Methodology:

Coring: Sediment cores were taken in March 2016 from the deepest part of Isla Hornos Lake (IHL; Zmax = 10.7 m) from a floating platform using both piston and gravity corers to create a composite core length of 4.13 m. Cores were packed onboard the Polarstern and brought back to Germany. At the Leibniz Institute for Baltic Sea Research the cores were split lengthwise, photographed and scanned for micro-XRF and magnetic susceptibility. The working halves were subsampled at 1.0 cm intervals for diatoms and other analyses.

Age-depth model: Age determinations were made using 14C radiometric methods on a range of sediment fractions. Age-depth models were generated using the software package BACON in R (Blaauw and Christen, 2011).

Diatoms: Diatoms were isolated from sediments following standard digestion protocol for large sample numbers, using heat and oxidation with 30% hydrogen peroxide and mounted on plain glass coverslips using Naphrax mounting medium. At least 400 valves were identified per sample at 1000x using Subantarctic and South American reference literature.

TEX86: Lipids were extracted from sediments with a mixture of dichloromethane and methanol. After adding a C46 GDGT as internal standard for quantification, the polar fraction containing GDGTs was isolated by microscale flash column chromatography. The fraction was then filtered. GDGTs were analysed by high performance liquid chromatography atmospheric pressure chemical ionization mass spectrometry (HPLC APCI-MS; Thermo Scientific(TM)). The global lake calibration with error bars of +/-3.7°C was used to convert TEX86 values into mean annual lake surface temperature estimates (Powers et al., 2010).

Magnetic Susceptibility: High-resolution magnetic susceptibility data was accomplished using a Bartington MS2E/1 spot-reading sensor integrated into an automatic logging system. Data shown are 5 point averages.

CNS: Bulk sediment TC, TN and TS were measured at the IOW, Germany, with a EA1110 CHN (CE instruments) elemental analyser.

XRF: Semi quantitative geochemical element data of the sediment core was acquired using an ITRAX X-ray fluorescence (XRF) core scanner at the Leibniz Institute for Baltic Sea Research Warnemünde (IOW). The XRF scanner was equipped with a chromium (Cr) tube and operated at 30 kV and 30 mA, with an exposure time of 15 seconds, and a scanning resolution of 1.0 mm.

ICP MS/OES: The concentrations of major elements (Ti, Al, Fe, Mg, Ca, Na, K, P and S) in sediment acid digestions were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES, iCAP 7400 Duo, Thermo Fisher Scientific) using external calibration and Sc (2 ppm) as internal standard. Following automated online dilution (5-fold) of the digestions and addition of the internal standards Be, Rh and Ir by a PrepFAST system (Elemental Scientific), the trace metal (As, Ba, Cd, Co, Cr, Cu, Mn, Mo, Ni, Rb, Sc, Se, Sr, Th, U, V, Zn and REEs) concentrations were measured by inductively coupled plasma mass spectrometry (ICP-MS, iCAP Q, Thermo Fisher Scientific).

The detailed methodology for diatoms, bulk sediment TC, TN and TS, XRF, magnetic susceptibility, ICP MS/OES, and GDGT/TEX-86 analysis can be found in Perren et al. (2025)

Data collection:

microXRF