Production Of Glycerol Dialkyl Glycerol Tetraether Research Articles

Environmental controls on the distribution of GDGT molecules in Lake Höglwörth, Southern Germany

Tetraether lipids, such as glycerol dialkyl glycerol tetraethers (GDGTs), are an increasingly used proxy for reconstructing paleoclimatic and paleoenvironmental dynamics. On a global scale, environmental drivers controlling the distribution of GDGTs in lake systems have been described extensively. However, the drivers that control down-core sedimentary GDGT composition within single lake systems are still poorly constrained. Here, we analysed GDGTs in a sediment core collected from Lake Höglwörth (Bavaria, Germany) covering the last ∼1100 years. The increase in ratio ΣIIIa/ΣIIa values (a ratio which captures the variation of 3 structural isomers of penta- and hexamethylated brGDGTs respectively) reflects a higher contribution of branched (br) GDGTs from aquatic sources after 1800 CE. This is synchronous with a high nutrient input as a result of increased local anthropogenic activities. The climate proxy methylation of branched tetraethers (MBT'5ME) reveals a pattern that deviates from the anticipated temperature changes during the last millennium. Instead, high MBT'5ME values from ∼880 to 1120 CE coincide with a period of increased soil input, resulting from lake damming and/or construction of the Höglwörth monastery. Decreased values after 1800 CE until present date correspond to a higher contribution of aquatic brGDGTs, with changes in the dependency between Ia and IIa suggesting a direct influence of bacterial community changes on the MBT'5ME. The proxy cyclisation of branched tetraethers (CBT') shows a linear increase with time, and a stepwise increase at 1700 CE, when a creek stream inflow channel was rerouted. Over time, MBT’5ME decreases gradually, while CBT’ and ΣIIIa/ΣIIa increase. This is hypothesized to be potentially caused by decreasing water depth over time and/or post- or syn-sedimentary production of GDGTs. The degree of cyclization (DC') shows variations on shorter timescales, possibly driven by the lake mixing regime and water column redox conditions. Our results indicate that caution must be taken when interpreting GDGTs for paleoclimate and paleoenvironmental reconstruction considering the possible influence of shifts in the provenance of brGDGTs. Further studies focusing on both sedimentary and intact polar lipids are highly recommended to constrain the source (water column, or sedimentary production) of GDGTs in lake sediments.

Spatial and temporal distributions of glycerol dialkyl glycerol tetraethers in suspended materials in the northern East China Sea: Applicability as a paleotemperature proxy

The spatial and temporal distributions of the glycerol dialkyl glycerol tetraether (GDGT) concentrations in suspended materials were investigated to determine whether the GDGT-based TEX86 can be used as a proxy for annual mean sea surface temperature in the northern East China Sea (ECS). Seawater samples were collected seasonally at three-month intervals from the surface and subsurface of the northern ECS. GDGT in the surface seawater was detected consistently across all seasons. However, the TEX86 temperatures of the surface seawater samples differed significantly from the in situ temperatures over the entire study area; in particular, TEX86 temperatures were higher (4–7 °C) than in situ temperatures in winter, and lower (9–12 °C) than in situ temperatures in summer. The occurrence of a high concentration of GDGTs near the bottom, with marked differences between TEX86 and in situ temperatures in the shallow water (~30 m) supports the possibility of sediment resuspension near the bottom in the western part and subsequent transportation eastward. GDGTs may have been resuspended owing to vertical mixing by strong winds, such as those from the monsoon (winter) and typhoons (summer), and increased turbulence near the bottom caused by tides across all seasons in the shallow waters (30–50 m). In contrast, the vertical distribution of GDGT concentration in the eastern part of the study area deeper than 100 m indicated that maximum GDGT concentrations occurred at depths of between 20 m and 50 m in all seasons. The TEX86 temperatures of the subsurface samples were similar to or warmer than in situ temperatures between 20 m and 50 m, suggesting that the main depth at which GDGT was produced in the eastern area could be the subsurface in that same depth range. A new local calibration equation was established based on the subsurface samples from the east. Our results indicate that sediment resuspension and subsurface production of GDGTs can have a significant impact on the use of GDGT-based TEX86 as a SST proxy in continental shelf area.

A four-year record of UK′37- and TEX86-derived sea surface temperature estimates from sinking particles in the filamentous upwelling region off Cape Blanc, Mauritania

Lipid biomarker records from sinking particles collected by sediment traps can be used to study the seasonality of biomarker production as well as processes of particle formation and settling, ultimately leading to the preservation of the biomarkers in sediments. Here we present records of the biomarker indices U37K′ based on alkenones and TEX86 based on isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs), both used for the reconstruction of sea surface temperatures (SST). These records were obtained from sinking particles collected using a sediment trap moored in the filamentous upwelling zone off Cape Blanc, Mauritania, at approximately 1300 water depth during a four-year time interval between 2003 and 2007, and supplemented by U37K′ and TEX86 determined on suspended particulate matter collected from surface waters in the study area. Mass and lipid fluxes are highest during peak upwelling periods between October and June. The alkenone and GDGT records both display pronounced seasonal variability. Sinking velocities calculated from the time lag between measured SST maxima and minima and corresponding index maxima and minima in the trap samples are higher for particles containing alkenones (14–59md−1) than for GDGTs (9–17md−1). It is suggested that GDGTs are predominantly exported from shallow waters by incorporation in opal-rich particles. SST estimates based on the U37K′ index correspond to the amplitude observed fluctuations in SST during the study period. Temperature estimates based on TEX86 show smaller seasonal amplitudes, which can be explained by either predominant production of GDGTs during the warm season, or a contribution of GDGTs exported from deep waters, which are in this region known to carry GDGTs in a distribution that translates to a high TEX86 signal.

Sources of glycerol dialkyl glycerol tetraethers (GDGTs) in catchment soils, water column and sediments of Lake Rotsee (Switzerland) – Implications for the application of GDGT-based proxies for lakes

We analysed glycerol dialkyl glycerol tetraether (GDGT) distributions in the water column, sediment and catchment soils of the Swiss Lake Rotsee to determine the sources of GDGTs in the lake sediment and to determine the implications for GDGT-based palaeoclimate proxies. The branched GDGT (brGDGT) distribution in the soils surrounding the lake showed significant heterogeneity, which may be partly explained by vegetation cover and soil moisture. One group of soils seems to provide the largest contribution of soil-derived GDGTs to the lake, or the distribution of GDGTs in these soils is affected by the relatively high soil moisture availability, creating lake-like conditions and GDGT distributions. Comparison of GDGT distributions in soils, water column and sediments indicated that brGDGTs and crenarchaeol in the sediment are partly soil derived, but that in situ production in the water column and/or sediment also takes place. Eutrophication seems to affect the distributions of brGDGTs by dilution of the supply of soil derived brGDGTs to the lake and by changing the degree of in situ production of brGDGTs in the water column. Furthermore, the eutrophic conditions in the lake promote methanogenic activity and subsequently cause a contribution of isoprenoid GDGTs (isoGDGTs) of methanogenic origin to the sediments. The aquatic production of GDGTs has implications for the reliability of GDGT-based proxy results. In particular, the application of the BIT and TEX86 indices is hampered by the mixed sources of the GDGTs in the lake. In contrast, global lake-specific brGDGT-based temperature calibrations resulted in temperature estimates that resemble measured mean annual and summer air temperatures. CBT-derived pH values agreed well with measured soil and water column values. Our results demonstrate that understanding the source of GDGTs in lake sediments is important for the robust interpretation of palaeoclimate records obtained from downcore proxy applications.

Distribution of glycerol dialkyl glycerol tetraethers, alkenones and polyunsaturated fatty acids in suspended particulate organic matter in the East China Sea

We investigated the spatial distribution of glycerol dialkyl glycerol tetraethers (GDGTs), alkenones, and polyunsaturated fatty acids in particulate organic matter collected at four sites along a depth transect from the continental shelf to the Okinawa Trough in the East China Sea during the spring bloom in 2008. The maximum alkenone concentration appeared in the top 25 m at all sites and the $$ U_{37}^{{{\text{K}}'}} $$ values were consistent with in situ water temperatures in the depth interval, suggesting that the alkenones were produced mainly in surface water. At the slope and shelf sites, GDGTs in the water column showed a concentration maximum at 74–99 m depth, and the $$ {\text{TEX}}_{86}^{\text{H}} $$ agreed with in situ water temperatures, suggesting the in situ production of GDGTs in the depth interval. The low-salinity surface water above 20 m depth was characterized by low GDGT concentrations and low $$ {\text{TEX}}_{86}^{\text{L}} $$ -based temperatures, suggesting either the production of GDGTs in winter season or the lateral advection of GDGTs by an eastward current. At the slope and Okinawa Trough sites, TEX86-based temperatures were nearly constant in the water column deeper than 300 m and corresponded to temperatures at the surface and near-surface waters rather than in situ temperatures. This observation is consistent with a hypothesis that Thaumarchaeota cells produced in surface waters are delivered to deeper water and also indicates that the residence time of suspended GDGTs in the deep-water column is large enough to mix the GDGTs produced in different seasons.

Multi-proxy reconstruction of terrigenous input and sea-surface temperatures in the eastern Gulf of Guinea over the last ~ 35 ka

The tight coupling between the atmospheric and oceanic circulation in the equatorial Atlantic region makes this area an important region for paleoclimatic research. Previous studies report the occurrence of large amounts of terrigenous material and soil organic carbon (SOC) within the marine sediments of the eastern Gulf of Guinea. We use the accumulation rates (AR) of branched glycerol dialkyl glycerol tetraethers (GDGTs) to identify variations in SOC delivery to the Niger Fan over the last 35ka, and compare these records to long-chain n-alkanes as a proxy for higher plant material, to an inorganic proxy for terrigenous input (aluminum AR) and to indicators for the marine productivity (AR of carbonate and crenarchaeol). In addition, sea surface temperatures (SSTs) are calculated based on the TEX86H index and environmental factors affecting the SST-reconstructions are discussed.Our results indicate that Al AR are closely connected to the rate of mean sea level change after 15kaBP, with an additional influence of the increased monsoonal precipitation and extended vegetation cover corresponding to the African Humid Period (14.8–5.5kaBP). Branched GDGT AR appears to be determined by shelf erosion in addition to the interplay of monsoonal precipitation and vegetation cover controlling soil erosion. Long-chain n-alkane concentrations clearly show a different trend than the other proxies, which might be due to their predominant eolian transport.Paleo-SSTs show a clear shift from colder temperatures during the last glacial period (20–22°C) to warmer temperatures during the Holocene (24–26°C). However, TEX86H-based SSTs are cold-biased compared to recent SSTs and Mg/Ca-based SST reconstructions, which is probably caused by a high seasonality of the Thaumarchaeota, with a maximum productivity of these organisms during the cold summer months. However, a sub-surface production of GDGTs and/or a potential bias of SST reconstruction by terrestrial input could not be completely excluded.

Applicability and calibration of the TEX 86 paleothermometer in lakes

We have conducted a global survey of archaeal glycerol dialkyl glycerol tetraether (GDGT) lipids in lake sediments in order to develop the TEX 86 paleotemperature proxy for application in continental systems. Surface sediments of 46 globally distributed lakes were analyzed for GDGT, but isoprenoid GDGT derived from aquatic Crenarchaeota, were only unambiguously detected in 20 of the 46 lakes analyzed. Aquatic crenarchaeotal GDGT were detected mainly in sediments from large lakes (>4000 km 2) and hydrothermal or volcanic lakes, suggesting that in some (mostly smaller) lakes either aquatic Crenarchaeota are present at low abundance resulting in sedimentary lipids below levels of detection, or they are absent. Branched GDGT, thought to be derived primarily from soil bacteria, were identified in all lake sediments analyzed. Correlation of the TEX 86 in those lakes with sufficient amounts of putative crenarchaeotal GDGT with annual mean lake temperature is reasonably good ( r 2 = 0.68, N = 20). In order to reduce the influence of soil derived isoprenoid GDGT on the TEX 86 lake temperatures, we have applied a filter based on relative soil derived inputs (as determined by the BIT [Branched and Isoprenoid Tetraether] index) which results in a calibration relationship with r 2 = 0.86 ( N = 12) and an estimated temperature error of 3.6 °C. Our results suggest that the TEX 86 should be applied only in lakes with sufficient production of GDGT by aquatic Crenarchaeota relative to isoprenoid GDGT derived from soil in the watershed or other aquatic sources.

Distributions of branched GDGTs in a tropical lake system: Implications for lacustrine application of the MBT/CBT paleoproxy

Branched glycerol dialkyl glycerol tetraether (GDGT) lipids are abundant and ubiquitous in lake sediments, potentially allowing for a paleolimnological application of the so-called MBT/CBT proxy (methylation index of branched tetraethers/cyclization ratio of branched tetraethers). To investigate the origin and characteristics of these compounds in lacustrine environments, we examined the distributions of GDGTs in soils, river sediments and lake sediments from Lake Towuti on the island of Sulawesi, Indonesia. We found significant differences in the degree of methylation and cyclization (expressed by way of the MBT and CBT indices) between the soil samples and the aquatic samples, suggesting that there may be in situ production of GDGTs in the aquatic environment. Based on these findings, we urge caution in the application of the MBT/CBT paleoproxy to lake sediments and encourage more rigorous study of these compounds in freshwater environments.

Archaeal tetraether membrane lipid fluxes in the northeastern Pacific and the Arabian Sea: Implications for TEX86 paleothermometry

The newly introduced temperature proxy, the tetraether index of archaeal lipids with 86 carbon atoms (TEX86), is based on the number of cyclopentane moieties in the glycerol dialkyl glycerol tetraether (GDGT) lipids of marine Crenarchaeota. The composition of sedimentary GDGTs used for TEX86 paleothermometry is thought to reflect sea surface temperature (SST). However, marine Crenarchaeota occur ubiquitously in the world oceans over the entire depth range and not just in surface waters. We analyzed the GDGT distribution in settling particulate organic matter collected in sediment traps from the northeastern Pacific Ocean and the Arabian Sea to investigate the seasonal and spatial distribution of the fluxes of crenarchaeotal GDGTs and the origin of the TEX86 signal transported to the sediment. In both settings the TEX86 measured at all trap deployment depths reflects SST. In the Arabian Sea, analysis of an annual time series showed that the SST estimate based on TEX86 in the shallowest trap at 500 m followed the in situ SST with a 1 to 3 week time delay, likely caused by the relatively low settling speed of sinking particles. This revealed that the GDGT signal that reaches deeper water is derived from the upper water column rather than in situ production of GDGTs. The GDGT temperature signal in deeper traps at 1500 m and 3000 m did not show a seasonal cyclicity observed in the 500 m trap but rather reflected the annual mean SST. This is probably due to a homogenization of the TEX86 SST signal carried by particles as they ultimately reach the interior of the ocean. Our data confirm the use of TEX86 as a temperature proxy of surface ocean waters.