Biogeochemical variations at the Porcupine Abyssal Plain sustained Observatory in the northeast Atlantic Ocean, from weekly to inter-annual timescales

Abstract. We present high-resolution autonomous measurements of carbon dioxide partial pressure p(CO₂) taken in situ at the Porcupine Abyssal Plain sustained Observatory (PAP-SO) in the northeast Atlantic (49° N, 16.5° W; water depth of 4850 m) for the period 2010–2012. Measurements of p(CO₂) made at 30 m depth on a sensor frame are compared with other autonomous biogeochemical measurements at that depth (including chlorophyll a fluorescence and nitrate concentration data) to analyse weekly to seasonal controls on p(CO₂) flux in the inter-gyre region of the North Atlantic. Comparisons are also made with in situ regional time series data from a ship of opportunity and mixed layer depth (MLD) measurements from profiling Argo floats. There is a persistent under-saturation of CO₂ in surface waters throughout the year which gives rise to a perennial CO₂ sink. Comparison with an earlier data set collected at the site (2003–2005) confirms seasonal and inter-annual changes in surface seawater chemistry. There is year-to-year variability in the timing of deep winter mixing and the intensity of the spring bloom.

The 2010–2012 period shows an overall increase in p(CO₂) values when compared to the 2003–2005 period as would be expected from increases due to anthropogenic CO₂ emissions. The surface temperature, wind speed and MLD measurements are similar for both periods of time. Future work should incorporate daily CO₂ flux measurements made using CO₂ sensors at 1 m depth and the in situ wind speed data now available from the UK Met Office Buoy.