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The End of an Interglacial

The Holocene and Stage 5e Compared

· How Stable was the Last Interglacial Period (MIS 5e/d)?
· How and when did it end?
· Comparisons to the Holocene?

Note: Marine isotope Stage 5e was not equivalent to the warm Eemian period of NW Europe (sidebar)

Abstract
ODP Site 658 off Cap Blanc, West Africa (20N, 18W) is located within the core of the wind-driven West African upwelling region, and has high sedimentation rates (15 cm/kyr) due to high biogenic and terrigenous (eolian dust) sediment supply. We quantified regional sea-surface temperature (SST) variability during the Last Interglacial period (ca. 130-115 ka BP) by performing full foraminiferal assemblage counts on this core at ca. 200 yr resolution. The calculated SST variations at Site 658C document large amplitude (2-4 degC) variations in sea-surface temperatures throughout marine isotope Stage 5e corresponding to the Last Interglacial period. The faunal changes suggest that the millennial-scale cool events reflect either increased local wind-driven upwelling or enhanced southward advection of cool subpolar waters. The Stage 5e SST variations closely resemble those previously described for the Holocene warm period both in pacing (1-3 kyr quasiperiod) and amplitude. The Site 658C SST record also documents a very abrupt termination of the MIS 5e warm period at ca. 118 ka BP, as indicated by an abrupt (~200-400 year) shift to much cooler SSTs. When compared to the Holocene sequence at Site 658C, the results suggest we are overdue for an abrupt transition to cooler climates, however orbital configurations These results are consistent with other high-resolution records of the Last Interglacial from the North Atlantic and support the view large-scale climatic reorganizations can be achieved within centuries.

Reference: deMenocal, P.B., J. Adkins, J. Ortiz, T. Guilderson, Millennial-Scale Sea-Surface Temperature Variability During The Last Interglacial and its Abrupt Termination, AGU Abstracts, Fall 2000.

Research Strategy High sedimentation rate core off NW Africa (ODP 658). Holocene and MIS 5e have similar tilt and precession alignments, however MIS 5e has significantly greater summer insolation (+10%) than the early Holocene (+7%). Sediments record both high- and low-latitude climate responses to insolation forcing. High-latitude ice volume (d18O) Low-latitude African monsoon response to insolation forcing (dust, upwelling) Compare SST and African climate evolution for MIS 2-1 (20-0 ka) and MIS 6-5 (135-110 ka).

A SeaWIFS image of a summer eolian dust plume event emanating from northwest Africa. The Sahara and Sahel regions of NW Africa are largest sources of mineral aerosol, supplying nearly 500 million tons of dust to the ocean annuallly. During the summer months, turbulent easterly waves entrain fine particles from the Sahara and Sahel regions which are convectively lifted to mid-tropospheric levels and transported westward by the African Easterly Jet. Geochemical and mineralogical studies of marine sediments off west Africa indicate that this eolian vector is the dominant supply of terrigenous sediment to eastern Atlantic sediments. ODP Site 658 (2200 m) is located off the Mauritanian coast, just south of the lower left of the image.

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

Holocene Evolution of High- and Low-Latitude Climate Holocene punctuated by African Humid Period (14.8-5.5 ka). Peak African monsoon at ca. 8.5-5.5 ka coeval with Mediterrean sapropel S1 (responses to increase JJA insolation). Wet conditions (8.5-5.5 ka) were associated with reduced terrigenous sediment fluxes (J. Adkins, unpub. data), increased upwelling, cooler SSTs, and great foram productivity. The Holocene was punctuated by irregular 1500±500 year cooling events which have correlatives in the North Atlantic (deMenocal et al., 2000; Bond et al., 1997).

Holocene climatic evolution off NW Africa Abrupt deglacial at 14.8 ka 14.8-8.5 ka (6.3 ka interval): warming and wet 8.5-5.5 ka (3 ka interval): peak wet conditions 5.5-0 ka (5.5 ka interval): warm and dry conditions

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Stage 6/5e Evolution of High- and Low-Latitude Climate Evident millennial-scale SST variations spanning MIS 5e, ~1000-2000 year cooling events (resolution can be increased by factor of two). Following MIS 6/5 deglacial transition (130 ka), Stage 5e interglacial also punctuated by an African Humid Period (126-122 ka) coeval with Mediterrean sapropel S5 (monsoon responses to increased JJA insolation). Age of S5 constrained by U-Th dates (±2 ka precision) of a highly detailed and replicated oxygen/carbon isotopic record from Israeli Soreq Cave (Bar-Matthews et al., 1999). Wet conditions (126-122 ka) also associated with reduced terrigenous sediment , increased upwelling, cooler SSTs, and greater foram productivity.

MIS 6/5e climatic evolution off NW Africa 132-126 ka (6 ka interval): warming and wet 126-122 ka (4 ka interval): peak humid conditions 122-117 ka (5 ka interval): warm and dry conditions post-117 ka: ABRUPT cooling associated with onset of MIS 5d glacial period

Synchronous changes in N. Atlantic climate and deep circulation near 118 ka Adkins et al. (1997) document very abrupt change in deep circulation (Cd/Ca) and sediment fluxes near 118 ka. Abrupt change in CaCO3 % near 118 ka attributed to increased terrigenous sediment focusing at this time due to change in deep circulation. Parallel changes in foram fragmentation suggest shift to more corrosive water chemistry. Transition occured within "a few centuries", perhaps less (Adkins et al., 1997).

The Holocene and Stage 5e climate evolution compared The Holocene and MIS 5e share similar climatic evolutions Both show millennial-scale climate (SST) variability at the ca. 1500±500 year quasiperiod Both exhibit a ~6 ka interval from the glacial termination to the onset of the humid period Both have a well-defined ~3-4 ka African Humid Period Both have a 5-6 ka interval of warm&dry conditions following the Humid Period HOWEVER, Stage 5e was abruptly terminated by an abrupt return to cool conditions which occurred within a few centuries or less.

Caveat Emptor

• Holocene and MIS 5e orbital configurations were different: Maximum JJA insolation at 20N was ~7% greater than present for the early Holocene (S1), whereas it was ~10% greater for MIS 5e (S5).
• We are presently witnessing exponential growth of anthropogenic CO2 which present a very different, new boundary condition to the end of the Holocene.
• These results do, however, underscore the importance of understanding the mechanisms and signatures of abrupt climate transitions. Transitions between climatic states has most often been abrupt despite linear or gradual forcing.