Abstract
Tropical waters show different regional aspects due to specificities in their nutrient biogeochemical cycles, which can affect the carbon system and influence their regional role as sinks or sources of CO2. This study was performed on particular tropical areas that present a different seasonal behaviour related to the carbon cycle observed in the late rainy season (July 2013). Understanding the CO2 drawdown and outgassing potential in these areas is needed to call attention to more long-term monitoring efforts and protect understudied tropical coastal systems more efficiently. This study is focused on nutrient values, hydrological data, biogeochemical carbon behaviour linked to the carbonate system and includes estimates of CO2 fluxes in three contrasting areas off the northeastern Brazilian shelf: 1) an urbanised estuary (Recife-REC), 2) a coastal Island (Itamaracá-ITA) and 3) an oceanic archipelago (Fernando de Noronha-FN). In general, REC acted as a source, while ITA and FN as carbon sinks. In ITA, despite the high DIC and Total Alkalinity observed (mean ~2360 μmol·kg-1), the sink is associated with an effective cascading of atmospheric CO2 associated with turbulent shallow waters coupled with biogenic removal of and precipitation of CaCO3 by coralline algae. FN acted as a sink, linked to minor decreases in Total Alkalinity (mean~2295 μmol·kg-1) influenced by ammonium-based primary production, nitrogen fixation and sporadic entrainment of nutrient rich waters in the upper thermocline. More studies in different western tropical Atlantic coastal systems can improve the knowledge of tropical shelf seas and their contribution to the ocean carbon budget under specific regional trophic regimes.
Highlights
Recent cumulative CO2 data from Friedlingstein et al (2019) showed that around 25% of all anthropogenic carbon emissions (160 ± 20 GtC) had been taken up by the global ocean since the preindustrial era, i.e. from 1850 and 2019
Results for pCO2sw in ITA showed a narrower range when compared to REC (Figure 4(c)), which reflects its oceanic and more oligotrophic conditions supported by little remineralization of organic matter inducing a new injection CO2aq
The relatively high total alkalinity (TA) and DIC signals in ITA indicate a water column prone to CO2(aq) saturation, leading to shifts in the carbonate system towards the formation of HCO3– and CO32−. These results suggest that ITA is the system that sits at the optimal part of the spectrum for CO2 take-up when compared to the other regions investigated
Summary
Recent cumulative CO2 data from Friedlingstein et al (2019) showed that around 25% of all anthropogenic carbon emissions (160 ± 20 GtC) had been taken up by the global ocean since the preindustrial era, i.e. from 1850 and 2019. The other 2 species, bicarbonate and carbonate ions ( HCO3– and CO32− , respectively), are proton acceptors and form the major acid-base system controlling seawater total alkalinity (TA) (Wolf-Gladrow et al, 2007). These DIC forms can participate in processes within the ocean, and be transported away from the regions of enhanced gas exchange (such as zones of deep water formation—a carbon sinking process known as the solubility pump), and are eventually removed by chemical precipitation with Ca2+/Mg2+ or take part in biological processes such as carbon fixation into living tissue and calcification of rigid structures. The biogeochemical processes that remove C from the DIC pool into other compartments (e.g., particulate matter by assimilation, precipitation, mineralization) form a gradient between the surface and subsurface waters, enabling the action of carbonate and the biological pumps
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