Mantle heterogeneity beneath the Antarctic–Phoenix Ridge off Antarctic Peninsula

Abstract   We determined the Sr, Nd and Pb isotopic compositions of basalts recovered from the Antarctic–Phoenix Ridge (APR), a fossil spreading center in the Drake Passage, Antarctic Ocean, in order to understand the nature of the subridge mantle source. There are no known hotspots in close proximity to the site. We observe that small-scale isotopic heterogeneity exists at a shallow level in the subaxial mantle of the APR. Enriched (E-type) mid-ocean ridge basalts (MORB) coexist with normal (N-type) MORB in this region. The E-type basalts are: (i) relatively young compared to the N-type samples; (ii) were erupted after the extinction of the APR; and (iii) have been generated by low-degree partial melting of an enriched mantle source. Extinction of the APR likely caused the extent of partial melting in this region to decrease. We interpret that the geochemically enriched materials dispersed in the ambient depleted mantle were the first fraction to melt to form the E-type MORB.     

Thermally induced brittle deformation in oceanic lithosphere and the spacing of fracture zones

Brittle deformation of oceanic lithosphere due to thermal stress is explored with a numerical model, with an emphasis on the spacing of fracture zones. Brittle deformation is represented by localized plastic strain within a material having an elasto-visco-plastic rheology with strain softening. We show that crustal thickness, creep strength, and the rule governing plastic flow control the formation of cracks. The spacing of primary crack decreases with crustal thickness as long as it is smaller than a threshold value. Creep strength shifts the threshold such that crust with strong creep strength develops primary cracks regardless of crustal thicknesses, while only a thin crust can have primary cracks if its creep strength is low. For a thin crust, the spacing of primary cracks is inversely proportional to the creep strength, suggesting that creep strength might independently contribute to the degree of brittle deformation. Through finite versus zero dilatation in plastic strain, associated and non-associated flow rule results in nearly vertical and V-shaped cracks, respectively. Changes in the tectonic environment of a ridge system can be reflected in variation in crustal thickness, and thus related to brittle deformation. The fracture zone-free Reykjanes ridge is known to have a uniformly thick crust. The Australian-Antarctic Discordance has multiple fracture zones and thin crust. These syntheses are consistent with enhanced brittle deformation of oceanic lithosphere when the crust is thin and vice versa.     

Origin and evolution of two contrasting thermal groundwaters (CO2-rich and alkaline) in the Jungwon area,South Korea: Hydrochemical and isotopic evidence

In the Jungwon area, South Korea, two contrasting types of deep thermal groundwater (around 20–33 °C) occur together in granite. Compared to shallow groundwater and surface water, thermal groundwaters have significantly lower δ¹⁸O and δD values (> 1‰ lower in δ¹⁸O) and negligible tritium content (mostly < 2 TU), suggesting a relatively high age of these waters (at least pre-thermonuclear period) and relatively long subsurface circulation. However, the hydrochemical evolution yielded two distinct water types. CO₂-rich water (P_CO2 = 0.1 to 2 atm) is characterized by lower pH (5.7–6.4) and higher TDS content (up to 3300 mg/L), whereas alkaline water (P_CO2 = 10^− 4.1–10^− 4.6 atm) has higher pH (9.1–9.5) and lower TDS (< 254 mg/L). Carbon isotope data indicate that the CO₂-rich water is influenced by a local supply of deep CO₂ (potentially, magmatic), which enhanced dissolution of silicate minerals in surrounding rocks and resulted in elevated concentrations of Ca²⁺, Na⁺, Mg²⁺, K⁺, HCO₃⁻ and silica under lower pH conditions. In contrast, the evolution of the alkaline water was characterized by a lesser degree of water–rock (granite) interaction under the negligible inflow of CO₂. The application of chemical thermometers indicates that the alkaline water represents partially equilibrated waters coming from a geothermal reservoir with a temperature of about 40 °C, while the immature characteristics of the CO₂-rich water resulted from the input of CO₂ in Na–HCO₃ waters and subsequent rock leaching.     

Glauconite grains in continental shelf sediments around the Korean Peninsula and their depositional implications

Glauconite grains are an important component of the surficial sediments on the Korean continental shelf. Relatively high glauconite contents (>20%) occur near Huksan Island in the Yellow Sea and on the outer shelf of the southern East Sea where they are associated with relict, sandy sediments. By contrast, glauconite grains are rare (<1%) in clay-dominated sediments. The grains can be classified into four major categories: (1) very abundant, green to dark green, spheroidal and lobate grains displaying bulbous and honeycomb exteriors with well-developed surface cracks; (2) dark green, discoidal grains with poorly defined cracks; (3) scarce, moderately dark green, accordion-shaped grains characterized by series of closely spaced, parallel surface ridges; (4) scarce, green to dark green, foraminifer-shaped grains. Microprobe analyses as well as optical microscopy and SEM observations show that most grains are composed of mineral mixtures (e.g., quartz, mica, feldspar) rather than a single mineral species. The mineralogy, morphology, and textural properties suggest that the grains may have formed mainly by replacement of fecal pellets, and the alteration of mica and clays which have filled foraminifera tests. The high potassium contents, rosette-shaped clay structures, and bulbous shapes reflect an “evolved” (mature) stage of glauconitization. Glauconite grains in Korean shelf sediments are presumably relict, and have been produced by the reworking of older glauconitic sediments during the Holocene sea-level transgression. Received: 4 October 1999 / Revision accepted: 18 May 2000     

Evolution of the Taebaeksan Basin,Korea: II,late Paleozoic sedimentation in a retroarc foreland basin and assembly of the proto‐Korean Peninsula

The Taebaeksan Basin comprises the lower Paleozoic Joseon Supergroup and the upper Paleozoic Pyeongan Supergroup, which are separated by a disconformity representing a 140 myr‐long hiatus. This paper deals mainly with the late Paleozoic paleogeographical and tectonic evolution of the Taebaeksan Basin on the basis of updated stratigraphy, sedimentation, and geochronology of the Pyeongan Supergroup. Late Paleozoic sedimentation in the Taebaeksan Basin recommenced at ~ 320 Ma and formed a thick siliciclastic succession of marginal marine and non‐marine alluvial deposits, the Pyeongan Supergroup. The Pyeongan Supergroup was deposited in a retroarc foreland basin formed by build‐up of a magmatic arc along the northern margin of the Sino‐Korean Craton. The formation of sedimentary deposits ceased at ~ 250 Ma due to the collision of the Sino‐Korean Craton and South China Craton that generated the Triassic Songnim orogeny in Korea. Diverse tectonic models have been proposed for assembly of the proto‐Korean Peninsula, but the indented wedge model is considered to best explain the geological features of the peninsula. The indented wedge model entails northward subduction of the central block of the Korean Peninsula (part of the South China Craton) beneath the northern block of the Korean Peninsula (part of the Sino‐Korean Craton) along the Sulu‐Imjingang Belt.     

Numerical simulations using momentum source wave-maker applied to RANS equation model

For Navier-Stokes equation model using the VOF scheme, Lin and Liu (Lin, P. and Liu, P.L.-F. (1999). Internal wave-maker for Navier-Stokes equations models. J. Waterw. Port Coast. Ocean Eng., 125 (4), 207–215.) developed an internal wave-maker method for which a mass source function of the continuity equation was used to generate target wave trains. Using this internal wave-maker method, various numerical experiments have been conducted without any problems due to waves reflected by a wave-maker. In this study, an internal wave-maker method using a momentum source function was proposed. Various numerical simulations in two and three dimensions were performed using the momentum source wave-maker applied to the RANS equation model in a CFD code, FLUENT. To verify their applicability in 2 dimensions, the computational results obtained using the momentum source wave-maker in a channel of constant depth were compared with the results obtained by using the mass source wave-maker and with the analytical solutions. And the results of the present numerical simulations of hydraulic experiments, which represent nonlinear waves on a submerged shoal and breaking waves on a plane beach, were compared with measurements. The comparisons showed good agreements between them. To see their applicability into 3-dimensional cases, the present results in a basin of constant depth were compared with the analytical solutions, and they agreed well with each other. In addition, vertical variation of longshore current was presented by using the 3-dimensional simulation results.     

Evolution of waves and currents over a submerged laboratory shoal

The vertically-integrated effect of interaction between waves and wave-induced currents on wave transformation over a submerged elliptic shoal was investigated based on numerical simulations of the Vincent and Briggs experiment [Vincent, C.L., Briggs, M.J., 1989. Refraction- diffraction of irregular waves over a mound. Journal of Waterway, Port, Coastal and Ocean Engineering, 115(2), pp. 269–284.]. The numerical simulations were performed using two numerical wave-current model systems: one, a combination of the wave model SWAN and the current model SHORECIRC, and the other, a combination of the wave model REF/DIF and the same current model. A time-dependent, phase-resolving wave and current model, FUNWAVE, was also utilized to simulate the experiment. In the simulations, the developed wave-induced currents defocused waves behind the shoal and brought on a wave shadow zone that showed relatively low wave height distributions. For the breaking case of monochromatic waves, the wave heights computed using FUNWAVE showed good agreement with the measurements and the resulting wave-induced currents showed a jet-like velocity distribution in transverse direction. And the computed results of the two model combinations agreed better with the measurements than the computed results obtained by neglecting wave-current interaction. However, it was found that for the case in which transverse interference pattern caused by refracted waves was strong, REF/DIF-SHORECIRC did not correctly evaluate radiation stresses, the gradients of which generate wave-induced currents. SWAN-SHORECIRC, which cannot deal with the interference patterns, predicted a jet-like wave-induced current. For breaking random wave cases, the computed results of the two model combinations and FUNWAVE agreed well with the measurements. The agreements indicate that it is necessary to take into account the effect of wave-induced current on wave refraction when wave breaking occurs over a submerged shoal.     

Seasonal Variation of Submesoscale Flow Features in a Mesoscale Eddy-dominant Region in the East Sea

Seasonal changes in the distribution of submesoscale (SM) flow features were examined using a fine-resolution numerical simulation. The SM flows are expected to be strong where mesoscale (MS) eddies actively develop and also when the mixed layer depth (MLD) is deep due to enhanced baroclinic instability. In the East Sea (ES), MS eddies more actively develop in summer while the MLD is deeper in winter, which provided the motivation to conduct this study to test the effects of MLD and MS eddies on the SM activity in this region. Finite-scale Liapunov exponents and the vertical velocity components were employed to analyze the SM activities. It was found that the SM intensity was marked by seasonality: it is stronger in winter when the mixed layer is deep but weaker in summer - despite the greater eddy kinetic energy. This is because in summer the mixed layer is so thin that there is not enough available potential energy. When the SM activity was quantified based on parameterization, (MLD × density gradient), it was determined that the seasonal variation of MLD plays a more important role than the lateral density gradient variation on SM flow motion in the ES.