Pedogenic Carbonate Research Articles

The Process of Soil Carbon Sequestration in Different Ecological Zones of Qingtu Lake in the Arid–Semi-Arid Region of Western China

As a vital component of the global carbon pool, soils in arid and semi-arid regions play a significant role in carbon sequestration. In the context of global warming, increasing temperatures and moisture levels promote the transformation of barren land into wetlands, enhancing carbon sinks. However, the overdevelopment of oases and excessive extraction of groundwater lead to the opposite effect, reducing carbon sequestration. This study examines two soil types—meadow soil (MS) and swamp soil (SS)—from Qingtu Lake, an arid lake in western China. It analyzes the sources of soil inorganic carbon, the composition and origin of dissolved organic matter (DOM), and the relationships between microbes, soil organic carbon (SOC), soil inorganic carbon (SIC), mineral composition, and soil texture. The results indicate that inorganic carbon in the study area consists of both primary carbonate minerals and secondary pedogenic carbonates. The DOM primarily consists of two components, both identified as terrestrial humic substances. In meadow soils, bacterial activity drives the weathering of plagioclase, which releases Ca2+ necessary for the formation of pedogenic carbonates. Plagioclase also provides colonization sites for microbes and, along with microbial activity, participates in the soil carbon cycle. Within the soil community, bacteria appear to play a more critical role than fungi. In contrast, microbial contributions to the carbon cycle in swamp soils are weaker, with minerals predominantly interacting with organic carbon to form mineral-associated organic matter, thus promoting the soil carbon cycle. These findings have important implications for understanding soil carbon sinks under different micro-ecological conditions in arid and semi-arid regions. Through targeted human intervention, it is possible to enhance carbon sequestration in these areas, contributing to the mitigation of global climate change.

Inorganic Carbon Pools and Their Drivers in Grassland and Desert Soils.

Inorganic carbon is an important component of soil carbon stocks, exerting a profound influence on climate change and ecosystem functioning. Drylands account for approximately 80% of the global soil inorganic carbon (SIC) pool within the top 200 cm. Despite its paramount importance, the components of SIC and their contributions to CO2 fluxes have been largely overlooked, resulting in notable gaps in understanding its distribution, composition, and responses to environmental factors across ecosystems, especially in deserts and temperate grasslands. Utilizing a dataset of 6011 samples from 173 sites across 224 million hectares, the data revealed that deserts and grasslands in northwestern China contain 20 ± 2.5 and 5 ± 1.3 petagrams of SIC in the top 100 cm, representing 5.5 and 0.76 times the corresponding soil organic carbon stock, respectively. Pedogenic carbonates (PIC), formed by the dissolution and re-precipitation of carbonates, dominated in grasslands, accounting for 60% of SIC with an area-weighted density of 3.4 ± 0.4 kg C m-2 at 0-100 cm depth, while lithogenic carbonates (LIC), inherited from soil parent materials, prevailed in deserts, constituting 55% of SIC with an area-weighted density of 7.1 ± 1.0 kg C m-2. Soil parent materials and elevation determined the SIC stocks by regulating the formation and loss of LIC in deserts, whereas natural acidification, mainly induced by rhizosphere processes including cation uptake and H+ release as well as precipitation, reduced SIC (mainly by PIC) in grasslands. Overall, the massive SIC pool underscores its irreplaceable role in maintaining the total carbon pool in drylands. This study sheds light on LIC and PIC and highlights the critical impact of natural acidification on SIC loss in grasslands.

Crop diversity significantly enhances soil carbon sequestration via alleviating soil inorganic carbon decline caused by rhizobium inoculation

Increasing crop diversity and nitrogen (N) fertilizer application have been identified as effective strategies for enhancing productivity and soil organic carbon (SOC) storage in agroecosystems. However, the impact of these management practices on soil inorganic carbon (SIC) in agroecosystems remains unclear. At present, we evaluated the effects of maize/faba bean intercropping, N application rates, and inoculation rhizobia of faba bean on SIC in the top 20 cm of soil depth using a 13-year crop diversity field experiment. Our results showed that the soil total carbon (TC) content increased significantly by 5.9 % and 7.0 % compared to faba bean monoculture and maize monoculture, respectively, after 13 years of continuous intercropping. Intercropping increased the pedogenic carbonate (PIC) content by 36.7 %, resulting in an 8.9 % higher SIC content compared to faba bean monoculture. Additionally, intercropping significantly reduced the dissolution of lithogenic carbonate (LIC) by 17.5 %, leading to a 7.6 % higher SIC content compared to maize monoculture. The formation of PIC was associated with an increase in soil available cations especially Ca2+ in intercropping. The conservation of LIC was related to the higher soil available Mg2+ in intercropping than monoculture. Faba bean inoculated with rhizobia significantly decreased SIC content due to soil acidification after 13 years of continuous cropping. Intercropping also significantly increased SOC and C3-derived SOC content compared to maize monoculture and increased C4-derived SOC content compared with faba bean monoculture. Soil organic carbon showed a positive correlation with SIC across all cropping systems, and the SOC fractions could affect the neoformation of PIC and dissolution of LIC. Our results demonstrate that intercropping can increase SIC content, which further promotes soil carbon sequestration. This study highlights the significance of increasing crop diversity on cropland carbon sequestration and provides practical implications for mitigating carbon emissions.

Composition and storage of soil inorganic carbon as well as the controlling factors in coastal area of the northern Jiangsu, China.

The sequestration of soil inorganic carbon (SIC) especially pedogenic carbonate (PC) is one of the important pathways reducing the concentration of atmospheric carbon dioxide and thus mitigating climate change in coastal areas. Using the technology of 13C stable isotope, we analyzed the differences in the composition and storage of SIC, and explored the key physicochemical properties influencing soil PC storage in different horizons (0-10, 11-20, 21-40, 41-60, 61-80 and 81-100 cm) from Suaeda salsa wetland (SS), Spartina alterniflora wetland (SA), young poplar plantation (YP), and mature poplar plantation (MP) in coastal area of the northern Jiangsu Province. The results showed that except for the surface (0-10 cm) soil in MP, the SIC content was higher than SOC in all soil horizons. Overall, neither the soil PC to SIC ratio nor the SIC storage were significantly different in SA and SS soils. Compared to wetland soils (0-40 cm), the soil PC to SIC ratio was reduced by 32.7% and 54.1% and the PC storage was reduced by 40.5% and 59.2%, the lithogenic carbonate (LC) storage changed little, while the SIC storage was reduced by 21.0% and 17.9%, respectively in the YP and MP soils. Compared to the YP soils (0-100 cm), both the soil PC to SIC ratio and the PC storage were significantly reduced while the LC storage was significantly increased, especially at the 41-100 cm soil horizons, meanwhile, the SIC storage was not significantly changed in the MP soils. Results of the structural equation modeling (SEM) indicated that key factors influencing soil PC storage were the ratio of PC to SIC, followed by the SOC content and bulk density. SOC could inhibit the formation of soil PC. Generally, the coastal wetlands have greater SIC storage and sequestration potential than poplar plantations, and the PC sequestration can be regulated by modulating the ratio of PC to SIC and SOC content.

Origin and evolution of soil materials of the Paleo-Chadian linear-dune landscape in the Far North Region of Cameroon

The mineralogy and micromorphology of associated red Arenosols and both yellow and grey Gleysols of the Paleo-Chadian linear-dune landscape of the Far North Region of Cameroon were studied in order to obtain information about depositional environments and pedogenic processes. Differences between the red soils and both others include a significant difference in clay content, which is much higher in the grey and yellow soils. The composition of the clay fraction is also different, with kaolinite and illite as main mineral phases in the red soils, whereas smectite is strongly predominant in the grey soils and a major phase in the yellow soils. Mössbauer spectroscopy analysis reveals a predominance of hematite as iron oxide mineral in the red soils, in contrast to goethite-predominance in the yellow soils. Thin section observations reveal poor sorting of the coarse fraction, differences in feldspar content, and an absence of reworked clay aggregates, as well as the presence of pore-related carbonate features in the grey soils and iron oxide nodules in the grey and yellow soils. The obtained results indicate that the red soils are derived from a different parent material than the other soils. The grey soils developed on flood-related fluvial deposits in interdune depressions, whereas the red soils formed on aeolian sands and the yellow soils show a mixed origin. In the interdune depressions, hydromorphic conditions led to iron reduction and pedogenic carbonate enrichment.

Soil development trajectories, chemical weathering and pedogenic mineral (trans)formation on glauconitic calcarenites in a Mediterranean area

Glauconite forms in shallow marine basins under oxic to suboxic conditions. It is an Fe-rich sheet silicate often found in calcareous sandstones. How soils evolve on glauconitic sandstones (calcarenite) is poorly addressed: therefore, we investigated the evolutional trajectories. These soils should be enriched in Fe, depleted in K and contain kaolinite and smectite resulting from weathering. We investigated 11 soil profiles on glauconitic calcarenite in a Mediterranean area. The elemental contents, fractions of Fe and Al, soil mineralogy, magnetic susceptibility and stable isotopic composition (C, O) of the bulk soil and carbonates were analysed to explore pedogenesis.The carbonates reflect a former marine basin having onshore organic matter resulting from freshwater input. With soil formation, the primary carbonates were replaced by pedogenic carbonates having a lower δ13C and δ18O signal. Several sites were influenced by human activity since ancient times, reflected by the magnetic susceptibility. The shallow soils had a low weathering degree (CIA of 55 to 70). The better developed Vertisols exhibited a CIA value of up to 85. All soils contained smectite and quartz and, depending on the weathering degree, calcite. Kaolinite formation was connected to high Ca, K, Mg and Na losses. The high oxalate- and dithionite-extractable Fe contents are explained by glauconite weathering. Kaolinite is inherited from the parent material but also actively forms in the soils probably through precipitation from soil water. Due to the presence of smectite and oxyhydroxides the soils sequester a high amount of soil organic carbon (up to 30 kg/m2). Topography exerts a distinct influence on weathering and soil formation. Deepest soils and an advanced weathering stage were measured on footslope sites and shallow soils having a low weathering degree on crest/shoulder sites. Soils on glauconitic calcarenite represent a unique weathering environment that should be investigated even more at the micro-scale.

U‐Pb Geochronology of Paleosol Carbonate Cements by LA‐ICP‐MS: A Proof of Concept and Strategy for Dating the Terrestrial Record

AbstractThis study investigates the potential of laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) U‐Pb dating for carbonate nodules in the Late Triassic Ischigualasto Formation of northwestern Argentina. We establish a fully characterized paragenetic sequence to guide the analysis of three pedogenic carbonates and compare the U‐Pb ages with published geochronology from volcanic ashes within the sedimentary succession. Our findings demonstrate the importance of interpreting U‐Pb data within a well‐defined paragenetic framework for accurate age interpretation of pedogenic carbonates. We observe variations in U‐Pb isotopic signatures across different generations of carbonate precipitates and identify syn‐pedogenic and early burial calcite cements as most suitable for precise dating. Respectively, these two calcite cements are interpreted as microcodium and crack‐lining calcite cements formed early in the paragenetic sequence during pedogenesis to early burial of the paleosols as they transitioned from the unsaturated vadose to saturated phreatic zone below the water table. The U‐Pb ages obtained from the carbonate nodules agree with the radioisotopic ages of volcanic ashes, supporting the validity of our dating strategy. These results contribute to advancing U‐Pb carbonate geochronology and highlight its increased potential for dating sedimentary records in the terrestrial realm. Future research should focus on replicating similar work on different carbonate nodules within the Ischigualasto Fm and expanding the application of LA‐ICP‐MS U‐Pb dating to other carbonate‐bearing formations, especially in successions with limited absolute ages or where volcanic ashes are sparse or absent.

Microstructure and quantitative tracing of carbonate for calcareous root tubes in dune soil from the Tengger Desert of Northwestern China: Implications for its formation age and paleoenvironmental reconstruction

Calcareous root tubes (CRTs), which are formed by the encrustation of roots with pedogenic carbonate, serve as an archive for paleoclimate reconstructions. However, controversies remain regarding the formation age of CRT carbonate and the quantitative tracing of its sources in the hinterland of the desert. In this study, 13 CRT samples were obtained from 11 sites in the Tengger Desert of northwestern China. Samples were described using X-ray micro-tomography Scanning (Micro-CT), and their 14C ages, δ13C, and 87Sr/86Sr values were measured. The results show that more than 99.84 % of pore diameters in the precipitated carbonate layer were less than 1 mm, which was much smaller than the plant roots in this region. Moreover, the root residues from poor cementation CRTs were dated at 151 − 431 cal yr BP, which was significantly younger than the carbonate age of other complete cemented CRT. Thus, most of the root residues found within completely cemented CRTs may invade after the CRTs formation and their 14C ages cannot represent the CRTs formation period, while the carbonate 14C dating results can indicate the CRTs formation period. 87Sr/86Sr values of CRTs occurred at 0.71152 − 0.71189, indicating that the main source of calcium in CRTs was the dissolution of carbonate in rhizosphere soil. δ13C values of CRTs varied between −5.4 ‰ and −3.2 ‰. Using the end-member mixing model, carbon sources from CRTs can be divided into rhizosphere soil respiration CO2 (81 − 82 %) and atmospheric CO2 (18 − 19 %). Hence, δ13C values of CRTs may derive the δ13C signal from soil and record paleoenvironment conditions. This study enhances our understanding of the formation mechanism of pedogenic carbonate and provides an important aspect in assisting paleoenvironmental reconstructions.

Clumped Isotopes Record a Glacial‐Interglacial Shift in Seasonality of Soil Carbonate Accumulation in the San Luis Valley, Southern Rocky Mountains, USA

AbstractClumped isotope paleothermometry using pedogenic carbonates is a powerful tool for investigating past climate changes. However, location‐specific seasonal patterns of precipitation and soil moisture cause systematic biases in the temperatures they record, hampering comparison of data across large areas or differing climate states. To account for biases, more systematic studies of carbonate forming processes are needed. We measured modern soil temperatures within the San Luis Valley of the Rocky Mountains and compared them to paleotemperatures determined using clumped isotopes. For Holocene‐age samples, clumped isotope results indicate carbonate accumulated at a range of temperatures with site averages similar to the annual mean. Paleotemperatures for late Pleistocene‐age samples (ranging 19–72 ka in age) yielded site averages only 2°C lower, despite evidence that annual temperatures during glacial periods were 5–9°C colder than modern. We use a 1D numerical model of soil physics to support the idea that differences in hydrologic conditions in interglacial versus glacial periods promote differences in the seasonal distribution of soil carbonate accumulation. Model simulations of modern (Holocene) conditions suggest that soil drying under low soil pCO2 favors year‐round carbonate accumulation in this region but peaking during post‐monsoon soil drying. During a “glacial” simulation with lowered temperatures and added snowpack, more carbonate accumulation shifted to the summer season. These experiments show that changing hydrologic regimes could change the seasonality of carbonate accumulation, which in this study blunts the use of clumped isotopes to quantify glacial‐interglacial temperature changes. This highlights the importance of understanding seasonal biases of climate proxies for accurate paleoenvironmental reconstruction.

Long-term variations in terrestrial carbon cycles and atmospheric CO2 levels: Exploring impacts on global ecosystem and climate in the aftermath of end-Cretaceous mass extinction

The short-term climate and environmental consequences (<∼1.0–10 Kyr) immediately following the end-Cretaceous extinction have been attributed to transient effects originating from Deccan volcanism and the Chicxulub impact. These two events have received significant attention for their role in precipitating extreme climate conditions. In contrast, the long-term (> ∼100 Kyr) climate and environmental changes of the post-extinction have been extensively studied in the marine systems, yet their ramifications within terrestrial ecosystems are less well-characterized and require further elucidation. This study presents a carbon isotope analysis of pedogenic carbonates from matured calcisols in the Nanxiong Basin, South China, to reconstruct carbon cycles and atmospheric CO2 concentrations (pCO2) spanning from 76.0 Ma to 62.0 Ma, aiming to decipher the long-term terrestrial environmental and climatic conditions. Results show that δ13C values range from −12.4‰ to −5.0‰ (mean − 9.0‰) and pCO2 varies between ∼250 ppmV and ∼ 2200 ppmV (mean 920 ppmV). When combined with δ13C data from Songliao Basin, NE China, the carbon cycle variation exhibits a 13C collapse and smooth towards subsequent rebound, indicating a process of ∼600 Kyr deterioration, ∼600 Kyr stabilization and ∼ 400 Kyr recovery in the terrestrial ecosystem. The fluctuation of estimated pCO2 aligns with the change pattern of sea surface temperatures, attesting to the strong perturbation of climate and environment at the same pace as the carbon cycle variations. It is proposed that ecosystems and environments in both terrestrial and surface ocean experienced a more unstable, difficult and erratic recovery process and were much more sensitive to climatic changes than in deep ocean for ∼1.5 million years in the aftermath of the end-Cretaceous mass extinction. In addition, the divergence of proxy variations from expected effects implies the Deccan eruption and Chicxulub impact could not have played a long-term role in governing the climatic and environmental perturbations following the Cretaceous-Paleogene Boundary.

Pliocene CO2 rise due to sea-level fall as a mechanism for the delayed ice age

Global sea level fell progressively during the Pliocene in response to the growth of ice sheets at high northern latitudes. However, not until ∼2.7 Ma did the climate system actually generate and maintain major ice sheets in the Northern Hemisphere. A decline in atmospheric CO2 levels (pCO2) is thought to be responsible for the Pleistocene ice ages. However, the link between pCO2 and Pliocene glaciations, or their delay, is not well understood, mainly because the magnitude of pCO2 remains uncertain beyond the maximum age of ice cores (∼0.8 Ma). Here, we develop a carbonate clumped isotope (∆47) geothermometer specifically for pedogenic carbonate in Chinese loess, which we use to reconstruct temperatures in northern China, and pCO2, for the past 7.24 Ma. The palaeotemperature results show an oscillatory cooling trend over time in the East Asian monsoon region, in good agreement with coeval tropical sea surface temperature records. The pCO2 reconstruction using the pedogenic carbonate palaeobarometer reveals low values (150–325 ppmv) during the intervals of 7.24–4.3 Ma and 2.6–0 Ma. In contrast, there is a three-step rise in pCO2 up to 535 ppmv from 4.3 to 2.6 Ma, coincident with an oscillatory decrease in sea level and in benthic δ13C. We suggest that the Pliocene CO2 rise was causally related to the sea-level fall and associated organic carbon oxidation on exposed continental shelves, thereby delaying the onset of the ice age. This implies a negative feedback mechanism induced by long-term cooling since the Late Miocene, which may have delayed the onset of the ice age but did not prevent it.

Where did the water come from? Wetlands and shallow lakes in semi‐arid dunefields from South America during the Pleistocene–Holocene transition

ABSTRACTStabilized and active dunes and sand sheet deposits abound in a small lake‐dotted semi‐arid region of the Western Pampean Dunefield, Argentina. Here, a multi‐scale and multi‐proxy study of three sites, across a hydrologic gradient from lakes to a dryland with groundwater levels at more than 25 m depth, analyzes calcareous and ferruginous rhizoliths, calcareous crusts, hypocoatings, pedogenic carbonate and amorphous Mn‐oxide precipitates within blowout dunes. These palustrine‐related features indicate significantly wetter conditions that allowed the development of shallow lakes and expanding wetlands during the Pleistocene–Holocene transition, limited by associated optically stimulated luminescence ages between ca. 14.7 and 11.6 ka. These wetter conditions, also identified in other nearby proxy records, may be associated with a strengthened South American Monsoon System, potentially during the Younger Dryas Chronozone, though other geological, ecological and climatic forcings cannot be ruled out with available data. Such a scenario lacks a modern analogue, since current hydrologic excess, evidenced in the formation of lakes and new rivers, is not observed in the localities which record paleolakes. This study underlines the variable conditions for pronounced hydrologic excess in semi‐arid eolian environments in western Argentina with complex ecological, anthropogenic and climatic linkages.

Paleoclimatic and paleoenvironmental significance of late Pleistocene pedogenic carbonates (‘Caliche Cordão’) from the coastal Pampa of southern Brazil

The pedogenic carbonates found in Pleistocene units of the coastal Brazilian Pampa, informally designated as ‘Caliche Cordão', include rhizocretions, soft masses and isolated or coalescent nodules, either isolated within the sediment or forming discrete horizons, the most developed horizons (Ca-01 and Ca-02) stretching for hundreds of meters along the banks of Chuy Creek. Petrographic analyses show micritic (Alpha-type) matrix in most samples, with few signs of recrystallization as circumgranular spar rims or cracked sand grains. Stable carbon (δ13C) and oxygen (δ18O) isotope values in samples from the lower horizon Ca-01 exhibit small variation and indicate C3-dominated plant assemblages and carbonate precipitation from relatively 18O-enriched water, whereas the δ13C and δ18O in samples from the upper horizon (Ca-02) exhibit wider variation, higher proportion of C4 plants and precipitation from 18O-depleted water in some samples. The δ18O variations can be explained by variable degrees of evaporative enrichment, or to relative contributions of 18O-depleted (tropical) or 18O-enriched (subtropical) rainfall, based on the similarity with estimates from fossils of mastodons. Nodules from other sites are similar, but larger isotopic variations indicate carbonate precipitation at distinct times and environmental conditions, and a few exhibit partial recrystallization as larger spar calcite crystals. Luminescence ages of ∼74.7 and ∼71.3 ka of quartz grains in two nodules from Ca-01 indicate precipitation during MIS 4, whereas ages of ∼52 ka from Ca-02, and ≥30 ka in nodules from the shores of Mirim Lagoon indicate carbonate formation between MIS 3 and MIS 2. The probable source of carbonate was calcium released from eolian-sourced silty dust produced by Andean glaciers and transported from loess deposits in Argentina during cold and dry intervals of the last glacial, when arid/semiarid environments expanded from Patagonia across the Pampas up to southern Brazil. The variations of δ13C in calcretes formed between MIS 4 and 2 point to changes in plant assemblages that could have contributed for the local extinction of the mammalian megafauna.

Nitrification inhibitor 3,4-dimethylpyrazole phosphate alleviates the dissolution of soil inorganic carbon caused by nitrogen fertilization

In carbonate soils, soil inorganic carbon (SIC) dissolves and releases CO2 due to exogenous application of nitrogen fertilizer, which makes the soil an important carbon source. Application of the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) is likely to be an effective measure to alleviate SIC dissolution. A 5-year field experiment was conducted to observe SIC dynamics under three treatments: No N fertilizer (CK), N fertilizer (NF), and N fertilizer combined with DMPP (N + NI). Soil samples were collected from 0 to 20, 20–40, 40–60, 60–80, and 80–100 cm in depth, and soil organic carbon (SOC), SIC, total carbon (TC), total nitrogen (TN), and carbon isotope composition (δ13C) of the soil were analyzed. The proportion of pedogenic carbonate (PIC) in SIC was estimated using an isotopic mass balance equation. Results showed that: (1) compared with the application of N only, DMPP had no significant effect on SOC content and δ13CSOC in the 0–100 cm soil profile; (2) the application of N fertilizer significantly reduced the SIC content by 10.0 g·kg−1 (58.3 %) in the 0–80 cm soil profile. In the 0–20, 20–40, and 60–80 cm soil layers, the SIC content was reduced by 2.9, 1.9, and 3.8 g·kg−1, respectively. However, the SIC content significantly increased by 3.2 g·kg−1 (58.9 %) in the 80–100 cm soil layer. DMPP application significantly increased the SIC content in the 0–20 and 20–40 cm soil layers by 1.3 g·kg−1 (50.9 %) and 1.2 g·kg−1 (48.2 %), respectively; (3) DMPP combined with N significantly alleviated PIC dissolution in the 0–40 cm soil layers and increased PIC stock in 0–100 cm soil by 6.2 Mg·ha−1 (64.6 %). Applying DMPP to farmland soil containing carbonate significantly alleviated the dissolution of N fertilizer on PIC, which may decrease CO2 emissions in the field.

Seasonality of C4 plant growth and carbonate precipitation in the Chinese Loess Plateau may cause positive carbon isotope anomalies in pedogenic carbonates

Seasonality of C₄ plant growth and carbonate precipitation in the Chinese Loess Plateau may cause positive carbon isotope anomalies in pedogenic carbonates

High‐Latitude, Indian Ocean, and Orbital Influences on Eastern African Hydroclimate Across the Plio‐Pleistocene Boundary

AbstractTerrestrial‐marine dust fluxes, pedogenic carbonate δ13C values, and various paleovegetation proxies suggest that Africa experienced gradual cooling and drying across the Pliocene‐Pleistocene (Plio‐Pleistocene) boundary (2.58 million years ago [Ma]). However, the timing, magnitude, resolution, and relative influences of orbitally‐driven changes in high latitude glaciations and low latitude insolation differ by region and proxy. To disentangle these forcings and investigate equatorial eastern African climate across the Plio‐Pleistocene boundary, we generated a high‐resolution (∼3,000‐year) data set of compound‐specific n‐alkane leaf wax δ2H values—a robust proxy for atmospheric circulation and precipitation amount—from the HSPDP‐BTB13‐1A core, which spans a ∼3.3–2.6 Ma sequence in the Baringo‐Tugen Hills‐Barsemoi Basin of central Kenya. In combination with the physical sedimentology, our data indicate that precipitation varied strongly with orbital obliquity, not precession, during the late Pliocene, perhaps imparted by variations in the cross‐equatorial insolation gradient. We also observe a marked shift toward wetter conditions beginning ∼3 Ma that corresponds with global cooling, drying in western Australia, and a steepening of the west‐east zonal Indian Ocean (IO) sea surface temperature (SST) gradient. We propose that northward migration of the Subtropical Front reduced Agulhas current leakage, warming the western IO and causing changes in the IO zonal SST gradient at 3 Ma, a process that has been observed in the latest Pleistocene‐Holocene but not over longer timescales. Thus, the late Cenozoic moisture history of eastern Africa is driven by a complex mixture of low‐latitude insolation, the IO SST gradient, and teleconnections to distal high‐latitude cooling.

Carbonate morphological features of vineyard soils in a semiarid Mediterranean environment

AbstractMany vineyard soils in semiarid Mediterranean environments develop from carbonated materials and often show very diverse forms of pedogenic calcium carbonate accumulations. Based on 275 vineyard soil profiles in the Castilla La Mancha (CLM) territory (Spain), this paper presents an in‐depth comprehensive study of a large number of morphological expressions of these carbonate distributions, while exploring various pedogenesis formation hypotheses. The interactions among the calcareous nature of parent material, surface topography and climate are the factors that most affected their morphology and pedogenesis. The occurrence and distribution of carbonate features in CLM vineyard soils can be more complex than in other areas. Many accumulations are relatively ancient, which is the case of petrocalcic horizons, and in‐keeping with conservative means, which reveals the occurrence of successive carbonate accumulation cycles. More recently dissolved and precipitated seasonally pedogenic carbonates (sometimes as coatings) have appeared, which are nothing more than a simple recent redistribution of carbonates. A structural imprint is observed: pedogenic carbonates appeared much more abundantly on the lowermost horizons, which can be related to the repeated redistribution phases that occurred in different pedogenesis stages. The data provided herein help to improve the understanding of the nature of viticultural calcareous soils in semiarid Mediterranean regions.

Braided-river architecture of the Triassic Swartberg member, Katberg Formation, South Africa: assessing age, fluvial style, and paleoclimate after the end-Permian extinction

ABSTRACT The Triassic Katberg Formation has played a central role in interpreting the end-Permian ecosystem crisis, as part of a hypothesis of aridification, vegetation loss, and sediment release in continental settings. We use drone images of an inaccessible cliff near Bethulie to investigate the Swartberg member, a braided-fluvial body 45 m thick, describing remote outcrop facies to identify geomorphic units and using spatial analysis to estimate their proportions in 2-D sections. Here the Swartberg member comprises three channel belts within shallow valleys, the lowermost of which is ∼500 m wide and incised into lacustrine deposits. The component channel bodies consist mainly of trough cross-bedded sand sheets (48%) and channel-scour fills (28%). Recognizable bars (15%) comprise unit bars with high-angle slipfaces and mounded bar cores (components of mid-channel compound bars), bars built around vegetation, and bank-attached bars in discrete, probably low-sinuosity conduits. Abandoned channels constitute 8% and 16% of flow-parallel and -transverse sections, respectively. When corrected for compaction, the average thalweg depth of the larger channels is 3.9 m, with an average bankfull width of 84 m, scaling broadly with the relief of the bars and comparable in scale to the Platte and South Saskatchewan rivers of North America. The fluvial style implies perennial but seasonably variable flow in a vegetated landscape with a humid paleoclimate. The northward paleoflow accords with regional paleoflow patterns and deposition on a megafan sourced in the Cape Fold Belt, where the Swartberg member represents the avulsion of a major transverse-flowing river. U-Pb dating of in situ and reworked pedogenic carbonate nodules from below the base of the Swartberg member yielded Anisian to Ladinian ages (Middle Triassic), younger than the previously assumed Early Triassic age and implying that considerable gaps in time exist in the succession. An assessment of the interval spanning the lower to mid Katberg Formation is needed to reevaluate the inferred unidirectional trend in fluvial style, aridification, and fossil distributions in this condensed, disjunct succession.

Fate and migration of enhanced rock weathering products through soil horizons; implications of irrigation and percolation regimes

Terrestrial enhanced rock weathering (ERW) is reckoned as a robust method to contribute to stabilizing atmospheric carbon and combat global warming. The method is particularly a matter of interest in agricultural soil, offering a vast opportunity for the application of crushed silicate minerals and secondary carbonate accumulation and migration. Although the applicability of this practice is well acknowledged, its impact on the geochemical alteration of soil vertical profile, particularly deeper layers, is less discussed in the literature. Moreover, the effect of irrigation and percolation regimes, as influenced by ambient conditions, is also not well-reported and characterized. In the present study, we utilized a high amendment rate (50 t/ha) of wollastonite skarn, a fast-weathering silicate mineral, to uncover the alterations in topsoil and subsoil as well as effluent water over a through a soil column experiment over five months, under more controlled (indoor) and less controlled (outdoor) ambient and irrigation conditions. The results indicate more conspicuous modification of parameters in shallow layers of soil compared to deeper ones, explicitly in the rain-fed setup. Silicate amendment induced an increase of 6.53 and 2.85 tCO2/ha (in the form of pedogenic carbonate (PC)) over the profile of soil (0–60 cm) under periodic and rain-fed water regimes, respectively. The silicate treatment was also found to be effective toward rising pH and electrical conductivity of topsoil (0–15 and 15–30 cm layers) and leachate, and a measurably greater release of Ca2+ and Mg2+ in the column effluent. We estimate a carbon drawdown ratio (carbonate/bicarbonate) of ≈15:1 in this study, showing that PC formation is a more significant weathering indicator than leachate bicarbonate export in the shorter term for a fast-weathering mineral in pH-neutral soils and moderate irrigation regime. Such observations are valuable for developing carbon drawdown quantification methods suitable for a variety of agricultural lands and global regions.

U-Pb dating of pedogenic calcite near the Permian−Triassic boundary, Karoo Basin, South Africa

We report U-Pb age determinations of carbonate nodules from an in situ paleosol horizon in the Upper Permian Balfour Formation and from several horizons of pedogenic nodule conglomerate (PNC) in the Triassic Katberg Formation, Karoo Basin, South Africa, using laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS). The paleosol sample yields an age of 252 ± 3 Ma, which overlaps with a previous high-precision U-Pb zircon date from a volcanic ash deposit 2 m above the paleosol. This relationship demonstrates the reliability of using LA-ICP-MS dating techniques on terrestrial pedogenic calcite. Two PNC samples collected at the base of the Katberg Formation within the same sandstone unit yield ages of 255 ± 3 Ma and 251 ± 3 Ma. The age of 251 ± 3 Ma overlaps with the high-precision U-Pb zircon date below the PNC and is a maximum age estimate of deposition for the base of the Katberg Formation. Our results show that reworked nodules in the same concentrated conglomerate lag can be of different ages, but that similarly aged nodules are spatially associated. In addition, two PNC samples collected higher in the section yield ages of 249 ± 3 Ma and 241 ± 3 Ma, providing maximum depositional ages for the lower to middle Katberg Formation for the first time. We demonstrate that pedogenic carbonate nodules can be dated with meaningful precision, providing another mechanism for constraining the age of sedimentary sequences and studying events associated with the Permian−Triassic transition in the central Karoo Basin, even though the extinction boundary may not be preserved in this area.