5.1. Origin of Red Clay Protodolomite: Indications From SEM and C-O Isotopes
 Micromorphological characteristics of Red Clay protodolomite reveal that it is drastically different from the Pleistocene loess dolomite. Pleistocene loess dolomites are detrital, originating from rock fragments [Li et al., 2007]. The loess dolomites are >45 μm, well ordered crystals that lack sharp corners or edges; most of them are well rounded suggesting they have endured abrasion due to long distance wind transportation [Li et al., 2007]. Thus, detrital dolomite grains in loess contain information about material in the loess source region. Red Clay protodolomites, in contrast, are poorly ordered and exhibit a perfect rhombic crystal shape (Figures 3f and 3g) that differs from the rounded, transport-influenced shape of dolomite in Pleistocene loess indicating that Red Clay protodolomite is authigenic.
 Besides the implications as to the origin of the protodolomite, SEM analysis also provides direct information about paleoclimate. The existence of micritic rhombic euhedral carbonate crystals (Figures 3f and 3g) indicates pedogenesis [Deutz et al., 2002] and is characterized by the dissolution and recrystallization of carbonate minerals in Red Clay. Additionally, SEM analysis indicates that detrital eolian carbonates are absent in the Red Clay where they were likely dissolved and reprecipitated. In contrast, Pleistocene carbonates are composed of both detrital grains >45 μm and well crystallized, authigenic, fine carbonate grains <2 μm [Sheng et al., 2008]. Hence, the preservation and types of carbonates suggests a stronger primary pedogenic effect in Red Clay than in Pleistocene loess.
 Detrital and authigenic dolomites have contrasting C–O isotopic values [Rabenhorst et al., 1984]. According to previous research on Pleistocene loess from the CLP, loess dolomite was inherited from the mountain belts of northwestern China, whose δ13CPDB isotope ratios range from 0 to 4‰ [Li et al., 2007]. However, our study reveals that Red Clay protodolomite δ13CPDB isotopic ratios range from −4.1 to −10.4‰ (Figure 7). The strong negative δ13CPDB of the Red Clay protodolomite differs significantly from inherited, detrital dolomite in Pleistocene loess and modern desert sand.
 Red Clay protodolomite δ18OPDB isotopic ratios average −9.9‰ in DJP and −8.5‰ in BJZ. The depletion of Red Clay δ18OPDB and their value range show a striking similarity to Pleistocene loess primary pedogenic carbonate (Figure 7), which is mostly calcite. Typical loess authigenic carbonate δ18OPDB ranges from −4 to −10‰ [Chen et al., 1996; Han et al., 1997; Sheng et al., 2008]. The very close δ18OPDB isotopic composition suggests that Red Clay protodolomite and loess primary pedogenic carbonate have a similar origin. Red Clay protodolomite is a primary pedogenic carbonate.
 Red Clay primary pedogenic protodolomite's C–O isotope ratio ranges plot in the meteoric water zone according to the classification of Warren  because both δ13CPDB and δ18OPDB ratios are highly negative. Protodolomite δ18OPDB ratios for DJP averaged −9.9‰ (n = 26, σ = ±1‰), and −8.5‰ for BJZ (n = 17, σ = ±0.65‰). However, under moderately evaporative conditions, typical pedogenic dolomite (that is <2 μm) in the saline soils of Alberta Canada [Kohut et al., 1995], the δ18OPDB ratios were enriched and increased to −3.3‰ (n = 3). In the highly evaporative environments as Qinghai salt lakes [Yu and Kelts, 2002] or South African hypersaline pans [Mauger and Compton, 2011], authigenic dolomite exhibits extremely positive δ18OPDB ratios, averaging about +5.3‰ (n = 2) and +4.1‰ (n = 8), respectively (Figure 7). Thus, as the negative δ18OPDB ratios indicate, Red Clay protodolomite is isotopically different from those formed under highly evaporative conditions and is more closely related to those formed from meteoric water and seasonally dry conditions.
5.2. Formation of Red Clay Protodolomite
 Dolomite is a common occurrence in ancient strata, but is only rarely present in modern carbonate sediments [Holland and Zimmerman, 2000]. Moreover, attempts to synthesize dolomite under the earth's surface temperature and pressure have been unsuccessful [Land, 1998]. These observations produced the long standing geological mystery - the “dolomite problem” - that has attracted geologists' attention for over 100 years. As currently viewed, the principal methods of low temperature dolomitization are the mixing zone model [Badiozamani et al., 1977; Cander, 1994; Humphrey and Quinn, 1989], the sabkha model [Gunatilaka, 1991], and bacterial mediation models [Sánchez-Román et al., 2008].
 However, in spite of the many models of dolomitization, there is a general consensus that the control on dolomitization is a kinetic barrier and not thermodynamic state [Arvidson and Mackenzie, 1999]. From a thermodynamics point of view, the formation of secondary dolomite could be expressed by the reaction equation:
The equation above refers to the direct precipitation of dolomite from solution. Thermodynamically, the basic chemical reaction requirements should be that the IAP of dolomitization fluid is >KSPDolomite. For the equation above, IAPDolomite = [Ca2+][Mg2+][HCO31−]4/[pCO2]2[pH2O]2. Factors such as higher temperature and seasonally dry conditions will decrease soil pCO2 and pH2O and promote the reaction toward right side. Other factors including fluid chemical composition of Mg2+, Ca2+, HCO3−/CO32− and ionic strength will also affect the rate of dolomitization. According to Hardie , the disordered Ca-rich dolomite has a KSPdolomite = 10−16.5 and [Mg2+]/[Ca2+] = 3.31 at 25°C and 1 atm pressure. Fluid supersaturated with Mg suitable for dolomitization is widely distributed on CLP. The largest river on CLP, the Yellow River, has an IAP = 10−10.7 using [Mg2+], [Ca2+] and [HCO3−] data from Gaillardet et al.  and Roy et al.  and an IAP = 10−10.5 [Zhang et al., 1995]. These values are orders of magnitude larger than KSPdolomite = 10−16.5 and therefore thermodynamically favorable. But, dolomitization on CLP mainly depends on kinetic factors, not aqueous thermodynamic equilibrium.
 Among the kinetic factors it has been suggested that elevated Mg/Ca ratio could disrupt the hydration spheres around Mg2+ when the solution is hypersaturated and highly saline [Burton and Machel., 1992]. In addition, temperature is a key factor; dolomite nucleation and crystal growth relies on solution temperature [Arvidson and Mackenzie, 1999]. Other factors, such as microbial mediation are possible. Typical spherical, highly crystalline phase dolomite was synthesized through microbial mediation from 25–45°C [Sánchez-Román et al., 2008].
 One of the striking features of Red Clay protodolomite are the concentrated layers of dolomite (dolocrete) separated by thick horizons of highly concentrated calcite (calcrete) (Figure 6). At low temperatures, the precipitation of calcite and dolomite is competitive [Arvidson and Mackenzie, 1999], and the Mg/Ca ratio determines which species will precipitate [Folk and Land, 1975]. Calcrete or dolocrete commonly forms in arid soils by soil solution/precipitation. In arid areas, Mg2+ and Ca2+ are leached downward from the upper soil layers during rainfall, whereas during the dry season, intense evaporation causes calcite to precipitate and calcrete to form. The calcrete horizon is crucial for dolocrete formation for two reasons. First, the compact calcite occurs as cementation thereby lowering the permeability of the soil [Mowers and Budd, 1996], preventing solutions from percolating downward and allowing stagnant soil solutions to form above the calcrete. Second, during seasonal dry and Ca precipitation in calcrete, the Mg2+concentration increases markedly in soil solution and the solutions become Mg-rich increasing the Mg/Ca ratio.
 The favorable Mg/Ca ratio for dolomite formation is discussed extensively in the literature. For disordered, Ca-rich dolomite to form,Folk and Land  proposed that an Mg/Ca ratio >1 is required, but others have suggested that values as high as 3.31 are required [Hardie, 1987]. In Hawaiian soil, dolomite develops on basalt and precipitates when the Mg/Ca ratio of its soil solution is ∼1 [Capo et al., 2000; Whipkey and Hayob, 2008; Whipkey et al., 2002]. Stream flux on central CLP has a Mg/Ca ratio of 2.34 and the largest river on CLP, the Yellow River has an Mg/Ca ratio of ∼0.85 [Gaillardet et al., 1999; Zhang et al., 1995]. Compared to stream flux, local rainwater has only a minor amount of Mg2+ and Ca2+, with Mg/Ca ratio <0.1. Local precipitation is the only major supply of water to soil. The contrast in Mg/Ca ratio values between rainwater and soil water provides evidence for downward leaching and weathering of Mg-containing minerals, like detrital dolomite, Mg-rich calcite and chlorite which is present in the eolian dust contribution to the soil.
 Compared to the overlying Pleistocene loess-paleosol, the Red Clay displays a higher silicate MgO % (wt) [Xiong et al., 2010] and higher chlorite mineral concentration [Gylesjö and Arnold, 2006], both of which are unstable during weathering. Moreover, the Pleistocene loess deposits on Loess Plateau are characterized by a high content of detrital dolomite [Li et al., 2007], but detrital dolomite is absent in the Red Clay. Because it is unlikely that the composition of eolian dust changed significantly from the Pliocene to Pleistocene [Wang et al., 2007], the lack of dolomite is likely the result of dissolution of carbonate minerals during weathering of the Red Clay. This weathering would provide a Mg2+ source for the formation of protodolomite in Red Clay. Therefore, during weathering more Mg2+ is expected to be released from detrital carbonates and chlorite, resulting in a higher a concentration of Mg2+ and Ca2+and a more highly evolved, Mg-rich soil solution than expected in the overlying loess.
 Red Clay protodolomite is present mainly in voids of about 100 μm (Figure 3e). The rhombic euhedral dolomite crystal groups suggest direct precipitation from soil solution [Whipkey et al., 2002], especially during dry and warm seasons. Due to seasonally aridity, soil water could evolve to have a high Mg/Ca ratio and high salinity resulting in a crystal size on the order of micrometers because of rapid crystallization (Figure 3f). The existence of voids in the soil provided spaces for precipitation and channels for the dolomitizing fluid and the delivery Mg2+, Ca2+ and HCO3− involved in the dolomitization reaction. From a dynamic point of view, besides void volumes the amount of dolomite depends on Mg flux, which is indispensible in any dolomitization model [Compton and Siever, 1986]. In the Red Clay Mg flux is proportional to the weathering contribution from detrital Mg-containing minerals, like detrital dolomite, Mg-rich calcite and chlorite, relying significantly on sufficient rainfall leaching by episodic monsoon events. The underlying calcrete horizons cemented Red Clay soil grains, lowered the permeability of soil layers, prevented further water percolation and guaranteed enough residence time for dolomitization. This resulted in alternating soil layers of dolocrete and calcrete.
 In addition, palygorskite accompanies dolomite in the Red Clay. The occurrence of palygorskite is commonly reported in soils from warm and arid regions, where soil alkalinity is high and Mg2+ and SiO2 (aq) in pore water are active [Singer, 1984]. The Mg2+ and SiO2 (aq) are often the product of local in situ weathering and pedogenesis [Hong et al., 2007]. Hence, the observation that the woven, silky palygorskite aggregates grew on the surface of Red Clay protodolomite in void spaces (Figure 3g) provides an important precipitation condition indicator. Palygorskite formation needs strict chemical conditions and its precipitation strongly depends on [Al3+], [H+] and [SiO2 (aq)] of soil solutions. According to Hong et al. , increasing the alkalinity of soil solutions is crucial for palygorskite to precipitate and higher alkalinity solutions only need lower [Al3+] and [SiO2 (aq)] to precipitate palygorskite. Thus, the existence of palygorskite as a woven coating on the surface of Red Clay protodolomite also indicates arid and alkaline conditions [Singer, 1984], which likely are seasonal when the two minerals precipitated from soil solution in Red Clay.
 Dolocretes as a groundwater cement or a groundwater replacement of primary carbonates are commonly reported in some soil profiles [Nash and McLaren, 2003; Watts, 1980]. But, in the Red Clay on the CLP, dolocretes are primary pedogenic in origin, not groundwater. Primary pedogenic and groundwater carbonates are different in many aspects [Nash and McLaren, 2003; Pimentel et al., 1996]. First, the groundwater replacement would produce carbonate that has meso-crystals or sparry crystals, whereas pedogenesis produces micrites (<5μm in diameter). Second, horizontal elongate carbonate bodies are produced by deep groundwater or non-pedogenic calcretes, but primary pedogeneic carbonates produce pendent, laminar and pisolitic fabrics for carbonate calcretes or dolocretes [Colson and Cojan, 1996]. Third, the horizontal groundwater carbonates are larger and thicker, up to 10 m or more in thickness; primary pedogenic calcretes are typically 0.5–2 m in thickness [Wright and Tucker, 1991]. Fourth, groundwater calcretes and dolocretes are associated with more permeable lithologies [Fu et al., 2004]. Cracks, cavities and channels are necessary in host sediments for groundwater carbonates to precipitate [Khalaf and Gabler, 2008]. Fifth, the groundwater environment lacks biogenic relics such as rhizogenic calcretes and root traces [Khalaf and Gabler, 2008; Pimentel et al., 1996].
 The following observations suggest that Red Clay calcretes and dolocretes are primary pedogenic carbonates: (1) In Red Clay soils porosities were low, ∼3–5% by volume [Guo et al., 2001]. Carbonates of groundwater origin need higher porosities, locally >25%. (2) Carbonate nodules in the Red Clay soil matrix are a few centimeters in diameter, appearing as irregular intervals and dispersed individual nodules in the groundmass. But, typical groundwater carbonates display uniformly massive horizons with a gradational top and base. (3) The carbonate nodules in the Red Clay are composed of micritic crystals (<5 μm), contrasting to phreatic sparry crystals typical of groundwater deposits. (4) The macrostructure of Red Clay carbonates displays laminar, nodular, pisolitic and pendent fabrics. But, groundwater carbonates never show pisolitic or pendent fabrics. (5) Moreover, dark Fe-Mn films (∼10% in volume) were also abundant in lower portions of Red Clay chronosequences. Their reddish oxidation color is similar to Fe-Mn films in the upper portion of Red Clay. This brick red color is distinct from the strongly reduced gray-green color of iron minerals affected by groundwater [Pipujol and Buurman, 1994]. In summary, we propose that the Red Clay dolocretes we studied are primary pedogenic and not phreatic/groundwater carbonates. The development of Red Clay dolocretes is closely related to climatic factors through their influence on pedogenesis.
 Our protodolomite records show that the occurrence of protodolomite gradually decreases upward in both the BJZ and DJP sections (Figure 6) and this decrease is especially prominent in the southern DJP section. This trend is broadly consistent with the global cooling as recorded in oxygen isotopes of marine sediments since the late Miocene [Zachos et al., 2001].