28th International Meeting on Organic Geochemistry

17 – 22 September 2017, Florence, Italy

MULTI-PROXY RECONSTRUCTION OF HYDROCLIMATE FROM TWO

INTERGLACIAL PERIODS IN NORTHWEST GREENLAND

M. Osburn1, J. McFarlin1, G. Lasher1, M. Kelly2, E. Osterberg2, and Y. Axford1 1Northwestern University, Evanston, Illinois, USA 2Dartmouth College, Hanover, New Hampshire, USA In order to better understand how hydroclimate will change in Arctic regions as a result of global warming, we seek to reconstruct that of past warm periods. Compound-specific organic isotopes record information on water isotopes through the lens of biological fractionations (Sachse et al., 2012). One limitation of this approach is the necessity of making large assumptions when interpreting an individual dataset, measuring one component (e.g. hydrogen isotopes on short-chain waxes) in order to interpret another variable/system (e.g. lake water) (e.g. Balascio et al., 2013; Thomas et al., 2016). Here we present a multi-proxy record of late Quaternary interglacial hydroclimate from northwest Greenland with independent measures of summer air temperature, lake water δ2H, lake water δ18O, and growth season precipitation δ2H. This diverse assemblage of data limits necessary assumptions and allows for direct comparison between these climatic variables, revealing unexpected divergence between lake water hydrogen and oxygen isotopes. Our data identifies key differences in hydrological conditions between two past warm periods, the last interglacial (Eemian) and the early to middle Holocene thermal maximum. This work was conducted at a small, non-glacial lake in northwest Greenland (76°5’N, 66°5’W). Despite close proximity to the Greenland Ice Sheet, this lake preserves organic-rich sediments deposited prior to the last glaciation in additional to a complete Holocene record. These older sediments were recovered in multiple cores and are interpreted to be Eemian in age (see McFarlin et al. in prep). Temperature from this record is inferred from insect assemblages (chironomid-inferred temperatures) (Francis et al., 2006). Lake water isotopic composition is reconstructed from δ18O of chironomid head capsules (remains of aquatic larvae, predominantly chitin) and compound-specific δ2H of short chain n-alkanes (21, 23). Longer-chain waxes (27, 29) are used to reconstruct growth-season precipitation. In addition, studies are underway to assess the chain length attribution and associated biosynthetic fractionation of various plant groups from Greenland.

Our record can be divided into three primary intervals with distinct temperature and isotope patterns: the late Holocene, the early Holocene, and the Eemian (Figure 1). The late Holocene is cool with depleted lake water δ18O and modestly enriched δ2H relative to measured δ18O and δ2H of modern lake water (August 2014). Precipitation δ2H is strongly enriched relative to modern lake water. In contrast, the early Holocene was warm and lake water δ2H was strongly depleted relative to modern lake water and early Holocene precipitation (as inferred from long-chain n-alkanes. This trend was observed previously in lower-latitude West Greenland by Thomas et al. (2016). Lake water δ18O is instead slightly more enriched in the early Holocene compared to the late Holocene, opposite of the trends in δ2H. The Eemian was warmer still, with lake water δ18O strongly enriched relative to modern, lake water δ2H approximating modern, and precipitation δ2H gradually becoming more enriched. Notably, this is the first record of Eemian hydroclimate from northern Greenland and the observed trends diverges significantly from even the warmest millennia of the Holocene.

28th International Meeting on Organic Geochemistry

17 – 22 September 2017, Florence, Italy

Independently reconstructing both hydrogen and oxygen isotopes allows us to place paleowater isotopes onto the global meteoric water line and compare to local evaporation lines from around Greenland (Figure 2). This interpretive framework suggests clear differences in hydroclimate between the three time periods including differences in the source and seasonality of moisture as well as landscape humidity. Quantitative stable isotope mass balance modelling is underway to more robustly interpret these variations.

Figure 1 Reconstruction of temperature, lake water isotopes, and precipitation isotopes from Wax Lips Lake.

Figure 2 Water isotopes of modern lake, meteoric and Greenland Ice Sheet melt waters in northwest, west and southwest Greenland (left) compared to reconstructed values from the Late Holocene, Early Holocene, and Eemian (right) figure after Henderson and Shuman (2009)(Henderson and Shuman, 2009). References

Balascio, N.L., et al., 2013. The Holocene 23, 1428–1439. Francis, D.R., et al., 2006. Palaeogeography, Palaeoclimatology, Palaeoecology 236, 107–124. Henderson, A.K., Shuman, B.N., 2009. Geol Soc America Bull 121, 1179–1189. Sachse, D., et al., 2012. Annu. Rev. Earth Planet. Sci. 40, 221–249. Thomas, E.K., et al., 2016. GRL 43, 5302–5308.