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Article

The Carbon Sink in the Mesoproterozoic Ocean and Its Implications for Marine Carbon Storage Pathways

by
Chaokun Zhang
1,
Wei Tian
1,* and
Yanxin He
2
1
School of Earth and Space Sciences, Peking University, Beijing 100871, China
2
School of Mining Engineering, Jiangxi University of Science and Technology, Ganzhou 341099, China
*
Author to whom correspondence should be addressed.
Sustainability 2026, 18(13), 6851; https://doi.org/10.3390/su18136851
Submission received: 21 May 2026 / Revised: 25 June 2026 / Accepted: 3 July 2026 / Published: 6 July 2026
(This article belongs to the Special Issue CO2 Capture and Utilization: Sustainable Environment)
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Abstract

Anthropogenic CO2 emissions have perturbed the global carbon cycle and increased atmospheric carbon concentrations to critical levels, making carbon capture and storage (CCS) a key strategy for mitigating climate warming. Natural carbon sequestration has operated continuously in marine environments throughout Earth history. Here, we investigate the growth mechanisms and carbon-sink significance of calcite concretions in the Mesoproterozoic Xiamaling Formation from the Zhaojiashan section and the Zhenzhuquan section in the North China Craton, using petrographic, elemental geochemical and C-O-Re-Os isotopic evidence. The presence of erosional surfaces and local truncation of host-rock laminae suggests that these concretions formed synsedimentarily or during early diagenesis near the sediment-water interface. The δ13C values (−5.05‰ to 1.54‰) of samples, together with δ18O-δ13C relationships, indicate a marine carbonate affinity and suggest that dissolved inorganic carbon was the dominant carbon source. In addition, the concretions display initial 187Os/188Os ratios as low as 0.136, close to the mantle Os end-member, implying a contribution from mantle-derived material during concretion formation. The middle rare earth element and yttrium (MREYs)-enriched patterns and slight positive Ce anomalies further indicate that concretion growth occurred mainly within the Mn- and Fe-reduction zones. We estimate that the calcite-concretion-bearing interval of the Xiamaling Formation sequestered 70.24 Gt C, equivalent to 257.56 Gt CO2, serving as an archive of marine carbon burial in the Mesoproterozoic ocean. Microbially mediated carbonate precipitation may represent an effective carbon immobilization mechanism in marine sediments and has potential implications for the development of subseafloor carbon storage strategies, especially where biocatalysts and/or brine could accelerate seawater CO2 mineral trapping to industrially relevant rates.

1. Introduction

Carbon is exchanged among the biosphere, hydrosphere, atmosphere, pedosphere, and geosphere. Its long-term redistribution among these reservoirs is modulated by the carbon cycle, one of the fundamental geochemical cycles sustaining the Earth system [1,2]. Since the Industrial Revolution, anthropogenic activities, including fossil-fuel combustion, cement and steel production, and deforestation, have substantially increased carbon emissions to the atmosphere [3,4,5]. The resulting intensification of the greenhouse effect has accelerated global warming at a rate far exceeding natural climatic variability [6,7]. These impacts include frequent and severe heatwaves and droughts, heightened risks of ecosystem degradation and food insecurity, ocean warming and acidification, and sea-level rise driven by ice-sheet loss [8,9,10]. To curb greenhouse gas emissions, carbon capture and storage (CCS) has been increasingly implemented worldwide. Achieving the Paris Agreement target of limiting global warming to 1.5 °C is expected to require the deployment of more than 2500 large-scale CCS facilities globally by 2040 [10,11,12,13].
Marine settings have been estimated to possess a carbon storage capacity of ca. 3.8 × 103 Gt [14,15,16,17]. Among the available CO2 sequestration approaches, offshore geological storage is increasingly regarded as a safer and more efficient alternative to conventional onshore geological sequestration, owing to its vast storage potential, isolation from freshwater aquifers, and relatively stable temperature and pressure conditions [18]. However, most marine CCS projects are focused on enhanced oil recovery and solubility trapping in saline aquifers [19,20] (Figure 1). Engineered in situ mineral carbonation, whereby dissolved CO2 is ultimately converted into carbonate minerals, remains largely restricted to basaltic formations and typically requires costly offshore infrastructure and long-term monitoring systems [21].
Natural carbon sequestration operates continuously in marine environments. The formation and burial of carbon-bearing minerals represent one of the key long-term (102~104 yr) feedbacks in Earth’s carbon cycle [22,23]. Carbonate concretions are authigenic mineral aggregates that constitute an important reservoir for mineral carbon storage [24,25,26]. In modern marine sediments, a range of microbially mediated reduction processes can promote concretion formation [27,28]. The dominant pathways are governed by the availability of electron acceptors and the composition of organic matter [28,29,30,31]. As a result, distinct diagenetic zones are characterized by different biogeochemical reactions, and porewater chemistry varies systematically with sediment depth. These variations can be traced through pore-fluid profiles in the sediment column [32] and have been commonly recorded in the petrographic and geochemical characteristics of carbonate concretions [30]. Moreover, coupled mineral-microbial processes in modern seas can sustain high carbon accumulation rates and longer carbon turnover times than those of terrestrial ecosystems [33], further underscoring the importance of investigating marine carbon-sink mechanisms. Elucidating the processes governing such carbonate mineralization not only improves our understanding of the marine carbon cycle [34], but also provides natural analogues and theoretical constraints for integrating geological time-scale natural carbon sinks with engineered marine CCS.
In this study, we investigate the petrographic and geochemical characteristics of Mesoproterozoic carbonate concretions from Unit 1 of the Xiamaling Formation. Our primary objectives are to reconstruct the growth mechanisms of these concretions and the diagenetic conditions under which they formed. On this basis, we further assess their implications for in situ carbon mineralization in modern marine environments and for the development of engineered CCS technologies and strategies.
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Figure 1. Global distribution of marine CCS. (Modified after [18,34]).
Figure 1. Global distribution of marine CCS. (Modified after [18,34]).
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2. Geological Background

The Mesoproterozoic-Neoproterozoic sedimentary succession overlies the crystalline basement in the northeastern margin of the North China Craton (NCC). This includes the Changcheng System (Changzhougou, Chuanlinggou, Tuanshanzi, Dahongyu and Gaoyuzhuang Formations), the Jixian System (Yangzhuang, Wumishan, Hongshuizhuang and Tieling Formations), the Daijian System (Xiamaling Formation), and the Qingbaikou System (Changlongshan and Jingeryu Formations) [35,36,37,38]. The Xiamaling Formation (Pt2/x) is regarded as the final sedimentary record prior to the breakup of the Columbia supercontinent [39,40]. It overlies the Tieling Formation (Pt2/t) and is underlain by the Changlongshan Formation (Pt3/c) (Figure 2).
The Xiamaling Formation is deposited in an extensional setting within the Yanliao Basin [39,40,41,42,43,44] and is subdivided into six units that collectively record a major transgressive-regressive cycle, with maximum transgression occurring in Unit 2 [45,46,47,48,49]. Palaeomagnetic data suggest that the NCC was located at ca. 10° N~30° N during the formation of the Pt2/x [50], and palaeogeographic reconstructions indicate its deposition in an open-marine environment [51,52,53] (Figure 2e). The geochronological data of a bentonite layer in the basal Unit 6 constrain the Pt2/t~Pt2/x boundary to ca. 1.40 Ga (1418 ± 14 Ma) [53]. Additional constraints are provided by zircon U-Pb ages of 1392 ± 1 Ma from bentonite in Unit 3 and 1384 ± 1 Ma from tuff in Unit 2 [48] (Figure 2a), as well as ages of 1353 ± 14 Ma and 1345 ± 12 Ma for diabase sills intruding the upper part of the formation [41]. Because the studied concretions were collected from the middle part of Unit 1, their depositional age is inferred to be ca. 1.36 Ga [54,55,56].
The lower to middle part of Unit 1 is characterized by interbedded black and greenish shales with minor limestone interlayers, and it grades upward into a regressive succession from silty shale to yellowish siltstone [54] (Figure 2a). This Unit is interpreted to have been deposited in a deep-water setting below storm wave base (>100 m). However, the occurrence of hummocky stratification and small-scale cross-bedding in its upper interval indicates sporadic and weak storm-wave reworking [39,45].
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Figure 2. Geological background of the study area. (a) integrated stratigraphic column of the Mesoproterozoic Xiamaling Formation (modified from [55]); (b) geological overview of the northern margin of the North China Craton; (c) location map of the study area; (d) distribution of Mesoproterozoic to Neoproterozoic strata in the North China Craton (modified from [36,56]); (e) paleogeographic reconstruction of the Yanliao Basin at 1.4~1.3 Ga (modified from [53]).
Figure 2. Geological background of the study area. (a) integrated stratigraphic column of the Mesoproterozoic Xiamaling Formation (modified from [55]); (b) geological overview of the northern margin of the North China Craton; (c) location map of the study area; (d) distribution of Mesoproterozoic to Neoproterozoic strata in the North China Craton (modified from [36,56]); (e) paleogeographic reconstruction of the Yanliao Basin at 1.4~1.3 Ga (modified from [53]).
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3. Methodology

Carbonate concretions analyzed in this study were collected from Unit 1 of the Xiamaling Formation exposed in freshly cut road sections at Zhaojiashan (40°28.7′ N, 115°23.4′ E) and Zhenzhuquan (40°31.5′ N, 116°21.43′ E), North China Craton (Figure 2c). During the deposition of the Xiamaling Formation, both sections were located in the depocenter of the Yanliao Basin [57]. Persistent subsidence and sediment focusing promoted the development of a continuous sedimentary record. Combined with a relatively low-energy depositional setting and minimal subsequent erosion or modification, these conditions enabled a well-preserved record of basin-scale geological events and rendered the sections suitable for paleoenvironmental reconstruction [58].
Hand specimens were cut into chips, and only the fresh, unweathered interiors were used for petrographic and geochemical analyses. The chips were cleaned, air-dried and powdered to ca. 200 mesh by an agate mortar to avoid metal contamination. Macroscopic features were documented in the field and polished slabs were prepared by Chenshuo Geological Service Company, Langfang, China. Petrographic observations were carried out using a Nikon Eclipse LV100NPOL polarizing microscope (Nikon, Shanghai, China) at the Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University.
Major and trace element compositions of six samples were analyzed at Nanjing FocuMS Technology Co., Ltd. (Nanjing, China) by X-ray fluorescence spectrometry (XRF) and inductively coupled plasma mass spectrometry (ICP-MS), respectively. For major element analysis, 0.5 g of powdered sample was mixed with 3.6 g Li2B4O7, 0.4 g LiF, 0.3 g NH4NO3 and a small amount of LiBr in a Pt crucible, and fused into glass discs prior to XRF measurement. For trace element analysis, powdered samples were digested in an HF-HNO3 mixture in high-pressure Teflon bombs at 190 °C for 48 h prior to ICP-MS analysis. Analytical precision and accuracy were generally better than 5% for major elements and better than 10% for most trace elements.
Carbon and oxygen isotope compositions of 24 samples were analyzed at the State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum (Beijing). Sample surfaces were removed prior to analysis, and the fresh interiors were powdered in an agate mortar. Carbonate powders were reacted with 10% HCl at 55 °C to release CO2, which was subsequently analyzed using a MAT 253 isotope ratio mass spectrometer (Thermo Fisher Scientific, Bremen, Germany) [59]. Isotope values are reported in per mil (‰) relative to the Vienna Pee Dee Belemnite (V-PDB) standard. Analytical precision was better than ±0.2‰ for both δ13C and δ18O.
Re-Os isotopic analyses of 10 samples were carried out at Nanjing FocuMS Technology Co. Ltd., China, using an Agilent 7700x quadrupole ICP-MS (Agilent Technologies, Hachioji, Tokyo, Japan). Rhenium and osmium concentrations, as well as Os isotopic compositions, were determined following Carius tube digestion, carbon tetrachloride extraction, and microdistillation [60]. Procedural blank corrections were applied to both Re and Os isotopic analyses. Total blanks were 8 ± 2 pg for Re and 3 ± 2 pg for Os, respectively. The blank 187Os/188Os ratio was 0.191 ± 0.025. The precision of 187Os/188Os measurements was better than 0.4% (2σ).

4. Result

The carbonate concretions are interbedded with silty shale and lie parallel to bedding (Figure 3a). Laterally, they grade wedge-shaped into the host silty shale. The samples deflect and locally cross-cut host-rock laminae, indicating syndepositional or early diagenetic formation and the development of compaction-resistant structures [24]. Erosional surfaces on the concretions are consistent with formation near the sediment-water interface (Figure 3c,f).
Macroscopically, the concretions are elliptical, blocky, or pancake-shaped (Figure 3), and range from 5 to 25 cm in height and 5 to 45 cm in width (Figure 3). They comprise yellowish-green replacive growth horizons and grey displacive growth horizons, with the latter forming the concretion core [54] (Figure 3d–f). Microscopically, the calcite makes up as much as 60% of the concretions, further indicating that the samples are calcite concretions. The sparry calcite crystals (50~300 μm) are surrounded by green-yellow clay minerals and authigenic quartz (Figure 4), and display a homogeneous texture and concentric growth fabric [25].
Major element compositions of samples are dominated by CaO (average 42.69 wt%), SiO2 (average 10.97 wt%), Fe2O3T (average 2.84 wt%), and MgO (average 2.59 wt%) (Table S1). Following Seredin and Dai [61], the rare earth element and yttrium (REYs) are subdivided into LREYs (La to Sm), MREYs (Eu to Dy plus Y), and HREYs (Ho to Lu). PAAS-normalized REYs patterns [62] show MREYs enrichment (Figure 5). The Eu/Eu* values (2∙EuSN/(SmSN + GdSN)) range from 0.93 to 1.07, the Ce/Ce* values (2∙CeSN/(LaSN + PrSN)) from 0.95 to 1.11, and the Pr/Pr* values (2∙PrSN/(CeSN + NdSN)) from 0.92 to 0.98.
The concretions yield δ13CPDB values of −5.05‰ to 1.54‰, within the reported range of Mesoproterozoic carbonate δ13Ccarb values (−5‰ to 5‰) [63,64]. δ18OSMOW values range from 19.17‰ to 24.74‰, equivalent to δ18OPDB values of −11.34‰ to −5.94‰ (Table S2).
Re concentrations range from 0.05 to 1.78 ng/g and Os concentrations range from 0.05 to 0.22 ng/g. Measured 187Os/188Os ratios range from 0.141 to 0.846, and 187Re/188Os ratios range from 0.234 to 14.494 (Table S3). Initial 187Os/188Os ratios, calculated using 1360 Ma as the reference age [54,55], range from 0.136 to 0.802.

5. Discussion

5.1. Source of Carbon

The carbon sources of calcite concretions commonly included dissolved inorganic carbon (DIC) derived from the overlying seawater and bicarbonate generated during the degradation of organic matter. These two reservoirs differ isotopically, with seawater DIC generally showing δ13C values near 0‰, whereas organic matter can be substantially 13C-depleted [65,66,67]. The δ13C values of our samples range from −5.05‰ to 1.54‰, with a mean of 0.21‰, falling within the range of Mesoproterozoic seawater carbonates (−5‰ to 5‰) [63,64]. This indicates that the dominant carbon source for concretion growth was marine DIC (Figure 6).
Combined with previously published carbon and oxygen isotope data (n = 21) [54] for calcite carbonates from the Xiamaling Formation, the δ18OSMOW-δ13CPDB cross-plot further supports a marine carbonate affinity for the calcite concretions in this study. Specifically, several lines of evidence indicate that post-depositional alteration of the studied concretions was negligible and that the oxygen isotope data are reliable: (i) δ18OPDB and δ13CPDB show no strong covariation (R2 = 0.19), arguing against pervasive coupled modification by late diagenetic fluids [68] (Figure 7a); (ii) Mn/Sr ratios are consistently <10, with an average of 4.04, which implies relatively good preservation of primary isotopic signals [69,70,71] (Figure 7b); (iii) the average δ18OPDB value (−8.96‰) is close to reconstructed values for coeval seawater (−8.4‰) [72], suggesting that the samples represent typical Mesoproterozoic marine carbonates.
Given that Mesoproterozoic volcanism [43,44,47] also have released CO2 with δ13C values of around −3.5‰ [73,74], hydrothermal input and a substantial mantle-derived component could have served as additional carbon sources during concretion formation. Integration of the previously published geochemical data (n = 8) for calcite carbonates from the Xiamaling Formation [54], the Eu/Sm-Sm/Yb binary mixing model indicates that the hydrothermal contribution to calcite carbonate formation was limited to less than 1% (Figure 8) [75]. However, the initial 187Os/188Os ratios of samples range from 0.136 to 0.802, with the lowest values approaching the mantle end-member (ca. 0.128) [76]. In particular, an extraterrestrial source of the unradiogenic Os values can be ruled out based on the low Ir concentrations and elevated Pt/Ir and Pd/Ir ratios of the Xiamaling Formation [44]. The absence of a correlation between Al and Os contents (R2 = 0.016; Figure 9a) indicates that the Os isotopic shift cannot be explained by reduced detrital input and a corresponding decrease in radiogenic crustal Os. The positive correlation between Ce/Ce*SN values and Os contents (R2 = 0.58; Figure 9b) suggests that the Ce/Ce*SN depletion, which reflects more oxygenated atmospheric conditions and enhanced oxidative weathering [77], did not affect Os enrichment in the samples. The most plausible interpretation is therefore enhanced input of unradiogenic Os from mantle-derived magmatism. Accordingly, despite the limited Re-Os isotopic dataset (n = 10), the sample distribution suggests that magmatic activities facilitated the transfer of deep-Earth material into the Mesoproterozoic ocean.
The Os end-member mixing model assumes a subcontinental lithospheric mantle (SCLM)-derived parental magma with Os = 2.9 ng/g and 187Os/188Os = 0.12 [78,79], an upper continental crust with Os = 10−4~3 × 10−2 ng/g and 187Os/188Os = 0.8~1.5, and an oceanic crust with Os = 4.5 × 10−2 ng/g and 187Os/188Os = 0.15 [80]. Relative to the estimated Mesoproterozoic seawater Os reservoir (Os = 0.33 ± 0.03 ng/g, 187Os/188Os = 0.38 ± 0.07) [44], the results indicate that the concretions incorporated an additional crustal component, but that mantle-derived Os could also have accounted for >60% of the total Os budget (Figure 10). Mantle-derived material could have made a significant contribution to the Mesoproterozoic marine system. Thus, mantle-derived carbon may have acted as a potential carbon source for concretion growth [81], although it may not dominate the bulk carbon isotopic composition.

5.2. Origin of Calcite Concretion

Calcite concretion precipitation is linked to microbial respiration of organic matter. During this process, organic matter is oxidized while electron acceptors are reduced [30]. These reactions generate HCO3− and CO32−, which increase pore-water alkalinity and/or pH and subsequently promote carbonate precipitation through association with cations [27,28]. As oxidants progressively accumulate, the depositional position of calcite concretions can be divided into six diagenetic zones: oxic zone, nitrogenous zone, manganous zone, ferruginous zone, sulfidic zone, and methanic zone. Different metabolic pathways within each zone generate distinct pore-water compositions and leave characteristic geochemical signatures [30]. In particular, REYs patterns, as a fingerprint of carbonate precipitation, can be used to reconstruct the redox fluctuation of the water column and the evolution of concretion diagenesis [82,83,84].
Calcite concretions in the Xiamaling Formation contain erosional surfaces and bent clay-rich laminae surrounding authigenic carbonate phases (cardhouse structures) [54]. These features indicate rapid concretion growth near the sediment-water interface during syngenetic to early diagenetic stages and suggest that the associated geochemical signatures may reliably record biogeochemical processes and, indirectly, local seawater chemistry [25,54]. Previous studies proposed that nitrate reduction and Mn reduction contributed to concretion formation in Unit 1, based on negative δ15Norg values and the elevated Mn values from concretion nuclei to rims [54,85]. In this study, the lack of clear relationships between (Eu/Eu*)SN and (La/Nd)SN, and between (Ce/Ce*)SN, (Pr/Yb)SN, (Y/Ho)SN and Al contents (Figure 9c–f), indicates that neither late burial diagenesis nor detrital input substantially modified the REYs patterns of the concretions [86,87]. Combined with the 8 previously reported REYs datasets for calcite carbonates from the Xiamaling Formation [54], the REYs patterns show not only the slight positive Ce anomalies previously interpreted as evidence for calcite concretion growth influenced by Mn reduction, but also a more pronounced enrichment in MREYs (Figure 5) in the samples analyzed in this study. This implies that concretion formation may have extended into the Fe-reduction zone (Figure 11). Under oxic conditions, Fe2+ and Mn2+ are oxidized to Fe-Mn oxides/hydroxides, which preferentially scavenge MREYs from solution [88]. Moreover, Ce is transported across the redoxcline at a rate more than four times higher than that of the other REYs [89]. Beneath the Fe-Mn redoxcline in anoxic conditions, reductive dissolution releases previously adsorbed MREYs and Ce, potentially generating local MREYs enrichment and positive Ce anomalies in porewaters [86,89,90,91] (Figure 11).
However, the pronounced negative δ34Spy values (<0‰) and low HI values reported for Unit 1 also imply that sulfate reduction and methanogenesis were active [85]. Both processes can generate substantial HCO3− and are widely regarded as major mechanisms promoting carbonate precipitation in the modern ocean [25,27,31,54]. While sulfate reduction and methanogenesis result in the HREY-enriched patterns and markedly δ13C-depleted compositions (−110‰ to −30‰) [65]. Such scenario is not supported by the isotopic and REYs characteristics of the studied samples. This may reflect a concretion growth rate that remained close to the rate of siliciclastic deposition, allowing the concretions to grow vertically while the reaction front likely remained within the Fe- and Mn-reduction zone [54]. Alternatively, it may indicate the expansion of the Mn- and Fe-reduction zones associated with a ferruginous ocean, as indicated by the high FeHR/FeT ratio (the ratio of highly reactive iron and iron speciations) in Unit 1 of the Xiamaling Formation [45].

5.3. Estimated CO2 Sink of Carbonate Concretions

Previous work suggests that the carbonate-concretion-bearing interval in the middle part of Unit 1 of the Xiamaling Formation has an average thickness of ca. 6 m across the North China Craton [54]. Given the areal extent of the Xiamaling Formation of ca. 6 × 104 km2 [92], the total volume of this concretion-bearing interval is estimated to be 360 km3. If the carbonate concretions in Unit 1 throughout the Yanliao Basin contained calcite in proportions similar to those of the studied samples (with calcite comprising ca. 60% of total concretion volume), then the total volume of calcite within the concretion-bearing interval would be 216 km3. Based on a calcite density of 2.71 g/cm−3 [93], the carbonate concretions of the Xiamaling Formation would contain approximately 585.36 Gt CaCO3, corresponding to 70.24 Gt C, or 257.56 Gt CO2 equivalent. Furthermore, siderite-dominated iron formations in Units 4 to 6 of the Xiamaling Formation contain at least 111 Gt C [92], equivalent to 407 Gt CO2. Collectively, carbonate concretion interval and Fe-carbonate mineralization in the Xiamaling Formation account for a total CO2 sequestration capacity of 664.56 Gt CO2 equivalent.
However, the devolatilization related to the 1.32 Ga magmatic episode alone may have generated up to 4.58 × 104 Gt CO2 equivalent [94]. Relative to this flux, the carbon sink represented by the Xiamaling Formation was unlikely to have played a first-order control on Mesoproterozoic atmospheric CO2 on a global scale. Nevertheless, the estimated sequestration capacity exceeds total anthropogenic CO2 emissions in 2024 (37.4 Gt CO2) [95] by more than an order of magnitude, highlighting the magnitude of natural geochemical carbon sequestration over geological timescales.

5.4. Implications for Marine CO2 Storage

Carbon burial in marine sediments as carbonate minerals also represents the largest long-term pathway for carbon removal from the modern Earth’s surface system [96,97,98]. Rau [99] modelled that, in principle, a 10% increase in dissolved CO2 in seawater can be re-equilibrated with the marine system within minutes, and that approximately 75% of the captured CO2 may subsequently be converted into carbonate minerals. This highlights the fundamental potential of marine carbonate formation as an efficient geochemical sink for carbon.
In the Xiamaling Formation, calcite carbonate formation occurred in a ferruginous ocean and was closely associated with Fe-Mn reduction. By contrast, the modern ocean generally contains much lower abundances of reactive Fe and Mn oxides [100]. The Mn reduction zone is commonly only a few centimetres thick [101,102,103], and bacterial Fe reduction accounts for no more than ca. 15% of global anaerobic organic carbon oxidation [104]. Because marine sulfate concentrations are relatively high compared with those of other terminal electron acceptors [105], sulfate reduction has become the primary process promoting carbonate precipitation. Once sulfate is exhausted, residual organic carbon may be converted to methane [106], and the subsequent anaerobic oxidation of methane can further promote subsurface carbonate formation. Despite these mechanistic differences, both pathways are forms of microbially mediated carbonate mineralization.
However, relying exclusively on natural marine carbon sinks for industrial-scale carbon sequestration remains impractical because the process is slow and the sequestration flux is limited (1.62 ± 0.6 × 106 t C yr−1) [100]. Consequently, engineered in situ CO2 mineral trapping has been explored most extensively in basaltic formations [107]. In such engineering, CO2-charged aqueous fluids are injected into mafic rocks, where dissolved CO2 forms carbonic acid that dissociates to H+, HCO3− and CO32−. These dissolved carbon species then react with divalent cations released during basalt dissolution to form carbonate minerals. Although conceptually attractive, this approach remains constrained by the high costs of CO2 capture, transport, injection and monitoring, with offshore basalt mineralization estimated at US$200~400 per tonne of CO2 [21]. Its broader implementation is further limited by the uneven geographic distribution of suitable host lithologies. In addition, leakage risks during storage raise environmental concerns, including ocean acidification, geohazards and potential impacts on marine ecosystems [18,108].
Against this backdrop, natural marine carbon sinks represented by carbonate precipitation should not be overlooked in the CCS strategies [109] and shallow subseafloor settings, where microbial processes can promote the coupling of Mg2+, Ca2+ and Fe2+ with HCO3− and CO32− to form stable carbonate phases such as calcite, magnesite and siderite [34] (Figure 12). These minerals are thermodynamically favourable, with strongly negative Gibbs free energies of formation (ΔG < −100 kJ mol−1) [107], and therefore have the potential to sequester carbon on timescales of at least 104 years.
Several approaches could enhance subseafloor carbonate mineral trapping. The addition of anaerobic methanotrophs and sulfate-reducing bacteria can strengthen microbially mediated carbonate formation. Urea-producing bacteria and halophilic carbonate-forming bacteria facilitate carbonate mineral precipitation through two principal mechanisms. Firstly, they regulate the density of carboxyl groups on carbon surfaces, which promotes carbonate nucleation under conditions of high carboxyl-group density and sharp biogeochemical gradients. Secondly, they decrease the kinetic barriers to carbonate formation and lower the temperature required for the precipitation of minerals such as calcite and siderite, thereby accelerating CO2 mineral trapping [23,110,111]. Moreover, biocatalysts such as co-injected carbonic anhydrase (CA) further enhance this process by catalyzing the hydration of CO2 to HCO3−, with the conversion rate increased by as much as 107-fold, followed by reaction with OH− and cations to induce carbonate precipitation in a microbially mediated alkaline environment [111,112,113]. The addition of appropriate amounts of brine also improves subseafloor wettability and facilitates CO2 immobilization through mineralization [111]. Nevertheless‌, extensive carbonate concretion formation may induce wellbore scaling (Figure 12c) and reduce porosity and permeability in the near-well region. Accordingly, during the injection of CO2 and cations, mitigation measures such as controlled backflow of fluids without additional fluid injection, or the use of chemical treatments (e.g., HCl or HF) to wash the near-well region, may be necessary to restore porosity and permeability [114,115].
Despite these prospects, substantial knowledge gaps remain regarding subseafloor carbonate mineral trapping. In our view, dedicated laboratory experiments under realistic field conditions are needed to better evaluate the feasibility of applying natural marine carbon sinks in CCS engineering, particularly with the assistance of biocatalysts and brine. Improved simulation tools should also be developed to predict and manage CO2 mineral trapping processes and to assess the effects of thermal, chemical, geological, and geomechanical factors on subseafloor carbonate precipitation.

6. Conclusions

(i)
The calcite-concretion-bearing interval in the middle part of Unit 1 of the Mesoproterozoic Xiamaling Formation records synsedimentary to early diagenetic concretion formation, as indicated by local truncation of host-rock laminae and the presence of erosional surfaces.
(ii)
Geochemical evidence, including a mean δ13C value of 0.21‰, indicates that marine dissolved inorganic carbon was the principal carbon source, while the low initial 187Os/188Os ratios point to a mantle-derived contribution to the concretions.
(iii)
MREYs-enriched patterns and slight positive Ce anomalies further suggest that concretion growth was associated with Fe-Mn reduction, with an estimated total sequestration of up to 70.24 Gt C.
(iv)
Microbially mediated carbonate precipitation may provide an efficient pathway for carbon immobilization and a complementary strategy for subseafloor CO2 storage if biocatalysts and/or brine could accelerate seawater CO2 mineral trapping to industrially relevant rates.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/su18136851/s1, Table S1: Elemental Geochemistry; Table S2: C-O isotopic Geochemistry; Table S3: Re-Os isotopic Geochemistry.

Author Contributions

Conceptualization, C.Z. and Y.H.; methodology, C.Z.; software, Y.H.; validation, W.T. and Y.H.; formal analysis, C.Z.; investigation, C.Z.; resources, W.T.; data curation, W.T.; writing—original draft preparation, C.Z.; writing—review and editing, W.T.; visualization, C.Z.; supervision, W.T.; project administration, C.Z.; funding acquisition, W.T. All authors have read and agreed to the published version of the manuscript.

Funding

This research is funded by the National Key Research and Development Program of China (2022YFF0800301) and the National Natural Science Foundation of China (Grant No. 42502306).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data will be made available on request.

Acknowledgments

We greatly thank for the assistance provided by Mimi Chen, Xin Xu and Xi Huang from the School of Earth and Space Sciences of Peking University, Tiantian Li from the College of Earth Sciences of China University of Petroleum-Beijing in the experimental testing and language polishing. We remain sincerely grateful to the anonymous reviewers, whose insightful comments have significantly improved the quality of this manuscript.

Conflicts of Interest

Authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 3. Macroscopic features of calcite concretions. (a–c) field occurrence of calcite concretions; (d–f) polished calcite concretions; (a) the concretions deflect and locally cross-cut host-rock laminae; (b,c) the concretions with erosional surfaces are elliptical, blocky, or pancake-shaped; (d–f) the concretions with erosional surfaces are composed of yellowish-green replacive growth horizons and grey displacive growth horizons.
Figure 3. Macroscopic features of calcite concretions. (a–c) field occurrence of calcite concretions; (d–f) polished calcite concretions; (a) the concretions deflect and locally cross-cut host-rock laminae; (b,c) the concretions with erosional surfaces are elliptical, blocky, or pancake-shaped; (d–f) the concretions with erosional surfaces are composed of yellowish-green replacive growth horizons and grey displacive growth horizons.
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Figure 4. Photomicrographs showing the petrographic characteristics of calcite concretions. (a) petrographic characteristics under plane-polarized light; (b) petrographic characteristics under cross-polarized light. Sparry calcite crystals (50~300 μm) are filled with green-yellow clay minerals and authigenic quartz.
Figure 4. Photomicrographs showing the petrographic characteristics of calcite concretions. (a) petrographic characteristics under plane-polarized light; (b) petrographic characteristics under cross-polarized light. Sparry calcite crystals (50~300 μm) are filled with green-yellow clay minerals and authigenic quartz.
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Figure 5. PAAS-normalized REYs patterns of calcite concretions [54].
Figure 5. PAAS-normalized REYs patterns of calcite concretions [54].
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Figure 6. The δ18O versus δ13C diagram of calcite concretions. (modified after [67]) [54].
Figure 6. The δ18O versus δ13C diagram of calcite concretions. (modified after [67]) [54].
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Figure 7. Validity assessment of sample’s oxygen isotope data in this study. Blue dots represent the sample data from this study. (a) cross-plot of δ18OPDB versus δ13CPDB of samples; (b) cross-plot of Sr versus Mn contents of samples.
Figure 7. Validity assessment of sample’s oxygen isotope data in this study. Blue dots represent the sample data from this study. (a) cross-plot of δ18OPDB versus δ13CPDB of samples; (b) cross-plot of Sr versus Mn contents of samples.
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Figure 8. Cross plot of Eu/Sm versus Sm/Yb and binary mixing modelling of calcite concretions. (modified after [75]) [54].
Figure 8. Cross plot of Eu/Sm versus Sm/Yb and binary mixing modelling of calcite concretions. (modified after [75]) [54].
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Figure 9. Element correlation plot of samples in this study. Blue dots represent the sample data from this study. (a) cross-plot of Al versus Os contents of samples; (b) cross-plot of Ce/Ce*SN versus Os contents of samples; (c) cross-plot of Eu/Eu*SN versus La/NdSN of samples; (d) cross-plot of Al versus Ce/Ce*SN of samples; (e) cross-plot of Al versus Pr/YbSN of samples; (f) cross-plot of Al versus Y/HoSN of samples.
Figure 9. Element correlation plot of samples in this study. Blue dots represent the sample data from this study. (a) cross-plot of Al versus Os contents of samples; (b) cross-plot of Ce/Ce*SN versus Os contents of samples; (c) cross-plot of Eu/Eu*SN versus La/NdSN of samples; (d) cross-plot of Al versus Ce/Ce*SN of samples; (e) cross-plot of Al versus Pr/YbSN of samples; (f) cross-plot of Al versus Y/HoSN of samples.
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Figure 10. Os isotopic binary mixing of calcite concretions. Blue dots represent the sample data from this study. (modified after [81]).
Figure 10. Os isotopic binary mixing of calcite concretions. Blue dots represent the sample data from this study. (modified after [81]).
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Figure 11. Schematic illustration of diagenetic environment with redox zones, the modelled aqueous REYs patterns, and the growth model for the calcite concretions in Mesoproterozoic oceans (modified after [30,85]). The yellow field represents the REY patterns of the studied samples (details in Figure 5), together with the associated microbial respiration processes and the corresponding chemical zones involved in their formation.
Figure 11. Schematic illustration of diagenetic environment with redox zones, the modelled aqueous REYs patterns, and the growth model for the calcite concretions in Mesoproterozoic oceans (modified after [30,85]). The yellow field represents the REY patterns of the studied samples (details in Figure 5), together with the associated microbial respiration processes and the corresponding chemical zones involved in their formation.
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Figure 12. Mechanisms and applications of marine carbon mineralization. (a) mechanism of carbon mineralization; (b) relationship between carbon storage duration and carbon trapping mechanism (modified after [34]); (c) carbonate concretions on the water-sampling pipeline of the CarbFix CCS (modified after [109]).
Figure 12. Mechanisms and applications of marine carbon mineralization. (a) mechanism of carbon mineralization; (b) relationship between carbon storage duration and carbon trapping mechanism (modified after [34]); (c) carbonate concretions on the water-sampling pipeline of the CarbFix CCS (modified after [109]).
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Zhang, C.; Tian, W.; He, Y. The Carbon Sink in the Mesoproterozoic Ocean and Its Implications for Marine Carbon Storage Pathways. Sustainability 2026, 18, 6851. https://doi.org/10.3390/su18136851

AMA Style

Zhang C, Tian W, He Y. The Carbon Sink in the Mesoproterozoic Ocean and Its Implications for Marine Carbon Storage Pathways. Sustainability. 2026; 18(13):6851. https://doi.org/10.3390/su18136851

Chicago/Turabian Style

Zhang, Chaokun, Wei Tian, and Yanxin He. 2026. "The Carbon Sink in the Mesoproterozoic Ocean and Its Implications for Marine Carbon Storage Pathways" Sustainability 18, no. 13: 6851. https://doi.org/10.3390/su18136851

APA Style

Zhang, C., Tian, W., & He, Y. (2026). The Carbon Sink in the Mesoproterozoic Ocean and Its Implications for Marine Carbon Storage Pathways. Sustainability, 18(13), 6851. https://doi.org/10.3390/su18136851

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