Abstract The Neogene expansion of C 4 grasslands transformed terrestrial ecosystems with marked influence on mammalian evolution, including hominins. However, the asynchronous C 4 expansion on different continents makes it difficult to identify the environmental drivers, especially for higher latitudes. Here we show that rainfall seasonality governed extratropical Plio-Pleistocene C 4 distributions in East Asia. Rainfall oxygen isotope ratios and clumped isotope soil temperatures exhibit coupled variations on the Chinese Loess Plateau (CLP) from 7 to 2.5 million years ago, indicating more spring rain during warmer times when the subtropical westerly jet was further poleward, and more concentrated summer rain under cooler climates. We attribute these changes to meridional shifts of a summer rain band on orbital and longer timescales. The most C 4 -rich ecosystems, as identified by organic carbon δ 13 C records, tracked this summer rain band, eventually eclipsing the southern CLP margin during the late Pleistocene cooling. Our model refines the East Asian paleomonsoon concept and explains the equatorward migration of extratropical C 4 ecosystems, highlighting the tight coupling between regional rainfall seasonality and vegetation.

Continental weathering regulates long-term climate via atmospheric CO2 consumption. Understanding how weathering regimes respond to tectonic and climatic forcing is therefore essential for clarifying the weathering-climate relationships. The Plio-Pleistocene reorganization of the Yellow River drainage provides an ideal setting to investigate such changes. While accumulating evidence suggests establishment of the modern-like drainage system no later than similar to 1 Ma, how this geomorphic transformation affected weathering patterns remains unexplored. Here, we address this question using new Li isotopic data (delta Li-7) from a marginal sea sediment core, integrated with Nd isotopes and geochemical records. Our multi-proxy record reveals a coupled provenance and weathering regime shift during the Mid-Pleistocene Transition (MPT). The epsilon Nd values shift from cratonic (-12 to -16) to orogenic (-10 to -12) signatures, while delta Li-7 values of weathering products transition from -2.7 +/- 0.9 parts per thousand to -0.9 +/- 0.6 parts per thousand. Combined with mineralogical and geochemical weathering indices, these data document a shift from more incongruent weathering dominated by continental margin floodplains to more congruent weathering governed by highlands of the NE Tibetan Plateau and Loess Plateau. We interpret this transition as a direct response to Yellow River reorganization, which replaced proximal cratonic sources with detritus from rapidly eroding orogenic and loess regions. Our study thus demonstrates that the balance between floodplain and mountain weathering set by large river drainage integration, maybe an important control on the type and intensity of silicate weathering over geologic timescales.

Non-rainfall water inputs (NRWI) are important sources of water in arid regions. The least studied NRWI is the direct sorption of atmospheric water vapor. It occurs near the surface in sandy desert soils but has not been investigated in wetter climates, where it could be important during seasonal or episodic drought, or in clay-rich soils, where it might be expected to occur more extensively. We report soil temperatures and soil water isotopic compositions indicating sorption of atmospheric water vapor deep within floodplain Vertisols in Texas, USA where annual rainfall exceeds 1000 mm. Profiles during the summer show that soil water delta 18O and delta D values above 40 cm plot to the right of the local meteoric water line (LMWL), consistent with liquid water undergoing evaporation, whereas those below 40 cm plot to the left of the LMWL (by as much as 2 parts per thousand in delta 18O space), confirming the presence of a diffused vapor component. Spatial and temporal coincidence at 100 cm during JuneJuly of 1) maximum latent heating, 2) maximum deviation to the left of the LMWL and 3) agreement between independent hydrogen and oxygen isotope mass balance calculations indicate that as much as 5 % of the soil water at this depth is condensed atmospheric vapor. The downward vapor flux is probably facilitated by the high smectite content, which makes these soils highly hydrophilic and results in deep cracking during the summer. These cracks provide air-filled conduits through which water vapor diffuses and sorbs in the cool subsurface. The downward flux of water vapor during the summer may help mitigate drought by partially replacing transpired water. Plain Language Summary: In most soils, water addition occurs exclusively through precipitation. In some soils, fog and dew are substantial water sources. In dry climates, some soils have been shown to remove water vapor directly from the air as it sticks to the surfaces of soil particles overnight. Although this water source is small in comparison to rainfall, the process of atmospheric vapor sorption helps moderate how much the land surface heats up during the day and may help sustain organisms during drought. In this study, we report observations that are consistent with an atmospheric water vapor source to clay-rich soils at deeper depths and in a wetter climate than has been previously recognized. We use isotopic compositions of water and soil temperatures as indicators of this process and provide a conceptual model in which the clay-rich soils of interest crack when dry providing conduits for vapor to penetrate to about 1 m depth where it is absorbed by the clay. This process may buffer the effects of drought.

Paleo-CO2 reconstructions are integral to understanding the evolution of Earth system processes and their interactions given that atmospheric CO2 concentrations are intrinsically linked to planetary function. Furthermore, past periods of major climate change provide unique insights into the response of land-atmosphere-ocean interactions to warming-induced climate change, particularly for times of pCO2 comparable to those projected for our future. How well the past can inform the future, however, depends on how well paleo-CO2 estimates areconstrained. CO2 estimates exist for much of the past half-billion years (the Phanerozoic), but proxies differ in their assumptions and degree of understanding, and there is substantial uncertainty and inconsistency in existing paleo-CO2 estimates. Here, we introduce a community initiative, CO2PIP, focused on advancing the science of paleo-CO2 reconstruction through critically evaluating and modernizing existing records and building a statistically robust multi-proxy atmospheric CO2 record for the Phanerozoic. CO2PIP builds on the previous work of the Cenozoic CO2 Proxy Integration Project (CenCO2PIP) Consortium (Hönisch et al., 2023) and takes a multi-step approach to building the next generation Phanerozoic CO2 record. We are building a standardized paleo-CO2 proxy data repository that includes all metadata and updated chronology and meets FAIR (findable, accessible, interoperable, reusable) data standards. Existing terrestrial-based CO2 estimates are being modernized through additional analyses and measurements, and a set of forward proxy system models are being developed to provide a quantified representation of proxy sensitivities to environmental and ecophysiological conditions and processes that govern the CO2 signals. Ultimately, statistical inversion analysis of the simulated and modernized proxy datasets will be used to produce quantitative, data-driven CO2 reconstructions for individual records and to generate a robust, quantitative reconstruction of atmospheric CO2 concentrations through the Phanerozoic. Digital infrastructure for presenting and archiving the CO2 compilation and project outputs (https://paleo-co2.org/) ensures full accessibility to the scientific community and the public.Hönisch, B. Royer, D., Breecker, D. O., et al., 2023, Towards a Cenozoic history of atmospheric CO2. Science, v. 382 (6675), DOI: 10.1126/science.adi5177).

Soils comprise the largest terrestrial carbon pool. Therefore, understanding processes that control soil carbon stabilization and release is vital to improving our understanding of the global carbon cycle. Heterotrophic respiration is the main pathway by which soil organic carbon is returned to the atmosphere; however, not all carbon utilized by heterotrophs shares this fate, as some portion is retained in the soil as biomass and biosynthesized extracellular compounds. The fraction of carbon consumed by microbes that is used for biomass growth (the carbon use efficiency or CUE) is an important variable controlling soil carbon stocks but is difficult to measure. Here we show that CUE can be continuously monitored in laboratory glucose-amended soil incubations by measuring CO2 and O2 gas concentrations, allowing instantaneous estimates of microbial biomass growth. We derive a theoretical relationship between the respiratory quotient (RQ), the ratio of carbon dioxide produced to oxygen consumed during respiration, and CUE that recognizes the influence of both substrate and biosynthesized product oxidation states on RQ. Assuming the biosynthesized product has the stoichiometry of an average microbe and that the substrate is primarily the glucose used for amendment, we measure RQ and use our theoretical relationship to calculate CUE and from that biomass production. Extractions of microbial biomass carbon at the end of the experiments reveal minimal net increases in standing biomass across all amended treatments, suggesting that much of this newly produced biomass is likely converted to necromass as substrate availability declines, and this results in a net storage of new soil organic matter. Carbon budgets compiled from measurements of relevant pools account for the amended carbon and suggest that with larger carbon amendments, increases in C:N ratios lead to increases in the relative portion of the amendment acutely lost from the soil. These findings demonstrate that soil RQ values may be used to monitor changes in CUE and that studies that monitor soil RQ values should consider CUE as a key factor when changes in RQ are observed, for instance, with changing environmental conditions or changes in production of plant-derived compounds. This new approach may be leveraged to provide information on the storage of soil organic matter. These findings demonstrate how measurements of soil RQ may be leveraged to understand soil carbon transformations, specifically the fate of fresh carbon inputs.

The semi-arid western region of South Africa hosts extensive earthen mounds known as heuweltjies, which are inhabited by Microhodotermes viator termites and play a critical role in soil biogeochemical cycling. These mounds accumulate significant stores of soil organic and inorganic carbon (C), including pedogenic calcium carbonate, which may form through microbially induced calcite precipitation. In this study, the effects of land use change on C dynamics in heuweltjie soils were assessed by examining soil biogeochemistry and apparent respiratory quotient (ARQ, based on soil pore gas composition). We investigated the oxalate-carbonate pathway (OCP) as a potential mechanism of C sequestration. Topsoils were collected from one pristine and one cultivated termite mound in a semi-arid region of South Africa and incubated for one week. The carbon dioxide (CO2) and oxygen concentrations of soil pore gas as well as chemical properties of soils treated with termite frass (excrement) or calcium oxalate (CaOx) were monitored. Increases in pH and the calcite saturation index in both CaOxand frass-treated soils suggested the potential occurrence of the OCP. The ARQ values did not reflect geochemical changes associated with OCP due to competing metabolic pathways, such as potential lignin degradation in frasstreated soils. Higher ARQ values in uncultivated versus cultivated CaOx-treated soils may indicate higher carbon use efficiency in uncultivated soils or destabilization of existing C in cultivated soils. Respiration in frass-treated soils was higher than control and CaOx-treated soils and resulted in production of bicarbonate (via dissociation of carbonic acid formed by dissolution of respired CO2 in water). This implies that termite-affected landscapes may sequester C in inorganic form. Increased total C in both cultivated and uncultivated soils treated with frass suggests that microbial CO2-fixation may occur in termite-affected landscapes, necessitating further investigation of pathways responsible for this process.

Subterranean termite mounds are key contributors to soil carbon storage in the semi-arid landscapes of the Greater Cape Floristic Region of South Africa. This study assessed soil organic carbon (SOC) and soil inorganic carbon (SIC) stocks in cultivated and uncultivated mounds in the Swartland region of the Western Cape. Radiocarbon activities of organic matter and soil-respired CO2 were measured to evaluate vertical carbon mixing rates within uncultivated mound soil profiles and to gain insight into microbial SOC utilization in topsoils of cultivated and uncultivated mounds. Cultivation significantly decreased total C stocks in mound topsoils. Spatial distribution of SOC, allocation of SOC to different stability fractions and SOC radiocarbon signatures in uncultivated mound soils showed that termite bioturbation facilitated the downward vertical movement of organic material and subsequent stabilization by organo-mineral interactions in subsoils. Spatial association of SOC with SIC provided evidence of biogenic carbonate formation in both cultivated and uncultivated mounds, which may serve as an additional C stabilization mechanism. Cultivation altered SOC stability, as soil-respired CO2 with lower radiocarbon activity (Delta 14C = -18.6 %o) in cultivated mound topsoils compared to uncultivated mounds (Delta 14C = 15.7-100.45 %o) indicates microbial exhaustion of labile SOC and degradation of recalcitrant pools in cultivated mound soils. The findings highlight termite mounds as critical reservoirs of subsoil SOC and SIC in a semi-arid region of South Africa. Cultivation destabilizes SOC stores in mound topsoils, therefore sustainable management of termite-affected landscapes is essential to maintain their carbon storage potential.

Quantifying climate sensitivity is essential for future climate projections, yet it varies with major Earth system changes. We present a glacial CO₂ reconstruction using paleosols from the Chinese Loess Plateau, covering 2580 to 800 thousand years ago. A stepwise decline in glacial CO₂ levels from ~300 ppm to <200 ppm is observed. Our paleosol-based CO₂ estimates support the key role of atmospheric CO₂ in driving major climate transitions during the Pleistocene, such as the long-term global cooling and the amplification of the glacial cycles. Based on compiled glacial and interglacial CO2 records, Earth system sensitivity, defined as the global temperature change for a doubling of CO2 once the whole Earth system has reached equilibrium, is estimated to be ~6.2-7.4 K (3.2-12.0 K, 95% confidence). Equilibrium climate sensitivity, after accounting for the different efficacy between ice-sheet and CO2 forcing and other slow feedbacks, is estimated to be 3.3 K (2.1-6.3 K, 95% confidence) and 3.7 K (1.7-6.3 K, 95% confidence), respectively. The lack of a significant difference between these values suggests no apparent state-dependency of climate sensitivity between glacial and interglacial climate states.

Anthropogenic carbon emissions contribute significantly to the greenhouse effect, resulting in global warming and climate change. Thus, addressing this critical issue requires innovative and comprehensive solutions. Silicate weathering moderates atmospheric CO2 levels over geological time, but it occurs too slowly to counteract anthropogenic emissions effectively. Here, we show that the microorganism Bacillus subtilis strain MP1 promotes silicate weathering across different experimental setups with various levels of complexity. First, we found that MP1 was able to form a robust biofilm in the presence of feldspar and significantly increased (p < 0.05) silicate dissolution rates, pH, and calcium carbonate formation in culture experiments. Second, in mesocosm experiments, we found that MP1 enhanced the silicate weathering rate in soil by more than six times compared to the untreated control. In addition, soil inorganic carbon increased by 20%, and the concentrations of ions, including calcium, magnesium, and iron, were also elevated under the MP1 treatment. More importantly, when applied as a seed treatment on eight soybean fields, we found that MP1 significantly (p < 0.05) boosted soil inorganic carbon, leading to a gross accrual of 2.02 tonnes of inorganic carbon per hectare annually. Our findings highlight the potential of enhancing native silicate weathering with microorganisms in agricultural fields to increase soil inorganic carbon, contributing to climate change mitigation.

Silicate weathering moderates atmospheric CO2 levels over geologic time but proceeds naturally at rates too slow to substantially mitigate anthropogenic emissions. Microorganisms can accelerate silicate weathering and may thereby augment carbon dioxide removal. Here, we demonstrate that Bacillus subtilis strain MP1 can couple silicate dissolution with carbonate precipitation to capture CO2. In culture, MP1 formed biofilm in the presence of feldspar and increased silicate dissolution rates, pH and calcium carbonate formation relative to MP1-free controls. In a soil column experiment, MP1 enhanced inorganic carbon accumulation and the levels of available Ca2+, Mg2+, Fe2+, and Al3+. These results suggest that MP1 was able to accelerate the weathering of silicates naturally present in the soil to release cations. In field experiments, application of MP1 increased soil inorganic carbon and exchangeable calcium, resulting in a gross accrual of 2.02 tonnes inorganic C ha-1 y-1 in the uppermost 30 cm of the soil column. Our results are consistent with a 'trans' calcification type model in which MP1 links silicate dissolution with calcite precipitation via cross-biofilm Ca2+ and proton transport. Furthermore, our results demonstrate the potential for microbially-enhanced silicate weathering in agricultural soils to promote inorganic carbon accumulation and removal of atmospheric CO2.

The Cretaceous is characterized as a greenhouse climate from elevated atmospheric carbon dioxide concentrations, transgressive seas, and temperate ecosystems at polar paleolatitudes. Here we test the hypothesis that the early Cretaceous was a cold climate state with a new Aptian atmospheric carbon dioxide record from the C3 plant proxy and early Cretaceous sea level curve from stable oxygen isotopes of belemnites and benthic foraminifera. Results show that carbon dioxide concentrations were persistently below 840 ppm during the Aptian, validating recent General Circulation Model simulations of ice sheets on Antarctica at those concentrations. In addition, sea level was estimated to be within the ice sheet window for much of the early Cretaceous prior to the Albian. This background state appears to have been episodically interrupted by Large Igneous Province volcanism followed by long-term carbon burial from weathering. We hypothesize that the early Cretaceous was largely an icehouse punctuated by warm snaps. Atmospheric carbon dioxide concentrations remained below 840 parts per million and polar regions were glaciated throughout much of the Early Cretaceous except during episodic volcanism, according to an analysis of stable isotope composition of plants and biogenic carbonate data.

Abstract Quantitative isotopic paleoaltimetry has been applied in regions where Rayleigh distillation controls isotopic lapse rates. Air mass mixing and moisture recycling are viewed as complicating factors. We show here that, because of such effects, a cross‐Andean transect of meteoric water δD values precisely marks the geographic position of the Western Cordillera crest. This modern water signal is also recorded in Pliocene‐Pleistocene hydrated volcanic glass δD values. δD values between the Pacific coast and Western Cordillera exhibit no trend up to 2.5 km elevation and 100 km inboard, consistent with an arid climate in which Amazonian moisture is topographically blocked and Pacific moisture is efficiently recycled. The result is a large δD lapse rate (−98‰/km) and an abrupt horizontal δD shift (2‰/km) at the Western Cordillera crest. Therefore, we conclude that cross‐orogen δD transects could locate the ancient Western Cordillera crest.

Silicate weathering has long been considered to maintain the Earth's climate stability, yet how the weathering responds to the late Cenozoic cooling remains unclear, partly because of the complicated factors which obscure the weathering records. Large rivers in East Asia integrate continental weathering history, but how the source-tosink system evolution affected the weathering signals need to be clarified. We compile proxy data of Nd isotopes, zircon ages and Chemical Index of Alteration (CIA) in East Asian margin and source terranes, along with new proxy data from core CSDP-2 in the shelf region, to understand the large river evolution and assess their influence on weathering proxy records. The median epsilon Nd values of East Asian marginal sediments increased from -18.8 in pre-3.6 Ma period to -11.3 since the 1.0 Ma, corresponding to the isotopic signatures in North China Craton and Northeastern (NE) Tibetan Plateau/Loess Plateau terranes, respectively. The zircon ages further confirmed the provenance shift, hinting that the modern-like Huanghe (Yellow River) system has been fully integrated during early Pleistocene, no later than similar to 1.0 Ma. This integration facilitated the sediment transportation from plateaus into the continental margin. Consequently, the CIA records in marginal regions show a more significant decrease than other Asian regions, within the context of Plio-Pleistocene cooling. Our study found that the upland expansion of large river system and global cooling jointly controlled the decline in the weathering intensity records, highlighting the importance to understand the evolution of source-to-sink system before interpreting the weathering signals from the continental margin sediments.

Soil organic matter (SOM) is a critical player in the global carbon cycle and a common paleoclimate archive, yet the mechanisms governing its evolution, particularly in subsoil (>30 cm) systems, remain less well understood. To better characterize subsoil SOM dynamics, we investigated the isotopic compositions of SOM in fine (<2 mu m) and coarser (>2 mu m) grain-size fractions of the top 2 m of a soil profile developing in eolian sediments on the Chinese Loess Plateau. We found that in the subsoil, SOM in the fine fraction, characterized by lower C/N and higher delta C-13, exhibits consistently younger radiocarbon ages than those in the coarser fraction. Such offsets suggest that organo-mineral interactions facilitate fresh carbon input to the fine fraction rather than preventing SOM decomposition. We then developed a mass-balance model to interpret the radiocarbon data. The results suggest continual carbon input from the above-ground biosphere to the 2-m deep subsoil. The active subsoil organic carbon cycling can lead to significant variations in delta C-13(SOM) under an environment that experienced substantial vegetation changes, resulting in time-averaged signals when using delta C-13(SOM) for paleoenvironmental reconstructions. Our study underscores the dynamic nature of the subsoil carbon pool, emphasizing its potentially underestimated role in the contemporary carbon cycle.

The stable isotopic composition of pedogenic carbonates is central to many studies of past climate and topography, providing a basis for our understanding of Earth's terrestrial history. A core assumption of many applications of oxygen isotope values (δ18O) of pedogenic carbonate is that they reflect the δ18O value of precipitation (rain/snow). This assumption is violated if soil carbonates form in evaporated soil waters. In this work, we develop a means to identify evaporation in ancient soils using the triple oxygen isotope composition (16O-17O-18O) of pedogenic carbonates. Both theoretical predictions of isotope kinetics during evaporation and studies of triple oxygen isotopes in other geological materials show that the deviation in the relationship between δ17O and δ18O from a reference line, evaluated using the parameter Δ′17O, is sensitive to evaporation. As a first step in developing the use of Δ′17O in ancient pedogenic carbonates, we report Δ′17O values from 47 near-modern pedogenic carbonate samples from globally distributed environments that vary in aridity (hyper-arid to humid). The Δ′17O values of pedogenic carbonate range from −154 to −60 per meg (as CaCO3, measured via O2, VSMOW-SLAP), corresponding to calculated soil water values of −66 to +27 per meg (VSMOW-SLAP) (using a carbonate-water triple oxygen isotope fractionation exponent of 0.5250 and clumped isotope-derived carbonate growth temperatures). The Δ′17O values indicate that evaporative modification of soil water from which pedogenic carbonate forms is common, especially in arid environments. Arid environments host pedogenic carbonates formed from soil waters ranging from highly to minimally evaporated, while humid environments host pedogenic carbonates formed from waters that are only minimally evaporated. The variability in Δ′17O within environments classified by the same aridity may relate to the fact that pedogenic carbonates record soil conditions only during times of carbonate mineralization, which may deviate from annual conditions. Thus, Δ′17O may be useful in understanding the specific circumstances of pedogenic carbonate formation but may not provide incontrovertible evidence of the magnitude of environmental aridity. Evaporative modification of δ18O values of pedogenic carbonates can be detected with Δ′17O, thereby improving estimates of δ18O of unevaporated waters. Our data show that evaporation must be (re)considered for all paleoclimate inferences based on the δ18O of pedogenic carbonate. The addition of Δ′17O will re-energize paleoclimate studies that use (or have avoided using) δ18O of pedogenic carbonate.

Stalagmites being prepared for paleoclimate analysis should typically be slabbed along the central growth axis. This is an important first step because it allows for the highest resolution of sampling with minimal over- or under-sampling of the growth layers. Further, stable isotope ratios and trace element concentrations along the central growth axis most closely record climate variability. Choosing how to slab to best expose the central growth axis for geochemical sampling is challenging based on external morphology alone. High-resolution X-ray computed tomography (XRCT) can provide the ability to discern the internal growth morphology of stalagmites non-destructively, inexpensively, and rapidly. These data can inform selection of optimal slabbing plane(s) and can help identify locations for preliminary U-series dating. We develop a conceptual screening model to assess rapidly the internal morphologies of uncut stalagmites. The specifics of screening the internal morphologies through XRCT scans include investigating the internal porosity of the sample, the number and size of voids and hiatuses, and the presence and absence of growth layers and growth axes. We demonstrate that XRCT scans capture the migration of center of growth in uncut stalagmites of both simple and complex internal morphologies. XRCT scanning facilitates the investigation of stalagmites with complex internal growth banding, opening up avenues to work on such samples when stalagmites with simpler internal morphologies are not available. Further, screening stalagmites for paleoclimate reconstructions using XRCT improves the sustainability of speleothem science by helping researchers select which stalagmites should be returned to caves without destructive slabbing, thereby minimizing impact on caves.