Benthic Organic Matter Transformation Drives pH and Carbonate Chemistry
in Arctic Marine Sediments
Abstract
The carbonate chemistry of Arctic Ocean seafloor and its vulnerability
to ocean acidification remains poorly explored. This limits our ability
to quantify how biogeochemical processes and bottom water conditions
shape sedimentary carbonate chemistry, and to predict how climate change
may affect such biogeochemical processes at the Arctic Ocean seafloor.
Here, we employ an integrated model assessment that explicitly resolves
benthic pH and carbonate chemistry along a S—N transect in the Barents
Sea. We identify the main drivers of observed carbonate dynamics and
estimate benthic fluxes of dissolved inorganic carbon and alkalinity to
the Arctic Ocean. We explore how bottom water conditions and in-situ
organic matter degradation shape these processes and show that organic
matter transformation strongly impacts pH and carbonate saturation (Ω)
variations. Aerobic organic matter degradation drives a negative pH
shift (pH < 7.6) in the upper 2—5 cm, producing Ω
< 1. This causes shallow carbonate dissolution, buffering
porewater pH to around 8.0. Organic matter degradation via metal oxide
(Mn/Fe) reduction pathways further increases pH and carbonate saturation
state. At the northern stations, where Ω > 5 at around
10–25 cm, model simulations result in authigenic carbonate
precipitation. Furthermore, benthic fluxes of dissolved inorganic carbon
(12.5—59.5 µmol cm−2 yr−1) and alkalinity (11.3—63.2 µmol cm−2 yr−1)
are 2—3-fold greater in the northern sites due to greater carbonate
dissolution. Our assessment is of significant relevance to predict how
changes in the Arctic Ocean may shift carbon burial and pH buffering
into the next century.