A multidisciplinary mineralogical, geochemical and biomarker study of Indus Fan sediments cored during International Ocean Discovery Program (IODP) Expedition 355 to the Laxmi Basin was carried out ...to define the different compositional signatures of sand, silt and clay. Upper Pliocene - lower Pleistocene turbidites from sites U1456 and U1457 were selected as the best candidates for this study. The integrated dataset presented here was obtained by coupling traditional and innovative bulk-sediment and single-mineral techniques on the same samples. Turbiditic deposits mostly consist of medium to fine silt, including rich and diverse heavy-mineral assemblages. Such a fine grain size forced us to push the limits of high-resolution quantitative heavy-mineral analysis down to as low as 5 µm. Heavy-mineral analysis allowed us to establish a Himalayan origin of the detritus in the studied turbidites. Heavy-mineral concentrations are higher in channel-fill than in overbank deposits. Mineralogical and geochemical data concur in revealing that fast-settling ultradense minerals such as zircon are preferentially concentrated in channel-fill deposits, whereas the top of overbank deposits are notably enriched with slow-settling platy phyllosilicates. Biomarker analysis represents a most suitable complementary technique that is able to investigate the provenance signature of the finer sediment fraction, largely consisting of clay. This technique allowed us to identify a largely terrigenous origin of organic matter at Site U1456 and an open marine origin at Site U1457. The latter site lies closer to the Laxmi Ridge, where thermal maturity increases with depth to reach the early oil window (127°C at c. 320 m below the seafloor).
Peridotitic, dunitic, and wehrlitic rocks drilled at Atlantis Massif (Mid-Atlantic Ridge), where mantle-derived rocks were exhumed to the sea floor via detachment faulting, were investigated ...employing in situ geochemical analyses. Since dunitic and wehrlitic rocks are commonly interpreted as fossil melt channels, they likely transported parental melts of mid-ocean ridge basalt (MORB) beneath paleo-ridge axis. Although the rocks are severely serpentinized, primary olivines, chromian spinels, orthopyroxenes, and clinopyroxenes were found in several samples. The Cr/(Cr + Al) ratios (Cr#) of the chromian spinel show bimodal distribution, where one group records higher Cr# (0.35–0.5) and the other group is represented by lower Cr# (0.2–0.3). The extent of mantle melting is strongly correlated to the Cr# of the chromian spinel, thus the mantle-derived materials of the Atlantis Massif bear at least two end-members with regard to the extent of melting: refractory mantle material with higher Cr#, and less refractory mantle material with lower Cr#. To quantitatively evaluate mantle melting and melt transport mechanisms in the suboceanic mantle beneath the section of the Mid-Atlantic Ridge, a one-dimensional, steady-state decompressional mantle melting model was carried out employing rare-earth element concentrations of clinopyroxene. Our modeling results demonstrate the presence of refractory peridotites molten in the presence of garnet, but crustal materials are limited in the Atlantis Massif. We advocate that the refractory mantle materials underwent little recent melting beneath the paleo-ridge axis, and alternatively had been subjected to ancient partial melting. The refractory mantle materials probably contributed to the initiation of the detachment fault and affected the extreme variations in MORB chemistry in the Mid-Atlantic Ridge.
•Peridotites are divided into at least two end members in terms of melt depletion.•Refractory peridotites underwent little recent melting beneath paleo-ridge axis.•Wehrlite was crystallized from a depleted melt.•Detachment fault was initiated under melt-limited condition.•Refractory peridotites affect the extreme variation in MORB chemistry.
Palaeoenvironmental manifestations of the Paleocene–Eocene thermal maximum (PETM; ~
56
Ma) are relatively well documented in low- to mid-latitude settings and at high southern latitudes, but no ...documented high northern latitude sites record the entire hyperthermal event. We present high-resolution multi-proxy records from a PETM succession on Spitsbergen in the high Arctic (palaeolatitude ~
75
°N). By comparing our results with those from Integrated Ocean Drilling Program Site 302-4A, we document regional palaeoenvironmental variations in the expression of the PETM, with evidence for major differences in basin-margin vegetation and water column oxygen depletion. Sedimentological, palynological and geochemical data demonstrate a pre-PETM sea level rise in Spitsbergen before the −
4‰ δ
13C
TOC excursion, which culminated in maximum flooding during the peak of the event. The appearance of the dinoflagellate cyst
Apectodinium before the onset of the carbon isotope excursion (CIE) corroborates that environmental change in the Arctic had begun prior to the CIE. Sedimentological and palynological evidence indicate that elevated terrestrial runoff resulted in water column stratification, providing further evidence for an intensification of the hydrological cycle during the PETM.
► Pre-PETM sea level rise in Spitsbergen, with maximum flooding during peak CIE. ► Pre-CIE
Apectodinium indicate environmental change before isotope excursion. ► Elevated terrestrial runoff resulted in water column stratification during PETM. ► Evidence for an intensification of the hydrological cycle during the PETM.
The IODP‐ICDP Expedition 364 drilling recovered a 829 m core from Hole M0077A, sampling ˜600 m of near continuous crystalline basement within the peak ring of the Chicxulub impact structure. The bulk ...of the basement consists of pervasively deformed, fractured, and shocked granite. Detailed geochemical investigations of 41 granitoid samples, that is, major and trace element contents, and Sr–Nd isotopic ratios are presented here, providing a broad overview of the composition of the granitic crystalline basement. Mainly granite but also granite clasts (in impact melt rock), granite breccias, and aplite were analyzed, yielding relatively homogeneous compositions between all samples. The granite is part of the high‐K, calc‐alkaline metaluminous series. Additionally, they are characterized by high Sr/Y and (La/Yb)N ratios, and low Y and Yb contents, which are typical for adakitic rocks. However, other criteria (such as Al2O3 and MgO contents, Mg#, K2O/Na2O ratio, Ni concentrations, etc.) do not match the adakite definition. Rubidium–Sr errorchron and initial 87Sr/86Srt=326Ma suggest that a hydrothermal fluid metasomatic event occurred shortly after the granite formation, in addition to the postimpact alteration, which mainly affected samples crosscut by shear fractures or in contact with aplite, where the fluid circulation was enhanced, and would have preferentially affected fluid‐mobile element concentrations. The initial (ɛNd)t=326Ma values range from −4.0 to 3.2 and indicate that a minor Grenville basement component may have been involved in the granite genesis. Our results are consistent with previous studies, further supporting that the cored granite unit intruded the Maya block during the Carboniferous, in an arc setting with crustal melting related to the closure of the Rheic Ocean associated with the assembly of Pangea. The granite was likely affected by two distinct hydrothermal alteration events, both influencing the granite chemistry: (1) a hydrothermal metasomatic event, possibly related to the first stages of Pangea breakup, which occurred approximately 50 Myr after the granite crystallization, and (2) the postimpact hydrothermal alteration linked to a long‐lived hydrothermal system within the Chicxulub structure. Importantly, the granites sampled in Hole M0077A are unique in composition when compared to granite or gneiss clasts from other drill cores recovered from the Chicxulub impact structure. This marks them as valuable lithologies that provide new insights into the Yucatán basement.
Although spherical video-based virtual reality (SVVR) has gained attention as an effective tool for technology-enhanced learning, little research has been conducted on its integration in formal ...education settings and the role of teachers in facilitating its pedagogical use. In this study, a design-based research (DBR) approach was used to develop teacher-facilitated tactics for SVVR-supported learning based on virtual inquiry fieldwork in formal senior secondary geography education in Hong Kong. Over a period of 2 academic years, we engaged with nine teachers (from nine schools across the upper, middle, and lower academic bands in Hong Kong) and their Grade-10 classes, which comprised a total of 504 students. In addition to reporting and discussing the pedagogical effectiveness of the teacher facilitation practices designed, enacted, analysed, and refined in the iterative DBR process, this paper presents the teacher facilitation principles deduced from these practices. Overall, the findings and implications of this DBR are academically and practically relevant for researchers and educational practitioners who seek to utilise SVVR for educational purposes and strive to enhance its pedagogical effectiveness.
•Research on the integration of SVVR into formal education and the pedagogical role of teachers therein is in its infancy.•This DBR formulated apt pedagogic tactics for scaffolding students in SVVR-supported inquiry fieldwork-based learning.•A theory-driven teacher-facilitated pedagogic framework for harnessing SVVR in formal schooling was developed.•An empirical reference for introducing SVVR-supported inquiry fieldwork-based learning into formal schooling was presented.
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 stre
ss
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.
Geodynamics of the South China Sea Sibuet, Jean-Claude; Yeh, Yi-Ching; Lee, Chao-Shing
Tectonophysics,
12/2016, Letnik:
692, Številka:
Part.B
Journal Article
Recenzirano
Odprti dostop
The beginning of seafloor spreading in the South China Sea (SCS) is now established from IODP drilling Leg 349 at 33Ma. Chron 12 (32Ma) is the oldest chron identified in the SCS. The nature of the ...crust of the northeastern part of the SCS located north of chron C12, where chrons 15 to 17 were previously identified, is not oceanic but thinned continental crust intruded by volcanic elongated features emplaced 17–22Ma ago. Based on magnetic anomaly identifications, the end of the SCS spreading could be either 15.5, 20.5Ma (Briais et al., 1993; Barckhausen et al., 2014) or something else. However, as post-spreading magmatic activity (~13–3.5Ma) largely masks the spreading fabric in particular near the axis of the east sub-basin, published locations of the axial magnetic anomaly (extinct spreading axis) and spreading rates are not reliable. A contoured map of the extremely dense set of magnetic data shows that a few magnetic lineations belonging to the magnetic seafloor spreading fabric are still preserved and parallel to the N055° bathymetric seafloor spreading trends identified on swath-bathymetric maps in the central part of the SCS, suggesting that the extinct ridge axis is N055° trending with potential N145° transform faults. Based on published swath-bathymetric data, oceanic domains with different seafloor spreading lineaments have been delimited (N055°, N075° and N085°) and provide important constraints used to propose a kinematic sketch of the SCS opening. As a consequence, the Zhongnan faults zone, located between the east and southeast sub-basins, acted as a major fracture zone system during the SCS opening, with horizontal offsets varying from ~50km to ~140km.
The flow-line pattern defined from the seafloor spreading lineaments and the few identified FZs have been used to highlight conjugate segments of continental margins. During the first phases of opening of the SCS, from the fit of continents to chron C10 (30Ma), the N175° extension observed in the Xisha trough and eventually south of the Macclesfield Bank extends to the Qui Nhon ridge, located along the eastern margin of Vietnam, in the southward prolongation of the Red River fault system. Normal faults curve toward the south with a horsetail geometry interpreted as evidence for a few tens of kilometers of dextral motion along the Qui Nhon ridge. Since chron 10 and until the end of SCS opening, the plate boundary located between the southern South China Sea (SSCS) and EU plates jumped westward several times from the location of the Ulugan fault near Palawan to the western limit of the southwest basin, explaining the progressive formation of the SCS from east to west and giving the characteristic V-shape of the SCS. The opening of the whole SCS is linked and occurred simultaneously with the northward subduction of the proto-SCS whose suture is located south of Palawan and extends westwards in north Borneo.
•Northeastern South China Sea crust is thinned continental crust intruded by volcanics.•Post-spreading magmatic activity (<13Ma) largely masks the spreading fabric.•N055°, N075° and N085° seafloor spreading oceanic domains are identified.•Extension in Xisha trough and south of Macclesfield Bank extends to Qui Nhon ridge.•South China Sea (SCS) opening is compensated by northward proto-SCS subduction.
Post-mega-earthquake geochemical and microbiological properties in subseafloor sediments of the Japan Trench accretionary wedge were investigated using core samples from Hole C0019E, which was ...drilled down to 851 m below seafloor (mbsf) at a water depth of 6,890 m. Methane was abundant throughout accretionary prism sediments; however, its concentration decreased close to the plate boundary decollement. Methane isotope systematics indicated a biogenic origin. The content of molecular hydrogen (H2) was low throughout core samples, but markedly increased at specific depths that were close to potential faults predicted by logging-while-drilling analyses. Based on isotopic systematics, H2 appeared to have been abundantly produced via a low-temperature interaction between pore water and the fresh surface of crushed rock induced by earthquakes. Subseafloor microbial cell density remained constant at approximately 105 cells mL–1. Amplicon sequences revealed that predominant members at the phylum level were common throughout the units tested, which also included members frequently found in anoxic subseafloor sediments. Metabolic potential assays using radioactive isotopes as tracers revealed homoacetogenic activity in H2-enriched core samples collected near the fault. Furthermore, homoacetogenic bacteria, including Acetobacterium carbinolicum, were isolated from similar samples. Therefore, post-earthquake subseafloor microbial communities in the Japan Trench accretionary prism appear to be episodically dominated by homoacetogenic populations and potentially function due to the earthquake-induced low-temperature generation of H2. These post-earthquake microbial communities may eventually return to the steady-state communities dominated by oligotrophic heterotrophs and hydrogenotrophic and methylotrophic methanogens that are dependent on refractory organic matter in the sediment.
PDF
