Diagenetic Fate of Biogenic Soft and Hard Magnetite in Chemically Stratified Sedimentary Environments of Mamanguá Ría, Brazil

Abstract

Magnetotactic bacteria (MTB) synthesize magnetite and greigite crystals under low oxygen conditions in the water column or uppermost sediment (greigite‐producing bacteria are found below the oxic‐anoxic transition). Dissolved iron and oxygen contents in local environments are known to be limiting factors for the production and preservation of biogenic magnetite. Understanding the processes that link MTB to their living environments is fundamental to reconstructing past chemical variations in the water column and sediment, and for using the magnetic properties of biogenic magnetite as environmental proxy indicators. Previous studies have suggested that the frequently identified biogenic soft (BS) and biogenic hard (BH) magnetite types are associated with equant and more elongated morphologies, respectively, and that their abundance varies in accordance with sedimentary oxygen content, where MTB that produce the BH component live in less oxygenated environments. We test this hypothesis in a high‐resolution integrated environmental magnetic and geochemical study of surface sediments from Mamanguá Ría, SE Brazil. Based on magnetic and pore water profiles, we demonstrate that both the BS and BH components occur within microaerobic environments and that as sediment oxygen content decreases with depth, the BS component disappears before the BH component. With continued burial into the sulfidic diagenetic zone, both components undergo progressive dissolution, but the BH component is more resistant to dissolution than the BS component. Our observations confirm previous inferences about the relative stability of these phases and provide a firmer basis for use of these two types of biogenic magnetite as paleoenvironmental proxies.

Reductive dissolution of biogenic magnetite

“…Thus, magnetofossil morphology in sediments can be used as a proxy for past OATZ (Usui et al 2017;Yamazaki et al 2020). However, when studying reducing sedimentary environments (Rodelli et al 2019), care is needed because of possible alteration of the original magnetofossil morphological compositions in the dissolution process.…”

Magnetostratigraphic Chronology of a Cenozoic Sequence From DSDP Site 274, Ross Sea, Antarctica

“…The two middle-coercivity components show values consistent with single domain (SD) magnetite (mean coercivity of 23 mT, dispersion parameter 0.23 in samples 218.92 mbsf and 45 mT, dispersion parameter 0.19 in sample 285.41 mbsf, respectively). Those components are assumed to represent low-coercivity SD magnetite and high-coercivity SD magnetite, probably of biogenic origin (Egli, 2004;Jovane et al, 2012;Yamazaki , 2012;Heslop et al, 2004;Rodelli et al, 2019). The component characterized by mean coercivities of 100 and 280 mT (dispersion parameter 0.15 and 20, respectively), are interpreted as hematite (Kruiver et al, 2001;Abrajevitch et al, 2009;Rodelli et al, 2019).…”

Carbon Flow for Plankton Metabolism of Saco do Mamanguá Ría, Bay of Ilha Grande, a Subtropical Coastal Environment in the South Brazil Bight

“…The Ría is ∼11 km long and ∼2 km wide inlet running northeastsouthwest and connected to the ocean only on the northeast side ( Figure 1A). It is characterized by shallow waters with mean water depth about 5 m, reaching a maximum of 20 m at the outer área, and steep coastlines made of old crystalline rocks (Rodelli et al, 2019).…”