Formation process of Pliocene cold seep carbonates from the southern Western Foothills, Southwestern Taiwan: A synthetical rare earth element and C–O–Sr–Nd isotope study

In this work, the Pliocene dolomitic carbonate concretions and pipes from Chiahsien of the Western Foothills, southwestern Taiwan were studied. We conducted a synthetical analysis of stable carbon and oxygen isotopes, rare earth element (REE) contents, and Sr and Nd isotopes, to trace the compositions and sources of pore fluids from which carbonates have precipitated and reconstruct the carbonate formation processes. The carbonates with low δ13C values (as low as −48.6‰) are promoted by the anaerobic oxidation of biogenic methane. The 87Sr/86Sr ratios of samples reflect the mixing of coeval bottom seawater and seep fluid enriched in 87Sr. The Nd isotopic signatures of studied carbonates indicate multiple Nd sources, including detrital components, Fe–Mn oxyhydroxides, organic matter, and seawater. The MREE-bulge patterns in samples are typical of anoxic conditions, which are supported by Ce anomalies. Our results demonstrate that the seepage intensity played a key role in the formation processes of concretions and pipes by controlling the depth of the sulfate-methane transition zone (SMTZ). The concretions characterized by the lowest δ13C values formed at a deeper depth in sediments under low methane flux, which suffered no/less influence of seawater. The representative pipe with obvious concentric lamination grew in a rim-to-core pattern. In the early stage, the variable flux of highly channelized seepage caused the fluctuation of carbon and oxygen isotopes. During the later stage, the modification of isotopic compositions in the core part were resulted from meteoric alteration. Our work provides insights into the formation process related to seepage evolution in ancient cold seeps on Taiwan Island.