Hf-Nd Isotopic Variability in Mineral Dust from Chinese and Mongolian Deserts: Implications for Sources and Dispersal

Mineral dust provenances are closely related to the orogenic processes which may have distinct Hf-Nd isotopic signatures. Here we report the clay-sized (\u3c2 μm) Hf-Nd isotope data from Asian dust sources to better constrain the source and transport dynamics of dust deposition in the North Pacific. Our results show that there is a more positive radiogenic Hf isotopic composition with clay-sized fractions than the corresponding bulk sample and a decoupling of the Hf-Nd couplets in the clay formation during the weathering process. The clay-sized Hf-Nd isotopic compositions of the desert samples from the Sino-Korean-Tarim Craton (SKTC) are different from those of the Gobi and deserts from the Central Asian Orogeny Belt (CAOB) due to varying tectonic and weathering controls. The Hf-Nd isotopic compositions of dust in the North Pacific central province (NPC) match closely with those from the Taklimakan, Badain Jaran and adjacent Tengger deserts, implying that the NPC dust was mainly transported from these potential sources by the westerly jet. Our study indicates that dusts from the CAOB Gobi deserts either didn\u27t arrive in NPC or were quantitatively insignificant, but they were likely transported to the North Pacific margin province (NPM) by East Asian winter monsoon

Detection of abrupt changes in East Asian monsoon from Chinese loess and speleothem records

Abrupt climate changes, especially at millennial timescales, have become a key topic in paleoclimatology because of their link with the tipping point theory and their extensive impacts on future climate scenarios. Although best documented in ice-cores and marine sediments, they are also recorded in different archives on the land, among which Chinese loess and speleothem records are of particular interest. These records effectively document orbital-to-millennial monsoon variability during the Pleistocene, but the dominant periodicity revealed by proxy records from these two types of archives is different. Nevertheless, millennial-scale oscillations recorded in Chinese loess and speleothem records seem highly similar over the last several glacial cycles. Such millennial fluctuations can be of varying amplitudes, and either more or less abrupt depending on the type of record. The observed abruptness usually depends on the sedimentation rates and sampling resolution in the loess and speleothem records. However, it can also be refined by comparisons with multiple physicochemical and isotopic proxies. Here, we apply a robust statistical method to detect abrupt changes in loess grain size and speleothem δ18O records. As a first step, we compared two reference records (i.e. the NGRIP ice-core and the Hulu speleothem δ18O records) of abrupt climate change to verify the robustness of our method in detecting the well-recognized abrupt events during the last glaciation. This result allows us, in a second step, to compare two high-resolution loess grain-size stacks (i.e. the CHILOMOS, and the LGS640 datasets) with the Chinese speleothem composite record built from Hulu and Sanbao records. Although visually observed rapid grain-size variations were previously interpreted as representing millennial-scale variations, our statistical analysis confirms that whether these abrupt climate changes can be robustly identified is highly dependent on the time resolution of the studied records. In a third step, we detected abrupt transitions in the MGSQ grain size stack, covering the last 3.6 Myrs. Our results also show that both winter and summer monsoons co-varied at glacial-interglacial to millennial timescales, reflecting a three-stage evolution of increasing intensity: (1) from 3.6 Ma to 2.6 Ma, (2) from 2.6 Ma to 1.2 Ma, and (3) from 1.2 Ma to present. Our results finally show that the identified abrupt climate events can be employed for synchronizing climate records on a global scale

Centennial- to millennial-scale monsoon changes since the last deglaciation linked to solar activities and North Atlantic cooling

Rapid monsoon changes since the last deglaciation remain poorly constrained due to the scarcity of geological archives. Here we present a high-resolution scanning X-ray fluorescence (XRF) analysis of a 13.5 m terrace succession on the western Chinese Loess Plateau (CLP) to infer rapid monsoon changes since the last deglaciation. Our results indicate that Rb∕Sr and Zr∕Rb are sensitive indicators of chemical weathering and wind sorting, respectively, which are further linked to the strength of the East Asian summer monsoon (EASM) and the East Asian winter monsoon (EAWM). During the last deglaciation, two cold intervals of the Heinrich event 1 and Younger Dryas were characterized by intensified winter monsoon and weakened summer monsoon. The EAWM gradually weakened at the beginning of the Holocene, while the EASM remained steady till 9.9 ka and then grew stronger. Both the EASM and EAWM intensities were relatively weak during the Middle Holocene, indicating a mid-Holocene climatic optimum. Rb∕Sr and Zr∕Rb exhibit an antiphase relationship between the summer and winter monsoon changes on a centennial timescale during 16–1 ka. Comparison of these monsoon changes with solar activity and North Atlantic cooling events reveals that both factors can lead to abrupt changes on a centennial timescale in the Early Holocene. During the Late Holocene, North Atlantic cooling became the major forcing of centennial monsoon events

Monsoonal control on a delayed response of sedimentation to the 2008 Wenchuan earthquake

Infrequent extreme events such as large earthquakes pose hazards and have lasting impacts on landscapes and biogeochemical cycles. Sediments provide valuable records of past events, but unambiguously identifying event deposits is challenging because of nonlinear sediment transport processes and poor age control. Here, we have been able to directly track the propagation of a tectonic signal into stratigraphy using reservoir sediments from before and after the 2008 Wenchuan earthquake. Cycles in magnetic susceptibility allow us to define a precise annual chronology and identify the timing and nature of the earthquake’s sedimentary record. The grain size and Rb/Sr ratio of the sediments responded immediately to the earthquake. However, the changes were muted until 2 years after the event, when intense monsoonal runoff drove accumulation of coarser grains and lower Rb/Sr sediments. The delayed response provides insight into how climatic and tectonic agents interact to control sediment transfer and depositional processes.This work was funded by the 2nd Tibetan Plateau Scientific Expedition and Research (2019QZKK0707) and CAS programs (QYZDJ-SSW-DQC033, XDA2007010202, and 132B61KYSB20170008) grants to Z.J. and SKLLQG grant (SKLLQGPY1603) to F.Z

Preferential dust sources: a geomorphological classification designed for use in global dust-cycle models

We present a simple theoretical land-surface classification that can be used to determine the location and temporal behaviour of preferential sources of terrestrial dust emissions. The classification also provides information about the likely nature of the sediments, their erodibility and the likelihood that they will generate emissions under given conditions. The scheme is based on the dual notions of geomorphic type and connectivity between geomorphic units. We demonstrate that the scheme can be used to map potential modern-day dust sources in the Chihuahuan Desert, the Lake Eyre Basin and the Taklamakan. Through comparison with observed dust emissions, we show that the scheme provides a reasonable prediction of areas of emission in the Chihuahuan Desert and in the Lake Eyre Basin. The classification is also applied to point source data from the Sahara to enable comparison of the relative importance of different land surfaces for dust emissions. We indicate how the scheme could be used to provide an improved characterisation of preferential dust sources in global dust-cycle models

Apparent timing and duration of the Matuyama-Brunhes geomagnetic reversal in Chinese loess

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; The Matuyama-Brunhes (MB) geomagnetic reversal in Chinese loess has been studied extensively as an important boundary for land-ocean stratigraphic and paleoclimatic correlations. However, the apparent timing and duration of the MB boundary remain controversial in Chinese loess deposits due to its inconsistent stratigraphic position and the uncertain chronologies. Here we synthesized high-resolution paleomagnetic records from four loess sequences in the central Chinese Loess Plateau and synchronized the loess-paleosol chronology by matching the grain-size variations to orbitally tuned grain-size time series. The synthesized paleomagnetic results reveal consistent features of the MB transition in Chinese loess, including the stratigraphic position (L8/S8 transition), timing (&sim;808&ndash;826 ka), duration (&sim;14&ndash;16 ka), and rapid directional oscillations. Compared with the MB transition in marine records (770&ndash;775 ka), the timing of the MB transition is relatively older and longer in Chinese loess, due to a complex interplay between different remanence acquisition mechanisms which occurred during the course of postdepositional physical and chemical processes.</p

Processes controlling the geochemical composition of the South China Sea sediments during the last climatic cycle

Author Posting. © Elsevier B.V., 2008. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Chemical Geology 257 (2008): 240-246, doi:10.1016/j.chemgeo.2008.10.002.Sediments of the upper 28.2 meters of Ocean Drilling Program (ODP) Site 1145 from the northern South China Sea (SCS) were analyzed for their geochemical composition. Most of the major and trace elements exhibit significant fluctuations at glacial-interglacial scales, implying a close relation with regional and global climate change. Al-normalized elemental ratios can be subdivided into three principal components (PC). PC1 (e.g., Ca/Al, Ba/Al, Sr/Al) displays significant glacial-interglacial variation and is related to paleoproductivity in the northern SCS. PC2 (e.g., K/Al, Mg/Al, Rb/Al) is associated with the degree of chemical weathering in the source regions and shows little glacial-interglacial variation. PC3 (e.g., Ti/Al, Zr/Al) reflects the relative contribution of coarse- and fine-grained materials in the terrigenous components of the SCS sediments, likely associated with changes in sea level and monsoon-induced fluvial input. Spectral analyses indicate that paleoproductivity (i.e., Ba/Al) in the South China Sea lags Hulu/Sanbao speleothem δ18O record (a indicator of annual average meteoric precipitation) by 102° and Indian summer monsoon (multi-proxy stack) by 23° at the precession band, indicating a close relationship with the Indian summer monsoon. However, the chemical weathering degree in the source area (PC2) is not sensitive to monsoon-related changes at the precession band during the last climatic cycle.This study was supported by the NSFC to Y.B. Sun and the US NSF to D.W. Oppo (OCE 0502960) and S.C. Clemens (OCE 0352215)

Paleoenvironmental change in the middle Okinawa Trough since the last deglaciation : evidence from the sedimentation rate and planktonic foraminiferal record

Author Posting. © The Authors, 2006. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Palaeogeography, Palaeoclimatology, Palaeoecology 243 (2007): 378-393, doi:10.1016/j.palaeo.2006.08.016.Well-dated, high-resolution records of planktonic foraminifera and oxygen isotopes from two sediment cores, A7 and E017, in the middle Okinawa Trough reveal strong and rapid millennial-scale climate changes since ~18 to 17 thousand years before present (kyr B.P.). Sedimentation rate shows a sudden drop at ~11.2 cal. kyr B.P. due to a rapid rise of sea-level after the Younger Dryas (YD) and consequently submergence of the large continental shelf on the East China Sea (ECS) and the retreat of the estuary providing sediment to the basin. During the last deglaciation, the relative abundance of warm and cold species of planktonic foraminifera fluctuates strongly, consistent with the timing of sea surface temperature (SST) variations determined from Mg/Ca measurements of planktonic foraminifera from one of the two cores. These fluctuations are coeval with climate variation recorded in the Greenland ice cores and North Atlantic sediments, namely Heinrich event 1 (H1), Bølling-Allerød (B/A) and YD events. At about 9.4 kyr B.P., a sudden change in the relative abundance of shallow to deep planktonic species probably indicates a sudden strengthening of the Kuroshio Current in the Okinawa Trough, which was synchronous with a rapid sea-level rise at 9.5-9.2 kyr B.P. in the ECS, Yellow Sea (YS) and South China Sea (SCS). The abundance of planktonic foraminiferal species, together with Mg/Ca based SST, exhibits millennial-scale oscillations during the Holocene, with 7 cold events (at about 1.7, 2.3-4.6, 6.2, 7.3, 8.2, 9.6, 10.6 cal. kyr BP) superimposed on a Holocene warming trend. This Holocene trend, together with centennial-scale SST variations superimposed on the last deglacial trend, suggests that both high and low latitude influences affected the climatology of the Okinawa Trough.This study was supported by the National Natural Science Foundation of China (Grant Nos. 40206007, 40106006, 90211022 and 40506027), the Chinese Academy of Sciences innovation program (KZCX3-SW-220), and the NSF (OCE05-29600 to DWO)

Decoupled Land and Ocean Temperature Trends in the Early-Middle Pleistocene

Record of long-term land temperature changes remains ephemeral, discontinuous, and isolated, thus leaving the common view that Pleistocene land temperature evolution should have followed ocean temperatures unconfirmed. Here, we present a continuous land surface temperature reconstruction in the Asian monsoon region over the past 3.0 Myr based on the distribution of soil bacterial lipids from the Chinese Loess Plateau. The land temperature record indicates an unexpected warming trend over the Pleistocene, which is opposite to the cooling trend in Pleistocene ocean temperatures, resulting in increased land-sea thermal contrast. We propose that the previously unrecognized increase of land-sea thermal contrast during much of the Pleistocene is a regional climate phenomenon that provides a likely mechanism in favor of the long-term enhancement of the Pleistocene East Asian summer monsoon

Last deglaciation in the Okinawa Trough : subtropical northwest Pacific link to Northern Hemisphere and tropical climate

Author Posting. © American Geophysical Union, 2005. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 20 (2005): PA4005, doi:10.1029/2004PA001061.Detailed deglacial and Holocene records of planktonic δ18O and Mg/Ca–based sea surface temperature (SST) from the Okinawa Trough suggest that at ∼18 to 17 thousand years before present (kyr B.P.), late spring/early summer SSTs were approximately 3°C cooler than today, while surface waters were up to 1 practical salinity unit saltier. These conditions are consistent with a weaker influence of the summer East Asian Monsoon (EAM) than today. The timing of suborbital SST oscillations suggests a close link with abrupt changes in the EAM and North Atlantic climate. A tropical influence, however, may have resulted in subtle decoupling between the North Atlantic and the Okinawa Trough/EAM during the deglaciation. Okinawa Trough surface water trends in the Holocene are consistent with model simulations of an inland shift of intense EAM precipitation during the middle Holocene. Millennial-scale alternations between relatively warm, salty conditions and relatively cold, fresh conditions suggest varying influence of the Kuroshio during the Holocene.Funding for this research was provided by NSFC (grants 40106006 and 40206007), SKLLQG (grant LLQG0204), and the NSF (OCE-020776 to DWO). Y.S.'s visit to WHOI was supported via a NSF START Fellowship