One century of intermediate water masses temperature variability of the West Indian Ocean reconstructed by Li/Mg-thermometer in scleractinian cold-water corals

As a link between the surface waters and the deep ocean, intermediate water masses play a key role in transmitting atmospheric variations into the deep sea. The paleo-oceanographic reconstructions of intermediate water mass variability are therefore essential to comprehend the pace and extent with which shallow marine changes are transferred into ocean basins. Cold water corals (CWC) thriving in intermediate water depths have been identified as an adequate geochemical proxy archive to reconstructed temporal changes in water mass properties, due to the sustained growth of their carbonate skeleton and their long lifespan (several hundred years).

Two living CWC colonies of Enallopsammia rostrata (Pourtalès, 1878) have been collected in the northern part of the Mozambique Channel around the island of Mayotte, during the research cruise SO306 with the RV Sonne in August 2024. The corals were collected with a ROV from water depths between 600 to 900 meters within the transition zone of South Indian Central Water (SICW) and the underlying Red Sea Water (RSW). The chemical composition (Ca, Li, Mg) of different branches from each colony was analysed using line scan laser ablation inductively coupled mass spectrometry (LA-ICP-MS). U/Th dating enables the determination of ages and calculation of growth rates for the individual colonial parts.

The sclerochronological aligning of the geochemical data along the U/Th-based growth rates enabled a reconstruction of Li/Mg_coral variations until the end of the penultimate century. Pronounced cyclic variabilities in ranges of duration from years to decades could be identified within the Li/Mg-records, due to the spatially high-resolution LA-ICP-MS measurements. However, a significant trend in Li/Mg_coral, that would indicate a continuous change of water temperatures, could not be identified within the two colony records over the reconstructed time period. Water temperatures derived from mean Li/Mg_coral by employing Li/Mg-temperature calibration (Montagna et al. 2014) match well with observed water temperature values between of 6.6°C and 9.4°C, respectively. The reconstructed temperature variability for both colonies show variations on an average range of 3°C (2SD) over multi-year intervals, which can be attribute to a changing extent of influence of the differently temperate water masses around Mayotte.

Our reconstruction shows no long-term temperature increase in the intermediate water masses of the West Indian Ocean during the last century contrary to the anthropogenic warming of the atmosphere and surface ocean. The found temperature variability, however, points to a dynamic and periodic shifting of the different water masses, which suggests a more lateral exchange within intermediate water depths in the northern entry area of the Mozambique Channel.

 

 

• Montagna, M. McCulloch, E. Douville, M. L. Correa, J. Trotter, R. Rodolfo-Metalpa, D. Dissard, C. Ferrier-Pagès, N. Frank, A. Freiwald, S. Goldstein, C. Mazzoli, S. Reynaud, A. Rüggeberg, S. Russo, M. Taviani (2014): Li/Mg systematics in scleractinian corals: Calibration of the thermometer. Geochim. Cosmochim. Acta 132, 288–310