ALBERT

Description: Marked by the expansion of ice sheets in the high latitudes, the intensification of Northern Hemisphere glaciation across the Plio/Pleistocene transition at ~ 2.7 Ma represents a critical interval of late Neogene climate evolution. To date, the characteristics of climate change in North America during that time and its imprint on vegetation has remained poorly constrained because of the lack of continuous, highly resolved terrestrial records. We here assess the vegetation dynamics in northwestern North America during the late Pliocene and early Pleistocene (c. 2.8–2.4 Ma) based on a pollen record from a lacustrine sequence from paleo-Lake Idaho, western Snake River Plain (USA) that has been retrieved within the framework of an International Continental Drilling Program (ICDP) coring campaign. Our data indicate a sensitive response of forest ecosystems to glacial/interglacial variability paced by orbital obliquity across the study interval, and also highlight a distinct expansion of steppic elements that likely occurs during the first strong glacial of the Pleistocene, i.e. Marine Isotope Stage 100. The pollen data document a major forest biome change at ~ 2.6 Ma that is marked by the replacement of conifer-dominated forests by open mixed forests. Quantitative pollen-based climate estimates suggest that this forest reorganisation was associated with an increase in precipitation from the late Pliocene to the early Pleistocene. We attribute this shift to an enhanced moisture transport from the subarctic Pacific Ocean to North America, confirming the hypothesis that ocean-circulation changes were instrumental in the intensification of Northern Hemisphere glaciation.

Description: Deutsche Forschungsgemeinschaft (DE)

Description: Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659

Description: The intensification of Northern Hemisphere glaciation (iNHG) and uplift of the Tibetan Plateau have been argued to be among the main drivers of climate change in midlatitude Central Asia during the Pliocene/Pleistocene. While most proxy records that support this hypothesis are from regions outside the Tibetan Plateau (such as from the Chinese Loess Plateau), detailed paleoclimatic information for the plateau itself during that time has yet remained elusive. Here we present a temporally highly resolved (~500 years) sedimentological record from the Qaidam Basin situated on the northeastern Tibetan Plateau that shows pronounced glacial‐interglacial climate variability during the interval from 2.7 to 2.1 Ma. Glacial (interglacial) intervals are generally characterized by coarser (finer) grain size, minima (maxima) in organic matter content, and maxima (minima) in carbonate content. Comparison of our results with Earth's orbital parameters and proxy records from the Chinese Loess Plateau suggests that the observed climate fluctuations were mainly driven by changes in the Siberian High/East Asian winter monsoon system as a response to the iNHG. They are further proposed to be enhanced by the topography of the Tibetan Plateau and its impact on the position and intensity of the westerlies.

Description: Key Points: Pronounced glacial‐interglacial climate fluctuations on the NE Tibetan Plateau during the latest Pliocene and early Pleistocene. Changes in East Asian Winter Monsoon and the position of the westerlies influenced sediment transport on the NE Tibetan Plateau. Intensification of Northern Hemisphere glaciation amplified climate fluctuations on the NE Tibetan Plateau.

Description: German Research Foundation (DFG

Description: The continental expression of global cooling during the Miocene Climate Transition in Central Asia is poorly documented, as the tectonically active setting complicates the correlation of Neogene regional and global climatic developments. This study presents new geochemical data (CaSO4 content, carbonate δ13C and δ18O) from the endorheic alluvial‐lacustrine Aktau succession (Ili Basin, south‐east Kazakhstan) combined with findings from the previously published facies evolution. Time series analysis revealed long‐eccentricity forcing of the paleohydrology throughout the entire succession, split into several facies‐dependent segments. Orbital tuning, constrained by new laser ablation U‐Pb dates and a preexisting magnetostratigraphy, places the succession in a 5.0 Ma long interval in the middle to late Miocene (15.6 to 10.6 Ma). The long‐term water accumulation in the Ili Basin followed the timing of the Miocene Climate Transition, suggesting increased precipitation in the catchment area in response to climate cooling and stronger westerly winds. This was paced by minima of the 2.4 Ma eccentricity cycle, which favored the establishment of a discharge playa (~14.3 Ma) and a perennial lake (12.6 to 11.8 Ma). Furthermore, low obliquity amplitudes (nodes) caused a transient weakening of the westerlies at ~13.7 to 13.5 Ma and at ~12.7 Ma, resulting in negative hydrological budgets and salinization. Flooding of the windward Ili Basin coeval with aridification in the leeward basins suggests that the Tian Shan was a climate boundary already in the middle Miocene. Our results emphasize the impact of climate fluctuations on the westerlies' strength and thus on Central Asian hydrology.

Description: Plain Language Summary: The global climate changed from an exceptional warm to a colder state in the middle Miocene epoch, representing a milestone in the evolution of today's climate. This study focuses on the, so far fragmentary, understanding of the Central Asian climate response to this global climate transition by investigating deposits of a former (salt) lake in the Ili Basin, southeast Kazakhstan. Regular sediment alternations represent cycles of low and high water level, overprinted by a long‐term lake expansion. Time series analysis of climate sensitive geochemical and environmental parameters, together with the determination of absolute rock ages, enabled the identification of sedimentary cycles (405 ka and 1.2 Ma long), which are equivalent to climate influencing variations of the Earth's orbit and tilt angle. We conclude that water level maxima are linked to periods of low seasonal climate differences reoccurring every 405 ka. The lake expansion is caused by more precipitation due to strengthened westerly winds, in response to global cooling. Westerly winds were transiently weakened during periods of low variability of the Earth's tilt angle, promoting high evaporation and salinization. Our results emphasize the impact of climate change on the westerlies' strength and thus on Central Asian moisture supply.

Description: Key Points: The endorheic Miocene Ili Basin features orbital control of its hydrological budget by long eccentricity and obliquity amplitude modulation. Obliquity amplitude modulation affected the westerlies' strength during the Miocene Climate Transition. The Miocene global cooling led to strengthening of the westerlies reflected by groundwater accumulation and lake expansion in the Ili Basin.

Description: Deutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/501100001659

Description: Soil and lake sediments are important paleoclimate archives often forming a source-sink setting. To better understand magnetic properties in such settings, we studied red soil on low-magnetic bedrock and subrecent sediments of Caohai Lake (CL) in Heqing Basin, China. Red soil is the only important source material for the CL sediments, it is highly magnetic with susceptibilities (χ) of ~10−5 m3/kg. The red soil is dominated by pedogenic nano-magnetite (~10–15 nm) arranged in aggregates of ~100 nm, with particle interaction that causes a wide effective grain size distribution in the superparamagnetic (SP) range tailing into stable single-domain behavior. Transmission electron microscopy and broadband frequency χ(f) suggest partial disintegration of the aggregates and increased alteration of the nanoparticles to hematite during transfer of red soil material to CL. This shifts the domain state behavior to smaller effective magnetic grain sizes, resulting in lower χfd% and χ values, and a characteristic change of χ(f). The SP-stable single-domain distribution of the aggregates in red soil could be climate dependent, and the ratio of saturation remanence to χ is a potential bedrock-specific paleoclimate proxy reflecting it. Magnetic properties of the CL sediments are controlled by an assemblage of nanoparticle aggregates and larger-sized bedrock-derived magnetite. The results challenge the validity of the previous paleoclimate interpretation from the 168-m-long Core-HQ (900–30 ka) in Heqing Basin. Disintegration of aggregates could lead to SP behavior with low χfd% without extinction of individual magnetite nanoparticles, and the χfd%-based assumption of SP magnetite dissolution may be wrong.

Description: Thermomagnetic curves of magnetic susceptibility (κ) are key to characterizing magnetic properties. We report hump‐shaped κ‐T curves of magnetite‐bearing basalt during heating‐cooling cycles to ∼340°C, with a large thermal hysteresis and similar starting and ending values, even in multiple repeated cycles, ruling out changes in magnetic mineralogy. Based on FORC diagrams and published results of engineered materials, we propose that thermal hysteresis arises from configurations of magnetic moments in clusters of single‐domain particles due to dipolar coupling, with different collective behavior during heating and cooling. This effect modifies the hump‐shaped thermal relaxation behavior of the individual nanoparticles. FORC and κ‐T results indicate an increase in effective particle sizes after 700°C‐heating. Our results are a warning against premature interpretation of a decreasing trend in κ‐T curves by maghemite inversion. Instead, fine particle behavior should be considered when a hump‐shaped κ‐T behavior is detected.

Description: Plain Language Summary: Thermomagnetic curves of magnetic susceptibility (κ) are key to characterizing magnetic properties. A marked drop in κ‐T curves at ∼300–400°C is often considered to indicate the inversion of maghemite to hematite. Such a drop is often preceded by an increase in κ, creating a hump shape that is rarely noted in discussions. We report hump‐shaped κ‐T curves in magnetite‐bearing basalt. When heating up to ∼340°C and cooled subsequently, a large thermal hysteresis was observed. This hump shape and the thermal hysteresis behavior occur in a very similar way in repeated κ‐T cycles, ruling out changes in magnetic mineralogy. We hypothesize that the thermal hysteresis arises from configurations of coupled magnetic moments in clusters of fine particles, which is partly irreversible upon cooling. This effect modifies the hump‐shaped thermal relaxation behavior of the individual particle moments. When heated to 700°C, grain boundaries may weld and internal stress effects are reduced, increasing the effective particle sizes and shifting the hump‐peak to a higher temperature. Our results indicate that fine particle behavior should be considered for all types of natural materials when a hump‐shaped κ‐T curve is observed rather than interpreting the drop in κ as maghemite inversion.

Description: Key Points: We observed reversible thermal hysteresis behavior in hump‐shaped partial magnetic susceptibility cycles of magnetite‐bearing basalts. The thermal hysteresis may be caused by blocked states of coupled nanoparticle moments modulating thermal activation. Descending susceptibility in hump‐shaped curves is often due to single‐domain thermal relaxation rather than maghemite inversion.

Description: National Natural Science Foundation of China http://dx.doi.org/10.13039/501100001809

Description: Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659

Description: http://dx.doi.org/10.17632/zx7y6g9rdv.2

Description: Anisotropy of magnetic susceptibility (AMS) combined with structural analysis are used in this work with the aim to characterize the tectonic evolution of the Triassic flysch within the eastern Tethyan Himalaya Thrust Belt in SE Tibet. The attitude of the magnetic foliation and lineation are concordant with the planar and linear structures of tectonic origin defined by the preferred orientation of the iron-bearing silicates. Two different tectonic domains can be defined: (a) the southern domain is controlled by the Eohimalayan tectonic foliation (S1) recorded in the magnetic foliation which trends east–west and dips to the north; (b) the northern domain is dominated by the Neohimalayan magnetic foliation with WNW–ESE strike and dips to the south opposite to the vergence of the main structures. A slightly prolate magnetic ellipsoid has been found in between the two domains recording the intersection of S1 and the subtle development of the S2 tectonic foliation. Hinterland propagation of the deformation lead to the Great Counter backthrust generation, pointed out by the SSW steeply plunging magnetic lineation. Furthermore different orientations of magnetic foliation may indicate an Early Miocene c. 20° clockwise vertical-axis rotation.

Description: The youngest deformation structures on the Tibet Plateau are about NNE-trending grabens. We first combine remote-sensing structural and geomorphological studies with structural field observations and literature seismological data to study the Muga Purou rift that stretches at c. 86°E across central Tibet and highlight a complex deformation field. ENE-striking faults are dominated by sinistral strike–slip motion; NNE-striking faults have normal kinematics and outline a right-stepping en-echelon array of grabens, also suggesting sinistral strike–slip; along NW-striking fault sets, the arrangement of grabens may indicate a dextral strike–slip component. Thus, in central Tibet, rifts comprise mostly grabens connected to strike–slip fault zones or are arranged en-echelon to accommodate sinistral wrenching; overall strain geometry is constrictional, in which NNE–SSW and subvertical shortening is balanced by WNW–ESE extension. The overwhelmingly shallow earthquakes only locally outline active faults; clusters seem to trace linkage or propagation zones of know structures. The earthquake pattern, the neotectonic mapping, and the local fault–slip analyses emphasize a distributed, heterogeneous pattern of deformation within a developing regional structure and indicate that strain concentration is weak in the uppermost crust of central Tibet. Thus, the geometry of neotectonic deformation is different from that in southern Tibet. Next, we use structural and palaeomagnetic data along the Zagaya section of southern central Tibet to outline significant block rotation and sinistral strike–slip SE of the Muga Purou rift. Our analysis supports earlier interpretations of reactivation of the Bangong–Nujiang suture as a neotectonic strike–slip belt. Then, we review the existing and provide new geochronology on the onset of neotectonic deformation in Tibet and suggest that the currently active neotectonic deformation started c. 5 Ma ago. It was preceded by c. north–south shortening and c. east–west lengthening within a regime that comprises strike–slip and low-angle normal faults; these were active at c. 18–7 Ma. The c. east-striking, sinistral Damxung shear zone and the c. NE-trending Nyainqentanghla sinistral-normal detachment allow speculations about the nature of this deformation: the ductile, low-angle detachments may be part of or connect to a mid-crustal décollement layer in which the strike–slip zones root; they may be unrelated to crustal extension. Finally, we propose a kinematic model that traces neotectonic particle flow across Tibet and speculate on the origin of structural differences in southern and central Tibet. Particles accelerate and move eastwards from western Tibet. Flow lines first diverge as the plateau is widening. At c. 92°E, the flow lines start to converge and particles accelerate; this area is characterized by the appearance of the major though-going strike–slip faults of eastern-central Tibet. The flow lines turn southeastward and converge most between the Assam–Namche Barwa and Gongha syntaxes; here the particles reach their highest velocity. The flow lines diverge south of the cord between the syntaxes. This neotectonic kinematic pattern correlates well with the decade-long velocity field derived from GPS-geodesy. The difference between the structural geometries of the rifts in central and southern Tibet may be an effect of the basal shear associated with the subduction of the Indian plate. The boundary between the nearly pure extensional province of the southern Tibet and the strike–slip and normal faulting one of central Tibet runs obliquely across the Lhasa block. Published P-wave tomographic imaging showed that the distance over which Indian lithosphere has thrust under Tibet decreases from west to east; this suggests that the distinct spatial variation in the mantle structure along the collision zone is responsible for the surface distribution of rift structures in Tibet.Supplementary material: Containing supporting data is available at http://www.geolsoc.org.uk/SUP18446.