A Review of Recent Advances in Red-Clay Environmental Magnetism and Paleoclimate History of the Chinese Loess Plateau

The red-clay sequence on the Chinese Loess Plateau (CLP) was deposited during the late Miocene-Pliocene and is encoded with important information of past climate changes. However, it has received much less study in comparison to the overlying Pleistocene loess-paleosol sequence. In this paper, we review recent progress in characterizing the environmental magnetic parameter-based paleoclimate history recorded by the red-clay sequence. Several key conclusions are as follows. (1) the red-clay and the loess-paleosol sequences have similar magnetic enhancement mechanisms but magnetic minerals in the red-clay sequence have experienced a higher degree of oxidation than in the loess-paleosol sequence. (2) The CLP experienced a cooling and wetting trend from 4.5 to 2.7 Ma, caused by ice sheet expansion and East Asian summer monsoon intensification, respectively. (3) The above conclusions benefit from backfield remanence curve unmixing and comparison of magnetic grain size/concentration records, which are particularly useful in separating the temperature from the precipitation signal. A clear need in future studies is to explore the concentration and the grain size variations of hematite and goethite in the red-clay sequence and their formation mechanisms. The payback would be a clear understanding of climate history during the late Miocene-Pliocene, a possible analog for future warmer climate

A major change in precipitation gradient on the Chinese Loess Plateau at thePliocene-Quaternary boundary

Spatiotemporal variations in East Asian Monsoon (EAM) precipitation during the Quaternary have been intensively studied. However, spatial variations in pre-Quaternary EAM precipitation remain largely uninvestigated, preventing a clear understanding of monsoon dynamics during a warmer climatic period. Here we compare the spatial differences in heavy mineral assemblages between Quaternary loess and pre-Quaternary Red Clay on the Chinese Loess Plateau (CLP) to analyze spatial patterns in weathering. Prior studies have revealed that unstable hornblende is the dominant (&sim;50%) heavy mineral in Chinese loess deposited over the past 500 ka, whereas hornblende content decreases to &lt; 10% in strata older than &sim;1 Ma in the central CLP because of diagenesis. In the present study we found that hornblende is the dominant heavy mineral in 2&ndash;2.7 Ma loess on the northeastern CLP (at Jiaxian), which today receives little precipitation. Conversely, hornblende content in the upper Miocene-Pliocene Red Clay at Jiaxian is &lt; 10%, as in the central CLP. The early Quaternary abundance of hornblende at Jiaxian indicates that the current northwestward-decreasing precipitation pattern and consequent dry climate at Jiaxian must have been initiated since &sim;2.7 Ma, preventing hornblende dissolution to amounts &lt; 10% as observed in the central CLP. By contrast, the 7 Ma and 3 Ma Jiaxian Red Clay hornblende content is significantly less than that of the Xifeng samples, despite the fact that today Xifeng receives more precipitation than Jiaxian, with expected enhanced hornblende weathering. This suggests that the northeastern CLP received more precipitation during the Late Miocene-Pliocene than at Xifeng, indicating that the precipitation gradient on the CLP was more east&ndash;west during the Late Miocene-Pliocene rather than northwestsoutheast as it was in the Quaternary. A comparison of magnetic susceptibility records for these sections confirms this inference. We attribute this major change in climatic patterns at &sim;2.7 Ma to decreased northward moisture transportation associated with Northern Hemisphere glaciation and cooling in the Quaternary. This study therefore demonstrates the potential usefulness of employing heavy mineral analysis in both paleoclimatic and paleooceanographic reconstructions.<br style="line-height: normal; text-align: -webkit-auto; text-size-adjust: auto;" /

Editorial: Tibetan Plateau uplift and environmental impacts: new progress and perspectives

No abstract available

Dominant 100,000-year precipitation cyclicity in a late Miocene lake from northeast Tibet

East Asian summer monsoon (EASM) precipitation received by northern China over the past 800 thousand years (ky) is characterized by dominant 100-ky periodicity, mainly attributed to CO2 and Northern Hemisphere insolation–driven ice sheet forcing. We established an EASM record in the Late Miocene from lacustrine sediments in the Qaidam Basin, northern China, which appears to exhibit a dominant 100-ky periodicity similar to the EASM records during the Late Quaternary. Because evidence suggests that partial or ephemeral ice existed in the Northern Hemisphere during the Late Miocene, we attribute the 100-ky cycles to CO2 and Southern Hemisphere insolation–driven Antarctic ice sheet forcing. This indicates a >6–million year earlier onset of the dominant 100-ky Asian monsoon and, likely, glacial and CO2 cycles and may indicate dominant forcing of Northern Hemisphere climate by CO2 and Southern Hemisphere ice sheets in a warm world.This work was funded by the national key research and development program of China (2016YFE0109500), the (973) National Basic Research Program of China (grant no. 2013CB956400), the Strategic Priority Research Program of the Chinese Academy of Sciences (grant no. XDB03020400), the National Natural Science Foundation of China (grant nos. 41422204, 41172329, and 41290253), and the U.S. NSF (grant no. 1545859)

Joint insolation and ice sheet/CO2 forcing on northern china precipitation during pliocene warmth

We demonstrate that the precipitation record on the Chinese Loess Plateau during the middle Piacenzian (3.264–3.025 Ma) has strong 20-kyr precessional cycles, challenging past knowledge about East Asian monsoon variations at the orbital timescales

Late Pliocene-early Pleistocene 100-ka problem

Little is known about the 100-ka ice volume cycles immediately before 1 Ma except that they are coherent with the 100-ka eccentricity cycles. Here we show that ice volume and ocean circulation change/global carbon mass balance are phase-locked and highly coupled regarding amplitude envelopes at the 100-ka band, but neither is phase-locked to and amplitude-coupled to the 100-ka eccentricity orbital forcing signal between 3-1 Ma. We describe this phasing and amplitude mismatch at the 100-ka band between 3-1 Ma between eccentricity forcing and paleo-ice volume records as the late Pliocene-early Pleistocene 100-ka problem. A free 100-ka oscillation internal to our climate system, especially those related to oscillations of atmospheric CO2 concentration, operating on an Earth characterized by a high-inertia deep thermohaline ocean that can store carbon and heat, might hold the key for the late Pliocene-early Pleistocene 100-ka problem. Copyright 2008 by the American Geophysical Union

Link between benthic oxygen isotopes and magnetic susceptibility in the red-clay sequence on the Chinese Loess Plateau

Recent rock magnetic work on the red-clay sequence on the Chinese Loess Plateau (CLP) convincingly demonstrates that the enhancement mechanisms of low-frequency magnetic susceptibility (i.e., measured at 470 Hz; χ1f) in the red-clay sequence are similar to those in the loess-paleosol sequence. Therefore, χ1f in the red-clay sequence should indicate precipitation intensity received by the CLP, as is the case in the overlying loess-paleosol sequence. Based on this result, we compared χ1f in the red-clay sequence with benthic oxygen isotope records. We infer that the primary precipitation source on the CLP varies over time in three phases: during 8.1-4.5 Ma, the East Asian summer monsoon dominates; during 4.5-4 Ma, the Polar Westerlies dominate; during 4-0 Ma, the East Asian summer monsoon dominates. We attribute these precipitation source shifts on the CLP to the closure of the Panama Seaway around 4.5 Ma and the Tibetan uplift during the interval 4-2.6 Ma. Copyright 2008 by the American Geophysical Union

Enhancement mechanisms of magnetic susceptibility in the Chinese red-clay sequence

Little is known about the mechanisms of magnetic susceptibility (χ) enhancement for the red-clay sequence on the Chinese Loess Plateau (CLP), in comparison to the overlying loess-paleosol sequence. Here we present a rock magnetic study of the red-clay sediments from the central CLP. Our results show that frequency dependence of χ (χfd = χlf - χhf, Where χlf and χhf are χ measured at 470 Hz and 4700 Hz, respectively), χlf, and susceptibility of anhystereticremanent-magnetization (χARM) are linearly correlated within the red-clay sequence. This linear correlation indicates that the pedogenic magnetic minerals of the red-clay have a rather uniform grain size distribution as in the loess-paleosol sequence, and the grain size is independent of the degree of pedogenesis. Nevertheless, red-clay sediments are slightly more enriched in superparamagnetic magnetic particles than the overlying loess-paleosol sediments as indicated by the higher slope of the regression line between χfd and χlf. Copyright 2007 by the American Geophysical Union