ALBERT

Description: This dataset compiles soil carbonate (i.e., soil inorganic carbon or SIC) content (% C) up to 7.8 m depth under natural vegetation (grassland or woodland) and cropland (rain-fed or irrigated). The dataset was collected to examine whether SIC content changes with decades-old agricultural conversion of natural vegetation. SIC represent more than a quarter of the terrestrial carbon pool and are often considered to be relatively stable, with fluxes significant only on geologic timescales. However, given the importance of climatic water balance on SIC accumulation, we tested the hypothesis that increased soil water storage and transport resulting from cultivation may enhance dissolution of SIC, altering their local stock at decadal timescales. We compared SIC storage to 7.3 m depth in eight sites across the Great Plains of the United States of America and the Pampas grasslands of Argentina, each site having paired plots of native vegetation and rain-fed croplands, and half of the sites having additional irrigated cropland plots. We took soil samples down to 8.5 m depth using a direct-push coring rig in the US sites and hand augers at the Argentinean sites. Sampling increments were every 0.3 m in the top 0.61 m of the soil and every 0.61 m thereafter in the US sites, and every 0.2 m to 1 m depth, then every 0.3 m to 4 m depth, and every 0.5 m thereafter in the Argentina sites. Sieved and homogenized soil samples were oven-dried at 60°C for for SIC measurement with a Carlo Erba Elemental Analyzer using the two-temperature combustion method. SIC contents are expressed as %C by weight; we note that this differs from carbonate contents reported by local soil surveys, which are %CaCO3 by weight. Inorganic carbon contents (%C) of the soil and carbonate nodules by depth were multiplied by soil and nodule weights and summed to estimate SIC storage.

Description: The MBT-CBT proxy for the reconstruction of paleotemperatures and past soil pH is based on the distribution of branched glycerol dialkyl glycerol tetraether (brGDGT) membrane lipids. The Methylation of Branched Tetraether (MBT) and the Cyclisation of Branched Tetraether (CBT) indices were developed to quantify these distributions, and significant empirical relations between these indices and annual mean air temperature (MAT) and/or soil pH were found in a large data set of soils. In this study, we extended this soil dataset to 278 globally distributed surface soils. Of these soils, 26% contains all nine brGDGTs, while in 63% of the soils the seven most common brGDGTs were detected, and the latter were selected for calibration purposes. This resulted in new transfer functions for the reconstruction of pH based on the CBT index: pH = 7.90-1.97 × CBT (r**2 = 0.70; RMSE = 0.8; n = 176), as well as for MAT based on the CBT index and methylation index based on the seven most abundant GDGTs (defined as MBT'): MAT = 0.81-5.67 × CBT + 31.0 × MBT' (r**2 = 0.59; RMSE = 5.0 °C; n = 176). The new transfer function for MAT has a substantially lower correlation coefficient than the original equation (r**2 = 0.77). To investigate possible improvement of the correlation, we used our extended global surface soil dataset to statistically derive the indices that best describe the relations of brGDGT composition with MAT and soil pH. These new indices, however, resulted in only a relatively minor increase in correlation coefficients, while they cannot be explained straightforwardly by physiological mechanisms. The large scatter in the calibration cannot be fully explained by local factors or by seasonality, but MAT for soils from arid regions are generally substantially (up to 20 °C) underestimated, suggesting that absolute brGDGT-based temperature records for these areas should be interpreted with caution. The applicability of the new MBT'-CBT calibration function was tested using previously published MBT-CBT-derived paleotemperature records covering the last deglaciation in Central Africa and East Asia, the Eocene-Oligocene boundary and the Paleocene-Eocene thermal maximum. The results show that trends remain similar in all records, but that absolute temperature estimates and the amplitude of temperature changes are lower for most records, and generally in better agreement with independent proxy data.