Benthic carbon cycling on the Antarctic continental shelf

Abstract

The sedimentation of pelagic production makes continental shelf sediments important sites for organic matter (OM) remineralization and nutrient regeneration in the ocean. Consequently, shelf sediments play an important role in the bentho-pelagic coupling by providing essential nutrients for algal growth and maintaining the high primary production of shelf areas and the adjacent open ocean. In Antarctica, changes in sea ice cover have a major impact on surface primary production and the subsequent sinking of organic carbon to the seafloor. Recent observations indicate that global warming has led to substantial changes in sea ice cover, with a significant reduction of one million square kilometers in the annual maximum sea ice extent around Antarctica. These changes in sea ice conditions are expected to trigger significant changes in the pelagic ecosystem, with potentially profound effects on the benthic ecosystem. The imprint of climate-sensitive variables such as sea ice cover can be best studied in shelf sediments, where shallow water depths result in increased OM supply and tighter bentho-pelagic coupling. Therefore, a comprehensive understanding of the current carbon cycle on the Antarctic continental shelf is crucial for assessing the vulnerability of the ecosystem to climate change and for predicting the future trajectory of the carbon cycle in Antarctic waters. This thesis aims to quantify benthic carbon remineralization rates on the Antarctic continental shelf and the release of nutrients, particularly iron, that limit primary production in the Southern Ocean. In addition, the thesis aims to identify the main pathways of OM degradation and associated microbial communities, while contextualizing the above variables within the prevailing sea ice conditions.

For this purpose, I first studied the geochemistry of shelf sediments along a gradient of sea ice cover on the eastern shelf of the Antarctic Peninsula (AP) (manuscript 1). The main focus was on carbon and iron fluxes within the sediment and between the water column and the sediment. The results were interpreted in the context of sea ice cover. An increase in carbon remineralization rates was observed as one moved from heavily ice-covered to moderately ice-covered stations where light availability and water column stratification increased. Conversely, the ice-free station displayed lower carbon remineralization rates and was subject to wind-driven mixing of the water column, which can deepen the mixed layer depth below the critical depth, resulting in reduced surface production. In summary, a positive correlation was found between moderate sea ice cover and increased carbon fluxes to the sediment, which followed an exponential increase. The study also revealed significant iron cycling in sediments with increased carbon remineralization, resulting in high dissolved iron fluxes. This finding highlights the importance of sediments underlying the moderate ice cover as a source of limiting nutrients for primary production in this region.

A complementary study of benthic microbial communities along the AP transect was conducted using 16S ribosomal RNA (rRNA) gene sequencing (manuscript 2). The results indicate that sea ice cover and its effect on organic carbon fluxes are the main drivers of changes in benthic microbial communities. As sea ice cover decreases, the benthic microbial community shifts towards anaerobic communities of iron and sulfate reducers. These communities were more abundant at low ice cover stations than at high ice cover stations. Furthermore, an increase in the relative abundance of Sva1033, a Desulfuromonadia clade, with dissolved iron concentration at low ice cover stations suggests a putative role for Sva1033 in dissimilatory iron reduction in surface sediments. In addition to Sva1033, this study successfully identified other taxa that could potentially contribute to dissimilatory iron reduction or have syntrophic partnerships and/or common metabolic preferences with iron reducers.

The focus was extended towards the southern shelf of the Weddell Sea, a region characterized by heavy sea ice cover (manuscript 3). Benthic oxygen uptake rates were measured at stations with different water depths and sediment compositions. Benthic measurements also revealed a dependence of carbon fluxes on sediment grain size and water depth. In general, diffusive oxygen uptake (DOU) rates on the southern Weddell Sea shelf were low. DOU showed a positive correlation with preserved total organic carbon (TOC) at stations with fine-grained sediments, whereas stations with typical of coarse-grained sediments showed a markedly different correlation of DOU with TOC. Common to all stations is that dissolved iron and manganese concentrations in pore water were found only at greater depths, suggesting very limited release of these nutrients back into the water column.

The strong dependence of benthic carbon fluxes on sea ice cover and water depth was then combined to derive a simple empirical model, which was validated by all available DOU measurements reported in the literature for the Antarctic seasonal ice zone (manuscript 4). The model allows extrapolation and budgeting of benthic carbon remineralization for the entire seasonal ice zone (16 million km²), yielding a total of 46 Tg C yr-1. Notably, although the Antarctic continental shelf represents only 15% of the total area, it contributes a significant 71% (33 Tg C yr-1) of the total benthic carbon remineralization. Furthermore, the total organic carbon supply to the sediments and the carbon burial in the sediments were estimated to amount to 52 Tg C yr-1 and 6 Tg C yr-1, respectively.

Overall, the thesis highlights the pivotal role of sea ice cover in controlling the benthic carbon and iron cycling on the Antarctic continental shelf. Through extensive data correlation and empirical modeling, the thesis has provided, for the first time, a quantitative framework for the relationship between sea ice cover and benthic carbon fluxes. It also emphasizes the substantial contribution of Antarctic shelf sediments to the marine carbon remineralization, allowing a better assessment of the carbon cycling and related CO2 sequestration across the Southern Ocean. Furthermore, it has improved our understanding of the main drivers of change in benthic microbial communities and, ultimately, nutrient fluxes across the sediment-water interface. These findings make a significant contribution to our understanding of the complex Antarctic ecosystem, which is necessary to assess the future trajectory of the Southern Ocean and its impact on the global carbon cycle.