Coupled microbial bloom and oxygenation decline recorded by magnetofossils during the Palaeocene–Eocene Thermal Maximum

Abstract

Understanding marine environmental change and associated biological turnover across the Palaeocene–Eocene Thermal Maximum (PETM; ~56 Ma)—the most pronounced Cenozoic short-term global warming event—is important because of the potential role of the ocean in atmospheric CO2 drawdown, yet proxies for tracing marine productivity and oxygenation across the PETM are limited and results remain controversial. Here we show that a high-resolution record of South Atlantic Ocean bottom water oxygenation can be extracted from exceptionally preserved magnetofossils—the bioinorganic magnetite nanocrystals produced by magnetotactic bacteria (MTB) using a new multiscale environmental magnetic approach. Our results suggest that a transient MTB bloom occurred due to increased nutrient supply. Bottom water oxygenation decreased gradually from the onset to the peak PETM. These observations provide a record of microbial response to the PETM and establish the value of magnetofossils as palaeoenvironmental indicators.

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

“…Overall, our results provide valuable insights into the stability of biogenic magnetite under reducing conditions and are consistent with a well‐known broader framework for mineral magnetic changes that occur in sediments during progressive diagenesis from oxic through to methanic conditions (e.g., Roberts, ). The relative stabilities of the frequently identified BS and BH components of Egli (, , ) have been discussed frequently in relation to varying diagenetic conditions (e.g., Chang et al, , ; Egli, , , ; Kodama et al, ; Usui et al, ; Yamazaki, ; Yamazaki & Shimono, ). However, inferences about the stability of these magnetite phases is based in all cases on diagenetic zonations based on magnetic properties with none of these studies including downcore pore water chemistry profiles.…”

“…Magnetite produced by MTB has specific characteristics in terms of stoichiometry, crystal structure, morphology, size, and spatial organization (Chen et al, ; Egli, ; Kopp & Kirschvink, ; Mann et al, ; Yan et al, ). The magnetic properties of biogenic magnetite have been studied extensively (Chang et al, ; Egli, , 2004b, 2004c; Egli et al, ; Jovane et al, ; Kruiver & Passier, ; Mohamed et al, ; Moskowitz et al, ; Pan, Petersen, Davila, et al, , Pan, Petersen, Winklhofer, et al, ; Roberts et al, ; Savian et al, , ), and a range of techniques exist to recognize and characterize such crystals. Environmental magnetic studies can be a rapid and reliable way to track the spatial distribution of MTB, which have the potential to enable reconstruction of oxygen gradients in natural environments (Chang et al, ), where other proxies or methods of study are less readily applicable.…”

“…The magnetic properties of biogenic magnetite have been studied extensively (Chang et al, ; Egli, , 2004b, 2004c; Egli et al, ; Jovane et al, ; Kruiver & Passier, ; Mohamed et al, ; Moskowitz et al, ; Pan, Petersen, Davila, et al, , Pan, Petersen, Winklhofer, et al, ; Roberts et al, ; Savian et al, , ), and a range of techniques exist to recognize and characterize such crystals. Environmental magnetic studies can be a rapid and reliable way to track the spatial distribution of MTB, which have the potential to enable reconstruction of oxygen gradients in natural environments (Chang et al, ), where other proxies or methods of study are less readily applicable. For example, 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 (Chang et al, , ; Egli, , , ; Kodama et al, ; Usui et al, ; Yamazaki, ; Yamazaki & Shimono, ).…”

“…Environmental magnetic studies can be a rapid and reliable way to track the spatial distribution of MTB, which have the potential to enable reconstruction of oxygen gradients in natural environments (Chang et al, ), where other proxies or methods of study are less readily applicable. For example, 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 (Chang et al, , ; Egli, , , ; Kodama et al, ; Usui et al, ; Yamazaki, ; Yamazaki & Shimono, ). Despite the potential value of such proxy information, magnetic proxies for BS and BH magnetite have not been assessed adequately in relation to direct pore water determinations of sedimentary oxygenation.…”

Domain State Diagnosis in Rock Magnetism: Evaluation of Potential Alternatives to the Day Diagram

“…Likewise, identifying the magnetic domain state for some magnetic mineral configurations presents challenges, and contrasting results can be obtained when analyzed with different methods, such as double or multiple magnetosome chain bundles even though individual magnetosome crystals have stable SD properties. Micromagnetic simulations of frustrated systems (Harrison, ), particles with complex geometries (Lascu et al, ; Williams et al, , ), and strongly interacting particle assemblages/magnetofossil chains (Chang et al, ; Evans et al, ; Harrison & Lascu, ; Muxworthy et al, ) have improved our theoretical understanding of these issues and are enabling more nuanced interpretations of domain states, which takes us beyond the simple SD‐PSD‐MD designation. These challenges should be grappled with when relevant; nevertheless, routine domain state diagnosis of geological materials remains fundamentally important in paleomagnetism and environmental magnetism.…”