15 research outputs found
Mercury records covering the past 90 000 years from lakes Prespa and Ohrid, SE Europe
The element mercury (Hg) is a key pollutant, and much insight has been gained by studying the present-day Hg cycle. However, many important processes within this cycle operate on timescales responsive to centennial- to millennial-scale environmental variability, highlighting the importance of also investigating the longer-term Hg records in sedimentary archives. To this end, we here explore the timing, magnitude, and expression of Hg signals retained in sediments over the past ∼ 90 kyr from two lakes, linked by a subterranean karst system: Lake Prespa (Greece, North Macedonia, and Albania) and Lake Ohrid (North Macedonia and Albania). Results suggest that Hg fluctuations are largely independent of variability in common host phases in each lake, and the recorded sedimentary Hg signals show distinct differences first during the Late Pleistocene (Marine Isotope Stages 2–5). The Hg signals in Lake Prespa sediments highlight an abrupt, short-lived peak in Hg accumulation coinciding with local deglaciation. In contrast, Lake Ohrid shows a broader interval with enhanced Hg accumulation and, superimposed, a series of low-amplitude oscillations in Hg concentration peaking during the Last Glacial Maximum, which may result from elevated clastic inputs. Divergent Hg signals are also recorded during the Early and Middle Holocene (Marine Isotope Stage 1). Here, Lake Prespa sediments show a series of large Hg peaks, while Lake Ohrid sediments show a progression to lower Hg values. Since ∼ 3 ka, anthropogenic influences overwhelm local fluxes in both lakes. The lack of coherence in Hg accumulation between the two lakes suggests that, in the absence of an exceptional perturbation, local differences in sediment composition, lake structure, Hg sources, and water balance all influence the local Hg cycle and determine the extent to which Hg signals reflect local- or global-scale environmental changes
Aromaticity and degree of aromatic condensation of char
The aromatic carbon structure is a defining property of chars and is often expressed with the help of two concepts: (i) aromaticity and (ii) degree of aromatic condensation. The varying extent of these two features is assumed to largely determine the relatively high persistence of charred material in the environment and is thus of interest for e.g. biochar characterization or carbon cycle studies. Consequently, a variety of methods has been used to assess the aromatic structure of chars, which has led to interesting insights but has complicated the comparison of data acquired with different methods. We therefore used a suite of seven methods (elemental analysis, MIR spectroscopy, NEXAFS spectroscopy, 13C NMR spectroscopy, BPCA analysis, lipid analysis and helium pycnometry) and compared 13 measurements from them using a diverse sample set of 38 laboratory chars. Our results demonstrate that most of the measurements could be categorized either into those which assess aromaticity or those which assess the degree of aromatic condensation. A variety of measurements, including relatively inexpensive and simple ones, reproducibly captured the two aromatic features in question, and data from different methods could therefore be compared. Moreover, general patterns between the two aromatic features and the pyrolysis conditions were revealed, supporting reconstruction of the highest heat treatment temperature (HTT) of char
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Aromaticity and degree of aromatic condensation of char
The aromatic carbon structure is a defining property of chars and is often expressed with the help of two
concepts: (i) aromaticity and (ii) degree of aromatic condensation. The varying extent of these two features
is assumed to largely determine the relatively high persistence of charred material in the environment
and is thus of interest for, e.g., biochar characterization or carbon cycle studies. Consequently, a
variety of methods has been used to assess the aromatic structure of chars, which has led to interesting
insights but has complicated the comparison of data acquired with different methods. We therefore used
a suite of seven methods (elemental analysis, MIR spectroscopy, NEXAFS spectroscopy, ¹³C NMR spectroscopy,
BPCA analysis, lipid analysis and helium pycnometry) and compared 13 measurements from them
using a diverse sample set of 38 laboratory chars. Our results demonstrate that most of the measurements
could be categorized either into those which assess aromaticity or those which assess the degree
of aromatic condensation. A variety of measurements, including relatively inexpensive and simple ones,
reproducibly captured the two aromatic features in question, and data from different methods could
therefore be compared. Moreover, general patterns between the two aromatic features and the pyrolysis
conditions were revealed, supporting reconstruction of the highest heat treatment temperature (HTT) of
char.Keywords: Aromaticity,
Stability,
Aromatic condensation,
Pyrolysis,
Char,
Biochar,
Pyrogenic organic matter,
Heat treatment temperatur
Chemical and Isotopic Thresholds in Charring: Implications for the Interpretation of Charcoal Mass and Isotopic Data
Charcoal plays a significant role
in the long-term carbon cycle,
and its use as a soil amendment is promoted as a C sequestration strategy
(biochar). One challenge in this research area is understanding the
heterogeneity of charcoal properties. Although the maximum reaction
temperature is often used as a gauge of pyrolysis conditions, pyrolysis
duration also changes charcoal physicochemical qualities. Here, we
introduce a formal definition of charring intensity (CI) to more accurately
characterize pyrolysis, and we document variation in charcoal chemical
properties with variation in CI. We find two types of responses to
CI: either linear or threshold relationships. Mass yield decreases
linearly with CI, while a threshold exists across which % C, % N,
and δ<sup>15</sup>N exhibit large changes. This CI threshold
co-occurs with an increase in charcoal aromaticity. C isotopes do
not change from original biomass values, supporting the use of charcoal
δ<sup>13</sup>C signatures to infer paleoecological conditions.
Fractionation of N isotopes indicates that fire may be enriching soils
in <sup>15</sup>N through pyrolytic N isotope fractionation. This
influx of “black N” could have a significant impact
on soil N isotopes, which we show theoretically using a simple mass-balance
model