Towards quantitative interglacial fire-vegetation-climate feedbacks: linking sedimentary fire proxy composition of marine isotope stage 11 (Lake El’gygytgyn) and modern lake surfaces (E Siberia)

The ongoing intensification of forest fires in the Siberian Arctic (larger areas, longer durations, higher intensities) raises concerns if these fires might lead to biome shifts from tundra to summergreen or evergreen boreal forest – with consequences for regional to global biophysical land properties and biogeochemical cycles. Given the short time span of instrumental observations, it is unknown if fire can initiate or support biome shifts under the ongoing amplified warming or if climate drives fire regime and biome changes independently. Lake El’gygytgyn in the Russian Far East is currently surrounded by tundra, but pollen data (ICDP sediment core 5011-1A) suggests that during late Marine Isotope Stage (MIS) 12 and “superinterglacial” MIS 11, c. 375-440 kyrs ago biomes changed several times:  from a glacial steppe to interglacial summergreen and evergreen boreal forest. Here, we investigate if and which type of fire regime shifts accompanied these biome shifts.

To enable a quantitative reconstruction of changes in fire intensities and the type of biomass burnt, we used multiple fire proxies. The monosaccharide anhydrides (MAs) are specific biomass burning residues from low-temperature fires analyzed with ultra-high-performance liquid chromatography coupled to a high-resolution mass spectrometer. Sedimentary charcoal reflects mid-to-high intensity fires and was analyzed in sieved fraction > 150 μm and from pollen slides using a microscope. MA isomer ratios and charcoal morphotypes were used to reconstruct the type of biomass burnt. We established links between fire proxy composition and pollen-based vegetation composition for the MIS 11 using core 5011-1A sediments considering dating uncertainties.

To link fire proxy composition with fire regime properties, we used the same fire proxies in modern lake surface sediments from three lakes in Eastern Siberia. We assessed modern charcoal and MA source areas by modelling lake-sedimentary fire proxy transport from modern fires using fire radiative power data of the MODIS Thermal Anomalies product, plume injection heights from the MODIS Terra and Aqua MCD19A2 data and hourly wind fields from ERA5 climate data.

We find clear differences in sedimentary fire proxy composition depending on source area of charcoals and MAs in modern lake sediments. Modern types of fire regime-fire proxy-vegetation-relationships are linkable to the derived past interglacial relationships indicating that fire regime change played a role during some, but not all biome shifts. Overall, we provide new understanding of Siberian sedimentary fire proxies, crucial for a sound, i.e. quantitative reconstruction of long-term fire regime change, allowing to assess the role of fire regime intensification in biome changes during periods of stark warming.