Global climate and the atmospheric partial pressure of carbon dioxide (...) are correlated over recent glacial cycles, with lower ... during ice ages, but the causes of the ... changes are unknown. ...The modern Southern Ocean releases deeply sequestered CO2 to the atmosphere. Growing evidence suggests that the Southern Ocean CO2 'leak' was stemmed during ice ages, increasing ocean CO2 storage. Such a change would also have made the global ocean more alkaline, driving additional ocean CO2 uptake. This explanation for lower ice-age ..., if correct, has much to teach us about the controls on current ocean processes. PUBLICATION ABSTRACT
Biological nitrogen fixation constitutes the main input of fixed nitrogen to Earth’s ecosystems, and its isotope effect is a key parameter in isotope-based interpretations of the N cycle. The ...nitrogen isotopic composition (δ ¹⁵N) of newly fixed N is currently believed to be ∼–1‰, based on measurements of organic matter from diazotrophs using molybdenum (Mo)-nitrogenases. We show that the vanadium (V)- and iron (Fe)-only “alternative” nitrogenases produce fixed N with significantly lower δ ¹⁵N (–6 to –7‰). An important contribution of alternative nitrogenases to N ₂ fixation provides a simple explanation for the anomalously low δ ¹⁵N (<–2‰) in sediments from the Cretaceous Oceanic Anoxic Events and the Archean Eon. A significant role for the alternative nitrogenases over Mo-nitrogenase is also consistent with evidence of Mo scarcity during these geologic periods, suggesting an additional dimension to the coupling between the global cycles of trace elements and nitrogen.
John H. Martin, who discovered widespread iron limitation of ocean productivity, proposed that dust-borne iron fertilization of Southern Ocean phytoplankton caused the ice age reduction in ...atmospheric carbon dioxide (CO2). In a sediment core from the Subantarctic Atlantic, we measured foraminifera-bound nitrogen isotopes to reconstruct ice age nitrate consumption, burial fluxes of iron, and proxies for productivity. Peak glacial times and millennial cold events are characterized by increases in dust flux, productivity, and the degree of nitrate consumption; this combination is uniquely consistent with Subantarctic iron fertilization. The associated strengthening of the Southern Ocean's biological pump can explain the lowering of CO2 at the transition from mid-climate states to full ice age conditions as well as the millennial-scale CO2 oscillations.
Nitrate persists in eastern equatorial Pacific surface waters because phytoplankton growth fueled by nitrate (new production) is limited by iron. Nitrate isotope measurements provide a new constraint ...on the controls of surface nitrate concentration in this region and allow us to quantify the degree and temporal variability of nitrate consumption. Here we show that nitrate consumption in these waters cannot be fueled solely by the external supply of iron to these waters, which occurs by upwelling and dust deposition. Rather, a substantial fraction of nitrate consumption must be supported by the recycling of iron within surface waters. Given plausible iron recycling rates, seasonal variability in nitrate concentration on and off the equator can be explained by upwelling rate, with slower upwelling allowing for more cycles of iron regeneration and uptake. The efficiency of iron recycling in the equatorial Pacific implies the evolution of ecosystem-level mechanisms for retaining iron in surface ocean settings where it limits productivity.
Dust has the potential to modify global climate by influencing the radiative balance of the atmosphere and by supplying iron and other essential limiting micronutrients to the ocean. Indeed, dust ...supply to the Southern Ocean increases during ice ages, and 'iron fertilization' of the subantarctic zone may have contributed up to 40 parts per million by volume (p.p.m.v.) of the decrease (80-100 p.p.m.v.) in atmospheric carbon dioxide observed during late Pleistocene glacial cycles. So far, however, the magnitude of Southern Ocean dust deposition in earlier times and its role in the development and evolution of Pleistocene glacial cycles have remained unclear. Here we report a high-resolution record of dust and iron supply to the Southern Ocean over the past four million years, derived from the analysis of marine sediments from ODP Site 1090, located in the Atlantic sector of the subantarctic zone. The close correspondence of our dust and iron deposition records with Antarctic ice core reconstructions of dust flux covering the past 800,000 years (refs 8, 9) indicates that both of these archives record large-scale deposition changes that should apply to most of the Southern Ocean, validating previous interpretations of the ice core data. The extension of the record beyond the interval covered by the Antarctic ice cores reveals that, in contrast to the relatively gradual intensification of glacial cycles over the past three million years, Southern Ocean dust and iron flux rose sharply at the Mid-Pleistocene climatic transition around 1.25 million years ago. This finding complements previous observations over late Pleistocene glacial cycles, providing new evidence of a tight connection between high dust input to the Southern Ocean and the emergence of the deep glaciations that characterize the past one million years of Earth history.
We report the first measurements of coupled nitrogen (N) and oxygen (O) isotope fractionation of nitrate by laboratory cultures of denitrifying bacteria. Two seawater strains (Pseudomonas stutzeri, ...Ochrobactrum sp.) and three freshwater strains (Paracoccus denitrificans, Pseudomonas chlororaphis, Rhodobacter sphaeroides) were examined. Among four strains of facultative anaerobic denitrifiers, N and O isotope effects were variable, ranging from 5‰ to 25‰, with evidence for a drop in the isotope effects as nitrate concentrations approached the half- saturation constant for nitrate transport. O isotope effects were similar to their corresponding N isotope effect, such that the progressive increase in nitrate δ¹⁸O, when plotted against that in¹⁵N (where${\rm{\delta }}{}^{18}{\rm{O}}_{{\rm{sample}}} = ({}^{18}{\rm{O:}}\,{}^{16}{\rm{O)}}_{{\rm{sample}}} /({}^{18}{\rm{O:}}\,{}^{16}{\rm{O)}}_{{\rm{reference}}} - 1 \times \,1000$, and ${\rm{\delta }}^{15} {\rm{N}}_{{\rm{sample}}} = ({}^{15}{\rm{N:}}\,{}^{14}{\rm{N)}}_{{\rm{sample}}} /({}^{15}{\rm{N:}}\,{}^{14}{\rm{N}})_{{\rm{reference}}} - 1 \times 1000)$, yielded slopes of 0.86 to 1.02, with a mean value of 0.96. R. sphaeroides, a photo-heterotroph that possesses only a periplasmic (nonrespiring) dissimilatory nitrate reductase, showed less variability in nitrate N isotope effects, between 13‰ and 20‰, with a modal value of ~15‰. In contrast to the respiratory denitrifiers, R. sphaeroides consistently showed a distinct ratio of δ¹⁸O to δ¹⁵N change of 0~.62. We hypothesize that heavy N and O isotope discrimination during respiratory denitrification occurs during the intracellular reduction of nitrate by the respiratory nitrate reductase, and the observed magnitude of fractionation is likely regulated by the ratio of cellular nitrate efflux relative to uptake. The data for R. sphaeroides are consistent with isotope discrimination directly reflecting the N and O isotope effects of the periplasmic nitrate reductase NAP, without modification by nitrate uptake and efflux.
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