ALBERT

Beschreibung: Chlorofluorocarbons-11 (CFC-11), CFC-12, and sulfur hexafluoride (SF 6 ) were measured during the December 2007 – February 2008 CLIVAR/Repeat Hydrography (RH) P18 section along ∼103ºW in the Southeast Pacific Ocean. Transit-time distributions (TTDs) of 1-D transport that matched all three tracers were consistent with high Peclet number flow ventilating the subtropical mode water and the main subtropical thermocline (30ºS-42ºS, 200-800 m). In the subtropics, TTDs with predominantly advective transport predicted decadal increases in CFC-12 and CFC-11 consistent with those observed comparing 1994 WOCE with 2007/8 CLIVAR/RH data, indicating steady ventilation in this region, and consistent with the near-zero changes observed in dissolved oxygen. The mean transport timescales from the tracer-tuned TTDs were used to estimate apparent oxygen utilization rates (OURs) on the order of 8-20 μmol kg −1 yr −1 at ∼200 m depth, attenuating to ∼2 μmol kg −1 yr −1 typically by 500 m depth in this region. Depth-integrated over the thermocline, these OURs implied carbon export rates from the overlying sea surface on the order of ∼1.8 moles C m −2 yr −1 from 30°S to 45°S, 2 – 2.5 moles C m −2 yr −1 from 45°S to 52°S, and 2.5 – 3.5 moles C m −2 yr −1 from 52°S to 60°S. This article is protected by copyright. All rights reserved.

Beschreibung: Nitrification, the oxidation of ammonium (NH 4 + ) to nitrite (NO 2 - ) and to nitrate (NO 3 - ), is a component of the nitrogen (N) cycle internal to the fixed N pool. In oxygen minimum zones (OMZs), which are hotspots for oceanic fixed N loss, nitrification plays a key role because it directly supplies substrates for denitrification and anaerobic ammonia oxidation (anammox), and may compete for substrates with these same processes. However, the control of oxygen and substrate concentrations on nitrification are not well understood. We performed onboard incubations with 15 N-labeled substrates to measure rates of NH 4 + and NO 2 - oxidation in the eastern tropical South Pacific (ETSP). The spatial and depth distributions of NH 4 + and NO 2 - oxidation rates were primarily controlled by NH 4 + and NO 2 - availability, oxygen concentration, and light. In the euphotic zone, nitrification was partially photoinhibited. In the anoxic layer, NH 4 + oxidation was negligible or below detection, but high rates of NO 2 - oxidation were observed. NH 4 + oxidation displayed extremely high affinity for both NH 4 + and oxygen. The positive linear correlations between NH 4 + oxidation rates and in situ NH 4 + concentrations and ammonia monooxygenase subunit A ( amoA ) gene abundances in the upper oxycline indicate that the natural assemblage of ammonia oxidizers responds to in situ NH 4 + concentrations or supply by adjusting their population size, which determines the NH 4 + oxidation potential. The depth distribution of archaeal and bacterial amoA gene abundances and N 2 O concentration, along with independently reported simultaneous direct N 2 O production rate measurements, suggests that AOA were predominantly responsible for NH 4 + oxidation, which was a major source of N 2 O production at oxygen concentrations 〉 5 µM. This article is protected by copyright. All rights reserved.