J Paleolimnol (2008) 40:1185–1192 DOI 10.1007/s10933-008-9219-1
COMMENT
Seismic evidence for the Pleistocene depositional changes in Lake Hovsgol, Mongolia, and implications for the age model and the sediment grain size record of KDP-01 drill core Alexander A. Prokopenko Æ Christopher St. G. C. Kendall
Received: 17 December 2007 / Accepted: 25 April 2008 / Published online: 21 May 2008 Ó Springer Science+Business Media B.V. 2008
Electronic supplementary material The online version of this article (doi:10.1007/s10933-008-9219-1) contains supplementary material, which is available to authorized users.
Therefore, the drilled sediment section cannot represent continuous sediment accumulation and the Brunhes age model across the unconformity cannot be nearly linear; the time interval representing a hiatus remains to be determined. The assumed nearly linear age/depth relationship in the upper 23 m above the angular unconformity is also an unlikely relationship, given the evidence of repeated changes in lake level, and hence in the depositional setting and sedimentation rates. We further propose a qualitative reference model for changes in the Lake Hovsgol depositional setting (presented as a step-by-step animation – see supplementary material) based on manually ‘backstripping and rebuilding’ the seismic pattern. We argue that this model provides a useful template of the likely sediment facies changes in the deep axial part of the Hovsgol basin: our crude model in fact captures the major depositional trends in the KDP-01 drill core section located some 10 km NW along the seismic line. We contend that changes in the depositional setting provide the first-order control on sediment grain size in the Hovsgol record. Our study provides important new constraints on the nature of sedimentary proxy records in Lake Hovsgol and on their interpretation as a record of Mongolian glaciation history.
A. A. Prokopenko (&) C. St. G. C. Kendall Department of Geological Sciences, University of South Carolina, Columbia, SC 29208, USA e-mail:
[email protected]
Keywords Central Asia Mongolia Lake Hovsgol Paleoclimate Paleo-lake Glaciations Pleistocene Lake Baikal
Abstract This paper seeks to arrive at a consistent interpretation of (1) the age model, (2) the grain size record, and (3) seismic reflection data from Lake Hovsgol (a.k.a Khubsugul or Ho¨vsgo¨l), Mongolia, reported by Fedotov et al. (2007, earlier by Fedotov et al. 2002, 2004). In their most recent contribution, the grain size record of the KDP-01 drill core is interpreted as a climatic signal while little consideration is given to lake-level changes and hence to basin-wide changes in depositional setting evident from seismic profiles; also, a nearly linear age model is at odds with the seismic evidence for a major angular unconformity in the sediment strata. The lack of regional seismic stratigraphic analysis has thus led to an improbable interpretation of the Lake Hovsgol sediment grain size record and ultimately to an improbable scenario of Mongolian glaciation history. Using the available seismic profiles, here we show that the drill core penetrated several transgressive/regressive sedimentary sequences and a major angular unconformity.
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Reconstructing history of Mongolian glaciation from lacustrine grain size records
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The Pleistocene glaciation history of continental Asia remains a hotly debated subject that is fueled by the scarcity of robust physical evidence. The record of each new section has the potential to significantly improve the understanding of glacial history or, conversely, confuse it by adding contradictory evidence to the overall pool of data. A new 450-ka
record of glaciation in the region is potentially a critical piece of the puzzle. As such, it deserves strict scrutiny. In the online issue of the Journal of Paleolimnology Fedotov et al. (2007) reported their estimates of clay/silt ratios in decalcified bulk sediment of the upper 23-m portion of the 53-m drill core KDP-01 from the deep axial part of Lake Hovsgol (Khubsugul) (Fig. 1a, b). The authors interpreted this ratio as a climate indicator presuming that ‘‘low clay/silt
Fig. 1 The structure of the sedimentary strata of Lake Hovsgol. (a) Bathymetric map (modified from Fedotov et al. 2007). (b) Location of the profile and sampling sites referred to in the current study. (c) The array of Lake Hovsgol 2001 seismic lines; highlighted portions 1–4 mark the published profiles (Fedotov et al. 2002, 2007). (d) SE-NW seismic profile across the deepest portion of Lake Hovsgol basin (modified from Fedotov 2007). (e) Preliminary interpretation of sediment geometries based on tracing individual reflections; present-day water depth is specified and major angular unconformities are highlighted with dashed lines. KDP-01 is located on the profile,
st. 10 and 12 gravity core locations (Prokopenko et al. 2005) are shown based on the reported water depths. Distances between cored sites are approximate because these sites do not exactly lie on the seismic line. Depth of penetration of KDP-01 drill core is estimated based on acoustic velocities (1,430 m/s in water, 1,650 m/s in sediments); it is clear that the drill core penetrated a major angular unconformity at ca. 24–27 m core depth and the age model should not be nearly linear across this boundary. The highlighted portion in panel E (lighter shading) shows the location of a higher-resolution segment analyzed in Fig. 2
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ratios indicate warm climates, while a high clay/silt ratio reflects ... cold climate.’’ From this assumption, they proposed what they believe to be an ‘‘improved reconstruction of glacial history for the last 450 ka’’ in northern Mongolia. The logic path they proposed is simple: (1) clay is assumed to be mainly of glacial origin; (2) clay abundance (recalculated into flux using measured specific gravity and assumed ages) is believed to represent ‘‘meltwater volume’’ and hence glacial ‘‘ice volume’’ in the Hovsgol basin; (3) changes in clay abundance per unit time (based on the accepted age model) are assumed to represent the ‘‘intensity of the deglacial events’’, i.e. the rate of changes in meltwater/ice volumes. This logical path, however, is misleading in that it ignores seismic evidence reported earlier by the same group of authors. Seismic evidence is critical for evaluating both the changes in depositional settings of the lake and the proposed age model. For the sake of a balanced evaluation of the Lake Hovsgol sedimentary record in the context of regional glaciation history, this evidence is reviewed here.
Seismic evidence for changes in depositional setting in Lake Hovsgol Of over 390 km of Lake Hovsgol seismic data collected in 2001, only four small segments were made available recently: profiles 1, 2 were reported by Fedotov et al. (2002) and profiles 3, 4 by Fedotov et al. (2007) (Fig. 1c). Seismic profiles contained evidence for dramatic changes in lake-level: three major sequences bounded by erosional surfaces were distinguished in the upper 23 m interval (Fedotov et al. 2002). Later, units were re-labeled I–IV distinguishing the topmost layer, which is 1–1.5-m thick, as a separate sequence (Fedotov 2007); here we adopt the latter (I–IV) version of sequence enumeration. In any lake or basin, base level changes affect the depositional setting and hence the sediment lithology. Here we use a higher-resolution profile 1 (Fig. 2) to qualitatively reconstruct successive changes in the Lake Hovsgol depositional setting evident from onlap/offlap patterns and erosional truncations. The interpretation of the profiles was made by manually tracing the reflections to enhance the general patterns of the sediment body geometries. We then discuss how depositional changes have likely affected the
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lithologic succession in the upper 24 m section of Lake Hovsgol sediments. Profile 1 is a higherresolution portion of a recently released long diagonal SE-NW profile (Fedotov 2007) (Fig. 1e). Despite low resolution, this long profile reveals the general structure of Lake Hovsgol sediment strata and allows comparing the base-level changes in the Hovsgol basin reconstructed from its SE portion with the grain size record of the drill core located some 10 km NW (Fig. 1e). Even though individual reflections cannot be traced reliably across the basin to the drill site, the reasonable basis for such comparison is (1) the general continuity of sequences in the upper ca. 24-m section across the axial part of the basin; and (2) comparable thickness of the respective units in the area where a higher-resolution fragment of 2001 seismic data is available (Fig. 2) and in the area where the KDP-01 drill site is located. A qualitative reference model for facies/grain size changes in the deep part of Lake Hovsgol (Fig. 2) was constructed here, based on the subsurface stratigraphy and sediment stacking order in the higher-resolution segment 1 of a seismic line (Fig. 1c, e). This model, derived from manually ‘backstripping and rebuilding’ the seismic pattern, was then compared with the grain size record of the upper 23-m sediment section discussed by Fedotov et al. (2007) to test if modeled and actual depositional trends are consistent. No attempts were made to match respective peaks and/or amplitudes between data and model. To help track the depositional changes, a step-by-step animation of the stacking order and base level change was created (See supplementary material). The reference model interpreting seismic patterns in terms of the likely sediment facies/grain size changes downdip was derived by making the following assumptions: (1) the entire sequence was presumed to be subaqueous; (2) given the limited lateral extent of the stacked clinoforms, the geometric relationships of the strata were interpreted to be primarily controlled by changes in base level and only to a lesser extent by sediment input; (3) the past base level was presumed to be constrained by the apparent toplap surfaces. In cases when updip terminations of sediment bodies are eroded, likely changes of base level were inferred from onlap/ offlap relationships downdip; (4) base level was assumed to correspond to the wave base ca. 15 m below lake level, a conservative middle of the 10–25-m range for wave action depths in Lake Baikal (Galazy 1993);
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Fig. 2 Preliminary analysis of subsurface stratigraphy and the overall stratal stacking pattern based on the higher-resolution SE portion of diagonal seismic line from Fedotov et al. (2002). Bottom panel summarizes likely changes in facies/grain size in the deep part of the Lake Hovsgol basin at the NW termination of the profile, as suggested by geometric relationships of sediment bodies updip (see text). For step-by-step animation of stacking order and base level change refer to animation (See supplementary material). Shading marks strata of uniform thickness which form the distinct stillstand sequence within unit IV; this geometry is suggestive of basin-wide deposition of abundant suspended material
(5) forced regressions were expected to cause the deposition of progressively coarser sediment facies in the deeper parts of the basin; (6) erosional truncation updip was interpreted to produce a ‘spike’ of coarser grain size downdip; (7) transgressions were interpreted as trends towards finer facies/grain size; (8) major transgressions were assumed to produce basin-wide drapes of fine sediment and were believed to have resulted in a rapid decrease in the average grain size. From the above basic assumptions the following reference model for facies/grain size change in the upper 24-m section of the Lake Hovsgol sediments
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was derived (Fig. 2, also, refer to animation (See supplementary material)): (1)
(2)
At the base of the section, the initial fill above the major angular unconformity was interpreted to consist of relatively coarse sediments that were directly affected by wave action; two coarse intervals are predicted at ca. 23 m and 20 m correlative with erosional truncations updip. The sediment of a relatively uniform composition and grain size was interpreted to correspond
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to the interval ca. 19–16 m, which accumulated during a stillstand phase (shading in Fig. 2). The geometry of the shaded sequence indicates that abundant suspended material settled basinwide in layers of uniform thickness. When compared with the overlying stacked clinoforms, this sediment accumulation pattern is unusual; this pattern is consistent with a scenario of sediment deposition in a glacially-fed lake. The ca. 16–15 m interval corresponds to a series of small clinoforms of limited lateral extent, which are interpreted to be a product of reduced sediment input. In the deep part of Lake Hovsgol, this interval was likely characterized by slowed sedimentation rates and was interpreted to consist of finer sediments. Above, a relatively uniform fine hemipelagic sediment was expected in the interval 15–12 m that corresponded to the onlapping generally transgressive sequences. In the interval ca. 12–5 m the sediment is likely coarser than in the underlying transgressive sequence. The basal layer at ca. 12 m was interpreted to be particularly coarse, since it is correlative with a major erosional truncation updip (this truncation was recognized as a boundary between units IV and III by Fedotov (2007). The likely general trend in sediment grain size over this 6-m interval is to become finer up the section. However, two coarser intervals are predicted at ca. 8 m and ca. 5 m, which are both correlative with erosional boundaries updip within unit II (Fig. 2). The sediment of the uppermost 4–5 m interval corresponds to a draped transgressive sequence (unit I of Fedotov et al. 2002, units I + II of Fedotov 2007). This likely makes these strata the finest sediment within the 23-m section, since it accumulated during the most prominent transgressive episode. It is important to note that the present-day status of the hydrologicallyoverfilled Hovsgol basin is a fairly recent phenomenon, given the accumulation history reconstructed in our model.
ca. -100 m (Prokopenko et al. 2003, 2005). The seismic profile shows that this sediment has limited lateral extent, and is unlikely to have strongly affected the lithology of the hemipelagic section. Consistent with this interpretation are abundant sandy laminae and lenses observed at 93–138 cm in the gravity core from st. 10 (Fig. 1); such laminae were not observed at a deeper site of st. 12 gravity core (Prokopenko et al. 2005). The most recent Holocene highstand is reflected by a basin-wide draping of a thin 20–80-cm layer of diatomaceous mud (Prokopenko et al. 2005) that is practically invisible in the seismic profile (Fedotov et al. 2002). As seen in Fig. 2, our independent reference model for trends in facies/grain size change within the upper 24-m sequence of Lake Hovsgol sediments appears to capture the general depositional trends seen in the actual mean grain size and specific gravity records of the KDP-01 drill core discussed by Fedotov et al. (2007). Denser and coarser sediments observed at the bottom and in the middle of the section, as well as spikes in mean grain size at ca. 12 m and 9 m can actually be predicted from seismics alone (Fig. 2, bottom). Further detailed comparison may even be warranted. For instance, the interval of slowed sedimentation rates/non-deposition and hence finer sediments atop the stillstand sequence may in fact correspond to a distinct grain size and specific gravity minima at ca. 17 m in KDP01 (arrows, Fig. 2). Overall, our exercise (refer to animation (See supplementary material)) illustrates that although limited, seismic patterns provide compelling evidence of a primary control of depositional setting over silt/clay grain size variations and lithologic succession of the KDP-01 section. Input of glacial clay may have modified this overall succession, yet it was unlikely the key factor affecting sediment composition.
The uneven-shaped body of sediment on slope (refer to animation (See supplementary material)) may be interpreted to have accumulated during the documented LGM lowstand of Lake Hovsgol at
Seismic evidence reviewed above indicates that dramatic changes in the depositional setting of Lake Hovsgol were associated with lake-level fluctuations during the time period represented by the upper 23-m
(3)
(4)
(5)
Discussion Clay/silt ratio in Lake Hovsgol sediments as a proxy
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portion of the KDP-01 drill core. References to extreme lowstands are found in the text by Fedotov et al. (2007), yet no connection with the grain size record was attempted by the cited authors, even though several episodes of major erosion and redeposition of older lake sediments must have left imprints on density and grain size records. The interpretation of the clay/silt ratio in sediments as a climatic index suggested by Fedotov et al. (2007) is most certainly incorrect under such circumstances, and so is the proposed ‘‘glaciation history’’ with ice/ meltwater volumes estimated from clay/silt ratios. Fedotov et al. (2007) further argued that the 30–50 km distance of transport would be ‘‘too far for the silt fraction to travel in suspension’’ from the glaciated northern part of the Lake Hovsgol catchment and hence only the clay fraction had ‘‘the capacity to reach the KDP-01 drill site’’. The distance between stacked clinoforms constraining the proximity of the shoreline and the drill site is on the order of 10 km (Fig. 1). This proximal depositional setting thereby provides a direct, alternative, nearby source of silt to the study site (refer to animation (See supplementary material)). In addition, the argument of the authors that the ‘distance’ is limiting the supply of silt to the study site is at odds with (a) the mean particle size above 30 lm reported by Fedotov et al. (2007) in Fig. 4(a) and (b) with clay/silt ratios of 0.4–1.6 in Fig. 5. Each of these estimates suggests that silt is a major component of Lake Hovsgol sediments throughout the studied interval. A cautionary note regarding the use of clay and silt reported by the cited authors pertains to the fact that in their Fig. 5, silt fraction (1.5–5 vol%) and clay fraction (1–4.5 vol%) do not add up to even 10% of the sediment volume. There is an apparent error in these numbers since the lithology in their Fig. 3 suggests that the bulk of the sediment is lake mud (i.e., silt and clay). Age model for the Lake Hovsgol sediment strata The only absolute age determinations in the Hovsgol sediment section so far are radiocarbon dates of the uppermost layer across the basin dating back to the last glacial (e.g., Prokopenko et al. 2005 and references therein). Paleomagnetic reversals at ca. 40 m and 53 m in the KDP-01 drill core were reasonably interpreted by Fedotov et al. (2007) as Brunhes/
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Matuyama and Matuyama/top Jaramillo reversals. In between the last glacial and Brunhes/Matuyama reversal, however, the age model is not as straightforward as presented by the cited authors. A nearly linear continuous age model throughout the Brunhes chron was repeatedly suggested by Fedotov et al. (2004; also, 2007, Fig. 2a) and is most certainly incorrect given an angular unconformity between units IV and V (Fig. 1e). The time interval represented by this hiatus remains to be determined. Low-amplitude inclination spikes do not provide absolute age control because matching inclination signals with global geomagnetic excursions is merely an interpretation (Table 1 and Fig. 2 in Fedotov et al. 2007). Such interpretation is always contingent on understanding sediment deposition processes. The nearly linear age model of Fedotov et al. (2007) cannot be accepted as a template for the 450-ka history of Mongolian glaciation for two main reasons. First, even in the upper 23 m above the angular unconformity, linearity is the least likely age/depth relationship given the major changes in the architecture of sedimentary strata in the Hovsgol basin and hence in sedimentation rates (Fig. 2, also, refer to animation see supplementary material). Second, there is a clear mismatch between the total number, relative amplitudes and the assigned names of the ‘excursions’ in KDP-01 and in Lake Baikal records (Oda et al. 2002 and references therein): excursions identified in Lake Baikal records were not represented in KDP-01 and vice versa. In addition, none of the reported 23-m records of Fedotov et al. (2007) really helps recognizing four glacial or four interglacial intervals from some repetitive quasi-cyclic proxy response. This is another indirect indication that their nearly linear age model is unlikely correct. Given the lack of a cyclic pattern in the behavior of ‘proxies’ reported by Fedotov et al. (2007) from Lake Hovsgol, it is most likely that the parameters they measured reflect facies changes, which are related to lake-level variations and in part to redeposition of older sediments. If so, their drill core grain size record was largely controlled by local depositional changes and not by the ice/meltwater volume of Mongolian glaciation. This notion is disregarded in the work by Fedotov et al. (2007) who instead attribute changes in clay abundance to some combination of the three: (1) ‘‘specific conditions in regional distribution of moisture during glaciation’’;
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(2) ‘‘glacial ice volumes’’, and (3) ‘‘solar insolation intensity’’. Matching the lithologic record with geologic structures Matching clay maxima with the largest terminal moraines should have been attempted by Fedotov et al. (2007) as an exercise to test their conceptual model and/or the age model. For instance, massive terminal moraines indicative of extensive glaciation are assigned the MIS 6 age by the cited authors in Fig. 6. Accordingly, MIS 6 must be the time of maximum relative clay abundance. Instead of the expected maximum, however, the authors reported a minimum of clay/silt ratio during MIS 6 (Fig. 5). Thus, their proxy interpretation is clearly at odds with their age model. The maximum clay/silt ratio associated with elevated water content and minimum specific gravity of sediment was reported by Fedotov et al. (2007) between ca. 17 and 19 m KDP-01 core depth, (one would need to convert age scale in their Fig. 5 to the depth scale in their Fig. 4). This is therefore the most likely maximum glaciation interval from the standpoint of the conceptual model of Fedotov et al. (2007). A noteworthy observation is that this interval corresponds to a distinct uniform stillstand sequence (Fig. 2, shaded). In case maximum glaciation did occur during MIS 6, the upper 23-m section may represent not the past 450 ka of sediment deposition but about half as much. Additional stratigraphic data and age determinations are necessary to estimate the basal age of sequence IV above the angular unconformity.
Conclusions Successive changes in the depositional settings of Lake Hovsgol and lithofacies in the central part of the basin can be reconstructed from sediment body geometries visible in seismic profiles. Qualitative reconstruction based on a single available highresolution profile within ca. 10 km from the KDP-01 drill site captures the general depositional trends in the reported grain size record of the drill core. Input of glacial clay is unlikely the primary control on lithology and hence the use of clay/silt ratios primarily as a climatic proxy suggested by Fedotov
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et al. (2007) is at odds with their own seismic data. Nearly linear age models of sediment accumulation also contradict the depositional history evident from the seismic data. The Lake Hovsgol story presented by Fedotov et al. (2007) is incomplete without a consideration of changes in depositional setting. This incompleteness results in a misleading interpretation of (a) clay/silt ratios, (b) the age model and ultimately of the regional glacial history. More work is needed to decipher the history of Pleistocene glaciations in continental Eurasia and to separate global and hemispheric climate signals from the local lacustrine depositional signal in the Hovsgol basin. A more productive approach would involve seismic stratigraphic analysis instead of relying on the simplistic assumption of sediment properties as climate indices and on selective use of primary evidence. Acknowledgments The study of Lake Hovsgol sediment cores was supported by NSF awards ATM-0402341 and ATM0402351. We thank M. Brenner and an anonymous reviewer for guidance and suggestions that helped improving the manuscript.
References Fedotov AP, De Batist M, Shapron E, De Ryker K, Pauls T (2002) Seismic profiling of the sediments of Lake Khubsugul. Doklady Earth Sci 382:261–263 (in Russian) Fedotov AP, Kazansky AYu, Tomurhuu D, Ziborova GA, Zheleznyakova TO, Vorobyova SS, Phedorin MA, Goldberg EL, Oyunchimeg TS, Narantsetseg TS, Vologina EG, Yuldashev AA, Kalugin IA, Tomurtogoo O, Grachev MA (2004) A 1 My record of paleoclimates from lake Khubsugul, Mongolia. Eos Trans 85:387–390 Fedotov AP (2007) Structure and composition of sediment strata of the Khubsugul basin as a record of tectonicclimatic evolution of Northern Mongolia in Late Cenozoic. Doctor of Sciences Thesis, Kazan State University, 42 pp (in Russian) Fedotov AP, Phedorin MA, De Batist M, Ziborova GA, Kazansky AYu, Semenov MYu, Matasova GG, Khabuev AV, Kugakolov SA, Rodyakin SV, Krapivina SM, Pouls T (2007) A 450-ka long record of glaciation in Northern Mongolia based on studies at Lake Khubsugul: high-resolution reflection seismic data and grain-size variations in cored sediments. J Paleolim. doi:10.1007/s10933-0079107-0 Galazy GI (ed) (1993) Atlas of Lake Baikal. Federal Service of Geodesy & Cartography, Moscow, 160 pp (in Russian) Oda H, Nakamura K, Ikehara K, Nakano T, Nishimura M, Khlystov O (2002) Paleomagnetic record from Academician Ridge, Lake Baikal: a reversal excursion at the base of marine oxygen isotope stage 6. Earth Planet Sci Lett 202:117–132
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1192 Prokopenko AA, Kuzmin MI, Kalmychkov GV, Gelety VF, Gvozdkov AN, Solotchin PA (2003) Changes in the composition of bottom sediments of Lake Hovsgol as an indication of climatic change in the Baikal region 15–14 ka BP. Doklady Earth Sci 390:1–4 (in Russian)
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J Paleolimnol (2008) 40:1185–1192 Prokopenko AA, Kuzmin MI, Williams DF, Gelety VF, Kalmychkov GV, Gvozdkov AN, Solotchin PA (2005) Basin-wide sedimentation changes and deglacial lakelevel rise in the Hovsgol basin, NW Mongolia. Quater Int 136:59–69
