GoKawiilIce core measurements
Fifty-one discrete CH 4 samples between 40–60 g were cut below the firn to ice transition (about 41 m) at depths ranging from 44.01 to 67.40 m (out of the total 69.33 m) in Huascarán’s SCA. The samples were shipped frozen to Oregon State University, where they were analysed for CH 4 concentration using the standard and well-documented wet extraction procedure5,49. The samples were evacuated for 90 min in a vacuum flask at −60 °C in a refrigerated bath. Once evacuated, the samples were submerged in a warm water bath (50 °C) to melt the ice and release the gas into the headspace above the water. After the samples were completely melted, they were refrozen in the refrigerated bath (−60 °C). The CH 4 concentration in the headspace was measured with a gas chromatograph7,50. Calculations were performed using methane peak area and headspace pressure, compared with daily measurements of calibrated air standards. The air from each sample was measured four times and the mean standard deviation of these measurements across all samples was 3.7 ppb. All of the CH 4 concentration measurements were conducted in the same laboratory at Oregon State University, removing any potential issues for scale offsets. Not enough ice was available for replicate measurements.
Five more samples (from 47.24, 57.53, 60.76, 63.22 and 64.11 m depth) were analysed for their stable carbon isotopic composition of methane (δ13C-CH 4 ) at Oregon State University following the procedure detailed in previous studies50,51. Ice samples for δ13C-CH 4 analysis were sealed in a glass extraction chamber held at −60 °C while the residual air was evacuated. The sample was melted in an ultrapure helium atmosphere and the air released was transferred to an extraction line using a helium carrier50. A series of gas traps separated methane and other hydrocarbons from bulk air before a gas chromatographic column isolated CH 4 from any residual gases (for example, CO and CO 2 ). The CH 4 was then oxidized to CO 2 using a Thermo GC IsoLink interface and analysed for δ13C-CO 2 using a Thermo Delta V isotope ratio mass spectrometer in continuous-flow mode. δ13C-CO 2 was translated to δ13C-CH 4 by bracketing each sample with reference air standards of known isotopic composition and presented relative to the Vienna Pee Dee Belemnite scale. Besides the ice-melting steps, air standard measurements were treated identically to those of ice samples. Data were not corrected for gravitational settling52 or diffusive isotopic fractionation34 during gas transport in the firn column, as the required data to do so did not yet exist. However, we expect these corrections to be small relative to the measured difference in δ13C-CH 4 between SCA and other polar records. We note an expected diffusive fractionation in which CH 4 growth rates are highest (for example, the oldest and youngest samples), data may be depleted (between 0.29 and 0.43‰ from diffusive fractionation)34. Sample depths were selected to correspond to PI and post-industrial revolution dates (that is, before and after 1850 CE, respectively) and sample masses used (between 130 to 300 g) were chosen on the basis of the total air content and CH 4 concentration of the two closest samples measured during the initial CH 4 concentration analysis. A careful determination of the sample size was important to limit linearity effects that are caused by differences in the amount of CH 4 within each sample relative to that in each standard air measurement (7 cc of 1,900 ppb CH 4 in air). Linearity corrections were generally lower than 0.02% and much smaller than the longer-term reproducibility of the system (±0.14‰).
All of the sample depths noted above were analysed to catalogue the past 2,000 years of CH 4 concentration and more recent δ13C-CH 4 fluctuations. Sampling was limited to the past 2,000 years owing to: (1) the high rate of thinning (Extended Data Fig. 7) below 55 m, which particularly limited sampling below 67.40 m, and (2) limited availability of sufficient ice below 67.40 m owing to ice required for other analyses. Ice availability was further limited for the δ13C-CH 4 samples as a result of the low air content in SCA owing to high elevation. The sample size below 64.11 m required for the isotope analysis was approximately 380 g and the ice that remained after other analyses was not sufficient to collect a reliable sample.
Ice core timescale and ice conditions
The timescale for the top 55 m of SCA (2019 to 1910 CE) was established by annual layer counting using seasonal variations in δ18O and concentrations of dust and nitrate (NO 3 −) to identify the annual cycles (Extended Data Fig. 8). The requirement for a year to be assigned is the alignment of at least two of the three parameters. The uncertainty in the assigned years at 55 m is ±2 years. Owing to compression, stratigraphy below 55 m thins rapidly (Extended Data Fig. 7), such that annual layer counting becomes difficult. To construct a timescale for this lower portion of SCA, the δ18O record was matched to the δ18O record from the Quelccaya summit core (Quelccaya Ice Cap, QIC) from southern Peru53. The QIC is located 910 km southeast of Huascarán in the Cordillera Vilcanota just above the Amazon basin. Both Quelccaya and Huascarán receive most of their precipitation from the tropical Atlantic, carried by easterlies over the Amazon, and it is reasonable to conclude that they have broadly similar δ18O profiles. Using the AnalySeries software54, δ18O records below the 1910 CE horizon (55.5 m in SCA, 75 m in QIC) in the two cores were matched by depth, allowing the QIC timescale to be transferred to SCA (Extended Data Fig. 9).
The linear correlation coefficient of the δ18O match between the two cores is +0.63 (P < 0.001). The difficulty of the matching increases with depth, increasing the potential for increasing timescale uncertainties with depth. The much higher stratigraphic thinning rate in SCA compared with QIC means that there is increasingly more time encompassed in each δ18O sample with depth relative to QIC. Also, the QIC δ18O data were smoothed with a 41-sample running mean, whereas the shorter SCA record was smoothed using a 9-sample running mean. The resulting timescale has the expected CH 4 rise in the early 1500s, occurring between 63.9 and 64.3 m in SCA, which corresponds to approximately 1480 to 1520 CE ice age (Δage: approximately 19 years; approximately 1500 to 1540 CE gas age), which lies within the time span of the same fluctuation in polar records7.
The ice samples were assessed for any potential alteration that may have occurred before or after coring that could have affected the CH 4 concentrations. After inspection, it was concluded that the SCA contains no indication of visible melt layers and all borehole temperatures recorded during drilling were between −9 and −15 °C between 0 and 10 m and around −9 °C from 10 m and below (Extended Data Fig. 4a). Following drilling and during transport, continuous temperature measurements were recorded with thermistors placed inside two randomly selected core section containment tubes. The thermistors indicate adequately low temperatures (consistent and below freezing) after drilling and throughout transport from the field to the freezer at BPCRC (Extended Data Fig. 4b,c). Furthermore, air content of SCA is consistent with what is expected on the basis of the site elevation and air content collected from other cores in both mid-latitude and polar regions (Extended Data Fig. 5). These assessments indicate that modern atmosphere has not influenced the sample concentrations38,39,40,41,42.
Box model
Our model is based on a previously constructed four-box model used to constrain CH 4 source distribution5 but adapted to use the SCA CH 4 record as an extra constraint on the signal for the ‘tropical south’ box (0–30° S) instead of relying only on inputs for the northern and southern boxes from polar ice cores. The model consists of four boxes: northern (NH), 30–90° N; tropical north (TN), 0–30° N; tropical south (TS), 0–30° S; and southern (SH), 30–90° S. A Monte Carlo simulation (1,000 iterations) is used to vary CH 4 inputs. True tropical boundaries are at 23.5° N and 23.5° S but 30° N to 30° S are used so that each latitudinal box has an equal air mass.
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