ALBERT

Description: The discovery that foraminifera are able to use nitrate instead of oxygen as energy source for their metabolism has challenged our understanding of nitrogen cycling in the ocean. It was evident before that only prokaryotes and fungi are able to denitrify. Rate estimates of foraminiferal denitrification were very sparse on a regional scale. Here, we present estimates of benthic foraminiferal denitrification rates from six stations at intermediate water depths in and below the Peruvian oxygen minimum zone (OMZ). Foraminiferal denitrification rates were calculated from abundance and assemblage composition of the total living fauna in both, surface and subsurface sediments, as well as from individual species specific denitrification rates. A comparison with total benthic denitrification rates as inferred by biogeochemical models revealed that benthic foraminifera account for the total denitrification on the shelf between 80 and 250 m water depth. They are still important denitrifiers in the centre of the OMZ around 320 m (29-56% of the benthic denitrification) but play only a minor role at the lower OMZ boundary and below the OMZ between 465 and 700 m (3-7% of total benthic denitrification). Furthermore, foraminiferal denitrification was compared to the total benthic nitrate loss measured during benthic chamber experiments. Foraminiferal denitrification contributes 1 to 50% to the total nitrate loss across a depth transect from 80 to 700 m, respectively. Flux rate estimates ranged from 0.01 to 1.3 mmol m-2 d-1. Furthermore we show that the amount of nitrate stored in living benthic foraminifera (3 to 705 µmol L-1) can be higher by three orders of magnitude as compared to the ambient pore waters in near surface sediments sustaining an important nitrate reservoir in Peruvian OMZ sediments. The substantial contribution of foraminiferal nitrate respiration to total benthic nitrate loss at the Peruvian margin, which is one of the main nitrate sink regions in the world oceans, underpins the importance of previously underestimated role of benthic foraminifera in global biochemical cycles.

Description: One of the objectives of the SFB 754 was the reconstruction of the factors controlling the intensity and the spatial extent of the OMZ in the Eastern Tropical Pacific, specifically off Peru, since the Last Glacial Maximum (21000 years ago). For the purpose of these paleoceanographic studies, long gravity cores were recovered during four scientific expeditions (M77/1, M77/2, M92, and M135; see Figure 4). During the cruises M77/1 and M77/2 in 2008, 51 sediment cores were retrieved below and in the centre of the OMZ, from ~17° S to the equator (Pfannkuche et al., 2011; see Figure 4). Most of the records collected in the core of the OMZ (i.e. ~200 to ~500 m depth), from ~8 to 15° S, show sedimentary discontinuities during the Holocene (last 11700 years), which preclude high resolution paleoceanographic reconstructions in this area (Erdem et al., 2016; Salvatteci et al., 2014, 2016). Based on the information collected during M77/1 and M77/2 and also on the scientific literature, cruise M135 aimed specifically at finding the most complete Holocene sequence in the Eastern Tropical South Pacific. For this purpose, a detailed paleoceanographic survey took place at ~17° S, an area that is less affected by processes that can produce sediment discontinuities. Six sediment cores were retrieved, two of which contained the most complete sediment sequences for the last 10 000 years (Salvatteci et al., 2019). In addition, core tops of near sediment surface cores from multi-corers (MUCs) have been used to establish local calibrations for several paleoproxies, such as redox-sensitive elements in foraminifera (i.e. Mn/Ca, I/Ca and Fe/Ca), foraminiferal assemblages, and stable Mo and N isotopes