Aquatic moss δ18O as a proxy for seasonally resolved lake water δ18O northwest Greenland

Reconstructing past climate seasonality is fundamental to understanding the nature of past climate changes. This is especially true in the Arctic, where climate is intensely seasonal and proxies that can distinguish climate conditions of multiple seasons in a single year are relatively rare. We propose that submerged aquatic mosses, which are abundant subfossils in some Arctic lake sediments and have distinctive seasonal growth morphologies, can be used to estimate past lake water oxygen isotope composition (delta O-18(lw)) across multiple seasons. Aquatic mosses are abundant, well preserved, and grow continuously in Arctic lakes whenever light is available, with some species displaying unique seasonal morphologies influenced by water temperature. Although Greenland paleorecords suggest that aquatic moss oxygen isotope values (delta(18)Oom) reflect the delta O-18 values of lake water, no modern calibration between delta(18)Oom and delta O-18(lw) exists in Greenland. We present a modern delta(18)Oom vs. delta O-18(lw) calibration using multiple moss species or morphotypes from eight lakes and ponds near Pituffik (Thule), northwest Greenland. We find strong linear relationships (r2 = 0.76-0.85) between the delta(18)Oom and delta O-18(lw) values of multiple species or morphotypes across the range of relatively low delta O-18(lw) values at Pituffik, and our results indicate isotopic fractionations are similar to those found previously at lower latitudes. To assess the potential of mosses as archives of seasonal delta O-18(lw) values, we analyzed delta(18)Oom in season-specific segments of moss strands, with seasons identified based upon growth morphology. Moss inferred lake water delta O-18 values (delta O-18(lwom)) are higher in autumn than spring or summer, likely due to increasing contributions of O-18 enriched precipitation and the cumulative effects of lake water evaporation throughout the ice-free season. For moss subsampled throughout summer, delta O-18(lwom) values generally increased through the season in parallel with observed delta(18)Olw values. Potential temperature dependent fractionation effects during biosynthesis, however, remain unconstrained and should be further addressed with future research. Overall, these findings suggest that aquatic mosses from lake sediments could be used to directly resolve climate seasonality of the past.