Dataset: McMurdo sediment

Samples were collected by SCUBA divers during the austral spring between September and November each year. Organisms identified in colocated images can be found in the dataset "McMurdo epifauna" https://www.bco-dmo.org/dataset/745874.

Two 4 cm diameter, 5 cm deep cores were collected, one for grainsize analysis which was refrigerated until processing, and one for carbon and nitrogen analysis that was frozen until analysis. Sediment grainsize analyses were carried out with a Beckman-Coulter LS 13 320 laser particle size analyzer attached to an aqueous module equipped with a pump and a built-in ultrasound unit. The measured size distributions were analyzed from 0.04 µm to 2 mm. Measurements of such a wide particle size range are possible because the particle sizer is composed of two units: a laser beam for conventional (Fraunhofer) diffraction (from 0.4 µm to 2 mm) and a polarized intensity differential scatter (PIDS) unit, which measures particles based on the Mie theory of light scattering (0.04 µm; Beckman Coulter Inc., 2003). The sediment samples were dispersed in de-ionized water, and increasing amounts of the sediment solution were added to the aqueous module of the particle sizer until obscuration values of 8%–12% and PIDS obscuration values of 48%–52% were obtained. Data interpolation and statistical analyses were obtained with the laser particle sizer proprietary software (Beckman Coulter Inc., 2003). Because all samples analyzed tend to log-normal grain size distributions in the 0.04 µm to 2 mm spectrum, geometric rather than arithmetic statistics were applied to the values obtained by the logarithmically spaced size channels of the particle sizer. For total organic carbon and nitrogen analysis, frozen samples were thawed at room temperature, stirred, and approximately 5 g equivalent dry weight subsample treated with 5-10 ml 0.5-1.0 M reagent grade H2SO4 as needed to remove all inorganic carbon. Samples were then dried at 60-70°C, visually inspected to ensure complete removal of any carbonate shell fragments, and homogenized in a ball-mill. A modification of the high temperature combustion method, utilizing a Wheatstone bridge current differential, was used in a 440 Elemental Analyzer (Control Equipment Co.). Combustion was at <800°C, below the CaCO3 decomposition temperature. The manufacturers-suggested procedures were followed on duplicate or triplicate 5-10 mg aliquots, with sample variance >10% leading to re-homogenizing and re-running analysis. Calibration was with known standards using Acetanilide or L-Cystine every 20-30 machine sample runs, with ±2% acceptable limits, and detection limits were 0.2 µg/mg carbon and 0.01 µg/mg nitrogen, dry weight. Quality control was maintained by analysis of National Research Council of Canada Marine Sediment Reference Material BCSS-1 at the beginning and end of each sample analysis set. All analyzed values were within the suggested criteria of ±0.09% carbon, ±0.008% nitrogen. NA values indicate data not available. ND indicates Not Detected.