Branched And Isoprenoid Tetraether Index Research Articles

Size-fractionated distribution of glycerol dialkyl glycerol tetraether core lipids in surface sediments of a large-river delta-front estuary

Glycerol dialkyl glycerol tetraether core lipids (GDGTs) are microbial biomarkers ubiquitously distributed in terrestrial and marine environments. Dispersal and fate of GDGTs in an estuary largely depends on sediment grain size, however, their size distribution patterns remain poorly understood. Here, surface sediments collected from the Changjiang Estuary were separated into <20, 20–32, 32–63, 63–125 and >125 μm fractions, and analyzed for GDGTs as well as total organic carbon (TOC), stable isotopic composition (δ13C) of TOC and lignin phenols, to investigate the size and spatial distributions of GDGTs and the particle size effects on GDGTs proxies in this large river delta-front estuary. The concentrations of isoprenoidal GDGTs (isoGDGTs) were higher in the finest fractions and in off-estuary sites. On the contrary, branched GDGTs (brGDGTs) were high not only in the finest fractions but in coarser fractions (>32 μm fractions), and thus at both near- and off-estuary sites. The branched and isoprenoid tetraether (BIT) index increased with increasing grain size, and decreased sharply from the estuary (~0.52) to the shelf (~0.16). BrGDGTs were positively correlated with crenarcheaol in both high and low BIT regions. The brGDGTIIIa/IIa ratios in all size fractions were <0.59, further indicating that the brGDGTs were mainly derived from terrestrial input with minimum in-situ production. Fractional TOC source assignments derived from the BIT index was significantly positively correlated with the fractions of terrestrial OC from a mixing model based on δ13C-TOC and lignin contents, indicating that BIT may track a broader pool of terrestrial OC than just soil OC. This work provides novel, yet preliminary insights into the size fractionated distribution characteristics of GDGTs and the applicability of BIT as a proxy for OC sources in estuarine sediments. More work is needed to further clarify the particle size effects on other GDGTs proxies in estuarine systems.

Seasonal and multi-annual variation in the abundance of isoprenoid GDGT membrane lipids and their producers in the water column of a meromictic equatorial crater lake (Lake Chala, East Africa)

Isoprenoid glycerol dialkyl glycerol tetraethers (isoGDGTs) are membrane lipids of Archaea. Organic biomarker proxies associated with these lipids, such as the TEX86 paleothermometer and Branched and Isoprenoid Tetraether (BIT) index, are often used in paleoenvironmental reconstructions for the marine environment, but their general applicability in lacustrine settings is hampered by limited understanding of the biological sources and environmental drivers influencing isoGDGT production. To validate the use of isoGDGT proxies in lakes, we studied the occurrence of isoGDGTs in Lake Chala, a permanently stratified (meromictic) crater lake in equatorial East Africa. We analyzed the abundance and distribution of isoGDGTs in 17 depth profiles of suspended particulate matter (SPM) collected monthly between September 2013 and January 2015, and compared this with the abundance and composition of archaea based on 16S rRNA gene and quantitative PCR analysis. Both isoGDGTs and archaeal abundance in the SPM were exceptionally low throughout the study period. In the oxygenated part of the water column, higher fractional abundances of crenarchaeol are matched by a predominance of the ammonia-oxidizing Thaumarchaeota I.1b that are known to produce this GDGT, whereas deep anoxic water layers are characterized by high fractional abundances of GDGT-0 as well as the anaerobic heterotrophic Group C3 MCG Bathyarchaea and specific euryarchaeotal methanogens. Analysis of intact polar lipid (IPL) isoGDGTs using SPM depth profiles from three months representing distinct seasons during the study period revealed the presence of several IPLs of GDGT-0 in the anoxic lower water column, which are rarely found in natural settings. IPL GDGT-0 with a phosphatidylglycerol (PG-), monohexose-phosphatidylglycerol (MH-PG-) and dihexose-phosphatidylglycerol (DH-PG-) head-group was typically only present just above the lake bottom at 90 m depth and probably reflect specific communities of anaerobic archaea. We also determined the flux and distribution of isoGDGTs in settling particles collected monthly between November 2006 and January 2015 from a sediment trap suspended at 35 m water depth to assess seasonal and inter-annual variability in surface-water isoGDGT production, and compared this with the temporal distribution of isoGDGTs in the 25,000-year long sediment record from Lake Chala. Monthly variation of isoGDGTs in the 98-month settling-particles record did not show a strong annual pattern related to seasonal water-column mixing and stratification, likely because the oxycline was regularly situated below sediment-trap depth. Episodes of high GDGT-0 concentrations relative to crenarchaeol in the settling particles can therefore be linked to periods of exceptionally shallow oxycline depth, which suppresses the thaumarchaeotal bloom. During such intervals, TEX86-based paleotemperatures are not reliable because isoGDGT input from other archaeal sources disproportionally influences TEX86 values and creates a cold-temperature bias. Additionally, the abundance of the crenarchaeol isomer relative to crenarchaeol (f[CREN']) gradually increases during such episodes of high GDGT-0/crenarchaeol ratio, suggesting increasing dominance of Group I.1b over Group I.1a Thaumarchaeota, and might prove a good marker for prolonged shallow-oxycline conditions. Most importantly, the associated near-absence of crenarchaeol during times of strong upper-water-column stratification results in high BIT-index values. We propose that this suppression mechanism may be the principal driver of BIT-index variation in the sediment record of Lake Chala, and the main source of observed congruence between the BIT index and climate-driven lake-level variation on long time scales.

Source, composition, and distributional pattern of branched tetraethers in sediments of northern Chinese marginal seas

Branched glycerol dialkyl glycerol tetraethers (brGDGTs), biomarkers for organic carbon source and paleoenvironment, can be produced in both land and sea. The source assignment is prerequisite for the accurate application of brGDGTs-based indicators. Here, we reported brGDGTs in surface sediments of northern Chinese Marginal Seas (CMS), including the Bohai Sea, Yellow Sea and East China Sea. Two source indicators, ∑IIIa/∑IIa and #ringstetra, have an average of 0.86 ± 0.18 and 0.74 ± 0.08 in the Bohai Sea, 0.61 ± 0.34 and 0.73 ± 0.16 in the East China Sea, and 1.32 ± 0.45 and 0.87 ± 0.12 in the Yellow Sea, suggesting variable contributions of terrestrial and marine-derived brGDGTs in the Bohai Sea and East China Sea, but the predominance of marine in-situ brGDGTs in the Yellow Sea. The distribution of brGDGTs presents a zonal pattern from nearshore to offshore in the Bohai Sea/East China Sea, but a complex and patched pattern in the Yellow Sea. This difference is attributed to different influences of terrestrial brGDGTs from the Yangtze River and Yellow River. However, the branched and isoprenoid tetraether (BIT) index, an indicator for terrestrial organic carbon in marine environments, has low values (an average of <0.1) in all three marginal seas, demonstrating the limitation of the BIT index in high productive marginal seas. Of those samples containing pure marine derived brGDGTs (∑IIIa/∑IIa ≥ 0.92), significant correlations between ∑IIIa/∑IIa and CBT5ME and between water temperature and MBT′5ME suggest that the production of brGDGTs by marine bacteria is also affected by ambient pH and temperature, although a global calibration is still lacking.

Establishing a terrestrial proxy based on fluorescent dissolved organic matter from sediment pore waters in the East China Sea

Terrestrial organic matter occupies an important position in the oceanic organic carbon pool. Some terrestrial proxies, like the Branched and Isoprenoid Tetraether (BIT) index, have been applied successfully to indicate the relative abundance of terrestrial organic matter in marine sediments. A new terrestrial proxy derived from sediment pore water fluorescent dissolved matter (fluorescent dissolved organic matter (FDOM)) was developed in this study. Surface sediment samples were collected from forty-two sites in the coastal region of the East China Sea (ECS) to examine the distributional patterns of FDOM. Three protein-like components (C1, C4 and C5) and two humic-like components (C2 and C3) of FDOM were identified using fluorescence excitation-emission matrices parallel factor analysis (EEMs-PARAFAC). Spatially, the intensity of these five components generally increased from the coast to the ocean with protein-like components showing a more obvious trend, which suggested that all five components had autochthonous contribution. However, the C2 and C3 proportions, especially C2 that mainly corresponds to the proportion of peak A in fluorescence excitation-emission matrices, significantly decreased from the coast to the ocean and significantly correlated with the BIT index from corresponding solid fractions. We posit that part of the humic-like components from terrestrial organic matter in sediments are released into the C2 and C3 pools in pore waters, which may be constrained by specific environmental conditions. Thus, the FDOM from pore waters can be integrated with BIT index to validate the nature of FDOM and use it as a biomarker to reflect the terrestrial input of organic matter mediated by different biogeochemical processes in coastal oceans. The proportion of peak A responsible for the fluorescence of C2 was suggest as a new terrestrial derived from FDOM.

Holocene organic geochemical record from the Western Indus continental shelf (northern Arabian Sea)

Primary production on the Western Indus continental shelf has been linked to the large quantities of nutrients delivered to the shelf by the Indus River. Multiple geochemical tracers and biomarker records, including stable isotopes (δ13C and δ15N), total organic carbon (TOC), total nitrogen (TN), molar carbon/nitrogen (C/N) ratio, the branched and isoprenoid tetraether (BIT) index, and glycerol dialkyl glycerol tetraether (GDGT), have been analyzed from the Indus-23AP sediment core recovered from the northern Arabian Sea. Our records show evidence of a mixture of marine and terrestrially derived organic matter (OM) during the last 14,000 years, as indicated by the C/N ratio, δ13C, δ15N, and the BIT index. The three sterol biomarkers (brassicasterol, dinosterol, and cholesterol) show concurrent enrichments during the last 3 millennia reflecting increased phytoplankton abundance because of increased Indus river discharge of nutrients during the summer monsoon. GDGT crenarchaeol enrichment is related to the BIT index. The TEX86-derived sea surface temperature (SST) record is shifted toward the summer season because Crenarchaeota are more abundant and active during periods of high primary production. SSTs indicate a long-term warming trend during the Holocene related to increasing winter insolation in the low latitudes northern Hemisphere.

Assessment of sedimentary heterocyst glycolipids as tracers of freshwater input to the Changjiang Estuary and East China Sea

Heterocyst glycolipids (HGs) are biomarkers of important freshwater and marine diazotrophs known as heterocystous cyanobacteria. Recent research has found that HGs with hexose moieties (C6) are derived from free-living, mostly freshwater species while those with pentose moieties (C5) are derived from diatom-symbiont, mostly marine species. Here, we analyzed 39 surface sediments from a study area ranging from the Changjiang Estuary to the offshore East China Sea for HGs to investigate distribution patterns at this freshwater-seawater interface. We detected both C5 and C6 HGs in these samples; however, they had different spatial distributions: the C6 HG abundance was highest at the freshwater end of the estuary and decease offshore, whereas the abundance of C5 HGs displayed almost inverse pattern. We hypothesized that such distributions were likely caused by the mixing of freshwater and seawater, leading to a mixture of HGs derived from inland waters and/or soils and the marine environment. The proportion of C6 HGs to total HGs, defined as the FHG index, decreased with distance offshore, possibly reflecting the delivery of freshwater into the ocean. The FHG index changed curvilinearly with salinity, likely resulting from concentration-based weighting of end-member contributions. The FHG index correlated linearly (r2 = 0.76) with δ13Corg, suggesting that FHG could be used to indicate relative contributions of freshwater-delivered terrestrial organic matter in this estuarine and coastal region. However, the FHG index did not linearly correlate with the branched and isoprenoid tetraether (BIT) index, another proxy used for terrestrial input, likely because the BIT index is predominantly controlled by variations in marine crenarchaeol production.

Sources and burial fluxes of sedimentary organic carbon in the northern Bering Sea and the northern Chukchi Sea in response to global warming

The Arctic and subarctic seas are the major CO2 sink areas on earth. In this study, the vertical variation characteristics of organic carbon, total nitrogen and their ratio (Corg/Nt), stable isotopes δ13C and δ15N, and BIT (branched and isoprenoid tetraether) index of GDGTs (glycerol dialkyl glycerol tetraethers) in combination with 210Pb-dating were used to analyze the changes in the marine and terrestrial sources of organic carbon in the northern Bering Sea (site 1), western Beaufort Sea slope (site 2) and northern Chukchi Sea (site 3). Organic carbon burial fluxes (OCBF) in the context of global warming were also explored at sites 1 and 3. The results showed that organic matter in these sediments were a mixed input of marine and terrestrial sources, and the BIT index and δ13C of site 2 suggested that the terrestrial soil organic matter was dominant. Based on a combination of 210Pb dating and Corg, the sedimentary OCBF at site 1 was 2.29–3.65 mg cm−2 y−1, and at site 3 was 0.00–0.41 mg cm−2 y−1. The temperature anomalies and sea ice changes in the Arctic in the past 100 years were compared with the burial fluxes of the terrestrial organic carbon. At site 1, the results indicated that fast melting of seasonal sea ice led to earlier arrival of ice algae bloom, enhanced zooplankton feeding and reduced carbon burial from 1947 to 2010, and the sudden increase in carbon burial after 2010 was attributed to an increase in primary productivity and terrestrial organic matter input due to an accelerated melting of sea ice. There was a smaller change in marine organic carbon content in site 3, but OCBF increased after a pre-1965 decrease, mainly controlled by terrestrial organic matter input associated with temperature rising and sea ice melting during recent decades.

Change in provenance of branched glycerol dialkyl glycerol tetraethers over the Holocene in the Baltic Sea and its impact on continental climate reconstruction

The Baltic Sea is an enclosed basin that experienced a number of different salinity phases during the Holocene corresponding to the establishment of a connection with the North Sea. Branched glycerol dialkyl glycerol tetraethers (brGDGTs) in surface sediments and Holocene sedimentary successions from the Gotland and Arkona Basins were analyzed to examine their potential applicability as indicators for soil organic matter input, as well as their suitability for paleoclimate reconstructions. Our results show a marked change in brGDGT distributions and the branched and isoprenoid tetraether (BIT) index. The transition of the Ancylus Lake (fresh) to the Littorina Sea (brackish) phase is revealed by a large drop in the BIT index and an increase in the MBT′5Me, which reveals a large shift in provenance of the sedimentary brGDGTs. During the lake phase, brGDGTs are presumed to be primarily produced by in situ production in the water column. During the brackish phase, in situ brGDGT production in the alkaline pore waters of the surface sediments (as revealed by their high degree of cyclization) was predominant although there was evidence of occasional input of soil-derived brGDGTs. The predominant aquatic autochthonous production of brGDGTs does not allow the use of brGDGTs for continental air temperature reconstructions but they can be used for bottom water and lake temperature reconstructions during some intervals. The results from this study demonstrate that geological changes and concomittant salinity variations can be revealed by the provenance of brGDGTs.

The C&amp;lt;sub&amp;gt;32&amp;lt;/sub&amp;gt; alkane-1,15-diol as a proxy of late Quaternary riverine input in coastal margins

Abstract. The study of past sedimentary records from coastal margins allows us to reconstruct variations in terrestrial input into the marine realm and to gain insight into continental climatic variability. There are numerous organic proxies for tracing terrestrial input into marine environments but none that strictly reflect the input of river-produced organic matter. Here, we test the fractional abundance of the C32 alkane 1,15-diol relative to all 1,13- and 1,15-long-chain diols (FC32 1, 15) as a tracer of input of river-produced organic matter in the marine realm in surface and Quaternary (0–45 ka) sediments on the shelf off the Zambezi and nearby smaller rivers in the Mozambique Channel (western Indian Ocean). A Quaternary (0–22 ka) sediment record off the Nile River mouth in the eastern Mediterranean was also studied for long-chain diols. For the Mozambique Channel, surface sediments of sites most proximal to Mozambique rivers showed the highest F1, 15 − C32 (up to 10 %). The sedimentary record shows high (15–35 %) pre-Holocene F1, 15 − C32 and low (&lt; 10 %) Holocene F1, 15 − C32 values, with a major decrease between 18 and 12 ka. F1, 15 − C32 is significantly correlated (r2 = 0.83, p &lt; 0.001) with the branched and isoprenoid tetraether (BIT) index, a proxy for the input of soil and river-produced organic matter in the marine environment, which declines from 0.25 to 0.60 for the pre-Holocene to &lt; 0.10 for the Holocene. This decrease in both FC32 1, 15 and the BIT is interpreted to be mainly due to rising sea level, which caused the Zambezi River mouth to become more distal to our study site, thereby decreasing riverine input at the core location. Some small discrepancies are observed between the records of the BIT index and FC32 1, 15 for Heinrich Event 1 (H1) and the Younger Dryas (YD), which may be explained by a change in soil sources in the catchment area rather than a change in river influx. Like for the Mozambique Channel, a significant correlation between FC32 1, 15 and the BIT index (r2 = 0.38, p &lt; 0.001) is observed for the eastern Mediterranean Nile record. Here also, the BIT index and FC32 1, 15 are lower in the Holocene than in the pre-Holocene, which is likely due to the sea level rise. In general, the differences between the BIT index and FC32 1, 15 eastern Mediterranean Nile records can be explained by the fact that the BIT index is not only affected by riverine runoff but also by vegetation cover with increasing cover leading to lower soil erosion. Our results confirm that FC32 1, 15 is a complementary proxy for tracing riverine input of organic matter into marine shelf settings, and, in comparison with other proxies, it seems not to be affected by soil and vegetation changes in the catchment area.

Source, transport and fate of soil organic matter inferred from microbial biomarker lipids on the East Siberian Arctic Shelf

Abstract. The Siberian Arctic contains a globally significant pool of organic carbon (OC) vulnerable to enhanced warming and subsequent release by both fluvial and coastal erosion processes. However, the rate of release, its behaviour in the Arctic Ocean and vulnerability to remineralisation is poorly understood. Here we combine new measurements of microbial biohopanoids including adenosylhopane, a lipid associated with soil microbial communities, with published glycerol dialkyl glycerol tetraethers (GDGTs) and bulk δ13C measurements to improve knowledge of the fate of OC transported to the East Siberian Arctic Shelf (ESAS). The microbial hopanoid-based soil OC proxy R′soil ranges from 0.0 to 0.8 across the ESAS, with highest values nearshore and decreases offshore. Across the shelf R′soil displays a negative linear correlation with bulk δ13C measurements (r2 = −0.73, p = &lt; 0.001). When compared to the GDGT-based OC proxy, the branched and isoprenoid tetraether (BIT) index, a decoupled (non-linear) behaviour on the shelf was observed, particularly in the Buor-Khaya Bay, where the R′soil shows limited variation, whereas the BIT index shows a rapid decline moving away from the Lena River outflow channels. This reflects a balance between delivery and removal of OC from different sources. The good correlation between the hopanoid and bulk terrestrial signal suggests a broad range of hopanoid sources, both fluvial and via coastal erosion, whilst GDGTs appear to be primarily sourced via fluvial transport. Analysis of ice complex deposits (ICDs) revealed an average R′soil of 0.5 for the Lena Delta, equivalent to that of the Buor-Khaya Bay sediments, whilst ICDs from further east showed higher values (0.6–0.85). Although R′soil correlates more closely with bulk OC than the BIT, our understanding of the endmembers of this system is clearly still incomplete, with variations between the different East Siberian Arctic regions potentially reflecting differences in environmental conditions (e.g. temperature, pH), but other physiological controls on microbial bacteriohopanepolyol (BHP) production under psychrophilic conditions are as yet unknown.

A 12-kyr record of microbial branched and isoprenoid tetraether index in Lake Qinghai, northeastern Qinghai-Tibet Plateau: Implications for paleoclimate reconstruction

Branched and Isoprenoid Tetraether (BIT) index was considered as a proxy for terrestrial organic matter input in lake sediments, based on the assumption that branched glycerol dialkyl glycerol tetraethers (bGDGTs) are mainly derived from terrestrial soils. However, mounting evidences have showed that the in situ production of bGDGTs is widespread in lakes, challenging BIT as a reliable terrestrial input proxy. Recently, BIT has been proven to be a reliable proxy for paleohydrology in a small crater lake (Lake Challa) in accordance with a different mechanism. However, the response of BIT to paleohydrology variation may differ for different lakes. In this study, we investigate the variations in the BIT index and the concentrations of its related GDGTs in a 12-ka sediment core from Lake Qinghai, in combination with our previous results for surface sediments. We find that variations in BIT strongly depend on the concentration of crenarchaeol in both surface and ancient sediments of this lake, whereas bGDGT concentration varies much less remarkably. Considering that crenarchaeol production is positively correlated with water depth in Lake Qinghai, water depth may exert negative control on the BIT index in this lake. This case is inconsistent with the positive relationship between BIT and lake levels or rainfall intensity reported for Lake Challa, suggesting that the response of BIT to local paleohydrology is site specific in lacustrine systems. Hence, the application of sedimentary BIT as a paleohydrological proxy in a specific lake requires caution before confirming the environmental controls on BIT in that lake.

Drastic changes in the distribution of branched tetraether lipids in suspended matter and sediments from the Yenisei River and Kara Sea (Siberia): Implications for the use of brGDGT-based proxies in coastal marine sediments

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 (δ13Corg, C/N), confirming the use of the BIT index to trace the delivery of river-transported and coastal cliff-derived terrigenous organic matter.

Distributions of bacterial and archaeal membrane lipids in surface sediments reflect differences in input and loss of terrestrial organic carbon along a cross-shelf Arctic transect

Enhanced climate warming affecting the Arctic region could have a dramatic impact on the terrigenous organic carbon (terrOC) stored in the Eurasian permafrost and could increase the amount of OC remobilized to the Arctic shelves. An improved understanding of the fate of this remobilized soil OC is essential for better understanding of the consequences for the Arctic and global carbon cycle. In this study, glycerol dialkyl glycerol tetraethers (GDGTs) and bacteriohopanepolyols (BHPs) in surface sediments along a 500km cross shelf transect from the mouth of the Kolyma River to the middle of the vast East Siberian Sea were analysed to assess their potential and that of the associated branched and isoprenoid tetraether (BIT) and R′soil indices for tracing terrOC in Arctic systems. Both BHP and GDGT contributions indicated the greatest contribution of terrOC close to the river mouth, while the associated indices showed declining trends in an offshore direction, supporting an increasing marine OC input and/or a decrease in terrOC. However, while the BHPs indicated a dominance of terrOC at the start of the transect, the GDGTs suggested a much larger, almost 50%, marine OC input at this point. In addition, the BIT index displayed an exponential decline, controlled mainly by a substantial contribution of marine GDGTs, while R′soil revealed a linear trend governed primarily by the removal of soil marker BHPs.These field results suggest that both biomarker approaches could be used to trace terrigenous derived OC in the Arctic environment. However, using a single proxy approach is not recommended and may lead to an under or over estimation of the relative importance of terrOC. Using a multi-proxy approach is valuable for fully understanding the fate of terrigenous derived OC along Arctic land-ocean transects.

Impact of river channel shifts on tetraether lipids in the Rhône prodelta (NW Mediterranean): Implication for the BIT index as an indicator of palaeoflood events

We tested the applicability of the BIT (branched and isoprenoid tetraether) index as a proxy for palaeoflood events in the river-dominated continental margin of the Gulf of Lions (NW Mediterranean). We compared the concentrations of branched glycerol dialkyl glycerol tetraethers (br GDGTs) and crenarchaeol in suspended particulate matter (SPM) collected downstream in the Rhône River, as well as in surface sediments and a ca. 8m piston core from the Rhône prodelta. The core covered the last 400yr, with four distinct intervals recording the river influence under natural and man-induced shifts in four main channels of the river mouth (Bras de Fer, Grand Rhône, Pégoulier and Roustan). The results indicate that there are mixed sources of br GDGTs and crenarchaeol in the prodelta, complicating application of the BIT index as an indicator of continental organic carbon input and thus as a palaeoflood proxy. However, the sedimentary BIT record for the period when continental material was delivered by the river more directly to the core site (Roustan phase; 1892 to the present) mimics the historical paleoflood record. This shows the potential of the BIT index as a palaeoflood proxy, provided that the delivery route of the continental material by rivers to the core sites remains constant over time. The study also highlights the idea that shifts in river channels should be taken into account for the use of the BIT index as a palaeoflood proxy.

Branched and isoprenoid tetraether (BIT) index traces water content along two marsh-soil transects surrounding Lake Qinghai: Implications for paleo-humidity variation

The BIT index represents the relative abundances of branched glycerol dialkyl glycerol tetraethers (bGDGTs) and the isoprenoid GDGT, crenarchaeol. While bGDGTs are produced mainly by soil (anaerobic) bacteria, crenarchaeol is known to be a biomarker for aerobic ammonia oxidation by chemolithoautotrophic Thaumarchaeota, particularly in the open ocean or lakes. Thus, the index in marine and lacustrine settings has been widely used as a proxy for soil input. Here, we have investigated GDGT distribution along two transects extending from the lake shore marsh to upland soils on the NE Qinghai-Tibetan plateau. The results show that soil water content (SWC) correlates positively with concentration of bGDGTs and negatively with concentration of crenarchaeol; consequently, there is a significant positive correlation between SWC and the BIT index. Our study highlights a new potential application of BIT as a humidity proxy in loess/soil and peat deposits.

Tracing soil organic carbon in the lower Amazon River and its tributaries using GDGT distributions and bulk organic matter properties

In order to trace the transport of soil organic carbon (OC) in the lower Amazon basin, we investigated the distributions of crenarchaeol and branched glycerol dialkyl glycerol tetraethers (GDGTs) by analyzing riverbed sediments and river suspended particulate matter (SPM) collected in the Solimões-Amazon River mainstem and its tributaries. The Branched and Isoprenoid Tetraether (BIT) index, a proxy for river-transported soil OC into the ocean, was determined from the distributions of these GDGTs. The GDGT-derived parameters were compared with other bulk geochemical data (i.e. C:N ratio and stable carbon isotopic composition). The GDGT-derived and bulk geochemical data indicate that riverine SPM and riverbed sediments in the lower Amazon River and its tributaries are a mixture of C3 plant-derived soil OC and aquatic-derived OC. The branched GDGTs in the SPM and riverbed sediments did not predominantly originate from the high Andes soils (>2500m in altitude) as was suggested previously. However, further constraint on the soil source area of branched GDGTs was hampered due to the deficiency of soil data from the lower montane forest areas in the Andes. Our study also revealed seasonal and interannual variation in GDGT composition as well as soil OC discharge, which was closely related to the hydrological cycle. By way of a simple binary mixing model using the flux-weighted BIT values at Óbidos, the last gauging station in the Amazon River, we estimated that 70–80% of the POC pool in the river was derived of soil OC. However, care should be taken to use the BIT index since it showed a non-conservative behaviour along the river continuum due to the aquatic production of crenarchaeol. Further investigation using a continuous sampling strategy following the full hydrological cycle is required to fully understand how soil-derived GDGT signals are transformed in large tropical river systems through their transport pathway to the ocean.

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.

Contribution of river‐borne soil organic carbon to the Gulf of Lions (NW Mediterranean)

We determined the spatial distribution patterns of the Branched and Isoprenoid Tetraether (BIT) index, a specific proxy of riverborne soil organic carbon (OC) input from land to the ocean, by analyzing soils from the Têt watershed (France) and surface sediments from the Gulf of Lions (NW Mediterranean) into which the Têet and Rhône Rivers flow. The BIT index pattern in the Têt watershed is heterogeneously distributed, varying between 0.2 and 1.0, with an average BIT value of 0.84 ± 0.14 (n = 58). Soil pH is the major factor controlling soil BIT values among the environmental factors considered on the basis of the statistical analysis of environmental parameters and glycerol dialkyl glycerol tetraether concentrations. In the Gulf of Lions, BIT values range from 0.01 to 0.85, with higher values along the coast compared with those offshore. The soil OC contributions to the total organic carbon (TOC), estimated on the basis of the BIT index, are &gt; 10% along the coast, whereas those from the outer shelf and the continental slope are &lt; 10%.

Transport and depositional process of soil organic matter during wet and dry storms on the Têt inner shelf (NW Mediterranean)

River floods and storm waves are major processes for the dispersal and deposition of terrestrial organic matter (OM) in river-dominated coastal areas. A “wet storm” is connected to a flood with a high river discharge, while a “dry storm” is not associated with a flood. To better understand the sedimentation dynamics of terrestrial OM, especially soil OM, at the land–ocean interface during wet and dry storms, we studied sediment trap and core material collected on the Têt inner shelf (NW Mediterranean) using multiple organic proxies in combination with hydrodynamic parameters. The proportion of soil OM to the total OM in the trap material calculated based on the BIT (Branched and Isoprenoid Tetraether) index was higher during both wet (~ 40%) and dry (~ 30%) storms than during non-storm periods (~ 10%). However, only surface sediments (1-cm thick layers) recovered after December wet and moderate storms in 2003 showed enhanced soil OM percentages compared to the deeper sediments deposited during the last century. Given that the wet storm eroded 4-cm of seabed at the study site, flood-induced fresh soil OM was not deposited on the Têt inner shelf during the wet storm. However, the following moderate storm caused resuspension of flood-associated soil OM which was previously trapped nearshore. Accordingly soil OM was transported to the Têt inner shelf and stored there until the dry storm occurred.

Sources and transport of terrestrial organic matter in Vancouver Island fjords and the Vancouver‐Washington Margin: A multiproxy approach using d13Corg, lignin phenols, and the ether lipid BIT index

The Branched and Isoprenoid Tetraether (BIT) index (a ratio of marine and terrestrial glycerol dialkyl glycerol tetraether [GDGT] membrane lipids) was recently introduced as a proxy for the fraction of fluvially derived organic carbon (OC) buried in marine sediments. We investigate the source and distribution of terrestrial OC in marine sediments in a large spatial data set that encompasses the Washington‐Vancouver Island Margin, Vancouver Island fjords, and Puget Sound with the goal of comparing the BIT index to other traditional measures of terrestrial OC (δ13C of OC [δ13Corg] and lignin phenols). δ13Corg values and lignin concentrations indicate that the fjords are a significant trap for undegraded, wood‐derived terrestrial material, with surface sediments containing up to 76% terrestrial OC. These proxies also show significant terrestrial OC contributions to margin sediments (up to 54%), with most likely an angiosperm source from the Columbia River. In contrast to δ13Corg values and lignin concentrations, BIT index values indicate extremely low terrestrial carbon throughout the study area (BIT index values range from 0 to 0.28) and are uncorrelated with the other proxies. Though the BIT index did not correlate with other proxies, margin GDGT concentrations varied offshore similarly to other terrestrially derived OC and are well‐correlated with other proxies. We suggest that the BIT index is best interpreted as a proxy for soil‐ or peat‐derived OC. The majority of terrestrially derived OC delivered to sediment in this area has a lignin‐rich, nonsoil source. The disparity between the BIT index and other proxies implies that the BIT index significantly underestimates terrestrial relative to marine OC input to sediments in areas in which riverine OC input is not soil or peat derived.