Impact Of Reductive Diagenesis On Magnetic Records Of Marine Sediments And Its Environmental Implications

Reductive diagenesis is an important phenomenon in environmental magnetismof marine sediments, because it may induce iron oxides to deplete rapidly and thusweaken, even delete magnetic records of paleoclimatic and paleoenvironmentalchanges. Magnetic susceptibility data from 421 sites from Ocean Drilling Programwere collected, and sharp downcore decreases of susceptibilityes at different depthsare discovered from 24 sites. It has been reported that 5 sites among the 24 sitessuffered from serious reductive diagenesis.Reductive diagenesis is generally favored by the occurrence of iron sulphides(often pyrite). But when pyrites in marine sediments are trace, its identificationbecomes difficult. In this thesis, detailed measurements of temperature-dependenceof magnetic susceptibilities (κ/T curves) on pure pyrites have been made in both anargon and an air atmosphere. The results provide two magnetic characters for theidentifacation of pyrite: (1) the susceptibility peak of magnetite at ~500℃ duringheating;(2) the susceptibility peak of pyrrhotite at ~320℃ during cooling, whensamples are heated to 560℃ or higher temperature. The two peaks result from twopossible processes: (1) magnetite firstly appears around the powder of pyrite byoxidation caused by oxygen adsorbed on the powder below ~400℃, and then thenewly formed magnetite reacts with sulphur atoms, released from the pyrite lattice,and converts to pyrrhotite at higher temperature;(2) pyrite directly changes topyrrhotite by desulphidation. This provides a useful method for the identification ofa few pyrites in sediments.Detailed environmental magnetic measurements were carried out on CoreMD77-181 in the Bay of Bengal, Core MD98-2172 in the Eastern Timor Sea andCore MD98-2182 in the Caroline Basin in the western Pacific ocean, includinganhysteretic remanent magnetization, isothermal remanent magnetization, κ/T curvesand magnetic hysteresis parameters, compiled with mineralogical analyses and grainsize measurements. The results discover different effects of reductive diagenesis onthe three cores.The environmental magnetic measurements of Core MD77-181 suggested that(1) the magnetic minerals of the upper 9.82 m sediments (the last 160 ka) isdominated by pseudo-single domain (PSD) magnetite, with more abundant andcoarser grains in glacial periods than in interglacial periods, resulting from the weakIndian summer monsoon;(2) the uppermost 6.50 m sediments (the last 70 ka) showsignificant temporal variations in grain sizes of magnetic minerals, possiblyreflecting the teleconnection between variations of the Indian summer monsoon andrapid cold events of the North Atlantic (Heinrich events);(3) the sediments below9.82 m are dominated by paramagnetic minerals, including pyrites, suggesting astrong effect of reductive diagenesis. The magnetic records of Core MD77-181 arecomparable to those of Core MD77-180 in the Bay of Bengal, and ODP Hole 722Bin the Arabian Sea. The environmental magnetic study of Core MD77-181 suggestthat the paleoceanographic event at ~160 ka had significantly changed the redoxstate of the northern Indian Ocean, from a reductive environment before the event toan oxic or suboxic environment after the event.The process of reductive diagenesis could be traced by the magnetic records ofCore MD98-2172. (1) The upper 3.85 m sediments are little affected by reductivediagenesis. The magnetic minerals of this section are dominated by PSD magnetite,with little variations of its content and grain size. (2) The 3.85-5.35 m sediments arewithin the stage of iron oxide reduction. PSD magnetite also mainly contributes tosediments of this section, with less abundant and coarser grains downcore. (3) The5.35-15 m sediments are within the stage of sulphate reduction. Paramagneticminerals contribute to sediments of this section, including pyrites. The detrital grainsize analysis and its comparation with susceptibility suggest that susceptibility canbe used as a proxy for the content of clay detrital grains for Core MD98-2172, underthe strong effect of reductive diagenesis. The spectral analysis of the susceptibilityshows a good climatic cycle with a period of 43 ka.The sediments of Core MD98-2182 are dominated by PSD magnetite. Thevariations of the content and grain size of magnetite in the upper 1.01 m sedimentsmay record paleoenvironmental changes. But the changes of the content and grainsize of magnetite in the sediments below 1.01 m may be controlled by changes ofsource areas and/or reductive diagenesis.