Stable carbon isotopes from the Eastern Atlantic Ocean: anthropogenic and biological signals

Abstract

Stable isotope analyses of environmental samples provide information on the source of materials and the processes responsible for their formation and conversion, and consequently offer information on processes and dynamics in ecosystems. The stable carbon isotopic composition (δ13C) of carbon species in the ocean are characterised by specific isotopic signatures, which vary temporally and regionally, and are thus widely used to understand the marine carbon cycle. However, some important coastal and open ocean regions: upwelling systems, subtropical gyres and regions between subpolar and subtropical gyres, have scant isotopic data available, limiting our understanding of the carbon cycle and influences from anthropogenic CO2 uptake. This thesis utilises the δ13C of dissolved inorganic carbon (DIC) and particulate organic carbon (POC) to better understand carbon cycle processes and the influence of anthropogenic CO2 in the Benguela Upwelling System (BUS), the Porcupine Abyssal Plain (PAP) observatory site and the eastern part of the subtropical gyre of the northeast Atlantic Ocean. In the BUS, the δ13CDIC values and DIC concentrations vary spatially: higher δ13CDIC and low DIC at the Lüderitz Upwelling Cell (LUC) and lower δ13CDIC and high DIC at the southern and northern BUS. The δ13CDIC, DIC and nutrient results indicate that increased biological productivity overrides the effect of low δ13CDIC upwelling waters on the δ13CDIC and DIC distribution at the LUC, whereas the upwelling waters and associated organic matter remineralisation dominate over the effect of biological productivity and control the δ13CDIC and DIC distribution at the NBUS and SBUS. The observed decoupling of δ13CDIC-phosphate and δ13CDIC-apparent oxygen utilization confirm that upwelling waters and the associated remineralisation of organic matter are the main sources and controlling processes of DIC cycling in the BUS. The δ13CPOC distribution in the water column at the PAP site is strongly controlled by biological processes and decreased with depth. A decrease of δ13CPOC in the upper 200 m is attributed to increasing inputs of 13C-depleted POC from the lower euphotic layer, whereas the decrease below 200 m is a result of a greater and preferential degradation of labile, 13C-enriched proteins and carbohydrate components, leaving more refractory, 13C-depleted lipid and lignin components. This study shows that surface water δ13CPOC values are isotopically lower compared to values reported in earlier studies in the northeast Atlantic Ocean (JGOF’S site in 1989 and PAP site in 1997) and are also lower relative to surface sediments, which is attributable to the 13C Suess effect resulting from the uptake of isotopically light anthropogenic CO2. The air-sea exchange signature of δ13C (δ13Cas) negatively deviates in surface waters of the subtropical gyre region studied. The relationship of δ13Cas versus temperature derived for the upper 200 m in this gyre in 2020 (δ13Cas = -0.027T - 1.773) deviates negatively from that observed in the same region in 1990 (δ13Cas = -0.042T - 1.084). These observations confirm that the surface ocean is influenced by anthropogenic CO2. The δ13CDIC in this region has decreased (-0.86 ‰, surface waters; -0.83 ‰, upper 200 m) during the 27-year period 1993-2020. δ13CDIC has decreased at a faster rate during the 2013-2020 period (0.056 ‰ per year, surface waters; 0.048 ‰ per year, upper 200 m) compared to the 1993-2013 period (0.023 ‰ per year, surface waters; 0.024 ‰ per year, upper 200 m). This implies that the uptake of anthropogenic CO2 in the upper ocean of this gyre region not only intensified during 1993-2020, but that it accelerated during 2013-2020. More broadly, this study highlights the utility of stable carbon isotopes in studies of short time carbon cycling in upwelling regimes and thus their role in modulating CO2 levels in the ocean-atmosphere system.

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