Assessing genesis and early diagenesis of siderite concretions at Paola Ridge (southern Tyrrhenian Sea) using stable isotopes and REE composition

Carbonate concretions formed in shallow sub-bottom sediments of the Paola Ridge (southern Tyrrhenian Sea) have been investigated for their geochemical and isotopic composition. These carbonates have been collected within an... [ view full abstract ]

Carbonate concretions formed in shallow sub-bottom sediments of the Paola Ridge (southern Tyrrhenian Sea) have been investigated for their geochemical and isotopic composition. These carbonates have been collected within an area of intense gas venting, occurring on top of two domed morphologies, and seepage, occurring alongside three diapirs. The entire area is dotted by pockmarks and dissected by normal faults. This area is known for having been a site of fluid seepage for a long time (at least 40 ka). The present-day venting is characterized by fluids with a prevalent CO2 composition.
Here we use stable isotopes composition (carbon and oxygen) of carbonates coupled with rare earth elements (REE) composition of different carbonate and non-carbonate phases for tracing fluids migration and early diagenesis of sea bottom sediments. We analyzed a series of calcite/aragonite and siderite concretions together with sulfide and Fe-oxyhydroxide nodules and siliciclastic sediments. Authigenic aragonite (and calcite) concretions, found in surficial sediments, show depleted 13C isotopic composition (average 13C = -39.9‰) and positive 18O values (average 4.5‰). Particular emphasis is for the siderite concretions retrieved within the core catchers of the two domed morphologies in the first 6 meters of sediment. Siderite concretions yielded positive 13C and 18O values with average values of +7.9‰ and +9.3‰, respectively. Flat REE pattern and lack of Ce anomaly of aragonite/calcite concretions suggest that the carbonates incorporated high amounts of organic matter and precipitated within highly alkaline pore water. REE pattern of siderite shows consistent LREE fractionation and positive Gd and La anomalies suggesting that organic matter has been removed from the system, probably through a combination of bacterial metabolism and fluid migration.
The formation of authigenic carbonate at Paola Ridge was fueled by mixed-composition fluid circulation through a system of normal faults imaged by the available seismic lines. Aragonite/calcite concretions were formed at sites of methane seepage as shown by the 13C-depleted composition of the concretions and the association with chemosymbiotic mollusks, while authigenic siderite formed in equilibrium with heavy carbon and heavy oxygen water. The geochemical composition of siderite may suggest the past presence of gas hydrates. In this scenario the formation of 18O–enriched siderite would have been fostered by the release of heavy oxygen from the decomposition of gas hydrates, even though, on the basis of the present-day chemo-physical parameters, their presence is considered to be unlikely in this part of the Mediterranean Sea.