Carbon export fluxes and export efficiency in the central Arctic during the record sea‐ice minimum in 2012: a joint ²³⁴Th/²³⁸U and ²¹⁰Po/²¹⁰Pb study

Abstract

The Arctic sea‐ice extent reached a record minimum in September 2012. Sea‐ice decline increases the absorption of solar energy in the Arctic Ocean, affecting primary production and the plankton community. How this will modulate the sinking of particulate organic carbon (POC) from the ocean surface remains a key question. We use the 234Th/238U and 210Po/210Pb radionuclide pairs to estimate the magnitude of the POC export fluxes in the upper ocean of the central Arctic in summer 2012, covering time scales from weeks to months. The 234Th/238U proxy reveals that POC fluxes at the base of the euphotic zone were very low (2 ± 2 mmol C m−2 d−1) in late summer. Relationships obtained between the 234Th export fluxes and the phytoplankton community suggest that prasinophytes contributed significantly to the downward fluxes, likely via incorporation into sea‐ice algal aggregates and zooplankton‐derived material. The magnitude of the depletion of 210Po in the upper water column over the entire study area indicates that particle export fluxes were higher before July/August than later in the season. 210Po fluxes and 210Po‐derived POC fluxes correlated positively with sea‐ice concentration, showing that particle sinking was greater under heavy sea‐ice conditions than under partially ice‐covered regions. Although the POC fluxes were low, a large fraction of primary production (>30%) was exported at the base of the euphotic zone in most of the study area during summer 2012, indicating a high export efficiency of the biological pump in the central Arctic.

Algal Export in the Arctic Ocean in Times of Global Warming

“…Relatively high annual PPC fluxes at the LR, in the northern LS, and in the NB likely reflected higher nutrient availability in these regions than in the AB and ESS (Table ; Lalande et al, ). In contrast, POC fluxes were much higher near the LS shelf (Figure ; Table ), in agreement with previous POC flux measurements in the Central Arctic Ocean (Cai et al, ; Lalande et al, ; Roca‐Martí et al, ). Annual PPC fluxes ranged from 0.7 to 14.5 % of annual POC fluxes in the Eurasian Arctic Ocean, clearly indicating that algal cells contribute to a small portion of the POC fluxes and that POC export must be considered with caution and even discarded as an indicator of primary production (Figure and Table ).…”

Utilizing 210Po deficit to constrain particle dynamics in mesopelagic water, western South China Sea

“…In the euphotic zone, 210 Po is usually deficient with respect to 210 Pb due to their rapid removal induced by plenty of biogenic particles [ Nozaki et al ., ; Stewart et al ., ]. Below the euphotic zone, 210 Po quickly reaches an equilibrium status with 210 Pb (i.e., 210 Pb equals to 210 Pb) [ Murray et al ., ; Roca‐Martí et al ., ]. Sometimes, 210 Po activity concentrations are higher than 210 Pb at the bottom of the euphotic zone or in the upper mesopelagic zone as a result of 210 Po release from sinking particles during particle remineralization (Figure ) [ Bacon et al ., ; Cochran et al ., ; Yang et al ., ].…”

“…In fact, similar deficiency occurred at the SEATS station in the northern SCS [Wei et al, 2014]. This deficiency provided a contrast to the equilibrium status between 210 Po T and 210 Pb T below the remineralization Geochemistry, Geophysics, Geosystems 10.1002/2017GC006899 layer in general mesopelagic water [Bacon et al, 1976;Roca-Mart ı et al, 2016]. At the same time, the average activity concentration of 210 Po P increased from 0.69 6 0.08 dpm 100 L 21 in the euphotic zone to 0.85 6 0.11 dpm 100 L 21 in the mesopelagic layer at station 105A, and from 0.53 6 0.06 to 0.65 6 0.11 dpm 100 L 21 at station 91.…”

“…In the euphotic zone, abundant biogenic particles usually introduce a quick sorption and sinking of 210 Po and 210 Pb on a short time scale, leading to insufficient time for 210 Po accumulation and resulting in an observable deficiency of 210 Po with respect to 210 Pb [ Bacon et al ., ; Nozaki and Tsunogai , ; Cochran et al ., ]. Due to the tight link between particulate organic components and 210 Po or 210 Pb adsorption [ Stewart et al ., ; Yang et al ., ; Chuang et al ., ; Rigaud et al ., ], the disequilibria between 210 Po and 210 Pb have increasingly been used to trace the export flux of particulate organic carbon (POC) [ Buesseler et al ., ; Verdeny et al ., ; Stewart et al ., ; Roca‐Martí et al ., ; Su et al ., ], biogenic silica [ Friedrich and Rutgers van der Loeff , ], and nitrogen [ Yang et al ., ] out of the euphotic zone in the last decade. The 210 Po‐ 210 Pb pair is also listed as one of the important proxies for investigating the oceanic particle dynamics in the ongoing GEOTRACES program [ Church et al ., ; Rigaud et al ., ].…”

Models of Plankton Community Changes during a Warm Water Anomaly in Arctic Waters Show Altered Trophic Pathways with Minimal Changes in Carbon Export

“…Similarly, the ubiquitous Micromonas sp. in Arctic waters has been detected in this region by sediment traps, and associated with increased 234 Thorium adsorption in the Central Arctic (Charles Bachy, pers. commun., Roca-Marti et al, 2016).…”