Gemma Rueda

Analytical Considerations for the Use of the Paleothermometer Tetraether Index86 and the Branched vs Isoprenoid Tetraether Index Regarding the Choice of Cleanup and Instrumental Conditions

The tetraether index of tetraethers consisting of 86 carbons (TEX(86)) is a novel proxy applied to obtain paleotemperature reconstructions from marine and lacustrine settings. It is usually applied alongside the branched vs isoprenoid tetraether (BIT) index, which provides paleoenvironmental information as well as information on the reliability of TEX(86). Both indices are calculated via the analysis of glycerol dialkyl glycerol tetraethers or GDGTs by means of high-performance liquid chromatography/atmospheric pressure chemical ionization-mass spectrometry (HPLC/APCI-MS). Here we test the performance of alternative methods for sample cleanup and instrumental analysis. In particular, we evaluate using alkaline hydrolysis as an alternative cleanup step to alumina column fractionation and show that the resulting TEX(86) and BIT are statistically equivalent. We also test two different adsorbents in the activated or deactivated state for preparative column fractionation and show that any of them can be used to measure TEX(86) but that a certain discrimination between GDGTs used in the BIT index can occur. Regarding the mass spectrometer design, an ion-trap is shown to be as precise as a quadrupole mass spectrometer for GDGT analysis. Some differences are observed for TEX(86) and especially for BIT values obtained from both MS designs. We provide evidence that the APCI conditions are at least partly responsible for these differences. We recommend caution when comparing BIT values among laboratories as this index seems to be especially sensitive to analytical conditions.

Coupling of air and sea surface temperatures in the eastern Fram Strait during the last 2000 years

The Arctic has undergone significant changes in the last 20–30 years. The understanding of these changes is limited by the time span or resolution of existing climate records. The main gateway of heat to the Arctic Ocean is the Fram Strait, between Greenland and Svalbard. We have reconstructed past air and sea surface temperatures and relative variations in freshwater extent in a marine core from eastern Fram Strait which spans the last 2000 years. The results show a progressive increase in air temperature and a decrease in the upper mixed water layer temperature, together with an increase in freshwater content during the last 1000 years. Considering the oceanographic features of the eastern Fram Strait, these results suggest that there might have been a progressive increase in the inflow of Atlantic water during the last 1000 years, accompanied by an increase in atmospheric heat transport, causing progressive sea-ice melting and subsequent cooling and freshening of the upper mixed water layer in this area. We suggest that our data show long-scale average changes in the modes of positive and negative Arctic multidecadal variability over the last 2000 years.