MONSOON RELATED VARIATIONS IN ZAIRE (CONGO) SEDIMENT LOAD AND INFLUENCE OF FLUVIAL SILICATE SUPPLY ON MARINE PRODUCTIVITY IN THE EAST EQUATORIAL ATLANTIC DURING THE LAST 200,000 YEARS
Rr. Schneider et al., MONSOON RELATED VARIATIONS IN ZAIRE (CONGO) SEDIMENT LOAD AND INFLUENCE OF FLUVIAL SILICATE SUPPLY ON MARINE PRODUCTIVITY IN THE EAST EQUATORIAL ATLANTIC DURING THE LAST 200,000 YEARS, Paleoceanography, 12(3), 1997, pp. 463-481
Two sediment cores from the Zaire Fan and the Angola Margin have been
investigated for their composition of terrigenous and biogenic constit
uents, respectively. For the late Quaternary, kaolinite/feldspar ratio
s and variations of terrigenous element ratios of Zr, Ti, K, Rb, and A
l reveal that the composition of Zaire River sediment load has fluctua
ted in tune with precessional variations of boreal summer insolation.
In particular, the correspondence of high kaolinite/ feldspar and Al/K
ratios with low-latitude insolation maxima strongly corroborates the
assumption that west African monsoonal precipitation and chemical weat
hering was enhanced during periods of increased central African heatin
g. The most striking feauture observed is that opal accumulation has b
een 2 to 10 times higher in Zaire Fan sediments than outside in contin
ental margin sediments off Angola, although biogenic Ba and C-org fluc
tuations from both areas indicate that changes of total paleoproductiv
ity were of the same magnitude in the Zaire River plume and off Angola
. From this we infer that the contribution of biogenic opal production
to total paleoproductivity has been significantly higher within the Z
aire River plume than in the oceanic upwelling regime farther to the s
outh off Angola over the last 200,000 years. The pattern of opal accum
ulation rates with respect to that of marine organic carbon implies th
at enhanced opal production off the Zaire River to a great extent was
the result of additional fluvial supply of dissolved silica during hum
id climates characterized by more intense chemical weathering on the c
ontinent, while total paleoproductivity created by oceanic upwelling w
as high in periods of increased zonal trade wind intensity at precessi
onal insolation minima and during cold, more arid glacial climate cond
itions. We presume that paleoproductivity off the Zaire was controlled
by the following two sources of nutrients: (1) marine nutrients inclu
ding nitrate and phosphate as well as the uptake of Ba on particulate
C-org caused by upwelling, and (2) silica, mainly delivered by the Zai
re River dissolved silicate. Hence our results underline the importanc
e of dissolved silicate in large rivers for marine silicate cycling in
the tropical to subtropical Atlantic at a millenial timescale.