Carbonate Rhizoliths Research Articles

Outcrop distribution and formation of carbonate rhizoliths in Badain Jaran Desert, NW China

Carbonate rhizoliths and related features are environmental and ecological indicators of not only palaeosols, but also modern soils. They designate subaerial vadose and pedogenic diagenesis environments. Rhizoliths are commonly used as a tool for paleoenvironmental reconstructions, but uncertainty remains regarding their formation mechanisms. Field characteristics of the dune rhizoliths in the Badain Jaran Desert, NW China were analyzed to investigate formation mechanisms and interpret paleoenvironmental significance. Systematic field sampling and intensive examination showed that the tube-like rhizoliths without morphological structures of plant roots have been placed at the dune soil surface following erosion and weathering but the underground rhizoliths are still connected to the dead Artemisia roots. Rhizoliths occurred only along the windward, long gentle slopes of mega-dunes. Their distribution patterns of both types in connection with plant distribution and landscapes have been studied. Their petrological and mineralogical characters were performed in detail using microscopic and spectroscopic techniques (cathodoluminescence, scanning electronic microscopy and energy dispersive X-ray spectroscopy) in laboratory. Altogether, field and laboratory analyses showed that rhizolith cement is calcium carbonate and despite the high porosity of soil and high aridness of climate, soil moisture plays an important role in sustaining Artemisia growth and rhizolith formation. Desert rhizoliths can therefore be used as a proxy to infer local soil moisture and plant communities. Moreover, as soil moisture content is related to climate and landscape position, rhizoliths could indirectly be indicators of aboveground environmental conditions, e.g. annual precipitation or humidity.

Characterization and formation of the pristine rhizoliths around Artemisia roots in dune soils of Tengeri Desert, NW China

Rhizoliths are the products of mineralization, petrification, or fossilization around and/or within plant roots. Among them, carbonate rhizoliths are the most common. Pristine carbonate rhizoliths with co-existing plant root relicts in the Tengeri Desert, NW China were studied, with a combination of intensive field observations and laboratory methods such as microscopy, scanning electronic microscopy, energy dispersive X-ray spectra, radiocarbon dating, and isotope mass spectrometer. The field observations revealed that the pristine rhizoliths are only present at the sites where Artemisia sphaerocephala Krasch are growing i.e. in swales among sand dunes. Soil moisture of the swales is the main controlling factor of rhizoliths formation. It is in turn affected by the soil physical properties, landscape position, and climate variability, consistent with the locations of sampled rhizoliths in the swales where Artemisia plants are mostly distributed. The 14C AMS dating indicated that the rhizoliths are much older (4000–5000 years) than their co-existing modern plant root relicts in agreement with field observations. Morphological, mineralogical and isotopic analyses revealed that carbon sources used for the rhizoliths formation were partially derived from decomposing plant roots but with significant contribution from dissolution of lithogenic carbonates. The calcium sources were suggested to be the in situ weathering of minerals (mostly lithogenic carbonates) and the pressure-dissolution of carbonates. Enough CO2 from the root decomposition have triggered carbonate accumulation around the root to form rhizoliths. Other minor chemical components of the root are S, N, P, which produce acidic water with the negative ions of SO42−, NO3–, PO43-, have also favored acidic soil environment and enhanced carbonate dissolution and mineral weathering. Redox environment around Artemisia roots were also observed to be a key factor for the pristine rhizolith formation. The pristine rhizoliths were preferentially formed in semi-closed redox condition with water nearly always available at intermediate depths. In addition, they were formed through carbonate epidiagenesis in shallow soils of the desert. Altogether, our results showed that the formation of the pristine rhizoliths was affected by the combination of several environmental factors. This led us to propose a conceptual model of rhizolith formation in desert soils.

Carbonate Rhizoliths in Dune Sands of the Belaya River Valley (Upper Angara Region)

Results of investigating carbonate rhizoliths formed in the Early Holocene dune sands in the Belaya River valley are presented. Carbonate accumulations are characterized by large sizes (2 to 7 cm in diameter, and about 1 m in length), which is associated with a high content of carbonates in the enclosing sediments and long-lasting rhizoliths formation. Morphology, mineralogical and isotopic composition of accumulations attest to their origin related to roots and root microorganisms activity. The redistribution of carbonates from the surrounding sediments and their concentration near the roots increased the carbonate content there to more than 30%. With the distance from the roots, the amount of carbonates decreases to 10–12% causing a concentric structure of rhizoliths. In the cross-section of the accumulations, the central and peripheral parts have distinct differences: in the central parts, carbonates are completely recrystallized and represented by pure calcite with insignificant inclusions of quartz and feldspars; on the periphery, coarse-grain silicate material prevails, and it is weakly cemented by clay-carbonate plasma bridges. The δ13C values of carbonates range from –7.4 to –1.5‰ and increase to the periphery owing to recrystallization of primary carbonates. The δ13C value of the rhizoliths’ organic residues is –22.15‰ indicating the formation of carbonate accumulations around the roots of C3-vegetation. The carbonate δ18O values of rhizoliths vary from ‒10.34 to –11.99‰ demonstrating the trend towards 18O enrichment from the inner to outer layers. Calculation of the annual temperatures using the δ18O values of the inner layers of rhizoliths consisting only of secondary carbonates, showed significant temperature deviations during the formation of carbonate accumulations from modern ones. The radiocarbon age of rhizoliths carbonate cementations (7160 ± 100 kyr BP) is slightly less than the age of enclosing deposits. Organic residues from the central parts of the rhizoliths are much younger (1770 ± 40 kyr BP) which is explained by the penetration of organic matter of soils and plant biomass through the cavities which apparently did not cause contamination of the carbonate component of the rhizoliths with younger carbon.

Early Pleistocene lacustrine sedimentation and diatom stratigraphy at Munya wa Gicheru, southern Kenya Rift Valley

This research develops a detailed palaeoenvironmental history for sediments at Munya wa Gicheru (southern Kenya Rift) and explores the roles of tectonics and climate in the formation of palaeolakes and their use in regional climate correlation. The lacustrine facies of the Munya wa Gicheru deposits were formed from ~1.9 to 1.65Ma and consist of massive and laminated diatomites and diatomaceous silts. Fresh water diatoms include Stephanodiscus transilvanicus, Stephanodiscus niagarae, Aulacoseira agassizi, Aulacoseira ambigua, Aulacoseira goetzeana, Aulacoseira granulata var. valida, A. granulata var. angustissima and Pseudostaurosira zeilleri. One moderately saline phase is marked by the dominance of Cyclotella meneghiniana and by the presence of rare Thalassiosira rudolfii. Twenty-five diatom-based zones and subzones can be distinguished, representing eight high-lake periods and eight shallow-water phases. Erosion surfaces and desiccation cracks indicate two periods of emergence. Lake waters ranged in pH from 7.2 to 9, with conductivities of ~200–11,000μScm−1, but mostly <1000μScm−1. In contrast, the uppermost sediments are comprised of fluvial silts, sands and gravels formed in fluvial, sheetwash and terrestrial settings and represent termination of lacustrine conditions. REE data suggest that the Limuru Trachytes are the dominant source of siliciclastics materials. Geochemical information indicates that catchment weathering was moderate and that CaO concentrations in the sediments are very low. Carbonate rhizoliths are absent. SEM images show minimal corrosion or alteration of the diatom silica.

Carbonate rhizoliths in loess and their implications for paleoenvironmental reconstruction revealed by isotopic composition: δ 13C, 14C

Loess–paleosol sequences are important terrestrial archives for studying Quaternary climate changes. They often contain secondary carbonates including e.g. rhizoliths (calcified roots). These secondary carbonates are precipitated in isotopic equilibrium with root-derived CO 2 and are therefore used to reconstruct the vegetation present during their formation based on stable carbon isotopic composition (δ 13C). Usually, the chronological context of secondary carbonates in general is not mentioned, because it is assumed that they are formed synsedimentary with loess deposition. The loess–paleosol sequence at Nussloch, SW Germany, contains in its youngest part (Upper Würmian) large carbonate rhizoliths with diameters of up to 5 cm and lengths of up to 1 m and more, which have not been described in this profile so far. We investigated rhizoliths as well as loess adjacent to and distant from rhizoliths for carbonatic carbon (C carb) and organic carbon (C org) contents, as well as their isotopic composition (δ 13C, radiocarbon dating), to identify the rhizolith origin and the time frame of their formation. Considering the 13C fractionation by carbonate precipitation, the δ 13C carb values (− 10.9 ± 0.1‰) revealed C 3 plant origin of the rhizolith carbonate and the absence of large amounts of occluded primary loess carbonate. Similar 14C ages of rhizolith C carb and C org (3788 ± 59 years BP and 3150 ± 59 years BP, respectively) argued for the absence of postsegregational alteration. Therefore they are suitable for the reconstruction of paleoenvironmental conditions after loess sedimentation. The 14C ages clearly indicate that rhizoliths did not form synsedimentary. Roots penetrated the loess at Nussloch after the deposition had ceased at ~ 15 ka BP. Even in the loess adjacent to the rhizoliths (up to a distance of 5 cm), δ 13C carb values indicate the presence of secondary carbonate deriving from postsedimentary organic matter of origin other than that of the reference loess material. Hence, this postsedimentary input of younger root biomass might have masked the initial plant signal in loess–paleosol sequences, which could cause uncertainties for paleoenvironmental reconstructions based solely on loess organic matter.

Wetland sedimentation and associated diatoms in the Pleistocene Olorgesailie Basin, southern Kenya Rift Valley

The Olorgesailie Basin in the southern Kenya Rift Valley contains a variety of lacustrine, wetland and terrestrial facies that were laid down during the last 1.2 million years. This study integrates diatom, ichnofossil, and rhizolith (silica and carbonate) evidence in order to identify and characterize shallow wetland deposition at the top (Member 14; ca. 493 ka) of the Olorgesailie Formation. A key feature of these deposits is the presence of facies with aquatic indicators (diatoms) that are overprinted by ichnofossils and rhizoliths that suggest drier conditions. Diatom floras include several fresh water Aulacoseira spp. and saline indicators such as Thalassiosira rudolfi, T. faurii, Cyclotella meneghiniana and Anomoeoneis sphaerophora. The main indicators of shallow fresh to mildly saline waters include Epithemia argus, Rhopalodia gibberula, Encyonema muelleri, and Synedra spp. Three main suites of ichnofossils indicate varying water table depths during their formation. These consist of 1) horizontal tunnels that suggest saturated substrates; 2) vertical burrows formed in substrates with relatively lower water tables; and 3) termite, hymenopteran, and beetle ichnofossils that indicate dry substrates. Member 14 of the Olorgesailie Formation also contains carbonate and siliceous rhizoliths, which occur both together and separately. Four possible models are suggested to explain the silica and carbonate rhizolith formation: 1) a complex of spring-fed marshes and dry areas; 2) ephemeral wetlands and floodplains; 3) wetlands with abundant organic decay, and 4) initial carbonate rhizoliths that were replaced by silica. Preservation of cellular structures and the reworking and incorporation of rhizoliths in overlying younger strata suggest that they formed early during diagenesis.

The influence of hydrology and climate on the isotope geochemistry of playa carbonates: a study from Pilot Valley, NV, USA

AbstractCarbonates often accompany lake and lake‐margin deposits in both modern and ancient geological settings. If these carbonates are formed in standing water, their stable isotope values reflect the aquatic chemistry at the time of precipitation and may provide a proxy for determining regional hydrologic conditions. Carbonate rhizoliths and water samples were collected from a playa lake in eastern Nevada. Pilot Valley (∼43°N) is a closed‐basin, remnant playa from the Quaternary desiccation of palaeo‐Lake Bonneville. Water is added to the playa margin by free convection of dense brines to the east and forced convection of freshwater off the alluvial fan to the west. Both freshwater and saline springs dot the playa margin at the base of an alluvial fan. Water samples collected from seven springs show a range from −16 to −0·2‰ (Vienna Standard Mean Ocean Water), and are consistent with published values. The δ18Ocalcite values from rhizolith samples range from −18·3 to −6·7‰ (Vienna Pee Dee Belemnite), and the average is −12‰ V‐PDB (1 − σ SD 2‰). With the exception of samples from Little Salt Spring, the range in the δ18Ocalcite values collected from the rhizoliths confirms that they form in equilibrium with ambient water conditions on the playa. The initial geochemical conditions for the spring waters are dictated by local hydrology: freshwater springs emerge in the northern part of the basin to the east of a broad alluvial fan, and more saline springs emerge to the south where the influence of the alluvial fan diminishes. Rhizoliths are only found near the southern saline springs and their δ13Ccalcite values, along with their morphology, indicate that they only form around saltgrass (Distichlis sp.). As the residence time of water on the playa increases, evaporation, temperature change and biological processes alter the aquatic chemistry and initiate calcite precipitation around the plant stems. The range in δ18Ocalcite values from each location reflects environmental controls (e.g. evaporation and temperature change). These rhizoliths faithfully record ambient aquatic conditions during formation (e.g. geochemistry and water depth), but only record a partial annual signal that is constrained by saltgrass growth and the presence of standing water on the playa margin.

Reconstructing the response of C 3 and C 4 plants to decadal-scale climate change during the late Pleistocene in southern Illinois using isotopic analyses of calcified rootlets

The δ 13C and δ 18O values of well-preserved carbonate rhizoliths (CRs) provide detailed insights into changes in the abundance of C 3 and C 4 plants in response to approximately decadal-scale changes in growing-season climate. We performed stable isotope analyses on 35–40 CRs sampled at 1-cm intervals from an 18-cm-thick paleosol formed in southern Illinois during Wisconsin interstadial 2. Minimum δ 13C values show little variation with depth, whereas maximum values vary dramatically, and average values show noticeable variability; maximum δ 18O values vary less than the minimum δ 18O values. These findings indicate that a diverse and stable C 3 flora with a limited number of C 4 grass species prevailed during this interval, and suggest that the maximum growing-season temperatures were relatively stable, but minimum growing-season temperatures varied considerably. Two general patterns characterize the relationships between the δ 13C and δ 18O values obtained from the 1-cm samples. In some cases, low δ 13C values correspond to low δ 18O values and high δ 13C values correspond to high δ 18O values, suggesting that cooler growing-season temperatures favored C 3 and warmer growing-season temperatures favored C 4 plants. In other cases, low δ 13C values correspond to high δ 18O values, likely suggesting that wetter growing-season conditions were favorable to C 3 plants. The high density of well-preserved CRs in this paleosol provides a unique opportunity to study detailed ecological responses to high-resolution variability in growing-season climate.

Evidence of long‐term seasonal climate forcing in rhizolith isotopes during the last glaciation

High density carbonate rhizoliths were found from a loess‐paleosol succession from the late Wisconsin period (21–11 ka) in Illinois. Their morphology shows that they formed in a close contact with living and decomposing roots, suggesting a root/microbial respiration origin. Carbon (δ13C) and oxygen (δ18O) isotopic analyses were performed on 36 and 37 individual rhizoliths of two separate 10 cm intervals and 98 bulk rhizoliths of all 10 cm intervals. The results of the individual rhizolith δ13C and δ18O analyses suggest that the carbon source was largely derived from respiring C3, C4 and microbial biomass, and that meteoric water was controlled mainly by warm‐season precipitation. The results of bulk rhizolith δ13C and δ18O analyses show that warm‐season proxies varied in phase with glacial fluctuations at submillennial scales, suggesting long‐term seasonal forcing may have played an important role on climate change during the late Wisconsin glaciation in North America.

Zeolitic diagenesis of late Quaternary fluviolacustrine sediments and associated calcrete formation in the Lake Bogoria Basin, Kenya Rift Valley

ABSTRACTLate Quaternary fluviolacustrine siltstones, mudstones and claystones (Loboi Silts) on the northern margins of the saline, alkaline Lake Bogoria in the Kenya Rift Valley contain up to c. 40% authigenic analcime and minor natrolite. The zeolitic sediments are reddish brown and up to 1 m thick. The amount of analcime increases upward in the profile, but decreases with distance from the lake. The altered sediments show many pedogenic features including zeolitic root mats, rootmarks, concretions and carbonate rhizoliths. Residual patches of calcrete locally cap the zeolitic rocks. The profile is interpreted as an exhumed palaeosol and land surface on the former margins of the lake.The analcime occurs as submicroscopic (0–5–2–5 μ.m) subhedral and euhedral crystals, which have an average Si/A 1 ratio of 2–33 (as determined by X‐ray microanalysis) or 2–18 (d‐value of 639 analcime peak). The analcime formed in lake marginal sediments (soils) by reaction of silicate detritus with Na2CO3 rich pore waters concentrated close to the land surface by evaporative pumping and evapotranspiration. Poorly ordered clay minerals were probably the main reactants. Authigenic illite may have been a by‐product of the reactions. Chemical analyses suggest that pore waters supplied some of Na+, and possibly K+ and SiO2. The associated calcrete and rhizoliths were formed during or shortly after the main period of zeolitic alteration. The Ca2+ may have originated from infiltrating dilute runoff and groundwater. Authigenic smectite was precipitated in open porosity following analcime formation. The zeolitic alteration at Lake Bogoria provides a relatively recent analogue for lake marginal zeolites found in many ancient saline, alkaline lake sediments.