New constraints on the provenance of hopanoids in the marine geologic record: BHPs in marine suboxic and anoxic environments

New constraints on the provenance of hopanoids in the marine geologic record

Sáenz, J. P.; Wakeham, S. G.; Eglinton, T. I.; Summons, R. E. (2011) Organic Geochemistry, 42(11), 1351–1362. DOI: 10.1016/j.orggeochem.2011.08.016

Summary

We examined the abundance and structural diversity of Bacteriohopanepolyols (BHPs) in suspended particulate matter, sinking particles, and sediments from three marine pelagic environments with strong vertical redox gradients: the Arabian Sea oxygen minimum zone, the Peru Margin OMZ, and the Cariaco Basin anoxic basin. BHP abundance and structural diversity were highest at suboxic and anoxic depths, indicating that anaerobic bacteria are important sources of hopanoids in the marine water column. We identified an isomer of bacteriohopanetetrol (BHT II) that is consistently associated with suboxic and anoxic conditions across all three sites. Based on its absence from terrigenous and oxic marine environments, we proposed BHT II as a promising candidate biomarker for water column suboxia and anoxia in the marine geologic record.

Key Findings

• BHP abundance in suspended particulate matter was highest at suboxic and anoxic depths across all three study sites (Arabian Sea, Peru Margin, Cariaco Basin), suggesting anaerobic bacteria as a major source.
• Six BHPs were identified in Arabian Sea SPM; BHT was the most abundant (1.8–107.8 ng/L). Surface and subsurface samples had distinct BHP molecular fingerprints.
• An isomer of BHT, designated BHT II, was detected at all three sites exclusively in association with suboxic or anoxic conditions — absent from oxic surface waters and from terrigenous environments studied to date.
• The molecular fingerprint of BHPs in suspended particles, sinking particles, and core-top sediments indicates that water column BHPs are exported to sediments, most likely associated with settling particles rather than the free-water phase.
• BHP structural diversity in marine pelagic environments is much lower than in terrigenous environments, consistent with lower phylogenetic diversity of hopanoid producers in the ocean (estimated 5–10% of marine bacteria).
• Hopanoids produced within OMZs and anoxic basins likely represent an important input to the marine sedimentary hopanoid inventory, particularly in upwelling environments.

Methods

• Sites: Arabian Sea (stations MS1, MS3, MS4), Peru Margin (ISP 1, ISP 2), Cariaco Basin time series site.
• SPM collection: large-volume in situ filtration (500–3000 L) through GFF filters (0.7 μm pore size) using Challenger Oceanic and McLane WTS-LV pumps.
• Sinking particles: moored sediment traps (Arabian Sea). Sediments: multicorer.
• BHP extraction: Soxhlet (Arabian Sea, Cariaco) or Bligh and Dyer (Peru); analysis by HPLC-APCI/MSⁿ with acetylation.
• Quantification: external BHT standard curve; PD internal standard.
• BHT II structural identity confirmed by HT-GC-MS from one Cariaco Basin sample.

Significance

We provided critical new constraints on the environmental distribution of hopanoids in the ocean, demonstrating that suboxic/anoxic water column environments are hotspots for BHP production. The identification of BHT II as a potential redox biomarker has practical value for paleoceanographic reconstructions. The data also showed that hopanoid provenance in marine sediments is more complex than previously assumed — multiple water column sources, with depth-dependent compositions, contribute to the sedimentary record.