Abstract:
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).
