Does Arctic warming reduce preservation of organic matter in Barents Sea sediments?

Abstract

Over the last few decades, the Barents Sea experienced substantial warming, an expansion of relatively warm Atlantic water and a reduction in sea ice cover. This environmental change forces the entire Barents Sea ecosystem to adapt and restructure and therefore changes in pelagic-benthic coupling, organic matter sedimentation and long-term carbon sequestration are expected. Here we combine new and existing organic and inorganic geochemical surface sediment data from the western Barents Sea and show a clear link between the modern ecosystem structure, sea ice cover and the organic carbon and CaCO₃ contents in Barents Sea surface sediments. Furthermore, we discuss the sources of total and reactive iron phases and evaluate the spatial distribution of organic carbon bound to reactive iron. Consistent with a recent global estimate we find that on average 21.0 ± 8.3 per cent of the total organic carbon is associated to reactive iron (fOC-Fe_R) in Barents Sea surface sediments. The spatial distribution of fOC-Fe_R, however, seems to be unrelated to sea ice cover, Atlantic water inflow or proximity to land. Future Arctic warming might, therefore, neither increase nor decrease the burial rates of iron-associated organic carbon. However, our results also imply that ongoing sea ice reduction and the associated alteration of vertical carbon fluxes might cause accompanied shifts in the Barents Sea surface sedimentary organic carbon content, which might result in overall reduced carbon sequestration in the future. This article is part of the theme issue 'The changing Arctic Ocean: consequences for biological communities, biogeochemical processes and ecosystem functioning'.

Keywords: Arctic Ocean; Barents Sea; carbon cycle; geochemical sediment composition; marine surface sediments; organic carbon bound to reactive iron.

Figure 1.

Map of the western Barents Sea and sampling locations (red dots). The northern Barents Sea is seasonally ice-covered and winter maximum and median sea ice coverage over the past 40 years [2] are shown as white area and blue line, respectively. The boundary between the relatively warm northward flowing North Atlantic Current and the southward flowing cold Arctic currents forms the oceanographic Polar Front (yellow line). (Online version in colour.)

Figure 2.

Published linear sedimentation rates (LSR) in the Barents Sea. Data and references are provided in electronic supplementary material, table S1. (Online version in colour.)

Figure 3.

Spatial distribution of CaCO₃ (left) and total organic carbon (right) in Barents Sea surface sediments. For further legend details see figure 1. (Online version in colour.)

Figure 4.

Grain size distribution in Barents Sea surface sediments in (a) decarbonated and (b) bulk sediment samples. (Online version in colour.)

Figure 5.

Spatial distribution of iron in Barents Sea surface sediments. Data from this study and Knies et al. [56]. (Online version in colour.)

Figure 6.

Distribution of (a) TOC, (b) bulk Fe, (c) reactive ion, (d) reactive iron fraction of total iron (fFe_R), (e) organic carbon bound to reactive iron (OC-Fe_R) and (f) the organic carbon fraction of total organic carbon bound to reactive iron (fOC-Fe_R) in Barents Sea surface sediments (0–1 cm). Circles mark stations which are seasonally sea ice covered and crosses are stations which are ice free during winter. Station locations (B1–B18) and ice coverage is shown in figure 1. (Online version in colour.)