Diagnosing CO<sub>2</sub> fluxes in the upwelling system off the Oregon–California coast

Abstract

Abstract.It is generally known that the interplay between the carbon and nutrients supplied from subsurface waters via biological metabolism determines the CO 2 fluxes in upwelling systems. However, quantificational assessment of such interplay is difficult because of the dynamic nature of both upwelling circulation and the associated biogeochemistry. We recently proposed a new framework, the Oceandominated Margin (OceMar), for semi-quantitatively diagnosing the CO 2 source/sink nature of an ocean margin over a given period of time, highlighting that the relative consumption between carbon and nutrients determines if carbon is in excess (i.e., CO 2 source) or in deficit (i.e., CO 2 sink) in the upper waters of ocean margins relative to their off-site inputs from the adjacent open ocean. In the present study, such a diagnostic approach based upon both couplings of physics-biogeochemistry and carbon-nutrients was applied to resolve the CO 2 fluxes in the well-known upwelling system off Oregon and northern California of the US west coast, using data collected along three cross-shelf transects from the inner shelf to the open basin in spring/early summer 2007. Through examining the biological consumption on top of the water mass mixing revealed by the total alkalinity-salinity relationship, we successfully predicted and semi-analytically resolved the CO 2 fluxes showing strong uptake from the atmosphere beyond the nearshore regions. This CO 2 sink nature primarily resulted from the higher utilization of nutrients relative to dissolved inorganic carbon (DIC) based on their concurrent inputs from the depth. On the other hand, the biological responses to intensified upwelling were minor in nearshore waters off the Oregon-California coast, where significant CO 2 outgassing was observed during the sampling period and resolving CO 2 fluxes could be simplified without considering DIC/nutrient consumption, i.e., decoupling between upwelling and biological consumption. We reasoned that coupling physics and biogeochemistry in the OceMar model would assume a steady state with balanced DIC and nutrients via both physical transport and biological alterations in comparable timescales.

Air–sea fluxes of greenhouse gases and oxygen in the northern Benguela Current region during upwelling events

“…CO 2 fluxes were positive for all upwelling events, with an average flux density of 0.70 ± 0.42 µmol m −2 s −1 and a maximum value of 2.0 µmol m −2 s −1 . During upwelling conditions, it is not uncommon to see such strong outgassing of CO 2 in a coastal upwelling region; the biological response to new nutrients takes some days to draw down DIC levels (Torres et al, 1999;Loucaides et al, 2012;Cao et al, 2014). Santana-Casiano et al (2009) used underway systems on cargo ships and weekly wind speeds to arrive at a mean flux between ca.…”

“…Coastal margins, particularly those associated with the upwelling of nutrient-rich subsurface waters, are biogeochemically active regions (Levin et al, 2015). The air-sea fluxes of greenhouse gases (GHGs, referring to the long-lived greenhouse gases CO 2 , N 2 O, and CH 4 ) from or to such systems can vary markedly, both spatially and temporally (Torres et al, 1999;Naqvi et al, 2010;Evans et al, 2011;Reimer et al, 2013;Capone and Hutchins, 2013). This is because both the occurrence and intensity of coastal upwelling events are variable in nature, as they are forced by surface winds that occur under specific synoptic conditions; even large events happen only on a timescale of days (Blanke et al, 2005;Goubanova et al, 2013;Desbiolles et al, 2014a, b).…”

“…For O 2 , the ventilation of deeper water masses can drive a net flux into the ocean, or net productivity can create oversaturation of dissolved O 2 . Hence, coastal upwelling regions can oscillate between being sources and sinks of O 2 and CO 2 (Torres et al, 1999;Santana-Casiano et al, 2009;Gregor and Monteiro, 2013;Cao et al, 2014;Evans et al, 2015). Most coastal upwelling systems are also known to be regional hotspots of N 2 O emissions (Bange et al, 2001;Lueker et al, 2003;Cornejo et al, 2006;Bianchi et al, 2012;Arévalo-Martínez et al, 2015;Babbin et al, 2015).…”

Air–Sea Fluxes of Greenhouse Gases and Oxygen in the Northern Benguela Current Region During Upwelling Events

“…CO 2 fluxes were positive for all upwelling events, with an average flux density of 0.64 ± 0.4 µmol m −2 sec −1 and a maximum value of 1.6 µmol m −2 sec −1 . During upwelling conditions, it is not uncommon to see such strong outgassing of CO 2 in a coastal upwelling region; the biological response to new nutrients takes some days to draw down DIC levels (Torres et al, 1999;Loucaides et al, 2012;Cao et al, 2014). Santana-Casiano et al (2009) used underway systems on cargo ships and weekly wind speeds to arrive at a mean flux between ca.…”

Biological carbon pump affected by CO2 uptake in the Benguela Upwelling System

“…Loaded with preformed nutrients, these mode waters support primary productivity in upwelling systems at lower latitudes and thus could potentially restore the CO2 uptake efficiency of the biological carbon pump. Although EBUS could act as regional CO2 sinks through the utilization of preformed nutrients and low sea water temperatures which increase the solubility of CO2 [21][22][23] , global models suggest that upwelling systems (in particular ones at lower latitudes) act as net CO2 sources to the atmosphere 24,25,26 . This was also found to be the case in a modelling study of the Benguela Upwelling System (BUS), which is located in the southeast Atlantic Ocean (Fig.…”