Abstract
Abstract. Long-term secular variation in the isotopic composition of seawater fixed nitrogen (N) is poorly known. Here, we document variation in the N-isotopic composition of marine sediments (δ15Nsed) since 660 Ma (million years ago) in order to understand major changes in the marine N cycle through time and their relationship to first-order climate variation. During the Phanerozoic, greenhouse climate modes were characterized by low δ15Nsed (∼−2 to +2‰) and icehouse climate modes by high δ15Nsed (∼+4 to +8‰). Shifts toward higher δ15Nsed occurred rapidly during the early stages of icehouse modes, prior to the development of major continental glaciation, suggesting a potentially important role for the marine N cycle in long-term climate change. Reservoir box modeling of the marine N cycle demonstrates that secular variation in δ15Nsed was likely due to changes in the dominant locus of denitrification, with a shift in favor of sedimentary denitrification during greenhouse modes owing to higher eustatic (global sea-level) elevations and greater on-shelf burial of organic matter, and a shift in favor of water-column denitrification during icehouse modes owing to lower eustatic elevations, enhanced organic carbon sinking fluxes, and expanded oceanic oxygen-minimum zones. The results of this study provide new insights into operation of the marine N cycle, its relationship to the global carbon cycle, and its potential role in modulating climate change at multimillion-year timescales.
Highlights
Nitrogen (N) plays a key role in marine productivity and organic carbon fluxes and is a potentially major influence on the global climate system (Gruber and Galloway, 2008)
Lower δ15N during greenhouse intervals and higher δ15N during icehouse intervals. This pattern suggests that long-term variation in the marine N cycle is controlled by first-order tectonic cycles, and that it is linked to long-term climate change
The Phanerozoic δ15Nsed curve shows a strong relationship to first-order climate cycles, with low values during the greenhouse climate modes of the mid-Paleozoic and mid-Mesozoic and high values during the icehouse climate modes of the Late Paleozoic and Cenozoic (Fig. 1)
Summary
Nitrogen (N) plays a key role in marine productivity and organic carbon fluxes and is a potentially major influence on the global climate system (Gruber and Galloway, 2008). Variation in marine sediment N-isotopic compositions during the Quaternary (2.6 Ma to the present) has been linked to changes in organic carbon burial and oceanic denitrification rates during Pleistocene glacial–interglacial cycles (François et al, 1992; Altabet et al, 1995; Ganeshram et al, 1995; Haug et al, 1998; Naqvi et al, 1998; Broecker and Henderson, 1998; Suthhof et al, 2001; Liu et al, 2005, 2008) At this timescale (i.e., ∼ 105 yr), the marine N cycle is thought to act mainly as a positive climate feedback, but negative feedbacks involving the influence of both N fixation and denitrification on oceanic fixed-N inventories have been proposed as well (Deutsch et al, 2004). We document variation in δ15Nsed from 660 Ma to the present, demonstrating a strong relationship to first-order climate cycles, with
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