Seasonal marine carbon system processes in an Arctic coastal landfast sea ice environment observed with an innovative underwater sensor platform

Abstract

Studying carbon dioxide in the ocean helps to understand how the ocean will be impacted by climate change and respond to increasing fossil fuel emissions. The marine carbonate system is not well characterized in the Arctic, where challenging logistics and extreme conditions limit observations of atmospheric CO2 flux and ocean acidification. Here, we present a high-resolution marine carbon system data set covering the complete cycle of sea-ice growth and melt in an Arctic estuary (Nunavut, Canada). This data set was collected through three consecutive yearlong deployments of sensors for pH and partial pressure of CO2 in seawater (pCO2sw) on a cabled underwater observatory. The sensors were remarkably stable compared to discrete samples: While corrections for offsets were required in some instances, we did not observe significant drift over the deployment periods. Our observations revealed a strong seasonality in this marine carbon system. Prior to sea-ice formation, air–sea gas exchange and respiration were the dominant processes, leading to increasing pCO2sw and reduced aragonite saturation state (ΩAr). During sea-ice growth, water column respiration and brine rejection (possibly enriched in dissolved inorganic carbon, relative to alkalinity, due to ikaite precipitation in sea ice) drove pCO2sw to supersaturation and lowered ΩAr to < 1. Shortly after polar sunrise, the ecosystem became net autotrophic, returning pCO2sw to undersaturation. The biological community responsible for this early switch to autotrophy (well before ice algae or phytoplankton blooms) requires further investigation. After sea-ice melt initiated, an under-ice phytoplankton bloom strongly reduced aqueous carbon (chlorophyll-a max of 2.4 µg L–1), returning ΩAr to > 1 after 4.5 months of undersaturation. Based on simple extrapolations of anthropogenic carbon inventories, we suspect that this seasonal undersaturation would not have occurred naturally. At ice breakup, the sensor platform recorded low pCO2sw (230 µatm), suggesting a strong CO2 sink during the open water season.

High interannual surface pCO2 variability in the Southern Canadian Arctic Archipelago's Kitikmeot Sea

“…A similar pattern was observed in the summer of 2018, except notably, when pCO 2 began to fall in late August the region did not revert all the way back into a sink (Butterworth et al, 2022). The third source of carbonate system measurements are provided by Duke et al (2021) who report autonomous pCO 2 measurements at a depth of 7 m from an instrument installed on the Ocean Networks Canada (ONC) underwater sensor mooring in Cambridge Bay between August 2015 and August 2018. The sensor measurements from Cambridge Bay indicate that pCO 2 is oversaturated in winter and undersaturated by the start of June at the onset of sea ice melt (Duke et al, 2021).…”

“…The breakdown of stratification at the end of the ice-free summer period and over the winter (Xu et al, 2021) may explain the good agreement between the EC tower and the ONC mooring at these times. In June 2017 the two systems diverge, the pCO 2 (sw) at the ONC mooring decreases due to a spring bloom (Duke et al, 2021) whereas pCO 2 (sw) from the EC tower does not. As the bloom in Cambridge Bay is caused by wastewater discharge (Back et al, 2021) it might be expected that this signal would not detectable at the EC tower.…”

“…The third source of carbonate system measurements are provided by Duke et al (2021) who report autonomous pCO 2 measurements at a depth of 7 m from an instrument installed on the Ocean Networks Canada (ONC) underwater sensor mooring in Cambridge Bay between August 2015 and August 2018. The sensor measurements from Cambridge Bay indicate that pCO 2 is oversaturated in winter and undersaturated by the start of June at the onset of sea ice melt (Duke et al, 2021). Their measurements show that there is a short period of oversaturation in the middle of August coinciding with increased sea temperature, the ocean then quickly returns to a sink and remains undersaturated up until freeze-up (Duke et al, 2021).…”

“…The sensor measurements from Cambridge Bay indicate that pCO 2 is oversaturated in winter and undersaturated by the start of June at the onset of sea ice melt (Duke et al, 2021). Their measurements show that there is a short period of oversaturation in the middle of August coinciding with increased sea temperature, the ocean then quickly returns to a sink and remains undersaturated up until freeze-up (Duke et al, 2021).…”

A mobile observatory powered by sun and wind for near real time measurements of atmospheric, glacial, terrestrial, limnic and coastal oceanic conditions in remote off-grid areas

“…Recently, a few smaller coastal undersea observatories have been deployed in communities of the Canadian Arctic. A typical community observatory installation includes an underwater instrument platform located on the ocean floor and linked by cable to a nearby wharf connection [ 8 ]. Presently, there is need for development of mobile autonomous off-grid observatories that can monitor the atmosphere-land–ocean transition.…”