Abstract
This study investigated the vertical and temporal distribution of Thaumarchaeota derived core isoprenoid glycerol dialkyl
glycerol tetraether (GDGT) lipids through sampling and analysis of both suspended particulate matter from the water column
at different times in the annual cycle and a 3 year long record of settling particles in two sediment traps
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at different depths at
an open lake location in Lake Superior. Results from these analyses suggest that Thaumarchaeota were present throughout the
water column during times of overturning, but mainly resided below the depth of the thermocline (20–40 m) during the period
of thermal stratification. Fluxes of thaumarchaeotal produced GDGTs were highly periodic and mainly occurred during two
periods of the annual cycle (winter and late spring/early summer). A covariance of both branched and isoprenoid GDGT
fluxes with the mass accumulation flux combined with the observation that those periods of maximum fluxes were associated
with increased BIT index values, however, suggest that these two periods of elevated fluxes may be related to an influx of
resuspended particles transported from shallower near shore regions of Lake Superior. During all sampling periods TEX86
inferred temperatures from SPM were in good agreement with in situ water temperatures of the depths at which the SPM
was sampled. The observed range of TEX86 inferred temperatures in 3 years of settling particles is relatively small and does
not show significantly higher inferred temperatures during the thermally stratified period, indicating that the sedimentary
TEX86 signal during the summer thermally stratified period mainly originated from depths below the relatively shallow thermocline.
Additionally, TEX86 values during the winter period of increased fluxes did not capture the decrease in water temperatures
observed throughout the water column during this period, and thus may be a further indication that the
thaumarchaeotal lipid flux was the result of sediment focusing. Flux-weighted TEX86 inferred temperatures from both sediment
traps were in good agreement with TEX86 temperatures from surface sediments from the same location in Lake Superior.
Both flux weighted TEX86 temperatures from the sediment traps and average TEX86 temperatures from surface
sediments were similar to averaged measured water temperatures at below 40 m depth within the error of the lacustrine TEX86 calibration. Based on the observed depths of Thaumarchaeota in the water column, TEX86 values in sediments of Lake
Superior likely reflect a combination of mixed-season and sub-thermocline temperatures. This is effectively the same as the
annual averaged water temperature observed at depths below 40 m in Lake Superior. Thus, trends in TEX86 inferred temperatures
in sediment records of Lake Superior, and similar lakes, are likely to reflect subsurface temperature variability rather
than that of surface temperatures
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