Local and global perspectives on carbon and nitrogen cycling during the Hirnantian glaciation
Abstract The impact of climate and ocean circulation changes on marine carbon and nitrogen cycling was investigated for a period covering the Late Ordovician Hirnantian glaciation. Three sections were studied: two from Nevada, one proximal and one distal; and one from the Yukon. The opportunity to compare δ 13 C carb , δ 13 C org and δ 13 C grap profiles between ocean and epeiric platform settings enabled a more thorough assessment of environmental and diagenetic factors influencing the preservation of seawater δ 13 C values at each stratigraphic section. Using the best preserved sedimentary components from the two Nevada sections, a lowstand sedimentary gradient of ∼ 4‰ was found between platform and ocean settings during the Hirnantian glaciation. Gradients of similar magnitude have been reported from Hirnantian platforms in the Canadian Arctic and Baltic basins. Taking the outboard sections as the most representative of the ocean carbon pool, a mean, positive δ 13 C shift of just 2.7 ± 0.4‰ is calculated, which is significantly smaller than the 5–7‰ positive shifts recorded in epeiric platform settings. The discrepancy is attributed to the effects of local carbon cycling in tropical and subtropical epeiric seas. It is hypothesized that increased carbonate weathering caused δ 13 C values of local carbon weathering fluxes to increase, which in turn caused seawater δ 13 C values to increase in basin proximal settings. In addition, nutrient dust fluxes may have contributed to increased levels of photosynthetic activity in proximal settings, driving sedimentary δ 13 C values even higher—much higher than sediment deposited in outboard settings where carbon exchanges with the surface ocean were less restrictive. A δ 15 N profile reconstructed from deposits of the distal Vinini Creek section records a 1.5‰ positive shift through the Hirnantian glacial interval. Interpreted as a major Late Ordovician upwelling zone, high organic carbon (> 1 wt.%) and high hydrogen indices (400–600) make the Vinini Creek section a suitable candidate for the preservation of original sedimentary δ 15 N records. To explain the results, a conceptual model of ocean nitrogen cycling under greenhouse climate conditions is presented that highlights the role of cyanobacteria as both primary producers and nitrogen fixers. It is hypothesized that in greenhouse climates, the fixed nitrogen inventory of the abyssal ocean was low due to efficient denitrification in the ocean's expansive oxygen minimum zones. This caused algal productivity to become strongly dependent on continuous supplies of fixed nitrogen from cyanobacteria in the photic zone, yielding sedimentary δ 15 N values between 0 and − 2‰. By contrast, the cooler icehouse climate of the Hirnantian led to increased ocean ventilation, greater partitioning of atmospheric oxygen into downwelling surface waters, and oxygen minimum zone shrinkage. As a result, fixed nitrogen levels climbed in the ocean's abyssal depths due to the lower denitrification rates. This in turn allowed nitrogen with high δ 15 N values to recycle back into the photic zone, driving sedimentary δ 15 N values higher during the Hirnantian glaciation.