|Description||This thesis assesses the utility of various sedimentary lipids as indicators of short-term
changes in depositional conditions and sedimentary organic matter sources in the
coastal upwelling regime off of Peru. A variety of lipids (n-alkanes, n-alkanols, C37
alkenones, hopanoids, keto-ols, lycopane, phytol, stenols, stanols, sterenes, and
tetrahymanol) were quantified in Peru margin sediments from both the oxygen minimum
zone (OMZ) and several near-shore locations. I discuss the utility of the lipid profiles, the
alkenone-Uk37 "paleothermometer", the n-alkane CPI, and several stanol/stenol ratios as
indicators of short-term changes in depositional conditions. This work also provides the
first assessment of the influence of Thioploca, a genus of sulfur-oxidizing bacteria, on
organic compound distributions in upwelling regime sediments.
The potential of the alkenone-Uk37 as a sedimentary marker for El Nino/Southern
Oscillation (ENSO) events was assessed by comparing the historical ENSO record with
detailed Uk37 profiles for 210Pb-dated cores from the OMZ. Sediments from the center of
the OMZ sectioned at intervals ≤ the yearly sedimentation rate have the greatest potential for
holding a Uk37 record of El Nino events. The Uk37 signals of individual El Nino events were
substantially attenuated in the sediments examined, and periods of frequent ENSO activity
(e.g., 1870-1891) were more readily identified than isolated ENSO events in periods of
less frequent ENSO activity. Detailed depth profiles of the C37 alkenones in a core, SC3,
from ≈253 m (02 <0.1 ml/1 bottom water) suggest significant alkenone degradation and/or
alteration (≈30%) in the 0-1 cm interval, despite the dysoxic depositional conditions.
However, the similarity of the Uk37 values for the five 2 mm sections from 0-1 cm suggest
that the Uk37 may be unaffected by the alkenone loss. Correlation between the C37 alkenone
concentration profiles in two cores from ≈15°S collected nine years apart are consistent
with the use of these compounds for "molecular stratigraphy."
The utility of sedimentary hydrocarbons and alcohols as indicators of short-term
changes in depositional conditions was determined in core SC3 by (1) a multivariate factor
analysis of the lipid data and (2) a consideration of individual source-specific biomarkers.
In this core, profiles of odd-carbon-number n-alkanes (C2s-C33) and even-carbon number
n-alkanols (C24-C28) reflect changes in the input of terrigenous sediment relative to marine
sediment during deposition, as indicated by the correlations between these lipids and
inorganic indicators of terrigenous clastic debris. The n-alkane carbon preference index
(CPI) provides a less-sensitive record of fluctuations in the terrestrial input than the
concentration profiles of the individual n-alkanes and n-alkanols, and these lipids are not
well-correlated with the historical El Nino record. The similarity of all the stenol profiles
measured and the lack of concordance between these profiles and inorganic indicators of
terrigenous input suggest that fluctuations in the abundance of higher plant stenols are
obscured by the larger marine contribution of these compounds. Similarities between the
profiles of total organic carbon (TOC) and cholestanol/cholesterol are consistent with stenol
hydrogenation being influenced by the sediment redox conditions.
Profiles of sterols and sterol alteration products illustrate that the rapid downcore
decreases in sterol concentrations do not simply represent conversion of sterols into other
steroidal compounds, but must also involve steroid degradation and possibly formation of non-solvent-extractable steroids. In OMZ sediments, the ratio [cholesterol alteration
products] I [cholesterol+ cholesterol alteration products] substantially increases from 0-4
cm, but at deeper depths, there is no systematic change in the ratio. This suggests that if
there is a progressive conversion of cholesterol to these degradation products below 4 cm,
then it is obscured by an equally rapid removal of these compounds from the sediments.
Stenol profiles in surface sediments suggest a range of degradation rates for these
compounds. Differential remineralization of steroids can cause the relative steroid
abundances in ancient sediments to bear little resemblance to the relative abundances of the
sterols from which these compounds were derived. This limits the use of steroids as
indicators of the relative importance of the original organic matter inputs. However,
important quantitative statements can be made concerning the depositional environment,
based on the presence, rather than relative abundance, of certain steroids derived from
In SC3, the down core increase in burial time (100 cm ≈ 310 years b.p.) and the
downcore changes in sediment chemistry did not result in accumulation of cholesterol
alteration products from more advanced portions of the alteration pathways than are
achieved in the 0-1 cm interval. The absence of cholest-4-ene, cholest-5-ene and cholestane
suggests that cholestadiene reduction to cholestenes and cholestene reduction to cholestane
do not begin to occur over the time scale and under the sedimentary conditions encountered
in the surface 100 cm of Peru margin OMZ sediments.
Although the hopanoids found in the Peru sediments can be related relatively easily to
compounds found in ancient sediments and oils, these hopanoids are not as useful as
steroids for reconstruction of organic matter sources and paleoenvironmental conditions.
This is because the bacterially-derived precursors of these compounds are generally not
specific to any particular type of bacteria.
This thesis provides the first assessment of the influence of Thioploca on organic
compound distributions in upwelling regime sediments. Thioploca, a genus of colorless,
sulfur-oxidizing, filamentous bacteria, constitutes as much as 80% of the biomass in
surface sediments from the OMZ. Since marine species of Thioploca have been found only
in dysaerobic surface sediments of upwelling regimes, biomarkers for this organism may
be useful in identifying similar depositional conditions in the sedimentary record.
Thioploca (dry) was found to be ≈3.8-4.1 wt% lipid. Three fatty acids: cis 16:1Δ9, 16:0
and cis 18:1Δ11 accounted for 69-72% of this lipid. Hydroxy fatty acids, hopanoids and
hydrocarbons were conspicuously absent from the Thioploca. This organism was found to
contain cyclolaudenol, a C31 sterol with an unusual structure; diagenetic alteration products
of this sterol may serve as markers for Thioploca input to sedimentary organic matter, and
hence as markers for paleo-upwelling depositional environments in the sedimentary record.
No quantitatively significant alteration products of cyclolaudenol were identified in the Peru
Thioploca was found to be 9-10 dry wt% protein, and the THAA composition of the
Thioploca contained no unusual amino acids that might serve as Thioploca markers. The
Thioploca TIIAA composition was similar to surface sediment from core SC3 (0-1 cm),
but differed significantly from the TIIAA composition of surface sediments (0-3 cm) from
the Peru margin analyzed in a previous study.||en_US