Indian Summer Monsoon Intensity Research Articles

Unstable relationship between tree-ring δ18O in the transitional zone of the Asian summer monsoon and the Indian summer monsoon

The Asian Summer Monsoon system (ASM) affects a large population in the monsoonal Asia. Previous studies have thoroughly discussed the temporally unstable response of tree-ring oxygen isotope (δ18O) to the regional climate variables and ocean–atmosphere interaction modes, however, the response to the ASM remains ambiguous. In this study, an annually resolved tree-ring δ18O chronology is developed from eastern Yunnan-Guizhou Plateau, in the transitional zone of the ASM. Based on the results from response analyses, the climatic controls of tree-ring δ18O is considered to be the moisture-related variables of the monsoon season. Spatial correlations suggest that in the study area, the water vapor is mainly originated from the Bay of Bengal and transported by the Indian Summer Monsoon (ISM). Furthermore, the temporal stability in relationships between the tree-ring δ18O and the ISM variability are explored. Our findings suggest that on interannual to decadal timescales, the tree-ring δ18O is significantly anti-correlated, but temporally unstable with respect to the ISM. The strength of interannual relationship is affected by the ISM intensity, which is modulated by the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) variability. Moreover, the strength of decadal relationship is associated with the phase shift of the Pacific Decadal Oscillation (PDO). Additionally, the decreasing trend of tree-ring δ18O after the late 1970 s that diverges from the simultaneous normal level of the ISM is caused by the isotopically-depleted trend in upstream precipitation of the moisture source. The opposite patterns of correlations between the tree-ring δ18O and the ENSO and IOD in recent decades demonstrate that the regional hydroclimate in the study region has been undergoing significant secular variations that are characterised by an increasing effect of the IOD and a decreasing effect of the ENSO. The findings from this study further improve our understanding of the tree-ring δ18O response to the ASM, at multi-time scales, and shed light on the impact of the ASM on regional hydroclimate variability, across the transitional zone of the ASM.

Holocene Indian Summer Monsoon variations inferred from end-member modeling of sediment grain size in the Andaman Sea

Whether the Indian Summer Monsoon (ISM) Maximum started in the early Holocene or middle Holocene remains a controversial issue. Grain-size measurements were conducted on 157-cm-long core ADM-C1 from the Andaman Sea; the core spanned the last 11.2 kyr BP. Four end-members (EMs) were unmixed using lognormal parametric end-member modeling. Genetic analyses of the grain-size EMs suggested the EM1 and EM2 may correlate with suspension in the upper layers and transportation in the benthic nepheloid layer, respectively, while the EM3 and EM4 were tied with deposition affected by monsoonal currents and sedimentation under extreme events, respectively. The EM3 was dominated by ISM intensity and transported by summer monsoon currents, with increase in the proportion of EM3 reflecting increasing ISM intensity, and vice versa. Variations in EM3 were used to define three stages of ISM evolution, as follows: (1) During 11.2–9.1 kyr BP, EM3 was relatively low, indicating a weak ISM; (2) during 9.1–4.5 kyr BP, EM3 increased substantially, indicating a strengthened ISM; (3) during 4.5-0 kyr BP, EM3 decreased gradually, suggesting a gradual weakening of the ISM. The Holocene ISM Maximum started at ~9.1 kyr BP rather than in the early Holocene, which may have been partly due to the slowdown of the Atlantic meridional overturning circulation (AMOC) during the early Holocene that resulted in a decreased land-sea thermal contrast between the landmass and the Indian Ocean. An additional cause may have been the remnant Northern Hemisphere ice sheets that impeded the northward shift of the Intertropical Convergence Zone (ITCZ). Seven relatively brief decreases in the ISM intensity (events 7, 6, 5, 4, 3, 2, 1) occurred at ca. 10.3, 9.7, 7.3, 5.7, 4.2, 2.4, and 0.8 kyr BP, respectively. They corresponded, within the age uncertainties, to the increased supplies of ice-rafted detritus to North Atlantic sediments, implying that changes in the ISM intensity on millennial scale were dominated by climatic processes in northern high latitudes.

A 338-year tree-ring oxygen isotope record from Thai teak captures the variations in the Asian summer monsoon system

A 338-year oxygen isotope record from teak tree-ring cellulose collected from Mae Hong Son province in northwestern Thailand was presented. The tree-ring series preserves the isotopic signal of the regional wet season rainfall and relative humidity. Tree-ring δ18O correlates strongly with regional rainfall from May to October, showing coherent variations over large areas in Southeast Asia. We reconstructed the summer monsoon season (May to October) rainfall based on a linear regression model that explained 35.2% of the actual rainfall variance. Additionally, we found that in the 19th century, there was a remarkable drought during many years that corresponded to regional historic drought events. The signals of the June to September Indian summer monsoon (ISM) for the period between 1948 and 2009 were clearly found. Spatial correlations and spectral analyses revealed a strong impact of the El Niño-Southern Oscillation (ENSO) on tree-ring δ18O. However, ENSO influenced the tree-ring δ18O more strongly in the 1870–1906, 1907–1943, and 1944–1980 periods than in the 1981–2015 period, which corresponded to periods of weaker and stronger ISM intensity.

An ~130 kyr Record of Surface Water Temperature and δ18O From the Northern Bay of Bengal: Investigating the Linkage Between Heinrich Events and Weak Monsoon Intervals in Asia

AbstractMillennial‐scale reductions in monsoon precipitation, so‐called Weak Monsoon Intervals (WMIs), have been identified in numerous paleoclimate records across the Afro‐Asian monsoon domain throughout the last glacial‐interglacial cycle. These are considered the regional response to cooling during Heinrich events in the North Atlantic realm and several mechanisms have been suggested to explain this hemisphere‐scale climatic teleconnection. In particular, reductions in Indian Ocean sea surface temperature (SST) have been proposed as the linking element between Heinrich events and WMIs. However, the validity of this relationship has only been demonstrated for the last ~20 kyr, leaving unresolved whether it also holds on longer time scales. Here we present a new paired record of planktonic foraminifera‐based δ18Osw‐ivc and UK'37‐based SST from the northern Bay of Bengal, covering the last ~130 kyr. The δ18Osw‐ivc record clearly reflects orbitally paced changes of Indian Summer Monsoon intensity superimposed by centennial‐ to millennial‐scale WMIs that occurred synchronously to North Atlantic Heinrich events. Comparison with the UK'37‐based SST reconstruction reveals, however, that WMIs in most cases were not paralleled by ocean surface cooling, questioning whether Indian Ocean SST lowering was the linking element between Heinrich events and reductions in monsoon precipitation in Asia also during the last glacial period.

Landscape dynamics and human-environment interactions in the northern foothills of Cho Oyu and Mount Everest (southern Tibet) during the Late Pleistocene and Holocene

Here we present an integrated earth surface process and paleoenvironmental study from the Tingri graben and the archaeological site of Su-re, located on the southern rim of the Tibetan plateau, spanning the past ca. 30 ka. The study area is characterized by cold climate earth surface processes and aridity due to its altitude and location in the rain shadow of the Mount Everest–Cho Oyu massif and is thus sensitive to climatic and anthropogenic perturbations. In this highly dynamic geomorphic environment, paired-cosmogenic nuclide results from boulders on a massive hummocky moraine in the southern Tingri graben reveal complex exposure histories that limit our capability of directly dating the corresponding glacial advance, and shed a note of caution on previously published single-nuclide-based exposure ages along the northern Himalaya. Based on geomorphic considerations, however, the moraine clearly represents the local last glacial maximum, and likely coincided with a ∼344 ± 109 m drepression of discontinuous permafrost zone relative to today during the global last glacial maximum (gLGM). This greatly intensified permafrost and periglacial hillslope processes and led to fluvial aggradation of the valley floors of ≥12 m. We observe formation of a thick (≥50 cm) pedo-complex starting at ca. 6.7 ka before present (BP) and erosional truncation at ca. 3.9 ka BP. Widespread landscape instability and erosion characterize the region subsequent to 3.9 ka and intensifies in the 15th century AD. Several lines of (geo)archaeological evidence, including the presence of pottery sherds, sling-shot projectiles and hammer stones within the sedimentary record, indicate human presence at Su-re since ca. 3.9 ka BP. Our data suggest that in the Su-re-Tingri area climatic conditions were warm and moist enough to allow vegetation expansion and soil formation only from ca. 6.7–3.9 ka, followed by weakening of the Indian summer monsoon (ISM) strength between ca. 4.2 and 3.9 ka, which is a prominent climatic event in the wider Asian monsoon region, and reflected in the investigation area by the 3.9 ka erosional boundary. Merging our Holocene landscape reconstruction with the geoarchaeological evidence, we speculate that the combined effect of Little Ice Age (LIA) cooling and an anthropogenic overuse of the landscape led to climatically induced landscape degradation and ultimately to an anthropogenically triggered ecological collapse in the 15th century. Such a scenario is in-line with regional historical data on declining monastery construction and migration of the ethnic group of the Sherpas.From an earth surface dynamics perspective, we find that transient landscape processes on the southern rim of the Tibetan plateau are strongly linked to millennial scale changes in the ISM intensity and duration. We identify three types of unidirectional non-linear ISM-landscape interactions. Given that the Tibetan plateau is the largest high-altitude landmass on our planet and our limited understanding of several of the key earth surface processes on the plateau, we pinpoint the need for more long-term (Quaternary scale) empirical data particularly on permafrost and periglacial processes and human-environment interactions.

ENSO-related droughts and ISM variations during the last millennium in tropical southwest China

Severe El Nino-Southern Oscillation (ENSO) droughts that occurred in southwest China during the last millennium have exerted important impacts on natural ecosystems and human society. Fossil phytoliths in high-resolution (~ 7 year) fluvial sediments in the monsoon rainforest are studied to reveal the influence of ENSO activity on tropical southwest China. The phytolith records together with multiproxy analysis indicate that three phases of distinctively warm-dry and cool-wet climate pattern have exerted profound influences on the sedimentary environment and tropical monsoon rainforest, which is sensitive to the occurrences of severe droughts and fires. Integrated comparisons demonstrate that climatic variations rebuilt by Ic (temperature proxy) are consistent with the temperature change in the Northern Hemisphere, Indian Summer Monsoon (ISM) variations in the northern Indian Ocean and southwest China, and aridity index (Iph) is linked to ENSO activity in the equatorial Pacific Ocean. At decadal timescale, there is an internal connection between ISM variability and ENSO activity in the tropical southwest China. Due to the modulation of ENSO activity, there is an antiphase relationship between temperature and ISM intensity, with a remarkably dry Medieval Warm Period (MWP), Modern Warming (MW) and a mostly wet Little Ice Age (LIA) during the last millennium. Spectral analysis of the climatic indices (Ic and Iph) indicated that the dominant cycle (~ 88 year) is a significant component of periodic solar activity variations. Our results confirmed that the possible forcing mechanism of such a “warm-dry and cool-wet” climate pattern is related to the coupling of solar periodic variations and ENSO variance spanning the tropical Indian Ocean and the “Indian subcontinent-southwest China”. This is significant of understanding the relationship among droughts, ENSO variance and ISM variation in the tropical southwest China under the background of global warming.

Long-term history of sediment inputs to the eastern Arabian Sea and its implications for the evolution of the Indian summer monsoon since 3.7 Ma

AbstractWe present a new set of clay mineral and grain-size data for the siliciclastic sediment fraction from International Ocean Discovery Program (IODP) Site U1456 located in the eastern Arabian Sea to reconstruct the variabilities in the continental erosion and weathering intensity in the western Himalaya, elucidate the sediment source-to-sink processes and discuss the potential controls underlying these changes since 3.7 Ma. The clay minerals mainly consist of smectite (0–90%, average 44%) and illite (3–90%, average 44%), with chlorite (1–26%, average 7%) and kaolinite (0–19%, average 5%) as minor components. The compositional variations in the clay minerals at IODP Site U1456 suggest four phases of sediment provenance: the Indus River (phase 1, 3.7–3.2 Ma), the Indus River and Deccan Traps (phase 2, 3.2–2.6 Ma), the Indus River (phase 3, 2.6–1.2 Ma) and the Indus River and Deccan Traps (phase 4, 1.2–0 Ma). These provenance changes since 3.7 Ma can be correlated with variations in the Indian summer monsoon intensity. The siliciclastic sediments in the eastern Arabian Sea were mainly derived from the Indus River when the Indian summer monsoon was generally weak. In contrast, when the Indian summer monsoon intensified, the siliciclastic sediment supply from the Deccan Traps increased. In particular, this study shows that the smectite/(illite+chlorite) ratio is a sensitive tool for reconstructing the history of the variation in the Indian summer monsoon intensity over the continents surrounding the Arabian Sea since 3.7 Ma.

Link between climate and catchment erosion in the Himalaya during the late Quaternary

Few studies using geochemistry of sediments from plains and delta of the Himalayan river system have reported contrasting results on the shift in erosion of different lithotectonic units, across the strike of Himalayan orogen, in response to changes in the intensity of Indian summer monsoon (ISM) and glacial cover during the late Quaternary. Here we present records of the 143Nd/144Nd (εNd) and 87Sr/86Sr ratios and δ13C of sediment organic matter (δ13CSOM) in bulk sediment samples from two ~45 m long alluvial cores, collected from a paleo-Sutlej channel (in Sirhind, Punjab) at an upstream connection of a large river (Ghaggar-Hakra) with present-day Sutlej River in NW India. The isotopic variations are compared with the available proxy records that document changes in the ISM and Himalayan glacial extent. Significant covariation in down-core profiles of εNd (−14.1 to −22.2) and 87Sr/86Sr (0.72251–0.79456) reveals changing contributions of sediment derived from the Higher and Lesser Himalaya end-member sources in the catchment. Higher εNd (and lower 87Sr/86Sr) during early marine isotope stage (MIS) 1 and late MIS3 reflects increased contribution from the Higher Himalaya as a result of receding glacial cover as well as intense ISM that penetrates into the interior of the Higher Himalaya during warm periods. The δ13CSOM varies from −28.5‰ to −21.3‰ and shows a 5–7‰ increase from colder MIS2 to warmer early MIS1, implying an increase in C4 plant abundance during the same period in response to intensification of the ISM in the post last glacial maximum period. These variations in sediment provenance and vegetation pattern thus highlight the pronounced influence of climate on distribution of erosion and vegetation types in the NW Himalaya.

Indian summer monsoon variability and vegetation changes in the core monsoon zone, India, during the Holocene: A multiproxy study

We present the results of sediment texture, δ13C values, TOC, TN, TOC/TN, and magnetic susceptibility (χlf) of a 1.54-m deep sediment trench recovered from the core monsoon zone (CMZ) of central India, to understand the paleovegetational history and Indian summer monsoon (ISM) variability during the Holocene. The lower δ13C values, TOC/TN ratio, and magnetic susceptibility (χlf) from ~11.4 to 9.5 ka BP suggest enhanced ISM intensity, which is well correlated with other available ISM records from both terrestrial as well as marine archives. A gradual stepwise expansion of C4 plants during ~8.1 and 6.3 ka BP, ~6.3 to 4.7 ka BP, and ~3.0 to 2.0 ka BP suggests a gradual weakening of ISM. The highest δ13C values (–18.7‰) recorded at ~2.0 ka BP indicate the dominance of C4 plants suggesting the weakest phase of ISM in the study area. The expansion of C3 plants from ~2.0 to 1.6 ka BP indicates a sudden increase in ISM intensity. Subsequently, three stages of enhanced ISM have been recorded between ~1.6 and 0.93 ka BP, ~0.76 and 0.42 ka BP, and ~0.28 ka BP to present.

Drainage geomorphic evolution in response to paleoclimatic changes since 12.8 ka in the eastern Kunlun Mountains, NE Qinghai-Tibetan Plateau

Investigation of the geomorphologic evolution of drainage basins can provide insight into the response of mountain-basin systems to paleoclimate fluctuations. This study investigates the evolution of geomorphic units and their connectivity, from upstream piedmont to downstream salt lake, since the late Quaternary in the Golmud River catchment of the eastern Kunlun Mountains (KLM). The study aims to elucidate the debate over linkages between: (1) landform evolution and paleoclimatic change and (2) dynamic geomorphic processes and lake evolution in the lower basin. Based on geomorphic and sedimentological observations and an optically stimulated luminescence (OSL) chronology, we show three main phases of geomorphic processes: (1) upstream piedmont alluvial fans and an alluvial delta fan at the salt lake margin have surface sediment OSL ages of 12.8–11.4 ka, suggesting a valley aggradation stage in which mass periglacial detritus were eroded and redeposited in the drainage basin; (2) incision terraces in alluvial valley fills provide ages between 11.4 and 8.8, 8.1–6.2, and 4.9–3.9 ka. Incision is ascribed to the reduction in supply of detritus owing to glacial recession and enhanced precipitation in the early to middle Holocene; and (3) widely distributed swamp sediments in the distal delta fan date to 3.2–1.8 ka, suggesting a wetter stage within the late Holocene climatic deterioration. Loess, indicative of aridity, was deposited intermittently during the middle to late Holocene. We reconstructed a wetness index, based on wetness scores of drainage geomorphic units, which is consistent with published moisture proxies. This association implies that (1) landform processes in KLM responded well to precipitation fluctuations in Indian summer monsoon (ISM) dominant regions, indicating their climatic forcing and (2) precipitation variations in the eastern KLM have been mainly controlled by ISM intensity since 12.8 ka. Linking the geomorphologic evolution of mountain-basin systems to downstream arid basin evolution shows that the periods of fluvial terrace incision (early to middle Holocene) were probably important sources of lake salt mineral enrichment. The landscape evolution patterns of mountain-basin systems in the eastern KLM offer a model for assessing the linkages between geomorphic processes, paleoclimatic changes, and lake evolution in other arid regions.

Decreasing Indian summer monsoon on the northern Indian sub-continent during the last 180 years: evidence from five tree-ring cellulose oxygen isotope chronologies

Abstract. We have constructed a regional tree-ring cellulose oxygen isotope (δ18O) record for the northern Indian sub-continent based on two new records from northern India and central Nepal and three published records from northwestern India, western Nepal and Bhutan. The record spans the common interval from 1743 to 2008 CE. Correlation analysis reveals that the record is significantly and negatively correlated with the three regional climatic indices: all India rainfall (AIR; r = −0.5, p < 0.001, n = 138), Indian monsoon index (IMI; r = −0.45, p < 0.001, n = 51) and the intensity of monsoonal circulation (r = −0.42, p < 0.001, n = 51). The close relationship between tree-ring cellulose δ18O and the Indian summer monsoon (ISM) can be explained by oxygen isotope fractionation mechanisms. Our results indicate that the regional tree-ring cellulose δ18O record is suitable for reconstructing high-resolution changes in the ISM. The record exhibits significant interannual and long-term variations. Interannual changes are closely related to the El Niño–Southern Oscillation (ENSO), which indicates that the ISM was affected by ENSO in the past. However, the ISM–ENSO relationship was not consistent over time, and it may be partly modulated by Indian Ocean sea surface temperature (SST). Long-term changes in the regional tree-ring δ18O record indicate a possible trend of weakened ISM intensity since 1820. Decreasing ISM activity is also observed in various high-resolution ISM records from southwest China and Southeast Asia, and may be the result of reduced land–ocean thermal contrasts since 1820 CE.

Variations of the Indian summer monsoon over the last 30 000 years inferred from a pyrogenic carbon record from south‐west China

ABSTRACTThe carbon isotope composition of pyrogenic carbon (δ13CPyC) can be used to identify changes in terrestrial vegetation and consequently climate. We analyzed the δ13CPyC values in a continuous lacustrine sediment core from the south‐east edge of the Qinghai–Tibetan Plateau that spans the last 30k cal a BP, to improve our understanding of changes in regional vegetation and the Indian summer monsoon (ISM) on centennial and millennial timescales. Our data show that a mixed C3/C4 plant community and a dry climate prevailed from 30 to 14.6k cal a BP. Monsoonal precipitation abruptly increased at the glacial–Holocene transition, and C3 plants growing under a humid climate dominated during the Holocene. Our record also reveals that ISM intensity decreased significantly during Heinrich Events and the Younger Dryas Chron. These findings suggest that changes in regional vegetation in south‐west China are primarily controlled by ISM intensity, which in turn is related to solar insolation and high‐latitude climate. This is the first δ13CPyC record covering the last glacial maximum from the region, and provides insight into orbital‐scale climate change and abrupt climate events that occurred in south‐western China's geological past.

Implications of palaeohydrological proxies on the late Holocene Indian Summer Monsoon variability, western India

In this paper we identify changes in the Indian Summer Monsoon (ISM) intensity during the late Holocene as revealed by palaeohydrological proxies in the fluvial archives of western India. Proxy indices, including the geomorphological, sedimentological, geochemical and biological evidence along with chronology have been employed to highlight the trend in ISM intensity over the late Holocene. It is inferred that on a longer timescale, the Indian Summer Monsoon weakened during the late Holocene however; short pulses of ameliorated monsoon occurred around 3, 1.6 and 0.3 cal ka BP. Clustering of flood events is seen to occur at 3–2.8, 2.2–1.6, 1.3–1.1 and 0.65–0.2 cal ka BP not necessarily coinciding with strengthening of the ISM. Palaeoflood data with regard to the ISM variability shows that the two most prevalent flood events at 0.5 and 1.7 cal ka BP coincide with the weak monsoon period/climatic transition in this region. The occurrence of high magnitude flood events during weak monsoon periods is also reflected in the modern flood and rainfall data. It is surmised that high magnitude flood events may represent peak monsoon periods however, cannot be singularly considered as indicators for enhanced monsoon conditions in a region. Only a more robust database of palaeohydrological proxies can further validate their implications on the monsoonal changes over western India.

Post-glacial climate forcing of surface processes in the Ganges–Brahmaputra river basin and implications for carbon sequestration

Climate has been proposed to control both the rate of terrestrial silicate weathering and the export rate of associated sediments and terrestrial organic carbon to river-dominated margins – and thus the rate of sequestration of atmospheric CO2 in the coastal ocean – over glacial–interglacial timescales. Focused on the Ganges–Brahmaputra rivers, this study presents records of post-glacial changes in basin-scale Indian summer monsoon intensity and vegetation composition based on stable hydrogen (δD) and carbon (δ13C) isotopic compositions of terrestrial plant wax compounds preserved in the channel–levee system of the Bengal Fan. It then explores the role of these changes in controlling the provenance and degree of chemical weathering of sediments exported by these rivers, and the potential climate feedbacks through organic-carbon burial in the Bengal Fan. An observed 40‰ shift in δD and a 3–4‰ shift in both bulk organic-carbon and plant-wax δ13C values between the late glacial and mid-Holocene, followed by a return to more intermediate values during the late Holocene, correlates well with regional post-glacial paleoclimate records. Sediment provenance proxies (Sr, Nd isotopic compositions) reveal that these changes likely coincided with a subtle focusing of erosion on the southern flank of the Himalayan range during periods of greater monsoon strength and enhanced sediment discharge. However, grain-size-normalized organic-carbon concentrations in the Bengal Fan remained constant through time, despite order-of-magnitude level changes in catchment-scale monsoon precipitation and enhanced chemical weathering (recorded as a gradual increase in K/Si⁎ and detrital carbonate content, and decrease in H2O+/Si⁎, proxies) throughout the study period. These findings demonstrate a partial decoupling of climate change and silicate weathering during the Holocene and that marine organic-carbon sequestration rates primary reflect rates of physical erosion and sediment export as modulated by climatic changes. Together, these results reveal the magnitude of climate changes within the Ganges–Brahmaputra basin following deglaciation and a closer coupling of monsoon strength with OC burial than with silicate weathering on millennial timescales.

Holocene Indian Summer Monsoon variation inferred from geochemical and grain size records from Lake Ximenglongtan, southwestern China

Identifying Holocene Indian Summer Monsoon (ISM) variability and its possible mechanism is critical for predicting future environmental changes. In this study, we present a well-dated, ca. 9.4kyr geochemical and grain size record derived from Lake Ximenglongtan, southwestern China. The record depicts the Holocene ISM evolution and multi-decadal to centennial abrupt precipitation change events superimposed on the long-term climate changes. The results indicate that precipitation was high during the first half of the Holocene from ca. 9.4 to 4.5calkyr B.P. but decreased over the later Holocene. On the multi-decadal to centennial time scale, our results show that a pronounced and prolonged drought event occurred between 7.6 and 7.0calkyr B.P. In addition, several other low precipitation events occurred at about 8.9, 6.3, 5.2, 4.2, 3.7, 2.5 and 1.7calkyr B.P., which correspond well with reported intervals of abrupt ISM weakening events. Our findings suggest that changes in solar activity may have primarily controlled the fluctuations of ISM intensity on the multi-decadal to centennial time-scale. Cooling of the North Atlantic climate and the interactions between tropical-ocean and atmosphere may have amplified the solar signal.

Sedimentology, geochemistry and OSL dating of the alluvial succession in the northern Gujarat alluvial plain (western India) - A record to evaluate the sensitivity of a semiarid fluvial system to the climatic and tectonic forcing since the late Marine Isotopic Stage 3

The alluvial successions in the northern Gujarat alluvial plain (western India) have been investigated for reconstructing the climatic fluctuations during the last 40ka. Alluvial architecture and geochemical proxies indicate prevalence of a strengthened Indian Summer Monsoon (ISM) with fluctuations between the late Marine Isotopic Stage 3 (MIS 3; 37ka) to the early MIS 2 (27ka). A gradual onset of aridity (declining ISM) after 27ka with peak aridity at ~22ka is observed. A gradual strengthening of ISM at around 18 and >12ka followed by a short reversal in ISM intensity between 12 and 11ka, is attributed to the Younger-Dryas (YD) cooling event. The aeolian sand sheet dated to 6 and 3.5ka represents the onset of regional aridity. Following this, a short-lived humid phase was observed after ~2ka, which includes the Medieval Warm Period (MWP). The study suggests that the variability in the ISM to the latitudinal migration of the Inter Tropical Convergence Zone was caused by insolation-driven cooling and warming events in the North Atlantic.The incision of the valley fill alluvium occurred in two distinct phases. The older incision phase occurred after 11ka and before 6ka, whereas the younger incision phase that led to the development of present day topography is bracketed between 3.5ka and before 1ka. The older incision phase is ascribed to the early to mid-Holocene enhanced ISM (climatically driven), whereas the younger incision seems to be modulated by the activation of basement faults (tectonically driven).

Effects of intensification of the Indian Summer Monsoon on northern Andaman Sea sediments during the past 700 years

Recent studies have revealed an intensification of the Indian Summer Monsoon (ISM) over the past 700 years, but its influence on the sediments in the northern Andaman Sea is unclear. To examine this influence, we used radiocarbon-dated sediment core StMY6, acquired 100 km offshore in the northern Andaman Sea from the Ayeyarwady River mouth, and obtained a 700-year-long record of the grain-size distribution and geochemistry of the sedimentary sequence. The ISM influences precipitation surrounding the Ayeyarwady River, and thus the river water and sediment discharges to the northern Andaman Sea near the river mouth in the Ayeyarwady River and the weathering intensity in the river catchment. Based on the system, we propose that higher sedimentation rates and larger modal grain size (the most abundant size in the grain-size distribution of sediments) and lower carbonate concentrations after about AD 1600 can be attributed to higher ISM intensity. Profiles of total organic carbon (TOC) content and carbon/nitrogen (C/N) ratios, however, did not show any trends after about AD 1600, which suggests that the ISM has had little influence on these proxies at this site.

Impacts of SW Monsoon on Phytoplankton Community Structure Along the Western Coastal BOB: an HPLC Approach

The major Indian rivers bring significant amount of freshwater along with inorganic nutrients and sediment load in to the northern Bay of Bengal (BOB) during the southwest monsoon (SWM); the southern bay does not experience equal freshening. This contrasting pattern may considerably impact the physicochemical features and phytoplankton community composition in this bay and was investigated during a coastal cruise during the SWM covering eight river plumes from both northern and southern bay; phytoplankton pigments and physicochemical parameters were analysed from different depths (0, 10, 25, and 50 m). Significant freshening, stratification and warmer waters were noticed in the northern bay relative to its southern part. Phytoplankton pigment analysis and diagnostic pigment-based size class analysis revealed the dominance of microphytoplankton (mainly diatoms) in the northern bay and were mostly confined to the surface waters. Their abundance was positively correlated with dissolved silicate (DSi) concentrations and inversely with salinity. Nanophytoplankton and picophytoplankton (prymnesiophytes, chrysophytes and cyanophytes) were mostly noticed in the subsurface waters and dominated the southern bay. This finding suggests that the dominance of microphytoplankton in the northern bay may significantly contribute to higher particle flux which has been reported earlier. Therefore, any modification in future river discharge, which is in turn related to the intensity of Indian summer monsoon, will alter the phytoplankton community structure in the coastal BOB and may be further cascaded to the other vital ecosystem components like fisheries resources, organic carbon export flux and benthic production.

Oscillations in the Indian summer monsoon during the Holocene inferred from a stable isotope record from pyrogenic carbon from Lake Chenghai, southwest China

A robust, well-dated record of centennial-scale abrupt changes in the Asian summer monsoon is crucial for understanding the potential forcing factors and their environmental effects. In this study, we analyzed the stable carbon isotopes of pyrogenic carbon (δ13CPC) in a 556-cm long sediment core retrieved from Lake Chenghai in the Yunnan Plateau, China. The results provide a continuous 7660-year precipitation record of the Indian summer monsoon (ISM). They indicate that from ∼7600calyrBP precipitation in the Lake Chenghai catchment gradually increased until 5030calyrBP, and then subsequently decreased in the second half of the Holocene. In addition, at least six centennial-scale droughts occurred at about 7300, 6300, 5500, 3400, 2500 and 500calyrBP. Our findings suggest that ISM intensity is primary controlled by variations in solar irradiance on a centennial time scale. This external forcing may be amplified by North Atlantic cooling events and El Niño-Southern Oscillation activity in the eastern tropical Pacific, which shift the intertropical convergence zone further southwards.

Late-Holocene Indian summer monsoon variability revealed from a 3300-year-long lake sediment record from Nir’pa Co, southeastern Tibet

Sedimentological and geochemical results from Nir’pa Co, an alpine lake on the southeastern Tibetan Plateau, detail late-Holocene Indian summer monsoon (ISM) hydroclimate during the last 3300 years. Constrained by modern calibration, elevated silt and lithics and low sand and clay between 3.3 and 2.4 ka and 1.3 ka and the present indicate two pluvial phases with lake levels near their current overflow elevation. Between 2.4 and 1.3 ka, a sharp increase in sand and corresponding decrease in lithics and silt suggest drier conditions and lower lake levels at Nir’pa Co. Hydroclimate expressions in the sedimentological proxies during the Medieval Climate Anomaly (MCA) and ‘Little Ice Age’ (LIA) are not statistically significant, suggesting that these events were minor compared to the millennial scale variability on which they were superimposed. However, decreasing sand and increasing lithics and silt during the MCA between 950 and 800 cal. yr BP may suggest briefly wetter conditions, while increasing sand and reduced lithics and silt from 500 to 200 cal. yr BP suggest potentially drier conditions during the LIA. Similarities with regional records from lake sediment and ice cores and speleothem records from the central and eastern Tibetan Plateau, India, and the Arabian Sea, suggest generally coherent late-Holocene ISM variability in these regions. Increased late-Holocene ISM intensity occurred during times when Tibetan Plateau surface air temperatures were warmer, Indo-Pacific sea surface temperatures were elevated, and the tropical Pacific was in a La Niña–like mean state. Conversely, aridity between 2.4 and 1.3 ka occurred in concert with cooling on the Tibetan Plateau and in the Indo-Pacific with more El Niño–like conditions in the tropical Pacific. Differences with western Tibetan records may reflect a weakened ISM and stronger westerlies in this region during the late-Holocene.