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Paleo-nutriet dynamics in the Eurasian Arctic Ocean

Periodic Reporting for period 1 - PaNDA (Paleo-nutriet dynamics in the Eurasian Arctic Ocean)

Reporting period: 2016-09-01 to 2018-08-31

The project Paleo-Nutrient Dynamics in the Eurasian Arctic Ocean (PaNDA) aimed at understanding the cycling of biologically critical chemical elements (specifically nutrients required by primary producers) at the seafloor and in the sediments of the Arctic Ocean north of Svalbard, both at present and during intervals of past global warmth. As the Arctic Ocean is one of the fastest warming regions on the planet, understanding how its carbon cycle functions - including the uptake of atmospheric carbon dioxide by primary producers and its burial into the underlying sediments - is of critical importance for predicting whether carbon burial in an Arctic Ocean covered by less and less sea ice might serve as a negative feedback mechanism to counteract global warming. To address these issues, sediment cores and extracted pore waters from north of Svalbard, recovered during the 2015 TRANSSIZ expedition on board the German icebreaker Polarstern, were investigated using a range of chemical methods to understand modern as well as past nutrient cycling and burial. The main objectives were to understand (a) whether the modern Arctic seafloor acts as a source or a sink of nutrients to the overlying water column, (c) if nutrient and carbon cycling during periods of past global warmth (i.e. the last interglacial, ~125,000 years before present) functioned significantly different from today, and (c) if these processes were dependent on the relative location within the Arctic Ocean in terms of water depth, proximity to land, or sea ice cover.
Hundreds of sediment and pore water samples were analysed in the context of this study. The sediment samples contain a record of nutrient and carbon cycling through time, while the pore waters inform us about the current cycling of nutrients and carbon within the seafloor (driven by microbial communities living in the sediment) and the potential recycling of bioavailable nutrients back to the water column.
Analyses included bulk chemical analyses of the sediments as well as specific extraction methods to quantify different chemical species of the nutrients nitrogen, phosphorus and iron in the sediment (their different speciation being related to their input process into the ocean in the first place, but also changes that occurred after they were buried into the seafloor). Pore waters were analysed for dissolved nutrients, metals, and other elements that are involved in sedimentary carbon cycling.
From these data, the depositional environment of sediments back to and during the last interglacial was reconstructed (in terms of organic matter export to the seafloor, sea ice cover, and in particular the availability of different nutrient species), and modern-day fluxes of nutrients back to the water column were calculated.
The main findings from this project so far are the following:
Since the last interglacial, several periods were identified when substantial amounts of bioavailable iron (an important micronutrient) was delivered to the study site north of Svalbard, which is the first time that significant and sustained amounts of iron oxides were reported that far north of an Arctic landmass. This has important implications for the potential to deliver nutrients from land to the open Arctic Ocean during what we assume were meltwater events on Svalbard associated with globally recognised sub-millennial warming events. The exciting results are currently being written up for publication in a peer-reviewed international journal.
Regarding nutrient cycling at the modern seafloor, we recognise that the sediments at all sites are important sources of silicic acid and ammonium to the bottom waters, with shallower sites close to Svalbard being more active than both deep and shallow water sites on the more distal Yermak Plateau. Phosphate and iron, in contrast, are fairly efficiently retained in the sediment due to iron oxides precipitating at the oxic seafloor, and phosphate adsorption onto these newly forming iron oxides. These data are currently being written up for publication in a peer-reviewed international journal.
This project is the first one to perform a high resolution pore water study in the part of the Arctic Ocean that is an important gateway to the Atlantic Ocean, and is the first to show how certain nutrients are recycled into the water column, while others are efficiently retained. This will have implications on the development of phytoplankton communities growing in these parts of the Arctic Ocean, as well as on the export of nutrients out of the Arctic basin.
While iron oxide layers were noted in marine sediment reaching back to the last interglacial close the Svalbard already, our study is the first to indicate that these potential fertilisation events can spread much further from the source on land than initially assumed. As glaciers and ice sheets around the world are melting at ever faster rates, our study contributes important new information about how far bioavailable iron can be shuttled across the open ocean and potentially increase phytoplankton blooms in these pelagic areas.
German research icebreaker FS Polarstern during TRANSSIZ expedition