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Lower temperature as the main cause of C4 plant declines during the glacial periods on the Chinese Loess Plateau
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Earth and Planetary Science Letters 214 (2003) 467^481 www.elsevier.com/locate/epsl
Lower temperature as the main cause of C4 plant declines during the glacial periods on the Chinese Loess Plateau Zhaohui Zhang a; , Meixun Zhao a , Huayu Lu b , Anthony M. Faiia a b
a Department of Earth Sciences, 6105 Fairchild Hall, Dartmouth College, Hanover, NH 03755, USA State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, 10 South Fenhui Road, Xi’an 710075, PR China
Received 3 December 2002; received in revised form 13 June 2003; accepted 10 July 2003
Abstract The distribution of C3 and C4 plants changed in regionally contrasting ways during the last glacial period. C4 plant expansion in low-latitude Africa and America coincided with C4 plant decreases in Mesoamerica and the US Great Plains. This C4 plant expansion has been attributed to lower pCO2 and increased aridity and the decline in C4 plants is believed to have been caused by increased winter precipitation and lower temperatures. However, it is still difficult to generalize whether pCO2 , temperature, or aridity was mainly responsible for C3 vs. C4 vegetation changes during glacial periods. Paleoclimate conditions on the Chinese Loess Plateau (CLP) offer opportunities to further evaluate the role of temperature in the C3 vs. C4 competition. Detailed climate records have shown that the CLP region was drier and colder during glacial periods, with aridity favoring C4 expansion and lower temperatures favoring C3 expansion. Here, we present two high-resolution compound-specific carbon isotope records of n-alkanes from the CLP as vegetation biomarkers to estimate the relative abundance of C4 plants during the last two glacial/interglacial transitions. More negative N13 C values during the glacial periods from two CLP sites clearly show that, in contrast with low-latitude sites, C4 plants declined. This decrease in C4 abundance was caused by lower temperature, despite the lower pCO2 and increased aridity on the CLP during glacial time. 5 2003 Elsevier B.V. All rights reserved. Keywords: Chinese Loess Plateau; C3 /C4 plants; n-alkanes; paleoclimate; carbon isotope
1. Introduction Knowledge of the response of terrestrial veg-
* Corresponding author. Present address: Department of Earth, Atmospheric and Planetary Sciences, E34-174, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA. Tel.: +1-617-253-8352; Fax: +1-617-253-8630. E-mail address: [email protected] (Z. Zhang).
etation to climate change is central to the prediction of the biosphere’s response to anthropogenically forced changes in the future and to the reasonable employment of land resources. Although temperature, precipitation, and pCO2 have been identi¢ed as the important variables controlling C3 /C4 vegetation shifts [1], the resulting interaction of these climatic variables is not well understood. Plants use two principal biosynthetic pathways to ¢x carbon. The C3 or Calvin^Benson pathway
0012-821X / 03 / $ ^ see front matter 5 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0012-821X(03)00387-X
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is the most common photosynthetic pathway used by plants and is characterized by an initial CO2 carboxylation to form phosphoglyceric acid, a 3-carbon acid [2]. The C4 or Hatch^Slack pathway has evolved a CO2 -concentrating mechanism in which CO2 initially combines with phosphoenol pyruvate to form a 4-carbon acid, oxaloacetate [2]. This CO2 -concentrating mechanism gives C4 plants a competitive advantage under low pCO2 conditions [1]. It is also generally agreed that C4 plants have a greater water-use e⁄ciency than C3 plants [2]. Thus, modern C4 plants are commonly distributed in hot and dry environments. Warmseason grasses and sedges use the C4 pathway. Virtually all trees, most shrubs, herbs, cool-season grasses and sedges use the C3 pathway. One approach to understanding the relationship between vegetation and climate is the reconstruction of the relative abundance of C3 and C4 plants in geologic records, and the correlation of their abundance with environmental proxies. During the last glacial period, expansions of C4 plants in low-latitude Africa and Mesoamerica have been attributed to lower pCO2 [3,4], to aridity, or to both [4^6]. The C4 plant decline in Babicora, Mesoamerica was explained by increased winter precipitation and lower temperature [5]. However, it is di⁄cult to generalize whether pCO2 , temperature, or aridity was mainly responsible for altering C4 abundance during the last glacial period. Here, we present two high-resolution, compound-speci¢c carbon isotope records of n-alkanes from the Chinese Loess Plateau (CLP) to estimate the relative abundance of C4 plants during the last two glacial/interglacial cycles. Our aim is to employ the climate conditions on the CLP to di¡erentiate between the competing roles of lower temperature and increased aridity under lower atmospheric pCO2 . Climate in the CLP today is characterized by seasonal alternations of the East Asian summer and winter monsoons. In summer, the warm-humid southeast monsoons of tropical/subtropical Paci¢c Ocean origin brings relatively high precipitation. In winter, the cold-dry northwest monsoon of subarctic origin prevails across the region resulting in high aridity [7^11] (Fig. 1). The CLP
contains a complete and highly resolved archive of Quaternary climate change [8,9]. Several decades of studies have yielded numerous records, consistently suggesting that the climate in central China was colder and drier during the glacial periods, due to an increased dominance of the winter monsoon [7^10]. In spite of extensive studies of such non-biological proxy indicators as magnetic susceptibility [12^15] and grain size [16,17], very little has been learned about the glacial/interglacial vegetation changes on the CLP. Pollen, an important tool for reconstructing paleovegetation, is present in exceedingly low concentration in the CLP loess strata [12], and is of limited use in evaluating shifting plant coverage. A newer technique employs the use of carbon isotope ratios. Terrestrial plants produce leaf waxes in which the carbon isotopic signatures of photosynthetic pathways are retained [18]. Leaf waxes from terrestrial plants have a characteristic odd-over-even number predominance in the longchain (C27 ^C35 ) n-alkane series. This distribution makes the odd number long-chain n-alkanes a good proxy for terrestrial plants [19]. n-Alkanes are highly resistant to biochemical degradation and diagenesis in the sedimentary record [20]. Long-chain n-alkanes produced by C3 and C4 plants show characteristically di¡erent N13 C values, 332 to 339x and 318 to 325x, respectively [18]. N13 C values of long-chain n-alkanes are more diagnostic than bulk organic matter in differentiating C3 and C4 plant contribution [3,5], as the latter has a mixed carbon input with di¡erent carbon isotope values. In addition, microbial degradation of organics with di¡erent qualities and carbon isotope values would change the N13 C value of bulk organic matter [21].
2. Samples and methods 2.1. Study area and sample strategy Xunyi and Luochuan are both located on the central CLP about 200 km from each other (Fig. 1). Present-day climate of these two sites is semihumid with an annual average precipitation of
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Fig. 1. A map showing the CLP (shaded area) and sampling locations (F for both Xunyi and Luochuan) (modi¢ed from Porter and An [15]). A: January sea-level pressure (mbar, in dashed lines), dominant wind vectors (arrows) and average southern limit of polar front (bold line). B: July sea-level pressure (mbar, in dashed lines), dominant wind vectors (arrows) and average landward limit of monsoonal front (bold line); L and H: Low and high pressure cells for the winter (A) and summer (B).
620 mm. The majority of rain falls in the summer season. The annual average temperature is 9.4‡C and the summer average temperature is 20‡C. Luochuan is steppe dominated and Xunyi is a mixture of forest and steppe. The loess tablelands (called ‘Yuan’) are well developed at both sites and complete loess-paleosol stratigraphic sequences allow for a systematic investigation of paleoclimate and paleovegetation [12,16]. For the Xunyi sequence, samples were taken every 5 cm from the surface to a depth of 220 cm and then every 10^20 cm over the interval 230^1580 cm, corresponding to approximately 500^2000 years resolution. At Luochuan, samples were collected every 5 cm throughout the 500 cm section. Each sample was divided into three aliquots for magnetic susceptibility, grain size and biomarker/total organic carbon (TOC) analysis. Samples were dried in an oven at 40‡C and stored in aluminium foil bags at room temperature. 2.2. Magnetic and grain size measurements About 10 g of each sample were measured for magnetic susceptibility using a Bartinton MS2 magnetic meter (values are given in units of 1038 m3 /kg). For non-carbonate grain size measurements, about 1.5 g of bulk sample was weighed into a 600-ml beaker. 10 ml of 10% H2 O2 was added and the beaker was heated until the organic matter oxidation reaction was com-
pleted. After cooling to room temperature, 10 ml of 10% HCl was added to the beaker to dissolve carbonate under boiling conditions. Then, the beaker was ¢lled with distilled water, which was siphoned o¡ after sitting for 24 h. Finally, 10 ml of 0.05 N NaPO3 was added to the beaker and sonicated for 10 min to separate ¢ne particles. A laser particle size analyzer (Mastersizer-S s) was used to measure grain sizes in the range of 0.05^ 900 Wm with a standard deviation less than 5% (based on 12 or more replicate analyses of several loess samples). The equipment is calibrated using an international standard. The percentage by volume of the coarse grain fraction ( s 30 Wm) is reported here. 2.3. TOC An aliquot of each sample was acidi¢ed with 6 M HCl and dried in an oven at 90‡C. TOC was measured on a Carlo Erba NA 1500 Series 2 elemental analyzer. About 20^60 mg of each sample was packaged in a silver or tin capsule and introduced into a 1020‡C combustion column containing chromium oxide on alumina, and silvered cobaltous/cobaltic oxide. Oxygen was then introduced into the helium carrier gas passing through the combustion column, which rapidly oxidized the sample. The oxidation products CO2 and NOx from the sample then passed through a copper reduction column at 650‡C
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where the NOx is reduced to N2 . The CO2 and N2 in the helium carrier gas were separated using a gas chromatograph column and measured using a thermal conductivity detector. Accuracy of the analytical method was monitored with the analysis of control standards and materials along with the samples. Reproducibility of the analytical method as determined by replicate analysis of samples was 0.05%. 2.4. Compound-speci¢c N 13 C of n-alkanes Samples of about 50V100 g were ground and weighed into an Alundum thimble and soxhlet extracted overnight with dichloromethane and methanol (3:1). The solvent was evaporated on a rotor-evaporator, and the total lipid extract was separated into three fractions by silica gel £ash-column chromatography. Elution with hexane gave the aliphatic hydrocarbon fraction, which contains the long-chain n-alkanes. The hexane fraction was analyzed on an HP 6890 gas chromatograph (GC) with an oven temperature program of 85‡C for 1 min, ramping to 200‡C at a rate of 20‡C/min, then increasing at 2‡C/ min to 300‡C and holding for 5 min, and increasing to 315‡C and holding for 8 min. The purity and structure of the n-alkanes were con¢rmed using a Varian 3800/Saturn 2000 GCMS. Compound-speci¢c carbon isotopic compositions of n-alkanes were measured by gas chromatography-isotope ratio-mass spectrometry (GCIRMS). An HP 5890 GC was connected to a Finnigan MAT delta plus XL mass spectrometer via a GC-C III interface. n-Alkanes passing through the GC column were oxidized at 940‡C and converted to CO2 . The gas stream was introduced to the mass spectrometer via an open split. Five pulses of standard CO2 gas, pre-calibrated against a commercial reference CO2 , were injected via the GC-C III interface to the IRMS for the computation of N13 C values of sample compounds. A set of 15 n-alkanes with known N13 C values acquired from Indiana University were measured daily to ensure accuracy of the machine. The standard deviation for duplicate analyses of this standard was 6 W 0.3x. The N13 C values are reported with reference to the PDB standard.
3. Results 3.1. Stratigraphy, magnetic susceptibility and age control The general stratigraphy of the last climatic cycle spanning 170 kyr in the Chinese loess sequence has been described in many studies [12,22]. It consists of a succession of loess and soil complexes denoted by L or S respectively. Typical loess is light colored, porous, calcareous and non-bedded whereas typical paleosols are dark gray brown to reddish brown and decalci¢ed to various degrees. The sequence consists of loess L2, soil complex S1, loess L1 and soil S0. Loess L1 is subdivided into subunits L1LL2, L1SS1 (weakly weathered), and L1LL2. S0 is the black loam of the Holocene [12,22]. We have only identi¢ed the major soil/loess boundaries in our pro¢les. The Xunyi section covers L2 to S0 (Fig. 2) and the Luochuan section contains L1 to S0 (Fig. 3). At both sites, anthropogenic disturbance of the very top portion of S0 (ca. 40 cm) can be observed in the ¢eld (shards, charcoal). In both the Xunyi and Louchuan sequences, magnetic susceptibility was low in the loess portions of the sequence and high in the paleosols (Figs. 2A and 3A). Variations in the magnetic susceptibility of Chinese loess deposits have been shown to be excellent proxies of the East Asian summer monsoon intensity [7,8,13]. Magnetic mineral production occurs during pedogenesis, and on the CLP, soil development is mainly controlled by precipitation [8,13,14]. Higher susceptibility values are typically associated with paleosol horizons and are interpreted as indicating warmer and wetter phases, while lower values are found in the loess layers and are thought to represent cooler and drier phases (Figs. 2A, 3A). Correlation of magnetic susceptibility records with dated marine oxygen isotope records has also enabled magnetic susceptibility to be used as a major stratigraphic tool for continental climatic reconstruction [12^15]. Intervals of rapid changes in magnetic susceptibility have been correlated with SPECMAP marine oxygen isotope stage (MIS) boundaries [15,23,24]. The magnetic susceptibility loess-paleosol boundaries L1/S0, S1/
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EPSL 6768 2-9-03 Cyaan Magenta Geel Zwart Fig. 2. Proxy records of Xunyi site for the last 170 kyr. A: Magnetic susceptibility. B: Percent of coarse grain fraction ( s 30 Wm). C: TOC content (%). D: N13 C of C27 , C29 , C31 and C33 n-alkanes (x). E: Weighted mean average N13 C of n-alkanes (x) and C4 plant percentage estimated based on a binary model. F: Corrected weighted mean average N13 C of n-alkanes (x) and C4 plant percentage. G: Vostock ice core CO2 concentration (ppmv) [42]. On the left are the loess/paleosol boundaries identi¢ed by magnetic susceptibility and ¢eld observation. On the right are the MIS and climate conditions on the CLP, identi¢ed based on the MS and coarse grain fraction records. LGM: last glacial maximum.
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L1, and L2/S1 are correlated with the boundaries of MIS 2/1 (12.05 ka), MIS 5/4 (73.91 ka) and MIS 6/5 (128.84 ka), respectively [15,23,24]. These transitions are used as age control points. Ages for other samples are established by linear interpolation.
the highest values in the Holocene, a trend similar to that of the Xunyi record (Fig. 3C). 3.4. Carbon isotopes of n-alkanes
In the Xunyi sequence, the coarse grain fraction ( s 30 Wm) ranges from 13.8 to 34.9% (Fig. 2B). The curve of the coarse grain fraction forms a mirror image of that for magnetic susceptibility. A greater percentage of coarse grains is seen in glacial periods, with the highest in MIS 6, indicating stronger wind due to a strengthened winter monsoon [16,17]. Lower values are seen in interglacial periods, with MIS 5 even lower than the Holocene value, re£ecting weak winter monsoons. In the Luochuan sequence, the percentage of coarse grains ( s 30 Wm) ranges from 22.0 to 43.4 (Fig. 3B). Surprisingly, the highest values are observed from middle to late Holocene. It is likely that these values re£ect human activities in the Luochuan region [25]. Deeper sections of the pro¢le mirror the magnetic susceptibility record as for the Xunyi sequence. The percentage of coarse grains increased as expected in the LGM (last glacial maximum).
Typical n-alkane distribution patterns from the Xunyi section are shown in Fig. 4A (for LGM) and Fig. 4B (for MIS 5). Detailed n-alkane distributions will be published elsewhere. In all the loess-paleosol sequences analyzed, the n-alkane distributions share three characteristics: they all have signi¢cant concentration in the C24 to C34 range; they all have strong odd-over-even preference; and C31 is always the dominant n-alkane (Fig. 4). These characteristics are consistent with a higher plant origin for the n-alkanes. Regressions between the C27 and C31 n-alkane concentrations and between C29 and C31 n-alkane concentrations have R2 values of 0.74 and 0.83, respectively, suggesting that these alkanes are all from higher plant waxes. The C27 , C29 , C31 , and C33 n-alkane concentrations are su⁄cient for carbon isotope ratio measurement using GCIRMS. The C27 , C29 , C31 and C33 n-alkanes show a reasonably coherent stratigraphic trend of N13 C variations (Figs. 2D, 3D), re£ecting their common source in higher plants. We have selected to use the weighted mean average N13 C of C27 , C29 , C31 and C33 n-alkanes, in order to reconstruct vegetation change:
3.3. TOC
weighted mean average N 13 C ¼
3.2. Percentage of coarse grain fraction ( s 30 Wm, non-carbonate)
TOC content in the Xunyi sequence ranges from 0.06 to 0.67% over the last 170 kyr with generally lower values measured in the loess sections and higher values measured in the paleosol sections (Fig. 2C). The lowest TOC content was found in L2 (MIS 6) and the highest in the Holocene. The timing of the changes in TOC values generally coincides with changes in the loess/paleosol lithology as well as with changes in magnetic susceptibility and the coarse grain fraction. However, the increase of TOC in MIS 5 lags that of magnetic susceptibility. TOC content in the Luochuan sequence ranges from 0.10 to 0.58% over the last 35 kyr with the lowest values recorded in the LGM (MIS 2) and
N 13 C 27 C 27 þ N 13 C 29 C 29 þ N 13 C 31 C 31 þ N 13 C 33 C 33 C 27 þ C 29 þ C 31 þ C 33
The weighted mean average N13 C values of n-alkanes (C27 -C33 ) from the Xunyi section ranges from 333.2 to 330.8x over the last 170 kyr (Fig. 2E), with generally more negative values during glacial periods and less negative values during interglacials. The most negative value of 333.2x was during the LGM. N13 C reached the highest value in the Holocene. The weighted mean average N13 C values of n-alkanes (C27 ^C33 ) from the Luochuan section range from 332.9 to 329.3x over the last 36 kyr (Fig. 3E), showing a similar glacial to interglacial oscillation. Overall, the two records reveal a shift of 2^3x
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EPSL 6768 2-9-03 Cyaan Magenta Geel Zwart Fig. 3. Proxy records of Luochuan site for the last 38 kyr. A: Magnetic susceptibility. B: Percent of coarse grain fraction ( s 30 Wm). C: TOC content (%). D: N13 C of C27 , C29 , C31 and C33 n-alkanes (x). E: Weighted mean average N13 C of n-alkanes (x) and C4 plant percentage estimated based on a binary model. F: Corrected weighted mean average N13 C of n-alkanes (x) and C4 plant percentage. G: Vostock ice core CO2 concentration (ppmv) [42]. On the left are the loess/paleosol boundaries identi¢ed by magnetic susceptibility and ¢eld observation. On the right are the MIS and climate conditions on the CLP, identi¢ed based on the MS and coarse grain fraction records. LGM: last glacial maximum.
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in the weighted mean average N13 C value of n-alkanes (C27 ^C33 ) with the more negative values during glacial periods. Because agricultural activity can be traced back to 7500 BP [25], the late Holocene values may have been in£uenced by anthropogenic activities. For the past several thousand years, the main crop in this region has been wheat, a C3 plant. Any agricultural contamination would lead to more negative N13 C values. Therefore, the late Holocene values are not used in the following discussion, which deals with natural vegetation response to climate.
are in accord with the climate of this region being colder and drier during glacial periods, and warmer and wetter during interglacials. This conclusion has been reached previously by many other studies [7^11,15^17,22]. The lower magnetic susceptibility and higher percent of coarse grains in L2 compared to sediment from the LGM seems to suggest a much drier climate in MIS 6 with stronger winter monsoons than the LGM. Furthermore, the higher magnetic susceptibility and smaller percent of coarse grains in S1 compared to S0, suggests a much wetter climate in MIS 5 with a stronger summer monsoon than in the Holocene.
4. Discussion 4.2. Plant productivity 4.1. Monsoon climates over the last 170 kyr The main focus of this section is the exploration of relationships between climate and vegetation during glacial and interglacial cycles on the CLP. Climatic proxy records for the CLP have been documented by numerous researchers over the last few years. We present a summary below. Detailed studies of Chinese loess sequences have established that magnetic susceptibility is correlated with precipitation, and hence can be used as a proxy of summer monsoon intensity [7,8,13^15]. Particle size, especially of the quartz fraction, has been used as a measure of the strength of the winter monsoon winds, which are responsible for transporting most of the dust building up as loess [15^17]. During periods of strong winter monsoon activity, the percentage of coarse grains is high, while magnetic susceptibility remains low. When the summer monsoon is dominant, magnetic susceptibility is high and the mean grain size is ¢ner. These relationships are re£ected in the opposite trending curves of magnetic susceptibility and coarse grain percent for both the Xunyi (Fig. 2A and B) and the Luochuan sequences (Fig. 3A and B). We have already mentioned that higher grain size values than expected in the Luochuan pro¢le during the late Holocene are most likely an anthropogenic e¡ect. The magnetic susceptibility and grain size data
Paleovegetation productivity estimation is more challenging since only a small percentage of plant organic matter is preserved in sediment. However, TOC in Chinese loess samples has been used as a proxy for paleoplant productivity. Soil organic matter content from desert and sandy soil regions of northern China is positively correlated with annual precipitation [26], suggesting that heavier precipitation increases productivity. Hence, TOC content re£ects the balance between organic matter production and decay. Sun and Liu (2000) also show that TOC and magnetic susceptibility are generally positively correlated over the last two glacial/interglacial cycles, indicating higher plant productivity during the wetter interglacial periods than the drier glacial periods [27]. For both the Xunyi and Luochuan records, TOC contents generally parallel magnetic susceptibility data (Figs. 2A and C, and 3A and C). This agreement clearly indicates that both proxies are controlled by precipitation, which re£ects the summer monsoon strength. TOC values are higher during MIS 5 and the Holocene, corresponding to warm/humid climates and lower during MIS 6 and the LGM, re£ecting cold/dry conditions. The high TOC ( s 0.5%) that is seen at the uppermost pro¢le (Figs. 2C and 3C) is largely due to the presence of highly decomposable organic matter from plant roots and addition of manure to the surface soils [25].
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4.3. Glacial/interglacial C3 /C4 vegetation changes on the CLP Using a binary model with endpoints for C4 and C3 plant wax n-alkanes, the relative contribution of C4 plants can be calculated. Long-chain n-alkanes produced by C3 and C4 plants have characteristically di¡erent N13 C values: 332 to 339x and 318 to 325x, respectively [7]. We chose endpoints of 321x for C4 plant n-alkanes and 336x for C3 plant n-alkanes. These values are well accepted and used for similar calculations [28]. The percent of C4 plant contribution (y) is calculated from the following formula : yUð321xÞ þ ð13yÞUð336xÞ ¼ N 13 Cmeasured
In the Xunyi pro¢le, the calculated C4 plant percentages varied from 21 to 35% during the Holocene and from 25 to 31% in MIS 5. C4 percent ranges for the glacial periods were 20^25% during the LGM and 19^24% during MIS 6 (Fig. 2E). The Luochuan pro¢le showed a similar sequence of vegetation change but the C4 plant percentage reached a maximum at 12^8 ka (Fig. 3E), a little earlier than the maximum in the Xunyi pro¢le at 11^5 ka. LGM C4 percentages at Luochuan ranged from 20 to 30% and Holocene C4 percentages ranged from 25 to 44%. These calculations indicate that the percent of C4 plants declined by ca. 10^20% during periods of glaciation. However, changing environmental factors can cause changes in the N13 C values of C3 and C4 plants without changing their relative abundance [29]. Tieszen [30] generalized six factors of primary relevance : (1) irradiance; (2) water stress; (3) osmotic stress; (4) nutrient supply; (5) temperature; and (6) altitude (associated with pCO2 decrease). The major factors which could have a¡ected the N13 C of plant waxes produced on the CLP without changing the C3 /C4 balance are pCO2 , aridity, and N13 C of atmospheric CO2 . A negative relationship has been established between atmospheric CO2 concentration and N13 C in C3 plants [31^34], but theories and observations suggested that N13 C values in C4 plants are not
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a¡ected by atmospheric CO2 concentration [35,36]. Feng and Epstein (1995) quanti¢ed the relationship between N13 C of C3 plants and pCO2 . For every decrease of 100 ppm in atmospheric CO2 concentration, N13 C values become heavier by 2.0 W 0.1x [33]. Hence, the lowering of pCO2 by 80 ppm during glacial periods would cause the N13 C of C3 plants to become 1.6x more positive. A negative relationship also exists between N13 C of C3 plants and water availability [33,37^39]. Stewart et al. (1995) estimated a decrease of 0.33 W 0.07x in N13 C of C3 plants per 100 mm increase in precipitation by examining eastern Austrian vegetation over a large range of precipitation (350^1500 mm) [38]. However, leaf N13 C of Atriplex canescens (a C4 plant) seems insensitive to variations in precipitation over the range of 100^250 mm [40]. During the last glacial period, the estimated precipitation in our study region was lower than the Holocene by up to 200 mm [8], which could have increased the N13 C of C3 plant n-alkanes by 0.6x. Changes in the N13 C of atmospheric CO2 would a¡ect the N13 C of plant material. Atmospheric CO2 was lighter by ca. 0.3x during the LGM [41] and would cause a similar decrease in the N13 C of n-alkanes for both C3 and C4 plants. This small decrease has little e¡ect on the calculation outcome and is not corrected for. We have used the relationships discussed above to estimate the e¡ect of pCO2 and aridity on the N13 C values of leaf wax n-alkanes. The pCO2 for each sample was determined from Vostock ice core record (Figs. 2G and 3G) [42] and precipitation values on the CLP were calculated from magnetic susceptibility using the equation: PA mm/ yr = 222+199Ulog10 (MS 1038 m3 /kg) [43]. In the correction, we assumed all vegetation to be C3 since the raw C4 % calculation (Fig. 3E) shows that C3 plants have been dominant over the last 170 kyr. Hence, our corrected weighted mean N13 C shows the maximum amount of o¡set possible. The corrected C4 % (Figs. 2F, 3F) and the raw C4 % (Figs. 2E, 3E) trend similarly but corrected C4 % is lower, with almost zero values during the glacial maxima (Figs. 2F, 3F). In addition, we need to consider the e¡ect of
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Fig. 4. Partial gas chromatograms of two samples from the Xunyi site on the CLP. Odd carbon number n-alkanes are marked by their numbers atop the peaks. The 200 cm sample is from the LGM (A) and the 940 cm sample is from the peak interglacial period (MIS 5, B).
dust and aerosol transported n-alkanes on our N13 C records. It is well established that leaf wax components can be transported via dust over thousands of kilometers [44,45]. For the CLP, most loess deposits originated as dust carried in from the arid desert and Gobi region in northwest China by the winter monsoons [46^50]. Since the vegetation of the cooler higher latitude regions
would be primarily C3 plants, variations in the winter monsoon could explain the small N13 C variations in our records for the LGM. However, plant productivity is very limited in the desert regions, and would have been lower during the colder and drier glacial periods, as indicated by lower TOC in both the Xunyi and Luochuan records (Figs. 2C, 3C).
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Fig. 5. Detailed comparison of N13 C of C31 n-alkane (solid line) and magnetic susceptibility (dotted line) across glacial/interglacial boundaries. A: Termination I for Xunyi site. B: Termination II for Xunyi site. C: Termination I for Luochuan site.
During interglacial periods, summer monsoon dominates and might increase the input of leaf waxes from low-latitude continental regions. However, dust de£ation and transport are limited for this humid and vegetation-covered region. Similar long-chain n-alkane distributions during glacial and interglacial periods indicate no major change of wax sources (Fig. 4). Also, data from Leizhou Peninsula in southeast China (unpublished) show that vegetation in this area was dominated by C4 plants during the LGM and by C3 plants during interglacial times. If there were input of leaf waxes from southeast China, then this would be in contrast to our observations. We conclude that our N13 C records re£ect the relative abundances of the locally growing C3 and C4 plants. Future work will need to analyze the n-alkanes born by dust transport. Our results o¡er new insights into the environmental factors controlling C3 /C4 shifts during the glacial/interglacial transition. At both the Xunyi and Luochuan sites, the expansions of C4 plants occurred when pCO2 was high and the climate was warmer and wetter. On the other hand, the
shift to more C3 plants occurred when pCO2 was low and the climate was colder and drier. This result clearly identi¢es lower temperature as the main cause of C4 plant decline during glacials on the CLP. 4.4. Increased/decreased temperature initiated the C3 /C4 vegetation change during the transitions on the CLP Upon close examination of the Xunyi and Luochuan records, C31 n-alkane (the dominant alkane) N13 C change leads the change in magnetic susceptibility by 1^2 kyr during the MIS 2/1 transition (Fig. 5A, C). Similarly, the shift to less C4 vegetation during the interglacial to glacial transition (MIS 5/MIS 4) led the change in magnetic susceptibility (Fig. 2A, F). This timing is in agreement with the ¢ndings of Wu et al. [10]. They concluded that changes in temperature and precipitation on the CLP were not in phase during the last 30 kyr [10]. During the last glacial period, temperature reached its coldest in the interval 19^ 17 kyr BP, whereas precipitation reached a mini-
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mum at 16 kyr BP [10]. Temperature was also the ¢rst to increase. In some areas of China, Holocene precipitation did not reach a maximum until 3 kyr BP following the maximum in temperature at 9^6 kyr [51]. One possible explanation for temperature change leading precipitation change lies in the interaction of the winter and summer monsoons over this region. During the LGM, when winter monsoon circulation was enhanced, cold air from high latitudes was brought to the Loess Plateau and caused rapid land surface temperature decreases because of the relatively small heat capacity of land [52]. However, the major source of water vapor to the CLP, the East Asian summer monsoons, might have responded more slowly to this change due to the relatively large heat capacity of the oceans [10]. N13 C of n-alkanes and magnetic susceptibility increases seem to be synchronous during the MIS 6/5 transition (Fig. 5B), probably due to low sampling resolution. 4.5. Temperature threshold for C4 plant growth Most other studies of C3 /C4 vegetation change concentrated on tropical and subtropical regions [3^6]. In the tropics, today’s vegetation type is mostly controlled by precipitation. Temperature during the last glacial period decreased by only 4‡C in equatorial Africa [53] and the growing season temperature was still high enough to support C4 plant expansion. Driven by lower pCO2 and increased aridity, the physiological advantage afforded by C4 metabolism presumably outweighed the negative e¡ect of the small temperature decrease. On the CLP, however, the summer temperature decrease from 20 to 13‡C during the LGM [54] was too harsh for C4 plant growth, even with lower pCO2 and increased aridity. Botanic studies show that with today’s climate, the distributions of C4 plants are positively correlated with growing season temperature in North America [55]. Laboratory and ¢eld data also indicate that photosynthesis (as measured by chlorophyll) is inhibited below 16‡C in corn and two other tropical C4 grasses [56,57]. Similarly, there seems to be a turnover point of minimum growing
season temperature for C4 plant expansion during the LGM. N13 C stratigraphy of long-chain n-alkanes in Lake Baikal sediments provides another piece of collaborative evidence. The N13 C values during the last glacial/Holocene transition are remarkably constant, well within the range for leaf wax n-alkanes synthesized by C3 plants [58]. Today’s July temperatures at Lake Baikal are ca. 11V14‡C [59], which is close to the LGM temperature on the CLP. The dominance of C3 plants today is due to cold growing season temperature. Lower temperature during the last glacial must have inhibited C4 plant growth in the Lake Baikal area despite lower pCO2 . Most likely, there exists a threshold of growing season temperature of 13^15‡C for C3 /C4 plant competition. Above it, lower pCO2 and aridity favored C4 expansion as seen in Africa [3] and Quexil, Mesomerica [5]; below this limit, C4 plants were out-competed by C3 plants in spite of lowered pCO2 and increased aridity, on the CLP and at Lake Baikal [58]. We can infer that C4 plant decline in Babicora was also mainly due to the lower growing season temperature, rather than increased winter precipitation. Our results provide another piece of evidence that low pCO2 during the LGM was insu⁄cient to trigger the expansion of C4 plant coverage without favorable climatic conditions [5].
5. Concluding remarks Multiple proxies from loess/paleosol sequences on the CLP allow comprehensive evaluation of paleoclimate and paleovegetation changes for the last 170 kyr. Magnetic susceptibility and grain size results indicate that climate over the last 170 kyr has oscillated between winter monsoon-dominated cold/dry glacials and summer monsoon-dominated warm/wet interglacials, in agreement with numerous previous studies. These conclusions have also been corroborated by measurements of TOC, which is mainly controlled by precipitation. During glacial periods, globally lower pCO2 and regionally increased aridity on the CLP would have favored expansion of C4 vegetation,
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but lower temperatures in the growing season caused the decline of C4 plants. Conversely, increasing temperature was essential to initiate the expansion of C4 grasses on the CLP during the Holocene. Our results suggest a turnover point of minimum growing season temperature of ca. 13^ 15‡C, below which C4 plants were out-competed by C3 plants. Only when temperature was greater than this threshold, could pCO2 and aridity play a signi¢cant role in the growth of the C4 plant community, as in Africa and Quexil, Mesoamerica. We can conclude that C4 vs. C3 vegetation changes during the last glacial/Holocene transition were controlled by regional climate variations. The results reported herein for the CLP have important implications in predicting vegetation response to future climate change. For example, global warming could cause C4 expansion in several areas where the current mean growing season temperature is near the threshold.
[4]
[5]
[6]
[7]
[8]
[9]
[10]
Acknowledgements [11]
We would like to thank Dr. Xiahong Feng for GCIRMS facility support and discussion and Dr. Geo¡ Eglinton for constructive comments. We are grateful to Drs. Steve Colman and Peter Sauer for their thoughtful reviews and suggestions. We thank Hua Wang and Xiaodong Miao for help during ¢eld work. This research was supported by Dartmouth College, the National Sciences Foundation (EAR111403 to X. Feng) and the National Sciences Foundation of China (NSFC40121303).[BOYLE]
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Lower temperature as the main cause of C4 plant declines during the glacial periods on the Chinese Loess Plateau Zhaohui Zhang a; , Meixun Zhao a , Huayu Lu b , Anthony M. Faiia a b
a Department of Earth Sciences, 6105 Fairchild Hall, Dartmouth College, Hanover, NH 03755, USA State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, 10 South Fenhui Road, Xi’an 710075, PR China
Received 3 December 2002; received in revised form 13 June 2003; accepted 10 July 2003
Abstract The distribution of C3 and C4 plants changed in regionally contrasting ways during the last glacial period. C4 plant expansion in low-latitude Africa and America coincided with C4 plant decreases in Mesoamerica and the US Great Plains. This C4 plant expansion has been attributed to lower pCO2 and increased aridity and the decline in C4 plants is believed to have been caused by increased winter precipitation and lower temperatures. However, it is still difficult to generalize whether pCO2 , temperature, or aridity was mainly responsible for C3 vs. C4 vegetation changes during glacial periods. Paleoclimate conditions on the Chinese Loess Plateau (CLP) offer opportunities to further evaluate the role of temperature in the C3 vs. C4 competition. Detailed climate records have shown that the CLP region was drier and colder during glacial periods, with aridity favoring C4 expansion and lower temperatures favoring C3 expansion. Here, we present two high-resolution compound-specific carbon isotope records of n-alkanes from the CLP as vegetation biomarkers to estimate the relative abundance of C4 plants during the last two glacial/interglacial transitions. More negative N13 C values during the glacial periods from two CLP sites clearly show that, in contrast with low-latitude sites, C4 plants declined. This decrease in C4 abundance was caused by lower temperature, despite the lower pCO2 and increased aridity on the CLP during glacial time. 5 2003 Elsevier B.V. All rights reserved. Keywords: Chinese Loess Plateau; C3 /C4 plants; n-alkanes; paleoclimate; carbon isotope
1. Introduction Knowledge of the response of terrestrial veg-
* Corresponding author. Present address: Department of Earth, Atmospheric and Planetary Sciences, E34-174, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA. Tel.: +1-617-253-8352; Fax: +1-617-253-8630. E-mail address: [email protected] (Z. Zhang).
etation to climate change is central to the prediction of the biosphere’s response to anthropogenically forced changes in the future and to the reasonable employment of land resources. Although temperature, precipitation, and pCO2 have been identi¢ed as the important variables controlling C3 /C4 vegetation shifts [1], the resulting interaction of these climatic variables is not well understood. Plants use two principal biosynthetic pathways to ¢x carbon. The C3 or Calvin^Benson pathway
0012-821X / 03 / $ ^ see front matter 5 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0012-821X(03)00387-X
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is the most common photosynthetic pathway used by plants and is characterized by an initial CO2 carboxylation to form phosphoglyceric acid, a 3-carbon acid [2]. The C4 or Hatch^Slack pathway has evolved a CO2 -concentrating mechanism in which CO2 initially combines with phosphoenol pyruvate to form a 4-carbon acid, oxaloacetate [2]. This CO2 -concentrating mechanism gives C4 plants a competitive advantage under low pCO2 conditions [1]. It is also generally agreed that C4 plants have a greater water-use e⁄ciency than C3 plants [2]. Thus, modern C4 plants are commonly distributed in hot and dry environments. Warmseason grasses and sedges use the C4 pathway. Virtually all trees, most shrubs, herbs, cool-season grasses and sedges use the C3 pathway. One approach to understanding the relationship between vegetation and climate is the reconstruction of the relative abundance of C3 and C4 plants in geologic records, and the correlation of their abundance with environmental proxies. During the last glacial period, expansions of C4 plants in low-latitude Africa and Mesoamerica have been attributed to lower pCO2 [3,4], to aridity, or to both [4^6]. The C4 plant decline in Babicora, Mesoamerica was explained by increased winter precipitation and lower temperature [5]. However, it is di⁄cult to generalize whether pCO2 , temperature, or aridity was mainly responsible for altering C4 abundance during the last glacial period. Here, we present two high-resolution, compound-speci¢c carbon isotope records of n-alkanes from the Chinese Loess Plateau (CLP) to estimate the relative abundance of C4 plants during the last two glacial/interglacial cycles. Our aim is to employ the climate conditions on the CLP to di¡erentiate between the competing roles of lower temperature and increased aridity under lower atmospheric pCO2 . Climate in the CLP today is characterized by seasonal alternations of the East Asian summer and winter monsoons. In summer, the warm-humid southeast monsoons of tropical/subtropical Paci¢c Ocean origin brings relatively high precipitation. In winter, the cold-dry northwest monsoon of subarctic origin prevails across the region resulting in high aridity [7^11] (Fig. 1). The CLP
contains a complete and highly resolved archive of Quaternary climate change [8,9]. Several decades of studies have yielded numerous records, consistently suggesting that the climate in central China was colder and drier during the glacial periods, due to an increased dominance of the winter monsoon [7^10]. In spite of extensive studies of such non-biological proxy indicators as magnetic susceptibility [12^15] and grain size [16,17], very little has been learned about the glacial/interglacial vegetation changes on the CLP. Pollen, an important tool for reconstructing paleovegetation, is present in exceedingly low concentration in the CLP loess strata [12], and is of limited use in evaluating shifting plant coverage. A newer technique employs the use of carbon isotope ratios. Terrestrial plants produce leaf waxes in which the carbon isotopic signatures of photosynthetic pathways are retained [18]. Leaf waxes from terrestrial plants have a characteristic odd-over-even number predominance in the longchain (C27 ^C35 ) n-alkane series. This distribution makes the odd number long-chain n-alkanes a good proxy for terrestrial plants [19]. n-Alkanes are highly resistant to biochemical degradation and diagenesis in the sedimentary record [20]. Long-chain n-alkanes produced by C3 and C4 plants show characteristically di¡erent N13 C values, 332 to 339x and 318 to 325x, respectively [18]. N13 C values of long-chain n-alkanes are more diagnostic than bulk organic matter in differentiating C3 and C4 plant contribution [3,5], as the latter has a mixed carbon input with di¡erent carbon isotope values. In addition, microbial degradation of organics with di¡erent qualities and carbon isotope values would change the N13 C value of bulk organic matter [21].
2. Samples and methods 2.1. Study area and sample strategy Xunyi and Luochuan are both located on the central CLP about 200 km from each other (Fig. 1). Present-day climate of these two sites is semihumid with an annual average precipitation of
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Fig. 1. A map showing the CLP (shaded area) and sampling locations (F for both Xunyi and Luochuan) (modi¢ed from Porter and An [15]). A: January sea-level pressure (mbar, in dashed lines), dominant wind vectors (arrows) and average southern limit of polar front (bold line). B: July sea-level pressure (mbar, in dashed lines), dominant wind vectors (arrows) and average landward limit of monsoonal front (bold line); L and H: Low and high pressure cells for the winter (A) and summer (B).
620 mm. The majority of rain falls in the summer season. The annual average temperature is 9.4‡C and the summer average temperature is 20‡C. Luochuan is steppe dominated and Xunyi is a mixture of forest and steppe. The loess tablelands (called ‘Yuan’) are well developed at both sites and complete loess-paleosol stratigraphic sequences allow for a systematic investigation of paleoclimate and paleovegetation [12,16]. For the Xunyi sequence, samples were taken every 5 cm from the surface to a depth of 220 cm and then every 10^20 cm over the interval 230^1580 cm, corresponding to approximately 500^2000 years resolution. At Luochuan, samples were collected every 5 cm throughout the 500 cm section. Each sample was divided into three aliquots for magnetic susceptibility, grain size and biomarker/total organic carbon (TOC) analysis. Samples were dried in an oven at 40‡C and stored in aluminium foil bags at room temperature. 2.2. Magnetic and grain size measurements About 10 g of each sample were measured for magnetic susceptibility using a Bartinton MS2 magnetic meter (values are given in units of 1038 m3 /kg). For non-carbonate grain size measurements, about 1.5 g of bulk sample was weighed into a 600-ml beaker. 10 ml of 10% H2 O2 was added and the beaker was heated until the organic matter oxidation reaction was com-
pleted. After cooling to room temperature, 10 ml of 10% HCl was added to the beaker to dissolve carbonate under boiling conditions. Then, the beaker was ¢lled with distilled water, which was siphoned o¡ after sitting for 24 h. Finally, 10 ml of 0.05 N NaPO3 was added to the beaker and sonicated for 10 min to separate ¢ne particles. A laser particle size analyzer (Mastersizer-S s) was used to measure grain sizes in the range of 0.05^ 900 Wm with a standard deviation less than 5% (based on 12 or more replicate analyses of several loess samples). The equipment is calibrated using an international standard. The percentage by volume of the coarse grain fraction ( s 30 Wm) is reported here. 2.3. TOC An aliquot of each sample was acidi¢ed with 6 M HCl and dried in an oven at 90‡C. TOC was measured on a Carlo Erba NA 1500 Series 2 elemental analyzer. About 20^60 mg of each sample was packaged in a silver or tin capsule and introduced into a 1020‡C combustion column containing chromium oxide on alumina, and silvered cobaltous/cobaltic oxide. Oxygen was then introduced into the helium carrier gas passing through the combustion column, which rapidly oxidized the sample. The oxidation products CO2 and NOx from the sample then passed through a copper reduction column at 650‡C
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where the NOx is reduced to N2 . The CO2 and N2 in the helium carrier gas were separated using a gas chromatograph column and measured using a thermal conductivity detector. Accuracy of the analytical method was monitored with the analysis of control standards and materials along with the samples. Reproducibility of the analytical method as determined by replicate analysis of samples was 0.05%. 2.4. Compound-speci¢c N 13 C of n-alkanes Samples of about 50V100 g were ground and weighed into an Alundum thimble and soxhlet extracted overnight with dichloromethane and methanol (3:1). The solvent was evaporated on a rotor-evaporator, and the total lipid extract was separated into three fractions by silica gel £ash-column chromatography. Elution with hexane gave the aliphatic hydrocarbon fraction, which contains the long-chain n-alkanes. The hexane fraction was analyzed on an HP 6890 gas chromatograph (GC) with an oven temperature program of 85‡C for 1 min, ramping to 200‡C at a rate of 20‡C/min, then increasing at 2‡C/ min to 300‡C and holding for 5 min, and increasing to 315‡C and holding for 8 min. The purity and structure of the n-alkanes were con¢rmed using a Varian 3800/Saturn 2000 GCMS. Compound-speci¢c carbon isotopic compositions of n-alkanes were measured by gas chromatography-isotope ratio-mass spectrometry (GCIRMS). An HP 5890 GC was connected to a Finnigan MAT delta plus XL mass spectrometer via a GC-C III interface. n-Alkanes passing through the GC column were oxidized at 940‡C and converted to CO2 . The gas stream was introduced to the mass spectrometer via an open split. Five pulses of standard CO2 gas, pre-calibrated against a commercial reference CO2 , were injected via the GC-C III interface to the IRMS for the computation of N13 C values of sample compounds. A set of 15 n-alkanes with known N13 C values acquired from Indiana University were measured daily to ensure accuracy of the machine. The standard deviation for duplicate analyses of this standard was 6 W 0.3x. The N13 C values are reported with reference to the PDB standard.
3. Results 3.1. Stratigraphy, magnetic susceptibility and age control The general stratigraphy of the last climatic cycle spanning 170 kyr in the Chinese loess sequence has been described in many studies [12,22]. It consists of a succession of loess and soil complexes denoted by L or S respectively. Typical loess is light colored, porous, calcareous and non-bedded whereas typical paleosols are dark gray brown to reddish brown and decalci¢ed to various degrees. The sequence consists of loess L2, soil complex S1, loess L1 and soil S0. Loess L1 is subdivided into subunits L1LL2, L1SS1 (weakly weathered), and L1LL2. S0 is the black loam of the Holocene [12,22]. We have only identi¢ed the major soil/loess boundaries in our pro¢les. The Xunyi section covers L2 to S0 (Fig. 2) and the Luochuan section contains L1 to S0 (Fig. 3). At both sites, anthropogenic disturbance of the very top portion of S0 (ca. 40 cm) can be observed in the ¢eld (shards, charcoal). In both the Xunyi and Louchuan sequences, magnetic susceptibility was low in the loess portions of the sequence and high in the paleosols (Figs. 2A and 3A). Variations in the magnetic susceptibility of Chinese loess deposits have been shown to be excellent proxies of the East Asian summer monsoon intensity [7,8,13]. Magnetic mineral production occurs during pedogenesis, and on the CLP, soil development is mainly controlled by precipitation [8,13,14]. Higher susceptibility values are typically associated with paleosol horizons and are interpreted as indicating warmer and wetter phases, while lower values are found in the loess layers and are thought to represent cooler and drier phases (Figs. 2A, 3A). Correlation of magnetic susceptibility records with dated marine oxygen isotope records has also enabled magnetic susceptibility to be used as a major stratigraphic tool for continental climatic reconstruction [12^15]. Intervals of rapid changes in magnetic susceptibility have been correlated with SPECMAP marine oxygen isotope stage (MIS) boundaries [15,23,24]. The magnetic susceptibility loess-paleosol boundaries L1/S0, S1/
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EPSL 6768 2-9-03 Cyaan Magenta Geel Zwart Fig. 2. Proxy records of Xunyi site for the last 170 kyr. A: Magnetic susceptibility. B: Percent of coarse grain fraction ( s 30 Wm). C: TOC content (%). D: N13 C of C27 , C29 , C31 and C33 n-alkanes (x). E: Weighted mean average N13 C of n-alkanes (x) and C4 plant percentage estimated based on a binary model. F: Corrected weighted mean average N13 C of n-alkanes (x) and C4 plant percentage. G: Vostock ice core CO2 concentration (ppmv) [42]. On the left are the loess/paleosol boundaries identi¢ed by magnetic susceptibility and ¢eld observation. On the right are the MIS and climate conditions on the CLP, identi¢ed based on the MS and coarse grain fraction records. LGM: last glacial maximum.
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L1, and L2/S1 are correlated with the boundaries of MIS 2/1 (12.05 ka), MIS 5/4 (73.91 ka) and MIS 6/5 (128.84 ka), respectively [15,23,24]. These transitions are used as age control points. Ages for other samples are established by linear interpolation.
the highest values in the Holocene, a trend similar to that of the Xunyi record (Fig. 3C). 3.4. Carbon isotopes of n-alkanes
In the Xunyi sequence, the coarse grain fraction ( s 30 Wm) ranges from 13.8 to 34.9% (Fig. 2B). The curve of the coarse grain fraction forms a mirror image of that for magnetic susceptibility. A greater percentage of coarse grains is seen in glacial periods, with the highest in MIS 6, indicating stronger wind due to a strengthened winter monsoon [16,17]. Lower values are seen in interglacial periods, with MIS 5 even lower than the Holocene value, re£ecting weak winter monsoons. In the Luochuan sequence, the percentage of coarse grains ( s 30 Wm) ranges from 22.0 to 43.4 (Fig. 3B). Surprisingly, the highest values are observed from middle to late Holocene. It is likely that these values re£ect human activities in the Luochuan region [25]. Deeper sections of the pro¢le mirror the magnetic susceptibility record as for the Xunyi sequence. The percentage of coarse grains increased as expected in the LGM (last glacial maximum).
Typical n-alkane distribution patterns from the Xunyi section are shown in Fig. 4A (for LGM) and Fig. 4B (for MIS 5). Detailed n-alkane distributions will be published elsewhere. In all the loess-paleosol sequences analyzed, the n-alkane distributions share three characteristics: they all have signi¢cant concentration in the C24 to C34 range; they all have strong odd-over-even preference; and C31 is always the dominant n-alkane (Fig. 4). These characteristics are consistent with a higher plant origin for the n-alkanes. Regressions between the C27 and C31 n-alkane concentrations and between C29 and C31 n-alkane concentrations have R2 values of 0.74 and 0.83, respectively, suggesting that these alkanes are all from higher plant waxes. The C27 , C29 , C31 , and C33 n-alkane concentrations are su⁄cient for carbon isotope ratio measurement using GCIRMS. The C27 , C29 , C31 and C33 n-alkanes show a reasonably coherent stratigraphic trend of N13 C variations (Figs. 2D, 3D), re£ecting their common source in higher plants. We have selected to use the weighted mean average N13 C of C27 , C29 , C31 and C33 n-alkanes, in order to reconstruct vegetation change:
3.3. TOC
weighted mean average N 13 C ¼
3.2. Percentage of coarse grain fraction ( s 30 Wm, non-carbonate)
TOC content in the Xunyi sequence ranges from 0.06 to 0.67% over the last 170 kyr with generally lower values measured in the loess sections and higher values measured in the paleosol sections (Fig. 2C). The lowest TOC content was found in L2 (MIS 6) and the highest in the Holocene. The timing of the changes in TOC values generally coincides with changes in the loess/paleosol lithology as well as with changes in magnetic susceptibility and the coarse grain fraction. However, the increase of TOC in MIS 5 lags that of magnetic susceptibility. TOC content in the Luochuan sequence ranges from 0.10 to 0.58% over the last 35 kyr with the lowest values recorded in the LGM (MIS 2) and
N 13 C 27 C 27 þ N 13 C 29 C 29 þ N 13 C 31 C 31 þ N 13 C 33 C 33 C 27 þ C 29 þ C 31 þ C 33
The weighted mean average N13 C values of n-alkanes (C27 -C33 ) from the Xunyi section ranges from 333.2 to 330.8x over the last 170 kyr (Fig. 2E), with generally more negative values during glacial periods and less negative values during interglacials. The most negative value of 333.2x was during the LGM. N13 C reached the highest value in the Holocene. The weighted mean average N13 C values of n-alkanes (C27 ^C33 ) from the Luochuan section range from 332.9 to 329.3x over the last 36 kyr (Fig. 3E), showing a similar glacial to interglacial oscillation. Overall, the two records reveal a shift of 2^3x
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EPSL 6768 2-9-03 Cyaan Magenta Geel Zwart Fig. 3. Proxy records of Luochuan site for the last 38 kyr. A: Magnetic susceptibility. B: Percent of coarse grain fraction ( s 30 Wm). C: TOC content (%). D: N13 C of C27 , C29 , C31 and C33 n-alkanes (x). E: Weighted mean average N13 C of n-alkanes (x) and C4 plant percentage estimated based on a binary model. F: Corrected weighted mean average N13 C of n-alkanes (x) and C4 plant percentage. G: Vostock ice core CO2 concentration (ppmv) [42]. On the left are the loess/paleosol boundaries identi¢ed by magnetic susceptibility and ¢eld observation. On the right are the MIS and climate conditions on the CLP, identi¢ed based on the MS and coarse grain fraction records. LGM: last glacial maximum.
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in the weighted mean average N13 C value of n-alkanes (C27 ^C33 ) with the more negative values during glacial periods. Because agricultural activity can be traced back to 7500 BP [25], the late Holocene values may have been in£uenced by anthropogenic activities. For the past several thousand years, the main crop in this region has been wheat, a C3 plant. Any agricultural contamination would lead to more negative N13 C values. Therefore, the late Holocene values are not used in the following discussion, which deals with natural vegetation response to climate.
are in accord with the climate of this region being colder and drier during glacial periods, and warmer and wetter during interglacials. This conclusion has been reached previously by many other studies [7^11,15^17,22]. The lower magnetic susceptibility and higher percent of coarse grains in L2 compared to sediment from the LGM seems to suggest a much drier climate in MIS 6 with stronger winter monsoons than the LGM. Furthermore, the higher magnetic susceptibility and smaller percent of coarse grains in S1 compared to S0, suggests a much wetter climate in MIS 5 with a stronger summer monsoon than in the Holocene.
4. Discussion 4.2. Plant productivity 4.1. Monsoon climates over the last 170 kyr The main focus of this section is the exploration of relationships between climate and vegetation during glacial and interglacial cycles on the CLP. Climatic proxy records for the CLP have been documented by numerous researchers over the last few years. We present a summary below. Detailed studies of Chinese loess sequences have established that magnetic susceptibility is correlated with precipitation, and hence can be used as a proxy of summer monsoon intensity [7,8,13^15]. Particle size, especially of the quartz fraction, has been used as a measure of the strength of the winter monsoon winds, which are responsible for transporting most of the dust building up as loess [15^17]. During periods of strong winter monsoon activity, the percentage of coarse grains is high, while magnetic susceptibility remains low. When the summer monsoon is dominant, magnetic susceptibility is high and the mean grain size is ¢ner. These relationships are re£ected in the opposite trending curves of magnetic susceptibility and coarse grain percent for both the Xunyi (Fig. 2A and B) and the Luochuan sequences (Fig. 3A and B). We have already mentioned that higher grain size values than expected in the Luochuan pro¢le during the late Holocene are most likely an anthropogenic e¡ect. The magnetic susceptibility and grain size data
Paleovegetation productivity estimation is more challenging since only a small percentage of plant organic matter is preserved in sediment. However, TOC in Chinese loess samples has been used as a proxy for paleoplant productivity. Soil organic matter content from desert and sandy soil regions of northern China is positively correlated with annual precipitation [26], suggesting that heavier precipitation increases productivity. Hence, TOC content re£ects the balance between organic matter production and decay. Sun and Liu (2000) also show that TOC and magnetic susceptibility are generally positively correlated over the last two glacial/interglacial cycles, indicating higher plant productivity during the wetter interglacial periods than the drier glacial periods [27]. For both the Xunyi and Luochuan records, TOC contents generally parallel magnetic susceptibility data (Figs. 2A and C, and 3A and C). This agreement clearly indicates that both proxies are controlled by precipitation, which re£ects the summer monsoon strength. TOC values are higher during MIS 5 and the Holocene, corresponding to warm/humid climates and lower during MIS 6 and the LGM, re£ecting cold/dry conditions. The high TOC ( s 0.5%) that is seen at the uppermost pro¢le (Figs. 2C and 3C) is largely due to the presence of highly decomposable organic matter from plant roots and addition of manure to the surface soils [25].
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4.3. Glacial/interglacial C3 /C4 vegetation changes on the CLP Using a binary model with endpoints for C4 and C3 plant wax n-alkanes, the relative contribution of C4 plants can be calculated. Long-chain n-alkanes produced by C3 and C4 plants have characteristically di¡erent N13 C values: 332 to 339x and 318 to 325x, respectively [7]. We chose endpoints of 321x for C4 plant n-alkanes and 336x for C3 plant n-alkanes. These values are well accepted and used for similar calculations [28]. The percent of C4 plant contribution (y) is calculated from the following formula : yUð321xÞ þ ð13yÞUð336xÞ ¼ N 13 Cmeasured
In the Xunyi pro¢le, the calculated C4 plant percentages varied from 21 to 35% during the Holocene and from 25 to 31% in MIS 5. C4 percent ranges for the glacial periods were 20^25% during the LGM and 19^24% during MIS 6 (Fig. 2E). The Luochuan pro¢le showed a similar sequence of vegetation change but the C4 plant percentage reached a maximum at 12^8 ka (Fig. 3E), a little earlier than the maximum in the Xunyi pro¢le at 11^5 ka. LGM C4 percentages at Luochuan ranged from 20 to 30% and Holocene C4 percentages ranged from 25 to 44%. These calculations indicate that the percent of C4 plants declined by ca. 10^20% during periods of glaciation. However, changing environmental factors can cause changes in the N13 C values of C3 and C4 plants without changing their relative abundance [29]. Tieszen [30] generalized six factors of primary relevance : (1) irradiance; (2) water stress; (3) osmotic stress; (4) nutrient supply; (5) temperature; and (6) altitude (associated with pCO2 decrease). The major factors which could have a¡ected the N13 C of plant waxes produced on the CLP without changing the C3 /C4 balance are pCO2 , aridity, and N13 C of atmospheric CO2 . A negative relationship has been established between atmospheric CO2 concentration and N13 C in C3 plants [31^34], but theories and observations suggested that N13 C values in C4 plants are not
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a¡ected by atmospheric CO2 concentration [35,36]. Feng and Epstein (1995) quanti¢ed the relationship between N13 C of C3 plants and pCO2 . For every decrease of 100 ppm in atmospheric CO2 concentration, N13 C values become heavier by 2.0 W 0.1x [33]. Hence, the lowering of pCO2 by 80 ppm during glacial periods would cause the N13 C of C3 plants to become 1.6x more positive. A negative relationship also exists between N13 C of C3 plants and water availability [33,37^39]. Stewart et al. (1995) estimated a decrease of 0.33 W 0.07x in N13 C of C3 plants per 100 mm increase in precipitation by examining eastern Austrian vegetation over a large range of precipitation (350^1500 mm) [38]. However, leaf N13 C of Atriplex canescens (a C4 plant) seems insensitive to variations in precipitation over the range of 100^250 mm [40]. During the last glacial period, the estimated precipitation in our study region was lower than the Holocene by up to 200 mm [8], which could have increased the N13 C of C3 plant n-alkanes by 0.6x. Changes in the N13 C of atmospheric CO2 would a¡ect the N13 C of plant material. Atmospheric CO2 was lighter by ca. 0.3x during the LGM [41] and would cause a similar decrease in the N13 C of n-alkanes for both C3 and C4 plants. This small decrease has little e¡ect on the calculation outcome and is not corrected for. We have used the relationships discussed above to estimate the e¡ect of pCO2 and aridity on the N13 C values of leaf wax n-alkanes. The pCO2 for each sample was determined from Vostock ice core record (Figs. 2G and 3G) [42] and precipitation values on the CLP were calculated from magnetic susceptibility using the equation: PA mm/ yr = 222+199Ulog10 (MS 1038 m3 /kg) [43]. In the correction, we assumed all vegetation to be C3 since the raw C4 % calculation (Fig. 3E) shows that C3 plants have been dominant over the last 170 kyr. Hence, our corrected weighted mean N13 C shows the maximum amount of o¡set possible. The corrected C4 % (Figs. 2F, 3F) and the raw C4 % (Figs. 2E, 3E) trend similarly but corrected C4 % is lower, with almost zero values during the glacial maxima (Figs. 2F, 3F). In addition, we need to consider the e¡ect of
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Fig. 4. Partial gas chromatograms of two samples from the Xunyi site on the CLP. Odd carbon number n-alkanes are marked by their numbers atop the peaks. The 200 cm sample is from the LGM (A) and the 940 cm sample is from the peak interglacial period (MIS 5, B).
dust and aerosol transported n-alkanes on our N13 C records. It is well established that leaf wax components can be transported via dust over thousands of kilometers [44,45]. For the CLP, most loess deposits originated as dust carried in from the arid desert and Gobi region in northwest China by the winter monsoons [46^50]. Since the vegetation of the cooler higher latitude regions
would be primarily C3 plants, variations in the winter monsoon could explain the small N13 C variations in our records for the LGM. However, plant productivity is very limited in the desert regions, and would have been lower during the colder and drier glacial periods, as indicated by lower TOC in both the Xunyi and Luochuan records (Figs. 2C, 3C).
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Fig. 5. Detailed comparison of N13 C of C31 n-alkane (solid line) and magnetic susceptibility (dotted line) across glacial/interglacial boundaries. A: Termination I for Xunyi site. B: Termination II for Xunyi site. C: Termination I for Luochuan site.
During interglacial periods, summer monsoon dominates and might increase the input of leaf waxes from low-latitude continental regions. However, dust de£ation and transport are limited for this humid and vegetation-covered region. Similar long-chain n-alkane distributions during glacial and interglacial periods indicate no major change of wax sources (Fig. 4). Also, data from Leizhou Peninsula in southeast China (unpublished) show that vegetation in this area was dominated by C4 plants during the LGM and by C3 plants during interglacial times. If there were input of leaf waxes from southeast China, then this would be in contrast to our observations. We conclude that our N13 C records re£ect the relative abundances of the locally growing C3 and C4 plants. Future work will need to analyze the n-alkanes born by dust transport. Our results o¡er new insights into the environmental factors controlling C3 /C4 shifts during the glacial/interglacial transition. At both the Xunyi and Luochuan sites, the expansions of C4 plants occurred when pCO2 was high and the climate was warmer and wetter. On the other hand, the
shift to more C3 plants occurred when pCO2 was low and the climate was colder and drier. This result clearly identi¢es lower temperature as the main cause of C4 plant decline during glacials on the CLP. 4.4. Increased/decreased temperature initiated the C3 /C4 vegetation change during the transitions on the CLP Upon close examination of the Xunyi and Luochuan records, C31 n-alkane (the dominant alkane) N13 C change leads the change in magnetic susceptibility by 1^2 kyr during the MIS 2/1 transition (Fig. 5A, C). Similarly, the shift to less C4 vegetation during the interglacial to glacial transition (MIS 5/MIS 4) led the change in magnetic susceptibility (Fig. 2A, F). This timing is in agreement with the ¢ndings of Wu et al. [10]. They concluded that changes in temperature and precipitation on the CLP were not in phase during the last 30 kyr [10]. During the last glacial period, temperature reached its coldest in the interval 19^ 17 kyr BP, whereas precipitation reached a mini-
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mum at 16 kyr BP [10]. Temperature was also the ¢rst to increase. In some areas of China, Holocene precipitation did not reach a maximum until 3 kyr BP following the maximum in temperature at 9^6 kyr [51]. One possible explanation for temperature change leading precipitation change lies in the interaction of the winter and summer monsoons over this region. During the LGM, when winter monsoon circulation was enhanced, cold air from high latitudes was brought to the Loess Plateau and caused rapid land surface temperature decreases because of the relatively small heat capacity of land [52]. However, the major source of water vapor to the CLP, the East Asian summer monsoons, might have responded more slowly to this change due to the relatively large heat capacity of the oceans [10]. N13 C of n-alkanes and magnetic susceptibility increases seem to be synchronous during the MIS 6/5 transition (Fig. 5B), probably due to low sampling resolution. 4.5. Temperature threshold for C4 plant growth Most other studies of C3 /C4 vegetation change concentrated on tropical and subtropical regions [3^6]. In the tropics, today’s vegetation type is mostly controlled by precipitation. Temperature during the last glacial period decreased by only 4‡C in equatorial Africa [53] and the growing season temperature was still high enough to support C4 plant expansion. Driven by lower pCO2 and increased aridity, the physiological advantage afforded by C4 metabolism presumably outweighed the negative e¡ect of the small temperature decrease. On the CLP, however, the summer temperature decrease from 20 to 13‡C during the LGM [54] was too harsh for C4 plant growth, even with lower pCO2 and increased aridity. Botanic studies show that with today’s climate, the distributions of C4 plants are positively correlated with growing season temperature in North America [55]. Laboratory and ¢eld data also indicate that photosynthesis (as measured by chlorophyll) is inhibited below 16‡C in corn and two other tropical C4 grasses [56,57]. Similarly, there seems to be a turnover point of minimum growing
season temperature for C4 plant expansion during the LGM. N13 C stratigraphy of long-chain n-alkanes in Lake Baikal sediments provides another piece of collaborative evidence. The N13 C values during the last glacial/Holocene transition are remarkably constant, well within the range for leaf wax n-alkanes synthesized by C3 plants [58]. Today’s July temperatures at Lake Baikal are ca. 11V14‡C [59], which is close to the LGM temperature on the CLP. The dominance of C3 plants today is due to cold growing season temperature. Lower temperature during the last glacial must have inhibited C4 plant growth in the Lake Baikal area despite lower pCO2 . Most likely, there exists a threshold of growing season temperature of 13^15‡C for C3 /C4 plant competition. Above it, lower pCO2 and aridity favored C4 expansion as seen in Africa [3] and Quexil, Mesomerica [5]; below this limit, C4 plants were out-competed by C3 plants in spite of lowered pCO2 and increased aridity, on the CLP and at Lake Baikal [58]. We can infer that C4 plant decline in Babicora was also mainly due to the lower growing season temperature, rather than increased winter precipitation. Our results provide another piece of evidence that low pCO2 during the LGM was insu⁄cient to trigger the expansion of C4 plant coverage without favorable climatic conditions [5].
5. Concluding remarks Multiple proxies from loess/paleosol sequences on the CLP allow comprehensive evaluation of paleoclimate and paleovegetation changes for the last 170 kyr. Magnetic susceptibility and grain size results indicate that climate over the last 170 kyr has oscillated between winter monsoon-dominated cold/dry glacials and summer monsoon-dominated warm/wet interglacials, in agreement with numerous previous studies. These conclusions have also been corroborated by measurements of TOC, which is mainly controlled by precipitation. During glacial periods, globally lower pCO2 and regionally increased aridity on the CLP would have favored expansion of C4 vegetation,
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but lower temperatures in the growing season caused the decline of C4 plants. Conversely, increasing temperature was essential to initiate the expansion of C4 grasses on the CLP during the Holocene. Our results suggest a turnover point of minimum growing season temperature of ca. 13^ 15‡C, below which C4 plants were out-competed by C3 plants. Only when temperature was greater than this threshold, could pCO2 and aridity play a signi¢cant role in the growth of the C4 plant community, as in Africa and Quexil, Mesoamerica. We can conclude that C4 vs. C3 vegetation changes during the last glacial/Holocene transition were controlled by regional climate variations. The results reported herein for the CLP have important implications in predicting vegetation response to future climate change. For example, global warming could cause C4 expansion in several areas where the current mean growing season temperature is near the threshold.
[4]
[5]
[6]
[7]
[8]
[9]
[10]
Acknowledgements [11]
We would like to thank Dr. Xiahong Feng for GCIRMS facility support and discussion and Dr. Geo¡ Eglinton for constructive comments. We are grateful to Drs. Steve Colman and Peter Sauer for their thoughtful reviews and suggestions. We thank Hua Wang and Xiaodong Miao for help during ¢eld work. This research was supported by Dartmouth College, the National Sciences Foundation (EAR111403 to X. Feng) and the National Sciences Foundation of China (NSFC40121303).[BOYLE]
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