Spatiotemporal variations of nitrogen isotopic records in the Arabian Sea

Abstract

Abstract. Available reports of dissolved oxygen, δ 15 N of nitrate (δ 15 N NO 3 ) and δ 15 N of total nitrogen (δ 15 N bulk ) for trap material and surface/downcore sediments from the Arabian Sea (AS) were synthesized to explore the AS' past nitrogen dynamics. Based on 25 µmol kg −1 dissolved oxygen isopleth at a depth of 150 m, we classified all reported data into northern and southern groups. By using δ 15 N bulk of the sediments, we obtained geographically distinctive bottom-depth effects for the northern and southern AS at different climate stages. After eliminating the bias caused by bottom depth, the modern-day sedimentary δ 15 N bulk values largely reflect the δ 15 N NO 3 supply from the bottom of the euphotic zone. Additionally to the data set, nitrogen and carbon contents vs. their isotopic compositions of a sediment core (SK177/11) collected from the most southeastern part of the AS were measured for comparison. We found a one-step increase in δ 15 N bulk starting at the deglaciation with a corresponding decrease in δ 13 C TOC similar to reports elsewhere revealing a global coherence. By synthesizing and reanalyzing all reported down core δ 15 N bulk , we derived bottom-depth correction factors at different climate stages, respectively, for the northern and southern AS. The diffusive sedimentary δ 15 N bulk values in compiled cores became confined after bias correction revealing a more consistent pattern except recent 6 ka. Such high similarity to the global temporal pattern indicates that the nitrogen cycle in the entire AS had responded to open-ocean changes until 6 ka BP. Since 6 ka BP, further enhanced denitrification (i.e., increase in δ 15 N bulk ) in the northern AS had occurred and was likely driven by monsoon, while, in the southern AS, we observed a synchronous reduction in δ 15 N bulk , implying that nitrogen fixation was promoted correspondingly as the intensification of local denitrification at the northern AS basin.

Variations in Denitrification and Ventilation Within the Arabian Sea Oxygen Minimum Zone During the Holocene

“…Weighed quantities (~20 mg) of dry, powdered samples filled into tin cups were analyzed for stable carbon and nitrogen isotopes using Finnigan Delta Plus isotope ratio mass spectrometer after combustion in a Flash 1112 EA elemental analyzer at the Centre for Marine Tropical Ecology, Bremen, Germany. The carbon and The open circle is location SS3201 of the water column profiles; the core locations include SK148/55 (present study), 905P (Ivanochko et al, 2005), MD-04 2876 (Pichevin et al, 2007), 111KL (Suthhof et al, 2001), SK 126/39 (Kessarkar et al, 2010), and SK177/11 (Kao et al, 2015). The gray shaded areas indicate highly productive zones.…”

A modeling study of physical controls on hypoxia generation in the northern Gulf of Mexico

“…Motivated by these previous studies, we assess whether very simple oxygen parameterizations coupled with the hydrodynamic model we have used in our previous, fully coupled modeling studies [Fennel et al, 2011Laurent et al, 2012;Feng et al, 2013;Laurent and Fennel, 2014;Yu et al, 2015] can simulate hypoxic conditions in the northern Gulf of Mexico. We find this to be the case and then use the simplified model to evaluate the role of different physical forcing factors in hypoxia formation, including river discharge, wind speed, and the seasonal shift in wind direction from upwelling-favorable in summer to downwelling-favorable during the rest of the year.…”

“…We evaluate the performance of the three simple oxygen models, which were ran from 1 January 2004 to 31 December 2007, by comparing the simulated hypoxic extent and oxygen concentrations with available observations from Rabalais et al [2007], Murrell et al [2013a], Nunnally et al [2013], and the Mechanisms Controlling Hypoxia (MCH) program. Considering that observations are spatially and temporally limited, we extended the validation by comparing output from the simple oxygen models to that of our full physicalbiogeochemical model, which has been shown to realistically reproduce the observed oxygen dynamics on the LA shelf [Yu et al, 2015]. Detailed parameterization and validation of the full biogeochemical model are described in Fennel et al [2011Fennel et al [ , 2013, Laurent et al [2012], Laurent and Fennel [2014], and Yu et al [2015].…”

“…Considering that observations are spatially and temporally limited, we extended the validation by comparing output from the simple oxygen models to that of our full physicalbiogeochemical model, which has been shown to realistically reproduce the observed oxygen dynamics on the LA shelf [Yu et al, 2015]. Detailed parameterization and validation of the full biogeochemical model are described in Fennel et al [2011Fennel et al [ , 2013, Laurent et al [2012], Laurent and Fennel [2014], and Yu et al [2015].…”

“…While the statistical modeling studies are instructive, they can only prove correlation not causation. Coupled physical-biogeochemical models are important complements that help build mechanistic understanding of the processes underlying hypoxia development and variability [Fennel et al, 2011Feng et al, 2013;Laurent and Fennel, 2014;Yu et al, 2015;Justić and Wang, 2014]. While the fully coupled physicalbiogeochemical models can mechanistically elucidate the complex interactions of physical and biological processes, they can be difficult to calibrate and their results are sometimes difficult to interpret.…”