Unique Quaternary Environment for Discoveries of Woolly Rhinoceroses in Starunia, Fore-Carpathian Region, Ukraine: Geochemical and Geoelectric Studies

Unique Quaternary environment for discoveries of woolly rhinoceroses in Starunia, fore-Carpathian region, Ukraine: Geochemical and geoelectric studies

Maciej J. Kotarba* Marek Dzieniewicz AGH-University of Science and Technology, Faculty of Geology, Geophysics and Włodzimierz J. Mo´scicki Environmental Protection, Mickiewicza Av. 30, 30-059 Kraków, Poland Henryk Sechman

ABSTRACT purpose of our geochemical and geoelectric studies was to investigate the In 1907, remnants of a mammoth and a woolly rhinoceros were geologic setting and environment of the Pleistocene sediments, as well as to discovered in the Pleistocene clays of an earth-wax mine in Starunia assess the possibility of new discoveries of large, extinct mammals. village. Then, in 1929, a nearly fully preserved woolly rhinoceros was found in the same mine. The unique combination of clays, oil, and FIELD AND ANALYTICAL PROCEDURES brine into which the animals had sunk is responsible for their almost For a near-surface geochemical survey, the patented sampling pro- perfect preservation. During the late Pleistocene winters, when the cedure of Sechman and Dzieniewicz (2007) was used. The sampling ice and snow cover was present in the tundra “paleoswamp,” areas of depth was ~1.2 m. Natural gas samples from wells and surface seeps were infl ow of brines, oils, and hydrocarbon gases had a higher tempera- placed into 500 mL glass vessels fi lled with saturated NaCl solution. The ture, which resulted in melting and cracking of the cover, and large molecular composition of gases was analyzed in a set of columns on gas mammals could be trapped. Geoelectric measurements, as well as chromatographs equipped with fl ame ionization (FID) and thermal con- molecular and stable isotope analyses of gases in the near-surface zone ductivity (TCD) detectors. Stable carbon isotope analyses were performed within the “paleoswamp” performed in 2004–2005, reveal a few places using a mass spectrometer, and results are presented in the δ notation favorable to the burial and preservation of Pleistocene vertebrates. relative to the Peedee belemnite (PDB) standard. The geoelectric method applied was DC resistivity sounding. A four-electrode Schlumberger array Keywords: woolly rhinoceros, ozokerite, Pleistocene, surface geochemi- with the largest spacing, AB/2 = 100 m, was used (Mościcki, 2005). cal survey, DC resistivity, stable carbon isotopes, Ukraine. GEOCHEMICAL STUDY INTRODUCTION In the Starunia area (Fig. 1), gas samples were collected from fi ve The discoveries of large Pleistocene mammals at the Starunia ozokerite wells in Oligocene and Eocene reservoirs and from two oil seeps. The (earth-wax) mine, situated in the fore-Carpathian region in Ukraine (Fig. 1), molecular composition, gas indices, and stable isotope ratios vary within were spectacular scientifi c events on a worldwide scale (e.g., Alexan drowicz, the following ranges (Kotarba et al., 2005a): CH4 85.6%–95.9%, C2H6

2005). In 1907, a partially preserved mammoth was found at a depth of 1.76%–4.78%, C3H8 0.41%–2.20%, N2 0.92%–2.40%, CO2 0.01%–3.49%,

12.5 m, and a woolly rhinoceros at a depth of 17.6 m. In 1929, Polish sci- hydrocarbon CHC index [CHC = CH4/(C2H6 + C3H8)] from 12 to 44, car- entists discovered a unique, nearly complete woolly rhinoceros carcass at a bon dioxide–methane (CDMI) index [CDMI = CO2/(CO2 + CH4)100(%)] δ13 − δ depth of 12.5 m. Radiocarbon dating determined two probable ranges for from 0.09% to 3.92%, C(CH4) from –46.6‰ to 27.1‰, D(CH4) from − − δ13 − − δ13 the age of the remnants of these mammals: one around 42 ka, and another 187‰ to 150‰, C(C2H6) from 33.1‰ to 26.5‰, and C(CO2) around 26 ka (Kuc et al., 2005). The woolly rhinoceros excavated in 1929 from −15.6‰ to +20.0‰. Moreover, slight concentrations of helium, up is displayed at the Natural History Museum in Kraków (Fig. 1). The fossil to 0.011% (N-1 well, Fig. 1) as well as hydrogen (0.25%) in the “upper fl ora and fauna excavated in Starunia in 1907 and 1929 were subjected to oil eye” are present. various scientifi c investigations for identifi cation, determination of ecology, Subsurface hydrocarbon gases accumulated in Oligocene and Eocene age, and other purposes (Alexandrowicz, 2005, and references therein). In reservoirs as well as gases connected with two oil seeps in the Starunia area 2004, Polish and Ukrainian scientists restarted studies in the Starunia area. were generated from type II kerogen during low-temperature thermogenic Quaternary deposits of the Velyky Lukavets River valley in Starunia processes (Figs. 2A and 2C). The thermogenic hydrocarbons migrated are developed as clays containing plant remains and large Pleistocene together with helium (Fig. 3A) to the surface along the “Rinne” fault zone mammals, as well as gravels, sands, and peats. The top of the Lower Mio- directly from deep accumulations (Kotarba et al., 2005a). Carbon dioxide cene Vorotyshcha Salt-bearing Formation of the Boryslav–Pokuttya Unit, is also of thermogenic origin, with the exception of gas from the “upper oil which underlies the Quaternary deposits, occurs at depths from 8 to 20 m. eye,” which was generated during microbial processes (Fig. 2B). The ozokerite deposit occurs in a Miocene molasse sequence. The near-surface geochemical studies were run along three measure- Pleistocene clays were saturated with brines that migrated from the ment lines (Fig. 1, lines P-0, P-2, and P-3). The analyzed gases are more underlying Lower Miocene Vorotyshcha beds rich in halite and sylvite, and variable in their molecular and isotopic compositions than gases from oil from Oligocene and Eocene strata of the Boryslav-Pokuttya Unit (Koltun deep accumulations and seeps (Kotarba et al., 2005a, 2005b). Concentra- et al., 2005; Kotarba et al., 2005a). Small oil seeps and ponds, mud vol canoes, tions of components of near-surface gases for 111 samples are presented in and saltwater springs are present in Starunia and nearby. The almost per- Table 1. Gas indices and stable carbon isotope ratios for 24 selected sam- fect preservation of skin and some soft tissues (e.g., tongue, trachea, palate; ples vary within the following ranges: hydrocarbon (CHC) index 12–4543, δ13 Kubiak, 1969) as well as collagen in bones (Kuc et al., 2005) of the woolly carbon dioxide methane (CDMI) index from 2.3%–100%, C(CH4) from − − δ13 − − δ13 rhinoceros discovered in 1929 was a result of natural processes, owing to 70.9‰ to 32.4‰, C(C2H6) from 29.9‰ to 25.4‰, and C(CO2) the brine- and oil saturation of the “spongy” microporous body structures. from −26.3‰ to +1.9‰. These ranges indicate that genetic history and Unfortunately, the DNA of the rhinoceros has not been investigated yet. The gas migration to the near-surface zone were complicated. The highest concentrations of analyzed gases occur in the central and southeastern *E-mail: [email protected]. parts of the study area, whereas in the northwestern and western parts,

© 2008 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, July July 2008; 2008 v. 36; no. 7; p. 567–570; doi: 10.1130/G24654A.1; 4 ﬁ gures; 1 table. 567 0 250 km Estonia Russia Oil and gas sample 1 METER Latvia Gas sample DC resistivity survey line Sweden Lithuania Oil sample E-2 BielarusBelarus Oil and gas seep: 00 4

Germany Poland OE-U “Upper oil eye” P-2 Ukraine OE-L “Lower oil eye” Czech

Republic 420 Slovakia Overthrust Austria HungarySlovenia Romania Study area OE-U

′″ 24° 25′ E 24o 30’ 24° 35′

48 41 20 MONASTYRCHANY 00 4 410 a

′ k ns STARUNIA tvy Solo Bystrytsya ° SAMBIR OE-L

48 42 N GV-200 UNIT am Place of discovery tre MARKOVA GV-B eS of woolly rhinoceroses inn a 410 R and mammoth k Mo-4 P-3 v a y E-3 n a M Ma-10 BORYSLAV- GVIZD POKUTTYA

Lukavets

lyky N Ve Ve UNIT P-0 N - 1 lyky S K ′″ Luk ′ IB A avet ka 420 U ° ° nyans sRive NIT BYTKIV vir 1

48 41 10 48 38 Nad NADVIRNA E- 0 24km Near-surface geochemical r PNIV 0 100 200 m measurement line Bystrytsya 24°29′″ 10 E 24° 29′″ 30 Figure 1. Location map of study area. Photo of unique woolly rhinoceros in Natural History Museum in Kraków by W.J. Mo´scicki.

2.0 A 20 B “Upper oil eye” C 1.8 1.6 8 3 –25 1.4 10 ) MIXING 1.2 MICROBIAL NII 10

METHANE (‰ ROGE II 2 MICROBIAL 1.0 2 KE THERMOGENIC H (‰) ) N MICROBIALGASES 26 –30 METHANE 10 0 GASES E r G MIXING III 0.8 R(%)

C (C C (CO ) “Lower oil eye” Gases:

13 13 –10 Near-surface 0.6 KERO

δ 1 MIGRATION δ C4263 10 –35 H KEROGEN NG Surface seep OXIDAT I

C CH = /(C H +C H ) THERMOGENIC IO MIX Deep accumulation N GASES –20 MIGRATION –70 –60 –50 –40 –30 –70 –60 –50 –40 –30 –70 –60 –50 –40 –30 δ13 δ13 δ13 C(CH4 ) (‰) C(CH4 ) (‰) C(CH4 ) (‰) δ13 Figure 2. Genetic characterization of natural subsurface and near-surface gases using (A) hydrocarbon index (CHC), (B) C(CO2), and δ13 δ13 (C) C(C2H6) versus C(CH4). Compositional fi elds in A are after Whiticar (1994), and compositional fi elds in B are after Kotarba and Rice (2001) modifi ed by Kotarba. Positions of vitrinite refl ectance curves for type II kerogen in C are after Berner and Faber (1996) and have been shifted based on average of δ13C values of Oligocene Menilite Formation type II kerogen (25.2‰) after Kotarba et al. (2005a). they decrease (Figs. 3A–3D). Eight anomalous methane concentrations High variability in the concentrations of gas components and mul- over 10% were found in the area. In the case of the sum of C2-C5 alkane tiple origins of individual gaseous components in the near-surface zone concentrations, the distribution of anomalous zones with concentrations provide insight into the lithological diversifi cation of Quaternary sedi- over 1% is similar. Sites of anomalous carbon dioxide concentrations over ments. These data suggest that the most favorable conditions for pres- 5% are generally in the same places. These relationships are most clearly ervation of large Pleistocene mammals and even for people who were visible on measurement line P-2 (Figs. 4A–4C). High helium concentra- living at that time and hunting for them (Matskevyj, 2005) occur at locations on the P-2 and P-3 lines were grouped near the “Rinne” fault zone tions associated with the fl ux of thermogenic gases to the near-surface (Figs. 3 and 4). Stable carbon isotope analyses of methane and ethane of zone from deep accumulations. These places are connected mainly with near-surface gases (Fig. 2) indicate that ethane and higher gaseous hydro- fault zones, which can also be proved by increased helium concentra- carbons from all sampled sites, and methane from a majority of the sites, tions as well as oil and brine fl uxes. are thermogenic in origin and migrated to the near-surface zone from deep accumulations. However, microbial methane or mixing of microbial and GEOELECTRIC STUDY thermogenic methanes also has occurred (Figs. 2 and 4), as indicated by DC resistivity soundings were grouped in three measurement lines the isotopic composition of the gases and the presence of hydrogen. The E-1, E-2, and E-3 (Fig. 1). The thickness of Quaternary deposits (natural stable carbon isotope composition of carbon dioxide (Fig. 2B) also sug- and/or human-altered), their electrical resistivity, and hydrogeologic con- gests a variety of origins: thermogenic (most frequent), recent microbial ditions in the area were studied. The thickness of the uppermost Quater- processes, and secondary microbial oxidation of hydrocarbons. nary deposits is ~10 m in places where no infl uences of human activity

568 GEOLOGY, July 2008 δ13 Methane concentration (vol%) Total C -C alkanes concentration (0.001 vol%) C(CH4 )(‰ ) He (ppm) 25 < –65 100100 to to 160 160 –65 to –55 5050 to to 100 100 0.001 0.01 0.1 1 10 0.001 0.01 0.1 1 10 100 1000 > –55 10 to 50 10 to 50 B Geochemical sampling point C A E-2

D AULT irt F 0 100 m roa d “RINNE”

′″

41 20

Ve Ve ly ly k k N-1 y N-1 y 3 L L E- u uk Ve k a a v 3 ly v P- e e k ts ts

yL N

N-1 well ″ uk

P-0 a 10 Place v ′ P-2 e of discovery ts E-1 48°41 DC resistivity sounding site 24° 29′″ 10 24° 29′″ 30 E Carbon dioxide concentration (vol%) Apparent resistivity (Ω m) Interpreted resistivity (Ω m)

0.05 0.1 0.25 0.5 1 2.5 5 10 1 1.6 2.5 4 6.5 10 16 25 40 65 100 0.25 0.4 0.65 1 1.6 2.5 4 6.5 10 16 25 D E F

Array spacing AB/2 = 2.15 m 2 m depth

Ve Ve Ve lyky Luka l ly y k k N-1 N-1 y N-1 y L L uk uk vets a a v v e e ts ts

Figure 3. Maps of geochemical indices and sediment resistivity, based on DC resistivity soundings in near-surface zone of Starunia area. A: Location of geochemical proﬁ les, stable carbon isotope composition of methane (lack of color dots in sampling point means no analyses of isotope composition were made) and helium concentration (helium concentration was measured in all sampling points. The lack of yellow

circles means that helium concentration is less than 10 ppm). B: Methane concentration. C: Sum of C2-C5 alkanes concentration. D: Carbon dioxide concentration. E: Apparent resistivity for 2.15 m spacing. F: Interpreted resistivity at 2 m depth. E-1, E-2, and E-3 are DC resistivity survey lines, and P-0, P-2, and P-3 are near-surface geochemical measurement lines.

TABLE 1. SELECTED STATISTICAL PARAMETERS FOR ALKANE, ALKENE, CARBON DIOXIDE, HYDROGEN, AND HELIUM CONCENTRATIONS MEASURED IN NEAR-SURFACE GAS SAMPLES Sum of Statistical Alkanes alkanes

parameters Unit CH4 C2H6 C3H8 iC4H10 nC4H10 iC5H12 nC5H12 C2-C5 CO2 H2 He Minimum values (ppm) 0.6 0 0 0 0 0 0 0 0.03* 0* 0 Maximum values (ppm) 824000.0 32700.0 3810.0 2120.0 2850.0 2210.0 1460.0 44800.0 22.0* 0.032* 110.0 Median values (ppm) 13.4 0.26 0.00 0.06 0 0 0 0.55 0.21* 0* 0 Mean values (ppm) 80300.0 1530.0 149.0 108.0 50.0 72.2 14.1 1930.0 1.89* 0.002* 2.72 Number of samples 217 178 108 114 87 92 26 181 11 7 44 Percentage of samples (%) 100.0 82.0 49.8 52.5 40.1 42.4 12.0 83.4 5.1 3.2 20.3

Note: C2-C5 = C2H6 + C3H8 + C4H10 + C5H12. *Values in vol%.

were noted. A very distinct low-resistivity anomaly (LRA), lower than one-dimensional (1-D) interpretation of the DC soundings—the distribution 0.5 Ωm, was registered in the central part of the area. of the interpreted resistivity at a depth of 2 m—shows a distinct LRA pattern Generally, the electrical resistivity of Quaternary deposits decreases (Fig. 3F). This pattern is probably caused by relatively salty water ﬁ lling the with depth. The top layer has a resistivity of 30 Ωm or higher and a thick- pores of unconsolidated Quaternary sediments at shallow depth. ness of 1–2 m. The next layer is 4–8 m thick and has 10–25 Ωm resistivity. The LRA zone is also readily visible on the proﬁ le data. The appar- Deeper sediments have very low resistivity, below 5 Ωm. The aforemen- ent resistivity pseudosection and results of 1-D quantitative interpretation tioned LRA is readily visible on apparent resistivity contours made for all are set together with geochemical data for survey line P-2 in Figure 4. The sounding spacings used (Mościcki, 2005). An example map corresponding interpreted resistivity within the LRA zone falls below 0.5 Ωm. The NE to penetration depths of not more than 1 m is shown in Fig. 3E. Quantitative, boundary of the anomaly is sharper than the SW one.

GEOLOGY, July 2008 569 erated. Unlike the places of helium and thermogenic (hydrocarbons and A NW SE –80 carbon dioxide) gas ﬂ uxes, such locations would be less likely locations M i c r o b i a l –70.9 4 –64.4 –65 for future discovery of well-preserved large vertebrates. –57.0 –57.7 M i x i n g –55.9 2 (‰) –48.6 C(CH ) –50

–44.1

–42.1 ACKNOWLEDGMENTS

–38.5

–37.1

–37. T h e r m o g e n i c –36.3 The Society of Research on Environmental Changes “Geosphere” in Kraków –30 B initiated the new research project in the Starunia area. We greatly acknowledge support 106 from the Authorities of the AGH University of Science and Technology in Kraków Methane 104 and the Ivano-Frankivsk National Technical University of Oil and Gas, Ukraine. Total C25 -C alkanes 102 REFERENCES CITED

(ppm) 100 Alexandrowicz, S.W., 2005, The history of “Starunia”—A palaeontologic site and old ozokerite mine, in Kotarba, M.J., ed., Polish and Ukrainian Geolog-

Concentration 10–2 ical Studies (2004–2005) at Starunia—The Area of Discoveries of Woolly 13

8.9

6.3

8.2

9.8

8.6

5.0

3.6

.1 δ .8 C(CO2 ) Rhinoceroses and Other Extinct Vertebrates: Warszawa-Kraków, Polish

–1

–2

–1

–2

–4

–8.6

–6

–13.6 C 1.9 (‰) Geological Institute and Society “Geosphere,” p. 21–35. 10 Berner, U., and Faber, E., 1996, Empirical carbon isotope/maturity relationships 71 for gases from algal kerogens and terrigenous organic matter, based on Helium (ppm) 61 dry, open-system pyrolysis: Organic Geochemistry, v. 24, p. 947–955, doi: 1 47 42

(vol%) 34 10.1016/S0146–6380(96)00090–3. Carbon 29 26 dioxide Koltun, Y.V., Dudok, I.V., Kotarba, M.J., Adamenko, O.M., Pavluk, M.I.,

Concentration 0.1 Burzewski, W., and Stelmakh, O.R., 2005, Geological setting and petroleum occurrence of the Starunia area, fore-Carpathian region, Ukraine, in –100 0 100 200 300 400 500 600 Kotarba, M.J., ed., Polish and Ukrainian Geological Studies (2004–2005) D “RINNE” FAULT at Starunia—The Area of Discoveries of Woolly Rhinoceroses and Other s20 s53 s19 s52 s18 s51 s17 s50 s16 s6 s60 s61 Extinct Vertebrates: Warszawa-Kraków, Polish Geological Institute and 100 20 Society “Geosphere,” p. 61–77. 10 ρ Kotarba, M.J., and Rice, D.D., 2001, Composition and origin of coalbed gases 50 5 a Ω m) in the Lower Silesian Basin, northwestern Poland: Applied Geochemistry,

- (m) AB/2 ( v. 16, p. 895–910, doi: 10.1016/S0883–2927(00)00058–5.

H 0 2 Kotarba, M.J., Więcław, D., Koltun, Y.V., Lewan, M.D., Marynowski, L., and Dudok, 1 E s20 s53 I.V., 2005a, Organic geochemical study and genetic correlations between source s20 s53 s19 115 s19 0.5 rocks and hydrocarbons from surface seeps and deep accumulations in the s52s52 s18s18 Starunia area, fore-Carpathian region, Ukraine, in Kotarba, M.J., ed., Polish and 110 s51 s51 s17 0.4 Ukrainian Geological Studies (2004–2005) at Starunia—The Area of Dis- s17 0.6 s50 s61 105 s50 s60 s61 1.0 coveries of Woolly Rhinoceroses and Other Extinct Vertebrates: Warszawa- s16s16 s6 s6 s60 1.6

(m) 2.5 Kraków, Polish Geological Institute and Society “Geosphere,” p. 125–145. 100 ρ H 4.0 Kotarba, M.J., Dzieniewicz, M., and Sechman, H., 2005b, Geochemical survey, 6.3 (Ω m) 95 10.0 molecular and isotopic compositions, and genetic identifi cation of near- 16.0 surface gases from the Starunia area, fore-Carpathian region, Ukraine, in 90 25.0 40.0 63.0 Kotarba, M.J., ed., Polish and Ukrainian Geological Studies (2004–2005) 0 100 200 (m) 100.0 at Starunia—The Area of Discoveries of Woolly Rhinoceroses and Other Extinct Vertebrates: Warszawa-Kraków, Polish Geological Institute and Figure 4. Results of research along P-2 measurement line. A: Stable Society “Geosphere,” p. 157–173. carbon isotope composition of methane. B: Distribution of methane Kubiak, H., 1969, Über die Bedeutung der Kadaver des Wollhaarnashorns von Starunia: Berichte der Deutschen Gesellschaft für Geologische Wissen- and sum of C2-C5 alkanes. C: Distribution of carbon dioxide and helium, and stable carbon isotope composition of carbon dioxide. D: Apparent schaften, ser. A: Geologie und Paläontologie v. 14, p. 345–347. resistivity pseudosection based on DC resistivity soundings. E: One- Kuc, T., Różański, K., Goslar, T., Kubiak, H., and Kotarba, M.J., 2005, Radio- dimensional (1-D) interpretation of resistivity soundings. carbon dating of remnants of woolly rhinoceroses and mammoth from Starunia, fore-Carpathian region, Ukraine, in Kotarba, M.J., ed., Polish and Ukrainian Geological Studies (2004–2005) at Starunia—The Area of Dis- coveries of Woolly Rhinoceroses and Other Extinct Vertebrates: Warszawa- A comparison of geoelectrical and geochemical maps shows a correla- Kraków, Polish Geological Institute and Society “Geosphere,” p. 195–203. Matskevyj, L.G., 2005, Archaeological sites in the Starunia area, fore-Carpathian tion between the two data sets (Fig. 3). We hypothesize that there is a geo- region, Ukraine, in Kotarba, M.J., ed., Polish and Ukrainian Geological logical trap here containing brines and hydrocarbons. That zone, in the area Studies (2004–2005) at Starunia—The Area of Discoveries of Woolly Rhi- where the “paleoswamp” developed in the past, likely had a tectonic origin. noceroses and Other Extinct Vertebrates: Warszawa-Kraków, Polish Geo- logical Institute and Society “Geosphere,” p. 45–51. CONCLUSIONS Mościcki, W.J., 2005, Characterization of near-surface sediments in the area of Starunia (fore-Carpathian region, Ukraine) based on DC resistivity Geochemical and geoelectric studies indicate that large mammals soundings, in Kotarba, M.J., ed., Polish and Ukrainian Geological Studies sank into a Pleistocene water reservoir (“paleoswamp”) that had devel- (2004–2005) at Starunia—The Area of Discoveries of Woolly Rhinoceroses oped in the fault zone and may have had a rectangular shape, 350 ×150 m, and Other Extinct Vertebrates: Warszawa-Kraków, Polish Geological Insti- extending NW-SE (Fig. 3). The herbivorous mammals migrating in tute and Society “Geosphere,” p. 103–114. Sechman, H., and Dzieniewicz, M., 2007, Infl uence of soil moisture on the search of food and water could be trapped in such places and drowned results of surface geochemical survey applied to petroleum exploration: in the clayey substance that was saturated with brines and oils. It seems Journal of Petroleum Science Engineering, v. 56, p. 267–282, doi: 10.1016/ possible that during the Pleistocene tundra winters, when a thick ice j.petrol.2006.09.007. and snow cover was present in the “paleoswamp,” areas of infl ow of Whiticar, M.J., 1994, Correlation of natural gases with their sources, in Magoon, brines, oils, helium, and thermogenic hydrocarbon gases had a higher L.B., and Dow, W.G., eds., The Petroleum System—From Source to Trap: American Association of Petroleum Geologists Memoir 60, p. 261–283. temperature, which resulted in melting and cracking of the cover. The identifi cation of several places within the “paleoswamp” where thermo- Manuscript received 17 December 2007 genic gases occur indicates sites favorable to burial and preservation of Revised manuscript received 18 March 2008 Manuscript accepted 22 March 2008 Pleisto cene large mammals and probably also human remains. Locally, in recent swamps, large quantities of microbial methane have been gen- Printed in USA

570 GEOLOGY, July 2008