Oxygen-isotope analysis

Sediment samples were dried at 50 °C, weighed and washed through a 63 μm mesh, dried and weighed again. The size fraction >250 μm was separated by dry sieving. On average, 10–12 specimens of the planktonic foraminifera species G. ruber (white) were picked from the >250 μm size fraction. Only clean foraminifera specimens of approximately the same size were selected. Measurements of stable oxygen and carbon isotopes were performed on a Thermo DeltaPlus mass spectrometer equipped with a GasBench 2 carbonate preparation device. The isotope values were calibrated versus NBS 19 (National Bureau of Standards) and the in-house standard ‘Standard Bremen’ (Solnhofen limestone). Isotope values are reported in per mil (‰) relative to the VPDB (Vienna Pee Dee Belemnite) scale. The analytical precision (1−sigma value) as obtained from 11 replicate Standard Bremen measurements was on average 0.058‰ for δ18O and 0.044‰ for δ18C. Only the oxygen-isotope data are reported in this paper.

Radiocarbon analysis

Eleven AMS 14C dates were obtained for core MD03-2614G principally within the period of Sporormiella decline (Supplementary Table 1). Sample KIA 22661 was comprised of Pterpoda taken from the >250 μm size fraction and was prepared at the Leibniz Laboratory for Radiometric Dating and Stable Isotope Research. The sample was treated with 15% H 2 O 2 in an ultrasonic bath followed by 100% H 3 PO 4 at 90 °C produce CO 2 . The CO 2 was reduced with H 2 at 600 °C using a Fe catalyst and the resulting ion–graphite mixture was packed into a target and measured. The following 10 samples were comprised of monospecific planktic foraminifera picked from the >250 μm size fraction of washed sediment. These 10 samples were prepared at the INSTAAR Laboratory for AMS Radiocarbon Preparation and Research (NSRL) before measurement by Accelerator Mass Spectrometry at the Keck Carbon Cycle AMS Laboratory at the UC Irvine (KCCAMS). Foraminifera were leached for 5 min in a 0.001 M solution of HCl. Each sample was then reacted with H 3 PO 4 and the CO 2 produced was cryogenically purified. The purified CO 2 was reduced with H 2 in the presence of a Fe catalyst and the resulting graphite was packed into AMS targets and measured.

Chronology

Tuning and tie-points. The age model for core MD03-2614G was established by tuning the G. ruber δ18O curve from MD03-2614G to nearby planktonic δ18O records from the Great Australian Bight45,46, Vostock ice core Deuterium (ref. 47) and the globally stacked benthic δ18O record48 (Supplementary Table 2). Reference curves were chosen for core intervals of comparable resolution49. The Globigerina bulloides δ18O curve was well in phase from the last glacial maximum (LGM) to 37 kyr ago but offset below. Radiocarbon dates were given preference to accomplish the graphic correlation with different reference curves.

Bayesian age modelling. The planktonic δ18O and radiocarbon ages were used to develop an age model using a P_sequence deposition model in OxCal 4.2 (refs 50, 51) with General Outlier analysis detection (probability=0.05)52. The 14C ages were calibrated using the Marine13 calibration data set17. To accommodate possible changes in the marine reservoir age, a Delta_R of 91±54 14C years was used (determined from the 14CHRONO Marine Reservoir Database at http://calib.org/marine/). Using Bayes’ theorem, the algorithms employed sample possible solutions with a probability that is the product of the prior and likelihood probabilities. Taking into account the deposition model and the actual age measurements, the posterior probability densities quantify the most likely age distributions; the outlier option was used to detect ages that fall outside the calibration model for each group, and if necessary, down-weight their contribution to the final age estimates. Modelled ages are reported here as thousands of calendar years BP, kyr or ka and plotted against depth (Supplementary Fig. 1).

Grain-size analysis

The terrigenous fraction of the sediment represents continental run-off and wind-blown dust in the <63 μm size fraction. The sand-sized fraction >63 μm is almost entirely composed of pelagic carbonate particles, such as planktonic foraminifera, pteropods and coccoliths, as well as benthic foraminifera and bryozoan debris. Bulk sediment samples were dried at 50 °C and finely ground in an agate mortar. Total organic and total carbon were analysed with a Leco CS200 furnace. The accuracy is 0.2% according to 52 parallel measurements of a commercial Leco standard. The proportion of organic carbon was subtracted from total carbon to calculate the carbonate content of the bulk sediment. The non-carbonate fraction was calculated as 100% CaCO 3 .

Pollen preparation and counting

A total of 39 samples were analysed in this study. Samples of ∼4–5 cm3 each were processed using the following method. Sub-sampled sediment was suspended in ∼40 ml of 10% sodium pyrophosphate followed by sieving over 210 and 7 μm mesh. The sediment fraction retained was treated with 10% HCl then acetolysis (10 min) and heavy liquid separation (sodium polytungstate S.G. 2.0, 20 min at 2,000 r.p.m.; 2 × ). A known amount of Lycopodium spores was added to the samples before processing to allow calculation of pollen and charcoal concentrations. Slides were mounted with glycerol and sealed with paraffin wax. Pollen was generally well preserved and abundant throughout the core. Pollen was counted along evenly spaced transects at magnification of × 630 on a Zeiss Axioskop. The average number of pollen counted per sample was ∼230 grains. Pollen percentages were calculated on a pollen sum consisting of all angiosperms and gymnosperms pollen grains counted (that is, excluding pteridophyte and fungal spores).

Charcoal analysis

Charcoal fragments larger than 10 μm were counted along three equally spaced transects of each sample slide and results presented as percentages of the dryland pollen sum.

Sporormiella identification and counting

Identification of Sporormiella was made according to the following criteria. They are ascospores with a sigmoid germinal aperture extending the entire length of the cell. Cells are 12–28 μm × 9–15 μm and are present as two types; end cells show one flattened and one round end, middle cells show two flattened ends53 (Supplementary Fig. 2). Sporormiella spores were calculated as a percentage of the pollen sum.

Palynological results and statistical analyses

A summary pollen diagram with the results of the palynological analysis is presented in Supplementary Fig. 3. Zonation of the pollen diagram was made using the outcome of stratigraphically constrained cluster analysis (CONISS routine)54. The 12 major pollen taxa (shown on Supplementary Fig. 3) were used in CONISS; all minor taxa, charcoal and Sporormiella were excluded. The main divisions of the diagram were placed at ∼70 and 15 kyr ago and the resulting zonation concurs with the per-eye interpretation of the pollen data and confirms that significant vegetation change took place at ∼70 kyr ago, coincident with the reduction in fire. The CONISS results also indicate that Sporormiella decline is not associated with major changes in vegetation.

Ramp function regression and bootstrap re-sampling (RAMPFIT)55 were applied to the Sporormiella data to quantify uncertainties. The RAMPFIT output confirms the per-eye interpretation of the Sporormiella curve and places the onset of the decline at 44.966±0.851 kyr ago and the completion at 43.087±0.645 kyr ago, with the number of Sporormiella spores decreasing from 9.694±0.412% to 2.041±0.729% (Supplementary Table 3, 4; Supplementary Fig. 5).

Data availability

Data on the physical properties of core MD03-2614G are archived in the PANGAEA database (doi:10.1594/PANGAEA.131767). Radiocarbon dating was supported by the NSF’s Archaeology and Archaeometry Program (BCS-0914821). All relevant data are available from the authors.