Biostratigraphic distribution of Miocene carbonates associated with gas hydrates in the northern Apennines (Italy) and their relationship with sea-level lowering

Gas hydrates are widespread in modern continental margins and their stability is related to temperature, pressure and availability of gas and water. Their sensitivity to tectonic activity and climate is still poorly constrained and more efforts are needed to understand how they respond to these forcing processes. Gas hydrates are frequently associated with carbonates, called clathrites, that may form bodies of large dimension and are marked by geochemical indicators such as 13C-depleted and enriched 18O values. A number of Miocene seep-carbonates outcrop in different geological settings of the northern Apennines (Italy) (Argentino et al., 2019a), showing characters suggesting paleo-gas hydrate occurrence and then can be considered as clathrites. Our biostratigraphic investigation on the sediment enclosing gas-hydrate associated carbonates has shown that they are roughly concentrated in three main intervals: in the Langhian (subzone MNN5a), in the upper Serravallian-lower Tortonian (subzone MNN6b to zone MNN7) and the upper Tortonian-lowermost Messinian (zones MNN10 and MNN11). By comparing seep distributions with 3rd order eustatic curves of Haq et al. (1987), they seem to match phases of sea-level lowering. The relationship among gas hydrate destabilization, climate change, sea-level variations, tectonic activity and fluid circulation, is particularly challenging in the fossil record. In the examined carbonates, a drop in the hydraulic pressure on the plumbing system during sea-level lowering would have shifted the base of the gas hydrate stability zone into shallower depths, inducing gas-hydrate destabilization. The uplift of the different sectors of the wedge-top foredeep system during tectonic migration could have increased the effect of the concomitant eustatic sea-level drop, reducing the hydrostatic load on the seafloor and inducing gas-hydrate decomposition. In this view, a precise biostratigraphic framework of paleo-clathrites in the sedimentary record may help to shed light into gas-hydrates long-term evolution and their relation with sea level changes and tectonics.