Although the TEX86 proxy has some advantages over the U k? 37 proxy, it also has significant weaknesses. , dos001 ]. Therefore, it seems unusual that the TEX86 index shows such a strong correlation with SST [ Huguet et al., 2006 ]. The cells of Thaumarchaeota are too small to sink to the ocean floor postmortem; therefore, the TEX86 signal must be transported to the ocean sediments in another way. A likely mechanism is that Thaumarchaeota are consumed and the TEX86 signal is incorporated into marine snow. As most food webs are active in the upper ocean, this would also explain why TEX86 is well correlated with SST [ Wuchter et al., 2005 , 2006 ; Huguet et al., 2006 ]. Support for this interpretation comes from sediment traps set up at different depths, with measurements from deeper sediment traps reflecting SST rather than the ambient ocean temperature [ Wuchter et al., 2005 , 2006 ]. A core top calibration using samples from multiple regions and ocean depths suggested that the TEX86 signal is strongly coupled to mixed layer temperatures, at depths of 0–30 m [ Kim et al., 2008 ]. However, another study suggested TEX86 temperatures cooler than actual SST, implying that for certain regions the TEX86 signal might originate in the subsurface [ Huguet et al., 2007 ].
Alkenones might be moved laterally and will be also reprocessed away from sediments, placing fossil alkenones or alkenones synthesized in almost any surroundings onto key passes and probably biasing U k?
Potential seasonal biases affect the TEX86 proxy as well as the U k? 37 proxy. Sediment trap studies suggest that the peak concentration of GDGTs occurs in the winter and spring months [ Wuchter et al., 2005 ], but when the TEX86 index is applied in sediment trap and core top studies the signal appears to be predominantly an annual mean [ Wuchter et al., 2005 ; Kim et al., 2008 ]. Both TEX86 and U k? 37 may be subject to alteration due to diagenesis [ Huguet et al., 2009 ] and contamination from secondary inputs [ Thomsen et al., 1998 ; Weaver et al., 1999 ; Weijers et al., 2006 ], although the diagenetic pathways differ [ Liu et al., 2009 ]. 37 temperature estimates [ Thomsen et al., 1998 ; Weaver et al., 1999 ]. GDGTs are also found in soils and can be transported to ocean basins by rivers, potentially affecting the TEX86 proxy for sites near river outflow [ Weijers et al., 2006 ]. Enclosed settings may show calibration lines that are offset from open ocean calibration lines, which suggests that different source populations ]. To improve SST estimates and to reduce the impact of secondary effects on temperature signals, it is desirable to use multiple proxies whenever possible [ Liu et al., 2009 ].
dos.dos.3. Temperatures Time Show
Figure 2 shows different temperature records generated using the proxies discussed above, including the Mg/Ca DST record of Lear et al. and high- and low-latitude SST records for the EOT [ Lear et al., 2008 ; Liu et al., 2009 ]. Although existing Mg/Ca DST records show a net cooling throughout the Eocene, at face value they show either no significant cooling or even warming at the EOT [ Lear et al., 2000 ; Billups and Schrag, 2003 ; Lear et al., 2004 ; Peck et al., 2010 ; Pusz et al., 2011 ]. This is not consistent https://datingranking.net/nl/bumble-overzicht/ with the cooling that might be expected during a period of rapid ice growth [ Coxall and Pearson, 2007 ]. The lack of cooling in the Mg/Ca records at the EOT initially led to the hypothesis that the majority of the oxygen isotope ? 18 O shift at the EOT is due to an increase in ice mass [ Lear et al., 2000 ] (also see section 2.3 on ? 18 O). This would necessitate the growth of a greater ice mass than could be accommodated on Antarctica, implying that Northern Hemisphere ice sheets formed much earlier in the Cenozoic than previously thought [ Coxall et al., 2005 ]. Additional evidence for Northern Hemisphere glaciation (albeit as isolated glaciers) much earlier in the Cenozoic was found in ice-rafted debris (IRD) deposits from the Arctic Ocean [ Moran et al., 2006 ] and off the coast of Greenland [ Eldrett et al., 2007 ]. However, it has also been shown that Antarctic land area at the EOT could have been greater than at present, meaning that more of the ? 18 O increase can be explained by the growth of Antarctic ice in combination with cooling [ Wilson and Luyendyk, 2009 ]. In addition, modeling studies suggest that atmospheric CO2 concentrations were above the threshold for bipolar glaciation at this time [ ].