Climate Change in Western Ireland During the Holocene As Indicated by Stable Isotope Rations of Carbon

CLIMATE CHANGE IN WESTERN IRELAND DURING THE HOLOCENE AS INDICATED BY STABLE ISOTOPE RATIOS OF CARBON AND OXYGEN IN LACUSTRINE SEDIMENTS JUSTIN DODD Beloit College, Beloit, WI Sponsor: Carl Mendelson, Beloit College INTRODUCTION Background Global climate change and the effect of Kirby et al. (2002) found a strong correlation increased amounts of “greenhouse” gases in between the winter position of the CPV and 18 the atmosphere have generated a great amount the δ Ocalcite values of the lacustrine sediments of scientific and public concern in recent in eastern North America. The CPV refers to years. Houghton et al. (1996) predict that over the polar front jet stream which causes a the next 50 years, global climate will warm as westerly circulation in the northern and much as 4.5 °C in response to anthropogenic southern hemispheres. Like the CPV, the doubling of atmospheric CO2. However, other NAO exerts a strong influence on North studies indicate that the “natural” climate Atlantic climate as standing low- and high- throughout the Holocene has been relatively pressure cells shift position (similar to El Niño unstable (O’Brien et al., 1995). Therefore, a in the Pacific). According to Lamb (1995), more complete understanding of Holocene the position of the CPV plays a significant role climate variability is necessary to accurately in weather patterns over northern Europe. assess potential future climate change. Therefore, the climatological data recorded by sediment in western Ireland are strongly The purpose of this research was to develop a dependent on the phase of the NAO. Ireland high-resolution record of climate change in and much of northwestern Europe also have western Ireland over the past 1000 to 2000 well-documented historical and years, by using stable isotope ratios of carbon meteorological records that offer yet another and oxygen in lacustrine sediment as proxies means of correlation. for climate change. This record can then be correlated with modern climatic data from METHODS meteorological stations to assess the sensitivity with which sediment responds to Core Sampling climate. Sediment data are also compared to Sediment samples were collected from Lough variations in the circumpolar vortex (CPV) Carra in western Ireland (Figure 1). This and North Atlantic Oscillation (NAO) indices, study focuses on two cores, LCAPC-03 (0.50 in order to develop greater understanding of m) and LCA-01 (2.05 m), which display the global forces and cycles that are continuous marl sequences up to the present responsible for Holocene climatic variability depositional surface of the lake. in western Ireland. This will, in turn, lead to a greater understanding of how and why climate Laboratory Analysis change will occur in the future. The cores were sampled at 2-mm intervals in LCAPC-03 and at 2-cm intervals in LCA-01. In order to measure the δ13C and δ18O values of the sediments, samples were roasted in vacuo at 200 ºC to remove all volatile organic initial concentration, also place further age matter constraints on the core sediments (Appleby and Oldfield, 1978). Marl accumulation rates from other lakes in western Ireland provided general temporal constraints for the Lough Carra cores (Diefendorf, unpublished data; O’Connell et al., 1999). RESULTS Both LCAPC-03 (Figure 2a) and LCA-01 (Figure 2b) represent continuous marl records up to the present. In LCA-01, 108 samples were analyzed. Seventy-eight samples from LCAPC-03 were analyzed at a resolution of 2 to 3 mm. The carbon and oxygen isotopic values for the cores display significant variability, indicating that minimal sediment mixing has occurred. Carbon isotope values are characterized by shifts of about 1‰ every 0.01 to 0.02 meters punctuated by larger excursions of greater Figure 1. Lough Carra is a shallow, medium- 13 sized marl lake in southwest county Mayo near than 1‰. In LCAPC-03, δ Ccalcite values the town of Partry. Lough Carra lies just to range from -2.9‰ to -0.0‰, with an average the northeast of the larger Lough Mask and value of -0.9‰ for the core. In the longer Lough Corrib, and is underlain by LCA-01 core, δ13C values range from Carboniferous limestone (adapted from King calcite and Champ, 2000). -2.2‰ to 2.2‰ and have an average value of 13 0.2‰. In both cores, the δ Ccalcite values become more negative from bottom to top, indicating that the sediments are becoming and water. The samples were then individually more enriched in 12C in the more recent reacted in a Kiel-III carbonate preparation sediments. A prominent excursion of -2.9‰ device directly attached to a Thermo Finnigan 13 occurs in δ Ccalcite values at 0.114 meters in MAT 253 stable isotope ratio mass LCAPC-03. The sediment above this point spectrometer in the University of yields a distinctive shift of about -1‰ in the Saskatchewan Isotope Laboratory, and values average δ13C values from -0.6‰ to -1.6‰. were corrected for acid fractionation and 17O calcite contribution. Isotopic data are reported The oxygen isotopes display trends similar to 13 relative to the Vienna Peedee belemnite the δ Ccalcite. LCAPC-03 has an average 18 (VPDB), and data are calibrated using NBS-18 δ Ocalcite value of -3.8‰, with a range from and NBS-19 carbonate standards. Precision for -5.4‰ to -2.6‰. LCA-01 has an average 18 both carbon and oxygen is better than ±0.1‰. δ Ocalcite value of -3.9‰ and ranges from 4.8‰ to -2.7‰. Cores were sampled for 137Cs and 210Pb to place age constraints on the sediment. DISCUSSION Atmospheric 137Cs is a by-product of the testing of nuclear weapons, and a “spike” in Stratigraphic age correlation was developed 137Cs concentration can accurately locate 1963 for lacustrine cores taken from four sites in (the peak year of aboveground testing) in the western Ireland from loss on ignition analysis sediment column (e.g. Huang and O’Connell, and accelerator mass spectrometry (AMS) 2000). 210Pb chronology models, which make radiocarbon dating (Diefendorf, unpublished use of constant rate of supply and constant data; O’Connell et al., 1999). From these ä13C -6.00 -4.00 -2.00 0.00 2.00 4.00 0.00 0 Figure 2a. Carbon and 500 oxygen isotopic data 0.50 1000 LCAPC-03. Age estimates are based on 1500 1.00 an accumulation rate 2000 of 17.0 years per 1.50 2500 centimeter of marl. Depth (m) The Suess effect can be 3000 13 seen in ä Ccalcite in the 2.00 3500 upper portion of the 4000 Estimated time (years B.P.) core. 2.50 -6.00 -4.00 -2.00 0.00 2.00 4.00 ä18O ä18Oä13C ä13C -7.00 -6.00 -5.00 -4.00 -3.00 -2.00 -1.00 0.00 0.01 0 Figure 2b. Carbon 100 and oxygen isotope 0.11 200 values for LCA-01. 13 ä Ccalcite values 300 0.21 become more negative 400 toward the top of the core, while ä18O 0.31 500 calcite Depth (m) values have a more 600 consistent average 0.41 700 with greater periodicity in 800 Estimated time (years B.P.) . excursions. -6.00 -5.00 -4.00 -3.00 -2.00 -1.00 0.00 1.00 ä18O data, an age correlation of 16.0 to 17.5 years intensive arable farming beginning in the early per centimeter of marl was obtained. Though 1800s, which causes greater runoff and the lake systems used in these analyses differ erosion. Large amounts of limestone were from the Lough Carra setting in size and burned to produce lime, which was then used geographic setting, an application of these as fertilizer. The additional calcium load from accumulation rates to the Lough Carra the slaked lime could have increased the samples presents some interesting carbonate precipitation rate, while the soil possibilities. disturbance supplied dissolved inorganic carbon with lower values to the lake basin. Assuming an accumulation rate of 17 years Another intriguing result is a corresponding per centimeter (0.59 cm/year) for the Lough 18 Carra marl, some historical and climatological negative excursion in δ Ocalcite values at about correlations can be made with the isotopic 187 years B.P., which could also indicate record represented in the cores. In the higher increased vegetation removal and drainage resolution LCAPC-03 core, there is a large, associated with increases in population and 13 arable farming. At 85 years B.P., or about negative δ C excursion at about 177 years calcite 1917, a shift of -1‰ in 13C values begins. B.P., or 1825AD. O’Connell et al. (1999) δ calcite describe a dramatic increase in This is consistent with a negative shift in 13 δ Ccalcite values between about 1900AD and isotopic data to be more rigorously the present called the Suess effect. This occurs constrained. as a result of increased atmospheric 12C due to burning of fossil fuels, which are considerably REFERENCES CITED enriched in 12C (e.g. Andres et al., 2000). At Andres, R.J., Marland, G., Boden, T., and Bischof, S., about the same time, Ireland experienced 2000, Carbon dioxide emissions from fossil fuel consumption and cement manufacture, 1751 -1991, significant expansion in arable farming, which and an estimate of their isotopic composition and meant more removal of trees and other latitudinal distribution, in Wigley, T.M.L., and vegetation, and therefore greater evaporation. Schimel, D.S., eds., The carbon cycle: Cambridge, The resulting increase in 18O would cause a Cambridge University Press, p. 53-62. 18 positive shift in δ Ocalcite values as seen in the Appleby, P.G., and Oldfield, F., 1978, The calculation Lough Carra cores.

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