Dataset: Duquette et al. 2018: Mg/Ca ratios

Echinoderms were collected during two National Science Foundation (NSF) research cruises: NBP12-10 aboard the U.S. R/V Nathanial B. Palmer (Jan 1 - Feb 9, 2013) and LMG13-12 aboard the U.S. R/V Laurence M. Gould (Nov 22 to Dec 20, 2013) around western Antarctica. Individuals were collected using a Blake Trawl (1.5 m width) and bottom temperatures were collected using Seabird SBE3 Oceanographic temperature sensors on a CTD rosette.

To prepare skeletal material, intact individuals or body components were placed in a drying oven for 24 hr at 35 °C and then digested in a 10% NaClO solution to remove all tissue (McClintock et al. 2011). Cleaned skeletal elements were rinsed generously with distilled water, vacuum dried, and then placed in a drying oven for 24 hrs. Skeletal material from intact individuals or body components was then ground into a fine powder using an agate mortar and pestle with the addition of several mL of 95% ethanol. The resulting slurry was air dried for 12 hrs and the subsequent powder used in mineralogical analysis.

Mg/Ca ratios were determined by X-ray diffraction (XRD) using a Philips X’Pert Analytical X-ray diffraction system (PANalytical B.V., Almelo, Netherlands). The system was set to run at 45kV and 40mV with a 2Θ angular scan range of 27˚ to 32˚. The scan speed was 2 s/step with a step size of 0.02 to obtain accurate measurements of the calcite peak occurring at 2Θ = 29.5 ˚-30.2 ˚. Resulting diffraction patterns was used to determine the Mg/Ca ratio following the equation given in Ries (2011), where “2Θ” is the measured location of the calcite peak on each sample’s x-ray diffraction patterns:

Mg/Ca = 0.17881(2Θ)2 – 10.20926(2Θ) + 145.59368