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Quaternary Science Reviews 25 (2006) 2251–2257

Does d13Ccarb of the Chinese loess indicate past C3/C4 abundance?A review of research on stable carbon isotopes of the Chinese loess

Zhiguo Raoa,b,c,�, Zhaoyu Zhua, Fahu Chenb, Jiawu Zhangb

aKey Laboratory of Marginal Sea Geology, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, ChinabSino-German Center for Arid Environment and Paleoclimate Research, Key Laboratory of Western China’s Environment Systems,

Ministry of Education, College of Resources and Environment, Lanzhou University, Lanzhou 730000, ChinacGraduate University of Chinese Academy of Sciences, Beijing 100039, China

Received 24 September 2005; accepted 30 March 2006

Abstract

Current research on stable carbon isotopes of the Chinese loess is reviewed. The stable carbon isotopic composition of soil organic

matter (SOM) (d13Corg) of paleosols formed during interglacials is more positive than that within loess accumulated during glacials.

Spatially, it becomes progressively more negative from the eastern Chinese Loess Plateau (CLP) to the western. However, stable carbon

isotopic composition of pedogenic carbonates (d13Ccarb) from the same sequence produces an opposing trend, both temporally and

spatially, and variation differs from expectations under conditions of equilibrium. This indicates that d13Ccarb in the Chinese loess cannot

be used to reconstruct the relative proportion of C3/C4 plants in past ecological communities. Furthermore, use of d13Ccarb to reconstruct

the expansion of C4 plants in older Tertiary red clay deposits is considered unreliable. We believe d13Ccarb may act as an indirect indicator

of vegetation density, following on changes in moisture supply controlled by the intensity of the Asian Summer Monsoon, but conclude

that full understanding of the paleoclimatic significance of d13Ccarb in the Chinese loess and underlying Tertiary red clay requires better

understanding of modern isotopic processes.

r 2006 Elsevier Ltd. All rights reserved.

1. Introduction

Plants assimilate carbon by one of three photosyntheticpathways, commonly called C3, C4 and Crassulacean acidmetabolism (CAM, Osmond, 1978) pathways. CAM plantsexist only in particular, specialized ecosystems such asdeserts. Pure C3 and C4 plants have d13C values rangingfrom ca �22% to �30% and �10% to �14%, respectively(Deines, 1980; Farquhar et al., 1989; O’Leary, 1981, 1988).Soil organic matter (SOM) preserves this isotopic distinc-tion with little or no isotopic fractionation (Melillo et al.,1989), and the relative proportion of C3 and C4 plants inlocal biomass can be inferred from the d13C compositionsof SOM (Jurgen Runge, 2002; Kingston et al., 1994;

e front matter r 2006 Elsevier Ltd. All rights reserved.

ascirev.2006.03.013

ing author. Key Laboratory of Marginal Sea Geology,

stitute of Geochemistry, Chinese Academy of Sciences,

640, China. Tel.: +86 20 8529 0061;

9 0130.

ess: zgrao@gig.ac.cn (Z. Rao).

Krishnamurthy and Bhattaharya, 1989; Krishnamurthyand DeNiro, 1982; Nordt et al., 1994; Schwartz et al., 1986;Stanley and Nancy, 1991). Cerling (1984) and Cerling et al.(1989) found that carbon isotopic compositions of modernsoil carbonates (d13Ccarb) and coexisting organic matter(d13Corg) differ systematically by 14–16% in undisturbedmodern soils. This difference is compatible with isotopicequilibrium between gaseous CO2 and aqueous and solidcarbonate species in the soil system, controlled by diffusivemass transfer of soil CO2 derived from irreversibleoxidation of SOM. Therefore, it was thought that d13Ccarb

could inform about C4/C3 abundance (Quade et al., 1989a),providing a foundation for employing d13Ccarb in paleoe-cological and paleoenvironmental reconstructions (Cerling,1992; Cerling and Hay, 1986; Cerling et al., 1988, 1993;Cole and Monger, 1994; Quade et al., 1989b, 1994; Quadeand Cerling, 1995).However, the present review indicates that the d13Corg

and d13Ccarb of the Chinese loess do not change

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conformably in both temporal and spatial sequences. Thedifference between these two phases is not limited to14–16%, and the value increases from eastern to westernChinese Loess Plateau (CLP), suggesting that the d13Ccarb

of the Chinese loess cannot be used to reconstruct therelative proportion of C3/C4 plants in paleoecosystems, asprevious researchers have done (Chen et al., 1996; Frakesand Sun, 1994; Han et al., 1995, 1996, 1997; Jiang et al.,2001).

Expansion of C4 vegetation in the late Miocene has beenwidely reported (Cerling et al., 1993, 1997; Fox and Koch,2003; Latorre et al., 1997; Morgan et al., 1994; Quade andCerling, 1995; Quade et al., 1989b, 1994). Aeolian-deposited Tertiary red clay that underlies the Chinese loessmay provide high resolution data regarding climate andvegetation change since the late Miocene (Guo et al., 2002).Heretofore, we have known little of the relationshipbetween Tertiary red clay d13Ccarb and d13Corg, but thepresent review of stable carbon isotopic studies of theChinese loess may help to reevaluate the use of Tertiary redclay d13Ccarb for past C3/C4 abundance reconstruction.Such studies of the d13Ccarb of the Tertiary red clay (Anet al., 2005; Chen and Zhao, 1997; Ding and Yang, 2000;Jiang et al., 2002; Yang et al., 1999) indicate that, similar tothe loess, it may be seen as an indicator of past C3/C4

abundance (An et al., 2005; Ding and Yang, 2000).

Qinghai Lake

Xining

Lanzhou

YuanbaoBaxie

Baoji

Lingtai

Qinghai-Tibet Plateau

Huanxian

Chinese Lo

Tengger desert

Yinchuan

Huining

C

Loess section forLoess section forLoess section for

N

E

Yellow River

Yangtse River

Beijing

34

36

38

40°

°

°

°

° °100 104

δ 13

δ13 Cδ 13 C

Fig. 1. The location of profiles for stable carbon i

2. Review of stable carbon isotopes of the Chinese loess

2.1. d13Corg studies of the Chinese loess

The d13Corg in more than ten Chinese loess profiles hasbeen examined (Fig. 1) (An et al., 1993; Gu et al., 2003; Heet al., 2002; Lin and Liu, 1992; Lin et al., 1991, 1992; Liuet al., 2002, 2005a, b; Rao et al., 2005; Vidic and Montanez,2004; Wang and Follmer, 1998; Wang et al., 1997; Zhanget al., 2003). Almost all available data show that d13Corg ofpaleosols developed during interglacials is more positivethan that of loess accumulated during glacials (Fig. 2). Guet al. (2003) compared the d13Corg of six profiles postdatingthe Last Glacial, located in the central and south CLP, andfound that the relative abundance of C4 plants increased ca40% at each sampling site from the Last Glacial Maximum(LGM) to the Holocene optimum, and increased south-eastward on the CLP during both the LGM and Holoceneperiods.The high resolution d13Corg record of the Yuanbao

profile (Fig. 2) (Rao et al., 2005) at the western edge of theCLP confirmed this trend; the d13Corg variations of theLast Glacial in this profile clearly showed pure C3

vegetation ranging from �24% to �27%. Other investiga-tions of the profiles, shown in Fig. 2, support the commonview that the abundance of C4 plants increased from

Xi'an

Taiyuan

Xunyi

river, lake

city

Lantian

WeinanYanshi Zhengzhou

JixianLuochuan

Xifeng

Jiaodao

ess Plateau

Mu us desert

Huhehaote

Yellow river

Shijiazhuang

hangwu

Fugu

researchresearch

and research

Red Clay Section for research

° ° °108 112 116

Corg

carb

org δ 13 Ccarb

δ 13Ccarb

sotopic research in the Chinese Loess Plateau.

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Fig. 2. The d13Corg records of Chinese loess profiles and their spatial

comparison. Profile locations are shown in Fig. 1. All records except

Yuanbao show more positive d13Corg in paleosols (interglacial) in

comparison with the loess (glacial). The Yuanbao d13Corg record of the

Last Glacial shows the characteristic change of pure C3 vegetation. The

comparison, based on stratigraphy, clearly shows a negative spatial shift

of d13Corg westward and northward in the CLP in both loess and

paleosols. d13Corg record of Xunyi modified from Liu et al. (2002), Lantian

and Luochuan from Liu et al. (2005a), Yanshi, Weinan and Baoji from Gu

et al. (2003), Yuanbao from Rao et al. (2005).

Z. Rao et al. / Quaternary Science Reviews 25 (2006) 2251–2257 2253

glacials to interglacials and from the northwestern tosoutheastern CLP (Liu et al., 2002, 2005a; Vidic andMontanez, 2004; Zhang et al., 2003).

2.2. d13Ccarb studies of the Chinese loess

The d13Ccarb variations of some profiles on the CLP havealso been studied (Chen et al., 1996; Frakes and Sun, 1994;Gu, 1991; Gu et al., 1992, 2000; Han et al., 1995, 1996,1997; Jiang et al., 2001; Li et al., 1995; Wang and Follmer,1998; Wang et al., 1997) (Fig. 1). As shown in Figs. 3 and 4,d13Ccarb of paleosols formed during the warm, moistinterglacials is negative, becoming more positive in loessaccumulated under cold, dry glacial conditions. Studies ofcarbonate nodules (pedogenic carbonate) from the upperfourteen paleosol horizons in the Xifeng and Luochuanprofiles indicate that the best developed paleosols have themost negative d13Ccarb values (Han et al., 1995, 1996,1997). In the S1 paleosol horizons, analysis of the carbonisotopic compositions of pedogenic carbonates reveals thatthe mean d13Ccarb value in the Xifeng profile (�6.7%, 3samples), located in the northwestern CLP, is more positivethan that in the Luochuan (�8.6%, 6 samples) and Lantianprofiles (�8.4%, 4 samples) in the eastern and southernCLP (Chen et al., 1996). Comparison of the d13Ccarb

records since the Last Interglacial of the Weinan andHuining profiles of the southeastern and western CLP,gives an apparent spatial trend, with d13Ccarb becomingmore positive northwestward, from ca �9% to �5%(in Weinan) and �6% to 0% (in Huining), respectively(Fig. 3). Thus, d13Ccarb data from the Chinese loess indicatethat when the climate became warmer and moister (fromwest to east and from glacial to interglacial periods), thed13Ccarb became more negative. This characteristic ofd13Ccarb, however, is completely opposite the variation ofd13Corg in the Chinese loess.

3. Discussion

The d13Ccarb data from the Chinese loess often have beenexplained by reference to past changes in the relativeabundance of C4/C3 vegetation (Chen et al., 1996; Frakesand Sun, 1994; Han et al., 1995, 1996, 1997; Jiang et al.,2001), although their variation trends are opposite ofd13Corg data. As a result, any estimation of past C4/C3

abundance based on these two classes of data will produceconflicting results. For example, the estimated abundanceof C4 plants increased by 40% from the LGM to theHolocene based on d13Corg data from the Luochuan profile(Gu et al., 2003), but decreased by 15% based on d13Ccarb

data from the same profile (Frakes and Sun, 1994) (Fig. 4).Because SOM derives directly from the local biomass,d13Corg has the advantage for reconstruction of C4/C3

ratios relative to d13Ccarb. Therefore, we should be verycautious about using d13Ccarb of the Chinese loess toreconstruct C4/C3 ratios.

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Dep

th (

m)

Depth (m

)

δ 13Ccarb (‰ VPDB) δ 18O( ‰ VPDB)-8 -6 -4 -2 0

0

2

4

6

8

10

12

14

16

18

20

22

24

-10 -9- 8 -7- 6 -5 -4 -10 -4-6-8

2

4

6

8

10

12

HuiningWeinan

Stra

tigra

phy

S0

L1

S1loess

paleosol

Fig. 3. The d13Ccarb and d18O records of Weinan profile (located at the southeastern edge of the CLP, Fig. 1) and d13Ccarb record of Huining profile

(located in the western CLP, Fig. 1). These two d13Ccarb records show that it becomes increasingly negative from loess (glacial) to paleosols (interglacial).

The comparison of this two d13Ccarb records shows a spatial positive shift in both loess and paleosol northwestward. There is an opposite trend between the

d13Ccarb and d18O records of Weinan profile which is modified from Han et al. (1996), and Huining from Jiang et al. (2001).

mordern soil weak paleosol losss paleosol

S0

L1LL1

L1LS1

L1LL2

S1

L2

(si unit)Magnetic susceptibility

01 00 200

13

11

9

7

5

3

0

Dep

th (m

)

-10 -8 -6 -4

1

Depth (m

)

12

10

8

6

4

2

0-22 -19-20-21-23-24

MIS1

MIS2

MIS3

MIS4

MIS5

δ 13Corg(‰ VPDB)δ 13Ccarb

(‰ VPDB) Stratigraphy

Fig. 4. The d13Ccarb, d13Corg and magnetic susceptibility (MS) records of the Luochuan loess profile. There is an opposing trend between d13Ccarb and

d13Corg along the temporal sequence; the difference between them is wide ranging and decreases from loess (glacial) to paleosol (interglacial). MS is a

traditional indicator of the intensity of Asian summer monsoon and soil development; high MS indicates strong Asian summer monsoon and well-

developed soil. It is clear that the relationship between climatic conditions and d13Ccarb is different to d13Corg; this may indicate they do not share the same

carbon source. d13Ccarb record modified from Frakes and Sun. (1994), d13Corg and MS record after Liu et al. (2005a).

Z. Rao et al. / Quaternary Science Reviews 25 (2006) 2251–22572254

Wang and Follmer (1998) and Wang et al. (2004)recognized the differences between d13Ccarb and d13Corg inthe Chinese and North America loess-paleosol sequence,

and have suggested that they are controlled mainly bymonsoon seasonality. In their source-control model (Wangand Follmer, 1998), increasing abundance of C4 biomass

ARTICLE IN PRESSZ. Rao et al. / Quaternary Science Reviews 25 (2006) 2251–2257 2255

will result in more positive d13Ccarb values and greaterdifferences between d13Ccarb and d13Corg. However,increasingly abundant C4 biomass seen in paleosols andin the southeastern CLP, known from d13Corg datacoupled with more negative d13Ccarb values and smallerdifferences between d13Ccarb and d13Corg, indicate theWang et al. model may not be suitable for observing theChinese loess.

Dever et al. (1987) discussed the stable isotopiccomposition of secondary calcite under semi-arid condi-tions, noting that enhanced evaporation (which wouldproduce a more positive d18O value) under drier climateconditions would result in increased rates of CO2 degassingfrom the soil moisture, and that this may produce a morepositive d13Ccarb value. On the other hand, consider thatmore positive d13Ccarb values in loess accumulated duringglacials, coupled with more negative d18O values, as shownby Fig. 3, may indicate that evaporation and degassing arenot the major factors controlling stable carbon isotopiccomposition of carbonate in the Chinese loess.

Coexisting d13Ccarb and d13Corg variations of NorthAmerica modern soils have been studied (Cerling, 1984;Cerling et al., 1989; Quade et al., 1989a). The relationshipof the two phases of isotopes shows a systematic differenceof 14–16% between them, possibly indicating that theyshare the same carbon source, i.e. the local plants. Thesame difference occurred in late Miocene-Pleistocenepaleosols in the Siwalik geological sequence in northernPakistan (Cerling et al., 1989) and paleosols of an alluvialfan system in the Chihuahua desert of New Mexico.In these cases, d13Ccarb data are seen as an indicator of pastvegetation type (Cerling et al, 1993; Cole and Monger,1994; Ehleringer et al., 1991; Quade and Cerling, 1995;Quade et al., 1989b). The difference between the twophases of a site in north-central Texas was reported torange from 8.88% to 12.87% (Humphrey and Ferring,1994), lower than that reported by Cerling et al. (1989).Nonetheless, an overall covariance between these tworecords suggests the d13Ccarb data could be used for C4

abundance reconstruction. The difference between d13Ccarb

and d13Corg in the Chinese loess, however, has largelyexceeded that reported by Cerling et al. (1989) Forinstance, calculations on 71 couplets of d13Ccarb andd13Corg data from the same samples from Chenjiawo,Liujiapo and Gongwangling profiles, located at the south-eastern edge of the CLP (Lantian), show that only 13couplets of data differ by 14–16%, with an overall range of9–23% ( Wang and Follmer, 1998; Wang et al., 1997).Comparison of d13Ccarb and d13Corg records since the LastInterglacial from the Luochuan profile (Fig. 4) show thatthe difference between the two phases lowered fromglacials (e.g., ca 30% for the Last Glacial) to interglacials(e.g., ca 10% for the Holocene). Due to spatial variation,the difference becomes lower southeastwards in the CLP.These characteristics of d13Ccarb and d13Corg in the Chineseloess demonstrate that they do not share the same carbonsource.

Cerling (1984) demonstrated mathematically that d13Cof soil CO2 is a function of the isotopic composition ofrespired CO2, the diffusion of CO2 through the soil, andatmospheric mixing. During periods of low biologicalactivity, d13C of the soil CO2 can differ greatly from that ofthe local vegetation. Thus, it is possible that d13C ofpedogenic carbonates in some environments may not bedirectly related to that of the respired CO2 and thevegetation. Studies on the relationship between the stablecarbon isotopic composition of the soil environment andthe modern climate and vegetation of the eastern MojaveDesert in Nevada (an environment similar to that of theCLP) indicate that d13Ccarb is controlled by differences inplant density and biological activity (Amundson et al.,1988). Investigation of the d13C of plants, SOM andmodern carbonates in the Palouse region of easternWashington State showed significant linear regression ford13Ccarb versus annual average precipitation, estimatedabove-ground productivity, and soil carbon content.Among them, the correlation between d13Ccarb andestimated above-ground productivity is highest (Stevensonet al., 2005). Based on this previous research, due to theregular changes in both time sequences and spatialgradients, we suggest that d13Ccarb in the Chinese loess isan indicator of vegetation density, but is not a direct proxyindicator of C4/C3 ratios.A simplified model for the d13Ccarb variation in the CLP

may be that under warm, moist climatic conditions (e.g.,interglacials in the time sequence, and southeastern CLP inthe spatial gradient), the vegetation density is relativelyhigh, with an increased influence of relatively negativecarbon isotopic composition of above-ground plants(dominated by C3 plants as known from d13Corg studies)in the d13Ccarb. Under cold, dry climatic conditions (e.g.,glacials in the time sequence, and northwestern CLP in thespatial gradient), the vegetation density is relatively lowand there is enhanced influence of a relatively positivecarbon isotopic composition of atmospheric CO2 diffusionon the d13Ccarb. Therefore, the d

13Ccarb in the Chinese loessmay be an indirect indicator of the Asian SummerMonsoon, due to the crucial control of the Asian SummerMonsoon on the moisture supply of the CLP, and to theutter dependence of CLP plants on monsoon-suppliedmoisture.

4. Conclusions

The present review of the stable carbon isotopic researchon Chinese loess highlights opposing trends betweend13Ccarb and d13Corg in both time and space sequence.The d13Ccarb in Chinese loess is not an indicator of relativeabundance of C4/C3 vegetation, but may be an indicator ofvegetation density and an indirect indicator of the intensityof the Asian Summer Monsoon. Similar to loess, theaeolian Tertiary red clay deposit of the CLP contains vastinformation about past environmental change. Recentstudy of d13Ccarb data from Tertiary red clay has

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reconstructed past C4/C3 abundance for a discussion of C4

vegetation expansion during the late Miocene (An et al.,2005; Chen and Zhao, 1997; Ding and Yang, 2000; Jiang etal., 2002; Yang et al., 1999), and we have validated suchreconstruction herein. Clearly, a full understanding ofd13Ccarb in both loess and Tertiary red clay demands morestudy in the CLP before isotopic compositions can be usedas any kind of proxy indicators.

Acknowledgements

This work is supported by the National Natural ScienceFoundation of China (Grant no. 40125001 and 40301050)and Innovation Project of Chinese Academy of Sciences(Grant No. KZCX2-SW-133). Thanks are due all theauthors cited in this paper for their pioneering work.We are grateful for thoughtful review of this paper byM. Stephens and Ian Candy, and for the editorialassistance of Cashion Callaway.

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