Planktic foraminiferal production stimulated by chlorophyll redistribution and entrainment of nutrients |
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| Institution: | 1. Department of Earth and Planetary Sciences, University of California Davis, Davis, CA 95616, USA;2. Research School of Earth Sciences, Australian National University, 142 Mills Rd, Acton, ACT 2601, Australia;3. College of Earth, Ocean and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, USA;4. Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA;5. ARC Centre of Excellence for Coral Reef Studies, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia;1. Department of Geology, Banaras Hindu University, Varanasi 221005, India;2. DST-Mahamana Centre of Excellence in Climate Change Research, Institute of Environmental and Sustainable Development, Banaras Hindu University, Varanasi 221005, India;3. Formerly at CSIR-National Institute of Oceanography, Dona Paula, Goa 403004, India;1. Center of Advanced Study in Geology, Banaras Hindu University, Varanasi, U. P.-221005, India;2. Instituto Portugues do Mar e Atmosfera (IPMA), Rua Alfredo Magalhães Ramalho, 6, 1449-006, Lisboa, Portugal;3. Centro de Ci?ncias do Mar (CCMAR), Universidade do Algarve, Campus de Gambelas, 8005-139, Faro, Portugal;4. Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing, PR China |
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| Abstract: | During September and October 1996 planktic foraminifers and pteropods were sampled from the upper 2500 m of the water column in the BIOTRANS area (47°N, 20°W), eastern North Atlantic, as part of the JGOFS program. Hydrography, chlorophyll fluorescence, and nutrient content were recorded at high spatial and temporal resolution providing detailed information about the transition time between summer and fall. At the beginning of the cruise a shallow pycnocline was present and oligotrophic conditions prevailed. Over the course of the cruise, the mixed layer depth increased and surface water temperature decreased by 1.5°C. Both chlorophyll-a dispersed in the upper 50 m by vertical mixing and chlorophyll-a concentrations at the sea surface increased. The nitracline shoaled and nutrient enriched waters were entrained into the mixed layer. Planktic foraminifers and pteropods closely reflected the changes in the hydrography by increased growth rates and changes in species composition. Three main groups of planktic foraminiferal species were recognized: (1) a temperate and low-productivity group dominated by Neogloboquadrina incompta characterized the shallow mixed layer depths. (2) A temperate and high-productivity group dominated by Globigerina bulloides characterized the period with wind-induced dispersal of chlorophyll-a and entrainment of nutrient-enriched waters. (3) A warm water group containing Globigerinoides sacculifer, Orbulina universa, Globigerinoides ruber (white), and Globigerinella siphonifera was most common during the first days of sampling. Synchronous with the hydrographic change from summer to fall, planktic foraminiferal and pteropod growth was stimulated by redistribution of chlorophyll-a and entrainment of nutrient-enriched waters into the mixed layer. In addition, the seasonal change in the eastern North Atlantic resulted in a transition of the epipelagic faunal composition and an increased calcareous particle flux, which could be used to trace seasonality in fossil assemblages and allow for better paleoceanographic interpretation of the boreal Atlantic. |
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