Ravi Bhushan

Evidence for solar forcing on the Indian monsoon during the last millennium

Abstract Solar influence on the intensity of the Indian monsoon is demonstrated using a sediment core from the eastern Arabian Sea dating back to 1200 yr, through pattern matching as well as spectral analysis of proxy records of monsoon and solar activity. The intensity of the Indian monsoon is found to have decreased during periods of solar minima during the last millennium. Periodicities of 200±20, 105±15 and 60±10 yr are observed in the proxy records coinciding with those known for solar cycles. The ∼60-yr periodicity observed in the instrumental rainfall data appears to be of solar origin and supports the hypothesis of solar control on the Indian monsoon on a multi-decadal time scale. Evidence for the presence of a coupled atmospheric forcing for the Indian and East African monsoons on a centennial time scale is also seen.

ΔR correction values for the northern Indian Ocean

Apparent marine radiocarbon ages are reported for the northern Indian Ocean region for the pre-nuclear period, based on measurements made in seven mollusk shells collected between 1930 and 1954. The conventional (super 14) C ages of these shells range from 693+ or -44 to 434+ or -51 BP in the Arabian Sea and 511+ or -34 to 408+ or -51 BP in the Bay of Bengal. These ages correspond to mean Delta R correction values of 163+ or -30 yr for the northern Arabian Sea, 11+ or -35 yr for the eastern Bay of Bengal (Andaman Sea) and 32+ or -20 yr for the southern Bay of Bengal. Contrasting reservoir ages for these two basins are most likely due to differences in their thermocline ventilation rates.

Sediment deposition rates on the continental margins of the eastern Arabian Sea using 210Pb, 137Cs and 14C.

Eight gravity cores from the active eastern continental margins of the Arabian Sea were dated using 210Pbxs, 137Cs and 14C. The short-term (< or = 100 years) sedimentation rates range from 0.06 to 0.66 cm/year where as the long-term (> or = 1000 years) ones using AMS 14C on planktonic foraminifera varied from 0.004 to 0.13 cm/year. For long-term chronology (< or = 50,000 years) AMS dating of well-cleaned planktonic foraminifera is most suited.

Radium isotopes and 222Rn in shallow brines, Kharaghoda (India)

The concentrations of Ra isotopes (224Ra, 223Ra, 228Ra and 226Ra) and 222Rn have been determined in brines from Kharaghoda, western India. The Ra isotopes concentrations in these samples are orders of magnitude higher than that reported in potable groundwaters. The 224Ra activities lie between 28 to 277 dpm per kg; about 4% to 96% of that expected from its production, calculated using 222Rn as a recoil flux monitor. The low [224Ra/222Rn] activity ratios are typical of less saline brines. Retardation factors for Ra in these brines, derived from the 222Rn-224Ra-228Ra system and based on a reversible exchange model, are in the range of about 0.3–114. These values are 3–4 orders of magnitude lower than that in potable groundwaters. The residence time of Ra in Kharaghoda brines is approximately one day. Our data suggest that in briny aquifers Ra is considerably less ‘particle-reactive’ than in potable groundwaters. The retardation factor for Ra shows a strong negative correlation with salinity. This anticorrelation suggests that salinity would play a crucial role in determining the mobility of Ra and its geochemical homologues through groundwater systems.

Temporal variations in 87Sr/86Sr and ɛNd in sediments of the southeastern Arabian Sea: Impact of monsoon and surface water circulation

Sr and Nd isotopic composition of silicate fractions of sediments have been measured in two well dated gravity cores from the eastern Arabian Sea archiving a depositional history of ∼29 and ∼40 ka. The 87 Sr/ 86 Sr and e Nd in the northern core (SS-3104G; 12.8°N, 71.7°E) ranges from 0.71416 to 0.71840 and −8.8 to −12.8; these variations are limited compared to those in the southeastern core (SS-3101G; 6.0°N, 74.0°E), in which they vary from 0.71412 to 0.72069 and −9.0 to −15.2 respectively. This suggests that the variation in the relative proportions of sediments supplied from different sources to the core SS-3104G are limited compared to core SS-3101G. The 87 Sr/ 86 Sr and e Nd profiles of SS-3101G exhibit two major excursions, ca. 9 ka and 20 ka, coinciding with periods of Holocene Intensified Monsoon Phase (IMP) and the Last Glacial Maximum (LGM) respectively with more radiogenic 87 Sr/ 86 Sr and lower e Nd during these periods. These excursions have been explained in terms of changes in the erosion patterns in the source regions and surface circulation of the Northern Indian Ocean resulting from monsoon intensity variations. The intensification of North-East (NE) monsoon and associated strengthening of the East Indian Coastal Current in southwest direction during LGM transported sediments with higher 87 Sr/ 86 Sr and lower e Nd from the western Bay of Bengal to the Arabian Sea. In contrast, enhanced South-West (SW) monsoon at ∼9 ka facilitated the transport of sediments from the northern Arabian Sea, particularly Indus derived, to the southeastern Arabian Sea. This study thus highlights the impact of monsoon variability on erosion patterns and ocean surface currents on the dispersal of sediments in determining the Sr and Nd isotopic composition of sediments deposited in the eastern Arabian Sea during the last ∼40 ka.

Distribution of natural and man-made radionuclides during the reoccupation of GEOSECS stations 413 and 416 in the Arabian Sea: temporal changes

Measurement of natural and man-made 14 C and 3 H and man-made 90 Sr, 239,240 Pu and 241 Am at two GEOSECS stations, GX 413 and 416 reoccupied two decades later are reported. Deeper penetration of nutrient and oxygen-poor intermediate waters along with man-made radionuclides, especially at GX 413 (near Bab el Mandab), are noticeable. The one-dimensional advection-diffusion model using pre-nuclear 14 C data supports this observation as the vertical advection velocity at GX 413 is 11.8 m/y compared to the 3.6 m/y in the central north Arabian Sea (GX 416).

Stable (δ 13 C and δ 15 N) Isotopes and Magnetic Susceptibility Record of Late Holocene Climate Change from a Lake Profile of the Northeast Himalaya

This study aims at reconstructing paleovegetation history in the northeast Himalaya during the past 2700 yrs using a 100 cm long sedimentary section selected near Anini village, Dibang valley, Arunachal Pradesh (India). Stable carbon and nitrogen isotopic composition (δ13C and δ15N); and total organic carbon (TOC) and total nitrogen (TN) content in organic matter as well as magnetic susceptibility were determined. Results obtained show a strong correlation between TOC-TN indicating similar source for organic matter. δ13C and TOC/TN ratio throughout the stratigraphic unit seems to be poorly correlated which suggests that the primary carbon isotope signatures are preserved in the sedimentary environment. Overall, δ13C values of the organic matter in the section vary in the range from–23.9 to–21.2 ‰ which is typical of mixed C3-C4 plants.. A distinguishable increasing trend in δ13C during 2700 to 1300 yrs indicates change in vegetation pattern from C3 to C4 plants due to significant climate change from wetter to drier conditions. After 1300 yrs δ13C values decrease rapidly by 1.3‰ and suggest increasing abundance of C3 plants in wetter climatic condition. The relatively small variation in δ13C values in the upper part of the section during 1200 yrs to present suggest a stable climatic condition.

Paleoproductivity variations in the equatorial arabian sea : Implications for east african and indian summer rainfalls and the el nino frequency

We analyzed a sediment core from the equatorial Arabian Sea, chronologically constrained by accurate accelerator mass spectrometry (AMS) radiocarbon dates on selected planktonic foraminiferal species, for paleoproductivity variations corresponding to the variations in the Indian Ocean Equatorial Westerlies (IEW). The IEW in turn are positively correlated to the Southern Oscillation Index (SOI), which is a measure of El Nio, Southwest monsoon (SWM), and east African rainfall (EAR). The productivity data show that Indian and east African rainfalls declined from 35,000 calendar yr BP up to the last glacial maximum (LGM), with the maximum El Nio frequency during the last glacial period. From ~14,500 to ~2000 calendar yr BP (i.e. core top), we find strengthening SWM and EAR along with declining El Nio frequency.

8000-year monsoonal record from Himalaya revealing reinforcement of tropical and global climate systems since mid-Holocene

We provide the first continuous Indian Summer Monsoon (ISM) climate record for the higher Himalayas (Kedarnath, India) by analyzing a 14C-dated peat sequence covering the last ~8000 years, with ~50 years temporal resolution. The ISM variability inferred using various proxies reveal striking similarity with the Greenland ice core (GISP2) temperature record and rapid denitrification changes recorded in the sediments off Peru. The Kedarnath record provides compelling evidence for a reorganization of the global climate system taking place at ~5.5 ka BP possibly after sea level stabilization and the advent of inter-annual climate variability governed by the modern ENSO phenomenon. The ISM record also captures warm-wet and cold-dry conditions during the Medieval Climate Anomaly and Little Ice Age, respectively.

Primary surface rupture of the 1950 Tibet-Assam great earthquake along the eastern Himalayan front, India

The pattern of strain accumulation and its release during earthquakes along the eastern Himalayan syntaxis is unclear due to its structural complexity and lack of primary surface signatures associated with large-to-great earthquakes. This led to a consensus that these earthquakes occurred on blind faults. Toward understanding this issue, palaeoseismic trenching was conducted across a ~3.1 m high fault scarp preserved along the mountain front at Pasighat (95.33°E, 28.07°N). Multi-proxy radiometric dating employed to the stratigraphic units and detrital charcoals obtained from the trench exposures provide chronological constraint on the discovered palaeoearthquake surface rupture clearly suggesting that the 15th August, 1950 Tibet-Assam earthquake (Mw ~ 8.6) did break the eastern Himalayan front producing a co-seismic slip of 5.5 ± 0.7 meters. This study corroborates the first instance in using post-bomb radiogenic isotopes to help identify an earthquake rupture.