Discovery and quantification of a widespread methane ebullition event in a coastal inlet (Baltic Sea) using a novel sonar strategy

A Lohrberg,O Schmale,I Ostrovsky,H Niemann,P Held,J Schneider Von Deimling

doi:10.1038/s41598-020-60283-0

A Lohrberg, O Schmale + Show 4 more

Open Access

PDF Available

https://doi.org/10.1038/s41598-020-60283-0

Copy DOI

Export

Save

Cite

Abstract
Highlights/Summary
Full-Text PDF
Similar Papers

Abstract

Listen

How much of the greenhouse gas methane is transported from the seafloor to the atmosphere is unclear. Here, we present data describing an extensive ebullition event that occurred in Eckernförde Bay, a shallow gas-hosting coastal inlet in the Baltic Sea, in the fall of 2014. A weak storm induced hydrostatic pressure fluctuations that in turn stimulated gas ebullition from the seabed. In a finely tuned sonar survey of the bay, we obtained a hydroacoustic dataset with exceptionally high sensitivity for bubble detection. This allowed us to identify 2849 bubble seeps rising within 28 h from the seafloor across the 90 km² study site. Based on our calculations, the estimated bubble-driven episodic methane flux from the seafloor across the bay is 1,900 μMol m−2 d−1. Our study demonstrates that storm-associated fluctuations of hydrostatic pressure induce bulk gas-driven ebullitions. Given the extensive occurrence of shallow gas-hosting sediments in coastal seas, similar ebullition events probably take place in many parts of the Western Baltic Sea. However, these are likely to be missed during field investigations, due to the lack of high-quality data acquisition during storms, such that atmospheric inputs of marine-derived methane will be highly underestimated.

Highlights

Methane is an important greenhouse gas, ranking second in radiative forcing by well-mixed greenhouse gases[1] and with an estimated global net atmospheric emission of about 592 Tg per year[2]
As shallow gas accumulations are a common feature in coastal settings, our study suggests that storm-induced pressure fluctuations at the seafloor and subsequent gas ebullitions are a global phenomenon, but one that is likely to be missed in standard field campaigns
Storm-associated and event-driven methane ebullitions can be overlooked by the ‘standard’ surveying strategies of marine acoustic research

Summary

Introduction

Methane is an important greenhouse gas, ranking second in radiative forcing by well-mixed greenhouse gases[1] and with an estimated global net atmospheric emission of about 592 Tg per year[2]. Methane inputs into the atmosphere from these regions are much more substantial than those from open waters[5,13,14,15] Among these coastal sites, the shallow gas-bearing sediments extensively found in river deltas, embayed coastal areas, and estuaries[16,17,18,19,20,21,22,23,24,25,26,27] are expected to expand due to global. An accurate estimate of the strength of the atmospheric methane source originating from seabed seepage requires a precise quantification of the methane gas bubble flux through the water column as well as measurements of the methane concentration at the sea surface, but in many cases both are unavailable in sufficient temporal and regional resolution[41]. This strategy necessarily leads to a mismatch of top-down and bottom-up estimations when upscaled for larger areas[42]

Methods

Results

Conclusion