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Biological carbon pumping (BCP) is a key process in which dissolved inorganic carbon in terrestrial aquatic ecosystems is utilized by aquatic autotrophs for photosynthesis and transformed into autochthonous organic matter (AOC). However, the mechanisms underlying BCP and the amount of generated AOC deposited effectively, are still poorly understood. Therefore, we conducted a systematic study combining modern hydrochemical monitoring and a sediment trap experiment in Fuxian Lake (Yunnan, SW China), the second-deepest plateau, oligotrophic freshwater lake in China. Temperature, pH, EC (electrical conductivity), DO (dissolved O2), [HCO3−], [Ca2+], SIc, partial CO2 (pCO2) pressure, and carbon isotopic compositions of HCO3− (δ13CDIC) in water from Fuxian Lake all displayed distinct seasonal and vertical variations. This was especially apparent in an inverse correlation between pCO2 and DO, indicating that variations of hydrochemistry in the lake water were mainly controlled by the metabolism of the aquatic phototrophs. Furthermore, the lowest C/N ratios and highest δ13Corg were recorded in the trap sediments. Analyses of the C/N ratio demonstrated that the proportions of AOC ranged from 30% to 100% of all OC, indicating that AOC was an important contributor to sedimentary organic matter (OC). It was calculated that the AOC flux in Fuxian Lake was 20.43 t C km−2 in 2017. Therefore, AOC produced by carbonate weathering and aquatic photosynthesis could potentially be a significant carbon sink and may have an important contribution to solving the lack of carbon sinks in the global carbon cycle. [ABSTRACT FROM AUTHOR]

• Calcites precipitated from the solutions with identical chemical and isotopic compositions have different δ 13 C values. • The DBL thickness affects the pH and the δ 13 C value of HCO − 3 at the mineral-solution interface. • Our finding offers a mechanistic interpretation on the variability of the ε calcite / DIC in riverine or cave environments. It is often assumed that the carbon isotope composition (δ 13 C) of carbonate minerals records that of the dissolved inorganic carbon (DIC) species with insignificant disequilibrium effect. However, results from field observations and laboratory experiments have shown that the δ 13 C difference between calcite and solution can vary up to 3‰ even under a similar set of solution composition and temperature, raising uncertainties on the δ 13 C's paleoclimate and paleoecology implications. One likely cause is the variable calcite precipitation rates and pH values induced by different thicknesses of the stagnant liquid layer between solid and well mixed bulk solution (i.e., diffusion boundary layer, DBL). To test this hypothesis, we selected a well-studied natural travertine deposit (Baishuitai, SW China) which consists of meter-scale travertine terraces. Calcite crystals at both the pool bottom and the rim of the terraces are inorganically precipitated from solutions identical in chemical, isotopic composition and temperature, but different only in hydrodynamics. This difference results in thicker DBL for water at the pool-bottom than for water flowing across the rim. We found that the δ 13 C and Mg/Ca ratios are higher while Sr/Ca ratios are lower for the pool-bottom calcites than for the rim calcites. By applying a mass transfer model, we quantitatively link the differences in carbon isotope and elemental compositions of the abiogenic calcites to the different hydrodynamic conditions. The inverse variation in Mg/Ca and Sr/Ca ratios in calcites arises from the different precipitation rates between at the pool-bottom and at the rim, while the consistently lower δ 13 C for calcites at the rim is due to their higher pH at the solid-solution interface than for calcites precipitated at the pool-bottom. In contrast, calcite precipitation rate has little influence on carbon isotope fractionation between calcite and HCO − 3 . Our results demonstrate the role of DBL thickness in governing the δ 13 C of HCO − 3 at mineral surface, which can assist to interpret the variable δ 13 C values of calcites in riverine or cave environments. [ABSTRACT FROM AUTHOR]

Microbial activity has been proposed to play an important role in the genesis of manganese (Mn) carbonate deposition in natural sediments, with microbial reduction of Mn(IV) oxides as a crucial factor that enhances the saturation state of Mn carbonates in pore waters. Recently, the nucleation of Mn carbonates on cellular templates has been suggested to represent a further microbial control. However, microstructural evidence for this model remains tentative and the Mn carbonate nucleation pathway is still unconstrained. Here we report petrographic and in-situ chemistry that supports the direct role(s) of microbes in the formation of Mn carbonates in the giant Malkansu Mn metallogenic belt at West Kunlun, Northwest China. The Mn carbonate ores exhibit negative δ13C values (−10.2 to −12.7‰) that indicate the microbial reduction of Mn oxide. Microscopic data show that the Mn carbonates exhibit spheroidal rod and dumbbell-like shaped morphologies with a well-defined core-shell structure. These carbonates are intimately associated with net-like organic matter. An incubation experiment was also carried out showing that biogenic Mn carbonate minerals share a strikingly similar crystal form with the natural Mn ore samples. Together, these results provide further evidence for a dual role of microbes in Mn‑carbonate ore formation: Mn(II) and carbonate produced by the microbial reduction of Mn(IV) oxides, and the nucleation of Mn carbonate minerals on cellular surfaces and/or organic matrices. Potentially, these Mn carbonates may have self-assembled to form rod- and dumbbell-like shaped morphologies, analogous to other carbonate systems. The unique core-shell structure observed herein also indicates that the formation of Mn‑carbonates mediated by microbes follows a heterogeneous nucleation pathway. [ABSTRACT FROM AUTHOR]