The amount of carbonate minerals washed into the Bengal Fan , a submerged river delta in the Bay of Bengal, from weathered rocks upstream peaks at 5—13 Ma in the stratigraphic record, a further suggestion that the monsoon was especially strong during this period. This discharge of carbonate-rich material coincides with erosion of the Tethyan Himalayas and initial unroofing tectonic and erosional removal of overlying units of the Lesser Himalayas after 8 Ma.
The Tethyan Himalayas, lightly metamorphosed sedimentary rocks that were deposited on the northern margin of India before it collided with Eurasia, are generally exposed in the rain shadow north of the highest ranges of the Greater Himalayas. The Lesser Himalayas are weakly metamorphosed rocks that structurally underlie the high-grade rocks of the Greater Himalayas and are mostly exposed in the foothills on the southern flank of the range.
Widespread erosion of the Lesser Himalayas occurred later, after 3 Ma in the western Indus Basin compared to farther east. The switch in the dominant sources of erosion from northern to southern areas likely reflects the raising of the High Himalayan barrier due to the ongoing tectonic collision of India and Asia, especially the break off of the subducting lithospheric slab. The lack of focused erosion in the High Himalayas promoted propagation of the Himalayan front southward after 10 Ma.
Dating of detrital zircons in the eastern Nicobar Fan a lobe of the Bengal Fan indicates that its sources of eroded material did not shift geographically as much as the sources of the main Bengal Fan did. The Nicobar Fan shows the influence of tectonically driven basin inversion as well as erosion of primary sources in supplying sediment to the deep sea. Although we often associate heavy rainfall and flooding with increases in erosion, drying of the Indian peninsula also led to enhanced erosion on various timescales.
A loss of stabilizing plant cover largely drove this erosion, implying that erosion does not have a simple linear relationship with precipitation. Attendees at the Chapman Conference raised doubts about whether chemical weathering of Himalaya-derived sediment controlled global climate through carbon dioxide drawdown because of a lack of a clear trend to increased weathering reconstructed from the new drilled record and linked to the global cooling that occurred during the Miocene and Pliocene epochs.
Instead, burial of organic matter, including woody debris, especially on the Bengal Fan, appears to have had global significance in drawing down carbon dioxide through the Miocene and Pliocene. During this period, erosion of the Himalayas accelerated as the ranges rose following progressive tearing and break off of the dense Indian lithosphere that had thrust below the southern margin of Eurasia after the start of the Indian collision, largely from west to east between 24 and 12 Ma.
Untangling these interacting influences remains a work very much in progress. Chapman Conference attendees identified several key future research goals. These goals include recovery of a complete record of erosion through the Paleogene 66—23 Ma ; key areas include the Murray Ridge in the Arabian Sea, the Bengal Fan, and the Red River delta. This time period is important because of recent suggestions that the monsoon may have strengthened much earlier than generally proposed, around 36 Ma.
Furthermore, if the formation of the Greater Himalayas after 23 Ma was climatically triggered, then an erosional record spanning their birth is required to test this hypothesis. In general, if we are to make progress in quantifying erosion in the sedimentary record, then we must understand the 3D structure of the submarine fans through seismic surveys linked to ocean drilling.
Concerning reconstruction of past continental environments, the research community aims to produce a regional Miocene vegetation cover map and to determine feedbacks between vegetation and monsoon climate. We also thank the U. Peter D. Clift pclift lsu. The portal can access those files and use them to remember the user's data, such as their chosen settings screen view, interface language, etc.
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The latter interpretation is consistent with paleobotanical evidence Antonie et al. These results suggest intensification of the ISM from 22 to 15 Ma. Thus, desertification over a wide area of the middle latitudes of the Asian interior seems to have commenced at or slightly before the OMB. In the Tarim Basin, pale yellowish loess-like siltstone continued to accumulate until at least ca. Qiang et al. A similar development of wet conditions was documented in the Jungger Basin between It is noteworthy that these wet periods coincide with the Miocene climatic optimum 17 to 15 Ma , which was most likely induced by higher atmospheric pCO 2 Foster et al.
Based on the pollen assemblage from fluvio-lacustrine sediments in Guyuan Fig. Further intensification of the warm and wet climate in East Asia occurred during the late Miocene to early Pliocene, suggesting intensification of the EASM e.
However, this dry—wet shift is diachronous with the earlier shift that occurred in northeast China Qiang et al. The magnetic susceptibility of the Red Clay Formation, considered to be a reflection of summer precipitation, increased after ca. This maximum occurred at approximately the same time as the mid-Pliocene warm period e. Pedogenesis weakened in the CLP after ca. Sun et al. At this time, the amplitude of orbital-scale variability further increased alongside a trend toward slightly increased magnetic susceptibility.
A further increase in magnetic susceptibility and the amplitude of orbital-scale variability occurred at ca. Based on analytical results from a drill core retrieved from the central Tengger Desert, Li et al. Pollen and organic carbon isotope records from the CLP An et al. It should be noted that the onset of the northern hemisphere glaciation NHG began ca. Furthermore, an age of ca. In an oxygen and carbon isotopic study of mammal teeth from central Pakistan, Martin et al.
The plant record from the Nepal Siwaliks during the middle Miocene indicates tropical evergreen forests with rare moist deciduous species flourished in these regions Prasad This was followed by a decrease from Even though deposition of the Red Clay Formation in the main part of the CLP only started between 8 and 7 Ma, this could have been a result of the slow uplift and erosion of the Ordos Platform before 8 Ma that caused erosion or non-deposition of eolian dust on the plateau Xu et al.
However, the strength and amplitude of the EAWM gradually decreased toward 4. The intensity gradually increased again toward the present with a relatively abrupt increase in amplitude at 1. Similar results were obtained from analysis of grain size in bulk samples and accumulation of loess from multiple sites in the CLP covering the last 7. Ding et al. They found a southward decrease in the grain size for loess—paleosol sequences, implying a strong influence from northerly EAWM winds after 2.
In contrast, there is no clear trend toward decreasing grain size for the Red Clay Formation. They suggested a significant reorganization of atmospheric circulation at 2. After 2. The occurrence of aeolian deposits provides direct evidence for the presence of deserts, and their provenance provides information on the location and extent of deserts. Chen and Li also reported that the relative contribution from the Gobi Altay Mountains recovers rapidly toward early Miocene levels at ca.
The reason for this is not certain, but considering that the loess—paleosol sequence in the CLP was derived primarily from the Tennger Desert during interglacials and from the Monglian Gobi Desert during glacials Sun et al. According to a new chronology from Zheng et al.
In summary, there is evidence for a summer monsoon during the Eocene, especially in East Asia. This was probably driven by higher atmospheric pCO 2 , although the latitudinal zonal distribution of climate was maintained Huber and Goldner ; Zhang et al.
Since ca. It seems that the EAWM was active since as long ago as 20 Ma, but was likely very weak until 13—12 Ma when it began to intensify. The second phase of EAWM intensification occurred at ca. The third phase of intensification occurred at 1. Superimposed on these stepwise intensifications of the EAWM are short-lived decreases in its intensity during ca.
The second phase of desertification probably occurred at ca. Further expansion of the dry area might have occurred at ca. Three major phases of the HTP uplift are described in this review see the third section and Fig.
The first pulse of the uplift in the south and central TP raised the area to close to its present height during the late Eocene ca. Unfortunately, no sedimentary sequences are available that record the late Eocene history of ISM. Deformation and exhumation started at the northern edge of the TP and Pamir soon after the collision and accelerated at ca. Evolution of Asian monsoons, desertification, HTP uplift, and their relation with global changes during Cenozoic.
In c and e , gray bars represent weak phases and black bars represent strong phases. In the Xining Basin in the northeastern TP, aridification started at ca. This cooling preceded the EOB and Antarctic ice sheet formation by more than 2 m. Coxall et al. Considering that strontium isotope ratios of seawater recorded in foraminifera started to increase at ca.
This weathering could, in turn, have caused the drawdown of atmospheric pCO 2 Pagani et al. This cooling and aridification would have occurred at least in Central East Asia, but was more likely hemispheric in its extent. The increase in oceanic lithium isotope ratios is also consistent with increasing silicate chemical weathering at ca. Furthermore, it is likely that global cooling, together with the opening of the Drake Passage, led to the formation of Antarctic ice sheets at the EOB DeConto and Pollard that further enhanced global cooling and aridification.
This uplift pulse was extensive and associated with a change in deformation mode, which is interpreted to have been related to the partial removal of lithospheric mantle under northern Tibet and subsequent weakening of the middle to lower crust Clark and Royden ; Royden et al. Initiation of south-dipping intracontinental subduction occurred between North Pamir and the Tien Shan. This process may have been triggered by a break-off of the western end of the north-dipping Indian slab Sobel et al.
According to climatic simulations of partial or phased uplift of the HTP, the uplift of the northern TP enhanced summer precipitation in northern East Asia and the southern and eastern TP Zhang et al. These predictions are generally consistent with recently published paleoclimatic data described in the fourth section.
In particular, a latitudinally zonal climate pattern during the Paleogene changed to a Neogene pattern. This pattern was characterized by arid zones restricted to the northwest of China and the development of a warm and wet climate in eastern China at the OMB ca.
There is also evidence of desertification in central Asia around the OMB or slightly earlier Guo et al. This contradiction can be explained by the effect of the concurrent uplift of the main part of Tibet that overwhelmed the effect of the northern TP uplift on rainfall in South Asia.
However, a complete understanding of this contradiction requires additional tectonic and paleoclimatic evidence. There is some evidence of decreasing atmospheric pCO 2 around 26 Ma Pagani et al. However, simultaneous development of a warm and wet climate in East Asia and central Pakistan cannot be explained by the decrease in pCO 2. The third pulse of the HTP uplift occurred at ca. This event is possibly related to the cessation of rapid Greater Himalayan exhumation.
It is also suggested that the LH sequence was uplifted beginning at ca. As described in the second section, climate simulations predict that the uplift of the northern TP would enhance the northwest penetration of the EASM front, the desertification of inland Asia, and strengthening of the EAWM.
There is some evidence that the EAWM strengthened at ca. These observations are consistent with climatic simulation predications. However, there is no clear evidence that the EASM intensified during this period. Rather, there is evidence of a drastic decrease in EASM precipitation from It should be noted that the time between It is possible that intensification of the EASM caused by the uplift of northern TP was canceled out by global cooling.
Alternatively, it is possible that desertification of inland Asia, intensification of the EAWM, and the reduction of ISM precipitation in northern India were caused by the decrease in pCO 2 and subsequent global cooling rather than by the uplift of the northern TP. There is evidence that tectonic activity in the northern TP has not been as intensive since ca.
For example, the period from ca. However, detailed examination of the magnetic susceptibility of the Red Clay Formation suggests the opposite result. Namely, the intensity of the EASM increased from 4. In fact, the interval between 4. The hypothesis that the uplift of the HTP intensified the Asian Monsoon has attracted attention from the geoscience community for more than 40 years. Yet it is still not proven that plateau uplift intensified the monsoon because we have insufficient knowledge of both the tectonic evolution of the HTP and the paleoclimatic evolution of the Asian monsoon during the Cenozoic.
In addition, climate models are not sophisticated enough to incorporate detailed topography and other boundary conditions at each stage of the uplift.
However, this situation is changing rapidly with drastic increase in new data with new and sophisticated analytical techniques, as well new simulation results with more advanced high-resolution models become available. In this review, we summarized recent progress in climate model simulations that examine the impact of the HTP uplift on the evolution of the Asian monsoon, the tectonic history of the HTP uplift, and paleoclimatic evolution of the Asian monsoon during the Cenozoic.
The uplift of the northern TP also reduces ISM precipitation in northern India, strengthens the western North Pacific subtropical high, and reduces precipitation in central Asia. The effect of the Paratethys on the Asian monsoon could also be significant, but the timing of this effect is restricted to the Eocene, since this water body largely disappeared during the Oligocene. The effect of pCO 2 on the intensity of the Asian monsoon is potentially significant and should be taken into account.
PCO 2 has opposing impacts on summer and winter monsoons, and its effects also contrast with the tectonically driven impact. Three major phases of the HTP uplift are recognized.
The first pulse involves the uplift of the southern and central TP at ca. The second pulse is the uplift of the northern TP at ca. The Greater Himalayan sequence was exhumed and thrusted over the Lesser Himalaya sequence at that time. The third pulse is the uplift of the northeastern and eastern TP starting at ca. The effect of the uplift of different parts of the HTP on climate was explored based on a comparison of climate model simulation results with paleoclimate records.
These predictions are tested by comparing the results with paleoclimatic data spanning critical time intervals. There are not enough paleoclimatic data to specify whether the ISM and Somali Jet intensified in association with the uplift of the southern and central TP at 40—35 Ma. However, it is possible that the uplift of the southern and central TP enhanced erosion and weathering of the HTP.
This weathering may have in turn resulted in a draw down of atmospheric CO 2 and global cooling, along with expansion the Antarctic ice sheets and a reduction in EASM intensity. The impact of the uplift of the northeastern and eastern TP on Asian monsoon strength at 15—10 Ma is difficult to evaluate because this interval is also a time of global cooling and Antarctic glaciation that might also have confounded Asian monsoon intensity.
In conclusion, the uplift of the northern TP at ca. However, as with the other two uplift phases that affected the HTP at ca. It is clear that other boundary conditions, especially the atmospheric pCO 2 level, also exert an influence on Asian monsoons. Thus, we must differentiate such effects from the effect of the HTP uplift to properly evaluate its impact. However, it is important to note that the uplift and subsequent erosion of the HTP may also affect the atmospheric pCO 2 level through chemical weathering.
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