The Role of Magmatism in Hydrocarbon Generation in Sedimented Rifts: a Nd Isotope Perspective from Mid-Cretaceous Methane-Seep Deposits of the Basque-Cantabrian Basin, Spain

Studies on the involvement of intrusive magmatism in hydrocarbon generation within sedimentary basins have gained momentum owing to increasing appraisal of the role that such processes may play in controlling global carbon cycle perturbations, and the exploration potential of the volcanic sedimentary basins. Nevertheless, for many areas the causal link between the intrusions and surrounding hydrocarbon systems remains disputed, encouraging a search for methods that could aid in identifying different hydrocarbon sources. Here, we have performed a multi-proxy geochemical study of the middle Cretaceous methane-seep deposits of the Basque-Cantabrian Basin, an early-stage, peri-cratonic rift marking the Mesozoic opening of the Bay of Biscay. Infilled by a thick sedimentary succession intruded by shallow-level igneous bodies, the basin shares analogies with modern young, sedimented rifts that sustain hydrocarbon seepage. We have applied a novel approach that uses the Nd isotope composition of the seep deposits to constrain the relationship between hydrocarbon seepage and igneous activity, and to explore the general potential of Nd isotopes to trace magmatic-influenced fluids in volcanic sedimentary basins. The Nd isotope data have been combined with rare earth element analyses and carbon and oxygen isotope measurements, providing broad insight into the former composition of the seeping fluids. For three out of four investigated seeps, the Nd isotope ratios observed in authigenic seep carbonates include signatures markedly more radiogenic than that reconstructed for background seawater-derived pore waters. The level of this Nd-143-enrichment varies both between and within individual deposits, reflecting spatial and temporal differences in fluid composition typical of seep-related environments. The radiogenic Nd isotope signals provide evidence of subseafloor interactions between the seeping fluids and mafic igneous materials, supporting the model of an igneous control on the mid-Cretaceous methane expulsion in the Basque-Cantabrian Basin. The thermogenic origin of the methane is in accord with the moderately negative delta C-13 values and paragenetic successions observed in the studied seep carbonates. For a single deposit, its relatively unradiogenic Nd isotope composition can be attributed to the smallest size and shallowest emplacement depth of the underlying intrusion, likely resulting in a short-lived character and limited hydrocarbon-generation potential of the associated contact metamorphism. The study demonstrates that Nd isotope analyses of seep carbonates offer a tool in disentangling methane fluxes from different organic matter alteration pathways for the numerous, both fossil and modern sedimented rifts for which the involvement of various methane sources remains insufficiently understood.This work was supported by the National Science Centre, Poland (grant No. 2016/23/D/ST10/00444; to MJ) , and the Eusko Jaurlaritza (Ikerketa Taldeak IT930-16) and the Spanish State Research Agency (project PID2019-105670GB-I00/AEI/10.13039/501100011033; both to LMA

Deep-sea paleoenvironmental evolution in the mid-Cretaceous of the Basque Pyrenees based on microfaunal analysis (Armintza section)

The mid-Cretaceous Black Flysch Group and Plentzia Formation constitute two key lithostratigraphic units in the evolution of the Pyrenean rift system but many of their paleoenvironmental characteristics are poorly known. In this work, a microfaunal analysis (planktonic and benthic foraminifera and ostracods) of these two units at the Armintza section has been carried out in order to interpret the paleoenvironmental conditions (paleodepth, oxygen level and water masses) in which they deposited, and to relate them to the contemporary tectono-sedimentary settings. Based on the high proportion of planktonic foraminifera with respect to total foraminifera and the assemblages of benthic foraminifera species, a paleodepth of ∼600 m is estimated for studied units. During syn-rift deposition of the organic-rich Black Flysch Gp. in middle Albian-early Cenomanian times, the relative abundance of planktonic foraminifera and the proportions of different shell morphotypes suggest the presence of a shallow water mass and the stagnation of deeper waters. Moreover, the presence of abundant hypoxia-tolerant benthic foraminifera species and the occurrence of only rare hypoxia-tolerant platycopid ostracods indicate severe hypoxia conditions (<1.5 ml/l) or even anoxia at the seafloor. A noticeable change is recorded at the contact between the Black Flysch Gp. and the Plentzia Fm. (lower-middle Cenomanian transition). During post-rift deposition of the Plentzia Fm. in earliest middle Cenomanian times, developed microfaunal assemblage shows an important increase in benthic foraminifera species, many of which indicate intermediate waters, and dominance of keeled planktonic foraminifera suggesting the end of water stagnation and the inflow of an intermediate water mass. In addition, the presence of hypoxia-tolerant species of ostracods and benthic foraminifera indicates the development of mild hypoxia conditions at the seafloor during this interval.Eusko Jaurlaritza (Ikerketa Taldeak IT1602-22), the Spanish State Research Agency (project PID2019-105670 GB I00/AEI/10.13039/501100011033; both to LMA), and HAREA-Coastal Geology Research Group (EJ/GV, IT 976-16; AP and JRL).This study was supported by Eusko Jaurlaritza (Ikerketa Taldeak IT1602-22), the Spanish State Research Agency (project PID2019-105670 GB I00/AEI/10.13039/501100011033; both to LMA), and HAREA-Coastal Geology Research Group (EJ/GV, IT 976-16; AP and JRL). The authors are grateful to two anonymous reviewers for their detailed, helpful suggestions and comments. The authors also thank for technical and human support provided by the SEM service of SGIker (UPV/EHU/ ERDF, EU)