LAKE LEVEL CHANGES IN A MIDDLE-TO-LATE HOLOCENE LAKE BOG DEPOSIT IN WESTERN IRELAND

Lough Monreagh, located in the Páirc Naisiúnta Bhoirne region of western Ireland, is 4 km SE of An Gort where lake sediments have been established as recorders of climate change adjacent to the North Atlantic. An 8.67 m core was recovered and the upper 5.0 m was subjected to loss on ignition, ostracode paleoecology, and pollen analysis to develop a millennial-scale climate record for the past ~7 kyr.

The Lough Monreagh core is marked by two contrasting facies associations. The basal facies (3.4–5.0 m) comprises carbonate mud with localized intervals of abundant shell material intercalated with finely laminated mud.  Charophytes and gastropods occur in association with ostracoda that include Metacypris cordata and Paracandona euplectella. Metacypris cordata are relatively abundant throughout the core with the exception of two intervals where Paracandona euplectella increase to as much as ~30% of the total ostracodes. The upper section of the core (0.0-0.4 m) comprises rich peaty mud with minor sandy intervals. Total organic carbon (%TOC) ranges from ~60-100% with two notable CaCO₃ peaks at the 2.1 and 2.8 m positions. Pollen analyses indicate a relative abundance of arboreal pollen types dominated by Pinus (30-40%). The peat-rich intervals (80-100% TOC) contain a relative abundance of disturbance weeds (Asteraceae), sedge (Cyperaceae), and grass (Poaceae). The uppermost peat is terminated by a sharp increase in CaCO₃ and marked increase of disturbance weeds, grasses, and cereal grains.

Changes in lake level and/or relative humidity can be estimated from physical and biotic data. During the carbonate mud deposition, lower lake levels are inferred from relative abundance of the littoral ostracode Metacypris cordata. In contrast, higher lake levels and wetter times are inferred by increases in Paracandona euplectella when the waters were rich with organic detritus. Following the onset of bog progradation during middle Holocene times, lower lake levels and increased aridity can be inferred from the increased abundance of arboreal pollen when the bog surface was forested. Overall, 5 changes in relative lake-level change can be recognized from the upper 5 m of the lake/bog core which indicates the potential of microfossils to identify millennial climate changes in lake deposits spanning the past several thousand years.