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 can be moved sideways and can also be recycled out-of sediments, setting traditional alkenones or alkenones synthesized in different environment onto core passes and you will probably biasing You 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 caribbeancupid-recensies 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 ].

2.dos.3. Temperatures Go out Series

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 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 [ ].