Introduction
During the evolution of the early Solar System, there was an interval from 4.1 to 3.8 billion years ago (Ga) called the Late Heavy Bombardment, during which the Earth, Moon, Mars, and other planetary surfaces were bombarded by large bolides at extremely high rates, leaving large craters on their surfaces1. Recent studies have indicated that the Late Heavy Bombardment did not end abruptly. Instead, high bombardment rates persisted until the end of the Archean at ~2.5 Ga, implying continuous or clustering impact crater formation over the early Earth’s surface2,3,4.
Stromatolite, laminated sedimentary structures accretionary away from appoint or limited surface5, is considered the oldest evidence of life on Earth, dating back to approximately 3.5 Ga in the early Archean6,7,8. Their laminated organo-sedimentary structures form through the trapping and binding of sediment grains by microbial activity or the precipitation of minerals driven by microbial metabolic processes, such as the photosynthetic uptake of CO2, which induces an increase in alkalinity and promotes carbonate precipitation9,10,11. A recent study of the 2.7 Ga Hartbeesfontein lacustrine stromatolites in South Africa indicated that the evolution of oxygenic photosynthesis may have fundamentally transformed the global environment of the early Earth; non-marine microbial mats served as localised “oxygen oases” for hundreds of millions of years before the Great Oxidation Event at approximately 2.4 Ga12. Stromatolites were prevalent throughout the Proterozoic but declined towards the Cambrian, partly due to widespread grazing by animals, which exerted trophic pressure on microbial mats13,14,15.
These ancient fossils persist on the modern Earth; present-day stromatolites have notably been identified in extreme environments where biofilms are protected from animal grazing, such as high-salinity coastal bays (e.g. Shark Bay in Australia)16, hot springs17, and hypersaline lakes18,19,20. This implies that stromatolites may serve as indicators of extreme palaeoenvironments21, although modern stromatolites are also found in open marine conditions9. The timing of relatively young stromatolite formation (< 55,000 years) can be determined through radiocarbon dating of their inorganic or organic components because stromatolite structures comprise numerous layers of carbonate minerals and organic microbial mats18,20,22,23,24.
This study presents the first report of stromatolites in the recently confirmed Hapcheon impact crater, Korea25. Analyses of the stromatolites’ 187Os/188Os ratios and rare earth elements (REEs) imply meteoritic influences and hydrothermal activity in the crater, respectively. Radiocarbon dating provides an estimate of their growth periods. This location offers a unique opportunity to investigate stromatolite formation and development within an impact crater.
Results and discussion
Hapcheon impact crater and the search for stromatolites
The study area is located in the Jeokjung–Chogye Basin (hereafter referred to as the Hapcheon impact crater) in Hapcheon Province, South Korea (Fig. 1). This site is notable for its bowl-shaped geomorphological landscape, surrounded by mountains with elevations of 200–700 m. The bedrocks in this area are Cretaceous and Mesozoic sedimentary rocks and consist mainly of greenish grey to dark grey sandstone, greenish brown sandstone, grey to dark grey shale, purple shale, sandy shale, mudstone, lime nodules, thin limestone beds and conglomerate. Mineralogical and textural evidence, including impact-induced shock-metamorphic features (e.g. shatter cones), has confirmed that the basin is an impact crater25. Subsequently, meteoritic components within the impact breccia (IB) deposits, impact modelling and the early post-impact sedimentary environments of the crater have been investigated26,27,28,29.
A Stromatolite sampling site (STR and MGS), slope deposit (SD) site, palaeolake (PL) boundary site, impact breccia (IB) sites, and drilling sites (20CR05, 20CR06, 20CR09, 20CR10, 20CR11, 20CR12 and 23HIC01) within the crater, based on a local digital elevation model (DEM). B Gravity anomaly features (see “Methods” for details). C SW–NE transect comparison between topography and gravity anomaly. D Enlargement of the shaded area in (C), displaying simplified lithological data from drilled cores. Numbers (e.g. 20,310 and 6670) indicate radiocarbon ages (cal yr BP)28,29.
To examine subsurface features of the Hapcheon impact crater, sedimentary cores were extracted from multiple sites, and a gravity survey was conducted. Sedimentary cores were collected along a northeast–southwest transect (20CR06, 20CR05, 20CR09, 20CR10, and 20CR11) and other sites (20CR12 and 23HIC01). These sedimentary cores contained fractured bedrock, IB, post-impact lake sediments and subaerial sediments (e.g. wetland and fluvial sediments) in ascending order25,28 (Fig. 1 and Supplementary Figs. 1−3). Age dating results for the sedimentary cores from sites 20CR05, 20CR09 and 20CR10 have been reported in the previous studies25,28,29. A gravity anomaly and drilling data indicate that the underlying structures are tilted in a northeast direction, aligning with the topography and providing evidence of rough subsurface features (Fig. 1). The 20CR05 coring site contained the thickest lake sediment deposits (approximately 65 m), implying that the depocentre of the palaeolake (PL) was located northeast of the crater’s centre.
Stromatolites were discovered in small valleys with intermittently flowing streams along the inner northwestern margin of the crater (Fig. 1 and Supplementary Fig. 4). At the first site where stromatolites were identified (STR) during a field survey in 2020, they were buried within muddy gravel deposits at a depth of approximately 20–40 cm, corresponding post-impact paleolake shorelines (Fig. 2 and Supplementary Fig. 4). Within an area of <1 m2, more than 20 stromatolites and their fragments (5–20 cm in diameter) were discovered, exhibiting a diverse range of morphologies (e.g. stratiform, domal and columnar). The stromatolites displayed a banded microstructure with wavy, wrinkled laminae predominantly 10–100 µm thick, as observed in thin sections (Fig. 2 and Supplementary Fig. 5). At a second stromatolite site in Misa-gol (or Misa Valley, MSG), two stromatolites (approximately 10 cm in diameter) were identified in a narrow dry channel during a field survey in 2021 (Supplementary Figs. 4−6).
For 14C dating, subsamples of stromatolites were obtained by crushing-grinding (A–C) and drilling processes (E–H). D Example for digitate stromatolite. E Yellow circles (n = 4) in the thin sections indicate subsampling points for osmium isotope analysis (187Os/188Os).
Biogenicity in the Hapcheon stromatolites
Stromatolites have served as valuable records of past microbial interactions with sediments and flowing water since their formation on early Earth30,31. In general, tracing biogenecity in old stromatolites is difficult due to the poor preservation of organic materials and cellular microfossils, which is attributed to early post-mortem and diagenetic nanogranula calcification32,33. Instead of finding cellular evidence, the combination of textual, mineralogical and spectroscopic analyses can provide microbial mat growth processes33,34,35. In this study, elemental mapping with Raman micro-spectroscopy and electron probe microanalysis (EPMA) was used to support such biogenic activity based on minerals and carbonaceous materials in the stromatolites (see the “Methods” for details).
The stromatolites from the Hapcheon impact crater reveal a clear organic-matter-enriched layer (OMEL), a quartz-enriched layer (QEL), and a calcite-enriched layer (CEL) (Fig. 3). As shown in the Raman spectrum, the Raman peaks are dominated by calcite (peaks at 279, 713 and 1095 cm−1), quartz (peak at 465 cm−1), and carbonaceous materials (G-band) (peaks at ~1034 and ~1600 cm−1), representing an organic matter layer (Fig. 3). Notably, quartz minerals are concentrated in the OMEL. Elemental maps acquired from the EPMA analysis provide high-resolution spatial information regarding the microstructure in the stromatolites. As shown in Fig. 4, the stromatolite matrix is dominated by Ca elements (mainly calcite) and terrestrial elements (e.g. Si and Al), which clearly describe the convex-upward growth pattern. The OMEL corresponds with the layers of enriched Si and Al elements. In contrast, the organic matter content of the Ca-dominated layers is very low. These mineral and elemental maps highlight the distribution of organic matter-rich and calcite-dominant laminations. Furthermore, these maps indicate that detrital minerals (e.g. quartz) are concentrated on the organic-rich layers and even beyond the angle of repose, as shown in the Si elemental map (Figs. 3 and 4). Previous studies have tested the distribution of grain size on the stromatolites and concluded that the existence of grains beyond the angle of repose represents trapping and binding processes by microbial mats on the stromatolites, supporting biogenicity31,36. Therefore, the existence of terrestrial elements and quartz minerals in the organic matter layer beyond the angle of repose may imply the same biogenetic processes on the stromatolites. Additionally, as shown in Fig. 3A, the laminae become thinner toward the margin. The thickening of laminations toward crests in the OMEL may indicate trophotactic behaviour30,34.
A Raman 2D mapping of carbonaceous material (G-band, ~1600 cm−1). OMEL is the organic matter-enriched layer. B Raman 2D mapping of quartz (465 cm−1). QEL and CEL are the QEL and CEL, respectively. C Representative Raman spectra of the CEL, QEL, and OMEL.
A Backscattered electron image for the thin section of the 20STR03 stromatolite. B–D 2D elemental maps of Ca, Si and Al in the thin section of the 20STR03 stromatolite. E Backscattered electron image for the 21MSG02 stromatolite. F–H 2D elemental maps of Ca, Si and Fe in the thin section of the 21MSG02 stromatolite.
Timeline of stromatolite growth
Radiocarbon dating may be used to determine the growth period of stromatolites that are less than 55,000 years old18,20,22,23,24. Radiocarbon (14C) dating (see Methods) was performed using organic matter in the stromatolites, which was recovered after HCl-treatment (Figs. 2 and 5, Supplementary Fig. 5, and Supplementary Table 1). The 14C ages along the growth line from the centre to the rim of a stromatolite exhibited an increasing–decreasing pattern, indicating an age-reversal trend before returning to a younger age (Fig. 5 and Supplementary Table 1). The age of the innermost part of stromatolite 20STR03_1 was 23,390 cal yr BP, increasing outward to 28,320 cal yr BP (20STR03_4) (Fig. 5). The age near the rim, representing the outermost growth position, was 14,660 cal yr BP (20STR03_6), implying that stromatolite 20STR03 grew between approximately 23,390 and 14,660 cal yr BP. Similar age-reversal patterns were observed in stromatolite STR04, with age reversing from 19,140 cal yr BP (20STR04_0) to 26,070 cal yr BP (20STR04_3) (Fig. 5), and in stromatolite STR05, where age reversed from 21,730 cal yr BP (20STR05_A) to 22,360 cal yr BP (20STR05_B) (Fig. 2). These age reversals indicate that the organic materials that formed (autochthonous) or became trapped on the stromatolite surface (allochthonous), as observed in the mineral and elemental maps (Figs. 3 and 4), were influenced by 14C-depleted carbon in the lake and older deposits on the crater slope28,29. This age-reversal pattern implies that these ages should be interpreted as an approximate estimation of the formation ages of the stromatolite layers in the context of 14C behaviour within the impact crater, rather than as the absolute ages of stromatolite growth or deposition. For example, stromatolite STR03 might have grown at a certain period, approximately between 23,390 and 14,660 cal yr BP.
A, B The 14C dates of stromatolites 20STR03 and 20STR04. To obtain the 14C ages, powdered subsamples of the stromatolites were collected by drilling holes into the samples using a micro-mill. Age reversals are indicated from 20STR03_1 to 20STR03_4 and from 20STR04_0 to 20STR04_3 along the growth lines. Based on the age reversal pattern, stromatolite 20STR03 grew approximately between 24,300 and 14,600 cal yr BP.
The age-reversal pattern in the 14C ages of stromatolites located at the lake margin resembled the reversal trend observed in lake sediments within the crater28 (Fig. 6A). After a meteorite impact event, which can cause extensive disturbance to surficial sediments and bedrock, the ages of organic matter (e.g. humic acids, humin, charcoal and plant fragments) within a crater would be influenced by old carbon effects. These effects result from the incorporation of pre-impact and syn-impact organic substances into slope deposits (SDs), as well as the addition of post-impact organic matter produced within the lake and surrounding areas. A previous study showed that the early post-impact lake environment was affected by frequent slumping turbidite events, leading to substantial infilling of the post-impact lake by reworked IBs and fallback deposits from the slopes and rims of the crater28. As shown in Fig. 6A, humic acids from approximately 71 m in depth, corresponding to the lowermost post-impact lake sediments (Stage 1), were dated to ~21,000 cal yr BP, whereas those from mid-depth lake sediments from approximately 50 m in depth (Stage 2) were dated to 35,000–43,000 cal yr BP. This age reversal has been attributed to the increased input of pre-impact and/or syn-impact allochthonous humic acids, based on the positive correlation between terrestrial mineral content (Fig. 6B) and humic acid ages29.
A 14C ages of humic acids25,26, humin25,26, and charcoal26 in lake sediments, along with 14C ages of plant fragments (100−500 µm) in IBs (this study). B Sum of terrestrial mineral content (M: muscovite, Q: quartz, Alb: albite, Chl: chlorite)26. C Calcite mineral content (%)26. D High-resolution XRF core scanning results, displaying semi-quantitative Ca variations as counts per second (cps). E, F 187Os/188Os ratios and Os concentrations in the lake sediments and IBs of the core 20CR05, indicating a meteoritic component in the Hapcheon impact crater. G Representative photographs of IBs from core 20CR05, showing sampling points for radiocarbon dating and the radiocarbon ages of plant fragments (<500 µm). Numbers indicate 14C ages (cal yr BP).
As shown in STR03 in Figs. 2 and 5, the age reversal from 23,390 to 28,320 cal yr BP in the stromatolites corresponded to Stage 1 of the lake sedimentary core (20CR05)(Fig. 6A). This similarity suggests that the stromatolite discovered in the crater could have formed in the early post-impact lake environment and that the humic acid component in the lake sediments during Stage 1 was significantly influenced by authigenic aquatic productivity. Notably, the presence of Spirogyra, a warm-water aquatic green alga, in the post-impact lake was identified through palynological analyses of sedimentary core 20CR0929. The age reversal may indicate a decline in in situ productivity and an increased influx of relatively old humic acids from the SDs of the crater. Similarly, as inferred from their laminated organo-sedimentary structures in the Raman 2D maps and EPMA elemental maps (Figs. 3 and 4), the age reversal observed in the stromatolites could have resulted from the trapping and binding of old organic debris transported from SDs through microbial activity on the stromatolites, although the uptake of old inorganic carbon from carbonates by microorganisms cannot be discounted.
Testing post-impact stromatolites
According to a recent study29, radiocarbon dating of plant fragments of different sizes (10−100 µm and 100−500 µm) in post-impact lake sediments from core 20CR09, located at the centre of the crater (Fig. 1), revealed variations in age. For example, the radiocarbon age of larger plant fragments (100−500 µm) at a depth of 20.85 m was 42,190 ± 250 cal yr BP; smaller plant fragments (10−100 µm) at the same depth were younger, at 39,310 ± 660 cal yr BP, likely due to the incorporation of post-impact freshwater algae and pollen (Supplementary Fig. 7A). The ages of plant fragments (100−500 µm) in a 33-m lake sediment core overlying the IBs ranged from 43,910 to 41,670 cal yr BP, implying that SDs, composed of IBs and fallback deposits containing syn-impact plant fragments, were continuously transported during this period. These findings support the reported range of ages for syn-impact fragments of the Hapcheon impact event (42,770 ± 1,030 cal yr BP)29, corresponding to the timing of the Hapcheon impact event.
In this study, the estimated timing of the impact event can be further tested by radiocarbon dating of plant fragments embedded within the underlying IBs from the deepest sedimentary cores (20CR05) in the crater (Fig. 6A and Supplementary Table 2). This approach is effective because IBs at greater depths in the central crater were deposited immediately after crater formation and were minimally affected by post-impact organic matter influx. As expected, plant fragments (100 − 500 µm) in IB deposits (87.8–140 m) (20CR05), isolated from the overlying lake sediments, mainly consist of charcoal, as shown in microphotographs taken at depths of 102.75 and 102.95 m within the IBs (Fig. 6A, G and Supplementary Figs. 7B, C). The radiocarbon ages of these fragments ranged from 43,220 to 41,240 cal yr BP. The presence of older ages at depths of 139.4 and 109.66 m implies an influence from pre-impact organic fragments, such as charcoal, predating the impact event29. The average age of the IB plant fragments (42,250 ± 990 cal yr BP or roughly 42,300 ± 1000 cal yr BP) was very similar to the humin ages in the overlying lake sediments, implying that these humin materials were transported from the SDs, which consisted of IBs and fallback materials (Fig. 6). Furthermore, this age of the IB plant fragments corresponds to the syn-impact age estimated in the post-impact lake sediments of core 20CR0929. Thus, considering this estimated timing (42,300 cal yr BP) of the impact event (Fig. 6), stromatolites found along the crater margins would have developed after formation of the impact crater, indicating post-impact stromatolite growth.
If the stromatolites identified in this study formed in early post-impact environments, meteoritic components may have been incorporated into their structure. Previous studies have successfully traced past impact cratering events using the osmium (Os) isotope fingerprint. For example, abnormally low 187Os/188Os ratios in sediments, IBs, and impact melt rocks, compared with those of target bedrocks, indicate the presence of meteoritic material in post-impact basins37,38. A meteoritic component has recently been identified in the IBs beneath the Hapcheon impact crater, and it has been reported that IBs contain platinum-group elements at concentrations approximately five-fold higher than those in the Cretaceous target rocks, along with a rhenium (Re) abundance two orders of magnitude higher26. Furthermore, the mean 187Os/188Os ratio in the underground IBs is 0.806, which is lower than that of the target rocks (1.123) but higher than that of a projectile meteorite (e.g. carbonaceous chondrite, Os = 459 ppb, 187Os/188Os = 0.126; Horan et al. 39), implying the presence of a meteoritic component26. These findings warrant further investigation regarding the possible incorporation of meteoritic material into the stromatolites found in the Hapcheon crater.
A comparison of 187Os/188Os ratios in stromatolites and various sites within the Hapcheon impact crater clearly indicates a meteoritic influence (Figs. 6 and 7 and Supplementary Tables 4–6). Average 187Os/188Os ratios in the STR03 stromatolite (n = 4) and MSG02 stromatolite (n = 4) were 0.637 and 0.762, respectively, and average Os concentration in the STR03 stromatolite (n = 4) and MSG02 stromatolite (n = 4) were 0.110 and 0.117 ppb, respectively (Fig. 7 and Supplementary Table 4). A recent study showed that 187Os/188Os ratios within target rocks in the crater, such as Jinju Formation shale, Jinju Formation sandstone and Chilgok Formation shale, were 0.1062, 0.894 and 1.416, respectively26. The average Os concentrations in these formations were 0.010, 0.0313 and 0.004 ppb, respectively26. Compared with these target bedrock values, the Os concentration in the stromatolites was 3.6–-28.4-fold higher, with depleted 187Os/188Os ratios of 0.2–0.8. In general, meteorites tend to exhibit low 187Os/188Os ratios, approximately 0.126, and a significantly higher Os concentration, with reported values reaching 459 ppb39. Even a small addition of meteoritic material to the target bedrock can therefore reduce 187Os/188Os ratios while increasing the Os concentration. Thus, the low 187Os/188Os ratios and elevated Os concentration in the STR03 stromatolite (e.g. 0.637 and 0.1104 ppb) can be attributed to the incorporation of a meteoritic component. As shown in Fig. 7, a binary mixing model based on target bedrock average values (Os = 0.0152 ppb, 187Os/188Os = 1.123; Choi et al. 26) and a projectile meteorite (carbonaceous chondrite, Os = 459 ppb, 187Os/188Os = 0.126; Horan et al. 39) was used to estimate the meteoritic component percentage. The analysis implied that the STR03 and MSG02 stromatolites contained approximately 0.021% and 0.022% meteoritic materials, respectively.
The curve represents the estimated mixing model between the target bedrock (Os = 0.0152 ppb, 187Os/188Os = 1.123; Choi et al.23) and a projectile meteorite (carbonaceous chondrite, Os = 459 ppb, 187Os/188Os = 0.126; Horan et al.29). All 187Os/188Os versus Os concentration data points for stromatolite samples (STR03 and MGS05), 20CR05 lake sediments, and IB are shown, along with the estimated meteoritic component percentage derived from the binary mixing model.
Similar meteoritic influences have been identified in various deposits within the crater (Fig. 7, Supplementary Figs. 8–12, and Supplementary Table 6). The 187Os/188Os ratios in surficial IB (n = 10) from the western and eastern areas range from 0.560 to 0.972, with an average of 0.834. PL marginal sediments (n = 4) exhibited a value of 0.854, corresponding to the average of the IBs, implying that the lake marginal sediments were transported from nearby IB deposits. Additionally, the northwest SDs (n = 6) had an average value of 0.863, which was similar to that of the PL marginal sediments. However, Os concentrations in various deposits from relatively high elevations were not as high as those in the stromatolites. High Os concentrations and low 187Os/188Os ratios were identified in the lake sediments of core 20CR05 (Fig. 6E, F and Supplementary Table 5). IBs at a depth of 87 m in core 20CR05 exhibited minimal meteoritic influence (Os = 0.02 ppb, 187Os/188Os = 1.034), whereas overlying lake sediments, such as those at 29.15 m depth (Os = 0.103 ppb, 187Os/188Os = 0.779), clearly contained a meteoritic component (Fig. 6E, F). Based on the binary mixing model (Fig. 7), the lake sediments in core 20CR05 (Os = 0.080 ppb, 187Os/188Os = 0.823) contained an average meteoritic component of 0.014%. Notably, the lake sediments between 72 and 6.5 m received a continuous, albeit small, influx of meteoritic components from SDs on the inner side of the crater, supporting the simultaneous transport of pre-impact and syn-impact organic materials (e.g. humic acids, humin, pollen, charcoal and plant fragments) and the influence of their old carbon content on the lake sediments29.
Considering the volume of the lacustrine deposits, a substantial amount of the materials overlying the sampling sites (e.g. IB and SD in Supplementary Figs. 8–12) in the crater should have been eroded and transported to the lake, resulting in the deposition of ~65 m of lacustrine sediments. The significant difference in the 187Os/188Os ratios and concentrations between CR05 lake sediments and other materials (IB and SD) may imply that the early surficial materials deposited shortly after the impact event had higher meteoritic components. Furthermore, compared with the coarser grain size in the PL, the CR05 lake sediments exhibited fine-grained laminations with a higher Os concentration. This may imply that the meteoritic signature was intensified during transportation, becoming enriched in fine sediments. This fractionation mechanism likely occurred during incorporation into the microbial mat through the trapping and binding of fine suspended materials, including the meteoritic component. This may have resulted in significant increases in Os concentrations compared to those in IB and SD.
Impact craters as a cradle for stromatolite growth and implications for the early Earth
After devastating impact events, the initial impact craters undergo several distinct stages, including the thermobaric phase, hydrothermal phase and post-impact succession phase40,41. Three primary heat sources have been proposed to drive impact-generated hydrothermal systems; impact melt rocks and impact melt-bearing breccias, elevated geothermal gradients in central uplifts and energy deposited in central uplifts due to the passage of the shock wave41,42. Notably, the hydrothermal phase is thought to persist over extended periods in various impact craters43,44,45,46. For example, the travertine mound within the 24-km Ries crater basin succession provides evidence for about 250,000 years of hydrothermal activity43.
To trace lacustrine environments and possible hydrothermal influence on stromatolite formation, the geochemical, mineralogical and biological characteristics of post-impact lake sediments were determined. High-resolution elemental data obtained through X-ray fluorescence (XRF) core scanning of the sediments revealed significant changes in allochthonous inputs from the SDs of the crater. Notable features in the elemental composition of the lake sediments included the presence of highly enriched elements (e.g. Ca, Sr and S) (Fig. 6 and Supplementary Fig. 13) in the lowermost post-impact lake sediments. Compared with terrestrial elements (Si and K), which exhibited stable levels over time, the Ca, Sr and S concentrations were elevated in the lower part of the core (Stage 1), corresponding to the early post-impact lake sediments. As shown in Fig. 6, Ca was detected at levels of 200,000 counts per second (cps) between 72 and 50 m, followed by a rapid decline to very low levels (<10,000 cps) in Stage 2. The variations in Ca abundance within lake sediments closely resemble fluctuations in calcite (CaCO3) mineral content (0–10%; Fig. 6). A similarly high calcite content (approximately 2.5%) was identified at 45.12–45.85 m in the basal lake sediments from a shallower site (core 20CR09) and at 34–42 m depth in the marginal site (core 23HIC01), which had a maximum calcite content of 12%28,29. This finding implies that Ca content in the lake sediments was primarily controlled by the formation of calcite through in situ precipitation in the sediments or in the water column, similar to the process responsible for stromatolite formation.
Comparable changes in elemental and mineral composition between IB and post-impact lake sediments have been reported from Lake El’gygytgyn, which formed in the Far East Russian Arctic after a meteorite collision at 3.58 Ma44. XRF core scanning of Lake El’gygytgyn sediments revealed significant clustering of high Ca peaks and calcite formation in the early history of the lake, between 3.58 and 3.3 Ma. These phenomena were attributed to the Ca ion flux driven by increased weathering in the catchment after the initial supply from underlying impactites and volcanic bedrock, followed by chemical weathering that resulted from ongoing hydrothermal activity during the early post-impact stage44. Similarly, in the Hapcheon impact crater, a high sediment flux may have occurred due to the steep relief of the young, bowl-shaped crater. As indicated by the high calcite content (8–10%) in the basal post-impact lake sediments (Fig. 6), the lake bottom water and sediment pore water were likely oversaturated with both Ca and carbonate ions.
Furthermore, the sum of terrestrial-origin mineral content, including quartz, muscovite, albite and chlorite—primarily transported from the crater slopes—exhibited an inverse relationship with calcite content and Ca levels (Fig. 6). Thus, the high calcite content in Stage 1 may have resulted from in situ precipitation processes in the lake, similar to those occurring during stromatolite formation. These variations in Ca and calcite content appear to co-vary with changes in 14C ages (Fig. 6). Older ages are associated with increased terrestrial mineral inputs to the lake, implying a greater influx of old organic substances (allochthonous influence) diluting the calcite precipitates in the sediments (Fig. 6 and Supplementary Fig. 13). The younger 14C ages in Stage 1 indicate that in situ productivity in the lake was higher during this period than during Stage 2, possibly due to stronger hydrothermal activity. These findings imply that the early post-impact lake environments in the crater were characterised by relatively high pH and salinity.
The influence of hydrothermal systems within impact craters can be inferred from REE and Europium (Eu) anomaly information. It has been suggested that, compared to other REEs, Eu is reduced to the more soluble Eu2+ under the interaction between hydrothermal fluids and host rocks47,48. These enriched Eu signals can be preserved in precipitates formed from hydrothermal fluids. Previous studies have provided strong evidence that this positive Eu anomaly found in stromatolites can reveal syndepositional hydrothermal influences. For example, the Paleoarchean stromatolites from the lowermost layers of the ca. 3.5-Ga Dresser Formation, Pilbara, Western Australia, clearly show a strongly positive Eu anomaly, implying that the stromatolites formed in hydrothermally influenced shallow-marine lagoonal environments34. Regarding impact-generated hydrothermal influences, a post-impact calcite vein from the fractured crystalline basement of the Ries impact crater showed a clear positive Eu anomaly with a range of 1.68–2.11, indicating post-impact hydrothermal fluid convection in the crater basement43.
As shown in Fig. 8 and Supplementary Tables 7 and 8, the normalised REE patterns in the two stromatolites from the Hapcheon impact crater revealed remarkable Eu enrichment. This positive Eu anomaly (>1) likely implies strong substitution of Eu for Ca in the crystals. Notably, the positive Eu anomaly decreased along the growth direction of the stromatolites. For example, the Eu anomalies in the early (inside) and last (outside) stages of 20STR03 stromatolite formation were 2.72 and 1.58, respectively, indicating a decreasing trend (Fig. 8). A similar pattern was found in the 21MSG02 stromatolite. These similar REE patterns and Eu anomaly changes imply a decrease in the Eu concentration in the shoreline lake water according to the gradual weakening of the hydrothermal activity in the crater. Consequently, the Eu anomaly recorded in these stromatolites serves as a proxy for past syndepositional hydrothermal influences. Although the exact timing of the stromatolites’ growth is difficult to define due to old carbon effects, it is clear that they grew at certain periods between the timing of the subaerial deposition (no lake environment) and the maximum age (20STR02) of the stromatolites, roughly 7000–30,000 years ago. This implies that the duration of the hydrothermal activity in the crater could have been more than ~27,000 years after the impact event at 42,300 cal yr BP, based on the minimum age of the stromatolites (14,660 cal yr BP).
If impact-driven hydrothermal activity occurred in the Hapcheon impact crater, the associated high temperatures should have influenced organisms present in the crater. These effects were investigated using microbial DNA obtained from post-impact lake sediments in core 20CR05. It is worth noting that these DNA results are not definitive, and may not represent the original microbiology present at the time of stromatolite formation, needing caution.
Information regarding the dominant microbial groups is presented in Supplementary Table 7. Among microbial DNA sequences retrieved from the lake sediments, the deeper samples (56.9 m and 71.18 m), corresponding to early post-impact lake sediments, yielded notable results. At a depth of 56.9 m, the dominant microorganism was Thiobacillus thioparus, a species capable of oxidising sulphides in anoxic environments49. At 71.18 m, the dominant operational taxonomic units (OTUs), including Annwoodia aquaesulis and Sulfuritortus calidifontis, were related to organisms isolated from geothermal environments. A. aquaesulis has been identified in geothermal water50, whereas S. calidifontis has been isolated from a hot spring microbial mat51. Both species are involved in sulphur oxidation and are moderate thermophiles. As indicated by elemental data obtained from XRF core scanning (Supplementary Fig. 13), sulphur was abundant in the lower lake sediments, implying hot spring microbial activity in the early post-impact lake environment of the crater. These microbial DNA findings are consistent with the freshwater algae assemblage in the lake sedimentary core (20CR09), which was dominated by Spirogyra, a warm-water aquatic green alga29.
Consequently, elemental and mineralogical data, as well as DNA analysis results, imply that hydrothermal activity was strong in the early post-impact lake environment of the Hapcheon impact crater. This hydrothermal influence was prominent around 20,000 years ago, as indicated by 14C ages obtained at a depth of 72 m, and remained active throughout Stage 1 (Fig. 6). This period coincides with the approximate estimation of stromatolite growth in the crater, as inferred from stromatolite ages—for example, stromatolite STR03 developed roughly between 23,400 and 14,600 cal yr BP. These findings demonstrate that stromatolites in the crater formed within an impact-driven hydrothermal environment during Stage 1. As hydrothermal activity declined, autochthonous productivity in the post-impact lake decreased, whereas the influence of allochthonous 14C-depleted organic matter increased during Stage 2, leading to the observed age reversals (Fig. 6).
It should be noted that the stromatolites found in the Hapcheon impact crater do not provide direct evidence of the evolution of oxygenic photosynthesis in the early Earth, including the Great Oxidation Event at approximately 2.4 Ga12. This is primarily because palynological data indicate that green algae were dominant in the post-impact lake, and the evolution of green algae postdates the Great Oxidation Event.
This study represents an attempt to trace stromatolite growth and development directly in a young impact crater in relation to impact-generated hydrothermal activity, and to assess the crater as a habitat for stromatolites. These evaluations were achieved through an integrated approach combining 14C dating, sedimentological and geochemical analyses, including REE patterns and the Eu anomaly, and the tracing of meteoritic components using 187Os/188Os ratios.
Hydrothermal systems have been proposed as potential sites for the origin and early evolution of life on Earth52,53,54,55 and possibly Mars56,57. The results presented here support this possibility and demonstrate that impact-generated hydrothermal activity in early post-impact lake environments could have facilitated stromatolite growth within impact craters. Thus, considering that asteroid collisions occurred frequently on the early Earth, stromatolite blooms in impact craters on the early Earth need to be investigated as active oxygen oases for the Great Oxidation Event at approximately 2.4 Ga12. Finally, craters with stromatolite-like sedimentary structures need to be considered as target areas for the origin and early evolution of life on Mars.
Methods and notes
Drilling cores
In 2020 and 2023, seven sedimentary cores (20CR05, 20CR06, 20CR09, 20CR10, 20CR11, 20CR12 and 23HIC01) were recovered using a rotary corer, which extracted 1-m-long, 60-mm-diameter core samples encased in plastic liners. After the cores had been photographed, subsamples were taken for radiocarbon dating and XRD-based mineralogical analysis25,28,29. In this study, simplified lithological features of the drilling cores were compared with the results of the gravity survey, based on a local digital elevation model (DEM) (Fig. 1).
Gravity survey
In total, 1071 gravity data points were collected through surveys conducted in November and December 2022 and April 2023. The gravity anomaly is displayed in Fig. 1. To improve spatial resolution, 387 data points were selected and supplemented with data from Choi et al. 58. Both the Bouguer anomaly and the isostatic anomaly (IA) were computed to reflect subsurface density distribution. The IA provides more detailed information regarding shallow layers through the removal of the gravitational effect caused by deep Moho topography. Based on 1458 data points, the IA ranged from 25.19 to 36.17 mGal, with an average of 31.79 mGal. Gravity reduction processes, including free-air, Bouguer, terrain, and isostatic corrections, are described in Lim et al. 59 and Choi et al. 58. In this study, a local DEM with improved resolution (1 arc-second) was utilised. As shown in Fig. 1, the computed IA clearly delineates the circular shape of the basin formed by the meteorite impact and corresponds to the thickness of the sedimentary layer (Fig. 1B), which is consistent with the drilling data (Fig. 1C, D). When the gravity anomaly was assessed in relation to surrounding topography, the anomaly within the crater was significantly lower than that of areas with similar elevations in the vicinity (Fig. 1C). The gravity anomaly and drilling data, which indicate a northeast-tilted structure consistent with the topography, imply that the meteorite impacted from a northeast to southwest direction.
Raman microspectroscopy
To determine the spatial characteristics of the stromatolite, Raman spectroscopy was performed on a thin section of the stromatolite (20STR03) at the Korea Institute of Geoscience and Mineral Resources (KIGAM), using a Renishaw InVia Qontor system (excitation wavelength of 532 nm). The Raman spectra were obtained with an acquisition time of 1 s and an acquisition number of 60. The setup consisted of a 3-s exposure time to ensure optimal mapping results. The step size for both 2D mappings was 2–4 μm. The Renishaw InVia Qontor system was operated using Wire 5.5 software. The calibration of the spectrometer was verified using a silicon wafer before each session.
EPMA
For quantitative analysis, thin sections of the stromatolites (20STR03 and 21MSG02) were carbon-coated to a thickness of 25 nm and examined using a field-emission electron probe microanalyser (JXA iHP-200F, Jeol LTD.) at the KIGAM. The EPMA analytical conditions were a working distance (WD) of 11 mm, an accelerating voltage of 15 kV, a probe current of 20 nA, a spot size of 0.6 − 7 μm, and a dwell time of 50 ms. The EPMA mapping was performed under an accelerating voltage of 15 kV, a probe current of 20 nA, a spot size of 0.6–7 μm, and a dwell time of 50 ms.
Radiocarbon dating of stromatolites
Eight stromatolites were prepared for radiocarbon dating (Fig. 2 and Supplementary Fig. 8). We applied a different approach to test possible contamination during the subsampling of the stromatolites. Stromatolites 20STR01 and 20STR02 were crushed and ground as a pre-treatment for radiocarbon dating (Fig. 2A–C). Stromatolite 20STR03 was slabbed and polished, then examined for potential contaminants, including extraneous post-depositional material from open cavities and other spaces, as well as diagenetic overgrowths. The material was then drilled using a micro-mill from the slab surface, and the resulting powder was collected for dating. The surfaces of stromatolites 20STR04, 20STR05, 21MSG01, 21MSG02 and 21STR01 were polished before drilling to obtain powdered subsamples. Especially the stromatolites 20STR03 and 20STR04 were sampled along the growth lines to test any old carbon effect.
To minimise the potential reservoir effect on inorganic carbon, which could result from the incorporation of older carbon through recrystallisation or leaching, organic matter derived from microbes was selected for dating. This organic matter was identified in thin sections using Raman spectrum (Fig. 3 and Supplementary Fig. 5). Approximately 2 g of powdered subsamples from the eight stromatolites were dissolved in 1 M HCl at room temperature for 1 day to isolate the organic fraction. Radiocarbon dating was performed on the recovered organic matter samples (n = 17) using the AMS facility at KIGAM. The 14C ages (i.e. conventional radiocarbon dates) were converted into calibrated ages (cal yr BP) using OxCal 4.4 software60,61 and the IntCal20 calibration curve62. The dating results are presented in Figs. 2 and 5 and Supplementary Table 1.
Radiocarbon dating of IBs in the sedimentary cores
To estimate the depositional ages of IBs in core 20CR05, radiocarbon dating was conducted using organic fragment components (<500 µm). IB samples were sieved into 10–100-µm and 100–500-µm fractions, then subjected to a series of chemical treatments (HCl, HF and NaOH). After graphitisation of the residues (mainly charcoal and other plant tissues), radiocarbon dating was performed using the AMS facility at KIGAM63,64. The 14C ages (i.e. conventional radiocarbon dates) were converted into calibrated ages (cal yr BP) using OxCal 4.4 software60,61 and the IntCal20 calibration curve62. The dating results are presented in Fig. 6 and Supplementary Table 2.
Osmium isotope and elemental analysis
To determine the potential relationship between stromatolites and the impact event, subsamples were collected from various environments within the crater for analysis of 187Os/188Os ratios. For example, surficial IB deposits were sampled from the low-lying area in the western part of the crater at the Gaggok site (Fig. 1 and Supplementary Figs. 8 and 9). The IB deposits primarily consisted of angular rock clasts ranging in size from a few millimetres to more than a few metres; they were distinctly different from the normal Cretaceous sedimentary bedrock found within the crater and its surrounding areas (Supplementary Fig. 8F–G). The polymictic breccias exposed at a construction site indicated that they were predominantly derived from target bedrock; fragments were intermixed with grey to greenish-grey sandstone and shale, dark grey to black shale, sandy shale, grey mudstone, and conglomerate (Supplementary Figs. 8 and 9). Additional IB deposits (IB8, IB9, and IB10) were sampled from near-summit areas of the eastern crater (Supplementary Fig. 10). Past lake shoreline sediments, characterised by coarsening-upward deposits with distinct layering, were identified in the downflow area adjacent to the IB deposit sampling site (Supplementary Fig. 11). SDs in the northwest margin of the crater, consisting of silty to pebble-sized sediments, were also sampled (Supplementary Fig. 12). The presence of meteoritic components was assessed in stromatolite samples (STR03 and MSG02) and a sedimentary core (20CR05) by comparing them with IBs and PL sediments.
The Os concentration and 187Os/188Os ratios in the stromatolites, lake sediments, and IBs from the crater were measured by the ALS Laboratory Group, ALS Scandinavia AB. Dry subsamples (2–5 g) were prepared and transferred to the laboratory for analysis. Os analysis was conducted using a high-pressure ashing digestion method, analogous to the Carius tube technique. This approach utilises an ID 192Os isotopic spike for Os quantification. Measurements were performed using either inductively coupled plasma-sector field mass spectrometry or inductively coupled plasma multicollector mass spectrometry, depending on the Os concentration in the sample. The optimisation of inductively coupled plasma-sector field mass spectrometry instrumental performance for Os isotope ratio measurements using an online sample digestion/OsO4 distillation technique has been described in previous studies65,66.
The REE contents of the stromatolites (20STR03 and 21MSG02) were determined using ICP-SFMS by the ALS Laboratory Group, ALS Scandinavia AB, following their internal standardisation protocols (see Rodushkin et al.67 for details of the sample preparation and instrumental analysis).
XRF core scanning of lake sediments (20CR05)
To assess past environmental changes in the impact crater, the elemental composition of sedimentary core 20CR05 was semi-quantitatively determined using an XRF core scanner (Avaatech B.V., Alkmaar, Netherlands) at KIGAM. This device provides non-destructive and near-continuous extraction of semi-quantitative elemental concentrations from sediment cores. XRF data were collected every 4 mm directly from the split-core surface of the archived half (downcore slit size: 1.2 cm) using generator settings of 10 and 30 kV, currents of 0.1 and 0.8 mA, and a sampling time of 10 s. Peak intensities of selected elements (Si, K, Ca, Sr and S) were expressed as counts per second (cps).
Microbial community analysis in the sedimentary core (20CR05)
Lake sediment samples (n = 15) from core 20CR05, weighing approximately 6–8 g each, were collected under sterile conditions and placed in sterile tubes for DNA extraction. DNA was extracted using a high-salt cetrimonium bromide method. Amplification of the V4 region of the 16S rRNA gene was performed using primers 515F and 806R68. For microbial community analysis, 15 samples were processed, of which nine did not amplify successfully. DNA libraries were constructed using the Herculase II Fusion DNA Polymerase Nextera XT Index Kit V2 and sequenced on the Illumina MiSeq platform at Macrogen. Raw sequencing reads were quality-checked and assembled using FLASH69. Representative OTUs were selected using the de novo CD-HIT-OTU picking method in QIIME270. Classification of representative OTUs was confirmed using BLASTn against the National Centre for Biotechnology Information (NCBI) 16S database. Raw sequencing reads were deposited in the NCBI Sequence Read Archive (SRR21116869–SRR21116864).
Data availability
Datasets used in this study are available at Mendeley Data (https://doi.org/10.17632/pzspd8rmxs.2).
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Acknowledgements
This research was supported by the Basic Research Project (GP2025-027, GP2025-042) of the KIGAM, funded by the Ministry of Science and ICT of Korea.
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Lim, J., Kim, Y., Park, S. et al. Discovery of stromatolite formation in post-impact hydrothermal lacustrine environments and its implications for early Earth. Commun Earth Environ 7, 334 (2026). https://doi.org/10.1038/s43247-026-03206-7
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DOI: https://doi.org/10.1038/s43247-026-03206-7