Evolution of (Bio‐)geochemical Processes and Diagenetic Alteration of Sediments Along the Tectonic Migration of Ocean Floor in the Shikoku Basin off Japan

The impact of depositional conditions on biogeochemical cycling of iron and stable iron signatures in sediments of the Argentina Continental Margin

10.5194/egusphere-egu21-8356 ◽

2021 ◽

Author(s):

Anne-Christin Melcher ◽

Susann Henkel ◽

Thomas Pape ◽

Anette Meixner ◽

Simone A. Kasemann ◽

...

Keyword(s):

Steady State ◽

Sulfate Reduction ◽

Pore Water ◽

Continental Margin ◽

Geochemical Processes ◽

Fe Oxides ◽

Diagenetic Alteration ◽

Fe Isotopes ◽

Oxic Zone ◽

Fe Reduction

The Argentina Continental Margin represents a unique geologic setting to study interactions between bottom currents and sediment deposition as well as their impact on (bio)geochemical processes, particularly the cycling of iron (Fe). Our aim was to determine (1) how different depositional conditions control post-depositional (bio)geochemical processes and (2) how stable Fe isotopes (&#948;56Fe) of pore water and solid phases are affected accordingly. Furthermore, we (3) evaluated the applicability of &#948;56Fe of solid Fe pools as a proxy to trace past diagenetic alteration of Fe, which might be decoupled from current redox conditions. Sediments from two different depositional environments were sampled during RV SONNE expedition SO260: a site dominated by contouritic deposition on a terrace (Contourite Site) and the lower continental slope (Slope Site) dominated by hemipelagic sedimentation. Sequentially extracted sedimentary Fe [1] and &#948;56Fe analyses of extracts and pore water [2,3] were combined with sedimentological, radioisotope, geochemical and magnetic data. Our study presents the first sedimentary &#948;56Fe dataset at the Argentina Continental Margin.The depositional conditions differed between and within both sites as evidenced by variable grain sizes, organic carbon contents and sedimentation rates. At the Contourite Site, non-steady state pore-water conditions and diagenetic overprint occurs in the post-oxic zone and the sulfate-methane transition (SMT). In contrast, pore-water profiles at the Slope Site suggest that currently steady-state conditions prevail, leading to a strong diagenetic overprint of Fe oxides at the SMT. Pore-water &#948;56Fe values at the Slope Site are mostly negative, which is typical for on-going microbial Fe reduction. At the Contourite Site the pore-water &#948;56Fe values are mostly positive and range between -0.35&#8240; to 1.82&#8240;. Positive &#948;56Fe values are related to high sulfate reduction rates that dominate over Fe reduction in the post-oxic zone. The HS- liberated during organoclastic sulfate reduction or sulfate-mediated anaerobic oxidation of methane (AOM) reacts with Fe2+ to form Fe sulfides. Hereby, light Fe isotopes are preferentially removed from the dissolved pool. The isotopically light Fe sulfides drive the acetate-leached Fe pool towards negative values. Isotopic trends were absent in other extracted Fe pools, partly due to unintended dissolution of silicate Fe masking the composition of targeted Fe oxides. Significant amounts of reactive Fe phases are preserved below the SMT and are possibly available for reduction processes, such as Fe-mediated AOM [4]. Fe2+ in the methanic zone is isotopically light at both sites, which is indicative for a microbial Fe reduction process.Our results demonstrate that depositional conditions exert a significant control on geochemical conditions and dominant (bio)geochemical processes in the sediments of both contrasting sites. We conclude that the applicability of sedimentary &#948;56Fe signatures as a proxy to trace diagenetic Fe overprint is limited to distinct Fe pools. The development into a useful tool depends on the refining of extraction methods or other means to analyse &#948;56Fe in specific sedimentary Fe phases.&#160;References:[1]Poulton and Canfield, 2005. Chemical Geology 214: 209-221. [2]Henkel et al., 2016. Chemical Geology 421: 93-102. [3]Homoky et al., 2013. Nature Communications 4: 1-10. [4]Riedinger et al., 2014. Geobiology 12: 172-181.

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Quantitative assessment of changes occurring in organic matter during early diagenesis

Philosophical Transactions of the Royal Society of London Series A Physical and Engineering Sciences ◽

10.1098/rsta.1993.0078 ◽

1993 ◽

Vol 344 (1670) ◽

pp. 101-111 ◽

Cited By ~ 4

Keyword(s):

Organic Carbon ◽

Quantitative Assessment ◽

Early Diagenesis ◽

Published Data ◽

Recent Sediment ◽

Geochemical Processes ◽

Diagenetic Alteration ◽

Bound Lipids ◽

Recent Sediments

It is perhaps a truism that sedimentary organic geochemical processes can best be quantified if the concentrations of the major organic chemicals are known. While numerous authors have reported the concentrations of one or other compound classes, relatively few studies have accounted for all of the organic carbon in a well-resolved depth sequence of Recent sediments. In the present study, we report a mass balance for organic carbon in 1 cm sections of Recent sediment (0-10 cm) from offshore Peru by measuring the major, operationally defined classes of biochemicals: proteins, carbohydrates and lipids. However, in deeper sediments in the same core (200 cm), the proportion of organic carbon that can be accounted for by present analytical methods is only about 60%, and recalculation of published data shows that this decreases to only a few percent at 100 m depth. We discuss briefly, methods which have proved useful for characterization of the uncharacterized fraction which some workers have termed humin or ‘proto-kerogen’. The smooth and rapid decreases in the proportions of ‘protein’ and ‘carbohydrate ’ carbon and ‘ bound ’ lipids in this core are attributed to diagenetic alteration rather than to variations in input, confirming many previous results. The overall proportions of free lipids, in contrast, did not decrease systematically in the shallowest sediments (0-25 cm). This is not reflected in most previous studies of individual lipids, many of which decrease rapidly with depth in these sediments. We suggest that interactions within the lipids may account for this apparent discrepancy.

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The influence of tectonic migration of ocean floor on (bio-)geochemical and diagenetic processes in subseafloor sediments from the Nankai Trough off Japan

10.5194/egusphere-egu21-4080 ◽

2021 ◽

Author(s):

Male Köster ◽

Myriam Kars ◽

Florence Schubotz ◽

Man-Yin Tsang ◽

Yuki Morono ◽

...

Keyword(s):

Organic Carbon ◽

Nankai Trough ◽

Sediment Cores ◽

Magnetic Data ◽

Geochemical Processes ◽

Energy Substrate ◽

Sediment Column ◽

Geochemical Conditions ◽

Carbon Burial ◽

Tectonic Migration

(Bio-)geochemical processes in subseafloor sediments are closely coupled to global element cycles. To gain an improved understanding of changes in (bio-)geochemical conditions on geological timescales, we investigate sediment cores from a 1180 m deep hole in the Nankai Trough offshore Japan (Site C0023). The sediment cores were taken during International Ocean Discovery Program (IODP) Expedition 370 (Temperature Limit of the Deep Biosphere off Muroto), which aimed at exploring the prerequisites and limits of deep microbial life [1]. Over the past 15 Ma, Site C0023 has moved ~750 km relative to its present-day geographic position from the central Shikoku Basin to the Nankai Trough due to motion of the Philippine Sea plate [2]. During its tectonic migration, Site C0023 has experienced significant changes in depositional and thermal conditions as well as resulting (bio-)geochemical processes.By combining a large set of complementary pore-water, solid-phase and rock magnetic data with sedimentation rates and sediment ages, our aim is to (1) reconstruct the evolution of (bio-)geochemical processes, especially the cycling of iron, along the tectonic migration, and to (2) investigate if iron(III) minerals are still available to serve as energy substrate for microbial respiration in the deep sediments. Our results demonstrate that a transition from organic carbon-starved conditions with predominantly aerobic respiration processes to an elevated carbon burial environment with increased sedimentation occurred at ~2.5 Ma. Higher rates of organic carbon burial as a consequence of an increased nutrient supply and primary productivity likely stimulated the onset of organoclastic iron and sulfate reduction, biogenic methanogenesis and anaerobic oxidation of methane. A significant temperature increase by ~50&#176;C across the sediment column associated with trench-style sedimentation since ~0.5 Ma potentially increased the bioavailability of organic matter and enhanced biogenic methane production. The resulting shifts in reaction fronts led to a diagenetic transformation of iron (oxyhydr)oxides into pyrite in the lower organic carbon-starved sediments several millions of years after burial. We also show that high amounts of iron(III), which were preserved in the deeply buried sediments due to carbon-starved conditions are still available as energy substrate for microbially mediated processes at Site C0023.Our study emphasizes that depositional and thermal changes ultimately driven by the tectonically induced migration have the potential to strongly influence and control geochemical conditions and (bio-)geochemical processes within the whole sediment column. Such studies are needed to gain a fundamental understanding of the coupling between depositional history, (bio-)geochemical processes and the resulting diagenetic overprint on geological timescales, thereby linking the sedimentary iron, sulfur and carbon cycles.References:[1] Heuer, V.B. et al., 2020. Science 370: 1230-1234.[2] Mahony, S.H. et al., 2011. Bulletin 123: 2201-2223.

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