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Indian-Atlantic interocean exchange as modulator of global climate

Periodic Reporting for period 1 - INDEXCLIMA (Indian-Atlantic interocean exchange as modulator of global climate)

Reporting period: 2019-10-01 to 2021-09-30

The INDEXCLIMA project (Indian-Atlantic interocean exchange as modulator of global climate) investigated the relationship between Atlantic deep water entering the Indian Ocean and Indian surface water flowing into the Atlantic Ocean over the last 630,000 years. The warm and salty surface Indian waters are transported into the Atlantic Ocean through the so-called Agulhas rings. These dense rings may contribute to maintain the strength of global thermohaline circulation, including the Gulf Stream that keeps a temperate climate in Europe. The circulation state affects the ocean capacity to sequestrate atmospheric CO2. The study period covers different glacials-interglacials characterised by distinct climate conditions. Even if the project results will attract attention of mainly scientists working on Earth and Climate Sciences, the carbon cycle and climate regulation by a large-scale oceanic circulation are of great interest of the society. In short, the project contributes to better understanding the interaction between Ocean and Climate by a unique approach reconstructing both surface and deep water variabilities from the same archives using geochemical proxies with microfossil assemblage.
The overall objectives of the project were:
(1) To reconstruct how the Atlantic deep water and southern-sourced deep water flowing into the Indian Ocean varied during the last 630,000 years, and evaluate how deep-water carbon storage varied with time and space.
(2) To analyse the relationship of Indian-Atlantic interocean exchange between surface and deep waters in order to evaluate the impact of the Indian surface waters entering Atlantic Ocean via the Agulhas rings to the Atlantic meridional overturning circulation under different climate conditions.
The work performed during the project was divided into 4 Work Packages (WP), including the project’s conclusions, the overview of the results and their exploitation and dissemination.
WP 1: Application of a new automated prototype machine to evaluate the preservation state of biogenic calcite
The fellow performed the analysis of the preservation of planktic foraminiferal shells distinguishing fragments of shells from complete shells using a prototype machine. This device is able to automatically recognise fragments and whole shells thanks to multiple images processed by artificial intelligence. Higher abundance of fragments, indicating possible dissolution of carbonate shells, occurred during warm interglacial periods. In addition, the fellow assessed visually the preservation of foraminiferal shells using a binocular magnifier. This helped to select only well-preserved foraminifera for chemical analyses. The fellow attended virtually a conference to present the first project results.

WP 2: Reconstruction of the Indian surface waters entering the Atlantic Ocean
For reconstructing the variation of Indian surface waters flowing into the Atlantic Ocean, a group of 5 species of planktic foraminifera were used. These species inhabit warm surface waters in the Indian Ocean and are found in Agulhas rings in the southeastern (SE) Atlantic Ocean. This group of species increased at the end of glacial periods, when Agulhas rings were estimated to be more abundant. Furthermore, the planktic foraminiferal species Globoconella inflata was used as proxy for temperate waters. Decreases of this species at the end of glacial periods indicated lower arrival of temperate waters from below 42°S. Based on these data, the most interesting time intervals over the last 630,000 years were selected.

WP 3: Atlantic deep-water circulation flowing into the Indian Ocean
To assess changes in Atlantic deep-water masses entering the southwestern (SW) Indian Ocean, Neodymium (Nd) isotopic composition and carbon isotopic composition were analysed. Nd isotopic composition is a tracer of water mass provenance while benthic foraminiferal carbon isotopic ratio is a sensitive tracer to water ventilation. Combination of the two geochemical tools revealed (i) decreases in northern-source Atlantic water contribution to the deep Indian water during glacial periods and (ii) storage of respired carbon that was enhanced in glacial waters below 2700 m. The new results suggest spatial extension deep-water with high carbon content to the SW Indian Ocean at least as far as 33ºS. The fellow attended an online international conference to present the final project results.

WP 4: Integration of project results to reconstruct influence of surface and deep Indian-Atlantic water mass exchange on global ocean circulation and climate
All project results were integrated and compared with available results to reconstruct spatiotemporal changes in surface and deep Indian-Atlantic water mass exchange. Two scientific articles will be submitted and published as open access articles with all data will be accessible (e.g. Pangaea data depository). The first article analyses the CO2 storage capacity of glacial Atlantic deep waters entering the SW Indian Ocean. The second article investigates the Indian-Atlantic subsurface- and deep-water mass exchange at the end of glacial periods. These two articles include the exploitable project results: planktic foraminiferal warm species and G. inflata abundances, fragmentation index data, foraminiferal Nd isotopic data, and benthic foraminiferal carbon isotope data.
The fellow also participated in an outreach activity (Science Festival) for Primary School students in France. He also supervised several students at the host institution, helping them to identify foraminiferal species. He performed scientific and administrative tasks related to project management, including budget expenses and organisation of meetings with supervisor and collaborators.
The completion of the INDEXCLIMA project allowed obtaining novel results from both surface and deep-water Indian-Atlantic water-mass exchange in the same cores for the first time. This reduced the uncertainty of age between the surface and deep-water records. These results helped to better understand vertical and horizontal extension, and temporal variation of CO2-rich Atlantic deep waters entering the Indian Ocean. Increased capacity of atmospheric CO2 sequestration was inferred for glacial periods. The project results were also important to know natural mechanisms governing surface and deep-water Indian-Atlantic water-mass exchange and its relationship with global ocean circulation and climate. A better knowledge of Ocean and Climate interactions is essential and the new data will be used for evaluating the performance of models to project future climate changes. Also, this project results might serve to raise awareness of young generation through teaching and outreach activities.
Location of studied cores and previously published cores, and oceanography