Description: Branched glycerol dialkyl glycerol tetraethers (brGDGTs), which are thought to be transported from soil to marine sediment by rivers, have been used to reconstruct the mean annual air temperature (MAAT) and soil pH of the drainage basin using the methylation index of branched tetraethers (MBT, recently refined as MBT') and cyclization index of branched tetraethers (CBT) from coastal marine sediment records. In this study we are tracing the brGDGTs from source to sink in the Tagus River basin, the longest river system on the Iberian Peninsula, by determining their concentration and distribution in soils, river suspended particulate matter (SPM), riverbank sediments, marine SPM, and marine surface sediments. The concentrations of brGDGTs in river SPM were substantially higher and their distributions were different compared to those of the drainage basin soils. This indicates that brGDGTs are mainly produced in the river itself. In the marine environment, the brGDGT concentrations rapidly decreased with increasing distance from the Tagus estuary. At the same time, the brGDGT distributions in marine sediments also changed, indicating that marine in-situ production also takes place. These results show that there are various problems that complicate the use of the MBT'/CBT for paleoreconstructions using coastal marine sediments in the vicinity of a river. However, if the majority of brGDGTs are produced in the river, it might be possible to reconstruct the environmental (temperature and pH) conditions of the river water using appropriate aquatic calibrations, provided that marine core locations are chosen in such a way that the brGDGTs in their sediments are predominantly derived from riverine in-situ production.

Description: 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 (0.02-0.95 µg/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 (0.01-0.20 µg/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.

Description: Our understanding of the continental climate development in East Asia is mainly based on loess-paleosol; sequences and summer monsoon precipitation reconstructions based on oxygen isotopes (?18O) of stalagmites from several Chinese caves. Based on these records, it is thought that East Asian Summer Monsoon (EASM) precipitation generally follows Northern Hemisphere (NH) summer insolation. However, not much is known about the magnitude and timing of deglacial warming on the East Asian continent. In this study we reconstruct continental air temperatures for central China covering the last 34,000 yr, based on the distribution of fossil branched tetraether membrane lipids of soil bacteria in a loess-paleosol sequence from the Mangshan loess plateau. The results indicate that air temperature varied in phase with NH summer insolation, and that the onset of deglacial warming at ~19 kyr BP is parallel in timing with other continental records from e.g. Antarctica, southern Africa and South-America. The air temperature increased from ~15 °C at the onset of the warming to a maximum of ~27 °C in the early Holocene (~12 kyr BP), in agreement with the temperature increase inferred from e.g. pollen and phytolith data, and permafrost limits in central China. Comparison of the tetraether membrane lipid-derived temperature record with loess-paleosol proxy records and stalagmite ?18O records shows that the strengthening of EASM precipitation lagged that of deglacial warming by ca. 3 kyr. Moreover, intense soil formation in the loess deposits, caused by substantial increases in summer monsoon precipitation, only started around 12 kyr BP (ca. 7 kyr lag). Our results thus show that the intensification of EASM precipitation unambiguously lagged deglacial warming and NH summer insolation, and may contribute to a better understanding of the mechanisms controlling ice age terminations.

Description: Soil-derived branched glycerol dialkyl glycerol tetraethers (brGDGTs) in marine river fan sediments have a potential use for determining changes in the mean annual temperature (MAT) and pH of the river watershed soils. Prior to their incorporation in marine sediments, the compounds are transported to the marine system by rivers. However, emerging evidence suggests that the brGDGTs in freshwater systems can be derived from both soil run-off and in situ production. The production of brGDGTs in the river system can complicate the interpretation of the brGDGT signal delivered to the marine system. Therefore, we studied the distribution of brGDGT lipids in suspended particulate matter (SPM) of the Yenisei River. Chromatographic improvements allowed quantification of the recently described hexamethylated brGDGT isomer, characterized by having two methyl groups at the 6/6' instead of the 5/5' positions, in an environmental dataset for the first time. This novel compound was the most abundant brGDGT in SPM from the Yenisei. Its fractional abundance correlated well with that of the 6-methyl isomer of the hexamethylated brGDGT that contains one cyclopentane moiety. The Yenisei River watershed is characterized by large differences in MAT (〉11 °C) as it spans a large latitudinal range (46-73°N), which would be expected to be reflected in brGDGT distributions of its soils. However, the brGDGT distributions in its SPM show little variation. Furthermore, the reconstructed pH values are high compared to the watershed soil pH. We, therefore, hypothesize that the brGDGTs in the Yenisei River SPM are predominantly produced in situ and not primarily derived from erosion of soil. This accounts for the absence of a change in the temperature signal, as the river water temperature is more stable. Using a lake calibration, the reconstructed temperature values agree with the mean summer temperatures (MST) recorded. The brGDGTs delivered to the sea by the Yenisei River during this season are thus not soil-derived, possibly complicating the use of brGDGTs in marine sediments for palaeoclimate reconstructions.

Description: Anaerobic oxidation of ammonium (anammox) is an important process in the marine nitrogen cycle. It has been estimated to contribute up to 50% of nitrogen loss from the ocean, and is especially prevalent within oxygen minimum zones (OMZs). Here we studied the presence and distribution of anammox in the extended OMZ of the Eastern Tropical North Pacific (ETNP) using ladderane fatty acids, specific biomarkers for anammox bacteria. The validity of ladderane fatty acids as proxies for anammox bacteria was demonstrated by their excellent correspondence with anammox 16S rRNA functional gene abundances and their expression and intact polar ladderane lipid concentrations in suspended particulate matter (SPM) from the Arabian Sea. In the ETNP, SPM was collected from various water depths at four stations along a northwest to southeast cruise transect and ladderane fatty acids were analyzed at each station. In all SPM samples where ladderane lipids were detected, C18 ladderane fatty acids were on average 5 fold more abundant than C20 ladderane fatty acids. Maximum concentrations in ladderane fatty acids (1.1 - 2.3 ng/l) were recorded at 400-600 m water depth, often corresponding to the depth of the secondary nitrite maximum. In one of the four stations, a second maximum in the ladderane fatty acid concentration was noted at a shallower depth (i.e. at 85 m), coinciding with higher nitrite concentrations at this water depth. The availability of nitrite probably limits anammox activity in the ETNP. Anammox lipids were abundant within the OMZ at all stations and concentrations were comparable to those in other OMZs, suggesting that anammox may be responsible for a significant loss of nitrogen in the OMZ of the ETNP.

Description: The distributions of branched glycerol dialkyl glycerol tetraethers (brGDGTs), which are transported from the soils where they are predominantly produced to marine sediments via rivers, have been applied in reconstructing mean annual air temperature (MAT) and pH of soils. However, paleoclimate reconstructions using sedimentary brGDGTs have proven difficult in arid regions, including the Iberian Peninsula. Recently, six novel 6-methyl brGDGTs have been described using new analytical methods (in addition to the nine 5-methyl brGDGTs previously used for climate reconstructions), and so new pH and MAT calibrations have been developed that were shown to improve the accuracy of reconstructions in a set of global soil samples, especially in arid regions. Because of this we decided to apply the new method to separate the 5- and 6-methyl isomers along with the novel calibrations to a sample set from the Iberian Peninsula to determine whether it improves paleoclimate reconstructions in this area. This set includes samples that run in a transect from source to sink along the Tagus River and out to the deep ocean off the Portuguese margin spanning the last 6000 years. We found that although pH reconstructions in the soils were improved using the new calibration, MAT reconstructions were not much better even with the separation of the 5- and 6-methyl brGDGTs. This confirmed the conclusion of previous studies that the amount of aquatically produced brGDGTs is overwhelming the soil-derived ones in marine sediments and complicating MAT reconstructions in the region. Additionally, the newseparation revealed a strong and until now unseen relationship between the new degree of cyclization (DC') of the brGDGTs and MAT that could be making temperature reconstructions in this and other arid regions difficult

Description: At the southern tip of Africa, the Agulhas Current reflects back into the Indian Ocean causing so-called "Agulhas rings" to spin off and release relatively warm and saline water into the South Atlantic Ocean. Previous reconstructions of the dynamics of the Agulhas Current, based on paleo-sea surface temperature and sea surface salinity proxies, inferred that Agulhas leakage from the Indian Ocean to the South Atlantic was reduced during glacial stages as a consequence of shifted wind fields and a northwards migration of the subtropical front. Subsequently, this might have led to a buildup of warm saline water in the southern Indian Ocean. To investigate this latter hypothesis, we reconstructed sea surface salinity changes using alkenone dD, and paleo-sea surface temperature using TEXH 86 and UK'37, from two sediment cores (MD02-2594, MD96-2080) located in the Agulhas leakage area during Termination I and II. Both UK'37 and TEXH 86 temperature reconstructions indicate an abrupt warming during the glacial terminations, while a shift to more negative dDalkenone values of approximately 14 per mil during glacial Termination I and II is also observed. Approximately half of the isotopic shift can be attributed to the change in global ice volume, while the residual isotopic shift is attributed to changes in salinity, suggesting relatively high salinities at the core sites during glacials, with subsequent freshening during glacial terminations. Approximate estimations suggest that dDalkenone represents a salinity change of ca. 1.7-1.9 during Termination I and Termination II. These estimations are in good agreement with the proposed changes in salinity derived from previously reported combined planktonic Foraminifera d18O values and Mg/Ca-based temperature reconstructions. Our results confirm that the dD of alkenones is a potentially suitable tool to reconstruct salinity changes independent of planktonic Foraminifera d18O.

Description: The distribution of branched glycerol dialkyl glycerol tetraethers (brGDGTs) in soils has been shown to correlate with pH and mean annual air temperature. Because of this dependence brGDGTs have found an application as palaeoclimate proxies in coastal marine sediments, based on the assumption that their distribution is not altered during the transport from soils to marine systems by rivers. To study the processes acting on the brGDGT distributions, we analysed the full suite of brGDGTs, including the recently described 6-Me brGDGTs, in both the suspended particulate matter (SPM) of the Siberian Yenisei River and the SPM and sediments of its outflow in the Kara Sea. The brGDGT distribution in the SPM of the Yenisei River was fairly constant and characterized by high abundances of the 6-Me brGDGTs, reflecting their production at the neutral pH of the river water. However, the brGDGT distribution showed marked shifts in the marine system. Firstly, in the Yenisei River Mouth, the fractional abundance of the 6-Me brGDGTs decreases sharply. The brGDGT signature in the Yenisei River Mouth possibly reflects brGDGTs delivered during the spring floods that may carry a different distribution. Also, coastal cliffs were shown to contain brGDGTs and to influence especially those sites without major river inputs (e.g. Khalmyer Bay). Further removed from the river mouth, in-situ production of brGDGTs in the marine system influences the distribution. However, also the fractional abundance of the tetramethylated brGDGT Ia increases, resulting in a distribution that is distinct from in-situ produced signals at similar latitudes (Svalbard). We suggest that this shift may be caused by preferential degradation of labile (riverine in-situ produced) brGDGTs and the subsequent enrichment in less labile (soil) material. The offshore distribution indeed agrees with the brGDGT distribution encountered in a lowland peat. This implies that the offshore Kara Sea sediments possibly carry a soil-dominated signal, indicating potential for palaeoclimate reconstructions at this site. Both in the river system and coastal cliffs, brGDGTs were much more abundant than crenarchaeol, an archaeal isoprenoid GDGT, resulting in high (〉0.93) Branched and Isoprenoid Tetraether (BIT) index values. Moving downstream in the marine sediments, a decrease in brGDGT concentrations, coeval with an increase in crenarchaeol, resulted in decreasing BIT index values. This decrease correlates with changes in bulk proxies for terrigenous input (d13Corg, C/N), confirming the use of the BIT index to trace the delivery of river-transported and coastal cliff-derived terrigenous organic matter.

Description: Bacteriohopanepolyols (BHPs) are ubiquitous bacterial membrane lipids, encountered in soils, river and marine suspended particulate matter (SPM) and sediments. Their abundance and distribution provides a direct means to identify bacterial inputs and can be used to trace soil-derived bacterial organic matter (OM) and in some cases the presence of bacterial groups and their activities in aquatic systems. We have studied the BHP distribution in the SPM of a major Siberian River (Yenisei River) that crosses a large latitudinal gradient, draining a large part of Mongolia and Siberian Russia. The Yenisei River is the main river to flow into the Kara Sea, a shelf sea of the Arctic Ocean. We show that the BHP distribution and concentration of SPM and surface sediments of the Yenisei Outflow in the Kara Sea allow to trace soil-marker BHPs and evaluate the performance of the R'soil index, a proxy developed to trace bacterial soil-derived OM. Soil-marker BHPs are present in the Yenisei River, and their concentration decreases from the Yenisei River Outflow into the offshore marine sediments. The R'soil correlates well with an independent proxy for bacterial OM, the BIT-index (r**2 = 0.82) and has a moderate correlation with the d13Corg values, a bulk OM proxy for terrigenous input (r**2 = 0.44). Consequently, the R'soil index performs well in the Kara Sea, strengthening its application for tracing bacterial OM in the Arctic Ocean, both in modern and downcore sediments. Furthermore, a suite of BHPs that are characteristic for methanotrophic bacteria, i.e. 35-aminobacteriohopane-30,31,32,33,34-pentol (aminopentol) and 35-aminobacteriohopane-31,32,33,34-tetrol (aminotetrol), is encountered in the Yenisei Outflow sediments. These components are partly sourced from terrigenous sources, but are likely also produced in-situ in the marine sediments. The distribution of the pentafunctionalized cyclitol ether BHP in the marine systems is noteworthy, and indicates that it can possibly be applied as a marker for cyanobacterial biomass in marine sediments.

Description: The Baltic Sea has had a complex salinity history since the last deglaciation. Here we show how distributions of alkenones and their dD values varied with past fluctuations in salinity in the Baltic Sea over the Holocene by examining a Holocene record (11.2-0.1 cal kyr BP) from the Arkona Basin. Major changes in the alkenone distribution, i.e., changes in the fractional abundance of the C37:4 Me alkenone, the C38:2 Et alkenone and a C36:2 Me alkenone, the latter which has not been reported in the Baltic Sea previously, correlated with known changes in salinity. Both alkenone distributions and hydrogen isotopic composition suggest a shift in haptophyte species composition from lacustrine to brackish type haptophytes around 7.7-7.2 cal kyr BP, corresponding with a salinity change that occurred when the connection between the basin and the North Sea was re-established. A similar salinity change occurred in the Black Sea. Previously published alkenone distributions and their d-values from the Black Sea were used to reconstruct Holocene changes in surface water salinity and, hence, it was shown that the unusual C36:2 alkenone dominates the alkenone distribution at salinities of 2-8 ppt (g/kg). This information was used to corroborate the interpretations made about salinity changes from the data presented for the Baltic Sea. Low and variable salinity waters in the Baltic Sea over the Holocene have led to a variable alkenone producing haptophyte community composition, including low salinity adapted species, hindering the use of the unsaturation ratios of long-chain alkenones for sea surface temperature reconstruction. However, these alkenone based indices are potentially useful for studying variations in salinity, regionally as well as in the past.