Two possible source regions for Central Greenland last glacial dust

Dust in Greenland ice cores is used to reconstruct the activity of dust-emitting regions and atmospheric circulation. However, the source of dust material to Greenland over the last glacial period is the subject of considerable uncertainty. Here we use new clay mineral and <10 µm Sr–Nd isotopic data from a range of Northern Hemisphere loess deposits in possible source regions alongside existing isotopic data to show that these methods cannot discriminate between two competing hypothetical origins for Greenland dust: an East Asian and/or central European source. In contrast, Hf isotopes (<10 µm fraction) of loess samples show considerable differences between the potential source regions. We attribute this to a first-order clay mineralogy dependence of Hf isotopic signatures in the finest silt/clay fractions, due to absence of zircons. As zircons would also be absent in Greenland dust, this provides a new way to discriminate between hypotheses for Greenland dust sources

Two possible source regions for central Greenland last glacial dust

Dust in Greenland ice cores is used to reconstruct the activity of dust-emitting regions and atmospheric circulation. However, the source of dust material to Greenland over the last glacial period is the subject of considerable uncertainty. Here we use new clay mineral and <10 µm Sr–Nd isotopic data from a range of Northern Hemisphere loess deposits in possible source regions alongside existing isotopic data to show that these methods cannot discriminate between two competing hypothetical origins for Greenland dust: an East Asian and/or central European source. In contrast, Hf isotopes (<10 µm fraction) of loess samples show considerable differences between the potential source regions. We attribute this to a first-order clay mineralogy dependence of Hf isotopic signatures in the finest silt/clay fractions, due to absence of zircons. As zircons would also be absent in Greenland dust, this provides a new way to discriminate between hypotheses for Greenland dust sources

Analysis of lipids and polycyclic aromatic hydrocarbons as indicators of past and present (micro)biological activity

Analysis of lipids and hydrocarbons is performed frequently in recent and ancient plant tissues, soils, sediments, peat deposits, oil, rocks, anthropogenic artifacts (archeological samples), and other materials to trace the contribution of different biological and anthropogenic sources of organic matter as well as environmental changes and the fate of organic matter like degradation. The approaches for the analysis of lipids and hydrocarbons strongly vary from traditional methodologies like thin-layer chromatography to universal approaches like pyrolysis, whereas the preparative separation of lipid fractions based on their polarity enables gas-chromatographic analyses of single fractions and compound-specific analysis of stable (2H/1H, 13C/12C) and radioactive (14C) isotope compositions. Often, lipid extraction operationally defines a subfraction of total lipids. On the one hand, free extractable lipids are obtained by extraction with organic solvents, whereas on the other hand, total samples or extraction residues are extracted for more polar lipid fractions using highly polar organic solvents and water, to release bound lipids. Procedures for extraction of free extractable lipids are diverse and mainly defined by the target of research and availability of instrumentation. In the current protocol, state-of-the-art techniques for the investigation of free extract- able lipids in various materials are explained, which can be applied even in laboratory environments with limited technical equipment. The protocols cover sample preparation, extraction, purification, analysis, as well as a brief overview of the data evaluation using lipid molecular proxies and compound-specific isotopes

Effect of temperature and rhizosphere processes on pedogenic carbonate recrystallization: Relevance for paleoenvironmental applications

In soils of arid and semiarid climates, dissolution of primary (lithogenic) carbonate and recrystallization with CO₂ from soil air leads to precipitation of pedogenic carbonates and formation of calcic horizons. Thus, their carbon isotope composition represents the conditions prevailing during their formation. However, the widespread use of the isotopic signature (δ¹³C, δ¹⁸O, Δ¹⁴C) of pedogenic carbonates for reconstruction of local paleovegetation, paleoprecipitation and other environmental conditions lacks knowledge of the time frame of pedogenic carbonate formation, which depends on climatic factors. We hypothesized that temperaturedependent biotic processes like plant growth and root and rhizomicrobial respiration have stronger influence on soil CaCO₃ recrystallization than abiotic temperature-dependent solubility of CO₂ and CaCO₃. To assess the effect of temperature on initial CaCO₃ recrystallization rates, loess with primary CaCO3 was exposed to ¹⁴CO² from root and rhizomicrobial respiration of plants labeled in ¹⁴CO₂ atmosphere at 10, 20 or 30 °C. ¹⁴C recovered in recrystallized CaCO₃ was quantified to calculate amounts of secondary CaCO₃ and corresponding recrystallization rates, which were in the range of 10⁻⁶–10⁻⁴day⁻¹, meaning that 10⁻⁴–10⁻²% of total loess CaCO₃ were recrystallized per day. Increasing rates with increasing temperature showed the major role of biological activities like enhanced water uptake by roots and respiration. The abiotic effect of lower solubility of CO₂ in water by increasing temperature was completely overcompensated by biotic processes. Based on initial recrystallization rates, periods necessary for complete recrystallization were estimated for different temperatures, presuming that CaCO₃ recrystallization in soil takes placemainly during the growing season. Taking into account the shortening effect of increasing temperature on the length of growing season, the contrast between low and high temperature was diminished, yielding recrystallization periods of 5740 years, 4330 years and 1060 years at 10, 20 and 30 °C, respectively. In summary, increasing CaCO₃ recrystallization rates with increasing temperature demonstrated the important role of vegetation for pedogenic CaCO₃ formation and the predominantly biotic effects of growing season temperature. Considering the long periods of pedogenic carbonate formation lasting to some millennia, we conclude that methodological resolution of paleoenvironmental studies based on isotope composition of pedogenic carbonates is limited not by instrumental precision but by the time frame of pedogenic carbonate formation and hence cannot be better than thousands of years

Carbonate recrystallization in root-free soil and rhizosphere of Triticum aestivum and Lolium perenne estimated by 14C labeling

Under arid and semiarid conditions, pedogenic (secondary) carbonates are formed in soil by precipitation of Ca²⁺ from soil parent material with dissolved CO₂ originating from root and rhizomicrobial respiration

Pedogenic carbonate formation: Recrystallization versus migration—Process rates and periods assessed by 14C labeling

Under arid to semihumid climatic conditions, dissolution of primary carbonate and recrystallization with carbon (C) from soil CO₂ leads to accumulation of significant amounts of pedogenic (secondary) carbonate. Most soils of arid and semiarid regions contain a carbonate accumulation horizon, the depth of which is related to climatic conditions and properties of parent material. It remains unclear whether this carbonate migrates from the upper horizons before or after recrystallization with soil CO₂. The aim of this study was to determine recrystallization rates during initial pedogenesis and to estimate the accumulation depth of secondary carbonate based on C isotopic exchange during secondary carbonate formation in an experiment with alternating moisture conditions. Maize grown on 1 m high loess-filled columns was pulse labeled in ¹⁴CO₂ atmosphere every 3 weeks. After 6 months, portions of secondary (recrystallized) CaCO₃3 were determined in 5 cm segments, based on ¹⁴C respired in the rhizosphere and subsequently incorporated into newly formed secondary carbonate. More than 80% of recrystallized carbonate (Ca¹⁴CO₃) was leached from the uppermost 15 cm of the loess column, and more than 70% of total secondary carbonate were accumulated in a depth between 15 and 50 cm. Based on the recrystallization rate calculated for the uppermost 15 cm of the loess column (1.77 ±� 0.26 ⋅ 10⁻⁵ day⁻¹), between 300 and more than 1,700 years are necessary for complete decalcification of the upper 15 cm. Our modeled data are consistent with formation of calcic horizons under relatively humid conditions