Mechanisms for a nutrient-conserving carbon pump in a seasonally stratified, temperate continental shelf sea

Continental shelf seas may have a significant role in oceanic uptake and storage of carbon dioxide (CO2) from the atmosphere, through a ‘continental shelf pump’ mechanism. The northwest European continental shelf, in particular the Celtic Sea (50°N 8°W), was the target of extensive biogeochemical sampling from March 2014 to September 2015, as part of the UK Shelf Sea Biogeochemistry research programme (UK-SSB). Here, we use the UK-SSB carbonate chemistry and macronutrient measurements to investigate the biogeochemical seasonality in this temperate, seasonally stratified system. Following the onset of stratification, near-surface biological primary production during spring and summer removed dissolved inorganic carbon and nutrients, and a fraction of the sinking particulate organic matter was subsequently remineralised beneath the thermocline. Water column inventories of these variables throughout 1.5 seasonal cycles, corrected for air-sea CO2 exchange and sedimentary denitrification and anammox, isolated the combined effect of net community production (NCP) and remineralisation on the inorganic macronutrient inventories. Overall inorganic inventory changes suggested that a significant fraction (>50%) of the annual NCP of around 3 mol-C m–2 yr–1 appeared to be stored within a long-lived organic matter (OM) pool with a lifetime of several months or more. Moreover, transfers into and out of this pool appeared not to be in steady state over the one full seasonal cycle sampled. Accumulation of such a long-lived and potentially C-rich OM pool is suggested to be at least partially responsible for the estimated net air-to-sea CO2 flux of ∼1.3 mol-C m–2 yr–1 at our study site, while providing a mechanism through which a nutrient-conserving continental shelf pump for CO2 could potentially operate in this and other similar regions

Bīrūnī, Abū Rayḥān

BĪRŪNĪ, ABŪ RAYḤĀN MOḤAMMAD b. Aḥmad (362/973- after 442/1050), scholar and polymath of the period of the late Samanids and early Ghaznavids and one of the two greatest intellectual figures of his time in the eastern lands of the Muslim world, the other being Ebn Sīn

The physical oceanography of Jones Bank: A mixing hotspot in the Celtic Sea

New measurements are presented of the currents and hydrography at Jones Bank in the Celtic Sea. These measurements, collected during the summer of 2008, identify a highly energetic internal wavefield generated by the local interaction of seasonally stratified flow with topography. Observations close to the bank crest reveal internal waves of up to 40 m amplitude; approaching 50% of the local water depth. These waves occur during spring periods and are predominantly associated with off-bank tidal flow. We provide evidence that these waves are the result of hydraulic control of the tidal currents which result in supercritical flow in the bottom mixed layer (bml) on the upper slopes of the bank. The waning tide produces a transition from super to subcritical flow, identifiable as a regular hydraulic jump on the bank slope. Microstructure measurements identify a turbulent ‘bore’ associated with such jumps that increases bml turbulence by several orders of magnitude and produces a regular burst of enhanced mixing at the base of the ever-present thermocline. Additional mixing in the surface mixed layer is provided during gale force conditions during the first of the two spring tides observed, occurring at the start of our three-week measurement period. The average thermocline mixing rate during stormy spring tide conditions is 1.9 (±1.0, 95%) × 10−3 m2 s−1, driven by both surface mixing and hydraulic jumps. During the later calm spring period the average thermocline mixing rate is 3.3 (±1.7) × 10−4 m2 s−1, dominated by jump associated events. By comparison, the intervening neap tide period produces no hydraulic jumps and is characterised by relatively calm weather. A strong shear layer is however maintained during this ‘quiet’ period sufficient to enhance thermocline turbulence by a factor 1000 above background levels. A short lived peak in thermocline turbulence during the neap period produces diapycnal mixing as high as 1.6 × 10−2 m2 s−1 however a more representative ‘background’ thermocline mixing rate is 2.8 (±0.6) × 10−5 m2 s−1