1.4. SEDIMENTARY SULFIDIZATION AND SULFURIZATION
Hydrogen sulfide produced during sulfate reduction in sediments is diffused from the microbial cell into the surrounding pore water and is trapped in the sediment as Fe sulfide minerals (sulfidization) and organosulfur compounds (sulfurization). A significant fraction of H2S may diffuse out of the sediment–water interface and be oxidized to SO4 2– or elemental sulfur (S0). Porewater H2S is oxidized to intermediate species such as S0, thiosulfate (polysulfides: Sx 2‐, HSx 2–) and sulfoxyanoins (sulfite, thiosulfate, and polythionates) (Chen and Morris, 1972; Berner and Westrich, 1985; Thamdrup et al., 1994; Canfield et al., 2005). Sulfide oxidation can be microbe‐mediated (e.g. Nielsen et al., 2010; Eckert et al., 2011; Rao et al., 2016) or abiotic, by reaction with oxides such as MnO2 or FeO(OH) (Berner and Westrich, 1985; Thamdrup et al., 1993; Yao and Millero, 1993, 1995, 1996; Canfield et al., 2005). The intermediate sulfur species produced from sulfide oxidation can undergo disproportionation reactions where the sulfur intermediates are simultaneously transformed to both H2S and SO4 2–; these reactions are often referred to as inorganic fermentation because they do not involve any other electron donor or acceptor (Bak and Cypionka, 1987, Canfield and Thamdrup, 1994; Habicht et al., 1998; Böttcher et al., 2001).The burial of Fe sulfide minerals and sulfurized organic matter depends on the availability of reactive Fe (Yucel et al., 2010; Zhu et al., 2016), sedimentation rate, reactive organic matter flux (Berner, 1985; Raiswell and Berner, 1985; Wilkin and Barnes, 1997; Werne et al., 2003; Markovic et al., 2015), and bottom water DO concentration. The degree of pyritization (DOP) values in sediments underlying the oxygenated waters are < 0.45, whereas, under dysoxic and euxinic waters the values range between 0.46 and 0.75 and > 0.75, respectively (Raiswell and Berner, 1985; Raiswell and Canfield, 2012). In the ASOMZ, pyritization is limited by the availability of reactive Fe oxides attributed to the reductive dissolution of Fe in the water column or at the sediment–water interface as a result of low DO concentration (Scheneau et al., 2002). The bottom water DO concentrations and detrital organic matter availability controls the spatial extent and the degree of bioturbation and bioirrigation. The activity of benthic organisms not only modifies the porewater concentration profiles it also plays an important role by oxidizing Fe‐sulfide minerals close to the sediment–water interface. The sediments underlying OMZs show the minimum influence of bioturbation (Levin, 2003, Cowie and Levin, 2009). The absence of burrowing/irrigation would minimize the exposure of early diagenetic Fe‐sulfides to oxidants such as NO3 –/NO2 – thereby enhancing the preservation of early sulfidization. The relative significance of organic matter sulfurization depends on the availability of labile organic molecules and the extent and rate of reactive Fe consumption (Zaback and Pratt, 1992; Passier et al., 1997; Werne et al., 2004). Loss of sedimentary reactive Fe through reductive dissolution and enhanced preservation of highly labile organic matter in the sediments underlying OMZs would promote enhanced organic matter sulfurization (Lückge et al., 2002; Schneau et al., 2002). The OBS constitutes an estimated 50% of the total sedimentary sulfur in the Peru margin following the polysulfide pathway (Mossmann et al., 1991). The incorporation of sulfur in organic molecules (sulfurization) occurs either intramolecularly (as cyclo sulfo groups such as thiolane, thiane, and thiophene) or intermolecularly through S2− or Sx 2− bonds between larger molecules (Sinninghe Damsté and de Leeuw, 1990; Abdulla et al., 2020). Lallier‐Verges et al. (1993) found pyrite infillings in pore spaces and pyrite framboids in sediment rich in autochthonous organic matter from the Oman margin. Furthermore, they suggested that 50% of organic matter degradation in organic poor sediments is supported by sulfate reduction and attributed it to the deposition of zooplankton debris (in addition to phytoplankton) with mineral skeletons, which increase porosity and enhance sulfate reduction. While in organic‐rich sediments, only 20 % of the organic matter degradation is supported by sulfate reduction. This may be caused by the deposition of purely organic phytoplankton with low porosity.
1.5. BENTHIC BIOLOGY
Oxygen Minimum Zones, which impinge on sediments of continental margins, have a strong influence on the abundance, diversity, and composition of benthic fauna (Levin, 2003). Low bottom‐water DO concentrations limit O2 penetration into the sediment, which creates unfavorable conditions for most benthic organisms to thrive. The abundance and diversity of benthic macrofaunal communities within the OMZs are typically low, and only a few tolerant benthic species are known to survive in these regions (Levin and Gage, 1998). However, small organisms (microbes, metazoan meiofauna, and foraminifera) may have a higher density in OMZs because of the abundance of detrital food and the absence of predators (e.g. Levin et al., 1991; Neira et al., 2001, 2018).
If sulfide is present in the upper sedimentary layers of OMZs, it inhibits aerobic respiration and can also react with trace amounts of O2 to form hydrogen peroxide (H2O2), which can cause cell damage (Bernhard and Bowser, 2008). Many benthic organisms living in OMZ environments develop morphological adaptations to cope with the O2 limitation. Low O2 availability in OMZs leads to the prevalence of hypoxia‐tolerant fauna such as nematodes, polychaetes, and some calcareous foraminifera. Most calcifying invertebrates tend to disappear in suboxic environments. However, some exceptions, such as the snail Alia permodesta and the mussel Amygdallum politum, have thin shells in suboxic or anoxic sediments (Levin, 2003; Moffitt et al., 2015). Some of the other commonly observed adaptations in benthic fauna colonizing OMZs include an increase in gill surface area in some crustaceans and polychaetes (Levin, 2003), elongation of branchiae in polychaetes (Lamont and Gage, 2000; Levin et al., 2009, 2010), test flattening of some foraminiferal species, and reduced body size in metazoan macrofauna (Gooday et al., 2010).
The lower transition zones of OMZs, where O2 concentrations begin to increase, are often regions of intense benthic biological activity characterized by a dramatic change in the population density of several benthic species (Levin, 2003; Gooday et al., 2009), an observation commonly termed as the ‘edge effect’ (Mullins et al., 1985; Levin, 2003). A zonation of faunal communities across O2 gradients has been observed on OMZ‐impinged continental margins, indicating the existence of substantial O2 tolerance thresholds for different benthic biota. These gradients are formed as a result of the specific O2 tolerances of each species and potentially also the absence of larger predators (Levin, 2003). Seasonal changes in oxygenation have also been observed to cause a compositional change in benthic communities (e.g. Sellanes and Neira, 2006; Woulds et al., 2007; Gutiérrez et al., 2008).
Bioturbation (particle mixing) in OMZ sediments is generally diminished owing to the reduction in species diversity and body size of benthic fauna. This usually leads to the formation of a generation of laminated or varved sediments underlying OMZs (e.g. Levin et al., 2009; Schimmelmann et al., 2016). However, some gutless symbiont taxa are capable of bioturbating OMZ sediments (Levin, 2003). Bioturbation generally leads to the churning of organic matter and particles deeper into the deposit, contributing to carbon burial and can fuel sub‐seafloor microbial processes. Laminated anoxic sediments typically contribute less to nutrient recycling. Thus, more labile organic material remains unused (Levin and Gallo, 2019).
1.6. MICROBIAL METABOLISM IN THE MARINE OXYGEN MINIMUM ZONE WATER COLUMN AND SEDIMENT
Marine phototrophic and chemolithoautotrophic microorganisms produce massive quantities of organic compounds that, in conjunction with other inorganic compounds, go through intricate
