B. L. K. Somayajulu

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.

High resolution Holocene monsoon record from the eastern Arabian Sea

Through stable oxygen and carbon analyses of rapidly accumulating sediment cores from the eastern Arabian Sea, we show that the excess of evaporation over precipitation (E−P) steadily appears to have decreased during the last ∼10 000 to ∼2000 years, most probably due to an increasing trend in the summer monsoon rainfall, contrary to the land-based paleoclimatic data from this region, which indicate onset of aridity around 4000 years ago. Our results are consistent with the hypothesis that significant spatial variability in the monsoon rainfall observed today was persistent during most of the Holocene. Alternatively, the trend can be seen as an adjustment between two phases, one between ∼10 000 and ∼6000 years ago of increasing precipitation and another between 3500 and 2000 years ago after the arid episode. We also report a significant ∼700 year periodicity, similar to that reported recently from the South China Sea, indicating that the centennial/millennial scale response of the Indian and Chinese monsoons to high latitude forcing may be alike.

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.

Late-quaternary biogenic productivity and organic carbon deposition in the eastern Arabian sea

Abstract Paleoproductivity variations in the eastern Arabian Sea, during the late Quaternary period (∼42 ka BP to present), have been studied using accumulation rates of sedimentary biogenic components: organic carbon (Corg), nitrogen (N), CaCO3, Sr and Ba. Such a multi-proxy approach reveals decreased surface productivity during the last glacial–interglacial transition. The observed change of surface water productivity during the last glacial–interglacial period is in antiphase to those observed in other low- and mid-latitude upwelling areas, however, consistent with some of the upwelling regions like NW Africa and NW Mexico. Sedimentary Corg and N are found to be decoupled from surface productivity trend, with significant enrichments in Corg and N during the Last Glacial Maximum (LGM). This is interpreted in terms of increase in sedimentation rates (by a factor of 3–4) resulting in the better preservation of Corg during the LGM.

Recent sedimentary records from the Arabian sea

An attempt is made to understand the redox conditions that prevailed in the north eastern continental margins of the Arabian Sea and in the nearby deep water regions during the past few centuries using short undisturbed sediment cores. The geochronology is accomplished using210Pb excess method and the proxy indicators chosen for productivity and associated redox changes are CaCO3, organic matter (OM), Mn and U along with major elements Fe and Al. Such changes in principle are related to high productivity in the overlying waters which in turn depend on monsoonal intensity that causes upwelling responsible for increase in productivity. Alongwith the published data on gravity cores from the same region, our measurements suggest the following:At ∼ 300 m water depth, south of 21°N, the sediment-water interface at depths of ∼ 300 m had been anoxic during the time span represented by the presently studied cores for approximately ∼ 700y as evidenced by low Mn/Al (< 0.7 × 10−2) and high U/Al (> 10−4) weight ratios. In some adjacent deeper regions, however, the environment turned oxic around ∼ 200 y BP. Whereas both Mn and Ra were lost to the overlying waters in the anoxic regions (depth ∼340m), the Mn that diffused from deeper sections appears to have mineralized at the sediment-water-interface. Studies of this type on long undisturbed cores from the margins of the Arabian Sea and the Bay of Bengal, involving several proxies and geochronology by more than one method are needed to understand short term environmental (and monsoonal intensity) changes of the recent past with high resolution.

10Be annual fallout in rains in India

The 10Be concentrations of annual rainfall collections during 1979–1981, at eight stations in India, ranged from 0.43 × 107 to 8.48 × 107 atoms/l and the corresponding 10Be fallouts are in the range of 0.31 × 10 6 to 2.73 × 106 atoms cm−2 a−1. The estimated 10Be global fallout based on the presently available data is 1.55 × 107 atoms−2 a−1 or 5 × 10−2 atoms cm−2 s−1. Most of the measured rates of fallout and deep sea deposition of 10Be are a factor of 2–3 lower than the present estimate.

GEOSECS Atlantic32Si profiles

Abstract Measurements of five cosmogenic 32 Si vertical profiles in Atlantic waters (27°N to 60°S) are presented. The amounts of dissolved SiO 2 extracted range from 2 to 54 g; the amounts of water from which SiO 2 was extracted range between 540 kg and 270, 000 kg. In additon, SiO 2 recovered from four surface particulate composites (64°N to 61°S) were also analyzed for 32 Si. 32 Si measurements were made by milking and counting the daughter activity, 32 P. The net 32 P activities range from 0.7 to 6.8 cph; typical errors in measurements of the 32 P activities are 20–30%. The 32 Si concentrations vary from 0.6 dpm/10 6 kg of water in the North Atlantic surface waters to 235 dpm/10 6 kg at 400 m depth in the circumpolar waters. The vertical profiles of 32 Si at the five Atlantic stations approximately follow the Si profiles but the depth gradients are different. This would be expected also considering the in-situ release mechanisms due to dissolution and advection/diffusion from the bottom waters. Except for the circumpolar station 89, where the Si and 32 Si profiles show the effect of marked vertical mixing (nearly depth independent profiles), the profiles show the following features: (1) specific activities of 32 Si ( 32 Si/SiO 2 ratios) are lowest at intermediate depths, and (2) on an average the surface specific activities are higher, by 2–4 times, than the bottom water values. These data are consistent with generation of the highest specific activity 32 Si waters at the surface, where Si concentrations are lowest and precipitation adds cosmogenic 32 Si scavenged from the troposphere. Rapid removal of biogenic silica to the water-sediment interface, without much dissolution during transit, leads to the second regime of high 32 Si specific activities. The 32 Si inventories in the water column in the latitude belt 27°N-27°S are in the range (1–1.4) × 10 −2 dpm 32 Si/cm 2 , which is consistent with the expected fallout of cosmogenic 32 Si. However, the 32 Si column inventories south of 40°S are higher by a factor of ∼ 5–7, whereas the corresponding Si inventories increase by only a factor of 3. This excess 32 Si in the Southern Ocean cannot be explained by direct fallout from the stratosphere or by melting of Antarctic snow and ice. Instead, this excess is maintained primarily by the southward deep-water transport of 32 Si dissolved from sinking particulates.

Annual fallout of32Si,210Pb,22Na,35S and7Be in rains in India

The concentration of radioisotopes7Be,35S were measured in Bombay since 1956 and22Na,210Pb,32Si since 1963. In Khandala and other stations such measurements have been made at irregular periods since 1961. In addition several measurements especially that of32Si were made in 1970. Data available todate from Indian stations is summarised and critically analysed.We conclude that appreciable amounts of35S,22Na and32Si, over and above their production by cosmic rays, were produced during the high yield Russian tests as evidenced by their fallout between 1962–66. Based on the bomb produced excess the half period for their removal from the stratosphere is deduced to be less than 1 year. The ‘excess’ contribution of32Si due to bombs is, however, small; about 1% of its inventory in the oceans.The present study shows that for stations where orogeny is the principal mechanism of precipitation, the annual fallout is independent of the annual rainfall.

GEOSECS Pacific and Indian Ocean 32Si profiles

Abstract Results of measurements of twelve 32 Si vertical profiles, nine from the Pacific Ocean at latitudes 45°N–58°S, and three from the Indian Ocean between the Equator and 38°S are presented. The amounts of in-situ extracted SiO 2 range from ∼ 1 to 25 g. The volumes of water from which dissolved silicon was extracted range from 200 to 9 × 10 5 kg. The net 32 P activities range from 0.7 to 3.8 cph. It is possible to measure accurately 32 Si ( 32 P) activities as low as 2 × 10 −2 dpm from 25 g SiO 2 with the present techniques. The 32 Si concentrations in water range from 0.1 dpm/10 6 kg seawater to 178 dpm/10 6 kg seawater. The overall pattern of 32 Si increase with depth in the oceans resembles that of Si but the two differ appreciably; the enrichment of the former is controlled by its relatively short half-life. The 32 Si/SiO 2 ratios vary from ∼ 1 in deeper waters ( > 1000 m) to 81.5 dpm/kg SiO 2 in the surface waters. Three depth variation patterns are observed in the Pacific and Indian oceans: (i) a low ratio, ∼ 10 dpm/kg SiO 2 varying within a factor of two between the surface and ∼ 5000 m depth; (ii) a monotonic increase from a high surface value (> 10 dpm/kg SiO 2 ) to a low value ( ∼ 5 dpm/kg SiO 2 ) at ∼ 3000 m beyond which the value either remains constant or increases slightly with depth; and (iii) in half of the cases, the ratios increase with depth in the depth interval 1–3 km. The above patterns do not show any latitudinal variation. The column inventories of 32 Si and Si in the north Pacific show a rather similar latitudinal variation, which is a coincidence. The column inventories of 32 Si show a pronounced peak at ∼ 35°N/S in the Pacific and Indian oceans that must be attributed to an increased tropospheric fallout in the spring injections in mid-latitudes. This is also expected in view of the short half-life of 32 Si. A similar feature was observed in the south Atlantic Ocean. Now that the 32 Si measurements are available for the three major oceans, the oceanic budget of 32 Si can be estimated. There are ∼ 920 × 10 14 dpm of 32 Si in the major oceans which corresponds to a global-average-production rate of ≥ 4.3 × 10 −4 atoms 32 Si/cm 2 s. Furthermore, since the atmospheric fall out of 32 Si has been determined, we can obtain a fairly accurate half-life of 32 Si. The geochemical half-life is estimated to be ≥ 120 yrs, in good agreement with the best value of 140 yrs based on other recent estimates. The lower limit reflects on the present uncertainty in the size and lifetime of the transient silicon pool at the water-sediment interface. From the observed decrease in the water column-averaged 32 Si/SiO 2 ratios from the Circumpolar Atlantic waters to their Pacific counterparts, we deduce an apparent velocity of 55 km/yr for the Circumpolar current. Using a one-dimensional model, the vertical advection velocity ( w ) and turbulent diffusion coefficient ( K ) have been deduced using both the 32 Si and 14 C data. The values of w range from 1.3 to 15 m/yr and of K from 0.5 to 8 cm 2 /s, except for the Somali basin where we obtain: w = 53m/yr and K = 16cm 2 /s . The 32 Si-based values are about 3 times higher than those based on 14 C.

Natural radionuclides in the Arabian Sea and Bay of Bengal: Distribution and evaluation of particle scavenging processes

Vertical and temporal variations in the activities of234Th,210Po and210Pb have been measured, in both dissolved and paniculate phases, at several stations in the eastern Arabian Sea and north-central Bay of Bengal. A comparative study allows us to make inferences about the particle associated scavenging processes in these two seas having distinct biogeochemical properties.A common feature of the234Th profiles, in the Arabian Sea and Bay of Bengal, is that the dissolved as well as total (dissolved + particulate) activity of234Th is deficient in the surface 200 m with respect to its parent,238U. This gross deficiency is attributed to the preferential removal of234Th by adsorption onto settling particles which account for its net loss from the surface waters. The scavenging rates of dissolved234Th are comparable in these two basins. The temporal variations in the234Th-238U disequilibrium are significantly pronounced both in the Arabian Sea and Bay of Bengal indicating that the scavenging rates are more influenced by the increased abundance of particles rather than their chemical make-up. In the mixed layer (0–50 m), the scavenging residence time of234Th ranges from 30 to 100 days.The surface and deep waters of both the seas show an enhanced deficiency of dissolved210Po relative to210Pb and that of210Pb relative to226Ra. The deficiencies of both210Po and210Pb in the dissolved phases are not balanced by their abundance in the particulate form indicating a net loss of both these nuclides from the water column. The scavenging rates of210Po and210Pb are significantly enhanced in the Bay of Bengal compared to those in the Arabian Sea. The mean dissolved210Po/210Pb and210Pb/226Ra activity ratios in deep waters of the Bay of Bengal are ∼ 0.7 and 0.1, respectively, representing some of the most pronounced disequilibria observed to date in the deep sea. The Bay of Bengal and the Arabian Sea appear to be the regions of most intense particle moderated scavenging processes in the world oceans. This is evidenced by the gross disequilibria exhibited by the three isotope pairs used in this study.