Northern Bohai Bay Research Articles

Characteristics and driving mechanisms of early Holocene weakened monsoon events: Evidence from northern Bohai Bay, China

The characteristics and driving mechanisms of early Holocene extreme climatic events can provide a basis for the prediction and assessment of such events under ongoing global warming. Coastal areas, with their unique geographical location and fragile ecological environment, are highly sensitive to climate change, making the study of extreme climatic events in these regions potentially providing a different understanding of the processes and mechanisms involved. We studied the Holocene sediments of core BXZK04 from northern Bohai Bay, China, combining 9 AMS 14C dates with high-resolution analyses of pollen, algal remains, and grain size, together with modern pollen databases, to quantitatively reconstruct mean annual precipitation. Our principal findings are: (1) The climate in northern Bohai Bay during 10,900–8600 cal yr BP showed a gradual trend towards increased humidity, with fluctuations. We propose that long-term climate change in the study area was dominated by Northern Hemisphere insolation and regional sea-level changes. (2) Two significant weak monsoon events occurred in the study area, during 9600–9300 cal yr BP and 9060–8950 cal yr BP, corresponding to the 9.4 ka and 8.9 ka climatic events, respectively. During these events, forest vegetation decreased, herbaceous vegetation expanded, and there was a significant reduction in monsoon precipitation (a decrease of ∼200 mm). These events may have been controlled by the weakening of solar activity and reduced Atlantic Meridional Overturning Circulation (AMOC) caused by freshwater influx to the North Atlantic, leading to the southward shift of the Intertropical Convergence Zone (ITCZ) and more frequent El Niño events.

Modeling the long-term fate of polycyclic aromatic hydrocarbons (PAHs) and public health risk in Bohai Bay Sea Area, China

The target of this study was to reconstruct the historical concentration, distribution, variation, and exposure risk evaluation for EPA PAHs to the whole sea of Bohai Bay and the coastal population, by employing a specific dynamic multimedia model during 1950–2050. The unsteady-state model, driven by temporal energy activities from 1950 and sustainable scenarios based on socioeconomic development, indicated the annual emission increased by 4.6 times (from 84.8 tons to 391 tons) until 2020 and resulted in concentrations up to 5.2 times in the atmospheric compartment, and 4.9 times in seawater. Two peak concentrations in 1997 and 2014, consistent with total PAHs input revealed significant regional anthropogenic input in northern Bohai Bay (Tianjin) and southern Bohai Bay (Hebei). The peak-to-peak values of the timing concentration revealed a notably alternative increase in the south (+109.4 %–128.6 %), instead of the rapid decline in the north (−21.5 %–44.5 %). The dominant processes at air-seawater interfaces were air-seawater molecular transfer (from 38.4 % to 51.8 %), and wet deposition (from 60.5 % to 47.5 %). Under 5 shared socioeconomic pathways, the optimal scenario (SSP1) achieved a 24.7 % emission decline, an atmospheric decrease of 15.1 %–31.1 %, and 24.8 %–41.2 % mitigation in seawater during 2020–2050, and each pathway exhibited a general lessening concave in the northern developed municipality, compared with convex in the southern developing regions. The inhalation risk assessment evaluated 10 generations living on Bohai Bay coasts, with an acceptable result, while the current sustainable conceive was with meager fruition in reducing risk.

Geochemical and Isotopic Characteristics of Two Geothermal Systems at the Nanpu Sag, Northern Bohai Bay Basin

The utilization of geothermal energy has gradually increased in northern China because of its unique advantages as a heating supply. However, the sustainable exploitation of geothermal energy usually requires a comprehensive investigation of the geothermal water circulation pattern prevailing at a proposed site. During the exploitation of geothermal energy resources at Nanpu Sag in northern China, thermal anomalies were found to exist in two adjacent regions: the Caofeidian and the Matouying. To reconcile the anomalies and to examine both the source of recharge water and the geothermal systems’ circulation dynamics, a comprehensive investigation was performed using multiple chemical and isotopic tracers (δ2H, δ18O, 87Sr/86Sr, δ13C, and 14C). The total dissolved solids (TDS) of the geothermal water are approximately 750 mg/L and 1,250 mg/L, respectively. The geothermal water isotopes at the two sites are also different, with average values of -9.3‰ and -8.2‰ for δ18O and -73.4‰ and -71‰ for δ2H, respectively. Moreover, the 87Sr/86Sr ratio of geothermal water at Matouying is 0.7185, which is much greater than that of Caofeidian, with an average value of 0.7088. All the results confirm the difference between the two geothermal systems and may explain the two circulation patterns of deep groundwater at Caofeidian and Matouying. The reservoir temperature obtained from theoretical chemical geothermometers is estimated to be 83–92°C at the Caofeidian and 107–137°C at the Matouying, respectively. The corrected 14C age implies a low circulation rate that would allow sufficient time to heat the water at Caofeidian. In addition, we propose a geothermal conceptual model in our study area. This model could provide key information regarding the geothermal sustainable exploitation and the effective management of geothermal resources.

Identification of annual routines and critical stopover sites of a breeding shorebird in the Yellow Sea, China

Pied Avocets (Recurvirostra avosetta) are common migratory shorebirds in the East Asian–Australasian Flyway. From 2019 to 2021, GPS/GSM transmitters were used to track 40 Pied Avocets nesting in northern Bohai Bay to identify annual routines and key stopover sites. On average, southward migration of Pied Avocets started on 23 October and arrived at wintering sites (mainly in the middle and lower reaches of the Yangtze River and coastal wetlands) in southern China on 22 November; northward migration started on 22 March with arrival at breeding sites on 7 April. Most avocets used the same breeding sites and wintering sites between years, with an average migration distance of 1124 ​km. There was no significant difference between sexes on the migration timing or distance in both northward and southward migration, except for the departure time from the wintering sites and winter distribution. The coastal wetland of Lianyungang in Jiangsu Province is a critical stopover site. Most individuals rely on Lianyungang during both northward and southward migration, indicating that species with short migration distances also heavily rely on a few stopover sites. However, Lianyungang lacks adequate protection and is facing many threats, including tidal flat loss. We strongly recommend that the coastal wetland of Lianyungang be designated as a protected area to effectively conserve the critical stopover site.

Identification of breeding grounds and annual routines of the newly discovered bohaii subspecies of Black-tailed Godwits

ABSTRACT The Bohai Black-tailed Godwit (Limosa limosa bohaii) is a newly discovered subspecies in the East Asian-Australasian Flyway. Based on satellite tracking of 21 individuals that were tagged in northern Bohai Bay, China, from 2016 to 2018, we here describe the annual cycle of this subspecies. All the birds had Thailand as their southernmost ‘winter’ destination. The spring departure was in late March during northward migration, Bohai Bay was the first stopping site where they spent on average 39 days (± SD = 6 d), followed by Inner Mongolia and Jilin province (stopping for 8 d ± 1 d). The arrival of the breeding grounds in the Russian Far East was centred in late May. Two breeding sites were detected, with average locations 1100 km apart; the eastern site was beyond the known Asian breeding distribution of the Black-tailed Godwit. Southward migration started in late June, with the godwits tending to make longer stops at the same two main stopping sites used in the spring, i.e. Inner Mongolia and Jilin province (32 ± 5 d) and Bohai Bay (44 ± 8 d), with some individuals making a third stop in the middle-lower reaches of the Yangtze River in southern China (12 ± 4 d). By the end of September, most tracked individuals had arrived in Thailand. Compared with the previously known subspecies, bohaii godwits have strikingly different schedules of migration and moult, this study thus adding to the knowledge about intraspecific diversity of black-tailed godwits in the East Asian-Australian Flyway.

Particulate and Dissolved Black Carbon in Bohai and Laizhou Bays, China: Distributions, Sources, and Contrasts Under Two Distinct Fluvial Hydrological Regimes

Coastal seas, including coastal bays, are the geographically critical transitional zone that links terrestrial and open oceanic ecosystems. Organic carbon cycling in this area is a dynamic and disproportionally key component in the global carbon cycle and budget. As the thermally-transformed organic carbon produced exclusively from the incomplete combustion of biomass and fossil fuels, the recalcitrance and resultant longer environmental residence times result in important implications of black carbon (BC) in the global carbon budget. However, the environmental dynamics of BC in coastal seas have not well been constrained. In this study, we conducted one seawater sampling campaign in the high-intensity BC emission influenced Bohai Bay (BHB) and Laizhou Bay (LZB) in 2013, and quantified both particulate and dissolved BC (PBC and DBC). We elaborated the distributions, sources, and associated influencing factors of PBC and DBC in BHB and LZB in 2013, and simultaneously contrasted the PBC and DBC quantity and quality under two distinct fluvial hydrological regimes of 2013 and 2014 [discussed in Fang et al. (Environ. Sci. Technol., 2021, 55, 788–796)]. Except for the overwhelmingly high PBC in northern BHB caused by anthropogenic point-source emission, horizontally, both PBC and DBC showed a seaward decreasing trend, suggesting that riverine discharge was the major source for PBC and DBC. Vertically, in contrast to the uniform concentrations of DBC between surface and bottom waters, the PBC levels in bottom waters was significantly higher than that in surface waters, which was primarily resulted from the intense sediment re-suspension process during this sampling period. The nearly simultaneous investigations in 2013 and 2014 revealed consistent spatial patterns of PBC and DBC quantity and quality. But significantly lower PBC and DBC quantity and quality were found in 2014 than in 2013, which were largely due to the significantly different climatic conditions (including the watershed hydrology and sunlit radiation) between these 2 years.

Geology and development of geothermal field in Neogene Guantao Formation in northern Bohai Bay Basin: A case of the Caofeidian geothermal heating project in Tangshan, China

Taking the Gaoshangpu-Liuzan geothermal field in the Nanpu sag of the Bohai Bay Basin as the research object, this paper discusses the geological conditions and potential of the geothermal resources of the Guantao Formation in the study area, and introduces the development practice of geothermal energy heating in Caofeidian. The average buried depth of the Guantao Formation is 1500–2500 m, the lithology is dominated by sandy conglomerate, and the average thickness of thermal reservoir is 120–300 m. The average porosity of thermal reservoir is 28%–35%, the permeability is (600–2000)×10−3 μm2, and the temperature of thermal reservoir is 70–110 °C. The formation has total geothermal resources of 13.79×1018 J, equivalent to 4.70×108 t of standard coal. Based on a large amount of seismic and drilling data from oil and gas exploration, this study carried out high quality target area selection, simulation of sandstone thermal reservoir, and production and injection in the same layer. The geothermal heating project with distributed production and injection well pattern covering an area of 230×104 m2 was completed in the new district of Caofeidian in 2018. The project has been running steadily for two heating seasons, with an average annual saving of 6.06×104 t of standard coal and a reduction of 15.87×104 t of carbon dioxide, achieving good economic and social benefits. This project has proved that the Neogene sandstone geothermal reservoir in eastern China can achieve sustainable large-scale development by using the technology of “balanced production and injection in the same layer”. It provides effective reference for the exploration and development of geothermal resource in oil and gas-bearing basins in eastern China.

Artificial Wetlands as Breeding Habitats for Shorebirds: A Case Study on Pied Avocets in China’s Largest Saltpan Complex

Artificial wetlands such as coastal saltpans have replaced a number of coastal natural habitats worldwide and may have accommodated specific waterbird populations in the East Asian–Australasian Flyway (EAAF). The role of saltpans in the EAAF as foraging grounds for shorebirds is widely recognized, although their role as breeding grounds for waterbirds is very limited and contradictory. The Nanpu saltpans in northern Bohai Bay, China, are one of the largest saltpan complexes in the world. In this study, we monitored the nesting success (852 nests) of pied avocets (Recurvirostra avosetta) during three breeding seasons (2015, 2016, and 2018) in the Nanpu saltpans. The nest daily survival rate (DSR) was 0.970; hence, nest survival over the 27 exposure days was 44%. The apparent nest success was 51%. Surprisingly, 55% of nests failed during the laying period. Flooding and nest abandonment were the main causes of nest failure during both the laying and incubation periods. We found a strong positive relationship between the DSR and nest age, with nests that approached hatching having a greater probability of survival than freshly started nests. We also found a strong negative relationship between the DSR and precipitation, with the highest DSR observed for nests that experienced no precipitation. The DSR decreased over the course of the 71-days nesting season and followed a linear trend. The DSR was also density dependent and decreased slightly when nests were denser. A literature review showed that nest survival in the Nanpu saltpans was average compared with that of other studies and that nest success in artificial wetlands was significantly higher than that in natural wetlands or both habitats. Nevertheless, nest success decreased with the study date, suggesting that that breeding conditions for the pied avocet are worsening with time. The loss of saltpans could negatively affect the population of avocets and other ground-nesting waterbird species; therefore, conservation actions and research efforts should be strengthened to understand and conserve these functional wetlands for waterbirds.

Discovery of a morphologically and genetically distinct population of Black‐tailed Godwits in the East Asian‐Australasian Flyway

Occurring across Eurasia, the Black‐tailed Godwit Limosa limosa has three recognized subspecies, melanuroides, limosa and islandica from east to west, respectively. With the smallest body size, melanuroides has been considered the only subspecies in the East Asian‐Australasian Flyway. Yet, observations along the Chinese coast indicated the presence of distinctively large individuals. Here we compared the morphometrics of these larger birds captured in northern Bohai Bay, China, with those of the three known subspecies and explore the genetic population structuring of Black‐tailed Godwits based on the control region of the mitochondrial genome (mtDNA). We found that the Bohai Godwits were indeed significantly larger than melanuroides, resembling limosa more than islandica, but with relatively longer bills than islandica. The level of genetic differentiation between Bohai Godwits and the three recognized subspecies was of similar magnitude to the differentiation among previously recognized subspecies. Based on these segregating morphological and genetic characteristics, we propose that these birds belong to a distinct population, which may be treated and described as a new subspecies.

Sediment mixing of extrabasinal and intrabasinal sources in a narrow elongated half-graben? Constraints on provenance and seismic geomorphological analysis of the Upper Eocene Liaoxi Sag in the offshore northern Bohai Bay Basin (China)

The interaction of provenance systems is very complicated in narrow elongated half-graben basins, such as some small sags in the Liaodong Bay region, offshore northern Bohai Bay Basin (China). Province identification was conducted through an integrated analysis of petrographic data, heavy minerals, detrital zircon U-Pb data, and three-dimensional (3D) seismic data from the upper Eocene Liaoxi Sag, offshore Bohai Bay Basin. The (light or heavy) mineral indices together with their spatio and temporal distribution show that the Mesozoic-dominated extrabasinal source and the Neoarchaean-dominated intrabasinal source contributions to provenance mixing can be distinguished in the oilfield area. The detrital zircons sampled from the potential sediment mixing area have ages from intrabasinal and extrabasinal sources, suggesting the potential evolution of the hanging wall drainages between the syn-rift and post-rift stages. The overlapping deltaic area between the western shoal margin braided-river delta and the eastern border-fault fan deltas, providing direct visualization of provenance mixing, is well imaged, particularly during the SQ3 lowstand period, but it is not apparent from seismic geomorphological evidence throughout the entire SQ4. Further investigation also reveals that this absence may be attributed to the gradually submerged Liaoxi Low Uplift and the more frequent delta autogenic shifting due to lake-level rise. Shallower water and more efficient wave transport, however, could be another cause of sediment mixing given the presence of widely distributed lacustrine beach bars between transverse deltas from both margins during the post-rift stage. This study enhances an integrated approach to provenance analysis for distinguishing extrabasinal and intrabasinal sources as well as provenance mixing in a narrow half-graben basin on an oilfield scale, even with limited cores and cuttings for instrumental analysis in the offshore region. The study also provides further implications for reservoir properties and connectivity from different source-to-sink systems.

Geochemistry and mineralogy of Quaternary sediments in the northern Bohai Bay Basin, North China: implications for provenance and climate change

The Bohai Bay Basin, located in the eastern North China Block, is an important sedimentary basin for studying the Quaternary climate and environment of northern China. Although sporo-pollen analyses have been used to study the paleoclimate, quantitative research via geochemistry had not been conducted. The sedimentary sequence, geochemistry, and mineralogy of Quaternary sediments from core GJ01 in the Yongding River fluvial plain, northern Bohai Bay Basin, were studied to determine their provenance and investigate climate change during the Quaternary. The stratigraphic sequence of the Quaternary was mainly formed by fluviation by a meandering river, showing a typical dual structure. The sediments have relatively high SiO2 and CaO concentrations, and low Al2O3, Na2O, and Fe2O3T concentrations. In terms of trace elements, most large ion lithophile elements are slightly enriched, and Rb and K concentrations are close to those of post-Archean Australian shale (PAAS); most high field strength elements are slightly to moderately depleted. Rare earth elements (REEs) vary significantly, with light rare earth elements (LREEs) obviously enriched relative to heavy rare earth elements (HREEs). A slightly negative Eu anomaly is present, with nearly no Ce anomaly. Mineralogical analysis shows that the main minerals in the fine-grained sediments are quartz, plagioclase, and K-feldspar. The geochemistry and mineralogy of the sediments indicate that the main provenances were recycled sedimentary rocks and felsic igneous rocks. The sediments have low chemical index of alteration (CIA) values, demonstrating that the source rocks underwent slight weathering. The mean annual precipitation and temperature of the studied area during the Quaternary varied from 531 to 875 mm and from 4.1 to 11.8 °C, respectively.

Transgressive wave- and tide-dominated barrier-lagoon system and sea-level rise since 8.2 ka recorded in sediments in northern Bohai Bay, China

Two sediment boreholes, DQ3 and DQ4, were drilled in northern Bohai Bay to investigate the genetic type and sedimentary evolution process of the Caofeidian barrier-lagoon system and the impact of relative sea level (RSL) rise. The core sediment was tested for grain size, microfossils (foraminifera and ostracods), and 14C dating. The Holocene formation of the delta-coast morphology and sedimentary types was closely associated with different phases of RSL rise. Fluvial facies, palaeosols and limnetic facies formed successively during the latest deglaciation but did not exhibit a direct relationship with RSL rise. These sediment facies were immediately followed by the littoral limnetic facies that formed during 9.5–8.2 cal. ka BP, when the Holocene sea level reached the study area. Then, offshore sand bars formed on the southern side, and tidal sand ridges formed on the eastern and western sides, forming a barrier island, which suggest that the Caofeidian barrier-lagoon system began to form under wave- and tide-dominated ocean dynamics at 8.2–7.8 cal. ka BP as the sea level rose at a rate of ca. 11.5 mm a−1. The Caofeidian barrier-lagoon system reached the maximum extent and was mainly influenced by wave-dominated ocean dynamics at 7.8–7.5 cal. ka BP, during which the sea level rose at a rate of ca. 9.0 mm a−1. The Caofeidian barrier-lagoon system began to erode and deteriorate at 7.5–6.7 cal. ka BP with a sea-level rise rate of ca. 2.6 mm a−1. Tidal sand ridges as barrier islands formed on the shore side under the influence of tide-dominated ocean dynamics. RSL rise was estimated at 2.3 m over the past 6.7 ka. Over this period, the sandy barrier islands were eroded by tides and the lagoon areas transitioned into an intertidal zone. Our reconstruction of RSL changes showed that the RSL rose abruptly in four stages, increasing by 1.7 m, 11.0 m, 1.7 m and 2.3 m at 9.5–8.7, 8.7–7.5, 7.5–6.7 and 6.7–0 cal. ka BP, respectively.

Geochemical characteristics and origin of natural gas in the Paleogene of northern Bohai Bay Basin, northeast China

As one of the most important proliferous basins in the northeast of China, the proven ration for natural gases in the Bohai Bay Basin is low. In order to figure out the genetic types and origins of natural gases in the Paleogene, a study on their geochemical characteristics (such as C1+, C1/C1+, C1/C2+3, δ13C1, δ13C2) and kinetic behaviour of the lacustrine source rock have been carried out. The relationship between carbon isotopes and components of gases shows that oil-associated gas and coal-derived gas with low mature are the most important types in our research area. The gases were mainly generated from the lacustrine source rocks with type II2-III kerogen. The pyrolysis experiment on the kerogen indicates that the Paleogene source rock entered the main gas window at the late Miocene.

Episodes of tidally-forced swale erosion on the inner shelf interspersed with millennial fluviodeltaic progradational interludes: Insights from northern Bohai Bay, China

Erosional swales are common features on the seafloor with vigorous bottom flow(s). On continental shelves with energetical tidal currents and limited river discharge, the initiation and evolution of tidally-induced erosional depressions and bedforms have been extensively discussed, but little is known about the evolution of such features when interspersed with active deltaic progradation over millennial timescales. A recent boomer seismic investigation revealed a swale erosional complex within and proximal to the subaqueous delta in northern Bohai Bay (NBHB), China, providing us with an opportunity to investigate such features. Here, using the same seismic dataset, we analyze the morphology, reflective characteristics and evolution of the erosional and depositional structures. The erosional swale complex consists of two episodes of tidally-induced incised channels within a northern progradational system (NPS; located in the northernmost Bohai Bay and prograde to the southeast), followed by an episode of extensive trough-and-bump formation (negative-and-positive relief) within the middle and northern NBHB, which are also erosional features. These features are all highly lineated, oriented WNW-ESE; we attribute their formation to the similarly-oriented tidal flows in this region. The multi-episode evolution of the swale erosional complex is predominately associated with progradation of the NPS, whose lobes cap the pre-existing erosional features and provide substrate for new erosion. The trough-and-bump complex was subject to long-term erosion, and may represent an evolution from the previous linear, tidally-forced marine incisions. The complex was subsequently buried in part by the step-wise progradation of a southern progradational system (SPS; developed within the southern 2/3 Bohai Bay and prograde to the north/northeast). In addition, we also observed clusters of longitudinal bedforms with convex internal reflectors superimposed on the erosional remnant (bumps) coeval with the erosion-dominated environment. They are observable on both sandy and muddy substrates.

Methyltrimethyltridecylchromans (MTTCs) in lacustrine sediments in the northern Bohai Bay Basin, China

Although methyltrimethyltridecylchromans (MTTCs) have been identified and studied for over 30 years in a large number of sediment extracts and crude oils, their origins and significance for palaeosalinity are still debated. Here, the geochemical features of MTTCs in 133 organic-rich lacustrine sediments obtained from the northern Bohai Bay Basin, China, are studied in detail. The sediment samples are in an immature to early mature stage and primarily enriched in Types I and II organic matter, which were deposited in lacustrine environments with relatively weakly stratified water columns and anoxic to suboxic conditions. The development of photic zone euxinia (PZE) in the water column was indicated by the occurrence of aryl isoprenoids. MTTCs were detected in all samples and characterized by low abundances of δ-MTTC and high abundances of α-MTTC. Variations in the relative abundance of MTTCs and calculated MTTCI values, suggest that selected lacustrine sediments were deposited in normal to semi-saline environments. A ternary diagram of mono-, di- and trimethyl-MTTCs was established, and shows potential for application as an indicator of changes in palaeosalinity. Variations in the abundance of MTTCs can effectively indicate palaeosalinity changes in lacustrine environments.

Temporal and spatial variations and impact factors of nutrients in Bohai Bay, China

The temporal and spatial distributions of dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), and dissolved silicate (DSi), and their long-term changes were investigated in Bohai Bay (BHB) in spring, summer, and autumn (2013–2014). The high DIN values were consistently distributed in the western inshore waters, mainly determined by terrestrial factors, e.g., riverine input, while DIP and DSi were mostly distributed in the southern coastal waters, the central BHB, or near the sea port Caofeidian in northern BHB, largely related to non-terrestrial factors, e.g., sediment release. Based on the nutrient distribution, BHB could be partitioned into western and eastern parts, with −15 m depth as the separation. The long-term variations of nutrients since 2000 showed an increase in DIN and decreases in DIP and DSi. Relatively slow changes in DIN and DIP and a rapid decrease in DSi were exhibited in summer, which was associated with precipitation and sediment release.

Seismic stratigraphy and morphology of the Holocene progradational system beneath Bohai Bay, Bohai Sea: Lobate evolution of a multi-sourced subaqueous fluviodeltaic complex

Since 6 kyr BP, the subaerial deltas of the Yellow, the Yanshan and the Taihang river systems evolved along the Bohai Bay coast, forming several lobate progradational systems associated with frequent avulsions of these river systems. Their subaqueous parts consist of a multi-sourced subaqueous deltaic complex in Bohai Bay, but little is known about the complex's subsurface structure, morphology and evolutionary history. Here we investigate these characteristics using a dense, high-resolution seismic survey. Using stratigraphic superposition, seismic geometry, and the results of previous studies, we identified nine seismic units (SUs) in the post-Last Glacial Maximum stratigraphy, which are interpreted, from bottom to top, as: (1) fluvial/lacustrine sediments (SU0); (2) marine sands combined with estuarine/coastal deposits (SU1); and (3) a subaqueous fluviodeltaic complex (SUs2–8); these units correspond to the lowstand system tract, the transgressive and/or highstand system tracts, and the normal regressive system tract, respectively, and evolved as a function of eustatic fluctuations. The subaqueous fluviodeltaic complex is characterized by a discrete and spatially-confined lobate architecture, which is indicative of step-wise switching of subaqueous lobes. Two sets of progradational systems (PS), a northern one (SUs2–3) and a southern one (SUs4–8), were observed within the complex, tapering in opposite directions and bounded by an erosional unconformity (R3). These characteristics indicate that the two PS were sourced from different river systems and prograded at different times. R3 forms the base of the southern PS but cross cuts the northern PS, indicating that it is a diachronous surface marks the preexisting seafloor, which was further reworked/eroded by the progradation of the southern PS. In each PS, the internal subaqueous lobes correlated well with the onshore lobes/superlobes of the subaerial deltas, suggesting the lobate evolution of the complex is driven predominantly by alternating erosion and deposition caused by the variation in fluvial supply, which appears to be linked to the frequent avulsions of the major river systems, in addition to the multi-sourced supply. Based on the dipping geometries and the locations of the depocenters, we hypothesize that (1) the northern PS is the seaward extension of the ancient Haihe River's northern branch; (2) the bulk of southern PS in the middle and southern Bohai Bay is correlated with the principal body of the Yellow River subaqueous delta; (3) the majority of the volume of the northern PS in northern Bohai Bay consists of the modern Haihe River subaqueous delta.

Alternative habitat: the importance of the Nanpu Saltpans for migratory waterbirds in the Chinese Yellow Sea

SummaryThe natural coastal wetlands of the East Asian-Australasian Flyway (EAAF) are disappearing at alarming rates, leading to rapid declines of many populations of waterbirds in the most species-rich flyway in the world. The identification and assessment of possible alternative habitats that may buffer the loss of natural wetlands should, therefore, be a priority for the conservation of migratory waterbirds using this flyway. Coastal saltpans are functional wetlands that support large numbers of waterbirds worldwide. The Nanpu Saltpans in the northern Bohai Bay of the Yellow Sea in China are one of the largest (290 km2) saltpan complexes in the world. In this paper, we document the value of the Nanpu Saltpans for supporting waterbirds. The surveys, carried out from 2013 to 2016, included waterbird counts in the saltpans (93 km2) at high and low tide and on the adjacent natural tidal flats (57 km2) at low tide. Of the 89 waterbird species recorded, 27 had maximum counts exceeding the 1% threshold value of estimated flyway populations. The maximum counts of waterbirds in northward migration and southward migration in the Nanpu Saltpans were 96,000 and 93,500, respectively, including both foraging and roosting birds; these figures do not account for turnover, so the total number of birds using the site is likely to be higher. The maximum counts on the adjacent tidal flats at low tide amounted to 73,000 and 20,000 waterbirds during northward and southward migration, respectively, and most of them were foraging birds. In the boreal winter, few birds fed in the saltpans, but several thousand fed on the tidal flats. Waterbirds used the inland ponds (2.0–18.0 km from the intertidal area) mainly for feeding both during low tide and high tide and used the nearshore ponds (0.3–4.3 km from the intertidal area) mainly for high-tide roosting. Some species, such as Black-tailed Godwit Limosa limosa, Marsh Sandpiper Tringa stagnatilis, Pied Avocet Recurvirostra avosetta, and Black-winged Stilt Himantopus himantopus, occurred mainly in the saltpans; other species preferred tidal flats, such as Red Knot Calidris canutus, Great Knot Calidris tenuirostris, Bar-tailed Godwit Limosa lapponica, Eurasian Curlew Numenius arquata, Relict Gull Larus relictus, and Grey Plover Pluvialis squatarola. This study clearly demonstrates the joint ecological function of the Nanpu Saltpan complex and adjacent tidal flats as a key staging area for waterbirds in the EAAF, and as such both urgently warrant protected status.

New insights into the distribution and evolution of the Cenozoic Tan-Lu Fault Zone in the Liaohe sub-basin of the Bohai Bay Basin, eastern China

As the largest strike-slip fault system in eastern China, the northeast-trending Tan-Lu Fault Zone (TLFZ) is a significant tectonic element contributing to the Mesozoic-Cenozoic regional geologic evolution of eastern Asia, as well as to the formation of ore deposits and oilfields. Because of the paucity of data, its distribution and evolutionary history in the offshore Liaohe sub-basin of the northern Bohai Bay Basin (BBB) are still poorly understood. Investigations of the strike-slip fault system in the western portion of the offshore Liaohe sub-basin via new seismic data provide us with new insights into the characteristics of the Cenozoic TLFZ. Results of this study show that Cenozoic dextral strike-slip faults occurred near the center of the Liaoxi graben in the offshore Liaohe sub-basin; these strike-slip faults connect with their counterparts to the north, the western part of the onshore Liaohe sub-basin, and have similar characteristics to those in other areas of the BBB in terms of kinematics, evolutionary history, and distribution; consequently, these faults are considered as the western branch of the TLFZ. All strike-slip faults within the Liaoxi graben merge at depth with a central subvertical basement fault induced by the reactivation of a pre-existing strike-slip basement fault, the pre-Cenozoic TLFZ. Data suggest that the TLFZ across the whole Liaohe sub-basin comprises two branches and that the Cenozoic distribution of this system was inherited from the pre-Cenozoic TLFZ. This characteristic distribution might be possessed by the whole TLFZ, thus the new understandings about the distribution and evolutionary model of the TLFZ in this study can be inferred in many research fields along the whole fault zone, such as regional geology, ore deposits, petroleum exploration and earthquake hazard.

Magnetostratigraphy of two deep boreholes in southwestern Bohai Bay: Tectonic implications and constraints on the ages of volcanic layers

Abstract The sediments of Bohai Bay Basin have recorded the processes of basin infilling and structural evolution, reflecting the destruction of the North China Craton during the late Mesozoic and the early Cenozoic. However, the absence of a reliable chronostratigraphic framework for the sedimentary sequences in the basin has precluded a comprehensive understanding of these processes. This is especially the case for southwestern Bohai Bay, the evolution of which has been affected by two Quaternary volcanoes, Dashan and Xiaoshan. In this study, we combine paleomagnetic and sedimentary analyses of the sediments from two boreholes (CK3 and G4) from southwestern Bohai Bay Basin in order to provide new insights into the sedimentary history and regional tectonic processes since the Pliocene. The main findings are as follows. (1) Magnetite and hematite are the main carriers of the characteristic remanent magnetization in these sedimentary sequences, with hematite the main component. (2) The two drill cores record the Brunhes, Olduvai and Gauss normal chrons, and the Matuyama and Gilbert reverse chrons. (3) Subsidence-related differences in the depths of the Matuyama/Brunhes (M/B) and Gauss/Matuyama (G/M) boundaries in boreholes across Bohai Bay, distributed within different structural units, reflect the fact that northern Bohai Bay is the regional subsidence center and that the development of the WNW-orientated structures of the basin has intensified during the Quaternary. (4) The trend of changes in the elevation of three marine layers is similar to that of the M/B and G/M boundaries across Bohai Bay. Based on magnetostratigraphic constraints of the boreholes, the ages of the second and third marine layers are assigned to Marine Oxygen Isotope (MIS) 3 and MIS 5, respectively. The WNW-orientated structures and ‘super interglacials’ resulted in the formation of transgressions in the Huanghua depression. (5) In borehole CK3, the ages of four volcanic layers at the depth intervals of 13.14–16.18 m, 33.37–48.02 m, 145.8–154.05 m and 222.16–233.80 m, are 10–18 ka BP, 80–90 ka BP, 2.2 Ma and 3.1 Ma, respectively. In borehole G4, the age of the volcanic layer at the depth of 46.4–55.8 m is 0.33–0.55 Ma. Comparative analysis of subsidence-related differences between the NEE and NE-orientated uplifts and depressions in western and southern Bohai Bay indicates that structural evolution in the vertical plane during the Quaternary was mainly controlled by the subduction of the Pacific Plate.