Search

The Bengal Fan covers the entire floor of the Bay of Bengal (BoB) and has accumulated erosional material from the Himalayas since the Early Eocene. The fan architecture is constructed by turbidity current deposits via channel levee systems during active fan progradation and hemipelagic sedimentation during periods of local fan inactivity. In the present study, we document the fan development, sedimentation history, and depositional processes in the lower Bengal Fan and present a site-to-site comparison of stratigraphy and channel migration since the Late Tortonian (ca. 7.5 Ma) from sedimentological and physical property records at Site U1451 of IODP Expedition 354 to 8°N in the lower Bengal Fan. Fine sediment (Sortable Silt, SS) textural and sorting records are used to reconstruct the current skinfriction shear stress in the Benthic Boundary Layer (BBL) of the BoB. Also, a distinction in shear stress environment between turbidity dominated active fan and background hemipelagic fan growth is presented. A criterion is set in the following paper to validate the use of SS records in hemipelagic deposits in the BoB to decipher the shear stress regimes of depositing flows on the basis of sand weight percentage and mean SS sizes (S͞S) variation. This study will enhance our understanding of the Bengal Fan deposition dynamics and fan development with the reconstructed shear stress regimes associated with various depositing flows (turbidity current and/or deep water circulation). It will provide a strong base to model fan internal processes and material flux to the BoB. [ABSTRACT FROM AUTHOR]

The evolution and driving mechanism of the South Asian summer monsoon (SASM) are still poorly understood. We here present a 12-Myr long SASM record by analyzing the strontium and neodymium isotopic composition of detrital components at IODP Exp. 359 Site U1467 from the northern Indian Ocean. The provenance investigation demonstrates that more dust enriched in εNd from northeastern Africa and the Arabian Peninsula was transported to the study site by monsoonal and Shamal winds during the summer monsoon season. A two-step weakening of the SASM wind since ~12 Ma is proposed based on the εNd record. This observational phenomenon is supported by climate modeling results, demonstrating that the SASM evolution was mainly controlled by variations in the gradient between the Mascarene High and the Indian Low, associated with meridional shifts of the Hadley Cell and the Intertropical Convergence Zone, which were caused by interhemispheric ice-sheet growth since the Middle Miocene.The εNd record from the IODP Exp. 359 Site U1467 in the northern Indian Ocean, along with climate modeling, reveals a two-step weakening of the South Asian summer monsoon (SASM) wind since 12 Ma. The SASM evolution was mainly caused by interhemispheric ice-sheet growth since the Middle Miocene. [ABSTRACT FROM AUTHOR]

Global overturning circulation underwent significant changes in the late Miocene, driven by tectonic forcing, and impacted the global climate. Prevailing hypotheses related to the late Miocene deep water circulation (DWC) changes driven by the closure of the Central American Seaways (CAS) and its widespread impact remains untested due to the paucity of suitable records away from the CAS region. Here, we test the hypothesis of the large-scale circulation changes by providing a high-resolution record of DWC since the late Miocene (11.3 to ~2 Ma) from the north-western Indian Ocean. Our investigation reveals a progressive shift from Pacific-dominated DWC before ~9.0 Ma to the onset of a modern-like DWC system in the Indian Ocean comprising of Antarctic bottom water and northern component water during the Miocene-Pliocene transition (~6 Ma) caused by progressive shoaling of the CAS and suggests its widespread impact. Deep ocean circulation plays a crucial role in controlling global climate. What caused on-set of modern like circulation in geological past remains unknown. New research finds constriction of the Central American Seaway caused on-set of modern-like circulation in Indian Ocean since the late-Miocene (~6 Ma). [ABSTRACT FROM AUTHOR]

Anthropogenic CO2 emission has resulted in lowering of surface ocean pH referred as ‘Ocean acidification (OA)’ which posed a serious threat to calcifying marine organisms. Several attempts have been made to assess the role of anthropogenic CO2 forcing against oceanographic factors/processes contributing to the recent OA trend; however, such attempts were hindered by the dearth of long-term pH records. Boron isotopic composition (δ11B) of corals has been used as a robust proxy for seawater pH records. In the present study, we have compiled available coral δ11B-pH records from the Indian, Pacific and Atlantic oceans and assessed regional trends, variability, forcing factors and their relative roles. Most of these δ11B-pH records show a discernable decline trend in recent decades with large variability. Our assessment of the Pacific records reveals that atmospheric CO2 forcing explains maximum pH variability followed by physical oceanographic factors/processes modulated by Pacific oscillations, e.g., ENSO and PDO. In addition, coral metabolic processes might control a large portion of the pH variability; however, they require detailed laboratory-based studies. Further, our investigation reveals a significant increase in pH variability (pH extremes) since ~1970s associated with ENSO events which might be critical for the resilience and adaptability of corals and other calcifiers. Research Highlights: Since the industrial era (~1850), Coral δ11B-pH records show a discernible decreasing trend and a rapid decline since 1970. Oceanographic processes control large inter-annual pH variability, whereas the long-term declining trend is driven by atmospheric CO2 forcing. The pH extremes are predicted to increase in future warming scenarios, a threat to coral ecosystem. [ABSTRACT FROM AUTHOR]

Following a high-end projection for mass loss from the Greenland and Antarctic ice-sheets, a freshwater forcing was applied to the ocean surface in the coupled climate model EC-Earthv2.2 to study the response to meltwater release assuming an RCP8.5 emission scenario. The meltwater forcing results in an overall freshening of the Atlantic that is dominated by advective changes, strongly enhancing the freshening due to dilution by Greenland meltwater release. The strongest circulation change occurs in the western North Atlantic subpolar gyre and in the gyre in the Nordic Seas, leaving the North Atlantic subpolar gyre the region where most advective salt export occurs. Associated with counteracting changes in both gyre systems, the response of the Atlantic Meridional Overturning Circulation is rather weak over the 190 years of the experiment; it reduces with only 1 Sv ( = 10 6 m 3 s - 1 ), compared to changes in the subpolar gyre of 5 Sv. This relative insensitivity of the AMOC to the forcing is attributed to enhanced convection in the Nordic Seas and stronger overflows that compensate reduced convection in the Labrador and Irminger Seas, and lead to higher sea surface temperatures (SSTs) in the former and lower SSTs in the latter region. The weakened subpolar gyre in the west also shifts the North Atlantic Current and the subpolar-subtropical gyre boundary; with the subtropical gyre expanding, and the western subpolar gyre contracting. The SST changes are associated with obduction of Atlantic waters in the Nordic Seas that would otherwise obduct in the western subpolar gyre. The anomalous SSTs also induce a coupled ocean-atmosphere feedback that further strengthens the Nordic Seas circulation and weakens the western subpolar gyre. This occurs because the anomalous SST-gradient enhances the westerlies, especially between 65 ∘ N and 70 ∘ N; the associated increase in windstress curl consequently enhances the gyre in the Nordic Seas. This feedback is driven by the Greenland mass loss; Antarctic meltwater discharge causes a weaker, opposite response and more particularly affects the South Atlantic salinity budget through northward advection of low-salinity waters from the Southern Ocean. This effect, however, becomes visible only hundred years after the onset of Antarctic mass loss. We conclude that the response to freshwater forcing from both ice caps can lead to a complex response in the Atlantic circulation systems with opposing effects in different subbasins. The relative strength of the response is time-dependent and largely governed by internal feedbacks; the forcing acts mainly as a trigger and is decoupled from the response. [ABSTRACT FROM AUTHOR]

The hydrochemistry of meltwater from the Sutri Dhaka Glacier, Western Himalaya, has been studied to understand the influence of the factors controlling the weathering processes of the glaciers during the peak ablation period. The high solar irradiance prompted intense melting, which has raised the stream flow of the glacier. The meltwater has been observed as slightly alkaline (mean pH 8.2) and contains the major anions (HCO > SO > NO > Cl) and cations (Ca > Mg > K > Na > NH ) with Ca (78.5%) and HCO (74.5%) as the dominant species. The piper diagram indicates the category of stream meltwater as Ca-HCO type. In addition, it is evident from the Gibbs diagram that the interaction between the meltwater and bedrock controls the ionic concentrations of the glacial meltwater. The high ratio value (~ 0.75) of HCO /(HCO + SO ) indicates that the carbonate weathering is dominant. Fe and Al followed by Mn, Sr, and Ti are the most dominant trace elements present in the meltwater. The significant negative correlation exhibited by the major ions and Sr with the discharge is recommended for the enrichment of these solutes during the lean discharge periods. However, the insignificant correlation of Fe, Al, Mn, and Ti with discharge suggests their physicochemical control. The principal component analysis (PCA) carried has highlighted three dominant composites, i.e., the water-rock interaction, atmospheric dust inputs, and physicochemical changes in the meltwater. Hence, the present study elucidates the export of geochemical solutes from Sutri Dhaka Glacier and factors governing the water chemistry, which helps in the better understanding of hydrochemical processes of the Himalayan glaciers and substantial improvement of our understanding about the glacio-hydrological environments and their response in the scenario of global warming. [ABSTRACT FROM AUTHOR]

The sediment cores collected from lakes L-8, L-10, and L-12 of Larsemann Hills, East Antarctica, were investigated for sediment components (sand, silt, clay, total organic carbon, total nitrogen, total phosphorus, elemental TOC/TN ratio, and biogenic silica or biogenic opal), major elements (Al, Fe, Mn, Ti, Mg, Ca), and trace metals (Cr, Co, Zn, Cd, Pb, Ba, Cu, Ni) to understand the source, processes, and productivity in the lacustrine sediments. In lake L-10, average sand content was higher than in lakes L-8 and L-12 which indicated the high intensity of mechanical weathering, resulted in releasing coarse-grained material from the rocks in the catchment area. High Ti/Al molar ratios (2.00-3.32) in all the three cores resulted from shorter transportation distance from different parent sources. Higher clay content near the surface in all the three lakes indicated deposition of fine-grained particles supplied by ice-melt water owing to ice-free conditions in the area in recent years. Relatively, higher biogenic silica along with high total organic carbon associated with high clay in the upper section of lakes L-10 and L-12 and middle section of core L-8 indicated deposition of finer particles from suspension which facilitated high primary productivity due to exposure of the lakes to the ice-melt water influx. Further, Mg/Ca ratio in all the three lakes was high near the surface indicating enrichment of biogenic sedimentation. C/N ratio was found to be much less than 10, indicating the major source of organic matter is autochthonous and exclusively derived from algae (C/N < 10) in all the cores. The metal concentration was found to be higher in core L-8 and was found to be associated with finer sediments, as compared to cores L-10 and L-12 where coarser sediments must have diluted the metal content. Ba was found to be of biogenic origin in cores L-8 and L-12 while in core L-10 it was of lithogenic origin. Cd, Zn, and Ni in all the three lakes were found to be mainly of biogenic origin, whereas all other metals studied were of lithogenic origin. Thus, the concentration of trace metals in Larsemann Hill lake sediments is entirely by natural processes regulated by lithology, catchment processes, and climatic conditions. [ABSTRACT FROM AUTHOR]