Vivianite formation in methane-rich deep-sea sediments from the South China Sea

Abstract

Abstract. Phosphorus is often invoked as the ultimate limiting nutrient, modulating primary productivity on geological timescales. Consequently, along with nitrogen, phosphorus bioavailability exerts a fundamental control on organic carbon production, linking all the biogeochemical cycles across the Earth system. Unlike nitrogen that can be microbially fixed from an essentially infinite atmospheric reservoir, phosphorus availability is dictated by the interplay between its sources and sinks. While authigenic apatite formation has received considerable attention as the dominant sedimentary phosphorus sink, the quantitative importance of reduced iron-phosphate minerals, such as vivianite, has only recently been acknowledged, and their importance remains underexplored. Combining microscopic and spectroscopic analyses of handpicked mineral aggregates with sediment geochemical profiles, we characterize the distribution and mineralogy of iron-phosphate minerals present in methane-rich sediments recovered from the northern South China Sea. Here, we demonstrate that vivianite authigenesis is pervasive in the iron-oxide-rich sediments below the sulfate–methane transition zone (SMTZ). We hypothesize that the downward migration of the SMTZ concentrated vivianite formation below the current SMTZ. Our observations support recent findings from non-steady-state post-glacial sedimentary successions, suggesting that iron reduction below the SMTZ, probably driven by iron-mediated anaerobic oxidation of methane (Fe-AOM), is coupled to phosphorus cycling on a much greater spatial scale than previously assumed. Calculations reveal that vivianite acts as an important burial phase for both iron and phosphorus below the SMTZ, sequestering approximately half of the total reactive iron pool. By extension, sedimentary vivianite formation could serve as a mineralogical marker of Fe-AOM, signalling low-sulfate availability against methanogenic and ferruginous backdrop. Given that similar conditions were likely present throughout vast swathes of Earth's history, it is possible that Fe-AOM and vivianite authigenesis may have modulated methane and phosphorus availability on the early Earth, as well as during later periods of expanded marine oxygen deficiency. A better understanding of vivianite authigenesis, therefore, is fundamental to test long-standing hypotheses linking climate, atmospheric chemistry and the evolution of the biosphere.

Niche Differentiation of Sulfate- and Iron-Dependent Anaerobic Methane Oxidation and Methylotrophic Methanogenesis in Deep Sea Methane Seeps

“…The slope of the northern SCS is one of the world's most active areas of modern marine sedimentary processes (Huang and Wang, 2007;Luan et al, 2019). A large amount of river-borne terrigenous sediment input leads to exceptionally high amount of iron oxides in the sediments of the northern SCS (Zhang et al, 2007;Liu Z. F. et al, 2016;Liu et al, 2018). Previous research had suggested that different kinds of iron oxides could serve as electron acceptors for Fe-AOM (Bar-Or et al, 2017).…”

“…The Jiaolong and Haima methane seeps are the two still-active seep sites in SCS. Previous studies on the seeps in SCS have led to many significant advancements including new insight into the methane seep structure ( Liu et al, 2018 ), mineral carbon/oxygen/sulfur isotopes ( Feng et al, 2018 ), macrobenthos ( Dong and Li, 2015 ; Gong et al, 2015 ), and microbial lipid biomarkers ( Guan et al, 2016 , 2018 ). In addition, the microbial distribution and diversity in SCS methane seeps were also investigated ( Jiang et al, 2007 ; Zhang et al, 2012 ), and a 16S rRNA gene-based survey indicated the presence of SRB and ANME.…”

The Effect of Early Diagenesis in Methanic Sediments on Sedimentary Magnetic Properties: Case Study From the SE Mediterranean Continental Shelf

“…challenged. In this zone only paramagnetic minerals as siderite (FeCO 3 ) and vivianite [Fe(II) 3 (PO 4 )•8H 2 O] were assumed to precipitate under elevated concentrations of dissolved ferrousiron, if pore waters are saturated with respect to carbonate or phosphate, respectively (Berner, 1980;Slomp et al, 2013;Liu et al, 2018). However, several studies have suggested magnetic modification and mineral precipitation just below the SMTZ, mainly monosulfides, pyrrhotite, greigite and pyrite (Dekkers, 1989;Peters and Dekkers, 2003;Riedinger et al, 2005Riedinger et al, , 2014Horng and Chen, 2006;Larrasoaña et al, 2007;Kars and Kodama, 2015;Shi et al, 2017;Horng, 2018;Roberts et al, 2018;Badesab et al, 2019).…”

Characteristics of Authigenic Minerals around the Sulfate-Methane Transition Zone in the Methane-Rich Sediments of the Northern South China Sea: Inorganic Geochemical Evidence

“…A low CRS content was observed only at Unit IV, which is coincidence with the average low TOC/TS ratio in this Unit. However, Liu et al [49] demonstrated that the main biogeochemical processes in Unit IV is controlled by iron mediated anaerobic oxidation of methane (Fe–AOM), which further confirms that of methane rich fluids are well developed in this site.…”

“…In fact, Unit III sediments revealed highly 13 C-depleted (extremely negative values) authigenic carbonates, which indicated AOM–SR. On the other hand Unit IV had relatively 13 C-depleted (−4.3‰) authigenic carbonates and was sulfate-poor due to iron–AOM, as demonstrated by Liu [49]. Nevertheless, the sulfate concentrations and δ 13 C TIC values may allow recognition of organoclastic sulfate reduction (OSR) in Unit I where the minimum δ 13 C TIC value was −2‰.…”