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Coherent High- and Low-Latitude Climate Variability During the Holocene Warm Period

Fig.3C Hole 658C terrigenous material content.  
Click for full figure including isotope data. Coherent High- and Low-Latitude Climate Variability During the Holocene Warm Period
Vol. 288 (5474), pp. 2198, 23 June 2000

Peter deMenocal and Joseph Ortiz
Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA.
Tom Guilderson
Center for Accelerator Mass Spectrometry, Lawrence-Livermore National Laboratory, Livermore, CA 94551, USA.

Michael Sarnthein
Institut für Geowissenschaften, Universität Kiel, Kiel, Germany.

Figure 3C. (Click image for full figure).
Late Glacial to Holocene variations in West African climate and subtropical Atlantic SST from Hole 658C compared with a record of glacial icerafted lithic grain concentrations from subpolar North Atlantic core V29-191. Calibrated radiocarbon age control points for both records are indicated by filled arrow symbols. The Hole 658C SST record documents a series of millennial- scale cooling events of 2° to 4°C amplitude, which are coherent with the Holocene series of North Atlantic ice-rafting events defined by Bond et al. within the dating uncertainties (6100 years). These millennial-scale SST variations were superimposed on a well-known shift in African monsoonal climate between 14.8 and 5.5 ka, when changes in the earth's orbit increased boreal summer insolation and invigorated the African monsoon, bringing much wetter conditions to subtropical Africa. The strengthened monsoonal wind field evidently supported stronger upwelling as documented by the greatly increased G. bulloides abundances (Fig. 2). The reduced terrigenous (eolian) concentrations at Hole 658C document the abrupt onset and termination of the African Humid Period at 14.8 and 5.5 ka, respectively, and reflect the increased humidity of African dust source areas at this time, when subtropical West Africa was nearly completely vegetated.

A faunal record of sea-surface temperature (SST) variations off West Africa documents a series of abrupt, millennial-scale cooling events, which punctuated the Holocene warm period. These events evidently resulted from increased southward advection of cooler temperate or subpolar waters to this subtropical location or from enhanced regional upwelling. The most recent of these events was the Little Ice Age, which occurred between 1300 to 1850 A.D., when subtropical SSTs were reduced by 3° to 4°C. These events were synchronous with Holocene changes in subpolar North Atlantic SSTs, documenting a strong, in-phase link between millennial-scale variations in high- and low-latitude climate during the Holocene.
To read or view the full study, please visit the Science website.
It was published in Science, Vol. 288 (5474), pp. 2198, 23 June 2000.

Download the Data from the WDC Paleo Archive.

From the end of the last ice age until 6,000 BP, the Sahara and adjacent Sahel regions were much greener than today. Using pollen data and a biome model, researchers in the BIOME 6000 project have reconstructed both vegetation and climatic conditions around 6,000 BP. Hoelzmann et al. 1998 found that much of what is desert today was covered in steppe vegetation, and that many small lakes and streams existed above 23° North latitude.

The rapid transition from a green Sahara to today's desert conditions began around 5,700 BP. This paper by deMenocal et al. (2000) provided records of ocean temperature and terrigenous dust in marine sediment cores off western Africa. DeMenocal and colleagues found that ocean temperatures off western Africa dropped around 5 °C between 5,700-5,000 BP, and that there was a dramatic increase in the amount of dust. The dust increase relates directly to the changes in vegetation as the Sahara expanded across northern Africa.

During that same period, Oppo et al. (2003) found a downward trend in NADW production around 6,500 BP and ending around 5,000 BP. This also corresponds to greater freshwater influx to the North Atlantic (Bond et al. 2001) and more winter-like conditions in Greenland (Alley et al. 1997). It is hypothesized that the climate and vegetation changes in Africa were related to changes in NADW formation.

Further back in time, the deMenocal et al. data support the vegetation reconstructions from pollen. From 6,000 BP to the end of the last glacial around 14,500, terrigenous sediments are low in the marine sediments. In fact, the terrigenous sediment content also reflects the Younger Dryas cooling with a move to more arid conditions. The transition from the last glacial is not pronounced in the sea surface temperature data. However, there is evidence of the abrupt cooling at 8,200 BP in addition to the abrupt warming at 5,700 BP.

References Cited:
Alley, R.B., P.A. Mayewski, T. Sowers, M. Stuiver, K.C. Taylor, and P.U. Clark. 1997. Holocene climatic instability: A prominent, widespread event 8,200 years ago. Geology 25(6): 483-486.

Bond, G., B. Kromer, J. Beer, R. Muscheler, M.N. Evans, W. Showers, S. Hoffmann, R. Lotti-Bond, I. Hajdas, and G. Bonani. 2001. Persistent Solar Influence on North Atlantic Climate During the Holocene. Science 294: 2130-2136.

Hoelzmann, P., D. Jolly, S.P. Harrison, F. Laarif, R. Bonnefille, and H.-J. Pachur. 1998. Mid-Holocene land-surface conditions in northern Africa and the Arabian Peninsula: A data set for the analysis of biogeophysical feedbacks in the climate system. Global Biogeochem. Cycles Vol. 12, No. 1, p. 35-52.

Oppo, D.W., J.F. McManus and J.L. Cullen. 2003. Palaeo-oceanography: Deepwater variability in the Holocene epoch. Nature, 422 (6929): 277-278 and 400 (correction).

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6 May 2004