Particulate organic carbon data in Antarctic waters

Brief description

The aim of the project was to determine the particulate organic carbon concentration in coastal polynyas and off-shelf sites south of the Polar Front. Data is collected from the CTD deployed at various stations. This record descripbes three datafiles.

(1) POC_data: station, pressure, temperature, conductivity, salinity, PAR, total beam attenuation coefficients (c), attenuation by particles (cp), site, particulate organic carbon concentration (from optical measurements), rho, longitude and latitude.
(2) Ammonium: station, pressure, site, ammonium, latitude, longitude
(3) POC_PON_ratio: site, depth, particulate organic carbon concentration (measured) particulate organic nitrogen concentration (measured), C:N ratio.

Lineage

Maintenance and Update Frequency: notPlanned

Statement: A total of 70 CTD (conductivity-temperature-depth, SeaBird 704, SeaBird Inc., Bellevue, USA) casts were undertaken within the Dalton (n = 20), Mertz (n = 24) and Ninnis (n = 14) polynyas, and north of the shelf moving away from the coastal polynyas (n = 12). Seawater samples were collected from Niskin bottles triggered at various depths during the CTD cast. Between 4 and 6 depths (10 – 200 m) were sampled for POC and PON based on the CTD Photosynthetic Available Radiation (PAR) and Chl-a profiles (Moreau et al. 2019). Seawater samples were filtered (475 – 1055 mL depending on organic matter content) onto pre-combusted (450°C for 12 h), 25 mm, quartz fiber filters (Advantec). All glassware in contact with POC samples were also pre-combusted prior to the voyage and subsequently rinsed with 2% v/v hydrochloric acid and ultra-high purity water between samples. Filters were then removed and placed into individual, sterile filter holders and wrapped in pre-combusted aluminium foil to limit light exposure. Filters were stored at −20°C until analysis (~6 months). Nine unused, pre-combusted filters were analyzed as ‘filter blanks’ to quantify background concentration of POC and PON on the filters. Particulate organic carbon and nitrogen were determined using standard carbon-hydrogen-nitrogen combustion methods (Knap et al. 1994). Filters were removed from storage, punched to 15.9 mm, and dried in an oven at 60°C overnight in muffled glass trays. 80 μL of 10% v/v hydrochloric acid was then added to each filter to remove any inorganic carbon. The acid-treated filters were subsequently dried in a desiccator overnight, folded and enclosed into 8x5 mm silver capsules (Elemental Microanalysis, UK) and analyzed at the Central Science Laboratory, University of Tasmania, using a Thermo Finnigan EA 1112 Series Flash Elemental Analyzer (estimated precision ~ 1%). The final sample POC and PON concentrations were calculated by subtracting the average carbon and nitrogen content measured in the filter blanks (2 ± 1 µg for carbon and 0.3 ± 0.1 µg for nitrogen), multiplying by the filter to punch area ratio and dividing the values by the volumes filtered. Particulate organic carbon measurements were not corrected for possible dissolved organic carbon adsorption on the GF/F filter. We obtained estimates of POC distribution along the water column by relating the discrete POC concentration to an in situ optical instrument - transmissometer at 660 nm (Wetlabs C-star serial no. 1421DR 25 cm pathlength). The up-cast transmissometer profile of the water column was used as it paired with the firing of the Niskin bottles for subsequent POC analysis in water. Baseline data were vertically averaged in 2dbar (~2 meter) bins. Factory calibration details are presented in Rosenberg and Rintoul (2017). The optical sensor output was delivered in percent units. The percent transmission (Tr) of light was then converted to total beam attenuation coefficients, c, using:

c = -(1 / r) * ln (Tr / 100)

where c = beam attenuation coefficient (m-1), r = beam path length (m), and Tr = beam transmission (%) (Gardner 1995). The beam attenuation coefficient (c) represents the sum of attenuation due to particles (cp), water (csw) and colored dissolved organic matter (CDOM). Scattering and absorption by CDOM is considered negligible in most open ocean waters, so attenuation by particles (cp) equals the total attenuation measured (c) minus the attenuation by water (csw):

cp = c - csw

where csw was derived from measurements of c at 2000 m depth at the off shelf sites, where particle concentrations can be assumed to be negligible.

The POC:cp relationship was assessed based on sites where POC and cp were simultaneously collected. Particulate attenuation, cp, of the profiling transmissometer were linearly correlated with discrete POC concentration (the coefficient of determination, R2 = 0.83). There was no evidence that the intercept of the relationship between the in situ POC concentration and particle attenuation varied by site (p = 0.1), which allowed for the POC concentrations to be pooled across all CTDs for each polynya, and the off-shelf stations. cp was transformed to POC concentration (mg m-3) via a single linear equation:

POC = 26.8 + 370.6cp

Data file contents are as follows:
(1) POC_data (from optical sensor on Seabird CTD): station, pressure, temperature, conductivity, salinity, PAR, total beam attenuation coefficients (c), attenuation by particles (cp), site, particulate organic carbon concentration (from optical measurements), rho, longitude and latitude.
(2) Ammonium (collected from individual CTD Niskin bottles): station, pressure, site, ammonium, latitude, longitude
(3) POC_PON_ratio (collected from individual CTD Niskin bottles): site, depth, particulate organic carbon concentration (measured) particulate organic nitrogen concentration (measured), C:N ratio.

Notes

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Antarctic Climate and Ecosystems CRC

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Australian Antarctic Division research projects AAS 4131 and 4291

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Institute for Marine and Antarctic Studies, University of Tasmania

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L. Ratnarajah also received support from BYONIC (ERC award number 724289 awarded to Alessandro Tagliabue)

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S. Moreau and C. Genovese were supported by the Australian Research Council’s Special Research Initiative for Antarctic Gateway Partnership (Project ID SR140300001)

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V. Puigcorbé was supported by Edith Cowan University through an Early Career Researcher Grant (G1003456) and an ECU-Collaboration Enhancement Scheme grant