四川贡嘎山国家级自然保护区南北坡小型兽类物种多样性

2022年3—10月,对贡嘎山国家级自然保护区开展小型兽类专项调查,布设了5条样线48个样方,总计下铗9226铗次,布设陷阱桶1687桶次,布设鼠笼60笼次。共采集标本1299号,经鉴定包含3目6科16属27种。较之以往研究新采集到小型兽类11种。对贡嘎山国家级自然保护区南、北坡小型兽类物种多样性进行了比较,结果表明:(1)贡嘎山国家级自然保护区南坡的小型兽类物种丰富度更高,北坡的小型兽类物种均匀度更高;(2)南、北坡小型兽类优势种有明显差异;(3)不同物种垂直分布区大小存在差异,相同物种在南、北坡垂直分布区大小亦有差异;(4)南、北坡小型兽类物种多样性差异主要体现在啮齿目和兔形目。

Bottom Currents Observed in and Around a Submarine Valley on the Continental Slope of the Northern South China Sea

Bottom currents at about 1000m depth in and around a submarine valley on the continental slope of the northern South China Sea were studied by a 14-month long experiment from July 2013 to September 2014. The observations reveal that bottom currents are strongly influenced by the topography, being along valley axis or isobaths. Power density spectrum analysis shows that all the currents have significant peaks at diurnal and semi-diurnal frequencies. Diurnal energy is dominant at the open slope site, which is consistent with many previous studies. However, at the site inside the valley the semi-diurnal energy dominates, although the distance between the two sites of observation is quite small （11 kin） compared to a typical horizontal first-mode internal tide wavelength （200 km）. We found this phenomenon is caused by the focusing of internal waves of certain frequencies in the valley. The inertial peak is found only at the open slope site in the first deployment but missing at the inside valley site and the rest of the de- ployments. Monthly averaged residual currents reveal that the near-bottom currents on the slope flow southwestward throughout the year except in August and September, 2013, from which we speculate that this is a result of the interaction between a mesoscale eddy and the canyon/sag topography. Currents inside the valley within about 10mab basically flow along slope and in the layers above the 10mab the currents are northwestward, that is, from the deep ocean to the shelf. The monthly mean current vectors manifest an Ek- man layer-like vertical structure at both sites, which rotate counter-clockwise looking from above.

Carbon and oxygen isotope characteristics of foraminiferan from northern South China Sea sediments and their significance to late Quaternary hydrate decomposition

Carbon and oxygen isotope and dating analyses of foraminiferan in sediment cores collected from three different areas of the northern slope of the South China Sea were conducted, in order to examine the records of the gas hydrate decomposition events since the late Quaternary under the conditions of methane seepage. The results show that： 1） the δ13C values of the benthic foraminiferan Uvigerina spp. （size range of 0.25-0.35 ram） are from -0.212% to -0.021% and the δ180 values of the planktonic foraminiferan Globigerinoides ruber （size range of 0.25-0.35 ram） are from -0.311% to -0.060%; 2） three cores （ZD2, ZD3 and ZS5） from the bottom of a hole are aged for 11 814, 26 616 and 64 090 a corresponding to the early oxygen isotope stage （MIS） Ⅰ, Ⅲ and Ⅳ final period, respectively; 3） a negative-skewed layer of carbon isotope corresponds to that of MIS II （cold period）, whose degree of negative bias is -0.2%0; and 4） the δ13C compositions of foraminiferans are similar to those of the Blake Ridge and the Gulf of Mexico sediments of the late Quaternary. According to the analysis, the reasons for these results are that the studied area is a typical area of methane seep environment in the area during MIS II due to the global sea-level fall and sea pressure decrease. Gas hydrate is decomposed and released, and a large number of light carbon isotopes of methane are released into the ocean, dissolved to inorganic carbon （DIC） pool and recorded in the foraminiferan shells. A pyrite layer developed in the negative bias layers of the foraminiferans confirms that the δ13C of foraminiferans is more affected by methane and less by the reduction of marine productivity and early diagenesis. The use of foraminiferan δ13C could accurately determine late Quaternary hydrate release events and provide evidence for both reconstructing the geological history of methane release events and exploring natural gas hydrate.

Observed near-inertial waves in the wake of Typhoon Hagupit in the northern South China Sea

Energetic near-inertial internal waves （NlWs） were observed on the continental slope of the northern South China Sea in September 2008. Characteristics of the observed near-inertial waves were examined based on current data recorded by a moored acoustic Doppler current profiler. Results of a simple slab model indicated that the NIWs were generated by the surface winds of Typhoon Hagupit. Following Hagupit＇s passage, the wave field was dominated by baroclinic NIWs. The near-inertial currents were surface-intensified with a maximum of 0.52 m/s but still reached 0.1 m/s at the depth of 210 m. Moreover, the near-inertial currents were clockwise-polarized and slightly elliptical. A depth-leading phase of the near- inertial currents was evident, which indicated downward energy propagation. However, the rotary vertical wavenumber spectra suggested that upward energy propagation also existed, which was consistent previous theoretical study. The frequency of the NIWs, modified by the positive background vorticity, was 0.714 2 cycles per day, which was 0.02f0 higher than the local inertial frequency （f0）. The near-inertial kinetic energy evolved exponentially and had an e-folding timescale of about 3 days. The vertical phase and group velocity were estimated to be 10 and 2.1 m/h, respectively, corresponding to a vertical wavelength of 340 m. The NlWs were dominated by the second mode with a variance contribution of 〉50%, followed by the third mode, while the first mode was insignificant.

Stability of submarine slopes in the northern South China Sea: a numerical approach

Submarine landslides occur frequently on most continental margins. They are effective mechanisms of sediment transfer but also a geological hazard to seafloor installations. In this paper, submarine slope stability is evaluated using a 2D limit equilibrium method. Considerations of slope, sediment, and triggering force on the factor of safety （FOS） were calculated in drained and undrained （4=0） cases. Results show that submarine slopes are stable when the slope is 〈16° under static conditions and without a weak interlayer. With a weak interlayer, slopes are stable at 〈18° in the drained case and at 〈9° in the undrained case. Earthquake loading can drastically reduce the shear strength of sediment with increased pore water pressure. The slope became unstable at 〉13° with earthquake peak ground acceleration （PGA） of 0.5 g; whereas with a weak layer, a PGA of 0.2 g could trigger instability at slopes 〉 10°, and 〉3 ° for PGA of 0.5 g. The northern slope of the South China Sea is geomorphologically stable under static conditions. However, because of the possibility of high PGA at the eastern margin of the South China Sea, submarine slides are likely on the Taiwan Bank slope and eastern part of the Dongsha slope. Therefore, submarine slides recognized in seismic profiles on the Taiwan Bank slope would be triggered by an earthquake, the most important factor for triggering submarine slides on the northern slope of the South China Sea. Considering the distribution of PGA, we consider the northern slope of the South China Sea to be stable, excluding the Taiwan Bank slope, which is tectonically active.

Sea surface height oscillation with quasi-four-month period along the continental slope in the northern South China Sea

The sea surface height oscillation with a quasi-four-month period （SSHO4） along continental slope in the northern South China Sea （NSCS） is detected using satellite altimeter data and an ocean model simulation. The SSHO4 is at southwest of Dongsha Island, and is characterized by a wavelength of-600 km and a southwestward phase speed of-0.1 m/s. Crossing the climatological background SST front, geostrophic currents corresponding to the SSHO4 generally induce sea surface temperature （SST） ＂tongues＂ during January-March. The cold and warm SST tongues appear southwest of cyclonic and anticyclonic eddies, respectively. The distance between the warm and cold SST tongues is about half the wavelength of the SSHO4. The geostrophic currents play an important role in lateral mixing, as manifested by the SST tongue phenomena in the NSCS.

Stratigraphic gaps at northern South China Sea margin reflect changes in Pacific deepwater inflow at glacial Termination Ⅱ

To constrain short-term changes of climate and oceanography in the northern South China Sea （SCS） over interglacial marine isotope stage （MIS） 5.5, we studied planktic and benthic δ18O records of seven marine sediment cores with a time resolution of 70-700 yr. Using 6-8 tie points the planktic records were tuned to the U/Th chronology of speleothem δ180 records in China and Europe. The last occurrence of pink Globigerinoides ruber marks the top of Heinrich stadial 11 （HS-11） near 128.4 ka. HS-11 matches a 2300-yr long positive δ180 excursion by 1.5/0.8‰ both in planktic and benthic δ18O records. Hence half of the planktic δ180 signal was linked to increased upwelling of δ18O- and 12C-enriched deep waters in the southwestern SCS. The increase was possibly linked to a strengthened inflow of Pacific deep waters through the Bashi Strait, that form a boundary current along the northern slope of the SCS, building a major sediment drift. At its lower margin near 2300-2400 m water depth （w.d.） Parasound records reveal a belt of modern erosion. At the end of glacial termination 2, stratigraphic gaps deleted HS-11 in core MD05-2904 and subsequent peak MIS 5.5 at ODP Site 1144. Likewise hiatuses probably earmarked all preced- ing glacial terminations at Site 1144 back to 650 ka. Accordingly, boundary current erosion then shifted -300 m upslope to ~2040-2060 m w.d. These vertical shifts imply a rise in boundary current buoyancy, that in turn may be linked to transient events of North Pacific deepwater formation similar to that traced in SCS and North Pacific paleoceanographic records over glacial termination 1.

Controlling factors for gas hydrate occurrence in Shenhu area on the northern slope of the South China Sea

Temperature and pressure on seafloor of the northern slope in the South China Sea are suitable for gas hydrate formation, but bottom simulation reflector (BSR), an indication of gas hydrate occurrence, only occurred in limited areas of the slope. Drillings in the BSR-distributed area (the District S) on the northern slope of the South China Sea suggested that gas hydrate only occurred at Sites SH2, SH3, and SH7 with high saturation (up to 20%-40%), and there is no hydrate at Sites SH1 and SH5 although the distance between SH1 to SH3 is only 500m. In this paper, we investigated seafloor gradient, fault development, temperature, and pressure in the District S on the northern slope of the South China Sea to understand the possible factors con- trolling BSR distribution and gas hydrate occurrence. The District S is a structurally fractured continental slope zone and its seafloor gradient varied greatly. The BSR-occurred areas have an average gradient of 19.89×10 2 whereas the BSR-free zone has the average gradient of 10.57×10 2 . The calculated relative structural intensities from fault densities and displacements show that the BSR-distributed areas tend to occur in the areas with a moderately high structural intensity, where faults frequently developed close to the seafloor that are possibly favored for lateral migration of gases. On the basis of temperatures and pressures at drilling sites, hydrate-occurred Sites SH2, SH3, and SH7 are located within the thermodynamically stable area for methane hydrate, and hydrate-absent Sites SH1 and SH5 are out of the thermodynamically stable area for methane hydrate formation, suggesting that both BSR and the thermodynamic stability are necessary for hydrate occurrence in the subsurface.