Pre-Snowball Earth ecosystems, insights from nitrogen isotopes in the Neoproterozoic Kwagunt Formation of the Chuar Supergroup, Grand Canyon.
Abstract
Reconstructing marine ecosystems prior to the Neoproterozoic Snowball Earth episodes is an important constraint to our understanding of these events and to the biogeochemical evolution of the Neoproterozoic. The mixed siliciclastic and carbonate sequence comprising the Chuar Supergroup, Grand Canyon, USA, encompasses the period in Earth history immediately prior to the first of the Snowball Earth episodes 740 million years ago and presents a unique opportunity explore this issue in relatively immature rocks. In the Phanerozoic, there is a consistent relationship between euxinic basins, widespread black shale deposition, and δ15N values below 0‰ that are indicative of nitrogen-fixation supported primary productivity. Fe-speciation data from black shales of the Walcott Member of the Kwagunt Formation, upper Chuar Supergroup suggest that the Chuar water column and the middle Neoproterozoic deep ocean was euxinic. Bulk δ15N values from the Walcott Member range from +1.7 to +4.7‰, which are not directly supportive of nitrogen fixation. Preliminary nitrogen isotope data from organic extracts suggest that organic nitrogen may be more significantly enriched than bulk isotopes suggest. These relatively 15N- enriched values contrast those from Phanerozoic episodes of black shale deposition. The Walcott primary producer community is interpreted to have utilized a nitrogen substrate that had an isotopic composition more like that of nitrate in modern, relatively oxygenated marine systems. On the basis of biomarker data many black shale sequences of the Phanerozoic (e.g. Devonian, Permo-Triassic and Cretaceous) record the presence of an active phototrophic sulfide oxidizer community. In contrast, our best efforts have yet to yield biomarker evidence of photic zone euxinia within the Walcott. We therefore suggest that the sulfidic chemocline during Walcott deposition was consistently below the photic zone. Oxygenic phototrophic primary producers utilized nitrate from the region above the sulfidic zone that is 15N-enriched due to water- column denitrification under dysoxic conditions. Stratigraphic maxima in total organic carbon (10.8%) and δ15N values (+4.7‰) co-occur and sedimentological parameters indicate basin deepening and reduced siliciclastic dilution as the primary factor in total organic carbon enrichments. Higher δ15N values during basin deepening may reflect a more dysoxic upper water column and a greater influence of denitrification on the δ15N of dissolved inorganic nitrate utilized by primary producers.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFMPP43A1513J
- Keywords:
-
- 0404 Anoxic and hypoxic environments (4802;
- 4834);
- 0469 Nitrogen cycling;
- 1041 Stable isotope geochemistry (0454;
- 4870);
- 4900 PALEOCEANOGRAPHY (0473;
- 3344);
- 4912 Biogeochemical cycles;
- processes;
- and modeling (0412;
- 0414;
- 0793;
- 1615;
- 4805)