Factors affecting 87Sr/86Sr preservation in conodont apatite: An example from the Ordovician of Baltoscandia

Despite frequent use of conodonts as an archive for seawater 87Sr/86Sr, available diagenetic screening tools are inconsistent predictors of primary seawater 87Sr/86Sr preservation. Here, we attempt to isolate variables affecting the preservation of seawater 87Sr/86Sr in conodonts. We present new Middle–Late Ordovician (∼470–450 Ma) conodont 87Sr/86Sr from mid-paleolatitude (30–45°S) carbonate successions from central Sweden (Fjäcka, Kårgärde) and Estonia (Uuga Cliff, Viki borehole) which have low Conodont Alteration Index (CAI) values of 1–1.5. Coeval 87Sr/86Sr measurements from Kårgärde and Estonian sections are offset from global seawater by ∼ +2 × 10−4 but preserve the overall structure of the seawater curve, suggesting minor diagenetic Sr exchange. 87Sr/86Sr measurements from the more argillaceous Fjäcka section show extensive alteration with highly radiogenic values as much as ∼6 × 10−4 greater than global seawater and a stratigraphic trend that diverges from the seawater curve. This may be explained by highly radiogenic Sr contribution from the Kinnekulle K-bentonite and Fjäcka shale units that occur at Fjäcka section. We compiled 1164 conodont 87Sr/86Sr measurements spanning ∼485–250 Ma and categorized measurements by host rock, sample preparation, paleolatitude, and CAI. Boxplots and Mann-Whitney U testing of residual values relative to the seawater curve (87Sr/86Srconodont – 87Sr/86Srseawater) reveal that paleolatitude and associated differences in sedimentation rate affect preservation of seawater 87Sr/86Sr values in conodonts. The relative abundance of clay minerals did not significantly correlate with residual values for conodonts from mid- or low-latitude deposits. We recommend that future investigations of seawater 87Sr/86Sr records using conodont apatite target low-latitude settings with high sedimentation rate and CAI ≤ 2.