the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 07 Nov 2023
Hydroclimate extreme events detected by a sub-decadal diatom oxygen isotope record of the last 220 years from Lake Khamra, Siberia
Boris K. Biskaborn
Ulrike Herzschuh
Jens Strauss
Luidmila Pestryakova
Hanno Meyer
Abstract. Northern latitudes have been significantly impacted by recent climate warming, which has increased the probability of experiencing extreme weather events. To comprehensively understand hydroclimate change and reconstruct extreme events such as droughts or floods, appropriate proxy records reaching further back in time are needed beyond meteorological measurements. Here we present a 220-year (2015–1790 CE), gapless stable oxygen isotope record of diatoms (δ18Odiatom) from Lake Khamra (59.99° N, 112.98° E) in Eastern Siberia, an area highly sensitive to climate change and with a demand for palaeohydrological data. From a 210Pb-137Cs-dated sediment short core, this high-resolution proxy record was analysed to reconstruct hydroclimatic extremes on a sub-decadal scale. The interpretation of the δ18Odiatom is supported by meteorological data, modern isotope hydrology, the ratio of planktonic-to-benthic diatom species and geochemical analyses of the same sediment indicative for the conditions in lake and catchment.
A comparison with meteorological data back to 1930 revealed that the δ18Odiatom record of Lake Khamra is primarily influenced by regional precipitation changes rather than air temperature. We identified winter precipitation, which enters the lake as isotopically-depleted snowmelt water, as the key process impacting the diatom isotope variability. We related the overall depletion of δ18Odiatom in recent decades to an observed increase in winter precipitation in the area, likely associated with the global air temperature rise, Arctic sea ice retreat and increased moisture transport inland. Available palaeoclimate proxy records, including a fire reconstruction of the same lake, support the new record as a valuable hydroclimate proxy indicative for precipitation deficits, tendentially excluding solar insolation and air temperature as driving forces even beyond meteorological recordings.
We identified two hydroclimatic extremes in the Lake Khamra δ18Odiatom record, one at the beginning of the 19th century and a second prominent event in the 1950s. Both were interpreted as drought periods, associated with enriched δ18Odiatom values likely caused by reduced winter precipitation and increased evaporation effects, which coincide with phases of reconstructed severe wildfires in the region. Despite the pristine lake area, we observed a triplication of mercury levels in the sediment record since the early 20th century, interpreted as an indication of human air pollution.
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Amelie Stieg et al.
Status: open (until 02 Jan 2024)
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RC1: 'Comment on cp-2023-85', Anonymous Referee #1, 13 Nov 2023
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The manuscript by Stieg et al is focuses on new paleoclimate data (mainly diatom d18O) from a small lake (Lake Khamra) in eastern central Siberia. Overall, there is currently a lack of paleo records from the region (especially over the last two hundred years), so this new data will make a valuable contribution to the discipline. However, there are a number of issues that the authors need to consider before the manuscript can be accepted.
- Currently the paper is very long and has a number of very short paragraphs that are closer to (long) bullet points rather than a normal paragraph. I encourage the authors to examine this and to also consider ways in which the manuscript could be written more concisely in all sections.
- The methodology describes the diatom taxonomy counts that were completed. However, only the results of one taxa are shown in the Results/Discussion sections. The full diatom record should be published and discussed, with the data made available in the Supplementary Information.
- The ecology of Aulacoseira subarctica is linked (indirectly) to wildfires. The case for this is not explained well in the manuscript. In places the ecology of the diatom is linked to Krammer et al., 1991, but this reference is a taxonomic guide. A more appropriate and detailed description of this taxa is needed.
- The Hg data should be expressed as an accumulation rate rather than as a concentration. The authors then need to make a clearer justification for how this data is linked (for some time periods) to regional processes such as wildfires, rather than global atmospheric deposition.
Minor points:
- The introduction could refer to Figure 1, to help individuals not familiar with locations discussed in that section.
- The three lines of text above Figure 2 can be removed and instead added to the caption.
- Lines 132-136: This information would be better if moved to the Methods.
- Line 159-160: Is this text needed?
- Line 229-231: Remove this text as no silicon isotope data is included in the manuscript.
- Line 235: Does the d18O data of 16.1‰ on the three samples refer to a 100% contamination sample obtained from the SPT residues? The text in this paragraph is not clear.
- Figures 3a and 6 could be combined.
Citation: https://doi.org/10.5194/cp-2023-85-RC1 -
RC2: 'Comment on cp-2023-85', Anson Mackay, 01 Dec 2023
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General comments
This study presents original, new diatom isotope data from a small Siberian lake in SW Yakutia. The quality of the isotope data is excellent; what few contaminants there were in the sediments, these have been taken account of robustly. The palaeolimnological record itself has been dated using 210Pb/137Cs analyses, and therefore the record presented is one of the few diatom isotope records that span the past couple of hundred years in relatively high resolution (compared to other Holocene records). The data therefore offer important insights in palaeohydrology in cold regions of the world.
The dating is robust, and I agree with their decision to the discard the radiocarbon dates. The interpretation of the likely control on the isotope values is methodical and supported by the data.
There were however a few instances where I thought the authors could have done more with some of the data presented, and where I thought the authors have over-interpreted their data especially in sections of the discussion that I detail below. My main suggestions given below are for the authors to both include more of the data they already have (diatoms) and to present some of the data in a form that is more useful for e.g. pollution histories, by taking into account sediment accumulation rates. Minor comments and corrections are included in the attached PDF
Specific questions / issues
Quality of the isotope data are excellent, both in terms of the purity of the samples obtained, but also the rigorous approach to determining contamination. Lines 249-250, the authors present data as z-scores to highlight any extreme values. But whilst this is very useful, I would not conflate an extreme value as being indicative of an extreme event, which is done in the discussion, notably Section 5.7.
Age model:
- As we can see from Table 2, another reason for omitting the 14C dates from the age model are that the samples get progressively younger (not older) the deeper in the core esp between 25-26 and 34-35, so either there has been in inversion (but that is not borne out by the 210Pb/137Cs record) or the dates are indeed affected by other processes as discussed.
TIC data:
- Fig 5 shows that the TIC data in the core vary between only 0.1% - 0.2%, yet values of up to 1.8% are quoted, so I think the scale used on Fig 5 is wrong for TIC. Nevertheless, even small increases to 1.8% show that the amounts of DIC present in the sediment are very low, and too low in my opinion, to make robust interpretations as to what changes in relative abundance means in terms of erosion of material into the lake. For example, an increase from 1 to 1.8% would not be robust enough.
- Erosion is probably better interpreted from other proxies such as % dry weight to magnetic susceptibility measurements (but these do not form part of this study).
Mercury data:
- as the authors have a robust radiometric age model, I’d recommend replotting the Hg data as fluxes as well, as these will be more directly comparable to other studies with respect to interpreting any increasing or decreasing trends in pollution.
Diatom data:
- Diatom methods are well accounted for.
- However, ideally a comprehensive diatom stratigraphy needs to be shown not just subarctica only. Remember that because these are relative abundance data, the decline or increase in any one species is also tied to other species in the dataset.
- For example, on lines 517-518, I don't think that this is a valid conclusion without knowing (i) what the other species are, and (ii) relative contributions of other abundant species to either total biomass (or preferably biomass accumulation rates). For example, in our paper Mackay et al. 2013 that you quote, a non-dominant species in terms of relative abundance (Stephanodiscus grandis) completely dominates the biomass of the diatom flora, leading to those authors concluding that changing assemblage composition alone in terms of relative abundance is likely to have little impact on the isotopic variation.
- Quite a lot of interpretation is made with respect to changing P/B ratios, and sometimes interpretations made are a bit circular. For example, in lines 659-660 the authors state “…overall rather good conditions for planktonic diatom species, comparable to the abundance of A. subarctica”, but isn’t subarcticaresponsible for most of the variation in P/B ratio in the first place?
- I’d be really interested to see if any fragilarioids are present in the assemblage – many of these will be tychoplanktonic so can live in both in littoral and deeper open waters, which would add in a different dimension to the interpretations given here.
Technical corrections & issues of interpretation
Lines 50-51: I think the statement about extreme events being linked to ocean currents and large-scale dynamics needs a source.
Line 53: the citation Ye et al. 1998 is perhaps too old for the statement "recent decades" given the timescales being considered here. Is there a more recent study that can be used instead of, or as well as?
Lines 275: the statement “…the uncertain species were counted once as benthic (ratio 1) and once as planktonic (ratio 2) species” is an odd thing to do. Could uncertain taxa be tychoplanktonic taxa? But as it goes, it’s not possible to evaluate this statement without knowing what the diatoms are. Most species habitats are actually known.
Line 355 (+ others), when quoting dates probably better to give these as approximate dates given uncertainty in the age model, e.g. c. 1940 CE, 1915 CE and 1875 CE (should dates be expressed as Common Era?)
Lines 395 / Figure 5: I wonder if it would not be better to align all the evidence for increased erosion either left to right (my preference) or right to left. If former chosen, then reverse x-axis for the diatoms. But for me it's not clear how changing abundance of A. subarctica (or P/B) could be both erosion and nutrients, and how the very small changes in P/B (1) can be anything really.
Lines 578-579: The statement “This signifies an offset in seasonality in our diatom record, as the winter season and its snowfall do not affect the lake water and the diatoms therein before the following summer” is unlikely to be true - where snow is falling there may be reduced biological activity over winter, but as Hampton et al. 2017* show, plankton under ice-covered lakes were more abundant than expected, and that winter conditions act as a strong antecedent to conditions the following summer (this might also give a more nuanced account for interpretations made in lines 628-629)
Line 600: This is not in the scope of this study, but future work might involve some electron microscopy to identify the carbonate minerals in the sediments. But I do agree that old carbon being in-washed into the lake would impact the radiocarbon dates as observed.
Lines 608-609: Might also be interesting to compare to Baikal (eg Roberts et al. 2020*) but also to represent the data as fluxes, taking into account SARs (see my comments above). This would give more nuance as to if Hg is coming from e.g. Russian or Asian sources.
Lines 610-620: I found this paragraph quite speculative (i) without showing a more detailed diatom stratigraphy, (ii) assuming that the really small changes in in DIC are related to erosion into the lake. I really don’t think the data allow for this interpretation.
Section 5.5: Overall in this section, I thought that there was over-interpretation and sometimes contradictory explanation of the diatom data in comparison to other proxies / archives selected.
Line 704: to be honest, I struggle to see a well-defined maximum here, leading to risk of over-interpreting what the datasets may indicate
Lines 757-758: doesn’t this statement contradict the interpretations given in Lines 705-710?
Section 5.6: this is a useful section and quite robustly discussed
Section 5.7: Given the nature of the data, I'm not convinced that a separate section on identifying extreme events is warranted, and much of the discussion could be absorbed into previous sections. Also, identifying extreme events is usually done using sometime kind of statistical analyses. It is the case that z-scores are used to highlight extreme values (e.g. = ± 3) but an extreme value is not an extreme event.
I found that some of the interpretations were not supported by the data. For example, on lines 870-871, sedimentary TIC (especially at these concentrations) is not a measure of water quality, and would erosion lead to an increase in salinity per se? This is not what's observed in other lakes. An increase in erosion may lead to an increase in sediment accumulation rates, but these are not estimated here.
Line 876: but is the charcoal peak really that pronounced at around 1800? It looks like a small gentle increase to be, and certainly not part of any extreme event
*Papers cited:
Hampton, S.E, Galloway, A.W.E., Powers, S.M., Ozersky, T., Woo, K.H., Batt, R.D., Labou, S.G., O’Reilly, C.M., Sharma, S., Lottig, N.R., Stanley, E.H., North, R.L., Stockwell, J.D., Adrian, R., Weyhenmeyer, G.A., Arvola, L., Baulch, H.M., Bertani, I., Bowman, L.L. Jr., Carey, C.C., Catalan, J., Colom-Montero, W., Domine, L.M., Felip, M., Granados, I., Gries, C., Grossart, H.-P., Haberman, J., Haldna, M., Hayden, B., Higgins, S.N., Jolley, J.C., Kahilainen, K.K., Kaup, E., Kehoe, M.J., MacIntyre, S., Mackay, A.W., Mariash, H.L., McKay, R.M., Nixdorf, B,. Nõges, P., Nõges, T., Palmer, M., Pierson, D.C., Post, D.M., Pruett, M.J., Rautio, M., Read, J.S., Roberts, S.L., Rücker, J., Sadro, S., Silow, E.A., Smith, D.E., Sterner, R.W., Swann, G.E.A., Timofeyev, M.A., Toro, M., Twiss, M.R., Vogt, R.J., Watson, S.B., Whiteford, E.J., Xenopoulos, M.A. (2017) Ecology under lake ice. Ecological Letters, 20, 98-111 doi: 10.1111/ele.12699
Roberts, S., Adams, J.K., Mackay, A.W., Swann, G.E.A., McGowan, S., Rose, N.L., Panizzo, V., Yang, H., Vologina, E., Sturm, S. Shchetnikov, A.A. (2020) Mercury loading within the Selenga River Basin and Lake Baikal, Siberia. Environmental Pollution 259, https://doi.org/10.1016/j.envpol.2019.113814
Amelie Stieg et al.
Amelie Stieg et al.
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