4 research outputs found
Characterization and Correction of Evaporative Artifacts in Speleothem Fluid Inclusion Isotope Analyses as Applied to a Stalagmite From Borneo
Fluid inclusion water isotope measurements in speleothems have great potential for paleoclimate
studies as they enable the reconstruction of precipitation dynamics and land temperatures. Several previous
observations, however, suggest that inclusion waters do not always reflect the isotopic composition of surface
precipitation. In such cases, dripwaters are thought to be modified by evaporation in the cave environment
that results in more positive Ī“ 2H and Ī“ 18O values and shallow Ī“ 2H/Ī“ 18O slopes. Although evaporation can
occur in cave systems, water can also be lost to evaporation during analysis but before water extraction.
Here, we examine the likelihood of this possibility with a stalagmite from Borneo. We demonstrate that
many samples lose water, and that water loss is controlled by the type and size of inclusions. With multiple
replicate measurements of coeval samples, we calculate an evaporative Ī“ 2H/Ī“ 18O slope of 1.0 Ā± 0.6 (2SE).
This value is consistent with model predictions of evaporative fractionation at high analytical temperature
and low humidity. Finally, we propose a method to correct for this effect. We find that fluidācalcite Ī“ 18O
paleotemperatures calculated with corrected Ī“ 18O data show excellent agreement with recent microthermometry
temperature estimates for Borneo, supporting the validity of our approach and implying limited stalagmite Ī“ 18O
disequilibrium variations. Corrected fluid inclusion Ī“ 18O and Ī“ 2H values follow the expected hydroclimate
response of Borneo to periods of reduced Atlantic Ocean meridional overturning circulation. Our results
suggest that careful petrographic examination and multiple replicate measurements are necessary for reliable
paleoclimate reconstructions with speleothem fluid inclusion water isotopes.The Norwegian
Research Council (Grant 262353/F20)European Research
Council (Grant 101001957 to A.N.M.)FARLAB (RCN Grant 245907)Juan de
la Cierva Fellowship (IJC2019040065-I)Spanish Ministry of
Science and Innovation and co-funded by
the European Development Fund and the
European Social Fun
Warm Middle Miocene Indian Ocean Bottom Water Temperatures: Comparison of Clumped Isotope and Mg/CaāBased Estimates
The middle Miocene is an important analogue for potential future warm climates. However, few independent deep ocean temperature records exist, though these are important for climate model validation and estimates of changes in ice volume. Existing records, all based on the foraminiferal Mg/Ca proxy, suggest that bottom water temperatures were 5ā8Ā°C warmer than present. In order to improve confidence in these bottom water temperature reconstructions, we generated a new record using carbonate clumped isotopes (Ī47) and compared our results with Mg/Caābased estimates for the Indian Ocean at ODP Site 761. Our results indicate temperatures of 11.0 Ā± 1.7Ā°C during the middle Miocene Climatic Optimum (MCO, 14.7ā17 Ma) and 8.1 Ā± 1.9Ā°C after the middle Miocene Climate Transition (MCT, 13.0ā14.7 Ma), values 6 to 9Ā°C warmer than present. Our record also indicates cooling across the MCT of 2.9 Ā± 2.5Ā°C (uncertainties 95% confidence level). The Mg/Ca record derived from the same samples indicates temperatures well within uncertainty of Ī47. As the two proxies are affected by different nonāthermal biases, the good agreement provides confidence in these reconstructed temperatures. Our Ī47 temperature record implies a ~0.6ā° seawater Ī“18O change over the MCT, in good agreement with previously published values from other sites. Our data furthermore confirm overall high seawater Ī“18O values across the middle Miocene, at face value suggesting ice volumes exceeding presentāday despite the warm bottom water temperatures. This finding suggests previously underappreciated additional influences on seawater Ī“18O and/or a decoupling of ice volume and ocean temperature
Cold spells in the Nordic Seas during the early Eocene Greenhouse
The early Eocene (c. 56 - 48 million years ago) experienced some of the highest global temperatures in Earthās history since the Mesozoic, with no polar ice. Reports of contradictory ice-rafted erratics and cold water glendonites in the higher latitudes have been largely dismissed due to ambiguity of the significance of these purported cold-climate indicators. Here we apply clumped isotope paleothermometry to a traditionally qualitative abiotic proxy, glendonite calcite, to generate quantitative temperature estimates for northern mid-latitude bottom waters. Our data show that the glendonites of the Danish Basin formed in waters below 5āĀ°C, at water depths of <300ām. Such near-freezing temperatures have not previously been reconstructed from proxy data for anywhere on the early Eocene Earth, and these data therefore suggest that regionalised cool episodes punctuated the background warmth of the early Eocene, likely linked to eruptive phases of the North Atlantic Igneous Province