Holocene linkages between high - and low-latitude climate

Millennial-Scale Linkages Between High- and Low-
Latitude Climate During the Holocene Warm Period

Abstract
A detailed record of West African climate and subtropical Atlantic sea-surface temperature (SST) variability was reconstructed at ODP Hole 658C off Cap Blanc, Mauritania. Age control was constrained by 31 AMS radiocarbon dates to 24 cal. ka BP; sedimentation rates are high (22 cm/ka) due to high biogenic and terrigenous (eolian) fluxes. Foraminiferal SST estimates at Hole 658C reveal a series of abrupt, millennial-scale cooling events which punctuated the Holocene warm period. During each of these events the coolings evidently resulted from increased southward advection of cooler temperate/subpolar waters and/or enhanced regional upwelling. The most recent of these events was the Little Ice Age between 1500-1870 AD, when subtropical SSTs were reduced by 3-4°C. These subtropical cooling events were synchronous with Holocene changes in subpolar North Atlantic SSTs, documenting a strong and in-phase linkage between millennial-scale variations in high- and low-latitude climate during the Holocene warm period.

deMenocal, P.B., J. Ortiz, T. Guilderson, and M. Sarnthein, Coherent High- and Low-Latitude Climate Variability During the Holocene Warm Period, Science, 288 (5474), 2198-2202 (2000) (PDF).

Space Shuttle photograph of Lake Chad (dotted yellow border, see right panel) in northwest Africa; Holocene Paloelake Chad extent is shown by the white solid border. This photo was taken from the north, looking across the arid sub-Sahara and Sahel regions and toward the (darker) tropical equatorial vegetation belt to the south.

Paleolake chad (white border) was roughly twice the size of New York state during its maximum expansion in the early Holocene (9-6 ka BP) associated with African Humid Period. These large perennial Holocene lakes lakes were subjected to abrupt aridification events associated with the Holocene cool events described here.

Site Location: ODP Hole 658C off West Africa (20°N, 18°W, 2200m) - Figure 1

Year-round upwelling occurs off West Africa due to coastal divergence resulting from the NE wind field.
The site is also positioned directly below the mid-troposphere summer African dust plume.
High accumulation rates (22 cm/ka) result from high biogenic and terrigenous sediment fluxes.
Sea-surface temperature seasonality is high here (18-24°C; 21°C mean) due to the winter penetration of the southward Canary Current and summer advance of warmer tropical waters.

Age Control - Figure 2

Age control was established with 31 AMS radiocarbon dates on G. bulloides and G. inflata to 24 cal. ka BP. (R=-500 yrs).
Sampling interval was 2 cm or ~50-150 years.
Terrigenous (eolian) percent calculated as residual after measurement of biogenic carbonate and opal percentages.

Holocene Variations in Subtropical Atlantic Sea-Surface Temperatures - Figure 3

Holocene sea-surface temperature variations were estimated using planktonic foraminiferal census counts. Between 500-1000 specimens were counted for each sample. SSTs were calculated using the F13' transfer function (±1.3-1.6°C std. error).
The Holocene faunal changes (SST) are large. LGM cooling at this latitude (20°N) was 6-9°C for cold/warm seasons, respectively.
The Holocene was punctuated by numerous large (3-4°C anomaly) millennial-scale cooling events.
These cool events are represented by increases in cool N. pachyderma (right-coiling) relative to warm G. inflata, suggesting increased southward Canary Current advection of cooler subpolar waters.

Millennial-Scale Linkages Between High- and Low-Latitude Climate - Figure 4

Subtropical Atlantic SST's varied significantly during the Holocene at a mean quasiperiod of 1500±500 yrs.
The subtropical Atlantic cool events, which were 2-4°C in amplitude, closely correspond with cooling events documented in the subpolar North Atlantic (V29-191 and V23-81; Bond et al., 1997).
The coldest interval since the LGM was the 8.2 cal. ka cooling event.
The Younger Dryas was (evidently) not anomalously cool here.
Note the increase in late Holocene (post-5 ka BP) SST variability, culminating in the Little Ice Age (0.1-0.7 ka BP; 1300-1850 AD).

The Last Climate Cycle: Little Ice Age - Medieval Warm Period - Figure 5

We identify two distinct cooling events which comprise the Little Ice Age (1300-1850 AD), following a broad warm interval (the Medieval Warm Period) between 500-1200 AD.
The "twin" LIA cooling events have also been observed at Greenland based on borehole temperature deconvolution analyses (Dahl-Jensen et al., 1998), and at the Bermuda Rise based on planktonic oxygen isotopic analyses (Keigwin, 1996).
We do not detect any systematic differences in the timing of these events between the Bermuda Rise, Greenland, and West Africa.
This indication of in-phase LIA cooling synchroneity throughout the N. Atlantic contrasts with the proposal for an out-of-phase LIA temperature signature recently proposed for the Laurentian Fan (Keigwin and Pickart, 1999).

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