Acoustic characteristics of cold-seep methane bubble behavior in the water column and its potential environmental impact

The amount of methane leaked from deep sea cold seeps is enormous and potentially affects the global warming,ocean acidification and global carbon cycle.It is of great significance to study the methane bubble movement and dissolution process in the water column and its output to the atmosphere.Methane bubbles produce strong acoustic impedance in water bodies,and bubble strings released from deep sea cold seeps are called"gas flares"which expressed as flame-like strong backscatter in the water column.We characterized the morphology and movement of methane bubbles released into the water using multibeam water column data at two cold seeps.The result shows that methane at site I reached 920 m water depth without passing through the top of the gas hydrate stability zone(GHSZ,850 m),while methane bubbles at site II passed through the top of the GHSZ(597 m)and entered the non-GHSZ(above 550 m).By applying two methods on the multibeam data,the bubble rising velocity in the water column at sites I and II were estimated to be 9.6 cm/s and 24 cm/s,respectively.Bubble velocity is positively associated with water depth which is inferred to be resulted from decrease of bubble size during methane ascending in the water.Combined with numerical simulation,we concluded that formation of gas hydrate shells plays an important role in helping methane bubbles entering the upper water bodies,while other factors,including water depth,bubble velocity,initial kinetic energy and bubble size,also influence the bubble residence time in the water and the possibility of methane entering the atmosphere.We estimate that methane gas flux at these two sites is 0.4×10~6–87.6×10~6 mol/a which is extremely small compared to the total amount of methane in the ocean body,however,methane leakage might exert significant impact on the ocean acidification considering the widespread distributed cold seeps.In addition,although methane entering the atmosphere is not observed,further research is still needed to understand its potential impact on increasing methane concentration in the surface seawater and gas-water interface methane exchange rate,which consequently increase the greenhouse effect.

Magnetic minerals in Mid-Pleistocene sediments on the Caiwei Guyot,Northwest Pacific and their response to the Mid-Brunhes climate event

Seamounts are ubiquitous topographic units in global oceans,and their influences on local oceanic circulation have attracted great attention in physical oceanography;however,previous efforts were less made in paleoclimatology and paleoceanography.The Caiwei Guyot in the Magellan Seamounts of the western Pacific is a typical seamount,and in this study,we investigate a well-dated sediment core by magnetic properties to reveal the relationship between deep-sea sedimentary processes and global climate changes.The principal results are as follows:(1)the dominant magnetic minerals in the sediments are low-coercivity magnetite in pseudo-single domain range,probably including a biogenic contribution;(2)the variabilities of magnetic parameters can be clustered into two sections at~500 ka,and the differences between the two units are evident in amplitudes and means;(3)changes in the grainsize-dependent magnetic parameters can be well correlated to records of global ice volume and atmospheric CO;in the middle Pleistocene.Based on these results,a close linkage was proposed between deep-sea sedimentary processes in the Caiwei Guyot and global climate changes.This linkage likely involves different roles of biogenic magnetite in the sediments between interglacial and glacial intervals,responding to changes in marine productivity and deep-sea circulation and displaying a major change in the MidBrunhes climate event.Therefore,we proposed that the sedimentary archives at the bottom of the Caiwei Guyot record some key signals of global climate changes,providing a unique window to observe interactions between various environmental systems on glacial-interglacial timescales.

Distinct BSRs and their implications for natural gas hydrate formation and distribution at the submarine Makran accretionary zone

To investigate the nature of gas hydrates in the Makran area,new high-resolution geophysical data were acquired between 2018-2019.The data collected comprise multibeam and two-dimensional multi-channel seismic reflection data.The multibeam bathymetry data show East-North-East(ENE)ridges,piggy-back basins,canyon and channel systems,and the morphology of the abyssal plain.Continuous and discontinuous bottom simulating reflectors(BSRs)occur in the piggy-back basins on most of the seismic profiles available.The BSRs cut the dipping layers with strong amplitude and reversed polarity.Discontinuous BSRs indicate a transition along a dipping high-permeable sand layers from gas-rich segment to the gas hydrate-bearing segment and sugge st alternating sediments of fine and relatively coarse grain size.Double BSRs are highly dynamic and attributed to slumps occurring in the study area.The BSRs induced by slumps are located both at deep and shallow depths,responding to the temperature or pressure variation.For the first time,BSRs are observed in the abyssal plain of the Makran area,being associated with anticline structures,which do not show large spatial continuity and are strongly conditioned by structural conditions such as anticlines and fluid migration pathways,including deep fault,gas chimney,and high-permeable sedimentary layer.Our results may help to assess the gas hydrate potential within the piggy-back basins and to determine the most promising target areas.Moreover,results about the abyssal plain BSR may help to locate hydrocarbon reservoirs in the deep ocean.