Exploring the potential of sedimentary lipid biomarkers combined with novel hyperspectral imaging techniques: regional paleoclimate variability inferred from southern Greenland lake sediments

Abstract:

The Arctic and Greenland are warming rapidly. To assess effects of the current warming, previous warm periods need to be investigated. High-resolution climate reconstructions from lakes help elucidate Greenland's past climate variability.

Lacustrine lipid biomarkers such as branched glycerol dialkyl glycerol tetraethers (brGDGTs) are useful for reconstructing past temperature. Their temporal resolution, however, is limited by the sample size required to obtain adequate material for analysis and the long sampling-procedures. Recent advances in spectral imaging techniques (VIS-RS, spatial resolution: 2mm) reported a successful proxy-to-proxy (PtP) calibration between a high-resolution spectral index (RABD_660:670) and a low-resolution biomarker (U^K₃₇) in lake sediments from W Greenland (von Gunten et al., 2012). This PtP-calibration increased the temporal resolution of the quantitative temperature reconstruction drastically. Here, we explore the potential of a PtP-calibration between a hyperspectral imaging index (HSI, RABD_590:730, spatial resolution: 70µm, reflecting paleo productivity) and brGDGT-derived temperatures, from Lakes Igaliku and 578, S Greenland. We also investigate hypolimnetic oxygen variations with the HSI index RABD_790:900 (bacteriopheophytin, anoxia).

During the 20^th century, HSI suggests both lakes experienced a rapid productivity increase, while Lake 578 also records increased productivity from ~1000 - 400 cal. yr BP. This stands in contrast to a decreasing temperature trend (derived from brGDGTs) and likely reflects changes in local land use (sheep farming in recent years and during the Norse settlement period). With the application of multivariate statistics on the uncalibrated brGDGTs and the hyperspectral signals we investigate these findings further.

The HSI index RABD_790:900 of Lake 578 shows rather anoxic conditions in the early-Holocene indicating a thermally stratified water column. We suggest this was either caused by extended lake-ice cover or a different prevailing wind-pattern. The downcore brGDGT-temperature reconstruction, in addition to quantifying other algal biomarkers, will improve our understanding of the environmental and human factors causing changes in primary productivity and hypolimnetic oxygen variations in S Greenland lakes.