Tracing Holocene Natural and Anthropogenic Signals in Iceland Based on Fire and Faecal Markers in Lacustrine Sediments

As a first step of an effort to accurately reconstruct a continuous history of climate across Iceland since the last deglaciation, we produced the first record of Holocene fire regime and human presence in Iceland based on pyrogenic PAHs (polycyclic aromatic hydrocarbons) and faecal sterols from a lake core in the northeast region.Paleoclimate research across Iceland provides a template for changes in climate across the northern North Atlantic. The role of orbitally driven cooling, volcanism, and human impact as triggers of local environmental changes is debated. However, the prevailing paradigm is that major losses of forest cover and a pervasive soil erosion began with the settlement of Iceland (~870 CE). While there are indications that human impact could have reduced environmental resilience in a context of deteriorating climatic conditions, it is still difficult to resolve to what extent human and natural factors affected Iceland landscape instability.PAHs can be formed during the incomplete combustion of biomass initiated by humans or natural wildfires, as well as by the microbial modification of biogenic precursors during low-temperature diagenesis. Factors such as fire temperature, biomass typology, and source distance can strongly affect pyrogenic PAH molecular weight and spatial distribution.Faecal sterols/stanols and their ratios have been used in archaeological and paleoclimate studies to detect human (5β-coprostanol) and/or livestock/herbivore waste (e.g., 5β-stigmastanol), and fertilisation by manure. The absence of large herbivorous mammals and humans in Iceland prior to settlement means that increases in the occurrence of faecal sterols and bile acids over natural background values should mark the arrival of humans and associated livestock in the catchment, which could be traced regionally.Our results indicate that the Icelandic fire regime during the Holocene followed four main phases. Among these, a very long period centred around the Holocene climatic optimum (ca 9.5 – 4.5 ka BP) was characterised by a generally low frequency fire regime, both in the lake catchment as in the whole north-eastern Iceland. This same period was also marked by relatively low background levels of faecal sterols/stanols.At 4.5 ka BP a new phase started, with a general increase of all PAHs values, denoting a probable increase in fire frequency in both the catchment and at a regional level, while a potential human presence based on sterol data seems to arrive only after 2 ka BP. According to our present chronology, there is no apparent human signal in the PAH curve around the 9th century C.E., where an increase in man-made fires would likely be expected in connection to the historical data on the Landnám (Viking colonisation of Iceland – traditionally in the 870s C.E.). Instead, the PAHs trend tends to match a general stepwise climatic “deterioration” previously highlighted by other lake proxies throughout Iceland, indicating that fire regimes have always primarily been controlled by natural factors, probably led by aridity/seasonality shifts.Overall, the data sustain the innovative view that early settlers did not quickly consume the already scarce wood and soil, but instead carefully planned the use of local resources.