Pliocene-Quaternary crustal melting in central and northern Tibet and insights into crustal flow

There is considerable controversy over the nature of geophysically recognized low-velocity-high-conductivity zones (LV-HCZs) within the Tibetan crust, and their role in models for the development of the Tibetan Plateau. Here we report petrological and geochemical data on magmas erupted 4.7-0.3 Myr ago in central and northern Tibet, demonstrating that they were generated by partial melting of crustal rocks at temperatures of 700-1,050°C and pressures of 0.5-1.5 GPa. Thus Pliocene-Quaternary melting of crustal rocks occurred at depths of 15-50 km in areas where the LV-HCZs have been recognized. This provides new petrological evidence that the LV-HCZs are sources of partial melt. It is inferred that crustal melting played a key role in triggering crustal weakening and outward crustal flow in the expansion of the Tibetan Plateau

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

Ambient-noise tomography of north Tibet limits geological terrane signature to upper-middle crust

We use ambient-noise tomography to map regional differences in crustal Rayleigh-wave group velocities with periods of 8–40?s across north Tibet using the International Deep Profiling of Tibet and the Himalaya phase IV arrays (132 stations, deployed for 10–24?months). For periods of 8–24?s (sensitive to midcrustal depths of ~5–30?km), we observe striking velocity changes across the Bangong-Nujiang and Jinsha suture zones as well as the Kunlun-Qaidam boundary. From south to north, we see higher velocities beneath the Lhasa terrane, lower velocities beneath the Qiangtang, higher velocities in the Songpan-Ganzi and Kunlun terranes, and the lowest velocities beneath the Qaidam Basin. Maps at periods of 34 and 40?s (sensitive to the middle and lower crust at depths of ~30–60?km) do not show evidence of changes across those boundaries. Any differences between the Tibetan terrane lower crusts that were present at accretion have been erased or displaced by Cenozoic processes and replaced almost ubiquitously by uniformly low velocities