Dataset: Carbonate chemistry data from water samples collected via ROV aboard Falkor (too) on cruises FKT230602 and FKT231202 in hydrothermally active areas off Costa Rica’s Pacific margin from Jun and Dec 2023

ROV Sampling

Discrete water samples were collected during two Octopus Odyssey expeditions aboard R/V Falkor (too) in June (FKT230602) and December (FKT231202) 2023. Sampling events were conducted predominantly at the bottom, with additional samples collected near the surface at Dorado Outcrop and Pampa Submarina, hydrothermally active areas off Costa Rica’s Pacific margin. 

Discrete water samples were collected from 2.5 L Niskin bottles mounted on the 4500 m ROV, and in situ physicochemical parameters (temperature, salinity) were measured with CTD. Water was collected at bottom, to characterize the carbonate chemistry of the sampling sites during each dive. Water samples were collected at the end of the dive.

In situ parameters measured with a CTD Sensor (Seabird FastCAT SBE49), oxygen sensor (Aanderaa 3841 O2 Optode) and temperature probe (PT100).

Laboratory analyses

Carbonate chemistry

Carbonate chemistry samples were collected first, as soon as the Niskin bottle was opened and before any other samples (e.g., phytoplankton, nutrients, microplastics, etc.) The sampling order was 1) dissolved inorganic carbon (DIC) and 2) total alkalinity (TA). DIC and TA samples were collected, poisoned and stored following procedures from SOP 3b (Dickson et al. 2007) and REMARCO-AO-P-01 (Sánchez-Noguera 2021). Water was collected from the Niskin bottle using a Tygon tubing. The water flow was constant during the process of rinsing the sample containers/syringes (x3) to remove bubbles from the draw Tygon tubing. Samples were stored in the dark at 4 ºC until analysis at Centro de Investigación en Ciencias del Mar y Limnología (CIMAR-UCR).

DIC: water was slowly drawn from the Niskin with a 60 mL syringe plugged into the Tygon tube. Sample from the syringe was filtered through a 0.45-micron filter and transferred with a smaller Tygon tube into a 40 mL amber borosilicate vial (acid-washed and combusted), rinsed three times with the sample. Sample was poisoned with 50 µL of saturated HgCl2 and vial closed with PTFE septum. DIC samples were analyzed using an automated nondispersive infrared sensor (analyzer multi N/C UV HS, Analytik Jena). DIC was determined by acidifying the samples with 10% H3PO4 and quantifying the extracted CO2 gas by NDIR spectrometry.

TA: 250 mL borosilicate bottles with glass stopper were rinsed (3x) with flowing water and filled directly from the Niskin. Each bottle was allowed to overflow approximately 1.5 times the filling time. Headspace was adjusted with a plastic syringe (10 mL), sample was poisoned with 100 µL of saturated HgCl2 and bottle sealed with a greased stopper (Apiezon-L). TA was determined by open-cell potentiometric titration with an 888 Titrando (Methrom) following procedures from SOP 3b (Dickson et al., 2007).

Instruments were calibrated against seawater certified reference material (CRM, Batch 127) prepared at the Scripps Institution of Oceanography of the University of California, San Diego.

Nutrients

Water was collected from the Niskin with a 60 mL syringe plugged into the Tygon tube. Sample from the syringe was filtered through a 0.45-micron filter and transferred with a smaller Tygon tube to a 60 mL plastic vial, rinsed three times with the filtered sample. Samples were frozen and stored at -20 ºC until analysis.