Active methanogenesis during the melting of Marinoan snowball Earth

Abstract

Geological evidence indicates that the deglaciation of Marinoan snowball Earth ice age (~635 Myr ago) was associated with intense continental weathering, recovery of primary productivity, transient marine euxinia, and potentially extensive CH₄ emission. It is proposed that the deglacial CH₄ emissions may have provided positive feedbacks for ice melting and global warming. However, the origin of CH₄ remains unclear. Here we report Ni isotopes (δ⁶⁰Ni) and Yttrium-rare earth element (YREE) compositions of syndepositional pyrites from the upper most Nantuo Formation (equivalent deposits of the Marinoan glaciation), South China. The Nantuo pyrite displays anti-correlations between Ni concentration and δ⁶⁰Ni, and between Ni concentration and Sm/Yb ratio, suggesting mixing between Ni in seawater and Ni from methanogens. Our study indicates active methanogenesis during the termination of Marinoan snowball Earth. This suggests that methanogenesis was fueled by methyl sulfides produced in sulfidic seawater during the deglacial recovery of marine primary productivity.

Fig. 1. The paleogeographic map of South China Block in late Neoproterozoic Era.

The green star marks the location of Huakoushan section (slope environment). The red stars mark the locations of Yazhai, Tongle, and Datan sections (basin environment).

Fig. 2. The stratigraphic profiles of geochemical data.

Stratigraphic variations of δ⁶⁰Ni, Ni concentration, Sm/Yb (normalized by post-Archean Australian shale, PAAS), and δ³⁴S of pyrite concretions in the upper Nantuo Formation. The δ³⁴S values are from Lang et. al.. DST here refers to Doushantuo Formation.

Fig. 3. The relationships between Ni concentration, Ni isotopes, and rare earth element pattern of the Nantuo pyrite samples.

a Cross-plot of Ni concentration versus δ⁶⁰Ni. There is an anticorrelation between Ni concentrations and δ⁶⁰Ni values. Dash lines represent the relationship between Ni concentration and isotopes in a theoretical mixing model. Here, δ⁶⁰Ni values of the two end-members are −0.5‰ and +1.5‰, respectively. The Ni concentration of the high-δ⁶⁰Ni end-member is fixed to 20 p.p.m., while that of low-δ⁶⁰Ni end-member is assigned to 50, 100, and 250 p.p.m., respectively. The anticorrelation between Ni concentration and δ⁶⁰Ni could be explained by a binary mixing model. b Cross-plot of Ni concentration versus Sm_N/Yb_N. Here, Sm and Yb are used to represent middle rare earth element (MREE) and heavy REE (HREE) respectively. An anticorrelation is shown between Ni concentration and MREE/HREE. REE data are normalized to post-Archean Australian shale (PAAS). Green triangles, red circles, red squares, and red diamonds represent the Huakoushan, Tongle, Yazhai, and Datan sections, respectively.

Fig. 4. Rare earth element pattern of the Nantuo pyrite.

Rare earth element (REE) data are normalized to post-Archean Australian shale (PAAS). Samples formed in the basin environment (Tongle, Yazhai, and Datan section) are strongly depleted in HREE. Samples from the slope region (Huakoushan section) enrich in MREE.

Fig. 5. Compilations of the Ni isotopes data.

a Boxplots showing the Ni isotopic compositions of different reservoirs^–,,,–. The box showing the range for δ⁶⁰Ni value between upper and lower quartiles. The vertical line inside the box represents the median value, while that outside the box represents the maximum and minimum value. The red and blue dash lines represent the upper continental crust (UCC) and modern seawater compositions, respectively^,. b Boxplot showing the Ni isotopic fractionations in various geochemical processes^,. The weathering process preferentially dissolves heavy Ni, and laboratory experiment shows preferential absorption of light Ni in ferrihydrite precipitation. Plants and methanogens preferentially absorb light Ni (ref. ).

Fig. 6. Compilations of rare earth element pattern.

All rare earth element (REE) data are normalized to post-Archean Australian shale (PAAS). a REE pattern of igneous rocks. All samples show Eu positive anomalies. All except for oceanic island basalt (OIB) show light REE (LREE)-depleted pattern; b REE pattern of porewater. REE pattern of the bottom water is similar to that of seawater. The middle REE (MREE) bulge type is found in Fe rich cores. The heavy REE (HREE)-enriched type is found in Fe lean cores. All samples slightly enrich in HREE; c REE pattern of hydrothermal fluids and precipitates. Most samples show a flat REE pattern, except for the pedestal slab near chimney showing a MREE-depleted pattern^,,. All samples show a positive Eu anomaly except for sulfide from ultraslow spreading ridge. Pedestal slab and Fe–Mn precipitation show a slight Y enrichment. d REE pattern for Black Seawater. Strong Ce negative anomaly could be characterized in shallow water. Deep water shows flat REE pattern with slight HREE enrichment. e REE pattern of Fe-oxide deposition. Modern samples are ferrihydrite precipitation on seafloor, while samples from Paleoproterozoic and Archean era are banded-iron-formation (BIF). Modern samples show flat pattern with slight Ce positive anomaly and negative Y anomaly. BIF samples show positive Eu and Y anomaly. f REE pattern for modern primordial plants including lichen, algae, and moss.^, All samples show a flat REE pattern, except for green algae that is slightly depleted Nd (ref. ). g Fractional coefficient for microbes. Different lines show fractional coefficient for different biomass concentration. Microbes preferentially absorb HREE. h REE fractionation during Fe(OH)₃ precipitation. All data show LREE-depleted pattern with a slight Y negative anomaly. Smaller fractionation is observed in acidic environment. i Comparison of REE pattern for fluid end-member and sulfide from Logatchev (mid-ocean ridge in Atlantic Ocean) and east Pacific rise (EPR). The REE pattern for sulfide and fluid end-member is generally similar. A smaller positive Eu anomaly and slight HREE enrichment might be caused by contamination of seawater.

Fig. 7. Schematic model showing the biogeochemical cycles during the termination of Marinoan snowball Earth in the Yangtze Block.

The recovery of primary productivity provided abundant organic matter in the euphotic zone by photosynthesis (PS), fueling microbial sulfate reduction (MSR) in the water column. Seawater MSR sustained sulfidic condition. Active methanogenesis (MG) might be fueled by methyl sulfide that was produced by H₂S-methylation in sulfidic seawater. Since methanogens preferentially absorb light Ni and heavy rare earth element (HREE), seawater was characterized by higher δ⁶⁰Ni and displayed a HREE-depleted pattern. Sinking of particulate biomass of methanogens into sediments sustained microbial iron reduction (MIR), which converted ferric Fe (Fe³⁺) to Fe²⁺, and released absorbed light Ni and HREE in porewater. Syndepositional pyrite precipitation in sediment porewater would incorporate both seawater and porewater signals, and various degree of degradation of methanogen-derived organic matter led to a negative correlation between Ni concentration and δ⁶⁰Ni.