A STABLE ISOTOPE PALEOCLIMATE RECORD OF THE LATE-GLACIAL/INTERGLACIAL TRANSITION FROM LOUGH INCHIQUIN, WESTERN IRELAND

AMALIA DOEBBERT Carleton College Sponsor: Dave Bice

INTRODUCTION documentation of their magnitude and distribution can improve predictions of the The shift from the Last Glacial Maximum to impact human activities may be having on the current Interglacial period is characterized global climate (Alley, 2000). New high- by several oscillations. Most prominent in resolution studies of continental paleoclimate Western European and ice core records, the in Europe, where the events are most clearly B∅lling-Aller∅d/Younger Dryas/Preboreal defined, contribute to developing an overall sequence of climate shifts is widely picture of the conditions that produced climate recognized. However even the most dramatic instability. and extensively studied of these climate swings, the Younger Dryas, has not been The purpose of this study is to develop a adequately characterized. paleoclimate record that will help to illuminate the nature of the glacial-interglacial transition The wide distribution of sites that indicate in western Ireland. climate fluctuation is significant because these climate shifts are thought to be closely tied to METHODS global ocean and atmospheric circulation An eight-meter piston core was recovered (Broecker, 2000). A greater understanding of from the shore of Lough Inchiquin, western these abrupt perturbations gained by better

Figure 1: Map showing locations of Lough Inchiquin, Tory Hill, Lough Gur, and Red Bog. Modified from (O'Brian 1962) Ireland (Fig.1). In this study the bottom 1.5 m RESULTS of the core were analyzed (LINC 1-7 and 1-8). The Younger Dryas event, marked by a Dates distinctive clay-rich section, is present in the AMS dates are not yet available from the part of the core that was analyzed. Sediments Lough Inchiquin (LINC 1) core. However, were sampled at ten cm intervals for loss on lithological correlation of LINC 1 to a core ignition and two mm intervals for isotope from Tory Hill (Fig.1) taken by O’Connell et analysis. al. (1999) indicates an age of 15 ka cal BP at the core’s base. The Younger Dryas period is Loss on Ignition (LOI) for the sediments was correlated to an age of 12 ka cal. BP (Fig. 2). run at the National University of Ireland, Galway. Samples were burned at 500° C to Loss on Ignition (LOI) combust organic plant matter (organics) and at Between 7.53 m and 7.45 m and subsequently 1000° C to combust carbonate. between 7.15 m and 7.05 m clay percentages Stable isotopes of carbon and oxygen were decrease and carbonate increases. A large analyzed at the University of Saskatchewan in increase in clays and a dramatic drop in Saskatoon, Canada. Prior to analysis samples carbonate takes place between 6.49 m and were heated in vacuo to 200 º C to remove 6.30 m. This shift, the most significant volatiles. Samples were reacted with 103% variation in the loss on ignition results, marks phosphoric acid at 70 º C by a Kiel III the Younger Dryas interval. The Younger carbonate preparation device coupled to a Dryas interval is also the location of the most Thermo Finnigan MAT 253 gas ratio mass dramatic increase in organic values. spectrometer. The results have been corrected Carbon Isotopes (Fig. 2) for 17O contribution, and have a 0.1 per mil or better standard deviation. Both δ13C and δ18O The most prominent trend in my carbon have been calculated relative to the Vienna isotopes is 6.6 ‰ overall shift towards more Peedee Belemnite (VPDB) international negative values. standard. Another significant trend is associated with

Figure 2: Stable isotope results from Lough Inchiquin. Tie lines correlate LINC 1 δ18O nearby Lough Gur and Red Bog (Ahlberg et al., 1996) as well as the GRIP ice core δ18O (Dansgaard et al., 1993) based on age and curve similarities. The B∅lling-Aller∅d (Bo/All), Younger Dryas (YD), and Older Dryas (OD) are marked on the GRIP and LINC 1 δ18O records. the Younger Dryas period between 6.61 m and δ13C. C3 plants contribute more C12 to DOC 6.34 m. Although no data was obtained than C4 plants. A shift from wet, warm between 6.58 m and 6.43 m due to lack of conditions to cold, dry conditions could force carbonate, values leading into the data gap a change in vegetation from C3 to C4 plants. make it clear that a positive shift in δ13C of at This adaptation would increase δ13C. least 2.06 ‰ takes place. However, high δ13C in the LINC 1 core Oxygen Isotopes (Fig. 2) implies that at Lough Inchiquin the proportion of C3 to C4 plants is less significant than the Between 6.60 m and 6.35 m, a negative shift is total vegetation around the lake. indicated by the values leading in and out of Younger Dryas section of the core. The The bedrock underlying Lough Inchiquin is change in δ18O associated with this period was also important to consider in interpretation of a minimum of 2.55 ‰. Most of this change is my isotopic record. Groundwater that enters the lake may acquire carbon from carbonate abrupt (between 6.60 m and 6.59 m) leading 13 into the Younger Dryas, but was more gradual bedrock. Given that the δ C values of this coming out of it. record are high, it is likely that the bedrock has influenced δ13C especially at the base of The base of the core, between 7.55 m and 7.48 the core. m, shows a dramatic negative peak. The minimum δ18O value of this peak (-7.53 ‰ at Interpretation of lacustrine oxygen isotopes 7.53 m) is the lowest value in the core. A rapid involves two primary sources of variation: shift leads out of this peak, raising values by temperature and the oxygen isotope value of 2.59 ‰ by 7.48 m. Negative departures from the lake water. Dansgaard (1964) first the normal values take place between 7.27- demonstrated that there is a positive linear 18 7.09 and 6.29-6.11 m. Throughout the course correlation of precipitation δ O values with of my core section, δ18O shows frequent air temperature. A 0.33 ‰ /°C rate of change, oscillations of about 1 ‰. as discussed by Ahlberg et al. (1996) is used to quantify temperature-based relationships at DISCUSSION Lough Inchiquin. Interpretation of Paleoclimate Near Ireland’s western coast, precipitation Proxies from westerly air masses has always Clay in lacustrine sediments is generally an dominated (Ahlberg et al., 1996). indicator of dry climate. During cold, dry Consequently, variations in the periods sparse vegetation allows clay to be precipitation/evaporation ratio at Lough easily washed or blown into the lake from Inchiquin and ocean salinity changes are the nearby. During dry periods windblown loess sources of water value variation significant to this study. These effects, which correlate can also bring clay-size particles from a 18 considerable distance. negatively with δ O, are considered subordinate influences at Lough Inchiquin Lacustrine carbon isotope records are often with respect to larger trends. Any effect they considered a measure of organic matter have serves to dampen temperature productivity in the lake. Productivity changes oscillations recorded. may be influenced by changes in aridity, among other factors. Kirby et al. (2002) have Paleoclimate at Lough Inchiquin hypothesized that when cloud cover is high Low δ18O values at the base of the LINC 1 (low aridity), productivity is reduced causing core suggest that climate was still cold when wet periods to result in low δ13C. sedimentation began in Lough Inchiquin. The 13 Because plants preferentially use C12, δ13C high clay content and δ C values in this basal values are also affected by the amount of section support an interpretation of cold, dry vegetation in the environment. Increases in climate. dissolved organic carbon (DOC) supplied by B∅lling/Aller∅d warming is shown to be rapid, decomposing organic matter could decrease recorded in less than 4.6 cm. The δ18O indicates that temperatures increased by up to isotope and LOI results from the LINC 1 core. 7.8° C. A brief negative δ18O shift, possibly The sequence obtained by looking at isotope the Older Dryas, suggests that there was a and loss on ignition results is validated by the short return to cold and/or dry conditions dates linked to the core. (Fig.2). A decrease in clays following this The presence of a dominant overall shift in interruption implies that the climate became 13 13 δ C, a feature not present in δ C records from more humid after the inferred Older Dryas. nearby areas, implies a strong and highly δ18O leading into and out of the Younger localized environmental change was also Dryas imply cooling of 7.7°C associated with taking place between the end of the last glacial the event. Interpretation of δ13C in the same maximum and the beginning of the Holocene. section implies a dry, low vegetation This may represent the development of environment around the lake. High clay extensive vegetation from a barren landscape content and relatively high levels of preserved left when the glaciers retreated. Increasing organics in this portion of the core imply a total vegetation in the area around the lake cold, dry climate as well. caused by wetter conditions and warmer 18 temperatures is the most plausible explanation Although an abrupt O increase marks the 13 δ for the change in δ C observed in my record. end of the Younger Dryas in this core, the δ18O values do not maintain the peak values REFERENCES CITED 13 reached at 6.35 m. Decreasing δ C values at Ahlberg, K., Almgren, E., Wright, H. E., Jr., Ito, E., and the top of the core imply that humidity Hobbie, S., 1996, Oxygen-isotope record of late- increased after the Younger Dryas. A brief glacial climatic change in western Ireland: Boreas, cold period indicated by the oxygen isotopes v. 25, p. 257-267. at 6.2 m corresponds to a slight increase in Alley, R. B., 2000, Ice Core Evidence of Abrupt δ13C. This excursion hints that a minor cold, Changes: Proceedings of the National Academy of dry period followed after the Younger Dryas, Sciences of the United States of America, v. 97, no. 4, p. 1331-1334. most likely the Preboreal Oscillation. Broecker, W. S., 2000, Abrupt climate change; causal 18 The frequent δ O shifts of 1 ‰ or greater constraints provided by the paleoclimate record: magnitude present the possibility of recurrent Earth-Science Reviews, v. 51, no. 1-4, p. 137-154. minor events. Local temperature variation may Dansgaard, W., 1964, Stable Isotopes in Precipitation: have taken place, but at this small scale Tellus, v. 16, no. 4, p. 436-468. precipitation patterns may play a role as well. Dansgaard, W., Johnsen, S. J., Clausen, H. B., Dahl- 18 Comparison to δ O curves from nearby Jensen, D., Gundestrup, N. S., Hammer, C. U., Lough Gur and Red Bog (Ahlberg et al., 1996) Hvidberg, C. S., Steffensen, J. P., Sveinbjörnsdóttir, shows that significant local variation does take A. E., Jouzel, J., and Bond, G., 1993, Evidence for place, even when larger trends are preserved. general instability of past climate from a 250 kyr ice-core record.: Nature (London), v. 264. LINC 1 shows remarkable similarity to the GRIP ice core δ18O record, illustrating that Kirby, M. E., Mullins, H. T., Patterson, W. P., and Burnett, A. W., 2002, Late glacial-Holocene large shifts are broadly regional (Fig. 2). atmospheric circulation and precipitation in the Consideration of the overall shift in δ13C as a northeast United States inferred from modern calibrated stable oxygen and carbon isotopes: GSA proxy for the amount of plant life nearby Bulletin, v. 114, no. 10, p. 1326-1340. indicates a gradual increase in the vegetation surrounding the lake. Using 13C as a proxy O'Brian, M. V., 1962, Geologic Map of Ireland: δ Director of the Ordiance Survey, Government of for aridity, it is inferred that climate at Lough Ireland, scale 1:750000. Inchiquin has gradually become wetter since O'Connell, M., Huang, C. C., and Eicher, U., 1999, the last glacial period. Multidisciplinary investigations, including stable- isotope studies, of thick late-glacial sediments from CONCLUSIONS Tory Hill, Co. Limerick, western Ireland: The well-documented Late-glacial/Interglacial Palaeogeography, Palaeoclimatology, transition sequence is evident in the stable Palaeoecology, v. 147, no. 3-4, p. 169-208.