Calcareous Nannofossils and their Relationship with Climate Variability
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2022-05-04
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Abstract
Calcareous nannofossils are what remains of calcareous phytoplankton after they die and find their way to the ocean floor. While these organisms are alive, they record changes in surface temperature of the ocean. For instance, Gephyrocapsa oceanica is strongly linked to warm ocean waters, whereas cooler waters are favorable for Gephyrocapsa muellerae. Calcareous nannofossils can also provide information about the availability of nutrients in surface waters. For example, Gephyrocapsa spp. (<3 µm) prefer higher nutrient availability. Conversely, Florisphaera profunda, a species that lives in the lower photic zone, is only abundant when nutrients are available deep in the water column, indicating stratified waters and decreased nutrients in the upper photic zone due to an absence of upwelling occurring in the region. Thus, nannofossil assemblages can be used to document how the climate has changed through Earth’s history. This study examined nannofossil assemblages in samples collected from piston core MD13-3504, obtained by the R/V Marion Defresne during research cruise MD13 to the Mozambique Channel. The Mozambique Channel is located between the island of Madagascar and Mozambique in East Africa and serves as a path for warm water from the equatorial Indian Ocean to enter the Southwestern Indian Ocean via the Agulhas Current. The Agulhas Current also injects warm, saline water into the South Atlantic Ocean via the Agulhas leakage, thereby influencing Atlantic Meridional Overturning Circulation. Calcareous nannofossil biostratigraphy using the appearance of Emiliania huxleyi and disappearance of Pseudoemiliania lacunosa indicates that the sediment core is dated to the middle to late Pleistocene, with the oldest sample examined >430 ka in age. Sea-surface temperatures were generally warm through this time interaval as the nannofossil assemblages are dominated by warm-water taxa such as Gephyrocapsa oceanica, Rhabdosphaera spp. and Helicosphaera spp. Cold-water species such as Coccolithus pelagicus and Gephyrocapsa muellerae appear for brief time periods throughout the studied interval. Surface water stratification increased through time, especially in the interval younger than 290 ka, based on high abundances of Florisphaera profunda and low N ratio values. These results suggest warmer and more oligotrophic waters dominated in the region over the last ~200 kyr.
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Calcareous Nannoplankton, Calcareous Nannofossils, Mozambique Channel, Pleistocene, Agulhas Current, Subtropical Front, Climate