Jonguk Kim

Metal-bearing molten sulfur collected from a submarine volcano: Implications for vapor transport of metals in seafloor hydrothermal systems

Gray, metal-bearing molten sulfur was collected from the central volcanic cone of an active off-axis caldera (MTJ-1) in the northeastern Lau Basin, southwestern Pacific Ocean, during an expedition for hydrothermal vents undertaken by the Korea Deep Ocean Study (KODOS) program in 2006. The molten sulfur enveloped the frame and chain bag of the dredge at a depth of ∼1700 m. Microscopically, the sulfur matrix contains numerous covellite (CuS) inclusions of various sizes and irregular shapes, indicating their formation via volcanic sublimation. The molten sulfur is enriched in elements commonly associated with magmatic input, including Cu, As, Au, Bi, Te, and Sb. The low sulfur isotope composition of the sulfur (δ 34 S = −7.5‰ to −8.2‰) indicates an origin via the magmatic degassing of SO 2 and disproportionation. The present results represent field-scale evidence for metal transport via sulfur-rich volcanic degassing in submarine massive sulfide deposits. The molten sulfur deposit plays an important role in enhancing the efficiency of the transfer of magmatic fluid to the hydrothermal system.

Tectonic and magmatic control of hydrothermal activity along the slow‐spreading Central Indian Ridge, 8°S–17°S

The complex geology and expansive axial valleys typical of slow-spreading ridges makes evaluating their hydrothermal activity a challenge. This challenge has gone largely unmet, as the most undersampled MOR type for hydrothermal activity is slow spreading (20–55 mm/yr). Here we report the first systematic hydrothermal plume survey conducted on the Central Indian Ridge (CIR, 8°S–17°S), the most extensive such survey yet conducted on a slow-spreading ridge. Using a combined CTD/Miniature Autonomous Plume Recorder (MAPR) package, we used 118 vertical casts along seven segments of the CIR (∼700 km of ridge length) to estimate the frequency of hydrothermal activity. Evidence for hydrothermal activity (particle and methane plumes) was found on each of the seven spreading segments, with most plumes found between 3000 and 3500 m, generally <1000 m above bottom. We most commonly found plumes on asymmetric ridge sections where ultramafic massifs formed along one ridge flank near ridge-transform intersections or nontransform offsets. The estimated plume incidence (ph) for axial and wall casts (ph=0.30, 35 of 118 casts) is consistent with the existing global trend, indicating that the long-term magmatic budget on the CIR is the primary control on the spatial frequency of hydrothermal venting. Our results show that the tectonic fabric of the CIR strongly determines where hydrothermal venting is expressed, and that using only near-axial sampling might underestimate hydrothermal activity along slow-spreading and ultraslow-spreading ridges. Serpentinization is a minor contributor to the plume inventory, based on 15 profiles with methane anomalies only, predominantly at depths above the local valley walls.

Relationship between polymetallic nodule genesis and sediment distribution in the KODOS (Korea Deep Ocean Study) Area, Northeastern Pacific

Polymetallic nodule and sediment characteristics were investigated for two blocks (KR2 and KR5) in the Korea Deep Ocean Study (KODOS) area in order to better understand nodule distribution and the potential effects of sediments on nodule genesis. The northern block (KR2) is dominated by hydrogenetic nodules, whereas the southern block (KR5) is dominated by diagenetic nodules. Sediments in the study area are assigned to three major lithologic units which are distinctive in color and texture. The northern block is characterized by a thick, metalpoor Unit 1 sediment, which is thin in the southern block, where metal-rich Units 2b and 3 occur close to the surface. The distribution of different nodule genetic types in the northern and southern blocks can be attributed to topographic variations (topographic high near seamounts in KR2 and abyssal plain in KR5) and different sedimentation rates (0.1 and 0.32 mm/kyr in blocks KR2 and KR5, respectively). The southern block has a geologic setting more conducive to diagenetic nodule formation, such as flat topography and sediment composition. Nodule distribution in the studied blocks might also be explained by the distribution of the sediment units of different metal contents. The northern block, in which Unit 1 is thicker, has more abundant hydrogenetic nodules, possibly because Unit 1 prevents metals that are remobilized from the underlying sediments from reaching the seabed where the nodules are forming.

Geochemistry and petrogenesis of mafic-ultramafic rocks from the Central Indian Ridge, latitude 8°–17° S: denudation of mantle harzburgites and gabbroic rocks and compositional variation of basalts

This study investigates the formation of lower oceanic crust and geochemical variations of basalts along the Central Indian Ridge (CIR, lat. 7°45′–17°10′ S). Harzburgites, various gabbroic cumulates, medium- to fine-grained oxide gabbros, diabases, and pillow basalts were recovered by dredging from segment ends such as ridge-transform intersections (RTIs), non-transform discontinuities (NTDs), and transform offset areas. The occurrence of both harzburgites and gabbroic rocks with minor basalts at all segments ends, and leucogabbro intrusive into harzburgite at the 12°45′ S NTD indicates that oceanic crust at segment ends exposes mantle-derived harzburgites and gabbroic intrusions with a thin basaltic cover due to sparse magmatic activity. Basalts collected along the entire ridge show wide compositional variations between N (normal)- and E (enriched)-mid-ocean ridge basalt (MORB). T (transitional)-MORBs with enriched affinities are more prominent than N-MORBs. There is no tendency of enrichment towards specific directions. (La/Sm)N variations in MORB along the CIR (8°–21°S) fluctuates at a regional scale with local high positive anomalies reflecting compositional heterogeneity of the sub-CIR mantle domain.

Textural and geochemical characteristics of Fe−Mn crusts from four seamounts near the Marshall Islands, western Pacific

Textural and geochemical properties of ferromanganese crust (Fe−Mn crust) samples from four adjacent seamounts near the Marshall Islands were investigated to delineate the paleoceanographic condition on their growth history. The Fe−Mn crust samples of this study show four distinct layers (layers 1 to 4 from top to bottom). The uppermost layer 1 is massive and black, and is enriched in hydrogenetic elements such as Mn, Co, Ni, and Mo. The next layer 2 is porous and filled with sediment. Detrital (Al, Rb, and Ti) and biogenic (Cu, Zn, and Ba) elements are enriched in layer 2. The layers 3 and 4 are phosphatized layers which are impregnated with carbonate fluorapatite (CFA), and therefore their primary mineralogy and geochemistry were not preserved. The property of layer 2 suggested that this layer had grown under the condition of high biogenic and detrital flux. Such a condition may be met in the Inter-tropical Convergence Zone (ITCZ) where the northeast and southeast trade winds meet. Considering the present location and paleotracking of seamounts, layer 2 appears to have formed when these seamounts were beneath the ITCZ. On the other hand, layer 1 may have started to grow after the seamounts moved out of the ITCZ with the northwestward movement of the Pacific plate. Our study indicates that the Fe−Mn crusts can be used to trace the paleolocation of ITCZ when precise age determination and information on the plate movement are provided.

Geophysical and Geological Exploration of Cobalt-rich Ferromanganese Crusts on a Seamount in the Western Pacific

Co-rich ferromanganese crusts (Fe-Mn crusts) distributed on the seamounts in the western Pacific are potential economic resources for cobalt, nickel, platinum, and other rare metals in the future. Regulations for prospecting and exploration of Fe-Mn crusts in the Area, which enables the process to obtain an exclusive exploration right for blocks of the fixed size, were enacted recently by the International Seabed Authority, which led to public attention on its potential for commercial development. Evaluation and selection of a mining site can be established based on abundance and grade of Fe-Mn crusts in the site as well as topography that should be smooth enough for mining efficiency. Therefore, acquisition of shipboard echo-sounding and acoustic backscatter data are prerequisite to select potential mine sites in addition to visual and sampling operations. Acoustic backscatter data can be used to locate crust-covered areas in a regional scale with the understanding of acoustic properties of crust through its correlation with visual and sampling data. KIOST had collected the topographic and geologic data to assess the resources potential for Fe-Mn crusts in the west Pacific region from 1994 to 2001. However, they could not obtain acoustic backscatter data that is crucial for the selection of prospective mining sites. Therefore, additional exploration surveys are required to carry out side scan sonar mapping combined with seafloor observation and sampling to decide the blocks for application of an exclusive exploration right.

Regional Variability of Manganese Nodule Facies in the KR1 Area in KODOS Area, Northeastern Equatorial Pacific

Research Vessel Building Unit, KIOST Ansan, P. O. Box 29, Seoul 425-600, KoreaHigh-resolution bathymetry and physico-chemical properties of manganese nodules were explored to identify therelationship between morphological features and nodule occurrences in the KR1, one of the Korean contract nodulefields located in the NE Pacific. The high-resolution seabed mapping showed that the southwestern sector of theKR1 (KR1-1) was relatively deeper than the northeastern sector (KR1-2) which is occupied by small-scale sea-mounts. In terms of nodule occurrence, manganese nodules in the KR1-1 were comparatively larger (2-4 cm) withrough surface (t-type) and discoidal shapes (D-type), while those in the KR1-2 were generally small (<2 cm) withsmooth surface (s-type) and irregular shapes (I-type). In addition, the nodules in the KR1-1 had higher contents ofCu, Mn and Ni. Such connections of water depths to nodule appearances and metal contents are commonlyobserved in the Pacific nodule fields. On the other hand, the nodules in the KR1-2 tend to be controled by morpho-logical features. The seamounts in the KR1-2 might continuously provide rock fragments as new nuclei of manga-nese nodules. As a result, the nodules could not grow over than 2 cm and showed the shapes of a newbie (i.e.,smooth surface and irregular shapes). As a result, our observations indicate that occurrence features of manganesenodules could be subjected to water depths and seabed morphology simultaneously.

Evaluation of Correction Parameter for the Free-fall Grab Based Mn Nodule Abundance in the Southern Sector of the KODOS

Quantitative estimate of manganese nodules based on the operation of a free fall grab (FFG) needs to be corrected because of its less retrieval ability. Previously, the correction parameter of the nodule abundance collected by FFG was calculated based on the image analysis of the photos of bottom sediment in the northern sector of the nodule exploration area of Korea in the NE equatorial Pacific. However, nodules in the southern sector are commonly covered by sediment, which may prevent the use of the correction parameter estimated by photographic techniques. In this study, we attempted dual nodule sampling at the same location by different equipments (i.e. box corer (BC) and FFG) to examine the previous correction parameter of nodule abundance for FFG operation. During the exploration cruises in 2007 to 2009, Mn-nodules were collected from 40 stations both by BC and FFG in the southern sector. The correlation analysis between BC and FFG operations revealed that the BC collected nodules 1.43 times larger than FFG. Our result suggests that the correction parameter of 1.43 can be applied for collection of FFG data to estimate Mn-nodule distribution in the southern sector. The obtained parameter is similar to the previous parameter (1.42~1.45) calculated by the image analysis method, indicating an usefulness of new correction parameter suggested by this study.

Chemical Speciations of Elements in the Fe-Mn Crusts by Sequential Extraction

Abstract : Sequential extraction was carried out on twenty two subsamples of three ferromanganese crustsfrom three seamounts (Lemkein, Lomilik, and Litakpooki) near the Marshall Islands in the western Pacific.The extraction was designed to fractionate Fe-Mn crust forming elements into four defined groups: (1)exchangeable and carbornate, (2) Mn-oxide, (3) Fe-oxyhydroxide, and (4) residual fraction. X-raydiffraction result shows that target material were well removed by each extraction step except for CFA inphosphatized crusts generation. According to chemical analysis of each leachate, most of elements in theFe-Mn crusts are bound with two major phases. Mn, Ba, Co, Ni, Zn, (Fe, Sr, Cu, and V) are stronglybounded with Mn-oxide (δ-MnO 2 ) phase, whereas Fe, Ti, Zr, Mo, Pb, Al, Cu, (V, P, and Zn) show chemicalaffinity with Fe-oxyhydroxide phase. This result indicates that significant amount of Al, Ti, and Zr can notbe explained by detrital origin. Ca, Mg, K, and Sr mainly occur as exchangeable elements and/or carbonatephase. Outermost layer 1 and innner layer 2 which are both young crusts generations are similar in chemicalspeciations of elements. However, some of Fe-oxyhydroxide bounded elements (Pb, Y, Mo, Ba, Al, and V)in phosphatized innermost layer 3 are released during phosphatization and incorporated into phosphate (Pb,Y, Mo, and Ba) or Mn-oxide phase (Al and V). Our sequential extraction results reveal that chemicalspeciations of elements in the hydrogenetic crusts are more or less different from interelemental relationshipcalculated by statistical method based on bulk chemistry.Key words : (sequential extraction), (Fe-Mn crusts), (phosphatization), (hydrogenetic elements)

Sinking Particle Flux in the Subtropical Oligotrophic Northwestern Pacific from a Short-term Sediment Trap Experiment

Time-series sediment traps were deployed in the subtropical oligotrophic northwestern Pacific (SONP) at three depths from August to September 2015 to better understand vertical flux of sinking particles. Sinking particles were collected at 5-day intervals over the sediment trap deployment period. The average total mass flux at water depths of 400 m, 690 m, and 1,710 m was 9.1, 4.4, and 4.1 mg m-2day-1, respectively. CaCO3 materials constituted 50 to 70% of sinking particles while in comparison particulate organic carbon (POC) constituted up to 20%. A synchronous variation of total mass flux was observed at the three depths, indicating that calcite-dominated particles sank from 400 to 1,710 m within a 5-day period. POC flux at these water depths was 2.4, 0.38, and 0.31 mg m-2day-1, respectively. Our results indicate low transfer efficiencies of 16% from 400 to 690 m and 13% for the 400 to 1,710 m depth range. The estimated transfer efficiencies were significantly lower than those observed at the K2 station in the northwest Pacific subarctic gyre, presumably because of a prevalence of pico-cyanobacteria in the SONP. Because cyanobacteria have a semi-permeable proteinaceous shell, they are more readily remineralized by bacteria than are siliceous phytoplankton in the northwest Pacific subarctic gyre. Continued surface water warming and expansion of the SONP will likely have a profound impact on ocean acidification in the northwest Pacific, possibly affecting the transfer efficiency of sinking POC to the deep-sea.