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GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 20, GB1009, doi:10.1029/2004GB002356, 2006

Biogeochemical processes as drivers of surface fCO₂ in contrasting provinces in the subarctic North Pacific Ocean

Melissa Chierici

Climate Change Research Project, National Institute for Environmental Studies, Tsukuba, Japan

Agneta Fransson

Climate Change Research Project, National Institute for Environmental Studies, Tsukuba, Japan

Yukihiro Nojiri

Climate Change Research Project, National Institute for Environmental Studies, Tsukuba, Japan

Abstract

The effect of temperature, biological processes, air-sea CO₂ exchange and vertical mixing as drivers of the seasonality of the surface water fugacity of CO₂ (fCO₂sw) were studied for the year 2000 in the subarctic North Pacific Ocean. The regional and seasonal variability of the surface water chemistry was studied using an extensive data set on surface water fCO₂ and nutrient concentrations in six contrasting provinces. We observed the largest seasonal amplitude for all parameters in the western provinces (Oyashio and Western Subarctic Gyre, WSG). Our study showed that biological processes and temperature were major controls for the monthly fCO₂sw change in all provinces. The magnitude and strength of the processes showed large temporal and spatial variability. The WSG showed larger influence by biological processes and vertical mixing than the Alaska gyre (AG), where the effect caused by temperature was larger, implying different forcing of the fCO₂ change in the two gyres. Biological activity, estimated from the monthly nitrate change corrected for addition induced by vertical mixing, resulted in a net annual CO₂ loss. The net carbon loss out of the top 50 m driven by biological activity was 3 times higher in the WSG (64 g C m⁻² yr⁻¹) than in the AG (23 g C m⁻² yr⁻¹). The annual sum of the fCO₂sw change based on all processes resulted in a CO₂ buildup in the surface waters for all provinces. Although the air-sea CO₂ exchange was of minor importance relative to the other considered processes (4 to 13%), all provinces showed a net annual uptake of atmospheric CO₂ from 1 to 23 g C m⁻² yr⁻¹ and an average for the whole study area of about 12 (±9) g C m⁻² yr⁻¹.

Received 16 August 2004; accepted 11 October 2005; published 15 February 2006.

Keywords: seasonal amplitudes; nitrate; physical and biological processes; mixed layer depth; sea-air CO₂ exchange.

Index Terms: 4227 Oceanography: General: Diurnal, seasonal, and annual cycles (0438); 4504 Oceanography: Physical: Air/sea interactions (0312, 3339); 4805 Oceanography: Biological and Chemical: Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 1615, 4912); 4806 Oceanography: Biological and Chemical: Carbon cycling (0428).

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Citation: Chierici, M., A. Fransson, and Y. Nojiri (2006), Biogeochemical processes as drivers of surface fCO₂ in contrasting provinces in the subarctic North Pacific Ocean, Global Biogeochem. Cycles, 20, GB1009, doi:10.1029/2004GB002356.

Copyright 2006 by the American Geophysical Union.