CONTROLLED PERTURBATIONS: OXYGEN AND FLOW MEASUREMENTS AT A DEEP-SEA FLUME

AND AT A WHALE CARCASS SITE

Eberhard Sauter ¹, Ulrich Hoge ¹, Christophe Ra-bouille ², Bruno Bombled²

1) Alfred-Wegener-Institut

2) LSCE

Objectives

One objective of this cruise was to quantify the effect of controlled perturbation of the system such as increased current in a flume or disposal of a whale carcass as an external food source on the overall functioning of the ecosystem.

Work at sea

ROV-based microprofile measurements inside and outside the flume

In total, three ROV dives were performed with the AWI microprofiler (MIC). The microprofiler was carried down to the seafloor on the ROV Porch which allowed a rapid start of the experiment after the exact target area had been reached. For the flume experiment, we decided to perform oxygen measurements at a reference station 5 meters south of the in-situ flume and to perform the controlled perturbation measurements in the central part of the flume. The first set of profiles was obtained during ROV dive #175 and lasted 5 hours. The microprofiler stayed overnight on the sea floor and the second set of profiles (inside flume) was achieved during the subsequent dive #176. The whale carcass was investigated during a single ROV dive (#178) using a shorter microprofiler programme which lasted 3 hours. The first set of measurement was achieved on the site of whale disposal, whereas the second set of profiles was measured 20 meters away from the former whale carcass. After all dives the MIC was recovered using the COLOSSOS lift. The MIC was equipped with 5 oxygen microelectrodes with tip diameter of 50 µm (provided by Unisense) and a resistivity electrode of 1 cm width. The oxygen sensors were calibrated using a two point approach: oxygen concentrations in bottom water were determined by Winkler titration and the zero was determined from the anoxic zone measured on dive #178 during the whale carcass set of profiles. Dithionite zeroes were tried at near in-situ temperature (2 °C), but showed poor adequation to bottom water zeroes.

ARK-XXII/1A-C

Tab. 38.1: Profiles of ROV drives

ROV dive Label Date Station Depth (m) Position

#175 PS70-171 13 July 2007 S3 2353m 78° 36,22' N 5° 05,13' E

#176 PS70-177 14 July 2007 S3 2353m idem

#178 PS70-215 20 July 2007 HG Central 2487m 79° 4,58' N 4° 08,54' E

Installation of long-term bottom current meters

An acoustic travel time current meter of the type MAVS3 (Nobska Instruments) was installed by the ROV Quest in the central part of the flume during dive #176 in order to record the current speed directly in the flume (Fig. 38.1 left). The deployment was programmed over a Period of one year with several 2 Hz measurements every 12 hours.

A similar current meter was installed at the same height above bottom (30 cm) outside the flume on a tripodal umbrella frame (Fig. 38.1 right). These two current meters are supposed to document the current speed and direction in and outside the flume.

Fig. 38.1: Placement of acoustic time current meters in side (left) and outside the deep-sea flume (right). Photographs © marum

Comparative microprofile measurements using push cores and onboard measurements Two push cores were collected inside and outside the flume in order to measure oxygen profiles on board ship. The cores outside the flume were collected on the southern side of the flume whereas the core in the flume was sampled 1.5 meter away from the MIC measurement.

Preliminary results O₂ Microprofiles

During the 3 ROV dives, 16 oxygen microprofiles and 4 resistivity microprofiles were acquired by the MIC. Concerning the flume experiment, 4 workable profiles were acquired in and outside the flume (one electrode broke during the first set of profiles).

The results of the in-situ experiment show a similar pattern in the flume and in the reference zone in the uppermost 2 centimetres and a difference in concentration in

38. CONTROLLED PERTURBATIONS: OXYGEN AND FLOW MEASUREMENTS

lower layers of the sediment, with an average concentration of 25 µmol/l lower in the flume than outside the flume. The diffusive oxygen uptakes (DOU) are similar in and outside the flume, whereas oxygen penetration depth is different. A drift in the sensor signal due to the sensor itself or the cooling of the electronics can be ruled out as several sensors showed little drift and the signal difference in the lower profile was much larger than the signal drift. These results are surprising, as the flume has been closed by a top lid for 4 years (since its deployment in 2003) with accelerated bottom current which should have lead to a reduction of vertical organic matter input and increased sediment resuspension in the flume. The oxygen fluxes show that diffusive uptake has remained at a high level in the flume, with potentially larger consumption in lower layers (below 2 cm).

With pushcores from the flume area, the comparison of in-situ and on board profiles showed a contrasted pattern. Indeed, the profiles in the flume had a similar pattern when measured on board and in-situ except the initial slope which was larger when measured on cores. This might be due to core compaction during sampling. On the contrary, the measurements outside the flume showed little discrepancy between both techniques in the upper part of the profile but lower concentration when measured on board compared to in-situ. This difference may be related to natural variability in sediment, as this same difference between techniques was observed on separate cores from a single cast.

Concerning the whale carcass, the visible remains of the whale were totally absent, except a rim of darker sediments around the former carcass. The profiles performed within this rim showed a large reduction of oxygen penetration compared to the reference site. This indicates that the sediment is still active below the former whale carcass two years after disposal.