Age and Correlation of Fossiliferous Late Paleocene–Early Eocene

Home , Bumbanian, Gashatan

PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY CENTRAL PARK WEST AT 79TH STREET, NEW YORK, NY 10024 Number 3474, 26 pp., 12 ®gures, 2 tables May 11, 2005

Age and Correlation of Fossiliferous Late Paleocene±Early Eocene Strata of the Erlian Basin, Inner Mongolia, China

GABRIEL J. BOWEN,1 PAUL L. KOCH,2 JIN MENG,3 JIE YE,4 AND SUYIN TING5

ABSTRACT

The Asian continent preserves a rich and diverse record of Paleogene mammal faunas and their evolution through time. The sequence of faunal succession is of key importance to our understanding of the origin and diversi®cation of modern mammal groups, as phylogenetic data suggest that many major modern clades may be rooted in Asia. By calibrating the Asian fauna sequence within a chronostratigraphic framework, we can begin to compare patterns of succession on a global scale and constrain models for the origination and dispersal of modern mammal groups in the early Paleogene. The Erlian Basin of Inner Mongolia preserves Early Paleogene strata and mammal fossils assignable to the Gashatan, Bumbanian, and Irdin Manhan Asian Land Mammal Ages (AL- MAs). We measured stratigraphic sections and analyzed the stable isotope composition of paleosol carbonates and paleomagnetic directions of rocks at three localities in the Erlian Basin. The data document patterns in lithology, carbon isotope composition, and magnetic polarity that are consistent at all three localities and allow us to present two constrained hypotheses for the correlation of the local stratigraphic sections. Within the resulting composite section, we are able to identify a secular decrease in the carbon isotope composition of paleosol carbonate that can be equated to a multimillion-year trend preserved in late Paleocene and

1 Current address: Department of Biology, University of Utah, Salt Lake City, UT, 84112 ([email protected]. edu). 2 Department of Earth Sciences, University of California, Santa Cruz, CA 95064 ([email protected]). 3 Division of Paleontology, American Museum of Natural History ([email protected]). 4 Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, P.O. Box 643, Beijing 100044, P.R. China ([email protected]). 5 Museum of Natural Science, Louisiana State University, Baton Rouge, LA 70803 ([email protected]).

early Eocene terrestrial and marine records. Using this trend and previously documented constraints on the age of the Bumbanian ALMA, the composite section is shown to correlate within the interval of time represented by chrons C26n±C24n of the Geomagnetic Polarity Timescale (GPTS). We outline three possible correlations of the sequence of magnetic polarity zones in our composite section to the GPTS and explore the biostratigraphic implications of these. All three possible correlations show that Gashatan faunas in Inner Mongolia occur within chron C24R, and the preferred correlation suggests that the Gashatan taxa may have persisted close to the Paleocene/Eocene boundary. If con®rmed through further sampling, this result would imply that the ®rst appearance of the modern mammal orders Primates, Artio- dactyla, and Perissodactyla in Asia at the base of the Bumbanian ALMA did not signi®cantly precede their ®rst appearances in Europe and North America at the Paleocene/Eocene boundary. Fossil sites in the Erlian Basin promise to be central to resolving the debate about whether these clades lived and diversi®ed in Asia before dispersing throughout the Northern Hemi- sphere at the Paleocene/Eocene boundary.

INTRODUCTION pearance of these groups and lack of closely allied fossil outgroups in early Paleogene The transition between the Paleocene and faunas led workers to conclude that the ap- Eocene epochs was one of the most dynamic pearances represent immigration from an un- intervals of the Cenozoic. Long-term trends seen locus of evolution (e.g., McKenna, in climate ushered in the warmest period of 1973). Phylogenetic analysis of recently dis- the Cenozoic (Zachos et al., 2001). Through- covered Paleocene mammal fossils from Chi- out this gradual shift in climate, faunas and na, however, has led some workers to con- ¯oras evolved in a series of abrupt events clude that Asian faunas include taxa closely leading to the establishment of the ®rst truly allied to the new groups, and that the origins modern terrestrial ecosystems evidenced in of the earliest Eocene debutants may be the fossil record of the Holarctic continents. traced to a low-latitude Asian Eden (Beard, Abrupt changes in climate and faunal and 1998; Beard and Dawson, 1999). ¯oral composition coincided at the Paleo- A simple test with the potential to verify cene/Eocene boundary, when transient global Asia as a source of emigration at the Paleo- warming (e.g., Kennett and Stott, 1991), dis- cene/Eocene boundary, if not as an evolu- persal of mammal groups throughout the tionary locus, would be to evaluate the rel- northern hemisphere (e.g., McKenna, 1973), ative age of the ®rst appearances of clades and shifts in ¯oral composition (Harrington, on the three Holarctic continents. If the early 2003) led to reorganization of terrestrial eco- representatives of one or more of the modern systems on geologically short timescales mammal groups were found to have greater (ϳ100 thousand years (ky); Rohl et al., 2000; antiquity in Asia than in Europe and North Bowen et al., 2001; Farley and Eltgroth, America, this would support Asia as a source 2003). of evolution or emigration. Conversely, ®nd- One of the most profound changes in ter- ing that the Asian ®rst appearances lagged or restrial ecosystems that occurred near the Pa- were synchronous with those on other con- leocene/Eocene boundary was the abrupt ®rst tinents would con¯ict with such a hypothesis. appearance of several modern groups of Recent work has signi®cantly clari®ed the mammals in faunas from Asia, Europe, and relative age of early Paleogene faunal turn- North America (Gingerich, 1989; Hooker, over events on the Holarctic continents. Of 1998; Ting, 1998). The P/E ®rst appearances key importance is a globally correlatable car- included the ®rst representatives of Primates, bon isotope excursion documented in marine Artiodactyla, and Perissodactyla, three extant and terrestrial sedimentary rocks (Koch et orders that went on to diversify and become al., 1992; Zachos et al., 1993) and recently important components of modern faunas, and chosen as the de®ning stratigraphic datum Hyaenodontidae, a family of carnivorous for the Paleocene/Eocene boundary (Ouda, mammals that suffered extinction in the Mio- 2003). The base of the ␦13C excursion has cene (e.g., Gingerich, 1989). The abrupt ap- been shown to correlate with the P/E ®rst 2005 BOWEN ET AL.: ERLIAN BASIN STRATA 3 appearances to within ϳ10 ky in North lithological, biostratigraphic, paleomagnetic, America (Bowen et al., 2001), where the new and isotopic data from three fossil-bearing groups appear at the base of the Wasatchian stratigraphic sections in the Erlian Basin. North American Land Mammal Age (NAL- These data allow re®ned stratigraphic corre- MA; Gingerich, 2001). Work in southern Eu- lation of rock units within the basin and sug- rope has shown that the P/E ®rst appearances gest correlations between these units and the there are unlikely to predate those in North GPTS. We then examine the implications of America (Cojan et al., 2000). In northern Eu- these correlations for the potential antiquity rope, taxa characteristic of the P/E ®rst ap- of the Bumbanian ALMA and associated pearance event ®rst appear at the Dormaal modern mammal groups in Asia. locality of Belgium (Smith, 2000). By correlation to other early Paleogene localities in GEOLOGICAL SETTING Belgium and northern France for which bulk organic carbon isotope data are available, Fossil mammals of the Erlian Basin occur this fauna is thought to occur within the CIE within a widely distributed package of sili- and be time-equivalent to the earliest Was- ciclastic sedimentary rocks. These rocks are atchian faunas (Steurbaut et al., 1999; Ma- largely undeformed and outcrop primarily in gioncalda et al., 2004). widely separated low mesas and in shallow In Asia, the ®rst representative of Hyaen- washes. As a result of their poor local ex- odontidae occurs in a fauna assigned to the posure and the relative isolation of outcrops Gashatan Asian Land Mammal age (AL- in this region, stratigraphic correlation of MAs; Meng et al., 1998), which has been rock units within the basin is dif®cult and no shown to be Late Paleocene in age (Bowen clear regional stratigraphy has been devel- et al., 2002). Other important appearance oped. Several stratigraphic units have been events, however, including the ®rst Asian pri- de®ned based on a combination of litholog- mates, perissodactyls, and perhaps artiodac- ical and faunal characteristics, and these tyls (Tong and Wang, 1998) occur within the were recently reviewed by Meng et al. (1998, subsequent Bumbanian ALMA (Dashzeveg, 2004). 1988; Ting, 1998). Earlier work (Bowen et The rocks at our study sites include red al., 2002; Ting et al., 2003) has shown that and tan mudstones, calcareous mudstones, the base of the Bumbanian is no younger and siltstones, commonly preserving milli- than the P/E boundary, at ϳ55 million years meter-scale laminations of probable lacus- ago (Ma; Wing et al., 1999), and that Gash- trine origin, massive, brick red mudstone atan index taxa persist at least until the end beds commonly preserving features indica- of chron C25n of the geomagnetic polarity tive of paleosol development (see below), timescale (GPTS) ϳ55.9 Ma (Cande and and laterally extensive sandstone and con- Kent, 1995). These correlations leave an un- glomerate beds of ¯uvial origin. These rocks constrained window of time some 900 ky have been previously assigned to the Nom- long during which modern groups may have ogen, Arshanto, and Irdin Manha Forma- evolved and diversi®ed in Asia or, alterna- tions, although the details of their assignment tively, archaic groups may have persisted be- has differed among studies (see Meng et al., fore the impending restructuring of their 2004). In many cases, these names refer to communities. units recognized on the basis of combined Here, our goal is to use magnetic and iso- lithostratigraphic and biostratigraphic char- tope stratigraphy to determine the age of acteristics, and as a result it is unclear how mammal faunas from the Erlian Basin of In- to assign many intervals in our local sections ner Mongolia (®g. 1). This region has pro- to these composite stratigraphic units. The duced several diverse collections of early Pa- stratigraphic nomenclature for the Erlian Ba- leogene mammals, including several assem- sin is in need of revision, but because of the blages assigned to the Gashatan ALMA, and limited stratigraphic and geographic cover- new fossiliferous levels and important spec- age of the present study we will not attempt imens continue to be discovered (Meng et al., such a revision here. Instead, we will refer 2004; Smith et al., 2004). We present new throughout to informal lithostratigraphic 4 AMERICAN MUSEUM NOVITATES NO. 3474

Fig. 1. Locality map for fossiliferous Early Paleogene sites in the Erlian Basin (dashed line), Inner Mongolia, China. Sites discussed in the text are Erden Obo (EO), Bayan Ulan (BU), and Huheboerhe (HH). All localities are to the southwest of the boarder town of Erlianhot (square). Digital relief from the GTOPO30 30Јϫ30Ј digital elevation model (U.S. Geological Survey, 1996). units which we found to be useful in corre- traces, burrows, depletion channels, and car- lating among our local sections, with the in- bonate nodules. Carbonate nodules were col- tention that our ®ndings might be incorpo- lected from paleosol beds on freshly exposed rated into a comprehensive revision of the rock surfaces where there was no evidence stratigraphy of the Erlian Basin in the future. of modern soil formation (e.g., modern roots, weathered textures). METHODS Paleomagnetic analyses were conducted at We collected lithological data, fossils, ori- the University of California paleomagnetics ented rock samples, and paleosol carbonate laboratory using a three-axis cryogenic su- nodules from measured stratigraphic sections perconducting magnetometer. Samples were at three localities (®gs. 1, 2). Sections were incrementally demagnetized in a shielded Њ measured using a Jacob staff and described oven at temperatures between 0 and 680 C. in the ®eld from freshly exposed surfaces. Directions were ®t to the stepwise demag- Cores and blocks were collected for paleo- netization data using least-squares analysis of magnetic analysis and oriented using a Brun- vector component plots. ton compass. Because exposure was poor, Samples of paleosol carbonate for stable vertical horizination characteristic of soil de- carbon and oxygen isotope analysis were velopment was not commonly observable in drilled from polished surfaces under magni- the ®eld. Paleosols were identi®ed as ®ne- ®cation using a dental drill. Microcrystalline grained siliciclastic rock units lacking inter- calcite was sampled exclusively; care was nal strati®cation and containing features in- taken to avoid void-®lling macrocrystalline dicative of soil formation, including root phases which are likely to have formed dur- 2005 BOWEN ET AL.: ERLIAN BASIN STRATA 5

Fig. 2. Lithostratigraphic columns, key taxa, and faunal levels of the Erlian Basin sections. Bio- stratigraphic af®liation of taxa is shown to the left of each section. ing post-burial diagenesis (e.g., Koch et al., glomerate. Fragmentary fossil vertebrate re- 1995; Bao et al., 1998; Bowen et al., 2001). mains, including specimens of the anagalid

Samples were reacted with 100% H3PO4 in a Pseudictops, arctostylopid Palaeostylops, Њ common acid bath at 90 C, and the CO2 and multituberculates Lambdopsalis and evolved was cryogenically puri®ed in an au- Sphenopsalis, occur in the lower ϳ5mofthe tomated preparation device. The puri®ed CO2 section (®g. 2). At the 21-m level there is a was analyzed using a VG Optima or Prism transition to ®ne-grained rocks, and from 21 gas source mass spectrometer, and sample to 55 m there is a series of variegated red- values corrected relative to analyses of an in- brown to gray-green unfossiliferous mud- house calcite standard. Isotope values are re- rocks. These are capped by a thin (ϳ0.5 m) ported in per mil (½) units and ␦ notation medium-coarse-grained sandstone with a relative to the V-PDB standard, where ␦ϭ sharp, undulatory basal contact. The sand- Ϫ (Rsample Rstandard)/Rstandard * 1000, and R in- stone contains quartz pebbles lenses and dicates the ratio 13C/12Cor18O/16O. Measure- abundant fragmentary vertebrate fossils. Be- ment precision was typically Ͻ0.05½ for tween 55.5 m and the top of the section re- carbon and oxygen, based on repeated anal- ported here (78 m) is a series of interbedded yses of the calcite standard. brown and tan siltstones and ®ne- to medium-grained sandstones. These are typically RESULTS Ͻ3.5 m thick, and produce fossils at several levels, including the tapiroid perissodactyl LITHOSTRATIGRAPHY AND FAUNAL LEVELS Teleolophus at ϳ65 m. At Erden Obo (42Њ73ЈN, 111Њ21ЈE), the The sampled section at Bayan Ulan basal ϳ21 m of the section consist of medi- (43Њ08ЈN, 111Њ35ЈE), some 40 km NNE of um to coarse-grained sandstone, soft, pink, Erden Obo (®g. 1), consists primarily of non- laminated siltstone, and lenses of pebble con- bedded red, tan, and brown siltstones and 6 AMERICAN MUSEUM NOVITATES NO. 3474

Fig. 3. Field photograph of mud-®lled channels at Bayan Ulan. The patchily distributed light-colored rock in the foreground of the photo is soft, white, silty mudstone containing pockets of rounded clayball conglomerate, which ®lls a sinuous channel incised into the surrounding hard, orange-red mudrock. The top of this channel could not be located in the ®eld area. An upper channel, ϳ5 m deep, can be seen on the ridge (black arrows). This feature cuts perpendicular to the plane of the photograph and is ®lled with white mudstone. A geologist is standing at the left of the photo (white arrow) in a plane between the two channels, providing an approximate indication of the scale of these features. mudstones. Within the lower 48 m of the level, where nonbedded red mudstones are measured section, red-brown mudstone beds erosionally truncated by a stacked complex up to 7.5 m thick are dominant and are in- of crossbedded to laminated, gray-white silt- terbedded with thinner, white, tan and brown stone and ®ne-grained sandstone. Tapiroid siltstone beds (®g. 2). Near the base of the fossils were collected from a bed just below section these beds are fossiliferous, and the this transition. Bayan Ulan fauna of Meng et al. (1998) was Within the mudstone interval comprising collected from the basal ϳ3 m of our section. the basal 48 m of the Bayan Ulan section we The dominant taxon at this level is the mul- observed several scours and ®ne-grained tituberculate Lambdopsalis. Other ``index'' sand or mud-®lled channels (®gs. 2, 3). fossils from this level include Palaeostylops, These appear to cut several meters into the the dinoceratid Prodinoceras, and Tribos- underlying strata, and add considerable phenomys (a gliroid). During our ®eldwork stratigraphic complexity to the section, and of 2002, we found a fossiliferous level at 8± in some cases these beds may produce mam- 9 m of the section (BU-01-01-04 was taken mal fossils that, without careful consider- at 8 m; this is the fossil site 02±02), from ation for their stratigraphic context, might which numerous specimens of small mam- lead to erroneous interpretation of the bio- mals including Palaeostylops, Pseudictops, stratigraphy of the section. We found it to be and a new basal gliroid (Meng et al., 2005) necessary to run our measured section were collected. There is no specimen of through one mud-®lled channel, and took Lambdopsalis from this level, and this fossil note of its position. Although section meter assemblage may represent a slightly younger levels reported for Bayan Ulan do not take fauna than the previously described Bayan into account the depth of this erosional sur- Ulan fauna (Meng et al., 2005). Slightly face, they do re¯ect the proper superposi- above this level, two calcanea that are similar tional order of the sites considering their to those of Gomphos were found (Meng et stratigraphic context. al., 2004). The main lithologic transition in Our third section was measured at Huhe- the Bayan Ulan section occurs at the 48 m boerhe (43Њ22ЈN, 111Њ45ЈE), and is a revised 2005 BOWEN ET AL.: ERLIAN BASIN STRATA 7 version of the section of Meng et al. (2004) permost red mudstones. Particular care must re¯ecting new observations made during the be taken in interpreting the provenance of 2004 ®eld season. At Huheboerhe, the basal fossils collected from these mudstone beds. 24 m of section consist of lens-shaped, cross- The biostratigraphic af®liations of many bedded, ®ne- to medium-grained sandstones, index taxa from the Erlian Basin sections are soft, pink, laminated sandy siltstones, and shown in ®gure 2. Prodinoceras, Palaeosty- brechiated to homogeneous, red-weathering lops, Tribosphenomys, and the multituber- mudstones. Within this package, ®ner culates are all indicative of the Gashatan grained rock types become increasingly dom- ALMA, and most of the collections from the inant up-section. The uppermost sandy silt- Erlian Basin sections that bear these taxa rep- stone bed produced a specimen of Prodino- resent assemblages that are compositionally ceras (23 m). The interval between 24 and similar to other assemblages of Gashatan age 47.5 m is predominately mudstone. Beds (Ting, 1998). The only taxon in our sections near the base of this interval are tan and that is considered diagnostic of the Bumban- brown, with a transition to gray mudstone ian ALMA is G. elkema (Dashzeveg, 1988; and a single, thick (7.5 m) red mudstone oc- Meng et al., 2004). Fossil tapirs are charac- curring at the 40-m level. Specimens of the teristic of subsequent ALMAs, which have basal gliroid Gomphos elkema (Meng et al., been termed the Arshantan and Irdin Man- 2004), a small perissodactyl, possibly Hom- han. These biostratigraphic units are weakly ogalax, and a hyaenodontid smaller than de®ned based on a combination of litholog- Prolimnocyon chowi (Meng et al., 1998) ical and faunal associations, and are in need were recovered from this red bed during of review. Because the tapir specimens and ®eldwork in 2002; very large (up to 10 cm associated fossils referred to here were col- diameter) paleosol carbonate nodules were lected from strata overlying the Bumbanian also found near the base of the bed. Screen- and Gashatan levels that have been described washing sediments from this bed in 2004 as the ``Irdin Manha beds'', we will describe yielded teeth of at least two species of small them as representing the Irdin Manhan rodents. These rodents are generally similar ALMA. to those from the Bumban beds of Mongolia (Shevyreva, 1989; Dashzeveg, 1990). How- PALEOMAGNETICS ever, because the rodent specimens from the Bumban beds are fragmentary and their tax- Samples from the Erlian Basin sections (N onomic identi®cations are currently uncertain ϭ 183) displayed a wide array of magnetic (Averianov, 1996), we do not provide pre- behaviors during stepwise demagnetization, liminary identi®cations of the Huheboerhe but many siltstone samples and some mud- specimens in this study to avoid further com- stones exhibited relatively stable behavior. plication of the situation. Demagnetization of these samples typically Lower in the mudstone interval, at 28.5 m, revealed multiple directional components pods of hard, brittle, carbonate grainstone up (®g. 4). Most samples preserved a low-tem- to ϳ50 cm in diameter were observed weath- perature component, which was removed at ering out in a discrete horizon. These are temperatures below 300ЊC and exhibits an comprised primarily of round, carbonate- average declination (D) of 355Њ and inclina- coated clayballs several millimeters in di- tion (I)of75Њ (N ϭ 177; ®g. 5A). We inter- ameter, and are tentatively interpreted as fos- pret this component to be a modern ®eld sil cold spring deposits. At the top of the overprint. Stable samples also typically pre- section, a coarse-grained sandstone and peb- served a second, characteristic direction, ble conglomerate overlies the thick red mud- which was removed between 300 and 580ЊC stone with a sharp basal contact. The con- (®g. 4). This component exhibits directions glomerate contains abundant fragmentary indicative of both normal and reversed ®elds, vertebrate fossils, including tapiroid perisso- and the average directions for normal and re- dactyls. Fragmentary fossils from the con- versed ®eld samples are roughly antipodal (D glomerate bed have washed down and can be ϭ 10.6Њ, I ϭ 48.0Њ, N ϭ 84, k ϭ 5.614, and found on the surface of the underlying, up- D ϭ 191.5Њ, I ϭϪ42.7Њ, N ϭ 58, k ϭ 6.215, 8 AMERICAN MUSEUM NOVITATES NO. 3474

Fig. 4. Vector component diagrams for the thermal demagnetization of Erlian Basin samples. Crosses show the horizontal magnetization component and circles the vertical component projected on the N±S vertical plane. Labels indicate the thermal temperature step (ЊC) for each vertical plane point. Note the relatively stable demagnetization behavior of samples with a wide range of remanence intensities. A, BU020109A and B, BU020111C, samples characterized by normal ®eld directions. C, BU020101C and D, HH020101E, samples preserving reversed ®eld directions. All samples depicted here show the removal of a normal ®eld overprint at temperatures Ͻ300ЊC and isolation of a characteristic component between 300 and 580ЊC. 2005 BOWEN ET AL.: ERLIAN BASIN STRATA 9

Fig. 5. Equal-area projections of sample and site mean remanence directions. Mean directions for each polarity are shown with stars, and circles represent ␣95. A, Low temperature component, isolated below 300ЊC, for individual samples. The mean direction of this component (D ϭϪ5Њ, I ϭ 75Њ)is similar to the expected modern-®eld direction for the Erlian Basin (D ϭ 0Њ, I ϭ 62Њ), with the somewhat steeper inclination likely re¯ecting the partial removal of primary reversed polarity magnetization from some samples at low temperature. B, Characteristic component, isolated between 300 and 580Њ C. C, Site mean directions, calculated from sample characteristic component directions. Mean normal and reversed site directions are similar to previously reported directions from the Tertiary of Inner Mongolia (Zhao et al., 1994). respectively; ®g. 5B). The unblocking tem- sites with multiple stable samples (table 1). perature of this component suggests that it These were classed as alpha or beta sites ac- was carried by magnetite, and we interpret it cording to data quality, with alpha sites hav- to be depositional remanent magnetization ing four or more samples clustered at 99% carried by detrital magnetite grains. con®dence and beta sites having two sites Site average directions were calculated for clustered at 99% con®dence or three or more 10 AMERICAN MUSEUM NOVITATES NO. 3474

TABLE 1 Paleomagnetic Data

Sitea Level (m) D siteb I siteb ␣95 NR k Erden Obo EO-01±27 67.5 12.1 Ϫ61.8 1 1.00 EO-01-25 62 2.8 44.2 40.2 3 2.81 10.46 EO-01-23 57.5 44.8 69.4 1 1.00 EO-01-22 54.8 78.8 66 1 1.00 EO-01-18 43 88.6 44 28.6 4 3.73 11.30 EO-01-15 36.5 253.9 Ϫ51.2 46.9 2 1.97 30.52 EO-01-07 24.5 237 Ϫ36.6 11 5 4.92 49.57 EO-01-05 23.9 215.3 Ϫ48.7 15.1 4 3.92 37.93 EO-01-04 17.2 193.8 Ϫ69.7 180 2 1.68 3.13 EO-01-02 13.5 32 48.5 23.4 4 3.82 16.33 EO-01-01 9 354.8 56.1 117 3 2.11 2.26 Bayan Ulan BU-02-01-13 48 20.2 17.8 16.2 5 4.83 23.33 BU-02-01-11 46.5 315.7 54.8 10.3 5 4.93 55.86 BU-02-01-10 46 126.6 58.8 41.9 4 3.48 5.77 BU-02-01-09 44 303.4 37.2 43.3 3 2.78 9.19 BU-02-02-01 42.5 1.3 59.1 8.3 5 4.95 86.97 BU-02-01-08 39.5 5.5 59.7 23.4 5 4.66 11.68 BU-02-01-07 34.5 23.5 72.6 16.7 5 4.82 21.90 BU-02-01-03 27 165.7 Ϫ34.3 19.9 4 3.87 22.29 BU-02-01-04 25 179.6 Ϫ29.1 180 2 1.77 4.32 BU-02-01-02 22 185.4 Ϫ58.9 31.1 4 3.69 9.72 BU-02-01-01 15 195.3 Ϫ64 17.6 5 4.80 19.83 BU-01-01-06 13 228 Ϫ51.2 48 2 1.97 29.17 BU-01-01-04 8 234.5 Ϫ3.3 13.4 5 4.88 33.62 BU-01-01-03 5 256.1 Ϫ70.8 12.3 3 2.98 101.63 BU-01-01-02 3.2 219.8 Ϫ54.1 33.9 5 4.34 6.06 BU-01-01-01 1.5 168.2 Ϫ39 8.8 4 3.97 111.08 Huheboerhe HH-02-01-10 53.8 191.1 Ϫ15.4 40.4 3 2.81 10.36 HH-02-01-08 44.8 212 Ϫ22.6 18.4 4 3.88 25.94 HH-02-01-05 40.3 181.4 Ϫ23.4 21.9 5 4.70 13.17 HH-02-01-04 36.8 42.9 19.7 18.4 5 4.78 18.28 HH-02-01-03 35.3 202.1 Ϫ30.1 26.7 4 3.77 12.82 HH-02-01-02 32.2 200.3 Ϫ28.1 19.1 5 4.76 17.01 HH-02-01-01 30.3 167.2 Ϫ18.6 13.3 5 4.88 34.04 a Sites are classed as alpha (bold), beta (italics), or poorly clustered (normal font) according to data quality as described in the text. b Site average declination (D) and inclination (I).

sites clustered at 95% con®dence, according coherent polarity intervals in all three sec- Watson's test for randomness (Watson, tions (®g. 6). At Erden Obo, two polarity 1956). Site mean directions for normal and transitions are preserved. A basal normal po- reversed sites are roughly antipodal (D ϭ larity zone is indicated by data from a single 18.8Њ, I ϭ 57.3Њ, N ϭ 11, k ϭ 5.551, and D alpha site, but supported by data from two ϭ 202.4Њ, I ϭϪ41.7Њ, N ϭ 17, k ϭ 8.108, other sites with poor clustering that suggest respectively; ®g. 5C). When plotted strati- this zone encompasses at least the 9- to 16.5- graphically, the site average directions de®ne m levels of our section. Sites from the middle 2005 BOWEN ET AL.: ERLIAN BASIN STRATA 11

Fig. 6. Virtual geomagnetic pole (VGP) latitudes calculated from site mean directions and interpreted pattern of polarity reversals for the three study sections. VGP data are classed by data quality, as described in the text; here, closed symbols represent alpha sites and open symbols represent beta sites. Lithostratigraphic symbols as in ®gure 2.

ϳ20 m of the section demonstrate reversed (N ϭ 63), and for multiple nodules from a polarity, and those above the 40 m level are single sampling level is 0.94 and 0.93½ (N characterized by normal polarity. The Bayan ϭ 29), respectively. No pervasive pattern of Ulan section contains two relatively well-de- covariation between carbon and oxygen iso- ®ned polarity zones, with a transition from topes is observed in the raw data set (®g. reversed to normal polarity occurring be- 7A), but covariation is observed between the tween 27 and 34.5 m. Our sampling at this mean ␦13C and ␦18O values of nodules from site shifts into a ϳ10 m deep, mud-®lled single sampling levels (®g. 7B). When site channel between 25 and 27 m, and an un- mean ␦13C values are plotted against strati- known amount of time is represented by ero- graphic level, all three localities show up- sional surface at the base of this channel. Site section decreases in ␦13C (®g. 8). The trend BU-02-01-03, within the lowermost channel- is particularly well de®ned within the Bayan ®lling beds, preserves a reversed direction Ulan section. At Erden Obo, there is an ϳ35- similar to that of the adjacent site BU-02-01- m sampling gap in the middle of the section, 04, which lies outside the channel. The most but samples near the top of the section have parsimonious interpretation of this observa- ␦13C values 2±3½ lighter than those at ϳ20 tion is that the time represented by the ero- m. Samples from Huheboerhe have relatively sional surface occurred within a single inter- variable ␦13C values, but level averages sug- val of reversed polarity. With the exception gest that the sampled interval may be char- of one alpha site at 28.5 m, all sampled sites acterized by a weak decrease in ␦13C up-sec- at Huheboerhe preserve well-constrained, re- tion. versed ®eld directions. DISCUSSION STABLE ISOTOPES FIDELITY OF THE PALEOMAGNETIC RECORD Analyses of the ␦13C and ␦18O values of 73 carbonate nodules from 30 sampling levels Several lines of evidence suggest that are shown in table 2 and ®gure 7. The av- characteristic directions isolated in Erlian erage range for multiple ␦13Cor␦18O analy- Basin paleomagnetic samples preserve infor- ses from a single nodule is 0.15 and 0.17½ mation about the magnetic ®eld at the time 12 AMERICAN MUSEUM NOVITATES NO. 3474

TABLE 2 Stable Isotope Data

Sample Nodule Level Level Nodule Sample (m) ␦13C ␦18O ␦13C ␦18O ␦13C ␦18O Erden Obo EO-01-27ϩ0.5-1 A 68.0 Ϫ6.84 Ϫ10.48 Ϫ6.86 Ϫ10.41 Ϫ7.01 Ϫ10.65 B Ϫ6.89 Ϫ10.33 EO-01-27ϩ0.5-2 A Ϫ7.10 Ϫ10.85 Ϫ7.15 Ϫ10.89 B Ϫ7.21 Ϫ10.92 EO-01-26ϩ0.5-1 A 64.5 Ϫ7.48 Ϫ9.07 Ϫ7.52 Ϫ8.91 Ϫ7.51 Ϫ9.26 B Ϫ7.56 Ϫ8.74 EO-01-26ϩ0.5-2 A Ϫ7.51 Ϫ9.65 Ϫ7.51 Ϫ9.62 B Ϫ7.50 Ϫ9.59 EO-01-25ϩ0.5-1 A 62.5 Ϫ7.11 Ϫ10.11 Ϫ7.14 Ϫ10.07 Ϫ7.20 Ϫ10.10 B Ϫ7.16 Ϫ10.03 EO-01-25ϩ0.5-2 A Ϫ7.03 Ϫ9.96 Ϫ7.25 Ϫ10.13 B Ϫ7.48 Ϫ10.31 EO-01-12-1 A 27.0 Ϫ5.79 Ϫ8.93 Ϫ5.72 Ϫ8.97 Ϫ5.67 Ϫ8.95 B Ϫ5.66 Ϫ9.01 EO-01-12-2 A Ϫ5.63 Ϫ8.86 Ϫ5.62 Ϫ8.92 B Ϫ5.61 Ϫ8.98 EO-01-06ϩ1-1 A 24.9 Ϫ6.12 Ϫ10.18 Ϫ6.12 Ϫ10.09 Ϫ6.10 Ϫ10.06 B Ϫ6.11 Ϫ10.00 EO-01-06ϩ1-2 A Ϫ6.10 Ϫ10.13 Ϫ6.08 Ϫ10.02 B Ϫ6.05 Ϫ9.91 EO-01-06ϩ0.5-1 A 24.4 Ϫ6.21 Ϫ10.24 Ϫ6.21 Ϫ10.24 Ϫ6.20 Ϫ10.27 EO-01-06ϩ0.5-2 A Ϫ6.17 Ϫ10.23 Ϫ6.20 Ϫ10.31 B Ϫ6.22 Ϫ10.39 EO-01-03-1 A 16.5 Ϫ5.80 Ϫ10.18 Ϫ5.78 Ϫ10.04 Ϫ5.92 Ϫ10.06 B Ϫ5.76 Ϫ9.90 EO-01-03-2 A Ϫ6.02 Ϫ10.15 Ϫ6.05 Ϫ10.08 B Ϫ6.08 Ϫ10.02 Bayan Ulan BU-02-02-01-1 A 42.5 Ϫ7.82 Ϫ9.40 Ϫ7.81 Ϫ9.35 Ϫ7.66 Ϫ9.58 B Ϫ7.81 Ϫ9.30 BU-02-02-01-2 B Ϫ7.90 Ϫ10.15 Ϫ7.90 Ϫ10.15 BU-02-02-01-3 A Ϫ7.31 Ϫ9.24 Ϫ7.25 Ϫ9.25 B Ϫ7.19 Ϫ9.25 BU-02-01-13-1 A 48.0 Ϫ9.23 Ϫ9.35 Ϫ9.15 Ϫ9.37 Ϫ10.54 Ϫ8.68 B Ϫ9.06 Ϫ9.40 BU-02-01-13-2 A Ϫ12.01 Ϫ8.05 Ϫ11.94 Ϫ7.99 B Ϫ11.87 Ϫ7.93 BU-02-01-12-1 A 47.5 Ϫ8.76 Ϫ7.79 Ϫ8.77 Ϫ7.61 Ϫ8.99 Ϫ8.60 B Ϫ8.79 Ϫ7.43 BU-02-01-12-2 A Ϫ8.44 Ϫ9.91 Ϫ8.68 Ϫ9.97 B Ϫ8.92 Ϫ10.02 BU-02-01-12-3 A Ϫ9.59 Ϫ8.23 Ϫ9.53 Ϫ8.23 B Ϫ9.47 Ϫ8.17 BU-02-01-11-1 A 46.5 Ϫ8.92 Ϫ10.54 Ϫ9.01 Ϫ10.65 Ϫ8.52 Ϫ10.20 B Ϫ9.10 Ϫ10.75 BU-02-01-11-2 A Ϫ8.05 Ϫ9.68 Ϫ8.03 Ϫ9.75 B Ϫ8.01 Ϫ9.82 BU-02-01-10-1 A 46.0 Ϫ8.03 Ϫ12.56 Ϫ8.05 Ϫ12.50 Ϫ8.05 Ϫ11.58 B Ϫ8.06 Ϫ12.44 2005 BOWEN ET AL.: ERLIAN BASIN STRATA 13

TABLE 2 (Continued)

Sample Nodule Level Level Nodule Sample (m) ␦13C ␦18O ␦13C ␦18O ␦13C ␦18O BU-02-01-10-2 A Ϫ8.17 Ϫ10.61 Ϫ8.05 Ϫ10.65 B Ϫ7.92 Ϫ10.69 BU-02-01-09-1 A 44.0 Ϫ9.15 Ϫ9.30 Ϫ9.15 Ϫ9.08 Ϫ8.70 Ϫ9.75 B Ϫ9.16 Ϫ8.86 BU-02-01-09-2 A Ϫ7.90 Ϫ11.54 Ϫ7.90 Ϫ11.34 B Ϫ7.90 Ϫ11.15 BU-02-01-09-3 A Ϫ9.03 Ϫ8.80 Ϫ9.04 Ϫ8.82 B Ϫ9.06 Ϫ8.83 BU-02-01-08-1 A 39.5 Ϫ9.67 Ϫ7.74 Ϫ9.63 Ϫ7.75 Ϫ9.19 Ϫ8.54 B Ϫ9.60 Ϫ7.76 BU-02-01-08-2 A Ϫ8.72 Ϫ9.30 Ϫ8.75 Ϫ9.33 B Ϫ8.79 Ϫ9.37 BU-02-01-07-1 A 34.5 Ϫ9.68 Ϫ9.10 Ϫ9.73 Ϫ9.11 Ϫ9.90 Ϫ9.27 B Ϫ9.78 Ϫ9.13 BU-02-01-07-2 A Ϫ10.44 Ϫ9.04 Ϫ10.37 Ϫ9.07 B Ϫ10.31 Ϫ9.10 BU-02-01-07-3 A Ϫ9.59 Ϫ9.62 Ϫ9.59 Ϫ9.62 BU-02-01-07-4 A Ϫ9.46 Ϫ7.32 Ϫ9.46 Ϫ7.32 BU-02-01-06-1 A 33.5 Ϫ9.68 Ϫ10.18 Ϫ9.80 Ϫ10.28 Ϫ9.32 Ϫ10.41 B Ϫ9.91 Ϫ10.37 BU-02-01-06-2 A Ϫ8.78 Ϫ10.62 Ϫ8.78 Ϫ10.62 BU-02-01-06-3 A Ϫ9.40 Ϫ10.32 Ϫ9.40 Ϫ10.32 BU-02-01-06-4 A Ϫ9.66 Ϫ10.58 Ϫ9.66 Ϫ10.58 BU-02-01-05-1 A 31.5 Ϫ8.03 Ϫ7.37 Ϫ8.06 Ϫ7.36 Ϫ8.22 Ϫ7.58 B Ϫ8.09 Ϫ7.35 BU-02-01-05-2 A Ϫ8.44 Ϫ7.82 Ϫ8.39 Ϫ7.79 B Ϫ8.33 Ϫ7.76 BU-02-01-04-1 A 25.0 Ϫ6.55 Ϫ8.47 Ϫ6.40 Ϫ8.38 Ϫ6.91 Ϫ8.38 B Ϫ6.25 Ϫ8.29 BU-02-01-04-2 A Ϫ9.50 Ϫ8.15 Ϫ9.57 Ϫ8.11 B Ϫ9.64 Ϫ8.07 BU-02-01-04-3 A Ϫ4.80 Ϫ8.67 Ϫ4.77 Ϫ8.64 B Ϫ4.73 Ϫ8.60 BU-02-01-02-1 A 22.0 Ϫ7.95 Ϫ7.12 Ϫ7.74 Ϫ7.07 Ϫ7.93 Ϫ7.86 B Ϫ7.54 Ϫ7.01 BU-02-01-02-2 A Ϫ6.93 Ϫ8.62 Ϫ6.97 Ϫ8.65 B Ϫ7.01 Ϫ8.67 BU-02-01-02-3 A Ϫ9.09 Ϫ7.53 Ϫ9.09 Ϫ7.53 BU-02-01-01-1 A 15.0 Ϫ7.81 Ϫ10.20 Ϫ7.62 Ϫ10.51 Ϫ7.74 Ϫ9.14 B Ϫ7.43 Ϫ10.81 BU-02-01-01-2 A Ϫ7.50 Ϫ9.00 Ϫ7.66 Ϫ8.81 B Ϫ7.82 Ϫ8.62 BU-02-01-01-3 A Ϫ8.04 Ϫ8.20 Ϫ7.93 Ϫ8.12 B Ϫ7.82 Ϫ8.03 BU-01-01-05-1 A 12.4 Ϫ7.32 Ϫ8.88 Ϫ7.29 Ϫ8.90 Ϫ7.18 Ϫ8.59 B Ϫ7.25 Ϫ8.91 BU-01-01-05-2 A Ϫ7.06 Ϫ8.39 Ϫ7.07 Ϫ8.29 B Ϫ7.08 Ϫ8.18 BU-01-01-04-1 A 8.0 Ϫ7.38 Ϫ8.13 Ϫ7.38 Ϫ8.47 Ϫ7.30 Ϫ8.58 B Ϫ7.38 Ϫ8.82 14 AMERICAN MUSEUM NOVITATES NO. 3474

TABLE 2 (Continued)

Sample Nodule Level Level Nodule Sample (m) ␦13C ␦18O ␦13C ␦18O ␦13C ␦18O BU-01-01-04-2 A Ϫ7.22 Ϫ8.73 Ϫ7.22 Ϫ8.68 B Ϫ7.22 Ϫ8.64 BU-01-01-02-1 A 3.2 Ϫ6.68 Ϫ10.46 Ϫ6.77 Ϫ10.62 Ϫ6.82 Ϫ10.62 B Ϫ6.85 Ϫ10.78 BU-01-01-02-2 A Ϫ6.73 Ϫ10.57 Ϫ6.79 Ϫ10.61 B Ϫ6.86 Ϫ10.66 BU-01-01-02-3 A Ϫ6.83 Ϫ10.64 Ϫ6.91 Ϫ10.63 B Ϫ6.98 Ϫ10.63 BU-01-01-01-1 A 1.5 Ϫ6.30 Ϫ8.61 Ϫ6.37 Ϫ8.71 Ϫ6.37 Ϫ8.71 B Ϫ6.43 Ϫ8.80 Huheboerhe HH-02-01-10-1 A 53.8 Ϫ7.26 Ϫ9.13 Ϫ7.26 Ϫ9.21 Ϫ9.22 Ϫ8.87 B Ϫ7.28 Ϫ9.25 C Ϫ7.23 Ϫ9.19 D Ϫ7.28 Ϫ9.26 HH-02-01-10-2 A Ϫ9.50 Ϫ8.17 Ϫ9.53 Ϫ8.16 B Ϫ9.53 Ϫ8.03 C Ϫ9.55 Ϫ8.28 HH-02-01-10-3 A Ϫ11.22 Ϫ7.15 Ϫ11.22 Ϫ7.15 HH-02-01-10-4 A Ϫ10.43 Ϫ8.67 Ϫ10.43 Ϫ8.67 HH-02-01-10-5 A Ϫ7.66 Ϫ11.18 Ϫ7.66 Ϫ11.18 HH-02-01-09-1 A 49.3 Ϫ7.18 Ϫ8.53 Ϫ7.08 Ϫ8.54 Ϫ7.28 Ϫ8.88 B Ϫ6.99 Ϫ8.55 HH-02-01-09-2 A Ϫ7.50 Ϫ9.21 Ϫ7.48 Ϫ9.22 B Ϫ7.46 Ϫ9.22 HH-02-01-11U-1 A 48.3 Ϫ7.06 Ϫ8.93 Ϫ7.09 Ϫ8.97 Ϫ7.14 Ϫ9.00 B Ϫ7.03 Ϫ8.87 C Ϫ7.35 Ϫ8.99 D Ϫ6.91 Ϫ9.09 HH-02-01-11U-2 A Ϫ7.16 Ϫ8.92 Ϫ7.20 Ϫ9.04 B Ϫ7.06 Ϫ8.94 C Ϫ7.21 Ϫ9.03 D Ϫ7.35 Ϫ9.26 HH-02-01-11L-1 A 47.8 Ϫ9.99 Ϫ9.65 Ϫ10.43 Ϫ9.84 Ϫ8.13 Ϫ10.15 B Ϫ10.24 Ϫ9.83 C Ϫ10.39 Ϫ9.93 D Ϫ11.10 Ϫ9.97 HH-02-01-11L-2 A Ϫ6.16 Ϫ10.45 Ϫ5.82 Ϫ10.45 B Ϫ5.87 Ϫ10.05 C Ϫ5.42 Ϫ10.58 HH-02-01-08-1 A 44.8 Ϫ8.65 Ϫ8.81 Ϫ8.74 Ϫ9.03 Ϫ8.75 Ϫ9.02 B Ϫ8.83 Ϫ9.24 HH-02-01-08-2 A Ϫ8.63 Ϫ9.01 Ϫ8.76 Ϫ9.02 B Ϫ8.89 Ϫ9.03 HH-02-01-07-1 A 43.3 Ϫ5.13 Ϫ9.05 Ϫ5.07 Ϫ9.13 Ϫ5.46 Ϫ9.44 B Ϫ5.00 Ϫ9.22 HH-02-01-07-2 A Ϫ5.87 Ϫ9.77 Ϫ5.86 Ϫ9.75 B Ϫ5.84 Ϫ9.73 2005 BOWEN ET AL.: ERLIAN BASIN STRATA 15

Fig. 7. Carbon and oxygen isotope compositions of paleosol carbonate. A, All data, with values for individual analyses shown. No pervasive pattern of covariation between ␦13C and ␦18O is apparent. B, Nodule average values for three sites with atypically large ␦13Cor␦18O variability. All sites exhibit positive covariation between ␦13C and ␦18O. At BU-02-01-04 and HH-02-01-11L variability is much greater for ␦13C than for ␦18O, whereas at HH-02-01-10 variablility for ␦18O exceeds that for ␦13C. In the former case, ␦13C variation among carbonate nodules likely re¯ects spatial or temporal variation in the ␦13C of carbon in the soil system. In the latter, diagenesis may have altered the isotopic composition of some nodules. of sediment deposition or early diagenesis, samples exhibits normal ®eld directions, the and are thus useful for magnetostratigraphy. high-temperature component displays both Most samples carried multicomponent natu- normal and reversed ®eld polarities. Mean ral remanent magnetization, and whereas the normal and reversed directions for alpha and low-temperature component isolated in most beta sites are roughly antipodal, and pass the

Fig. 8. Average carbon isotope values from paleosol carbonate nodules by level. Error bars are ranges for nodule average ␦13C at each level. All three sections preserve a similar range of values, and show a decrease in ␦13C through time. 16 AMERICAN MUSEUM NOVITATES NO. 3474 reversal test of McFadden and McElhinny genesis of soil carbonate isotope signatures (1990) at 95% con®dence. The somewhat has been observed (e.g., Budd et al., 2002), different inclinations for the normal and re- however, and the potential for diagenetic ar- versed polarity averages may re¯ect shallow- tifacts must be addressed before attempting ing of the reversed ®eld directions due to the to use our paleosol carbonate ␦13C record as presence of a minor, secondary magnetiza- a tool for correlation. tion that was not fully removed during ther- Careful examination of the variation and mal demagnetization (e.g., Scott and Hotes, covariation of ␦13C and ␦18O values provides 1996). Our average normal and reversed site a useful method for identifying diagenetic directions are not statistically different at overprinting in many cases (e.g., Bowen et 95% con®dence from previously reported al., 2001), although such an approach may normal (D ϭ 359.6Њ, I ϭ 53.7Њ, N ϭ 8, k ϭ fall short for soils formed in environments 65.9) and reversed (D ϭ 175.8Њ, I ϭϪ67.4Њ, with high diagenetic potential (Budd et al., N ϭ 5, k ϭ 67.1) directions of Tertiary vol- 2002). We can present two expectations for canic rocks in Inner Mongolia (Zhao et al., changes in isotopic signatures during diagen- 1994). Again, the inclination for our reversed esis that might help identify cases where ␦13C sites is somewhat lower than in the Zhao et values have been altered. First, diagenesis al. (1994) study, suggesting the persistence should lead to characteristic changes in intra- of a minor magnetic overprint in these sam- and internodule isotopic variability, the na- ples. Normal and reversed sample polarities ture of which will depend on the degree of are consistent among multiple samples at alteration. If alteration is partial, it should in- most sites, and site mean polarities de®ned crease intranodule isotopic variability, and in coherent stratigraphic polarity intervals. In some cases internodule variability, through addition to these considerations, strong sup- the precipitation of diagenetic phases with port for the ®delity of our paleomagnetic po- different ␦13C and/or ␦18O values than the larity record is provided by the consistency preserved, primary material. In contrast, of magnetic, isotopic, lithologic, and bio- complete alteration would produce very low stratigraphic constraints on the correlation of intra- and internodule variability, as all val- the three measured sections, as discussed be- ues would be shifted to that of the diagenetic low. phase. Second, because diagenetic ¯uids have much more oxygen than carbon, post- ␦18 FIDELITY OF THE ISOTOPIC RECORD burial should alter O preferentially relative to ␦13C. Carbon isotopes in authigenic soil carbon- The narrow range of within-nodule isoto- ␦13 ate re¯ect the C of soil CO2, which below pic variation observed in Erlian Basin sam- ϳ50 cm depth within the soil is largely de- ples is similar to that seen in other studies of rived from the decomposition of soil organic well-preserved paleosol carbonate micrite, matter (e.g., Cerling, 1984). The ␦13C value but the within-level variation for many pa- of CO2 respired from soil organic matter is leosols is somewhat greater (e.g., Koch et al., similar to that of carbon ®xed from the at- 1995; Bowen et al., 2001). Examination of mosphere by plants living on the soil (Bowen within-level variation on a case-by-case basis and Beerling, 2004), meaning that carbonate shows that there is no strong relation be- formed at depth within a soil provides an in- tween ␦13C and ␦18O variation within individ- direct proxy of atmospheric ␦13C. Because at- ual paleosols (®g. 9), and that many of those mospheric CO2 is well mixed over the time- paleosols with the greatest internodule vari- scales on which soil carbonate form (1±100 ability for ␦13C have relatively low ␦18O rang- ky; e.g., Birkeland, 1999), large changes in es (®gs. 7B, 9). The one exception is site the ␦13C of the atmosphere through time HH-02-01-10, where the very high range re- should be re¯ected in aggrading sequences ported for ␦18O results from a single analysis of carbonate-bearing paleosols and can be of a small nodule, which gave a ␦18O value used to correlate widely separated terrestrial almost 2½ lower than for any other sample sedimentary sections (Koch et al., 1992; Co- from that level. In general, these observa- jan et al., 2000; Bowen et al., 2002). Dia- tions are not consistent with the expectations 2005 BOWEN ET AL.: ERLIAN BASIN STRATA 17

Fig. 9. Ranges of ␦13C and ␦18O values (maximum±minimum) measured for multiple carbonate nodules from individual paleosols. Labels indicate the position of high variability sites that are shown in ®gure 7B. put forward for the effects of diagenetic al- and paleosols with high internodule variabil- teration, and it seems likely that many of our ity may have formed during such an event. samples preserve the isotopic signature of the Only one such event is known during the ear- soils in which they formed. ly Paleogene (Koch et al., 1992); therefore, Although we are reasonably con®dent that this explanation could only apply to one pa- diagenesis has not dramatically changed the leosol or stratigraphic interval of high ␦13C ␦13C values of our paleosol carbonate sam- variability in each of our sections, and would ples, it would still be useful to understand imply that those levels were time-equivalent. the possible causes of atypically large ␦13C Third, it is possible that nodules in the high- variation in some of the paleosols sampled. variability paleosols were formed over a We cannot choose among alternatives with range of depths in the fossil soils. Soil CO2 the data currently available, but we can sug- near the soil surface includes a greater com- gest at least three possible sources of varia- ponent of atmospheric CO2, giving it much ␦13 tion. First, it is possible that different nodules higher C than CO2 deeper within the soil. Because paleosol horizons and boundaries from these paleosols are sampling landscape- between stacked paleosols could not be level variation in plant ␦13C values. The ␦13C clearly distinguished in the poorly exposed of plants using the C3 photosynthetic path- ϳ Erlian Basin sections, it is possible that the way varies by 11½ in response to environ- ␦13C values of some of the nodules sampled mental differences and variation in physio- in our study re¯ect a relatively large atmo- logical tolerance among taxa (O'Leary, spheric contribution. In this case, only the 1995). If the paleosols exhibiting high inter- lowest ␦13C values from these levels should nodule ␦13C variability represent abnormally be used for chemostratigraphy to ensure that long periods of soil formation or formed in measurements re¯ect vegetation-derived CO2 atypically dynamic environments, they may consistently throughout the stratigraphic sec- have precipitated carbonate nodules that tion. sampled a range of natural plant/soil ␦13C values. Second, the history of atmospheric INTRABASINAL CORRELATION ␦13C includes several short-lived, transient Using the lithostratigraphic, magnetostra- shifts (Koch et al., 1992; Jahren et al., 2001), tigraphic, and biostratigraphic data presented 18 AMERICAN MUSEUM NOVITATES NO. 3474 above, we propose two alternative correla- val would not signi®cantly affect the main tions among the three local sections studied conclusions of our study. here (®g. 10). The two alternatives differ Both correlations are consistent with the only in how the Huheboerhe section is cor- majority of our stratigraphic data, and each related to the Bayan Ulan and Erden Obo has its own strengths and weaknesses. The sections. In alternative correlation A, we main strengths of alternative correlation A place the greatest weight on our lithologic are that it aligns the transition from sand- data and correlate the base of the mudstone stone to mudstone deposition at Erden Obo unit at Huheboerhe to that at Erden Obo (®g. and Huheboerhe and that it pairs the rela- 10A). In alternative B, we correlate the Hu- tively low (Ϫ8toϪ9½) ␦13C values from heboerhe section to Erden Obo based on the the upper mudstone beds at Huheboerhe with single normal polarity site at Huheboerhe. the similar ␦13C values in the middle of the For alternative B, the placement of the Hu- mudstone units at the other sites. Under cor- heboerhe section relative to the others is relation B, these low paleosol carbonate ␦13C some ϳ15 m lower than for correlation A values would correlate to an interval in the (®g. 10B). The composite stratigraphic sec- other two sections where ␦13C values in the tion implied by correlation A includes ®ve other sections are between 1 and 3½ higher. magnetic polarity zones, which we designate In contrast, the primary shortcoming of al- Aϩ through Eϩ, whereas the composite sec- ternative A is that it correlates the single nor- tion implied by alternative correlation B in- mal polarity magnetostratigraphy site at Hu- cludes four polarity zones, here designated heboerhe within a relatively well-sampled re- aϪ,Aϩ,BϪ, and Eϩ (®g. 10). versed zone at Bayan Ulan. Further magne- Both alternative correlations presented tostratigraphic sampling will be necessary to here assume that the main (mudstone) nor- clearly distinguish between these two alter- mal polarity zones at Huheboerhe and Bayan native correlations. Ulan represent continuous deposition under Using correlation A, we propose a prelim- normal polarity conditions (e.g., no reversals inary composite stratigraphic section for the are missed). Although it is possible that our interval of study in Erlian Basin (®g. 11). sampling may not capture a reversed polarity The composite section shows an up-section zone occurring during the time of deposition decline in paleosol carbonate ␦13C, ®ve mag- of this rock package in one of these sections, netic polarity zones, and three distinct bio- we feel that our dense magnetostratigraphic stratigraphic intervals. Gashatan fossils occur sampling of these units and the consistency from the base of the composite section, just of lithostratigraphic and biostratigraphic pat- below the base of polarity zone Aϩ, to ap- terns in the two sections supports the inter- proximately the midpoint of polarity zone pretation that this is a distinct polarity inter- DϪ. Bumbanian fossils range from the mid- val that can be correlated among the sites. dle of zone to the top of DϪ, and Irdin Man- Furthermore, no signi®cant erosional surfac- han fossils ®rst appear well above the base es were observed in the mudstone interval at of zone Eϩ. Paleosol carbonate ␦13C values Huheboerhe (with the exception of the major decrease by ϳ3½ up-section, with the high- sandstone scour at the top of the section) or est measured values occurring near the base in the lower, fossiliferous part of the Bayan of polarity zone BϪ and the lowest within Ulan mudstone interval. We believe that it is polarity zone Eϩ. There is some suggestion unlikely that a reversed polarity interval was that ␦13C values may increase slightly at the removed from this part of our sections as a top of the section, but these data come ex- result of erosion. A greater degree of chan- clusively from the poorly sampled Erden neling was observed in the middle part of the Obo section, and it is not clear that this pat- Bayan Ulan mudstone interval, and we can- tern represents stratigraphic and not spatial not rule out the possibility that polarity zones variation. If alternative correlation B for the may have been lost in that part of the section. Huheboerhe section was adopted, the impli- Those eroded beds occur above the fossilif- cations for the composite section would be erous levels considered here, though, and the relatively minor. In this case the poorly de- loss of magnetostratigraphic from that inter- ®ned polarity zone shown as Cϩ in ®gure 11 2005 BOWEN ET AL.: ERLIAN BASIN STRATA 19

Fig. 10. Proposed correlation of measured sections and Erlian Basin composite magnetic polarity stratigraphy. A and B present two alternate hypotheses for the correlation of the Huheboerhe section. Correlation tie points in A and B include (1) base of the normal-polarity, mudrock-dominated interval; (2) base of the Bumbanian ALMA, as indicated by the ®rst appearance of Gomphos; (3) normal to reversed polarity transition within the mudrock-dominated interval; and (4) erosional base of the upper sand/siltstone unit. In B, 5 represents correlation of the normal polarity site at Huheboerhe to the basal normal polarity zone at Erden Obo. Lithostratigraphic symbols are as in ®gure 2. 20 AMERICAN MUSEUM NOVITATES NO. 3474

Fig. 11. Composite section and preferred correlation of Early Paleogene rocks in the Erlian Basin (left panel) to the Geomagnetic Polarity Timescale (GPTS; Cande and Kent, 1995), the sequence of North American Land Mammal Ages (NALMA), and biozones and paleosol carbonate ␦13C record of the Bighorn Basin, Wyoming (right panel). Data from the local sections have been combined using correlation A (®g. 10A, see text). From left to right, left panel: generalized stratigraphy, composite carbon isotope stratigraphy, composite magnetostratigraphy, stratigraphic distribution of biozones, and magnetic polarity zones for the composite section. Symbols in the isotopic and magnetic time series represent data from the Erden Obo (squares), Bayan Ulan (circles), and Huheboerhe (diamonds) local sections, and alpha and beta magnetostratigraphy sites are indicated by closed and open symbols, respectively. Biozones include the Gashatan (G), Bumbanian (B), and Irden Manha (I) ALMAs, and letters indicate the actual stratigraphic position of fossil collections. Right-hand panel is after Koch et al. (2003), with a generalized schematic carbon isotope curve based on the Bighorn Basin record shown here. Question marks indicate uncertainty in the correlation of the uppermost part of our section given that polarity zone Eϩ may be a composite representing more than one normal polarity chron. would merge with zone Aϩ and reversed po- our composite section (®g. 11), we propose larity zones BϪ and DϪ would be joined correlations between the Erlian Basin strata (see ®g. 10B). The basal reversed polarity and the GPTS (Cande and Kent, 1995). We sites at Huheboerhe would then de®ne a re- can constrain the correlation of our polarity versed polarity zone at the base of the com- reversals to the GPTS based on the trend in posite section (aϪ). The Gashatan fossil P. paleosol carbonate ␦13C values in the Erlian matyr from Huheboerhe would be the only Basin section, which we believe re¯ects a fossil taxon known de®nitively to occur secular decrease in the ␦13C of Earth's sur- within that zone, although the Gashatan as- face carbon reservoirs from the Late Paleo- semblage from the base of the Erden Obo cene to the Early Eocene. This trend has been section is unconstrained and could correlate previously documented in marine (e.g., Za- either within aϪ or the base of Aϩ. chos et al., 2001) and terrestrial carbonates (e.g., Koch et al., 1992, 2003), and is of ap- AGE AND GLOBAL CORRELATION proximately the same magnitude as that seen Based on the isotope, faunal, and magne- in our new record. The only other secular tostratigraphic information summarized in trend of comparable magnitude and the same 2005 BOWEN ET AL.: ERLIAN BASIN STRATA 21 direction that has been documented in Early tions which cannot be ruled out given the Paleogene ␦13C records is the rapid, transient current data (®g. 12). Using intrabasin cor- decline in ␦13C at the Paleocene/Eocene relation A, we can present the alternative hy- boundary (Zachos et al., 2001). The entirety pothesis that polarity zone Cϩ does represent of the Paleocene/Eocene boundary event, chron C25n (®g. 12A). In this case, zones however, lies within chron C24r of the Aϩ and BϪ would correlate to chrons C26n GPTS, making it a poor match for the ␦13C and C25r, respectively. A second alternative, shift preserved in the Erlian Basin section. based on intrabasin correlation B, is identical The Late Paleocene ␦13C high occurs with- to the preferred global correlation with the in chron C26r of the GPTS, and ␦13C de- exception that the additional, basal, reversed creases steadily through time until reaching polarity zone (designated aϪ) is correlated to a minimum within chron C24n. In our Erlian chron C25r. Given the durations of these Basin record, this decline spans polarity chrons in the GPTS, both alternatives are zones BϪ through Eϩ of the composite sec- reasonable in that they give fairly constant tion (®g. 11). Previous work has con®rmed sediment accumulation rates for the bounded that the boundary between the Gashatan and polarity zones of the composite section. Test- Bumbanian ALMAs occurs within the lower ing these hypotheses will require further part of C24r (Bowen et al., 2002), and be- magnetostratigraphic sampling around and cause both Gashatan and Bumbanian fossils below the normal polarity site at Huheboerhe Ϫ occur in polarity zone D we correlate this to identify the pattern and duration of polar- zone to C24r. We consider it to be unlikely ϩ ity zones near and below the base of the cur- that polarity zone C , de®ned by a single rent composite section. normal polarity site tightly bounded by re- The proposed correlations to the GPTS versed sites, represents the 500-ky chron suggests that our section from the Erlian Ba- C25n, and prefer the interpretation that it sin represents more than 3 million years of represents one of the many short-lived ex- time spanning the Paleocene/Eocene bound- cursions of the magnetic ®eld (cryptochrons) ary. Our isotopic sampling does not appear which have been identi®ed within C24r to have recovered evidence of the 100-ky, 3± (Cande and Kent, 1995). We therefore sug- 6½ carbon isotope excursion marking the gest that zones BϪ through DϪ correlate to Paleocene/Eocene boundary (®g. 11). This C24r and that Aϩ represents the top of C25n. This lowermost polarity zone is again de- result is not surprising given the very low ®ned by only one alpha site, but is supported sediment accumulation rates indicated for by poorly clustered normal directions from our sections by the proposed magnetostrati- two other sites at Erden Obo (not shown). graphic correlations. For example, the polar- The stratigraphic extent of this zone is un- ity interval representing C24r according to constrained because it occurs at the base of the preferred global correlation seems to be the Erden Obo and Bayan Ulan sections, but most expanded in the Bayan Ulan section, it is possible that further sampling down-sec- where it is represented by at least 30 m of tion at Huheboerhe could allow us to identify section. Given the estimated duration of C24r ϳ this polarity zone and constrain its base. Fi- ( 2.6 m.y.; Cande and Kent, 1995), this nally, we correlate the base of the uppermost gives an average sediment accumulation rate polarity zone Eϩ to the C24r±C24n bound- of Ͻ1.2 cm/ky. If we assume that sedimen- ary. Based on the evidence for signi®cant tation was continuous, the P/E boundary erosion within this part of our section and the event would be represented by only ϳ1.2 m, presence of fossils thought to be of middle a thickness of rock much less than our av- Eocene age (Dashzeveg and Hooker, 1997) erage sampling interval. Further, there is am- in the upper part of Eϩ, however, we feel ple evidence for signi®cant sedimentary hi- that it is likely that this is a composite po- atuses and erosional down-cutting in all three larity zone including the normal subchrons of our local sections, making it even more within C24n and subsequent normal polarity likely that the P/E boundary ␦13C excursion zones. interval was not present or not sampled in We also present two alternative correla- our sections. 22 AMERICAN MUSEUM NOVITATES NO. 3474

Fig. 12. Two alternative correlations of the Erlian Basin composite magnetic polarity zones and faunal zones (letter designations as in ®g. 11) to the Geomagnetic Polarity Timescale. A, Alternative global correlation based on intrabasin correlation A (see ®g. 10A). Polarity zone Cϩ, de®ned by the single normal polarity site at Huheboerhe, is considered to represent chron C25n, and polarity zones Aϩ and BϪ are shifted to correspond to C26n and C25r, respectively. Only one Gashatan collection, the upper Gashatan level at Bayan Ulan containing Palaeostylops, Pseudictops, and Palaeomylus, falls within C24r, and the range of Gashatan faunas is extended downward into C26n. B, Alternative global correlation based on intrabasin correlation B (see ®g. 10B). This is identical to the preferred global correlation (®g. 11) with the exception that the lower reversed polarity sites in the Huheboerhe section constitute a basal reversed polarity zone (designated aϪ), which is correlated to chron C25r. The range of Gashatan faunas in the composite section is extended downward into this zone by the Prodinoceras- bearing locality at Huheboerhe.

CONCLUSIONS: THE AGE OF EARLY Eocene boundary represented an interval PALEOGENE ALMAS within which the nature of Asian faunas was The new data from the Erlian Basin place unconstrained (Bowen et al., 2002; Ting et additional constraints on the age of Early Pa- al., 2003). This window of time represented leogene ALMAs and their correlation to the the potential extent to which Bumbanian fau- land mammal ages of North America and Eu- nas, and the Asian ®rst appearances of rope. These data are preliminary, and further ``new'' groups of mammals therein, might sampling will help to resolve the exact have predated these key ®rst appearances on chronostratigraphic placement of the Erlian the other Holarctic continents. If modern Basin faunas, but our current ®ndings pro- mammals evolved and diversi®ed in an vide evidence that faunas typical of the Asian Eden before dispersing throughout the Gashatan ALMA persist into chron C24r. Northern Hemisphere, they did so within this Previously, the ϳ900-ky interval of time be- interval. Under all three possible correlations tween the base of C24r and the Paleocene/ of the Erlian Basin composite section to the 2005 BOWEN ET AL.: ERLIAN BASIN STRATA 23

GPTS, Gashatan fossils are shown to range boundary is only now being realized. Close into the base of C24r, although the extent of ties between climatic events at the P/E their persistence within this chron varies sub- boundary and modernization of mammal stantially among correlations. According to faunas in North America and Europe have the preferred global correlation and the al- been established, but the nature of Asian fau- ternative based on intrabasin correlation B, nal modernization near the P/E boundary has Gashatan faunas are found throughout more remained a topic of debate. By constraining than half of the interval of the Erlian Basin the timing of taxonomic turnover for this re- composite section representing chron C24r, gion, we can begin to understand whether the implying that the Gashatan ALMA must ex- evolutionary modernization of faunas on the tend quite near to the Paleocene/Eocene Asian continent operated at its own pace, tied boundary. If the alternative global correlation to the rest of the Holarctic only by episodic shown in ®gure 12A proves correct, how- pulses of opportunistic interchange, or if ever, only a single Gashatan level would lie these communities developed in pace with, within C24r, and signi®cant uncertainty and presumably in response to the same ex- would remain regarding the placement of the trinsic or intrinsic forcing as, other Northern Gashatan/Bumbanian boundary relative to Hemisphere faunas. Our preliminary results the Paleocene/Eocene boundary. presented here suggest that modernization of Though they have received less scrutiny Asian faunas near the P/E boundary may than the Gashatan/Bumbanian boundary, the have been nearly synchronous with turnover ages of most other ALMA boundaries are al- in Europe and North America, and that mam- most entirely unconstrained by independent mals on these three continents may have data. If either alternative correlation shown evolved in close step through this climati- in ®gure 12 proves correct, the known range cally and environmentally dynamic interval. of Gashatan faunas will be extended downward into chron C25r and possibly C26n. ACKNOWLEDGMENTS This would imply that Gashatan faunas span an interval of time including the boundary We thank W. Clyde, R. Coe, G. Dickens, between the Tiffanian and Clarkforkian Thierry Smith, and Paul White for helpful NALMAs, which occurs within C25n, and reviews of this manuscript, and R. Coe, C. that the boundary between the Gashatan and Pluhar, and X. Zhao of the University of Cal- the earlier Nonshanian ALMA correlates ifornia Paleomagnetics laboratory for their within the Tiffanian NALMA. At this point, assistance. Funding for this work was pro- it is dif®cult to say anything concrete about vided by a U.S. National Science Foundation the upper boundary age of the Bumbanian grant to Koch, Meng, and collaborators ALMA based on our new data. Under intra- (EAR-0120727), National Natural Science basin correlation A, the Gomphos level at Foundation of China grants to Meng Huheboerhe is likely somewhat younger than (G200007707 and 49928202), and by the In- that at Bayan Ulan. Although the correlation stitute of Vertebrate Paleontology and Paleo- of this level relative to the C24r/C24n anthropology (IVPP) of the Chinese Acade- boundary preserved in the other two local my of Sciences. Bowen was supported by a sections is not well constrained, it appears National Science Foundation Graduate Re- that the Bumbanian level at Huheboerhe may search Fellowship during part of the project. lie within the upper part of C24r, making it Field work in Inner Mongolia would not somewhat younger than other Bumbanian have been possible without the logistical and faunas (Bowen et al., 2002; Ting et al., intellectual support of the staff and faculty of 2003). As noted above, Irdin Manhan fossils the IVPP. occur within what is likely to be a composite polarity interval, and further up-section sam- REFERENCES pling will be required in order to constrain Averianov, A. 1996. Early Eocene Rodentia of their age and correlation. Kyrgyzstan. Bulletin du MuseÂum National The full nature and magnitude of global d'Histoire Naturelle, 18: 629±662. change occurring at the Paleocene/Eocene Bao, H., P.L. Koch, and R.P. Hepple. 1998. He- 24 AMERICAN MUSEUM NOVITATES NO. 3474

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a This paper meets the requirements of ANSI/NISO Z39.48-1992 (Permanence of Paper).