Diagenetic sensitivity of paleoenvironmental proxies: A rock magnetic study of Australian continental margin sediments

Abstract

[1] A rock magnetic study of the upper 40 m of the Ocean Drilling Program Site 133-820A recovered at the outer edge of the northeastern Australian continental margin shows that downcore variations in magnetic parameters are diagenetically driven and correlate with the changes in global sea level. We identified intervals enriched in single-domain (SD) magnetite in the studied section. Unlike previous studies that postulated a detrital source, we show that the bulk of the SD fraction is biologically produced and is likely to be authigenic. The abundance of SD magnetite thus cannot be used as an indicator for provenance or sediment transport mechanisms. The biogenic magnetite was preserved during the high sedimentation rate periods, likely due to a short residence time in the corrosive zone of active iron reduction. The presence of the biogenic magnetite thus can be used as an indicator for a low degree of reductive dissolution. More advanced dissolution during the periods of slow sedimentation, coincident with sea level highstands, resulted in significant changes in the composition of the detrital assemblage, particularly in relative abundances of different mineral phases. The overall stability of the detrital magnetic minerals toward dissolution varies in the studied section as hematite > magnetite > goethite. Due to the higher stability of hematite, reductive diagenesis in general will lead to the lowering of the goethite/hematite (G/H) ratio, which can be mistaken for an increased aridity in the sedimentary source areas in the conventional interpretation for this proxy. The sensitivity of the G/H proxy to diagenesis should be taken into account in paleoenvironmental studies.Components: 9300 words, 11 figures.

Source-to-sink magnetic properties of NE Saharan dust in Eastern Mediterranean marine sediments: review and paleoenvironmental implications

“…These results contrast with the reactivity of iron-bearing minerals to sulfide, where magnetite has a similar reactivity to goethite and hematite (Poulton et al, 2004). It also contrasts with other studies in which goethite has been reported to be more sensitive to reductive dissolution than hematite (Abrajevitch and Kodama, 2011). This discrepancy can be explained by the grain size of different magnetic minerals in Eastern Mediterranean marine sediments.…”

“…Once deposited in sediments, the signal of airborne magnetic minerals can be overprinted by other detrital or biogenic sources (e.g., Hesse, 1994;Roberts et al, 2011). Moreover, loss of detrital magnetic minerals or authigenic growth of new magnetic phases can occur during sedimentary diagenesis (Hounslow and Maher, 1999;Abrajevitch and Kodama, 2011;Liu et al, 2012a;Roberts, 2015, submitted). Adequate evaluation of the reliability of magnetic minerals as recorders of eolian dust deposition, therefore, demands careful, case-specific investigation of the processes that drive the formation, transportation, accumulation and preservation of magnetic minerals in sediments (Maher, 2011;Liu et al, 2012a).…”

A New Tool for Separating the Magnetic Mineralogy of Complex Mineral Assemblages from Low Temperature Magnetic Behavior

“…These approaches are increasingly undertaken to characterize changes in contributions to the total remanent magnetization and in magnetic grain size of sedimentary archives of various origins from past and present continents and oceans (e.g., Abrajevitch and Kodama, 2011;Lascu et al, 2012;Chen et al, 2014;Nie et al, 2014;Hyland et al, 2015;Fabian et al, 2016;Maxbauer et al, 2016c;Zhang et al, 2016). However, obtaining mineral compositions of an ensemble of components via these methods is only possible indirectly through a series of assumptions or a priori knowledge.…”

Atmospheric halogen and acid rains during the main phase of Deccan eruptions: Magnetic and mineral evidence

“…Absence of biomagnetite is also observed in the low-MS interval, but its origin still needs to be confi rmed in the future. Variations in detrital titanomagnetite content at Bidart can be explained by several causes, such as (1) a rapid sea-level rise (e.g., dilution and remoteness to the source), (2) diagenetic reduction (Abrajevitch and Kodama, 2011), or (3) annihilation of iron oxides at or close to their continental source. The third hypothesis is tested here as a consequence of the huge Deccan volcanic gas emissions and resulting acid rains, which could have dissolved the continental magnetite grains before any riverine transfer to the oceanic basin.…”