Climate and atmospheric circulation during the Early and Mid-Holocene inferred from lake-carbonate oxygen-isotope records from western Ireland

The Early to Mid-Holocene experienced marked climate change over the northern hemisphere mid-latitudes in response to changing insolation and declining ice volume. Oxygen isotopes from lake sediments provide a valuable climate proxy, encoding information regarding temperature, hydroclimate and moisture source. We present oxygen-isotope records from two lakes in western Ireland that are strongly influenced by the North Atlantic. Excellent replication between the records suggests they reflect regional, not local, influences. Carbonate oxygen-isotope values peaked at the start of the Holocene, between 11.2 and 11.1 cal ka bp, and then decreased markedly until 6 cal ka bp at both sites. Palaeoecological evidence supports only modest change in temperature or hydroclimate during this interval and we therefore explain the decrease primarily by a reduction in the oxygen-isotope composition of precipitation (delta O-18(ppt)). We show a similar decrease in delta O-18 values in a forward model of carbonate isotopes between 12-11 and 6-5 cal ka bp. However, the inferred reduction in delta O-18(ppt) between the Early and Mid-Holocene in the model is mainly linked to a decrease in the delta O-18 of the ocean source water from ice sheet melting whereas the lake carbonate isotope records are more consistent with changes in the transport pathway of moisture associated with atmospheric circulation change as the dominant cause.