Prior to initial human settlement c. AD 1280, fluxes of charcoal and biomarkers in Lake Kirkpatrick and Lake Diamond were nearly undetectable (Fig. 2). The lack of large charcoal particles (>125 µm in diameter) testifies to the near-absence of fire in the surrounding Lophozonia-podocarp forests which is typical of records from the region5. The presence of extremely low values of molecular fire tracers (PAH total flux < 1 ng cm−2 yr−1, MA total flux < 4 ng cm−2 yr−1) prior to human arrival is attributed to small fires in drier settings in New Zealand or to background atmospheric deposition from distant fires in southeastern Australia and post-depositional processes21,22,23.

Figure 2 Multi-proxy comparison. (a–c) Fecal sterols, total PAH and total MA fluxes (ng cm−2 yr−1) in Lake Kirkpatrick (this study). (d,e) Charcoal flux (pieces cm−2 yr−1) and pollen percentages in Lake Kirkpatrick (McWethy et al.6). Full size image

The fluxes of all MAs and PAHs abruptly and simultaneously increase in sediments at c. AD 1345–1365 and mark a period of intense or multiple fire events. The peaks occur shortly after Māori arrival and during the Initial Burning Period, which has been identified from charcoal records from Lake Kirkpatrick and other South Island lakes5,24,25. Levoglucosan, mannosan and galactosan records clearly indicate that combustion of plant biomass reached a maximum at c. AD 1350 with fluxes of 390, 278 and 66 ng cm−2 yr−1, respectively. Relative proportions of the three isomers are consistent with emission factors typical of conifer burning26.

The PAH pattern (Fig. S1) is consistent with typical PAH profiles obtained from the combustion of biomass including several types of hardwood27 and softwood28,29,30. Low molecular weight compounds (Table S1), such as naphthalene, acenaphthylene and acenaphthene (128–154 g mol−1), are poorly represented, which is not surprising as they are commonly present in the gaseous phase and relatively more water-soluble and prone to biodegradation than heavier PAHs, which absorb to atmospheric particles31. The PAH distribution is dominated by 3- and 4-ring molecules (166–228 g mol−1), in particular phenanthrene, fluorene, fluoranthene and pyrene (Fig. S1). These PAH compounds are produced during biomass burning and are involved in the formation of atmospheric particles, eventually incorporated into lake sediments as a result of aerial deposition and surface runoff17. At Lake Kirkpatrick, phenanthrene accounted for 43% of total PAHs on average, with concentrations ranging from a few nanograms per gram (dry weight) to a maximum of 212 ng g−1, corresponding to a flux of 180 ng cm−2 yr−1, during the IBP. Heavier compounds (252–278 g mol−1) were present only in small to negligible concentrations, and this was not surprising as they are mainly produced by higher temperature processes (e.g. fossil fuel combustion) and associated with coarse particles, less likely to travel far from the source area17,32,33.

The abundance and distribution of medium-weight PAHs during the Initial Burning Period is consistent with sustained fires characterized by low oxygen availability and a high flaming to smoldering combustion ratio34,35. Based on the scarcity of high molecular weight PAHs, on the thermal stability of detected compounds36 and on burning experiments37,38, we infer a maximum combustion temperature averaging 400–500 °C. This temperature range was found to maximize the production of 3–4 ring PAHs from biomass combustion32,39. Concentrations and trends detected for medium-weight PAHs thus suggest an infrequent low-intensity natural fire regime before the arrival of humans and high-intensity or high-frequency fires during the Initial Burning Period.

The peak in retene, a tracer of combusted coniferous wood and the associated degradation of abietic acid30, in the L. Kirkpatrick core (101–112 cm depth) implies an abundance of softwood fuel (Fig. S2). However, the levoglucosan to mannosan ratio suggests an increased hardwood to softwood fuel ratio40 at ~AD 1348–1394 (101–112 cm), ~AD 1790–1805 (33–36 cm) and ~AD 1924–1949 (9–13 cm) (Fig. S2). The observed retene record is consistent with the combustion and/or the post-depositional reduction of diterpenoids from softwood species41.

After c. AD 1350, the pollen data5,6 suggest a shift in the composition of vegetation from native Lophozonia-podocarp forest towards more fire-adapted shrubs (e.g., Leptospermum), grasses and bracken fern (Pteridium esculentum). The biomarker reconstruction agrees with the new fuel types that replaced the podocarp-hardwood forests and the change in fire regime characterized by infrequent or smaller low-intensity fires based on low levels of macroscopic charcoal (Fig. 2)6. Such fires would result in lower temperatures and less oxygen depleted conditions, limiting the production of MAs and PAHs, which is strongly dependent on fuel and burning conditions30,35, as observed in the post-settlement record.