ALBERT

Description: Biomass burning on the African continent is widespread and interactions with climate, vegetation dynamics and biogeochemical cycling are complex. To obtain a better understanding of these complex relationships, African fire history has been widely studied, although mostly on relatively short time-scales (i.e. yrs to kyrs) and less commonly on long-term scales. Here, we present a 192-kyr, continuous biomass-burning record from sub-Saharan Northwest Africa based on the fire biomarker levoglucosan in a marine sediment core offshore Guinea. Notable features of our record include an increase in levoglucosan accumulation at 80 ka and two peaks at 50-60 ka. The event at 80 ka is likely related to an overall increase in sedimentation rates rather than an increase in biomass burning in the Northwest African savanna region. Our record indicates that glacial/interglacial changes in regional climate and vegetation composition (C3 vs. C4 plants) were not a major influence on biomass burning over the last 192 kyrs. However, we suggest that the burning events at 50-60 ka might be caused by increased occurrence of C3 vegetation and human settlement in this region. At this time, the savanna region became wetter and fuel loads likely increased. Therefore, the region was more hospitable for humans, who likely used fire for hunting activities. Collectively, we hypothesize that on longer (glacial/interglacial) timescales, biomass burning, regional climate and African vegetation are not necessarily coupled, while around 50-60 ka, higher fuel loads and human fire-use may have influenced fire occurrence in sub-Saharan Northwest Africa.

Description: Long chain n-alkanes are terrestrial higher plant biomarkers used to reconstruct continental paleoclimatic and paleohydrological conditions with marine sedimentary archives. Latitudinal variation in their concentration and distribution in marine sediments relatively close to the continent has been widely studied, but little is known on how far this continental signal extends into the ocean. Furthermore, no studies have examined the seasonal variation in the deposition of these biomarkers in marine sediments. Here we studied longitudinal variation in the composition of long chain n-alkanes and two other terrestrial higher plant biomarkers (long chain n-alkanols and long chain fatty acids) in atmospheric particles, as well as longitudinal and seasonal variation in long chain n-alkanes in sinking particles in the ocean at different water depths and in surface sediments, all collected along a 12°N transect across the tropical North Atlantic Ocean. The highest abundance of all three biomarker classes was closest to the African coast, as expected, because they are transported with Saharan dust and the largest part of the dust is deposited close to the source. At this proximal location, the seasonal variability in long chain n-alkane flux and the chain length distribution of the n-alkanes in sinking particles was most pronounced, due to seasonal change in the dust source or due to change in vegetation composition in the source area, related to the position of the Intertropical Convergence Zone (ITCZ). In contrast, in the open ocean the seasonal variability in both the long chain n-alkane flux and chain length distribution of the n-alkanes was low. The abundance of the alkanes was also lower, as expected because of the larger source-to-sink distance. At the western part of the transect, close to South America, we found an additional source of the alkanes in the sinking particles during spring and autumn in the year 2013. The d13C of the alkanes in the surface sediment closest to the South American continent indicated that the isotope signal is likely derived from C3 vegetation from the Amazon, implying an input from the Amazon River, as there is no significant aeolian input from South America there since the prevailing wind direction is from the east. Finally, the concentration of the alkanes was similar in the material collected from the atmosphere, the particles collected while settling through the marine water column, and in the surface sediments, providing evidence that degradation of long chain n-alkanes from the atmosphere to settling at the sediment-water interface at deep open ocean sites is minimal.

Description: Here we provide the data set of fire proxies from the 77 ha, 32 m deep Lake Czechowskie (53°52′27″N 18°14′12″E, 109 m a.s.l.), northern Poland. The sediment core JC11-K5 was recovered in 2011 in 30 m water depth using an UWITEC gravity corer. JC11-K5 was dated by correlating ten macroscopically visible layers with counted annual layer sequences of adjacent cores and tephra shards related to the Askja eruption in 1875 CE. As a conservative estimate, we assigned a 2σ error of 10 years to the marker layers that we used for calculating the age-depth model in OxCal v. 4.2, a Bayesian age-depth modelling approach that provides posterior age uncertainties. For sedimentary macroscopic charcoal analysis, 1 cm3 of wet sediment was dissolved in water, sieved through a 150-µm mesh. Under a stereomicroscope, macroscopic charcoal of three size classes (150-300, 300-500, and ≥500 µm) was counted continuously throughout the core. To estimate a proxy error that combines sampling, preparation and macrocharcoal counting uncertainties, we continuously sampled short core JC11-K2 between 35-55 cm core depth (n = 20), i.e., interval 1840-1875 CE, that could be linked to core JC11-K5 by four marker layers as determined from varve counting. Samples were processed in the same way as for JC11-K5. The numbers of absolute particles cm-³ were compared with the JC11-K5 samples of the same time interval (n = 31) to determine an overall mean relative standard deviation of 0.8 % of each sample. The topmost 75 samples (1780-2010 CE) were also analyzed for monosaccharide anhydrides (MAs) (n = 75, 1780-2011 CE): 125-250 mg dry sediment were extracted with a DIONEX Accelerated Solvent Extractor (ASE 200, 100 °C, 7.6×106 Pa) using a 9:1 solvent mixture of dichloromethane (DCM):methanol (MeOH). As an internal standard, 2.5-5 ng deuterated levoglucosan (dLVG) was added. The total lipid extracts were separated on an unactivated SiO2 gel column (Merck Si60, grade 7754) using sequential elution with DCM:MeOH (9:1) and DCM:MeOH (1:1). The 1:1 fractions were re-dissolved in 95:5 acetonitrile:H2O and filtered using a 0.45 µm polytetrafluoroethylene filter before analysis. The MAs were analyzed by ultra-high pressure liquid chromatography-high resolution mass spectrometry. Authentic standards for LVG, GAL and MAN were obtained from Sigma Aldrich, and that for dLVG (C6H3D7O5) from Cambridge Isotope Laboratories, Inc. Integrations were performed on mass chromatograms within 3 ppm mass accuracy. Concentrations were corrected for relative response factors to dLVG of 0.997, 0.822, and 2.137 for LVG, MAN, and GAL, respectively. Instrumental (standard) errors for LVG, MAN, and GAL were 4 ± 3, 14 ± 15, and 28 ± 38% (1σ), respectively.

Description: Processed monosaccharide anhydrides records based on the raw data of sediment core JC11-K5 of Lake Czechowskie, N Poland using a robust Monte Carlo based approach that includes age and proxy measurement uncertainties in equally spaced time windows. For details and code see Dietze et al. (2019).

Description: Processed charcoal records based on the raw data of sediment core JC11-K5 of Lake Czechowskie, N Poland using a robust Monte Carlo based approach that includes age and proxy measurement uncertainties in equally spaced time windows. For details and code see Dietze et al. (2019).

Description: JC11-K5 was dated by correlating ten macroscopically visible layers with counted annual layer sequences of adjacent cores and tephra shards related to the Askja eruption in 1875 CE. As a conservative estimate, we assigned a 2σ error of 10 years to the marker layers that we used for calculating the age-depth model in OxCal v. 4.2, a Bayesian age-depth modelling approach that provides posterior age uncertainties.