Sampling Procedure

Seven short sediment cores from the Peruvian OMZ were considered for the present study (Fig. 3.1, Table 3.1). The cores were recovered by using multicore technology during R.V.

Meteor cruise M77/1 in October and November 2008. One additional core was sampled during R.V. Meteor cruise M77/2 in November and December 2008. Within a couple of minutes after the multicorer came on deck, one tube was chosen from the array, and brought to a laboratory with a constant room temperature of 4°C. Supernatant water of the core was carefully removed. Then the core was gently pushed out of the multicorer tube and cut into 10-mm-thick slices for benthic foraminiferal analysis. The samples were transferred to Whirl-Pak™ plastic bags and transported at a temperature of 4°C. One core was completely frozen, and later sliced and subsampled at IFM-GEOMAR, Kiel. The samples from these eight cores were used to collect the specimens for the analysis of PD. Core-doubles from six locations were sampled for supernatant water and pore-water for the analysis of nitrate-concentrations in the same temperature-controlled laboratory like the other cores. Supernatant water was

54 A

Figure 3.1. A Study area and B oxygen distribution at 11°S off Peru (water-column oxygen profile from M77/1-424/CTD-RO-9); C Nitrate distribution between 10°30'−11°15'S (data from six CTDs at different

water depths).

55

collected with a syringe and filtrated via a syringe filter (0.2-nm cellulose-acetate filters) into a small PTFE-bottle.

The core-tube was placed in a glove bag flooded with argon to preserve pore water from oxygen contamination. The core was pushed out of the multicorer tube and was cut into 10-mm-thick slices and transferred into centrifuge tubules. The pore water was separated from sediments with a cooled centrifuge. The supernatant pore-water was also filtrated with a syringe-filter (0.2 nm cellulose-acetate filters) under argon atmosphere.

For 12 stations one separate multicorer tube was chosen to study benthic foraminifera that were living at the time of sampling (Tab. 3.2). The cores were sliced in 2- or 5-mm intervals from 0−10 mm, in 5-mm intervals from 10−40 mm, and in 10 mm intervals from 40−50 mm sediment depth in order to resolve the microhabitat depth structure. The samples were filled in PVC bottles, stained and preserved with a solution of 2 grams rose Bengal per liter of ethanol.

Afterwards the samples were stored and transported at 4°C to IFM-GEOMAR, Kiel.

Table 3.1. Locations sampled for pore-density determination. (* indicates that [NO3−]BW was interpolated from the closest measured data points in the profile of [NO₃⁻]_BW.).

Site

The surface sediment samples corresponding to the top centimeter were washed over a 63-µm mesh sieve. The residues were collected in ethanol to prevent corrosion and dried at 50°C.

They were further subdivided into the grain-size fractions of 63−125, 125−250, 250−315, 315−355, 355−400, and >400 µm. Up to 40 specimens of the shallow infaunal species Bolivina spissa and B. seminuda Cushman were picked from the 125−250 µm fraction. The larger fractions were considered when a sufficient number was not found in the 125−250 µm fraction. The 3 specimens shown in Figure 3.4 were mounted on aluminum stubs, sputter-coated with gold, and photographed with a CamScan-CS-44 scanning electron microscope (SEM) at the Christian-Albrecht-University in Kiel. All other specimens were mounted on

56

aluminum stubs and photographed with a Zeiss™ Leo VP 1455 SEM at the Geological-Palaeontological Institute of Hamburg University. Samples were not coated with a conductive layer to save them for further chemical analyses. To compensate charging effects on the specimens, the SEM was operated in a high pressure mode. Areas on the tests of the specimens were determined with Zeiss™ AxioVision 4.7. The pores from 232 specimens of B. spissa were counted on SEM photographs, and the pore diameter was assessed for a single specimen. Only megalospheric specimens were used for the analysis. All visible pores of a specimen were recorded. Because of the spatulate shape of the tests of B. spissa, we consider the pores recorded as half of all pores the specimen had. Additionally the PD and P of a single specimen of B. seminuda were determined. In this case pores were counted solely in an area of 10,000 µm² because of the strong homogeneity of pore distribution and the clavate shape of B. seminuda.

The rose Bengal-stained samples were washed on sieves with mesh sizes of 2,000 µm and 63 µm. The 63−2,000-µm size fraction was oven-dried at 50°C and weighed afterwards. The dried residues were divided with an Otto microsplitter until at least 300 well-stained individuals were counted. Specimens were picked onto Plummer cell slides, sorted by species, glued and tallied. The lengths of Living specimens were photographed taken with a MPX2050 CCD-camera from AOS™ coupled with a Navitar™ 6.5× zoom. The lengths of only well-preserved specimens were determined from these images with Zeiss™ AxioVision 4.7.

Table 3.2. Locations sampled for species distribution and test length determinations.

Site M772-005-1 St.635 12°05.66’ 77°40.07’ 214

57 3.2.3Environmental Parameters

Oxygen concentrations and temperatures in the water column were measured with a CTD-sensor. The oxygen sensor of the CTD was calibrated by Winkler titration of water samples taken during the respective CTD casts. Immediatly after retrieval, the CTD water was carefully subsampled into Winkler (~10 ml) flasks and the oxygen content was fixed immediatlely using 0.1 ml manganese (II) chloride and 0.1 ml alkaline iodide (Grasshoff et al., 1983). Oxygen concentrations were determined within 12 h by Winkler titration.

Nitrate concentrations in pore and bottom waters were measured onboard using a Metrohm 761 compact ion-chromatograph equipped with a Methrom/Metrosep A SUPP5 anion-exchange column (150/4.0 mm) and solution of Na2CO3 (3.2 mM) with NaHCO3 (1.0 mM) as eluent. The IAPSO seawater standard was used for calibration. For two sampling sites (M77/1-565/MUC-60 and M77/1-604/MUC-74) [NO3–

]BW was interpolated from the closest station on the 11°S transect, where [NO3−]BW was measured. The nitrate profile (Fig 3.1c) implies an error of ~2 µmol/L for the interpolated data.

Figure 3.2. A Depth distribution of the relative abundance of Bolivina spissa in the study area (Resig, 1990). Brackets indicate the center of the OMZ, where no specimens were found in this study; B

Depth distribution of B. seminuda in the study area.

58 3.3 Results