Organic Geochemistry 89-90 (2015) 56–60

Contents lists available at ScienceDirect

Organic Geochemistry

journal homepage: www.elsevier .com/locate /orggeochem

Note

Global calibration of a novel, branched GDGT-based soil pH proxy

http://dx.doi.org/10.1016/j.orggeochem.2015.10.0050146-6380/� 2015 Elsevier Ltd. All rights reserved.

⇑ Corresponding author at: Hadal Science and Technology Research Center,College of Marine Sciences, Shanghai Ocean University, Shanghai 201306, China.

E-mail address: yunpingxu@pku.edu.cn (Y. Xu).

Wenjie Xiao a, Yunping Xu a,b,⇑, Su Ding a, Yinghui Wang a, Xinyu Zhang a, Huan Yang c, Guoan Wang d,Juzhi Hou e

aMOE Key Laboratory for Earth Surface Process, College of Urban and Environmental Sciences, Peking University, Beijing 100871, ChinabHadal Science and Technology Research Center, College of Marine Sciences, Shanghai Ocean University, Shanghai 201306, ChinacKey Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, ChinadDepartment of Environmental Science and Engineering, College of Resources and Environmental Sciences, China Agricultural University, Beijing 100193, ChinaeKey Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100085, China

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

Article history:Received 3 June 2015Received in revised form 6 October 2015Accepted 16 October 2015Available online 24 October 2015

Keywords:6-Methyl brGDGTsSoil pHCBTIBTGDGTs

Recently, 6-methyl branched glycerol dialkyl glycerol tetraethers (brGDGTs) were separated from5-methyl brGDGTs, which are used in brGDGT-based proxies. Here we analyzed brGDGTs in 27 soil sam-ples along the 400 mm isoline of mean annual precipitation in China by using tandem 2D liquid chro-matography. The fractional abundance of 6-methyl brGDGTs showed a positive correlation with soilpH, while that of 5-methyl brGDGTs decreased with increasing soil pH. The abundance ratio of 6-/5-methyl brGDGTs, namely the isomerization of branched tetraethers (IBT), was calculated. The correlationof IBT with pH (pH = 6.33 � 1.28 � IBT; R2 0.89; root mean squared error, RMSE, 0.24) was much strongerthan that of the traditionally used cyclization index of branched tetraethers (CBT) with pH (R2 0.52; RMSE0.49) and comparable with that of CBT0 with pH (R2 0.88; RMSE 0.25). Compiling all available data from319 soil samples resulted in a global calibration: pH = 6.53 � 1.55 � IBT (R2 0.72; RMSE 0.65), which has abetter correlation than the CBT5ME-pH proxy (R2 0.63; RMSE 0.78), but a weaker correlation than theCBT0-pH proxy (R2 0.85; RMSE 0.52). Our result suggests that the IBT is a promising indicator for soilpH, particularly in cases when some compounds in the CBT0 index cannot be determined.

� 2015 Elsevier Ltd. All rights reserved.

1. Introduction MAT (ca. 5.5 �C) and soil pH (ca. 0.9), suggesting the existence of

Branched glycerol dialkyl glycerol tetraethers (brGDGTs; Fig. 1),containing alkyl chains with four to six methyl side branches andzero to two cyclopentyl moieties, are produced by bacteria in soiland peat including, but not necessarily limited to, Acidobacteria(Weijers et al., 2006; Sinninghe Damsté et al., 2011). By analyzingglobally distributed soils, Weijers et al. (2007) found that thedegree of methylation of branched tetraethers (MBT) is related tomean annual air temperature (MAT) and, to a lesser extent, soilpH, whereas the cyclization index of branched tetraethers (CBT)is related only to soil pH, as corroborated in subsequent studies(e.g. Sinninghe Damsté et al., 2008; Peterse et al., 2012; Yanget al., 2014). The MBT, or its modified form (MBT0), and CBT havebeen increasingly used for paleoenvironmental reconstruction(Schouten et al., 2013; Sanchi et al., 2015 and references therein).However, the global calibration of the MBT(MBT0)/CBT proxy hasrelatively large scatter (Peterse et al., 2012) for reconstructed

other factors influencing brGDGT composition.Recently, De Jonge et al. (2013) identified 6-methyl brGDGTs

eluting shortly after the known 5-methyl brGDGTs by using tan-dem liquid chromatography (LC). The separate quantification of5- and 6-methyl brGDGTs in 239 globally distributed soils led toa recalibration (De Jonge et al., 2014a) of the MBT-CBT proxiesand the development of new indices (e.g. MBT5ME, CBT5ME andCBT0). The number of studies reporting 6-methyl brGDGTs is lim-ited (De Jonge et al., 2013, 2014a,b; Ding et al., 2015; Weberet al., 2015; Yang et al., 2015). More studies are therefore neededto understand the distributions of 6-methyl brGDGTs and theirrelationship with environmental conditions. Here, we analyzedsurface soils along the 400 mm isoline of mean annual precipita-tion (MAP) in China. Since all sampling sites receive an equalamount of MAP, the effect of precipitation on GDGT distributionscan be excluded (Dirghangi et al., 2013; Menges et al., 2014).

2. Material and methods

Samples (88.98–122.15�E; 29.06–49.94�N) were collectedalong the 400 mm isoline of MAP, extending from the southern

Fig. 1. Structures of brGDGTs. The compounds have one or two methyl branches at the a5 and/or x5 position or at the a6 and/or x6 position, and are referred to as 5-methylbrGDGTs and 6-methyl brGDGTs, respectively.

W. Xiao et al. / Organic Geochemistry 89-90 (2015) 56–60 57

slope of Daxingan Mountain to the Tibet Plateau (Fig. 2; Wanget al., 2013); of these, 27 were used for GDGT analysis (Table S1).The MAP represents an average value from > 30 yr (http://www.escience.gov.cn/metdata/page/index.html). The pH measurementand GDGT analysis followed the procedure of Yang et al. (2015).The separation of 5- and 6-methyl brGDGTs was achieved withtwo silica LC columns in sequence (150 mm � 2.1 mm; 1.9 lm,Thermo Finnigan; USA). The software SPSS 20.0 (IBM, USA)was used for the statistical analysis with a significant level ofp < 0.01.

3. Results and discussion

The 5- and 6-methyl brGDGTs were successfully separated forall samples (Fig. 3). The 5-methyl brGDGTs were dominated byIIa and IIIa, while the 6-methyl brGDGTs were dominated by IIa0,IIIa0 and IIb0. The proportion of 6-methyl brGDGTs varied from 11to 80% (Table S2), showing a strong positive correlation with soilpH (R2 0.82, p < 0.001). Such a correlation is consistent with thefindings for soils from global locations (De Jonge et al., 2014a),the Qinghai-Tibet Plateau (Ding et al., 2015) and Mt. Shennongjia(Yang et al., 2015).

Considering the correlation between the relative content ofcyclopentyl moieties in brGDGTs and soil pH, Weijers et al.(2007) proposed the CBT index. De Jonge et al. (2014a) modified

this to CBT5ME, based on 5-methyl brGDGTs alone, which also cor-related with soil pH (n = 221; R2 0.60). Following an objective sta-tistical approach, De Jonge et al. (2014a) proposed a modified CBTindex, namely CBT0, which has the strongest correlation with soilpH (n = 221; R2 0.85; root mean standard error, RMSE 0.52):CBT0 ¼ 1g IcþIIa0þIIb0þIIc0þIIIa0þIIIb0þIIIc

IaþIbþIc .In our study, a slightly weaker, but still significant, correlation

was observed between CBT5ME and soil pH (R2 0.52; RMSE 0.49).The fractional abundance of 6-methyl brGDGTs such as IIIa0 and IIa0

correlated positively with soil pH, whereas that of 5-methylbrGDGTs such as IIIa and IIa correlated negatively with soil pH(Fig. 3). Considering this, we calculated the abundance ratio of6- to 5-methyl brGDGTs, namely isomerization of branched tetra-

ethers index: IBT ¼ � log IIa0þIIIa0IIaþIIIa

� �(Ding et al., 2015). Fig. 4 shows

that the linear correlation of the IBT vs. soil pH (R2 0.89; RMSE0.24) is stronger than that of the CBT5ME vs. soil pH (R2 0.52,RMSE 0.49) and comparable to that of the CBT0 vs. soil pH (R2 0.87;RMSE 0.25). To test whether the IBT-pH proxy is valid at a globalscale, we combined all the available data for 6-methyl brGDGTs,including 239 soils (3 outliers) from globally distributed locations(De Jonge et al., 2014a), 27 soils from the Qinghai-Tibet Plateau (Dinget al., 2015), 26 soils from Mt. Shennongjia (Yang et al., 2015) and27 soils in this study (Fig. 4). The global calibration of the IBT pHproxy (pH = 6.53 � 1.55 � IBT; R2 0.72; RMSE 0.65) showed astronger correlation than the CBT5ME-pH proxy (R2 0.63; RMSE

Fig. 2. Map showing soil sampling locations mentioned in the text. Pink circles refer to this study. Orange, blue and black circles refer to Ding et al. (2015), Yang et al. (2015)and De Jonge et al. (2014a), respectively (for interpretation of the references to color in this figure legend, the reader is referred to the web version of the article).

Fig. 3. Extracted ion chromatograms (EICs, m/z 1050, 1036) showing separation of IIIa � IIIa0 and IIa � IIa0 in selected soil samples with different pH. (A) MH-1; (B) KDE-3;(C) FZ-2.

58 W. Xiao et al. / Organic Geochemistry 89-90 (2015) 56–60

Fig. 4. Scatterplots of A and B: measured soil pH vs. IBT; a and b: offset between measured and calculated pH values vs. IBT; C and D: measured soil pH vs. CBT5ME; c and d:offset between measured and calculated pH values vs. CBT5ME; E and F: measured soil pH vs. CBT0; e and f: offset between measured and calculated pH values vs. CBT0 . Thecompiled dataset includes samples from this study (pink circles), Ding et al. (2015; orange circles), Yang et al. (2015; blue circles) and De Jonge et al. (2014a; black circles) (forinterpretation of the references to color in this figure legend, the reader is referred to the web version of the article).

W. Xiao et al. / Organic Geochemistry 89-90 (2015) 56–60 59

0.78), but a weaker correlation than the CBT0 pH proxy (R2 0.83;RMSE 0.53). Because the IBT index contains only major 5-methyland 6-methyl brGDGTs (IIa, IIIa, IIa0, IIIa0; Table S2), we recommendthe use of the IBT-soil pH proxy in settings where some minorcompounds in the CBT0 index (e.g., Ic, IIb0, IIc0, IIIb0 and IIIc0) arebelow detection limit or coelute on LC–MS, or the IBT-soil pH proxyfunctions as well as the CBT0-soil pH proxy such as in the Chinesesoil dataset.

Our results suggest that changing the position(s) of methylsmay be another strategy for brGDGT-producing bacteria to adjust

the proton permeability of their membrane besides the previouslyrecognized cyclization. However, De Jonge et al. (2014a) arguedthat the slight structural difference between 5- and 6-methylbrGDGTs was unlikely to have a strong impact on the physico-chemical properties of cellular membranes. If so, the correlationbetween soil pH and IBT can be attributed to the changes insoil microbial community with pH, in that bacteria producing6-methyl brGDGTs are more abundant under alkaline conditionsthan those producing 5-methyl brGDGTs. Either way, the IBT indexis controlled by soil pH.

60 W. Xiao et al. / Organic Geochemistry 89-90 (2015) 56–60

4. Conclusions

By using two silica columns in tandem, 5- and 6-methylbrGDGTs were successfully separated from soils along the400 mm isoline of MAP in China. The IBT index, based on the6-/5-methyl brGDGT ratio, has a stronger correlation with soil pHthan the previously defined CBT(CBT5ME) index at both regionaland global scale. Although the IBT index has a slightly weaker cor-relation with soil pH than the CBT0 index at the global scale, the IBTindex has much simpler format and only contains major brGDGTs.We therefore recommend the application of the IBT index for thereconstruction of soil pH in cases when some compounds used inthe CBT0 index cannot be determined.

Acknowledgements

The work was financially supported by the National BasicResearch Program of China (2014CB954001) and the NationalScience Foundation of China (41476062). We are grateful to C. DeJonge and an anonymous reviewer for constructive comments.X. Dang is thanked for sample analysis.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.orggeochem.2015.10.005.

Associate Editor—S. Schouten

References

De Jonge, C., Hopmans, E.C., Stadnitskaia, A., Rijpstra, W.I.C., Hofland, R., Tegelaar, E.,Sinninghe Damsté, J.S., 2013. Identification of novel penta- and hexamethylatedbranched glycerol dialkyl glycerol tetraethers in peat using HPLC–MS2, GC–MSand GC–SMB–MS. Organic Geochemistry 54, 78–82.

De Jonge, C., Hopmans, E.C., Zell, C.I., Kim, J.-H., Schouten, S., Sinninghe Damsté, J.S.,2014a. Occurrence and abundance of 6-methyl branched glycerol dialkylglycerol tetraethers in soils: implications for palaeoclimate reconstruction.Geochimica et Cosmochimica Acta 141, 97–112.

De Jonge, C., Stadnitskaia, A., Hopmans, E.C., Cherkashov, G., Fedotov, A., SinningheDamsté, J.S., 2014b. In situ produced branched glycerol dialkyl glyceroltetraethers in suspended particulate matter from the Yenisei River, EasternSiberia. Geochimica et Cosmochimica Acta 125, 476–491.

Ding, S., Xu, Y., Wang, Y., He, Y., Hou, J., Chen, L., He, J.S., 2015. Distribution ofbranched glycerol dialkyl glycerol tetraethers in surface soils of the Qinghai-Tibetan Plateau: implications of brGDGTs-based proxies in cold and dry regions.Biogeosciences 12, 3141–3151.

Dirghangi, S.S., Pagani, M., Hren, M.T., Tipple, B.J., 2013. Distribution of glyceroldialkyl glycerol tetraethers in soils from two environmental transects in theUSA. Organic Geochemistry 59, 49–60.

Menges, J., Huguet, C., Alcañiz, J.M., Fietz, S., Sachse, D., Rosell-Melé, A., 2014.Influence of water availability in the distributions of branched glycerol dialkylglycerol tetraether in soils of the Iberian Peninsula. Biogeosciences 11, 2571–2581.

Peterse, F., van der Meer, J., Schouten, S., Weijers, J.W.H., Fierer, N., Jackson, R.B.,Kim, J.-H., Sinninghe Damsté, J.S., 2012. Revised calibration of the MBT–CBTpaleotemperature proxy based on branched tetraether membrane lipids insurface soils. Geochimica et Cosmochimica Acta 96, 215–229.

Sanchi, L., Ménot, G., Bard, E., 2015. Environmental controls on paleo-pH at mid-latitudes: a case study from Central and Eastern Europe. Palaeogeography,Palaeoclimatology, Palaeoecology 417, 458–466.

Schouten, S., Hopmans, E.C., Sinninghe Damsté, J.S., 2013. The organic geochemistryof glycerol dialkyl glycerol tetraether lipids: a review. Organic Geochemistry 54,19–61.

Sinninghe Damsté, J.S., Ossebaar, J., Schouten, S., Verschuren, D., 2008. Altitudinalshifts in the branched tetraether lipid distribution in soil from Mt. Kilimanjaro(Tanzania): implications for the MBT/CBT continental palaeothermometer.Organic Geochemistry 39, 1072–1076.

Sinninghe Damsté, J.S., Rijpstra, W.I.C., Hopmans, E.C., Weijers, J.W.H., Foesel, B.U.,Overmann, J., Dedysh, S.N., 2011. 13,16-Dimethyl octacosanedioic acid (iso-diabolic acid), a common membrane-spanning lipid of acidobacteriasubdivisions 1 and 3. Applied and Environmental Microbiology 77, 4147–4154.

Wang, G., Li, J., Liu, X., Li, X., 2013. Variations in carbon isotope ratios of plantsacross a temperature gradient along the 400 mm isoline of mean annualprecipitation in north China and their relevance to paleovegetationreconstruction. Quaternary Science Reviews 63, 83–90.

Weber, Y., De Jonge, C., Rijpstra, W.I.C., Hopmans, E.C., Stadnitskaia, A., Schubert, C.J.,Lehmann, M.F., Sinninghe Damsté, J.S., Niemann, H., 2015. Identification andcarbon isotope composition of a novel branched GDGT isomer in lakesediments: evidence for lacustrine branched GDGT production. Geochimica etCosmochimica Acta 154, 118–129.

Weijers, J.W.H., Schouten, S., Hopmans, E.C., Geenevasen, J.A.J., David, O.R.P.,Coleman, J.M., Pancost, R.D., Sinninghe Damsté, J.S., 2006. Membrane lipids ofmesophilic anaerobic bacteria thriving in peats have typical archaeal traits.Environmental Microbiology 8, 648–657.

Weijers, J.W.H., Schouten, S., van den Donker, J.C., Hopmans, E.C., Sinninghe Damsté,J.S., 2007. Environmental controls on bacterial tetraether membrane lipiddistribution in soils. Geochimica et Cosmochimica Acta 71, 703–713.

Yang, H., Pancost, R.D., Dang, X., Zhou, X., Evershed, R.P., Xiao, G., Tang, C., Gao, L.,Guo, Z., Xie, S., 2014. Correlations between microbial tetraether lipids andenvironmental variables in Chinese soils: optimizing the paleo-reconstructionsin semi-arid and arid regions. Geochimica et Cosmochimica Acta 126, 49–69.

Yang, H., Lü, X., Ding, W., Lei, Y., Dang, X., Xie, S., 2015. The 6-methyl branchedtetraethers significantly affect the performance of the methylation index (MBT0)in soils from an altitudinal transect at Mount Shennongjia. OrganicGeochemistry 82, 42–53.