DSi values are an order of magnitude lower than the discharge weighted global mean riverine value of 158 μM25 (Table 1), and suggest low silica denudation rates compared with nonglacial river catchments when viewed in isolation. DSi concentrations, SI Qtz and SI ASi track the evolution of subglacial drainage (Figs 2 and 4). Higher concentrations (∼40 μM) were found at the onset of the melt season, when an inefficient drainage system was present at the glacier bed and dilution by dilute supraglacial meltwater was lower36. As the melt season progressed, lower DSi concentrations and electrical conductivity, as efficient drainage pathways opened up, suggest greater dilution by supraglacial meltwater (Fig. 2), with waters becoming increasingly undersaturated (SI Qtz and SI ASi minimums are −1.71 and −3.21, respectively).

Glacial silica export is dominated by the ASi fraction (Table 1). The ASi fraction of SPM (0.51–1.21 wt.%) was comparable to those measured in the Ganges basin (mean 1.2% by weight)37, which is characterized by high sediment yields38, and is higher than the estimated global river SPM ASi of 0.6 wt.%33. The corresponding mean concentrations of ASi in glacial meltwaters were nearly six times higher than the concentrations measured in the Ganges basin (68 μM)37 and far exceed the mean concentration of ASi in river waters given by Conley39 of 28 μM that is used in recent ocean silica budget estimates6. This difference is caused by both higher mean SPM concentrations (∼1 versus ∼0.1 g l−1)38 and the higher mean ASi (∼0.9 versus ∼0.6 wt.%)33 in glacial runoff. ASi was mostly associated with the fringes of larger platy material (Fig. 3), suggesting it is a product of aluminosilicate mineral weathering40 and/or mechanical grinding41,42. EDS of most ASi identified the incorporation of other elements into the ASi nanostructures, most commonly Al and Fe (Fig. 3a,b). This is not unexpected as naturally occurring mineral ASi incorporates less soluble elements as impurities during formation, because of its loose structure and high water content43.

It has previously been demonstrated that iceberg-rafted debris is likely a large source of reactive nanoparticulate iron to the euphotic zone16. Our results indicate that icebergs also have the capacity to supply ASi to surface waters. Iceberg ASi concentrations (Table 1) fall at the lower end of concentrations reported for glacial and some nonglacial waters, but exceed the Conley39 estimated concentration of ASi in river waters. Significantly lower concentrations of ASi in iceberg-rafted debris compared with our glacial meltwaters reflect unsorted sediments in iceberg sediment bands versus finer more reactive material carried as SPM in glacial meltwaters, and lower SPM concentrations in icebergs versus meltwaters (∼0.5 versus ∼1 g l−1).

Recent studies have exploited the use of modern nano-observation technologies to study ASi formation on mineral surfaces from aqueous weathering processes40. Two chemical weathering mechanisms have previously been identified. First, the dissolution–reprecipitation mechanism, where ASi forms as a precipitated weathering crust on freshly ground and leached particles. This has been observed even in solutions that are significantly undersaturated with respect to silica. Second, the leached surface layer hypothesis44, where preferential removal of weakly bonded ions (for example, Na+ and K+) from the mineral surface leave an amorphous crust rich in more insoluble ions such as silica. Both mechanisms invoke higher chemical weathering rates in subglacial environments than previously realized20, as ASi concentrations are high.

The comminution of bedrock by glaciers and ice sheets is also likely to be important in producing structural change to mineral surfaces41,42,45. Grinding of quartz produces a disturbed amorphous surface layer42 that is much more soluble than the primary mineral41. For example, Henderson et al.41 found freshly ground silica particles were more than an order of magnitude more soluble (115 p.p.m.) than ‘cleaned’ crushed quartz particles (11 p.p.m.) at pH 8. Silicate minerals that have been freshly abraded by glacial action are therefore likely to be substantially more soluble than unaltered mineral surfaces.

There is considerable uncertainty around the lability of ASi before long-term burial in fjords and near coastal regions. We believe ASi associated with glacial sediments will be highly labile downstream for three main reasons. First, glacial rock flour is potentially highly reactive because of a disturbed surface layer and large surface area per unit mass35,45, and because it has been observed to form buoyant flocs on contact with salt water46. Second, the extraction protocol used is designed to capture the silica that will likely dissolve in sea water (that is, the highly labile component)33,47. Last, ASi (and unreactive silicate minerals) dissolution is catalysed by the presence of alkali metals (for example, Na+) and alkali earth metals (for example, Ca2+)48. ASi is therefore expected to dissolve much more rapidly in marine waters than fresh waters49,50, generating DSi for diatom uptake. ASi has been found to be up to two orders of magnitude more soluble in saline waters than fresh waters49,51 and is expected to dissolve relatively rapidly (on timescales of days to weeks)50. For example, Kato and Kitano50 found complete dissolution of 50 mg of synthetic ASi in 1 litre of artificial sea water in <22 days.

We performed a simple seawater leach on Leverett Glacier SPM to determine the lability and therefore potential bioavailability of ASi (see Methods for details). This demonstrated rapid release of DSi from ASi over a period of days to weeks (Fig. 5). Treated sediment, with ASi removed before leaching (pre-extracted with 0.1 M Na 2 CO 3 ), showed only minor Si dissolution over a period of 672 h (28 days; Fig. 5) compared with untreated sediment. Untreated sediments displayed up to 25% ASi dissolution over the same time period, indicating ASi will likely dissolve relatively rapidly in high salinity waters. The DSi (measured as silicic acid) released into solution is bioavailable to marine diatoms. We propose two possible scenarios for longer-term dissolution of ASi in saline waters (>28 days). The first uses a linear dissolution function derived from the final two time points (306 and 672 h; dashed black line in Fig. 5). Under this scenario complete ASi dissolution would occur within 259 days (∼9 months). The second uses a more conservative power fit function derived from all time points (dotted black line in Fig. 5). Under this scenario, at least 60% of ASi dissolves within a year. We therefore hypothesize that 60 to 100% of SPM ASi will dissolve within a year in saline waters. Benthic processing of glacial material, and delivery back into the euphotic zone, is likely to be important on longer timescales, as has been demonstrated in other fjord environments52.

Figure 5: Percentage dissolution of amorphous silica from Leverett Glacier suspended particulate matter in low Si seawater leach. Points indicate the mean of four replicate leaches, with bars showing the minimum and maximum values attained. The dashed and dotted lines show the dissolution fits used to estimate complete amorphous silica (ASi) dissolution time. The percentage total ASi in sediments used in the seawater leach was calculated in triplicate extractions using the 0.1 M Na 2 CO 3 extraction, documented in the Methods. Full size image

We found further evidence of rapid ASi dissolution from Greenlandic meltwaters, with more than an order of magnitude increase in DSi concentrations across a buoyant SPM-rich glacial meltwater plume mixing with saline fjord waters downstream of Leverett Glacier (Fig. 6)53. Our data set represents a limited number of observations and a snapshot in time, but the positive association between DSi and salinity is contrary to what is usually observed in nonglacial estuaries and deltas, where there is removal of ∼25% DSi because of reverse weathering and diatom uptake6. The DSi concentration observed at S5 (21.1 μM; ∼40 km from S1) is an order of magnitude greater than oceanic surface water DSi concentrations in the North Atlantic (generally <2 μM), despite the high diatom productivity observed in west Greenland fjords19,53. Recent studies have also recorded higher concentrations (mean concentration of 2.22 μM) of surface DSi in coastal and open ocean waters on the Greenland Shelf54 compared with the North Atlantic. In a similar manner, iceberg ASi will also likely provide an important source of DSi as iceberg-rafted debris melts out in marine waters. Enhanced primary production has been recorded in the wake of icebergs, through observation of surface chlorophyll concentrations55,56, and diatom communities have been observed growing on the underside of icebergs in the Southern Ocean57. This is consistent with icebergs being a primary source of nutrients, including silica, to ocean surface waters.

Figure 6: Søndre Strømfjord transect of surface dissolved silica concentrations. (a) Satellite image of the Leverett Glacier study region. The position of Leverett Glacier terminus, at the head of Watson River, is given. S1 indicates the point at which Watson River exits the settlement of Kangerlussuaq. S2–S5 indicate sampling points along Søndre Strømfjord. (b) Concentrations of dissolved silica (DSi) plotted against salinity at sampling points S1–S5. The shaded region indicates the approximate range of regional sea surface dissolved silica concentrations from Painter et al.54. The plot x axis is reversed to reflect site positioning in (a). The satellite image in (a) is from Google, Landsat, USGS/NASA. Full size image

The glacial impact on the marine Si cycle and associated budgets will depend on the magnitude of the glacial flux and the lability of the exported Si. High rates of physical weathering29 and the presence of a labile ASi solid-phase indicative of subglacial silicate mineral chemical and/or physical weathering mean that ice sheets are likely a significant source of DSi to downstream fjords and near coastal regions. Concentrations derived from Leverett Glacier are likely to be typical of other large land-terminating outlet glaciers that export large quantities of meltwater from the GrIS following drainage across the glacier bed58. There are clear limitations to using a single glacier to estimate Si meltwater export from the GrIS and we acknowledge there may be large uncertainties in our estimates because of the extrapolations we have made. However, Leverett Glacier is significantly larger (by almost two orders of magnitude) than any other glaciated catchment reported thus far in the literature (both in Greenland and worldwide). The underlying geology59 and catchment hydrology60 are likely typical of other large land-terminating outlet catchments of the GrIS and therefore the values we derive are a reasonable first-order approximation of GrIS fluxes, until more data become available.

GrIS dissolved and amorphous silica fluxes are comparable to the total estimated input from atmospheric deposition (0.5 Tmol year−1), groundwater (0.6 Tmol year−1) and hydrothermal sources (0.6 Tmol year−1)6. We find it is likely to be the most dominant single source of dissolved and amorphous silica to the pan-Arctic region if we compare the GrIS Si flux with Arctic rivers. Using DSi estimates from Durr et al.25, and an upper ASi of ∼1.2 wt.% for riverine SPM (an upper estimate derived from Frings and co-workers33,37 as there are no data for Arctic rivers), we estimate an Arctic riverine Si input of 0.35 Tmol year−1 (Table 1). The GrIS could therefore provide up to ∼37% of total DSi+ASi input into the coastal regions of Arctic seas >60°N (∼50% of the total nonglacial riverine flux). The wider impact of these fluxes will depend on physical oceanographic factors around the GrIS that may not favour significant off-shelf export61. Processing of dissolved and amorphous silica may also limit the flux of silica out of long fjord systems. However, glaciated fjords harbour highly productive microbial ecosystems53, are important feeding grounds for seabird and marine mammals9 and have been identified as regions of high carbon burial62.

The Antarctic Ice Sheet (AIS) may also be a significant source of dissolved and amorphous silica to the Southern Ocean. Previous published estimates indicate the AIS DSi flux is in the region of ∼0.1 Tmol year−1 (ref. 6), but it neglected the potential export of ASi attached to SPM and iceberg-hosted sediments. We make a comparison with these original estimates using results from more recent research combined with our data of GrIS ASi concentrations to provide a revised approximation of the silica flux from the AIS. The only meltwaters to be sampled from the basal environment of the AIS come from subglacial Lake Whillans27. These waters indicate that Antarctic meltwaters are enriched in DSi compared with GrIS meltwaters, likely because of the long residence time of waters, and lack of dilution by incoming supraglacial melt (as in the GrIS). This study suggests that DSi concentrations in subglacial Antarctic meltwaters may be between 130 and 210 μM27, similar to the mean nonglacial global riverine estimate25. We estimated the DSi contribution from AIS subglacial meltwater using modelled basal melt rates of 65 km3 year−1 (ref. 63). This gives a meltwater DSi flux of ∼0.01 Tmol year−1, similar to the GrIS DSi flux (Table 1). AIS meltwater sediment flux is highly uncertain as no measurements exist. We therefore use a conservative ASi concentration estimate of 120 μM (the lowest value recorded at Leverett Glacier) with the above meltwater flux63. This gives a total AIS DSi+ASi meltwater flux of ∼0.02 Tmol year−1 that is a similar order of magnitude to the flux of Treguer8 (0.04 Tmol year−1), but substantially less than the GrIS (0.2 Tmol year−1). Iceberg calving fluxes are significantly higher from the AIS than the GrIS. Depoorter et al.64 estimate an iceberg calving flux of 1,321±144 km3 year−1 from the AIS. If we assume a similar sediment loading (0.5 g l−1) and ASi wt.% to our GrIS iceberg estimates (Table 1), this gives a AIS iceberg flux of ∼0.06 Tmol year−1. Our estimated AIS dissolved and amorphous silica flux is therefore in the region of ∼0.08 Tmol year−1, around half that of the GrIS, and similar to the previous AIS estimate (∼0.1 Tmol year−1)8. We estimate that the total ASi+DSi flux from the AIS and GrIS is therefore ∼0.3 Tmol year−1, ∼3% of the global Si budget (Table 1). However, the AIS DSi and ASi flux estimate remains speculative because of uncertainties in subglacial meltwater discharge and DSi concentrations, as well as no data on ASi concentrations for SPM in AIS meltwaters or iceberg-hosted sediments.

Studies postulate a link between the supply of Si to the ocean and the efficiency of the biological carbon pump1,2,3. Diatoms dominate the phytoplankton community during periods where the silica flux to the oceans is high, and are likely more efficient exporters of carbon than other primary producers2. Peaks in diatom abundance in marine sediment records from the last deglaciation have previously been explained by enhanced surface supply of DSi as a result of changes in ocean circulation and upwelling65,66,67,68. However, here we suggest that glacial runoff and iceberg-entrained debris may deliver an additional high DSi+ASi flux during deglaciation, especially during meltwater pulse events and Heinrich events. We construct crude estimates of palaeo ice sheet fluxes of DSi and ASi to the oceans using recent model estimates for meltwater release during the last deglaciation69. These calculations indicate that meltwater pulse event 1a (∼15,000 to 14,500 years before present) contributed meltwater discharge of at least 15,000 km3 year−1, equivalent to sea level rise of >4 cm year−1. A crude calculation indicates ice sheets would have delivered on the order of 5.7 Tmol year−1 of DSi+ASi to the oceans, assuming a similar SPM, ASi and DSi concentration to modern-day Leverett Glacier (Table 1). Nonglacial riverine discharge was likely significantly lower during the Last Glacial Maximum compared with present day (by at least 20–25%)70. Our estimated palaeo ice sheets flux is therefore similar to the approximate DSi+ASi flux for palaeo rivers (∼5.5–5.8 Tmol year−1, assuming nonglacial riverine silica fluxes broadly scale with discharge). The impact of the palaeo ice sheet Si flux will be felt for an extended period after input1, given the long residence time of Si in the oceans of >10,000 years33.

Our findings indicate that ice sheets play a more significant role in the global Si cycle than previously recognized, mainly via export of large quantities of potentially labile amorphous silica. This phase dominates the glacial dissolved and amorphous silica meltwater flux, with ASi concentrations up to 627 μM and yields of >36,000 kg Si km−2 measured at a large ice sheet catchment. Our flux estimates of dissolved and amorphous silica for the GrIS demonstrate that meltwater and iceberg discharge are significant and may provide similar amounts to the oceans as dust deposition, groundwater discharge and hydrothermal input. Hence, the GrIS likely contributes a large proportion of the dissolvable silica in the productive fjord and near coastal regions, where diatoms make up a large proportion of the phytoplankton community. These results indicate that glaciated regions play a more important role in the Si cycle than previously appreciated, and should be considered in future marine dissolved and amorphous silica budgets. Our findings have significant implications for the understanding of the Si cycle in the past, with globally significant fluxes of silica into the oceans likely during catastrophic melting of the large palaeo ice sheets that covered nearly 30% of land surface area. Large ice sheet pulses of dissolved and amorphous silica during these periods are a viable driver of deglacial diatom-dominated phytoplankton communities as observed in core records, in turn potentially enhancing the efficiency of the biological pump.