Inorganic geochemistry of Arctic Ocean sediments

Ann-Katrin Meinhardt Institute for Chemistry and Biology of

the Marine Environment, Oldenburg

Objectives

The goal of the geochemical program was a detailed investigation of Arctic Ocean sediments and pore waters. The main focus will be to investigate the formation mechanism and geochemical expression of repetitively occurring brown coloured

8.3 Sediment cores

sediment layers (“manganese cycles”). In Arctic Ocean sediments distinct dark brown intervals, which are rich in Mn and Fe compared to other hemipelagic sediments, alternate with light olive/yellowish brown intervals, which are comparably poor in these elements. There are different explanations for the formation mechanism of these colour cycles. Primary sources of Mn and Fe in the Arctic Ocean are the circum-Arctic rivers, which drain extensive peat bogs rich in both, Mn and Fe.

The input of metals to the deeper ocean basins is controlled by climatic features, i.e., higher riverine input during warmer conditions and lower input during colder conditions, or changes in ventilation of the bottom waters. Another explanation for the cyclicity may be diagenetic overprinting after deposition. During organic matter degradation, microbially mediated dissolution of metal (oxyhydr)oxides may occur in the absence of free oxygen under suboxic to anoxic conditions. The reduced metal species are then liberated to the pore water. Upon diffusion to the oxic/suboxic boundary, the reduced metal species may again precipitate and form a new sedimentary Mn/Fe peak. Multiple dark brown Mn- and Fe-rich intervals may be formed in this way after deposition. Our work focuses at a combination of both, sediment and pore water analyses to obtain information about the importance of organic matter degradation in Arctic Ocean sediments and the diagenetic mobility of metals like Mn and Fe.

Work at sea

Sampling of pore water was performed at 18 stations (Tab. 8.3) shortly after sediment recovery. The sediment cores (MUC or SL) were transported into the 4 °C laboratory, where the pore waters were sampled by the rhizon technique.

Pore water sampling of giant box corers was not performed due to problems during ARK-XXIII/3 (Jokat, 2009). A rhizon is a polymer filter with 0.1 µm pore size, which is attached to a PE/PVC tube with a luer lock. The MUC tubes were prepared with pre-drilled 3.8 mm holes in 1 cm resolution, which were taped during the coring process. In the cooling laboratory, the rhizons were stuck into the tube at variable resolution (~1 to 5 cm). A syringe was attached to every rhizon and vacuum was applied with the help of a spacer (Fig. 8.7). Pore waters of the SL were sampled in a similar way by drilling holes into the liner at ~20 cm resolution and inserting the rhizons (Fig. 8.8). Variable amounts of pore water were retrieved, mostly ~10 ml, depending on sediment features like porosity and sampling time. After sampling, the rhizons were removed, the cores were sealed and the pore water was filled in PP-tubes. Several fractions were obtained (1. untreated, for the determination of labile components and nutrients; 2. acidified, for the determination of metals after the cruise; 3. oxygenated [for reactive Mn, see below]) and stored at 4 °C. Rhizons were cleaned with 10% HCl and Milli-Q-Water for re-use.

Sediment samples of the MUC cores were taken at 1 cm resolution with a plastic spatula and stored in plastic bags. Sediment samples of a GC (PS78/248-6) were taken with plastic spatula at variable resolution, depending on discernible sediment properties, and stored in plastic bags.

For the determination of reactive Mn, a fraction of the pore water was left in the syringe and allowed to oxidise for ~48 hours. After this time the pore water was filled in PP-tubes with a syringe filter and was acidified with HNO₃.

Directly after dividing the pore water into the different fractions, analyses of ammonium and total alkalinity were performed. Both parameters were determined via photometric methods using a microtiter plate reader, which only consumes

onshore for several dissolved ions and metals (e.g. nitrate, phosphate, manganese, iron and other main and trace elements) via photometric methods, ICP-OES and ICP-MS. The sediment samples will be freeze-dried, ground and analysed for major and minor elements and bulk parameters using X-Ray Fluorescence, coulometry, combustion analyses, ICP-OES and ICP-MS.

Tab. 8.3: List of pore water samples, sediment samples and measured parameters on board

Station Gear Sediment Alkalinity Ammonium Acidified split Reactive Mn split

PS78/201-7 MUC X X X X

PS78/206-2 MUC X X X X

PS78/206-3 GC X X X X

PS78/208-1 MUC X X X X

PS78/211-1 MUC X X X X

PS78/217-1 MUC X X X X

PS78/220-6 MUC X X X X

PS78/220-7 GC X X X X

PS78/225-4 MUC X X X X

PS78/231-2 MUC X

PS78/231-2 GC X X X X

PS78/237-1 MUC X X X X

PS78/237-3 GC X X X X

PS78/248-4 MUC X X X X

PS78/248-6 GC X X X X X

PS78/275-1 MUC X X X X

PS78/277-2 MUC X X X X

PS78/280-6 MUC X X X X

PS78/285-6 MUC X X X X

8.3 Sediment cores

Fig. 8.7: Pore water sampling of a MUC

Fig. 8.8: Pore water sampling of GC segments

Fig. 8.9: Pore water profile of sediment core PS78/248-6 (SL)  

Preliminary results

In most of the cores, pore water ammonium values are low, ranging close to the detection limit of the method and ~10 µM, and show no trends with depth. One exception is core PS78/248-6 (Fig. 8.9, Lomonosov Ridge) with increasing values from ~470 cm core depth up to ~44 µM at 585 cm.

Both ammonium and inorganic carbon species (=alkalinity) are metabolites of microorganisms. Increasing ammonium values with depth are indicative for the increasing degradation of organic matter in the sediment. But compared to other marine settings, the observed values are rather low. Alkalinity values of all cores show little variability with depth and range mostly between 2 and 3 mM (Fig. 8.9) In the reactive Mn experiment, the fraction that was allowed to oxidise showed no precipitation of solid, oxidised Mn. This visual observation needs to be tested by quantitative Mn measurements of both fractions.

These very first results illustrate the general low productivity in the Arctic Ocean.

In one of the sediment cores (PS78/248-6) the increase of ammonium with depth indicates that diagenetic processes may occur deeper in the sediment. Further analyses of other parameters are necessary for a comprehensive study.