Local and temporal variability of biogeochemical trace metal cycling in marine surface sediments

Trace metals such as Mo, U, and V are useful paleo-redox proxies because of their sensitivity to redox conditions and unique incorporation pathways into marine sediments. However, microbial processes can change the primary signal of specific trace metals that can record the environment at the time of deposition. To investigate the impact of microbial activity on trace metals during early diagenesis, geochemical analysis was performed via bag-incubations on samples collected from two giant box corers during RV SONNE Expedition SO260, funded by the MARUM-Center for Marine Environmental Sciences at the University of Bremen. The first core was taken at the head of the Mar del Plata Canyon, consists of mud to fine sand, and was sampled at five depths. The second core was taken on a coral mound, contains larger grains and abundant coral fragments, and was sampled at four depths. Four splits were taken at each depth; the first split was processed on-board, while splits 2-4 were stored in argon flushed aluminum bags at 4°C for processing and analysis every 4 months on-shore. Our data show strong changes in the pore-water trace metal concentrations in the first box core samples, with Mo increasing more than 5000 nM within 8 months. In samples from the second box core, pore-water Mo increases by more than 1000 nM in the first 4 months before decreasing likely due to the onset of sulfate reduction and, consequently, the formation of hydrogen sulfide leading to the (co-)precipitation of Mo. Our data indicate that the dissolution of iron and manganese oxides leads to the release of associated trace metals at different time points for each site. With comparable amounts of organic carbon at both sites, the observed changes are likely related to sediment composition and physical properties. The sediments sampled at the coral mound have a higher overall porosity and thus provide more space for fluid circulation and host microbial communities. This could explain the faster cycling of iron minerals, potential onset of sulfate reduction, and thus changes in the concentration of relevant trace metals. Therefore, our data suggest that trace metal cycling is not only related to the overall sediment composition, but also to physical properties including pore space, permeability, and grain size which affect how much area is available for microbial communities

Impact of physical properties on biogeochemical trace metal cycling in modern marine surface sediments of the Argentine Basin

Specific trace metal contents that can record the environment at the time of deposition are commonly applied as tracers (proxies) to reconstruct ancient oceanic conditions. However, microbial processes can alter the primary trace metal signal of the sediments during sediment burial. To investigate trace metal cycling during early diagenesis, geochemical analyses were performed via bag-incubations on samples collected from two giant box corers retrieved during RV SONNE Expedition SO260, funded by the MARUM-Center for Marine Environmental Sciences at the University of Bremen. The cores were retrieved off-shore Argentina, one from the head of the Mar del Plata Canyon and the other from a coral mound. Collected sediments are dominantly silt to fine grained sand but include dropstones and coral fragments as well. Our data show strong changes in the pore-water trace metal concentrations in the samples from the Mar del Plata Canyon. For example, molybdenum (Mo) increases by more than 5000 nM within 8 months. In samples from the coral mound box core, pore-water Mo increases by more than 1000 nM in the first 4 months before decreasing again likely due to the onset of sulfate reduction and, consequently, the formation of hydrogen sulfide leading to the (co-)precipitation of Mo. Our data indicate that the reductive dissolution of iron and manganese oxides leads to the release of associated trace metals at different time points for each site. The observed changes are likely related to sediment composition and physical properties. Sediments sampled at the coral mound site include coral fragments, increasing overall porosity and permeability providing more space for fluid circulation and to host microbial communities. Therefore, our data suggest that trace metal cycling is closely related to physical properties including pore space, permeability, and grain size that affect how much area is available for microbial communities

Impact of depositional regimes on biogeochemical cycling of iron in sediments of the Argentina Continental Margin – First results of RV SONNE expedition SO260

At the Argentina Continental Margin fundamental interactions between bottom currents, sediment deposition and how these processes control biogeochemical reactions and element cycling including iron can be studied. The focus of my thesis was to determine (1) the consequences of depositional environments (lower continental slope vs. contourite terrace) on biogeochemical processes and the (2) preservation and diagenetic cycling of solid Fe phases in the sediments. Additionally it was assessed (3) how sedimentary stable Fe isotope signatures (δ56Fe) are affected during early diagenesis. A sequential extraction protocol was applied to determine four reactive Fe phases including Fe carbonates, easily reducible Fe (oxyhydr)oxides, reducible Fe (oxyhydr)oxides and hardly reducible Fe oxides. Purification for sedimentary δ56Fe signals of Fe carbonates and easily reducible Fe (oxyhydr)oxides followed. The dataset was combined with pore-water data from RV SONNE cruise SO260 and provided stable carbon isotope data. Different extents of the redox zonation and location of the sulfate-methane-transition (SMT) were found. It is suggested that sedimentation rates are low at the lower continental slope with steady state conditions, causing a strong diagenetic overprint of Fe phases unlike the contourite site, where high sedimentation rates are evident and a diagenetic overprint is missing. Fe extraction data indicate that reactive Fe oxides are subject to reductive dissolution at the SMT. Yet significant amounts of reactive Fe oxides are preserved below for deep Fe reduction (potentially through Fe mediated anaerobic oxidation of methane), which is evidenced by dissolved Fe in methanic sediments. The δ56Fe of reactive Fe phases suggest significant microbial fractionation during deep Fe reduction at the lower continental slope, whereas at the contourite site microbial fractionation is not reflected in δ56Fe. It is concluded that the applicability of δ56Fe signatures as tracer of microbial Fe reduction might be dependent on the depositional regime (limited in high sedimentation areas)

Impact of depositional regimes on biogeochemical cycling of iron and stable Fe signatures in sediments from the Argentina Continental Margin

The Argentina Continental Margin represents a unique geologic setting where fundamental interactions between bottom currents and sediment deposition as well as their impact on biogeochemical processes and element cycling, in particular iron, can be studied. The aims of this study were to investigate 1) the consequences of different depositional conditions on biogeochemical processes and 2) diagenetic cycling of Fe mineral phases in surface sediments. Furthermore, it was 3) studied how sedimentary stable Fe isotope signatures (δ56Fe) are affected during early diagenesis and finally 4) evaluated, under which conditions δ56Fe might be used as proxy for microbial Fe reduction in methanic sediments. During RV SONNE expedition SO260, carried out in the framework of the DFG-funded Cluster of Excellence “The Ocean in the Earth System”, surface sediments from two depositional environments were sampled each using gravity corer and multi corer. One study site is located on the lower continental slope at 3605 m water depth (Biogeochemistry Site), while the other site is situated in a contourite system on the Northern Ewing Terrace at 1078 m water depth (Contourite Terrace Site). Sequential Fe extractions were performed on the collected sediments to determine four operationally defined reactive Fe phases targeting Fe carbonates, (easily) reducible Fe (oxyhydr)oxides and hardly reducible Fe oxides [1]. Purification of extracts for δ56Fe analysis of the Fe carbonates and easily reducible Fe (oxyhydr)oxide fractions followed [2]. The dataset was combined with pore-water data obtained during the cruise and complemented by concentrations and stable carbon isotope signatures of dissolved methane determined post-cruise. The extent of the redox zonation and depth of the sulfate-methane-transition (SMT) differ between the two sites. It is suggested that sedimentation rates at the Biogeochemistry Site are low and that steady state conditions prevail, leading to a strong diagenetic overprint of sedimentary Fe phases. In contrast the Contourite Terrace Site is characterized by high sedimentation rates and a lack of pronounced diagenetic overprint [3]. Reactive Fe phases are subject to reductive dissolution at the SMT. Nevertheless, significant amounts of reactive Fe phases are preserved below the SMT as evidenced by the presence of dissolved Fe in the methanic sediments, and are available for deep Fe reduction possibly through Fe-mediated anaerobic oxidation of methane [4]. In this study, δ56Fe signatures of reactive Fe phases in methanic sediments were determined for the first time. These data suggest significant microbial fractionation of Fe isotopes during deep Fe reduction at the Biogeochemistry Site, whereas at the Contourite Terrace Site the δ56Fe signatures do not indicate remarkable microbial Fe isotope fractionation. It is concluded that the applicability of δ56Fe signatures as tracer for microbial Fe reduction might be sensitive to the depositional regime, and thus may be limited in high sedimentation areas. References: [1]Poulton SW and Canfield DE, 2005. Chemical Geology 214: 209-221. [2]Henkel, S. et al., 2016. Chemical Geology 421: 93-102. [3]Riedinger, N. et al., 2005. Geochimica et Cosmochimica Acta 69: 4117-4126. [4]Riedinger, N. et al., 2014. Geobiology 12: 172-181

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

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 (δ56Fe) of pore water and solid phases are affected accordingly. Furthermore, we (3) evaluated the applicability of δ56Fe 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 δ56Fe analyses of extracts and pore water [2,3] were combined with sedimentological, radioisotope, geochemical and magnetic data. Our study presents the first sedimentary δ56Fe 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 δ56Fe values at the Slope Site are mostly negative, which is typical for on-going microbial Fe reduction. At the Contourite Site the pore-water δ56Fe values are mostly positive and range between -0.35‰ to 1.82‰. Positive δ56Fe 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 δ56Fe 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 δ56Fe in specific sedimentary Fe phases. 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

Strong changes in depositional conditions during the Late Glacial and the Holocene along the northern Argentina Continental Margin: a multiproxy approach.

We investigated sediments from three different depositional environments along the northern Argentine continental margin to assess the main processes controlling sediment deposition since the last glacial period. Further, we evaluated how different depositional conditions affect (bio)geochemical processes within sediments. Sediment cores were collected during expedition SO260 in 2018[1]. Two sites are located at ~1100 m water depth north and south of the Mar del Plata Canyon (N- and S-Middle Slope Site). Another site is situated at the lower continental slope at 3600 m water depth (Lower Slope Site). Reliable age constraints of sediments deposited during the last glaciation at the Argentine margin are difficult to obtain due limited amounts of carbonate. We overcame this issue by combining radio-isotope analyses (14C,230Thex) with sedimentological, geochemical and magnetic data demonstrating that all sites experienced distinct changes over time. Both, N- and S-Middle Slope Sites, record at least the last 30 ka. The S-Middle Slope Site is dominated by continuously organic carbon-starved and winnowed sandy deposits, which according to geochemical and magnetic data leads to insignificant sulfate reduction and sulfidation of iron (oxyhydr)oxides. Glacial sedimentation rates at the Middle Slope increase northwards suggesting a decrease in bottom-current strength. The N-Middle Slope Site records a transition from the last glacial period, dominated by organic carbon-starved sands, to the early deglacial period when mainly silty and organic carbon-rich sediments were deposited between 14-15 ka BP. Concurrently, glacial sedimentation rates of ~50 cm/ka significantly increased to 120 cm/ka. We propose that this high sedimentation rate relates to lateral sediment re-deposition by current-driven focusing as response to sea level rise. Towards the Holocene, sedimentation rates strongly decreased to 8 cm/ka. We propose that the distinct decrease in sedimentation rates and change in organic carbon contents observed at the N-Middle Slope Site caused the nonsteady-state pore-water conditions and deep sulfate-methane-transition (SMT) at 750 cm core depth. The Lower Slope Site records the last 19 ka. Continuously high terrigenous sediment input (~100 cm/ka) prevailed during the Deglacial, while sedimentation rates distinctly decreased to ~13 cm/ka in the Holocene. Here, pore-water data suggest current steady-state conditions with a pronounced SMT at 510 cm core depth. Our study confirms previous geochemical-modelling studies at the lower slope, which implied that the observed SMT fixation for ~9 ka at specific depth relates to a strong decrease in sedimentation rates at the Pleistocene/Holocene transition[2]. During the Holocene, total organic and inorganic carbon contents, inorganic carbon mass accumulation rates and XRF Si/Al ratios (preserved diatom flux) increase at our sites. We relate this to increased primary production in surface waters and less terrigenous input along the continental margin. Our multidisciplinary approach presents improved age constraints at the northern Argentine Margin and demonstrates that lateral/vertical sediment transport and deposition was strongly linked to Glacial/Interglacial variations in bottom currents, seafloor morphology, sea level and sediment supply. The dynamic depositional histories at the three sites still exert a significant control on modern sedimentary (bio)geochemical processes. [1]Kasten et al. (2019). Cruise No. SO260. Sonne-Berichte. [2]Riedinger et al. (2005). Geochim. Cosmochim. Acta. 69