International Conference on Gas Hydrates (ICGH) (6th : 2008)

REDOX VARIATIONS AT COLD SEEPS RECORDED BY RARE EARTH ELEMENTS IN SEEP CARBONATES Feng, Dong; Chen, Duofu; Lin, Zhijia; Peckmann, Jörn; Bohrmann, Gerhard; Roberts, Harry H.

Abstract

Understanding the formation conditions of seep carbonate is crucial to better constrain the dynamic fluid flow and chemical fluxes associate with cold seeps on the seafloor. Rare earth element (REE) in seep carbonates collected from modern cold seeps of Gulf of Mexico, Black Sea, Congo Fan, ancient seeps of Beauvoisin (Oxfordian, J3, Southeastern France) and Marmorito (Miocene, Northern Italy) were studied. Our focus has been on 5% HNO3-treated solution (authigenic carbonate minerals) of carbonates. Several crystalline forms of carbonate minerals have been selected for analysis. Total REE (ΣREE) contents in seep carbonates varies widely, from 0.068 to 43.655 ppm, but the common trend is that the ΣREE in microcrystalline phases is highest and lowest of in sparite, suggesting that the ΣREE of seep carbonates may be a function of diagenesis. The shale-normalized REE patterns of the seep carbonates show varied Ce anomalies across several seep sites and even within one site, suggesting that the formation condition of seep carbonate is variable and complex. Overall, our results show that apart from anoxic, oxic formation condition is also common at hydrocarbon seep environments.

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