Age estimation of the Solomon Sea based on heat flow data

Several heat flow measurements were made during the NAT83 cruise in the central part of the Solomon Sea Basin. The average value of 87 mW/m² (2.08 HFU) calculated from these and other data indicates that the age of the Solomon Sea Basin may range from 24 to 44 Ma. This is supported by the water depth, of approximately 4,500 m, versus age relationship. There is a possibility that the Solomon Sea Basin is not a back-arc basin associated with an arc but was formerly a relatively large oceanic plate. The agreement in age from both heat flow and water depth data favors the latter hypothesis.     

Age estimation of the Solomon Sea based on heat flow data

Several heat flow measurements were made during the NAT83 cruise in the central part of the Solomon Sea Basin. The average value of 87 mW/m² (2.08 HFU) calculated from these and other data indicates that the age of the Solomon Sea Basin may range from 24 to 44 Ma. This is supported by the water depth, of approximately 4,500 m, versus age relationship. There is a possibility that the Solomon Sea Basin is not a back-arc basin associated with an arc but was formerly a relatively large oceanic plate. The agreement in age from both heat flow and water depth data favors the latter hypothesis.     

Heat flow and quantity of methane deduced from a gas hydrate field in the vicinity of the Dnieper Canyon,northwestern Black Sea

Seismic reflection data document for the first time the existence of a BSR in a limited area west of the Dnieper Canyon in the northwestern Black Sea. Seismic wide-angle data suggest that gas hydrates occupy in average 15±2% of the pore space in a zone of 100 m in thickness. A conservative quantification of the amount of methane associated with this gas hydrate occurrence is about 12±3×10¹¹ m³ (0.6±0.2 Gt of methane carbon). Conductive heat flow deduced from the BSR depth is in the range of 21±6 to 55±15 mW m^–2.     

Organic carbon accumulation and sulfate reduction rates in slope and basin sediments of the Ulleung Basin,East/Japan Sea

This study investigated the organic carbon accumulation rates (OCARs) and sulfate reduction rates (SRRs) in slope and basin sediments of the Ulleung Basin, East/Japan Sea. These sediments have high organic contents at depths greater than 2,000 m; this is rare for deep-sea sediments, except for those of the Black Sea and Chilean upwelling regions. The mean organic carbon to total nitrogen molar ratio was estimated to be 6.98 in the Ulleung Basin sediments, indicating that the organic matter is predominantly of marine origin. Strong organic carbon enrichment in the Ulleung Basin appears to result from high export production, and low dilution by inputs of terrestrial materials and calcium carbonate. Apparent sedimentation rates, calculated primarily from excess ²¹⁰Pb distribution below the zone of sediment mixing, varied from 0.033 to 0.116 cm year⁻¹, agreeing well with previous results for the basin. OCARs fluctuated strongly in the range of 2.06–12.5 g C m⁻² year⁻¹, these rates being four times higher at the slope sites than at the basin sites. Within the top 15 cm of the sediment, the integrated SRRs ranged from 0.72 to 1.89 mmol m⁻² day⁻¹, with rates approximately twice as high in the slope areas as in the basin areas. SRR values were consistently higher in areas of high sedimentation and of high organic carbon accumulation, correlating well with apparent sedimentation rates and OCARs. The sulfate reduction rates recorded in the basin and slope sediments of the Ulleung Basin are higher than those reported for other parts of the world, with the exception of the Peruvian and Chilean upwelling regions. This is consistent with the high organic carbon contents of surface sediments of the Ulleung Basin, suggesting enhanced organic matter fluxes.     

A comparative study between present and palaeo-heat flow in the Qiangtang Basin,northern Tibet,China

The Qiangtang Basin is a significant prospective area for hydrocarbon and gas hydrate resources in the Tibetan Plateau, China. However, relatively little work has been performed to characterise heat flow in this basin, which has restricted petroleum and gas hydrate exploration. In this study, we compare present and palaeo-heat flow in the Qiangtang Basin to provide information on geothermal regime, hydrocarbon generation and permafrost that is necessary for further petroleum and gas hydrate exploration. We base our study on temperature data from a thermometer well, thermal conductivity tests, vitrinite reflectance data, homogenisation temperature data from fluid inclusions, stratigraphic information and a time-independent modelling approach. Our results indicate that in the central Qiangtang Basin, the present thermal gradient is approximately 15.5 °C/km, and heat flow is approximately 46.69 mW/m². Heat flow in the Qiangtang Basin is not relatively stable since the Early Jurassic, as previous research has suggested, and it is generally decreasing with time. Additionally, there is a clear difference between the hottest thermal regime of the southern and northern Qiangtang Depressions during Cretaceous to Pleistocene time. In the southern Qiangtang Depression, the palaeogeothermal gradient is approximately 32.0 °C/km, and palaeo-heat flow is approximately 70 mW/m². However, in the northern Qiangtang Depression, the palaeogeothermal gradient exceeds 81.8 °C/km, and palaeo-heat flow is greater than 172.09 mW/m². The high thermal regime in the northern Qiangtang Depression is driven mainly by hydrothermal convection. Gas reservoirs are possible targets for hydrocarbon exploration in this depression. Currently, the northwestern part of the northern Qiangtang Depression is the most favourable area for gas hydrate exploration in the Qiangtang Basin.     

Heat flow derived from BSR and its implications for gas hydrate stability zone in Shenhu Area of northern South China Sea

Bottom simulating reflectors (BSRs), known as the base of gas hydrate stability zone, have been recognized and mapped using good quality three-dimensional (3D) pre-stack migration seismic data in Shenhu Area of northern South China Sea. Additionally, seismic attribute technique has been applied to better constrain on the distribution of gas hydrate. The results demonstrate that gas hydrate is characterized by “blank” zone (low amplitude) in instantaneous amplitude attribute. The thickness of gas hydrate stability zone inferred from BSR ranges from 125 to 355 m with an average of 240 m at sea water depth from 950 to 1,600 m in this new gas hydrate province. The volume of gas in-place bound in hydrate is estimated from 1.7 × 10⁹ to 4.8 × 10⁹ m³, with the most likely value of around 3.3 × 10⁹ m³, using Monte Carlo simulation. Furthermore, geothermal gradient and heat flow are derived from the depths of BSRs using a conductive heat transfer model. The geothermal gradient varies from 35 to 95°C km⁻¹ with an average of 54°C km⁻¹. Corresponding heat flow values range from 43 to 105 mW m⁻² with an average of 64 mW m⁻². By comparison with geological characteristics, we suggest that the distribution of gas hydrate and heat flow are largely associated with gas chimneys and faults, which are extensively distributed in Shenhu Area, providing easy pathways for fluids migrating into the gas hydrate stability zone for the formation of gas hydrate. This study can place useful constraints for modeling gas hydrate stability zone from measured heat flow data and understanding the mechanism of gas hydrate formation in Shenhu Area.     

Basin modeling of the Late Miocene Zeit source rock in the Sudanese portion of Red Sea Basin: Implication for hydrocarbon generation and expulsion history

The Late Miocene Zeit Formation is exposed in the Red Sea Basin of Sudan and represents an important oil-source rock. In this study, five (5) exploratory wells along Red Sea Basin of Sudan are used to model the petroleum generation and expulsion history of the Zeit Formation. Burial/thermal models illustrate that the Red Sea is an extensional rift basin and initially developed during the Late Eocene to Oligocene. Heat flow models show that the present-day heat flow values in the area are between 60 and 109 mW/m². The variation in values of the heat flow can be linked to the raise in the geothermal gradient from margins of the basin towards offshore basin. The offshore basin is an axial area with thick burial depth, which is the principal heat flow source.The paleo-heat flow values of the basin are approximately from 95 to 260 mW/m², increased from Oligocene to Early Pliocene and then decreased exponentially prior to Late Pliocene. This high paleo-heat flow had a considerable effect on the source rock maturation and cooking of the organic matter. The maturity history models indicate that the Zeit Formation source rock passed the late oil-window and converted the oil generated to gas during the Late Miocene.The basin models also indicate that the petroleum was expelled from the Zeit source rock during the Late Miocene (>7 Ma) and it continues to present-day, with transformation ratio of more than 50%. Therefore, the Zeit Formation acts as an effective source rock where significant amounts of petroleum are expected to be generated in the Red Sea Basin.     

“Uniform geothermal gradient” and heat flow in the Qiongdongnan and Pearl River Mouth Basins of the South China Sea

The Pearl River Mouth Basin (PRMB) and Qiongdongnan Basin (QDNB) are oil and gas bearing basins in the northern margin of the South China Sea (SCS). Geothermal survey is an important tool in petroleum exploration. A large data set comprised of 199 thermal conductivities, 40 radioactive heat productions, 543 measured geothermal gradient values, and 224 heat flow values has been obtained from the two basins. However, the measured geothermal gradient data originated from diverse depth range make spatial comparison a challenging task. Taking into account the variation of conductivity and heat production of rocks, we use a “uniform geothermal gradient” to characterize the geothermal gradient distribution of the PRMB and QDNB. Results show that, in the depth interval of 0–5 km, the “uniform geothermal gradient” in the PRMB varies from 17.8 °C/km to 50.2 °C/km, with an average of 32.1 ± 6.0 °C/km. In comparison, the QDNB has an average “uniform geothermal gradient” of 31.9 ± 5.6 °C/km and a range between 19.7 °C/km and 39.5 °C/km. Heat flows in the PRMB and QDNB are 71.3 ± 13.5 mW/m² and 72.9 ± 14.2 mW/m², respectively. The heat flow and geothermal gradient of the PRMB and QDNB tend to increase from the continental shelf to continental slope owing to the lithosphereic/crustal thinning in the Cenozoic.     

Deep flow and transport through the Ulleung Interplain Gap in the Southwestern East/Japan Sea

Deep circulation in the southwestern East/Japan Sea through the Ulleung Interplain Gap (UIG), a possible pathway for deep-water exchange, was directly measured for the first time. Five concurrent current meter moorings were positioned to effectively span the UIG between the islands of Ulleungdo to the west and Dokdo to the east. They provided a 495-day time series of deep currents below 1800 m depth spanning the full breadth of the East Sea Deep and Bottom Water flowing from the Japan Basin into the Ulleung Basin. The UIG circulation is found to be mainly a two-way flow with relatively weak southward flows directed into the Ulleung Basin over about two-thirds of the western UIG. A strong, persistent, and narrow compensating northward outflow occurs in the eastern UIG near Dokdo and is first referred to here as the Dokdo Abyssal Current. The width of the abyssal current is about 20 km below 1800 m depth. The low-frequency variability of the transports is dominated by fluctuations with a period of about 40 days for inflow and outflow transports. The 40-day fluctuations of both transports are statistically coherent, and occur almost concurrently. The overall mean transport of the deep water below 1800 m into the Ulleung Basin over the 16.5 months is about 0.005 Sv (1 Sv=10⁶ m³ s^?1), with an uncertainty of 0.025 Sv indicating net transport is negligible below 1800 m through the UIG.     

Surface heat fluxes during hot events

We selected surface flux datasets to investigate the heat fluxes during “hot events”; (HEs), defined as short-term, large-scale phenomena involving very high sea surface temperature (SST). Validation of the heat fluxes against in-situ ones, which are estimated from in-situ observation in HE sampling conditions, shows the accuracies (bias ± RMS error) of net shortwave radiation, net long wave radiation, latent heat and sensible heat fluxes are 20 ± 45.0 W m⁻², −9 ± 12.3 W m⁻², −2.3 ± 31.5 W m⁻² and 1.5 ± 5.0 W m⁻², respectively. Statistical analyses of HEs show that, during these events, net solar radiation remains high and then decreases from 246 to 220 W m⁻², while latent heat is low and then increases from 100 W m⁻² to 124 W m⁻². Histogram peaks indicate net solar radiation of 270 W m⁻² and latent heat flux of 90 W m⁻² during HEs. Further, HEs are shown to evolve in three phases: formation, mature, and ending phases. Mean heat gain (HG) in the HE formation phase of 60 W m⁻² is larger than the reasonably estimated annual mean HG range of 0–25 W m⁻² in the Indo-Pacific Warm Pool. Such large daily HG in the HE formation phase can be expected to increase SSTs and produce large amplitudes of diurnal SST variations during HEs, which have been observed by both satellite and in-situ measurements in our previous studies.     

Mean flow and variability in the southwestern East Sea

The Ulleung Basin is one of three deep basins that are contained within the East/Japan Sea. Current meter moorings have been maintained in this basin beginning in 1996. The data from these moorings are used to investigate the mean circulation pattern, variability of deep flows, and volume transports of major water masses in the Ulleung Basin with supporting hydrographic data and help from a high-resolution numerical model. The bottom water within the Ulleung Basin, which must enter through a constricted passage from the north, is found to circulate cyclonically—a pattern that seems prevalent throughout the East Sea. A strong current of about 6 cms⁻¹ on average flows southward over the continental slope off the Korean coast underlying the northward East Korean Warm Current as part of the mean abyssal cyclonic circulation. Volume transports of the northward East Korean Warm Current, and southward flowing East Sea Intermediate Water and East Sea Proper Water are estimated to be 1.4 Sv (1 Sv=10⁻⁶ m³ s⁻¹), 0.8 Sv, and 3.0–4.0 Sv, respectively. Deep flow variability involves a wide range of time scales with no apparent seasonal variations, whereas the deep currents in the northern East Sea are known to be strongly seasonal.     

Mapping gas hydrate and fluid flow indicators and modeling gas hydrate stability zone (GHSZ) in the Ulleung Basin,East (Japan) Sea: Potential linkage between the occurrence of mass failures and gas hydrate dissociation

The Ulleung Basin, East (Japan) Sea, is well-known for the occurrence of submarine slope failures along its entire margins and associated mass-transport deposits (MTDs). Previous studies postulated that gas hydrates which broadly exist in the basin could be related with the failure process. In this study, we identified various features of slope failures on the margins, such as landslide scars, slide/slump bodies, glide planes and MTDs, from a regional multi-channel seismic dataset. Seismic indicators of gas hydrates and associated gas/fluid flow, such as the bottom-simulating reflector (BSR), seismic chimneys, pockmarks, and reflection anomalies, were re-compiled. The gas hydrate occurrence zone (GHOZ) within the slope sediments was defined from the BSR distribution. The BSR is more pronounced along the southwestern slope. Its minimal depth is about 100 m below seafloor (mbsf) at about 300 m below sea-level (mbsl). Gas/fluid flow and seepage structures were present on the seismic data as columnar acoustic-blanking zones varying in width and height from tens to hundreds of meters. They were classified into: (a) buried seismic chimneys (BSC), (b) chimneys with a mound (SCM), and (c) chimneys with a depression/pockmark (SCD) on the seafloor. Reflection anomalies, i.e., enhanced reflections below the BSR and hyperbolic reflections which could indicate the presence of gas, together with pockmarks which are not associated with seismic chimneys, and SCDs are predominant in the western-southwestern margin, while the BSR, BSCs and SCMs are widely distributed in the southern and southwestern margins. Calculation of the present-day gas-hydrate stability zone (GHSZ) shows that the base of the GHSZ (BGHSZ) pinches out at water depths ranging between 180 and 260 mbsl. The occurrence of the uppermost landslide scars which is below about 190 mbsl is close to the range of the GHSZ pinch-out. The depths of the BSR are typically greater than the depths of the BGHSZ on the basin margins which may imply that the GHOZ is not stable. Close correlation between the spatial distribution of landslides, seismic features of free gas, gas/fluid flow and expulsion and the GHSZ may suggest that excess pore-pressure caused by gas hydrate dissociation could have had a role in slope failures.     

Gravity crustal models and heat flow measurements for the Eurasia Basin,Arctic Ocean

The Gakkel Ridge in the Arctic Ocean with its adjacent Nansen and Amundsen Basins is a key region for the study of mantle melting and crustal generation at ultraslow spreading rates. We use free-air gravity anomalies in combination with seismic reflection and wide-angle data to compute 2-D crustal models for the Nansen and Amundsen Basins in the Arctic Ocean. Despite the permanent pack-ice cover two geophysical transects cross both entire basins. This means that the complete basin geometry of the world’s slowest spreading system can be analysed in detail for the first time. Applying standard densities for the sediments and oceanic crystalline crust, the gravity models reveal an unexpected heterogeneous mantle with densities of 3.30 × 10³, 3.20 × 10³ and 3.10 × 10³ kg/m³ near the Gakkel Ridge. We interpret that the upper mantle heterogeneity mainly results from serpentinisation and thermal effects. The thickness of the oceanic crust is highly variable throughout both transects. Crustal thickness of less than 1 km dominates in the oldest parts of both basins, increasing to a maximum value of 6 km near the Gakkel Ridge. Along-axis heat flow is highly variable and heat flow amplitudes resemble those observed at fast or intermediate spreading ridges. Unexpectedly, high heat flow along the Amundsen transect exceeds predicted values from global cooling curves by more than 100%.     

Depositional conditions and organic matter distribution in the Bornholm Basin,Baltic Sea

Information on grain-size distribution and total organic carbon (TOC) content of surface sediment and cores from the Bornholm Basin, together with dating of cores using the ²¹⁰Pb method and shallow seismic chirp profiling, has been analysed to elucidate long-term accumulation patterns. The presence of non-depositional areas with lag sediments and low TOC content below the wave base indicates that inflows of dense bottom water originating in the North Sea and associated near-bottom currents have strong influence on the depositional patterns of bulk sediment and organic matter in this deep basin. The general fining in mean grain size towards the northeast corresponds to the direction of inflow currents and prevailing winds. Recent and previously found ²¹⁰Pb-based mean accumulation rates vary greatly within the basin, between 129 and 1,144 g m⁻² year⁻¹. The accumulation rate may vary by a factor of three even between stations located only 3–4 km apart. Rates recorded close to a seismic profile are consistent with the variation in Holocene sediment thickness. This variation reflects a depositional system controlled by near-bottom inflow currents, consisting of a large-scale channel and a wedge-formed sediment package. The spatial variation in TOC content depends partly on water depth, presumably due to generally poorer degradation in the deepest part of the basin because of less frequent oxygen supply by inflow water. Moreover, there is a tendency of higher TOC contents in the southern part of the basin, which may be due to the input of sediments originating from the Oder River. Compared to values for the central, deep Baltic Sea, TOC contents show lower values of 4–6% and insignificant temporal variations. This may be due to the Bornholm Basin being located much closer to the source of the more oxic inflow water, resulting in more favourable degradation conditions.     

Heat flow and mass wasting in the Wilmington Canyon Region: U.S. Continental Margin

The average corrected heat flow in the Wilmington Canyon region, an area of inferred slope instability, is 35 ± 10 mW/m². This average heat flow is marginally consistent with the 46 ± 9 mW/m² measured at other North Atlantic sites over 160 m.y. old. High topographic relief causes most of the variability in surface heat flow and may lower the mean surface heat flow. There is no significant difference between the average corrected heat flow of 35 ± 10 mW/m² in sediment slide areas and the average corrected heat flow of 34 ± 10 mW/m² in undisturbed sediments.     

Strong heat flow variability in an active shallow gas environment, Dnepr palaeo-delta, Black Sea

The Dnepr palaeo-delta on the north-western continental slope of the Black Sea is a prolific seepage zone characterised by a highly variable heat flow ranging from 24 to 88 mW/m². New thermal data were collected at 33 closely spaced stations in order to better understand the apparent relation between heat flow variability and seepage features. Strong relief gradients and associated landslide shaping may explain first-order heat flow variability but, locally, thermal parameters appear to be controlled by fluid and gas migrations. High heat flow anomalies are found at sites where faults and diapirs offer pathways for warm fluid flow from deeper sedimentary levels. Low heat flow is most strongly expressed at ridge crests near seepage sites but is also found where seeps are absent. Reduced heat flow and nonlinear temperature-depth variations are interpreted to result from natural or induced gas ebullition of saturated shallow gas covered by a thin, relatively impermeable sedimentary seal. The presence of sealed gas pockets, in particular at ridge crests, is supported by methane pore-water analysis and a shallow gas front widely observed in the study area.     

Geothermal modeling of the gas hydrate stability zone along the Krishna Godavari Basin

A wide-spread bottom simulating reflector (BSR), interpreted to mark the thermally controlled base of the gas hydrate stability zone, is observed over a close grid of multichannel seismic profiles in the Krishna Godavari Basin of the eastern continental margin of India. The seismic data reveal that gas hydrate occurs in the Krishna Godavari Basin at places where water depths exceed 850 m. The thickness of the gas hydrate stability zone inferred from the BSR ranges up to 250 m. A conductive model was used to determine geothermal gradients and heat flow. Ground truth for the assessment and constraints on the model were provided by downhole measurements obtained during the National Gas Hydrate Program Expedition 01 of India at various sites in the Krishna Godavari Basin. Measured downhole temperature gradients and seafloor-temperatures, sediment thermal conductivities, and seismic velocity are utilized to generate regression functions for these parameters as function of overall water depth. In the first approach the base of gas hydrate stability is predicted from seafloor bathymetry using these regression functions and heat flow and geothermal gradient are calculated. In a second approach the observed BSR depth from the seismic profiles (measured in two-way travel time) is converted into heat flow and geothermal gradient using the same ground-truth data. The geothermal gradient estimated from the BSR varies from 27 to 67°C/km. Corresponding heat flow values range from 24 to 60 mW/m². The geothermal modeling shows a close match of the predicted base of the gas hydrate stability zone with the observed BSR depths.     

Heat flow measurements on the Lomonosov Ridge, Arctic Ocean

Heat flow was measured on the Lomonosov Ridge during the 5th Chinese National Arctic Expedition in 2012. To derive the time-temperature curve, resistivity data were transformed to temperature by the resistivity- temperature program. Direct reading and linear regression methods were used to calculate the equilibrium temperature, which were regressed against the depth of the probes in sediment to derive the geothermal gradient. Then, heat flow was calculated as the product of geothermal gradient and thermal conductivity of sediments. The heat flow values on the basis of the two methods were similar （i.e., 67.27 mW/m2 and 63.99 mW/m2, respectively）. The results are consistent with the measurements carried out at adjacent sites. The age of the Lomonosov Ridge predicted by the heat flow-age model was 62 Ma, which is in accordance with the inference that the ridge was separated from Eurasia at about 60 Ma.     

Gas hydrates in the western deep-water Ulleung Basin, East Sea of Korea

Geophysical surveys and geological studies of gas hydrates in the western deep-water Ulleung Basin of the East Sea off the east coast of Korea have been carried out by the Korea Institute of Geoscience and Mineral Resources (KIGAM) since 2000. The work included a grid of 4782 km of 2D multi-channel seismic reflection lines and 11 piston cores 5–8 m long. In the piston cores, cracks generally parallel to bedding suggest significant in-situ gas. The cores showed high amounts of total organic carbon (TOC), and from the southern study area showed high residual hydrocarbon gas concentrations. The lack of higher hydrocarbons and the carbon isotope ratios indicate that the methane is primarily biogenic. The seismic data show areas of bottom-simulating reflectors (BSRs) that are associated with gas hydrates and underlying free gas. An important observation is the numerous seismic blanking zones up to 2 km across that probably reflect widespread fluid and gas venting and that are inferred to contain substantial gas hydrate. Some of the important results are: (1) BSRs are widespread, although most have low amplitudes; (2) increased P-wave velocities above some BSRs suggest distributed low to moderate concentration gas hydrate whereas a velocity decrease below the BSR suggests free gas; (3) the blanking zones are often associated with upbowing of sedimentary bedding reflectors in time sections that has been interpreted at least in part due to velocity pull-up produced by high-velocity gas hydrate. High gas hydrate concentrations are also inferred in several examples where high interval velocities are resolved within the blanking zones. Recently, gas hydrate recoveries by the piston coring and deep-drilling in 2007 support the interpretation of substantial gas hydrate in many of these structures.     

High primary productivity and f-ratio in summer in the Ulleung basin of the East/Japan Sea

To better understand the cause of high summer primary productivity in the Ulleung Basin located in the southwest part of the East/Japan Sea, the spatial dynamics of primary, new, and regenerated productivities (PP, NP, and RP) were examined along the path of the Tsushima Warm Current system in summer 2008. We compared hydrographic and chemical parameters in the Ulleung Basin with those of the Kuroshio Current in the Western Pacific Ocean and the East China Sea. In summer, integrated primary productivity (IPP, 0.37–0.96 g C m⁻² d⁻¹) and integrated new productivity (INP, 26–221 mg N m⁻² d⁻¹) within the euphotic zone in the Ulleung Basin were higher than those in the East China Sea and the Western Pacific Ocean (0.17–0.28 g C m⁻² d⁻¹, 2−5 mg N m⁻² d⁻¹, respectively). In contrast, there was no pronounced spatial variation in integrated regenerated productivity (IRP, 43–824 mg N m⁻² d⁻¹). Strong positive correlations between IPP and INP (also the f-ratio), and between nitrate uptake rate in the mixed layer and nitrate upward flux through the top of pycnocline in summer in the Ulleung Basin imply that the high IPP was mainly supported by supply of nitrate from the underlying water in the euphotic zone. Shallowing of the pycnocline depth as the current enters the East/Japan Sea facilitates nitrate supply from the nutrient-replete cold water immediately below the pycnocline through nitrate upward flux. A subsurface maximum in PP at or above the pycnocline and a high f-ratio further support the importance of this source of nitrate for maintaining the high summer PP in the Ulleung Basin. In comparison, the high PP layer was observed at the surface in the following fall and spring in the Ulleung Basin. Our results demonstrate the importance of hydrographic features in enhancing PP in this oligotrophic Tsushima Warm Current system.