Glycerol Dialkyl Glycerol Tetraethers Concentrations Research Articles

GDGT distributions on the East Siberian Arctic Shelf: implications for organic carbon export, burial and degradation

Abstract. Siberian permafrost contains a globally significant pool of organic carbon (OC) that is vulnerable to enhanced warming and subsequent release into the contemporary carbon cycle. OC release by both fluvial and coastal erosion has been reported in the region, but the behaviour of this material in the Arctic Ocean is insufficiently understood. The balance between OC deposition and degradation on the East Siberian Arctic Shelf (ESAS) influences the climate–carbon cycle feedback in this area. In this study we couple measurements of glycerol dialkyl glycerol tetraethers (GDGTs) with bulk geochemical observations to improve knowledge of the sources of OC to the ESAS, the behaviour of specific biomarkers on the shelf and the balance between delivery and removal of different carbon pools. Branched GDGT (brGDGT) concentrations were highest close to river mouths, yet low in "ice complex" permafrost deposits, supporting recent observations that brGDGTs are mostly delivered by fluvial erosion, and may be a tracer for this in complex sedimentary environments. BrGDGT concentrations and the branched and isoprenoidal tetraether (BIT) index reduced quickly offshore, demonstrating a rapid reduction in river influence. Stable carbon isotope ratios changed at a different rate to the BIT index, suggesting not only that OC on the shelf is sourced from fluvial erosion but also that erosion of coastal sediments delivers substantial quantities of OC to the Arctic Ocean. A model of OC export from fluvial, coastal and marine sources is able to recreate the biomarker and bulk observations and provide estimates for the influence of fluvial and coastal OC across the whole shelf. The model shows that coastal erosion delivers 43 % of the OC and 87 % of the mineral sediment to the ESAS, but that rivers deliver 72 % of brGDGTs, indicating that brGDGTs can be used as a proxy for river-derived sediment.

Occurrence and distribution of glycerol dialkanol diethers and glycerol dialkyl glycerol tetraethers in a peat core from SW Tanzania

Isoprenoid and branched glycerol dialkanol diethers (iso GDDs and br GDDs) have recently been detected in various environments, including sediment, soil and peat deposits. Their structures strongly resemble those of glycerol dialkyl glycerol tetraethers (GDGTs). Nevertheless, the origin of GDDs and their link to GDGTs remain unclear. Here we examined the concentration and distribution of iso GDDs and br GDDs, together with those of iso GDGTs and br GDGTs along a 4m peat core from Tanzania. Whereas br GDDs have only been detected to date in their core lipid (CL) form, we report here, for the first time, br GDDs in both CL and intact polar lipid (IPL) forms, suggesting a biosynthetic origin for br GDDs. Concentrations of br GDGTs and br GDDs on one hand and of iso GDGTs and iso GDDs on the other hand, were observed to significantly co-vary (R2 0.49–0.58; p<0.05), both for the CL and IPL fractions. Moreover, the fractional abundance of each GDD correlated significantly with that of its GDGT analogue (R2 0.33–0.97; p<0.05). Taken together, the data show that GDDs and GDGTs are closely related and likely take part in common metabolic pathways, although the hypothesis of GDDs being degradation products of GDGTs cannot be excluded.

Distribution of ether lipids and composition of the archaeal community in terrestrial geothermal springs: impact of environmental variables.

Archaea can respond to changes in the environment by altering the composition of their membrane lipids, for example, by modification of the abundance and composition of glycerol dialkyl glycerol tetraethers (GDGTs). Here, we investigated the abundance and proportions of polar GDGTs (P-GDGTs) and core GDGTs (C-GDGTs) sampled in different seasons from Tengchong hot springs (Yunnan, China), which encompassed a pH range of 2.5-10.1 and a temperature range of 43.7-93.6°C. The phylogenetic composition of the archaeal community (reanalysed from published work) divided the Archaea in spring sediment samples into three major groups that corresponded with spring pH: acidic, circumneutral and alkaline. Cluster analysis showed correlation between spring pH and the composition of P- and C-GDGTs and archaeal 16S rRNA genes, indicating an intimate link between resident Archaea and the distribution of P- and C-GDGTs in Tengchong hot springs. The distribution of GDGTs in Tengchong springs was also significantly affected by temperature; however, the relationship was weaker than with pH. Analysis of published datasets including samples from Tibet, Yellowstone and the US Great Basin hot springs revealed a similar relationship between pH and GDGT content. Specifically, low pH springs had higher concentrations of GDGTs with high numbers of cyclopentyl rings than neutral and alkaline springs, which is consistent with the predominance of high cyclopentyl ring-characterized Sulfolobales and Thermoplasmatales present in some of the low pH springs. Our study suggests that the resident Archaea in these hot springs are acclimated if not adapted to low pH by their genetic capacity to effect the packing density of their membranes by increasing cyclopentyl rings in GDGTs at the rank of community.

A mineralogical and organic geochemical overview of the effects of Holocene changes in Amazon River flow on three floodplain lakes

A synthesis of the impacts of the Amazon River hydrological changes on the sedimentation process of organic matter (OM) in three different floodplain lakes (Santa Ninha, Maracá, and Comprido lakes) is presented in this study. Today the Santa Ninha and Maracá lakes are directly and permanently connected with the main channel of the Amazon River, in contrast to Comprido Lake, which is indirectly and periodically influenced by the Amazon River due to its greater distance from the main channel. All three lake sediment records showed a reduced river inflow due to dry climatic conditions during the early and middle Holocene followed by an increased fluvial input during the wetter late Holocene. In Santa Ninha and Maraca Lakes, the reduced river inflow period was recorded by sediments with a low abundance of smectite (on average ~20wt.%), a clay mineral mainly transported by the fluvial system, high total organic carbon (TOC) contents (on average ~8.2wt.%), and a predominantly acidic soil OM input evidenced by high concentrations (on average 180μggTOC−1) of branched glycerol dialkyl glycerol tetraethers (GDGT). During the late Holocene, a higher smectite abundance (on average ~43 wt.%) and a lower TOC content (on average ~1.4 wt.%) pointed to greater dilution by riverine lithogenic matter. This change was accompanied by a proportional increase in the aquatically produced crenarchaeol, suggesting a higher lake water level. In Comprido Lake, a sedimentation gap occurred during the early and middle Holocene. The wetter late Holocene, since 3000calyears BP, was characterized by high TOC values (on average ~9wt.%) and a sharp increase in soil OM input as recorded by an increase in branched GDGT concentrations (on average ~81μggTOC−1), but the smectite content was low (on average ~14%). This combination suggests that the soil OM input to Comprido Lake from the local catchment area became dominant during the wet-climate late Holocene due to the large distance of the lake from the Amazon River main channel. Consequently, our study shows that the sedimentation processes of OM in Amazonian floodplain lakes were strongly influenced by variations in the hydrodynamic regime of the Amazon River during the Holocene. However, the impacts of the variations on the three floodplain lakes were different depending on the distance of each lake from the main channel of the Amazon River.

Nutricline shoaling in the eastern Pacific warm pool during the last two glacial maxima

Isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs) and alkenones were analyzed in sediment samples retrieved from Ocean Drilling Program Site 1241 covering the last 150000 years to understand the hydrological evolution of the eastern Pacific warm pool (EPWP). GDGT and alkenone concentrations showed higher values in marine isotope stage (MIS)-2 and MIS-6, which suggests the enhancement of primary production at glacial maxima. \( {\text{TEX}}_{86}^{\text{H}} \)- and \( U_{ 3 7^\prime }^{\text{K}} \)-derived temperature depicted different temperature evolutions. \( U_{ 3 7^\prime }^{\text{K}} \)-derived temperature was marked by small variation during the glacial–interglacial cycles, whereas \( {\text{TEX}}_{86}^{\text{H}} \)-derived temperature showed pronounced glacial–interglacial variation that was similar to Mg/Ca-derived temperature records from nearby cores in the EPWP. Given that enhanced primary production during glacial maxima suggests nutricline shoaling, unchanged \( U_{ 3 7^\prime }^{\text{K}} \) over glacial–interglacial cycles can be interpreted as the shift of alkenone production depth. \( {\text{TEX}}_{86}^{\text{H}} \) seems not to be influenced by glacial–interglacial changes in nutricline depths, recording an integrated temperature in surface and thermocline water. The shallow nutricline in the EPWP during glacial maxima most likely reflected the intense formation of Antarctic intermediate water.

Source and distribution of glycerol dialkyl glycerol tetraethers along lower Yellow River-estuary–coast transect

To assess the source of glycerol dialkyl glycerol tetraethers (GDGTs) and their usefulness as proxies for terrestrial organic matter inputs and temperature in the Yellow River-dominated margin, we measured isoprenoid and branched GDGT concentrations in surface sediments along a lower Yellow River-estuary–coast transect. Branched GDGTs dominated over isoprenoid GDGTs in the riverbed sediments and had similar compositions from river to coast. In contrast, isoprenoid GDGTs displayed an increasing abundance and a decreasing GDGT-0 to crenarchaeol ratio (1.6 to 0.6) toward the sea. Such distribution patterns of GDGTs, combined with the result from a principal component analysis (PCA), confirmed the different origin of branched and isoprenoid GDGTs with branched GDGTs being primarily from soil erosion of the Chinese loess plateau (CLP) whereas, in addition to allochthonous terrestrial inputs, aquatic Thaumarchaeota partially contributes to the isoprenoid GDGT pool in estuarine and coastal areas. The branched GDGT-derived temperature (avg. 11°C) is consistent with the annual mean air temperature (MAT) of the CLP in the middle river basin, a major source region for the Yellow River sediments, whereas the isoprenoid-derived temperature (12.7 to 28.4°C) deviated widely from the annual mean temperature in the study region. Application of a binary mixing model based on δ13C, the branched and isoprenoid tetraether (BIT) index and branched GDGT concentrations showed consistent decreases in the relative amount of terrestrial organic carbon toward the sea, but estimates from the latter two proxies were lower than those from the δ13C.

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.

Constraints in the application of the Branched and Isoprenoid Tetraether index as a terrestrial input proxy

[1] Determination of the relative inputs of aquatic autochthonous and terrestrial allochthonous organic matter into marine and lacustrine environments is essential to understanding the global carbon budget. A variety of proxies are used for this purpose, including the Branched and Isoprenoid Tetraether (BIT) index. This is calculated from the concentrations of branched glycerol dialkyl glycerol tetraethers (GDGTs), derived from unidentified terrestrial bacteria, and crenarchaeol, a marker for aquatic mesophile Thaumarchaeota (Crenarchaeota group I). As the index is a ratio, its value depends on both the crenarchaeol aquatic in situ production and the soil-derived branched GDGT input. Therefore, the BIT index reflects not only changes in the input of terrestrial or soil organic matter but also relative variations in aquatic Thaumarchaeota abundance in the water column. In fact, we show that in oceanic and lacustrine settings, the BIT index can be dominated by the aquatic end-member of the ratio. Consequently, the BIT index by itself can be an unreliable proxy to compare the input of terrestrial matter between sites and over time, and we propose that the quantification of branched GDGT fluxes or concentrations may instead be a better indicator of soil terrestrial inputs.

Distributions of branched GDGTs in soils and lake sediments from western Uganda: Implications for a lacustrine paleothermometer

Bacterially produced branched glycerol dialkyl glycerol tetraethers (GDGTs) are ubiquitous in soils and lake sediments and can potentially be used to reconstruct past temperatures. In lakes, however, it is still unclear if these compounds are derived from eroded soils or if they are produced in situ. To better understand environmental controls on the distributions of these compounds and the sources of branched GDGTs to lake sediments, we compare branched GDGT distributions and concentrations in lake sediments and catchment soils within a 3600m altitudinal transect in western Uganda. Reconstructed mean annual air temperature (MAAT), determined from the degree of methylation (MBT) and cyclisation (CBT) of branched GDGTs in soils, decreases with increasing altitude, as is expected from the air temperature gradient in our transect. However, we observe significant offsets between observed and reconstructed temperatures in soils from wet, high elevation soils but not in most dry, low elevation soils. Branched GDGT distributions differ significantly between lake sediments and soils at all elevations, with greater differences at low elevations than at high elevations. These data support previous hypotheses that branched GDGTs are produced in situ in lakes and suggest that the abundance of water in soil environments may play a role in controlling the distribution of branched GDGTs. While branched GDGTs in lacustrine sediments can be used to reconstruct temporal temperature variations in lakes, we urge caution in utilizing this proxy in lacustrine systems with high soil loadings, especially if there is evidence of changing clastic fluxes through time.

Effects of long term oxic degradation on the [formula omitted], TEX 86 and BIT organic proxies

One of the primary prerequisites for the application of organic proxies is that they should not be substantially affected by diagenesis. However, studies have shown that oxic degradation of biomarker lipids can affect their relative distribution. We tested the diagenetic stability of the U 37 K ′ and TEX 86 palaeothermometers upon long term oxygen exposure. For this purpose, we studied the distributions of alkenones and glycerol dialkyl glycerol tetraethers (GDGTs) in different sections of turbidites at the Madeira Abyssal Plain (MAP) that experienced different degrees of oxygen exposure. Sediments were deposited anoxically on the shelf and then transported by turbidity currents to the MAP, which has oxic bottom water. This resulted in partial degradation of the turbidite organic matter as a result of long term exposure to oxic bottom water. Concentrations of GDGTs and alkenones were reduced by one to two orders of magnitude in the oxidized parts of the turbidites compared to the unoxidized parts, indicating substantial degradation. High-resolution analysis of the Pleistocene F-turbidite showed that the U 37 K ′ index of long chain alkenones increased only slightly (0.01, corresponding to <0.5 °C) in the oxidized part of the turbidite, suggesting minor preferential degradation of the C 37:3 alkenone, in agreement with previous studies. TEX 86 values showed a small increase (0.02, corresponding to ∼2 °C) in the F-turbidite, like U 37 K ′ , while for other Pliocene/Miocene turbidites it either remained unchanged or decreased substantially (up to 0.06, corresponding to ∼6 °C). Previous observations showed that the BIT index, a proxy for the contribution of soil organic matter to total organic carbon, was always substantially higher in the oxidized part in all the turbidites, as a result of preferential degradation of marine-derived GDGTs. This relative increase in soil-derived GDGTs affects TEX 86, as the isoprenoid GDGT distribution on the continent can be quite different from that in the marine environment. Our results indicate that the organic proxies are affected by long term oxic degradation to different extents; this should be taken into account when applying these proxies in palaeoceanographic studies of sediments which have been exposed to prolonged oxic degradation.

Constraints on the application of the MBT/CBT palaeothermometer at high latitude environments (Svalbard, Norway)

Branched glycerol dialkyl glycerol tetraethers (GDGTs) are membrane lipids of unknown bacteria that are ubiquitous in soil and peat. Two indices based on the distribution of these lipids in soils, the Cyclization of Branched Tetraethers (CBT) and the Methylation of Branched Tetraethers (MBT) indices have been shown to correlate with soil pH, and mean annual air temperature (MAT) and soil pH, respectively, and can be used to reconstruct MAT in palaeoenvironments. To verify the extent to which branched GDGTs in marine sediments reflect the distribution pattern on land and whether these proxies are applicable for palaeoclimate reconstruction in high latitude environments with a MAT of <0°C, we compared the branched GDGT distribution in Svalbard soils and nearby fjord sediments. Although branched GDGT concentrations in the soil are relatively low (0.02–0.95μg/g dry weight (dw)) because of the cold climate and the short growing season, reconstructed MATs based on the MBT/CBT proxy are ca. −4°C, close to the measured MAT (ca. −6°C). Concentrations of branched GDGTs (0.01–0.20μg/g dw) in fjord sediments increased towards the open ocean and the distribution was strikingly different from that in soil, i.e. dominated by GDGTs with one cyclopentane moiety. This resulted in MBT/CBT-reconstructed MAT values of 11–19°C, well above measured MAT. The results suggest that at least part of the branched GDGTs in marine sediments in settings with a low soil organic matter (OM) input may be produced in situ. In these cases, the application of the MBT/CBT palaeothermometer will generate unrealistic MAT reconstructions. The MBT/CBT proxy should therefore only be used at sites with a substantial input of soil OM relative to the amount of marine OM, i.e. at sites close to the mouth of rivers with a catchment area where sufficient soil formation takes place and the soil thus contains substantial amounts of branched GDGTs.

Changes in terrestrial organic matter input to the Mendeleev Ridge, western Arctic Ocean, during the Late Quaternary

Hydrocarbons and glycerol dialkyl glycerol tetraethers (GDGTs) were analyzed in Late Pleistocene sediments of Core HLY0503-08JPC collected at the Mendeleev Ridge during the Healy-Oden Trans Arctic Expedition 2005 (HOTRAX'05) to investigate environmental changes in the western Arctic Ocean during the last full glacial cycle, ca. 130 kyr. Variations in long-chain n-alkane and GDGT concentrations correspond to alternated color banding, brown (interglacial/interstadial) and grayish (glacial/stadial) layers. Grayish layers are characterized by abundant higher-plant n-alkanes and branched GDGTs, implying larger contribution of terrestrial plant and soil organic matter (OM) in glacial environments, possibly due to the deposition of fine-grained products of glacial erosion in the Amerasian basin. Lithic n-alkanes derived from mature OM show pronounced peaks, which can be classified into six types presumably indicative of various sediment sources. Some peaks are correlated to events of iceberg discharge and freshwater outbursts from proglacial lakes of the Eurasian and, possibly, Laurentide ice sheets, suggesting that other peaks may correspond to similar events.

An experimental field study to test the stability of lipids used for the TEX 86 and [formula omitted] palaeothermometers

We report a one year field experiment to study sedimentary degradation of isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs) and long-chain alkenones, lipids used for the TEX 86 and U 37 K ′ sea surface temperature (SST) proxies, respectively. Homogenised surface sediments from the Namibian upwelling region were set out on moorings in oxic (Oder Bay and Arkona Bay; Baltic Sea) and sub-oxic to anoxic (Gottland Basin; Baltic Sea) waters and in oxic Atlantic waters off Cape Blanc for one year to assess the influence of oxygen exposure on GDGTs and alkenones. No selective degradation effect on both TEX 86 and U 37 K ′ proxies was evident under anoxic and sub-oxic conditions. Under oxic conditions, GDGT concentrations and the TEX 86 did not significantly change as well. However, selective degradation of alkenones occurred at all oxic sites resulting in a small but significant increase of the U 37 K ′ index by up to 0.06 units, corresponding to a temperature increase of up to 1.8 °C. Our results demonstrate that selective alkenone degradation is a relatively fast process and complement earlier evidence for selective degradation of alkenones in the laboratory, water column and sediments. In contrast, our results suggest that the TEX 86 is relatively resistant to selective oxic degradation at least on relatively short (years) time scales.

Selective preservation of soil organic matter in oxidized marine sediments (Madeira Abyssal Plain)

In ocean margin sediments both marine and terrestrial organic matter (OM) are buried but the factors governing their relative preservation and degradation are not well understood. In this study, we analysed the degree of preservation of marine isoprenoidal and soil-derived branched glycerol dialkyl glycerol tetraethers (GDGTs) upon long-term oxygen exposure in OM-rich turbidites from the Madeira Abyssal Plain by analyzing GDGT concentrations across oxidation fronts. Relative to the anoxic part of the turbidites ca. 7–20% of the soil-derived branched GDGTs were preserved in the oxidized part while only 0.2–3% of the marine isoprenoid GDGT crenarchaeol was preserved. Due to these different preservation factors the Branched Isoprenoid Tetraether (BIT) index, a ratio between crenarchaeol and the major branched GDGTs that is used as a tracer for soil-derived organic matter, substantially increases from 0.02 to 0.4. Split Flow Thin Cell (SPLITT) separation of turbidite sediments showed that the enhanced preservation of soil-derived carbon was a general phenomenon across the fine particle size ranges (<38 μm). Calculations reveal that, despite their relatively similar chemical structures, degradation rates of crenarchaeol are 2-fold higher than those of soil-derived branched GDGTs, suggesting preferential soil OM preservation possibly due to matrix protection.