the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 May 2024
Antarctic Subglacial Trace Metal Mobility Linked to Climate Change Across Termination III
Abstract. Antarctic meltwater is a significant source of iron that fertilizes present-day Southern Ocean ecosystems and may enhance marine carbon burial on geologic timescales. However, it remains uncertain how this nutrient flux changes through time, particularly in response to climate, due to an absence of geologic records detailing trace metal mobilization beneath ice sheets. In this study, we present a 25 kyr record of aqueous trace metal cycling beneath the East Antarctic Ice Sheet measured in a subglacial chemical precipitate that formed across glacial termination III (TIII). The deposition rate and texture of this sample describe a shift in basal meltwater flow following the termination. Alternating layers of opal and calcite deposited in the 10 kyr prior to TIII record centennial-scale subglacial flushing events, whereas reduced basal flushing resulted in slower deposition of a trace metal-rich (Fe, Mn, Mo, Cu) calcite in the 15 kyr after TIII. This sharp increase in calcite metal concentrations following TIII indicates that diminished subglacial meltwater flow restricted the influx of oxygen from basal ice melt to precipitate-forming waters, causing dissolution of redox-sensitive trace metals from the bedrock substrate. These results are consistent with a possible feedback between orbital climate cycles and Antarctic subglacial iron discharge to the Southern Ocean, whereby heightened basal meltwater flow during terminations supplies oxygen to subglacial waters along the ice sheet periphery, which reduces the solubility of redox sensitive elements. As the climate cools, thinner ice and slower ice flow reduce basal meltwater production rates, limiting oxygen delivery and promoting more efficient mobilization of subglacial trace metals. Using a simple model to calculate the concentration of Fe in Antarctic basal water through time, we show that the rate of Antarctic iron discharge to the Southern Ocean is highly sensitive to this heightened mobility, and may therefore, increase significantly during cold climate periods.
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RC1: 'Comment on egusphere-2024-1359', Jon Hawkings, 04 Sep 2024
reply
The role of ice sheets in global biogeochemical cycles has received increased attention in the last decade, in part due to the sensitivity of these and their connected environments to climatic warming. There is large uncertainty in the role of the Antarctic Ice Sheet in regional biogeochemical cycles, in part because of the difficult-to-access subglacial drainage system where the majority of chemical weathering (and release of biologically important elements) is expected to occur. Associated with this is an almost complete lack of data on how meltwater and elemental fluxes from the Antarctic Ice Sheet might change under different climate scenarios. Data elucidating these temporal subglacial processes is extremely rare. Piccione et al. use the geochemical composition of accurately dated subglacial precipitates collected from an ice marginal moraine to glean information about chemical weathering conditions at the ice sheet bed, and explore how these conditions might have changed during a period of climate transition. The study uses multiple complementary and leading-edge analysis to reinforce the findings, including isotopic dating, elemental mapping and elemental speciation. The authors hypothesize that opal and calcite precipitation record centennial subglacial flushing events and that meltwater trace element export (in this case Fe, Mn, Co and Mo) is related to reconfigurations in subglacial drainage associated with ice thickness (and therefore climate). The highest flux of trace elements is assumed to be following TIII, where a thinner ice sheet and cooling climate reduces basal melting, and therefore delivery of oxygen, instead promoting mobilization of redox sensitive trace metals in a hypothesized subglacial brine.
General comments
I really enjoyed reading this manuscript – it’s well written, has a really novel and high-quality dataset, and uses innovative approaches to add evidence to the main hypothesis. This is only one precipitate sample, but I appreciate the attempt to upscale processes to the ice sheet at large, given the paucity of data available. There’s not much that I would recommend changing beside some suggestions, clarifications and typos in my specific comments below. My general comments below are more to stimulate some additional discussion, rather than requests for any substantial changes to the text.
My main general comment is more of a question. In my mind, I would have expected that these trace elements (Fe and Mn anyway) would likely originate from silicate bedrock weathering, and should therefore be enriched in opal bands, which presumably indicate elevated silicate bedrock weathering (and supersaturation of amorphous silica) and therefore longer residence waters. The argument here is that opals precipitate from freeze concentration (cryoconcentration), but it would also require a non-trivial amount of dissolved silicon. Within this context, it might also be useful to have a little additional discussion on how these precipitates might/might not incorporate these metals (i.e. are the metals retained in an opal precipitating environment). I think I just need some more clarity in my mind.
Similarly, calcite accumulation is assumed minimal during periods of low CO2 delivery, presumably because there is less CO2 supplied from melting basal ice? The assumption here is that there are no other proton sources in the subglacial environment, which may/may not be correct (e.g. mineralization of organic matter, sulfide weathering etc…), and that there are no alternative mechanisms of calcite precipitation, but this probably needs a little discussion, even if it is mostly signposting.
Another general clarification needed is that precipitation is slow and likely only occurs in low physical weathering intensity environment, as the authors comment. How do we know this is representative of subglacial environments across the EAIS? What kind of flow rates are required to sustain long term calcite/opal precipitation? Does this tell us anything about the efficiency of the subglacial hydrological system in the location that these precipitates formed? Although some of these questions are outside the scope of the paper, others are quite important for interpretations of e.g. iron export to the Southern Ocean.
My final general comment is based on the assumption that the parent waters are Fe-poor, based on Whillans Subglacial Lake data from Vick-Majors et al. (2020). Although I don’t think is a bad assumption, our other data (Hawkings et al., 2020), suggests that the filterable Fe concentrations could be much higher than Whillans (even if you consider different smaller pore size filtered – i.e. 0.22 um vs 0.45 um) and Mercer Subglacial Lake has higher measured O2 concentrations than Whillans Subglacial Lake (see cited Priscu paper). Perhaps the argument here is that Mercer might contain more of the brine that is being postulated to occur in some subglacial environments here for some reason? A related argument could be that the Mercer data is more representative of conditions beneath the East Antarctic Ice Sheet, given that waters entering Mercer include those sourced from East Antarctica. Either way, it would be an interesting experiment to include this data at the other endmember for the modelling experiment, given it’s our only other datapoint for these environments.
Specific comments:
L293: Is the suggestion that these are part of the carbonate structure, rather than as e.g. oxides within the carbonate precipitate? Is siderite present – doesn’t look like from the XANES?
L439: I would probably use the term “subglacial meltwater” rather than “basal meltwater” as its more of a catch all term
L449: Could you quote some values here for comparative purposes
L449: I think this needs more thought. Typically, you would assume Mo (and Cu to a degree) to be more soluble and more mobile in oxygenated environments, and less mobile in hypoxic environments, which is a reason it’s used as a redox palaeo-proxy (e.g. Boothman and Coiro, 2009; Boothman et al 2022). In this way it is expected to behave differently to Fe and Mn, which would be less mobile in oxic waters. So there’s a little bit of conundrum here is that you really need a redox boundary for Mo to accumulate onto Fe and Mn oxyhydroxides.
L474: typo “may have occurred”
L488-491: I think this should probably focus on Fe, as that’s the primary limiting micronutrient in the Southern Ocean.
Figure 4: Can you include Si on panel c? Is it possible to include a scale bar on A?
Figure 5: needs labelling of panels (A-C)
Figure 1 or 4: Is it possible to indicate where XANES were collected on the precipitate?
Citation: https://doi.org/10.5194/egusphere-2024-1359-RC1
Data sets
U-series Geochronology, Isotope, and Elemental Geochemistry of a Subglacial Precipitate that Formed Across Termination III Gavin Piccione https://doi.org/10.15784/601781
Model code and software
Simplified model of thermal energy balance beneath the Antarctic ice sheet Slawek Tulaczyk https://doi.org/10.5281/zenodo.11126839
Modeled Antarctic subglacial iron discharge across glacial termination III Gavin Piccione https://doi.org/10.5281/zenodo.11126883
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