INTRODUCTION Ostracodes have been used to reconstruct climate histories from a variety of continental interior lake settings (Schwalb et al. 1999; Grafenstein et al. 1999a; Grafenstein 2002; Ito 2002). The cos- mopolitan distribution of lacustrine Ostracoda in Ireland (Doug- las and McCall 1992; Meisch 2002) facilitate faunal-based pa- leoenvironmental reconstructions, while their calcite carapaces are well-suited for stable oxygen (δ 18 O) and carbon (δ 13 C) isotope studies that further substantiate faunal-based paleohydrological histories (e.g., Holmes and Chivas 2002; Ito et al. 2003). The primary objective of this research is to evaluate ostracode assemblages along with δ 18 O and δ 13 C values to develop a Holo- cene climate record for western Ireland. We present an ostracode and isotope geochemistry proxy record of temperature variabil- ity that identifies global climatic trends reported in western Ire- land (Mitchell et al. 1996). STUDY AREA Regional Climate and Lake Hydrology Ireland has a temperate, maritime climate with seasonal tem- perature variation moderated by adjacent North Atlantic waters (Jordan 1997). Atmospheric temperatures range between 2.5 and 19.0°C annually with a mean temperature of 9.5°C. Relative hu- midity in Ireland ranges from 70-90%, while precipitation rang- es between 800-2800mm per year with much of the rain falling on the western coast during the winter months (MET 2003). The oligo-to mesotrophic, carbonate producing lakes in western Ireland are hydrologically open (Allot 1986). Lake water tem- peratures vary from 4.2°C to 24.0°C seasonally. Lakes are well- buffered by the Carboniferous marine carbonate bedrock geol- ogy of the region (Mitchell et al. 1996). Carbonate precipitation in the lakes has generated thick sequences of calcite mud, known throughout Ireland as marl (65-95 wt % CaCO 3 ), that are often superseded by peat deposits (Diefendorf et al. 2006). At pres- ent, carbonate precipitates both in basin centers and along the littoral benches that rim the bog margins. Carbonate production is especially enhanced during summer months coincident with maximum solar irradiance and biological productivity. Lough Corrib (9°15’W, N53°30’) has a relatively large, open sur- face area (178km 2 ) and it is known to turnover once annually (Allot 1986). The lake is surrounded by an expansive blanket bog where peat accumulation has been relatively unfettered over the last 4000 years (Mitchell et al. 1996). The lake is situated on karstic limestone draining into Galway Bay via the southward flowing River Corrib (text-fig 1). The broad carbonate platform that fringes the modern lake ranges in depth from 5 to 10m; how- ever, deeper areas of the lake approach 46m. The Global Network for Isotopes in Precipitation (GNIP) main- tains a hydrological monitoring site in Valentia, south western Ireland. Oxygen isotope values at Valentia are lowest during the winter months (-6.0precip‰ VSMOW mean) and highest during summer months (-4.0precip‰ VSMOW mean). Diefendorf and Patterson (2005) report Burren lake water δ 18 O values that range between -4.1 to -6.1‰ VSMOW during summer months. During the summer of 2002, nearby Lough Inchiquin yielded δ 18 O lake water values of -5.9‰ VSMOW, similar to precipitation values. Bedrock samples collected from the Burren Limestone yield- ed carbon isotope values that range between 3.2-3.6‰ VPDB (Diefendorf et al. 2006). Holocene temperature variability in western Ireland: Evidence from limnic ostracode assemblages and stable isotope values Neil E. Tibert 1 , William P. Patterson 2 , Aaron F. Diefendorf 3 , Anna Martini 4 , and Christopher Stanton 5 1 Department of Earth & Environmental Sciences, University of Mary Washington, 1301 College Avenue, Fredericksburg VA 22401 2 Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, SK S7N 5E2 Canada 3 Department of Geosciences, Pennsylvania State University, 503 Deike Building, University Park, PA 16802 4 Department of Geology, Amherst College, Amherst MA 01002 5 Department of Geology, East Carolina University, Room 101, Greenville, NC 27858 email: ntibert@umw.edu, Bill.patterson@usask.ca ABSTRACT: Western Ireland is ideally situated to receive maritime precipitation that has uncomplicated trajectory his- tories. Biota in Irish lakes therefore have the potential to divulge temperature and precipitation records for the Holocene. Blanket bog cores adjacent to Lough Corrib contain abundant ostracoda whose calcite shells were analyzed for stable oxy- gen and carbon isotope values. The earliest Holocene sediments are characterized by the cool water, oligotrophic ostracode Limnocytherina sanctipatricii that displays relatively high δ 18 O and δ 13 C values, indicative of cool, arid conditions. Middle Holocene sediments are characterized by cool, humid conditions differentiated by abundant Metacypris cordata that thrived on productive carbonate benches surrounding the lake perimeter. Relatively low oxygen isotope values suggest cool, wet conditions. Carbon isotope values are relatively low as DIC was dominated by terrestrial organic matter. Late Holocene warm, humid conditions are differentiated by decreased Metacypris cordata abundance and relatively high δ 18 O values in response to increased atmospheric warmth and increased rates of vegetative production. Possible climate excursions include the 8.2 kyr cold event and the Middle Holocene Thermal Maximum. 353 stratigraphy, vol. 4, no. 4, pp. 353-361, text-figures 1-5, plates 1-2, tables 1-2, 2007