Insoluble prokaryotic membrane lipids in continental shelf sediments offshore Cape Town: Implications for organic matter preservation

Abstract

The largest organic carbon (OC) reservoir on Earth is in the geosphere, mainly comprising insoluble organic matter (IOM). IOM formation, therefore, plays an important role in the short and long-term carbon cycle, carbon bioavailability and formation of source rocks. To explore the mechanism of insolubilization of organic matter (OM), we have analysed soluble and IOM fractions of continental shelf marine sediments. We have applied sequential solvent-extractions followed by a selective chemical degradation of the post-extraction residue, specifically targeting prokaryotic membrane lipids (branched fatty acids -FAs, hopanoids, archaeol and glycerol dialkyl glycerol tetraethers -GDGTs). Up to 80% of prokaryotic membrane lipids are not solvent-extractable, and we observe compound-specific differences in partitioning between soluble and IOM fractions. Based on these observations, we propose a variety of mechanisms for the incorporation of prokaryotic lipids into IOM in marine sediments: First, OM association with authigenic carbonates; second, cross-linking via esterification reactions with time, which could be particularly relevant for FAs; third, competition between reactivity and loss of polar head groups, the latter rendering the OM less susceptible to incorporation; and finally, inherent solvent-insolubility of some lipids associated with prokaryotic cells.

Selective chemical degradation of silica sinters of the Taupo Volcanic Zone (New Zealand). Implications for early Earth and Astrobiology

“…It is also highly variable, as reflected by those ranges, amongst the three samples measured here. With the exception of archaeol, which has not been observed previously in IOM of peat and marine sediments, these findings are consistent with previous work from profoundly different settings (Chaves Torres & Pancost, ; Chaves Torres et al., ).…”

“…The preferential occurrence in the IOM of archaeal ether lipids, both archaeol and i‐GDGTs, over the bacterial DAGE analogues cannot be directly compared to previously studied peat and marine samples which lacked DAGEs. However, in those settings i‐GDGTs are preferentially incorporated into IOM fractions relative to the bacterial br‐GDGTs (Chaves Torres & Pancost, ; Chaves Torres et al., ), as was also observed in previous work (Tierney, Schouten, Pitcher, Hopmans, & Sinninghe Damsté, ; Weijers et al., ). This could be due to a range of factors in the marine setting, where br‐GDGTs were likely allochthonous (Zell et al., ), but the analogous observation in peat and now geothermal settings suggests that bacterial ether lipids are less prone to occurring in IOM fractions.…”

“…Here, we focus on the comparison of prokaryotic membrane lipids obtained from solvent extraction, BD and Sox, versus the analogues in IOM fractions, BHy and AMe. These findings are interpreted in the context of previous findings from a peat profile (Chaves Torres & Pancost, ) and coastal marine sediments (Chaves Torres et al., ), which were explored using the same methodology as in the current TVZ silica sinters. This allows us to explore the implications of insolubisation on the determination and interpretation of lipid biomarkers.…”

“…Alternatively or additionally, the preferential occurrence of certain prokaryotic biomarkers in the IOM could arise from different diagenetic fates associated with different IP moieties. For example, archaeol—which occurred only in solvent extracts of peat (Chaves Torres & Pancost, ) and marine sediments (Chaves Torres et al., )—typically contains phosphatidyl moieties (XV) (e.g., Lipp & Hinrichs, ; Peterse et al., ; Sturt, Summons, Smith, Elvert, & Hinrichs, ), whereas i‐GDGTs frequently occur as glycolipids (XVI) (Lengger, Hopmans, Sinninghe Damsté, & Schouten, ; Liu, Lipp, & Hinrichs, ; Schouten et al., ) and have been frequently reported in both soluble and IOM fractions (Chaves Torres & Pancost, ; Chaves Torres et al., ; Huguet, Fosse, Metzger, Fritsch, & Derenne, ; Tierney et al., ; Weijers et al., ). On the contrary, in hydrothermal environments archaeol has typically been reported not only as a phospholipid (XV)but also as a glycolipid (XVI) (Bradley, Fredricks, Hinrichs, & Summons, ), or only as a glycolipid (Gibson et al., ; Sollich et al., ), potentially an adaptation strategy in response to higher temperature or lower pH (Baba et al., ; Curatolo, ; Shimada et al., ; Sollich et al., ; Wang et al., ).…”

“…This represents only a small percentage (<3.5%) of the total organic carbon (TOC) of the sinters (see Supporting information), but it is crucial for understanding the partitioning of these biomarkers. Finally, the use of prokaryotic lipids as biomarkers in the current study enables a comparison to previous work on peat (Chaves Torres & Pancost, ) and marine sediments (Chaves Torres, Melbourne, Hernandez‐Sanchez, Inglis, & Pancost, ), and therefore, to assess whether previous findings apply in a setting where there is no wider OM matrix, that is whether previously observed patterns for IOM occurrence/formation are consistent across the geosphere.…”