Earth and Planetary Science Letters 389 (2014) 34–42

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Earth and Planetary Science Letters

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Aridification in continental Asia after the Middle EoceneClimatic Optimum (MECO)

Roderic E. Bosboom a, Hemmo A. Abels b,∗, Carina Hoorn c, Bas C.J. van den Berg a,ZhaoJie Guo e, Guillaume Dupont-Nivet a,d,e

a Paleomagnetic Laboratory Fort Hoofddijk, Department of Earth Sciences, Utrecht University, Budapestlaan 17, 3584 CD Utrecht, The Netherlandsb Department of Earth Sciences, Utrecht University, Utrecht, The Netherlandsc Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, Amsterdam, The Netherlandsd Géosciences Rennes, UMR 6118, Université de Rennes 1, Rennes, Francee Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, Peking University, Beijing 100871, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 20 November 2012Received in revised form 22 November 2013Accepted 10 December 2013Available online xxxxEditor: J. Lynch-Stieglitz

Keywords:AsiaXining BasinMiddle Eocene Climate Optimum (MECO)aridificationterrestrial sedimentspalynology

Global climate cooling from greenhouse to icehouse conditions occurred across an enigmatic transitionalinterval during the Eocene epoch characterized by incipient polar ice-sheet formation as well as short-lived warming events, of which the Middle Eocene Climatic Optimum (MECO) is most noticeable.Understanding this critical period requires high-resolution records that are being gathered in marinebasins, but are still lacking in the terrestrial realm. Here, we provide a precisely-dated terrestrial recordcrossing the MECO time interval from the Xining Basin (NW China). We document a rapid aridificationstep and the onset of obliquity-dominated climate cyclicity indicated by lithofacies and pollen recordsdated at 40.0 Ma at the base of magnetochron C18n.2n. This shift is concomitant – within error– with the MECO peak warming in Ocean Drilling Program Site 1258 for which we reassessed themagnetostratigraphic age at 40.0 Ma (also at base of magnetochron C18n.2n). The rapidity of the shiftobserved in the Xining Basin and the region-wide aridification and monsoonal intensification reportedaround 40 Ma suggests Asian paleoenvironments were responding to global climate changes associatedwith the MECO. However, the Xining records show only the permanent shift but not the transient peakwarming observed in marine MECO records. We thus relate this permanent aridification to occur duringthe post-MECO cooling. We propose the mechanisms linking global climate to Asian paleoenvironmentsmay be eustatic fluctuations driving the stepwise retreat of the proto-Paratethys epicontinental sea orsimply global cooling reducing moisture supply to the continental interior. In any case, Eocene globalclimate cooling from greenhouse to icehouse conditions seem to have played a primary role in shapingAsian paleoenvironments.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Ever since the first ocean sediments were drilled, hypothesesinvolving tectonic, oceanic and atmospheric dynamics have beenproposed to explain the dramatic shift from greenhouse to ice-house conditions during the Cenozoic (DeConto and Pollard, 2003).Better comprehension is arising from compiled deep-sea tempera-ture reconstructions from various ocean basins (Pälike et al., 2012)showing that after maximum temperatures in the early Eocene, en-suing long-term gradual cooling ultimately lead to major expansionof Antarctic ice-sheets at the Eocene–Oligocene Transition (EOT)33.9 million years ago (Ma; Zachos et al., 2008). This transitionalperiod of cooling is often referred to as the ‘doubthouse’ (Browning

* Corresponding author.E-mail address: h.a.abels@uu.nl (H.A. Abels).

0012-821X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.epsl.2013.12.014

et al., 1996) and is characterized by short-lived warming eventsand by periodic polar ice-sheet formations. Recently, this periodhas received considerable attention because it is believed to rep-resent the key interval to understand the long-term descent intothe icehouse (Bohaty and Zachos, 2003; Pälike et al., 2012). TheMiddle Eocene Climatic Optimum (MECO) is the most prominenttransient warming in this transitional period. The MECO was firstidentified in ocean drill cores from the Southern Ocean (Bohatyand Zachos, 2003). It has now been documented in stable iso-tope records worldwide, both in ocean drill sites in the Indian,Atlantic, Pacific and Southern Oceans and in land-based marinesections in Italy and the United Kingdom (Jovane et al., 2007;Villa et al., 2008; Luciani et al., 2010; Spofforth et al., 2010;Dawber et al., 2011). This clearly emphasizes the global im-pact of the MECO on the marine realm. The MECO is currentlyrecognized as a ∼500-kyr warming event between ∼40.5 and∼40 Ma characterized by gradual warming until sudden peak

R.E. Bosboom et al. / Earth and Planetary Science Letters 389 (2014) 34–42 35

Fig. 1. Study locations and geological setting. a. Location of the Xining Basin alongthe north-eastern margin of Tibetan Plateau. b. A simplified geological map of thefield area showing lithostratigraphic units, tectonic features and locations of thestudied sections.

warming (base of magnetochron C18n.2n) is followed by relativelyrapid post-MECO cooling within ∼50 kyr (Bohaty et al., 2009;Edgar et al., 2010). The MECO has been associated with short-livedshoaling of the calcite compensation depth (CCD; Lyle et al., 2005;Bohaty et al., 2009; Pälike et al., 2012), which is combined with atwo- to threefold increase of pCO2 (Bijl et al., 2010) not yet under-stood (Sluijs et al., 2013). The rise in pCO2 and temperature shouldlead to an enhanced hydrological cycle and increased weatheringrates, which should be visible in terrestrial settings. However, well-dated high-resolution paleoclimate records for this interval of timeare still lacking from the terrestrial realm.

Here, we present the first well-dated terrestrial record showingmajor paleoenvironmental change across the MECO event. Litho-stratigraphic and palynological analyses of the middle Eocene la-custrine successions of the Xining Basin in NW China reveal signif-icant paleoenvironmental change, while magnetostratigraphic dat-ing ensures accurate age calibration and comparison with marinerecords.

2. Regional paleoenvironmental context

The Xining Basin is a sub-basin in the western part of a broadPaleocene–Miocene basin complex along the north-eastern marginof the present-day Tibetan Plateau (Fig. 1a). After Mesozoic ini-tial basin development, Cenozoic reactivation followed at ∼50 and∼25 Ma due to the India–Asia collision (Dupont-Nivet et al., 2004;Horton et al., 2004). The basin provides a long and sub-completestratigraphic record from ∼50 to ∼15 Ma, which has been well-

dated using magnetostratigraphy (Dai et al., 2006; Xiao et al., 2010,2012; Abels et al., 2011).

Previous sedimentary and pollen analyses from this basinhave provided accurate records of stepwise regional aridificationassociated with global climate cooling culminating at the EOT(Dupont-Nivet et al., 2007, 2008; Xiao et al., 2010; Abels et al.,2011). Regular cyclicity of these sediments has been related to adominant obliquity-forcing signal with additional impact of shorteccentricity and interpreted as a sign for high-latitude climatescontrolling moisture supply to the area related to incipient polarice-volume fluctuations (Xiao et al., 2010; Abels et al., 2011). Inaddition to the stepwise aridification of dry desert to steppe pa-leoenvironmental conditions, pollen analysis indicates long-termeffect of regional uplift shown by the appearance of high-altitudePicea pollen in chron C16r at ∼36.9 Ma (Dupont-Nivet et al., 2008;Hoorn et al., 2012). In consequence, aridification has been in-terpreted as a combination of changing moisture supply to thecontinental interior in response to global oceanic cooling, retreatand desiccation of the shallow epicontinental proto-Paratethys Seato the west, as well as the long-term and stepwise regional upliftof various parts of the Tibetan Plateau (Dupont-Nivet et al., 2007;Abels et al., 2011; Bosboom et al., 2011, 2013).

3. Sections and sampling

Here, previously studied stratigraphic sections at Shuiwan andXiejia (Dai et al., 2006; Dupont-Nivet et al., 2007; Abels et al.,2011) are extended downward and complemented with the newTiefo and Dasigou sections (Figs. 1b and 2). Detailed lithologicallogs were made in the field. Samples were collected from evap-orite beds for X-Ray Diffraction (XRD) mineralogy and from var-ious lithologies throughout the sections for pollen analyses. XRDanalyses and pollen extraction were performed using standardizedtechniques, which are described in detail in the Supplementarymaterial.

Paleomagnetic sampling was performed using a standard por-table electric drill powered by a portable gasoline generator. Sam-ples were collected at an approximate resolution of 0.5 m andorientated with a magnetic compass mounted on an orientationstage. The field samples were cut into core specimens of approx-imately 10.5 cm3. Paleomagnetic analyses were carried out in theshielded Paleomagnetic Laboratory ‘Fort Hoofddijk’ of the Facultyof Geosciences at Utrecht University, the Netherlands. Represen-tative specimens of characteristic lithologies were powdered andanalyzed for their thermomagnetic behavior using a Curie balance(Mullender et al., 1993) and a KLY-3 susceptibility bridge. Singlespecimens were thermally demagnetized in a shielded oven by us-ing up to 22 temperature steps of 5 to 50 ◦C (Supplementary Fig.S3). The remanent magnetization at each temperature step wasmeasured by a 2G Enterprises DC SQUID cryogenic magnetometer.

4. Lithostratigraphy

The studied stratigraphic sections are characterized through-out the studied part of the basin by two distinct parts in termsof lithofacies, lithostratigraphic cyclicity and interpreted paleoen-vironments (Figs. 2 and 3). The upper part consists of regular al-ternations of massive reddish-brown clayey mudstone layers withgreenish muddy gypsum beds of basin-wide extent (Fig. 3b), previ-ously described and analyzed by Abels et al. (2011) at the Shuiwansection. The gypsum facies are interpreted as deposits of shallowephemeral to perennial saline lakes and the reddish-brown mud-stone layers as deposits of dry mudflat environments. Cyclostrati-graphic analyses based on the duration of dominant cyclicityusing magnetostratigraphically-constrained sediment accumulationrates from this upper part of the studied stratigraphic interval by

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Fig. 2. Litho- and magnetostratigraphy of the Dasigou, Tiefo and Xiejia sections with XRD mineralogy sample levels (a) and VGP latitude (b) with corresponding magnetic

polarity zones (c). The Xiejia section consists of two subsections which are separated by a non-exposed interval of unknown thickness.

Abels et al. (2011) has shown that the regular cyclicity is dom-inantly forced by obliquity (41-kyr) with additional influence ofshort eccentricity (100-kyr).

In the lower part, dry mudflat and saline lake lithofacies arealso dominant, but lithologies are much more variegated. The reg-ular cyclicity is absent and strong lateral variations preclude basin-wide bed-to-bed correlations. In particular, the prominent evap-oritic interval comprising laminated gypsum, anhydrite, glauberitebeds and thin interbedded clay layers is recognized as a peren-nial saline lake system with variable facies and age throughout thebasin (Fig. 3e; see Fig. 2 for XRD sample levels and SupplementaryFig. S1 for XRD bulk results). At the Dasigou section, this intervalincludes a several-meter thick massive sandstone bed indicative ofpoint-bar deposition in a fluvial channel. Also exclusively occur-ring in the lower part are decimetre-scale silt- and sandstone bedstypical of ephemeral fluvial deposits, centimetre-scale layers ofcalcium-carbonate interpreted as chemical precipitates in a lacus-trine environment and a hydromorphic paleosoil horizon showingcolor mottling, rootlets and bioturbation (Fig. 3c).

The transition from the lower to the upper part is character-ized by mudstone-gypsum alternations, which can only be partiallycorrelated across the basin due to lateral variation in depositionalenvironments. The start of the transitional interval is at cycle −23,while the first bed that can be straightforwardly correlated be-tween the four studied sections in the basin is cycle −15, markingthe beginning of the upper part of the studied stratigraphic inter-val (Figs. 2, 3a and 3d).

Lithofacies in the lower part clearly indicate wetter atmosphericconditions during which a more hydrodynamic depositional envi-

ronment led to higher morphological variability in the basin. Con-versely, in the upper part, drier conditions are in line with thedepositional environment becoming significantly more stable withindividual sedimentary cycles correlatable across the studied partof the basin.

Following previous interpretations of younger regional lithofa-cies steps recorded in the Xining Basin (Dupont-Nivet et al., 2007;Abels et al., 2011), we attribute this change to regional atmo-spheric aridification. Furthermore, the observed aridification stepis combined with the installation of obliquity-driven sedimentarycyclicity expressed up to the EOT, which have been previouslyattributed to high-latitude signal that could be enhanced by theoccurrence of incipient polar ice sheet formation at this time (Xiaoet al., 2010; Abels et al., 2011).

5. Palynology

To further ground our paleoenvironmental interpretation, wecompiled a composite palynological record from different litho-logical beds throughout the Shuiwan, Tiefo and Xiejia sections(Fig. 5; Supplementary Tables S1 to S3 and Fig. S2). Nearest liv-ing relatives are indicated where possible and taxa were groupedaccording to biome type following Yu et al. (2000) and Ni et al.(2010). Four main groups are distinguished labeled Steppe–Desert,Angiosperms–Gymnosperms, Broad-leaved Forest types, and Ferns.Gymnosperms are very rare and the Angiosperms–Gymnospermsgroup thus nearly consists of angiosperms only (see Supplemen-tary Table 1 for the composition of these groups).

The lower part of the succession is characterized by an av-erage 50% presence of the Steppe–Desert group, around 20–25%

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Fig. 3. Field photographs of studied sections. a. The shift from unstable pre-MECO tocyclic post-MECO lithofacies as recorded in the Dasigou section. b. Typical gypsum-mudstone cycles of 1 to 2 m thick at the top of the Dasigou section are character-istic of post-MECO lithostratigraphy. c. Paleosoils and fluvial sandstones define thepre-MECO lithofacies at the base of the Dasigou section (hammer for scale). d. Theshift from unstable pre-MECO to cyclic post-MECO lithofacies as recorded in theTiefo section. At the base lies the prominent white saline lake evaporite unit. e. Ra-dial glauberite crystals in a green mudstone matrix in the pre-MECO evaporite unitat the base of the Tiefo section (hammer for scale).

Angiosperms–Gymnosperms, 20–25% Broad-leaved Forest types,and on average 5% Ferns. Particularly taxa such as Ulmipollenites,but also Sapindaceidites, and to a lesser extent the Retitricolpitesgroup, are abundant in these levels. The nearest living relativeof Ulmipollenites is Ulmus (elm), a genus that presently occurs inthe Asian steppe–desert and is common in fluvial beds or whereground water is nearby (Dulamsuren et al., 2009; Wesche et al.,2011). Ulmipollenites (included in the Broad-leaved Forest group) isvery abundant at the top of the prominent evaporite interval in in-terbedded dark brown clay layers. Together, lithofacies and Ulmusabundance, are indicative of the wettest conditions in the basin.

The palynological composition noticeably changes from thelower to the upper part of the successions. In the upper part,Desert-Steppe vegetation dominates with 80% or more of thepollen sum. Angiosperms decrease to 5% and Broad-leaved Foresttypes reduce to 5–10%, while ferns are rare in the upper interval.This major change in vegetation marks a significant aridification ofthe paleoenvironment. The palynological change is observed in thedifferent sections and, despite a data gap due to poor preserva-tion of pollen, it coincides with the aridification deduced from thelithofacies and lithostratigraphic change (Fig. 5).

6. Magnetostratigraphic dating

To precisely date the identified lithofacies and pollen change inthe Xining record, we performed magnetostratigraphy on four par-allel sections that directly underlie and overlap the Shuiwan andTashan sections previously dated from ∼39 Ma up well into theOligocene ∼26 Ma (Abels et al., 2011; Xiao et al., 2012).

6.1. Rock magnetism

To determine the rock magnetic properties and carriers of thecollected samples and to devise the best procedure for subsequentthermal demagnetization, rock magnetic analyses were performedfirst. From these analyses two clearly distinct behavioral types canbe distinguished (Supplementary Fig. S3). Most of the samplesfrom dry mudflat deposits (red mudstones) generally show a de-crease in magnetization near the Curie temperature of magnetite(∼580 ◦C), maghemite (∼640 ◦C) and infrequently of hematite(∼680 ◦C), in agreement with previous studies (Dai et al., 2006;Xiao et al., 2010). Saline lake deposits (comprising muddy evap-orite beds and greyish-green mudstones) were also sampled. Theygenerally show a decrease in magnetization at temperatures con-siderably below or close to the Curie temperature of magnetite(∼580 ◦C). In some samples, an increase in magnetization around500 ◦C is interpreted to relate to the oxidation of iron-sulphidesinto magnetite (Passier et al., 2001).

6.2. Thermal demagnetization

Single specimen samples from the sections were thermally de-magnetized to reveal the remanent magnetization. For the prefer-ably sampled red mudstones, after progressive removal of an over-print component at temperatures of 100 to 300 ◦C, the trajectoriesof the characteristic remanent magnetization (ChRM) show lineardecay towards the origin from 300 ◦C to around 600 ◦C and infre-quently around 700 ◦C, supporting the combined presence of mag-netite and hematite as dominant ferromagnetic carriers in agree-ment with the rock magnetic results. Samples from evaporites alsohave an overprint up to 300 ◦C usually followed by decay towardthe origin around 500 ◦C, occasionally until 600 ◦C. This high tem-perature decay is often noisy because of the typically weak ChRMintensities of these samples and the apparent magnetic mineraltransformation. Therefore, samples with a high temperature re-versed polarity were clearly identified, but samples showing a hightemperature normal polarity component were sometimes ambigu-ous and rejected from further analysis if the secondary normaloverprint was not clearly separated from the higher temperaturecomponent.

6.3. ChRM direction analyses

For most samples, the ChRM directions were calculated by line-fits on orthogonal demagnetization diagrams using principal com-ponent analysis (Kirschvink, 1980) on a minimum of four temper-ature steps. Maximum angular deviations (MAD) on the line-fitswere usually below 15◦ , but MAD of up to 30◦ were accepted ifthe polarity could be clearly discerned. For a few weaker reversedsamples, typically from evaporite beds, the line-fits were forcedthrough the origin if directions clustered in a distinct reversed po-sition, and great circle ChRM analysis (McFadden and McElhinny,1988) was performed if the demagnetization path was non-linearbut along a great circle.

Virtual geomagnetic poles (VGPs) calculated from the ChRM di-rections show two distinct positive latitude (normal) and negativelatitude (reversed) groups. VGPs outlying more than 45◦ from themean normal and reversed VGP respectively were neglected fordetermining polarity zones (open points in Fig. 2). Using Fisherstatistics (Fisher, 1953) the means of remaining normal and re-versed ChRM directions were calculated (Supplementary Fig. S4).The Dasigou dataset passes the reversals test of McFadden andMcElhinny (1990) with classification C. The reversals test is neg-ative for Tiefo and Xiejia sections as the angle between the meansof normal and reversed polarity is larger than the critical angle.The offset is 4.4◦ for Tiefo and 24.8◦ for Xiejia and interpreted to

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Fig. 4. Magnetostratigraphy of Ocean Drilling Program (ODP) site 1051 in the Atlantic Ocean (Bohaty et al., 2009; Edgar et al., 2010) based on our reinterpretation of theoriginal data. The polarity zones defined by the calculated VGP latitudes are correlated to the Geological Time Scale 2012 (GTS12; Gradstein et al., 2012). The position of thepeak warming of the MECO is dated by linear interpolation assuming constant accumulation rate in C18n.2r. The thickness between the peak warming and the top of C18n.2ris (13 ± 4)% of the length of C18n.2r taken to represent 100% with uncertainties derived from the transitional intervals defining the base and top of C18n.2r indicated in greyshading. The peak warming is thus dated by linear interpolation at 40.01 ± 0.04 Ma when correlated to the GTS12 and assuming constant accumulation rates through theinterval. See Fig. 2 for the legend of magnetostratigraphic symbols.

be related to a partial overlapping normal overprint, expressed es-pecially in the weaker ChRM gypsiferous sediments of the salinelake type. This overprint, however, does in our view not affect thereliability of the magnetostratigraphy as previously noted for Xin-ing Basin records (Dupont-Nivet et al., 2007, 2008; Abels et al.,2011).

6.4. Correlation to GPTS

The polarity zones were defined based on at least two consec-utive VGP latitudes of identical polarity such that isolated direc-

tions within an interval of opposing polarity are neglected (Fig. 2).Apart from the lower Xiejia section consisting mainly of evaporites,the remarkably correlative polarity zonation between the Tiefo,Dasigou and upper Xiejia sections (Fig. 5) attests for the primaryorigin of the paleomagnetic signal and the reliability of the fol-lowing straightforward correlation to the Geologic Time Scale 2012(GTS12; Gradstein et al., 2012) from ∼44 Ma to ∼38 Ma (chronsC20 to C17).

The obtained pattern is the direct downward continuation ofthe previously correlated magnetostratigraphies of the Shuiwanand Tashan sections (Abels et al., 2011; Xiao et al., 2012) in

R.E. Bosboom et al. / Earth and Planetary Science Letters 389 (2014) 34–42 39

Fig. 5. Atmospheric aridification in the Xining Basin after the MECO event. Correlation of the four lithological sections in the Xining Basin by magnetostratigraphic correlationsto the GTS12. To the right, the composite pollen record is displayed against time, showing permanent aridification is coincident with the lithological aridification step. Tothe left, stable isotope stratigraphy and magnetostratigraphy from ODP Site 1051 (Edgar et al., 2010) are given, confirming the approximate coincidence of the MECO peakwarming with the aridification step on the Asian continent. See Fig. 2 for the legends of litho- and magnetostratigraphic symbols.

which a reliable magnetostratigraphic correlation is determinedfrom C8n.2n (26 Ma) down to C18n.2n (39 Ma). Accordingly,the upper two normal intervals are correlated to chrons C18nand C17n, like in the lithologically correlated lateral Shuiwansection. The long reversed interval below at Tiefo and Dasigoulikely represent the expected C18r and C19r. The expected oc-currence of the short C19n is represented by a few normal sam-ples consistently occurring at correlative levels of the Tiefo andDasigou sections. At the bases of Dasigou and Tiefo sections, thesubsequent long normal chron represents the next normal C20nunderlain by C20r at the Tiefo section (Fig. 2; SupplementaryFig. S5).

In the newly developed temporal framework, the environmentalshift and onset of cyclicity can be dated at 40.05±0.08 Ma assum-ing constant accumulation rates through C18r up to the positionof the lithofacies shift of cycle −15 and given the uncertaintiesin the positions of the top and base of C18r respectively. Alterna-tively, a very similar age of 40.00 ± 0.08 Ma is obtained for theshift if the top and base of C18n (rather than C18r) are used as areference for the calculation. This supports the robustness of ourage determination, although the real uncertainty range is likely tobe larger due to the assumption of constant sedimentation rates inthe Xining Basin over the duration of magnetochron C18r or C18n.Therefore, we place the absolute age for the onset of the cyclicity

40 R.E. Bosboom et al. / Earth and Planetary Science Letters 389 (2014) 34–42

(occurring in the base of chron C18n.2n) at 40.0 ± 0.1 Ma usingGTS12. Similarly, the start of the preceding transitional interval atcycle −23, in the top of chron C18r, is dated at 40.41 ± 0.11 Ma or40.41 ± 0.15 Ma for C18r and C18n, respectively, which leads us touse an age of 40.4 ± 0.2 Ma for further discussion (Fig. 5).

To compare our terrestrial record with global climate records,we reassess the age of the MECO event in Ocean Drilling Pro-gram (ODP) Site 1051, which is the only unambiguous record ofthe MECO directly coupled to high-resolution magnetostratigraphy(Fig. 4; Bohaty et al., 2009; Edgar et al., 2010). As uncertaintyintervals were not given in the original study, we re-interpretedthe paleomagnetic data from this site by calculating the ChRMdirections and VGPs from the majority of the original raw demag-netization data, similar to our own paleomagnetic analyses of theXining Basin sections. After cut-off of VGPs outlying more than 45◦from the mean, we obtain a polarity pattern very similar to that ofEdgar et al. (2010), but now including uncertainty intervals. As thetop of C18n.2n is lacking from the data, we calculated the positionof the MECO with respect to the base of C18r by again assum-ing constant accumulation rates through C18r up to the positionof the MECO and given the uncertainties in the positions of thetop and base of C18r respectively. Our magnetostratigraphic cali-bration gives a position within chron C18n.2n of 113.0 ± 3.6% withrespect to the base of chron C18r, yielding an age of the MECOpeak warming at 40.01±0.04 Ma assuming constant accumulationrates through the well-defined magnetochron C18r below. Directcomparison of this age with obtained ages for the environmentalshift in the Xining record (40.0 ± 0.1 Ma) suggests that the Asianaridification occurred roughly concomitant with the peak warm-ing of the MECO event at 40.01 ± 0.04 Ma (Fig. 5). However, aprobably real uncertainty of 0.2–0.4 million years, hampered bythe assumption of constant sediment accumulation rates over thelong chron C18r, are likely on these ages, preventing further deter-mination whether the Asian aridification occurred directly before,during, or after MECO peak warming.

7. Discussion

A permanent aridification step dated approximately concomi-tant in time with the transient peak warming of the MECO in themarine realm (Bohaty et al., 2009) is evident in our Asian terres-trial sedimentary records. However, a transient rather than a per-manent change would be expected in the Xining Basin records, be-cause the MECO has been associated to a peak in pCO2 (Bijl et al.,2010) that should affect continental paleoenvironments. This mayindicate that thresholds to changing depositional environments inthe Xining Basin were not reached during the MECO event, sug-gesting change in atmospheric climate during the MECO was notvery severe. However, the absence of a transient record may be at-tributed to the gap in our pollen record across the MECO eventand/or attributed to a depositional hiatus during the MECO tran-sient peak although accumulations are relatively constant through-out the interval from chrons C20n (41 Ma) up to C8n.2n (26 Ma)in this part of the Xining Basin.

The Xining Basin records have revealed further aridificationsteps similar to the one observed at the MECO. The most promi-nent occurs in the top of C17n.1n at ∼ 37.0 Ma where lithofa-cies and pollen indicate significant aridification (Abels et al., 2011;Hoorn et al., 2012), which has shown to be concomitant with high-altitude pollen appearance (Dupont-Nivet et al., 2008). The sub-sequent larger shift occurs at the onset of the Eocene–Oligocenegreenhouse to icehouse transition at ∼ 33.9 Ma (Dupont-Nivetet al., 2007; Xiao et al., 2010) with gypsum bed disappearanceinterpreted as atmospheric aridification. These aridification stepsoccurred within an depositional environment dominated by drymudflat and saline lakes experiencing changes from warmer and

wetter environments to colder and especially more arid conditionsin the Xining Basin. However, the aridification step at the MECO(∼40.0 Ma; base of chron C18n.2n) presented here marks the on-set of these dry mudflat and saline lacustrine paleoenvironmentsreplacing the underlying more hydrodynamic sedimentary environ-ments with wetter facies and palynology. This suggests this step isa prominent event, which is probably regionally extensive as ex-plained below.

Indeed, this well-dated record of climate deterioration in theXining Basin is regionally corroborated by other Asian sedimen-tary records with less age accuracy. Song et al. (2013) interpretclimate deterioration in the neighboring Qaidam Basin, with clearlong-term trends from warm and wet climate in the middle Eoceneto drier environments in the Oligocene. Here a drop of ther-mophilic palynological taxa (defined by Lu et al., 2010, as consist-ing of ferns like Pterisisporites, Polypodiaceaesporites, gymnospermssuch as Podocarpidites, Tsugaepollenites, and Rutaceoipollis) and de-crease in chemical weathering around the base of chron C18n.2nat 40 ± 1 Ma are interpreted as indicative of significant aridifica-tion. Similar to the Xining Basin, this major aridification step isreported to occur after a transient warmer and wetter intervalthat can be tentatively correlated to the transient peak warmingof the MECO and the aridification to the post-MECO cooling. In an-other section from the Qaidam Basin, aridification starting also ataround the base of chron C18n.2n at ∼40 Ma is indicated by heav-ier stable carbon isotopes of lake carbonate (Rieser et al., 2009).From Fushun, north-eastern China, Quan et al. (2011) report paly-nological data indicating East Asian monsoon intensification withan age attributed at 41–40 Ma after the late middle Eocene. Theauthors link the intensification to uplift of parts of the TibetanPlateau functioning as a rain shadow prohibiting eastern moistureto penetrate further west into the Asian continent.

However, the onset of Asian desertification has been associ-ated with monsoon intensification caused by the complex interplaybetween Tibetan Plateau uplift, global cooling and westward re-treat of the shallow epicontinental proto-Paratethys sea (Ramsteinet al., 1997; Guo et al., 2002; Dupont-Nivet et al., 2007, 2008;Bosboom et al., 2011; Huber and Goldner, 2011). The stepwisenature and the rapidity of the changes observed in the XiningBasin favors a swift mechanism such as global climate and/or epi-continental sea retreat rather than the gradual tectonic uplift ofthe Tibetan Plateau, although these probably worked in concertupon reaching threshold conditions (Dupont-Nivet et al., 2008;Hoorn et al., 2012). Oceanic global cooling may have had a di-rect influence on aridification by reducing moisture supply to thecontinental interior. More indirectly, successive sea-level loweringinduced by ice-sheet formation linked to global cooling may havegoverned the stepwise retreat of the shallow epicontinental seafrom Central Asia (Ramstein et al., 1997; Dupont-Nivet et al., 2007;Zhang et al., 2007). Recent study shows that the stepwise sea re-treat to the west and the stepwise aridification recorded in theXining Basin are closely correlated in time (Bosboom et al., 2011,2013), which suggests that the retreat has been an important forc-ing mechanism driving the desertification of Asia and may providethe mechanism linking global climate cooling to Asian aridification.

We therefore speculate that the observed shift in the Asianrecord is linked to the relatively rapid post-MECO cooling in themarine realm (Lyle et al., 2005; Villa et al., 2008; Bohaty et al.,2009; Pälike et al., 2012), rather than to the transient warm-ing during the MECO event, although both are within age uncer-tainty range. A link with global cooling is supported by the ob-served appearance of obliquity cycles after the aridification stepin the Xining record. This requires a climatic mechanism dom-inated by obliquity without significant influence of precession.However, at these lower latitudes, the direct influence of obliquityon solar insolation is small with respect to the influence of pre-

R.E. Bosboom et al. / Earth and Planetary Science Letters 389 (2014) 34–42 41

cession, which is mainly modulated by eccentricity (low latitu-dinal climate systems are otherwise only under significant obliq-uity influence at times of low eccentricity; Abels et al., 2009;Westerhold and Röhl, 2009). The prolonged occurrence of obliq-uity as observed in our records is thus more likely caused bya climate tele-connection to high-latitude climate cyclicity, whichcan be strongly and continuously sensitive to obliquity (Westerholdand Röhl, 2009; Xiao et al., 2010; Abels et al., 2011). Furthermore,modeling experiments show that ice volume variability forced byobliquity can be very large at times of incipient ice sheet for-mation (DeConto and Pollard, 2003) resulting in an even strongerinfluence of obliquity. Therefore, the sudden appearance of obliq-uity cycles after the MECO in the Xining records might relate toincipient polar ice-volume fluctuations long before the Eocene–Oligocene greenhouse–icehouse Transition (Abels et al., 2011).

8. Conclusions

Our results provide the first precisely-dated terrestrial recordacross the transitional late Eocene interval preceding the Eocene–Oligocene Transition in continental China. We find in lithofaciesand pollen of the Xining Basin the expression of a sudden per-manent aridification with a shift to obliquity-dominated climatecyclicity at the base of chron C18n.2n around 40 Ma. This promi-nent aridification step appears to be regionally correlative to anAsian paleoenvironmental transition associated with monsoonalintensification. We suggest that the stepwise nature of the Asianpaleoenvironmental changes indicates they are driven by globalclimate changes in concert with regional tectonic uplift, epiconti-nental sea retreat, and monsoonal intensity change. A link to globalcooling is supported by the appearance of obliquity-dominated cy-cles most likely related to high-latitude climate variability. Thismay be speculatively related to large ice volume variability dur-ing incipient polar ice-sheet formation reported to start at thistime. Rather than the transient MECO warming event, the observedaridification step could thus better be linked to post-MECO coolingmarking the onset of the final descent into the icehouse, ultimatelyleading to the Eocene–Oligocene greenhouse to icehouse transition.To ascertain these speculations, additional high-resolution recordsof Asian paleoenvironments as well as a better understanding ofice volume changes at this time are needed during this critical pe-riod in the greenhouse to icehouse transition.

Acknowledgements

The Netherlands Organization for Scientific Research (NWO) andthe Chinese Science Foundation (NCSF) supplied grants to R.B.,H.A., C.H., and G.D.N. We thank Marijn van Cappelle, Marijke Leb-bink, Julia Straathof, Dennis Terry, Huang Wentao, and Wang Yong-shen for field contributions and discussions, Mark Dekkers, MartineHagen, Martin Konert, Anita van Leeuwen, Tom Mullender, and An-nemarie Philip for laboratory assistance.

Appendix A. Supplementary material

Supplementary material related to this article can be found on-line at http://dx.doi.org/10.1016/j.epsl.2013.12.014.

References

Abels, H.A., Abdul Aziz, H., Ventra, D., Hilgen, F.J., 2009. Orbital climate forcing inmudflat to marginal lacustrine deposits in the Miocene Teruel Basin (NortheastSpain). J. Sediment. Res. 79, 831–847.

Abels, H.A., Dupont-Nivet, G., Xiao, G., Bosboom, R.E., Krijgsman, W., 2011. Step-wise Asian paleoenvironmental changes preceding the Eocene–Oligocene Tran-sition (EOT) in the terrestrial Xining Basin, China. Palaeogeogr. Palaeoclimatol.Palaeoecol. 299, 399–412.

Bijl, P.K., Houben, A.J.P., Schouten, S., Bohaty, S.M., Sluijs, A., Reichart, G.J., SinningheDamsté, J.S., Brinkhuis, H., 2010. Transient middle Eocene atmospheric CO2 andtemperature variations. Science 330, 819–821.

Bohaty, S.M., Zachos, J.C., 2003. Significant Southern Ocean warming event in thelate middle Eocene. Geology 31, 1017–1020.

Bohaty, S.M., Zachos, J.C., Florindo, F., Delaney, M.L., 2009. Coupled greenhousewarming and deep esa acidification in the middle Eocene. Paleoceanography 24,1–16.

Bosboom, R.E., Dupont-Nivet, G., Grothe, A., Brinkhuis, H., Villa, G., Mandic, O., Sto-ica, M., Huang, W., Yang, W., Guo, Z., Krijgsman, W., 2013. Linking Tarim Basinsea retreat (west China) and Asian aridification in the late Eocene. Basin Res.http://dx.doi.org/10.1111/bre.12054.

Bosboom, R.E., Dupont-Nivet, G., Houben, A.J.P., Brinkhuis, H., Villa, G., Mandic,O., Stoica, M., Zachariasse, W.-J., Guo, Z., Li, C., 2011. Late Eocene sea retreatfrom the Tarim Basin (West China) and concomitant Asian paleoenvironmentalchange. Palaeogeogr. Palaeoclimatol. Palaeoecol. 299, 385–398.

Browning, J.V., Miller, K.G., Pak, D.K., 1996. Global implications of lower to mid-dle Eocene sequence boundaries on the New Jersey coastal plain: The icehousecometh. Geology 24, 639–642.

Dai, S., Fang, X., Dupont-Nivet, G., Song, C., Gao, J., Krijgsman, W., Langereis, C.G.,Zhang, W., 2006. Magnetostratigraphy of Cenozoic sediments from the XiningBasin: Tectonic implications for the northeastern Tibetan Plateau. J. Geophys.Res. 111.

Dawber, C.F., Tripati, A.K., Gale, A.S., MacNiocaill, C., Hesselbo, S.P., 2011.Glacioeustasy during the middle Eocene? Insights from the stratigraphy of theHampshire Basin, UK. Palaeogeogr. Palaeoclimatol. Palaeoecol. 300, 84–100.

DeConto, R.M., Pollard, D., 2003. Rapid Cenozoic glaciation of Antarctica induced bydeclining atmospheric CO2. Nature 421, 245–249.

Dulamsuren, C., Hauck, M., Nyambayar, S., Bader, M., Osokhjargal, D., Oyungerel,S., Leuschner, C., 2009. Performance of Siberian elm (Ulmus pumila) on steppeslopes of the northern Mongolian mountain taiga: Drought stress and herbivoryin mature trees. Environ. Exp. Botany 66, 18–24.

Dupont-Nivet, G., Horton, B.K., Zhou, J., Waanders, G.L., Butler, R.F., Wang, J., 2004.Paleogene clockwise tectonic rotation of the Xining–Lanzhou region, northeast-ern Tibetan Plateau. J. Geophys. Res. 109.

Dupont-Nivet, G., Krijgsman, W., Langereis, C.G., Abels, H.A., Dai, S., Fang, X., 2007.Tibetan plateau aridification linked to global cooling at the Eocene–Oligocenetransition. Nature 445, 635–638.

Dupont-Nivet, G., Hoorn, C., Konert, M., 2008. Tibetan uplift prior to the Eocene–Oligocene climate transition: evidence from pollen analysis of the Xining Basin.Geology 36, 987–990.

Edgar, K.M., Wilson, P.A., Sexton, P.F., Gibbs, S.J., Roberts, A.P., Norris, R.D., 2010.New biostratigraphic, magnetostratigraphic and isotopic insights into the Mid-dle Eocene Climatic Optimum in low latitudes. Palaeogeogr. Palaeoclimatol.Palaeoecol. 297, 670–682.

Fisher, R.A., 1953. Dispersion on a sphere. Proc. R. Soc. Lond. Ser. A, Math. Phys.Sci. 217, 295–305.

Gradstein, F.M., Ogg, J.G., Schmitz, M., Ogg, G., 2012. The Geologic Time Scale 2012.Elsevier.

Guo, Z.T., Ruddiman, W.F., Hao, Q.Z., Wu, H.B., Qiao, Y.S., Zhu, R.X., Peng, S.Z., Wei, J.J.,Yuan, B.Y., Liu, T.S., 2002. Onset of Asian desertification by 22 Myr ago inferredfrom loess deposits in China. Nature 416, 159–163.

Hoorn, C., Straathof, J., Abels, H.A., Yadong, X., Utescher, T., Dupont-Nivet, G., 2012.Late Eocene palynological record of climate change and Tibetan Plateau uplift(Xining Basin, China). Palaeogeogr. Palaeoclimatol. Palaeoecol. 344–345, 16–38.

Horton, B.K., Dupont-Nivet, G., Zhou, J., Waanders, G.L., Butler, R.F., Wang, J.,2004. Mesozoic–Cenozoic evolution of the Xining–Minhe and Dangchang basins,northeastern Tibetan plateau: Magnetostratigraphic and biostratigraphic results.J. Geophys. Res. 109.

Huber, M., Goldner, A., 2011. Eocene monsoons. J. Asian Earth Sci. 44, 3–23.Jovane, L., Florindo, F., Coccioni, R., Dinarès-Turell, J., Marsili, A., Monechi, S., Roberts,

A.P., Sprovieri, M., 2007. The middle Eocene climatic optimum event in theContessa Highway section, Umbrian Apennines, Italy. Geol. Soc. Am. Bull. 119,413–427.

Kirschvink, J.L., 1980. The least-square line and plane and the analysis of paleomag-netic data. Geophys. J. R. Astron. Soc. 62, 699–718.

Lu, J.F., Song, B.W., Chen, R.M., Zhang, J.Y., Ye, H., 2010. Palynological assemblageof Eocene–Oligocene pollen and their biostratigraphic correlation in Dahonggou,Daqaidam Regions, Qaidam Basin. Earth Sci. J. China Univ. Geosci. 35, 839–848.

Luciani, V., Giusberti, L., Agnini, C., Fornaciari, E., Rio, D., Spofforth, D.J.A., Pälike, H.,2010. Ecological and evolutionary response of Tethyan planktonic foraminiferato the middle Eocene climatic optimum (MECO) from the Alano section (NEItaly). Palaeogeogr. Palaeoclimatol. Palaeoecol. 292, 82–95.

Lyle, M., Olivarez-Lyle, A., Backman, J., Tripati, A., 2005. Biogenic sedimentation inthe Eocene equatorial Pacific—the stuttering greenhouse and Eocene carbonatecompensation depth. In: Wilson, P.A., Lyle, M., Firth, J.V. (Eds.), Proceedings ofthe Ocean Drilling Program, Scientific Results, pp. 1–35.

McFadden, P.L., McElhinny, M.W., 1988. The combined analysis of remagnetizationcircles and direct observations in palaeomagnetism. Earth Planet. Sci. Lett. 87,161–172.

42 R.E. Bosboom et al. / Earth and Planetary Science Letters 389 (2014) 34–42

McFadden, P.L., McElhinny, M.W., 1990. Classification of the reversal test in palaeo-magnetism. Geophys. J. Int. 103, 725–729.

Mullender, T.A.T., van Velzen, A.J., Dekkers, M., 1993. Continuous drift correctionand separate identification of ferromagnetic and paramagnetic contribution inthermomagnetic runs. Geophys. J. Int. 114, 663–672.

Ni, J., Yu, G., Harrison, S.P., Prentice, I.C., 2010. Paleovegetation in China during thelate Quaternary: biome reconstructions based on a global scheme of plant func-tional types. Paleogeogr. Paleoclimatol. Paleoecol. 289, 44–61.

Pälike, H., Lyle, M.W., Nishi, H., Raffi, I., Ridgwell, A., Kusali, Gamage, Klaus, A., Ac-ton, G., Anderson, L., Backman, J., Baldauf, J., Beltran, C., Bohaty, S.M., Busch,P.B.W., Channell, J.E.T., Chun, C.O.J., Delaney, M., Dewangan, P., Jones, T.D., Edgar,K.M., Evans, H., Fitch, P., Foster, G.L., Gussone, N., Hasegawa, H., Hathorne,E.C., Hayashi, H., Herrle, J.O., Holbourn, A., Hovan, S., Hyeong, K., Iijima, K.,Ito, T., Kamikuri, S.-i., Kimoto, K., Kuroda, J., Leon-Rodriguez, L., Malinverno, A.,Moore Jr., T.C., Murphy, B.H., Murphy, D.P., Nakamura, H., Ogane, K., Ohneiser, C.,Richter, C., Robinson, R., Rohling, E.J., Romero, O., Sawada, K., Scher, H., Schnei-der, L., Sluijs, A., Takata, H., Tian, J., Tsujimoto, A., Wade, B.S., Westerhold, T.,Wilkens, R., Williams, T., Wilson, P.A., Yamamoto, Y., Yamamoto, S., Yamazaki, T.,Zeebe, R.E., 2012. A Cenozoic record of the equatorial Pacific carbonate compen-sation depth. Nature 488, 609–614.

Passier, H.F., de Lange, G.J., Dekkers, M.J., 2001. Magnetic properties and geochem-istry of the active oxidation front and the youngest sapropel in the easternMediterranean Sea. Geophys. J. Int. 145, 604–614.

Quan, C., Liu, Y.S.C., Utescher, T., 2011. Paleogene evolution of precipitation in north-eastern China supporting the middle Eocene intensification of the East Asianmonsoon. Palaios 26, 743–753.

Ramstein, G., Fluteau, F., Besse, J., Joussaume, S., 1997. Effect of orogeny, plate mo-tion and land–sea distribution on Eurasian climate change over the past 30million years. Nature 386, 788–795.

Rieser, A.B., Bojar, A.-V., Neubauer, F., Genser, J., Liu, Y., Ge, X.-H., Friedl, G., 2009.Monitoring Cenozoic climate evolution of northeastern Tibet: stable isotope con-straints from the western Qaidam Basin, China. Int. J. Earth Sci. 98, 1063–1075.

Sluijs, A., Zeebe, R.E., Bijl, P.K., Bohaty, S.M., 2013. A middle Eocene carbon cycleconundrum. Nat. Geosci. 6, 429–434.

Song, B., Zhang, K., Lu, J., Wang, C., Xu, Y., Greenough, J., 2013. The middle Eocene

to early Miocene integrated sedimentary record in the Qaidam Basin and its im-plications for paleoclimate and early Tibetan Plateau uplift. Can. J. Earth Sci. 50,1–14.

Spofforth, D.J.A., Agnini, C., Pälike, H., Rio, D., Fornaciari, E., Giusberti, L., Luciani, V.,Lanci, L., Muttoni, G., 2010. Organic carbon burial following the middle Eoceneclimatic optimum in the central western Tethys. Paleoceanography 25.

Villa, G., Fioroni, F., Pea, L., Bohaty, S., Persico, D., 2008. Middle Eocene–lateOligocene climate variability: Calcareous nannofossil response at KerguelenPlateau, Site 748. Marine Micropaleontol. 69, 173–192.

Wesche, K., Walther, D., von Wehrden, H., Hensen, I., 2011. Trees in the desert:Reproduction and genetic structure of fragmented Ulmus pumila forests in Mon-golian drylands. Flora 206, 91–99.

Westerhold, T., Röhl, U., 2009. High resolution cyclostratigraphy of the earlyEocene – new insights into the origin of the Cenozoic cooling trend. Clim. Past 5,309–327.

Xiao, G., Abels, H.A., Yao, Z., Dupont-Nivet, G., Hilgen, F.J., 2010. Asian aridifica-tion linked to the first step of the Eocene–Oligocene climate Transition (EOT)in obliquity-dominated terrestrial records (Xining Basin, China). Clim. Past 6,627–657.

Xiao, G., Guo, Z., Dupont-Nivet, G., Lu, H., Wu, N., Ge, J., Hao, Q., Peng, S., Li, F., Abels,H.A., Zhang, K., 2012. Evidence for northeastern Tibetan Plateau uplift between25 and 20 Ma in the sedimentary archive of the Xining Basin, NorthwesternChina. Earth Planet. Sci. Lett. 317–318, 185–195.

Yu, G., Chen, X., Ni, J., Cheddadi, R., Guiot, J., Han, H., Harrison, S.P., Huang, C., Ke,M., Kong, Z., Li, S., Li, W., Liew, P., Liu, G., Liu, J., Liu, Q., Liu, K.-B., Prentice, I.C.,Qui, W., Ren, G., Song, C., Sugita, S., Sun, X., Tang, L., Van Campo, E., Xia, Y.,Xu, Q., Yan, S., Yang, X., Zhao, J., 2000. Palaeovegetation of China: a pollen data-based synthesis for the mid-Holocene and last glacial maximum. J. Biogeogr. 27,635–664. http://dx.doi.org/10.1046/j.1365-2699.2000.00431.x.

Zachos, J.C., Dickens, G.R., Zeebe, R.E., 2008. An early Cenozoic perspec-tive on greenhouse warming and carbon-cycle dynamics. Nature 451,279–283.

Zhang, Z., Wang, H., Guo, Z., Jiang, D., 2007. What triggers the transition of palaeoen-vironmental patterns in China, the Tibetan Plateau uplift or the Paratethys Searetreat?. Palaeogeogr. Palaeoclimatol. Palaeoecol. 245, 317–331.