Land-ocean linkages

To evaluate land–ocean interaction in southwest Africa during the last glaciation we compared land and ocean proxies measured on the same sediment from cores of the Southeast Atlantic Ocean. the following sections shortly describe the work on the different cores starting with the southernmost one.

Southeast trade wind variations during the last 135 kyr: evidence from pollen spectra in the eastern South Atlantic sediments
Shi, N., Schneider, R., Beug, H.-J., Dupont, L.M.
Earth and Planetary Science Letters 187 (2001) 311-321

The aeolian pollen influx in marine sediments of GeoB1711-4 (28°19'S 12°23'E, water depth 1967 m) reflects variations in SE trade wind intensity and/or zonality. The pollen influx of major elements (grasses, desert/semi-desert and Cape Reeds) shows prominent increases under glacial conditions. The influx of dry forest pollen as a minor component derived from far northeast inland indicates the input by the easterly winds and shows weak increases under interglacial conditions. Six trade wind events have been recorded during the last 135 ka. They correlate with the sea surface cooling, demonstrating the trade wind control of variations in Benguela upwelling intensity. The good correlation between the aeolian pollen record and the Vostok ice core temperature record suggests that cooling over the Antarctic strongly influenced the SE trade wind intensity in subtropical latitudes. During glaciation, the pollen influx maxima are coeval with low latitude summer insolation minima of Northern Hemisphere, suggesting that the increased SE trade wind zonality also enhances the aeolian pollen influx to the studied site.

On material of  GeoB 1023-5 (17°09'S 11°01'E, 1978 m water depth) situated just north of the Walvis Ridge in the vicinity of the mouth of the Cunene River, we studied the deglaciation period in more detail. During the earliest part of the deglaciation (20-15 ka), parallelism occurred between increasing sea surface temperatures and a southward shift of the savannah. Around 14.5 ka ago, corresponding to the beginning of the Antarctic Cold Reversal period (ACR, 14.0-12.9 ka), the grass pollen record declined and the record of desert and semi-desert vegetation increased. The inferred changes in the vegetation indicate drier conditions parallel to the slightly reduced sea surface temperatures. However, the grass pollen rose and the pollen from Tribulus (a typical desert plant) fell again around 12.6 ka (at the end of the ACR), indicating the return of more humid conditions, when the sea surface temperatures declined further to a minimum of 20°C during the Younger Dryas period (YD). The coupling between cold South Atlantic sea surface temperatures and dry southwest African vegetation, seemingly strong until 14 ka, ceased to exist after 13 ka. Stronger trade winds during the ACR and the YD probably caused increased upwelling off the coast of Angola. By comparing marine and terrestrial proxy records derived from the same marine sediment core, we infer that southeast Atlantic sea surface temperatures did not influence the vegetation development in the Kalahari during and after the YD.


Correlation between vegetation in southwestern Africa and oceanic upwelling in the past 21,000 years
Shi, N., Dupont, L.M., Beug, H.-J., Schneider, R.
Quaternary Research 54 (2000) 72-80

Abstract
Dinoflagellate cyst and pollen records from marine sediments off the southwestern African coast reveal three major aridification periods since the last glaciation and an environmental correlation between land and sea. Abundant pollen of desert, semi-desert, and temperate plants 21,000 –17,500 cal yr B.P. show arid and cold conditions in southwestern Africa that correspond to low sea surface temperatures and enhanced upwelling shown by dinoflagellate cysts. Occurrence of Restionaceae in the pollen record suggests northward movement of the winter-rain regime that influenced the study area during the last glacial maximum. Decline of Asteroideae, Restionaceae, and Ericaceae in the pollen record shows that temperate vegetation migrated out of the study area about 17,500 cal yr B.P., probably because of warming during the last deglaciation. The warming in southwestern Africa was associated with weakened upwelling and increased sea surface temperatures, 2000–2800 years earlier than in the Northern Hemisphere. Aridification 14,300 –12,600 cal yr B.P. is characterized by a prominent increase of desert and semi-desert pollen without the return of temperate vegetation. This aridification corresponds to enhanced upwelling off Namibia and cooler temperatures in Antarctica, and it might have been influenced by oceanic thermohaline circulation. Aridification 11,000–8900 cal yr B.P. is out of phase with the northern African climate. Reduction of the water vapor supply in southwestern Africa at that time may be related to northward excursions of the Intertropical Convergence Zone.

Southwest African climate independent of Atlantic sea surface temperatures during the Younger Dryas
Dupont, L.M., Kim, J.-H., Schneider, R.R., Shi, N.
Quaternary Research 61 (2004) 318-324

Abstract
To investigate land–sea interactions during deglaciation, we compared proxies for continental (pollen percentages and accumulation rates) and marine conditions (dinoflagellate cyst percentages and alkenone-derived sea surface temperatures). The proxies were from published data from an AMS-radiocarbon-dated sedimentary record of core GeoB 1023-5 encompassing the past 21,000 years. The site is located at ca. 2000 m water depth just north of the Walvis Ridge and in the vicinity of the Cunene River mouth. We infer that the parallelism between increasing sea surface temperatures and a southward shift of the savanna occurred only during the earliest part of the deglaciation. After the Antarctic Cold Reversal, southeast Atlantic sea surface temperatures no longer influenced the vegetation development in the Kalahari. Stronger trade winds during the Antarctic Cold Reversal and the Younger Dryas period probably caused increased upwelling off the coast of Angola. A southward shift of the Atlantic anti-cyclone could have resulted in both stronger trade winds and reduced impact of the Westerlies on the climate of southwestern Africa.

Land-sea linkages during deglaciation: High-resolution records from the eastern Atlantic off the coast of Namibia and Angola (ODP site 1078)
Dupont, L., Behling, H.
Quaternary International 148 (2006) 19-28

Abstract
The distribution of pollen in marine sediments is used to record vegetation change on the continent. Generally, a good latitudinal correspondence exists between the distribution patterns of pollen in the marine surface sediments and the occurrence of the source plants on the adjacent continent. To investigate land–sea interactions during deglaciation, we compare proxies for continental (pollen assemblages) and marine conditions (alkenone-derived sea surface temperatures) of two high-resolution, radiocarbon-dated sedimentary records from the tropical southeast Atlantic. The southern site is located West of the Cunene River mouth; the northern site is located West of the Angolan Huambe Mountains. It is inferred that the vegetation in Angola developed from Afroalpine and open savannah during the last Glacial maximum (LGM) via Afromontane Podocarpus forest during Heinrich Event 1 (H1), to an early increase of lowland forest after 14.5 ka. The vegetation record indicates dry and cold conditions during the LGM, cool and wet conditions during H1 and a gradual rise in temperature starting well before the Younger Dryas (YD) period. Terrestrial and oceanic climate developments seem largely running parallel, in contrast to the situation ca. 51 further South, where marine and terrestrial developments diverge during the YD. The cool and wet conditions in tropical West Africa, South of the equator, during H1 suggest that low-latitude insolation variation is more important than the slowdown of the thermohaline circulation for the climate in tropical Africa. r 2005 Elsevier Ltd and INQUA. All rights reserved.


Thirty thousand years of vegetation development and climate change in Angola (ODP Site 1078)
Dupont, L.M., Behling, H., Kim, H.-J.
Climate of the Past 4 (2008) 107–124

Abstract
ODP Site 1078 situated under the coast of Angola provides the first record of the vegetation history for Angola. The upper 11 m of the core covers the past 30 thousand years, which has been analysed palynologically in decadal to centennial resolution. Alkenone sea surface temperature estimates were analysed in centennial resolution. We studied sea surface temperatures and vegetation development during full glacial, deglacial, and interglacial conditions. During the glacial the vegetation in Angola was very open consisting of grass and heath lands, deserts and semi-deserts, which suggests a cool and dry climate. A change to warmer and more humid conditions is indicated by forest expansion starting in step with the earliest temperature rise in Antarctica, 22 thousand years ago. We infer that around the period of Heinrich Event 1, a northward excursion of the Angola Benguela Front and the Congo Air Boundary resulted in cool sea surface temperatures but rain forest remained present in the northern lowlands of Angola. Rain forest and dry forest area increase 15 thousand years ago. During the Holocene, dry forests and Miombo woodlands expanded. Also in Angola globally recognised climate changes at 8 thousand and 4 thousand years ago had an impact on the vegetation. During the past 2 thousand years, savannah vegetation became dominant.

We compared alkenone derived sea surface temperatures, dinoflagellate cyst assemblages, pollen data, and Aluminium/Kalium ratios recorded in core GeoB 1008-3 (06°35'S, 10°19'E, 3124m water depth) off the Congo River mouth. Spectral analysis was used to unravel timely relationships of the land-ocean linkages. We concluded that strong monsoon rains, in phase with precession (maximum summer insolation over the northern tropics), lead to lower salinity of the surface waters of the tropical Southeast Atlantic, expansion of rain forest, and intensified chemical weathering in tropical Africa.

 Dupont_et_al_1999.pdf

Dupont, L.M., Schneider, R., Schmüser, A., Jahns, S., 1999. Marine-terrestrial interaction of climate changes in West Equatorial Africa of the last 190,000 years. Palaeoecology of Africa, 26: 61-84.

A wide variety of proxies has been applied to study the terrestrial input in the Congo deep-sea fan area, the composition of its overlying waters, and the land–ocean interactions of the past 1 to 2 million years. These proxies include stable isotopes of foraminifers, total organic carbon (TOC), alkenone-derived sea-surface temperatures (SST), biomarker content and compound-specific stable isotopes, element composition, clay minerals, pollen and spores, dinoflagellate cysts, diatom valves, and opal. Not only the sedimentation in the deep-sea fan but also the productivity of the overlying waters is strongly influenced by the Congo River discharge and its fluctuations depending on the strength of the monsoon. SST and marine productivity are further affected by wind- and river-induced upwelling. A direct relation between SST, precipitation in the Congo Basin, vegetation cover, chemical weathering, and runoff could be established for the past 200 thousand years. Increase of mean global ice volume between 1000 and 500 ka suppressed the precession forcing of trade-wind zonality and monsoonal river runoff, leading to a higher production of nonsilica marine organisms compared to diatoms, and increased eolian transport of terrigenous material.

 Dupont_2009.pdf

Dupont, L.M., 2009. The Congo deep-sea fan as an archive of Quaternary change in Africa and the eastern tropical South Atlantic. In: Kneller, B., Martinsen, O.J. & McCaffrey, B. (eds.) External Controls on Deep-Water Depositional Systems. Society for Sedimentary Geology, Special Publication 92: 78-88.

Mid-Pleistocene environmental change in tropical Africa began as early as 1.05 Ma
Dupont, L.M., Donner, B., Schneider, R.R., Wefer, G.
Geology 29 (2001) 195-198

The pollen record of Ocean Drilling Program Site 1075 (Leg 175) situated just north of the Congo canyon and covering the period between 1.35 and 0.62 Ma (million years) revealed that 1.05 Ma ago, a large environmental change occurred in equatorial Africa. This is much earlier than the well-known climatic shift of the Middle Pleistocene at 0.9 Ma. Glacial-interglacial fluctuations of the African vegetation become apparent only after 1.05 Ma when Podocarpus trees spread in the mountains of west central Africa and Congo River discharge decreased. The sequence of vegetation change associated with the glacials and interglacials of the Middle Pleistocene probably differed from that observed during the last glacial-interglacial cycle. It seems that, between 0.9 and 0.6 Ma, the early interglacials were characterized by warm dry conditions (with extensive lowland forests but also with open grasslands) and the early glacials were characterized by cool humid conditions (with extensive Afromontane Podocarpus forests). On the other hand, the glacials during the past 200,000 years were cold and dry (extension of open grasslands) and the interglacials were warm and humid (lowland forest or Afromontane forest extension).

Sediments of a box core (GIK 16867-1), a gravity core (GIK 16867-2), and a piston core (GIK 16867-3) provided a record of the past 700,000 years. The site is located in the northern Angola Basin (02°12'S, 05°06'E) at 3890m water depth, rather far from the coast and probably deprived of river input. During the past 700,000 years, the vegetation of equatorial Africa adjacent to the Gulf of Guinea mainly consisted of lowland rain forest at lower altitudes and Afromontane forest at higher altitudes. Grass-rich vegetation zones or savannahs were probably not extensive in western Cameroon, southwestern Congo, and Gabon. During cool and cold periods, the Afromontane forest spread and expanded to lower altitudes. Correlation of the abundance curves of neritic dinoflagellate cysts with that of the lowland rain forest illustrates the salinity decrease in the eastern Gulf of Guinea as a result of increased precipitation and intensified fresh-water discharge from the African continent.


Land-sea correlation by means of terrestrial and marine palynomorphs from the equatorial East Atlantic: phasing of SE trade winds and the oceanic productivity
Dupont, L.M., Marret, F., Winn, K.
Palaeogeography, Palaeoclimatology, Palaeoecology 142 (1998) 51-84

Abstract
Sporomorphs and dinoflagellate cysts from site GIK16867 in the northern Angola Basin record the vegetation history of the West African forest during the last 700 ka in relation to changes in salinity and productivity of the eastern Gulf of Guinea. During most cool and cold periods, the Afromontane forest, rather than the open grass-rich dry forest, expanded to lower altitudes partly replacing the lowland rain forest of the borderlands east of the Gulf of Guinea. Except in Stage 3, when oceanic productivity was high during a period of decreased atmospheric circulation, high oceanic productivity is correlated to strong winds. The response of marine productivity in the course of a climatic cycle, however, is earlier than that of wind vigour and makes wind-stress-induced oceanic upwelling in the area less likely. Monsoon variation is well illustrated by the pollen record of increased lowland rain forest that is paired to the dinoflagellate cyst record of decreased salinity forced by increased precipitation and run-off.

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