Magnetic Mineral Assemblages of Diagenetically Reduced Sediments and Their Contributions to Paleomagnetic Signals

Magnetic Mineral Assemblages of Diagenetically Reduced Sediments and Their Contributions to Paleomagnetic Signals

Abstract During early diagenesis of marine sediments, iron-bearing minerals undergo a series of redox reactions until they reach equilibrium with reactive chemical components. Paleomagnetic records in the sediments subjected to severe diagenesis can be distorted or lost due to iron mineral dissolution, and hence these sediments were often excluded from paleomagnetic studies without detailed examination. Silicate-hosted magnetic inclusions and hematite are likely more resistive to reductive diagenesis compared with unprotected magnetite. Thus, these minerals have potential for preserving paleomagnetic records in reduced sediments. To better understand this issue, we conducted a paleo- and rock magnetic study of a sediment core taken from the Ontong Java Plateau in the western equatorial Pacific Ocean using various techniques including isothermal remanent magnetization component analyses, first-order reversal curve diagrams, low-temperature magnetic measurements, thermal demagnetization of three-component isothermal remanent magnetization, chemical separation, and electron microscopy. The magnetite dissolution front occurs at five point seven meters in depth in the studied core. Below this horizon, silicate-hosted magnetic inclusions and hematite carry forty-six to sixty-three percent and twenty-one to thirty-four percent of saturation isothermal remanent magnetization, respectively. Depositional remanent magnetization acquisition efficiency of silicate-hosted magnetic inclusions is estimated to be relatively low in the studied core based on the grain sizes of silicate hosts and concentration of magnetic inclusions. It was revealed that magnetostratigraphy and relative paleointensity correlative to the global stacks could still be recovered from the sediments below the dissolution front. Relict hematite may be an important carrier of the paleomagnetic records.

Plain Language Summary Marine sediments potentially preserve continuous records of past geomagnetic field variations, but these precious records can be lost during reductive diagenesis ubiquitous in sediments, because magnetic minerals carrying the records dissolve below a certain depth within sediments. Our study using a sediment core taken from the western equatorial Pacific Ocean revealed that even in sediments suffered from severe reductive diagenesis, hematites as well as magnetites embedded in silicate minerals, which protect the inclusions from dissolution, survive, whereas unprotected magnetite is lost, and that these relict magnetic minerals can preserve past geomagnetic records including variations of the strength of the geomagnetic field. Our study implies that large numbers of legacy sediment cores that were excluded from geomagnetic studies by reason of reductive diagenesis may be useful for future research.

One. Introduction

Marine and lacustrine sediments have potential to preserve continuous records of geomagnetic field variations in the past including relative paleointensity. Sediments ubiquitously suffer early diagenesis, and at a certain depth in a sedimentary column, reductive dissolution of magnetic minerals starts, as introduced below. This process compromises the preservation of paleomagnetic records.

In early diagenesis, sediments undergo a series of chemical changes associated with oxidative degradation of organic matter. Oxidants in sediments are utilized for the oxidation of organic matter according to the decreasing order of free energy yielded by each reaction: oxygen, nitrate, manganese oxides, iron oxyhydr-oxides, sulfate, and methane. These oxidants are depleted sequentially until the diagenetic system reaches an equilibrium where either all oxidants or all reactive organic matter are consumed. When more energy-producing oxides are depleted, organic matter degradation proceeds with the reduction of iron three-bearing minerals to dissolved iron two+. Ferruginous diagenesis,

causing dissolution of readily reactive iron minerals such as hydrous ferric oxide, ferrihydrite, and lepidocrocite at the depth known as the iron-redox boundary. The loss of these weakly magnetic minerals has little effect on remanent magnetization. Below the ferruginous zone, organic matter degradation proceeds with the reduction of sulfate, releasing dissolved sulfide as a by-product into sedimentary pore water. Sulfate reduction causes dissolution of well-crystallized iron-bearing minerals like magnetite and precipitation of iron sulfides (e.g., pyrite and greigite). The onset of the pervasive dissolution of magnetite and replacement by paramagnetic pyrite in the sulfate reduction cause a sudden drop in magnetic concentration and remanent magnetization intensity, which define the magnetite dissolution front. The reductive dissolution of iron-bearing minerals also leads to an increase in their average grain size because finer grains are expected to be dissolved earlier than coarser grains due to their higher surface-to-volume ratio.

Recently, potential of silicate-hosted magnetic inclusions of detrital origin has become recognized as a carrier of the remanent magnetization of the sediments suffered from reductive dissolution of magnetic minerals. In such sediments, host silicates protect magnetic inclusions from reductive dissolution, whereas unprotected detrital magnetite and magnetofossil, which are the major carriers of remanent magnetization of marine sediments, are susceptible to dissolution.

The reductive dissolution of magnetic minerals destroys paleomagnetic records at least partly. Sediments with large variations in magnetic mineralogy, concentration, and grain size associated with reductive diagenesis were considered to be unsuitable for relative paleointensity estimations in general. On the other hand, rock-magnetic records of diagenetically reduced sediments can help us better understand a sedimentary diagenetic history and underlying information related to paleoceanographic conditions including paleoproductivity. The extent to which diagenetic reaction proceeds and the corresponding depth at which it occurs are influenced by sedimentation rates and organic matter input, which may be controlled by factors such as orbitally controlled productivity cycles and glacial-interglacial changes. These factors can lead to a cycling of sedimentary environments between oxic and anoxic conditions, resulting in non-steady state diagenesis.

Previously, sediment cores suffered from reductive dissolution of magnetic minerals were often excluded from paleomagnetic studies from a preconception that paleomagnetic records would have been lost. It is, however, necessary to seek potential for utilizing such sediments in order to obtain paleomagnetic records of higher time resolution or older ages; reductive diagenesis is usually progressed in sediments with a high sedimentation rate, which usually have a large input of organic matter, or in sediments buried deeply. In this study, we first document in detail compositional changes of magnetic mineral assemblages in sediments associated with reductive dissolution of magnetic minerals. Next, we examine contributions of individual magnetic components to remanent magnetization of the sediments. We show potential of relict hematite for paleomagnetic recording in sediments with reductive diagenesis. We then attempt RPI estimates to investigate the possibility of recovering meaningful paleomagnetic signals from diagenetically reduced sediments.