Vol.:(0123456789) Aquatic Geochemistry https://doi.org/10.1007/s10498-020-09387-7 1 3 ORIGINAL ARTICLE Efects of Bioirrigation and Salinity on Arsenic Distributions in Ferruginous Concretions from Salt Marsh Sediment Cores (Southern Brazil) Larissa Costa 1  · Nicolai Mirlean 1  · Guilherme Quintana 1  · Segun Adebayo 2  · Karen Johannesson 2,3,4 Received: 26 January 2020 / Accepted: 10 July 2020 © Springer Nature B.V. 2020 Abstract Arsenic (As), iron (Fe), and manganese (Mn) contents were measured in sediment nodules and associated pore waters obtained from sediment cores collected from a salt marsh on Pólvora Island (southern Brazil). Sediment cores were obtained when brackish water domi‑ nated the estuary, at two diferent environments: an unvegetated mudfat colonized by crabs (Neohelice granulata), and a low intertidal stand vegetated by Spartina alternifora. We determined the percentage of nodules in each depth interval of the cores, along with redox potential, and As, Fe, and Mn contents of the nodules. The mineralogy of the nodules was investigated, and results showed they are mainly composed by quartz, phyllosilicates, and amorphous Fe–Mn oxides/oxyhydroxides. Pore water results showed that bioturbation by local crabs supports oxygen penetration to depths of ca. 25 cm below the salt marsh sur‑ face, with lower Fe contents in pore water associated with the brackish period. However, S. alternifora growth appears to have a greater impact on sediment geochemistry of Fe, Mn, and possibly As due to sulfate reduction and the associated decrease in pore water pH. Higher Fe concentrations were observed in the pore waters during the period of brackish water dominance, which also corresponded to the S. alternifora growth season. The study demonstrates that diferences in geochemical conditions (e.g., Fe content) that can develop in salt marsh sediments owing to diferent types of bioirrigation processes (i.e., bioirriga‑ tion driven by crabs versus that related to the growth of S. alternifora) play important roles in the biogeochemical cycling of As. Keywords Arsenic · Salt marshes · Bioturbation · Iron–manganese oxides/oxyhydroxides * Larissa Costa larissap.costa@hotmail.com 1 Oceanography Institute, Federal University of Rio Grande, Av. Itália km 08 Campus Carreiros, Rio Grande, RS 96201‑900, Brazil 2 Department of Earth and Environmental Sciences, Tulane University, New Orleans, LA 70118‑5698, USA 3 School for the Environment, University of Massachusetts Boston, Boston, MA 02125‑3393, USA 4 Intercampus Marine Science Graduate Program, University of Massachusetts System, Boston, MA 02125‑3393, USA