Institutional Repository of South China Sea Institute of Oceanology, CAS
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Sediment microbes mediate the impact of nutrient loading on blue carbon sequestration by mixed seagrass meadows
No full textRecent studies have reported significant variability in sediment organic carbon (SOC) storage capacity among seagrass species, but the factors driving this variability are poorly understood, limiting our ability to make informed decisions about which seagrass types are optimal for carbon offsetting and why. Here we show that differences in SOC storage capacity among species within the same geomorphic environment can be explained (in part) by below-ground processes in response to nutrient load; specifically, differences in the activity of microbes harboured by morphologically-different seagrass species. We found that increasing nutrient load enhanced the relative contribution of seagrass and algal sources to SOC pools, boosting sediment microbial biomass and extra cellular enzyme activity within mixed seagrass meadows composed of Thalassia hemprichii and Enhalus acoroides, and thus possibly weaken the seagrass blue carbon sequestration capacity. The relative contribution of seagrass plant material to sediment bacterial organic carbon (BOC) and the influencing SOC-decomposing enzymes in E. acoroides meadows were half that of T. hemprichii meadows living side-by-side, even though the mixed seagrass meadows received SOC from the same sources. Overall this research suggests that microbial activity can vary significantly among seagrass species, thereby causing fine-scale (within-meadow) variability in SOC sequestration capacity in response to nutrient load. (C) 2017 Elsevier B.V. All rights reserved
Seasonal variations in the barrier layer in the South China Sea: characteristics, mechanisms and impact of warming
No full textA new observational dataset, the South China Sea Physical Oceanographic Dataset 2014, is examined to investigate the seasonal characteristics, formation mechanisms, and warming effects of the barrier layer (BL) in the South China Sea (SCS). Statistical analysis reveals that the BL is thicker and occurs more frequently during summer and early autumn, while in winter it often coexists with temperature inversions. The formation mechanisms are discussed from the perspective of the controlling regime and the net turbulent energy required for BL evolution. In the initial stage (March-May), the BL is absent due to weak mixing, scarce rainfall and surface warming. In the formation and maintenance stage (June-September), the BL grows in summer and persists into the transition season. The BLs can be classified into three regimes: the flux regime (in the Luzon Strait), the combined regime (in the eastern basin) and the wind regime (southeast of Vietnam). In the attenuation stage (October-February), associated with the winter monsoon, the BL mainly occurs in the combined regime (along the path of western boundary current) and the flux regime (in the southeast corner). The characteristics and generation mechanisms of the temperature inversions near the south Chinese coast, east of Vietnam, and in the Gulf of Thailand are also discussed. Our analysis further demonstrates that the BL has a significant warming effect on upper ocean temperature and heat content in the SCS
Spatial distribution of turbulent mixing in the upper ocean of the South China Sea
No full textThe spatial distribution of the dissipation rate (epsilon) and diapycnal diffusivity (kappa) in the upper ocean of the South China Sea (SCS) is presented from a measurement program conducted from 26 April to 23 May 2010. In the vertical distribution, the dissipation rates below the surface mixed layer were predominantly high in the thermocline where shear and stratification were strong. In the regional distribution, high dissipation rates and diapycnal diffusivities were observed in the region to the west of the Luzon Strait, with an average dissipation rate and diapycnal diffusivity of 8.3 x 10(-9) W kg(-1) and 2.7 x 10(-5) m(2) s(-1), respectively, almost 1 order of magnitude higher than those in the central and southern SCS. In the region to the west of the Luzon Strait, the water column was characterized by strong shear and weak stratification. Elevated dissipation rates (epsilon > 10(-7) W kg(-1)) and diapycnal diffusivities (kappa > 10(-4) m(2) s(-1)), induced by shear instability, occurred in the water column. In the central and southern SCS, the water column was characterized by strong stratification and weak shear and the turbulent mixing was weak. Internal waves and internal tides generated near the Luzon Strait are expected to make a dominant contribution to the strong turbulent mixing and shear in the region to the west of the Luzon Strait. The observed dissipation rates were found to scale positively with the shear and stratification, which were consistent with the MacKinnon-Gregg model used for the continental shelf but different from the Gregg-Henyey scaling used for the open ocean
Marine phytoplankton biomass responses to typhoon events in the South China Sea based on physical-biogeochemical model
No full textMost previous studies confirmed that marine phytoplankton biomass can increased dramatically and then formed blooms along the typhoon track after typhoon passage. However, many events of no significant responses to typhoons were neglected. In order to figure out the most important factor for bloom formation, a coupled physical-biogeochemical model was used to study the biogeochemical phytoplankton responses to all 79 typhoon events that affected the South China Sea (SCS) during 2000-2009. The major factors investigated included typhoon intensity and translation speed, Chlorophyll a concentration (Chl-a) and vertical nitrate transport in the euphotic zone before and after typhoon passage. The results revealed that phytoplankton blooms were triggered after 43 typhoon events, but no significant blooms were found after 36 other typhoons. Of the 43 typhoon events that triggered blooms, 24 were in the open ocean and 19 were on the coast. Subsurface blooms were detected after five typhoon events that did not trigger surface blooms. Over half of the typhoons that affect the oligotrophic SCS can trigger phytoplankton blooms, and contribute to the marine primary productivity. The mechanism of the above results were surveyed, we found that (1) an increased nitrate concentration is the basic and key precondition for phytoplankton blooming in the oligotrophic SCS; (2) typhoon intensity, and translation speed control the upward flux of nitrates together, and translation speed has more effect than intensity; (3) uplifted nitrates could trigger phytoplankton bloom, and Chl-a levels reached a peak 3 days later than nitrate levels; (4) mesoscale eddies and the nutricline depth before a typhoon's arrival also affects bloom genesis; and (5) the composition of phytoplankton functional groups in the coast was adjusted by typhoon, which have more complex mechanism of bloom formation than that in the open ocean. In summary, the physical driving force that modulates blooms is vertical nutrient transportation in the SCS. (C) 2017 Elsevier B.V. All rights reserved
Evolution and statistics of thermal plumes in tilted turbulent convection
No full textWe present a systematic experimental study of thermal plumes within tilted turbulent thermal convection using the shadowgraph technique. The measurements are performed in a rectangular cell (aspect ratio Gamma(x) =1 and Gamma(y) = 0.25) over a wide range of tilt angles (0 <= beta <= pi/2 rad) at constant Prandtl (Pr similar or equal to 10) and Rayleigh (Ra similar or equal to 6.80 x 10(10)) numbers. It is found that the plume width, lambda, is distributed log-normally in space and the time-averaged most probable plume width, <lambda(p)>, has similar scale with the thermal boundary layer thickness, lambda(T). <lambda(p)> and lambda(T) slowly increase as tilt angle, beta, increases from 0 to 1 rad and they broaden rapidly as beta > 1 rad. The average plume area density, (Phi), deduced from the image intensity, is proposed to represent the heat flux of thermal plumes. Its beta dependence is consistent with that of the Nusselt number, Nu, over the tilt angle range. lambda(p) and Phi exhibit oscillation of convective flow. The oscillation intensity and period strongly depend on beta, but are less affected by Ra and Pr. The plume velocity exhibits a different pattern from that of the two-dimensional convective flow measured at Pr similar or equal to 6.3 and Ra similar or equal to 4.42 x 10(9). The magnitude of plume velocity peak is weakly affected by the cell tilting. The position of the plume velocity peak increases linearly with increasing beta. (C) 2017 Elsevier Ltd. All rights reserved
Alcohol compounds in Azolla imbricata and potential source implication for marine sediments
No full textThis study investigated the composition of long-chain alkyl diols, triols, sec-alcohols, hydroxyl acids, and other hydroxylated compounds in Azolla imbricata and compared the organic alcohol components of Azolla filiculoides, Azolla microphylla, and South China Sea (SCS) sediments in order to investigate the possible indication of Azolla being the biological source of diols and triols in SCS sediment. Large amounts of diols, monohydroxy acids, and sec-alcohols with internal hydroxy groups at omega 20 were detected in the three types of Azolla. Among these, 1,omega 20-diol and omega 20-hydroxy acid exhibited strong even-odd predominance distribution, whereas omega 20-sec-alcohol exhibited strong odd-even predominance distribution. In addition, small amounts of diols, triols, and dihydroxy acids with internal hydroxy groups at 9, 10 or omega 9, omega 10 were detected, among which the chain length of C-29 was predominate. Compounds having similar structures as those in Azolla reflected a similar biosynthetic pathway: omega 20-hydroxy acid exhibiting even-odd predominance distribution is decarboxylated to omega 20-sec-alcohol exhibiting odd-even predominance distribution and converted to 1, omega 20-diol with even-odd predominance distribution by acyl reduction; omega 9, omega 10-hydroxy acid is converted to 1,20,21(1, omega 9, omega 10)-triol by acyl reduction, and then converted to 9,10-diol by hydrogenation and dehydration. The alcohol components in A. imbricata were clearly not the biological source of 1,13/1,14/1,15-C-28,C- 30,C- 32 diols and 1,3,4-C27-29 triols in the SCS sediment. Certain marine diatoms might be the source of 1,14-C-28,C- 30 diol in inshore sediment, but the biological source of diols and triols in the SCS sediment requires further investigation
Distinct Bacterial Communities Associated with Massive and Branching Scleractinian Corals and Potential Linkages to Coral Susceptibility to Thermal or Cold Stress
No full textIt is well known that different coral species have different tolerances to thermal or cold stress, which is presumed to be related to the density of Symbiodinium. However, the intrinsic factors between stress-tolerant characteristics and coral-associated bacteria are rarely studied. In this study, 16 massive coral and 9 branching coral colonies from 6 families, 10 genera, and 18 species were collected at the same time and location (Xinyi Reef) in the South China Sea to investigate the bacterial communities. The results of an alpha diversity analysis showed that bacterial diversities associated with massive corals were generally higher than those with branching corals at different taxonomic levels (phylum, class, order, and so on). In addition, hierarchical clustering tree and PCoA analyses showed that coral species were clustered into two large groups according to the similarity of bacterial communities. Group I consisted of massive Goniastrea, Plesiastrea, Leptastrea, Platygyra, Echinopora, Porites, and Leptoria, and group II consisted of branching Acropora and Pocillopora. These findings suggested that both massive corals and branching corals have their own preference for the choice of associated bacteria, which may be involved in observed differences in thermal/cold tolerances. Further analysis found that 55 bacterial phyla, including 43 formally described phyla and 12 candidate phyla, were detected in these coral species. Among them, 52 phyla were recovered from the massive coral group, and 46 phyla were recovered from the branching coral group. Formally described coral pathogens have not been detected in these coral species, suggesting that they are less likely to be threatened by disease in this geographic area. This study highlights a clear relationship between the high complexity of bacterial community associated with coral, skeletal morphology of coral and potentially tolerances to thermal or cold stress
Millennial-scale ocean dynamics controlled export productivity in the subtropical North Pacific
No full textThe integrated effects of ocean-climate dynamics on export production in the North Pacific have remained elusive. We present a 91 k.y. export productivity (EP) record based on sedimentary reactive phosphorus from the western subtropical North Pacific. On a millennial time scale, EP decreased during Northern Hemisphere cold events when atmospheric dust loading was high, and increased during warm episodes. The inferred antiphase relation between dust and EP suggests that the supply of macronutrients to the sunlit surface ocean, modulated by the penetration depth of North Pacific Intermediate Water and not eolian Fe, exerted a major control on EP in the subtropical North Pacific. A compilation of global EP records suggests that eolian Fe most likely played a role in stimulating EP regionally only in the Subantarctic zone of the Southern Ocean. Over the past 91 k.y., during the cold-south-warm-north phase of the bipolar seesaw, the biological pump in both hemispheres was enhanced synchronously, yet by different drivers; atmospheric Fe input for the Subantarctic and subsurface macronutrient supply for the North Pacific, including the tropical and/or subtropical Pacific, and the Antarctic zone of the Southern Ocean
南海东部古扩张脊的俯冲机制
No full text南海东部古扩张脊处于欧亚板块和太平洋板块的汇聚地带,其东侧为马尼拉海沟、北吕宋海槽和西吕宋海槽,由于受到多个构造单元的相互作用,使其处于复杂的构造环境中。南海东部古扩张脊俯冲过程的研究对深入理解南海海盆构造演化、火山及地震活动等具有重要意义,同时也是今后南海构造研究的重要方向之一。在总结前人研究基础之上,探讨南海东部古扩张脊俯冲时间、俯冲深度及动力学过程。南海板块在16 Ma之后,由于菲律宾板块NW向仰冲的作用,使南海东部古扩张脊被动地沿马尼拉海沟进行俯冲,形成了现今马尼拉海沟中段的构造格局。古扩张脊俯冲深度为200~300km,并且在约100km处发生板片撕裂,造成古扩张脊两侧俯冲角度的不同
南海的“山根拆沉成因观”——南海成因新议
No full text南海成因问题至今仍众说纷纭。作者以寻求南海形成演化的内因为着眼点,在大量地震剖面地质解释的基础上,运用构造解析法对南海南北陆缘前新生代基底地质构造变形形迹进行了系统的构造恢复和组合分析,发现了残存于南海南北陆缘中生代基底中的断层-褶皱具有碰撞造山带特有的背冲断褶构造特征,指出该古造山带的中轴位置大致沿现今双峰-笔架海山连线分布,并称之为古双峰-笔架碰撞造山带。结合南海地区新生代岩浆岩地球化学特征分析和南北陆缘新生代伸展构造特征分析,提出该造山带在新生代发生了山根拆沉的解体过程,并明确提出了该"山根拆沉"机制是诱发南海形成的根本原因