GOLDSCHMIDT CONFERENCE EDINBURGH 1994 The 100,000 year cyclicity in seawater 87Sr/86Sr: an enviro-analytical artifact? 179 S.C. Clemens L.P. Gromet J.W. Farreil Geological Sciences, Box 1846, Brown University, Providence, RI. 02912-1846, USA. Oceanography, Univ. of British Columbia, 6270 University Blvd. Vancouver V6TIZ4, Canada. ln~oducfion Time-series analysis of single-species planktonic foraminifera has resulted in the apparent resolu- tion of 100-kyr cyclicity in seawater STSr/S6Sr and its covariation with changes in global ice volume over the past 450 kyrs (Dia et al. 1992; Clemens et al. 1993). Both records indicated ~0.000020 (20 ppm) variability between glacial- and interglacial- age seawater STSr/S6Sr. Although this variability is at or near analytical resolution at the ls level, the persistence of the signal over several 100-kyr cycles and a strong in-phase covariation with global ice volume lent support to possible interpretation as an environmental signal driven by changes between glacial and interglacial climates. We sought to further test the global nature of this signal by developing an independent high- resolution record based on benthic foraminifera from a North Atlantic site. These analyses also displayed a 100-kyr isotopic signal and an in-phase relationship with ice volume (rlSo). However, 0,709195 t 0.700185 0.7O9175 0.709165 0:/09195 0.709185 ] 0.709175 0.7091~ 0.70919S 0.709185 ~ o.'/o9175 0.709165 0.709155 I 17 $r/16 Sr ~APd II O ( ! } ODP~ln 7SS (O. ~ IIId~) -2.5 ~ -1.5 0.5 1.5 r 12-24-4 PC (c,wunmtml; ^tlln~) -zs ~ v[ 1.5 Va-231 O*. ehliquilecahaa; ]~r~) -9.5 -0.5 0.5 I,$ 50 100 1SO 200 250 ~0 350 'leo 4~0 ICy~ FiG. 1. Time series records of 87Sr/S6Sr and planktonic 6lSo (global ice volume). Sr isotopic data are plotted relative to an NBS-987 value of 0.710257. Error bars for the Sr data represent lc. improved analytical precision associated with these data allowed resolution of a systematic, positive relationship between instrumental fractio- nation and S7Sr/S6Sr that accounts for the 20 ppm range of variability. Thus the 100-kyr cyclicity in the Sr isotopic composition of seawater from this site appears to stem from variations in instru- mental fractionation (possibly of environmental origin) and is therefore an analytical artifact. 0.709225 0.709205 ~ 0,709185 d~ ~ 0.7~)165 0,709145 0.709125 0.9960 0.710300 -~ 0.7107.80 O~ 0.710260 Ub aa 0.710240 dn ~3 0.710220 0.710200 0.99~ s,. Chain 82-24-4 PC (r : 0.72) | . | , t - 9 _ o Samples o ~~ -o ~176176 Oo o . o. 9 o o o o o 9 . , - , - 0.9980 1.000 1.002 1.004 1.006 si. , ch~? s2-2~ PC (, 7 o5~ , Standards o o o o Q o~ go o @ o o ~ o - , - , - , - , - 0.9980 1.000 1.002 1.004 1.006 Fractionation Factor (1 + a) FIG. 2. Chain 82-24-4 PC and Site 758 S7Sr/S6Sr vs. fractionation for both samples (upper) and standards (lower) 9 Data are plotted relative to an NBS-987 value of 0.710257. ~ = fractionation per atomic mass unit. The large number of Site 758 data points in the upper plot reflect duplicate and triplicate anlayses from raw sample as well as replicate runs of individual filaments.