15,000 years of black carbon deposition – A post-glacial fire record from maar lake sediments (Germany)

• Black carbon (BC) influx was recorded in two Eifel maar lakes (15–0 ka BP).
• BC influx was elevated from Late Pleistocene grass- and shrublands.
• BC influx increased from 7.5 ka BP with the onset of Neolithic land use.
• BC indicated burning of arable sites by low burning temperatures (6.5 and 5 ka BP).
• High burning temperatures occurred since 4 ka BP with beginning metallurgy.

Fires accompanied human development throughout the Holocene, leaving behind black carbon (BC) as residues from incomplete biomass burning. Here we used molecular fire markers, benzene polycarboxylic acids (BPCAs), to reconstruct fire history in two Eifel maar lakes, Germany. We hypothesized to find indications for (i) changes in BC related to ecosystem changes, (ii) an increase in BC influx at the onset of agriculture until modern times, and (iii) a change in BC quality due to technical progress in combustion, e.g., at the beginning of agriculture and at the onset of the Bronze Age. To calculate absolute BC influx into the maar lakes, we multiplied BC contents with sedimentation rates. The BC influx rates were elevated during tundra-like vegetation in the Late Pleistocene (up to 7.7 g BC m−2 a−1), followed by relatively constant 2.5 g BC m−2 a−1 from the Bølling interstadial (>13.7 kilo years before present, ka BP) until the early Atlantic when forest began to develop. Thereafter, BC influx increased with the onset of land use of Neolithic cultures in the region from 7.5 ka BP to rates of 7–9 g BC m−2 a−1. Noteworthy, also the quality of BC changed: higher ratios of five-to six-times carboxylated benzenes (B5CA/B6CA) pointed at colder, arable fires approximately 1000 years after first Neolithic activity from 6 to 4 ka BP (B5CA/B6CA increased from 1.0 to 2.0). From 4 ka BP (Bronze Age) to modern times increasing burning temperatures as indicated by dropping B5CA/B6CA ratios (from 2.0 to 1.0) were related to metallurgy and industrialization. Between 2.5 and 1 ka BP maximum BC influx rates were reached with ca 15 g BC m−2 a−1. With increasing combustion efficiency and a reduction of wild fires during the last centuries, total BC influx decreased, suggesting that fossil fuel combustion contributed less to total BC input into the lake sediments than former vegetation fires.