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Abstract. Oxygen-deficient zones (ODZs) are major sites of net natural nitrous oxide (N2O) production and emissions. In order to understand changes in the magnitude of N2O production in response to global change, knowledge on the individual contributions of the major microbial pathways (nitrification and denitrification) to N2O production and their regulation is needed. In the ODZ in the coastal area off Peru, the sensitivity of N2O production to oxygen and organic matter was investigated using 15N tracer experiments in combination with quantitative PCR (qPCR) and microarray analysis of total and active functional genes targeting archaeal amoA and nirS as marker genes for nitrification and denitrification, respectively. Denitrification was responsible for the highest N2O production with a mean of 8.7 nmol L−1 d−1 but up to 118±27.8 nmol L−1 d−1 just below the oxic–anoxic interface. The highest N2O production from ammonium oxidation (AO) of 0.16±0.003 nmol L−1 d−1 occurred in the upper oxycline at O2 concentrations of 10–30 µmol L−1 which coincided with the highest archaeal amoA transcripts/genes. Hybrid N2O formation (i.e., N2O with one N atom from NH4+ and the other from other substrates such as NO2-) was the dominant species, comprising 70 %–85 % of total produced N2O from NH4+, regardless of the ammonium oxidation rate or O2 concentrations. Oxygen responses of N2O production varied with substrate, but production and yields were generally highest below 10 µmol L−1 O2. Particulate organic matter additions increased N2O production by denitrification up to 5-fold, suggesting increased N2O production during times of high particulate organic matter export. High N2O yields of 2.1 % from AO were measured, but the overall contribution by AO to N2O production was still an order of magnitude lower than that of denitrification. Hence, these findings show that denitrification is the most important N2O production process in low-oxygen conditions fueled by organic carbon supply, which implies a positive feedback of the total oceanic N2O sources in response to increasing oceanic deoxygenation.

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Understanding intermediate water circulation across the last deglacial is critical in assessing the role of oceanic heat transport associated with Atlantic Meridional Overturning Circulation variability across abrupt climate events. However, the links between intermediate water circulation and abrupt climate events such as the Younger Dryas (YD) and Heinrich Event 1 (H1) are still poorly constrained. Here, we reconstruct changes in Antarctic Intermediate Water (AAIW) circulation in the subtropical North Atlantic over the past 25 kyr by measuring authigenic neodymium isotope ratios in sediments from two sites in the Florida Straits. Our authigenic Nd isotope records suggest that there was little to no penetration of AAIW into the subtropical North Atlantic during the YD and H1. Variations in the northward penetration of AAIW into the Florida Straits documented in our authigenic Nd isotope record are synchronous with multiple climatic archives, including the Greenland ice core δ18O record, the Cariaco Basin atmosphere Δ14C reconstruction, the Bermuda Rise sedimentary Pa/Th record, and nutrient and stable isotope data from the tropical North Atlantic. The synchroneity of our Nd records with multiple climatic archives suggests a tight connection between AAIW variability and high‐latitude North Atlantic climate change.

Much uncertainty exists about the state of the oceanic and atmospheric circulation in the tropical Pacific over the last glacial cycle. Studies have been hampered by the fact that sediment cores suitable for study were concentrated in the western and eastern parts of the tropical Pacific, with little information from the central tropical Pacific. Here we present information from a suite of sediment cores collected from the Line Islands Ridge in the central tropical Pacific, which show sedimentation rates and stratigraphies suitable for paleoceanographic investigations. Based on the radiocarbon and oxygen isotope measurements on the planktonic foraminifera Globigerinoides ruber, we construct preliminary age models for selected cores and show that the gradient in the oxygen isotope ratio of G. ruber between the equator and 8°N is enhanced during glacial stages relative to interglacial stages. This stronger gradient could reflect enhanced equatorial cooling (perhaps reflecting a stronger Walker circulation) or an enhanced salinity gradient (perhaps reflecting increased rainfall in the central tropical Pacific).

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<p><strong>Abstract.</strong> Oxygen deficient zones (ODZs) are major sites of net natural nitrous oxide (N<sub>2</sub>O) production and emissions. In order to understand changes in the magnitude of N<sub>2</sub>O production in response to global change, knowledge on the individual contributions of the major microbial pathways (nitrification and denitrification) to N<sub>2</sub>O production and their regulation is needed. In the ODZ in the coastal area off Peru, the sensitivity of N<sub>2</sub>O production to oxygen and organic matter was investigated using <sup>15</sup>N-tracer experiments in combination with qPCR and microarray analysis of total and active functional genes targeting archaeal <i>amoA</i> and <i>nirS</i> as marker genes for nitrification and denitrification, respectively. Denitrification was responsible for the highest N<sub>2</sub>O production with a mean of 8.7 nmol L<sup>−1</sup> d<sup>−1</sup> but up to 118 ± 27.8 nmol L<sup>−1</sup> d<sup>−1</sup> just below the oxic-anoxic interface. Highest N<sub>2</sub>O production from ammonium oxidation (AO) of 0.16 ± 0.003 nmol L<sup>−1</sup> d<sup>−1</sup> occurred in the upper oxycline at O<sub>2</sub> concentrations of 10–30 µmol L<sup>−1</sup> which coincided with highest archaeal <i>amoA</i> transcripts/genes. Oxygen responses of N<sub>2</sub>O production varied with substrate, but production and yields were generally highest below 10 µmol L<sup>−1</sup> O<sub>2</sub>. Particulate organic matter additions increased N<sub>2</sub>O production by denitrification up to 5-fold suggesting increased N<sub>2</sub>O production during times of high particulate organic matter export. High N<sub>2</sub>O yields of 2.1 % from AO were measured, but the overall contribution by AO to N<sub>2</sub>O production was still an order of magnitude lower than that of denitrification. Hence, these findings show that denitrification is the most important N<sub>2</sub>O production process in low oxygen conditions fueled by organic carbon supply, which implies a positive feedback of the total oceanic N<sub>2</sub>O sources in response to increasing oceanic deoxygenation.</p>