Paleohydrological data comprising pollen assemblages and leaf‐wax hydrogen isotopes (δDwax) from paleolake sediments in the Qaidam Basin (China) provide evidence for a link between increased moisture ...availability on the Tibetan Plateau and global cooling during the Mid‐Pleistocene Transition. Notably, they document the persistence of humid and cold conditions during Marine Isotope Stages 24–22 (936–866 ka) suggesting that boundary conditions favorable for extended glaciation on the Tibetan Plateau first developed at ~900 ka. Our δDwax results indicate a strong influence of proximal (monsoonal) moisture sources during that glacial, in agreement with the intensification of the interhemispheric moisture transport resulting from Antarctic ice volume increase at ~900 ka. The consistency of our results with other marine and terrestrial climate data sets suggests that extended glaciation on the Tibetan Plateau may have initiated ~500 ka earlier than previously assumed, implying that midlatitude ice sheets actively contributed to global cooling during the Mid‐Pleistocene Transition.
Plain Language Summary
The Mid‐Pleistocene Transition (MPT) marks an intensification of global cooling associated with an expansion of Earth's ice sheets. The Tibetan Plateau—today the most extensively glaciated region outside the high latitudes—may have played a pivotal role in this process. However, because its glaciation history is yet poorly constrained, testing this hypothesis has remained difficult. Here we assess the climatic boundary conditions for glacier development on the Tibetan Plateau during the MPT using pollen and organic biomarker data from paleolake sediments (Qaidam Basin, China). Our results suggest a link between increased moisture availability on the Tibetan Plateau and global cooling during Marine Isotope Stages 24–22. We infer that favorable conditions for extended glaciation on the Tibetan Plateau first developed at ~900 ka, that is, ~500 ka earlier than previously assumed, suggesting that ice sheet formation on the Tibetan Plateau indeed played an active role in global cooling during the MPT.
Key Points
Link between increased moisture availability on the Tibetan Plateau and global cooling during Marine Isotope Stages 24‐22
Favorable conditions for extended glaciation on the Tibetan Plateau first developed at ~900 ka
Ice sheet formation on the Tibetan Plateau played an active role in global cooling during the Mid‐Pleistocene Transition
The synthesis of the macrolactone 23 is described. The synthesis features a diastereoselective hydroboration of the chiral alkene 17 followed by a Suzuki cross-coupling reaction with the benzoate 5. ...The resulting seco acid 21 was converted to the macrolactone 23 by a Mitsunobu lactonization using immobilized triphenylphosphine. The stereogenic centers in the alkene 17 were established by a Noyori reduction of the β-keto ester 8 and an Evans aldol reaction. The synthesis illustrates the conversion of a syn aldol product to the corresponding anti product by inversion of the methyl-bearing center.
How sediment grain size corresponds to both climate change and tectonics is increasingly the focus of debate. The shrinkage and migration of the huge paleo-lake in the western Qaidam Basin (WQB), NE ...Tibetan Plateau (TP) provide an excellent case study. We present a nearly 6Ma well-dated high-resolution grain size record from the 723-m-deep drill core SG-1b (located in the Jianshan Anticline in the WQB) to show how sediment grain size responds to anticline growth and climate change. The results show that variations in grain size represent three distinct phases. Phase I (7.3–3.6Ma) is characterized by fine sediments with good sorting, with a predominance of clay and fine silt. During this phase, the drilling site was in a deep to semi-deep lake environment, the Jianshan Anticline has undergone relatively weak tectonics and the climate was dry, but still much wetter than that in the Quaternary; grain size variation and lacustrine deposition were principally controlled by climate. Phase II (~3.6–3.3Ma) was a transitional period characterized by a rapid shift toward consistently coarser sediments with increased volumes of medium-coarse silt and sand. During this phase, the drilling site underwent a dramatic shift to a shallow lake environment, the anticline experienced rapid uplift and the climate was in an episode of rapid drying; grain size variation and lacustrine deposition were principally controlled by tectonics in the Jianshan Anticline. Phase III (~3.3–1.6Ma) was a period exhibiting a long-term fining trend in mean grain size relative to Phase II, but still much coarser than that of Phase I, and with a distinct and prolonged increase in very fine sediments accompanied by poor sorting. During this phase, the drilling site was in a shallow lake environment between 3.3Ma and 1.9Ma, and finally became a lakeshore-like environment between 1.9Ma and 1.6Ma; the anticline experienced rapid and continuous uplift and the climate was in a long-term stepwise drying and cooling trend. A conceptual model was drawn to show how grain size and lacustrine deposition responded to uplift of the Jianshan Anticline and climate change in the Qaidam Basin.
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•A 723m deep core located at the top of an anticline in western Qaidam Basin has been studied.•7.3–1.6Ma records of grain-size, lithofacies and growth strata sequence are presented.•Tectonics and climatic change control the anticline development and sedimentary records.•The long-term shrinkage of the paleolake at the anticline area started at 3.6Ma.•The dramatic shift from a semi-deep to a shallow lake happened between 3.6Ma and 3.3Ma.
The Qaidam Basin is an ideal archive to study long-term climate and erosion histories at the NE Tibetan Plateau. We present a magnetostratigraphic study of the 723 m deep drill-core SG-1b of ...lacustrine sediments at the Jianshan anticline in the western Qaidam Basin. The polarity sequence shows 18 normal and 19 reverse polarity zones which can be readily correlated with chrons C1n-C3Br of the Geomagnetic Polarity Time Scale 2004 (GPTS 2004), dating the core at about 7.3-1.6 Ma. The resulting mean sediment accumulation rate (SAR) between polarity boundaries ranges from 6.5 to 30.4 cm ka−1. High SARs occur within the intervals of >7.3-6.0, 5.2-4.2 and 3.6-2.6 Ma indicating three episodic phases of higher erosion. From the derived variation of SARs and previous results, we conclude that growth strata at the Jianshan anticline started to develop at ∼1.6 Ma by limb rotation. All this we relate to pulse tectonic uplift of the NE Tibetan Plateau and fault-propagation-folding in the Qaidam Basin.
The lacustrine deposits in Heqing basin provide an excellent archive for long-term high-resolution palaeoclimate studies in the monsoon-dominated southeastern Tibetan Plateau region. In this study, ...we investigate the climatic significance of magnetic parameters for analysing the variability of the past climate. For this, we performed comprehensive time-series and statistical analysis on previously published proxy data from a 168-m-long drill core (Core-HQ) that spans ∼900–30 ka, comprising mainly magnetic parameters and carbonate content (CC). Moreover, we investigated magnetic properties of modern soil in the catchment that predominantly formed on limestone bedrock. Key findings are: (1) modern soils and sediments of Core-HQ both contain a mixture of magnetite (Mt), maghemite (Mgh) and hematite (Ht), but magnetic concentration of the soils is one order higher; (2) a superparamagnetic (SP) fraction of Mt/Mgh dominates in the soils whereas in Core-HQ the SP contribution is generally very low; (3) a larger grain-size fraction of Mt/Mgh and Ht is also present in the soils. We explain variations of magnetic concentration and CC in Core-HQ by an increased wind transport of soil and a decreased surface water transport of carbonate and soil in less humid periods. Low-temperature oxidation of magnetite in the catchment is as another crucial process that reflects weathering conditions and is likely sensitive to humidity; the degree of LTO can be semi-quantified by the magnetic parameters ARM/SIRM and S-ratio. Combining CC, ARM/SIRM and S-ratio values, we derive a weathering intensity (WI). The WI index variation along Core-HQ shows strong fluctuations on a 100-kyr eccentricity scale in the lower part, especially during ∼630–380 ka, followed by a long period (∼320–80 ka) of persistently weaker weathering (drier?) conditions with low variability, and a rapid return of much stronger weathering (wetter?) conditions at ∼80 ka. We suggest that a reduced influence of the Indian summer monsoon accounts for less moisture supply to the region and lower climatic variability.
The application of magnetic proxies in paleoclimate studies has the advantage of fast and non-destructive measurement procedures, which is an important precondition for obtaining high-resolution ...datasets within a manageable time frame. Here we demonstrate the advantage of using magnetic proxies for a ∼940-m-long drill core comprising lacustrine sediments from the Qaidam Basin (NE Tibetan Plateau), which was previously dated by magnetostratigraphy and optical stimulated luminescence (OSL) dating. Comparison of three independent mass-specific magnetic susceptibility (χ) datasets confirms the horizontal homogeneity of the drill core at scales of several cm3. The potential linkage of χ to astronomical forcing is suggested by spectral analysis. Magnetic grain size- and mineralogy-dependent parameters were used to evaluate possible scenarios explaining the χ variation. Moreover, using additional palynological results we demonstrate the correlation of χ with climatic constraints, thereby showing the climate sensitivity of χ. Higher and lower χ values are related to dry and more humid conditions, respectively. Our results lead us to conclude that χ variations are caused by the interference of two driving mechanisms: low-temperature oxidation in the catchment area and a changing catchment area. In summary, this study confirms the informative value of magnetic proxies for detecting paleoenvironmental change in high resolution.
Total Synthesis of Salicylihalamides A and B Herb, Christian; Bayer, Alexander; Maier, Martin E.
Chemistry : a European journal,
November 19, 2004, Letnik:
10, Številka:
22
Journal Article
Recenzirano
Odprti dostop
The paper illustrates two efficient routes to macrolactone 19 containing a 3‐(para‐methoxybenzyloxy)propyl side chain at C‐15. The chiral center at C‐15 was introduced by a Noyori reduction of keto ...ester 5. The intermediate common to both routes, aldehyde 8, was prepared from keto ester 5. The subsequent chain extension utilized Evans aldol reactions. The first route leads to the alkene 14, which was used, after hydroboration, for a Suzuki cross‐coupling reaction with vinyl iodide 15. The derived seco acid 18 was converted into the macrolactone 19 by a Mitsunobu lactonization by using immobilized triphenylphosphine. Alternatively, an aldol reaction of 8 with the 4‐pentenoyl derivative 20 was used to prepare alkene 26. This building block led to ester 28, which could also be converted into macrolactone 19 by the classical ring‐closing metathesis. After conversion of the C‐15 side chain to the corresponding aldehyde, the enamide was introduced through hemiaminal formation and formal elimination of water. Separation of the double‐bond isomers and removal of the silyl protecting groups provided salicylihalamides A (E)‐1 and B (Z)‐1.
Running rings around enamides: By use of a new enamide synthesis, salicylihalamide 2 was prepared from the macrolactone 1. This macrolactone could either be reached by Mitsunobu macrolactonization or classical ring‐closing metathesis.
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