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Icehouse-Greenhouse Variations in Marine Denitrification

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Icehouse-Greenhouse Variations in Marine Denitrification EGU Journal Logos (RGB) Open Access Open Access Open Access Advances in Annales Nonlinear Processes Geosciences Geophysicae in Geophysics Open Access Open Access Natural Hazards Natural Hazards and Earth System and Earth System Sciences Sciences Discussions Open Access Open Access Atmospheric Atmospheric Chemistry Chemistry and Physics and Physics Discussions Open Access Open Access Atmospheric Atmospheric Measurement Measurement Techniques Techniques Discussions Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Open Access Biogeosciences Discuss., 10, 14769–14813, 2013 Open Access www.biogeosciences-discuss.net/10/14769/2013/ Biogeosciences Biogeosciences BGD doi:10.5194/bgd-10-14769-2013 Discussions © Author(s) 2013. CC Attribution 3.0 License. 10, 14769–14813, 2013 Open Access Open Access This discussion paper is/has been under review for the journal BiogeosciencesClimate (BG). Climate Icehouse-greenhouse Please refer to the correspondingof the final Past paper in BG if available. of the Past Discussions variations in marine denitrification Open Access Icehouse-greenhouse variations in marineOpen Access Earth System Earth System T. J. Algeo et al. Dynamics Dynamics denitrification Discussions 1 2 3 4 5 Title Page Open Access T. J. Algeo , P. A. Meyers Geoscientific, R. S. Robinson , H. Rowe , and G.Geoscientific Q. Jiang Open Access 1 Instrumentation Instrumentation Abstract Introduction Department of Geology, University of Cincinnati, Cincinnati, Ohio 45221-0013, USA Methods and Methods and 2Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, Data Systems Data Systems Conclusions References Michigan 48109-1005, USA Discussions Open Access 3Graduate School of Oceanography, UniversityOpen Access of Rhode Island, Narragansett, RI 02882, USA Tables Figures 4 Geoscientific Bureau of Economic Geology,Geoscientific University of Texas at Austin, Austin, Texas 78713-8924, USA 5 Model Development Department of Geoscience,Model Development University of Nevada at Las Vegas, Las Vegas, NV 89154-4010, J I USA Discussions J I Open Access Open Access Received: 12 August 2013Hydrology – Accepted: and 19 August 2013 – Published:Hydrology 6 September and 2013 Back Close Correspondence to: T. J. AlgeoEarth ([email protected]) System Earth System Published by Copernicus PublicationsSciences on behalf of the European GeosciencesSciences Union. Full Screen / Esc Discussions Open Access Open Access Printer-friendly Version Ocean Science Ocean Science Discussions Interactive Discussion Open Access Open Access 14769 Solid Earth Solid Earth Discussions Open Access Open Access The Cryosphere The Cryosphere Discussions Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Abstract BGD 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 10, 14769–14813, 2013 15 sediments (δ Nsed) since 660 Ma (million years ago) in order to understand major 5 changes in the marine N cycle through time and their relationship to first-order climate Icehouse-greenhouse variation. During the Phanerozoic, greenhouse climate modes were characterized by variations in marine 15 15 low δ Nsed (∼ −2 to +2 %) and icehouse climate modes by high δ Nsed (∼ +4 to denitrification 15 +8 %). Shifts toward higher δ Nsed occurred rapidly during the early stages of ice- house modes, prior to the development of major continental glaciation, suggesting a T. J. Algeo et al. 10 potentially important role for the marine N cycle in long-term climate change. Reser- 15 voir box modeling of the marine N cycle demonstrates that secular variation in δ Nsed was likely due to changes in the dominant locus of denitrification, with a shift in fa- Title Page vor of sedimentary denitrification during greenhouse modes owing to higher eustatic Abstract Introduction (global sea-level) elevations and greater on-shelf burial of organic matter, and a shift Conclusions References 15 in favor of water-column denitrification during icehouse modes owing to lower eustatic elevations, enhanced organic carbon sinking fluxes, and expanded oceanic oxygen- Tables Figures 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 mod- ulating climate change at multimillion-year timescales. J I J I 20 1 Introduction Back Close Nitrogen (N) plays a key role in marine productivity and organic carbon fluxes and Full Screen / Esc is thus a potentially major influence on the global climate system (Gruber and Gal- loway, 2008). Variation in marine sediment N-isotopic compositions during the Quater- Printer-friendly Version nary (2.6 Ma to the present) has been linked to changes in organic carbon burial and Interactive Discussion 25 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., 14770 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 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 posi- BGD tive climate feedback, but negative feedbacks involving the influence of both N fixation 10, 14769–14813, 2013 and denitrification on oceanic fixed-N inventories have been proposed as well (Deutsch 15 5 et al., 2004). Although pre-Quaternary δ Nsed variation has been reported, including highly 15N-depleted (−4 to 0 %) Jurassic-Cretaceous units (Rau et al., 1987; Jenkyns Icehouse-greenhouse et al., 2001; Junium and Arthur, 2007) and highly 15N-enriched (+6 to +14 %) Car- variations in marine boniferous units (Algeo et al., 2008), the Phanerozoic record of marine N-isotopic vari- denitrification ation and its relationship to long-term (i.e., multimillion-year) climate change have not T. J. Algeo et al. 10 been systemically investigated to date (Algeo and Meyers, 2009). Additional study of the marine N cycle is needed to better understand its relationship to organic carbon burial and long-term climate change and to more accurately parameterize N fluxes in 15 Title Page general circulation models. In this study, we document variation in δ Nsed from 660 Ma to the present, demonstrating a strong relationship to first-order climate cycles, with Abstract Introduction 15 15 15 lower δ N during greenhouse intervals and higher δ N during icehouse intervals. This pattern suggests that long-term variation in the marine N cycle is controlled by Conclusions References first-order tectonic cycles, and that it is linked to – and possibly a driver of – long-term Tables Figures climate change. J I 2 Methods J I 20 This study is based on the N-isotope distributions of 153 marine units ranging in age Back Close from the Neoproterozoic (660 Ma) to the early Quaternary (∼ 2 Ma) (Fig. 1). Among these units are 35 that were analyzed specifically for this study (see isotopic methods, Full Screen / Esc Appendix A), 33 that were taken from our own earlier research publications, and 85 that were taken from other published reports. For each study unit, we determined the Printer-friendly Version 25 median (50th percentile), standard deviation range (16th and 84th percentiles), and full Interactive Discussion range (minimum and maximum values) of its δ15N distribution (Supplement Table 1). We also report organic δ13C distributions as well as means for %TOC (total organic 14771 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | carbon), %N, and molar Corg : N ratios, where available (Supplement Table 1). The ages of all units were adjusted to the 2012 geologic timescale (Gradstein et al., 2012), BGD as were the age ranges of the Phanerozoic icehouse and greenhouse climate modes in 10, 14769–14813, 2013 Fig. 1 (Montañez et al., 2011). A LOWESS curve was calculated for the entire dataset 5 per the methods of Appendix B. Icehouse-greenhouse variations in marine 3 Results denitrification 15 Our δ Nsed dataset exhibits a mean of +2.0 ± 3.1 % with a range of −5.2 to +10.4 % T. J. Algeo et al. 15 (Supplement Table 1; n = 153). δ Nsed values are mostly intermediate (0 to +3 %) during the late Cryogenian to early Ediacaran, low (−5 to 0 %) during the late Edi- 10 acaran to mid-Ordovician, intermediate during the late Ordovician to early Mississip- Title Page pian, high (+3 to +10 %) during the late Mississippian to Pennsylvanian, intermediate during the Triassic to early Cretaceous, low during the mid-Cretaceous, and intermedi- Abstract Introduction ate to high during the late Cretaceous to Recent (Fig. 1). The modeled LOWESS curve Conclusions References for the Phanerozoic exhibits a minimum of −2.8 % in the Cambrian and a maximum Tables Figures 15 of +8.0 % in the Mississippian; its variance ranges from 1.8 to 5.8 % but is mostly 15 2–4 % . The most abrupt changes in δ Nsed are associated with a ∼ 6 % rise dur- 15 J I ing the mid-Mississippian and a ∼ 5 % rise during the late Cretaceous. The δ Nsed dataset shows a strong relationship to first-order climate cycles, with low values dur- J I ing the greenhouse climate modes of the mid-Paleozoic and mid-Mesozoic and high 20 values during the icehouse climate modes of the Late Paleozoic and Cenozoic (Fig. 1). Back Close The δ15N dataset exhibits pronounced secular variation (i.e., a range of > 10 %) sed Full Screen / Esc and strong secular coherence
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