1100 and 600 year periodicities in the effective precipitation record from a full Holocene ombrotrophic peat sequence.
by
Paul Hughes
Palaeoecology Laboratory (PLUS), Department of Geography, University of Southampton, SO17 1 BJ, UK.
Coauthors: Keith Barber (PLUS), Pete Langdon (Department of Geography, University of Exeter, EX4 4RJ, UK)

The aim of this project was to develop techniques for extending the plant macrofossil record of effective precipitation from raised bogs back into the early-Holocene. Sedges and cotton grasses rather than Sphagnum, which usually forms the basis for plant macrofossil-based climate reconstructions, dominate the lower strata of many British raised bogs. The Quadrat and Leaf Count (QLC) macrofossil technique established by (Barber et al. 1994) was refined to include species level identifications of various taxa in the Cyperaceae and Ericaceae as well as fungal remains and charred plant fragments. Both Sphagnum leaf counts and monocotyldon rootlet/leaf epidermis counts were completed to produce estimates of the abundance of each species encountered in raised bog peats. The extended plant macrofossil analyses enabled oscillations in sedges, indicative of 'mud-bottom sites' surrounding pools, to be separated from members of the Cyperaceae typical of low and high bog hummocks.

The refined plant macrofossil-based reconstruction of raised mire water tables was completed on a 10 metre peat core (lab code: WLM11) retrieved from Walton Moss, Cumbria, UK. Radiocarbon dating demonstrated that the peat sequence accumulated at an almost constant rate, averaging 10.3 years/cm. The basal ombrotrophic peats have a date of ca. 10500 cal BP.

The appearance of the sedge Rhynchospora alba at ca. 8000 cal BP. provides evidence for the first major climatic 'wetshift' registered at Walton Moss. The sedge is later replaced by ombrotrophic Sphagna at c.7800 cal. BP marking an extended period of very wet mire conditions. This event coincides with a now well know climatic cooling phase recorded in Greenland ice cores. Other raised bogs from as far afield as Galway also show the same distinctive phase of high water levels at ca. 8000 cal. BP (Hughes 1997). The Sphagnum-rich peats overlying the Rhynchospora horizon at Walton Moss provide a continuous record of fluctuations in mire surface wetness, which is closely controlled by changes in effective precipitation. This proxy-palaeoclimate signal, registers 'climatic deteriorations' commencing at ca. 5300, 4410-3990 (2? range), c.3500, 3170-2860 (2? range), 2320-2040 (2? range), ca.1750, ca.1450, ca.300 and ca.100 cal. BP.. Time series analysis undertaken on the full climatic archive from Walton Moss revealed statistically significant periodicities of ca.1100 years and c.600 years between 'wet shifts'. These two cycles are remarkably close to those identified by Chapman and Shackleton (2000) in ocean sediments from the North Atlantic. The apparent correspondence between the two records may indicate that the millennial-scale terrestrial climate signal is modulated by changes in North Atlantic Deep Water (NADW) formation.

Date received: April 2, 2001

Copyright © 2001 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # cagc-90.