Discovery and biogeochemistry of asphalt seeps in the North S\xe3o Paulo Plateau, Brazilian Margin

Kai Jiang,Jing Zhang,Akihiro Sakatoku,Shota Kambayashi,Vivian Helena Pellizari,Toshiro Yamanaka,Toshiyuki Kanehara,Katsunori Fujikura

doi:10.1038/s41598-018-30928-2

Abstract

An initial multiple biogeochemical dataset was acquired from the first discovered asphalt seeps in the Brazil margin during deep-sea dive surveys in 2013 using a manned submersible. These surveys were conducted on the outer escarpment of the North São Paulo Plateau. Sediment cores taken from the submersible were processed for pore water and sediment biogeochemistry. The silica concentration, as a chemical geothermometer, showed a steep gradient in the pore water, which indicates the possibility of an active brine system operating in the seepage area. Rare earth elements were used as powerful tracers of chemical processes. Low rare earth element concentrations in both asphalt and Fe-Mn oxyhydroxide-phase sediments suggests that rare earth elements were released during the oil fractionation and biodegradation processes and further depleted under the reducing environment. The main bacterial communities of the sediment were Proteobacteria in the asphalt sites, while at non-asphalt sites, the main bacterial communities of sediment were Firmicutes. Stable carbon and nitrogen isotopes were used to determine the food sources of the heterotrophs, and results suggest that asphalt probably provides a carbon source for these benthic animals. This study may provide useful information to clarify the impact of heavy hydrocarbon seepage on the marine ecosystem.

Highlights

The significance of research on petroleum seeps has been rapidly and widely recognized, and large numbers of petroleum seeps have been discovered in the world’s oceans[1]
The δ13C and δ15N values of asphalt ranged from −26.9‰ to −25.8‰ and 0.3‰ to 0.5‰, respectively, in dive #1345 and from −28.4‰ to −27.3‰ and 1.1‰ to 1.3‰, respectively, in dive #1346. This is the first discovery of asphalt seeps located in the Brazilian margin and the Southeastern Atlantic
Asphalt seeps discovered along the escarpment indicate that the salt diapirism is connected to deep oil reservoirs or to the source rock

Summary

Introduction

The significance of research on petroleum seeps has been rapidly and widely recognized, and large numbers of petroleum seeps have been discovered in the world’s oceans[1]. On the opposite coast of the South Atlantic, the Brazilian margin has large oil and gas reserves and abundant diapiric salt structures[13]. This margin is a place where petroleum seeps may occur. We eventually determined that the black material was asphalt according to later hydrocarbon composition analysis[14] This was the first discovery of natural asphalt seeps in the Brazilian margin, which is one of the relatively less well-studied types of heavy hydrocarbon seepages. The deep-sea survey in this study confirmed previous study results indicating that the surface of the São Paulo Plateau is irregular largely because of abundant diapiric structures[15]. The asphalt shows a high C35 homohopane index, indicating that it is a carbonate source rock[16]

Methods

Results

Conclusion