ALBERT

Description: Two Slocum gliders (units 331 and 439) were deployed during RRS Discovery expedition DY081 on July 17th 2017 at 62.9°N, 52.6°W, approximately 40 km off the Greenland shelf break, travelled North along the coast in a zig-zag pattern between the shelf and deep waters, and were recovered 8 days later from 63.7°N, 53.1°W and 62.9° N, 52.7°W respectively on July 24th 2017. Gliders profiled from the surface to 1000 m, except during the two excursions onto the shelf, once south and once north of the Godthåb Trough, where they followed the bathymetry. Each glider was fitted with a pumped CTD and bio-optical sensors (WET Labs puck). These bio-optical sensors measure optical backscattering (in the form of volume scattering function), chlorophyll fluorescence, and UV fluorescence for fluorescing dissolved organic matter (FDOM), a subset of coloured organic matter (CDOM). This dataset contains raw and processed/gridded data files from the glider deployments. The raw data are contained as .dbd and .ebd files in ***_raw_data.zip folders for each of glider unit 331 and 439. The data were processed using the SOCIB glider toolbox (https://github.com/socib/glider_toolbox) and saved as a NetCDF (processed_***.nc) with the following variables: longitude, latitude, time (Julian Day), pressure, eastward velocity, northward velocity, temperature (not thermally corrected), salinity (not thermally corrected), chlorophyll fluorescence (not corrected for quenching), coloured organic matter (cdom), backscatter (volume scattering function) and oxygen concentration. The expedition report is provided and metadata can be found in the processed/gridded data files.

Description: Physical, chemical and biogeochemical measurements derived from CTD-rosette deployments during three visits to site P3 (November to December, 2017) in the South Atlantic. Measurements were made during COMICS cruise DY086 on the RRS Discovery using a trace metal free Titanium Rosette (events 4, 7, 15, 19, 24, 26, 29) and a Stainless Steel Rosette (all other events). Physical parameters include temperature, salinity, density, photosynthetically active radiation and turbulence; chemical parameters include dissolved oxygen, dissolved oxygen saturation, nitrate, phosphate and silicate; biogeochemical parameters include turbidity, beam transmittance, beam attenuation, fluorescence, particulate organic carbon (POC), dissolved organic carbon (DOC), chlorophyll-a, net primary productivity (NPP), ambient leucine assimilation and bacterial cell count. To determine turbulence, a downward facing lowered acoustic doppler current profiler (LADCP, Teledyne Workhorse Monitor 300 kHz ADCP) was attached to the CTD frame. Shear and strain, which are obtained from velocity and density measurements, were used to estimate the dissipation rate of turbulent kinetic energy and the diapycnal eddy diffusivity from a fine-scale parameterisation. Estimates are calculated by parameterising internal wave-wave interactions and assuming that wave breaking modulates turbulent mixing. A detailed description of the method for calculating diffusivity from LADCP and CTD can be found in Kunze et al. (2006). Two datasets with different vertical resolutions were produced: one in which the shear is integrated from 150 to 300 m and the strain over 20-150 m, and one in which the shear is integrated from 70 to 200 m and the strain over 30-200 m. Nutrients (nitrate, phosphate, silicate) were determined via colourimetric analysis (see cruise report, Giering and Sanders, 2019), POC was determined as described in Giering et al. (2023), DOC and DOC flux were determined as described in Lovecchio et al. (2023), NPP was determined as described in Poulton et al. (2019), and ambient leucine assimilation and bacterial cell count were determined as described in Rayne et al. (2024). Bacterial abundance and leucine assimilation were made from bottle samples of six CTD casts of the stainless-steel rosette. Water was collected at six depths (6 m, deep-chlorophyll maximum, mixed layer depth + 10, 100, 250 and 500 m). Acid-cleaned HDPE carboys and tubing were used for sampling. Samples were then stored in the dark and at in-situ temperature prior to on-board laboratory sample preparation or analysis. Flow cytometry was used to measure bacterial abundance. Room temperature paraformaldehyde was used to fix 1.6 ml samples for 30 minutes. Then, using liquid nitrogen, the samples were flash frozen and stored at -80°C. Samples were then defrosted before being stained using SYBR Green I and run through the flow cytometer (BD FACSort™). The method of Hill et al. (2013) was applied to determine prokaryotic leucine assimilation using L-[4,5-³H] leucine which has a specific activity of 89.3 Ci/mmol­. In the mixed and upper layers of the water column, the protocol in Zubkov et al. (2007) was followed. Below the mixed layer, adaptions to the method included reducing the concentration of ³H-Leucine to 0.005, 0.01, 0.025, 0.04 and 0.05 nM; increasing experimental volumes to 30 ml; enhancing incubation times to 30, 60, 90 and 120 min. These adaptions were made to improve accuracy where lower rates of leucine assimilation were expected. Data were provided by the British Oceanographic Data Centre and funded by the National Environment Research Council.

Description: Discrete measurements of particulate organic carbon (POC) concentration and flux were made on the RRS Discovery during COMICS cruise DY086 at site P3 in the South Atlantic from November to December, 2017 (Giering et al. 2023). Data is from a variety of equipment including marine snow catchers, neutrally-buoyant sediment traps (PELAGRA) and a stand-alone pump system. Marine snow catchers settled on-deck for 2 hours. Slow sinking particles were collected from the base and fast sinking particles were collected from the tray. These data were used along with bottle POC data to calibrate glider backscatter data from the GOCART project.

Description: Binned median of Slocum G2 glider data from 0 to 1000 m depth. The data were collected in the northern Benguela region between 14 February 2018 and 19 June 2018 at a site approximately 100 km from the coast. The glider sampled continuously following a triangular path of ~12 km per side roughly centred on 10.8°E, 18.1°S. The glider sampled only on the upward dive with a vertical resolution of ~20 cm, emerging 5 to 6 times per day. Temperature, Conductivity and Depth were measured with a standard Slocum Glider Payload CTD (pumped) from Seabird (SN 9109). Dissolved oxygen was measured with an Aanderaa optode, model 4831 (SN286). Depth-averaged currents (DAC) for each 1000 m downward and upward dive were estimated from the difference between the glider's actual and predicted surfacing locations. Glider surface currents were also estimated at each surfacing via linear regression of GPS location with respect to time. Salinity and oxygen data were calibrated against shipboard CTD bottle samples. The data have been binned (median) into 6 hourly, 2 m depth bins for all variables while the currents timeseries were binned daily (median). 1D variables consist of: time (seconds since 00:00:00 on 1 January 0000), depth (meters), longitude (degrees East), latitude (degrees South), zonal and meridional glider surface currents (U_surf and V_surf, m/s), and zonal and meridional glider depth averaged currents (U_dac and V_dac, m/s). 2D variables consist of: conservative temperature (°C), absolute salinity (g/kg), potential density (kg/m³), and dissolved oxygen concentration (µmol/kg).

Description: The biological carbon pump, which transports particulate organic carbon (POC) from the surface to the deep ocean, plays an important role in regulating atmospheric carbon dioxide (CO2) concentrations. We know very little about geographical variability in the remineralization depth of this sinking material and less about what controls such variability. Here we present previously unpublished profiles of mesopelagic POC flux derived from neutrally buoyant sediment traps deployed in the North Atlantic, from which we calculate the remineralization length scale for each site. Combining these results with corresponding data from the North Pacific, we show that the observed variability in attenuation of vertical POC flux can largely be explained by temperature, with shallower remineralization occurring in warmer waters. This is seemingly inconsistent with conclusions drawn from earlier analyses of deep-sea sediment trap and export flux data, which suggest lowest transfer efficiency at high latitudes. However, the two patterns can be reconciled by considering relatively intense remineralization of a labile fraction of material in warm waters, followed by efficient downward transfer of the remaining refractory fraction, while in cold environments, a larger labile fraction undergoes slower remineralization that continues over a longer length scale. Based on the observed relationship, future increases in ocean temperature will likely lead to shallower remineralization of POC and hence reduced storage of CO2 by the ocean.

Description: The North Atlantic is a substantial sink for anthropogenic CO2. Understanding the mechanisms driving the sink's variability is key to assessing its current state and predicting its potential response to global climate change. Here we apply a time series decomposition technique to satellite and in situ data to examine separately the factors (both biological and nonbiological) that affect the sea-air CO2 difference (ΔpCO2) on seasonal and interannual time scales. We demonstrate that on seasonal time scales, the subpolar North Atlantic ΔpCO2 signal is predominantly correlated with biological processes, whereas seawater temperature dominates in the subtropics. However, the same factors do not necessarily control ΔpCO2 on interannual time scales. Our results imply that the mechanisms driving seasonal variability in ΔpCO2 cannot necessarily be extrapolated to predict how ΔpCO2, and thus the North Atlantic CO2 sink, may respond to increases in anthropogenic CO2 over longer time scales. ©2018. The Authors.

Description: Ocean color sensors are crucial for understanding global phytoplankton dynamics. However, the limited life spans of sensors make multisensor data sets necessary for estimating long-term trends. Discontinuities may be introduced when merging data between sensors, potentially affecting trend estimates and their uncertainties. We use a Bayesian spatiotemporal model to investigate the presence of discontinuities and their impacts on estimated chlorophyll trends. The discontinuities considered are the introduction of Medium Resolution Imaging Spectrometer, Moderate Resolution Imaging Spectroradiometer-Aqua, and Visible Infrared Imaging Radiometer Suite and the termination of Sea-Viewing Wide Field-of-View Sensor. Discontinuities are detected in ~70% of regions, affecting trend estimates (~60% of regions have statistically different trends) and potentially even biasing trend estimates (opposite sign in ~13% of regions). Considering a single discontinuity increases trend uncertainty by an average of 0.20%/year (0.59%/year for two discontinuities). This difference in trend magnitude and uncertainty highlights the importance of minimizing discontinuities in multisensor records and taking into account discontinuities when analyzing trends. ©2018. American Geophysical Union. All Rights Reserved.

Description: The Sentinel-3 tandem project represents the first time that two ocean colour satellites have been flown in the same orbit with minimal temporal separation (~30 s), thus allowing them to have virtually identical views of the ocean. This offers an opportunity for understanding how differences in individual sensor uncertainty can affect conclusions drawn from the data. Here, we specifically focus on trend estimation. Observational chlorophyll-a uncertainty is assessed from the Sentinel-3A Ocean and Land Colour Imager (OLCI-A) and Sentinel-3B OLCI (OLCI-B) sensors using a bootstrapping approach. Realistic trends are then imposed on a synthetic chlorophyll-a time series to understand how sensor uncertainty could affect potential long-term trends in Sentinel-3 OLCI data. We find that OLCI-A and OLCI-B both show very similar trends, with the OLCI-B trend estimates tending to have a slightly wider distribution, although not statistically different from the OLCI-A distribution. The spatial pattern of trend estimates is also assessed, showing that the probability distributions of trend estimates in OLCI-A and OLCI-B are most similar in open ocean regions, and least similar in coastal regions and at high northern latitudes. This analysis shows that the two sensors should provide consistent trends between the two satellites, provided future ageing is well quantified and mitigated. The Sentinel-3 programme offers a strong baseline for estimating long-term chlorophyll-a trends by offering a series of satellites (starting with Sentinel-3A and Sentinel-3B) that use the same sensor design, reducing potential issues with cross-calibration between sensors. This analysis contributes an important understanding of the reliability of the two current Sentinel-3 OLCI sensors for future studies of climate change driven chlorophyll-a trends.