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Final Report Summary - OCEANQUANT (Quantification of Past Ocean Circulation)

Project objectives
In order to predict the climatic evolution it is essential to disentangle the relevant, often interwoven, driving processes of the climate system. One of the most promising approaches is to investigate the climate system under of different boundary conditions of the past. The last glacial-interglacial cycle offers a wide array of natural experiments during which the climate changed, sometimes abruptly, thereby allowing deciphering the triggers and feedback mechanisms, which affected climate variability on various time scales. In particular, the reconstruction of past changes in ocean circulation has been identified as one of the most pressing tasks in paleoclimatology. The importance of ocean circulation and associated inter-hemispheric heat transport in the climate system is paramount (the probably best known example for the impact of ocean-circulation on climate is the Gulfstream system). This project has focused on investigating the strength of past ocean circulation, as gleaned from the testimony of deep sea sediments. These records extend beyond the noise of recent decadal oscillations and provide an independent perspective on the links between climate and ocean dynamics. Reconstructing the dynamics of a long-gone past ocean circulation patterns seems to be an unrealistic task at first glance. But nature has provided Earth scientists with a difficult to access - but unique - system of natural isotopes (231Pa/230Th), which allows quantitative reconstructions of past ocean circulation from marine sediments. Low ratios of 231Pa/230Th in the sediments indicate stronger advection of water masses above the seafloor, due to the more mobile behaviour of 231Pa compared to 230Th. In contrast high 231Pa/230Th ratios are found when circulation was stagnant or sluggish. In addition, new developments in the 14C-dating technique allow the applicability of radiocarbon to estimate past changes in ocean ventilation in areas and time periods formerly excluded from the scope of this proxy. The scientific goal of this project was to provide quantitative estimates of past ocean circulation based on 231Pa/230Th supported by 14C measurements and complemented by the applications of semi-complex to complex models.

Work performed
During the two years of the project a huge measurement program was performed: After setting up the lab in Bern a very effective measurement line for determining U-, Th-, and Pa- isotope concentrations has been established. The new sedimentary 231Pa/230Th data set from the Pacific and Atlantic Ocean will crucially improve our understanding of past ocean circulation and is unprecedented in its spatial coverage for the last 30,000 years. Further experimental efforts aimed at combining 231Pa/230Th and 14C measurements from identical sample material. This has been accomplished for a number of sediment cores with emphasis on the Pacific. On important aspect of the results obtained by measuring 231Pa/230Th are the as-well obtained “side-products” of Uranium- and Thorium-isotopes. These isotopes hold information on e.g. paleo-dust, particle-flux and redox situations in the sediment-water interface. Quite a number of publications dealing with these questions have been supported by the new data made available through this project. Besides performing measurements of the radioactive circulation proxies 231Pa/230Th and 14C, I also gained results from measuring Nd isotopes and opal concentrations, which provide complementary information on water mass provenance and biological productivity.
Work on the project not only comprised measurements on deep sea sediments but also included model work. Two different levels of complexity have been applied. First, a 2D box model was used to better identify the patterns of 231Pa/230Th for various circulation modes and locations. In a second step the 231Pa/230Th data base has been used to disentangle the effects of particle scavenging and circulation on both radionuclides. Using the Bern3D ocean model, a model of intermediate complexity with a resolution of 36x36 grid cells and 32 depth layers, the validity of sedimentary 231Pa/230Th in order to report ocean circulation has been constrained.

Main results and forthcoming expected results
Leading a co-operation with scientists from Heidelberg, Kiel and Zurich, we found that the circulation of the Atlantic Ocean in the past was more stable than once thought. We analysed the circulation of the deep waters of the North Atlantic over the past 140,000 years and were able to demonstrate that the current “warm” mode of the Atlantic’s circulation played a key role even during the earth’s last cold period, contrary to prior assumptions. In the warm mode, water masses close to the surface at the northern latitudes sink down and then form the deep North Atlantic waters flowing south. In phases in which the circulation is in ‘cold’ mode, the water masses in the north reach a lesser depth. At the same time, Antarctic bottom waters from the south penetrate further north. Contrary to previous assumptions, we found that the warm mode predominated not only during warm cycles, but even during the last cold cycle. Only at the glacial peaks of the last two cold periods the circulation pattern of the cold mode were observed. Only when the ice sheets had fully extended during the cold period peaks did such fresh water intrusions cause the deep water accumulation in the North Atlantic to collapse over several hundred years, resulting in a further drop in average temperatures in the northern hemisphere. This suggests that significant deviations from a deep and active Northern Overturning cell were restricted to peak glacial conditions, which were very different from today’s or greenhouse climate. The risk of a significant slow-down of the Atlantic Meridional Overturning Circulation with all its deleterious consequences is thus considered to be rather unlikely. Because the implications of these results are quite far reaching, affecting future predictions of changes in ocean circulation and therewith future climate under global warming. The study was published in “Nature”:
Other main results cover e.g. the quantitative estimate of the ocean circulation in the Arctic Ocean. By a combined approach of modelling and measuring 231Pa and 230Th the behaviour of both radionuclides in this almost enclosed marine system has been better understood and little differences between glacial and interglacial circulation strength has been found. Further, based on a comprehensive mapping of circulation strength and deep-water provenance from several Atlantic Ocean sediment cores the evolution of water mass distribution has been reconstructed back to the Last Glacial Maximum. Increased influence of Southern Sourced Waters and northward advection of deep waters during the peak glacial and deglaciation has been found, but also evidence for an active but shallower northern overturning cell during these times. Another published main result was the role of ocean ventilation in triggering CO2 releases and deglacial Ocean circulation in the Atlantic.
One main preliminary result, which still needs some more modelling efforts, affects the role of 231Pa/230Th as a circulation proxy in the Pacific. While 231Pa/230Th and 14C revealed a surprisingly strong signal of weaker circulation (231Pa/230Th) and older deep water (14C) during the last Glacial in the Southern Pacific, and according signal is missing in the North so far. The hypothesis about glacial deep water formation in the North Pacific has not been solved, but it can be expected, that the finalized new data-model approach from this project will bring strong evidence.
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