Air‐Sea CO2 Flux Estimates in Stratified Arctic Coastal Waters: How Wrong Can We Be?

Abstract

Summer near-surface seawater sampling in the Canadian Arctic revealed potential for significant errors (nearly 0.1 μmol·(m s)) in CO2 fluxes calculated from measured air-sea CO2 gradients. River runoff and sea ice melt strongly stratify these waters, often resulting in surface mixed layers only a few meters thick and isolated from waters sampled by shipboard underway systems. Samples collected with the underway system, rosette, and small boats exposed substantial near-surface gradients in CO2 partial pressure (pCO2) over the top 7 m at many stations. Distributions of temperature, salinity, and fluorescence indicated that the sources of the CO2 system gradients varied between stations, precluding simple corrections to align subsurface data with shallower conditions. Overall, the strong summertime sink of atmospheric CO2 implied by the underway data was not supported by shallower data. Plain Language Summary Large quantities of sea ice melt and river runoff in the Arctic Ocean form thin layers of fresh water at the surface that are isolated from deeper water. However, standard methods of sampling surface waters from ships draw water from 2 to 7 m below the surface, which can cause errors in air-sea CO2 fluxes calculated from measured seawater CO2 concentrations. We have quantified the potential error for waters of the coastal Arctic Ocean by measuring CO2 concentrations in samples collected near the surface using different methods from both large ships and small boats. We found that large errors could result from shipboard sampling at some stations. In particular, measuring the CO2 concentration from automated instruments that drawwater from below the ship’s hull systematically overestimated atmospheric CO2 absorption by the ocean. 1. Arctic Ocean Stratification and Estimating Air-Sea CO2 Fluxes From Bulk Parameterizations Extreme, widespread stratification, similar to what is observed on smaller scales in temperate and tropical estuaries and coastal waters, is one of the most striking characteristics of the summer surface Arctic Ocean. Both sea ice melt and river runoff result in shallow, low-salinity surface mixed layers in both coastal and deepwater regions of the Arctic (e.g., Burt et al., 2016; Yamamoto-Kawai et al., 2009). These relatively fresh surface layers are often no more than a few meters thick and can persist for extended periods of time, because of the strong stratification at the interface with themore saline waters below, as well as the generally moderate winds during the melt season (e.g., Polyakov et al., 1999). As a whole, the Arctic Ocean currently appears to serve as a net atmospheric CO2 sink (e.g., Evans et al., 2015; Land et al., 2013; Yasunaka et al., 2018). However, that net sink is likely sensitive to the seasonal progression of ice coverage (e.g., Else, Galley, et al., 2012; Evans et al., 2015) and presumably river runoff, which appears to be increasing (e.g., Dai et al., 2009; Déry et al., 2016). The sink is also punctuated by instances of strong outgassing, most notably in coastal and river-influenced waters where heterotrophic respiration of terrestrial organic matter and sedimentary methane can release large quantities of CO2 (e.g., Anderson et al., 2011; Else et al., 2008; Graves et al., 2015). In addition, as sea ice breaks up and retreats from the coasts, upwelling may increasingly bring high CO2 waters to the surface, where they can outgas (Else, Papakyriakou, et al., 2012; Mathis et al., 2012; Williams & Carmack, 2015), and as summertime surface waters grow warmer, summer outgassing is likely increasing (Else et al., 2013; Else, Papakyriakou, et al., 2012; Geilfus et al., 2018; Land et al., 2013). MILLER ET AL. 235 Geophysical Research Letters