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International Ocean Discovery Program Expedition 355 drilled Sites U1456 and U1457 in Laxmi Basin (eastern Arabian Sea) to document the impact of the South Asian monsoon on weathering and erosion of the Himalaya. We revised the chronostratigraphic framework for these sites using a combination of biostratigraphy, magnetostratigraphy and strontium isotope stratigraphy. The sedimentary section at the two sites is similar and we divided it into six units bounded by unconformities or emplaced as a mass-transport deposit (MTD). Unit 1 underlies the MTD, and is of early–middle Miocene age at Site U1456 and early Paleocene age at Site U1457. An unconformity (U1) created by emplacement of the MTD (unit 2) during the late Miocene Epoch (at c. 9.83–9.69 Ma) separates units 1 and 2 and is identified by a marked change in lithology. Unit 3 consists of hemipelagic sediment with thin interbeds of graded sandstone of late Miocene age, separated from unit 4 by a second unconformity (U2) of 0.5–0.9 Myr duration. Unit 4 consists of upper Miocene interbedded mudstone and sandstone and hemipelagic chalk deposited between c. 8 and 6 Ma. A c. 1.4–1.6 Myr hiatus (U3) encompasses the Miocene–Pliocene boundary and separates unit 4 from unit 5. Unit 5 includes upper Pliocene – lower Pleistocene siliciclastic sediment that is separated from unit 6 by a c. 0.45 Myr hiatus (U4) in the lower Pleistocene sediments. Unit 6 includes a thick package of rapidly deposited Pleistocene sand and mud overlain by predominantly hemipelagic sediment deposited since c. 1.2 Ma. [ABSTRACT FROM AUTHOR]

The Indian summer monsoon (ISM) is an important conveyor in the ocean-atmosphere coupled system on a trans-regional scale. Here we present a study of a sediment core from the northern Oman margin, revealing early to mid- Holocene ISM conditions on a near-20-year resolution. We assess multiple independent proxies indicative of sea surface temperatures (SSTs) during the upwelling season together with bottom-water conditions. We use geochemical parameters, transfer functions of planktic foraminiferal assemblages and Mg= Ca palaeothermometry, and find evidence corroborating previous studies showing that upwelling intensity varies significantly in coherence with solar sunspot cycles. The dominant80-90-year Gleissberg cycle apparently also affected bottom-water oxygen conditions. Although the interval from 8.4 to 5.8 ka BP is relatively short, the gradually decreasing trend in summer monsoon conditions was interrupted by short events of intensified ISM conditions. Results from both independent SST proxies are linked to phases of weaker oxygen minimum zone (OMZ) conditions and enhanced carbonate preservation. This indicates that atmospheric forcing was intimately linked to bottom-water properties and state of the OMZ on decadal timescales. [ABSTRACT FROM AUTHOR]

Abstract: The Australian–Indonesian monsoon has a governing influence on the agricultural practices and livelihood in the highly populated islands of Indonesia. However, little is known about the factors that have influenced past monsoon activity in southern Indonesia. Here, we present a ∼6000 years high-resolution record of Australian-Indonesian summer monsoon (AISM) rainfall variations based on bulk sediment element analysis in a sediment archive retrieved offshore northwest Sumba Island (Indonesia). The record suggests lower riverine detrital supply and hence weaker AISM rainfall between 6000 yr BP and ∼3000 yr BP compared to the Late Holocene. We find a distinct shift in terrigenous sediment supply at around 2800 yr BP indicating a reorganization of the AISM from a drier Mid Holocene to a wetter Late Holocene in southern Indonesia. The abrupt increase in rainfall at around 2800 yr BP coincides with a grand solar minimum. An increase in southern Indonesian rainfall in response to a solar minimum is consistent with climate model simulations that provide a possible explanation of the underlying mechanism responsible for the monsoonal shift. We conclude that variations in solar activity play a significant role in monsoonal rainfall variability at multi-decadal and longer timescales. The combined effect of orbital and solar forcing explains important details in the temporal evolution of AISM rainfall during the last 6000 years. By contrast, we find neither evidence for volcanic forcing of AISM variability nor for a control by long-term variations in the El Niño-Southern Oscillation (ENSO). [Copyright &y& Elsevier]

The response of the tropical climate in the Indian Ocean realm to abrupt climate change events in the North Atlantic Ocean is contentious. Repositioning of the intertropical convergence zone is thought to have been responsible for changes in tropical hydroclimate during North Atlantic cold spells, but the dearth of high-resolution records outside the monsoon realm in the Indian Ocean precludes a full understanding of this remote relationship and its underlying mechanisms. Here we show that slowdowns of the Atlantic meridional overturning circulation during Heinrich stadials and the Younger Dryas stadial affected the tropical Indian Ocean hydroclimate through changes to the Hadley circulation including a southward shift in the rising branch (the intertropical convergence zone) and an overall weakening over the southern Indian Ocean. Our results are based on new, high-resolution sea surface temperature and seawater oxygen isotope records of well-dated sedimentary archives from the tropical eastern Indian Ocean for the past 45,000 years, combined with climate model simulations of Atlantic circulation slowdown under Marine Isotope Stages 2 and 3 boundary conditions. Similar conditions in the east and west of the basin rule out a zonal dipole structure as the dominant forcing of the tropical Indian Ocean hydroclimate of millennial-scale events. Results from our simulations and proxy data suggest dry conditions in the northern Indian Ocean realm and wet and warm conditions in the southern realm during North Atlantic cold spells. [ABSTRACT FROM AUTHOR]

Abstract: We use new and published data to discuss the East Asian winter monsoon (EAWM) development and the relationship between the EAWM and the East Asian summer monsoon (EASM) since the Last Glacial Maximum (LGM) with special emphasis on the Holocene. The depth of the mixed layer in the northern South China Sea (SCS) is strongly modulated by the seasonal reversing monsoon winds with deeper vertical mixing during the winter monsoon season and less mixing during the summer monsoon season. Reconstructing the relative depth of the mixed layer thus represents a sensitive diagnostic variable of past changes in the EAWM strength. We derived estimates of the vertical thermal structure from the difference (∆T) in Mg/Ca-based temperatures between surface- (Globigerinoides ruber) and thermocline-dwelling (Pulleniatina obliquiloculata) planktonic foraminifera, and the temperature difference between alkenone and P. obliquiloculata in core MD05-2904 located in the northern SCS. Our ∆T estimates suggest enhanced upper water column mixing due to strengthened EAWM winds during the last glacial, consistent with other marine and terrestrial records as well as model simulations that also suggest a dominance of the cold and dry northwesterly winter monsoon system during the last glacial. Our ∆T estimates reveal that the EAWM was weaker during the Holocene compared to the last glacial. We find that EAWM and EASM changes show an inverse behaviour during the last glacial that is consistent with previous studies but co-varied during the Holocene. It can be inferred from these findings that the variability of the East Asian monsoon during the Holocene is more complex than the simple anti-phase relationship between summer and winter monsoon as assumed previously. The early-mid Holocene strengthening of both the EAWM and the EASM is most likely attributed to the higher summer insolation that resulted in a stronger seasonal contrast. [Copyright &y& Elsevier]

Abstract: Millennial-scale variations of the East Asian winter monsoon (EAWM) remain elusive due to sparse and controversial reconstructions. By compiling a variety of alkenone-based sea surface temperature (SST) estimates, we find that the west–east SST gradient in the southern South China Sea (SCS) well documents the temporal dynamics of the winter “cold tongue” off the southern Vietnam and by inference, variations in the EAWM intensity over the past 26ka. Our results reveal that the winter “cold tongue” SSTs were significantly colder during Heinrich event 1 and the Younger Dryas event, resulting in an increased west–east SST gradient in the southern SCS due to a strengthened EAWM. Within dating uncertainties, an intensified EAWM during cold stadials was coeval with the shutdown or a reduction in strength of the Atlantic meridional overturning circulation (AMOC), exhibiting a strong linkage between the AMOC and the EAWM system. The west–east SST gradient also indicates an enhanced EAWM during the early Holocene, which may be induced by postglacial ice-sheet dynamics and a strong seasonal contrast in solar insolation. Our findings suggest that the EAWM was probably modulated by a complex interplay between the AMOC, solar insolation and ice-sheet dynamics on sub-orbital time scales. [ABSTRACT FROM AUTHOR]

Abstract: The monsoon system represents one of the basic elements of global atmospheric circulation. Its evolution and variability over long periods of geologic time play a significant role in our understanding of global climate. In this study, we focus on the Late Miocene interval from 10Ma to 6Ma, a period of postulated profound ecological and environmental shifts in East and South Asia. The combined approach of measuring planktonic foraminiferal Mg/Ca ratios and stable oxygen isotopes from Ocean Drilling Program (ODP) Site 1146 enabled us to reconstruct temperature independent seawater δ 18O (i.e. proxy for sea surface salinity) variations in order to reconstruct the hydrography in the northern South China Sea. Located offshore the Pearl (Zhujiang) River, or its predecessor, the location of ODP Site 1146 is considered to provide a most sensitive record for detecting potential changes in freshwater input/river run-off as a result of changes in continental humidity, and hence changes in East Asian summer monsoon (EASM) climate. Local seawater δ 18O reconstructions reveal that monsoon development during the Late Miocene can be regarded as an abrupt weakening in the EASM around 7.5Ma. We suggest that an EASM weakening was most likely the driving force for decreasing aridity in East and South Asia at 8–6Ma, leading to widespread ecosystem changes in East and South Asia. [Copyright &y& Elsevier]

Global atmospheric circulation experienced drastic changes during the Middle Miocene climate transition (MMCT∼14.7–13.0 Ma) possibly related to the glaciation on West-Antarctica. Palaeoceanographic reconstructions have found that upwelling in the western Arabian Sea due to summer South Asian monsoon (SAM) winds likely occurred since ∼14.7 Ma, with fully modern-like monsoonal wind patterns after the end of the MMCT at around ∼13 Ma. Whether the changes in monsoonal circulation since ∼14.7 Ma are also associated with upper ocean hydrographic changes in the eastern Arabian Sea (EAS) is currently not known for the middle Miocene. To this end, the difference in Mg/Ca-based temperatures (ΔT°C) of surface-dwelling with lower mixed layer/thermocline-dwelling and sub-thermocline-dwelling planktic foraminifera was reconstructed to estimate the upper ocean thermal gradient at Site NGHP-01-01 A in the EAS and thus changes in the upper surface water column structure, i.e., mixed layer depth (MLD) and the depth of thermocline (DOT). The Mg/Ca-temperature estimates from the upper mixed layer down to the sub-thermocline show a prominent cooling trend between ∼14.2 Ma and 13.2 Ma. The upper water column reconstructions reveal weaker mixing, a shallower thermocline, and therefore a well stratified upper water column in the EAS after ∼14.2 Ma. We suggest that the weaker mixing and shallower thermocline in the EAS after ∼14.2 Ma are most likely due to an intensification of the summer SAM. A strong salinity stratification and/or the formation of a barrier layer (BL) because of increased SAM rainfall and advection of low salinity water may also have contributed to a weaker mixing and shallower thermocline in the EAS during the investigated time period. The change in the upper ocean hydrography in the EAS after ∼14.2 Ma fits well into the emerging picture of monsoonal-driven upper ocean hydrographic changes in the equatorial and northern Indian Ocean due to an intensification of the summer SAM since ∼14.7 Ma. • Cooling of the mixed layer and sub-thermocline in the eastern Arabian Sea (EAS) between ∼14.2 Ma and 13.2 Ma • Weaker mixing, a shallower thermocline, and a well stratified upper water column occurred in the EAS after ∼14.2 Ma • A shallower thermocline in the EAS after ∼14.2 Ma is most likely due to an intensification of the summer SAM • A strong salinity stratification due to stronger summer SAM rainfall may also have contributed to a shallower thermocline • Results support monsoonal-driven hydrographic changes in the Arabian Sea due to an intensified summer SAM since ∼14.7 Ma [ABSTRACT FROM AUTHOR]

Abstract: The Late Pliocene is thought to be characterized by the simultaneous intensification of both the East Asian winter monsoon (EAWM) and East Asian summer monsoon (EASM). However, the evolution of the EASM during the Pliocene remains still controversial and only little is known about the dynamics of the EASM during the Pliocene on orbital time scales. Here we use clay mineral assemblages in sediments from Ocean Drilling Program (ODP) Site 1143 in the southern South China Sea (SCS) to obtain proxy records of past changes in the EASM climate during the Pliocene. Provenance analysis suggests that illite, chlorite and kaolinite originated mainly from the Mekong River drainage area. Smectite was derived mainly from the Indonesian islands. The kaolinite/illite ratio and the chemical index of alteration (CIA) of siliciclastic sediments allowed us to reconstruct the history of chemical weathering and physical erosion of the Mekong River drainage area and thus, the evolution of the EASM during the Pliocene. Our clay minerals proxy data suggests a stronger EASM during the Early Pliocene than during the Late Pliocene. We propose that the long-term evolution of the EASM has been driven by global cooling rather than the uplift of the Tibetan Plateau. Spectral analysis of kaolinite/illite ratio displays a set of strong periodicities at 100ka, 30ka, 28ka, 25ka, and 22ka, with no clear obliquity-related signal. Our study suggests that the Pliocene EASM intensity on orbital time scales is not only controlled by the Northern Hemisphere summer insolation, but also strongly influenced by equatorial Pacific ENSO-like ocean–atmosphere dynamics. [Copyright &y& Elsevier]