Branched And Isoprenoid Tetraether 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.

The dispersal of fluvially discharged and marine, shelf-produced particulate organic matter in the northern Gulf of Mexico

Abstract. Rivers play a key role in the global carbon cycle by transporting terrestrial organic matter (TerrOM) from land to the ocean. Upon burial in marine sediments, this TerrOM may be a significant long-term carbon sink, depending on its composition and properties. However, much remains unknown about the dispersal of different types of TerrOM in the marine realm upon fluvial discharge since the commonly used bulk organic matter (OM) parameters do not reach the required level of source- and process-specific information. Here, we analyzed bulk OM properties, lipid biomarkers (long-chain n-alkanes, sterols, long-chain diols, alkenones, branched and isoprenoid glycerol dialkyl glycerol tetraethers (brGDGTs and isoGDGTs)), pollen, and dinoflagellate cysts in marine surface sediments along two transects offshore the Mississippi–Atchafalaya River (MAR) system, as well as one along the 20 m isobath in the direction of the river plume. We use these biomarkers and palynological proxies to identify the dispersal patterns of soil–microbial organic matter (SMOM), fluvial, higher plant, and marine-produced OM in the coastal sediments of the northern Gulf of Mexico (GoM). The Branched and Isoprenoid Tetraether (BIT) index and the relative abundance of C32 1,15-diols indicative for freshwater production show high contributions of SMOM and fluvial OM near the Mississippi River (MR) mouth (BIT = 0.6, FC321,15 &gt; 50 %), which rapidly decrease further away from the river mouth (BIT &lt; 0.1, FC321,15 &lt; 20 %). In contrast, concentrations of long-chain n-alkanes and pollen grains do not show this stark decrease along the path of transport, and especially n-alkanes are also found in sediments in deeper waters. Proxy indicators show that marine productivity is highest close to shore and reveal that marine producers (diatoms, dinoflagellates, coccolithophores) have different spatial distributions, indicating their preferred niches. Close to the coast, where food supply is high and waters are turbid, cysts of heterotrophic dinoflagellates dominate the assemblages. The dominance of heterotrophic taxa in shelf waters in combination with the rapid decrease in the relative contribution of TerrOM towards the deeper ocean suggest that TerrOM input may trigger a priming effect that results in its rapid decomposition upon discharge. In the open ocean far away from the river plume, autotrophic dinoflagellates dominate the assemblages, indicating more oligotrophic conditions. Our combined lipid biomarker and palynology approach reveals that different types of TerrOM have distinct dispersal patterns, suggesting that the initial composition of this particulate OM influences the burial efficiency of TerrOM on the continental margin.

Glycerol dialkyl glycerol tetraether signatures in tropical mesotidal estuary sediments of Qua Iboe River, Gulf of Guinea

The continuous supply of land derived organic carbon by rivers is significant to the ocean carbon cycle. In river-estuary systems, the composition and distribution of sedimentary organic matter (OM) can provide detailed information on the balance of land-derived inputs and in situ productivity. This insight is significant for an improved understanding of anthropogenic activity and the changing climate. In the Gulf of Guinea (GoG), high rainfall, coupled with flooding and a warmer climate have direct impacts on OM distributions. Glycerol dialkyl glycerol tetraethers (GDGTs) have been used extensively to investigate OM. In this study, we focused on the widespread occurrence of isoprenoid GDGTs (isoGDGTs), branched GDGTs (brGDGTs), H-tetraethers (H-isoGDGTs and H-brGDGTs), hydroxyl GDGTs (OH-GDGTs), isoGDGT isomers, methylated isoGDGTs (Me-GDGTs), butanetriol and pentanetriol tetraethers (BDGTs and PDGTs), in short sediment cores collected from the inner and outer Qua Iboe River Estuary, located in the GoG. We further determined total organic carbon (Corg), total nitrogen (TN), long chain hydrocarbons (LCA; C27–C33n-alkanes), dinosterol, brassicasterol and phytol, to estimate OM sources, flooding activity, OM degradation, to reconstruct pH and to determine past tropical temperatures. The distribution patterns of GDGTs, use of principal component analysis (PCA), heatmap hierarchical cluster analysis, correlation models, the ratios of isoGDGTs/brGDGTs, ring index (RI), brGDGT-IIIa/IIa, branched and isoprenoid tetraether (BIT), Corg/TN and terrestrial-marine biomarker ratio (TMBR) were all sensitive for indicating terrestrial vs marine OM contributions to the total OM of the tropical estuarine sediments. Marine OM persisted from subsurface to bottom sediment layers (8–38 cm) in the outer estuary site, while a mixed OM source was observed in inner estuary sediments. The Cren/Cren’ ratio >58.5 (up to 83.8), pointed to a predominance of Thaumarchaeota group I.1a microbes. The Methane Index (MI) and isoGDGT-0/Cren ratio indicated significant imprints of a non-methanotrophic archaeal community. A positive linearity of total BDGTs and PDGTs plotted against MI and isoGDGT-0/Cren suggested that these compounds are produced by the same group of the non-methanogenic archaeal community in this tropical estuary. Thus, Methanomassiliicoccus luminyensis is not the only source of BDGTs and PDGTs in the marine environment. Further, the calculated CBT5me derived pH reflected in situ measured sediment pH (6.3). TEX86H-derived sea surface temperature (SST), within error, reflected water column temperature. Calculated brGDGT mean annual air temperatures (MAATs) reflected average annual atmospheric temperatures. Ring index OH-GDGTs temperature proxies (RI-OH SSTsummer and RI-OHꞌ SST) reflected mean dry season (November to February) temperatures. A newly derived MAAT proxy, calculated from H-brGDGT, also reflected the tropical annual air temperature (23–28 °C). Altogether, as expected, the OM sources are traced largely to both terrestrial and marine inputs. The terrestrial OM was deposited by excess flooding and surface runoff occasioned by torrential monsoon rains in recent years, triggered by climate change, while mesotidal currents introduced marine OM into the Qua Iboe River Estuary system. This study further contributes to our understanding of OM distributions in warmer climate conditions.

Carbon isotope excursions during the late Miocene recorded by lipids of marine Thaumarchaeota, Piedmont Basin, Mediterranean Sea

Abstract Group I mesophilic Thaumarchaeota fix dissolved inorganic carbon (DIC), accompanied by a biosynthetic fractionation factor of ~20‰. Accordingly, the δ13C signature of their diagnostic biomarker crenarchaeol was suggested as a potential δ13CDIC proxy in marine basins if input from nonmarine Thaumarchaeota is negligible. Semi-enclosed basins are sensitive to carbon-cycle perturbations, because they tend to develop thermohaline stratification. Water column stratification typified the semi-enclosed basins of the Mediterranean Sea during the late Miocene (Messinian) salinity crisis (5.97–5.33 Ma). To assess how the advent of the crisis affected the carbon cycle, we studied sediments of the Piedmont Basin (northwestern Italy), the northernmost Mediterranean subbasin. A potential bias of our δ13CDIC reconstructions from the input of soil Thaumarchaeota is discarded, since high and increasing branched and isoprenoid tetraether (BIT) index values do not correspond to low and decreasing δ13C values for thaumarchaeal lipids, which would be expected in case of high input from soil Thaumarchaeota. Before the onset of the crisis, the permanently stratified distal part of the basin hosted a water mass below the chemocline with a δ13CDIC value of approximately −3.5‰, while the well-mixed proximal part had a δ13CDIC value of approximately −0.8‰. The advent of the crisis was marked by 13C enrichment of the DIC pool, with positive δ13CDIC excursions up to +5‰ in the upper water column. Export of 12C to the seafloor after phytoplankton blooms and limited replenishment of remineralized carbon due to the stabilization of thermohaline stratification primarily caused such 13C enrichment of the DIC pool.

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.

Asian Winter Monsoon Imprint on the Water Column Structure at the Northern South China Sea Coast

Coastal regions of the northern South China Sea (SCS) strongly interact with the Asian monsoon circulation (AMC). Thus, variations of sea surface temperature (SST) here are newly suggested to document AMC changes in an effective manner, but additional physical parameters of oceanic conditions, probably also in relation to the AMC system, remain poorly understood. In this study, we analyzed glycerol dialkyl glycerol tetraethers (GDGTs) from a well-dated sediment core YJ, retrieved at the northern SCS coast, to further scrutinize the intrinsic response of water column to winter AMC strength. It shows that within the time frame of past ∼1,000 years, the tetraether index of lipids with 86 carbon atoms (TEX86) and published alkenone (U37K′) temperature records together confirm a reduced thermal gradient during the Little Ice Age (LIA), in comparison to that during the Medieval Climate Anomaly (MCA). Considering concurrent variations of the branched and isoprenoid tetraether (BIT) and the ratio of archaeol to caldarchaeol (ACE), for example, with decreased values (&amp;lt;∼0.3) for the former and relatively high values for the latter at the LIA, indicative of stratification and salinity changes, respectively, these multiple lines of evidence thereby call for well mixing of onsite water at site YJ correspondingly. Our results suggest that winter AMC strength is a critical factor for mixing subsurface waters and modifying thermal/saline conditions at the northern SCS coasts through the last millennium and also, perhaps, on longer timescales.

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.

The Sources of Organic Carbon in the Deepest Ocean: Implication From Bacterial Membrane Lipids in the Mariana Trench Zone

Hadal trenches have higher microbial carbon turnover rates as compared to adjacent abyssal plains. However, the source of organic carbon in the trench remains enigmatic. In this study, we show that a fraction of organic carbon is possibly derived in situ and correlated with chemoautotrophic communities supported by the fluid discharge of water-rock interaction in the trench wall, based on analysis of glycerol dialkyl glycerol tetraether (GDGT) membrane lipids, including archaeal isoprenoid GDGTs (IsoGDGTs) and bacterial branched GDGTs (BrGDGTs), in sediments and rocks of the Mariana and Yap Trenches, northwest Pacific Ocean. These trench sediments contained relative higher BrGDGTs ratios, which was a rare observation in the open ocean. The BrGDGT-to-IsoGDGT ratios ranged in 0.02–0.88 (mean = 0.10 ± 0.11) in sediments and 0.09–0.38 (mean = 0.17 ± 0.13) in altered rocks. The calculated values of branched and isoprenoid tetraether (BIT) index ranged from 0.02–0.73 (mean = 0.18 ± 0.11) in sediments and from 0.16–0.9 in altered rocks (mean = 0.37 ± 0.27). Moreover, these GDGTs exhibited similar characteristics to those of altered basalt rocks, indicating inputs of organic carbon from the trench subsurface environment. Thus, in addition to organic-rich material settling, we propose chemoautotrophic activity in oceanic crust could be an additional source of organic carbon in the deepest part of the ocean, with an important role in deep-sea carbon cycles.

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.

Miocene C4 Grassland Expansion as Recorded by the Indus Fan

AbstractIn the late Miocene, grasslands spread across the forested floodplains of the Himalayan foreland, but the causes of the ecological transition are still debated. Recent seafloor drilling by the International Ocean Discovery Program (IODP) provides an opportunity to study the transition across a larger region as archived in the Indus submarine fan. We present a multiproxy study of past vegetation change based on analyses of the carbon isotopic composition (δ13C) of bulk organic carbon, plant wax n‐alkanes and n‐alkanoic acids, and quantification of lignin phenols, charcoal, and pollen. We analyze the hydrogen isotopic composition (δD) of plant wax to reconstruct precipitation δD. We use the Branched and Isoprenoid Tetraether (BIT) index to diagnose shifts between terrestrial versus marine lipid inputs between turbidite and hemipelagic sediments. We reconstruct ocean temperatures using the TEX86 index only where marine lipids dominate. We find evidence for the late Miocene grassland expansion in both facies, confirming this was a regional ecosystem transformation. Turbidites contain dominantly terrestrial matter from the Indus catchment (D‐depleted plant wax), delivered via fluvial transport as shown by the presence of lignin. In contrast, hemipelagic sediments lack lignin and bear D‐enriched plant wax consistent with wind‐blown inputs from the Indian peninsula; these show a 7.4–7.2 Ma expansion of C4 grasslands on the Indian subcontinent. Within each facies, we find no clear change in δD values across the late Miocene C4 expansion, implying consistent distillation of rainfall by monsoon dynamics. Yet, a cooling in the Arabian Sea is coincident with the C4 expansion.

Predominance of hexamethylated 6-methyl branched glycerol dialkyl glycerol tetraethers in the Mariana Trench: source and environmental implication

Abstract. Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are useful molecular indicators for organic carbon (OC) sources and the paleoenvironment. Their application in marine environments, however, is complicated because of a mixed terrestrial and marine source. Here, we examined brGDGTs in sediments from the Mariana Trench, the deepest ocean without significant terrestrial influence. Our result shows a strong predominance of hexamethylated 6-methyl brGDGT (IIIa′) (73.40±2.39 % of total brGDGTs) and an absence of 5-methyl brGDGTs, different from previously reported soils and marine sediments that comprised both 5-methyl and 6-methyl brGDGTs. This unique feature, combined with high δ13COC (-19.82±0.25 %), low OC∕TN ratio (6.72±0.84), low branched and isoprenoid tetraether (BIT) index (0.03±0.01), and high acyclic hexa- ∕ pentamethylated brGDGT ratio (7.13±0.98), support that brGDGTs in the Mariana Trench sediments are autochthonous rather than terrestrial products. The compiling of literature data shows that the enhanced fractional abundance of hexamethylated 6-methyl brGDGTs is a common phenomenon in continental margins when the marine influence was intensified. The cross plot of acyclic hexa- ∕ pentamethylated brGDGT ratio and fractional abundance of brGDGT IIIa′ provide a novel approach to distinguish terrestrial and marine-derived brGDGTs.

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.

Decadal to interdecadal variability in TEX86 temperature on the inner shelf of the East China Sea over the last 3.7 kyr

To understand the long-term environmental evolution of the East China Sea (ECS) and adjacent areas, the variation in TEX86H-derived sea surface temperature (SST) at site T08 collected from the mud area on the inner continental shelf of the ECS over the last 3725 years was reconstructed. The core-top TEX86H SST is 23.2 °C, close to the annual mean SST (22.9 °C) at site T08. The branched and isoprenoid tetraether (BIT) index, which is a proxy for soil organic matter, ranges from 0.029 to 0.059, indicating that the influence of terrestrial input on the TEX86H SST proxy is negligible. The ratio between glycerol dialkyl glycerol tetraethers (GDGTs) GDGT-0 and GDGT-5 (R0/5), which is used to indicate the contribution of methanogenic archaea to the GDGT pool, varies between 0.04 and 0.62, suggesting a minor influence of methanogens on the TEX86H proxy. During the past 3725 years, variation in TEX86H SST can be divided into three periods with an overall increasing trend. During Period I (3725–2500 yr BP), TEX86H SST ranged from 19.0 to 20.6 °C with a mean of 20.1 °C. The values were generally relatively low but with marked fluctuation, attributed to increased input of cold coastal water caused by a strengthened East Asian winter monsoon (EAWM) and a weakened Kuroshio Current (KC). During Period II (2500–750 yr BP), after a low-temperature period at 2400–2200 yr BP, TEX86H SST increased gradually from 19.2 to 23.5 °C. During this period, an intensified East Asian summer monsoon (EASM) and a strengthened KC and Taiwan Warm Current (TWWC) transported warm water from low-latitude areas to ECS coastal areas. During Period III (750 yr BP to present), TEX86H SST increased gradually and reached a maximum (24.2 °C) at ∼565 yr BP. Subsequently, TEX86H SST decreased and reached a low value (21.9 °C) at ∼279 yr BP. During the period (<750 yr BP), higher TEX86H SST values were caused mainly by intensified KC, whereas lower values corresponded to the Little Ice Age. Spectral analysis reveals that variation in TEX86H SST exhibits periodicities of 175, 80, and 68 years, consistent with the periodicities of solar activity.

Simultaneous extraction and chromatographic separation of n-alkanes and alkenones from glycerol dialkyl glycerol tetraethers via selective Accelerated Solvent Extraction

A method is described for simultaneous extraction and chromatographic separation of saturated hydrocarbons (n-alkanes) and unsaturated long-chain ketones (alkenones) from glycerol dialkyl glycerol tetraethers (GDGTs) using selective accelerated solvent extraction (ASE). We show that the method can be used to successfully separate n-alkanes and alkenones from GDGTs in marine sediment samples and n-alkanes from GDGTs in lake sediments and soil samples. A comparison of our method with analysis of samples purified using classical silica column chromatography demonstrates that the new method does not affect the alkenone unsaturation index (U37K'), the TetraEther indeX (TEX86), the Methane Index (MI), the Ring Index (RI), the branched-GDGT-based paleotemperature and soil pH indices, the Branched and Isoprenoid Tetraether (BIT) index, the n-alkane average chain length (ACL) or the carbon preference index (CPI). The new procedure results in higher sample recoveries for n-alkanes and more precise U37K' estimates at low concentrations, and contributes to reduced analyst laboratory time.

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.

English

Contextually, precipitation fluctuated due to climate variability and evolved due to emissions of anthropogenic greenhouse gases and land-use changes since the industrial revolution in the 1880s. However, some studies problematize that there is little understanding of nexuses between precipitation changes and climate change and variability in tropical Africa. Therefore, this paper sought to assess such linkages in semi-arid lowlands of Mwanga District, Tanzania. The findings revealed statistically significant decrease of annual rainfall, Kendall’s tau rτ (44) = -.230, p = .019 and Kendall’s tau rτ (39) = -.223, p = .024, at Same Meteorological station (1970 to 2009 and 2012 and 2015) and Nyumba ya Mungu Meteorological stations (1977 to 2015), respectively; thus confirming occurrence of human-induced climate change in the study area. Also, the findings revealed statistically significant correlations between amounts of rainfall (September – February) and Nino 3.4 index and between amounts of rainfall (October – December) and dipole mode index at both stations, hence confirming that precipitation changes during short rainy seasons (Vuli) in the lowlands were significantly influenced by cycles of El Nino-southern oscillation and Indian Ocean dipole. Besides, branched and isoprenoid tetraether (BIT) indices revealed that wettest conditions, due to climate variability. This occurred from 650 to 950 CE (common era), 1550 to 1700 CE, 22 to 25 ka BP (before present, present defined as 1950). Whereas, driest conditions occurred from 1968 to 1974, 1780 to 1820 CE, 1170 to 1300 CE (between ca. 0.8 and 0.6 ka BP), 11.7 to 13.1 (the Younger Dryas), 15 to 18 (Heinrich 1 stadial) and 23.4 (Heinrich 2 stadial) ka BP. Lastly, the paper recommends enhancement of traditional and modern-day environmental knowledge systems with regard to weather forecasting and prediction. Key words: Branched and isoprenoid tetraether index, climate change, climate variability, El Nino-southern oscillation, Indian Ocean dipole, and short rainy season.

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.

Influence of human activities and organic matters on occurrence of polybrominated diphenyl ethers in marine sediment core: A case study in the Southern Yellow Sea, China

The Southern Yellow Sea (SYS) is an important reservoir of anthropogenic organic contaminants, such as polybrominated diphenyl ethers (PBDEs). To reconstruct the historical records of PBDEs and examine their relationships with the human activities and organic matters, a210Pb-dated sediment core was collected from the central mud area in the SYS. The concentrations of tri-to hepta-BDEs (∑7PBDEs) and BDE-209 ranged from 9.8 to 99.8 pg g−1 d.w. and from 12.1 to 855.4 pg g−1 d.w., respectively, both displaying the increasing trends from the bottom to the surface. More importantly, there was a faster increase for PBDEs since the 1990s, especially for BDE-209, which responded well with the rapid economic growth, and the increases of urbanization and industrialization in the local areas of the SYS. The analogously vertical patterns and significant relationships between PBDEs and total organic carbon (TOC) implied the TOC-dependent deposition of PBDEs in the core. Furthermore, multiple biomarker-based proxies of terrestrial organic matter (TOM) and marine organic matter (MOM) were introduced to systematically investigate the different effects of TOM and MOM on PBDE deposition in the SYS. The similarly down-core profiles and significant correlations were found between PBDEs and the MOM proxies (sum of rassicasterol, dinosterol and C37 alkenones (∑A + B + D) and marine TOC) as well as the branched and isoprenoid tetraether (BIT), but not for TOM proxies (∑C27+C29+C31n-alkanes, terrestrial and marine biomarker ratio (TMBR) and terrestrial TOC), indicating that MOM was an important factor driving PBDE deposition in the sediment core from the SYS.

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.