New SIMS U–Pb zircon age on the macroscopic multicellular eukaryotes from the early Mesoproterozoic Gaoyuzhuang Formation, North China

Abstract

Decimetre-scale carbonaceous macrofossils from the Mesoproterozoic Gaoyuzhuang Formation in the Yanshan Range are known as the current oldest unambiguous evidence of macroscopic multicellular eukaryotes. Here, we reported a new SIMS zircon age of 1588.8 ± 6.5 Ma from a volcanic tuff in the Qianxi County of Hebei Province, about 11 m above the macrofossil’s horizon. This new age provides a direct age constraint on the macroscopic eukaryotic fossils from the Gaoyuzhuang Formation. It indicates that macroscopic life with the moderate diversity and certain morphological complexity had already evolved at the beginning of the Mesoproterozoic, and implies a possibility of discovering macroscopic eukaryotes in earlier rocks. This study also calls for a stratigraphic framework to integrate biological and environmental studies in different regions for a better understanding of the evolution of multicellular organisms and environmental change during this important period.

1. Introduction

The emergence of multicellular organisms is a critical milestone in the evolution of life on Earth (Bonner, 1998; Niklas and Newman, 2020). The current oldest unambiguous evidence of macroscopic multicellular eukaryotes is the decimetre-scale carbonaceous macrofossils from the Mesoproterozoic Gaoyuzhuang Formation in the Yanshan Range, North China (Zhu et al. 2016; Chen et al. 2023). These macrofossils display multiple regular morphologies (cuneate, linear, oblanceolate and tongue-shaped) and large dimensions up to several centi- to decimetres, exhibiting resemblances to some living macroalgae (Zhu et al. 2016; Chen et al. 2023). Their age has been approximately constrained to 1560 Ma–1580 Ma by zircon U–Pb ages from two outcrops in Yanqing, Beijing (Li et al. 2010) and Jizhou, Tianjin (Tian et al. 2015, 2020) which are about one hundred kilometres apart (Fig. 1(a)).

Figure 1. Geological setting of the sample. (a) Distribution of Proterozoic outcrops in the Yanshan Range showing the reported tuff locations. (b) Correlation of the horizons of reported zircon U–Pb ages and fossils at (1) Yanqing, (2) Jizhou and (3) Qianxi. (c) Outcrop photograph of the tuff bed (yellow arrow) in the section. (d) Macrofossils (red arrows) from Member 3 of the Gaoyuzhuang Formation in Qianxi, Hebei.

The lack of direct age constraint on these macrofossils hinders our understanding of the timing of the origin and early evolution of macroscopic eukaryotes. Macrofossils with regularly repeated forms were mainly found in Qianxi County of Hebei Province (Zhu et al. 2016; Chen et al. 2023), but the host sections lack relevant geochemical and geochronological studies of Gaoyuzhuang Member 3. Considering the variational facies of the Gaoyuzhuang Formation in the Yanshan Range (Liang and Jones, 2021), direct age constraints for the macrofossil assemblages are necessary to understand the relationship between the origin of multicellular organisms and associated environmental conditions (Zhang et al. 2018).

Here, a new zircon age of a volcanic tuff several meters above the fossil horizon at the Qianxi section is reported. The precise SIMS U–Pb zircon age provides a direct age constraint for the important fossil assemblage.

2. Materials and methods

The Mesoproterozoic Gaoyuzhuang Formation is widely distributed in the Yanshan Range and is considered to be accumulated during a marine transgression (Tian and Zhai, 1996). Located between the sandstone of the Dahongyu Formation and the silty dolomite of the Yangzhuang Formation, the Gaoyuzhuang Formation is dominated by carbonate and is subdivided into four members (Tian and Zhai, 1996). Member 3 of the Gaoyuzhuang Formation constitutes a large portion of this rock unit, which is about 680 m thick in the type section at Northern Jizhou District, Tianjin Municipality, with the lower half dominated by medium to thick-bedded dolostones and the upper half dominated by dolomitic limestones. The dolomitic limestones bear small fossiliferous siliceous concretions (Shi et al. 2017) and molar-tooth structures (Mei and Tucker, 2011).

In the section at Qianxi County of Hebei Province, Member 3 is about 470 m thick, dominated by medium to thick-bedded dolostone and argillaceous dolostone, with subordinate siltstone and mudstone. The blade-like macrofossils were reported in muddy dolostone about 225 m above the bottom of this member (Zhu et al. 2016; Chen et al. 2023). Many smaller macrofossils have recently been discovered in the dolomitic mudstone about 435 m above the bottom (Chen et al. 2023).

About 11 m above the upper macrofossil horizon (Fig. 1(b)), the sample QXHY-6 was obtained from a tuff bed at an outcrop ca. 800 m west of the urban district of Qianxi County (Fig. 1(a), 40° 07′ 50″ N, 118° 16′ 32″ E).

Zircon crystals were separated from ca. 5 kg sample (QXHY-6) by the conventional density and magnetic separation methods. Mounted in an epoxy disk with the reference zircons of Plešovice and Qinghu, the grains were then polished to section the crystals in half for analysis. All zircon crystals were documented with transmitted and reflected light photomicrographs and cathodoluminescence (CL) images to reveal their external and internal structures.

U, Th and Pb isotopes were measured using a CAMECA IMS 1280-HR at the Beijing Research Institute of Uranium Geology, following conventional methods (Li et al. 2009). The O₂ ⁻ primary beam, accelerated at ∼13 kV with an intensity of ca. 10 nA, was used to bombard the zircon surfaces, resulting in ellipsoidal analysis spots with sizes of about 20 μm × 30 μm, respectively. Analyses of the unknown grains were interspersed with those of the reference zircon at a ratio of 3:1.

The ratios of U–Th–Pb are determined relative to the reference zircon of Plešovice (337 Ma) (Sláma et al. 2008). The measured compositions were corrected for common Pb using non-radiogenic ²⁰⁴Pb. Corrections were sufficiently small to be insensitive to the choice of common Pb composition and an average of present-day crustal composition was used for the common Pb assuming that the common Pb was largely surface contamination introduced during the sample preparation (Stacey and Kramers, 1975). Uncertainties on individual analyses are reported at 1 σ level. Weighted mean ages for pooled U/Pb (and Pb/Pb) analyses are quoted with a 95% confidence interval. The data reduction is done using the online program IsoplotR (Vermeesch, 2018).

Analyses of zircon reference Qinghu were interspersed with unknowns, and eight analyses yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 160.6 ± 2.1 Ma (MSWD = 0.9), consistent, within error, of the reported value of 159.5 ± 0.2 (Li et al. 2013).

3. Results

Zircon crystals from the tuff sample QXHY-6 are mostly subhedral to euhedral, which are 75–180 μm in length and 35–80 μm in width, respectively (Fig. 2(a)). Oscillatory zones are distinct in the CL images (Fig. 2(a)), and the ratios of Th/U are mostly higher than 0.4 (Table S1), suggesting the zircons are of magmatic origin.

Figure 2. SIMS zircon U–Pb age for the tuffs sample QXHY-6. (a) Images of representative dated zircons (transmission light and CL) show the SIMS analysis dots (white ovals) and the ²⁰⁷Pb/²⁰⁶Pb ages. (b) Wetherill U–Pb concordia diagram. (c) Weighted average of the ²⁰⁷Pb/²⁰⁶Pb ages.

Excluding two obvious outliers, 28 of 30 analyses yielded a relatively consistent ²⁰⁷Pb/²⁰⁶Pb age ranging from 1564 Ma to 1678 Ma, respectively. They lie on a discordia line going through zero in the U–Pb concordia plot with an upper intercept of 1588.8 ± 6.5 Ma (Fig. 2(b)), indicating that they suffered recent Pb loss. The upper intercept age of 1588.8 ± 6.5 Ma is consistent with the weighted mean age of ²⁰⁷Pb/²⁰⁶Pb within error (1589.1 ± 6.5 Ma, Fig. 2(c)), representing the crystallization age of these zircons.

4. Discussion and conclusion

These macrofossils are important for our understanding of early eukaryote evolution and associated biological innovation. In this study, we present a new SIMS U–Pb zircon age from a volcanic tuff above the macrofossil-bearing horizon in the same section. A discordia line going through zero in the U–Pb concordia plot indicates these zircons suffered recent Pb loss. The ²⁰⁷Pb/²⁰⁶Pb system was maintained during the stoichiometric loss of total lead, contributing to the consistent ²⁰⁷Pb/²⁰⁶Pb ages of these zircons (Spencer et al. 2016). The upper intercept age of 1588.8 ± 6.5 Ma, which is consistent with the weighted mean ²⁰⁷Pb/²⁰⁶Pb age within error, is interpreted as the depositional age of this tuff, and a direct age constraint on the macrofossil horizon.

Zircons from tuff layers in the Gaoyuzhuang Formation in other sections were previously dated by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) and sensitive high-resolution-ion microprobe (SHRIMP). Ages of 1582 ± 12 Ma (SHRIMP) and 1577 ± 12 Ma (LA-MC-ICPMS) were obtained from a volcanic tuff at the bottom of Member 3 of the Gaoyuzhuang Formation in the Jizhou section (Tian et al. 2015, 2020), and ages of 1560 ± 5 Ma (LA-MC-ICPMS) and 1559 ± 12 Ma (SHRIMP) were obtained from a tuff at the top of Member 3 in the Yanqing section (Li et al. 2010). These two sets of ages constrained Member 3 to ca. 1580–1560 Ma. Within the lithostratigraphic framework, the new dated horizon at the top of Member 3 at Qianxi section approximately corresponds to the ca. 1560 Ma volcanic ash layers in the Yanqing section.

At face value, the newly obtained date seems to be older than the previously published radio-isotopic date from the equivalent horizons in the Yanqing section. This apparent difference could be reconciled by the precision of 1–2% of these methods (Gehrels, 2014; Yang et al. 2017) or reflects the lithostratigraphic diachroneity. Within the wide distribution range of the Gaoyuzhuang Formation, the thickness of Member 3 varies greatly in the Yanshan Range (Fig. 1(b)). In Jizhou, Tianjin, the thickness is up to ca. 680 m (Shang et al. 2019), while it is ca. 470 m in Qianxi (Chen et al. 2023). Depending on the lithostratigraphic subdivision, the thickness of Member 3 was reported to be ca. 190 m (Shang et al. 2019) or ca. 300 m (Mei, 2007) in the Gan’gou section of Yanqing. The lithological characteristics of Member 3 also vary largely (Fig. 1(b)). Nodular limestones at the bottom are obvious at the Yanqing and Jizhou sections (Shang et al. 2019), but are missing at the Qianxi section (Chen et al. 2023). Limestone is also absent at the Qianxi section, but common in the Jizhou and Yanqing sections. With much wider distribution than the underlying Dahongyu Formation, the Gaoyuzhuang Formation is considered to have formed during a continued marine transgression (Huang, 2006). The erosion–deposition processes can lead to the spatial and temporal variations of the depositional sequences (Meng et al. 2011).

In most cases, lithostratigraphic boundaries are diachronous, and are not considered as precise markers for temporal correlation. The new SIMS U–Pb zircon age provides a direct age constraint on the macroscopic eukaryotic fossils, and; therefore, is critical for better understanding the evolution of multicellular organisms with environmental change (Yang et al. 2022).

At Jizhou section, evidence for a progressive oxygenation event was found in the lower part of Member 3, the approximate level of the macrofossil horizon at Qianxi section (Zhang et al. 2018). The evidence was verified by subsequent studies in other areas with different methods, which suggest an important link between the development of macroscopic life and oceanic oxygenation at this time (Shang et al. 2019; Tang et al. 2022; Xie et al. 2022; Ye et al. 2023; Xu et al. 2023). However, neither chronological nor geochemical work on Member 3 were carried out in the Qianxi area, where most of those macrofossils with regularly repeated forms were found. The new age calls for biological and environmental studies to be carried out on the same stratigraphic section. Furthermore, a stratigraphic correlation framework is urgently needed to integrate biological and environmental studies that are carried out in different areas of the Yanshan Range, to obtain a holistic perspective on the relationship between the evolution of life and the environment.

The early Mesoproterozoic was thought to be characterized by generally low environmental oxygen concentrations, which restricted the emergence and evolution of multicellular organisms (Holland, 2006; Lyons et al. 2014). The origin of eukaryotic multicellularity was thought to have a later occurrence after the oxygen levels increased. Molecular clocks estimate that multicellular eukaryotes originated at the transition between the Mesoproterozoic and Neoproterozoic (Sharpe et al. 2015). However, more and more fossil evidence indicates that multicellular eukaryotes made their appearances in the early Mesoproterozoic and may have appeared even earlier (Javaux and Lepot, 2018).

With the new direct age constraint reported herein, the macroscopic eukaryotic fossils from the Gaoyuzhuang Formation suggest that macroscopic life had already evolved in moderate diversity and certain morphological complexity at the beginning of the Mesoproterozoic. These new insights also emphasize the possibility of finding fossils of multicellular eukaryotes during this period and even earlier to the Paleoproterozoic.