Search Results (3)

Atmospheric pressure gradients determine the dynamics of the southwest monsoon (SWM) and northeast monsoon (NEM), resulting in rainfall in the Indian subcontinent. Consequently, the surface salinity, mixed layer, and thermocline are impacted by the seasonal freshwater outflow and direct rainfall. Moreover, seasonally reversing monsoon gyre and associated currents govern the northern Indian Ocean surface oceanography. This study provides an overview of the impact of these dynamic changes on sea surface temperature, salinity, and productivity by integrating more than 3000 planktonic foraminiferal censuses and bulk sediment geochemical data from sediment core tops, plankton tows, and nets between 25° N and 10° S and 40° E and 110° E of the past six decades. These data were used to construct spatial maps of the five most dominant planktonic foraminifers and illuminate their underlying environmental factors. Moreover, the cured foraminiferal censuses and the modern oceanographic data were used to test the newly developed artificial neural network (ANN) algorithm to calculate the relationship with modern water column temperatures (WCTs). Furthermore, the tested relationship between the ANN derived models was applied to two foraminiferal censuses from the northern Bay of Bengal core MGS29-GC02 (13°31′59″ N; 91°48′21″ E) and the southern Bay of Bengal Ocean Drilling Program (ODP) Site 758 (5°23.05′ N; 90°21.67′ E) to reconstruct the WCTs of the past 890 ka. The reconstructed WCTs at the 10 m water depth of core GC02 suggest dramatic changes in the sea surface during the deglacial periods (i.e., Bolling–Allerǿd and Younger Dryas) compared to the Holocene. The WCTs at Site 758 indicate a shift in the mixed-layer summer temperature during the past 890 ka at the ODP Site, in which the post-Mid-Brunhes period (at 425 ka) was overall warmer than during the prior time. However, the regional alkenone-derived sea-surface temperatures (SSTs) do not show such a shift in the mixed layer. Therefore, this study hypothesizes that the divergence in regional SSTs is most likely due to differences in seasonality and depth habitats in the paleo-proxies. Full article

The geochemical investigation of shales from the Early to Middle Cretaceous Chichali Formation in the Hazara Basin was conducted to determine the origin, tectonic setting and evolution, paleo-weathering conditions, and paleo-oceanographic reconstruction. The research included a comprehensive field survey, sample collection, and analysis of a variety of main, trace, and rare-earth elements using an X-ray fluorescence spectrometer (XRF). Bivariate plots and ternary diagrams were used to determine the provenance, tectonic setting, and paleo-weathering conditions that existed during the development of the Chichali Formation in the Hazara Basin. The values of Ba/Sc, Ba/Co, Th/Sc, Cr/Th, Cr/Zr, Th/Co, Th/Cr, Sc/Th, bivariate plots of Al₂O₃ vs. TiO₂, TiO₂ vs. Zr, TiO₂ vs. Ni, Df1–Df2, Zr vs. Nb, and La/Sc vs. Th/Co, and ternary diagram of K₂O–Fe₂O₃–Al₂O₃ were used to illustrate the passive continental margin setting of Chichali Formation shales. The detailed chemical analysis also provides an understanding of the marine geochemical cycle, which reflects the origin of these sediments. The average K₂O/Al₂O₃ value is less than 0.4, indicating that the shale contains clay minerals. The Chichali Formation’s Chemical Index of Alteration (mean = 71) and Index of Compositional Variation (mean = 1.12) values show a modest degree of chemical weathering in the source locations. From an environmental standpoint, the Chichali Formation is richer in toxic elements such as Ba, Zn, Ni, Cr, and Cu, which may be damaging to agricultural soils and drinking water when present in excess. These metals are incorporated into the formation during the weathering process. Full article

Rabbitfish (Siganidae) are coral reef fish that are distributed across diverse habitats that include estuaries, mangroves, reefs, and even seaweed mats. Given their ecological diversity and natural widespread distributions across the Indo-Pacific region, we were interested to investigate the evolutionary history of this group and patterns of divergence that have contributed to their present-day distributions. In the present study, samples were collected from the South China Sea to study taxonomic and phylogenetic relationships, and divergence times. We investigated the taxonomic relationships among modern rabbitfish species, reconstructed their molecular phylogeny, and estimated divergence times among selected lineages based on a fragment of the mtDNA cytochrome oxidase I (COI) and sequences of the nuclear rhodopsin retrogene (RHO). Our results indicate that modern rabbitfish likely originated in the Indo-West Pacific during the late Eocene [37.4 million years ago (mya)], following which they diverged into three major clades during the Pliocene/Pleistocene. Subsequent diversification and origins of the majority of siganids may likely be associated with episodes of paleo-oceanographic events, including greenhouse and glaciation events (Eocene–Miocene) as well as major plate tectonic events (Pliocene–Pleistocene). Some modern siganid species may naturally hybridize with congeneric species where their geographical ranges overlap. A comprehensive taxonomic analysis revealed that the phylogeny of Siganidae (cladogenesis of Clades I, II, and III) is characterized by divergence in several external morphological characters and morphometric parameters. Our study demonstrates that morphological characteristics, geographical heterogeneity, and environmental change have contributed to siganids’ historical diversification. Full article