Rainfall dependent transformations of iron oxides in a tropical saprolite transect of Hainan Island, South China: Spectral and magnetic measurements

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116,, doi: /2010jf001712, 2011 Rainfall dependent transformations of iron oxides in a tropical saprolite transect of Hainan Island, South China: Spectral and magnetic measurements Xiaoyong Long, 1,2 Junfeng Ji, 1 and William Balsam 3 Received 8 March 2010; revised 30 May 2011; accepted 8 June 2011; published 25 August [1] The iron oxide content of soils and sediments controlled by weathering and pedogenesis is generally considered a reasonable indicator of climate. Previous studies in temperate zones have established a positive correlation in aerobic soils between ferrimagnets and low to moderate rainfall; the correlation seems to be reversed under extreme climates with high rainfall. Here we present a transect of saprolitic soils from Hainan Island, South China, with high rainfall ( mm/yr), little temperature variation (23 24 C), and extreme weathering. Along this transect we observed that both hematite concentration and magnetic susceptibility decrease with increasing rainfall, whereas goethite concentration displays a large increase. However, there is no systematic trend in the total amount of iron oxides related to chemical weathering intensity along the transect. Goethite is the favored mineral phase of iron oxide with increasing rainfall and accumulates at the expense of hematite and maghemite through the dominance of rainfall driven processes. These pedogenic processes coincide with the fundamentals of previous nonmonotonic models of hematite, magnetic susceptibility and rainfall control of an inflection point. This study also verifies a common genetic relationship between hematite and pedogenic ferrimagnets across a wide climate range. A conceptual model considering both rainfall and temperature is proposed to help interpret the mechanism of ferrimagnet formation and changes in the rainfall inflection point. Citation: Long, X., J. Ji, and W. Balsam (2011), Rainfall dependent transformations of iron oxides in a tropical saprolite transect of Hainan Island, South China: Spectral and magnetic measurements, J. Geophys. Res., 116,, doi: /2010jf Introduction [2] Iron oxides are widespread in earth systems and are highly concentrated near the surface as a result of aerobic weathering of Fe bearing minerals [Hochella et al., 2008]. Iron oxides in soils and sediments can be divided into a magnetic group and a coloring group, distinct from each other according to their physical properties. The former includes magnetite (Mgt) and maghemite (Mgh) and is characterized by high magnetic susceptibility (MS), whereas the latter includes hematite (Hm) and goethite (Gt) and is characterized by striking colors. The distribution and formation of each group in soils and sediments is controlled by weathering and pedogenesis under specific climate conditions [Schwertmann, 1971, 1985; Singer and Fine, 1989; Singer et al., 1996; Maher, 1998]. 1 Institute of Surficial Geochemistry, College of Earth Sciences and Engineering, Nanjing University, Nanjing, China. 2 College of Geographical Science, Southwest University, Chongqing, China. 3 Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana, USA. Copyright 2011 by the American Geophysical Union /11/2010JF [3] Previous studies have demonstrated that the concentration of the magnetic group (also called ferrimagnets), which is often determined by MS, is positively correlated with low to moderate rainfall in modern surface soils on a large spatial scale [Han et al., 1996; Maher, 1998; Porter et al., 2001; Maher et al., 2002; Warrier and Shankar, 2009]. MS has been suggested as a proxy to reconstruct paleorainfall in temperate monsoon zones, especially for the eolian sediments on the Chinese Loess Plateau (CLP) [Kukla et al., 1988; Heller et al., 1993; Liu et al., 1995; Maher and Thomson, 1995; Maher, 1998]. However, a growing body of evidence suggests a negative correlation between ferrimagnets and rainfall in extreme climates with excessive rainfall [Begétetal., 1990; Han et al., 1996; Singer et al., 1996; Chlachula et al., 1998; Liu et al., 2001, 2003; Balsam et al., 2004]. As a result, a rainfall inflection point around 1200 mm/yr has been proposed to separate the opposite patterns of ferrimagnets across the monsoon regions of South China [Han et al., 1996]. In addition, a lower rainfall inflection point, around 650 mm/yr, has also been proposed in the interpretation of paleoclimate on the CLP [Balsam et al., 2004]. However, most previous studies of ferrimagnet formation under high rainfall were done in monsoon zones and mountain areas where a change in rainfall is coupled with a 1of15

2 Figure 1. (a) Locality map showing the study area and samples (indicated by red dots) used in the study. (b, c) Two representative profiles on the north and south of transect are illustrated. (d) The tropical ferrasol transect is underlain by basalt and exhibits a dramatic increase in rainfall and little variation in temperature from north to south as indicated by the monthly climate data (MMR = mean monthly rainfall; MMT = mean monthly temperature) in Haikou and Qionghai. change in temperature. Further, the spatial comparison of soil magnetism is often complicated by a change of parent material, soil duration, vegetation, topography, and fire history [Han et al., 1996; Maher, 1998; Lu, 2003; Eggleton and Taylor, 2008; Blundell et al., 2009]. The mechanism of magnetic reduction in high rainfall regions is often attributed to the reductive dissolution of ferrimagnets controlled by gleying processes, especially in cold regions with anaerobic soils and high rainfall [Liu et al., 2001]. However, the formation of a rainfall inflection point in aerobic soils cannot be explained by current mechanisms without strong redox process, especially in warm regions with high rainfall and high evaporation. [4] In contrast to ferrimagnets, the coloring iron oxides, Hm and Gt, contain more definitive climatic information than ferrimagnets and from a wider climate range [Cornell and Schwertmann, 2003]. Hm is favored by warmer, drier, and more seasonal climate, whereas Gt is favored by cooler, wetter, and a less seasonal climate [Schwertmann, 1971], although a change of ph and organic matter can also affect the distribution of Hm and Gt [Cornell and Schwertmann, 2003]. Moreover, a positive correlation between the concentration of Hm and pedogenic ferrimagnets commonly exists in aerobic soils, thereby making the coloring iron oxides useful in understanding the formation mechanism and climate implication of ferrimagnets [Ji et al., 2001; Balsam et al., 2004; Y. G. Zhang et al., 2007; Zhang et al., 2009; Kumaravel et al., 2010; Souza Junior et al., 2010; Torrent et al., 2007; Torrent et al., 2010a, 2010b]. Recently, Torrent et al. [2006] proposed a pathway where the neoformation of pedogenic Mgh precedes the formation of Hm to explain changes in the concentration of pedogenic ferrimagnets in aerobic soils. This abiotic mechanism was originally proposed on the basis of laboratory experiments on the dehydration of ferrihydrite (Fh) [Barrón and Torrent, 2002]. In contrast, a chemical kinetic model of ferrimagnets illustrated that the abiotic pathway can be neglected under temperate conditions [Boyle et al., 2010], and the latest TEM study on the formation of Hm suggests a biotic effect in the magnetic change of eolian sediments on the CLP [Chen et al., 2010]. However, studies based on eolian sediments and less weathered soils may be influenced by varying detrital ferrimagnet input in temperate regions. In contrast, the highly weathered tropical soils that have a high content of pedogenic iron oxides, such as those in this study, should help in understanding the relationship between both 2of15

3 Table 1. Iron Oxides Concentration and Distribution with Increasing Rainfall Along the Ferralsol Transect Location Sample Elevation (m) MAT ( C) MAR (mm/yr) Fe t Fe d Fe d /Fe t Redness Hm Gt Hm/(Hm+Gt) Hm/Fe d c lf (SI) Pedo c lf (SI) c fd (SI) c fd Hm/c fd (10 7 gm 3 ) HN01 Layer I Layer II HN02 Layer I Layer II HN03 Layer I Layer II HN04 Layer I Layer II HN05 Layer I Layer II HN06 Layer I Layer II HN07 Layer I Layer II HN08 Layer I Layer II HN09 Layer I Layer II HN10 Layer I Layer II HN11 Layer I Layer II HN12 Layer I Layer II HN13 Layer I Layer II HN14 Layer I Layer II Average coloring and magnetic group iron oxides during weathering and pedogenesis. [5] To explore the effect of excessive rainfall on the concentration and distribution of pedogenic iron oxides in aerobic soils, we present a tropical saprolite transect, not subjected to strong redox processes and undergoing extreme weathering on a basaltic parent material from Hainan Island, South China. Methodologically, in comparison with previous large scale iron oxide surveys [Han et al., 1996; Blundell et al., 2009; Viscarra Rossel et al., 2010], our study holds factors that control soil pedogenesis constant with the exception of rainfall. The temperature and soil age along the transect are nearly constant, whereas rainfall varies from 1440 to 2020 mm/yr. In addition, the parent material has a high content of Fe bearing silicate minerals and low ferrimagnet content. This climosequence, therefore, provides a good opportunity to understand the relationship between both groups of pedogenic iron oxides in aerobic soils under high rainfall and high evaporation conditions. 2. Study Area and Samples 2.1. Physiographic Setting [6] Hainan Island is located in the south of China (Figure 1a). Northern Hainan Island (NHI) is underlain by Quaternary basalt whereas Southern Hainan Island (SHI) is mainly granitic. Hainan Island has a typical tropical monsoon climate with a mean annual temperature (MAT) of approximately 24 C and a mean annual rainfall (MAR) of approximately 1600 mm/yr. The orographic effect of the south central highlands blocks water vapor from the South China Sea on the east side of the of island from reaching the western side and increases the rainfall differential between the western and the eastern sides of the island. Weathering crusts from NHI commonly have undergone extreme chemical weathering but little physical disturbance since Late Pleistocene [G. L. Zhang et al., 2007]. Thick soil coverage over the uppermost weathering crust derived from the oldest basalt is characterized by a high content of clay minerals cemented by the iron oxides, thereby preventing physical disturbance and retarding chemical weathering. A previous study of a chronosequence from NHI [Huang and Gong, 2000] documented the good correlation between thickness of the weathering crust and soil age. In addition, it has been documented that the chemical weathering intensity in the uppermost soils reaches a plateau when the soil age is greater than 1.3 Myr [G. L. Zhang et al., 2007]. Geochemical studies also indicate lower Si flux in the rivers of NHI than SHI, even though basalt is more susceptible to dissolution than granite [Chen and Chen, 1992]. As proposed by Stallard and Edmond [1987], NHI can be considered a typical transport limited region, whereas SHI is a typical weathering limited region. [7] Our transect lies in northeastern NHI, where the underlying basalt is restricted to the Quiongshan episode of the early Pleistocene with K Ar ages around 1.6 ± 0.3 Myr [Ge et al., 1989; G. L. Zhang et al., 2007]. The parent material is a typical quartz tholeiitic basalt with a high content of Febearing primary minerals and a small ferrimagnet content as indicated by a low MS, less than 120 (SI, 10 8 m 3 /kg), measured in the parent material along the transect. The MAT range along the transect is small ( C), whereas the variation in MAR is high ( mm/yr, Table 1). Rainfall is concentrated in the summer and autumn as indi- 3of15

4 Table 2. Chemical Composition and Chemical Weathering Intensity Measured in Layer I and Layer II Along the Transect Location Sample ph Clay N C Al 2 O 3 CaO Fe 2 O 3 K 2 O MgO MnO Na 2 O P 2 O 5 SiO 2 TiO 2 LOI CIA Sa HN01 Layer I Layer II HN02 Layer I Layer II HN03 Layer I Layer II HN04 Layer I Layer II HN05 Layer I Layer II HN06 Layer I Layer II HN07 Layer I Layer II HN08 Layer I Layer II HN09 Layer I Layer II HN10 Layer I Layer II HN11 Layer I Layer II HN12 Layer I Layer II HN13 Layer I Layer II HN14 Layer I Layer II Average cated by mean monthly temperature (MMT) and mean monthly rainfall (MMR) from Haikou and Qionghai, which are located on the north and south of this transect, respectively (Figure 1d). [8] Samples were taken from local highlands on the basalt plateau that had a slightly undulating terrain. The elevation difference between the samples was less than 200 m as a result of the gentle slope of the plateau, less than 8. Although the highest rainfall on the transect is close to that found in the Amazon Basin or Southeast Asia, this Hainan Island transect is dominated by seasonal rainfall and an undulating terrain that has good drainage. All the sampling locations are away from villages and cities to avoid recent disturbance by human activities, especially cultivation and irrigation. The majority of the vegetation along the transect is characterized by rubber trees, eucalyptus, and shrubs. Coverage by vegetation increases from low to high rainfall regions (Figures 1b and 1c). Wildfires, which have the potential to influence iron oxides, are not common because of high relative humidity (RH) throughout the year as a result of the tropical monsoon climate Soil Description [9] All soil profiles along the transect are similar in thicknesses and range from 2 to 3 m above the unweathered parent material. These profiles form under well drained conditions as indicated by the clear transition from the regolith, saprolite, and subsoil horizons to the topsoil horizon (Figures 1b and 1c). In addition, each profile has a relatively thin humus layer (1 2 cm) on the surface because of high temperatures and oxidizing conditions. The texture and color of the uppermost soils, including the subsoil and topsoil horizons at depths between 0 and 1 m, appears to be homogeneous vertically (Figures 1b and 1c). It is observed that there is no strong redox process and corresponding iron migration, which often results in soil color change or reticulated mottling. Both horizons have a high clay content, averaging about 62.3% along the transect (Table 2). The uppermost soils are assumed to have had enough time to mature and reach chemical equilibrium with the climate because a similar climate pattern has been sustained since the Late Pleistocene [Huang et al., 1999]. Twenty eight soil samples were collected from 14 localities. At each locality, a trench was dug through the topsoil and subsoil horizon and samples were collected at depths of 0 20 cm (layer I) and cm (layer II), roughly corresponding to topsoil and subsoil horizons (Table 2). Samples from each layer were thoroughly mixed to increase vertical representation and horizontal comparability. In contrast to the similar soil color of both horizons at each profile, the soils along the transect exhibit a distinct color change from red (2.5 YR 3/6) to yellow (10 YR 6/8). Soils along the transect are classified as rhodic ferralsol or xanthic ferralsol [IUSS Working Group WRB, 2007]. Clearly, the spatial color change should be correlated to the change of concentration and distribution of iron oxide phase controlled by climate. 3. Methods 3.1. Basic Soil Analysis [10] Soil ph was measured with a glass electrode in 1 M potassium chloride at a soil:solution of 1:2.5. Total carbon (C%) and nitrogen (N%) content were measured using a Euro EA3000 CHNS O analyzer. The free iron oxides, Fe d, 4of15

5 Table 3. Correlation Coefficient Matrix for the Parameters Along the Transect a MAR MAT Altitude Clay ph OC Fet Fed Redness Hm Gt Hm/(Hm+Gt) cclf Pedo clf ccfd ccfd CIA Sa MAR 1 MAT 0.954** 1 Altitude 0.642* 0.618* 1 Clay ph OC Fe t * Fe d * Redness 0.861** 0.716** 0.605* Hm 0.887** 0.755** 0.564* ** 1 Gt 0.744** 0.631* ** 0.889** 1 Hm/(Hm+Gt) 0.865** 0.739** ** 0.988** 0.943** 1 cc lf 0.855** 0.858** ** 0.766** 0.760** 0.801** 1 Pedo c lf 0.845** 0.852** * 0.730** 0.696** 0.758** 0.989** 1 cc fd 0.860** 0.878** ** 0.743** 0.742** 0.778** 0.994** 0.978** 1 ccfd 0.920** 0.936** 0.732** * ** 0.737** 0.608* 0.710** 0.779** 0.760** 0.814** 1 CIA Sa * a One asterisk indicates that correlation is significant at 0.05 level (two tailed); two asterisks indicates that correlation is significant at 0.01 level (two tailed). were determined on a JY38S inductively coupled plasma spectrometer after the citrate bicarbonate dithionite (CBD) procedure, that is, two extractions with sodium dithionite in a hot (75 ± 5 ) sodium citrate and sodium bicarbonate solution [Mehra and Jackson, 1960]. The residues of the deferrated samples were combined and ground to produce a natural soil matrix for the determination of Hm content (see below). [11] The elemental composition of the soils was determined by X ray fluorescence (XRF) with an ARL9800 XP+XRF spectrometer and expressed as an oxide. The total iron, Fe t, which is often expressed as ion weight percentage, was calculated from the weight percentage of iron oxide (Fe 2 O 3 ). The chemical index of alteration (CIA) is calculated as 100* Al 2 O 3 /(Al 2 O 3 + CaO + Na 2 O+K 2 O) by the molecular proportions of oxides [Nesbitt and Young, 1982]. The index Sa is calculated as SiO 2 /Al 2 O 3 by the molecular proportions of oxides [Huang et al., 1996] Magnetic Measurement [12] MS is a sensitive and effective method for estimating the minuscule weight concentration of ferrimagnets in soils and sediments. It has also been widely used to describe soils formed from different parent materials and to aid in soil taxonomy [Mullins, 1977; Maher, 1986; Singer and Fine, 1989]. Pedogenic ferrimagnets formed under extreme weathering conditions often have an extremely high MS value, close to 111,600 SI [Dearing, 1994]. Magnetic measurements can be sensitive and reliable, often reflecting small changes in the ferrimagnet content. The low frequency MS (c lf ) and high frequency MS (c hf ) were determined with a Bartington MS2B meter at 0.47 and 4.7 khz to estimate the total ferrimagnet content. Frequency dependent susceptibility, c fd and c fd %, are calculated by c lf c hf and (c lf c hf ) 100/c lf to determine the absolute and relative content of superparamagnetic ferrimagnets which is correlated with pedogenesis [Worm, 1998; Worm and Jackson, 1999]. Meanwhile, Pedo c lf is calculated by the difference of c lf measured before and after the CBD procedure which can effectively remove pedogenic ferrimagnets from soils and sediments [Heller et al., 1993; Fine et al., 1995; Vidic et al., 2000] and is commonly used to estimate the pedogenic ferrimagnet fraction Diffuse Reflectance Spectrophotometry (DRS) [13] Diffuse reflectance spectra of ground samples were analyzed in a Perkin Elmer Lambda 900 spectrophotometer from 400 to 2500 nm at 2 nm intervals. The color index, redness, was calculated using the standard color band [Judd and Wyszecki, 1975]. Hm and Gt are the two main coloring agents in soils and are characterized by distinct colors, red and yellow, respectively. Hm is a more intense coloring agent than Gt [Bigham et al., 1978]. Redness, which is calculated as a sample s percentage reflectance in the red color band ( nm) divided by the sample s total reflectance within the VIS ( nm), is used to estimate Hm content [Torrent et al., 1980; Barrón and Torrent,1986]. [14] In order to overcome the matrix effect in estimating Hm and Gt, a series of mixing experiments were conducted. For these experiments, we used the Hm standard, Pfizer R1599, pure red Fe oxide; for Gt we used Hoover Color Corporation Synox HY610 yellow. Both standards are fine 5of15

6 Table 4. Multivariate Regression Analysis of Fe d, Hm/(Hm+Gt), Hm, and lgc fd Averaged by Layer I and II Using MAR, MAT, Altitude, Clay Content, ph, and Organic Matter Along the Transect a B Standard Error Beta t Sig. Zero Order (r) Partial (r) Fe d : R 2 = 0.702, Adjusted R 2 = 0.446, RMSE = (Constant) MAR MAT Altitude Clay ph OC Hm/(Hm+Gt) : R 2 = 0.896, Adjusted R 2 = 0.807, RMSE = (Constant) MAR MAT Altitude Clay ph OC Hm : R 2 = 0.924, Adjusted R 2 = 0.859, RMSE = (Constant) MAR MAT Altitude Clay ph OC lg c fd (SI): R 2 = 0.958, Adjusted R 2 = 0.922, RMSE = (Constant) MAR MAT Altitude Clay ph OC a Regression coefficients (unstandardized and standardized, B and Beta), t statistic, and level of significance are displayed together with zero order and partial correlations coefficients between dependent variables and independent variables. grained (submicron), similar to iron oxides found in soils [Bigham et al., 1978]. First, known quantities of Hm were added to the deferrated soil matrix produced by the CBD procedure to establish a primary regression equation between Hm content and redness. From this equation, a rough estimate of Hm content in natural soil samples can be calculated directly. Because other iron compound phases such as Fh and ferrimagnets were dissolved by the CBD procedure and because they have a relatively low weight percentage [Ji et al., 2002; Liu et al., 2004; Torrent et al., 2007], a rough estimate of Gt content can be calculated by the following equation when we assign free iron oxides (Fe d ) expressed as iron ion concentration to the combination of iron in stoichiometric Hm (a Fe 2 O 3 ) and Gt (a FeOOH) [Torrent et al., 2007]: Gt ¼ 1:59 ðfe d Hm=1:43Þ ð1þ [15] Second, we produced a series of calibration samples by mixing known quantities of both Hm and Gt together with the same deferrated soil matrix. The weight concentration of the pure Hm and Gt we mixed with the deferrated samples is similar to the roughly estimated content of Hm and Gt in natural samples obtained from the primary regression equation and Fe d. Thus, the new data relating Hm content and redness was combined with the data attained in the first step to calibrate the primary regression equation. Finally, a more accurate estimate of the Hm content of natural samples was produced from the calibrated regression equation and the Gt content recalculated by equation (1) X Ray Diffraction [16] For ferralsol samples, which often have a high content of iron oxides after extreme weathering, the traditional X ray diffraction (XRD) method has also been used to detect and determine the concentration of magnetic and coloring iron oxides directly. The clay fractions of selected samples were extracted from the bulk soils and then boiled in 5 M NaOH for 2 h [Singh and Gilkes, 1991] to concentrate the iron oxides. The XRD patterns of the residues were obtained using a Rigaku D/max ra diffractometer by Cuka radiation with the goniometer from 20 to 45 stepped by The ratios of Hm and Gt were obtained from d (012) peak area ( 3.5) of Hm and d (110) peak area of Gt [Kampf and Schwertmann, 1982] Multivariate Statistical Analysis [17] To evaluate the potential effect of climate and the other soil factors on the distribution of pedogenic iron oxi- 6of15

7 Figure 2. (a, b) Relationship between the Hm content, Gt content, and redness of the CBD treated samples into which known quantities of Hm and Gt have been mixed. The samples indicated by red contain only Hm, whereas the calibration samples indicated by blue contain both Hm and Gt. The red trend line is a regression line from samples that contain only Hm; the black trend line is for all samples. (c) The ratios of Hm/(Hm+Gt) of natural samples estimated by DRS and XRD have shown good correlation with each other. (d) The transect exhibits significant change of redness with rainfall. des along the transect, both multivariate correlation and regression analysis were done on the averaged variables from layers I and II using SPSS 18.0 (Tables 3 and 4). In comparison with previous large scale studies [Blundell et al., 2009], five potential factors, MAT, altitude, clay content, ph, and organic matter (OC, C%) in addition to MAR, were used as independent variables. The dependent variables include the concentration and ratio of pedogenic iron oxides estimated by Fe d, Hm/(Hm+Gt), Hm, and log c fd (SI). These dependent variables were regressed against the above independent variables. 4. Results 4.1. Quantification of Coloring Iron Oxides [18] Figure 2a illustrates the Hm content of samples in the mixing experiment as a function of redness. For the samples containing only known quantities (0.5%, 1.25%, 2.5%, 5%, 7.5%, 10%, 15%, and 20%) of Hm a good correlation between Hm content and redness was obtained with a primary exponential regression equation with an R 2 = 0.99 as follows: Hm ð% Þ ¼ 0:0015 exp 0:193Redness ð2þ where Hm is the weight percentage in samples and redness is the percentage reflectance in the red color band. We also analyzed more than 20 calibration samples that contained both Hm (0 to 10%) and Gt (0 to 20%) in ratios similar to natural samples as estimated by equations (1) and (2). With the addition of calibration samples that contained both Hm and Gt, the correlation between the Hm content and redness remained high, R 2 = The two regression curves illustrated in Figure 2a almost coincide, and the final regression equation for all samples compared to equation (2) has undergone a minor adjustment in the parameters as follows: Hm ð% Þ ¼ 0:0012 exp 0:196Redness ð3þ [19] The results of the regression indicate that Gt does not significantly affect the determination of Hm by altering 7of15

8 Figure 3. rainfall. Relationship between iron oxides, (a) Fe d and Fe t, (b) Hm, (c) Gt, and (d) Hm/(Hm+Gt) and redness. The Hm content of natural soil samples can be accurately estimated directly from the final regression equation (3) by using redness; samples along the transect vary from 36.2% to 44.7% in redness, and Hm decreases with increasing rainfall (Figure 2d). Because the Gt content in the mixing experiments has no clear correlation with the redness and its color signature seems to be masked by Hm in the red color band (Figure 2b), the Gt content of natural samples has to be calculated indirectly by equation (1). [20] The combined method to determine Gt by redness and Fe d is validated by traditional XRD methods. The Hm/ (Hm+Gt) ratio estimated by two methods exhibits a high linear correlation with a slope close to 1 (Figure 2c). But the positive intercept around 20 in Figure 2c indicates that the Hm/(Hm+Gt) values estimated by redness and Fe d are lower by about 20% than the traditional method. In fact, the CBD procedure not only dissolves Hm and Gt but also ferrihydrite and ferrimagnets, although they make little weight contribution to Fe d in mature soils. The combined method with DRS and Fe d overestimates the content of Gt and underestimates the ratio of Hm/(Hm+Gt). On the other hand, the traditional method is often conducted after iron oxides in the samples are concentrated by clay extraction. Thus, the estimated Hm/(Hm+Gt) does not stand for the bulk sample, rather for Hm/(Hm+Gt) in the clay fraction of the sample. Although Hm and Gt are often nanosize in natural systems [Hochella et al., 2008], Hm is more concentrated in the clay fraction than Gt because Gt has a greater tendency to form coatings on the sand and silt grains [Hao et al., 2009]. As a result, the Hm/(Hm+Gt) of the clay fraction should be higher than in the bulk samples. Therefore, the combined methods can be used to estimate the Hm/(Hm+Gt) of soils effectively. In order to apply the method more directly, the Hm/(Hm+Gt) proxy can be replaced by Hm/Fe d or Redness/Fe d. These indices are intuitive and have the potential to be useful in soil surveys on a large scale in the future Concentration and Distribution of Iron Oxides Along the Transect [21] The measured concentrations of Fe d, Hm, and Gt as well as the values c lf, Pedo c lf, c fd and c fd % of the soils are reported in Table 1. Changes in the concentration and distribution of iron oxides with increasing rainfall along the transect are commonly coupled in layer I and layer II (Figures 3 and 4). The Fe d, which is used to estimate the total amount of pedogenic iron oxides, including magnetic 8of15

9 Figure 4. Relationship between (a) c lf, (b) Pedo c lf, (c) c fd, (d) c fd,and rainfall along the transect. group and coloring group, displays relatively little variation with rainfall and averages around 11.1%. Similarly, the Fe t also remains stable around an average of 15.5% (Figure 3a). [22] However, each group of iron oxide exhibits significant change along the transect. The coloring iron oxides show a remarkable linear correlation with the rainfall. Hm decreases from 8.7% to 1.6% (Figure 3b), and Gt increases from 6.9% to 18.9% (Figure 3c) as rainfall increases. The Hm/(Hm+Gt) diminishes from 55.3% to 9.5% (Figure 3d). Similarly, c lf and Pedo c lf show good negative correlations with the rainfall (Figures 4a and 4b); c lf decreases from 3170 to 358 SI and Pedo c lf from 2950 to 166 SI. c fd and c fd % also exhibit similar trends with rainfall as indicated by high correlation coefficients (Figures 4c and 4d). The c fd decreases from 574 to 22 SI and c fd % from 18.5% to 5.2% Chemical Composition of Soils on the Transect [23] Major element concentrations expressed as oxides are listed in Table 2. After extreme weathering the soil samples exhibit a homogeneous chemical composition. The weight percentage of Fe 2 O 3 varies from 20.2% to 25.3% with an average of 22.1%; Al 2 O 3 varies from 22.3% to 30.4% and averages 27.1%. The weight percentage of SiO 2 ranges from 25.2% to 33.6% and shows random variation around an average of 29.3%. The average weight percentage of K 2 O, Na 2 O, CaO, and MgO totals less than 1%. Therefore, the immobile elements such as Fe and Al are highly concentrated while the alkali and alkali earth elements such as K, Na, Ca, and Mg are found in low concentrations. In addition, soil chemistry analysis indicates a stable acidity with ph averaging about 4.7. Organic matter, indicated by total C and total N, also shows little variations around the averages of 1.2% and 0.1%, respectively Correlation Between Soil Factors, Iron Oxides and Chemical Weathering [24] The correlation matrix of soil factors, iron oxides, and chemical weathering indices is displayed in Table 3. Both the coloring iron oxides estimated by redness, Hm, Hm/(Hm+Gt), Gt, and the magnetic iron oxides estimated by c lf, Pedo c lf, c fd and c fd show higher correlation coefficients with MAR than with the other soil factors including MAT, altitude, clay content, ph, and organic matter. The chemical weathering intensity estimated by Fe d, Fe t,fe d/ Fe t, CIA, and Sa exhibit weak correlations with all the soil factors and each phase of the iron oxides. [25] However, the correlation analysis cannot evaluate the relative contribution of variables with a much different range of variation, instead, multiple regression analysis of 9of15

10 iron oxides and soil factors was performed. The regression analysis of Fe d, Hm, Hm/(Hm+Gt), and c fd with the main soil factors (Table 4) reveals that MAR has a dominant and significant effect (Sig < 0.05) on the change of Hm/(Hm+Gt), Hm, and c fd, but has no significant effect (Sig > 0.05) on Fe d, since Fe d exhibits little variation along the transect. In addition, MAT often has a secondary, but not a significant effect, on the change in Fe d because of the limited temperature variation. The other soil factors, including altitude, clay content, ph, and organic matter, also make no significant and direct contribution to the systematic monotonic change of iron oxides along the climosequence. These observations confirmed that rainfall is the dominant factor in controlling the change of both groups of pedogenic iron oxides along the transect. 5. Discussion 5.1. Rainfall Dependence of the Coloring Iron Oxides Distribution [26] The coloring iron oxides as a whole are highly concentrated in the uppermost soils as indicated by the high Fe d. More importantly, the competitive relationship between the distribution of Hm and Gt has been found to exhibit a strong dependence on rainfall along the transect (Figures 3b and 3c), even though the total amount of iron oxide remains relatively constant (Figure 3a). The Hm/(Hm+Gt) ratios also show a corresponding decrease as the rainfall increases. In addition, the MAT also has positive partial correlation with Hm and Hm/(Hm+Gt) as indicated in Tables 4b and 4c, although MAT exhibits little variation along the transect. Since Hm is favored by warmer, drier, and more seasonal climate, whereas Gt is favored by cooler, wetter, and a less seasonal climate [Schwertmann, 1971], the competitive distribution pattern of Hm and Gt with increasing rainfall conforms to the climatic requirement of pedogenic Hm and Gt, even though the temperature remains almost constant. Given that evaporation along the transect is relatively constant as it is controlled by MAT and MMT (Figure 1d), higher MAR and RH favors the recrystallization of Gt from Fh, but retards the dehydration of Fh to form Hm [Torrent et al., 1982; Cornell and Schwertmann, 2003]. Moreover, the elevated rainfall toward the southern end of the transect that occurs mostly in the cool seasons (autumn and winter), as indicated by the MMR and MMT of Qionghai and Haikou (Figure 1d), can also increase the formation efficiency of Gt with elevated rainfall. [27] In contrast to the rainfall dependent distribution of coloring iron oxides along the transect, the total amount of iron oxides as estimated by Fe t and Fe d appears to be controlled by chemical weathering and does not exhibit a systematic trend with high rainfall. Moreover, other chemical weathering indices including Fe d /Fe t, CIA, and Sa remain relatively stable, averaging around 72.0%, 98.4%, and 1.9%, respectively (Tables 1 and 2). These similar chemical weathering intensities are also confirmed by the homogeneous chemical composition (Table 2) and clay mineralogy (unpublished results in this study) of soil samples along the transect. These data indicate that because of elevated rainfall the chemical weathering rate has slowed and the high level of weathering in the uppermost soils is insensitive to additional changes in rainfall. In addition, the elevated rainfall without a comparable increase of temperature may also account for the insensitivity of Fe d in high rainfall regions. [28] Therefore, the ratio of Hm to Gt as a climate proxy is more sensitive to elevated rainfall than the total amount of coloring iron oxides in high rainfall regions. Coloring iron oxides can be helpful in understanding the climatic implication of ferrimagnets with high rainfall in warm regions. However, the distribution of Hm and Gt is sensitive to drainage conditions on both local and global scales [Cornell and Schwertmann, 2003; Viscarra Rossel et al., 2010]. In addition, the application of Hm/(Hm+Gt) as an indicator in paleoclimatic reconstructions should be confirmed to rule out systematic influence by local factors such as topography and parent materials Rainfall Dependence of the Magnetic Iron Oxides Distribution [29] Like the coloring iron oxides, the magnetic iron oxides also exhibit enrichment in the uppermost soils along the transect. Both total ferrimagnets (Figures 4a and 4b) and fine grained ferrimagnets (Figures 4c and 4d) as estimated by magnetic parameters are more concentrated in the soils than in the underlying saprolite and parent material whose c lf and c fd are less than 120 and 6 SI, respectively. As indicated by the synchronous change of c fd and c lf, the accumulation of fine grained ferrimagnets is responsible for the magnetic enhancement in the soils. [30] Surficial magnetic enhancement of soil profiles in warm regions is often correlated to pedogenesis [Singer et al., 1996; Lu, 2003; Lu et al., 2008], but the enhancement may be complicated by wildfires on the soil surface [Eggleton and Taylor, 2008]. Mgh has been considered as the dominant phase of ferrimagnets in aerobic soils [Goulart et al., 1998; Torrent et al., 2006]. Fire causes the dehydration of iron and aluminum hydroxide in topsoils, thereby forming more Mgh, Hm, and boehmite than is present in subsoils [Eggleton and Taylor, 2008]. However, Hainan Island has high RH throughout the year, and the occurrence of wildfires is not recorded. Moreover, the Hm/(Hm+Gt) ratio and clay mineralogy (mainly kaolinite, gibbsite and vermiculite) are similar in each of the two soil layers sampled along our transect, suggesting that fire has not been an important factor. Therefore, pedogenesis is the dominant process responsible for the accumulation of ferrimagnets in the uppermost soils along the transect. The dominance of pedogenesis is also indicated by the Pedo c lf accounting for about 90% of the c lf of bulk samples. [31] However, the reduction of ferrimagnets with increasing rainfall along the transect is opposite the classical temperate zone pattern with low to moderate rainfall [Liu et al., 1995; Maher, 1998; Maher et al., 2002]. We infer that there is a rainfall infection point at less than 1440 mm/yr separating these opposite patterns. Balsam et al. [2004] proposed a rainfall inflection point around 650 mm/yr for the temperate and monsoonal CLP. In addition, a rainfall inflection point around 1200 mm/yr has been suggested for a modern soil transect with moderate rainfall from temperate to tropical monsoon regions of China [Han et al., 1996]. In Han et al. s transect, increasing MAR ( mm) is accompanied by a remarkable increase in MAT ( 5 C 25 C). In contrast, a somewhat lower rainfall inflection point, around 10 of 15

11 1000 mm, has been estimated in modern soils in Hawaii that formed under mountain climate conditions, where the increasing MAR ( mm) is accompanied by a significant decrease in MAT (23 C 17 C) [Singer et al., 1996]. Although the comparison of magnetism in soils and sediments on a large scale is often complicated by a difference in parent material and pedogenic processes [Han et al., 1996; Maher, 1998; Blundell et al., 2009], it has been confirmed that there is indeed a rainfall inflection point controlling the trend of change in pedogenic ferrimagnets in both temperate and tropical regions. Moreover, the position of the rainfall inflection point seems to be variable and changes with mean annual temperature if other factors are held constant. [32] Therefore, although MS is widely used to reconstruct paleorainfall in temperate zones with low to moderate rainfall, we suggest that the reconstruction of rainfall based on the ferrimagnets alone should be employed tentatively and cautiously in regions with elevated rainfall, especially those above the rainfall inflection point around 1400 mm/yr. Moreover, in regions that exhibit great annual temperature differences, the effect of temperature needs to be taken into account Mechanism of the Magnetic Reduction With Excessive Rainfall [33] In cold regions with high rainfall, the mechanism for the magnetic reduction includes the effect of gleying processes that are characterized by the destruction of ferrimagnets under reducing conditions and the formation of limonite or iron sulfide with weak magnetism [Liu et al., 2001]. This mechanism provides a good explanation for magnetic reduction in anaerobic soils in colder regions with high rainfall and low evaporation. It has also been found in the paddy soils of subtropical and tropical regions [Lu, 2003]. However, it does not completely explain the magnetic reduction in aerobic soils in regions with high rainfall and evaporation. Recently, Balsam et al. [2004] proposed a nonmonotonic model characterized by a rainfall inflection point separating opposite trends in MS and Hm with rainfall in their interpretation of paleoclimate in the eolian sequences of the CLP, a temperate region. Our transect, formed in aerobic soils under highly oxidizing conditions, coincides with Balsam et al. s model of an inflection point for high rainfall in tropical regions. [34] In our climosequence with high rainfall, the concentration and distribution of iron oxides estimated by Hm, Gt, Hm/(Hm+Gt), and Hm/Fe d has a highly positive correlation with all the magnetic parameters (Table 3). The reduction in pedogenic ferrimagnets is often accompanied by a decrease in pedogenic Hm and an increase in Gt, whereas the total amount of iron oxides remains relatively constant. Since Mgh is considered the dominant phase of ferrimagnets in aerobic soils under highly oxidizing conditions [Goulart et al., 1998; Torrent et al., 2006], our data indicate that excessive rainfall above the inflection point can block the neoformation of Hm and Mgh and favors the neoformation of Gt. In addition, the leftward shift of the mixing peak of Hm (d110) and Mgh (d311) by XRD is detected in our data and indicates a deceasing ratio of Hm/ (Hm+Gt), although the content of Mgh cannot be directly detected by the its individual peak Mgh (d220) (Figure 5). This shift indicates a gradually decreasing amount of Mgh that is mixed into the peak of Hm (d110) as rainfall increases in our transect. This trend, based on the XRD method, independently validates the results estimated by magnetic and spectral analyses. The correlation between Hm content and pedogenic ferrimagnets in high rainfall regions is consistent with previous studies in regions where rainfall is low [Balsam et al., 2004; Torrent et al., 2006; Y. G. Zhang et al., 2007; Zhang et al., 2009; Kumaravel et al., 2010; Torrent et al., 2010a, 2010b], even if the ferrimagnets show the opposite correlation with rainfall in low and high rainfall regions. [35] These observations suggest that there is a common genetic connection between the neoformation of ferrimagnets and coloring iron oxides in aerobic soils. As proposed by Torrent et al. [2006], Mgh often acts as the intermediate product during the neoformation of Hm that forms by dehydration from the Fh. But Mgh is not involved in the neoformation of Gt, which is precipitated directly from aqueous solutions in soil. In addition, increasing organic matter, lower ph, and poor drainage conditions can favor the formation Gt over Hm [Cornell and Schwertmann, 2003]. Although there is no systematic change of ph or organic matter in the uppermost soils along the transect (Table 2), excessive rainfall during the cool seasons can still favor the neoformation of Gt at the expense of Hm and Mgh by higher soil moisture and less oxidizing conditions. In other words, although the less oxidizing conditions are controlled by relatively poor drainage and a small amount of organic matter does not commonly cause the reduction dissolution of iron oxides, the magnetic reduction in aerobic soils with elevated rainfall can also be attained by a change of chemical equilibrium between Hm and Gt, which is sensitive to soil RH [Torrent et al., 1982]. The amount of organic matter, which is often correlated to bacteria, may effectively promote the elevated rainfall driven processes [Cornell and Schwertmann, 2003], but the presence of organic matter is not a prerequisite. From the point of view of thermodynamics, the dissolution of Mgh can either precede or follow the dissolution of Hm, since Mgh and Hm are the least stable phases among the studied iron oxides and since these Fe 2 O 3 polymorphs have similar chemical and mineralogical characteristics [Cornell and Schwertmann, 2003; Navrotsky et al., 2008]. Therefore, this study partly verifies the abiotic mechanism of magnetic change by the genetic connection between Hm and Mgh in aerobic soils across a wide climate range. [36] Recently, the ratio of Hm and pedogenic ferrimagnets, which is often illustrated by Hm/c fd, was suggested to be a robust variable [Balsam et al., 2004; Torrent et al., 2006; Torrent et al., 2010a, 2010b], since synchronous changes between them are common. The proxy can reflect the relative accumulation rate between ferrimagnets and Hm in pedogenesis. However, Hm/c fd is a more inclusive proxy involving both the magnetic and coloring iron oxides than Hm/(Hm+Gt). It is controlled not only by the degree of weathering, but is also correlated to climate with different combinations of rainfall and temperature [Torrent et al., 2006]. In the temperate regions of CLP and Russian steppe, the Hm/c fd of weakly weathered soils is generally less than gm 3 [Torrent et al., 2006], whereas the ratio of moderately weathered Mediterranean soils is generally greater than gm 3 [Torrent et al., 11 of 15

12 Figure 5. Classic XRD patterns with different ratios of Hm/(Hm+Gt). The patterns are from the samples (a) HN01 I, (b) HN02 II, (c) HN08 I, (d) HN13 I,and (e) HN14 I taken along the transect with increasing rainfall. The values estimated by XRD are shown in the top right corner of the patterns and the corresponding values estimated by redness are listed in the parentheses. Hm, hematite; Gt, goethite; Mgh, maghemite; Qtz, quartz. 2010a, 2010b]. With the extreme weathering of the soils in our transect, the ratios range from 1.4 to gm 3 and average gm 3 (Table 1). This large variation is consistent with previous findings from Brazilian soils under high rainfall [Torrent et al., 2006]. Hence, our data and previously published data confirm that the relative formation rate between ferrimagnets and Hm in pedogenesis can change significantly with differing combinations of temperature and rainfall, even in soils with similar chemical weathering intensities. However, variation in the relative formation rate between Hm and ferrimagnets does not affect the synchronous reduction of Hm and Mgh with the increasing rainfall in this transect. [37] These geochemical processes, driven by elevated rainfall, are mainly transformations between different iron oxide phases and are independent of the total amount of iron oxides that is controlled by chemical weathering. Similar climosequences, from a red (hematitic) soil subtype to a yellow (goethitic) soil subtype, occur widely in zonal aerobic soils such as ferrosols, acrisols, alisol, lixisols, and luvisols under subtropical and tropical climate [Xiong and Li, 1987; Cornell and Schwertmann, 2003; Torrentetal., 2006; IUSS Working Group WRB, 2007] even though their chemical weathering intensities are much different. In particular, the distribution pattern in south China is quite clear and is controlled by a tropical and subtropical monsoon climate with different combinations of MAT and MAP [Xiong and Li, 1987]. Correspondingly, the response mechanism of ferrimagnets is not only a local process, but also a zonal process in aerobic soils controlled by climates with different combinations of rainfall, temperature, and other local factors Model of Magnetic Change with Climate [38] Previous studies have found that the magnetic change in aerobic soils is often correlated to chemical weathering intensity under a monsoon climate where a change in rainfall is often companied by a synchronous change of temperature [Huang et al., 1996;Ding et al., 2001;Liu et al., 2003; Lu, 2003; Lu et al., 2008]. It has been demonstrated that there is a positive relationship between chemical weathering indices estimated by Fe d,fe d /Fe t and magnetic parameters along a chronosequence weathered from basalts in eastern China [Huang and Gong, 2000; Lu et al., 2008]. A similar positive correlation has also been established between Fe t and MS in the study of topsoils across southern China, although the parent material and soil age is uncertain [Huang et al., 1996]. These reveal that the release of iron from primary Fe bearing minerals controlled by chemical weathering plays an important role in the accumulation of pedogenic ferrimagnets in aerobic soils. In contrast, in our climosequence where the increase in rainfall is not accompanied by a comparable increase of temperature, all the chemical weathering indices including Fe d show little variation, even though there is a dramatic reduction in MS and Hm. This suggests that the magnetic change in aerobic soils is not only correlated to the chemical weathering but is also correlated to the ratio of Hm and Gt. However, both 12 of 15

13 Figure 6. Two dimensional models relating the pedogenic production of iron oxides and climate. (a) Relationship between Fe d and both rainfall and temperature. (b) Relationship between Hm/(Hm+Gt) and both rainfall and temperature. (c) Relationship between Hm (or Mgh) content and rainfall and temperature resulting from their genetic relation in aerobic soils. The gradual color change from black to white indicates increasing values. The corresponding trend of Hm/(Hm+Gt) and Fe d is indicated by the dotted line. The direction of the arrow on the curve illustrates the direction of change with increasing rainfall if temperature is not considered. Note the relative change between increasing rate of Fe d and decreasing rate of Hm/(Hm+Gt) with rainfall. chemical weathering intensity estimated by Fe d and the ratio of Hm/(Hm+Gt) are controlled by climate with different combinations of temperature and rainfall. Previous models of ferrimagnets and climate have not systematically considered the effect of temperature change with rainfall [Liu et al., 2001; Balsam et al., 2004]. In order to understand the climatic response of pedogenic ferrimagnets more comprehensively, we propose a modified climate ferrimagnet/iron oxide model that considers both rainfall and temperature in aerobic soils. [39] This modified model is based on the common genetic relation between Mgh and Hm in aerobic soils across a wide climate range. The change in pedogenic ferrimagnets is assumed to be reflected by a change of Hm content in aerobic soils. In practice, it can be indirectly estimated from the product of total amount of iron oxides (Fe d ) and Hm/(Hm+Gt). Of the two variables, Fe d is controlled by chemical weathering and can be elevated by increasing rainfall accompanied by an increase in temperature (Figure 6a) [Ollier, 1969; White and Blum, 1995], whereas Hm/(Hm+Gt) is reduced with increasing rainfall accompanied by a decrease in temperature[cornell and Schwertmann, 2003] (Figure 6b). The changes of Fe d and Hm/(Hm+Gt) show opposite responses to rainfall if the effect of temperature is not considered. However, Fe d may increase more rapidly than Hm/(Hm+Gt) decreases in low rainfall regions (Figures 6a and 6b), and the content of Hm (or Mgh) as their product can increase (Figure 6c). In this case, we can reconstruct the paleorainfall by the positive correlation between Fe d, MS, and rainfall as shown in previous studies in temperate regions with low and moderate rainfall [Kukla et al., 1988; Liu et al., 1995; Maher and Thomson, 1995; Maher, 1998]. However, with increasing rainfall, Fe d levels off compared to the decrease in Hm/(Hm+Gt) (Figures 6a and 6b), resulting in a decrease in the content of Hm or Mgh (Figure 6c). Our climosequence is just such a case. As a result, there will be a rainfall inflection point separating the inverse change patterns of Hm or Mgh with rainfall. This coincides with Balsam et al. s [2004] model in temperate regions. [40] If the effect of temperature is considered, the rainfall inflection point becomes variable. When a rainfall climosequence is accompanied by higher mean annual temperature, the increase in Fe d with rainfall will be greater, whereas Hm/(Hm+Gt) will decrease with rainfall. Hm or Mgh will continue to increase and the rainfall inflection point rises correspondingly. Therefore, there will be a series of rainfall inflection points increasing with mean annual temperature, and a rainfall inflection line (or curve) at different latitudes across temperate, subtropical, and tropical regions (Figure 6c). However, in most climosequences, changes in rainfall are accompanied by a continuous change in temperature, either increasing or decreasing. The rainfall inflection point along a climosequence accompanied by increasing temperature is expected to be higher than the one accompanied by decreasing temperature. This is evidenced by a higher inflection point under a monsoon climate [Han et al., 1996] than a mountain climate [Singer et al., 1996]. [41] The conceptual model above provides a basis for the interpretation of regional changes in ferrimagnets controlled by climate in aerobic soils, especially for soil climosequences with distinct color change between red and yellow, if other factors are held constant. Together with Liu et al. s [2001] model in anaerobic soils, a complete evolutionary series of iron oxides can be depicted with increasing rainfall and soil moisture. Pedogenic ferrimagnets, therefore, have been widely employed to reconstruct paleorainfall in the regions with low and moderate rainfall, where the accumulation of ferrimagnets is correlated to chemical weathering and is highly sensitive to the supply of rainfall. However, in regions with excessive rainfall or where a change of rainfall is accompanied by highly varying temperature, we suggest that a combination of proxies including both magnetic and coloring iron oxides as well as other chemical and physical 13 of 15