deglaciation likely contributed amorphous silica and dissolved silica to the ocean. Scientists speculate that the annual glacial contribution could have reached 5.5 Tmol during that time, which is the same as the estimate for paleo-rivers. The input of silica from melting glaciers could have enabled increased diatom productivity observed over the last glacial-interglacial period and may explain increases in diatom rich layers in the sedimentary record.
This research is important because it touches upon a reservoir of silica that has been previously discounted based on assumptions that it was too small. It also introduces mechanisms that could explain pulses of diatom production in paleoceanographic records.
The article does not address that even though the melting of glaciers can make a significant contribution of silica to the oceans surface that may drive diatom production, there may be limitations by other nutrients, like iron, nitrate, or phosphorus.