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
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 -
RC2: 'Comment on egusphere-2024-1359', Marcus Gutjahr, 22 Oct 2024
This manuscript submitted by Gavin Piccione and co-authors presents a hydrochemical reconstruction of subglacial meltwater conditions under the East Antarctic Ice Sheet during Termination III. This study is largely of (isotope) geochemical nature but also involves a minor modelling component. I enjoyed reading this manuscript with all the various research methods involved, and was particularly intrigued by the discussed trace metal budgets as a function of prevailing subglacial water conditions and oxygen availability. It is well written and fits the journal well. I have several points which I would like to see addressed yet would suggest major revisions to the current version.
Major points
The description dealing with the generation of U-series data in the manuscript is a bit vague. The authors describe the chemical purification and analytical measurement. In lines 99-101 the authors write:
“Accuracy of 234U-230Th dates were tested using MIS 5e coral and compared to dates from (Hamelin et al., 1991), as well as a previously dated carbonate precipitate (Frisia et al., 2017).”
Matching previous results is good practice. It suggests the mass spectrometric and chemical approach is robust. However, are we dealing with calcitic or aragonitic material here? In the first case we could encounter the problem that the oxidation state of U in aqueous solutions is 6+ and the dominant aqueous speciation involves the uranyl moiety (UO2+). The authors discuss the oxygen deficit in the parent solution, but if U was present as a uranyl tricarbonate ion, it would not fit into a calcite matrix (Reeder et al., 2000 Environ. Science Techn.), raising concerns about the long-term retention of U in these special carbonates. Did the authors also obtain U concentrations? It would be very helpful for comparison with other carbonate matrixes, marine or non-marine. How significant is the uncertainty in initial 234U/238U in resulting U/Th ages? But the most important point – as far as I can tell – is whether these carbonates are calcite or aragonitic? Personally I made the painful experience that for example Antarctic cold water corals, which are calcitic, display pronounced U open system behaviour and even had quite variable 230Th/232Th (Gutjahr et al. 2013 Chem Geol).
The elemental analyses are very interesting. One point that I stumbled over, however, is the question how these major, minor or trace elements are incorporated into the sample. Are these assumed to be structurally incorporated, or rather scavenged into Fe oxides or other mineralogical phases in the precipitate? I am asking since I wondered about the partition coefficients of the presented elements into opal or carbonate material. In other words, could some of the variability in elemental concentrations shown throughout this manuscript just be controlled by mineralogy as opposed to subglacial water chemistry and elemental concentration? The authors should still properly discuss this potentially obscuring factor. Can some of the variation in elemental concentrations be exclusively ascribed to mineralogy? The implications laid out in section 3.4 for example depend heavily on this detail.
The authors should also spend a bit more time on discussing key parameters on oxygen availability in meltwater below the ice sheet. I have the impression that this is still not well enough laid out. I emphasize this point since it is an essential component on observed and inferred elemental subglacially dissolved metal concentrations. What is the most efficient process to provide oxygen to these subglacial water bodies?
Lines 263-264 and in general: Before quoting U isotopic ratios, please define these! I assume these are activity ratios, not elemental. And you have to introduce whether these are present-day or initial 234U/238U. In fact, if these are present-day 234U/238U, then you should also calculate initial 234U/238U since these will be substantially higher. If these ratios are modern 234U/238U it would provide evidence for substantial enrichment of 234U in these TIII meltwaters, which is worth a dedicated discussion section given apparently strong incongruent release of 234U during subglacial weathering.
Minor comments
Line 213 and the following reads: “calcite accumulation is i) minimal during times when subglacial flushing rates are slow because CO2 delivery is low”. I may be a bit slow here but as far as as I am aware higher CO2 delivery should lead to higher DIC that will lead to calcite undersaturation, not trending toward supersaturation, unless alkalinity equally increases alongside. This statement here hence seems quite speculative and not always necessarily the case. Could you clarify this point?
210-211: If opal precipitation is slow, is this growth substantially slower than the carbonate layers? If yes, what would this imply for the determined age model (and involved uncertainties) given the alternation of calcite and opal layers?
Fig. 4c: Why not simply show sample height in mm instead of micrometres?
Line 416 and elsewhere in the text: I thought a glacial termination is followed by an interglacial. It hence reads a little awkward if the authors discuss climatic cooling directly following TIII. Such phrasing is used at various places in the manuscript.
Lines 462-464: Here the authors state: “Total Fe flux increases by about 0.2 Gmol yr-1 for every 0.001% increase in the fraction of basal water made up of Fe-rich brine, and increasing by an order of magnitude when subglacial waters contain 0.005% brine (Fig. 6c).” This is quite a statement. The authors may be right but I cannot entirely follow the argument. Could the authors corroborate their suggestion in a little more detail? While they may be right, I remain a bit sceptical how realistic this may be, also with regard to the potential effect of differential elemental partitioning into the sample (see my major Kd comment above). And it would obviously first of all apply to the subglacial setting here.
Citation: https://doi.org/10.5194/egusphere-2024-1359-RC2 -
RC3: 'Comment on egusphere-2024-1359', Anonymous Referee #3, 01 Nov 2024
This is an excellent paper that provocatively provides an intriguing line of evidence for the Southern Ocean Fe fertilisation hypothesis as an explanation for the positive feedbacks in the glacial period carbon cycle. I have two substantive pieces of criticism.
First, the manuscript infers quite a lot from a single sample. An alternative hypothesis could be that the glacial period transition causes a reconfiguration of the local glacial hydraulic system, so that the sample became cut off from regular input of subglacial water. There is plenty of offshore evidence for shifts in the sources of glacial material between glacial and interglacial cycles (i.e. Licht and Hemming, 2017). Such reconfigurations of ice streams during glacial-interglacial transitions would presumably create all sorts of local changes to ice sheet basal hydrology, with geochemical consequences. But I do think publication should go ahead, just perhaps with a bit more acknowledgement of the limitations and backing away from the claim that all of Antarctica is represented by this single sample.
Second, the authors cannot propagate a surface temperature record instantaneously to the bed as they do in equation 3 of the appendix. The authors write: “By using a steady-state approximation in Equation (3), we are neglecting the time lags with which a climate signal affects the thermal energy balance of an ice sheet base. MacAyeal (1993) provides an insightful discussion of these lags.” MacAyeal’s “insightful discussion” presents a mathematical argument of why any perturbation in surface temperature would never make it to the bed in anything nearly as thick as an ice sheet (specifically he postulates no temperature perturbation could make it more than 314 m through ice over the timescale of Heinrich events). Indeed, the latest temperature perturbation caused by our present interglacial period has had absolutely no effect on the temperature gradients at ice sheet beds anywhere they’ve been measured in Greenland and Antarctica. If a change to the Q term has any appreciable effect on the meltwater generation model in the appendix, then Figure 1F is invalidated. The authors need to revise their model of basal melt so that Ts and a, and H are held constant at long-term averages for equation 3. There are no temperature, accumulation, or thickness perturbations of anywhere near sufficient duration to effect a change in the temperature gradient at the bed across the termination III period. The approach in the supplement is otherwise correct. But this teleportation of heat from the surface to the bed is physically impossible. The analysis needs to be redone, Figure 1F replotted, and possibly some of the data reinterpreted accordingly.
I have some line-by-line comments below.
Line 13: Consider adding a ka range, as not all readers will know when glacial termination III was.
Line 17-18: I might remove the phrase “that diminished subglacial meltwater flow” from this sentence. Restricted influx of oxygen is directly indicated by the data. Diminished subglacial meltwater flow is one possible explanation.
Line 19: I would say “glacial-interglacial cycles”, the degree to which these cycles are orbitally caused or orbitally regulated is a matter beyond the scope of the paper.
Line 25: Maybe state the positive feedback more explicitly here.
Line 27: Please don’t use SO for Southern Ocean. This is also sulfur oxidation (i.e. SO-CD) and unnecessarily adds confusion and uncertainty, particular for casual readers. Best practice is to only use abbreviations that are highly standard within the scientific literature.
Line 41: You define an abbreviation “MDV” that you never subsequently use.
Line 46: Remove the word “fresh”
Line 47: Maybe replace “lone” with “primary”. There could be chemical sources of oxygen.
Line 65: Remove “unique”.
Line 72: Presumably you could have quite a lot of Fe precipitation without reaching Blood Falls levels of Fe concentration.
Line 162-168: While I appreciate that the authors cannot precisely locate the formation location of their sample, some degree of local parameterisation would be appropriate here. They could certainly average the geothermal heat flux over the catchment of Elephant Moraine (though it actually seems like Elephant Moraine is fairly close to the continental average). For the other variables, the authors actually used the EPICA record, which is an interior East Antarctic record, appropriately close to their sampling location.
The authors should rewrite this paragraph to be less apologetic and say more of what they actually did (i.e. model the change in accumulation and slope based on the EPICA core). The implicit assumption is that EPICA represents interior Victoria Land to first approximation (which is reasonable enough given the options available), not that the Elephant Moraine catchment can be captured by modelling Antarctica as a whole (which is a careless approach, at best).
Line 172: Why yearly? In general, there must be homeostasis over a sufficiently long timescale, but either seasonal or centennial changes do perturb this. I would just remove the statement between the two commas.
Line 175: It sounds like you could test the sensitivity of your model to this assumption. Presumably there’s a whole range of below saturation conditions possible prior to reaching saturation. I don’t know why you need to pick Subglacial Lake Whillans, specifically.
Line 180: I find this sentence confusing. What is the force of “plus” grammatically? Perhaps it will make more sense below.
Line 214: I might say CO2 production, rather than delivery. CO2 in air bubbles is slight and we expect CO2 to mostly source from the oxidation of organic matter (as the -20 ‰ δ13C amply demonstrates).
Line 217: I think you mean figure 1D.
Line 256: I am not convinced by the mixing curve presented in figure 3A. Assuming there is two component mixing, why does the other endmember have to be all the way at zero δ13C? Why can’t you have a (-17,-50) end member? It seems odd to project out to hypothetical end member for which you have no observations. Also, a rise in δ 18O doesn’t necessarily mean a more peripheral source for the ice. It could mean a source formed during a warmer period. Or it could indicate ice that has been enriched through processes such as regelation.
Line 355: I might avoid the direct comparison to Blood Falls here. Yes, you have evidence of sufficiently high Fe concentrations to precipitate siderite, but Blood Falls has several other pertinent features, like mirabilite saturation and chloride concentrations 4x that of seawater, which you have no evidence for here.
Line 358: Without evidence of saturation for either sulphate or chloride minerals precipitating, it is doubtful whether this could truly be a brine.
Line 418: If the thermodynamic modelling is done correctly (per my comments above), renewed surface cooling cannot suppress meltwater production, only changes to strain heating are likely to cause these sort of changes.
Line 425: Remove n.d.
Line 440: Maybe replace “orbital-scale” with “global”
Line 495: Again, I don’t particularly like the use of “orbital” to describe the timescale of glacial interglacial cycles.
Lines 495-499: I would explicitly state the potential for a positive feedback here. I.e. that less hydrological active ice sheet could release reduced Fe, which in turn fertilises the biological carbon pump in the Southern Ocean, resulting in even colder temperatures.
Line 507: I would, again, pull back the comparison to Blood Falls.
Citation: https://doi.org/10.5194/egusphere-2024-1359-RC3
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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