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.

The Middle Eocene Climatic Optimum (MECO) is a global warming event that occurred at around 40 Ma and lasted about 500 kyr. We study this event in an abyssal setting of the Tasman Sea, using the IODP Core U1511B-16R, collected during the expedition 371. We analyse magnetic, mineralogical, and chemical parameters to investigate the evolution of the sea bottom conditions at this site during the middle Eocene. We observe significant changes indicating the response to the MECO perturbation. Mn oxides, in which Mn occurs under an oxidation state around +4, indicate a high Eh water environment. A prominent Mn anomaly, occurring just above the MECO interval, indicates a shift toward higher pH conditions shortly after the end of this event. Our results suggest more acid bottom water over the Tasman abyssal plain during the MECO, and an abrupt end of these conditions. This work provides the first evidence of MECO at abyssal depths and shows that acidification affected the entire oceanic water column during this event. The Eocene (~56-34 Ma) was characterized by a gradual climatic cooling, accompanied by decreasing atmospheric pCO 2 and culminating with the onset of the Antarctic glaciation in the early Oligocene (33 Ma) 1-5. This trend was interrupted during the middle Eocene by a warm period known as Middle Eocene Climatic Optimum (MECO), with duration of ~500 kyr and a warmth peak at ~40 Ma 6,7. The MECO has been identified in several sedimentary records around the globe, including the South Pacific Ocean 8,9. It is related to an increase in seawater temperature, from the surface to deep bathyal depths, and increasing pCO 2 in the atmosphere 10,11. Moreover, significant changes in atmospheric and oceanic circulation dynamics and in the patterns of continental rainfall are recorded 12,13. However, classic climatic models fail to explain how such conditions could persist for several hundreds of thousand years 14. Southern Ocean (SO) circulation is extremely important for understanding the climatic evolution during the Eocene, and particularly during the MECO. The separation of Australia from Antartica during the middle-late Eocene profoundly affected the circulation and made this region particularly sensitive to paleoceanographic changes 4,15. In this complex geological framework, the study of iron and manganese oxides in the sediments can provide important information, as they are strongly controlled by redox conditions and circulation 16. Manganese oxides typically occur as cryptocrystalline materials, in which Mn precipitates under different oxidation states: Mn 4+ , Mn 3+ and Mn 2+17. Moreover, Mn is more sensitive than Fe to pH, and requires more basic conditions to precipitate. Therefore, an environment may promote the oxidation and precipitation of iron and not of manganese, if the pH is not sufficiently high 18-21. Accordingly, relatively small shift in the redox conditions can change significantly the equilibrium solubility of these elements and thus their presence or absence in the geological record. Microorgani...

The Cananéia-Iguape system is a combined estuarine-lagoonal sedimentary system, located along the SE coast of Brazil. It consists of a network of channels and islands oriented mainly parallel to the coast. About 165 years ago, an artificial channel, the Valo Grande, was opened in the northern part of this system to connect a major river of the region, the Ribeira River, to the estuarine-lagoon complex. The Valo Grande was closed with a dam and reopened twice between 1978 and 1995, when it was finally left open. These openings and closures of the Valo Grande had a significant influence on the Cananéia-Iguape system. In this study we present mineralogical, chemical, palaeomagnetic, and geochronological data from a sediment core collected at the southern end of the 50 km long lagoonal system showing how the phases of the opening and closure of the channel through time are expressed in the sedimentary record. Despite the homogeneity of the grain size and magnetic properties throughout the core, significant variations in the mineralogical composition showed the influence of the opening of the channel on the sediment supply. Less mature sediment, with lower quartz and halite and higher kaolinite, brucite, and franklinite, corresponded to periods when the Valo Grande was open. On the other hand, higher abundance of quartz and halite, as well as the disappearance of other detrital minerals, corresponded with periods of absence or closure of the channel, indicating a more sea-influenced depositional setting. This work represented an example of anthropogenic influence in a lagoonal-estuarine sedimentary system, which is a common context along the coast of Brazil.

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