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Abstract. Oxygen-deficient zones (ODZs) are major sites of net naturalnitrous oxide (N2O) production and emissions. In order to understandchanges in the magnitude of N2O production in response to globalchange, knowledge on the individual contributions of the major microbialpathways (nitrification and denitrification) to N2O production andtheir regulation is needed. In the ODZ in the coastal area off Peru, thesensitivity of N2O production to oxygen and organic matter wasinvestigated using 15N tracer experiments in combination with quantitative PCR (qPCR) andmicroarray analysis of total and active functional genes targeting archaeal amoAand nirS as marker genes for nitrification and denitrification, respectively.Denitrification was responsible for the highest N2O production with amean 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 productionfrom ammonium oxidation (AO) of 0.16±0.003 nmol L−1 d−1occurred 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 dominantspecies, comprising 70 %–85 % of total produced N2O fromNH4+, regardless of the ammonium oxidation rate or O2concentrations. Oxygen responses of N2O production varied withsubstrate, but production and yields were generally highest below 10 µmol L−1 O2. Particulate organic matter additions increasedN2O production by denitrification up to 5-fold, suggesting increasedN2O production during times of high particulate organic matter export.High N2O yields of 2.1 % from AO were measured, but the overallcontribution by AO to N2O production was still an order of magnitudelower than that of denitrification. Hence, these findings show thatdenitrification is the most important N2O production process in low-oxygen conditions fueled by organic carbon supply, which implies a positivefeedback of the total oceanic N2O sources in response to increasingoceanic deoxygenation. 

The Line Islands Ridge (LIR), located south of the Hawaiian Islands between 7°N and 1°S, is one of the few large central Pacific regions shallower than the regional carbonate compensation depth. Thick sequences of carbonate sediments have accumulated around the LIR despite it being located in the sediment-starved central tropical Pacific. The LIR is an important source of carbonates to the surrounding region and deposition around the LIR has expanded the equatorial Pacific carbonate sediment tongue by about 5% of its total area. Furthermore, sediments on the ridge are potentially important paleoceanographic archives. A recent survey at the crest of the LIR finds evidence for high current activity, significant erosion, but overall net sediment deposition. Currents are strong enough to form sediment waves and lee drifts in the Palmyra Basin, at the northern terminus of the LIR. Sediments along the LIR are pelagic foraminiferal sands that are easily eroded and flow out into the surrounding abyssal plain in active submarine channel systems. As channels migrate, pelagic sediments fill in the abandoned channel arms. Despite significant sediment losses from the top of the ridge, 1.3 km of sediment has accumulated in the upper Palmyra Basin over basement formed 68 to 85 million years ago (Ma). Late Neogene erosion may be more extensive than earlier erosion cycles, in response to reduced sediment production as the Palmyra Basin exited the high productivity equatorial latitudes. Sediments with good stratigraphic order needed for paleoceanographic study are limited in this dynamic sedimentary environment, but can be found with proper survey. 

Abstract 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 foraminiferaGlobigerinoides ruber, we construct preliminary age models for selected cores and show that the gradient in the oxygen isotope ratio ofG. ruberbetween 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).