| Authigenic carbonates and chemosynthentic shell fragments are good archives for the evolution history of past methane seepage. The authigenic carbonates, loose and cemented chemosynthentic bivalve fragments were collected on the seafloor inside the pockmarks CN03, Tobic, Do Do and G11, located at N yegga pockmark field, mid-Norwegian continental margin. Petrology, mineralogy, isotope geochemistry, U-Th dating on authigenic carbonates were carried out, in order to:(1) understand the diagenetic environment where carbonates precipitated;(2) figure out composition of the methane-rich fluid, methane source and seep evolution characteristics;(3) determine the time of methane releasing;(4) understand possible interaction between methane seepage, hydrate dissociation and Storegga slide.Authigenic carbonates are characteristic of a high abundance of various voids and fissures, manifesting that carbonates are related to the gas venting. Meanwhile authigenic carbonates are composed of a siginifciant amount of detritus, especially of quartz, feldspar and clay, indicating that these carbonates were originally precipitated within sediments. Authigenic carbonate crusts pockmark Tobic are cemented with numerous chemosynthetic bivalves. The carbonate phases are predominated of micritic high- magnesium calcite and acicular aragonite, with little dolomite. Peloids and framboidal pyrites are abundant, indicating the reducing conditions and microbial mediated processes during the precipitation of the carbonates. The δ13C values are ranging from-58.67 to-47.46‰ PDB, clearly demonstrating that the carbonates are derived from the processes of anaerobic methane oxidation coupling with sulfate reduction, and microbial methane is the dominant carbon source. The δ13C values of bivalve shells are-3.8~-2.7‰ PDB, indicating their carbon source is primarily from the dissolved inorganic carbon(DIC) in sea water. The 87Sr/86 Sr ratios were determined with values of 0.70913~0.70918, proximal to the value of modern bottom seawater, suggesting that the fluid for the precipitation of carbonates is affected by mode rn bottom seawater, not from deep sources. It also verifies that the methane for the carbonate formation is microbial in origin. Meanwhile δ13C values of the carbonates effectively reflect the difference in δ13C values of methane in the correspondent pockmarks.The U-Th dating of authigenic carbonates was carried out. The preliminary results show that most carbonates precipitated between 18.5-12.3 ka, that is, methane seepage was intermittent since last glacial maximum(LGM). Based on carbonate precipitation equilibrium equations, the oxygen isotopic compositions of fluids from which carbonates precipitated were reconstructed, showing that carbonates precipitated during LGM are in or near isotopic equilibrium with the contemporary bottom water, not affected by heavy oxygen isotopic fluid released from gas hydrate dissociation. This might suggest that the authigenic carbonates precipitated either from the methane from somewhere, or from the methane derived from gas hydrate dissociated at depth due to the seale vel drop during the LGM. However, carbonates precipitated during Holoence show about +0.7‰ heavier than bottom water, this might be caused by decomposition of gas hydrates in proximity in sediments.In summary, the methane seepage has been active since the LGM. It is much ealier than the recent slide event occurring at 8 ka in the Storegga Slide. It might infer that gas hydratedissociation may be one of the factors trigger the Storegga submarine slide. |
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