Abstract Carbonate mounds and pockmarks are geologically and ecologically important features distributed worldwide in the world’s oceans. In the present study, we present a chemical characterization of deep-sea scleractinian coral skeletons collected in these geomorphological areas at the southeastern continental margin of Brazil. Coral samples were collected from ten sampling stations during cruises aboard the R/V Alpha Crucis, in 2019. Three species of scleractinian corals were studied: Enallopsammia rostrata, Solenosmilia variabilis, and Desmophyllum pertusum. Stable isotopes of carbon and oxygen (δ13C and δ18O), metals, and phosphorus present in the coral carbonate skeletons were analyzed. Corals are recognized as archives of physical-chemical variations in the marine environment, and the Element/Ca ratios, δ13C, and δ18O allowed for the characterization of the studied areas. Chemical composition found in pockmark areas indicated affinity to terrigenous and particulate materials input (Ba/Ca, Fe/Ca, Mn/Ca, Li/Ca, and Mg/Ca). Greater availability of nutrients and anthropogenic materials (Pb/Ca, Cd/Ca, Zn/Ca, and P/Ca) is also likely to occur in this region, with some elemental ratios higher than those measured in other oceans. These mounds can act as barriers for metals from land flows. Also, corals benefit from a higher food supply due to stronger currents. The corals at the top of the Alpha Crucis Carbonate Ridge receive significant marine influence. Most coral samples have carbonate of aragonitic origin, except for a specimen of D. pertusum, which presented carbonate of biogenic calcite and aragonite. The results demonstrate the potential of scleractinian corals in the chemical characterization of the deep ocean and the need for further investigation of carbonate mound areas from the SW Atlantic.

Abstract Pockmarks are geological features that often sustain hydrocarbon-related communities of microorganisms when active, seeping oil or methane. Microbial communities of inactive pockmarks have not been well studied until the present. Plenty of pockmarks in the Southwestern Atlantic Ocean have been discovered. However, no information is available about the organic characteristics in association with the microbial diversity related to methane and non-methane hydrocarbon consumption. This study examined the identity and potential ecology of the methane-related microbial community in an inactive SW Atlantic Carbonate-enriched Pockmark (SWACP). Undisturbed sediment cores were enriched with CH4 99.5% at 5°C, with samples harvested at 16h, 120h, 240h, 720h, and 960h, followed by metataxonomics functional prediction, and the correlation of microbial groups with incubation and enrichment types. The SWACP is depleted in organic compounds, and chemosynthetic production is dominant. Incubated mini-cores of sediment were affected by incubation time and enrichment type, which influenced the microbial composition. Although several taxa were shared among all sediment samples, specific groups per enrichment type and incubation time were observed. These communities comprised taxa previously reported in marine bottom waters, carbonate crusts, active cold-seeps, and inactive pockmarks. The methane-enriched taxa were predominantly related to aerobic methanotrophy, methylotrophy, aerobic and anaerobic non-methane hydrocarbon degradation, and fermentation. This study brings the first survey of the key microbial groups in methane fluxes of a Brazilian deep-sea pockmark, providing data for understanding the ecology surrounding the SW Atlantic gas field areas.

Abstract Although carbonate mounds have been investigated for 100 years, few studies focus on the giant variety. The Alpha Crucis Carbonate Ridge (ACCR), a ~17 x 12-km ring-shaped ridge formed by hundreds of mounded structures, located between the 300 and 800-m isobaths and reaching a maximum height of 340 meters above the adjacent seafloor, is the first giant carbonate mounded feature described for the SW Atlantic margin. This study provides the first multiproxy approach to investigate sediments covering the ACCR and its adjacencies. Most of the area is located under the Intermediate Western Boundary Current (IWBC) flow, which carries the nutrient-rich Antarctic Intermediate Water (AAIW). Radiocarbon aging shows pronounced differences for the shallow layers (MIS3 for the top of the mounds and late Holocene for the adjacencies). Grain size data indicate the prevalence of sandy fractions on top of the mounds and muddy sediments in the adjacent areas. Fe/Ca and Ti/Ca proxies allowed for identifying mainly biogenic sedimentation in the area. However, the input of allochthonous terrigenous sediment is necessary for mound buildup, and values of Fe and Ti collected on the top of the mounds are significant. End-Members distributions and metal concentrations also allowed for recognition of distinct sources of sediment. εNd and Ln(Fe/K) indicated two primary terrigenous sources, the Precambrian rocks of the Brazilian shield (Cabo Frio end-member) and the multiple lithologies drained by the Rio de la Plata basin. Redox condition proxies indicated that the area is submitted to oxic conditions, probably reflecting the action of the IWBC. This work provides the first insight into an integrated grain-size and geochemical characterization of the Alpha Crucis Carbonate Ridge (southwestern Atlantic margin).