Weak Summer Monsoon Research Articles

Agricultural adaptation and resilience through climatic shifts in semi-arid India: 2000 years of archaeobotanical evidence from Vadnagar, Gujarat

The relationship between historical climate change and past agricultural production contributes to a better understanding of the impacts of projected climate change by providing empirical data for resilient human responses. This study explores the periods of dynastic transitions and crop production at the urban site of Vadnagar, in semi-arid northwest India through several climate events, generally characterised by weakening summer monsoon precipitation during the Late Holocene. Artefacts from the site present an unbroken sequence of seven successive cultures from the first century BCE to the nineteenth century CE. Archaeobotanical data indicate the sufficient water availability during the Historic and Medieval periods, allowing crop production dominated large-grained cereals (C3 plants). However, during the Post-Medieval period (ca.1300−1850 CE) a resilient crop economy based on small-grained cereals (C4 plants) dominated, representing a human adaptation to prolonged weakening of monsoonal precipitation. Isotopic and phytolith data at the site present a clear signal of changing local environmental conditions over two millennia, consistent with regional palaeoclimate records, providing and interpretive context for agricultural evidence at Vadnagar. Despite long-term reduction in summer humidity, we argue that an adaptable agricultural package coupled with suitable water management systems allowed for the resilience of the urban settlement at Vadnagar.

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.

Different responses of precipitation patterns to the East Asian summer monsoon weakening: The 7.2 and 8.2 ka events

The 8.2 ka abrupt cooling event has been extensively studied, whereas the 7.2 ka event and their comparations rarely mentioned. Here, we present a high resolution (∼4 years) and precisely dated (±25 years) speleothem multi-proxy record from Zhangjia Cave (northern margin of Sichuan basin, China) spanning the 7.2 and 8.2 ka events within the transition of early to middle Holocene. The speleothem δ18O mainly reflects the dynamic facets of the East Asian summer monsoon (EASM) system, while its δ13C and trace elements ratios serve as an indicator of local hydrological conditions. Our results reveal intense dry conditions in southwest China during the 7.2 ka event, despite the 7.2 event was characterized by a relatively stronger monsoon compared to the 8.2 ka event. Furthermore, the hydrological conditions during the 7.2 ka event and the middle Holocene appear to exhibit “north flood and south drought” pattern when compared to different paleoclimate records in EASM domains, while it can be summarized as “spatially consistent drought” in the 8.2 ka event. These different hydrological conditions are a response to the weakening of the EASM, a phenomenon also observed in modern climatic observations and empirical simulations, which underpins our understanding that the speleothem δ18O variations are mainly dynamical manifestations of the EASM system, likely different with its thermodynamical appearances.

Projections of mean and extreme precipitation over China and their resolution dependence in the HighResMIP experiments

Projected near-future changes in mean and extreme precipitation by the end of the mid-21st century over China are investigated using the latest CMIP6 HighResMIP simulations grouped by different categories of horizontal and vertical resolutions. Evaluation of the historical simulations demonstrates improvements in the realism of the simulated precipitation over the different sub-regions with increased model resolutions. More apparent improvements are displayed by models with increased vertical resolution than those with increased horizontal resolutions. The near-future scenario of SSP5–8.5 suggests general increases in mean and extreme precipitation across Northwest and East-Northeast China. Also, models with coarser horizontal resolutions project an intensified precipitation seasonality over China. Models at relatively high vertical resolutions project decreased precipitation and enhanced droughts over Southwest China, while these are less apparent in models with relatively coarse vertical resolutions. All the simulations project a northward shift of summer and winter monsoon flows in East Asia. Meanwhile, a weaker summer monsoon across the tropics with a weakening of the monsoon trough over the South China Sea are projected. This indicates a less favorable environment for precipitation across the southern sub-regions. The complicated responses of precipitation to these environmental mechanisms are partly linked to the presented resolution-dependent uncertainties. Such uncertainties should be recognized and treated with caution in understanding the near-future hydrological changes in China.

Coral Paleoclimate Perspectives Support the Role of Low‐Latitude Forcing on the 4.2 ka BP Event

AbstractThe 4.2 ka BP Event is an abrupt climate change that might have contributed to the collapse of ancient civilizations and marks the transition between the mid‐ and late‐Holocene. Despite considerable research on this event, our understanding remains primarily based on terrestrial paleoclimate reconstructions, leaving a significant gap in understanding the role of the ocean in this event. Here, we present paired sea surface temperature (SST) and seawater δ18O reconstructions based on four fossil corals from the South China Sea. Our results demonstrate that the climate during the event was cooler, and there were meridional dry‐wet patterns in East Asia, indicating a weakened summer monsoon. Furthermore, our examination of additional coral records from the Pacific and Indian Oceans suggests that low‐latitude forcing (i.e., SST anomalies in the tropical Pacific) plays a crucial role in driving hydrology shifts in East Asia over the 4.2 ka BP interval.

Surface and thermocline ocean circulation intensity changes in the western Arabian Sea during ∼172 kyr

The relationship between monsoon-induced productivity and ocean circulation over millennial timescales is crucial. However, a study regarding the circulation of the western Arabian Sea (WAS) needs to be conducted. Understanding the seasonal, surface, and thermocline ocean circulation throughout the glacial-Interglacial period in the WAS is also elusive and essential. The long distant Antarctic Intermediate Water (AAIW) flow, and Sub-antarctic Mode Water (SAMW) penetration to the north side of the equator in the Indian Ocean needs to be understood and underlined using the micropaleontological (planktonic foraminiferal assemblages) and geochemical analysis such as oxygen isotope of Globigerinoides ruber and Neogloboquadrina dutertrei (δ18OG.ruber and δ18ON.dutertrei) in this study. Furthermore, understanding the variations in paleoproductivity, upper oceanic water column stratification, surface mixing, and thermocline layer variability in the WAS is also helpful. According to the results, the variation in the upwelling intensity during Glacial-Interglacial and monsoon variation suggest the upper water stratification/vertical mixing in the WAS. Here, we provide new evidence of poorly ventilated water mass in our region. Our results indicate that enhanced northward expansion of AAIW coincides with the weakening of summer monsoon during glacial periods except mid-MIS4, reflected by isotopic study and assemblages. In this context, depleted δ18ON. dutertrei value and high concentration of Globorotalia menardii (G. menardii) were due to lack/absence of AAIW ventilation and SAMW; therefore, warming occurred in the mid-MIS4.

Seasonal variations and controlling factors of speleothem multi-proxy in southeastern China: Implications for the reconstruction of precipitation seasonality

Precipitation in southeastern China exhibits strong seasonal variability, which significantly impacts local agricultural production and social development. However, the reconstruction of precipitation seasonality has been limited by the lack of precisely dated high-resolution paleoclimatic records. This study presents seasonal-scale multi-proxy records (trace elements: Mg/Ca, Sr/Ca Ba/Ca and stable isotopes: δ18O and δ13C) of a modern (1810–2009 AD) annually laminated stalagmite (EM1) from E’Mei Cave, Jiangxi Province in southeastern China. Comparative analysis of observation data with meteorological and simulation data shows that the seasonal variation of EM1 δ18O, derived from precipitation δ18O, is mainly controlled by the large-scale circulation and precipitation seasonality on an interannual timescale. The seasonal EM1 δ18O variation is controlled by the seasonal precipitation δ18O, however, the most negative values of EM1 δ18O are higher than those of simulated calcite δ18O and the average amplitude of the seasonal variation of EM1 δ18O (∼1.93‰) is much smaller than that of simulated calcite δ18O (∼9.72‰) because of evaporation and mixing of waters in the epikarst system. On the decadal timescales, variations of δ13C, Mg/Ca, Sr/Ca, and Ba/Ca were found to be strongly correlated, consistent with the variation of the local dry/flood index, indicating a common influencing factor of local hydroclimate change. However, their seasonal phase relationships vary between strong and weak summer monsoon conditions. We find a strong covariation between Mg/Ca and δ13C with Sr/Ca and Ba/Ca during the strong monsoon period (1951–1976), and they are in antiphase with seasonal δ18O variation. However, the seasonal variations of Sr/Ca and Ba/Ca transformed to dominantly antiphase with Mg/Ca and δ13C during the weak monsoon period (1977–1991 AD). Therefore, we suggest that Mg/Ca and δ13C are dominantly controlled by the local hydroclimate changes on seasonal timescales but Sr/Ca and Ba/Ca might have been affected by complex processes in the epikarst under different hydrothermal configuration conditions. The findings indicate the potential of the variation of the seasonal phase relationships between multi-proxy records in reconstructing precipitation seasonality changes under different hydrothermal backgrounds.

Arctic climate-Indian monsoon teleconnection during the last millennium revealed through geochemical proxies from an Arctic fjord

Some of the most visible signs of climate change can be found in the northern high latitude region. Due to dynamic glacial-marine contrast, sedimentary geochemical proxies from western Svalbard fjords may provide a detailed account of paleoenvironmental changes in the region. We constructed a multivariate sedimentary record from a high-Arctic fjord (Kongsfjorden) using the sediment core NP14-FM. The new multi-proxy record includes sedimentary organic matter (OM) concentrations and their carbon and nitrogen isotopic composition, covering a period between AD 1106 and AD 1967. Using an unsupervised learning technique, the multivariate data was partitioned into two groups, helping us identify occurrences of warm and cold Arctic spells at multi-decadal resolution during the last millennium. Individually, the δ15N variability forms a highly coherent pattern with surface temperature records while δ13C and elemental OM concentrations reveal changing mixing proportions between marine and terrestrial sources. Furthermore, the region's interchanging warm and cold episodes, as detected by the multivariate sedimentary record, were linked to stronger and weaker summer monsoon over India, respectively. Modulation of meridional thermal gradients over the Indian monsoon domain amid contrasting climatic conditions in the Arctic region likely facilitated the observed Arctic-Indian monsoon teleconnection during the last millennium.

Karst-ecological changes during the middle and late Holocene in Southwest China revealed by δ18O and δ13C records in a stalagmite

Rocky desertification is a major environmental problem in the karst regions of Southwest China and seriously impacts ecological and socioeconomic development. Human activities and climate change have been proposed as the main factors responsible for the development and expansion of rocky desertification, but more evidence is needed to support this hypothesis. In this study, the history of the Asian summer monsoon (ASM) since 6200 yr BP is reconstructed based on 230Th dating and δ18O and δ13C data of a stalagmite from Feilong Cave in Guizhou Province of Southwest China. Positive δ18O and δ13C excursions record eleven interdecadal–centennial weak summer monsoon events at 1000–870, 1660–1460, 2040–1900, 2780–2460, 3020–2860, 3250–3120, 3650–3370, 4273–3980, 4820–4480, 5370–5180, and 5840–5560 yr BP. These weak ASM events were controlled by a southward shift of the Intertropical Convergence Zone and frequent El Niño events. Changes in the δ13C values of this stalagmite were mainly driven by changes in vegetation coverage and type, soil microbial activity, and regional hydrogeological conditions. The simultaneous positive excursions of δ18O and δ13C at ∼5500, 4200, and 2800 yr BP indicate a degradation of the karst ecological environment in response to the weak summer monsoon. In addition, frequent dynasty changes in China since 3000 yr BP profoundly influenced the karst ecological environment. During the Medieval Warm Period, the Jing-Kang event (JK event) resulted in the fall of the Northern Song Dynasty and led to a large-scale migration into southern China. As a consequence, the population increase in the Yangtze River basin enhanced land use, accelerated the disappearance of forests, and finally resulted in the deterioration of the regional environment. We conclude that climate change and anthropogenic activity may have jointly triggered and/or exacerbated rocky desertification in Southwest China.

Enhanced Impact of Vegetation on Evapotranspiration in the Northern Drought-Prone Belt of China

Evapotranspiration (ET) is an essential component of the land–atmosphere water cycle. In this work, the trend of ET and its dominant factors during 1982 to 2011 are investigated in the northern drought-prone belt of China (NDPB) based on five datasets, including the gridded FLUXNET, using the Pearson correlation and linear regression methods. Specially, we focus on the increasing contribution of vegetation in the change of ET. During 1982–2011, summer ET significantly increased at the rate of 0.33 mm/year (p < 0.05) in the NDPB. However, similar to global-mean ET, the ET in NDPB also experienced a pronounced fluctuation during 1999 and 2002. The role of water supply differed remarkably before and after the fluctuation while the atmospheric demand maintained weak constraint on ET. Before the fluctuation (during 1982–2000), ET correlated significantly (p < 0.01) and positively with soil moisture, indicating ET was primarily limited by water supply. However, their correlation weakened remarkably after the fluctuation when soil moisture decreased to the lowest level for the past thirty years, indicating that neither moisture supply nor atmospheric demand dominated the ET during this period. In contrast, vegetation leaf area index (LAI) maintained consistent significant (p < 0.01) and positive correlation with ET before and after the fluctuation in the NDPB, and it reflected over 60% of the change in ET. Moreover, the LAI in NDPB increased by 19.6% which was more than double of the global-mean increase. The ET increase due to rising LAI offset the ET decrease due to reduction of soil moisture, and vegetation became the primary constraint on ET during 2001–2011. The expansion of vegetation may intensify the risk of drought and cause conflicting demands for water between the ecosystem and humans in the NDPB, especially in the case of weak summer monsoon.

Hydroclimate seasonality from paired coral Sr/Ca and δ18O records of Kikai Island, Southern Japan: Evidence of East Asian monsoon during mid-to late Holocene

The East Asian Monsoon (EAM) modulates the seasonal variability of sea surface temperature (SST), sea surface salinity (SSS), and rainfall in the region. However, seasonal-scale reconstructions showing the hydroclimate response to monsoon intensity shifts in Holocene are limited. We used paired coral Sr/Ca and oxygen isotope (δ18Ocoral) records from modern (1989–2015), 5.7 ka, 4.9 ka, and 3.2 ka corals from Kikai Island, Southern Japan to reconstruct past seasonal variability of hydroclimatic variables. The monthly δ18Ocoral-SSS relationship revealed a statistically significant positive linear correlation (r = 0.63; p < 0.000; n = 310) while the δ18Osw and rainfall showed a statistically significant negative linear correlation (r = 0.55; p < 0.000; n = 93).The large SST and SSS seasonality in 5.7 ka (ΔSSTmax-min = 9.5 °C; Δδ18Omax-min = 1.86‰) and 4.9 ka (ΔSSTmax-min = 10.2 °C; Δδ18Omax-min = 1.45‰) indicate an enhanced seasonal ocean warming which amplified the Western Pacific Subtropical High (WPSH) and the southwesterlies. In contrast, the small SST amplitude in 3.2 ka (ΔSSTmax-min = 7.8 °C; Δδ18Omax-min = 0.98‰) indicates a weakened WPSH and southwesterlies which led to reduced summer rainfall and positive SSS signal. The lower salinity records due to higher rainfall in 7.0 ka, 6.1 ka, 5.7 ka, and 4.9 ka coincide with the strong East Asian summer monsoon (EASM) from 8.2 to 4.7 ka in stacked sediment records. The δ18Ocoral record shows positive SSS driven by weakened summer monsoon rainfall in 4.4 ka, 4.2 ka, 3.8 ka, 3.4 ka, and 3.2 ka.The higher winter SSS (positive δ18Ocoral) and lower winter SST derived from 5.7 ka coral in Kikai Island agree with the strong East Asian winter monsoon (EAWM) intensity from 8.2 to 5.6 ka based on the stacked sediment record. The weak EAWM phase from 5.6 ka to 4.5 ka is consistent with the winter records of warm SST and lower SSS in 4.9 ka. The lower winter SSS and warmer winter SST in 3.2 ka support the evidence of a weak phase around 3.0 ka. The extratropical systems such as the intensified or weakened Siberian High and Aleutian Low have affected the northeasterly EAWM and consequently influenced the variability of winter SST and SSS in subtropical southern Japan in mid- and late Holocene.

Late quaternary clay mineral and grain-size records from northwest Australia and their implications for paleoclimate, ocean currents, and paleodrainage of the Bonaparte basin

High-resolution clay mineral assemblages and siliciclastic grain-size records were collected from the International Ocean Discovery Program (IODP) high sedimentation Site U1483 off the Scott Plateau of the northwest Australian margin and studied to identify the clay mineral sources, transport processes, and ocean current behavior, as well as to evaluate the Australian summer monsoon over the last 410 kyr. Clay minerals are primarily composed of smectite (41–70%), followed by kaolinite (10–28%), illite (13.5–25%), and trace chlorite (3–14%). The provenance study revealed that the primary sources of clay minerals at Site U1483 are the Victoria and Ord rivers in northwest Australia. Smectite is mainly derived from the mafic volcanic and smectite-rich Bonaparte Gulf, whereas kaolinite and illite are primarily derived from felsic igneous and metamorphic rocks, respectively, found in the drainage areas of these rivers. Chlorite is primarily contributed by the Indonesian Throughflow (ITF), with a minor contribution from the northwest Australian rivers. Variations in the clay mineral assemblages and grain-size records indicate strong glacial–interglacial cyclicity, with small grain sizes, high smectite content, and low kaolinite and illite content during glacial periods. During interglacial periods, grain size, kaolinite, and illite percentages increase. Smectite is the dominant clay mineral during both the glacial and interglacial periods. (Kaolinite + illite + chlorite)/smectite and kaolinite/smectite ratios are adopted as proxies for the Leeuwin Current strength and the Australian summer monsoon, respectively. During glacial periods, low kaolinite/smectite and (kaolinite + illite + chlorite)/smectite ratios indicate a weak summer monsoon, resulting in low river discharge and a weak Leeuwin Current, which can transport small smectite-sized sediments to the core location. High kaolinite/smectite and (kaolinite + illite + chlorite)/smectite ratios during interglacial periods correspond to intense Australian monsoons and a strong ITF and Leeuwin Current, which contributed relatively more kaolinite and illite than during glacial periods. The proxy records' spectral analysis indicates a strong eccentricity period of 100-kyr, an obliquity period of 41-kyr, and a precession period of 23-kyr, implying that the clay mineral input along the northwest Australian margin is influenced by both high-latitude ice sheet forcing and low-latitude tropical processes.

Quantitative reconstruction of the relative humidity by a coupled δ18O-δ2H approach during the Younger Dryas in central China

Paleoclimate records in central China reveal asynchronous hydroclimate changes to those from both northern and southern China during the Holocene. Whether such a spatial mode works for the Younger Dryas event (YD), a prominent abrupt climate change during the transition from the last glacial to the Holocene, is poorly constrained by quantitative records. In this study, we apply the novel coupled hydrogen isotope composition of leaf wax n-alkanes and oxygen isotope of hemicellulose (δ2Hn-alkane-δ18Osugar) paleohygrometer approach to reconstruct the paleo relative humidity (RH) on a peat layer spanning 13.1–10.0 cal ka BP in the Dajiuhu peatland, central China. The coupled δ2Hn-alkane-δ18Osugar approach reveals that the RH was relatively low in the period of 12.7–12.1 cal ka BP (RH avg. 79%, 1σ 4.9%) and then turned to relatively high values during 12.1–11.7 cal ka BP (RH avg. 88.6%, 1σ 3.9%). A dry interval happened from the end of the YD to 11.3 cal ka BP (RH< 80%). A general wetting trend during the YD period in central China contrasts with the prevailing drying in northern China and maybe also in southern China. The occurrence of a wetting trend during the YD period in central China probably results from the interplay between the westerlies and the East Asian summer monsoon under a context with weak summer monsoon intensity.

Comprehensive analysis of climate-related comfort in southern China: Climatology, trend, and interannual variations

This study investigated the variations of climate-related comfort in Guangdong Province by using a comprehensive comfort index during 1970–2021. The results showed that (1) the comfortable period (extremely comfortable, comfortable, and relatively comfortable types) is distributed from October to the subsequent April, and the uncomfortable period (relatively uncomfortable and uncomfortable types) starts from May to September. The most dominant climate-related comfort in Guangdong is the relatively uncomfortable type. Spatially, more extremely comfortable and comfortable days appear in southeastern Guangdong. However, more relatively uncomfortable and uncomfortable days appear in southwestern Guangdong. (2) The linear trends of annual uncomfortable days in coastal regions are the most significant, which are attributed to increased surface air temperature (SAT) induced by the weakened summer monsoon. (3) Additionally, the extremely comfortable days in central and eastern Guangdong have negative correlations with the East Asian Winter Monsoon (EAWM). However, the uncomfortable days across Guangdong have positive correlations with the East Asian Summer Monsoon (EASM). The former connection is mainly caused by strong EAWM-induced low SAT, and the latter connection is mainly affected by strong EASM-induced less precipitation. Our results could provide some scientific references for urban planning, improvement of the thermal environment, and resident health risk analyses.

A synthesis of monsoon exploration in the Asian marginal seas

Abstract. The International Ocean Discovery Program (IODP) conducted a series of expeditions between 2013 and 2016 that were designed to address the development of monsoon climate systems in Asia and Australia. Significant progress was made in recovering Neogene sections spanning the region from the Arabian Sea to the Sea of Japan and southward to western Australia. High recovery by advanced piston corer (APC) has provided a host of semi-continuous sections that have been used to examine monsoonal evolution. Use of the half-length APC was successful in sampling sand-rich sediment in Indian Ocean submarine fans. The records show that humidity and seasonality developed diachronously across the region, although most regions show drying since the middle Miocene and especially since ∼ 4 Ma, likely linked to global cooling. A transition from C3 to C4 vegetation often accompanied the drying but may be more linked to global cooling. Western Australia and possibly southern China diverge from the general trend in becoming wetter during the late Miocene, with the Australian monsoon being more affected by the Indonesian Throughflow, while the Asian monsoon is tied more to the rising Himalaya in South Asia and to the Tibetan Plateau in East Asia. The monsoon shows sensitivity to orbital forcing, with many regions having a weaker summer monsoon during times of northern hemispheric Glaciation. Stronger monsoons are associated with faster continental erosion but not weathering intensity, which either shows no trend or a decreasing strength since the middle Miocene in Asia. Marine productivity proxies and terrestrial chemical weathering, erosion, and vegetation proxies are often seen to diverge. Future work on the almost unknown Paleogene is needed, as well as the potential of carbonate platforms as archives of paleoceanographic conditions.

Timing and Structure of 10.9 and 10.3 ka BP Events Revealed by Annually Laminated Stalagmite Records From Shihua Cave, Northern China

AbstractStudies have shown that East Asia experienced several abrupt centennial‐scale weak monsoonal events in the early Holocene. However, the timing and structure of these climate events remain unclear due to the lack of precisely dated and highly resolved records. In this study, we present a well‐dated high‐resolution East Asia summer monsoon record from 11.32 to 10 ka BP based on two coeval stalagmites, one of which developed clear annual lamina, from Shihua Cave, northern China. Stalagmite δ18O records show 2 weak summer monsoon events at 10.9 and 10.3 ka. The 10.9‐ka event occurred from 10.93 to 10.79 ka BP for ∼140 years and features a “W”‐shaped structure. The 10.3‐ka event, also known as the ice‐rafted debris event 7 in the North Atlantic, lasted ∼260 years with an asymmetric double‐plunging structure. The two events are concurrent with the cold conditions in Greenland ice core record, indicating a close correlation between East Asia summer monsoon and North Atlantic climate. Spectral analyses on Shihua Cave stalagmite δ18O time series and annually lamina thickness sequence both show a statistically significant periodicity of 200 years, indicating that anomalous climate events may be triggered by solar activity. The 10.3‐ka event also coincides with three small freshwater outbursts in the high‐latitude North Atlantic. This agreement suggests that freshwater outbursts may have played a role in reinforcing this weak 10.3‐ka event in monsoonal China.

Late Miocene unroofing of the Inner Lesser Himalaya recorded in the NW Himalaya foreland basin

AbstractTesting models that link climate and solid Earth tectonics in mountain belts requires independent erosional, structural and climatic histories. Two well‐preserved stratigraphic sections of the Himalayan foreland basin are exposed in NW India. The Jawalamukhi (13–5 Ma) and Joginder Nagar sections (21–13 Ma) are dated by magnetostratigraphy and span a period of significant climate change and tectonic evolution. We combine sediment geochemistry, detrital zircon U–Pb dating and apatite fission track analyses to reconstruct changes in the patterns of erosion and exhumation in this area from the Early Miocene to Pliocene. The provenance of the foreland sediments reflects a mixture of Tethyan and Greater Himalayan sources from 21 to 11 Ma, with influx from the Inner Lesser Himalaya starting after 11 Ma, and a strong increase in Crystalline Inner Lesser Himalayan erosion after 8 Ma. This distinct shift in provenance most likely reflects exhumation of the Kullu‐Rampur Window, as well as the northward motion of the Jawalamukhi section towards the Himalayas, drainage reorganization in the foreland, and/or tectonically driven drainage capture in the mountains. Prior to 10.5 Ma sediment came from a large river whose sources were Greater Himalaya and Haimanta dominated, likely a palaeo‐Sutlej, while after 8 Ma the source river was dominated by a more local drainage. Our work is consistent with Nd isotope and mica Ar‐Ar constraints from the same sections that demonstrate initial Inner Lesser Himalayan unroofing in this region from 11 Ma, earlier than the 2 Ma implied from the marine record and during a period of summer monsoon weakening when fission track data indicate very rapid cooling and erosion of the Lesser Himalaya sources from no later than 10 Ma. Tectonically driven rock uplift coupled with southerly migration of the maximum rainfall belt during a time of drying, may have focused erosion over the Lesser Himalayan Duplex and created the Kullu‐Rampur Window.

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.

Delineation of terrestrial and marine productivity in the southwestern continental margin of India

• Terrestrial/marine paleoproductivity proxies show paleosummer monsoon fluctuations. • Pollen is more reliable and coherent climate proxy than marine ones. • Summer monsoon weakened along SW India during Younger Dryas . • Monsoon intensified from early to late Holocene with a break during 6.7–4.0 ka. Although there are numerous investigations on the reconstruction of palaeomonsoon, there are inconsistencies between the terrestrial and marine archives. In this study, we have made an attempt to unravel this problem from a multi-proxy study of a sediment core from the south-west continental margin of India (SWCI). Here, we characterize pollen and dinocyst, and compare it with the marine primary productivity indicators (organic carbon and CaCO 3 ) in an AMS 14 C-dated sediment core collected off the SWCI. The sedimentation rate decreases consistently from late-glacial through early Holocene to the late Holocene (31.16 cm/ka, 24.8 cm/ka and 15.4 cm/ka respectively). The down-core distribution of pollen shows the presence of Poaceae and Cyperaceae without mangroves and other terrestrial pollen around 13.2 ka BP − 11.7 ka BP, suggesting a weakened summer monsoon during the Younger Dryas. Increase in the abundance of mangroves and tropical moist/dry deciduous forest pollen along with the dinocyst during ∼ 11.7 ka to 8.5 ka BP indicates the growth of vegetation in the hinterland as a result of the intensification of monsoon during the early Holocene. This was followed by a maximum marine transgression phase along with a strengthened summer monsoon between ∼ 8.0 ka BP and 6.7 ka BP, as evidenced by the dominance of mangroves and presence of terrestrial pollen taxa. Further, a significant decline of vegetation during 6.7 ka BP to 4.0 ka BP indicates weakened summer monsoon. In contrast to the pollen profiles, however, a significant increase in the mangroves and herbaceous pollen from 4.0 ka BP to 3.2 ka BP suggests a gradual revival of the summer monsoon during the late Holocene. Above mentioned paleoclimatic reconstructions are in contrast to the marine CaCO 3 and organic carbon, as these are lower and fairly constant from Younger Dryas upto 8.0 ka BP, then gradually increased till 6.5 ka BP, and show little variations during the late Holocene. This study suggests that the pollen preserved in the marine sediments are not only reliable indicators, but they are also consistent with other palaeomonsoon proxies measured in archives from the South Asia for the reconstruction of climate.

Indian monsoon variability during the past ∼8.5 cal kyr as recorded in the sediments of the northeastern Arabian Sea

We analyzed benthic and planktic foraminifera, total organic carbon and stable isotopes ratios of carbon and oxygen from 100 samples of marine sediment Core ABP, 25/03, off Gujarat in the northeastern Arabian Sea. Factor and cluster analyses of thirty highest-ranked benthic foraminifer species enabled to identify seven biofacies in the study sequence characterizing major changes in deep-sea environments during the past ∼8.5 cal kyr. The benthic foraminiferal data has been combined with population abundances of upwelling indicator planktic foraminifer Globigerina bulloides and mixed layer species from Core ABP, 25/03 and compared with that from ODP Hole 723A and Core ABP, 25/02, western and eastern Arabian Sea, respectively. A major change in benthic foraminifera occurs during ∼8.5 to 6 cal kyr BP, when the summer monsoon was stronger whereas during 6 to 2 cal kyr BP, the data suggest a weak summer monsoon. Planktic foraminifer Globigerina bulloides does not show any significant trend in the study core, but deep thermocline species Neogloboquadrina dutertrei and mixed layer species show significant trend with an inverse correlation with the TOC. This core records high TOC from ∼6.5 to 4 cal kyr BP and during the last 2 cal kyr and low TOC during ∼8.5 to 6.5 cal kyr BP and 4 to 2 cal kyr BP. It is interesting to note intervals of high mixed layer species coincide with high Neogloboquadrina dutertrei percentages and low TOC values at Core ABP, 25/03 during 8.5 to 6.5 cal kyr BP.The faunal data is supported by carbon and oxygen isotope ratios (δ13C and δ18O) of shells of benthic foraminifer Cibicides spp. to strengthen our understanding of changes in the eastern Arabian Sea since ∼8.5 cal kyr BP.