Abstract
Abstract. The relative abundance of individual archaeal membrane lipids, namely of glycerol dialkyl glycerol tetraethers (GDGTs) with different numbers of cyclopentane rings, varies with temperature, which enables their use as a paleotemperature proxy index. The first GDGT-based index in marine sediments called TEX86 is believed to reflect mean annual sea surface temperature (maSST). The TEX86L is an alternative temperature proxy for “low-temperature” regions (<15 ∘C), where the original TEX86 proxy calibration shows a larger scatter. However, TEX86L-derived temperatures still display anomalous estimates in polar regions. In order to elucidate the potential cause of the disagreement between the TEX86L estimate and SST, we analyzed GDGT fluxes and TEX86L-derived temperatures in sinking particles collected with time-series sediment traps in high-northern- and high-southern-latitude regions. At 1296 m depth in the eastern Fram Strait (79∘ N), a combination of various transporting mechanisms for GDGTs might result in seasonally different sinking velocities for particles carrying these lipids, resulting in strong variability in the TEX86L signal. The similarity of flux-weighted TEX86L temperatures from sinking particles and surface sediments implies an export of GDGTs without alteration in the Fram Strait. The estimated temperatures correspond to temperatures in water depths of 30–80 m, where nitrification might occur, indicating the favorable depth habitat of Thaumarchaeota. In the Antarctic Polar Front of the Atlantic sector (50∘ S), TEX86L-derived temperatures displayed warm and cold biases compared to satellite-derived SSTs at 614 m depth, and its flux-weighted mean signal differs from the deep signal at 3196 m. TEX86L-derived temperatures at 3196 m depth and the surface sediment showed up to 7 ∘C warmer temperatures relative to satellite-derived SST. Such a warm anomaly might be caused by GDGT contributions from Euryarchaeota, which are known to dominate archaeal communities in the circumpolar deep water of the Antarctic Polar Front. The other reason might be that a linear calibration is not appropriate for this frontal region. Of the newly suggested SST proxies based on hydroxylated GDGTs (OH-GDGTs), only those with OH-GDGT–0 and crenarchaeol or the ring index (RI) of OH-GDGTs yield realistic temperature estimates in our study regions, suggesting that OH-GDGTs could be applied as a potential temperature proxy in high-latitude oceans.
Highlights
The knowledge that Thaumarchaeota, one phylum of Archaea, regulate the composition of their membrane lipids according to the surrounding water temperatures enabled the development of the paleothermometer TEX86 (Kim et al, 2010; Schouten et al, 2002)
Ibration did not occur with those OH-glycerol dialkyl glycerol tetraethers (GDGTs) calibrations at the deep trap (Fig. 6d)
We speculated in Sect. 5.2.2 that the warm-biased TEXL86 temperatures relative to sea surface temperature (SST) may be caused by GDGTs synthesized by Euryarchaeota dwelling in deep waters of the Antarctic Polar Front (APF)
Summary
The knowledge that Thaumarchaeota, one phylum of Archaea, regulate the composition of their membrane lipids according to the surrounding water temperatures enabled the development of the paleothermometer TEX86 (Kim et al, 2010; Schouten et al, 2002). TEX86 is calculated based on the relative abundance of GDGT containing zero to three cyclopentane (GDGT–0 ∼ 3) or four cyclopentane and one cyclohexane (crenarchaeol) ring (Fig. 1). E. Park et al.: Archaeal lipid flux and GDGT-based thermometry use of TEX86 in high-latitude regions (Bendle and RosellMelé, 2004; Ho et al, 2014). A logarithmic calibration of TEXL86, excluding the crenarchaeol regio-isomer, was suggested for regions where maSSTs are below 15 ◦C (see Eq 2; Kim et al, 2010). The authors speculated that the lack of correlation between the crenarchaeol regio-isomer and SST at low temperatures might be caused by genetically different GDGT producers. All TEXL86 calibrations for temperature include a rather large scatter, resulting in a calibration error of ±4 ◦C for the TEXL86 calibration (Kim et al, 2010)
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