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 observati...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.展开更多
In the 1940s,Wang Zhenduo王振铎re-created the sinan司南(southpointing loadstone spoon)mentioned in ancient texts.However,subsequent researchers were unsuccessful in their attempts to replicate this design,resulting in...In the 1940s,Wang Zhenduo王振铎re-created the sinan司南(southpointing loadstone spoon)mentioned in ancient texts.However,subsequent researchers were unsuccessful in their attempts to replicate this design,resulting in longstanding questions over its feasibility.Loadstones with high natural remanent magnetization acquired today resemble the magnetism of those recorded in premodern Chinese documents.By simulating ancient stoneware processing techniques,a loadstone was cut and polished into a spoon shape.It was found that during the course of processing the loadstone spoon,demagnetization was negligible,while after completion,the internal magnetic field was rebalanced due to the change in shape,and the magnetic moment decreased slightly then stabilized for the long term.Experiments of this sort carried out by the author have testified to the enduring and reliable directivity of the resulting loadstone spoon.Paleo-magnetic research indicates that over the past two millennia the horizontal component of the geomagnetic field intensity in northern China underwent a clear M-shaped development.The level in the Qin and Han dynasties is approximately double that of the current day.Using a simulated ancient geomagnetic field,experiments have revealed that multiple methods,such as the“water floating method”and“hanging method,”could manifest a degree of geo-directivity.The comprehensive studies carried out demonstrate that the loadstone spoon is the most optimal among these diverse means for attaining magnetic directivity from the Pre-Qin period(before 221 BCE)to the Tang dynasty(618–907 CE)in view of the availability of loadstone,the state of the geomagnetic field,and existing knowledge and techniques。展开更多
基金funded by China National Offshore Oil Corporation (CNOOC)sponsored by the National Natural Science Foundation of China (Nos.41406031 and 41376038)NSFC-Shandong Joint Fund for Marine Science Research Centers (No.U1406404)
文摘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.
文摘In the 1940s,Wang Zhenduo王振铎re-created the sinan司南(southpointing loadstone spoon)mentioned in ancient texts.However,subsequent researchers were unsuccessful in their attempts to replicate this design,resulting in longstanding questions over its feasibility.Loadstones with high natural remanent magnetization acquired today resemble the magnetism of those recorded in premodern Chinese documents.By simulating ancient stoneware processing techniques,a loadstone was cut and polished into a spoon shape.It was found that during the course of processing the loadstone spoon,demagnetization was negligible,while after completion,the internal magnetic field was rebalanced due to the change in shape,and the magnetic moment decreased slightly then stabilized for the long term.Experiments of this sort carried out by the author have testified to the enduring and reliable directivity of the resulting loadstone spoon.Paleo-magnetic research indicates that over the past two millennia the horizontal component of the geomagnetic field intensity in northern China underwent a clear M-shaped development.The level in the Qin and Han dynasties is approximately double that of the current day.Using a simulated ancient geomagnetic field,experiments have revealed that multiple methods,such as the“water floating method”and“hanging method,”could manifest a degree of geo-directivity.The comprehensive studies carried out demonstrate that the loadstone spoon is the most optimal among these diverse means for attaining magnetic directivity from the Pre-Qin period(before 221 BCE)to the Tang dynasty(618–907 CE)in view of the availability of loadstone,the state of the geomagnetic field,and existing knowledge and techniques。
