No abstract

Low δ15N values, ranging from +1.2‰ to −3.9‰, and atomic carbon/nitrogen (C/N) ratios of 25–50 are characteristic of “black shales” deposited during the Cenomanian‐Turonian boundary interval and Oceanic Anoxic Event II (OAE II). An observed antithetic relationship between C/N ratios and δ15N could suggest a predominance of terrestrially derived organic matter or a diagenetic control on δ15N variability shifting bulk δ15N values lower. However, Hydrogen Indices (HI) generally >450, and a positive correlation of HI with C/N mitigates against a significant terrestrial organic matter fraction. High C/N values are likely the result preferential degradation of labile, N‐rich compounds during early diagenesis and loss of N as ammonium from sediments through time. A hypothetical model that considers the degradation of a 15N‐enriched labile protein fraction yields only small, 1–2‰ negative shifts in δ15N. However, 15N depletion during diagenesis is contrary to normal isotope kinetics which should result in 15N enrichment of bulk organic matter. Therefore we conclude that the bulk δ15N values in this study reflect primary changes in the nitrogen cycle. The δ15N data support the hypothesis of expanded nitrogen fixation driven by upwelling of nutrient‐nitrogen poor, phosphorus replete waters during OAE II and from the mid‐Cenomanian to Santonian at Demerara Rise. The low δ15N values, which are significantly lower than bulk δ15N values in modern regions where nitrogen fixation is known to be important, probably result from a more significant fraction of dissolved inorganic nitrogen being produced by nitrogen fixation. During the peak of OAE II a marked shift to lower δ15N values is observed. This shift possibly reflects greater utilization of 15N‐depleted ammonium during a chemocline upward excursion (CUE). Dominance of low δ15N values from other periods of more widespread marine anoxia is likely the result of similar processes.

© 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. This is an author produced version of a paper published in Nature. Uploaded in accordance with the publisher's self-archiving policy.eprints@whiterose.ac.uk https://eprints.whiterose.ac.uk/ Reuse Items deposited in White Rose Research Online are protected by copyright, with all rights reserved unless indicated otherwise. They may be downloaded and/or printed for private study, or other acts as permitted by national copyright laws. The publisher or other rights holders may allow further reproduction and re-use of the full text version. This is indicated by the licence information on the White Rose Research Online record for the item. TakedownIf you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request. carbon (C) and phosphorus (P) for the formation of nucleic acids and proteins. As a result, N 49 and P are the principal limiting nutrients controlling autotrophic CO 2 fixation, which in turn 50 regulates climate, weathering, and the redox state of Earth's surface on geologic timescales. 51The marine nitrogen cycle is driven largely by biological processes. The primary 52 source of N to the biosphere is nitrogen fixation, the conversion of atmospheric N 2 to organic 53 nitrogen in its bioavailable form (ammonium, NH 4 + ). In the modern oceans, ammonium is 54 oxidized via the stepwise process of nitrification, producing nitrite (NO 2 -) and nitrate (NO 3 -). 55Nitrate (and nitrite) can be assimilated into organic matter, by both oxygenic 56 photoautotrophic bacteria (cyanobacteria) and eukaryotic phytoplankton. Fixed nitrogen is 57 mostly recycled in the water column, but some sinks to the sediments where it is buried 58 and/or remineralized. Some bioavailable nitrogen in the modern oceans is returned to the 59 atmosphere as N 2 via denitrification (the reduction of NO 3 -) and anaerobic ammonium 60 oxidation (anammox, the oxidation of NH sediments (~0 to 10‰) (Fig. 1). Our statistical treatment of the temporal or nitrogen redox cycling independent of surface oxygenation 4,18,19 . To date, however, no 92 records of contemporaneous shallow-water sediments linked directly to records of ocean or 93 atmospheric oxygenation have been available to test these alternatives. 94Here we examine the response of the nitrogen cycle to changing atmosphere and 95 ocean redox conditions during deposition of ~2.31 Ga siliciclastic rocks, filling a ~400 96 million-year gap in the temporal 15 N record (Fig. 1), in sediments contemporaneous with the 97 early stages of the GOE. We focus our analyses on the Rooihoogte and Timeball Hill (R-TH) 98 formations, present in drill core EBA-2 in the Potchefstroom Synclinorium, South Africa 99 (Extended Data Fig. 2 and 3). The R-TH form the basal part of the Pretoria Group in the 100Transvaal basin, and were deposited on a palaeo-delta slope open to the ocean 1 . U-Pb...

We describe a trapping and chromatography system that cryogenically removes CO(2) and N(2) generated from sample combustion in an elemental analyzer (EA) and introduces these gases into a low-flow helium carrier stream for isotopic analysis. The sample size required for measurement by this system (termed nano-EA/IRMS) is almost 3 orders of magnitude less than conventional EA analyses and fills an important niche in the range of analytical isotopic methods. Only 25 nmol of N and 41 nmol of C are needed to achieve 1.0 per thousand precision (2sigma) from a single measurement while larger samples and replicate measurements provide better precision. Analyses of standards demonstrate that nano-EA measurements are both accurate and precise, even on nanomolar quantities of C and N. Conventional and nano-EA measurements on international and laboratory standards are indistinguishable within analytical precision. Likewise, nano-EA values for international standards do not differ statistically from their consensus values. Both observations indicate the nano-EA measurements are comparable to conventional EA analyses and accurately reproduce the VPDB and AIR isotopic scales. Critical to the success of the nano-EA system is the procedure for removing the blank contribution to the measured values. Statistical treatment of uncertainties for this procedure yields an accurate method for calculating internal and external precision.

Global warming lowers the solubility of gases in the ocean and drives an enhanced hydrological cycle with increased nutrient loads delivered to the oceans, leading to increases in organic production, the degradation of which causes a further decrease in dissolved oxygen. In extreme cases in the geological past, this trajectory has led to catastrophic marine oxygen depletion during the so-called oceanic anoxic events (OAEs). How the water column oscillated between generally oxic conditions and local/global anoxia remains a challenging question, exacerbated by a lack of sensitive redox proxies, especially for the suboxic window. To address this problem, we use bulk carbonate I/Ca to reconstruct subtle redox changes in the upper ocean water column at seven sites recording the Cretaceous OAE 2. In general, I/Ca ratios were relatively low preceding and during the OAE interval, indicating deep suboxic or anoxic waters exchanging directly with near-surface waters. However, individual sites display a wide range of initial values and excursions in I/Ca through the OAE interval, reflecting the importance of local controls and suggesting a high spatial variability in redox state. Both I/Ca and an Earth System Model suggest that the northeast proto-Atlantic had notably higher oxygen levels in the upper water column than the rest of the North Atlantic, indicating that anoxia was not global during OAE 2 and that important regional differences in redox conditions existed. A lack of correlation with calcium, lithium, and carbon isotope records suggests that neither enhanced global weathering nor carbon burial was a dominant control on the I/Ca proxy during OAE 2.