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. 2024 Apr 25;9(18):19892–19903. doi: 10.1021/acsomega.3c09161

Reduction in Marine Primary Productivity in the Early Cambrian Nanhua Basin, South China

Chengsheng Jin †,*, Zihu Zhang ‡, Meng Cheng ‡, Guochang Wang §, Huajin Chang †, Zhengqi Cao †, Tao Zhang †,*
PMCID: PMC11080022  PMID: 38737017

Abstract

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The Cambrian explosion is represented by a rapid diversification of early animals in which the role of marine primary productivity remains obscure. In this study, we analyzed multiple geochemical data, including TOC, major, and trace elements, in the basinal Yuanjia section, South China. Covariations among TOC, P/Al, CuEF, and NiEF suggest that they could be taken as effective marine productivity proxies in the early Cambrian Nanhua Basin. The similarities of CdEF and Cd/Mo in the Nanhua Basin and modern upwelling settings suggest that they might be effective to track upwelling, where Cd and Mo were mainly controlled by plankton biomass and redox conditions, respectively. Our results indicate that CoEF and Co × Mn were invalid in evaluating upwelling because of the significant effects of water-column redox conditions on Co enrichments in the Nanhua Basin. The decreased TOC, P/Al, CuEF, and NiEF reflect a long-term decline in marine productivity from late age 2 to age 3. In comparison with the published results in the outer shelf (Jinsha, TZS drill core, YJK drill core, and GDM-1 well) and slope areas (TX-1 well), the fall in marine productivity might be common in the early Cambrian Nanhua Basin. Our results exhibit that the reduced marine productivity was accompanied by weakened upwelling, quiet hydrothermal activities, and enhanced local terrestrial fluxes, indicating that variations in marine productivity might be mainly driven by the development of upwelling in the early Cambrian Nanhua Basin. Comparison of marine productivity with fossil records suggests that food availability was sufficient to sustain the Cambrian explosion in the Nanhua Basin. We infer that marine productivity might indirectly stimulate early animal evolution through its significant impact on water-column oxygen levels in the early Cambrian Nanhua Basin.

1. Introduction

The early Cambrian witnessed the occurrences of almost all metazoan phyla for the first time in Earth’s history, which is widely known as the Cambrian explosion.1,2 It is well accepted that large, active animals have higher physiological requirement for oxygen relative to small, inactive animals.3 Therefore, the enhanced oxygenation of the oceanic systems has been widely invoked for metazoan evolution during the early Cambrian.3 On the other hand, a shift in primary producers from small, cyanobacteria-dominated phytoplankton to large, eukaryotic-dominated phytoplankton occurred in the early Cambrian Ocean, which might significantly enhance the complexity of food webs.4 Accordingly, marine productivity has been frequently taken as another important trigger for the Cambrian explosion.5 Moreover, marine redox conditions are closely connected with marine productivity on local and global scales.6,7 For example, local marine redox conditions are strongly affected by the availability of organic matter that might be driven by local marine productivity.6 Meanwhile, marine redox conditions globally might be controlled by atmospheric oxygen levels, which might possibly result from the burial of organic matter in association with marine productivity.7 For these reasons, the possible oxygen and productivity triggers can be strongly linked during the Cambrian explosion.

There are a large number of research studies on oceanic oxygen levels during the early Cambrian.8−10 In contrast, a few studies focused on marine productivity in the early Cambrian, yielding some controversial results. For example, compilations of total organic carbon (TOC) and δ15NTN in the sediment suggest a rise in marine productivity, resulting in the Cambrian explosion.5,11 However, δ15NTN data indicate that both Chengjiang and Qingjiang biotas thrive in low productivity even oligotrophic environments during the peak Cambrian explosion, making the influence of marine productivity on the Cambrian explosion unclear.12

The early Cambrian marine productivity has been considered to be controlled by many factors, particularly the terrestrial input, upwelling, and hydrothermal activity, which can separately supply nutrients from the continent, deep ocean, and hydrothermal fluids to the oceanic euphotic zone, respectively. The terrestrial fluxes can be evaluated by continental basement rocks, precipitation of carbonate sediments, proportion of glauconite-rich siliciclastic rock units, 87Sr/86Sr, and εNd.13 The developments of upwelling can be constrained by the proxies of Co × Mn and Cd/Mo in organic-rich sediments.14,15 The signals of hydrothermal fluids can be documented by multiple geochemical proxies, such as Eu/Eu*,16−18 Fe/Ti,19,20 Al/(Al+Mn+Fe),19−21 Hg contents, and Hg isotopes.17

In this study, we report geochemical data of paleoproductivity (CuEF, NiEF, and P/Al), hydrothermal activities (Fe/Ti, Al/(Al+Mn+Fe), and Eu/Eu*), and local terrestrial fluxes (Al and Th) from the basinal Yuanjia section in South China. In combination with previously published data from the same section and other contemporary sections in the Nanhua Basin, this study provides new insights into the fluctuations of marine productivity and their causes during the early Cambrian.

2. Geological Setting

2.1. Paleogeography and Study Sections

The South China craton was located in the northwest of the Gondwana supercontinent during the early Cambrian (Figure 1A).22,23 It included the Yangtze Block in the northwest and the Cathaysia Block in the southeast (Figure 1B).24 The early Cambrian Yangtze Block was composed of shelf, transitional, and slope–basinal settings from the northwest to the southeast (Figure 1B). The shelf facies comprised dolomite, limestone, and phosphorite from Cambrian Fortunian Age to early Age 2, whereas the coeval slope–basinal facies were mainly composed of chert.9 The transitional zone consisted of dolomite and limestone interbedded with shale during the same period.25 During late age 2, the whole Yangtze Block was almost covered by black shale due to a marine transgression.9

Figure 1.

Figure 1

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Locality and paleogeography of South China during the early Cambrian. (A) Global paleogeography.23 Copyright 2015 Elsevier Ltd. (B) Paleogeography of South China.24 Copyright 2007 Elsevier Ltd. Sections: 1—Jinsha, 2—TZS drill core, 3—JYK drill core, 4—GDM-1 well, 5—TX-1 well, and 6—Yuanjia.

The Yuanjia section (27°29′25″N, 110°14′46″E) is located in Huaihua City, Hunan Province, which is deposited in a basinal setting. In ascending order, the lower Cambrian strata at Yuanjia comprise the upper Liuchapo, Niutitang, and Jiumenchong formations (Figure 2). All samples are collected from the Niutitang Formation because the coeval strata record many geological events, such as the radiations of early animals, enhanced ocean oxygenation, and high marine productivity. The Niutitang Formation comprises black shale, a Ni–Mo layer, and two layers of phosphate nodules, where a few sponge spicules were preserved.26 In this study, two obvious members can be recognized by changes in TOC, P/Al, CuEF, NiEF, and Cd/Mo in the Niutitang Formation (Figures 3 and 4). The lower member (LM, 0–23.5 m) shows high TOC (average 14.9%), P/Al (average 50.4 × 10–3), CuEF (average 49.9), NiEF (average 49.9), and Cd/Mo (average 0.3437). The upper member (UM, 23.5–65 m) reveals moderate TOC (average 12.1%), P/Al (average 10.1 × 10–3), CuEF (average 11.6), NiEF (average 8.2), and Cd/Mo (average 0.0663).

Figure 2.

Figure 2

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Stratigraphic framework in the early Cambrian Nanhua Basin. Sections: 1—Jinsha,9 2—TZS drill core,12 3—JYK drill core,12 4—GDM-1 well,16 5—TX-1 well,16 6—Yuanjia (this study). The U–Pb age of 526.43 ± 0.54 Ma is from Yang et al.33 The Re–Os age of 521 ± 5 Ma for the Ni–Mo layer is from Xu et al.35 The age of 515.41 ± 1.91 Ma based on cyclostratigraphic analysis is from Zhang et al.39 Abbreviations: ED—Ediacaran; F-S2—Fortunian-Stage 2; DY—Dengying Formation; YJH—Yanjiahe Formation; MXS—Mingxinsi Formation; L—Liuchapo Formation; LB—Laobao Formation; JMC—Jiumenchong Formation; LM—Lower member; and UM—Upper member.

Figure 3.

Figure 3

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Geochemical results for productivity and redox conditions are given in the Yuanjia section. FeHR/FeT < 0.22 and >0.38 point to oxic and anoxic conditions, respectively.14 When anoxia is indicated, Fepy/FeHR < 0.7–0.8 and >0.7–0.8 reflect ferruginous and euxinic states, respectively.14 The redox interpretations of Fe speciation proxies are effective for sedimentary rocks with FeT >0.5%. TOC, Fe speciation, MoEF, and UEF are from Chen et al.14 and Cheng et al.40

Figure 4.

Figure 4

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Geochemical results for upwelling, terrigenous fluxes, and hydrothermal activity are given in the Yuanjia section. The distinguishment between hydrothermal activities (Al/(Al+Fe+Mn) < 0.4) and terrigenous fluxes (Al/(Al+Fe+Mn) > 0.4) are according to Sylvestre et al.61

2.2. Stratigraphic Correlations for the Study Sections

The stratigraphic correlations have been roughly established in the early Cambrian Nanhua Basin based on index fossils, lithostratigraphic markers, and U–Pb ages (Figure 2).9 The Ediacaran/Cambrian boundary at ∼539 Ma lies at the top of the Dengying Formation based on extensive occurrences of the Basal Cambrian Carbon Isotope Excursion (BACE) in the Nanhua Basin and other areas globally.27 This boundary is placed within the Liuchapo Formation based on (1) the appearances of small shelly fossils at Longbizui28 and (2) a series of similar U–Pb ages, for example, 545.76 ± 0.66 Ma at Longbizui,29 542.6 ± 3.7 Ma at Bahuang,30 and 541.48 ± 0.46 Ma at Pingying.31

The lithological change from dolostone or chert to black shale at the base of the Niutitang and Shuijingtuo formations has been taken as a correlation marker based on some similar U–Pb ages at the base of the organic-rich sequence, for example, 526.5 ± 1.1 Ma at Meishucun,32 526.43 ± 0.54 Ma at Yanjiahe,33 522.3 ± 3.7 Ma at Bahuang,30 and 522.7 ± 4.9 Ma at Taoying.34 The Ni–Mo layer has been regarded as another lithological marker for the boundary between Cambrian ages 2 and 3 (∼521 Ma), which is dated to 521 ± 5 Ma by Re–Os isotopes.35

An obvious drop in the TOC concentration is widely recognized in the middle Niutitang and Shuijingtuo formations (Figure 2). The TOC decline has been considered to be a correlation marker because it might respond to a fall in sea level in the Nanhua Basin. This stratigraphic correlation is supported by the appearances of the contemporary fossils in the Jinsha section, TZS drill core, and YJK drill core.36 For example, Zhenbaspis at Jinsha and Qingjiang fossils at both TZS drill core and YJK drill core appear in the Wutingaspis–Eoredlichia Assemblage Zone.37 The lithological shift from mudstone, silty shale, or shale to limestone at the end of the Niutitang Formation (or the upper Shuijingtuo Formation) has been taken as a correlation marker in the Nanhua Basin6,9,38 because it might be in response to a continued marine regression.9,38 This lithological transition is accompanied by a change in the trilobite zone from the Wutingaspis–Eoredlichia to Yunnanaspis–Yiliangella Assemblage Zone,36 which is dated to 515.41 ± 1.91 Ma based on the cyclostratigraphic analysis.39

3. Materials and Methods

A total of 47 samples were collected at Yuanjia for geochemical analyses. TOC, Al, redox proxies (Fe speciation, MoEF, and UEF), and upwelling proxies (Co × Mn and Cd/Mo) were discussed in the previous studies.14,40 Nutrient-related elements (P, Cu, and Ni) of 29 samples were analyzed in a previous study.40 In this study, P for 17 new samples and Ti for all samples were conducted at the China University of Geosciences (Wuhan). Trace elements (Th, Cu, Ni, Ba, and Nd) and rare earth elements (REEs) were measured for 17 new samples at Nanjing FocuMS Technology Co.

For Ti and P analyses, approximately 0.6 mg of sample powder was weighed into a porcelain crucible. Approximately 6 g of Li2B4O7 was added to the platinum crucible, which was well mixed with the sample powder by a porcelain spoon. The mixture was transferred into a platinum crucible, which was dissolved at 1080 °C. The platinum crucible was cooled to form a glass disk for Ti and P analyses using X-ray fluorescence (XRF) spectrometry. The long-term analyses of two standards (i.e., GBW07107 and GBW07108) yield an analytical error of less than 10%.

For nutrient elements and REEs, approximately 0.3 g of sample powder was heated at 150 °C for 12 h in order to remove volatiles. Approximately 50 mg of the dried sample was weighed into a Teflon bomb. Following the successive additions of 1 mL HNO3 and 1 mL HF, the Teflon bomb was put into a steel sleeve, which was heated at 190 °C for above 24 h. The Teflon bomb was opened after cooling and was heated at 140 °C on a hot plate. When the solution was completely evaporated, 1 mL HNO3 was added to the Teflon bomb. In order to make the solution entirely evaporate again, the Teflon bomb was heated at 140 °C. The Teflon bomb was put into a steel sleeve again with the successive additions of 1 mL HNO3, 1 mL MQ water, and 1 mL ln, which was heated at 190 °C for above 12 h. The final solution was transferred to a polyethylene bottle, which was diluted to 100 g through 2% HNO3. The diluted solution was analyzed using an Agilent 7500a quadrupole inductively coupled plasma mass spectrometer. The long-term tests of two standards (i.e., BCR-2 and GBW07108) produce an analytical error of less than 5%. Enrichment factors (EFs) are calculated as the ratios of elements to Al in samples compared to the average upper continental crust (AUCC), as XEF = (X/Al)sample/(X/Al)AUCC, where X refers to the interested elements (e.g., Cu and Ni), and elemental values of AUCC are from McLennan.41

4. Results

All geochemical data are listed in the Supporting Information, and the key proxies are summarized in Figures 3 and 4. In the study section, TOC exhibits a general decrease from the LM to the UM, varying between 6.4 and 35.5% (average 13.1%). CuEF ranges from 1.3 to 196 (average 24.4), which is high in the LM (8.6–196, average 49.9) compared to the UM (1.3–196, average 24.4). NiEF varies between 2.0 and 74.4 (average of 13.5), which shows a negative shift from the LM to the UM. P/Al declines from the LM to the UM, ranging from 1.7 × 10–3 to 256 × 10–3 (average 24.5 × 10–3). Al ranges from 0.9 to 5.9% (average 3.6%), which is low in the LM (0.9–5.9%, average 2.9%) relative to the UM (2.2–5.4%, average 3.9%). Th varies between 2.7 and 8.5% (average 6.2%), which is low in the LM (2.7–8.5%, average 4.9%) compared to the UM (4.1–8.4%, average 6.7%). Al/(Al+Fe+Mn) ranges from 0.40 to 0.93 (average 0.61), which shows a slight increase from the LM to the UM. Fe/Ti varies between 0.60 and 18.1 (average 8.88), which increases from the LM to the UM. Eu/Eu* is stable from the LM to the UM, ranging from 0.89 to 1.46 (average 1.05). Ce/Ce*varies between 0.38 and 1.04 (average 0.74). Y/Y* ranges from 1.15 to 1.75 (average of 1.47).

5. Discussion

5.1. Reconstruction for Marine Productivity in the Early Cambrian Nanhua Basin

5.1.1. Productivity Proxies

TOC is frequently used to evaluate marine productivity owing to its direct relationship with primary producers.42 In the photic zone, primary producers absorb carbon dioxide to yield organic matter through photosynthesis. A part of the organic matter in the photic zone is consumed by producer respiration and animal predation, and the rest is sunk into the deep ocean. In general, a majority of organic matter is degraded through bacterial respiration from the surface ocean to the sediment–water interface,43 causing a small portion of organic matter to reach the sediment.44 The residual organic matter is substantially consumed by diagenetic processes, which can be preserved in the sedimentary rock.45 Therefore, the analyzed TOC values might be affected by other factors (e.g., detrital dilution, redox conditions, and sedimentation rates) except for marine productivity.46 Strong detrital input can dilute organic matter in the sediment.6 Organic matter tends to be well preserved in reducing conditions rather than under oxic conditions, which is because of the substantial loss through aerobic relative to anaerobic respiration.42 Rapid sedimentation accelerates the isolation of organic matter from the oxic seawater or porewater, enhancing the preservation of organic matter.47

As a nutrient element, P is taken as an effective productivity proxy.48 P is an indispensable nutrient for the growth of marine phytoplankton, which is an important composition of each organism. It is sunk to the sediment–water interface with organic remains after the death of organisms, and thus, P in the sediment is strongly associated with marine productivity in the surface ocean.42 The fate of P in the sediment is primarily controlled by bottomwater redox conditions. In oxic states, P released via the remineralization of organic matter is adsorbed onto Fe-oxyhydroxides or biologically sequestrated into polyphosphates.49 In reducing states, Fe-oxyhydroxides are dissolved.50 For these reasons, under reducing conditions, P is generally released from the sediments to the overlying water columns.

As micronutrient elements, Cu and Ni are usually applied to evaluate primary productivity.51,52 Cu and Ni are absorbed by organisms to yield organic matter along with the photosynthesis, and they have a good relationship with organic matter in the surface ocean.53 However, Cu and Ni are also profoundly affected by the water-column redox conditions. Under oxic conditions, a majority of Cu exists in the form of organic–metal ligands, and the rest is present as CuCl+.49 Cu is retained in the sediment through both processes of adsorption onto Fe–Mn-oxyhydroxides and complexation with organic matter.54 Under anoxic conditions, Cu2+ is reduced to Cu+, which is deposited as CuS, Cu2S, or incorporated in FeS2.49 In oxic states, Ni exists as Ni2+ or NiCl+.54 It is retained in the sediment through the complexation with organic matter. In mildly reducing states, Ni can diffuse from the sediments to the overlying seawaters owing to the lack of Mn oxides and sulfides.49 In strong reducing states, Ni is precipitated as NiS.55

5.1.2. Primary Productivity at Yuanjia

The productivity proxies can be significantly affected by local water-column redox conditions (see Section 5.1.1). Accordingly, local redox states should be taken into consideration before the exploration of the primary productivity. Fe speciation (i.e., FeHR/FeT and Fepy/FeHR) and redox-sensitive elements (i.e., MoEF and UEF) suggest persistently euxinic conditions at Yuanjia (Figure 3).14 The relatively stable redox conditions are accompanied by the general decreases in productivity proxies from the LM to the UM (Figure 3), suggesting that local redox conditions play minor roles in the fluctuations of the productivity proxies.

The covariations in TOC and P/Al are usually applied to distinguish redox and productivity effects. In redox-dominated systems, TOC is generally negatively correlated with P/Al, which mainly reflects their reverse behaviors under reducing conditions.42 In contrast, TOC tends to show a positive correlation with P/Al in productivity-dominated systems because they are commonly controlled by marine productivity.42 In the Yuanjia section, a weakly positive relationship between TOC and P/Al may indicate that marine productivity is first-order control on their variations (R2 = 0.27; Figure 5A). Therefore, the generally downward trends in the TOC and P/Al record a long-term decrease in marine productivity from the LM to the UM at Yuanjia (Figure 3).

Figure 5.

Figure 5

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TOC versus P/Al (A) and CuEF (B), P/Al versus CuEF (C), TOC versus NiEF (D), P/Al versus NiEF (E), and CuEF versus NiEF (F) in the Yuanjia section.

Micronutrients (Cu and Ni) can track marine productivity based on their relationships with TOC and P/Al. In the Yuanjia section, the TOC shows no correlation with CuEF (R2 = 0.06; Figure 5B), suggesting that Cu accumulation is not directly controlled by sedimentary organic matter. P/Al exhibits a slightly positive relationship with CuEF (R2 = 0.16; Figure 5C), implying that marine productivity has significant effects on Cu accumulation. The weakly positive correlation between TOC and NiEF (R2 = 0.32; Figure 5D) is indicative of productivity control on Ni accumulation, which is consistent with the moderately positive relationship between P/Al and NiEF (R2 = 0.58; Figure 5E). A strong positive correlation is recorded between CuEF and NiEF (R2 = 0.80; Figure 5F), given that they are mainly associated with marine productivity. For these reasons, the reduced TOC, P/Al, CuEF, and NiEF point to a decline in marine productivity from the LM to the UM (Figure 3).

5.1.3. Marine Productivity at Other Coeval Sections in the Nanhua Basin

To reconstruct marine productivity at a broader scale, previously reported productivity results from the outer-shelf Jinsha section,6 TZS drill core,12 YJK drill core,12 GMD-1 well,16 and slope TX-1 well16 are compiled (Figure 6). These data provide a potential to evaluate the temporal changes in marine productivity across the early Cambrian Nanhua Basin. Different proxies are used to explore marine productivity of the compiled sections because of the data availability, i.e., TOC and organic carbon accumulation rate (OCAR) at Jinsha,6 TOC and δ15NTN in TZS and YJK drill cores,12 TOC, P/Al, CuEF, and NiEF in GMD-1 and TX-1 wells16 (Figure 6). It should be noted that OCAR is frequently used to assess marine productivity because of their positive relationship in modern oceans.42 Sedimentary δ15NTN data can possibly trace marine productivity owing to its strong connection with marine nitrogen cycle.12

Figure 6.

Figure 6

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Comparisons of marine productivity in the Yuanjia section to other sections in the Nanhua Basin. Productivity and redox sources: 1—Jinsha,6 2—TZS drill core,12 3—JYK drill core,12 4—GDM-1 well,16 5—TX-1 well,16 and 6—Yuanjia (this study).

In the Jinsha section, the LM exhibits high productivity on the basis of elevated TOC (2.2–8.6%, average 4.6%) and OCAR (57.9–230 mg cm–2 kyr–1, average 123 mg cm–2 kyr–1), but the UM shows low productivity according to negligible TOC (0–0.6%, average 0.1%) and OCAR (5.0–158 mg cm–2 kyr–1, average 28.2 mg cm–2 kyr–1).6 Therefore, the TOC and OCAR suggest a fall in marine productivity from the LM to the UM.

In the TZS drill core, the LM is characterized by high TOC (2.0–9.8%, average 4.1%) and low δ15NTN (−4.2 to +1.0 ‰, average −0.7 ‰), whereas the UM reveals low TOC (0.3–3.3%, average 1.3%) and δ15NTN (−3.3 to −0.1‰, average −1.6‰).12 In the YJK drill core, the LM shows high TOC (3.0–8.4%, average 4.5%) and low δ15NTN (−4.4 to +1.1‰, average −1.1‰), but the UM exhibits low TOC (0.5–6.7%, average 1.7%) and δ15NTN (−0.9 to +1.1‰, average 0.2‰).12 In both TZS and YJK drill cores, low δ15NTN data of anoxic samples in the LM suggest that the marine nitrogen cycle is dominated by quantitative assimilation of upwelled NH4+ and intense nitrogen fixation, which results from expansions of anoxic waters into the photic zone. The developments of widespread anoxic conditions are driven by elevated marine productivity as evidenced by high TOC. On the contrary, low δ15NTN data of oxic–suboxic samples in the UM suggest that the marine nitrogen cycle is dominated by biological N2 fixation, which is caused by the limited nitrate availability in association with low marine productivity based on low TOC. In summary, TOC and δ15NTN suggest a decrease in marine productivity from the LM to the UM.

In the GDM-1 and TX-1 wells, TOC, P/Al, CuEF, and NiEF can reflect changes in marine productivity according to their positive covariations, which are similar to those in the Yuanjia section (Figure 6). In the GDM-1 well, the LM exhibits high productivity based on elevated TOC (4.0–10.7%, average 5.9%), P/Al (10.6 × 10–3–46.1 × 10–3, average 16.5 × 10–3), CuEF (2.6–33.2, average 7.5), and NiEF (2.2–19.3, average 7.6), but the UM reveals low productivity as documented by low TOC (0.7–3.7%, average 2.4%), P/Al (6.3 × 10–3–11.5 × 10–3, average 8.9 × 10–3), CuEF (1.9–5.4, average 3.9), and NiEF (0.8–3.4, average 1.9).16 In the TX-1 well, the LM shows high productivity according to elevated TOC (4.1–6.8%, average 5.5%), P/Al (10.0 × 10–3–494 × 10–3, average 81.2 × 10–3), CuEF (4.6–48.3, average 15.5), and NiEF (3.5–19.5, average 8.3), but the UM reveals low productivity on the basis of low TOC (0.7–1.7%, average 1.3%), P/Al (2.2 × 10–3–5.1 × 10–3, average 3.9 × 10–3), CuEF (1.9–3.0, average 2.4), and NiEF (1.0–1.8, average 1.3).16 In short, the decreases in TOC, P/Al, CuEF, and NiEF suggest a decrease in marine productivity from the LM to the UM.

5.2. Possible Factors Controlling Marine Productivity in the Early Cambrian Nanhua Basin

5.2.1. Upwelling

Cd and Cd/Mo are usually used to distinguish marine upwelling from restricted environments. Cd and Mo are, importantly, influenced by water-column redox conditions. Under oxic conditions, Cd and Mo are present as Cd2+ and molybdate (MoO42–), respectively.49 In the presence of H2S, Cd is present in the form of CdS, and Mo forms particle-reactive thiomolybdates (MoO4–xSx2–, x = 1–4).54 On the other hand, Cd is also linked to marine productivity, which reveals a nutrient-type profile from surface to deep waters.51 However, Mo is conservative in seawater, which exhibits an obvious contrast to nutrient-like elements (e.g., Cd).56 Hence, the modern upwelling sediments show high CdEF as a consequence of a huge supply of plankton material in rich of Cd under elevated productivity,57 which also exhibits elevated Cd/Mo owing to more effective fluxes of Cd to the sediments compared to Mo.56 In contrast, the modern restricted environments reveal low CdEF and Cd/Mo because of the limited Cd availability and similar fluxes of Cd and Mo to the sediment, respectively.56,57

Co, Mn, and Co × Mn are frequently applied to assess marine upwelling and restricted environments. Co and Mn are associated with water-column redox conditions. Under oxic conditions, Co is soluble as Co2+, but Mn is insoluble as Mn(III) or Mn(IV) hydroxides or oxides.49 Under anoxic conditions, Co is present as CoS, and Mn is converted to Mn2+ or MnCl+.54 Co and Mn behaviors are different in marine upwelling and restricted environments. In upwelling environments, Co and Mn enrichments mainly depend on detrital minerals, which are generally depleted in deep waters.56 Thus, the upwelling environments are characterized by low CoEF, MnEF, and Co × Mn owing to the limited fluxes of Co and Mn from deep to surface waters.56,57 In restricted settings, the supplies of Co and Mn are mainly controlled by high riverine fluxes.56 Hence, the restricted settings show high CoEF, MnEF, and Co × Mn due to enough supplies of both Co and Mn from river waters.56,57

The upwelling proxies are significantly associated with water-column redox conditions, and thus, the redox effects should be explored prior to the applications of the upwelling proxies. Fe speciation and redox-sensitive elements suggest relatively stable redox states (i.e., persistently euxinic conditions) in the Yuanjia section (Figure 3). This redox variation is consistent with the decreased trends of CdEF and MnEF (Figure 4), suggesting that marine redox conditions have minor effects on changes in CdEF and MnEF. However, water-column redox conditions play key roles in CoEF because of their similar changes (Figures 3 and 4), suggesting that CoEF and Co × Mn are not valid to assess marine upwelling.

Our results suggest the weakened upwelling from the LM to the UM. In the LM, euxinic samples are characterized by high CdEF (37.2–3030, average 904) and low MnEF (0.01–0.92, average 0.23) (Figure 7), which have been discovered in modern coastal upwelling environments instead of modern restricted environments.54 Strong upwelling is supported by high Cd/Mo (0.0154–1.1885, average 0.3437) (Figure 4). In the UM, euxinic samples show moderate CdEF (2.4–1694, average 161) and low MnEF (0.04–0.62, average 0.18), suggesting weak upwelling (Figure 7). The reduced upwelling is supported by low Cd/Mo (0.005–0.7532, average 0.0663) (Figure 4).

Figure 7.

Figure 7

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Patterns of elemental enrichments in the Yuanjia section. Elemental enrichments of costal upwelling environments and restricted environments are sourced from Brumsack.57

The appearance of an upwelling system in the early Cambrian Nanhua Basin might be linked to paleogeographic and climatic conditions. The Nanhua Basin was suited in the low to midpaleolatitudes (0–30° N) of the northern hemisphere (Figure 1A). This range of paleolatitudes might allow the northeaster trade winds in the subtropical arid zone to develop in the Nanhua Basin, where the cool and arid climate is supported by low chemical index of alteration (CIA)58 and high Ti/Al.59 A rise of trade winds stimulates the developments of the oblique offshore currents, leading to the occurrence of extensive upwelling in the Nanhua Basin.59

5.2.2. Hydrothermal Activity

Al/(Al+Mn+Fe) and Fe/Ti are commonly applied to evaluate hydrothermal activity.20 In general, elevated Fe and Mn are associated with hydrothermal activity, whereas high Al and Ti are attributed to terrigenous fluxes.19 Hence, Al/(Al+Mn+Fe) < 0.4 and Fe/Ti > 20 are generally indicative of hydrothermal activity, but Al/(Al+Mn+Fe) > 0.4 and Fe/Ti < 20 usually point to terrigenous fluxes.60,61 More visually, an Fe–Al–Mn ternary diagram can distinguish the hydrothermal and nonhydrothermal origins.62

The REE pattern is another effective proxy to track hydrothermal activity. Hydrothermal fluids show depletion in heavy REEs (HREEs), positive Eu anomaly, and negligible negative Ce anomaly.63 In contrast, REE patterns of seawaters are characterized by enrichment in HREEs, significantly negative Ce anomalies, no obvious Eu anomalies, and positive Y anomalies.64,65 Positive Eu anomaly can be caused by the Ba interference during analysis, which can be tracked by the relationship between Ba/Nd and Eu anomalies.66,67

In this study, all samples are characterized by high Al/(Al+Mn+Fe) (0.40–0.93, average 0.61) and low Fe/Ti (0.60–18.11, average 8.67) (Figure 4), suggesting a minor influence from hydrothermal input. This explanation is consistent with a Fe–Al–Mn ternary diagram indicative of nonhydrothermal activities (Figure 8). All samples show seawater-like REE patterns, as suggested by enrichment in HREE, negative Ce anomalies (0.38–1.04, average 0.74), and positive Y anomalies (1.15–1.75, average 1.47) (Figure 9). Given the weakly positive correlation between Ba/Nd and Eu/Eu* (R2 = 0.29; Figure 10), the positive Eu anomalies (up to 1.46) of some samples might be affected by Ba interference. In summary, there are almost no hydrothermal fluids in the study section.

Figure 8.

Figure 8

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Al–Fe–Mn ternary diagram in the Yuanjia section. The distinguish between hydrothermal and nonhydrothermal origins is according to Yamamoto.62

Figure 9.

Figure 9

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PAAS-normalized REE patterns are in the Yuanjia section.

Figure 10.

Figure 10

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Cross plot of Ba/Nd versus Eu*/Eu in the Yuanjia section.

5.2.3. Terrigenous Input

Al and Th are frequently used to assess local terrigenous fluxes in marine systems. Al and Th are insoluble in seawater, which are immobile during chemical weathering and diagenesis.68 In this study, the moderately positive correlation between Al and Th (R2 = 0.74) suggests that both Al and Th are mainly controlled by terrigenous inputs (Figures 4 and 11). Our samples show low Al (0.9–5.9%, average 2.9%) and Th (2.7–8.5 ppm, average 4.9 ppm) in the LM, reflecting low terrigenous fluxes. Our samples reveal high Al (2.2–5.4%, average 3.9%) and Th (4.1–8.4 ppm, average 6.7 ppm) in the UM, pointing to high terrigenous fluxes. The enhanced terrigenous fluxes from the LM to the UM possibly result from an increase in continental erosion as indicated by 87Sr/86Sr values at the same time.69

Figure 11.

Figure 11

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Cross plot of Al vs Th in the Yuanjia section.

5.3. Drivers of the Decreased Marine Productivity in the Nanhua Basin

Changes in marine productivity are generally assumed to be controlled by terrigenous fluxes, where high marine productivity is connected with elevated fluxes of terrigenous nutrients.70,71 Nevertheless, our result of decreased marine productivity is consistent with the enhanced terrigenous fluxes in the Nanhua Basin from late age 2 to late age 3 (∼526–515 Ma), which does not support the terrigenous controls on marine productivity (Figures 3 and 4).

Fluctuations in marine productivity are considered to be caused by hydrothermal activities.16,17 Hydrothermal fluids can release a large number of nutrients, resulting in high marine productivity.72 Thus, marine productivity is expected to be higher in the hydrothermal active areas than in other normal sections. For example, marine productivity based on TOC contents is high near the hydrothermal active area (e.g., Tongren section) relative to nonhydrothermal sections in the Nanhua Basin.17 However, our finding of low TOC values (average 1.4%) at Tongren and high TOC values (average 13.1%) at Yuanjia do not support a significant influence of hydrothermal activities on marine productivity, where hydrothermal and nonhydrothermal activities are recognized in the Tongren17 and Yuanjia sections (this study), respectively. This inference is in agreement with the existence of a decrease in marine productivity in nonhydrothermal areas (e.g., Yuanjia section; this study).

Variations in marine productivity are supposed to be contributed to upwelling.6,14,40 Strong upwelling can bring massive nutrients from the deep ocean to the photic zone of shallow waters, leading to high marine productivity. In contrast, the diminished upwelling weakens nutrient availability from deep waters to surface waters, resulting in low marine productivity. This significant effect of upwelling on marine productivity is evidenced by the good relationship between them in this study. For example, high marine productivity and strong upwelling appear simultaneously in the LM, but low marine productivity coexists with diminished upwelling in the UM (see Sections 5.1.3 and 5.2.1).

5.4. Implications for the Animal Radiations during the Early Cambrian

The Cambrian explosion is known for the first appearance of metazoan phyla and the establishment of a complex marine ecosystem dominated by metazoans during the early Cambrian. During Fortunian and age 2, the evolution of early animals is represented by the occurrence of small shelly fauna globally.1,2 In the transition from late age 2 to early age 3 (∼526–518 Ma), small shelly fauna is replaced by panarthropods in the inner-shelf settings of the Nanhua Basin, but Niutitang sponge fauna dominated by sponges appears from outer shelf to slope to basinal settings.2,73 In late age 3 (∼518–514 Ma), the Cambrian explosion reached its peak as evidenced by the appearances of Chengjiang and Qingjiang biotas.2,37

Our reconstruction of marine productivity provides an opportunity to explore its relationship with metazoan evolution during the early Cambrian. Our results show a decreased marine productivity from late age 2 to age 3 (Figure 6), while the evolution of life exhibits an increase in metazoan complexity and diversity.2,37 The reverse trends of marine productivity and metazoan evolution suggest that food was not a major factor controlling the evolution of early animals during the Cambrian explosion. Such an inference conforms to no direct relationships between marine productivity and the occurrences of early animals (e.g., Niutitang sponge fauna and Qingjiang Biota). In the modern ocean, sponges can thrive in an oligotrophic environment because they can uptake dissolved organic matter and nutrients from the surrounding seawater.74 However, Niutitang sponge fauna appears in high-productivity environments based on high OCAR at Jinsha6 and ε112Cd at Zunyi.75 It is well accepted that a large amount of foods are needed for microphagous suspension feeders.76 For example, modern whales, whale sharks, and basking sharks live in high latitudes with elevated marine productivity.77 Nevertheless, Qingjiang biota occurs in an oligotrophic environment based on δ15NTN in both TZS and YJK drill cores.12

Marine productivity might stimulate the radiation of early animals through its effect on atmospheric-oceanic oxygen levels. Low marine productivity inhibits organic matter burial, resulting in low oxygen levels in the atmosphere-ocean system. In contrast, high marine productivity enhances organic matter burial, and thus, a huge amount of O2 is released into the atmosphere, causing the oxygenation of the oceanic systems. The significant effect of high marine productivity on oceanic oxygen levels is evidenced by episodes of oceanic oxygenation events based on U and Mo isotopes in the LM.78,79 Correspondingly, the Niutitang sponge fauna widely appears in the Nanhua Basin. On the other hand, low marine productivity reduces water-column oxygen consumption, leading to local oxic conditions. The important influence of low marine productivity on redox conditions is documented by an expansion of local oxic conditions in the UM. As a consequence, complex faunas such as Chengjiang and Qingjiang biotas appear in the shelf areas of the Nanhua Basin.9

6. Conclusions

TOC, major, and trace elements were measured and compiled at Yuanjia to investigate changes in primary productivity and its relationship with the radiation of early animals in the early Cambrian. Our finding of synchronous changes in TOC, P/Al, CuEF, and NiEF suggests that they were effective to reconstruct marine productivity in the early Cambrian Nanhua Basin. CdEF and Cd/Mo can be regarded as the solid upwelling proxies in the Nanhua Basin owing to the significant effects of plankton biomass and redox conditions on Cd and Mo enrichments, respectively. CoEF and Co × Mn failed to assess marine upwelling in the Nanhua Basin because Co enrichments were controlled by water-column redox conditions. Integrated with the existing productivity data from the outer shelf (Jinsha, TZS drill core, YJK drill core, and GDM-1 well) and slope areas (TX-1 well), our study reflects a decreased marine productivity from Age 2 to Age 3 in the Nanhua Basin. The decline in marine productivity was accompanied by weakened upwelling, quiet hydrothermal activity, and enhanced local terrestrial flux, suggesting that fluctuations in marine productivity might be mainly connected with upwelling. In comparison to fossil distributions, our study suggests that there might be enough food for the radiation of early animals in the early Cambrian Nanhua Basin. Marine productivity might indirectly trigger animal complexity and diversity by regulating water-column oxygen levels in the early Cambrian Nanhua Basin.

Acknowledgments

This study was supported by the NSFC Program (41902027, 41903032, and 42002027).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.3c09161.

  • Geochemical data in the Yuanjia section (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao3c09161_si_001.pdf (106.9KB, pdf)

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