| Cold seeps are characterized by the migration of reducing gases,primarily methane and hydrogen sulfide,from the seafloor to sediments,seawater,and even the atmosphere.As these gases ascend,they chemically react with the sediments and seawater,altering the properties of both the seawater and the seafloor environment,resulting in the formation of various cold seep byproducts.Research has shown that in cold seep regions,which feature low temperatures,high pressure,and limited food resources,diverse biological communities thrive.These organisms derive the energy necessary for their survival from processes such as anaerobic oxidation of methane(AOM)and sulfate reduction(SR).The flux of methane in cold seep environments experiences continuous fluctuations over time and space.Factors such as the natural dynamics of fluid flow at methane seepage sites,along with increasing human activities,impact the biogeochemical properties of the microenvironment.Consequently,these factors shape the activity and evolution of chemoautotrophic organisms within these ecosystems.While our understanding of the mechanisms driving long-term changes in cold seep fluids has advanced considerably,the methods and carriers for examining shortterm changes remain scarce.Cold seep bivalves,with lifespans ranging from a few years to several decades,serve as ideal carriers for capturing short-term environmental fluctuations.Bivalve shells engage in biomineralization and other vital processes,incorporating elements from their surroundings and preserving them within carbonate shells.Consequently,it is highly plausible to reconstruct cold seep system biogeochemical processes and investigate organismal elemental responses to various cold seep environmental conditions using the elemental geochemical data stored in bivalve shells.This research holds significant value for deciphering short-term cold seep fluid alterations and the evolutionary traits of life under extreme environmental circumstances.This thesis undertakes experimental analysis of the mineral composition,microstructure,and trace element linear scanning in the shells of two chemoautotrophic bivalve species from the Haima cold seep seepage area in the South China Sea: the sulfide-nourished clam Archivesica marissinica and the methanenourished mussel Gigantidas haimaensis.A Lab X XRD-6100 X-ray diffractometer system(Shimadzu,Japan)was used to analyze the mineral composition of bulk shells(individual microstructures could not be separated since the extremely thin thickness of shell).The X-ray source was a Cu anode using Cu Ka radiation equipped with a diffracted beam graphite monochromator.A Coxem EM30 PLUS SEM system(South Korea)was used to observe the microstructures of A.marissinica and G.haimaensis.Shell fragments from several sections along the cross sections of each species were sputtercoated with a gold layer prior to observation,and the magnification varied from 969 to 2600 times.The concentrations of Li,Mg,Ca,Ti,Mn,Co,Cu,Zn,Sr,Zr,Mo,Ba,Th,and U were determined by means of laser ablation-inductively coupled plasma-mass spectrometry(LA-ICP-MS)in spot analysis mode along the growth axis of the shells near their outside layer.The distance between the two laser spots is about 1mm.According to the sample size,the number of laser spots is 153(C2015),111(C2020),116(M2020)and144(M2021),respectively.The findings from X-ray diffraction(XRD)and scanning electron microscope(SEM)analyses suggest that there are notable differences in the mineral composition and microstructure of shells from bivalves with different feeding types.The mineral composition of shells from bivalves containing methane-oxidizing bacteria symbionts is primarily composed of aragonite and calcite,and the shell’s microstructure is distinctly layered.Conversely,the shells of clams containing sulfide-oxidizing bacteria symbionts are mainly composed of aragonite and consist of a series of layer structures oriented in the growth direction.The results from laser ablation-inductively coupled plasma-mass spectrometry(LA-ICP-MS)and high-resolution elemental geochemistry analysis of trace element/Ca ratios(Li/Ca,Mg/Ca,Ti/Ca,Mn/Ca,Co/Ca,Cu/Ca,Zn/Ca,Sr/Ca,Zr/Ca,Mo/Ca,Ba/Ca,Th/Ca,and U/Ca)reveal that the complex distribution patterns of some trace elements(Mg/Ca,Sr/Ca,Mo/Ca,and U/Ca)in bivalve carbonate shells are mostly influenced by mineral composition or age.The Co/Ca and Ba/Ca ratios suggest that physiological and environmental factors strongly influence the absorption of trace elements during the biomineralization process in bivalves.Furthermore,the distribution patterns of other trace elements provide essential information for discussing unresolved issues such as the loss of trace elements after the death of bivalves and the possible effects of human activities such as sediment disturbance.Overall,this study highlights the importance of combining high-resolution elemental geochemistry information,mineral information,and morphological characteristics of bivalve shells to uncover the physiological and environmental factors that control bivalve growth and evolution.It also explores the potential of bivalve shells in revealing short-term-scale cold seep system micro-environmental changes. |
