Paleo-perspectives for the future circulation and carbon cycle dynamics in the Arctic Ocean

The Arctic Ocean is rapidly shifting to a new state in unforeseen ways. This may have serious implications for Earth's climate, the marine ecosystems, and the management of our ocean's resources. One important example is the shrinking of the Arctic sea ice thickness and extent. A second, relatively overlooked aspect is the ongoing changes in the Arctic marine carbon cycle, including rapid acidification of seawater and changes in the ability of the ocean's to absorb CO2 from the atmosphere. A broader perspective into the past history of our climate allows us to investigate a wider range of climate boundary conditions that provide a geological context to the present-day climate change. In this context, there has been major progress in the use of the elemental and isotopic composition of marine microfossils to reconstruct past oceanographic conditions. However, these methods have been seldom applied to the Arctic Ocean mainly because of the limited availability of microfossils and diagenetic alterations. In this project, we aimed to study the Arctic Ocean circulation-carbon cycle-climate feedbacks during some major past climate events/transitions using the geochemical signals stored in fossil planktic (surface dwellers) and benthic (seabed dwellers) foraminiferal shells. We have critically assessed some previous ocean circulation reconstructions from the Arctic Ocean and the Nordic Seas and highlighted significant caveats that must be avoided in future studies. For example, we have resolved ambiguous issues related to the use of different benthic foraminiferal species in reconstructing deep ocean conditions during the last glacial period. Our refined results enabled us to identify the role of the Arctic Ocean and northern Nordic Seas in the climate and carbon cycle evolution during the last glacial, a period that was characterized by abrupt regional climate changes associated with rapid changes (at centennial scale) in atmospheric pCO2 and gradual global climate changes associated with gradual (at multi-millennial scale) changes in atmospheric pCO2. The project results have revealed persistent water exchange between the Arctic Ocean, the Nordic Seas and the North Atlantic Ocean during the last glacial and that the Arctic Ocean was not isolated from the global ocean during the last glacial period, as previously suggested. Our results reveal that short-term changes in the CO2 storage and release in/from the deep Arctic Ocean and the Nordic Seas were likely contributors to the small, but abrupt, changes in atmospheric pCO2 during the last glacial period. In addition, our carbon cycle modelling results suggest a possible contribution from terrestrial Arctic carbon reservoirs in some of the abrupt changes in atmospheric pCO2 during the last glacial, in particular ~23,500 years ago. Furthermore, deep ocean storage/release of heat in this region was fundamental in setting regional abrupt climate changes during the last glacial as well as the glacial-interglacial global changes. We also identified significant latitudinal shifts in the deepwater formation sites within the Nordic Seas during the last glacial period. These new findings identify the role of the Arctic Ocean and the Nordic Seas in the climate and carbon cycle evolution regionally and globally during the last glacial period and provide invaluable datasets to constrain climate models. In addition, new results (manuscript in prep.) of the investigation of changes in sea-ice extent and sea surface temperatures in the Nordic Seas during the last interglacial period (~130,000-115,000 years ago before present, a semi-analogue for our warming future) shows significantly enhanced oceanic heat flow to the Nordic Seas, confirming the amplified effects of climate change at high latitudes. Furthermore, we have discovered a new mussel species that lived in the deep Arctic Ocean during the time period between 19,000 and 15,000 years before present. This newly discovered species, Acharax svalbardensis, no longer lives today and its occurrence in the past could be linked to warmer-than today deep ocean temperatures and intensive methane gas emissions at the seafloor in the Arctic Ocean. This finding is an example that showcases the impact of climate on the marine ecosystem. Finally, this project has substantially enhanced the cooperation between the host institution in the UK (University of Cambridge) and the home institution in Norway (UiT the Arctic University) e.g., through master and PhD student research visits from UiT to Cambridge and setting new collaborative and long-lasting projects between the paleoclimate research groups in both institutions. More generally, the project has enabled the project leader to establish a wide international collaboration network that has been instrumental for the project leader to secure additional substantial funding to establish a new research group and new infrastructure at UiT the Arctic University of Norway.

- The project findings enabled us to reconstruct important aspects of ocean circulation and carbon cycle feedbacks in the Arctic Ocean and the Nordic Seas and their impacts on climate. For example, our results showed that regional changes in the deep ocean storage of heat and CO2 were contributors to the abrupt changes in regional climate, sea ice extent and atmospheric pCO2 during the last glacial period. We also documented an enhanced oceanic heat flow to the Nordic Seas and diminished sea ice extent during the last interglacial period (a semi-analogue for our warming future), confirming the amplified effects of climate change at high latitudes. The project results therefore provide invaluable datasets to constrain climate models. - The project has enabled the project leader to establish a wide international collaboration network that has been instrumental to secure additional substantial funding to establish a new research group and infrastructure at UiT.

The Arctic Ocean is shifting to a new state in unexpected ways with serious implications for earth's climate, resource managements, and the marine ecosystems. One important example is the ongoing shrinkage of the Arctic sea ice thickness and extent, which is by far more rapid than predicted by climate models. A second, relatively overlooked aspect is the ongoing changes in the Arctic marine carbon cycle, including rapid acidification of seawater (i.e., decrease of seawater pH) and changes in the CO2 exchange with the atmosphere. A broader paleoclimatic perspective allows a wide range of boundary conditions to be investigated, which has the potential to improve our understanding of the interplay and feedback processes between climate, Arctic hydrography and the carbon cycle. In this context, there has been major progress in the use of the elemental and isotopic composition of calcium carbonate parts of foraminifera, which are unicellular organisms, to reconstruct past oceanographic conditions. However, these methods have been seldom applied to the Arctic Ocean mainly because of the limited availability of foraminifera and diagenetic alterations. To overcome these complications, we will use novel micro-analytical techniques that are capable of measuring highly resolved elemental- and isotopic compositions of single foraminifera. Doing so, our project represents the first effort to quantify the Arctic's carbon cycle-climate feedbacks beyond the observational records by combining proxies for Arctic Ocean hydrography (e.g., temperature, salinity and sea ice cover), primary productivity and seawater carbonate chemistry (e.g., pH and concentration of CO2) for most of the past two million years. Ultimately, we expect that the results will decipher the role of the Arctic Ocean as a regulator of the past changes in climate and atmospheric CO2; knowledge that will help improving our ability to project future changes in the Arctic Ocean climate system and their consequences.