Sedimentological and palynological analysis of sediment cores from the northeastern Vietnamese Mekong River Delta

Three radiocarbon-dated sediment cores from the northeastern Vietnamese Mekong River Delta have been analysed with a multiproxy approach (grain size, pollen and spores, macro-charcoal, carbon content) to unravel the palaeoenvironmental history of the region since the mid Holocene. During the mid-Holocene sea-level highstand a diverse, zoned and widespread mangrove belt (dominated by Rhizophora) covered the extended tidal flats. The subsequent regression and coeval delta progradation led to the rapid development of a back-mangrove community dominated by Ceriops and Bruguiera but also represented locally by e.g. Kandelia, Excoecaria and Phoenix. Along rivers this community seems to have endured even when the adjoining floodplain had already shifted to freshwater vegetation. Generally this freshwater vegetation has a strong swamp signature but locally Arecaceae, Fabaceae, Moraceae/Urticaceae and Myrsinaceae are important and mirror the geomorphological diversity of the delta plain. The macro-charcoal record implies that natural burning of vegetation occurred throughout the records, however, the occurrence of the highest amounts of macro-charcoal particles is linked with modern human activity.

Stable hydrogen and carbon isotope records and concentration data for four long chain n-alkanes from core GIK16160-3 derived from the Zambezi delta in the Mozambique Channel during the last 37,000 years

Hydrogen isotope values (dD) of sedimentary terrestrial leaf wax such as n-alkanes or n-acids have been used to map and understand past changes in rainfall amount in the tropics because dD of precipitation is commonly assumed as the first order controlling factor of leaf wax dD. Plant functional types and their photosynthetic pathways can also affect leaf wax dD but these biological effects are rarely taken into account in paleo studies relying on this rainfall proxy. To investigate how biological effects may influence dD values we here present a 37,000-year old record of dD and stable carbon isotopes (d13C) measured on four n-alkanes (n-C27, n-C29, n-C31, n-C33) from a marine sediment core collected off the Zambezi River mouth. Our paleo d13C records suggest that each individual n-alkanes had different C3/C4 proportional contributions. n-C29 was mostly derived from a C3 dicots (trees, shrubs and forbs) dominant vegetation throughout the entire record. In contrast, the longer chain n-C33 and n-C31 were mostly contributed by C4 grasses during the Glacial period but shifted to a mixture of C4 grasses and C3 dicots during the Holocene. Strong correlations between dD and d13C values of n-C33 (correlation coefficient R2 = 0.75, n = 58) and n-C31 (R2 = 0.48, n = 58) suggest that their dD values were strongly influenced by changes in the relative contributions of C3/C4 plant types in contrast to n-C29 (R2 = 0.07, n = 58). Within regions with variable C3/C4 input, we conclude that dD values of n-C29 are the most reliable and unbiased indicator for past changes in rainfall, and that dD and d13C values of n-C31 and n-C33 are sensitive to C3/C4 vegetation changes. Our results demonstrate that a robust interpretation of palaeohydrological data using n-alkane dD requires additional knowledge of regional vegetation changes from which nalkanes are synthesized, and that the combination of dD and d13C values of multiple n-alkanes can help to differentiate biological effects from those related to the hydrological cycle.

Phytoplankton abundance measured on water samples from rivers of the Lena Delta in 2010

The Lena Delta in Northern Siberia is one of the largest river deltas in the world. During peak discharge, after the ice melt in spring, it delivers between 60-8000 m**3/s of water and sediment into the Arctic Ocean. The Lena Delta and the Laptev Sea coast also constitute a continuous permafrost region. Ongoing climate change, which is particularly pronounced in the Arctic, is leading to increased rates of permafrost thaw. This has already profoundly altered the discharge rates of the Lena River. But the chemistry of the river waters which are discharged into the coastal Laptev Sea have also been hypothesized to undergo considerable compositional changes, e.g. by increasing concentrations of inorganic nutrients such as dissolved organic carbon (DOC) and methane. These physical and chemical changes will also affect the composition of the phytoplankton communities. However, before potential consequences of climate change for coastal arctic phytoplankton communities can be judged, the inherent status of the diversity and food web interactions within the delta have to be established. In 2010, as part of the AWI Lena Delta programme, the phyto- and microzooplankton community in three river channels of the delta (Trofimov, Bykov and Olenek) as well as four coastal transects were investigated to capture the typical river phytoplankton communities and the transitional zone of brackish/marine conditions. Most CTD profiles from 23 coastal stations showed very strong stratification. The only exception to this was a small, shallow and mixed area running from the outflow of Bykov channel in a northerly direction parallel to the shore. Of the five stations in this area, three had a salinity of close to zero. Two further stations had salinities of around 2 and 5 throughout the water column. In the remaining transects, on the other hand, salinities varied between 5 and 30 with depth. Phytoplankton counts from the outflow from the Lena were dominated by diatoms (Aulacoseira species) cyanobacteria (Aphanizomenon, Pseudanabaena) and chlorophytes. In contrast, in the stratified stations the plankton was mostly dominated by dinoflagellates, ciliates and nanoflagellates, with only an insignificant diatom component from the genera Chaetoceros and Thalassiosira (brackish as opposed to freshwater species). Ciliate abundance was significantly coupled with the abundance of total flagellates. A pronounced partitioning in the phytoplankton community was also discernible with depth, with a different community composition and abundance above and below the thermocline in the stratified sites. This work is a first analysis of the phytoplankton community structure in the region where Lena River discharge enters the Laptev Sea.