Sedimentary lipids as indicators of depositional conditions in the coastal Peruvian upwelling regime

Abstract

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 specific sources. 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 surface sediments. 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.