Seawater DIC analysis: The effects of blanks and long‐term storage on measurements of concentration and stable isotope composition

Abstract

Dissolved inorganic carbon (DIC) concentration and stable isotope composition measurements (d 13 C) are central to the study of biological and chemical processes in the ocean. We examined a small volume (1 mL) method using continuous-flow isotope ratio mass spectrometry (CF-IRMS) for making both measurements in a single sample run. Prior exploration of this approach has yielded variable results. Effects of concentration, isotopic composition, storage time, and the gas used to flush the sample tubes were tested using standard solutions. Multiple isotopic composition standards permitted calculation of an apparent blank that develops due to slow rates of gas exchange through the septum. A blank correction is applied to the d 13 C measurements, giving precision and accuracy better than 0.1& after 6 months of storage and 0.2& after 18 months of storage. CF-IRMS concentration measurements on field samples collected near Tatoosh Island, off the Pacific coast of Washington, U.S.A., were compared to independent measurements (Dickson, CDQC Laboratory, UCSD) on the same water, the latter approach involving collection of 500 mL of seawater fixed with mercuric chloride. Concentration measurements by CF-IRMS on seawater samples (precision 1-2%) are correctable to the Dickson results, though scatter in this relation exceeds analytical precision. The DIC and d 13 C measurements on coastal seawater, in conjunction with salinity measurements, indicated strong effects of biological activity on carbon dynamics relative to possible effects of mixing water masses. In sum, this small volume methodology has application to field situations and laboratory experiments where larger volumes are logistically challenging and storage space is limited.

Kelp beds and their local effects on seawater chemistry, productivity, and microbial communities

“…Over multiple dates, the aragonite saturation state was below 1.0, and the calcite saturation state was typically below 3.0. The partial pressure of CO 2 (as pCO 2 ) was 167 latm lower inside the kelp bed when estimated with pH and TA values using "seacarb," but did not differ when we attempted to measure it directly (Olack et al 2018), possibly because of the lower sample size for our direct measurement of DIC (n = 32) compared with twice the number of instrumental measures (n = 64). Notes: *0.001 < P < 0.05, **P < 0.001, § 0.05 < P < 0.10.…”

“…The d 18 O of seawater was analyzed from seawater collected and stored in airtight 4-mL vials (borosilicate glass, screw cap with polyethylene cone liner) for later analysis at the University of Chicago (Olack et al 2018). The d 18 O of seawater was analyzed from seawater collected and stored in airtight 4-mL vials (borosilicate glass, screw cap with polyethylene cone liner) for later analysis at the University of Chicago (Olack et al 2018).…”

“…To further characterize seawater chemistry in the kelp bed, we analyzed the isotopes of oxygen and carbon in seawater. The d 18 O of seawater was analyzed from seawater collected and stored in airtight 4-mL vials (borosilicate glass, screw cap with polyethylene cone liner) for later analysis at the University of Chicago (Olack et al 2018). The isotopes of carbon in the dissolved inorganic carbon of seawater (d 13 C DIC ) were determined from 1-mL seawater samples injected into 12-mL Exetainer tubes TM with phosphoric acid and evacuated with helium gas (Olack et al 2018).…”

Dead in the Water: The Vicious Cycle of Blanks During Natural Level 14 C Manipulation of Marine Algal Cultures

“…Hence, it is more likely that the shift in both pre-and post-growth DIC 14 C values has occurred due to the contribution of some unknown blank source during the experiment or during sample storage until 14 C analysis. A significant sample storage effect has been observed for DIC δ 13 C values due to gas exchange through the septum even in the absence of an atmospheric headspace (Olack et al, 2018). As discussed above, an atmospheric blank C source cannot explain 14 C-depletion of DIC during the "Intermediate" E. huxleyi and all T. pseudonana experiments or subsequent sample storage, but likely has to derive from material produced from petroleum sources.…”

A simple CO2 equilibration method for measuring blood oxygen isotope compositions

“…37 Measured δ 18 O values from outgassed CO 2 in the method presented here are stable for incubation times up to a few days. More controlled sample acquisition, storage, and processing methods may be required for reliable measurement of δ 13 C using an analogous approach (e.g., 38 ).…”