Northern East China Sea Shelf Research Articles

Terrestrial and marine POC export fluxes estimated by 234Th–238U disequilibrium and δ13C measurements in the East China Sea shelf

The use of 234Th–238U disequilibrium has been widely employed to estimate the sinking flux of particulate organic carbon (POC) from the upper sea and ocean. Here, the deficits of 234Th relative to 238U in the water column and the carbon isotope signature (δ13C) of POC in the East China Sea (ECS) Shelf were measured, which was used to distinguish the fraction of marine and terrestrial POC export fluxes. In the ECS Shelf, very strong deficits of 234Th relative to 238U were observed throughout the water column, with 234Th/238U activity ratios ranging from 0.158 ± 0.045 to 0.904 ± 0.068 (averaging 0.426 ± 0.159). The residence times of particle reactive radionuclide 234Th (τTh–T) in the ECS shelf water varied between 9 and 44 days, which is significantly shorter than that in the continental slope area or the basin area. This phenomenon indicates that there is a more rapid particle scavenging process in the ECS shelf water compared to the continental slope and basin upper water. By applying a two-end-member mixing model based on the δ13C, the fraction of terrestrial POC was estimated to be 0 to 74% (mean: 30 ± 22%) and the fraction of marine POC was in the range of 25% to 100% (mean: 70 ± 22%). Fluxes of marine and terrestrial POC settling to the seafloor exhibited significant spatial differences among different stations, ranging from 11 to 129 mmol C/m2/day and from 2.6 to 38 mmol C/m2/day, respectively. The averaged terrestrial POC fluxes in the southern and northern ECS Shelf were similar (~ 21 to 24 mmol C/m2/day), while the marine POC fluxes in the north (86 ± 37 mmol C/m2/day) were approximately four times higher than those in the south (26 ± 20 mmol C/m2/day). Interestingly, the estimated export flux of both marine and terrestrial POC were approximately one order of magnitude higher than the previously reported burial fluxes of POC (ranging from 1.1 ± 0.1 to 11.4 ± 1.1 mmol C/m2/day) in the underlying bottom sediments, indicating that the majority (> 90%) of both terrestrial and marine POC exported from the upper water column are degraded in the sediments of the ECS Shelf. This “carbon missing” phenomenon can greatly be attributed to rapid decomposition by other processes (including microbial reworking, cross-shelf transport, and possible consumption by benthic organisms). Our findings highlight the dynamic nature of carbon cycling in the continental shelf and the need for further research to understand these processes and improve carbon budget assessments.

Clay minerals and elemental composition of sediments on different sedimentary units in the northern East China Sea shelf: provenance tracing and genetic mechanism analysis

Clay minerals and elemental composition of sediments on different sedimentary units in the northern East China Sea shelf: provenance tracing and genetic mechanism analysis

Developing sediment provenance discrimination tracers using major elements in East China Sea mud deposits

Discriminating shelf sediment provenances using sediment trace metal and rare earth element (REE) concentration and isotope ratios and clay mineral contents has several unavoidable hurdles, such as the effect of sediment grain size on metal concentrations, ubiquitous inclusion of REE-rich heavy minerals in sediments, and meager and inconsistent compositional differences. Furthermore, to date, no study has analyzed and determined a convincing tracer that can differentiate between Huanghe (HH) and Changjiang (CJ) sediment provenances, especially quantitatively. In this study, we quantitatively discriminated the provenances of the southwestern Cheju Island mud deposit (SWCIM) in the northern East China Sea (ECS) shelf, using seven major elements (Si, Al, Fe, K, Na, Ca, and Mg), and further determined the spatial variations of the contributions from the HH and CJ rivers to the northern ECS sediments. The results indicate that the SiO2/Al2O3 and Fe2O3/Al2O3 ratios of the HH, CJ, and Korean (KR) river sediments can be considered as reliable tracers for differentiation of the northern ECS shelf sediment provenance. Furthermore, the quantitative contributions of HH, CJ, and KR to the northern ECS sediment budget were approximately 80, 20, and 0%, respectively. The systematic spatial variations of the HH and CJ river contributions to the sediment provenances in the study area, such as the significant contributions of the HH river in the northwestern part of the SWCIM and the CJ river in the southwestern part, support the availability and applicability of the Fe2O3/Al2O3 ratio as a sediment provenance discrimination tracer. We propose that these major element concentration ratios can be applied to distinguish other siliciclastic shelf sediment provenances.

Effects of water masses on the zooplankton community structure in the northern East China Sea during the East Asian Summer Monsoon in 2020

The prolonged rainy season in 2020 was classified as the second-largest rainfall event since 1961. In this study, we conducted a survey to assess fluctuations in the zooplankton community structure of the northern East China Sea (nECS) after the rainy season in August 2020. We examined 24 stations and observed several environmental factors, including water temperature, salinity, chlorophyll-a concentration, suspended particulate matter (SPM), and phytoplankton density. Zooplankton samples were collected from 12 stations using MOCNESS to target the surface mixed layer (SML), pycnocline layer (PNC), and lower layer (LL). In regions with low water temperatures, the Yellow Sea and northern East China Sea shelf mixed water community in each layer exhibited a decrease in the relative abundance of Calanus sinicus, whereas the incidence of Paracalanus parvus s. l. showed an increasing trend. Similarly, in areas with low salinity, the CDW and northern East China Sea shelf mixed water community in SML and PNC displayed comparable distributions, with a tendency for the relative abundance of Acartia pacifica and Paracalanus parvus s. l. to show a replacement pattern. Moreover, the Tsushima Warm Current (TWC) community in each layer, characterized by high water temperature and salinity, showed a high relative abundance of Oncaea spp. and a diverse species distribution, mainly in the eastern part of the study area. Our study identified multiple vertically distributed communities, providing detailed information on water mass structures through cluster analysis. This approach enhanced previous studies by offering novel insights into the vertical structure of water masses and their association with zooplankton communities in the nECS.

Structural evolution of two‐stage rifting in the northern East China Sea Shelf Basin

This study focuses on the stratigraphic correlation and structural evolution of the Ieodo and Jeju basins in the northern East China Sea Shelf Basin (ECSSB), which are compared to the Changjiang and Xihu depressions, respectively. Based on multi‐channel seismic reflection data, Cenozoic sedimentary successions resting on the acoustic basement in both basins can be divided into multiple syn‐ and post‐rift units. Basement‐involved structures exhibit different orientations, that is, E–W (ENE–WSW) and NE–SW trending structures in the Ieodo and Jeju basins, respectively. They represent the development of two‐stage rifting. The first rift stage began in the Palaeocene and was only restricted to the Ieodo Basin located west of the Hupijiao Rise. The second rift stage subsequently occurred in the Jeju Basin located east of the Hupijiao Rise during the late Eocene to Miocene. The Eocene post‐rift unit of the Ieodo Basin was uplifted while the Jeju Basin was subsided by bounding faults. The sequential development of rift structures in the basins suggests eastward migration of the Cenozoic rifting in the northern ECSSB. The NE‐trending rift structure of the Jeju Basin parallel to the subduction zone of the Pacific Plate suggests that the second rift stage possibly evolved under the influence of the back‐arc extensional regime. The Hupijiao Rise between the Ieodo and Jeju basins was possibly a geologic basement in the Palaeocene and was an uplifted remnant caused by footwall exhumation during the second rift stage of the Jeju Basin.

Post-bomb coral Δ14C record from Iki Island, Japan: possible evidence of oceanographic conditions on the northern East China Sea shelf

In this study, a sea-surface water Δ14C record of AD 1966–2000 (i.e., after the atmospheric nuclear-bomb testing period of the mid-1950s to early 1960s) was reconstructed from a coral sample collected from Iki Island, western Japan. The island is located in the Tsushima Strait where the Tsushima Current flows from the East China Sea (ECS) continental shelf into the Sea of Japan, indicating a strong influence of the ECS shelf water on the island. It is widely accepted that the Tsushima Current originates in the area between the ECS shelf break and the Nansei Islands further offshore as a branch of the Kuroshio Current, although another possible origin is the Taiwan–Tsushima Current System. The Δ14C record from Iki Island shows the following evidence of a response to the atmospheric nuclear testing: (1) an increase from ~55‰ in 1966 to ~133‰ in 1970, (2) a plateau ranging between ~123 and ~142‰ during the 1970s to the late 1980s, and (3) a gradual decrease from ~115‰ in 1990 to ~83‰ in 2000. Comparison of this record with coral Δ14C records from the Nansei Islands (Okinawa Island, Ishigaki Island and Kikai Island), located ~160–280 km off the ECS shelf break and little influenced by the shelf water, suggests that the surface-water Δ14C around Iki Island was ~30–45‰ lower than that of the Nansei Islands from the mid-1960s to late 1970s, and that the Δ14C difference between Iki Island and the Nansei Islands decreased from the end of the 1970s to ~0–15‰ in the mid-1980s to 2000. The lower Δ14C around Iki Island can be explained as follows: (1) in contrast to the Nansei Islands area, the ECS shelf area is a vertically mixed, highly concentrated carbon reservoir significantly connected to subsurface and deeper waters outside the shelf area, strongly suggesting that the surface-water Δ14C of the shelf area (perhaps excepting very shallow innermost shelf areas) was significantly less sensitive to the atmospheric nuclear-bomb 14C spike than that of the Nansei Islands area; (2) the shelf-surface water is conveyed from the northern ECS to Iki Island by the Tsushima Current. Thus, it can be suggested that the post-bomb coral Δ14C record from Iki Island reflects oceanographic conditions of the northern ECS shelf.

Air–sea CO<sub>2</sub> fluxes in the East China Sea based on multiple-year underway observations

Abstract. This study reports the most comprehensive data set thus far of surface seawater pCO2 (partial pressure of CO2) and the associated air–sea CO2 fluxes in a major ocean margin, the East China Sea (ECS), based on 24 surveys conducted in 2006 to 2011. We showed highly dynamic spatial variability in sea surface pCO2 in the ECS except in winter, when it ranged across a narrow band of 330 to 360 μatm. We categorized the ECS into five different domains featuring with different physics and biogeochemistry to better characterize the seasonality of the pCO2 dynamics and to better constrain the CO2 flux. The five domains are (I) the outer Changjiang estuary and Changjiang plume, (II) the Zhejiang–Fujian coast, (III) the northern ECS shelf, (IV) the middle ECS shelf, and (V) the southern ECS shelf. In spring and summer, pCO2 off the Changjiang estuary was as low as < 100 μatm, while it was up to > 400 μatm in autumn. pCO2 along the Zhejiang–Fujian coast was low in spring, summer and winter (300 to 350 μatm) but was relatively high in autumn (> 350 μatm). On the northern ECS shelf, pCO2 in summer and autumn was > 340 μatm in most areas, higher than in winter and spring. On the middle and southern ECS shelf, pCO2 in summer ranged from 380 to 400 μatm, which was higher than in other seasons (< 350 μatm). The area-weighted CO2 flux on the entire ECS shelf was −10.0 ± 2.0 in winter, −11.7 ± 3.6 in spring, −3.5 ± 4.6 in summer and −2.3 ± 3.1 mmol m−2 d−1 in autumn. It is important to note that the standard deviations in these flux ranges mostly reflect the spatial variation in pCO2 rather than the bulk uncertainty. Nevertheless, on an annual basis, the average CO2 influx into the entire ECS shelf was 6.9 ± 4.0 mmol m−2 d−1, about twice the global average in ocean margins.

Dinoflagellate cyst assemblages from the northern shelf sediments of the East China Sea: An indicator of marine productivity

Abstract The dinoflagellate cyst assemblages from the northern shelf of the East China Sea were examined to assess their potential use as indicators of marine productivity in shelf environments. A characteristic disparity in the species compositions of dinoflagellate cysts between coastal sediments and outer-shelf sediments was observed. Coastal areas affected by fluvial freshwater input adjacent to the Changjiang River mouth were dominated by protoperidinioid species, whereas open sea shelf areas were dominated by gonyaulacoid species. The rarity of protoperidinioid cysts in the shelf sediment is primarily attributable to the deficit of silicate in the waters overlying the northern East China Sea shelf, associated with low supplies of suspended particulate matter from the adjacent continent. This would limit the growth of the protoperidinioid species, which feed on diatoms. In contrast, a constant supply of nitrogen generated by microbial nitrogen fixation appeared to have played an important role in supporting the production of gonyaulacoid species in this shelf region. Thus it was determined that dinoflagellate cyst assemblages in marine sediments are closely linked to significant changes in nutrient composition, leading to a selective advantage for the growth of favored taxa. This result suggests that gonyaulacoid dinoflagellate cysts are a potentially useful proxy indicator for low nutrient concentration levels, especially of silicate, in the shelf environment.

Timing of trap formation and petroleum generation in the northern East China Sea Shelf Basin

Basin modeling and cross-section restoration from the northern East China Sea Shelf Basin reveal the timing of trap formation and petroleum generation. Cross-section restoration suggest that extension started in the Late Cretaceous and was interrupted by inversion at the end of Miocene that created large anticline structures, providing numerous petroleum traps. One-dimensional basin modeling of the JDZ-VII-I well show that the main phase of oil generation in the synrift fluvial shales in the Jeju Basin occurred during the Early Oligocene–middle Miocene period, predating the regional inversion. Thus, potential for large oil accumulations in the southern part of the basin is probably limited. Most of the petroleum are likely to have flowed toward the basin margin as well as basin center until the Late Miocene because no structures were available to capture them. The formation of anticline structures overlapped and/or postdated the main phase of gas generation in the Jeju Basin. This is probably why the JDZ-VII-1 is gas-prone with the gas trapped mainly in the anticlinal structures formed by the tectonic uplift. The source rocks in the southwestern part of the Domi Basin are immature for petroleum generation.

Structural characteristics of the northern Okinawa Trough and adjacent areas from regional seismic reflection data: Geologic and tectonic implications

Analysis of regional multi-channel seismic data from the northern Okinawa Trough and adjacent shelf provides some important constraints on the structural development of the area. The sedimentary strata in the northern East China Sea shelf basin, separated from the Okinawa Trough by the Taiwan–Sinzi belt, are affected by the Miocene compressional tectonism and truncated by the Late Miocene unconformity. In contrast, those in the western margin of the northern Okinawa Trough are cut by numerous normal faults and the Late Miocene horizon forms a conformable surface. This suggests that the Taiwan–Sinzi belt acted as a buttress for the northern Okinawa Trough against the compressional tectonism. Our data also reveal the Ho Basin in the western margin of the northern Okinawa Trough, previously known only from proprietary industry data. The Longwan Ridge, lying between the Ho Basin and the northern Okinawa Trough, may be the youngest of the relict arcs or buried ridges in the area. The abrupt along-strike change in the fault polarity near the northern margin of the depocenter of the Ho Basin suggests a NW-trending left-lateral transfer zone or fault. The western boundary of the northern Okinawa Trough is characterized by the gently deepening seafloor, whereas that of the southern Okinawa Trough is marked by steep border faults, suggesting more rapid subsidence in the south. This, together with the well-developed symmetric axial faults in the south and the greater width and the lack of well-developed axial zone in the north, may suggest focused rifting/extension in the southern Okinawa Trough and diffuse rifting/extension in the northern Okinawa Trough. We postulate that the diffuse rifting/extension in the northern Okinawa Trough is due to the tectonic perturbation, caused by the convergence or subduction of the topographically high and buoyant Amami Plateau at the northern Ryukyu arc.

Neotectonic Movement of the Northern East China Sea Shelf Basin

On the base of comprehensive analysis of some new single-track seismic profiles and logging data, the neotectonic movement characteristics of the northern East China Sea Shelf Basin are researched. In the study area, there are intensive neotectonic movement. The main manifestations of the neotectonics include vertical movement, fracture activity and magma activity. The research indicate that the sedimentation centers shift from west to east since Miocene. NE-NNE-striking fault and EW-striking fault are the main active faults. The activity of these faults are gradually later from west to east. The magma activity are frequent and can be divided into three phases in the neotectonic period . They are mainly distribute along active faults. From west to east, the magma activity phases are gradually later. The intrusion gradually weaken and eruption gradually strengthen.

Structural evolution of the northern East China Sea Shelf Basin interpreted from cross-section restoration

The northern East China Sea Shelf Basin consists of three depressions (the Domi, Jeju, and Socotra Depressions), separated by basement highs or rises. Reconstruction of depth-converted seismic reflection profiles from these depressions reveals that the northern East China Sea Shelf Basin experienced two phases of rifting, followed by regional subsidence. Initial rifting in the Late Cretaceous was driven by the NW-SE crustal stretching of the Eurasian plate, caused by the subduction of the Pacific plate beneath the plate margin. Major extension (~15 km) took place during the early phase of basin formation. The initial rifting was terminated by regional uplift in the Late Eocene-Early Oligocene, which was probably due to reorganization of plate boundaries. Rifting resumed in the Early Oligocene; the magnitude of extension was mild (<1 km) during this period. A second phase of uplift in the Early Miocene terminated the rifting, marking the transition to the postrift phase of regional subsidence. Up to 2,600 m of sediments and basement rock were removed by erosion during and after the second phase of uplift. An inversion in the Late Miocene interrupted the postrift subsidence, resulting in an extensive thrust-fold belt in the eastern part of the area. Subsequent erosion removed about 900 m of sediments. The regional subsidence has dominated the area since the Late Miocene.

Seismic stratigraphy and structural analysis of the northern East China Sea Shelf Basin interpreted from multi-channel seismic reflection data and cross-section restoration

Analysis of multi-channel seismic data from the northern East China Sea Shelf Basin (ECSSB) reveals three sub-basins (Socotra, Domi, and Jeju basins), separated by structural highs (Hupijiao Rise) and faulted basement blocks. These sub-basins show a typical rift-basin development: faulted basement and syn-rift and post-rift sedimentation separated by unconformities. Four regional unconformities, including the top of acoustic basement, have been identified and mapped from multi-channel seismic data. Faults in the acoustic basement are generally trending NE, parallel to the regional structural trend of the area. The depths of the acoustic basement range from less than 1000 m in the northwestern part of the Domi Basin to more than 4500 m in the Socotra Basin and 5500 m in the Jeju Basin. The total sediment thicknesses range from less than 500 m to about 1500 m in the northwest where the acoustic basement is shallow and reach about more than 5500 m in the south. Interpretation of seismic reflection data and reconstruction of three depth-converted seismic profiles reveal that the northern ECSSB experienced two phases of rifting, followed by regional subsidence. The initial rifting in the Late Cretaceous was driven by the NW-SE crustal stretching of the Eurasian Plate, caused by the subduction of the Pacific Plate beneath the Eurasian Plate. Extension was the greatest during the early phase of basin formation; estimated rates of extension during the initial rifting are 2%, 6.5%, and 3.5% in the Domi, Jeju, and Socotra basins, respectively. A regional uplift terminated the rifting in the Late Eocene-Early Oligocene. Rifting and extension, although mild, resumed in the Early Oligocene; while fluvio-lacustrine deposition continued to prevail. The estimated rates of extension during the second phase of rifting are 0.7%, 0.8%, and 0.5% in the Domi, Jeju, and Socotra basins, respectively. A second phase of uplift in the Early Miocene terminated the rifting, marking the transition to the post-rift phase of regional subsidence. Regional subsidence dominated the study area between the Early Miocene and the Late Miocene. An inversion in the Late Miocene interrupted the post-rift subsidence, resulting in an extensive thrust-fold belt in the eastern part of the area. Uplift and subsequent erosion were followed by regional subsidence.

The distribution and characteristics of the igneous complexes in the northern East China Sea Shelf Basin and their implications for hydrocarbon potential

This paper presents results of two-dimensional seismic mapping of the northern East China Sea Shelf Basin. Various igneous features such as sills, volcanic edifices and stocks were identified by the geophysical exploration. The sills are most common, and are observed at more than 90 locations. Most mapped sills in the study area are characterized by high-amplitude continuous reflections with distinct terminations. Saucer- and cup-shaped sills are observed locally. The stocks are discordant (nearly vertical) igneous bodies and they are characterized by seismic transparency, with upturned host rocks and uplifted overburden. The volcanic edifices and/or necks consist of irregular mounds and peaks and are characterized by strong positive top reflections with chaotic internal facies. The oldest igneous activity in the northern East China Sea Shelf Basin is Early Cretaceous (123.3 ± 3.7). This igneous activity coincides with those observed in eastern China which has been related mainly to the subduction of the Pacific Plate beneath Eurasia Plate. The Miocene igneous activity is well constrained based on seismic stratigraphic relationships within the folded stratigraphy, age dating, and the occurrence of igneous sills. The timing of this intrusion is coincident with the intensive igneous activity as previously suggested for the eastern China. Igneous rocks can produce hydrocarbon traps, reservoirs and they can act as a seal, and therefore are of great importance in petroleum study.

Geochemical and isotopic characteristics of volcanic rocks from the northern East China Sea shelf margin and the Okinawa Trough

Volcanic rocks both from the northern East China Sea (NECS) shelf margin and the northern Okinawa Trough are subalkaline less aluminous, and lower in High Field Strength Elements (HFSE). These rocks are higher in Large Ion Lithophile Elements (LILE), thorium and uranium contents, positive lead anomalies, negative Nb-Ta anomalies, and enrichment in Light Rare Earth Elements (LREE). Basalts from the NECS shelf margin are akin to Indian Ocean Mid-Ocean Ridge Basalt (MORB), and rhyolites from the northern Okinawa Trough have the highest (207)Pb/(208)Pb and (208)Pb/(204)Pb ratios. The NECS shelf margin basalts have lower (87)Sr/(86)Sr ratios, epsilon(Nd) and sigma(18)O than the northern Okinawa Trough silicic rocks. According to (40)K-(40)Ar isotopic ages of basalts from the NECS shelf margin, rifting of the Okinawa Trough may have been active since at least 3.65-3.86 Ma. The origin of the NECS shelf margin basalt can be explained by the interaction of melt derived from Indian Ocean MORB-like mantle with enriched subcontinental lithosphere. The basalts from both sides of the Okinawa Trough may have a similar origin during the initial rifting of the Okinawa Trough, and the formation of basaltic magmas closely relates to the thinning of continental crust. The source of the formation of the northern Okinawa Trough silicic rocks was different from that of the middle Okinawa Trough, which could have been generated by the interaction of basaltic melt with an enriched crustal component. From the Ryukyu island arc to East China, the Cenozoic basalts have apparently increasing trends of MgO contents and ratios of LREE to Heavy Rare Earth Elements (HREE), suggesting that the trace element variabilities of basalts may have been influenced by the subduction of the Philippine Sea plate, and that the effects of subduction of the Philippine Sea plate on the chemical composition of basaltic melts have had a decreasing effect from the Ryukyu island arc to East China.

Geologic evolution and aspects of the petroleum geology of the northern East China Sea shelf basin

Analysis of multichannel seismic reflection profiles reveals that the northern East China Sea shelf basin experienced two phases of rifting, followed by regional subsidence. The initial rifting in the Late Cretaceous created a series of grabens and half grabens, filled by alluvial and fluviolacustrine deposits. Regional uplift and folding (Yuquan movement) in the late Eocene–early Oligocene terminated the initial rifting. Rifting resumed in the early Oligocene, while alluvial and fluviolacustrine deposition continued to prevail. A second phase of uplift in the early Miocene terminated the rifting, marking the transition to the postrift phase. The early postrift phase (early Miocene–late Miocene) is characterized by regional subsidence and westward and northwestward marine transgression. Inversion (Longjing movement) in the late Miocene interrupted the postrift subsidence, resulting in an extensive thrust-fold belt in the eastern part of the area. The entire area entered a stage of regional subsidence again and has become a broad continental shelf. Source rocks include synrift lacustrine facies, fluvial shales, and coal beds. Synrift fluvial, lacustrine, and deltaic deposits, postrift littoral and/or shallow-marine sandstones, and fractured basement have the potential to provide reservoirs. Various types of hydrocarbon traps (e.g., faulted anticlines, overthrusts, rollover anticlines, faults, unconformity traps, combination structural-stratigraphic traps, weathered basement, and stratigraphic traps) are recognized, but many of these traps have not been tested.