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(a) Mo EF versus U EF for the Member IV ORM. The dashed lines are equivalent to the molar Mo/U ratio for modern seawater (1Â SW) and for a fraction of modern seawater (0.3Â SW), following Algeo and Tribovillard (2009). Green squares = Jiulongwan outcrop section. Red triangles = Site 1 drill core section. Large filled symbols = samples with highest d 98 Mo from each section. (b) d 98 Mo versus d 238 U for the Member IV ORM, showing no correlation between the two isotope systems (R 2 <0.3 for both sections). Symbols are the same as in (a). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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To improve estimates of the extent of ocean oxygenation during the late Ediacaran Period, we measured the U and Mo isotope compositions of euxinic (anoxic and sulfidic) organic-rich mudrocks (ORM) of Member IV, upper Doushantuo Formation, South China. The average δ238U of most samples is 0.24 ± 0.16 ‰ (2SD; relative to standard CRM145), which is sl...
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Context 1
... by euxinic bottom waters during deposition of Member IV, consistent with prior data (Table 1 High EF values for both Mo (35-512; average = 142) and U (5-52; average = 14) indicate a dominance of hydroge- nous over detrital contributions of Mo and U to Member IV ORM. The Mo/U ratios of Member IV ORM consis- tently exceed those of modern seawater (Fig. 3a). This observation is consistent with the occurrence of locally eux- inic bottom waters, and unrestricted exchange between the Table 1 Geochemical data for Member IV, Doushantuo Formation, Three Gorges region, South China. local depositional basin and the open ocean (Algeo and Tribovillard, 2009). High Mo/U ratios in ORM can also be ...
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... data were not corrected for detri- tal contributions because of the dominance of hydrogenous over detrital U and Mo in the Member IV ORM. Despite the persistence of euxinic bottom waters at the studied localities, the stratigraphic trends of d 238 U and d 98 Mo are significantly different (Fig. 2), with no significant correla- tion between them (Fig. 3b) In contrast, the Mo isotope data show a more complex pattern. The exceptionally low d 98 Mo at the base gives way upsection to d 98 Mo values between À0.4& and +0.4&. This trend is followed up sec- tion by an excursion to high d 98 Mo (up to 2.0&) near the top of Member IV. A return to low d 98 Mo values marks the top of Member ...
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... oxyhydroxide particles below the sediment-water interface releases Mo, which can then be sequestered by Fe sulfide minerals and organic particles, thus preserving a low d 98 Mo signature in sulfidic sediments ( Herrmann et al., 2012;Scholz et al., 2013). In both sections, the samples with highest d 98 Mo have among the lowest Mo/U ratios (see Fig. 3a), suggest- ing a decreased shuttle effect, along with increased bottom water sulfide concentrations, enabled better capture of sea- water d 98 Mo in these ...
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Introduction:
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Citations
... However, the timing in which different marine habitats became oxygenated, and therefore the causal relationship between such oxygenation and evolutionary steps are intensely debated 6,7 . Hypothesized timelines of marine oxygenation range from a step-wise increase in O 2 between the Neoproterozoic and Cambrian , parallel to the radiation of animal life 4,[8][9][10][11] (possibly reaching values as high as modern oceans by the early Cambrian 8 ); to minor (0.5-10% of the modern ocean levels [12][13][14], local, and/or episodic oxygenation [15][16][17][18][19][20] , prior to the late Paleozoic-early Mesozoic eras, when evolutionary steps such as the expansion of vascular plants or the proliferation of phytoplankton drove major global redox changes [17][18][19][20][21][22][23] . To test these hypotheses, reconstructions of the timeline and extent of oxidation of marine environments from redox sensitive geochemical proxies are imperative. ...
... Proxy-based reconstructions of the oxygenation of marine environments typically follow one of three patterns leading to persistent oxygenation of the modern ocean: (1) A step increase during the NOE (0.8-0.55 Ga) e.g, 6,11,60 ; (2) Episodic oxygenation between the NOE and Cambrian e.g. 15,16 ; and (3) gradual or episodic increase in oxygenation during the mid-late Paleozoic e.g.17,21-23 . These differences may reflect the complex structure of the early Paleozoic oxycline, in which shallow remineralization of organic material reduced continental shelves and left deeper basins relatively oxidized 17 , or possibly different sensitivities of elements and the records that contain them to postdepositional alteration 25 . ...
Understanding causal relationships between evolution and ocean oxygenation hinges on reliable reconstructions of marine oxygen levels, typically from redox-sensitive geochemical proxies. Here, we develop a proxy, using dolomite U–Pb geochronology, to reconstruct seawater U/Pb ratios. Dolomite samples consistently give U–Pb dates and initial ²⁰⁷Pb/²⁰⁶Pb ratios lower than expected from their stratigraphic ages. These observations are explained by resetting of the U–Pb system long after deposition; the magnitude of deviations from expected initial ²⁰⁷Pb/²⁰⁶Pb are a function of the redox-sensitive U/Pb ratios during deposition. Reconstructed initial U/Pb ratios increased notably in the late-Paleozoic, reflecting an increase in oxygenation of marine environments at that time. This timeline is consistent with documented shifts in some other redox proxies and supports evolution-driven mechanisms for the oxygenation of late-Paleozoic marine environments, as well as suggestions that early animals thrived in oceans that on long time scales were oxygen-limited compared to today.
... However, sediments deposited underneath strongly euxinic water columns tend to record the Mo isotopic composition of seawater because seawater Mo is quantitatively converted to MoS 4 2− without fractionation (Barling et al., 2001;Neubert et al., 2008;Kendall et al., 2017). Particulate shuttle processes on the other hand can lead to light Mo isotope compositions of sediments owing to the fractionation during absorption of Mo on iron and manganese oxyhydroxides (Herrmann et al., 2012;Kendall et al., 2015). The different behavior of Mo isotopes makes this isotope system a promising tool for clarifying the mechanism of Mo enrichment at seeps. ...
Methane release from marine sediments strongly influences the local seafloor environment and ecosystems, and may impact Earth’s climate system. Recent studies revealed anomalous molybdenum (Mo) enrichment in seep sediments, which was linked to methane release events. Marine seep sediments are a potential sink of Mo for the global ocean, while the mechanisms leading to local Mo enrichment are not fully understood. The sediments from a gas hydrate-bearing area of the South China Sea analyzed herein reveal authigenic Mo (Moauth) contents ranging from 0 and 31.4 µg/g and δ98Moauth values ranging from 0.18‰ to 3.31‰. The range of δ98Moauth values of seep sediments is therefore similar to values of modern iron-rich sediments with low concentrations of dissolved porewater hydrogen sulfide and sediments deposited under weakly euxinic sediments. Among the obtained South China Sea data, the more positive δ98Moauth values (> ca. 1.5‰) are interpreted to reflect diffusion of seawater Mo into the sediment at moderate seepage rates and Mo isotope fractionation during the formation of thiomolybdates in the sulfidic seep environment. The lower δ98Moauth values (< ca. 1.5‰), coinciding with high Fe/Al and Mn/Al ratios of sediments, are interpreted to represent the release of Mo upon reductive dissolution of iron and manganese oxyhydroxides, following the delivery of the oxyhydroxides to the sediment during prominent particulate shuttle processes at high seepage rates. Our study suggests a prominent role of particulate shuttle processes in Mo sequestration at marine methane seeps, and sets the stage for the development of a new Mo isotope proxy that may help to constrain past methane seepage intensity.
... For instance, significant negative carbon isotope excursions have been observed after the Snowball Earth glaciation, indicating rapid oxidation of the dissolved organic carbon reservoir [20]. However, there are still geochemical evidences suggesting that deep ocean remains anoxic, such as iron speciation [23], redox-sensitive elements [7,8,34], and trace elements [9,19]. ...
After the end of the Cryogenian Snowball Earth glaciations, the Ediacaran Ocean experienced rapid oxidation and a subsequent increase in marine sulfate concentration. This led to a significant negative excursion in pyrite sulfur isotope values, particularly observed in the Doushantuo Formation of the slope facies in the South China. However, the extent of this oxidation event remains unclear. In order to address this, we carried out geochemical analysis of the Doushantuo Formation in the Cenpiaokou section of the Jiangkou County, Guizhou Province. In the Cenpiaokou section, the pyrite is mainly euhedral-subhedral and framboidal in crystal morphology. The pyrite contents are highest at the base of the Doushantuo Formation and decrease towards the middle and upper parts. Similarly, the proportion of framboidal pyrite also decreases upward, ranging from 93 % at the bottom to 9 % in the upper part of the Doushantuo Formation. Additionally, the sulfur isotope of pyrite (δ34S) shows sig- nificant differences compared to other slope facies sections. There is no negative excursion in δ34S at the base of the Doushantuo Formation. The lower part of the Doushantuo Formation exhibits frequent oscillations with generally high δ34S values (+6.6 ‰ ~ +37.3 ‰), while the upper part shows a decreasing trend. The organic carbon isotope (δ13Corg) displays an inverse correlation with δ34S, with stable values in the lower part and gradually heavier values in the upper part. The nitrogen isotopes (δ15N) range from -1.6 ‰ to +2.1 ‰, suggesting a generally anoxic state with strong nitrogen fixation at the Cenpiaokou section. These findings suggest that the deep ocean oxidation during the early Ediacaran was not a global event, but rather a regional event, with anoxia still dominant overall.
... Lu et al., 2017), indicating a modern-like oceanic budget for osmium and hence, a modern-like riverine composition and flux. This observation, however, may not be strictly valid as δ 98 Mo river values are regulated by multiple factors such as incongruent weathering, δ 98 Mo value of exposed rocks, oxidative weathering rate, and Mo adsorption, which may have varied in the past (Kendall et al., 2015). ...
The Precambrian‐Cambrian (Pc‐C) boundary marks significant biological, atmospheric, and oceanic changes. These changes include extinction of the Ediacaran fauna, initiation of complex lifeforms, and oxygenation of the atmosphere and oceans. In this contribution, elemental and Mo‐S isotopic compositions of organic‐rich shales overlying the Pc‐C boundary from the Tal Formation, Lesser Himalaya, have been investigated. These datasets are used to reconstruct past oceanic redox state and sulfate concentrations. The principal component analysis of the elemental dataset identifies six major factors, with factors associated with organic matter and sulfide phases accounting for about half of the total variance. Iron speciation and Mo/U data suggest that the shales were deposited in anoxic and ferruginous deep water conditions. The δ⁹⁸Mo data (1.5 ± 0.2‰) and their mass balance calculations indicate that the areal extent of sulfidic waters and pyrite burial rates were about 2–4 times higher during the Pc‐C transition than in the modern ocean. The pyrite‐δ³⁴S values (3.6–8.3‰) for the Tal shales are isotopically heavier compared to modern‐day sedimentary pyrites (∼−21‰). Calculations involving earlier‐reported δ³⁴S values for early Cambrian seawater and our measured pyrite‐δ³⁴S data estimate the seawater sulfate concentration (8 ± 3 mM) during their deposition. This sulfate value for the Tal basin is higher than that reported for the late Neoproterozoic ocean (<5 mM), attributable to increasing oxygen availability and continental supply during this time. The observed basinal conditions and high terrestrial input may have influenced metazoan diversification.
... It has been suggested that a Neoproterozoic oxygenation event (NOE) occurred following the Tonian, during which oxygen accumulated and ventilated the deep oceans 2 . Multiple oceanic oxygenation events (OOEs) during the Ediacaran have been inferred on the basis of various geochemical proxies, including carbon and sulfur isotopes 1,5,12 , Fe speciation data 13 , nitrogen isotopes 14 , uranium isotopes 15 , molybdenum isotopes 16 , thallium isotopes 17,18 , chromium isotopes 19 , and redox-sensitive element enrichments 7,11 . The scope of these OOEs is debated, with effects either extending into the deep ocean 7,11,14 or limited to continental shelves 9,16,19,20 . ...
Increasing oxygenation of the early Ediacaran Ocean is thought to have been responsible for the emergence of early animals. Although geochemical studies have suggested periods of oceanic oxygenation in the Ediacaran, direct evidence for seafloor oxygenation has been lacking. Here, we report frequent occurrences of distinctive, sub-millimetric, and early diagenetic pyrite-marcasite rosettes in phosphorites from the lower Ediacaran Doushantuo Formation (Weng’an, South China). They typically consist of a nucleus of framboidal pyrite, a cortex of radiating marcasite blades intergrown with quartz, and a rim of second-generation pyrite, recording partial oxidative dissolution of pyrite and co-precipitation of marcasite and quartz. This inference is further supported by near-zero carbon isotope values of the host dolostone, similarly low sulfur isotope values for pyrite and marcasite, and evident Fe-isotope fractionation between marcasite and pyrite. Collectively, our findings reveal intermittent bottom-water and porewater oxygenation events, providing direct evidence of high-frequency oxygenation of Ediacaran continental shelves.
... Mo isotopes in organic-rich sedimentary rocks have been widely used in reconstructing local and global marine redox states because it exhibits high sensitivity to redox conditions [65,67,75,86,[88][89][90][91]. There are two main modes of Mo output in the ocean. ...
Studying the accumulation rules of organic matter (OM) in paleo-ocean sediments can not only enhance our understanding of how OM becomes enriched in ancient oceans but also provide guidance for the exploration of shale gas in unconventional shale strata. A breakthrough has been made in shale gas exploration in the early Cambrian Qiongzhusi Formation in South China. However, less attention has been paid to the intraplatform basin of the Yangtze Platform, and the factors controlling organic matter enrichment in this special region remain unclear. This study focuses on a continuous drilling core across the full well section of the Qiongzhusi Formation in the intraplatform basin of the Yangtze Platform. Through the comprehensive analysis of total organic carbon (TOC), major and trace elements, and Mo isotopes, this study investigates the controlling factors for OM enrichment with δ98/95Mo ratios utilized to identify the existence of euxinic bottom water. The examined 240 m long core can be divided into four units, where the TOC values of the lower Units 1 and 2 (0.2–5.0 wt.%) average higher than the upper Units 3 and 4 (0.2–2.5 wt.%). Redox indicators (U/Th, Ni/Co, EF(Mo)—EF(U)) indicate an increasing oxidation of bottom waters from the bottom upwards. δ98/95Mo data further confirm the presence of weakly euxinic conditions in Units 1 and 2, addressing the ongoing controversy surrounding bottom water redox environments. Primary productivity indicators (Ni/Al, Cu/Al) suggest a relatively low average productivity level within the intraplatform basin. The upwelling indicators EF(Co) * EF(Mn) of different profiles in the Yangtze Platform suggest that low productivity within the intraplatform basin can be mainly attributed to the absence of upwelling. Consequently, this study proposes an organic matter enrichment mechanism for the Qiongzhusi Formation in the intraplatform basin, which emphasizes the significance of the redox environment in the formation of high-quality hydrocarbon source rocks in restricted environments that lack upwelling, setting it apart from the deep ocean. These findings have the potential to provide valuable insights for the exploration of high-quality hydrocarbon source rocks in other similar regions.
... The Precambrian-Cambrian transition represents a dramatic change of the hydrosphere and biosphere in Earth history, with the Cambrian explosion of animal diversity which likely was linked to increased oxygen concentration in the oceans (Knoll and Carroll 1999;Sperling et al. 2013). However, the state of oxygenation of the oceans during the Early Cambrian is not well constrained and may have been variable (Kendall et al. 2015). Marine black shales provide a possibility to reconstruct local and global ocean redox conditions. ...
... Therefore, Mo isotope compositions of euxinic sediments from the past may serve as proxies of paleoceanic redox conditions. Mo isotope data with high values of up to 2.5‰ δ 98/95 Mo for black shales and organic-rich cherts point to modern-like oxygenated seawater in the Early Cambrian (Chen et al. 2015;Wen et al. 2015;Cheng et al. 2020) and already in the Late Ediacaran (Kendall et al. 2015). This finding is in conflict with the interpretation of earlier Mo isotope data which suggested an overall much less oxygenated Early Cambrian Ocean with about 1.1 ± 0.1‰ δ 98/95 Mo, based on Mo isotope data from extremely sulfide-and metal-rich black shales (Lehmann et al. 2007;Wille et al. 2008;Xu et al. 2012Xu et al. , 2013. ...
... The negative correlation between δ 98/95 Mo and δ 238/238 U indicates changes in the local depositional environment, different from the expected positive correlation for global ocean redox variations (Kendall et al. 2020). Such an inverse correlation of Mo and U isotope compositions is not only observed in the Early Cambrian black shales in South China but also in organic-rich shales that formed in local (semi-)restricted basins throughout geological history (Asael et al. 2013;Kendall et al. 2015Kendall et al. , 2020Andersen et al. 2020;Brüske et al. 2020;Li et al. 2022). The inverse correlation could be attributed to changes in dissolved sulfide concentrations and sulfate reduction rates related to deep water renewal rates and metal removal to sediment (Bura-Nakić et al. 2018). ...
The basal unit of the Early Cambrian black shale sequence of South China hosts sulfide-rich polymetallic units, non-sulfidic vanadium-rich black shales, sapropelic alginite (combustible shale), barite, and phosphorite. This rock spectrum occurs in a paleoceanographic similar, and stratigraphically correlated, transgressive upwelling setting on the passive continental margin of the Neoproterozoic Yangtze Platform. Several centimeter-thick polymetallic sulfidic units (3–13 wt% Mo+Ni, 100–600 ppm U) have relatively light Mo (δ98/95Mo = 1.1 ± 0.2‰) and relatively heavy U isotope composition (δ238/235U = 0.2 ± 0.1‰). Several meter-thick V-rich shales with multiple ore-grade layers (0.1–0.8 wt% V, < 100 ppm U, Mo and Ni ~ 100 ppm) have isotopically lighter Mo (δ98/95Mo = 0.3 ± 0.4‰) and heavier U composition (δ238/235U = 0.4 ± 0.2‰ and up to 0.7‰). The inverse Mo versus U isotope correlation suggests that both metals were enriched by removal from anoxic to strongly euxinic bottom water in restricted basins along the rifted continental margin. Metal replenishment probably occurred via the cycling of Fe–Mn-oxyhydroxide particles across the redox boundary, with sorption/desorption of Mo (and likely Ni) in a stratified water column. In contrast, V enrichment with much lower Mo, Ni, and U contents, but more fractionated Mo and U isotope composition, reflects non-sulfidic anoxic depositional conditions in a partly open system with higher bottom water renewal rates. While Mo isotope fractionation likely occurred in the water column, U isotope fractionation may dominantly have occurred at the water-sediment interface, perhaps in a benthic organic flocculent layer. These findings indicate that local hydrodynamic control and stratified water column redox conditions may explain the observed variation of metal enrichment (Mo–Ni versus V) in the black shales. Furthermore, the high δ98/95Mo values up to 2.6‰ of the black shales studied and the correlated U and Mo isotope data suggest that Early Cambrian seawater was at least episodically broadly similar to modern seawater.
... Second, the δ 238 U OC fluctuations of this interval might be linked to marine redox fluctuations (e.g., Dahl et al., 2014;Kendall et al., 2015;Wei et al., 2018). The high amplitude δ 238 U fluctuations near the Є-O boundary (from ~0.2 to 1.0‰, Fig. 4), similar to or even greater than those associated with other globally recognized oceanic anoxic events, should have coincided with drastic oscillations of the global carbon cycle and severe oceanic mass extinctions (e.g., Brennecka et al., 2011;Bartlett et al., 2018;Dahl et al., 2014Dahl et al., , 2019Cheng et al., 2020). ...
The early Paleozoic witnessed two spectacular radiations of marine organisms—the Cambrian (Є) Explosion and the Great Ordovician (O) Biodiversification Event. However, the period between them was characterized by a marine biodiversity plateau attributed to recurring biocrises. In the current study, we present the I/Ca records from slope lime mudstones of the Green Point Formation in western Newfoundland, along with a three-sink U-isotope mass balance model and previously reported limestone δ238Ucarb signals of the same interval, to further explore oceanic redox conditions at the Є-O boundary. The I/Ca ratios of the lime mudstones, deposited along the eastern Laurentian (western Iapetan) continental slope, exhibit a narrow range between 0.02 and 0.33 μmol/mol. Poor correlations between the I/Ca values and their [Sr], δ18O, Mn/Sr, Fe/Sr, Mg/Ca, and δ13Corg counterparts, together with near-micritic textures of the limestones, argue against significant influences of postdepositional alterations on the I/Ca signatures. The iodine-depleted lime mudstones, with I/Ca values well below the Proterozoic Eon baseline I/Ca ratios (∼0.5–1 μmol/mol), suggest the presence of shallow marine oxic-anoxic interfaces along the regional continental margin. Substantially low limestone I/Ca ratios (<0.5 μmol/mol) and dysoxic to anoxic depositional conditions have also been reported from several other age-equivalent sections deposited along the shelf and slope of ancient Iapetan and Laurentian continental margins. As a result, seawaters surrounding the Iapetan and Laurentian continental margins at the Є-O transition might have been commonly poorly oxygenated with shallow oxyclines or expanded oxygen minimum zones in the shelf and/or slope areas. This interpretation is further supported by our three-sink U-isotope mass balance modeling, which predicts widespread marine anoxia at the Є-O boundary with anoxic to euxinic water covering 1.0–21.1% of the ocean floor, significantly higher than the modern day (∼0.2%). Furthermore, the wide range of the oceanic δ238UOC (−1.22 to −0.25‰) values, estimated from the limestone δ238Ucarb signals, might reflect oceanic redox oscillations during this period. However, the δ238UOC fluctuations could also be attributed to variable accumulations of the 238U-enriched authigenic U phases during early diagenesis. Overall, evidence from the lime mudstone I/Ca ratios and the estimated extent of marine anoxia at the Є-O boundary in this study aligns with earlier viewpoints that the slowness of marine biodiversity accumulation during the late Cambrian and the Early Ordovician was linked to widespread oceanic anoxia.
... Geochemical studies have shown strong links between increased oxygenation and the rise of metazoan life (Fike et al., 2006;Kendall et al., 2015;Sahoo et al., 2016;Ostrander et al., 2020); however, the link (s) between biodiversification of already established ecosystems and fluctuating oxygenation needs further investigation (Elmqvist et al., 2003;Sampaio et al., 2021). Several studies have attempted to quantify changes in atmospheric and marine oxygenation throughout the Ordovician. ...
... 1b-d and 3c). Other independent carbonate-hosted and siliciclastic-hosted redox proxies (for example, Tl and Mo isotopes 46,47 , carbonate-bound iodate 35 and Fe speciation 36 ) corroborate the increase after a previous decrease in ocean anoxia across the SE. Increasing ocean anoxia would decrease P burial, which, when coupled with continued P release from DOM and weathering, would lead to another increase in the P reservoir (Fig. 2c), as observed in the CAP data (P-2nd in Figs. ...
Phosphorus is a limiting nutrient that is thought to control oceanic oxygen levels to a large extent1–3. A possible increase in marine phosphorus concentrations during the Ediacaran Period (about 635–539 million years ago) has been proposed as a driver for increasing oxygen levels4–6. However, little is known about the nature and evolution of phosphorus cycling during this time⁴. Here we use carbonate-associated phosphate (CAP) from six globally distributed sections to reconstruct oceanic phosphorus concentrations during a large negative carbon-isotope excursion—the Shuram excursion (SE)—which co-occurred with global oceanic oxygenation7–9. Our data suggest pulsed increases in oceanic phosphorus concentrations during the falling and rising limbs of the SE. Using a quantitative biogeochemical model, we propose that this observation could be explained by carbon dioxide and phosphorus release from marine organic-matter oxidation primarily by sulfate, with further phosphorus release from carbon-dioxide-driven weathering on land. Collectively, this may have resulted in elevated organic-pyrite burial and ocean oxygenation. Our CAP data also seem to suggest equivalent oceanic phosphorus concentrations under maximum and minimum extents of ocean anoxia across the SE. This observation may reflect decoupled phosphorus and ocean anoxia cycles, as opposed to their coupled nature in the modern ocean. Our findings point to external stimuli such as sulfate weathering rather than internal oceanic phosphorus–oxygen cycling alone as a possible control on oceanic oxygenation in the Ediacaran. In turn, this may help explain the prolonged rise of atmospheric oxygen levels.
