Aarhus Bay spring cruise 2003 with St.F. Henry

Field trip: 10 / 13 March 2003 Sample party in NERI labs: 11 - 14 March 2003 Research Area: Aarhus Bay Port: Aarhus Chief Scientist: Henrik Fossing (NERI) - Participants

Cruise objectives

The objective for this METROL cruise to Aarhus Bay was to estimate the depth of the sulfate-methane transition zone by direct measurements along a 7.4-km long line streching from the central part of the bay towards the east.

Five stations were selected for sediment sampling based on seismic measurements performed before by METROL partner GEUS. Station M1 was positioned in the central part of the bay where the seismic studies had revealed free methane gas (i.e. CH4 bubbles) in 3.5-4 m depth below seafloor (bsf). In this part of the bay, the seismic survey showed a quite homogeneous picture with respect to the depth contour of free methane gas over hundreds of square meters. The four other stations (M2 - M5) were chosen along a transect with gradually more methane towards the sediment surface, with no free CH4 in the upper 4 m bsf at station M2 to presence of free gas in <1m bsf at station M5.

Sediment was sampled by gravity and Rumohr Lot coring. Gravity cores were split onboard into 1-m sections, which were transferred to the NERI laboratory in Silkeborg for analyses of the sulfate and methane porewater profiles. The loss of surface sediments in the gravity cores was estimated by aligning the sulfate profiles with those measured in undisturbed Rumohr Lot cores from the same station.