DOI: 10.5285/df50e30a-0672-461a-928e-aafafb45fd55
title: Chronological and biomarker reconstructed mean summer air temperatures (MSAT) for the past 6,000 years from lake sediments on Annenkov Island (near South Georgia) and the South Shetland Islands

description: The dataset contains chronological and biomarker compound and brGDGT (branched Glyceryl Dialkyl Glyceryl Tetraether) mean summer temperature (MSAT) data for the last c. 6,000 years from sediments extracted from Fan Lake on Annenkov Island (near South Georgia) and Yanou Lake, King George Island, South Shetland Islands. Temperature was reconstructed using the Pearson et al. (2011) global calibration and the Foster et al. (2016) Antarctic calibration. For the latter, we studied 32 lakes from Antarctica, the sub-Antarctic Islands and Southern Chile to: 1) quantify their GDGT composition and investigate the environmental controls on GDGT composition; and 2) develop a GDGT-temperature calibration model for inferring past temperatures from Antarctic and sub-Antarctic lakes. The downcore temperature reconstruction data produced using the new Antarctic brGDGT-temperature calibration were tested on Fan Lake and Yanou Lake to provide a proof of concept for the new calibration model in the Southern Hemisphere. This study is an output of the British Antarctic Survey (BAS) Natural Environment Research Council (NERC) funded Science Program, and was funded by NERC Studentship NE/J500173/1 to LF (BAS and Newcastle University) with additional support from: the European Commission under the 7th Framework Programme through the Action - IMCONet (FP7 IRSES, action No.319718 and the ESF-funded IMCOAST project AP6 to SJR, both coordinated by the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Germany). Additional funding from the Natural Environmental Research Council (NERC-CASS), and the German Research Foundation (DFG project no. BR 775/25-1). Logistic support from the NERC-British Antarctic Survey (BAS), HMS Endurance and 892 Naval Air Squadron, the Alfred Wegner Institute (AWI) and the Instituto Antartico Argentino (IAA).
description: Reconstructed Temperature datasets: Sediment cores were taken from Fan Lake in 2005/6 and from Yanou Lake in 2006/7. GDGT data were collected between 2007-2016. Samples were kept frozen at BAS, Cambridge and GDGT compounds extracted using standard procedures described in detail in Foster et al. (2016). Temperatures were produced from the GDGT compounds included in the datasets as follows: Pearson et al. (2011) - Global calibration MSAT = 20.9 + (98.1xGDGT-1b) - (12.0xGDGT-II) - (20.5xGDGT-III) Foster et al. (2016) - Antarctic calibration MSAT = 18.7 + (80.3×GDGT-Ib) - (25.3 ×GDGT-II) - (19.4×GDGT-III) + (369.9×GDGT-IIIb) Chronology datasets: For Fan Lake, Lead-210 (210Pb) and Caesium-137 (137Cs) dating analysis was undertaken on the upper 11 cm of the surface core using dried and homogenised samples packed into a 40-mm tube to 40 mm depth and left to stand for at least 21 days to allow 226Ra and 214Pb to reach equilibrium. Samples were measured on an Ortec J-shape ultra-low background germanium well detector and remote preamplifier. Data analysis and dating model calculations were undertaken following standard procedures as defined in Appleby (2001). We undertook Cs-137 dating but concentration levels were at or below background/detection limits meaning peaks/ages were unreliable using this method. This is important information to retain as these two measurements are independent of each other and often undertaken together to reinforce confidence in the recent (i.e., 1850-present day) chronology. However, Cs-137 in particular, and often Pb-210 levels are often below detection limits and/or sedimentation rates are too low in Antarctic lake systems for these methods to be used with confidence. Thus, the downcore sequentially-increasing Pb-210 chronology from Fan Lake is very encouraging and improves confidences in our age-depth model for the key C20th section of this record which overlaps with observational meteorological records from South Georgia. Unsupported 210Pb estimates were derived from both the constant initial concentration (CIC) and constant rate of supply (CRS) method (Appleby and Oldfield, 1978). CRS ages were used in whole core age-depth modelling. A total of 32 samples for radiocarbon dating were taken from the Fan Lake core. Calibration of terrestrial radiocarbon ages was carried out in OXCAL v.4.2 using the SHCal13 Southern Hemisphere atmosphere dataset. For post-bomb samples, absolute percentage of modern carbon (pMC) data were corrected according to 13C/12C isotopic ratios from measured pMC, where a 'modern' pMC value is defined as 100% (ad 1950 CE),and the 'present day' pMC value is defined as 107.5% (2010 CE), and calibrated using the SHCal13 SH Zone 1-2 Bomb curve in CALIBomb. As all radiocarbon age errors are less than 50 years, calibrated ages are rounded to the nearest 5 years. A master age-depth model was constructed using a combination of all 210Pb CRS ages and 32 radiocarbon ages in a Bayesian age-depth model undertaken in BACON v2.2. All ages quoted in text are weighted mean ages produced from the BACON age-depth model rounded to the nearest 10 years. Chronological data from Fan Lake were first published in Strother et al. (2017). For Yanou Lake (YAN), 15 AMS radiocarbon (14C) ages from, in order of preference: (1) moss macrofossil layers (consisting of hand-picked fine strands of the aquatic moss Drepanocladus longifolius (Mitt.) Paris, but also occasional layers of Campylium polygamum (Schimp.) Lange & C.E.O. Jensen, and some unidentifiable/mixed species moss fragments-considered more likely to have been reworked; (2) terrestrial and/or lacustrine algae; (3) other intact macrofossils and sub-fossils, including bones (bone-collagen, where extractable); (4) other (macro)fossil fragments; (5) organic-rich bulk sediments and, near the base of each core and as a last resort, (6) bulk glaciolacustrine or glaciomarine sediments. Bulk sediments were only dated where macrofossils were absent, while paired macrofossil or bone-collagen and bulk sediment samples were measured in the surface sediment and wherever present to check for any systematic offsets between ages obtained from different carbon sources. Measured radiocarbon ages were calibrated using SH13 and MARINE13 calibration curves and age-depth models were generated using Bayesian age-depth modelling techniques as for Fan Lake. All the 'as measured' (uncalibrated) radiocarbon age data were used as input data for final age-depth model runs (YAN-M4 where model run number is indicated by the -M suffix). For marine sediments, the scale of the marine reservoir effect (MRE) on the Antarctic Peninsula is still debated and several values have been used. For consistency, we have recalibrated and updated all our previously published lake sediment ages using the latest 2013 calibration curves. For the marine-influenced sections of the YAN record, we recalibrated the published MARINE09 calibrated ages from Watcham et al. (2011) with the MARINE13 calibration curve. There were some differences to previously published results using a 100% marine carbon source for all marine-influenced sediments from 210-355 cm depth. We did not include terrestrial-marine partitioning (e.g., 50% marine: 50% terrestrial) used previously in Watcham et al. (2011) as we considered that the partition could vary in a non-systematic manner through time. There were also no significant differences between using a local MRE &#916;R value of 664 &#177; 10 years (Total MRE=1,064 &#177; 10 years minus the global marine reservoir of 400 years), and using a local MRE &#916;R value of 700 &#177; 50 years (Total MRE=1,100 &#177; 50 years), which is also commonly used in compilations of published penguin bone and eggshell ages. Initial chronological data for Yanou were published initially in Watcham et al. (2011) and then updated, with downcore age-depth models added, in Roberts et al. (2017). See original papers for references.

Notes on the ages used:
- Reservoir age and the uncertainty are both in 14C years BP rather than calibrated years -this is applied as an offset during the calibration process and makes the calibrated ages younger (e.g., if surface sediments, which should have a zero age, have an C14 age of c. 400 years the reservoir age is set to 400 14C years with the uncertainty based on repeated measurement data or, often as &#177;50 or 100 14C years, depending on the size of the offset)
- 14C years BP (or radiocarbon years before present, or conventional 14C age) is what are measured during the analytical process by Accelerated Mass Spectrometry (AMS) - this is the raw age data calculated from the half-life of C-14 (5568 years) and the proportion of C-14 measured in a sample
- Here, present in the before present (BP) refers to 1950 CE (common era, same as the now defunct term years AD) - this is radiocarbon dating convention as 1950 CE is immediately before the rapid increase in atmospheric nuclear bomb tests which messed up natural C14 in the atmosphere (and ages derived from samples deposited after that date)
- The atmospheric concentration of C14 changes with time which means C14 years do not equal calendar years and need to be calibrated by comparison with reference datasets of terrestrial annual tree-ring and varved (annually-laminated) sediments which have been radiocarbon dated from the northern (INTCAL13) and southern hemispheres (SH13). MARINE13 is the most recent calibration curve used for marine samples globally.
- Samples with >100% modern carbon are Post-bomb in age (i.e., post 1950 CE) and calibrated using separate calibration curve (CALIBOMB).
- All calibrated radiocarbon ages are cal yr BP, which stands for calibrated years before present and is on the same timescale as a regular calendar year.
- So our C14 ages in the tables are in 14C years BP (as listed). Measured C14 ages in the table and the age-depth model applied to the GDGT depth data have both been calibrated and are shown in cal years BP
- Our Pb-210 ages are years where present in this instance is before 2005 CE (the year in which the surface core was taken) - this can easily be converted to cal yr BP by subtracting from 1950 CE i.e., 1950-2005 = -55 cal yr BP
description: Instrumentation: From Foster et al. (2016) and Roberts et al. (2017): Data were analysed using an Acquity Xevo TQ-S (triple quadrupole with step wave; Waters Ltd.) LC-MS set up with an atmospheric pressure chemical ionisation (APCI) source (Ion saber II) operated in positive ion mode. Analytical separation was achieved using a Grace Prevail Cyano HPLC column (3μm, 150 x2.1mmi.d.) fitted with an in line filter (Waters Acquity UPLC in-line filter, 0.2μm) at 40 degC using a binary solvent gradient where eluent A was hexane and eluent B was propanol. The LC-MS settings were: source offset 50 V, capillary 1.5 KV, desolvation temperature 200 degC, cone voltage 30 V, desolvation gas (N2). Detection was achieved using selected ion monitoring (SIM) of targeted [M +H]+ions (dwell time 50ms).The target ions were m/z 1302, 1300, 1298, 1296 and 1292 for the isoGDGTs and 1050, 1048, 1046, 1036, 1034, 1032, 1022, 1020 and 1018 for the brGDGTs. Peak identification and integration was carried out using MassLynx software (version 4.1). All statistical analysis was undertaken in R 2.15.2 (R Foundation for Statistical Computing). Bayesian age-depth model and weighted mean (wmean), median ages and 95% confidence interval age ranges were constructed using BACON v. 2. and radiocarbon and Pb-210 dating analysis of both cores. Details can be found in Strother et al. (2015) and Roberts et al. (2017).
description: Reconstructed Temperature datasets: The Pearson et al. (2011) calibration covers a temperature range from 1.5°C to 31.2°C. It doesn't perform as well at the Antarctic sites since it has only 6 samples below 5°C. This results in an overestimation of the temperature in the lower end of the calibration range. The Foster et al. (2016) focused on increasing the number of lower temperature sites and creating a new regional calibration for Antarctic lakes. The Foster et al. (2016) calibration results should be used in preference for (sub)Antarctic lake data. Note temperatures are mean summer. For the Yanou Lake (YAN) record, whole YAN-GDGT data set has two values that are above the 5th and 95th percentile outliers (&lt; -0.33°C and &gt;8.61°C). These are 11.32°C and 13.84°C at 1,660 cal a BP and 3,140 cal a BP, respectively. The Fan Lake dataset (FAN) contains three values above the 95th percentile value of 12.87°C between 3,440 and 3,550 cal a BP. Note that any values greater than the 10.3°C maximum limit of the Foster et al. (2016) calibration dataset should be treated with caution. The Foster et al calibration is better suited for use in colder lakes such as Fan Lake and Yanou Lake. As a result, it reproduced the C20th observational MSAT data for Fan Lake and Yanou Lake well, and better than the Pearson et al (2011) calibration. However, it also produced some unexpectedly high temperatures for some, but not all, samples older than 2 ka in the Fan and Yanou lake reconstructions. We are currently investigating this. The most straightforward explanation is the need for more data in the upper end of the temperature gradient in the Antarctic (Foster et al. 2016) calibration dataset. To address this we are currently working on improving and updating the Antarctic, and global, lake GDGT-temperature calibrations.