<rahmann et al. 
but typically ranged from 7 to 18 1. To determine benthic 
solute fluxes, four (M77/1, M77/2) or eight sequential water 
samples (M92, M107, M136, M137, MSM17/4) were removed 
periodically with glass syringes (volume of each syringe — 
16 to 47 ml). The syringes were connected to the chamber 
using 1 m long Vygon tubes. Prior to deployment, these tubes 
were filled with distilled water and care was taken to avoid 
enclosure of air bubbles. An additional syringe water sampler 
(four or eight sequential samples) was used to monitor the 
ambient bottom water. The sampling ports for ambient bottom 
water were positioned about 30-60 cm above the sediment- 
water interface. 
For the measurement of the dinitrogen/argon ratio (N2/Ar), 
CO», and/or dissolved inorganic carbon (DIC) concentrations on 
cruises M92, M107, M136, M137, and MSM17/4, water samples 
were pumped into four (M92, M107, and MSM17/4) or eight 
(M136 and M137) 750 mm long glass tubes with an internal 
diameter of 4.6 mm (volume 12.5 ml) using self-constructed 
underwater peristaltic pumps. Prior to deployment, each glass 
tube was filled with distilled water that was completely replaced 
by the sample without dilution. Four (M92 and MSM17/4) or 
eight tubes (M136, M137, and M107) were used to sample each 
chamber and the ambient bottom water. During all cruises, the 
incubations at the sea floor were conducted for time periods of 
at least 24 h and to up 48 h, defined as the time interval between 
insertion of the chamber into the sediment and filling of the last 
syringe. Immediately after retrieval of the observatories, the water 
samples were transferred to the on-board cool room for further 
sample processing. 
Dissolved O, concentrations in each chamber and in the 
ambient bottom water were measured using optodes (Aanderaa 
Systems; Tengberg et al., 2006). As reported by Tengberg 
zt al. (2006) the precision of the sensors was +1 WM at 
higher O2 concentrations in the range of 300-500 WM and 
improved (40.5 WM) at O» concentrations below 300 WM. 
Salinity effects on the measured O2, concentration were 
corrected internally by the optode using a salinity of 35. O2 
concentrations were cross-calibrated with automated Winkler 
O2 measurements in parallel water samples. For the calculation 
of the total oxygen uptake (TOU), the linear part of the 
O2 time series after the start of the chamber incubation was 
used. In addition to O», fluxes of nitrate (NO3”7), nitrite 
(NO,”), ammonium (NH4*), phosphate (PO4°), and silicic 
acid (H4SiO4) were measured routinely. During some lander 
deployments, further biogeochemical parameters such as sulfide 
(e.g., M92), dissolved organic matter (M136/M137), or trace 
metals (M136/M137) were measured. 
Fluxes of routinely measured solutes were calculated from the 
linear increase or decrease of concentration versus time and the 
height of the water in each chamber. Starting with cruises M136 
and M137, a logistic function in addition to linear regression 
was used to capture the occasional sigmoidal temporal trend of 
solute concentrations. 
The landers were also equipped to recover the surface 
layer of the incubated sediment (10-15 cm), which serves as 
a check for sediment disruption during seafloor operations 
and chamber insertion. The sediment surface for most 
-rontiers in Marine Science | www.frontiersin.ore 
SFB754 Data Legacy 
deployments during the cruises was intact and undisturbed. 
The sediment was routinely subsampled for geochemical 
pore water analysis and, depending on the specific goals of 
the cruise, for biological analyses (e.g., foraminifera, sulfur 
bacteria, bacterial metagenomic analyses, and viruses). 
Details of sampling and processing of water and sediment 
samples, as well as their geochemical analysis, are presented 
ın the respective cruise reports (see Tables 1, 2) and specific 
publications (given above). 
As indicated above, in addition to standard flux measurements 
of the natural system, during Meteor cruise M137 a series of 
in situ experiments was conducted. During these incubations, 
NO3” and OO, concentrations inside the benthic chamber were 
experimentally manipulated (cf. cruise report from M137; see 
Table 1). During cruises M136 and M137, the BIGO lander 
was slightly modified to enable trace metal measurements in the 
benthic chambers and in the bottom water (cooperation with 
F. Scholz, GEOMAR). To determine gradients of nutrients and 
irace metals within the benthic boundary layer, the BIGO was 
2quipped with an extendable arm (cooperation with F. Scholz, 
GEOMAR). Subsequent to the placement of the lander on the 
seafloor, the arm unfolded and allowed water sampling at several 
heights above the seafloor. Water samples were collected in 
appropriate sampling bags. 
Near-Surface Sediment Coring 
Undisturbed sediment cores for the biogeochemical analysis of 
near surface sediment were retrieved using a multiple-corer 
(MUC) and using push-cores inserted into the sediment retrieved 
with the BIGO incubation chambers once on deck. The MUC 
was equipped with 6-8 Perspex liners, 60 cm long with an 
internal diameter of 10 cm. The MUC was lowered into the 
sediment with a speed of 0.3 m s7! in all deployments. Once 
on the sea floor, the liners were pushed into the sediment under 
gravity by a set of lead weights. Penetration ranged from 10 
to 50 cm depending on the sediment type. BIGO push-cores 
had a diameter of 10 cm and recovered around 5-20 cm of 
sediment. After retrieval, all cores were transferred to an on- 
board cool room set to the temperature of the bottom water 
and processed immediately. Supernatant bottom water of the 
MUC cores was sampled and filtered for subsequent analyses. 
[n general, at least one MUC and one BIGO sediment core 
was collected at the same site, but not necessarily on the 
same day. Sub-sampling for redox-sensitive parameters (e.g., 
dissolved Fe, nutrients) was mainly achieved by sectioning the 
sediment cores inside an argon filled glove bag. The sampling 
depth resolution increased from 0.5 or 1 cm at the surface to 
4 cm at larger depths. Sediment samples were then spun in 
a refrigerated centrifuge at 4000 g for 20 min to separate the 
porewater from the particulates. Subsequently, the porewater 
samples were filtered (0.2 wm cellulose-acetate syringe filters) 
under argon. In sandy sediments (MSM17/4, M107), rhizone 
samplers were used to extract porewaters. BIGO cores were 
sliced either under argon or ambient atmosphere depending on 
ıhe cruise. Standard analytes measured in porewater included 
nutrients, trace metals, total alkalinity, major ions, and dissolved 
hydrogen sulfide. 
Zeptember 2021 | Volume 8 | Article 72820