Dryland irrigation increases accumulation rates of pedogenic carbonate and releases soil abiotic CO2

Agricultural fields in drylands are challenged globally by limited freshwater resources for irrigation and also by elevated soil salinity and sodicity. It is well known that pedogenic carbonate is less soluble than evaporate salts and commonly forms in natural drylands. However, few studies have evaluated how irrigation loads dissolved calcium and bicarbonate to agricultural fields, accelerating formation rates of secondary calcite and simultaneously releasing abiotic CO2 to the atmosphere. This study reports one of the first geochemical and isotopic studies of such “anthropogenic” pedogenic carbonates and CO2 from irrigated drylands of southwestern United States. A pecan orchard and an alfalfa field, where flood-irrigation using the Rio Grande river is a common practice, were compared to a nearby natural dryland site. Strontium and carbon isotope ratios show that bulk pedogenic carbonates in irrigated soils at the pecan orchard primarily formed due to flood-irrigation, and that approximately 20–50% of soil CO2 in these irrigated soils is calcite-derived abiotic CO2 instead of soil-respired or atmospheric origins. Multiple variables that control the salt buildup in this region are identified and impact the crop production and soil sustainability regionally and globally. Irrigation intensity and water chemistry (irrigation water quantity and quality) dictate salt loading, and soil texture governs water infiltration and salt leaching. In the study area, agricultural soils have accumulated up to 10 wt% of calcite after just about 100 years of cultivation. These rates will likely increase in the future due to the combined effects of climate variability (reduced rainfall and more intense evaporation), use of more brackish groundwater for irrigation, and reduced porosity in soils. The enhanced accumulation rates of pedogenic carbonate are accompanied by release of large amounts of abiotic CO2 from irrigated drylands to atmosphere. Extensive field studies and modelling approaches are needed to further quantify these effluxes at local, regional and global scales.

The first Miocene fossils from coastal woodlands in the southern East African Rift

The Miocene is a key time in the evolution of African mammals and their ecosystems witnessing the origin of the African apes and the isolation of eastern coastal forests through an expanding biogeographic arid corridor. Until recently, however, Miocene sites from the southeastern regions of the continent were unknown. Here we report discovery of the first Miocene fossil teeth from the shoulders of the Urema Rift in Gorongosa National Park, Mozambique, at the southern East African Rift System. We provide the first 1) radiometric age determinations of the fossiliferous Mazamba Formation, 2) reconstructions of past vegetation in the region based on pedogenic carbonates and fossil wood, and 3) description of fossil teeth from the southern rift. Gorongosa is unique in the East African Rift System in combining marine invertebrates, marine vertebrates, terrestrial mammals, and fossil woods in coastal paleoenvironments. The Gorongosa fossil sites offer the first evidence of persistent woodlands and forests on the coastal margins of southeastern Africa during the Miocene, and an exceptional assemblage of fossil vertebrates including new species. Further work will allow the testing of hypotheses positing the formation of a northeast-southwest arid corridor isolating species on the eastern coastal forests from those elsewhere in Africa.

Pedogenic carbonates accumulation in a calcareous Mediterranean soil following introduction of irrigation

<p>In calcareous Mediterranean soils, pedogenic and lithogenic carbonates can be important constituents of the soil matrix. However, their relative proportion and their relation to soil functioning has been scarcely studied. The interest in determining the proportion of pedogenic carbonates relies on the fact that they can be related to the physical, chemical and biological properties of the soil and, therefore, affect plant growth and soil productivity. Carbonates dynamics can be affected by some farming management practices and land-use changes, such as the adoption of irrigation, due to changes in the soil water regime, the composition of the soil solution, the concentration of CO<sub>2</sub> in the soil atmosphere, and the changes related to fertilization.</p><p>To gain knowledge on the importance of the effect of the introduction of irrigation on carbonates dynamics in the tilled layer of agricultural soils, we studied the evolution of the proportion of pedogenic carbonates in a Mediterranean calcareous soil after seven years of irrigation. We used the isotopic signature of C in soil carbonates for these estimations. The study was conducted in two plots under contrasting agricultural management on the same soil unit: dryland wheat cropping, and irrigated corn for 7 consecutive years, in Enériz (Navarre, Spain).</p><p>Our results showed that the transformation of dryland wheat to irrigated corn, produced a preferential accumulation of pedogenic carbonates (31-56%) in the tilled layer (0-30 cm) of the irrigated soil only over 7 years after the land-use change. Therefore, the processes related to this land use change can alter the soil carbonates dynamics in a very short period of time, and they may have consequences in terms of plant nutrient dynamics and the soil structure. Future research on the origin of the soil carbonates (pedogenic or geogenic) in agricultural soils will help to understand the actual significance of carbonates dynamics in terms of the global C balance in these soils.</p>

Analysis of paleosol-based proxies from the Turkana Basin through paleo-landscape and paleoclimate reconstruction

<p>The Turkana Basin is world famous for its extensive outcrops that provide insights into the paleoclimate and paleolandscapes in which hominins evolved over the past ~4 Ma. The Nachukui Formation, part of the Omo Group, reflects basin-wide dynamic processes of interlaced sequences including floodplains, deltas, and river systems throughout the Plio-Pleistocene. Paleosols associated with floodplains of the fluvial systems provide a valuable window into better understanding key intervals within this record and are frequently associated with fossiliferous fluvial sequences. This study analyzed three paleosols taken from outcrops of the Kaitio and Natoo Members of the Nachukui Formation. In particular, the Kaitio Mmb was assumed to be simply a lacustrine environment deposited during the longest-lived part of Paleolake Lorenyang (~1.7-2 Ma). However, recent studies have worked to provide a more comprehensive understanding of this member, indicating it was a far more dynamic lacustrine margin than previously recognized. This research builds upon this stratigraphic framework to integrate paleosol-based geochemical proxies to better reconstruct the paleoclimate and paleoenvironment of West Turkana Kaitio (WTK). This includes 1) x-ray fluorescence (XRF) elemental analysis of bulk sediment, and 2) stable isotope analysis on both bulk sediment and pedogenic carbonates. These data allowed us to make estimates of mean annual precipitation (MAP), vegetation type, and paleotemperatures. Using the CalMag and CIA-K weathering indices, the MAP estimates range from 351-933 mm of rain/year, with the means for both proxies ranging from 351-917 mm with an average MAP of 761.75 mm. The CIA-K weathering index produced MAP values of 503-933 mm with an average 812.88 mm. Compared to modern average rainfall values in the basin (324.1-151.6 mm/yr), our MAP estimates indicate the basin experienced more precipitation in the Plio-Pleistocene than it does today. Pairing the geochemical data with our sedimentological assessment allowed us to better characterize these paleosols for a more in depth understanding of the depositional environment of the Kaitio Member. </p>

The δ13C, δ18O and Δ47 records in biogenic, pedogenic and geogenic carbonate types from paleosol-loess sequence and their paleoenvironmental meaning

Paleoenvironmental reconstructions are commonly based on isotopic signatures of a variety of carbonate types, including rhizoliths and land-snail shells, present in paleosol-loess sequences. However, various carbonate types are formed through distinct biotic and abiotic processes over various periods, and therefore may record diverging environmental information in the same sedimentological layer. Here, we investigate the effects of carbonate type on δ13C, δ18O, and clumped isotope-derived paleotemperature [T(Δ47)] from the Quaternary Nussloch paleosol-loess sequence (Rhine Valley, SW Germany). δ13C, δ18O, and T(Δ47) values of co-occurring rhizoliths (-8.2‰ to -5.8‰, -6.1‰ to -5.9‰, 12–32°C, respectively), loess dolls (-7.0‰, -5.6‰, 23°C), land-snail shells (-8.1‰ to -3.2‰, -4.0‰ to -2.2‰, 12–38°C), earthworm biospheroliths (-11‰, -4.7‰, 8°C), and “bulk” carbonates (-1.9‰ to -0.5‰, -5.6‰ to -5.3‰, 78–120°C) from three sediment layers depend systematically on the carbonate type, admixture from geogenic carbonate, and the duration of formation periods. Based on these findings, we provide a comprehensive summary for the application of the three isotopic proxies of δ13C, δ18O, and Δ47 in biogenic and pedogenic carbonates present in the same sediment layer to reconstruct paleoenvironments (e.g., local vegetation, evaporative conditions, and temperature). We conclude that bulk carbonates in Nussloch loess should be excluded from paleoenvironmental reconstructions. Instead, pedogenic and biogenic carbonates should be used to provide context for interpreting the isotopic signature for detailed site- and time-specific paleoenvironmental information.