A novel freeze corer for characterization of methane bubbles and assessment of coring disturbances

Abstract

Aquatic ecosystems with organic‐rich sediments are a globally significant source of methane to the atmosphere. In shallow waters, ebullition is often a dominant emission pathway of methane. Current knowledge on the processes controlling gas bubble formation and persistence in aquatic sediments is limited. An important prerequisite for accurate quantification of the structure and methane bubbles in sediment samples is to preserve the ambient in situ conditions during the withdrawal process and further analysis. A novel freeze corer has been developed that facilitates sampling of gas‐bearing soft sediments for X‐ray computer tomography. The sampler allows freezing sediment inside a double‐walled corer with a mixture of dry ice and ethanol. This corer has moderate costs and offers important advantages for gassy sediment sampling. Its simplicity and robustness allow to perform sampling from a small boat and the ability to characterize in situ sediment features. The applicability of this freeze coring technique for gas bubble quantification was validated during laboratory experiments aimed to investigate the effects of freezing on sediment gas content, bubble size distribution, and their geometry by comparing computer tomography scans of unfrozen vs. frozen cores. The performance of the corer was further evaluated during field conditions in Lake Kinneret (the Sea of Galilee, Israel). The results demonstrate the suitability of the freeze‐coring method for in situ preservation of gas‐bearing sediments. The sediment structure, however, showed some displacements of sediments layers and bubble abundance in some core regions. Future investigations are needed to address the nature of disturbances of the frozen sediment.

Laboratory and field investigations on freeze and gravity core sampling and assessment of coring disturbances with implications on gas bubble characterization

“…Due to the short time difference (approximately 36 h) between sampling and CT scan, the effect of increasing methane production can be assumed to affect θ , but to a minor extent. When comparing our results to those of Dück et al (), who showed that freezing of gassy clay sediments under laboratory conditions caused a contraction in θ by 27% ± 6%, it can be concluded that the greater differences in our study can be related to the changes in hydrostatic pressure and sample temperature, as described above.…”

“…This allows adjusting the core stratigraphy to remove the effect of shortening to their natural position to reduce the risk of an under‐ or overestimation of the core data. Dück et al () have shown that video imaging method for visualization of the freeze corer penetration into the sediment can bring a benefit in terms of quantification of shortening and is recommended to be used. In cases where the visibility at the benthic boundary layer is not good, the use of an echo‐sounding technique for the measurement of the penetration depth is recommended. Gravity coring is a relatively easy, cost‐efficient method and requires less effort than freeze coring.…”

“…) is a further development of the remote‐controlled freeze corer described by Lotter et al (). For a detailed description of the new corer design, see Dück et al (). The tripod is made of modular low‐weight aluminum, which can easily be assembled and extended.…”

Review and Analysis of Key Techniques in Marine Sediment Sampling

“…As the drive mechanism of flap seals is relatively simple, only a torsion spring is needed. At present, the ice seal structure is widely used in natural gas sampling [46][47][48][49].…”

Characterization of the gassy sediment layer in shallow water using an acoustical method: Lake Kinneret as a case study

“…Also, corers inevitably damage sediment during the sampling process, disrupting its structural integrity (e.g., see fig. 6 in Dück et al 2019). This significantly affects the free gas content and its quantitative estimates in cores.…”