Polycyclic Aromatic Hydrocarbons in tropical Australian stalagmites: a framework for reconstructing paleofire activity - Research data

Description

We investigate the possibility to use polycyclic aromatic hydrocarbons in stalagmites as proxies for paleofires at KNI-51, a shallow cave located in tropical Western Australia, where bushfire is a regular occurrence. In order to test links between the stalagmite PAHs and fire above the cave, we performed a series of experiments using PAH distributions in stalagmite aragonite, sediment from the cave and overlying soil. In addition, the possibility of surface contamination was evaluated by measuring PAH abundances and distributions in sequential digestions. PAHs were measured in soils above the cave, in sediments from the stalagmite chamber floor as possible sources of these organic compounds, and at near annual resolution in three aragonite stalagmites to evaluate the degree of deposition and conservation. Signal replication of PAHs was also tested in two coeval stalagmites. The results support the hypothesis that PAHs in KNI-51 stalagmite carbonate reflect paleofire activity within 3 km of the cave, and thus that stalagmites can serve as an important new high resolution proxy for landscape-scale fire activity. Given that karst is present in many fire-prone environments, and that stalagmites can be precisely dated and may grow continuously for millennia, the potential utility of a stalagmite-based paleofire proxy is high.

Steps to reproduce

SEQUENTIAL DIGESTION To test the possibility of external contamination from target compounds affecting stalagmites from cave KNI-51, we performed a sequential digestion test adapted from Wynn and Brocks (2014). Four rectangular sections averaging 10 g were cut from stalagmite slabs using a Dremel tool equipped with a cutting diamond wheel. In the organic cleanroom, the sections were cleaned in the sonic bath with n-hexane and dichloromethane, dried, and weighed. Sections were digested for 2 minutes in 20 mL of a 3M HCl solution (pre-cleaned three times by liquid-liquid extraction with dichloromethane) to remove only an external layer averaging 1.5 g from the sections. The process was repeated 3 times for each section to assess the distribution of organic proxies from the surface to the interior of the slabs. The residual aragonite from the interior was treated as described below. STALAGMITE SAMPLES Samples analyzed for PAH abundances were drilled using a Dremel handheld drill with a 1 mm diameter diamond-coated bit in order to obtain continuous samples in 2-3 mm intervals. Prior to drilling each sample, the outer surface of the slabs was thoroughly rinsed with pesticide grade n-hexane and dichloromethane. Powders were then transferred to a class 10,000 cleanroom for organic analyses and extracted. The extraction procedure is detailed in Argiriadis et al. (2019), although with slight modifications. Briefly, samples were spiked with 50 µL of the internal standard solution containing 13C6-acenaphthylene, 13C6-phenanthrene, 13C4-benzo(a)pyrene at 1 ng µL-1. Then, they were dissolved in 3M HCl and liquid-liquid extracted three times with 10 mL of DCM and 5 mL of n-hexane. Extracts were buffered with anhydrous sodium sulfate, reduced to ~200 µL under a gentle stream of nitrogen, spiked with 20 µL of 13C6-chrysene at 1 ng µL-1 as a recovery standard, and analyzed by gas chromatography-mass spectrometry (GC-MS). SOIL/SEDIMENT/FLOOD LAYER SAMPLES The sediment samples were removed from the stalagmite slab with the hand drill. Soil/sediment samples were freeze-dried and then ground in a hand mortar. Samples were dispersed with diatomaceous earth, spiked with internal standards, and extracted three times through an ASE 350 accelerated solvent extractor using a 1:1 (v/v) mixture of DCM and n-hexane at 100 °C and 1000 psi in 22 mL stainless steel cells. Extracts were concentrated to ~200 µL and analyzed by GC-MS. GC-MS/MS ANALYSIS PAHs were quantified through GC-MS/MS. The GC was equipped with a HP-5ms capillary column (60 m, 0.25 mm i.d., 0.25 µm film thickness, Agilent Technologies) and the temperature program was as follows: 70 °C (2 min), 10 °C min-1 to 200 °C (5 min), 8 °C min-1 to 280 °C (5 min), 5 °C min-1 to 310 °C (9 min). Helium was used as carrier gas (1 mL min-1), while argon was used as collision cell gas. The PTV injector was operated in splitless mode (70 °C to 320 °C at 14.5 °C s-1).

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