Organic geochemical evidence for nitrogen-limited oligotrophic seas in the Late Ordovician
Abstract
The Late Ordovician (~450-44 Ma) was a period of drastic environmental change, beginning in a hothouse climate with epeiric seaways near a Phanerozoic high and concluding with the Hirnantian glaciation, large positive carbon isotope excursion(s) (Hirnantian isotopic carbon excursion, HICE) and one of the Big Five mass extinctions. Warm sea temperatures and low oxidant concentrations relative to modern likely made intervals of the Late Ordovician particularly suited to the development of extensive denitrification in oxygen minimum zones (OMZs), contributing to nitrogen limitation of marine ecosystems. Mixed carbonate/siliciclastic deposits on Anticosti Island, Canada, provide an excellent opportunity to assemble stratigraphic lipid biomarker records of microbial community structure and response to environmental change associated with the Hirnantian glaciation in a tropical epeiric seaway. Lipid biomarkers extracted from Anticosti rocks yield low thermal maturities, consistent with the shallow burial history of the island and lack molecules likely derived from contamination (e.g. zero oleanane from angiosperms). In addition, Anticosti bitumens bear the C29 sterane predominance and low C28/C29 sterane ratio expected from Lower Paleozoic rocks. Anticosti biomarkers reveal unusual environmenal conditions throughout the Hirnantian at Anticosti. Despite the presence of unequivocally marine fossils, Anticosti bitumens contain no detectable 24-n-propylcholestane, usually a robust marker of marine environmental conditions. Anticosti bitumen have high abundances of 3β-methylhopanes (4-11% of C30 αβ-hopane), molecules which are commonly associated with Type I methanotrophic bacteria. Moderate abundances of 2α-methylhopane (2-4% C30 αβ-hopane) with only trace quantities of gammacerane are found. Hopane/sterane ratios, reflecting a highercontribution of bacteria relative to eukaryotes, range from 1.8 to 11.2 (average 4.8), higher than the Phanerozoic marine average of 0.5-2.0. H/St ratios reach a maximum below the main pulse of the HICE, and are at the lowest values during the maximum carbon isotope excursion. We propose that the biomarker distribution prior to the HICE reflects a nitrogen-limited oligotrophic ecosystem in which warm sea temperatures and nutrient limitation allowed marine bacterial communities to thrive relative to eukaryotes. Lower H/St ratios and declining 3β-methylhopane abundances during the main pulse of the HICE interval likely reflect microbial community response to cooler water temperature, contraction of OMZs, and increased ocean ventilation. Integration of the Anticosti record with data from the Hirnantian Vinini formation of Nevada, USA and Katyan formations from the Midcontinent region, USA, extends the temporal, spatial, and facies range of stratigraphic biomarker records of the response of marine microbial communities during the Late Ordovician transition from hothouse with extensive OMZs to ice-house with reduced OMZs.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.B41B0216R
- Keywords:
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- 1055 GEOCHEMISTRY / Organic and biogenic geochemistry;
- 4850 OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL / Marine organic chemistry;
- 4912 PALEOCEANOGRAPHY / Biogeochemical cycles;
- processes;
- and modeling