Abstract. Melting of the Greenland Ice Sheet (GrIS) is the largest
single contributor to eustatic sea level and is amplified by the growth
of pigmented algae on the ice surface, which increases solar radiation
absorption. This biological albedo-reducing effect and its impact upon sea
level rise has not previously been quantified. Here, we combine field
spectroscopy with a radiative-transfer model, supervised classification of
unmanned aerial vehicle (UAV) and satellite remote-sensing data, and runoff modelling to calculate
biologically driven ice surface ablation. We demonstrate that algal growth
led to an additional 4.4–6.0 Gt of runoff from bare ice in the
south-western sector of the GrIS in summer 2017, representing 10 %–13 %
of the total. In localized patches with high biomass accumulation, algae
accelerated melting by up to 26.15±3.77 % (standard error, SE). The year 2017
was a high-albedo year, so we also extended our analysis to the particularly low-albedo 2016 melt season. The runoff from the south-western bare-ice zone attributed to algae was much higher in 2016 at 8.8–12.2 Gt, although the
proportion of the total runoff contributed by algae was similar at 9 %–13 %. Across a 10 000 km2 area around our field site, algae covered
similar proportions of the exposed bare ice zone in both years (57.99 %
in 2016 and 58.89 % in 2017), but more of the algal ice was classed as
“high biomass” in 2016 (8.35 %) than 2017 (2.54 %). This interannual
comparison demonstrates a positive feedback where more widespread, higher-biomass algal blooms are expected to form in high-melt years where the
winter snowpack retreats further and earlier, providing a larger area for bloom
development and also enhancing the provision of nutrients and liquid water
liberated from melting ice. Our analysis confirms the importance of this
biological albedo feedback and that its omission from predictive models
leads to the systematic underestimation of Greenland's future sea level
contribution, especially because both the bare-ice zones available for algal
colonization and the length of the biological growth season are set to
expand in the future.
Here we report on a set of six apatite reference materials (chlorapatites MGMH#133648, TUBAF#38 and fluorapatites MGMH#128441A, TUBAF#37, 40, 50) which we have characterised for their chlorine isotope ratios; these RMs span a range of Cl mass fractions within the apatite Ca10(PO4)6(F,Cl,OH)2 solid solution series. Numerous apatite specimens, obtained from mineralogical collections, were initially screened for 37Cl/35Cl homogeneity using SIMS followed by δ37Cl characterisation by gas source mass spectrometry using both dual‐inlet and continuous‐flow modes. We also report major and key trace element compositions as determined by EPMA. The repeatability of our SIMS results was better than ± 0.10‰ (1s) for the five samples with > 0.5 % m/m Cl and ± 0.19‰ (1s) for the low Cl abundance material (0.27% m/m). We also observed a small, but significant crystal orientation effect of 0.38‰ between the mean 37Cl/35Cl ratios measured on three oriented apatite fragments. Furthermore, the results of GS‐IRMS analyses show small but systematic offset of δ37ClSMOC values between the three laboratories. Nonetheless, all studied samples have comparable chlorine isotope compositions, with mean 103δ37ClSMOC values between +0.09 and +0.42 and in all cases with 1s ≤ ± 0.25.
Here we report on the oxygen isotope compositions of four proposed apatite reference materials (chlorapatite MGMH#133648 and fluorapatite specimens MGMH#128441A, MZ-TH and ES-MM). The samples were initially screened for 18 O/ 16 O homogeneity using secondary ion mass spectrometry (SIMS) followed by δ 18 O determinations in six gas source isotope ratio mass spectrometry laboratories (GS-IRMS) using a variety of analytical protocols for determining either phosphate-bonded or "bulk" oxygen compositions. We also report preliminary δ 17 O and Δ' 17 O data, major and trace element compositions collected using EPMA, as well as CO 3 2and OHcontents in the apatite structure assessed using thermogravimetric analysis and infrared spectroscopy. The repeatability of our SIMS measurements was better than AE 0.25 ‰ (1s) for all four materials that cover a wide range of 10 3 δ 18 O values between +5.8 and +21.7. The GS-IRMS results show, however, a significant offset of 10 3 δ 18 O values between the "phosphate" and "bulk" analyses that could not be correlated with chemical characteristics of the studied samples. Therefore, we provide two sets of working values specific to these two classes of analytical methodologies as well as current working values for SIMS data calibration.
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