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Hg isotope evidence for oceanic oxygenation during the Cambrian Explosion
Haifeng Fan, Xuewu Fu, Ruofei Yang, Hanjie Wen, Chunlin Hu, Jack F.
Ward, Hongjie Zhang, Hui Zhang, Xingliang Zhang
PII: S0016-7037(23)00494-5
DOI: https://doi.org/10.1016/j.gca.2023.10.020
Reference: GCA 13200
To appear in: Geochimica et Cosmochimica Acta
Received Date: 9 April 2023
Accepted Date: 12 October 2023
Please cite this article as: Fan, H., Fu, X., Yang, R., Wen, H., Hu, C., Ward, J.F., Zhang, H., Zhang, H., Zhang,
X., Hg isotope evidence for oceanic oxygenation during the Cambrian Explosion, Geochimica et Cosmochimica
Acta (2023), doi: https://doi.org/10.1016/j.gca.2023.10.020
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© 2023 Published by Elsevier Ltd.
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Hg isotope evidence for oceanic oxygenation during
the Cambrian Explosion
Haifeng Fana, f*, Xuewu Fub, f, Ruofei Yanga, f, Hanjie Wenc, f, Chunlin Hud, Jack F.
Warde, Hongjie Zhanga, f, Hui Zhangb, f, Xingliang Zhangg
a. State Key Lab of Ore Deposit Geochemistry, Institute of Geochemistry, CAS,
Guiyang 550081, China
b. State Key Lab of Environment Geochemistry, Institute of Geochemistry, CAS,
Guiyang 550081, China
c. School of Earth Sciences and Resources, Chang’an University, Xi’an 710054,
China
d. State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of
Geology and Palaeontology, CAS, Nanjing 210008, China
e. School of the Environment, University of Queensland, Brisbane, Queensland 4072,
Australia
f. University of Chinese Academy of Sciences, Beijing 100049, China
g. State Key Laboratory of Continental Dynamics, Shaanxi Key Laboratory of Early
Life and Environments, Department of Geology, Northwest University, Xi’an
710069, China
*Corresponding author:
Haifeng Fan (fanhaifeng@mail.gyig.ac.cn)
2 / 28
ABSTRACT:
The Cambrian explosion is a critical evolutionary milestone in life history, but the
mechanistic relationship between the Cambrian explosion, rising atmospheric and
oceanic oxygen levels, and the primary productivity remains controversial. Here, we
present new mercury (Hg) isotope data from Cambrian marine sediments of the ~521–
514 Ma Yu’anshan and Shuijingtuo Formations (Nanhua Basin, South China), which
preserve the famous ~518 Ma Chengjiang and Qingjiang Cambrian Biotas, respectively.
We find evidence that, prior to Cambrian animal diversification, local terrestrial and/or
atmospheric inputs are important drivers of Hg enrichment in the shallow shelf
Yu’anshan and Shuijintuo Formations. Elevated Hg and total organic carbon (TOC)
concentrations of coeval deeper shelf and slope sections of the Niutitang Formation are
mainly attributed to upwelling of Hg associated with dissolved organic carbon (Hg-
DOC) from pelagic seawater, unrelated to locally high primary productivity. During the
deposition of fossiliferous strata, negative shifts of Δ199Hg and δ202Hg values in the
shallow water shelf shales from the Shuijingtuo Formation, combined with similar Hg
isotope characteristics in the deeper shelf and slope shales, demonstrate a significant
shallow and deep-water oxygenation event took place in the Nanhua Basin. A
comparable negative shift of Δ199Hg values occurs in coeval Indian Craton sediments,
suggesting this oxygenation event could be regional, or possibly even global, in scale.
Our newly-collected Hg isotope data provide strong evidence that rising molecular
oxygen levels in both surface and deep seawater are associated with enhanced marine
primary productivity and could be a critical driver of the Cambrian explosion. Finally,
we also argue that Hg isotopes are an emerging and promising redox proxy for studies
of Precambrian seawater, but the chemical response of Hg to atmospheric and ocean
oxygenation requires further calibration.
Key words: Cambrian explosion, Hg isotope, Increased oxygen levels
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1. Introduction
The Cambrian explosion, the rapid diversification of animal body plans and rise of
ecosystem complexity across the Ediacaran–Cambrian transition, is arguably the most
important evolutionary event in the history of Earth (Erwin et al., 2011). Despite its
significance, however, the triggers of the Cambrian explosion remain controversial. For
example, substantial increases of oceanic and atmospheric oxygen levels have been
suggested to drive extraordinary ecological expansion during Cambrian time (Cloud,
1968; Knoll and Carroll, 1999; Li et al., 2017). Other studies, however, have de-
emphasized the role of rising oxygen levels (Sperling et al., 2015), instead linking the
Cambrian explosion to ecological, developmental, or evolutionary mechanisms under
hypoxic conditions (Erwin et al., 2011; Hammarlund et al., 2018; Mills and Canfield,
2014; Sperling et al., 2013). Furthermore, Sperling and Stockey (2018), based on a
comprehensive shale total organic carbon (TOC) data set, highlighted the importance
of food supply for biotic radiation, suggesting that a congruent rise of primary
productivity and oxygen levels facilitated the Neoproterozoic–Cambrian increase in
animal size and diversity.
The Chengjiang and Qingjiang Biotas from South China, and the Sirius Passet
Biota from Greenland, represent three well-known Burgess Shale-type Biotas of
Cambrian Stage 3 age (~521–514 Ma; Fu et al., 2019a; Hammarlund et al., 2019; Saleh
et al., 2022), when the Cambrian explosion reached its summit (Paterson John et al.,
2019; Zhang and Shu, 2021). These biotas are important archives for understanding the
Cambrian explosion and associated ecosystem development, but the
paleoenvironmental conditions in which they occurred are unclear. Fe speciation, δ15N,
and trace element systematics suggest the strata-hosted Chengjiang and Sirius Passet
Biotas may have been deposited in an oxygen minimum zone (Hammarlund et al., 2017;
Hammarlund et al., 2019). A recent study, however, suggested that the Chengjiang
fauna lived in an oxygen-rich delta front environment (Saleh et al., 2022). For the
Qingjiang Biota, N isotopes have been interpreted to reflect paleoenvironmental water
column chemistry like that of Chengjiang Biota (Chang et al., 2022). It is also
noteworthy that organic-rich shales (TOC up to 20 wt.%) are widespread in the Nanhua
Basin below the Chengjiang and Qingjiang fossiliferous layers, and these have been
interpreted as consequences of local high primary productivity during early Cambrian
time (Cheng et al., 2020; Jin et al., 2016). However, very low quantities of TOC (<2
wt.%) are observed in sedimentary rocks containing the Chengjiang, Qingjiang, and
Sirius Passet fossils (Hammarlund et al., 2017; Hammarlund et al., 2019; Qi et al.,
2018). It remains uncertain, therefore, whether decreasing TOC concentrations reflect
a transition from a food-rich to a food-limited period or, alternatively, lower TOC due
to local oxygenation.
In marine sediments, mercury (Hg) is mainly derived from atmospheric
precipitation (sometimes with significant terrestrial input in nearshore environments),
but extreme Hg enrichment is mostly associated with volcanism (Fitzgerald et al., 2007;
Grasby et al., 2017; Grasby et al., 2019). These endmember sources are characterized
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by different mass-dependent and mass-independent fractionation signals (MDF and
MIF, respectively; Blum et al., 2014; Sun et al., 2019). Atmospheric Hg (II) commonly
shows positive MIF and negative MDF signals (Δ199Hg = 0.39 ± 0.28 ‰; Δ200Hg = 0.17
± 0.08 ‰; δ202Hg = -0.41 ± 0.48 ‰), unlike Hg (0) (Δ199Hg = -0.11 ± 0.12 ‰; Δ200Hg
= -0.04 ± 0.05 ‰; δ202Hg = 0.1 ± 0.56 ‰; Sun et al., 2019). Published data demonstrate
that modern marine sediments dominated by atmospheric Hg usually exhibit more
positive MIF signals (Δ199Hg = 0.12 ± 0.05 ‰; Δ200Hg = 0.04 ± 0.04 ‰) than sediments
with abundant volcanic (Δ199Hg = 0‰; Δ200Hg = 0‰) and terrestrial Hg contributions
(Δ199Hg = -0.25 ± 0.14 ‰; Δ200Hg = 0.02 ± 0.04 ‰), but MDF does not show obvious
systematic variation (Blum et al., 2014; Sun et al., 2019; Zheng et al., 2018). Δ200Hg
variation is limited to photochemical transformations of Hg in the tropopause or
stratosphere, while Δ199Hg variation is caused by photochemical transformations in the
troposphere and in surface waters (Blum et al., 2014). Moreover, recent studies have
suggested photochemical reduction of Hg (II) is redox sensitive, and opposite Δ199Hg
fractionation trends occur in anoxic and oxic conditions (Bergquist and Blum, 2007;
Motta et al., 2020; Zheng and Hintelmann, 2010).
In light of this sensitivity to environmental chemical conditions, Hg concentrations
and isotopes in marine sediments have been used to trace volcanic emissions (e.g.,
(Grasby et al., 2019 and references therein) and, recently, global Ediacaran oceanic
carbon cycling (Fan et al., 2021), late Archean atmospheric chemistry (Zerkle et al.,
2020), and the end-Archean “whiff” of oxygen (Meixnerová et al., 2021). During the
end-Archean (~2.5 Ga) “whiff” of oxygen, the oxygenated interval shows strong Hg
enrichment coupled with slightly negative Δ199Hg values, suggestive of increased
oxidative weathering (Meixnerová et al., 2021). A strong negative shift of Δ199Hg (from
0 ‰ to -0.12 ‰) recorded in deglacial banded iron formation could be associated with
the oceanic oxygenation during the end of the Sturtian glaciation (Sun et al., 2022).
Therefore, Hg isotopes, especially MIF, could potentially be utilized to trace
atmospheric and oceanic oxygenation events, but the chemical response of Hg to
atmospheric and oceanic oxygenation requires further investigation.
Here, we present Hg data from two Nanhua Basin drill cores that contain
Chengjiang and Qingjiang Biota fossils. In this contribution, we first estimate the utility
of Hg isotopes as a potential redox proxy by distinguishing Hg sources of the Nanhua
Basin sediments, and comparing these Hg data to Mn concentrations and TOC/P
(mol/mol) ratios (which are strongly controlled by benthic redox conditions; Algeo and
Ingall, 2007). We then evaluate correlations between oceanic/atmospheric redox
conditions and the Cambrian Explosion.
2. Geological setting
During the Cambrian Series 2 and Stage 3 (~526–514 Ma), organic-rich marine
shales were deposited widely across shelf to slope and basin locations in the Nanhua
Basin (Fig. 1A). The shallow shelf shales comprise the Yu’anshan Formation in
Yunnan province and the Shuijintuo Formation in Hubei province (Fig. 1A). The
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Yu’anshan and Shuijintuo Formations host the Chengjiang and Qingjiang Biotas,
respectively (Zhu et al., 2001; Fu et al., 2019a). The Chengjiang and Qingjiang Biotas
are separated by ~1050 km and contain different fossil assemblages, representing
communities developed in response to local environmental conditions (Fu et al., 2019a).
The Qingjiang Biota is equivalent to the Chengjiang Biota in age (~518 Ma; Yang et
al., 2018), and both correlate with the global Cambrian Series 2 and Stage 3 (Fu et al.,
2019a). The Qingjiang Biota, however, is preserved in a somewhat more distal setting
than that of the Chengjiang Biota (Fu et al., 2019a; Saleh et al., 2022).
Our Chengjiang Biota samples were collected from drill cores ZK-2808 (hereafter
named Chengjiang core) in the Kunyang area from Yunnan Province. Qingjiang Biota
samples were collected from drill core ZK-03 (hereafter named Qingjiang core) in the
Yichang area from Hubei Province (Fig. 1A). The Chengjiang core mainly contains
three members of the Yu’anshan Formation (Fig. 1B). Member I, representing sea level
highstand, is composed of carbonaceous black shale. Member II is a succession of
mudstones intercalated with sandstones that transitions upwards into shallower, wave-
influenced fluid mud deposits. Member III, a sandstone-dominated upper member, was
deposited in a storm wave-influenced shelf or distal shoreface setting (Saleh et al., 2022;
Zhu et al., 2001). The ~518 Ma Chengjiang Biota (e.g., Yang et al., 2018) occurs within
storm-flood-dominated delta sediments of Member II (Saleh et al., 2022; Zhu et al.,
2001) (Fig. 1B), which is the thickest and most lithologically complex member of the
Yu’anshan Formation. Our samples, including shales and mudstones, were only
collected from Members I and II.
The Qingjiang core mainly contains three members of the Shuijingtuo Formation
(Fig. 1B). Member I consists of a laminated carbonaceous black shale deposited during
sea level highstand and contains an ~521 ± 5 Ma Ni-Mo sulfide layer (at 355.89 m) that
correlates with Ni-Mo layers from Guizhou and Hunan provinces (Xu et al., 2011). The
Qingjiang Biota is preserved in Member II, which is a black calcareous shale with
subordinate sandstone deposited below storm wave base (Fu et al., 2019a). Member III
is a black, centimeter–decimeter bedded limestone with thin mudstone partings,
indicating a shallow water environment (Fu et al., 2019a). Our samples, including
shales and mudstones, were collected from all three members of the Shuijingtuo
Formation.
We also compared our investigated cores to existing Hg data from three sections
of organic-rich shales belonging to the contemporaneous Niutitang Formation: the shelf
Zhijin section, the slope Tongren section, and the slope Jishou section (Fig. 1A). The
three sections have been well investigated in previous studies, where the Niutitang
Formation is underlain by phosphorites of the Gezhongwu Formation in the Zhijin
section and by cherts of the Liuchapo Formation in the Tongren and Jishou sections
(Wang et al., 2020; Yin et al., 2017; Zhu et al., 2021a). Among them, slope sections
show extreme enrichment of organic materials (mostly >10 wt.%) compared to the shelf
sections (mostly < 4 wt.%).
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3. Methods
3.1 Hg concentrations and isotope compositions
A DMA-80 automatic Hg analyzer at the State Key Laboratory of Environmental
Geochemistry, Chinese Academy of Sciences (CAS), was used to measure Hg
concentrations. NIST SRM 2711a, procedural blanks, and sample duplicates were
analyzed to monitor accuracy and precision. Duplicate analysis of Hg concentrations,
overall, shows a small relative standard deviation (mean = ± 5.5%). In this study, the
mean of the duplicate tests is presented as the total Hg value.
The double-stage combustion method of Sun et al. (2013) was used to
preconcentrate Hg from individual samples into 7 mL of 40 % mixed acid solution (v/v,
HNO3/HCl = 2:1). The trapping bottle and impinger were rinsed three times with 10
mL of Milli-Q water immediately after preconcentration. The Milli-Q water was added
into the trapping solution, yielding a final acid concentration of ~20 %. The final
trapping solutions were stored at 2–4 °C until the Hg isotope analysis was performed.
A Nu Instruments cold vapor multi-collector inductively coupled plasma mass
spectrometer (CV-MC-ICPMS) was used to collect Hg isotope ratios at the State Key
Laboratory of Environmental Geochemistry, CAS, following the method of Fu et al.,
(2019b).
Standard-sample-standard bracketing using the National Institute of Standards and
Technology (NIST) Standard Reference Material (SRM) 3133 was performed to correct
Hg isotope mass bias. Hg isotopic MDF are reported as delta values (δ) in per-mil (‰)
relative to the mean ratios measured for the NIST SRM 3133 before and after each
sample. MIF is reported in capital delta values (Δ), ∆xHg (‰) = δxHgsample – β×δ202
Hgsample, where x refers to the mass of Hg isotope (199Hg, 200Hg, 201Hg, and 204Hg). The
corresponding β values of these Hg isotopes are 0.252, 0.502, 0.752, and 1.493,
respectively.
Repeated analysis of NIST SRM 8610 (n = 17) and NIST SRM 2711a (Montana
soil, n = 6) was performed to monitor analytical uncertainties of Hg isotopic
compositions. The 2SD of these two standard references with δ202Hg and Δ199Hg of
respectively 0.07–0.10 ‰ and 0.05–0.06 ‰ represent the typical analytical
uncertainties of our samples. The measured Hg isotopic compositions of NIST SRM
8610 (δ202Hg = -0.51 ± 0.07 ‰ and Δ199Hg = -0.02 ± 0.05 ‰) and NIST SRM 2711a
(δ202Hg = -0.18 ± 0.10 ‰ and Δ199Hg = -0.23 ± 0.06 ‰) agree with previously
published results and are presented in Table S1.
3.2 Other elements
To measure TOC content, samples were firstly leached in 2N HCl to remove
carbonate components. Ultrapure water (18.2 MΩ-cm) was then used to rinse the
leached residue three times, and the rinsed residue (representing the TOC component)
was analyzed for total carbon. TOC and total sulfur (TS) contents from the Qingjiang
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core samples were determined using a LECO C-S Elemental Analyzer at the Institute
of Geochemistry, CAS. A Flash EA 2000 Elemental Analyzer at Nanjing Institute of
Geology and Palaeontology, CAS (NIGPAS) was used to analyze samples from the
Chengjiang core. Manganese and phosphorous concentrations were measured via X-
ray fluorescence at the ALS Chemex facility in Guangzhou, China, and an Agilent 710
ICP-OES at NIGPAS. The relative standard deviations (RSD) for these elements were
better than ±5 %, based on the results of one standard reference shale (marine shale,
SBC-1; Fan et al., 2021).
4. Results
4.1 Chengjiang core preserving the Chengjiang Biota
At the bottom of the Chengjiang core, from 101 m to 72 m depth, total Hg
concentrations range from 111 to 190 ppb and Hg/TOC (ppb/%) varies from 64 to 112
(Fig. 2A). These samples show a narrow range of δ202Hg and Δ199Hg, from -0.88 ‰ to
-0.48 ‰ (-0.69 ± 0.15 ‰, mean ±1σ standard deviation) and -0.03 ‰ to 0.06 ‰ (0.01
± 0.04 ‰), respectively (Fig. 2A). Up section, from a depth of 70 m, total Hg gradually
declines from 47 ppb to 14 ppb through the fossiliferous layer (shaded blue in Fig. 2A).
Hg/TOC does change considerably due to varied TOC content upwards. δ202Hg values
increase from -0.81 ‰ to 0.10 ‰, and Δ199Hg values decrease from 0.08 ‰ to -0.06 ‰.
Throughout the whole section, Mn concentrations increase from 236 ppm to 672 ppm,
and then decline to 299 ppm. TOC/P ratios gradually decrease from 41 (in the
lowermost part of the core) to 3 (in the uppermost part of the core) (Fig. 2A). All data
are listed in Table S2.
4.2 Qingjiang core preserving the Qingjiang Biota
In the Qingjiang core, total Hg concentrations gradually decrease from 650 ppb at
the lowest part of the drill core to 40 ppb in the Qingjiang fossiliferous layers (shaded
blue in Fig. 2B), and then to 20 ± 16 ppb at 343–330 m depth (Fig. 2B). Like total Hg
concentrations, Hg/TOC decreases from ~150 at a depth of ~360 m depth to ~40 at
~343 m depth. In the Qingjiang core, Hg/TOC is lowest at ~339 m, where it reaches
~30, and Hg/TOC increases above this stratigraphic height. δ202Hg values show
relatively homogenous values (-0.53 ± 0.14 ‰) in the lowest part of the Qingjiang core
(357–351 m), and they gradually decline to the lowest recorded values in the Qingjiang
fossiliferous layer (-2.74 ± 0.20 ‰), before increasing above the fossiliferous layer to
approximately -0.67 ‰ (Fig. 2B). The Qingjiang core Δ199Hg trend is less clear, but,
overall, these values decrease from slightly positive to negative values up section (Fig.
2B). Mn concentrations increase up section from 150 ppm to 558 ppm or higher values
(Fig. 2B). TOC/P ratios increase from 74 to 224 in the lowermost three meters of core,
before gradually decreasing from 190 to ~14. All data are listed in Table S3.
5. Discussion
5.1 Widespread Hg enrichment in the Nanhua Basin (~521 Ma)
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In the Nanhua Basin, Hg enrichment comparable to that occurring in the lowest
part of the Yu’anshan (Chengjiang core) and Shuijingtuo (Qingjiang core) Formations
(i.e., below the fossiliferous layers; Fig. 2A and B) has been reported in deeper shelf
and slope sections (Zhijin, Tongren, and Jishou sections) of the equivalent Niutitang
Formation elsewhere in this Basin (Figs. 2C and 3; Wang et al., 2020; Wu et al., 2022;
Yin et al., 2017; Zhu et al., 2021a, b). At that time, seawater redox conditions in the
shallow shelf Chengjiang and Qingjiang settings (as well as the deeper shelf and slope
locations) were oxygen-depleted or anoxic/euxinic, as demonstrated by our high TOC/P
ratios (modern anoxic facies have TOC/P of ~110–200; Algeo and Ingall, 2007), low
Mn concentrations (< 400 ppm; Fig. 2A and B), and previous Fe speciation data (Li et
al., 2017; Li et al., 2021; Wei et al., 2021). Hg enrichment in marine sediments is tightly
correlated with degree of sequestration of Hg by organic materials (Grasby et al., 2019)
and Hg-sulfide in strongly anoxic conditions (Shen et al., 2020; Zheng et al., 2018).
The clear positive correlations of total Hg with TOC and/or TS concentrations (Fig. 3),
respectively, in the two investigated cores and other referenced slope sections indicate
that dissolved Hg was mainly sequestered by organic materials and/or Hg-sulfide from
an anoxic seawater column prior to the Cambrian explosion. However, the mean of
published Hg concentrations in marine sediments is 62 ppb (Hg/TOC ~75; Grasby et
al., 2019), which is significantly less than the Hg concentrations (~100–1000 ppb,
Hg/TOC mostly >70) of the sections discussed here. Sedimentation rates and local
redox conditions have been suggested to affect Hg-enrichment in sediments (Mazrui et
al., 2016; Selin, 2009). However, the estimated deposition rates during this time are
surprisingly similar to those found in modern euxinic basins (Lehmann et al., 2007).
Additional Hg sources are, therefore, required to explain the elevated Hg concentrations
in the Nanhua Basin samples (Wang et al., 2020; Wu et al., 2022; Zhu et al., 2021a).
Significant additional Hg sources in the Cambrian Nanhua Basin could possibly include
local terrestrial, volcanic, and pelagic (open ocean) seawater inputs. Lower Cambrian
strata were deposited before the evolution of land plants (Lenton et al., 2016), and thus
it is reasonable to assume that terrestrial Hg may be characterized by detrital near-zero
Δ199Hg and δ202Hg values of -0.1 ‰ to -0.6 ‰ (i.e., similar to the composition of silicate
rocks; Fan et al., 2021; Smith et al., 2008). The lower Chengjiang core sediments were
deposited in a low oxygen, low energy, hemipelagic environment sometimes affected
by higher-energy tractive events (Saleh et al., 2022). The relatively homogenous
Δ199Hg (0.01 ± 0.04 ‰) and δ202Hg values (-0.69 ± 0.15 ‰) in the lowermost
Chengjiang core (below 51 m depth; Fig. 2A) and Qingjiang cores (0.04 ± 0.04 ‰ and
-0.55 ±0.15 ‰, respectively, below 352 m depth; Fig. 2B) are highly consistent with
that reported for pre-anthropogenic marine sediments (Δ199Hg = 0.05 ± 0.01 ‰; δ202Hg
= -0.76 ± 0.16 ‰; Gehrke et al., 2009), which may therefore indicate that Hg
enrichment is mainly attributed to local atmospheric, and possibly terrestrial Hg
contributions, given higher TS concentrations at that time.
By contrast, recent studies have proposed large Hg contributions to the Nanhua
Basin by volcanic activity (Wang et al., 2020; Wu et al., 2022; Zhu et al., 2021a). Most
organic-rich shales in the Yu’anshan (Chengjiang core), Shuijingtuo (Qingjiang core),
and Niutitang (Zhijin, Tongren and Jishou sections) Formations, however, show low–
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moderate Hg/TOC (ppb/%) ratios (mostly < 150) and Hg/TS (ppb/%) ratios (mostly <
100, for samples with obvious correlation between Hg and TS), which are (1) much less
than those of sedimentary rocks containing abundant volcanic material (Hg/TOC > 200;
Grasby et al., 2019), and (2) similar to the Ordovician/Silurian marine black shales
(Hg/TS less than ~ 300) without volcanic Hg input (Shen et al., 2019). Nevertheless,
the extremely high Hg/TOC (~140–760) recorded by some Niutitang Formation slope
sections has been attributed to volcanic contributions (Wang et al., 2020; Zhu et al.,
2021b). Volcanic activity also commonly causes transient and extreme Hg enrichment
spikes in ancient stratigraphic records (Grasby et al., 2019). Assuming that volcanic Hg
was an overwhelming source for Hg enrichment in the Nanhua Basin, the homogenous
δ202Hg and near zero Δ199Hg values could be preserved in those offshore anoxic
organic-rich shales (Grasby et al., 2017). However, an obvious depth-gradient of Hg
isotopes is observed in the Nanhua Basin (Figs. 3 and 4), where shallow water
sediments in our investigated Chengjiang and Qingjiang cores commonly show
terrestrial-like δ202Hg and Δ199Hg signals, compared to deeper shelf and slope
sediments (δ202Hg < -1.3‰; Δ199Hg ~ 0.2‰ in the Zhijin and Tongren sections; Wu et
al., 2022; Yin et al., 2017; Zhu et al., 2021a). Therefore, the wide range of δ202Hg and
Δ199Hg values and low–middle Hg/TOC ratios (mostly <150) in the lower part of all
mentioned sections indicate that volcanic contributions could not play a significant role
in widespread Hg enrichment in the Nanhua Basin.
Transmission of pelagic-seawater (in the open ocean) by subsurface paleocurrents
has also been invoked to explain moderate Hg enrichment (Hg/TOC = 50–200) during
the Cretaceous Oceanic Anoxic Events (OAE1a and OAE2; Scaife et al., 2017). It
should be noted that the lowest parts of Shuijingtuo (Qinjiang core), Yu’anshan
(Chengjiang core), and Niutitang Formations represent a large-scale transgression event
(Jin et al., 2016), when the upwelling of a pelagic-seawater-enriched dissolved organic
carbon (DOC) reservoir have generated negative carbonate and organic carbon δ13C
excursions (Jiang et al., 2012). The transgression event may have also transported large
amounts of dissolved Hg associated with DOC (Hg-DOC) into the Nanhua Basin
because dissolved Hg is usually bonded to DOC in pelagic seawater (Fitzgerald et al.,
2007). The upwelling of a Hg-DOC-enriched reservoir and local precipitation of Hg
associated with organic matter or sulfide in anoxic conditions would result in moderate
Hg enrichment in local sediments. This mechanism has been proposed to explain Hg
enrichment (Hg ~400–1000 ppb, Hg/TOC ~80) in anoxic marine sediments from the
Nanhua Basin during the Ediacaran Shuram excursion (Fan et al., 2021). Furthermore,
higher total Hg concentrations, more positive Δ199Hg values, and more negative δ202Hg
values in the deeper shelf and slope shales (Zhijing and Tongren sections) than in the
shallow water delta sediments (the lower part of Chengjiang core, this study) and, to a
lesser extent, shallow shelf shales (the lower part of Qingjiang core, this study; Figs. 2,
3 and 4) indicate the enriched Hg in these shales could be sourced from different
endmember reservoirs. One endmember could be local/regional terrestrial plus
atmospheric deposition. Another could be upwelling Hg-DOC associated with pelagic
seawater in the open/global ocean, possibly characterized by mostly negative δ202Hg (-
2.08 ± 0.61 ‰) and positive Δ199Hg (0.21 ± 0.06 ‰; Fig. 4; Štrok et al., 2015). This
10 / 28
hypothesis is also supported by a negative correlation between δ13C of organic carbon
and Δ199Hg values in the lower part of the Niutitang Formation in the Tongren region
(R2 = 0.60, p<0.01; Wu et al., 2022), and it can easily explain elevated Hg (up to 1170
ppb) and TOC (mostly > 10%, up to 20%) in the lower Niutitang Formation deeper
shelf and slope shales. However, Hg isotopes of the lowest parts of the Shuijingtuo
(Qinjiang) and Yu’anshan (Chengjiang) Formations demonstrate lesser Hg-DOC
contributions from the open ocean (Fig. 4). Another possibility is that Hg isotope
signatures of Hg-DOC upwelling from pelagic ocean were overprinted by those of the
local terrestrial and atmospheric Hg. In summary, Hg enrichment in the deeper shelf
(Zhijin) and slope (Tongren) shales could be mainly attributed to Hg-DOC upwelling
from the open ocean, but Hg in the shallow shelf shales (Qingjiang, this study) could
be dominantly controlled by local atmospheric and possibly terrestrial inputs (Fig. 4).
5.2 Hg isotope excursions through the fossiliferous strata (~ 518 Ma)
Decreasing total Hg concentrations and negative Δ199Hg excursions after the Hg-
enrichment interval (i.e., through the fossiliferous strata, shaded blue in Fig. 2) are
observed in our investigated cores (Fig. 2A and B). These trends may reflect a change
of the dominant Hg source and/or the varying seawater redox conditions. For instance,
declines of total Hg and Δ199Hg (from 0.15 ‰ to -0.20 ‰) in the deeper shelf and slope
sections from the Nanhua Basin (Fig. 2C) and the Indian Craton have been attributed
to increasing terrestrial contributions (Liu et al., 2021b; Wu et al., 2022; Zhu et al.,
2021a). As mentioned earlier, Hg sourced from terrestrial weathering during
Precambrian time could be characterized by near-zero Δ199Hg and less negative δ202Hg
(e.g., -0.6 ‰ to -0.1 ‰) signatures due to the paucity of land plants (Meixnerová et al.,
2021). The fossiliferous strata in the Chengjiang core indeed correspond to a regression
episode, when periodic freshwater discharge transports important terrestrial material
into deltaic environments (Saleh et al., 2022). The up-section increase of δ202Hg and
negative or near-zero Δ199Hg values in the Chengjiang core (depth of 15–51 m) may
therefore indicate an increasing terrestrial contribution, which is compatible with
deposition in a very nearshore delta environment (Saleh et al., 2022). However, the
significant negative Δ199Hg excursions to values < -0.1 ‰ observed in the more distal
Qingjiang and slope Tongren sections (Fig. 2B and C; Wu et al., 2022; Zhu et al., 2021a)
could not be caused by the enhanced terrestrial contributions alone.
The occurrence of Hg isotope excursions in both the Nanhua Basin (this study) and
the Indian Craton (Liu et al., 2021b) may not be coincidental. Following existing
chemical stratigraphic correlations (Liu et al., 2021; Wu et al., 2022), these excursions
could be mostly isochronous. In turn, these excursions indicate a significant regional
and/or global change of Hg geochemical cycling could have taken place during this
time. Modern oceanic Hg biogeochemical cycling studies (Jiskra et al., 2021; Shah et
al., 2021) have demonstrated deposition of atmospheric Hg (0) and Hg (II) are the two
major sources of Hg in the open ocean. The mean Δ199Hg value of gaseous Hg (0) is
approximately 0.50 ‰ lower than that of atmospheric Hg (II) in modern environments
(Jiskra et al., 2021; Sun et al., 2019). Deposition of atmospheric Hg (0) in the ocean is
11 / 28
mainly controlled by water–atmosphere bi-directional diffusion and subsequent
photochemical, abiotic aphotic, and oxidation processes in surface waters, the latter of
which could be greatly enhanced in oxygenated seawater containing H2O2, chloride,
and Mn/Fe-oxides (Amyot et al., 2005; Lalonde et al., 2001; Siciliano et al., 2002;
Zhang et al., 2015). In addition, two studies suggested that the negative shifts of Hg
MIF in late Mesoproterozoic (∼1.1 Ga) and Neoproterozoic (635–551 Ma) shales were
most likely caused by the enhanced sequestration of atmospheric Hg (0) to sediments
by thiols and sulfide that were enriched in the surface ocean due to photic zone euxinia
(Zheng et al., 2018; Zheng et al., 2023). However, Fe species data indicated a
contraction of euxinic conditions and TOC/P ratios and Mn concentrations suggest an
expansion of oxic conditions in the Nanhua basin at that time (Li et al., 2021; this study).
Therefore, we propose that increased uptake of atmospheric Hg (0) by surface seawater
could be driven by enhanced oxidation of Hg (0) due to the transition from anoxic to
oxic conditions. During this process, Hg (0) was strongly adsorbed on the MnO2
surfaces via chemisorption mechanism and oxidized to Hg (I) and then Hg (II) (Zhang
et al., 2015; Zhang et al., 2014). Given the negative Δ199Hg signals of atmospheric Hg
(0) and the subsequent negative Hg (II) odd-mass independent fractionation (odd-MIF)
produced during oxidation processes (Zheng et al., 2019), enhanced adsorption of Hg
(0) by Mn-oxides and conversion to Hg (II) and, finally, removal to marine sediments
under oxidizing conditions is expected to contribute to the negative Δ199Hg excursions
recorded by fossiliferous strata. Total Hg concentrations remains at relatively low levels
because the sequestration of atmospheric depositional Hg (II) by organic matter could
have been strongly limited.
In addition to the precipitation pathways of atmospheric Hg (0), odd-MIF induced
by photochemical reduction of Hg (II) species, under anoxic or oxic conditions, is also
a potential trigger for changes in Δ199Hg values. Previous experimental studies show
that the photoreduction of Hg (II) complexed with S-donor ligands (Hg-SR) under
neutral-pH anoxic conditions, Hg (II) complexed with O-donor ligands (Hg-OR), and
methylmercury (MeHg) produce positive Δ199Hg in residual dissolved Hg (II) pools,
whereas photoreduction of Hg-SR under neutral pH oxic conditions and Hg (II)
complexed with Cl (Hg (II)-Cl) produce negative Δ199Hg in residual Hg pools
(Bergquist and Blum, 2007; Motta et al., 2020; Zheng and Hintelmann, 2010). In anoxic
seawater, Hg (II)-SR, Hg (II)-OR, and MeHg are the dominant Hg (II) species
(Fitzgerald et al., 2007), and the photoreduction of these Hg (II) species produces
positive odd-MIF in seawater (Bergquist and Blum, 2007; Motta et al., 2020; Zheng
and Hintelmann, 2010). By contrast, photoreduction of the increased Hg (II)-Cl
complexes and the relatively minor Hg (II)-SR complexes in neutral pH, oxic seawaters
would generate negative odd-MIF (Fitzgerald et al., 2007; Motta et al., 2020), which is
expected to offset or out-compete the positive odd-MIF generated by Hg (II)-OR
complexes during photoreduction, leading to less positive or even negative Δ199Hg in
seawater.
As a benthic redox proxy, decreasing TOC/P values to less than 10–20 (TOC/P
<30 in fully oxic Amazon shelf and Blake Plateau sediments; Algeo and Ingall, 2007),
12 / 28
together with increasing Mn concentrations, show a redox transition of both bottom and
surface water from anoxic towards oxic conditions after the Hg enrichment interval in
both the Chengjiang and Qinjiang cores (Fig. 2A and B). These trends are also
consistent with the decreasing Mo and U concentrations recorded in other shelf and
slope sections (Han et al., 2018; Li et al., 2021). Moreover, positive and negative
correlations of Δ199Hg values with TOC/P ratios and Mn concentrations, respectively,
in the Qingjiang core (Fig. 5A, B) could indicate increasing oxygen levels in local
seawater were an important factor controlling variations of Δ199Hg values. This can be
also evidenced by the same positive correlation between Δ199Hg and TOC/P ratios
recorded in the Chengjiang core (R2= 0.40, p<0.01). However, the absence of a negative
relationship between Δ199Hg and Mn concentrations in the Chengjiang core, could be
attributed to abundant terrestrial materials input (Mn-Al, R2= 0.52, p<0.01). The higher
concentrations of Mn-oxides (in the presence of Cl-) may stimulate the adsorption and
subsequently oxidation of Hg (0) to Hg (II) and removal into sediments (Zhang et al.,
2015; Zhang et al., 2014), although this mechanism could not induce extra Hg MIF.
During the transition from the anoxic period to the oxic period (evidenced by TOC/P
ratios and Mn concentrations in this study and Fe species, S, and U isotope data
reviewed by Li et al. (2021)), chemical effects of enhanced uptake of atmospheric Hg
(0) by oxygenated seawater and photoreduction of major Hg (II)-Cl and minor Hg (II)-
SR complexes would be greater than those of positive odd-MIF produced by
photoreduction of Hg (II)-OR complexes, causing an observed negative shift of Δ199Hg
in the Nanhua Basin and the Indian Craton. Therefore, an appreciable (i.e., 0.15–0.3 ‰)
negative Δ199Hg shift could reflect a significant oxygenation event after Hg enrichment
(i.e., during the deposition of fossiliferous strata), but the potential mechanism should
be investigated further.
We further speculated that this oxygenation hypothesis is supported by a
pronounced negative shift of δ202Hg values observed in most offshore sediments from
the Qingjiang (-0.53 ‰ to -2.74 ‰; Fig. 2B), Zhijin (-1.58 ‰ to -2.78 ‰) and Tongren
(-2.55 ‰ to -4.03 ‰; Fig. 2C) sections during deposition of the fossiliferous strata (Yin
et al., 2017; Zhu et al., 2021a). Although Hg MDF is complex and caused by different
physical, chemical, and biological pathways (e.g., Blum et al., 2014; Grasby et al.,
2019), clear correlations of δ202Hg with TOC/P ratios (R2= 0.58, p< 0.01) and Mn
concentrations (R2= 0.50, p< 0.01) are observed in the Qingjiang core (Fig. 5C, D).
These observations could indicate Hg MDF is also controlled by seawater redox
conditions, where the oxidation of Mn2+ and subsequent precipitation of Mn-oxides
could preferentially adsorb isotopically light Hg, resulting in more negative δ202Hg
values preserved by more oxic sediments. However, an experimental study suggested a
fractionation only up to approximately -0.4 ‰ during the adsorption of Hg (II) on Fe
oxides (Jiskra et al., 2012), which may be insufficient to induce a negative excursion of
-1.5 ‰ in the Qingjiang core. A recent study indicated that δ202Hg can be altered (0.3 ‰
to 0.7 ‰) during the thermal maturation process, and the direction of change can be
either positive or negative (Liu et al., 2022). Although the influence of the regional
thermal maturation processes could not be excluded, it is likely that Hg MDF could be
associated with the adsorption and oxidation of Hg (0) by Mn-oxides in the water
13 / 28
column and the desorption and re-adsorption (or reduction and re-oxidation) at the
water-sediments interface or in porewater. These processes have been invoked to
explain the extremely large Se isotope fractionation (-12.77 ‰ to 4.93 ‰) at the
chemocline (Wen and Carignan, 2011). The δ202Hg values in the Chengjiang core do
not show any correlation with TOC/P ratios and Mn concentrations (Fig. 2A), which
may suggest that the influence of terrestrial contributions to Hg MDF overwhelmed that
of redox conditions. Therefore, it is important to exercise caution when linking Hg
MDF to redox variations. No Mn concentrations and TOC/P ratios were reported for
the deeper water Zhijin and Tongren sections, but samples with the more negative
δ202Hg values usually show lower TOC concentrations in the Zhijin and Tongren
sections, suggesting oxidation of abundant organic matter by Mn-oxides or free oxygen
in the ancient water column. When this mechanism became the dominant fractionation
pathway, it would disrupt the pre-existing shallow water Hg isotope fractionation
system (atmosphere–ocean exchange layer), enhance adsorption and oxidation of Hg
(0) and out-compete photoreduction of Hg (II), during which less positive and even
negative Δ199Hg could be preserved in most offshore sediments.
5.3 Hg isotopes as a paleo-redox proxy of the Precambrian ocean and/or
atmosphere
Negative Δ199Hg values in Precambrian marine sediments have been explained by
the input of Hg via soil erosion (Deng et al., 2022a; Wu et al., 2022; Zhu et al., 2021a).
However, terrestrial Hg before the evolution of land plants could not show obviously
negative Δ199Hg values like modern soil (e.g., Fan et al., 2021). Instead, the negative
Δ199Hg values could be mainly produced by Hg cycling in seawater and atmosphere,
via the oxidation of Hg (0) by halogen compounds and/or H2O2 (Sun et al., 2022), and
the enhanced sequestration of atmospheric Hg (0) to the sediments by thiols and sulfide
(Zheng et al., 2018; Zheng et al., 2023). It is noteworthy that previous studies have
reported four negative excursions of Δ199Hg values during the Neoproterozoic,
including at ~ 850–760 Ma (0 ‰ to -0.21 ‰), ~ 660–650 Ma (0 ‰ to -0.28 ‰), ~ 570
Ma (0.01 ‰ to -0.35 ‰), and ~ 518–510 Ma (0 ‰ to -0.18 ‰) (Deng et al., 2022b; Fan
et al., 2021; Liu et al., 2021a; Sun et al., 2022). Among them, the negative Δ199Hg
values at ~ 660–650 Ma could be caused by oceanic and atmospheric oxygenation
through the release of photochemically produced H2O2 in melting glaciers in the
aftermath of the Sturtian glaciation, during that time abundant dissolved Fe was
oxidized to form banded iron formation (Sun et al., 2022). The ~ 570 Ma negative
Δ199Hg values are also corresponding to a global oceanic oxygenation event during the
Shuram carbon isotope excursion (Fan et al., 2021). The ~ 518–510 Ma negative Δ199Hg
values are associated with a local/global oceanic oxygenation event (This study). As
discussed in Section 5.2, δ202Hg values do not always show regular variations with
redox proxies such as Mn concentrations and TOC/P ratios, which could be due to
complex Hg MDF processes occurring in seawater and the atmosphere. However, most
Δ199Hg and δ202Hg values during these oxygenation events do not show normal source-
controlled negative correlations recorded in marine sediments. Therefore, we suggest
that Hg isotopes are a promising potential paleo-redox proxy of the Precambrian ocean
14 / 28
and/or atmosphere, but more further investigations are necessary.
5.4 Oceanic oxygenation stimulated Cambrian biodiversity development
In the Nanhua Basin, the occurrence of volcanic tuff at ~526 Ma in the Yunnan,
Guizhou, and Hubei Provinces indicates significant volcanism (Compston et al., 2008;
Okada et al., 2014). Extensive volcanism could release massive amounts of greenhouse
and reducing gases (such as CO2, H2S, and Hg) into oceans, causing high surface water
temperatures, ocean anoxia, and seawater acidification in shelf basins, all of which
could destroy oceanic ecosystems and possibly cause the extinction of small shelly
fauna in shallow waters, as well as localized, extreme Hg enrichment (Wang et al., 2020;
Wu et al., 2022). Volcanic Hg input could also result in near zero Δ199Hg and
homogenous δ202Hg signals (and extremely high Hg/TOC ratios) in relatively shallow
water sediments, which are not displayed by the data from the Chengjiang and
Qingjiang cores, nor the other deeper shelf and slope sections. However, it is important
to consider that upwelling currents could transport Hg-DOC-rich water masses from
pelagic seawater to clay-rich shallow seawater during transgressions, where clay
sediments efficiently mix with Hg-DOC, causing large amounts of Hg-DOC to be
adsorbed by clay sediments and preserved in deeper shelf and slope shales, such as the
Zhijin, Tongren, and Jishou sections (Fig. 6A). This mechanism has been recently
proposed as an alternative interpretation for the formation of organic-rich shales (Zhang,
2021), which would also stimulate and expand euxinic seawater conditions in the
Nanhua Basin and the Indian Craton through a complex interaction between continental
dissolved sulfate and upwelling DOC from the open ocean. The euxinic seawater would
inherently facilitate Hg precipitation as sulfide and inhibit biological activity. As such,
we argue (1) positive Δ199Hg and more negative δ202Hg signals in deeper shelf and slope
sediments (Zhijin and Tongren sections) could be mostly due to Hg-DOC upwelling,
and (2) high TOC concentrations (up to 20 %) recorded in most slope organic-rich
shales from the lowest part of the Niutitang Formation (~521 Ma) may not be directly
linked to locally elevated primary production, as proposed by previous studies (Cheng
et al., 2020; Jin et al., 2016). This hypothesis is also supported by low δ114Cd (~0.2 ‰)
values in the deeper shelf Zhijin section and species richness of acritarchs, which are
inconsistent with enhanced primary productivity during the Hg- and organic-
enrichment interval (Frei et al., 2021; Li et al., 2021).
In the aftermath of volcanism and upwelling of the Hg-DOC reservoir (Fig. 6B),
almost all redox sensitive trace elements (Mo, U, and V) and Hg show gradually
decreasing concentrations in marine sediments of the Shuijingtuo, Yu’anshan, and
Niutitang Formations. A possible cause of this is an oceanic oxygenation event during
the late Cambrian Age 2 to middle of Cambrian Age 3 (Li et al., 2017; Li et al., 2021),
which is also supported by decreasing TOC/P ratios and increasing Mn concentrations
in the Chengjian and Qingjiang cores. The negative shifts of Δ199Hg and δ202Hg through
the fossiliferous strata suggests enhanced adsorption and oxidation of Hg (0) by Mn-
oxides (as well as significant photochemical reduction of Hg (II) in near-neutral,
oxidizing shallow seawater and transport into deeper water sediments; Fig. 6B),
15 / 28
indicating a significant oceanic oxygenation event occurred in the Nanhua Basin and
the Indian Craton. Other redox proxy data (such as Mo and U isotopes) from elsewhere
in the world (i.e., Oman, Tarim, and Siberia) suggest this oxygenation event could be a
widespread, global period of oceanic oxygenation (reviewed in Wei et al., 2021).
Nevertheless, this oxygenation episode is not only coupled to the diversification of the
Chengjiang and Qingjiang Biotas in the Nanhua Basin, but it also coincides with a sharp
increase in global biodiversity of marine invertebrates during the Cambrian Age 3 (Na
and Kiessling, 2015). Furthermore, a positive δ114Cd shift of ~0.4 ‰ in the Niutitang
Formation in one shelf Zhijin section also indicates high primary productivity (Frei et
al., 2021). High primary productivity would also enhance free oxygen levels in the
shallow shelf waters and provide sufficient food and energy for animals (Sperling and
Stockey, 2018). However, the lower TOC could be caused by the strong degradation of
organic matter in an oxic seawater and/or the dilution of enhanced regional terrestrial
materials input. Finally, we argue that rising molecular oxygen in seawater, associated
with enhanced marine primary productivity, may have been a critical driver of the
Cambrian explosion.
6. Conclusions
In this contribution, we demonstrate the utility of coupled Δ199Hg and δ202Hg
signals in marine sediments (where Hg is dominantly sourced from the atmosphere) as
a seawater, and possibly atmospheric, paleoredox proxy, especially during periods
before the evolution of land plants. Furthermore, we provide compelling evidence for
a significant, large-scale, or even global oxygenation event during the summit of the
Cambrian explosion. We also propose oxygenation of surface to deep seawater, induced
by the enhanced primary productivity, may have facilitated the Cambrian explosion.
Although the extent of global oxygenation during this time needs to be examined further,
our data and interpretations add evidence and new details strengthening the mechanistic
relationship between rising oxygen levels and the Cambrian explosion.
ACKNOWLEDGMENTS
This research was funded by the NSFC (41890841, U1812402, 42073016, 92062221,
42121003), the CAS (ZDBS-LY-DQC029) and the Guizhou Provincial 2020 Science
and Technology Subsides (GZ2020SIG). We give thanks to Linhao Cui and Chao
Chang for their assistance during sampling, to Yan’an Shen and Maoyan Zhu for
improving manuscript and to editors and reviewers for their thoughtful and thorough
reviews of our manuscript.
Appendix A. Supplementary Material.
The supplementary material includes Tables S1–S3, which provide Hg isotope data and
16 / 28
other elemental concentrations (Hg, TOC, TS, Mn, P) from this study.
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Figure captions
Fig. 1 A. Early Cambrian geographic map of South China (Fu et al., 2019a) showing
the locations of investigated two shallow water cores (Chengjiang and Qingjiang), as
well as the deeper shelf Zhijin (Yin et al., 2017), slope Tongren (Zhu et al., 2021a), and
Jishou sections (Wang et al., 2020) discussed in this study. B. Simplified litho-, bio-
and chrono-stratigraphy of investigated cores.
Fig. 2 Hg concentrations and isotope compositions, Hg/TOC, Mn, and TOC/P
chemostratigraphy of the two newly-analyzed cores (A, Chengjiang; B, Qingjiang) and
another previous published section (C, Tongren, Zhu et al., 2021a) covering ~521–514
Ma. The legends of stratigraphic column can be found in Fig.1.
Fig. 3 Total Hg concentrations plotted against total organic carbon (TOC, A) and total
sulfur concentrations (TS, B) of organic-rich shales from the Nanhua Basin. All blue
lines for Chengjiang samples and all red lines for Qingjiang samples. For all samples,
total Hg positively correlates with TOC, but total Hg does not positively correlate with
total sulfur in most slope shales.
Fig. 4 Correlations of Δ199Hg and δ202Hg in Nanhua Basin shales. Relationships could
indicate local atmospheric and/or terrestrial contributions in the shallow water locations
during Hg-enrichment interval prior to the occurrence of Chengjiang and Qingjiang
Biotas, as well as Hg associated with dissolved organic carbon (Hg-DOC) upwelling
from the pelagic seawater to deeper shelf and slope regions. The Zhijin and Tongren
sections are taken from refs. (Wu et al., 2022; Yin et al., 2017; Zhu et al., 2021a).
Fig. 5 Δ199Hg and δ202Hg plotted against TOC/P ratios and Mn concentrations in the
Qingjiang core. The data indicate redox conditions of the seawater column have
strongly controlled Hg isotopic variations. Three samples with abnormal relation
between TOC/P ratios and Mn concentrations are not included.
Fig. 6 (A) Hg geochemical cycling and fractionation of Hg isotopes in an anoxic
seawater column before Cambrian radiation, accompanied by significant terrestrial
input in shallow water shelf and Hg-DOC upwelling from the pelagic ocean in deeper
shelf and slope locations, and (B) an oxidizing seawater column occurring during and
slightly before deposition of the fossiliferous strata. (↑) and (↓) indicate that the
processes and/or relative fractions of Hg (II) species in total Hg (II) are increasing and
decreasing, respectively.
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Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be
considered as potential competing interests:
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