Dolomite of detrital origin in carbonate‐rich soil can help indicate the intensity of carbonate dissolution and leaching processes. Using Fourier transform infrared spectroscopy, we generated a high‐resolution record of dolomite and calcite contents from eight sections on the Chinese Loess Plateau spanning the last 130 kyr. Based on the relative abundance of calcite and dolomite, four dissolution phases of carbonate minerals related to summer monsoon (SM) variations are identified as follows: (1) coexistence of dolomite and calcite indicating a very weak SM with a mean annual precipitation (MAP) < 610 mm; (2) calcite without dolomite indicating a weak SM, 610 < MAP < 690 mm; (3) both dolomite and calcite are absent, but there is no downward leaching indicating a strong SM, 690 < MAP < 725 mm; and (4) both dolomite and calcite are absent, and downward leaching of the underlying loess layer suggesting a very strong SM with MAP > 725 mm.

[1] The provenance of element Ti in Huguangyan maar lake (HGY) sediment has been under debate since Yancheva et al. (2007a) proposed its eolian source from the Chinese Loess Plateau (CLP) and applied it as a proxy for Asian winter monsoon. We performed a comprehensive study on Ti-carrying minerals and Ti fluxes in lake sediments, wall rocks, and loess from the CLP. Here we show that the chemical compositions and morphological features of magnetite, the most important Ti carrier in HGY surface sediments, are identical to those in wall rocks around the lake, but substantially different from those in CLP loess samples. Our calculated modern eolian Ti flux from the CLP is negligible compared to the average total Ti flux to HGY, and Ti flux from catchment erosion can provide enough Ti, indicating Ti in HGY sediment are predominantly from wall rock erosion. Inverse correlation between total organic carbon (TOC) (vegetation density) and Ti/Ca ratio (terrestrial input) in the Holocene suggests monsoon-induced vegetation density predominates over runoff in controlling Ti input to HGY. S-ratio represents TOC directly mediated reductive diagenesis of iron oxides (magnetite preservation), rather than anoxic/ oxic condition at the bottom water induced by winter monsoon. Our record shows a decreased vegetation density, increased Ti input and weakened reductive diagenesis through the Holocene driven by a weakened summer monsoon. Citation: Shen, J., X. Wu, Z. Zhang, W. Gong, T. He, X. Xu, and H. Dong (2013), Ti content in Huguangyan maar lake sediment as a proxy for monsoon-induced vegetation density in the Holocene, Geophys.

[1] High-resolution variations in carbonate minerals from the Jiaxian Red Clay section, located at the northern limit of the present East Asian summer monsoon (EASM) on Chinese Loess Plateau were quantified using Fourier transform infrared spectroscopy. We analyzed a large quantity of sediments dated from the late Miocene to Pliocene (8.2-2.6 Ma). The carbonates in this interval show high-frequency variations alternating between leached and calcareous horizons. The low carbonate contents and high values of magnetic susceptibility and high Rb/Sr ratios were found in the leached zones, a pattern that is consistent with that observed in the overlying Quaternary loess-paleosol sequences. This pattern suggests that East Asian Monsoon (EAM) rainwater enhanced leaching and accumulation processes of carbonate minerals in the Red Clay Formation in a way similar to the loess-paleosol sequence. Seven alternating leached and calcareous zones are identified, suggesting oscillations of the EASM and East Asian winter monsoon intervals. The calcareous zones were also found to have high Zr/Rb ratio. These indications of shifts from a strong EASM to East Asian winter monsoon dominance correlate well with the cooling transition indicated by deep sea d 18O isotopes. This evidence suggests that the EAM was active during the late Miocene and Pliocene and was similar to the Quaternary monsoon. The presence of a strong EAM during the Pliocene Warm Period also raises questions about the hypothesis that past and future warm climate conditions could produce a permanent El Niño-like state. Citation: He, T

[1] The Pliocene epoch is considered the most recent analog of modern warming because CO 2 levels were similar to the present. To explore the carbonate minerals formed in the warmer Pliocene epoch, we studied two continuous sections of the Red Clay Formation on the Chinese Loess Plateau (CLP) by X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM) and Stable Isotope Mass Spectrometry. The Red Clay Formation on the CLP exhibits diagnostic FTIR absorption features of calcite and protodolomite. This allowed quantification of the two carbonate minerals by the FTIR method. Using the FTIR method we estimate the average concentration of protodolomite in Bajiazui is 3.6% whereas the Duanjiapo section is 6.0%. Protodolomite occurrence is more consistent and the concentration is higher from $6.5-4.2 Ma B.P. and decreases markedly from 4.2-2.6 Ma B.P. Red Clay protodolomite is depleted in both d 13 C PDB and d 18 O PDB , ranging from À4.1‰ to À10.4 and from À6.7‰ to À11.6, respectively, and has a slightly higher d18 O value than the calcites. SEM observations show that Red Clay protodolomite is composed of euhedral rhombic crystals that range from 1-20 mm in diameter, grow into the soil voids and coexist with authigenic calcite and palygorskite. These observations imply that the protodolomite grew in situ and is authigenic from pedogenesis. Dolomitization in the Red Clay sequence appears to be the result of overcoming kinetic barriers. We propose that in the Red Clay a warm climate with seasonal dry conditions leads to the formation of calcrete from soil pore waters thereby enriching the pore solutions with respect to Mg 2+ and significantly increasing the Mg/Ca ratio bringing about the formation of protodolomite.Components: 9100 words, 7 figures, 8 tables.

Previous studies of the late Cenozoic erosion rate have yielded different views—long-term stable rates or a significant increase at climate transitions—leading to uncertainty concerning the hypothesized global erosion rate controlled by either tectonic uplift or climatic changes. Here, we present a seven-million-year hornblende mineral record along the Lingtai section of the Chinese Loess Plateau. By examining the spatial distribution of hornblende minerals in seven desert basins, which are potential loess source areas, we constructed a ratio of hornblende versus total heavy minerals to reflect past changes in physical/chemical weathering strength. Our results demonstrate that the ratio has generally increased since 7 Ma, with three significant shifts recorded at 2.6 Ma, 1.4 Ma and 0.5 Ma linked to the onset, continuation and expansion of the Northern Hemisphere glaciation, respectively. Given that chemical weathering during the diagenetic history produces a trend of smoothly increasing hornblende migrating upwards, the three shifts at these boundaries can be interpreted as changes in the bedrock erosion rate on the northern Tibetan Plateau, which may be related to tectonic uplift events and incision of the Yellow River. Evidence presented here supports the idea of coupling between climate change, tectonic uplift and regional erosion.