Primary productivity and ocean circulation changes on orbital and millennial timescales off Northwest Africa during the last glacial/interglacial cycle: Evidence from benthic foraminiferal assemblages, stable carbon and oxygen isotopes and Mg/Ca paleothermometry

Abstract

This PhD thesis examines climate-induced changes in the atmospheric circulation off and over southern Northwest Africa and its effect on marine primary productivity and sea surface temperature conditions during the Late Pleistocene. For this purpose two sediment cores from the continental slope off Northwest Africa at 12°N (GeoB9526-5 and GeoB9527-5) are investigated for benthic foraminiferal assemblages, stable oxygen and carbon isotopic compositions of benthic and planktic foraminiferal tests, Mg/Ca paleothermometry and sedimentary geochemical parameters. Additionally, Mg/Ca data obtained from sediment core GeoB95208-5 located at 15°N are included. The results of benthic foraminiferal assemblage (GeoB9526-5) and geochemical analyses (GeoB9526-5 and GeoB9527-5) reveal that high-latitude cold events as well as variations in low-latitude summer insolation had an impact on humidity, wind conditions and the location of the tropical rainbelt during the last 31 kyr, which in turn resulted in changes in the intensity and seasonality of primary productivity off southern Northwest Africa. High accumulation rates of benthic foraminifera, carbonate and organic carbon as well as the dominance of infaunal species and high numbers of deep infaunal species indicate high primary productivity and enhanced supply of refractory organic matter from the upper slope and shelf during melt water events (Heinrich stadial 1 and 2). A more southern tropical rainbelt as well as intensified Northeast trade winds are suggested to have provoked almost permanent upwelling. A phytodetritus-related benthic fauna indicates seasonally pulsed input of labile organic matter and low year-round primary productivity during the Last Glacial Maximum. Compared to Heinrich stadials, a more northward expanded rainbelt and lower Northeast trade wind intensity resulted in seasonal and weak upwelling. High productivity with seasonally high input of labile organic matter is supposed for the orbitally forced African Humid Period. An intensified African monsoon and the northernmost summer position of the tropical rainbelt resulted in enhanced river discharge stimulating intense phytoplankton blooms. During the last 4000 years strong carbonate dissolution likely caused by enhanced organic matter flux is associated with increasing aridity and stronger Northeast trade winds inducing intensive, seasonal coastal upwelling. The comparison of the Cibicidoides wuellerstorfi d13C record (GeoB9526-5) with d13C records from the eastern Atlantic between 45°N and 25°S (same water mass) provides evidence for productivity related overprints in the d13C signal off Northwest Africa at 12°N. During times of seasonal deposition of fresh phytodetritus, d13C values at site GeoB9526-5 were significantly lower than at reference sites. Only during Heinrich stadials 1 and 2, when the area off Northwest Africa at 12°N experienced year-round organic matter fluxes, d13C values at site GeoB9526-5 faithfully record deep-water ventilation. The Mg/Ca based sea surface temperature (SST) records from core sites GeoB9526-5 (12°N) and GeoB9508-5 (15°N) exhibit SST anomalies representing the thermal bipolar seesaw pattern in the tropical Northwest Atlantic during glacial periods of reduced Atlantic Meridional Overturning circulation (AMOC) associated with Heinrich stadials H1 to H6. The comparison of two SST records indicate that the SST zero-anomaly line, which separates the cooling in the Northern Hemisphere and warming in the Southern Hemisphere, was not at the same latitudinal position during all Heinrich stadials. The SST proxy data in combination with freshwater-hosing model results suggest that these variations of the tropical seesaw SST pattern were related to a long-term movement of the summer Intertropical Convergence Zone (ITCZ). Orbital forced changes of the meridional temperature gradient as well as the expansion of the Northern Hemisphere glaciation are supposed to drive this long-term movement of the ITCZ, which also affected the rainfall over West Africa.