Extracting Reliable Paleo-Ocean Temperatures at Southern Mid-Latitudes During the Greenhouse to Icehouse Transition: a LA-ICP-MS Study of the Trace Element Chemistry of Eocene Foraminifera from New Zealand
Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has been used to measure in situ elemental (Mg, Al, Mn, Zn, Sr, Ba/Ca) ratios of 13 species of variably preserved early to middle Eocene planktonic and benthic foraminifera from the mid-Waipara River section, north Canterbury, New Zealand. The sediments from Waipara River were deposited at bathyal depths (ca. 1000 m) on the northern margin of the east-facing Canterbury Basin at a paleo-latitude of ca. 55 dgrees S. LA-ICP-MS analysis yields trace element depth profiles through foraminifera test walls that can be used to identify and exclude zones of surficial contamination and infilling material resulting from diagenetic coatings, mineralisation and detrital sediment. Screened Mg/Ca ratios are used to calculate sea temperatures from late early to early middle Eocene (ca. 51 to 46.5 Ma), a time interval that appears to span the termination of the Early Eocene Climatic Optimum (EECO). During this time, sea surface temperatures (SST) varied from 30 to 24 degrees C and bottom water temperatures (BWT) from 21 to 14 degrees C. Comparison of Mg/Ca sea temperatures with published delta superscript 18 O and TEX subscript 86 temperature data from the same samples (Hollis et al., 2009) shows close correspondence, indicating that LA-ICP-MS can provide reliable Mg/Ca sea temperatures even where foraminiferal test preservation is less than ideal. Agreement between the three proxies also implies that Mg/Ca - temperature calibrations for modern planktonic and benthic foraminifera can generally be applied to Eocene species, although some species (e.g., V. marshalli) show significant calibration differences. The Mg/Ca ratio of the Eocene ocean is constrained by our data to be 35-50% lower than the modern ocean depending on which TEX86 - temperature calibration is used to compare with the Mg/Ca sea temperatures (Kim et al., 2008; Liu et al., 2009). Sea temperatures derived from oxygen isotope analysis of foraminifera from mid-Waipara show amplified variability relative to the Mg/Ca and TEX86 derived temperatures. While this difference might be attributed to the oxygen isotopes being more susceptible to diagenetic effects, the data may be consistent with the growth and collapse of significant global ice sheets during cool periods in the Eocene on timescales of ca. 0.5 Myr. The timing of the termination of the EECO in the reconstructed climate record from mid-Waipara is consistent with other published climate records (Tripati et al., 2003, 2005; Zachos et al., 2008). A large decrease in foraminiferal Mn/Ca ratios up the mid-Waipara section is observed with the youngest samples having Mn/Ca ratios similar to modern foraminifera. This does not appear to be a diagenetic fingerprint as foraminiferal preservation is generally poorer up-section. Global cooling following the EECO may have led to enhanced biological productivity and uptake of Fe and Mn, thereafter producing an ocean with Mn concentrations more similar to the present ocean. This hypothesis is consistent with that proposed to explain changes in the thallium isotope ratios of Fe and Mn crusts observed at this time (Nielsen et al., 2009).