Benthic foraminiferal response to changes in mining pattern: a case study from the Zuari estuary,Goa, India

Being sensitive to environmental changes, foraminifera have been extensively used to monitor pollution level in the marine environment, including the effect of mining in coastal areas. In the Goa state of India, the rejects from opencast mining on land largely find their way to the estuaries, as washout during monsoon. Additionally, the Mormugao Port at the mouth of the Zuari estuary is the hub of activities due to the transport of ore from hinterland areas by barges and its subsequent loading for export. On the directive of the Supreme Court of India, all the mining-related activities abruptly stopped throughout India, including that in Goa in 2012, and got reinstated in 2015. Therefore, it provided a fit case to test the effectiveness of benthic foraminifera as an indicator of environmental impact due to mining activities. A total of ten surface sediment samples from five locations in Zuari estuary were collected from a depth range of 4.5–8.5 m in the years of 2013 and 2016 and were analyzed for both the living (stained) and dead benthic foraminifera. The year 2013 represents a time interval immediately after the closure of extensive mining activity, and the sampling during 2016 represents minimal mining. The living benthic foraminiferal abundance was higher (19–54/g sediment) during 2013 and decreased substantially during 2016 (3–22/g sediment), suggesting an adverse effect of activities associated with mine closure on benthic foraminifera. Additionally, the relative abundance of Ammonia was also significantly low during the year 2016. The temporal variation in dead foraminifera was, however, different than that of the living foraminifera. The differential response was attributed to the terrigenous dilution as a result of change in sedimentation rate. Therefore, we conclude that living foraminifera correctly incorporate the changes in mining pattern and may be used as an effective tool to monitor the impact of mining. We further suggest that the potential counter effect of terrigenous dilution on total and living benthic foraminiferal population should be considered while interpreting temporal variations in foraminiferal abundance in marginal marine settings.     

Foraminifers as indicators of marine pollution: a culture experiment with Rosalina leei

In order to develop a viable foraminiferal proxy for heavy metal pollutants, juvenile specimens of Rosalina leei were subjected to different mercury concentrations (0-180 ng/l). Initially considerable growth was observed in specimens kept in saline water having a mercury concentration up to 100 ng/l. But with the gradual increase in concentration of mercury the growth rate started decreasing. Total growth achieved was significantly lower in case of specimens kept at relatively higher mercury concentrations then those maintained in normal saline water. The most significant result of this experiment was the addition of abnormal chambers in the specimens kept at higher mercury concentration. Later the specimens kept at highest concentration (180 ng/l) were subjected to progressively increasing concentration of mercury to see the further effects and it was found that the specimens were still living at as high a mercury concentration as 260 ng/l although there was no growth.     

Fluid-absent Melting of High-grade Semi-pelites: P-T Constraints on Orthopyroxene Formation and Implications for Granulite Genesis

Rocks of semi-pelitic composition are common in high-grade terranes.The first appearance of orthopyroxene in these rocks marks thetransition from amphibolite- to granulite-facies conditions,and is commonly attributed to the process of fluid-absent partialmelting. We have conducted fluid-absent melting experimentson two natural semi-pelitic rocks (quartz, plagioclase, alkalifeldspar, biotite and garnet) with the specific objective ofdetermining the pressure–temperature conditions necessaryto produce orthopyroxene. In contrast to previous experimentalstudies, our starting materials were obtained from a transitionalamphibolite–granulite terrane. Importantly, the high TiO₂(>5 wt %) and F (>1 wt %) contents of biotite in our experimentsare more representative of biotite found in rocks on the vergeof granulite-facies conditions than those used in earlier studies.Experiments were conducted in a piston-cylinder apparatus at800–1050°C and 7–15 kbar. We reversed the firstappearance of orthopyroxene in two-stage experiments at 7 and10 kbar. Fluid-absent melting of biotite began at     

Thermodynamic structure of the monsoon boundary layer under the influence of a large-scale depression

Using data from the MONsoon Trough Boundary Layer EXperiment (MONTBLEX), the thermodynamic structure of the atmospheric boundary layer (ABL) under the influence of a monsoon depression has been studied. When the depression was in the vicinity of the observing station, the soundings showed an increase in potential temperature, the sub-cloud layer was well mixed, the wind speed increased to 35 m/s, and the monsoon boundary layer was convectively more unstable at night than in the daytime. Cloud-top processes, which lead to an apparent breakdown of the boundary layer, seem to explain this.     

Abundance and size variation of Globorotalia menardii in the Northeastern Indian Ocean during the late quaternary

Variation in absolute abundance, size and coiling direction ratio of the planktic foraminifera Globorotalia menardii during the late Quaternary have been studied in three cores, collected along a north-south transect from the northeastern Indian Ocean. Increased abundance and larger size is observed during the warm interglacial period, whereas, smaller and fewer G. menardii specimens are noted during the cold glacial periods. A comparatively better match between absolute abundance, size and oxygen isotopic composition of foraminifera as compared to carbon isotopic composition at all the locations indicates that the abundance and size of G. menardii is more affected by seawater temperature and salinity as compared to productivity changes. The amplitude and frequency of temporal changes in the abundance and size of G. menardii vary from core-to-core, indicating the influence of site-specific physico-chemical conditions. In comparison to the abundance and size, the coiling direction of G. menardii does not show any systematic change in any of the cores. The majority of the specimens coil sinistrally. We conclude that the absolute abundance and variation in the average size of G. menardii can be used to infer past climatic changes. However, the variation in coiling direction of G. menardii appears to be unaffected by late Quaternary paleoclimatic variations in the north-eastern Indian Ocean.     

Relationship between abundance and morphology of benthic foraminifera <Emphasis Type="Italic">Epistominella exigua</Emphasis>: Paleoclimatic implications

The relationship between abundance (relative as well as absolute abundance) and morphology (size of the shell, number of chambers and proloculus size) of benthic foraminifera Epistominella exigua has been studied in a core to understand the influence of changing environmental conditions on its morphology and distribution. A total of 50 samples from the top 100 cm section of a gravity core collected from the southern Bay of Bengal Fan were used. The changes in abundance of E. exigua match well with variation in average proloculus size of E. exigua. An opposite relationship however, is observed between the abundance and size of E. exigua shells. The higher abundance corresponds with smaller E. exigua shells. Similarly, the increased E. exigua abundance matches with a decrease in the number of chambers. The increased abundance of E. exigua shows favourable conditions for its growth and survival. Thus the study indicates that during favourable conditions, while the number of chambers in the shells and the size of the shells of E. exigua decreases, more number of specimens have a larger proloculus. Since asexual reproduction results in megalospheric specimens with larger proloculus, smaller size and less number of chambers, it is inferred that E. exigua prefers an asexual mode of reproduction during favorable conditions. The findings can be used to apply morphological characteristics of E. exigua as a proxy to infer past climatic conditions.     

Linkage between Seasonal Insolation Gradient in the Tropical Northern Hemisphere and the Sea Surface Salinity of the Equatorial Indian Ocean during the Last Glacial Period

Paired stable oxygen isotope and Mg/Ca analyses in calcite tests of the mixed-layer-dwelling planktic foraminifer Globigerinoides ruber has been used to reconstruct equatorial Indian Ocean δ18O of seawater (δ18Osw) over the last ~137 thousand years. On the basis of ice-volume-corrected δ18Osw (δ18Osw–ivc), relative changes in sea surface salinity (SSS) have been estimated. The SSS estimates suggest three episodes of higher SSS (131–113 thousand years before present (kyr BP), 62–58 kyr BP, and 30–24 kyr BP) within the last glacial period as compared with the present. SSS comparison between interglacial episodes reveals that the surface seawater over the core site was significantly saltier during the penultimate interglacial than the Holocene. We suggest that the evolution of a seasonal insolation gradient between the Indian monsoon areas and the equator over the investigated time interval was instrumental in shaping the strength of the Indian winter and summer monsoons that left their imprints on the equatorial Indian Ocean SSS via freshwater input and wind-induced mixing. The study shows that the insolation difference between northern latitudes and the equator during winter affects monsoon strength in the Indian region, especially during cold intervals.     

Oxygen isotope records of Globigerina bulloides across a north-south transect in the south-western Indian Ocean

Along a north-south transect (9.69°N to 55.01°S) in the southwestern Indian Ocean during the Indian Pilot Expedition to Southern Ocean (PESO), the oxygen isotopic analysis of planktic foraminifera (Globigerina bulloides) from 23 surface sediment samples was carried out to assess the relationship between isotopic composition of G. bulloides and the prevailing physical (seawater temperature and salinity) conditions of the ambient seawater. An increasing trend in the δ¹⁸O value is noticed towards higher latitude. Apparently such an increase in δ¹⁸O values is inversely related to the temperature changes along the transect. However, slight mismatch is observed at a few stations due to calcification out of optimum conditions or due to the salinity changes. The preliminary results of the present study, if extended to the subsurface sediments coupled with other parameters, may contribute to the reconstruction of the paleohydrography of the region, especially the position of various seawater fronts during the geologic past albeit with areal limitation.     

Assessing the suitability of benthic foraminiferal morpho-groups to reconstruct paleomonsoon from Bay of Bengal

Temporal changes in benthic foraminiferal morpho-groups were suggested as an effective proxy to reconstruct past monsoon intensity from the Arabian Sea. Here, in order to test the applicability of temporal variation in morpho-groups to reconstruct past monsoon intensity from the Bay of Bengal, we have documented recent benthic foraminiferal distribution from the continental shelf region of the northwestern Bay of Bengal. Based on the external morphology, benthic foraminifera were categorized into rounded symmetrical (RSBF) and angular asymmetrical benthic foraminifera (AABF). Additionally, a few other dominant groups were also identified based on test composition (agglutinated, calcareous) and abundance (Asterorotalids and Nonions). The relative abundance of each group was compared with the ambient physico-chemical conditions, including dissolved oxygen, organic matter, salinity and temperature. We report that the RSBF are abundant in comparatively warm and well oxygenated waters of low salinity, suggesting a preference for high energy environment, whereas AABF dominate relatively cold, hypersaline deeper waters with low dissolved oxygen, indicating a low energy environment. The agglutinated foraminifera, Asterorotalids and Nonions dominate shallow water, low salinity regions, whereas the calcareous benthic foraminiferal abundance increases away from the riverine influx regions. Food availability, as estimated from organic carbon abundance in sediments, has comparatively less influence on faunal distribution in the northwestern Bay of Bengal, as compared to dissolved oxygen, temperature and salinity. We conclude that the factors associated with freshwater influx affect the distribution of benthic foraminiferal morpho-groups in the northwestern Bay of Bengal and thus it can be used to reconstruct past monsoon intensity from the Bay of Bengal.