Research paperThe tropical rainbelt and productivity changes off northwest Africa: A 31,000-year high-resolution record
Introduction
Variations in primary productivity affect the atmospheric CO2 concentration and play an important role in the marine biogeochemical cycle of organic carbon and carbonate. The documentation of past changes in marine productivity provides fundamental information for the reconstruction of global climate. Today, an extended oceanic upwelling system causes high primary production off Northwest Africa. Permanent, year-round upwelling prevails between 20° and 25°N. South of 20°N, modern primary production is highly seasonal due to variations in the direction and strength of NE trade winds, which force upwelling and fertilization through dust influx, and the intensity of the African monsoon, which drives fluvial nutrient input by the Gambia and Casamance Rivers (Fig. 1) (Schemainda et al., 1975, Lesack et al., 1984, Debenay et al., 1989, Debenay et al., 1994). Insolation determines intensity and migration of the trade winds and thus the position of the surface low pressure convergence zone, the Intertropical Convergence Zone (ITCZ), varying between about 20°N and 5°N (Fig. 1). Coastal upwelling occurs south of Cape Verde (14°53′N) only during winter, when the NE trade wind belt is in its southernmost position and the ITCZ is situated around 5°N. Most of the upwelling activity is concentrated above the 45–55 km wide shelf region and the upper continental slope (Mittelstaedt, 1991). However, upwelling filaments, caused by convergence of the northward flowing North Equatorial Countercurrent (NECC) and the southward flowing Canary Current (CC), carry nutrient-rich subsurface water and thereby stimulate phytoplankton growth several hundred kilometers offshore in the open ocean (Van Camp et al., 1991, Helmke et al., 2005). Beside the vertical organic matter flux, especially in ocean margin settings, lateral advection is a major factor responsible for the transport of particulate organic carbon to the ocean floor (Jorissen et al., 1998, Jorissen & Wittling, 1999, Karakas et al., 2006). In addition, turbulent mixing through intense and highly variable trade winds and currents, as prevailing in modern, year-round upwelling off Cape Blanc (21°N), induce lateral advection of re-suspended material, including refractory organic matter, from the shelf and the upper slope transported offshore to the lower slope and bathyal (Karakas et al., 2006).
Recent studies show that the position of the ITCZ over West Africa is only loosely linked to the latitudinal zone of precipitation known as the tropical rainbelt (Nicholson, 2009). Maximum rainfall is generated by a large core of ascending humid air between the mid-tropospheric African easterly jet (AEJ) and the upper-tropospheric tropical easterly jet (TEJ) (Fig. 2). Strength and latitudinal position of the AEJ mainly determine the intensity and latitudinal expansion of moisture uplift. Seasonal rainfall in West Africa is a function of the intensity, width and latitude of the rainbelt (Nicholson, 2008). The southward motion of the tropical rainbelt is accompanied by a southward expansion of dry conditions on the West African continent and enhanced eolian dust transport until far in the North Atlantic Ocean. The dust mobilized in the Sahara and Sahel is carried by continental trade winds (Fig. 1) (Swap et al., 1996, Stuut et al., 2005), and the easterly mid-tropospheric Saharan Air Layer (SAL) (Sarnthein et al., 1981), prevailing at altitudes between 1500 and 6000 m. Saharan dust contains minerals and elements, which stimulate phytoplankton growth (Goudie & Middleton, 2001, Eglinton et al., 2002). The fertilizing effect of dust on primary productivity is well established (e.g. Boyd et al., 2000, Bowie et al., 2001, Boyd et al., 2004). During boreal summer, the tropical rain belt is in its northernmost position, brings rain far into the Sahel region and initiates increased fluvial input of suspended and dissolved nutrients by rivers into the Atlantic Ocean (Fig. 1) (Kutzbach, 1981, Kattan et al., 1987, deMenocal & Rind, 1993).
Early studies suggested that cooling during the LGM increased the aridity and enhanced the intensity of NE trade winds on the NW African continent (Sarnthein et al., 1981, Hooghiemstra et al., 1987, Tiedemann et al., 1989). In offshore environments, this resulted in stronger upwelling and higher productivity compared to the Holocene (Sarnthein et al., 1988). More recently, however, it was shown that during the LGM latitudinal differences in upwelling and wind conditions induced more complex and small-scale productivity variability along the NW African margin (Bertrand et al., 1996, Martinez et al., 1999, Abrantes, 2000, Sicre et al., 2000, Ternois et al., 2000, Zhao et al., 2000, Sirce et al., 2001, Henderiks & Bollmann, 2004, Eberwein & Mackensen, 2006, Zhao et al., 2006, Eberwein & Mackensen, 2008, Romero et al., 2008).
Rapid high-latitude climate changes, such as Dansgaard–Oeschger and Heinrich events (Heinrich, 1998, Dansgaard et al., 1993), and precession-forced changes in low latitude summer insolation are supposed to be the main factors influencing past latitudinal changes of the ITCZ and humidity variations driven by the African monsoon (Schefuβ et al., 2003). It is believed that massive discharges of icebergs and the resulting melt water input to the North Atlantic during Heinrich events triggered perturbations of the Atlantic meridional overturning circulation (AMOC) (Bond et al., 1993, Keigwin & Lehman, 1994). A slow-down of the AMOC is then supposed to have reduced heat transport from the tropics to the northern North Atlantic (Charles et al., 1996, Rühlemann et al., 1999a, Vidal et al., 1999). Since the impacts of Heinrich events have been documented at low latitudes as well, a signal transport from northern high latitudes by either hydrological means or atmospheric teleconnections must have taken place (e.g. Schulz et al., 1998, Wang et al., 2001, Broecker, 2003, Rohling et al., 2003, Tjallingii et al., 2008).
Abrupt millennial-scale dry events in the tropics and subtropics of Africa and enhanced offshore transport of Saharian dust were associated with low North Atlantic sea surface temperatures during the Heinrich melt water events. A more southerly position of the ITCZ and the tropical rainbelt, as well as an intensification of trade wind belts, may have been the driving forces (Street-Perrott & Perrott, 1990, Adegbie et al., 2003, Zhao et al., 2006, Jullien et al., 2007, Mulitza et al., 2008, Itambi et al., 2009). However, orbitally forced long-term variation of monsoonal strength caused changes, from increased vegetation cover and expanded lakes during the early Holocene African Humid Period (AHP) to more arid present-day conditions (Jolly et al., 1998, deMenocal et al., 2000, Gasse, 2000, Lezine et al., 2005). Thus seasonality and intensity of river discharge forced by monsoonal precipitation, as well as upwelling have been affected by the relocation of the ITCZ and trade wind belts off Senegal/Guinea Bissau. Hence, the continental margin of southern NW Africa is well suited for a study of the interaction between high-latitude climate fluctuation and low-latitude ocean productivity.
To monitor the influence of climate variability on surface ocean productivity, we determined the elemental composition of marine sediments by x-ray fluorescence (XRF) scanning (e.g. Arz et al., 1998, Tjallingii et al., 2008, Itambi et al., 2009). Since Fe/K values of atmospheric dust samples increase from the Sahel–Saharan region to the tropics (Stuut et al., 2005), this ratio is suggested to vary with changes in humidity and monsoonal precipitation (Mulitza et al., 2008). According to that, increasing amounts of dust derived from deeply weathered terrains (Moreno et al., 2006) cause the increase of Fe/K values towards the tropics, because of its relatively high concentration of iron in comparison to more mobile potassium (Mulitza et al., 2008). In addition, Fe/K values of fluvial transported sediments are higher than those of the eolian transported sediments (Gac & Kane, 1986, Stuut et al., 2005, Mulitza et al., 2008). Highest water discharge and therefore highest sedimentary load of the rivers occurs during rainy seasons (Lesack et al., 1984, Kattan et al., 1987, Debenay et al., 1994). As element of heavy minerals, such as rutile, Ti is transported dominantly in Saharan dust, so that its concentration depends on wind strength (Glaccum, 1978, Schütz & Rahn, 1982). In relation to the high iron content of the terrestrial input we used the Ti/Fe ratio as an indicator of eolian transport and wind strength.
To monitor the influence of riverine nutrient supply and upwelled nutrients on surface ocean productivity we determined the composition of benthic foraminiferal faunas. On account of their wide distribution and high fossilization potential, benthic foraminifera are frequently used to reconstruct surface productivity changes and corresponding organic matter flux to the seafloor (e.g. Herguera & Berger, 1991, Loubere, 1991, Mackensen et al., 1994, Thomas et al., 1995, Perez et al., 2001). In general, abundance, distribution pattern and habitat depth of benthic foraminifera are controlled by dissolved oxygen concentrations of bottom and pore waters, and quality, quantity, and seasonality of organic matter supply (e.g. Mackensen, 1997, Sen Gupta, 1999, Gooday, 2003, Jorissen et al., 2007).
At the lower continental slope off NW Africa, the benthic foraminiferal assemblage composition is largely controlled by the availability of food (Lutze & Coulbourn, 1984, Lutze et al., 1986, Timm, 1992, Jorissen et al., 1998). Due to the seasonality of nutrient supply, phytoplankton blooms occur during periods of strong coastal upwelling and monsoonally forced river runoff (Schemainda et al., 1975). Such seasonal blooms are followed by rapid sinking of large, repacked organic particles that produce a fluffy layer of phytodetrital aggregates on the seabed (e.g. Lampitt, 1985, Rice et al., 1994, Beaulieu, 2002). The freshly deposited phytodetritus is a major source of easily digestible, high-quality food for benthic organisms, including benthic foraminifera (e.g. Thiel et al., 1989).
Here we present a millennial scale reconstruction of paleoproductivity and paleoclimate at the southern NW-African continental margin over the last 31 kyr. We used the benthic foraminiferal assemblage composition in combination with geochemical proxies to assess and infer past changes in intensity and seasonality of productivity and organic matter fluxes related to low latitude imprints of last glacial climate variability.
Section snippets
Material and methods
In 2005, sediment cores GeoB9526-5, GeoB9527-5, and corresponding surface sediment samples were recovered off West Africa at 12°26.1′N, 18°03.4′W in 3231 m water depth, and at 18°13.0′W, in 3671 m water depth, respectively. The 10.8 and 10.13 m-long sediment cores, and the surface sediment samples, obtained with the aid of a gravity corer and a multiple corer, respectively, were sampled at 5 cm spacing, wet sieved over 63 μm, and subsequently dry sieved over 125 μm. For stable isotope measurements,
Results
The ages of the oldest sediments in cores GeoB9526-5 and GeoB9527-5 are about 31.1 and 30.7 kyr BP, respectively (Fig. 3b–d), and linear sedimentation rates range from 5.2 to 27.4 cm kyr− 1 in GeoB9526-5, from 2.7 to 17.5 cm kyr− 1 in GeoB9527-5 (Fig. 4, Fig. 6f).
In Core GeoB 9526-5, 126 benthic foraminiferal species were identified and specimens were counted from the > 125 μm size fraction. BFN and BFAR range from 82 to 2792 N g− 1, and 375 to 30974 N cm− 2 kyr− 1, respectively, with highs in both between 6
Discussion
For the sake of clarity, the discussion starts with a short overview concerning the ecological significance of the four foraminiferal assemblages as defined by principal component analysis. Afterwards, we discuss results in greater detail organized according to selected time intervals of environmental changes, including Heinrich Event 2 (H2, 25.4–24.3 kyr BP) and Heinrich Event 1 (H1, 16.8–15.8 kyr BP), the Last Glacial Maximum (LGM, 23.5–18.3 kyr BP), the mid Holocene period of orbitally forced
Conclusions
Based on benthic foraminiferal analysis of sediment core GeoB9526-5 and additional geochemical studies of GeoB9526-5 and GeoB9527-5 we demonstrate that late Quaternary high-latitude cooling events and orbitally forced variations in low-latitude insolation strongly affected productivity in the coastal region off NW Africa, south of Cape Verde. Short- and long-term climatic changes resulted in variations in the latitudinal position of the ITCZ and the tropical rainbelt, which control wind
Acknowledgments
We thank Günter Meyer, Lisa Schönborn, Cornelia Saukel and Susanne Wiebe for technical support, and Stefan Mulitza for discussion. Critical reviews of Nikki Khanna, two anonymous referees and Ellen Thomas are much appreciated. This work was supported by the DFG Research Centre “The Ocean in the Earth System”.
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