Description: Thriving benthic communities were observed in the oxygen minimum zones along the southwestern African margin. On the Namibian margin, fossil cold-water coral mounds were overgrown by sponges and bryozoans, while the Angolan margin was characterized by cold-water coral mounds covered by a living coral reef. To explore why benthic communities differ in both areas, present-day environmental conditions were assessed, using conductivity–temperature–depth (CTD) transects and bottom landers to investigate spatial and temporal variations of environmental properties. Near-bottom measurements recorded low dissolved oxygen concentrations on the Namibian margin of 0–0.15 mL L−1 (≜0 %–9 % saturation) and on the Angolan margin of 0.5–1.5 mL L−1 (≜7 %–18 % saturation), which were associated with relatively high temperatures (11.8–13.2 ∘C and 6.4–12.6 ∘C, respectively). Semidiurnal barotropic tides were found to interact with the margin topography producing internal waves. These tidal movements deliver water with more suitable characteristics to the benthic communities from below and above the zone of low oxygen. Concurrently, the delivery of a high quantity and quality of organic matter was observed, being an important food source for the benthic fauna. On the Namibian margin, organic matter originated directly from the surface productive zone, whereas on the Angolan margin the geochemical signature of organic matter suggested an additional mechanism of food supply. A nepheloid layer observed above the cold-water corals may constitute a reservoir of organic matter, facilitating a constant supply of food particles by tidal mixing. Our data suggest that the benthic fauna on the Namibian margin, as well as the cold-water coral communities on the Angolan margin, may compensate for unfavorable conditions of low oxygen levels and high temperatures with enhanced availability of food, while anoxic conditions on the Namibian margin are at present a limiting factor for cold-water coral growth. This study provides an example of how benthic ecosystems cope with such extreme environmental conditions since it is expected that oxygen minimum zones will expand in the future due to anthropogenic activities.

Description: An exceptionally large cold-water coral mound province (CMP) was recently discovered extending over 80 km along the Namibian shelf (offshore southwestern Africa) in water depths of 160-270 m. This hitherto unknown CMP comprises 〉2000 mounds with heights of up to 20 m and constitutes the largest CMP known from the southeastern Atlantic Ocean. Uranium-series dating revealed a short but intense pulse in mound formation during the early to mid-Holocene. Coral proliferation during this period was potentially supported by slightly enhanced dissolved oxygen concentrations compared to the present Benguela oxygen minimum zone (OMZ). The subsequent mid-Holocene strengthening of the Benguela Upwelling System and a simultaneous northward migration of the Angola-Benguela Front resulted in an intensification of the OMZ that caused the sudden local extinction of the Namibian corals and prevented their reoccurrence until today.

Description: Tubular carbonate conduits (TCC) represent the termination of fluid plumbing systems in environments of hydrocarbon seepage and play a relevant role in the discharge of methane from sub-seafloor sediments to the water column. However, the biogeochemical reactions and biological activities involved in their formation are not fully understood. To address this, TCC samples were collected with a remotely operated vehicle from the seabed on the SW flank of the Athina mud volcano in the eastern Mediterranean Sea. Petrographic, mineralogical, stable carbon and oxygen isotope and lipid biomarker analyses were performed to elucidate the formation processes of the tubular carbonates. Clotted and fibrous aragonite form the internal lining of the cavities, while the outer portion of the tubes is formed by micritic Mg-calcite cementing hemipelagic sediment. 13C-depleted Mg-calcite and aragonite (as low as −14.4‰ V-PDB) and lipid biomarkers (archaeol, −89.8‰ V-PDB) indicate that carbonate precipitation was influenced by sulfate-dependent anaerobic oxidation of methane (AOM). AOM locally enhances aragonite precipitation, thereby facilitating early lithification of the conduits within the mud volcano sediments. The size and morphology of the TCC comparable with the buried portion of tubeworm colonies found in the proximity of the sampling site. However, our results suggest that TCC likely formed by the action of burrowing organism rather than being mineralizations of the tubeworm colonies. This study provides new insights into the interpretation and understanding of TCC, highlighting the role of macrofaunal activity in the formation of migration pathways for hydrocarbon-rich fluids on the flank of a mud volcano.

Description: Tubular carbonate conduits (TCC) are exposed on a secondary terrace at the SW Peak of Athina mud volcanoes Anaximander Mountains, eastern Mediterranean Sea. Three carbonatic phases compose the TCC sampled at Athina mud volcano: the micritic high-Mg-calcite, the clotted and fibrous aragonite and the detrital infilling. The stable isotopes of carbon and oxygen data from the sample studied show the influence of the anaerobic oxydation of methane in the formation of TCC.

Description: Alive and dead colony sections of Madrepora oculata cold-water corals were measured from underwater video transects carried out to characterize benthic communities. Data were collected from 17th to 25th January 2016 off Angola in 6 coral mounds. Video transects were recorded with the Remotely Operated Vehicle SQUID during the ANNA expedition (Hebbeln et al. 2016). The data included in this dataset have been published in Orejas et al. (2021).

Description: This dataset includes occurrence data for the cold-water coral Madrepora oculata obtained from underwater video transects performed to characterize benthic communities. Data were collected from 17th to 25th January 2016 off Angola on 6 coral mounds. Video transects were carried out with the Remotely Operated Vehicle SQUID during the ANNA expedition (Hebbeln et al. 2016). The information included in this dataset have been published in the paper Orejas et al. (2021).

Description: Occurence data and measurements of alive vs. dead colony sections of Madrepora oculata cold-water corals were obtained at six coral mounds off Angola from 17th to 25th January 2016 by recording underwater video transects with the Remotely Operated Vehicle SQUID during the ANNA expedition (Hebbeln et al. 2016). Video transects were primarily performed to characterize benthic communities. The data included in these datasets have been published in Orejas et al. (2021).

Description: Uranium-series age, isotope concentrations and ratios from Lophelia pertusa fragments from surface sediment samples collected during the M122 ANNA cruise off Namibia. *Measured 234U/238U activity ratios (δ234Um) are presented as deviation permil (‰) from the equilibrium value. Decay corrected 234U/238U activity ratios (δ234Ui) are calculated from the given ages and with λ234U: 2.8263 x 10-6 yr-1. Agecor Corrected ages, according to Frank et al. (2004) and Wefing et al., (2017). n.d.: not determinable because lower than the analytical blank (〈0.003 ng/g). For GeoB20572-4 and 20579-1, two L. pertusa fragments were collected from the bulk sample for dating: ' sample 1; '' sample 2.