The two studied thermokarst lakes mainly differ in their physical characteristics (bathymetry and presence of a talik) and topographic settings (presence or absence of strongly eroding shoreline). The larger and deeper floating-ice lake has mostly low-lying peaty shorelines, but a quarter of the lake borders an actively eroding yedoma permafrost cliff of up to 20 m height (Fig. 1, Supplementary Fig. S1a). This cliff slumps Late Pleistocene-aged yedoma soil into the lake during the summer months (Supplementary Fig. S1b). The shallow bedfast-ice lake is located on the northwestern edge of a drained lake basin and is fully surrounded by a flat, peaty and marshy tundra landscape (Supplementary Fig. S1c).
3.1 Core descriptions
The sediments of the two lakes noticeably differed visually: while the floating-ice lake cores showed layering, the bedfast-ice lake had very homogenous and dark-brown sediment colors reflecting a high abundance in organic material.
The floating-ice lake cores F1 and F3 both showed sporadic and indistinct layering due to slightly varying SOM contents and had dark grayish brown, grayish brown and olive brown colors (Fig. 2) based on the Munsell Soil Color Chart (2009). Core F2 was strongly layered and had clear, distinct boundaries with layer thicknesses from a few mm up to 2 cm, the layering was most prominent between 60 and 16 cm depth (Fig. 2). The sediment colors ranged from grayish brown, dark grayish brown, dark brown, very dark brown to black based on the Munsell Soil Color Chart (2009). The top 16 cm of core F2 were especially rich in organic material and visible plant remains. The lower half of the core (111 − 60 cm) was considerably more homogeneous and lacked the distinct layering of the upper 60 cm.
Core B1 contained two main sediment facies (Fig. 2): (i) a very homogenous, unstratified, organic-rich, dark brown lake mud at the top (36 − 0 cm) and bottom (69 − 59 cm) of the core and (ii) a gray, homogeneous deposit (59 − 36 cm) with sharp layer boundaries and strong color contrast to the brown lake mud above and below. The lake mud of core B1 was considerably more homogenous and of darker brown than the sediments of the cores F1-F3 from the floating-ice lake.
Plant remains (unidentified branches, strongly broken apart plant residues, plant roots, parts of Betula nana, brown mosses, Sphagnum spp.) as well as small shell fragments of bivalves and freshwater ostracods were found throughout all cores in varying abundances.
3.2 Physiochemical sediment properties
The sedimentological and physical analysis showed that mineral components in the lake sediments were generally more abundant in the floating-ice lake while a separate pocket of particularly mineral-rich material was also found in the bedfast-ice lake.
Core F2, from the deepest part of the floating-ice lake, recorded the highest average clay content (37.8 ± 9.8%, 1σ, n = 15) compared to the cores F1 (26.4 ± 5%, 1σ, n = 10) and F3 (22.5 ± 4.8%, 1σ, n = 10). The clay content within the F2 core profile more than doubled from the bottom (20.7%, 101 cm) to the top (50.4%, 0.5 cm). The shoreline samples S1 (30.4%), S2 (27.9%) and S3 (29.5%) also had considerably lower clay contents than core F2. Elevated sand contents within core F2 occurred at 101 cm (8.5%) and 80 cm depths (13.2%), such sand peaks were only found in the lower part of core F2 but not towards the top (Fig. 2). Within the floating-ice lake, the average silt content was the lowest at core site F2 (58.6 ± 7.4%, 1σ, n = 15). With proximity to the shorelines at core sites F1 (67.6 ± 4.8%, 1σ, n = 10) and F3 (71.2 ± 4.6%, 1σ, n = 10) silt contents increased noticeably. The latter was closest to the actively eroding yedoma permafrost cliff which had higher silt contents of 65.1% (S2) and 64.0% (S3) in comparison to the peaty, western shoreline sample S1 (51.7%). Average sand contents were overall low across the floating-ice lake with 5.9 ± 2.3% (F1, 1σ, n = 10), 3.6 ± 3.3% (F2, 1σ, n = 15) and 6.3 ± 5.5% (F3, 1σ, n = 10). This corresponded well with the sand contents of the shoreline samples S2 (7.0%) and S3 (6.7%) from the eroding yedoma permafrost cliff. The very high sand content of 17.9% (S1) at the peaty western shoreline was not reflected at the closest sediment core site F1 (5.9 ± 2.3%, n = 10). However, the high sand content at shoreline site S1 was likely due to the methodology of the grainsize measurement. For the grainsize analysis all organic material was stripped from the sample and only the remaining mineral components were analyzed. If the abundance of organic material would have been taken into account, too, the relative abundance of the sand component at site S1 would have been considerably lower.
In core B1 of the bedfast-ice lake the lake mud (36 − 0 cm and 69 − 59 cm) had an average clay content of 59.6 ± 2.2% (1σ, n = 7). This was almost twice as high as the average clay content at the three coring sites F1-F3 of the floating-ice lake (30.2 ± 10%, 1σ, n = 35). The average silt (37.2 ± 2.5%, 1σ, n = 7) and average sand content (3.2 ± 0.5%, 1σ, n = 7) in the lake mud of core B1 were much lower than the overall average numbers of the floating-ice lake (64.8 ± 8.2%, 1σ, n = 35 and 5.0 ± 4.0%, 1σ, n = 35, respectively). The pocket with the homogenous deposit from 59 − 36 cm depth in core B1 was dominated by silt (83.9 ± 1.0, 1σ, n = 3) while sand (6.3 ± 0.5%, 1σ, n = 3) and clay (9.8 ± 0.6%, 1σ, n = 3) were also present.
The average sediment porewater contents of the floating-ice lake cores (Fig. 2) were similar with 36 ± 2.3 wt% (F1, 1σ, n = 10), 41.7 ± 7.6 wt% (F2, 1σ, n = 15) and 30.4 ± 3.1 wt% (F3, 1σ, n = 10). The average porewater content in the lake mud of core B1 from the bedfast-ice lake (49.2 ± 8.1%, 1σ, n = 7) was noticeably higher than in the cores F1-F3. The sandy silt pocket in core B1 (59 − 39 cm) contained considerably less porewater with an average of only 21.7 ± 0.8% (1σ, n = 3).
The sediment density (1.63 ± 0.11 g cm-³, 1σ, n = 1353) and MS values (8.9 ± 5.1 cm³ g-1, 1σ, n = 1308) were on average higher in the floating-ice lake than in the lake mud of the bedfast-ice lake with 1.31 ± 0.16 g cm-³ (1σ, n = 201) and 7.9 ± 4.4 cm³ g-1 (1σ, n = 192), respectively. The sandy silt pocket in core B1 (59 − 39 cm) was clearly distinguishable from the surrounding lake mud by its considerably higher sediment density with an average of 1.89 ± 0.14 g cm-³ (1σ, n = 136). Particularly SOM-rich sections in core B1 (23 cm) and core F2 (80 cm) were also clearly visible in both, the density and MS profiles (Fig. 3).
The Si/Al and Mn/Fe ratios of all four sediment cores had low, stable downcore values with two main exceptions (Fig. 3). Firstly, the sandy silt pocket in core B1 of the bedfast-ice lake had a considerably higher Si/Al ratio with an average of 14.3 ± 0.8 (1σ, n = 26) in comparison to the surrounding core B1 lake mud with 7.7 ± 0.6 (1σ, n = 47). Secondly, there was a strong difference in the Mn/Fe ratio within core F2, the lower core half (110 − 57 cm) had an average of 0.02 ± 0.01 (1σ, n = 60) while the top half (57 − 0 cm) had a significantly higher average with 0.14 ± 0.08 (1σ, n = 58).
3.3 Sedimentary and dissolved organic matter
SOM content and porewater DOC concentrations were considerably higher in the bedfast-ice lake in comparison to the floating-ice lake. The LOI data provided an estimate of the SOM amount present in the analyzed sediment. The shoreline samples from the floating-ice lake significantly varied in their LOI values. Sample S1 from the peaty, western shoreline was almost entirely made up of SOM with a LOI value of 73.3 wt%. The LOI values from the samples S2 and S3 of the yedoma permafrost cliff were up to 15 times lower with 4.9 wt% and 5.3 wt%, respectively (Fig. 4). The proximity to the comparably SOM-depleted permafrost cliff was also reflected in the LOI values of the sediment cores F1-F3 of the floating-ice lake. Core F3 was the closest to the yedoma permafrost cliff and had the lowest average LOI value with 5.5 ± 1.1 wt% (1σ, n = 10). Core F2 had the highest average LOI value of the three floating-ice lake cores with 7.9 ± 2.4 wt% (1σ, n = 15) while core F1 had an average LOI value of 6.7 ± 0.6 wt% (1σ, n = 10). Core F2 marked the highest individual LOI value within the floating-ice lake of 12.2 wt% in an OM-rich layer (80 cm).
The lake mud of core B1 from the bedfast-ice lake had an average LOI value of 28 ± 4.9 wt% (1σ, n = 7) with a maximum LOI value of 36.2 wt % at the core top (0.5 cm). The sandy silt pocket in core B1 was low in SOM content which was reflected in an average LOI value of only 2.7 ± 0.5 wt% (1σ, n = 3).
The differing amounts of SOM in the two different lakes were even stronger pronounced in the sediment porewater DOC concentrations. The combined average porewater DOC concentration across all three cores (F1-F3) of the floating-ice lake was 56.2 ± 23.8 mg L-1 (1σ, n = 27). The average porewater DOC concentration in the lake mud of core B1 (35 − 5 cm) was almost four times higher with 220 ± 65 (1σ, n = 4), and a maximum concentration of 317 mg L-1 at 35 cm depth.
All four cores had in common that DOC concentrations increased with sediment depth. Core F3 had the highest average porewater DOC concentration (78.7 ± 24.1 mg L-1, 1σ, n = 9) and individual porewater DOC measurement (99.9 mg L-1, 85 cm) within the floating-ice lake (Fig. 4). The cores F1 and F2 had almost identical average porewater DOC concentrations with 44.9 ± 16.4 mg L-1 (1σ, n = 8) and 45.0 ± 10.6 mg L-1 (1σ, n = 10). DOC concentrations of the lake water were also measured at coring sites B1 (19.5 mg L-1) and F2 (11.6 mg L-1).
3.4 Stable carbon isotope ratios of DOC and SOC in lake sediments
The δ13CSOC values across the three cores of the floating-ice lake followed a general pattern from high values at the core bottom towards lower values at the core top (Fig. 4). The shoreline samples also differed strongly in their δ13CSOC values from each other. Sample S1 from the peaty, western shoreline had the lowest δ13CSOC value with − 27.9‰, while the two shoreline samples S2 and S3 from the yedoma permafrost cliff had considerably higher values with − 26.0‰ and − 25.8‰, respectively. The difference in δ13CSOC signatures between the shoreline samples S1 (peaty, west) and S2-S3 (yedoma, east) near the floating-ice lake was also reflected in the δ13CSOC values of the three sediment cores with proximity (lower values) and distance (higher values) to the different shorelines. The westernmost core F1 had the lowest average δ13CSOC value with − 28.6 ± 0.3‰ (1σ, n = 10) within the floating-ice lake. Core F2 in the center of the lake had a higher average δ13CSOC value of -28.2 ± 0.7‰ (1σ, n = 15). The highest average δ13CSOC value was measured at the easternmost core F3 with − 27.0 ± 0.4‰ (1σ, n = 10) closest to the actively eroding yedoma permafrost cliff and shoreline samples S2-S3. The average δ13CSOC value in the lake mud of core B1 from the bedfast-ice lake was considerably lower with − 30.1 ± 0.2‰ (1σ, n = 7) in comparison to the combined average δ13CSOC value of the cores F1-F3 with − 27.9 ± 0.8‰ (1σ, n = 35). The sandy silt pocket of core B1 had an average δ13CSOC value of -29.1 ± 0.6‰ (1σ, n = 3).
The porewater δ13CDOC profiles of the three cores in the floating-ice lake generally followed the same trend as the δ13CSOC profiles with high values at the core bottom and low values at the core top. The average porewater δ13CDOC values of the three cores did not follow the exact same pattern as the δ13CSOC averages though. The highest average porewater δ13CDOC value was also detected in core F3 with − 26.3 ± 0.7‰ (1σ, n = 9) while core F1 had an average of -28.6 ± 0.3‰ (1σ, n = 8) and core F2 the lowest average with − 27.7 ± 0.3‰ (1σ, n = 10). The average porewater δ13CDOC value in the lake mud of core B1 was − 28.8 ± 0.4‰ (1σ, n = 4), which was considerably lower than the combined floating-ice lake δ13CDOC average of -27.1 ± 1.0‰ (1σ, n = 27). Additionally, δ13CDOC lake water samples were analyzed for coring site F2 with − 28.2‰ and coring site B1 with − 29.3‰ (Fig. 4). Generally, an increase in SOM abundance appeared to result in higher DOC concentrations (Fig. 5a). Similarly, a direct relationship of higher δ13CDOC values with elevated δ13CSOC values was also observed (Fig. 5b).
The value range in δ13CSOC and δ13CDOC values in the floating-ice lake developed mostly independent of SOM (LOI) abundances and DOC concentrations (Fig. 6a-b). In the bedfast-ice lake, the opposite was the case, while it had a larger variability in SOC and DOC values, the accompanying δ13CSOC and δ13CDOC values remained at a stable value range in comparison to the floating-ice lake (Fig. 6a-b).
3.5 Radiocarbon ages of lake sediments
The most noticeable difference between the two lakes was the presence of young SOC in the bedfast-ice lake sediment, while the floating-ice lake sediment was dominated by considerably older SOC (Fig. 2, Table 2). The youngest bulk 14C age from the lakes was recorded at the top of sediment core B1 in the bedfast-ice lake with 1655 ± 60 year BP, while the bulk average 14C age from the three core tops of the floating-ice lake cores was much older at 4800 ± 490 year BP (1σ, n = 3).
Table 2
Overview of the radiocarbon (14C) analyses of the sediment cores B1 and F1-F3 and the shoreline samples S1-S3. The three calibrated 14C age ranges of core F2 were calculated with IntCal20 (Reimer et al. 2020) while the negative calibrated age ranges of core B1 were calculated with the NHZ1 data set (Reimer, Brown and Reimer 2004)
Location | Sample name | Lab no. ETH- | Core depth [cm] | Material | 14C age [yr BP] | Age uncertainty [yr] | Calibrated 14C age, 2-σ range [cal yr BP] |
Bedfast-ice lake | B1 | 97289 | 1 | Bulk sediment | 1655 | 60 | - |
B1 | 97302 | 1.5 | Brown moss leaves and stems | -725 | 20 | -50 to -51 |
B1 | 97305 | 32 | Brown moss leaves and stems | -115 | 20 | -67 to -69 |
B1 | 97303 | 67 | Brown moss leaves and stems | -5 | 20 | -5 |
Floating-ice lake | F1 | 97291 | 0.5 | Bulk sediment | 4950 | 70 | - |
F2 | 97290 | 0.5 | Bulk sediment | 4130 | 70 | - |
F3 | 97292 | 0.5 | Bulk sediment | 5315 | 80 | - |
F2 | 97300 | 1 | Brown moss leaves and stems | 1000 | 20 | 956 to 902 |
F2 | 97301 | 79 | Brown moss leaves and stems | 1440 | 20 | 1356 to 1300 |
F2 | 97298 | 111 | Brown moss leaves and stems | 1665 | 25 | 1611 to 1516 |
Shoreline samples | S1 | 97295 | (See Table 1 for sampling locations) | Bulk peat soil | 3475 | 70 | - |
S2 | 97294 | Bulk yedoma soil | 17590 | 130 | - |
S3 | 97293 | Bulk yedoma soil | 28170 | 290 | - |
Furthermore, two macrofossil 14C age profiles were collected from the cores F2 and B1 for comparison of the lake ages. Core F2 had a chronological macrofossil 14C age profile from the bottom to the top with age ranges of 1611 − 1516 cal yr BP (111 cm), 1356 − 1300 cal yr BP (79 cm) and 956 − 902 cal yr BP (1 cm). The macrofossil 14C age profile of core B1 from the bedfast-ice lake contained contemporary carbon, the yielded 14C ages and age ranges were of modern origin (post AD 1950) with − 5 cal yr BP (67 cm), -67 to -69 cal yr BP (32 cm) and − 50 to -51 cal yr BP (1.5 cm).
The bulk shoreline sample S1 from the peaty western shoreline of the floating-ice lake yielded a 14C age of 3475 ± 70 year BP. The two bulk samples S2 and S3 from the yedoma permafrost cliff along the eastern shoreline of the same lake resulted in significantly older 14C ages of 17590 ± 130 year BP and 28170 ± 290 year BP, respectively.
3.6 Microbial diversity of two contrasting lakes
The NMDS of community distance matrices showed that the bacterial and archaeal community compositions clearly differed between the two lakes (Fig. 7).
The bacterial community composition was most influenced by Mg (R2 = 0.58), water depth (R2 = 0.56), Al (R2 = 0.52), and Cr (R2 = 0.42) (Fig. 7, Supplementary Table S1). The environmental parameters with the strongest correlation for the archaeal community composition were Cl (R2 = 0.46), Mg (R2 = 0.45), Rh (R2 = 0.45) and Al (R2 = 0.42) (Fig. 7, Supplementary Table S1). Overall, water depth and elemental composition appeared as the strongest factors for inter-lake microbial community diversity. It is noteworthy, that the sediment depth also appeared to strongly impact the archaeal and bacterial community diversity in the floating-ice lake (especially cores F1 and F2) as indicated by the elongated cluster shapes which successively follow the sampling depth (Fig. 7). These observations were also supported by noticeable differences in the Shannon Diversity Index in both, the archaeal and bacterial communities, of the two lakes (Supplementary Fig. S2).
The distribution of the different archaeal taxonomic levels of phylum (Supplementary Fig. S3), class (Supplementary Fig. S4), order (Supplementary Fig. S5), family (Supplementary Fig. S6), and genus (Fig. 8) are also displayed in more detail. In addition, the distribution of the anaerobic methane oxidizing archaeon Candidatus ‘Methanoperedens’ (Supplementary Fig. S7) is shown separately. The distribution of the bacterial taxonomic levels of phylum (Supplementary Fig. S8), class (Supplementary Fig. S9), order (Supplementary Fig. S10), family (Supplementary Fig. S11) are also displayed in detail in the supplementary figures. In addition, the aerobic bacterial methane oxidizers of the Methylomonadaceae family (Supplementary Fig. S12) are shown separately.
We observed 1142 bacterial ASVs across all samples. The bacterial communities were dominated by Proteobacteria (44.0%), Bacteroidota (23.3%), Chloroflexi (5.9%), and Acidobacteriota (3.0%) across all sediment cores (Supplementary Fig. S8). In the three floating-ice lake sediment cores the most abundant bacterial orders were Burkholderiales (36.0%), Bacteroidales (13.0%), Sphingobacteriales (4.1%), Caulobacterales (3.1%), Spirochaetales (3.0%), and Campylobacterales (3.0%) while in the bedfast-ice lake the orders of Burkholderiales (31.5%), Bacteroidales (21.7%), Flavobacteriales (7.4%), Anaerolineales (4.9%), and Pseudomonadales (3.3%) were most dominant (Supplementary Fig. S10). We detected sequences of the aerobic methanotrophic genus Methylobacter of the Methylomonadaceae family in all sampled cores (Supplementary Figs. S11-12).
The archaeal community was dominated by Euryarchaeota (49.0%), Bathyarchaeota (26.7%), Woesearchaeota (DHVEG-6) (14.4%), Thaumarchaeota (3.8%), and Altiarchaeales (1.3%) (Supplementary Fig. S3). However, the bedfast-ice lake samples showed a significant difference in their archaeal community composition compared to the floating-ice lake (Figs. 7–8). We observed several methanogenic orders, including, Methanomicrobiales and Methanosarcinales (Supplementary Fig. S5). The abundance of these orders varied strongly, for example for Methanosarcinales, with 77.2% in core B1 at 50 cm depth (sandy silt), and 13.4% in sediment core F1 at 55 cm depth (Supplementary Fig. S5).
In both lakes, aceticlastic Methanosaeta, aceticlastic and methylotrophic Methanosarcina and hydrogenotrophic methanogens were observed in the methanogenic communities (Fig. 8). The methanogenic communities of the bedfast-ice lake and floating-ice lake differed in the most abundant methanogenic genera.
In the bedfast-ice lake the methanogenic community was dominated by the hydrogenotrophic Rice Cluster II, Methanosaeta, Methanosarcina and Methanobacterium, while the cores from the floating-ice lake showed a dominance of Methanoregula, Methanosaeta, Rice Cluster II and Methanomicrobiaceae (Fig. 8).
In the bedfast-ice lake core B1, the archaeal community profiling also showed the presence of the anaerobic methanotrophic archaeon Ca. ‘Methanoperedens’ with varying relative abundances of 1.6% (0.5 cm, lake mud) and 65.8% (50 cm, sandy silt) within the Euryarchaeota (Fig. 8, Supplementary Fig. S7). Ca. ‘Methanoperedens’ was also present in the top 10 cm of core F3, but especially in the top 30 cm of core F2 (Figs. 8 and S7). These archaea oxidize methane at the expense of nitrate, Fe(III) or Mn(IV) (Haroon et al. 2013; Cai et al. 2018; Leu et al. 2020); from the phylotype, however, it was not possible to deduce which substrate they used at this particular site. However, Ca. ‘Methanoperedens’ was mostly absent or present in low abundances (< 1%) in 48 out of 56 floating-ice lake samples (Figs. 8 and S7). No other archaeal methanotrophs were found.