Fifteen years since discovery: Group I alkenones yield the first organic geochemical proxy of cold season temperatures
Abstract
Starting >15 years ago, our discovery that freshwater alkenone producers belong to a separate, previously unknown group of Isochrysidales, and synthesize alkenone isomers distinct from those inhabiting ocean and saline waters, has led to novel continental paleotemperaure and paleosalinity proxies. This presentation will summarize our understanding of the occurrence, global distribution of Group I Isochrysidales, biosynthesis of alkenone isomers and give examples of paleo-reconstructions. Environmental data from lakes in Alaska, Canada, Greenland, China, Siberia, Iceland etc are subjected to statistical analyses to identify key factors affecting the abundance of Group I alkenones. Our results indicate that the phylogenetically diverse Group I Isochrysidales, including two separate clades ("Greenland" vs "EV"), thrive in lakes with an optimal pH of ~ 8, but exceptionally high abundance of certain metal ions may inhibit their growth. We found strong temperature correlations from in-situ suspended particulate matter (SPM) samples, independent of spatial and temporal variability or measured proportions of "Greenland" and "EV" clades in lake waters, indicating that Group I alkenones are immune to species-mixing effects that often complicate paleotemperature reconstruction in saline lakes and marginal ocean sediments. Systematic sampling of SPM over seasonal cycles and from different lakes indicates that Group I Isochrysidales' peak growth season is spring. Analyses of surface sediments from diverse lakes indicate that the UK37 index is a proxy for mean temperature of the spring isotherm (MTSI), and at certain high-latitude sites records winter-spring temperature through the effects of lake ice accumulation and the timing of melt. Taking advantage of the differences in alkenone structures in Group I and II Isochrysidales, we develop novel proxy calibrations for paleotemperature reconstruction (R3b, UK38methyl) that disentangle the mixing of the Group I and II Isochrysidales in low salinity waters, and demonstrate that the Group I-specific R3b proxy is most strongly correlated with winter temperature and maximum ice thickness. This presentation will also highlight examples of paleoclimate and paleoenvironmental applications of Group I alkenones from lakes in Alaska, NE China, and the Black Sea.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.B53E2443H
- Keywords:
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- 0424 Biosignatures and proxies;
- BIOGEOSCIENCES;
- 0473 Paleoclimatology and paleoceanography;
- BIOGEOSCIENCES;
- 4912 Biogeochemical cycles;
- processes;
- and modeling;
- PALEOCEANOGRAPHY;
- 4924 Geochemical tracers;
- PALEOCEANOGRAPHY