Strong Summer Monsoon Research Articles

Low-latitude forcing on 4.2 ka event indicated by records in the Asian monsoon region

The 4.2 ka event is an important climatic event of the Holocene that is expected to considerably influence the development of human civilizations. However, the forcing mechanism of the 4.2 ka event remains unclear, with current evidence being divided between high-latitude and low-latitude forcings. The Asian climate system contains strong summer and winter monsoon circulations, and thus the region is ideal for studying high- and low-latitude climate forcings on 4.2 ka event. In this study, we present a high-resolution pollen record of the past 5000 years from Wudalianchi Crater Lake in northeastern China in the East Asian monsoon region. An evident decrease in broadleaf trees occurred during the 4.2 ka event, suggesting a decrease in summer monsoon rainfall. Well-dated high-resolution pollen, stalagmite δ18O, and winter monsoon records were collected to study the possible transportation of high- and low-latitude signals. Most of the pollen records showed significant changes during the 4.2 ka event, while no apparent change was found in winter monsoon records. Meanwhile, more obvious changes in stalagmite δ18O records were observed in lower- rather than the higher-latitude regions around 4.2 ka. Our results suggest that the signal of the 4.2 ka event might be transported by low-latitude processes, thus corresponding to a low-latitude forcing.

Surface oceanographic changes from ∼ 25,000 to 3500 cal yr BP in the eastern Arabian Sea

Multiproxy data of pteropods and planktic foraminifera from Core SK291/GC17 suggest significant surface paleoceanographic shifts from ∼25,000 to 3500 calibrated years before present (cal yr BP) in the eastern Arabian Sea (EAS). Increased values of the global ice-volume free stable oxygen isotope ratio of seawater (δ18Osw IVF) and stable oxygen isotope ratio in pteropod (δ18OH. inflatus IVF), as well as lower percentages of planktic foraminifer Globigerina bulloides from ∼22,000 to 18,000 cal yr BP, correspond to the weak Indian summer monsoon (ISM) during the Last Glacial Maximum (LGM). In the early Holocene, the EAS witnessed an increase in upwelling driven by strong summer monsoon winds, and a decrease in salinity corresponding with enhanced precipitation in the Western Ghats region. From ∼4800 to 4000 cal yr BP, including the 4.2 ka event, a prominent increase in the δ18OH. inflatus IVF values suggest an enhanced intrusion of the Arabian Sea High Saline Water (ASHSW) to the relatively deeper part of the mixed layer. In the same interval, the low representations of G. bulloides and thermocline dwelling planktic foraminifera indicate weakening of the coastal upwelling and a reduction in surface productivity due to weak ISM across the Arabian Sea, with more severe conditions in the EAS.

High-resolution climate change during the Marine Isotope Stage 3 revealed by Zhouqu loess in the eastern margin of the Tibetan Plateau

A consensus has not yet been reached on effects of climate change and driving mechanisms between the Tibetan Plateau (TP) and adjacent monsoonal areas during the Marine Isotope Stage 3 (MIS 3). Loess–paleosol sequences from the TP provide valuable information about the MIS 3 environmental history. Detailed color index and a diffuse reflectance spectral (DRS) analysis of Zhouqu (ZQ) loess from the Western Qinling Mountains were conducted to investigate climate change on the eastern margin of the TP during the MIS 3. Our results show that the variations in color index and iron oxide content in ZQ loess are mainly caused by the pedogenesis and climate conditions. The lightness (L*) value and hematite (Hm) content were used to reconstruct the precipitation history and temperature changes, respectively. The reconstructed records revealed that climate change during the MIS 3 was characterized by high frequency and large amplitude, with millennial-scale changes superimposed on orbital-scale changes. Warm–humid climate occurred in the late MIS 3, while the early climate of MIS 3 was relatively cold–dry. The Indian summer monsoon (ISM) and temperature variations during the MIS 3 mainly occurred due to obliquity and precession. The North Atlantic cooling led to the southward movement of the Intertropical Convergence Zone, and the downstream cooling of the atmosphere by the westerly jet could result in events on a millennial-scale in the eastern margin of the TP. The interhemispheric forcing may play an important role in driving the strong summer monsoon in the late MIS 3.

Natural and anthropogenic controls on environmental change during the Holocene based on a multi-proxy record obtained from subalpine peatland in southern China

The interactions between past climate, human activity and environmental change in subtropical mountainous areas are poorly understood due to the lack of reliable records in South China. In this study, the evolution of the East Asian summer monsoon (EASM) during the Holocene, and the interactions between regional human activity and environmental change, were studied using multi-proxy records from a subalpine peat core recovered from South China. The chronology of this peat core has been well-constrained by 10 AMS 14C dates of peat stems. A series of proxy indicators, including carbon isotopes (δ13C), loss on ignition (LOI), magnetic susceptibility (MS), the chemical index of alteration (CIA), and geochemical elements from the Shiwangutian (SWGT) peatland were used to reconstruct the palaeohydrological changes during the Holocene. Regional moisture levels showed a generally arid–wet–arid pattern, and three phases of climatic change were detected as follows. 1) Between 11,600 and 9000 cal yr BP, the EASM was weak and a relatively dry climate developed. 2) Between 9000 and 4000 cal yr BP, the prevalence of humid climatic conditions was associated with a strong summer monsoon. 3) After 4000 cal yr BP, the climate shifted to relatively dry conditions. Further comparisons and analysis suggested that solar insolation, migration of the Intertropical Convergence Zone (ITCZ), and El Niño-Southern Oscillation (ENSO) activity played an important role in determining the variations in Holocene EASM intensity. In addition, the increase in both MS and heavy metal concentrations over the last 1000 years is consistent with an increase in the population of Hunan Province. Therefore, it can be inferred that population growth and the associated expansion of cropland and mining led to an increase in soil erosion and metal tool use. These findings suggest that the impact of human activity generally outweighed the natural climatic controls on the environment and landscape in the mountainous region of southern China over the last 1000 years.

Vegetation response to climate change and human activity in southwestern China since the Last Glacial Maximum

Southwestern China, a biodiversity hotspot, underwent dramatic climate change following the Last Glacial Maximum and intensive human activity also exerted a major impact since the late Holocene. In this paper, we report a pollen record from Yilong Lake in Yunnan Province, southwestern China, to understand vegetation response to long-term climatic variation and human activities at a regional-level over the last 27,000 years. Findings show that temperate and coniferous taxa were dominant before 18 cal kyr BP reflecting the glacial climate. Between 18 and 14 cal kyr BP, thermophilic taxa increased in response to global deglaciation. A notable shift from deciduous to evergreen forests occurred at approximately 14 cal kyr BP, after which pollen concentration increased but pollen diversity decreased. Pollen assemblages indicated dense forests, primarily composed of evergreen oaks developed under warm and strong summer monsoon conditions from 13 to 8 cal kyr BP. High rates of vegetation change occurred during the middle Holocene before vegetation diversity gradually increased during the Holocene Thermal Optimum, with high variability of precipitation in the Yilong Lake region. Anthropogenic forcing progressively became the main factor affecting vegetation composition and vegetation change rates over the past three millennia.

Late Quaternary evolution of the Belan River Basin, Central India

The Marginal Gangetic Plain is the southernmost segment of the Ganga Plain, formed by Himalayan Orogen thrust fold stress flexing the Indian lithosphere. Rivers originating in the Himalayas have received much attention (i.e., Ganga, Yamuna, Kosi, Ghagra, etc.). In contrast, the tributaries draining from the peripheral bulge of the foreland basin have not been studied in detail, although it bears signatures of seismic activity in the basement of the craton. The Belan River, a tributary of the Tons River and sub-tributary of the Ganga River, drains through the Marginal Gangetic Plain. Various tectonic-induced geomorphic signatures have been identified using Landsat Images, such as fluvial incision, river shifting, gorges channel formation, etc., in the Belan River Basin. Geomorphic indices of active tectonics (asymmetry factor, SL index, basin elongation ratio, and long profile), drainage pattern, and lineament study have been performed for the Belan River Basin. These parameters indicate slope adjustment due to the uplift and subsidence of the parts of the basin area. The most recent and extensive valley fills started accumulating at ∼51 ± 7 ka and persisted until around ∼22 ± 2 ka. The aggradation was accompanied by a weakening summer monsoon, followed by incision after 22 ± 2 ka, indicating a return to warmer and wetter conditions and a stronger summer monsoon after the Last Glacial Maximum (LGM). Tectonic exhumation in the southern and south-eastern parts of the Belan River Basin revealed by geomorphic indices. These findings indicate that the drainage networks of the Belan River Basin were built over a tectonically controlled valley and that denudational processes have been transforming the basin for a considerable period.

Pollen-based quantitative paleoclimatic record spanning the Mid-Brunhes Event in the Nihewan Basin, north China

The Middle Pleistocene is an important period of global climatic and environmental evolution during the Quaternary. We conducted pollen analyses of 144 fossil samples and combined 1436 modern surface samples to characterize the processes of vegetation succession and climate change in the Nihewan Basin during the Middle Pleistocene, especially the Mid-Brunhes Event. The results showed that the pollen assemblages during 0.8–0.3 Ma were dominated by herbaceous pollen (> 60%), indicating that the vegetation in the study area was mainly forest steppe. The quantitative climatic reconstruction indicates that annual average temperature (Tann) was generally >6 °C, and annual average precipitation (Pann) was generally >490 mm. The warm climatic conditions persisted for a long interval, and the interval of cold conditions was shorter. Compared with the warm period, during the interval of cold climate Tann decreased by ∼4 °C, and Pann decreased by ∼130 mm. The interval of 0.478–0.424 Ma, Picea dominated the pollen assemblages (> 80%), Tann was generally <2 °C, and Pann was generally <400 mm, which may be a manifestation of the Mid-Brunhes event in the Nihewan Basin, which is corresponded with MIS 12. During this period, Northern Hemisphere insolation and the atmospheric CO2 concentration were low, the Eurasian ice sheet expanded, the winter monsoon strengthened, and the regional climate was cold and dry. The intervals of 0.742–0.478 Ma and 0.424–0.266 Ma were two pronounced warm periods in the Nihewan Basin, which correspond well with MIS 13 and MIS 11, respectively. MIS 13 was the warmest and wettest period during the studied interval, and it may have been related to the unusually strong summer monsoon caused by the climatic asymmetry between the Northern and Southern Hemispheres.

Sea Surface Salinity Anomaly in the Bay of Bengal during the 2010 Extremely Negative IOD Event

Based on Soil Moisture and Ocean Salinity (SMOS) data and the Ocean Reanalysis System 5 (ORAS5) dataset, positive salinity anomalies exceeding 2 psu in the northern Bay of Bengal (BoB) and negative salinity anomalies with the peak of the freshening anomalies reaching −2 psu around Sri Lanka were observed in autumn 2010. Here, an analysis of the anomalous salt budget revealed that anomalous horizontal advection contributed most to the variability in salinity in the BoB. With the development of La Niña and negative Indian Ocean dipole (nIOD) in summer and autumn, the strong summer monsoon current and Wyrtki jet combined with the anomalous basin-scale cyclonic circulation led to more high-salinity water entering the northern BoB. In addition, more freshwater was transported southward along the eastern coast of India by east Indian coastal current (EICC) in autumn, resulting in extremely negative salinity anomalies around Sri Lanka and positive salinity anomalies in the northern BoB. Moreover, the freshwater around Sri Lanka was carried farther into the southeastern Arabian Sea by the west Indian coastal current (WICC) in November, which affected the salinity stratification in winter and then influenced the variation of the Arabian Sea Mini Warm Pool (ASMWP) in the following spring. The ASMWP could affect the Indian summer monsoon (ISM) through its influence on the monsoon onset vortex (MOV) over the southeast Arabian Sea (SEAS).

Late Miocene Tarim desert wetting linked with eccentricity minimum and East Asian monsoon weakening

Periodic wetting is an inherent feature of many monsoon marginal region deserts. Previous studies consistently demonstrate desert wetting during times of Earth’s high orbital eccentricity and strong summer monsoon. Here we report the first evidence demonstrating desert wetting during Earth’s low orbital eccentricity from the late Miocene strata of the northwestern Tarim Basin of northern China, which is commonly thought to be beyond the range of Asian monsoon precipitation. Using mechanisms for modern Tarim wetting as analogs, we propose that East Asian summer monsoon weakening enhanced westward moisture transport and caused opposite desert wetting pattern to that observed in monsoon marginal region deserts. This inference is supported by our model simulations. This result has far-reaching implications for understanding environmental variations in non-monsoonal deserts in the next few thousands of years under high atmospheric CO2 content and low eccentricity.

Modulation of Effective Precipitation by Temperature in the East Asian Monsoon Margins During Marine Isotope Stage 5

AbstractProjecting future changes in effective precipitation under global warming, which requires constraints on both the precipitation and evapotranspiration, is still challenging in the semi‐arid region of East Asia. Here we present proxies, based on Sr/Ca and 87Sr/86Sr of soil carbonate, to reconstruct the evolution of both effective precipitation and temperature from a high‐resolution loess profile in the East Asian monsoon marginal area covering the last interglacial period as a geological analog for future conditions. We find that intervals with a stronger summer monsoon are characterized by lower effective precipitation during Marine Isotope Stage (MIS) 5 due to a larger increase in evapotranspiration than in monsoonal precipitation in response to higher temperature. The greater response of the effective precipitation to temperature than summer monsoon intensity during MIS 5 thus suggests the risk of further aridification in the semi‐arid region of East Asia under climate warming.

Impacts of the 2019 Strong IOD and Monsoon Events on Indian Ocean Sea Surface Salinity

The impact of the 2019 super positive Indian Ocean Dipole (PIOD) event, the strongest in the last four decades, which also co-occurred with an El Niño and a strong summer monsoon, on Indian Ocean sea surface salinity (SSS) is examined using the Soil Moisture Active Passive satellite measurements. Salt budget estimation suggests a predominant, nearly ocean-wide influence by surface freshwater flux and horizontal advective terms. Subsurface ocean influence on the salt budget occurs mainly in the southeastern tropical Indian Ocean (SETIO). The PIOD event suppressed the influence of the El Niño, thereby causing anomalous high precipitation in western India, and leading to an unusual freshening in the southeastern Arabian Sea (AS), which is subsequently advected towards the equatorial Indian Ocean (EIO). In the western Bay of Bengal (BoB), following the waning of monsoon-influenced precipitation in the fall, SSS becomes anomalously salty and traverses towards the AS against the flow of anomalous surface currents. During the peak of the summer monsoon in August–September and the peak of the PIOD event in September–November, SSS in the EIO exhibited tendency for freshening, mainly driven by westward advection of freshwater from the eastern BoB. Conversely, in the SETIO, there was tendency for salinification due to suppression of precipitation, enhanced upwelling of high subsurface salinity, and northward advection of salty water. During December to January of the following year, these salinity tendencies reversed, with salinification in the EIO and freshening in the SETIO.

Monsoon and Tropical Climate Forcing on the Physicochemical and Thermocline Characteristics of the Maldives Inner Sea: Insights From Marine Isotope Stages 1–2 and 10–13

AbstractThe Maldivian archipelago, in the equatorial Indian Ocean, provides a unique location to assess long‐term basin‐wide South Asian Monsoon (SAM) processes as well as its response during climatic extremes. This insight is beneficial to better understand future SAM influences on the Maldives Inner Sea physicochemical characteristics and its diverse tropical ecosystems in a warming world. This study uses samples from International Ocean Discovery Program Expedition 359, drilled within the Inner Sea drift deposits. Multiple foraminiferal species (n = 15) and proxies (δ18O, δ13C, and Mg/Ca) are used to assess glacial‐interglacial SAM dynamics influencing the Inner Sea conditions across Marine Isotope Stages (MIS) 1–2 and 10–13. The summer SAM is modulated by insolation and atmospheric CO2 and has a predominant impact on the northern Indian Ocean surface salinity. As with present‐day observations, a strong summer monsoon resulted in large basin‐wide δ18Osw (salinity) gradients during the interglacials. Moreover, at the MIS11 minimum (MIS11c), a recognized analog for the present‐day, sea surface temperatures (SSTs) were warmer than the present with a stronger summer SAM. This led to an expanded surface mixed layer and strong thermocline, resulting in a highly stratified water column and prominent oxygen minimum zone in the Inner Sea during MIS11c. SSTs in the Maldives are projected to increase at the end of this century and based on the reality that current warming (anthropogenically driven) is much faster than seen during MIS11, the Maldivian tropical coral reef and benthic shoal ecosystems will be subject to increasing stress.

Impacts of Cumulus Parameterization Schemes on the Simulation of Interannual and Interdecadal Variation of Meiyu in the Middle and Lower Reaches of the Yangtze River by Using Regional Climate Model 4

This study has revealed the impacts of cumulus parameterization scheme (CPS) on the simulation of interannual and interdecadal variations of Meiyu in the middle and lower reaches of Yangtze River valley (YangtzeMeiyu) by using a regional climate model. A multiple 55-yr (1955-2009) simulation of YangtzeMeiyu is conducted using RegCM4.6 with different CPSs, including Emanuel scheme, Kuo scheme and Grell scheme. It is found that all the CPSs have good performances and strong convergence in simulating the interannual variation of YangtzeMeiyu rainfall amounts. However, large bias are found in the simulated interdecadal variation of YangtzeMeiyu rainfall amount by the three CPSs. Because the Emanuel CPS has good performance in the simulation of both interannual and interdecadal variations of YangtzeMeiyu, its overall performance in the simulation of total amount of YangtzeMeiyu is the best among the three CPSs. The second best CPS is the Grell scheme, and the worst is the Kuo scheme. The model convergence in simulating the distribution of YangtzeMeiyu shows an obvious characteristic of interdecadal variation. During the years with strong summer monsoon and northward rain belt, the simulated distributions of YangtzeMeiyu by the three CPSs are quite different, and the model convergence is weak. On the contrary, during the years with weak summer monsoon and southward rain belt, the simulated distributions of YangtzeMeiyu by the three CPSs are similar with each other, and the model convergence is strong. As a result, when the well-known interdecadal change happens in the late 1970s, the monsoon changes from strong to weak, whereas the model convergence changes from weak to strong.

Impacts of extremely asymmetrical polar ice sheets on the East Asian summer monsoon during the MIS-13 interglacial

Marine Isotope Stage (MIS) 13 (∼500 ka ago) was a relatively weak interglacial according to the benthic δ18O and Antarctic ice core records. However, proxy records from the Northern Hemisphere indicate that MIS-13 was at least as warm as or even warmer than more recent interglacials, with an extremely strong summer monsoon and possible melting of the Greenland ice sheet. In this study, assuming an asymmetric hemispheric climate, we investigate the response of the East Asian summer monsoon to various Greenland and Antarctic ice sheet sizes using a set of sensitivity experiments with the HadCM3 model through factor separation analysis. Results show that with no Greenland ice sheet and a large Antarctic ice sheet there is an enhancement of summer monsoon precipitation in the East Asian monsoon region of up to ∼7%, in agreement with the direction of change from proxy reconstructions for China. This enhancement is associated with a stronger land–sea thermal contrast, more water vapor transport to East Asia and a northward shift of the Intertropical Convergence Zone. The response to the Greenland ice sheet removal is related to thermodynamic and topographical effects that influence the East Asian monsoon through an orographically induced wave train. The response to the larger Antarctic ice sheet involves stronger upwelling of circumpolar deep water in the Southern Ocean melts the sea ice, leading to a warmer SST. This warming propagates to the North, affecting the East Asian summer monsoon.

Holocene pedostratigraphic records from the southern Chinese Loess Plateau and their implications for the effects of climate on human civilization

The relationship between Holocene climate change and the evolution of civilizations, both in China and elsewhere, is of increasing research interest. Here we present the results of a study of the particle size, CaCO3 content, magnetic susceptibility, and soil microstructure of the Holocene paleosol and loess sequence at the sites of Luochuan and Lintong in the southern Chinese Loess Plateau. The results reveal that the paleosol in the late stage of the mid-Holocene is less developed than that in the early stage, and that the corresponding average annual precipitation was at least 100 mm higher than today, due to the enhancement of the summer monsoon. Climatic deterioration occurred in the middle stage of the mid-Holocene, during 6000–5000 yr BP, which was the coldest and driest stage. Four stages of monsoon change occurred in the study area during the Holocene: at 8500, 6000, 5000, and 3100 yr BP. They enable the following climatic subdivision of this interval: three cold and dry periods with enhanced winter monsoon and dust storm activity, and two warm humid periods with a strong summer monsoon and weak dust storm activity. The transformation of the monsoon climate since 3100 yr BP indicates that the modern interglacial period, compared with the last interglacial, is not typical and possesses some of the characteristics of cold and dry climates. The observed pattern of climate change since the mid-Holocene has substantially affected the development of civilization in the region. This is manifested by the frequent migration of ancient capitals caused by the onset of a cold and dry climate: the location of capitals shifted from high altitudes in the Loess Plateau to loess tableland areas on the lower altitude plains, including to the first terrace in the low-altitude plain and then to the moderately-high second terrace of the Wei River. Our results contribute to an improved understanding of the climatic drivers of human civilization in China and provide a basis for predicting future environmental changes in the region.

10Be and OSL dating of Pleistocene fluvial terraces along the Hongshuiba River: Constraints on tectonic and climatic drivers for fluvial downcutting across the NE Tibetan Plateau margin, China

Accurate geochronologic constraints of fluvial surfaces along tectonically active mountain fronts can help clarify river responses to tectonic uplift and rapid climate change. In this study, we re-constrained abandonment ages of four fluvial terraces along the Hongshuiba River on the northern margin of the Qilian Shan, NE Tibet using 10Be depth profiles to date fluvial deposits, and OSL dating of capping loess. Comparison of chronological ages of the same surface derived from both boulders and depth profiles provides an opportunity to evaluate the suitability of different sampling methods for determining landform ages in arid climate. Correlating terrace ages with regional climate proxies suggests that terraces formed during an interglacial stage or during glacial to interglacial transitions, or during a period of strong summer monsoon or monsoonal weakening. The initial period of fluvial incision may primarily result from climate changes (e.g, monsoon evolution), and other factors that influence the ratio of sediment flux to water discharge. Similarities between fluvial incision rates and incremental tectonic uplift rates suggest a tectonic control on continuous river downcutting before ∼26 ka. In summary, climate forcing drives the initial abandonment of fluvial terraces in this region, while the amplitude of fluvial incision was mainly controlled by progressive tectonic uplift. However, comparison of regional fluvial incision rates (e.g. ∼6mm/a of the Hongshuiba River), exhumation rates from the Qilian Shan (∼0.5mm/a), denudation rates from the river catchments (0.3-0.8mm/a), and vertical slip rates of the northern Qilian Thrust (∼1mm/a) all suggest climate change driven forcing of incision since the Late Pleistocene to Holocene.

Mineralogical and isotopic evidence for the sediment provenance of the western South Yellow Sea since MIS 3 and implications for paleoenvironmental evolution

Changing sediment provenance and related paleoenvironmental evolution since marine isotope stages (MIS) 3 have been investigated based on accelerator mass spectrometry (AMS) 14C and optically stimulated luminescence (OSL) dating of core CSDP-01, recovered from the western South Yellow Sea (SYS). Based on our results, sea level fluctuations and monsoonal climate variations are identified as the principal factors driving these changes. Sea level fluctuations determined the accommodation space in the shelf area, while the monsoonal climate controlled the sediment supply capacity from river to sea. Mineral and SrNd isotopic records reveal the major contribution of the paleo-Changjiang River (Yangtze River) to the study core area during the early-mid MIS 3, which likely defined the northern limit of the paleo-Changjiang delta at that time. High sediment supply and limited accommodation space led to the development of sediment-fed systems on the SYS shelf, resulting in a high preservation potential for thick-stacking and overlapping deltaic sediments. Along with deltaic progradation, core sediment provenance gradually transformed from the paleo-Changjiang River to the paleo-Huanghe River (Yellow River) near the late MIS 3, implying that the paleo-Huanghe River likely emptied into the SYS and built a large delta during the MIS 3 interval. A strong summer monsoon during the early MIS 1 interval was favorable for the Changjiang to deliver abundant sediments to the study core area. A weak summer monsoon during the mid-late MIS 1 when the coastline had retreated to nearly its present-day position decreased the Changjiang and Huanghe supply to the study area. Low sediment supply and sufficient accommodation space gave rise to the development of sediment-starved systems during this period. ɛNd values fluctuated between the Changjiang and Huanghe sources at that time, suggesting that sea bed erosion materials are the main sediment contribution to the study area. In general, sedimentary evolution in the western SYS since MIS 3 is a good example of river-sea interaction in the marginal seas of Southeast Asia, in which sea level and monsoonal climate changes should play an important role.

Late Tortonian – Piacenzian multi-proxy record of Asian southwest monsoon intensification: evidence from Coastal Makran, southeast Iran

This study presents a long-term, multi-proxy reconstruction of the Asian southwest monsoon during the Tortonian to Piacenzian, based on a 4.78 Ma record from Coastal Makran, northwestern Gulf of Oman, southeast Iran. The integration of humidity proxies (clay minerals, Th/K, volume magnetic susceptibility, and grain size analysis), marine redox sensitivity (Th/U), total organic matter, carbonate content, 87Sr/86Sr ratio, and spectral gamma-ray data conducted here provide valuable information that fill the existing gap in marine palaeoclimate records. The results show that a strong winter monsoon condition associated with relatively low precipitation and subsequently low physical and chemical weathering dominated the region during late Tortonian – late Messinian (7.65–5.83 Ma). However, a few episodes of intense physical and chemical weathering related to high precipitation are observed during this period (6.23–6.01 Ma), which is consistent with increased organic matter input from continental reservoirs to the oceans. In addition, the data indicate that from the latest Messinian (5.82–5.33 Ma) to Zanclean–Piacenzian (5.33–2.87 Ma), a strong summer monsoon accompanied by a relatively wetter condition and higher physical and chemical weathering resulted in a high detrital input into the basin. This higher weathering period is associated with the highest rate of Himalayan uplift, causing enhanced precipitation. Wavelet analysis of spectral gamma-ray data revealed notable periodicities at 750 Ka and 1.7 Ma, with significant periodicities centered around 5.75–6.03 Ma over the latest Messinian – Zanclean. Comparison with palaeoclimate records from other sites indicates a teleconnection with respect to precipitation, weathering, and productivity, especially during the Messinian–Zanclean transition.

Cultural evolution and spatial-temporal distribution of archaeological sites from 9.5–2.3 ka BP in the Yan-Liao region, China

With basic information from 8353 archaeological sites, this study describes a holistic spatial-temporal distribution pattern of archaeological sites of the prehistoric culture sequence from 9.5 ka BP (ka BP = thousands of years before 0 BP, where “0 BP” is defined as the year AD 1950) to 2.3 ka BP in the region that extends from the Yanshan Mountains to the Liaohe River Plain (i.e., the Yan-Liao region) in northern China. Based on spatial statistics analysis–including the spatial density of the sites and Geographic Information System nearest- neighbour analysis, combined with a review of environmental and climatic data–this paper analyses cultural evolution, the spatial-temporal features of the archaeological sites and human activities against the backdrop of climatic and environmental changes in this region. The results reveal that prehistoric cultural evolution in the Yan-Liao region is extensively influenced by climatic and environmental changes. The Xinglongwa, Zhaobaogou and Fuhe cultures, which primarily developed during a habitable period from 8.5 ka BP to 6.0 ka BP with strong summer monsoons, have similar maximum density values, spatial patterns and subsistence strategies dominated by hunting-gathering. Significant changes occurred in the Hongshan and Lower Xiajiadian cultures, with a significant increase in numbers and densities of sites and a slump in average nearest-neighbour ratio when the environment began to deteriorate starting in 6.0 ka BP. Additionally, with the onset of a weak summer monsoon and the predominance of primitive agriculture, sites of these two cultures present a different type of concentric circle-shaped pattern in space. As the environment continuously deteriorated with increasing aridity and the spread of steppe, more sites were distributed towards the south, and primitive agriculture was replaced by livestock husbandry in the Upper Xiajiadian culture. The most densely populated areas of the studied cultures are centralized within a limited area. The Laohahe River and Jiaolaihe River basins formed the core area in which most archaeological sites were distributed during the strong summer monsoon period and the first few thousand years of the weak summer monsoon period.

Flexural subsidence analysis of the Laxmi Basin, Arabian Sea and its tectonic implications

AbstractTwo-dimensional flexural backstripping and thermal modelling (assuming laterally variable stretching) is applied along regional depth-converted interpreted seismic profiles from the Laxmi Basin in the Arabian Sea. Results from reverse post-rift flexural modelling reveal considerable basin-wide subsidence in response to the crustal geodynamics during and after the last extensional phase. Unloading of the stratigraphy allows us to estimate the degree of laterally varying extension, assuming thermal subsidence and pure shear. High degrees of extension in the basin centre predict considerable water depths at the time of rift cessation, consistent with deep drilling data. We suggest that regional extension prior to Paleocene time could have fuelled variable subsidence in the Laxmi Basin but that extension is less than seen in typical oceanic lithosphere. Volcanic loading by the seamounts shortly after extension has flexed the basin and implies an effective elastic thickness (Te) at that time of ∼6 km. Reconstruction of the seamount top near sea level at the end of emplacement indicates no major transient uplift potentially linked to the Deccan mantle plume activity. Backstripping of post-rift sediments from interpreted seismic profiles supports the presence of a hyper-thinned crust underneath the Laxmi Basin, with β factors reaching &gt;7 in the basin centre and ∼3 across much of the basin width. Computations of decompacted sediment accumulation rates in light of new results from IODP Expedition 355 show that basin sedimentation peaked during early–middle Miocene time, possibly coeval with uplift and erosion of the Himalayan–Tibetan Plateau driven by strong summer monsoon rains.