Different types of pockmarks, including single pockmarks, circular pockmarks, elongated pockmarks, chain-type pockmarks, and compound pockmarks, were identified in coastal areas around Fujian, China. The sediments associated with pockmarks were mainly silty clay to clay, with a small quantity of silt with fine sand. The sulfate content in the pore water in the sedimentary layers associated with pockmarks decreased with depth from the surface, whereas the free methane content increased with depth. The interaction between sulfate and methane is well known, but differences in the sulfate–methane transitional zone (SMTZ) were observed in different areas with different hydrologic characteristics. The sedimentary SMTZ of the offshore Zhe-Min mud wedge was shallow, at 50–70 cm below the seafloor. The sedimentary SMTZ was moderately deep (90–115 cm) in the central bay area and deep (180–200 cm) in the sandy area offshore. This variability in SMTZ depth reflects different amounts of free methane gas in the underlying formations, with a shallower SMTZ indicating a higher free methane content. The free methane had δ13C values of −26.47‰ to −8.20‰ and a biogenic hybrid genetic type. The flux of sedimentary gas from the pockmark surfaces, calculated according to Fick’s formula, was 2.89 to 18.85 L/m2·a. The shape, size, and scale of the pockmarks are directly related to the substrate type and the gas production of the underlying strata and thus vary with the sedimentary environment and development stage. Therefore, different types of pockmarks, in various phases of development, are associated with different sedimentary and dynamical conditions. A single circular pockmark is formed by a strong methane flux. As the intensity of methane flux weakens, the pockmark becomes elongated in the direction of the water flow because of long-term erosion induced by regular hydrodynamic forces. Finally, under a weak intensity of methane flux and the influence of complex hydrodynamic conditions, pockmarks merge to form large-scale compound pockmarks.
Abstract Knowledge of the morphological characteristics of coastal foredunes, particularly crest height, is of crucial importance in evaluating coastal vulnerability to marine disasters. However, the factors driving variation in foredune height and the corresponding mechanisms remain poorly understood, especially for regional, large‐scale landscapes. In this contribution, an extensive foredune–beach topographic survey and comprehensive data collection, including regional wind regime, wave and tide climate, were conducted along the ~400 km‐long western coast of Taiwan Strait. The results show that the factor with the most significant effect on the spatial variation in foredune height is wave power, followed by aeolian drift potential (shore‐normal component), while beach width and grain size appear to have only marginal impacts for our study sites. Under a relatively high‐energy wave climate, significant volumes of sandy sediments can be delivered to the beach–dune system, and the beach state tends to be more dissipative, thus contributing to a higher/larger foredune. In particular, the in‐phase synchronization of sediment supply (contributed by wave processes) and transport potential (controlled by wind processes) can lead to the greatest potential for foredune growth. Finally, the source (river) to sink (coastal foredune) dispersal of sediments along this ~400 km‐long strait coastal regime is summarized.
Abstract In this study, paleoproductivity on millennial scales was precisely reconstructed from core MD12‐3428cq in the northern South China Sea (SCS) over the past 24 kyr, based on a transfer function derived from the strong exponential negative correlation between relative abundance of Florisphaera profunda (%FP) in core top sediments and basin‐wide satellite‐based primary productivity (PP) in the SCS. To detect the potential driving mechanisms of PP, correlation analyses were carried out among our PP records and other paleoenvironment parameters. PP peaked during 18–15 ka in parallel with the strong East Asian Winter Monsoon (EAWM). From 15 ka to the early Holocene, a decrease in PP coincided with sea level progradation and weakening of EAWM, which ultimately reduced fluvial nutrient levels and wind‐driven upper water column mixing. Since the middle Holocene, gradually more frequent El Niño‐Southern Oscillation (ENSO) events have taken place, further decreasing PP by injecting oligotrophic Kuroshio water masses into the northern SCS. Associated findings conclusively indicated that the main controlling factors of PP in the northern SCS have shifted from EAWM (glacial) to ENSO (interglacial) over the past 24 kyr.
Abstract High‐resolution clay mineral assemblage combined with Nd and Sr isotopic compositions of Core MD12‐3434 located in the northern South China Sea was investigated to reveal terrigenous sediment response to the East Asian monsoon evolution during the last glaciation. Clay mineralogical variations indicate clear millennial‐scale oscillations that are mainly due to rapid changes in the proportion of smectite to illite and chlorite. Smectite is derived predominantly from rapid chemical weathering of volcanic rocks in Luzon under strong summer monsoon, while illite and chlorite are mainly sourced from Taiwan through reinforced physical erosion. Thus, the smectite/(illite + chlorite) ratio is adopted to reconstruct the East Asian summer monsoon evolution during the last glaciation. Rapid increases in the smectite/(illite + chlorite) ratio imply strengthened summer monsoon occurred during Dansgaard‐Oeschger and Bølling‐Allerød interstadials. In contrast, rapid decreases in the ratio indicate relatively weakened summer monsoon happened during Heinrich and Younger Dryas stadials. These millennial‐scale climatic signals documented by clay mineralogical compositions of deep‐sea sediments in the South China Sea can be better preserved in calm deeper‐water sedimentary environments. Our study highlights the prompt responses of the East Asian monsoon system to millennial‐scale climatic changes occurred in high‐latitude Northern Hemisphere through contemporaneous chemical weathering of volcanic rocks and/or sediment supply variations under strong physical erosion in the low‐latitude South China Sea, implying an atmospheric teleconnection from the North Atlantic to the Asian monsoon region.
The ecological environment and resource endowment of an island are more vulnerable compared to the mainland, and special assessment and measurement of the ecological suitability for development are significant. Pingtan Island (Fujian, China) was taken as a case study. Changes in ecosystem services value and the profit-and-loss balance between ecological footprint and biocapacity were assessed using land use/cover changes based on remote-sensing images taken in 2009, 2014 and 2017, and the ecological suitability of development was measured. Results show that island development led to a decrease in the ecosystem services value and an increase in ecological footprint and biocapacity. The key ecological factors restricting the scale of island development are topography, vegetation with special functions and freshwater. Biocapacity of islands can increase not only by changing from lower-yield land types to higher-yield construction land types but also by external investment. A new measurement framework was proposed that simply and clearly reveals the ecological suitability of island development and the underlying key constraints.
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