Dataset: In situ dissolution rates of biogenic calcites
Deployment: KM1712

The methodology for measuring dissolution rate follows published methods in Naviaux et al., 2019 and Dong et al., 2019. The ratio of the dissolving solid was measured following methods of Subhas et al., 2018.

Dissolution experiments were conducted in situ using modified Niskin incubators, described in detail by Naviaux et al. (2019) and Dong et al. (2019). In this study, we dissolved bleached, ¹³C-labeled E. huxleyi liths. A total of 20 coccolith dissolution experiments were conducted at depths between 240 to 1000 meters at Stations 2 to 5 with temperatures ranging from 2.4 to 4.8 degrees Celsius and Ωcalcite from 0.96 to 0.67. We conducted one experiment with a planktic foraminiferal assemblage, cultured and ¹³C -labeled as described in Subhas et al. (2018). Briefly, roughly 0.5 to 1.5 milligrams (mg) of labeled biogenic calcite was sealed in between 47mm "Nuclepore" polycarbonate membrane filters (0.8 um pore size). These packets were then mounted inside the Niskin incubators. The incubators were hung on a hydrowire, sent down to depth, and triggered closed. The Niskin reactors remained closed at depth for 24 to 58 hours and were sampled for silica, SRP, nitrate, alkalinity, pH, and δ¹³C -DIC upon recovery. Niskin data were quality checked by comparing SRP, silica, and nitrate to ambient water-column values obtained via CTD/rosette deployments on the same cruise. Saturation states in the Niskin reactors were determined from Alk-pH pairs, input into CO2SYS along with the temperature, salinity, depth, SRP, and silica concentrations at which the reactor was deployed. Dissolution rates were calculated by taking the difference between the final incubator and ambient water column ¹³C/¹²C ratios, multiplied by the [DIC] and the mass of seawater (1.126 kg) inside the incubators, and divided by the incubation time. Rates calculated in this way are in units of g Ca¹³CO₃ g CaCO₃⁻¹d⁻¹. Biogenic materials were not 100% labeled and rates were scaled for the extent of isotope labeling following Subhas et al. (2018). Briefly, when the amount of ¹³C in the dissolving material is substantially enriched above natural abundance (¹³C/¹²C~0.01), isotope ratio differences can be multiplied by a correction factor of (Rs+1)/Rs, where Rs is the ¹³C/¹²C ratio of the dissolving solid (i.e. a reduced form of Eq. 3 from Subhas et al. (2018) when Rs >> R₁, R₂). One batch of E. huxleyi was used for the majority of the dissolution rates shown here (Rs = 0.928). One experiment using the original batch of bleached E. huxleyi (Rs = 20, Subhas et al. 2018) was also run. The planktic foraminifera assemblage from Subhas et al. (2018) was used (Rs = 1.6). Dissolution rates were normalized further by the specific surface areas of E. huxlyei liths (10.5 m²g) and planktic foraminifera (4.3, Subhas et al., 2018).

Problem Report:
The saturation state of the planktic experiment is anomalously low (0.64). Based on co-located coccolith and calcite dissolution experiments, both the in situ pH and alkalinity appear lower than they should be for that depth. We applied a correction factor of 0.06 to the in situ Omega value to correct for this (i.e. Oca_use = Oca + Oca_corr).