Methane emissions from the upwelling area off Mauritania (NW Africa)

A. Kock,S. Gebhardt,H. W. Bange

doi:10.5194/bg-5-1119-2008

A. Kock, S. Gebhardt + Show 1 more

Open Access

https://doi.org/10.5194/bg-5-1119-2008

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Abstract

Abstract. Coastal upwelling regions have been identified as sites of enhanced CH4 emissions to the atmosphere. The coastal upwelling area off Mauritania (NW Africa) is one of the most biologically productive regions of the world's ocean but its CH4 emissions have not been quantified so far. More than 1000 measurements of atmospheric and dissolved CH4 in the surface layer in the upwelling area off Mauritania were performed as part of the German SOPRAN (Surface Ocean Processes in the Anthropocene) study during two cruises in March/April 2005 (P320/1) and February 2007 (P348). During P348 enhanced CH4 saturations of up to 200% were found close to the coast and were associated with upwelling of South Atlantic Central Water. An area-weighted, seasonally adjusted estimate yielded overall annual CH4 emissions in the range from 1.6 to 2.9 Gg CH4. Thus the upwelling area off Mauritania represents a regional hot spot of CH4 emissions but seems to be of minor importance for the global oceanic CH4 emissions.

Highlights

Atmospheric methane (CH4) is a greenhouse gas and plays an important role in the chemistry of the Earth’s atmosphere (WMO, 2007; IPCC, 2007)
On the basis of more than 1000 measurements of atmospheric and dissolved CH4 in the surface layer we present an area-weighted, seasonally adjusted CH4 flux estimate for the Mauritanian upwelling
The mean atmospheric CH4 dry mole fraction during P320/1 in March 2005 was 1.84±0.02 ppm CH4 and is in agreement with the monthly mean of 1.821 ppm measured in March 2005 at the NOAA/ESRL atmospheric baseline observatory Izana on Tenerife (28.3◦ N, 16.5◦ W, elevation 2300 m) (Dlugokencky et al, 2007)

Summary

Study site description

The eastern tropical North Atlantic Ocean is an area with strongly differing hydrographic and biological properties: On the one hand, the Canary and North Equatorial Currents form the eastern part of the North Atlantic Subtropical Gyre which is a zone of low nutrient-supply and low primary production (see e.g. Signorini et al, 1999). The eastern tropical North Atlantic Ocean is an area with strongly differing hydrographic and biological properties: On the one hand, the Canary and North Equatorial Currents form the eastern part of the North Atlantic Subtropical Gyre which is a zone of low nutrient-supply and low primary production Several attempts have been made to decipher characteristic biogeographic or oceanic provinces for the tropical Northeast Atlantic Ocean: On the basis of algal ecology, near-surface circulation patterns and chlorophyll data, Sathyendranath et al (1995) proposed the province Canary Current (CNRY) which includes both the upwelling areas of NW Africa (off Senegal, Mauritania and Morocco) and the upwelling off Portugal. An oceanic classification was introduced by Hooker et al (2000) on the basis of T-S data They defined the Canary Current Upwelling Province (CCUp) from 16◦ N to 20◦ N. The two coastal provinces (CBsl and CBsh) extend from 15◦ N to 22◦ N and are comparable with the CCUp province by Hooker et al (2000) but they are not comparable with the more extended CNRY province by Sathyendranath et al (1995)

Methods

Results and discussion

CH4 air-sea exchange