Description: In this study, we obtained concentrations and abundance ratios of long-chain alkenones and glycerol dialkyl glycerol tetraethers (GDGTs) in a one-year time-series of sinking particles collected with a sediment trap moored from December 2001 to November 2002 at 2200 m water depth south of Java in the eastern Indian Ocean. We investigate the seasonality of alkenone and GDGT fluxes as well as the potential habitat depth of the Thaumarchaeota producing the GDGTs entrained in sinking particles. The alkenone flux shows a pronounced seasonality and ranges from 1 µg m-2 d-1 to 35 µg m-2 d-1. The highest alkenone flux is observed in late September during the Southeast monsoon, coincident with high total organic carbon fluxes as well as high net primary productivity. Flux-weighted mean temperature for the high flux period using the alkenone-based sea-surface temperature (SST) index UK’37 is 26.7°C, which is similar to satellite-derived Southeast (SE) monsoon SST (26.4°C). The GDGT flux displays a weaker seasonality than that of the alkenones. It is elevated during the SE monsoon period compared to the Northwest (NW) monsoon and intermonsoon periods (approximately 2.5 times), which is probably related to seasonal variation of the abundance of Thaumarchaeota, or to enhanced export of GDGTs by aggregation with sinking phytoplankton detritus. Flux-weighted mean temperature inferred from the GDGT-based TEXH86 index is 26.2°C, which is 1.8 °C lower than mean annual (ma) SST but similar to SE monsoon SST. As the time series of TEXH86 temperature estimates, however, does not record a strong seasonal amplitude, we infer that TEXH86 reflects ma upper thermocline temperature at approximately 50 m water depth.

Description: Arctic rivers are known to export large quantities of carbon by discharge of dissolved and particulate organic carbon (DOC, POC), and in a warming and progressively moister Arctic, these exports may increase resulting in a reduction of arctic continental carbon stocks. These rivers have highly variable discharge rates with a pronounced maximum during the spring freshet associated with highest concentrations of DOC and POC. Most studies investigating the isotopic composition and quality of carbon exported by Arctic rivers rely on samples taken in summer during base flow, which is due to the logistical challenges associated with sampling in the remote Arctic permafrost regions. Here we present a record of δ13C and ∆14C of DOC and POC collected between late May during the freshet and late August 2014 in the Lena River Delta. POC ∆14C shows an initial trend towards older values in the spring samples, which is reversed in summer, associated with a shift towards more depleted δ13C values. We interpret this aging trend as reflecting progressive thawing throughout the ice-free season, resulting in mobilization of progressively older carbon from deeper thawed layers. The summer reversal indicates admixture of aquatic organic matter. DOC ∆14C, in contrast, remains at relatively modern levels with rather constant δ13C values throughout the sampling period. We furthermore analysed the biomarker composition of Lena Delta particulate OM collected in spring and summer. From spring to summer, we observe trends in abundance of individual leaf-wax derived biomarkers indicating higher abundance of algal biomass in the summer particles. Trends in soil microbial biomarkers and compound-specific δD of leaf-wax lipids suggest a shift in sources towards higher contributions from the southern catchment in summer. DOC composition investigated with FT-ICR-MS changes from spring with higher abundances of compounds with high H/C and low O/C ratios to late summer, when fewer compounds were found. Our results illustrate the seasonal variability in composition and sources of organic matter discharged by the Lena River. Paired with the strong seassonality of the hydrograph, this implies that total annual discharge of organic matter contains a disproportionally high contribution from the northern part of the catchment.

Description: During the last glacial termination atmospheric carbon dioxide (CO2atm) rose about 100 ppm and atmospheric radiocarbon activity (Δ14C) concurrently dropped by about ca. 400 ‰. Global warming likely triggered large-scale thawing of permafrost soils in the northern hemisphere resulting in release of 14C-depleted carbon which may have contributed to the changes in CO2atm and Δ14C1. However, the timing and duration of the thawing as well as regional differences regarding these points are poorly understood. In order to unravel the evolution of permafrost decomposition and its role within the glacial-interglacial climate change a profound understanding of the deglacial carbon-turnover and temperature development in subarctic and arctic regions are of great importance. Working with two sediment cores from the continental margin off Kamchatka Peninsula, western Bering Sea (WBS, site SO201-2-114KL) & Northwest Pacific (NW Pacific, site SO201-2-12KL) we establish Glacial to Holocene records in sea surface temperature (SST) and mean air temperature (MAT) using the TEXL86 (SST) and CBT/MBT (MAT) temperature proxies that are based on marine and terrigenous biomarkers (Glycerol dialkyl glycerol tetraethers). The hydrogen isotopic composition (δD) of plant-wax derived lipid biomarkers (long-chain n-alkanes and long-chain n-fatty acids) is another tool for reconstructing air temperature but can also provide information of the provenance of terrigenous organic matter (OM). The radiocarbon activity of the lipid biomarkers is applied to reconstruct changes in terrestrial residence times of terrigenous OM. Our records in SST and CBT/MBT-based MAT are interpreted as summer temperatures. From the Bølling/Allerød interstadial (B/A) to the present they show temperature fluctuations similar to Greenland ice core records including a warming at the onset of the B/A and a cooling during the Younger Dryas (YD) followed by a warming into the Preboreal (PB) suggesting an atmospheric coupling between N-Atlantic and N-Pacific, East Siberian/Kamchatka temperature development. However, during Heinrich Stadial 1 (HS1) where WBS SST and Kamchatka MAT cool down synchronously with Greenland temperatures, the NW-Pacific SST rises gradually and seems to be decoupled from the N-Atlantic. Since the gradual warming trend and the subsequent synchronization with Greenland ice core data during B/A is known from SST records from the Gulf of Alaska2 (GOA) surface conditions in the NW Pacific likely are controlled by the Alaskan Stream overprinting the atmospheric effect and the influence of the East Kamchatka Current. Asynchronous development of our TEXL86 record and the GOA records during the Holocene indicate that the AS weakenes over the deglaciation. For the time-span of the Last Glacial Maximum to the onset of the B/A interstadial, the CBT/MBT temperature proxy may be seasonally biased as it produces improbably high summer temperatures that level Holocene conditions. In contrast to the CBT/MBT-based temperatures the δD of n-fatty acids does not show clear stadial-interstadial fluctuations and remains on a stable level during the deglaciation instead. During the Holocene, δD increases progressively, which may have resulted from a gradual warming trend. Comparing the radiocarbon activity of the n-fatty acids to the Δ14C-signature of the atmosphere at the time of deposition the dimension of the terrestrial residence time prior to the deposition (ΔΔ14C) can be estimated. High ΔΔ14C values during deglaciation indicate that the plat-wax biomarkers are strongly pre-aged which may bias the δD-temperature record. Gradually decreasing ΔΔ14C imply declining terrestrial residence times from late glacial to late Holocene and argue for major changes in the relative contribution of weakly and strongly pre-aged OM. Possible sources for strongly pre-aged OM include permafrost decomposition and the congruent mobilization of 14C-depleted carbon but also the erosion of basal tills from the North-American or Kamchatka ice sheets that bear OM dating back to times prior to the glaciation. The n-C25/n-C25+n-C27 ratio, an indicator for the relative contribution of sphagnum spp.3, is on a stable level over most parts of the deglaciation indicating a constant composition of the vegetation and/or a constant carbon source. Between 16.5-14.6 kaBP the record shows a sharp excursions towards higher values congruent with a sharp increase in the δD of n-alkanes that is not evident in the δD of n-fatty acids. Coevally, lowered CPI-values (carbon preference index) point to a major contribution of fossil carbon at that time. Thus, our sites may be subject to a change in the carbon source. It coincides with melting events of the N-American ice sheets4 which may indicate that large amounts of OM from eastern Beringia accumulated in the WBS/NW Pacific during HS1. 1. S. A. Zimov, E. A. G.Schuur & F. S. Chapin III (2006). Permafrost and the global carbon budget.- Science, 312, p. 1612-1613 2. J. E. Vonk & Ö. Gustaffsson (2009). Calibrating n-alkanes Sphagnum proxies in sub-Arctic Scandinavia.- Organic Geochemistry, 40, p. 1085-1090. 3. S. K. Praetorius & A. C. Mix (2014). Synchronization of North Pacific and Greenland climates preceded abrupt deglacial warming.- Science, 345, p. 444-448. 4. I. L. Hendy & T. Cosma (2009).Vulnerability of the Cordilleran Ice Sheet to iceberg calving during late Quaternary rapid climate change events.- Paleoceanography, 23, p. PA2101

Description: The Northern Bay of Bengal (NBoB) is a globally important region for deep-sea organic matter (OM) deposition due to massive fluvial discharge from the Ganges-Brahmaputra-Meghna (G-B-M) rivers and moderate to high surface productivity. Previous studies have focused on carbon burial in turbiditic sediments of the Bengal Fan. However, little is known about the storage of carbon in pelagic and hemipelagic sediments of the Bay of Bengal over millennial time scales. This study presents a comprehensive history of OM origin and fate as well as a quantification of carbon sediment storage in the Eastern Bengal Slope (EBS) during the last 18 ka. Bulk organic proxies (TOC, TIC, TN, δ13CTOC, δ15NTN) and content and composition of total hydrolysable amino acids (THAA) in a sediment core (SO188-342KL) from the EBS were analyzed. Three periods of high OM accumulation were identified: the Late Glacial (LG), the Bölling/Alleröd (B/A), and the Early Holocene Climatic Optimum (EHCO). Lower eustatic sea level before 15 ka BP allowed a closer connection between the EBS and the fluvial debouch, favoring high terrestrial OM input to the core site. This connection was progressively lost between 15 and 7 ka BP as sea level rose to its present height and terrestrial OM input decreased considerably. Export and preservation of marine OM was stimulated during periods of summer monsoon intensification (B/A and EHCO) as a consequence of higher surface productivity enhanced by cyclonic-eddy nutrient pumping and fluvial nutrient delivery into the photic zone. Changes in the THAA composition indicate that the marine plankton community structure shifted from calcareous-dominated before 13 ka BP to siliceous-dominated afterwards. They also indicate that the relative proportion of marine versus terrestrial OM deposited at site 342KL was primarily driven by relative sea level and enlarged during the Holocene. The ballasting effect of lithogenic particles during periods of high coastal proximity and/or enhanced fluvial discharge promoted the export and preservation of OM. The high organic carbon accumulation rates in the EBS during the LG (18–17 ka BP) were 5-fold higher than at present and comparable to those of glacial upwelling areas. Despite the differences in sediment and OM transport and storage among the Western and Eastern sectors of the NBoB, this region remains important for global carbon sequestration during sea level low-stands. In addition, the summer monsoon was a key promotor of terrestrial and marine OM export to the deep-ocean, highlighting its relevance as regulator of the global carbon budget.

Description: Paleoenvironmental studies based on terrigenous biomarker proxies from sediment cores collected close to the mouth of large river systems rely on a proper understanding of the processes controlling origin, transport and deposition of biomarkers.Here, we contribute to the understanding of these processes by analyzing long-chain n-alkanes from the Amazon River system. We use the dD composition of long-chain n-alkanes from river bed sediments from the Amazon River and its major tributaries, as well as marine core-top samples collected off northeastern South America as tracers for different source areas. The d13C composition of the same compounds is used to differentiate between long-chain n-alkanes from modern forest vegetation and petrogenic organic matter. Our d13C results show depleted d13C values (-33 to -36‰) in most samples, indicating a modern forest source for most of the samples. Enriched values (-31 to -33‰) are only found in a few samples poor in organic carbon indicating minor contributions from a fossil petrogenic source. Long-chain n-alkane dD analyses show more depleted values for the western tributaries, the Madeira and Solimões Rivers (-152 to -168‰), while n-alkanes from the lowland tributaries, the Negro, Xingu and Tocantins Rivers (-142 to -154‰), yield more enriched values. The n-alkane dD values thus reflect the mean annual isotopic composition of precipitation, which is most deuterium-depleted in the western Amazon Basin and more enriched in the eastern sector of the basin. Samples from the Amazon estuary show a mixed long-chain n-alkane dD signal from both eastern lowland and western tributaries. Marine core-top samples underlying the Amazon freshwater plume yield dD values similar to those from the Amazon estuary, while core-top samples from outside the plume showed more enriched values. Although the variability in the river bed data precludes quantitative assessment of relative contributions, our results indicate that long-chain n-alkanes from the Amazon estuary and plume represent an integrated signal of different regions of the onshore basin. Our results also imply that n-alkanes are not extensively remineralized during transport and that the signal at the Amazon estuary and plume includes refractory compounds derived from the western sector of the Basin. These findings will aid in the interpretation of plant wax-based records of marine sediment cores collected from the adjacent ocean.

Description: When using biomarkers such as n-alkanes as tools for paleo-environmental reconstructions, it is imperative to determine their specific sources for each setting. Towards that goal, we analysed a set of various potential organic matter (OM) sources such as aquatic and terrestrial plants, dust, and soils from Laguna Potrok Aike (LPA) and surrounding areas in Southern Patagonia. We determined chain length distributions and hydrogen (δD) and carbon (δ13C) isotopic compositions of n-alkanes of different OM sources in order to quantify their relative contributions to lake sediments. Our results reveal that mid-chain n-alkane, n-C23, is predominantly produced by submerged aquatic plants, whereas long-chain n-alkanes (n-C29 to n-C31) are derived from various terrestrial sources. We estimated their relative contributions to the sediment using two approaches, i.e. based on the n-alkane distributions and their δD and δ13C values. Both approaches result in similar estimates of aquatic and terrestrial contributions for mid- and long-chain n-alkanes to the sediment. 62-73% of the mid-chain n-C23 alkanes originate from aquatic sources while 66-77% of the long-chain n-alkanes originate from dust and 14-30% from terrestrial plants. Our study shows that mid-chain n-alkanes such as n-C23 alkane in LPA are derived mainly from aquatic macrophytes and thus have the potential to record changes in lake-water isotopic composition. In contrast, the n-C29 alkane reflects the isotopic signal of various terrestrial sources from Southern Patagonia.

Description: Bulk organic parameters and stable and radiocarbon isotope compositions of organic carbon (δ13COC and Δ14COC) as well as various biomarkers (lignin phenols, plant waxes etc.) have been used to investigate the biogeochemical characteristics of organic carbon in the Amazon River system. However, the source, concentration and distribution pattern of lignin on the Amazon shelf and fan has not been assessed so far. In particular, the compound-specific stable carbon isotope compositions (δ13C) of lignin phenols have not been characterized in the Amazon River system. In order to study the distribution of lignin in the lower Amazon basin and its dispersal on the shelf and fan, we used riverbed sediments from the Amazon mainstream and its main tributaries and marine surface sediments on the Amazon shelf and fan. The samples were analyzed for particulate organic carbon content (POC), δ13COC, lignin phenol compositions and compound-specific δ13C of individual lignin phenols. The concentrations of aluminium and silicon (Al/Si) were used as a proxy for grain size[1]. The POC content in the main tributaries ranged from 0.13 to 3.99 wt-% and increased with Al/Si ratio in each tributary. δ13COC varied from -26.1‰ to -29.9‰ VPDB in riverbed sediments. Lignin content (represented by Λ8, sum of eight lignin phenols in OC, expressed as mg/100mg OC) ranged from 0.73 to 6.91 and is positively related with Al/Si ratio in the main tributaries except for the Xingu River, in which Λ8 decreased with Al/Si. Ratios of syringyl to vanillyl (S/V) and cinnamyl to vanillyl (C/V) varied from 0.70 to 1.51 and 0.08 to 0.47, respectively, suggesting that the dominant source of lignin is non-woody angiosperm tissue. The ratios of vanillic acid to vanillin (Ad/Al)v (0.26-0.71) and syringic acid to syringaldehyde (Ad/Al)s (0.15-0.57) indicated relatively fresh, non-degraded lignin. In marine sediments, the δ13COC ranged from -18.6‰ to -26.7‰ and is correlated with the Λ8 value (0.04-2.01). The decreasing Λ8 value along the coast from the Amazon River mouth towards the northwest implies that lignin is distributed by the North Brazil Current. A main plant source of non-woody angiosperm tissue was indicated by the S/V (0.59-1.62) and C/V (0.10-0.43) ratios on the marine samples. The agreement between riverbed and marine sediments suggests that processing of POC during transport from the basin to offshore does not change the plant source information of lignin. Highly degraded lignin on the Amazon fan and the southeast shelf is indicated by (Ad/Al)v (0.49-0.99). δ13C of lignin phenols of 9 marine sediments ranged from -28.6‰ to -33.3‰ and were consistently lower than δ13COC (-19.7‰ to -26.7‰). Depleted δ13C of lignin phenols indicate that the POC produced by the terrestrial biosphere is mainly derived from higher plants using C3 photosynthesis. References Bouchez, J., Galy, V., Hilton, R.G., Gaillardet, J., Moreira-Turcq, P., Pérez, M.A., France-Lanord, C., Maurice, L., 2014. Source, transport and fluxes of Amazon River particulate organic carbon: Insights from river sediment depth-profiles. Geochemica et Cosmochimica Acta 133, 280-298

Description: Author Posting. © The Author(s), 2016. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature Geoscience 9 (2016): 687-690, doi:10.1038/ngeo2778.

Description: The age of organic material discharged by rivers provides information about its sources and carbon cycling processes within watersheds. While elevated ages in fluvially-transported organic matter are usually explained by erosion of soils and sediments deposits it is commonly assumed that mainly young organic material is discharged from flat tropical watersheds due to their extensive plant cover and rapid carbon turnover. Here we present compound-specific radiocarbon data of terrigenous organic fractions from a sedimentary archive offshore the Congo River in conjunction with molecular markers for methane-producing land cover reflecting wetland extent. We find that the Congo River has been discharging aged organic matter for several thousand years with apparently increasing ages from the Mid- to the Late Holocene. This suggests that aged organic matter in modern samples is concealed by radiocarbon from atmospheric nuclear weapons testing. By comparison to indicators for past rainfall changes we detect a systematic control of organic matter sequestration and release by continental hydrology mediating temporary carbon storage in wetlands. As aridification also leads to exposure and rapid remineralization of large amounts of previously stored labile organic matter we infer that this process may cause a profound direct climate feedback currently underestimated in carbon cycle assessments.

Description: This study was supported by the Deutsche Forschungsgemeinschaft (grants SCHN 621/3-3, RU 458/29-3, GR 1845/2-3, SCHE 903/1), the US National Science Foundation (grant OCE-0137005), a Starting Grant from the European Research Council (ERC) awarded to HMT for project AMOPROX (No. 258734) and grants ICM-NC120066 and FONDAP15110009 to RDP-H. This work was supported by the DFG Research Center/Cluster of Excellence ‘The Ocean in the Earth System’ at MARUM – Center for Marine Environmental Sciences, University of Bremen.

Description: 2017-02-15