[Show abstract][Hide abstract] ABSTRACT: Rapid climatic shifts across the last glacial to Holocene transition are pervasive feature of the North Atlantic as well as low latitude proxy archives. Our decadal to centennial scale record of summer monsoon proxy Globigerina bulloides from rapidly accumulating sediments from Hole 723A, Arabian Sea shows two distinct intervals of weak summer monsoon wind coinciding with cold periods within Ållerød inerstadial of the North Atlantic named here as IACP-A1 and IACP-A2 and dated (within dating uncertainties) at 13.5 and 13.3 calibrated kilo years before the present (cal kyr BP), respectively. Spectral analysis of the Globigerina bulloides time series for the segment 13.6-13.1 kyr (Ållerød period) reveals a strong solar 208-year cycle also known as de Vries or Suess cycle, suggesting that the centennial scale variability in Indian summer monsoon winds during the Ållerød inerstadial was driven by changes in the solar irradiance through stratospheric-tropospheric interactions.
[Show abstract][Hide abstract] ABSTRACT: Uplift of the Himalayas and Tibetan Plateau (ca. 10 8 Ma) has been said
to be the main cause of the origin or intensification of the Indian
monsoon system, because mountains modulate the land-sea thermal
contrast. The intensification of the monsoons, in turn, is seen as the
cause of major changes in fauna and flora on land (as a result of
changing precipitation patterns) as well as in the Indian Ocean, where
the monsoons drive increased upwelling and thus increased productivity.
We argue that the interactions between the elevation of the Himalayas
and Tibetan Plateau, the onset of the monsoons, and their effects on the
Indian Ocean biota remain uncertain. The timing of these events (uplift,
monsoons, and biotic change) is not well constrained. Neogene deep-sea
benthic foraminiferal faunal and isotope records of the Ninetyeast Ridge
combined with published data show that a major increase in biogenic
productivity occurred at 10 8 Ma throughout the Indian Ocean, the
equatorial Pacific, and southern Atlantic. We suggest that this Indian
Ocean high-productivity event was not simply the result of
monsoon-induced upwelling or nutrient delivery from the weathering of
newly uplifted mountains, but may have been caused by strengthened wind
regimes resulting from global cooling and the increase in volume of the
Antarctic ice sheets.
[Show abstract][Hide abstract] ABSTRACT: We quantitatively analyzed deep-sea benthic foraminifera from >125 μm size fraction from Ocean Drilling Program (ODP) Site 757 to understand deep-sea paleoceanographic changes in the southeastern Indian Ocean during the late Oligocene–Miocene (26.5–5.35 Ma). We used the knowledge of the ecology of Recent deep-sea benthic foraminifera for environmental interpretations. Factor and cluster analyses were run using percentages of 46 highest-ranked species that helped identify six biofacies defining six clusters of samples. The faunal data document a major shift in deep-sea ventilation, organic carbon flux and productivity at ∼8.3 Ma, coinciding with the major intensification of the Indian Ocean monsoon system. This marks a change from cold and well-oxygenated deep waters with low and pulsed organic carbon flux during 26.5 to ∼8.3 Ma to oxygen-poor deep waters with sustained flux of organic matter. From 15.0 to 7.6 Ma, the deep-sea currents were stronger in the southeastern Indian Ocean probably due to major expansion of the East Antarctic ice sheet and increased production of deep waters and in turn increased deep-sea circulation. The productivity increased in the Indo-Pacific region in the late Miocene and the modern monsoon regime probably established at 8.3 Ma.
Marine Micropaleontology - MAR MICROPALEONTOL. 01/2004; 51(1):153-170.
[Show abstract][Hide abstract] ABSTRACT: Diversity parameters of Neogene deep-sea benthic foraminifera were measured at Ocean Drilling Program (ODP) Hole 752A, southeastern Indian Ocean (water depth of 1086.3 m) using Information Function (H), Equitability (E), number of species (S) and Sander's rarefaction values. These parameters combined with population abundance of dominant benthic foraminifera (Bulimina macilenta, Nuttallides umbonifera, Cibicides wuellerstorfi, Cibicides lobatulus, Bolivina pusilla, Ehrenbergina carinata, Gavelinopsis lobatulus, Cassidulina laevigata, Globocassiulina subglobosa) reveal significant paleoceanographic changes in the southeastern Indian Ocean during the Neogene. The values of all the diversity parameters show a decrease from 25 to 23 Ma and thereafter an increase with peak values at ~ 13.5 Ma. The Late Oligocene to Earliest Miocene was an interval of more unstable conditions at Hole 752A dominated by species characteristic of low organic carbon, well-ventilated, carbonate corrosive high energy conditions. The highest values of diversity parameters coincide with the early Middle Miocene climatic optimum. All these parameters show a declining trend and gradual decrease from 13.5 to 4.5 Ma coinciding with the major build up of ice sheets in the Antarctic region. Major increase in B. pusilla and E. carinata population during this time suggests high nutrient levels and low oxygen conditions at Hole 752A. This interval corresponds with the so-called “biogenic bloom” and an intense Oxygen Minimum Zone as observed throughout the Indo-Pacific region. Deep waters were warmer from 4.5 to 3 Ma marked by an increase in species diversity values, coinciding with the early middle Pliocene warmth. The species diversity values abruptly decreased in the younger interval, contemporaneous with the major Northern Hemisphere glaciation. During this time species characteristics of high-energy bottom currents and relatively cold deep water were dominant.
Palaeogeography Palaeoclimatology Palaeoecology s 361–362:94–103. · 2.75 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A significant component of domestic demand for water of urban areas located in the Gangetic plains is met by heavy pumping of groundwater. The present study is focused on the Patna municipal area, inhabited by 17 million people and spanning over 134 km2, where entire urban water demand is catered from pumping by wells of various capacities and designs. The present study examines the nature of the aquifer system within the urban area, the temporal changes in the water/piezometric level and the recharge mechanism of the deeper aquifers. The aquifer system is made up of medium-to-coarse unconsolidated sand, lying under a ~40-m-thick predominantly argillaceous unit holding 8- to 13-m-thick localised sand layers and continues up to 220 m below ground. Groundwater occurs under semi-confined condition, with transmissivity of aquifers in 5,500–9,200 m2 day−1 range. Hydraulic head of the deeper aquifer remains in 9–19 m range below ground, in contrast to 1–9 m range of that of the upper aquitard zone. The estimated annual groundwater extraction from the deeper aquifer is ~212.0 million m3, which has created a decline of 3.9 m in the piezometric level of the deeper aquifer during the past 30 years. Unregulated construction of deep tube wells with mushrooming of apartment culture may further exacerbate the problem. The sand layers within the aquitard zone are experiencing an annual extraction of 14.5 million m3 and have exhibited stable water level trend for past one and half decades. This unit is recharged from monsoon rainfall, besides contribution from water supply pipe line leakage and seepage from unlined storm water drains.
[Show abstract][Hide abstract] ABSTRACT: Seventy three core samples of 10 cc volume were analysed from Ocean Drilling Program (ODP) Site 756, Hole B with an average interval of 236 kyr per sample. Factor and cluster analyses of 37 high ranking species of benthic foraminifera enabled us to identify four clusters representing four biofacies. Biofacies Sl-Om is characterised by Stilostomella lepidula, Orthomorphina modesta, Robulus gibbus, Bulimina miolaevis and Cibicides cf. lucidus. This biofacies dominates the latest Oligocene to early Miocene interval (24–21 Ma and 18.75–16.5 Ma), indicating low to intermediate dissolved oxygen, organic carbon rich environment, warm and sluggish deep waters. During 21–18.75 Ma, the biofacies Gp-Nu (Globocassidulina pacifica, Nuttallides umbonifera, Trifarina angulosa, Quinqueloculina weaveri, Robulus gibbus and Pullenia bulloides) dominates the benthic assemblage at Hole 756B. This biofacies also has a short lived presence at ~ 16.5 Ma. This was an interval of intense circulation of cold and corrosive deep water similar to Antarctic Intermediate Water (AAIW), and intermediate to high flux of organic matter to Hole 756B. The Antarctic Circumpolar Current (ACC) initiated during this time. From 16.5 to 10.6 Ma, the benthic assemblage at Hole 756B is dominated by biofacies Ub-Gs (characteristic species Uvigerina buzasi, Globocassidulina subglobosa, Laticarina pauperata), indicating intermediate to high food supply and cold deep water. This interval coincides with the middle Miocene positive oxygen isotope shift and permanent build up of Antarctic ice sheets. The late Miocene interval (10.6–6.5) is characterised by biofacies Ec-Bs (dominant species Ehrenbergina carinata, Bolivina spathulata, Osangularia culter and Pullenia osloensis) indicating low oxygen condition with high food supply in the study area. This corresponds to an interval of global biogenic bloom.