Composition and sources of lipid compounds in speleothem calcite from southwestern Oregon and their paleoenvironmental implications |
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| Authors: | Ahmed I Rushdi Peter U Clark Alan C Mix Vasile Ersek Bernd R T Simoneit Hai Cheng R Lawrence Edwards |
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| Institution: | (1) Chair of Green Energy Research, College of Food and Agricultural Sciences, King Saud University, Riyadh, 11451, Kingdom of Saudi Arabia;(2) Department of Geosciences, Oregon State University, Corvallis, OR 97331, USA;(3) College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, USA;(4) Department of Chemistry, Oregon State University, Corvallis, OR 97331, USA;(5) Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455, USA;(6) Present address: Department of Earth Science, University of Oxford, Oxford, OX13PR, USA |
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| Abstract: | We analyzed speleothem calcite from the Oregon Caves National Monument, southwestern Oregon, to determine the preservation,
distribution, concentrations and sources of aliphatic lipid compounds preserved in the calcite. Maximum speleothem growth
rate occurs during interglaciations and minimum during glacial intervals. Concentrations of the total lipid compounds range
from 0.5 to 12.9 μg g−1. They increase at times of low speleothem growth rate, suggesting dilution, whereas the apparent accumulation rate of lipid
compounds tends to be highest during times of fastest speleothem growth rate. Such increased accumulation generally corresponds
to times of warm (interglacial) climate, suggesting either a greater source of organic materials during interglacial times
and/or greater efficiency of compound capture during more rapid calcite growth. Aliphatic lipid compounds include homologous
n-alkanoic acids, n-alkanols and methyl n-alkanoates and sterols with concentrations ranging from 0.3 to 7.8 μg g−1, 0.4 to 1.1 μg g−1, 0.5 to 9.6 μg g−1 and 0.1 to 2.7 μg g−1, respectively. Minor amounts of branched methyl n-alkanoates and dimethyl n-alkanedioates are also present. The high concentrations of methyl n-alkanoates are the result of esterification reactions of free fatty acids in alkaline solutions with high pH values associated
with the dripping cave waters. The distribution patterns and geochemical parameters and indices indicate that the major sources
of the aliphatic lipids involved leaching from higher plants and microbial residues derived from the soil zone above the cave
system. The estimated percentage of microbial inputs ranged from 42 to 90% of the total lipids and also showed an increase
in accumulation during warm climates. These well-preserved lipid compounds in speleothem calcite could be used as biomarkers
for paleoenvironmental study. |
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