National Centre for Earth Science Studies

Recurrent sea-level changes are reflected in the formation of coastal landforms; estuary, lakes, lagoons and barrier beaches during the late Quaternary period along the Kerala coast, SW India. The sediment core from these landforms was addressed for sediment lithofacies, geochemical parameters and associated foraminiferal assemblages to reconstruct the late Quaternary environment. The textural characteristics of the sediment indicate a shallow marine to lagoon and swampy/marsh environment. A layer of coarse sandy sediment (7–9 m thickness) is overlying the late Pleistocene (40 Kyrs BP) sediment sequence. The sediment was deposited under violent to the calm environment attributing its deposition in diverse energy regimes. This is also corroborated with the micro-textures of quartz grains. The geochemical elemental relationship (Rb vs K2O, Ni vs TiO2, K2O/Na2O vs SiO2/Al2O3, discriminant function) is established with paleoweathering, provenance and tectonic setting. An abundant population of Ammonia beccarii, A. tepida, E. discoidale, N. scaphum. A. beccarii in the sediments reveals their high tolerance and adaptability to the changing environment. The multiproxy studies on sediments, support a stronger monsoon in the early Holocene leading to the incursion of high-water levels, increased flow discharge and bottom scouring but, weak monsoon and arid climate prevailed during the Last Glacial Maximum (LGM). A conceptual model is proposed to depict the shoreline evolution and its climate.

Bibliographic Info: Varghese T. I., Prakash, T N., Sheela Nair, L., Sreenivasulu, G., Nagendra, R. [2021]. Reconstruction of the paleoenvironment of the late Quaternary sediments of the Kerala coast, SW India. Journal of Asian Earth Sciences, Vol. 222, Art. 104952. https://doi.org/10.1016/j.jseaes.2021.104952

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Tropical urbanized coastal regions are hotspots for the discharge of nutrient-enriched groundwater, which can affect sensitive coastal ecosystems. Here, we investigated how a beach modifies groundwater nutrient loads in southern India (Varkala Beach), using flux measurements and stable isotopes. Fresh groundwater was highly enriched in NO3 from sewage or manure. Submarine groundwater discharge and nearshore groundwater discharge were equally important contributors to coastal NO3 fluxes with 303 mmol NO3 m–1 day–1 in submarine and 334 mmol NO3 m–1 day–1 in nearshore groundwater discharge. However, N/P ratios in nearshore groundwater discharge were up to 3 orders of magnitude greater than that in submarine groundwater, which can promote harmful algae blooms. As groundwater flowed through the beach, N/P ratios decreased toward Redfield ratios due to the removal of 30–50% of NO3 due to denitrification and production of PO4 due to mineralization of organic matter. Overall, tropical beaches can be important natural biogeochemical reactors that attenuate nitrogen pollution and modify N/P ratios in submarine groundwater discharge.

Bibliographic Info: Oehler, T., Murugan, R., Mintu E. George, Suresh Babu, D. S., Dähnke, K., Ankele, M., Böttcher, M. E., Santos, I. R., Moosdorf, N. [2021]. Tropical beaches attenuate groundwater nitrogen pollution flowing to the ocean. Environmental Science and Technology, Vol. 55 (12), pp. 8432-8438. https://doi.org/10.1021/acs.est.1c00759

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Macrophysical and microphysical features of clouds and precipitation at coastal [20 m above Mean Sea Level (MSL)] and high-altitude cloud physics observatory (HACPO; 1820 m above MSL) sites on the windward side of Western Ghats are examined using ground (Ceilometer and Disdrometer) and satellite-based observations for the years 2017 and 2018 during the pre- and post-monsoon seasons. Low- and high-level clouds dominate over the coastal site, whereas at HACPO, middle-level clouds dominate with a prominent diurnal cycle. Stratiform and convective raindrop size distributions (RSDs) show that nearly 80% of total precipitation was convective in both the seasons at the coastal site. The relative increase in mass-weighted mean diameter (Dm), lower value of normalized intercept parameter (Nw) and higher mean cloud effective radius of ice and liquid phase imply deeper cloud formation in the site. But at HACPO, 54.85% of precipitation is evolved from the convective core and 45.15% from the stratiform core. Lower specific humidity, less liquid water content and weak convective available potential energy signify a dry environment which delimits vertical growth of orographic clouds in coherence with RSD variations having low Dm and high Nw with low rainwater content. Enhanced collision-coalescence in the deep convective clouds sustained by strong updraft results in precipitation with high concentration of mid and large size drops at coastal site, whereas in HACPO, middle-level clouds persist for longer periods favoring warm rain processes by shallow convection that cause smaller drops at the surface.

Fig. 1: The topography of the study area showing locations of the coastal and high-altitude cloud physics observatory (HACPO) sites.

Bibliographic Info: Sumesh, R. K., Resmi, E. A., Unnikrishnan. C. K., Dharmadas Jash, Ramachandran, K. K. [2021]. Signatures of shallow and deep clouds inferred from precipitation microphysics over windward side of Western Ghats. Journal of Geophysical Research: Atmospheres, Vol. 126 (10), Art. e2020JD034312. https://doi.org/10.1029/2020JD034312

Lightning characteristics in India are examined with satellite-based Lightning Imaging Sensor (LIS) and ground-based Indian Lightning Detection Network (ILDN). LIS observations indicated that synoptic weather systems are major contributors of lightning in Indian hotspots. Western disturbances (mid-tropospheric systems with extratropical origin) were the greatest contributor of lightning in the Himalayas (93%), tropical cyclonic storms and low-pressure systems (oceanic in origin) were key lightning contributors in parts of eastern India (43%), and lower tropospheric troughs were major contributors in other hotspots. For the first time, this study reported the occurrence of significantly high lightning activity before active monsoon spells in the Central India region (65–87° E, 18–27° N). Consequently, satellite-based lightning observations could be used to predict active monsoon spells. Harmonic analysis was used to study diurnal lightning-flash density. The maximum observed standardized diurnal amplitude of lightning activity was 0.35, and maximum explained diurnal variation was 15%. Also, for the first time, we compared the ILDN data and LIS observations, and found good agreement regarding lightning variability. LIS data showed an increase in annual lightning activity in tropical South-western India (SWI), and the results also suggested that during El Niño and negative Indian Ocean Dipole periods, SWI experiences above-average lightning activity.

Bibliographic Info: Unnikrishnan, C. K., Sunil Pawar, Gopalakrishnan, V. [2021]. Satellite-observed lightning hotspots in India and lightning variability over tropical South India. Advances in Space Research, Vol. 68 (4), pp. 1690-1705. https://doi.org/10.1016/j.asr.2021.04.009

Knowledge about long-term variation of the geomagnetic dipole field remains in its nascent stage because of the paucity of reliable experimental data over geological periods. Here, we present the first robust experimental data from the largest Cretaceous flood basalt province on Earth, the ~65–66 Ma Deccan basalt within a thick (1250 m) unbiased stratigraphic section down to the basement, recovered from a drill hole of the Koyna Deep Scientific Drilling Project in the Western Ghats, India. Critical analysis of the result along with similar results of the Cretaceous age find that (i) the dipole moment during the end Cretaceous Deccan eruption is the lowest in whole of Cretaceous (ii) dipole moment at the onset/termination of the Cretaceous Normal Superchron is apparently lower relative to that in mid-superchron, however, such differences cannot be deciphered in shorter polarities probably because of insufficient time to develop recognizable variations (iii) inverse relation between dipole moment and reversal rate is lacking and (iv) a cause and effect relation between core-mantle boundary heat flux and low dipole moment that appears to be the principle governing factor in forming the Large Igneous Provinces on the surface of earth.

Bibliographic Info: Radhakrishna, T., Asanulla R. Mohamed, Venkateshwarlu, M., Soumya, G. S. [2020]. Low geomagnetic field strength during End-Cretaceous Deccan volcanism and whole mantle convection. Scientific Reports, Vol. 10 (1), Art. 10743. https://doi.org/10.1038/s41598-020-67245-6