Description: The Maritime Continent, home to widespread tropical rainforest and millions of people, is the primary region of deep atmospheric convection on the Earth. However, debate exists whether the isotopologues of water reflect rainfall amount during the Last Glacial Maximum (LGM), resulting in different interpretations of the LGM climate of the Maritime Continent. Here we present paired leaf wax δ13C and δD records together with pollen data from a sediment core retrieved off East Java dating back to 22,000 years before present. We use three n-alkane homologues (n-C29, n-C31 and n-C33) in order to reconstruct past changes in vegetation types and seasonal rainfall. Our results suggest that in East Java, evergreen rainforest remained the dominant vegetation type in montane regions since the seasonality there remained relatively unaltered over the entire period. In contrast, the East Javanese lowlands were characterised by C4 grass expansion and an extended dry season but a wetter rainy season, thus stronger seasonality, during the LGM.

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, it is commonly assumed that mainly young organic material is discharged from flat tropical watersheds due to their extensive plant cover and high 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 in the watershed. We find that the Congo River has been discharging aged organic matter for several thousand years with increasing ages from the mid- to the Late Holocene. This suggests that aged organic matter in modern samples is concealed by radiocarbon from 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: Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are membrane lipids produced by soil bacteria and occur in near coastal marine sediments as a result of soil organic matter input. Their abundance relative to marine-derived crenarchaeol, quantified in the BIT index, generally decreases offshore. However, in distal marine sediments, low relative amounts of brGDGTs can often still be observed. Sedimentary in situ production as well as dust input have been suggested as potential, though as yet not well constrained, sources. In this study brGDGT distributions in dust were examined and compared with those in distal marine sediments. Dust was sampled along the equatorial West African coast and brGDGTs were detected in most of the samples, albeit in low abundance. Their degree of methylation and cyclisation, expressed in the MBT' (methylation index of branched tetraethers) and DC (degree of cyclisation) indices, respectively, were comparable with those for African soils, their presumed source. Comparison of DC index values for brGDGTS in global soils, Congo deep-sea river fan sediments and dust with those of distal marine sediments clearly showed, however, that distal marine sediments had significantly higher values. This distinctive distribution is suggestive of sedimentary in situ production as a source of brGDGTs in marine sediments, rather than dust input. The presence of in situ produced brGDGTs in marine sediments means that caution should be exercised when applying the MBT'-CBT palaeothermometer to sediments with low BIT index values, i.e. 〈 0.1, based on our dataset.

Description: Geochemical and mineralogical proxies for paleoenvironmental conditions have the underlying assumption that climate variations have an impact on terrestrial weathering conditions. Varying properties of terrigenous sediments deposited at sea are therefore often interpreted in terms of paleoenvironmental change. Also in gravity core GeoB9307-3 (18° 33.99' S, 37° 22.89' E), located off the Zambezi River, environmental changes during Heinrich Stadial 1 (HS 1) and the Younger Dryas (YD) are accompanied by changing properties of the terrigenous sediment fraction. Our study focuses on the relationship of variability in the hydrological system and changes in the magnetic properties, major element geochemistry and granulometry of the sediments. We propose that changes in bulk sedimentary properties concur with environmental change, although not as a direct response of climate driven pedogenic processes. Spatial varying rainfall intensities on a sub-basin scale modify sediment export from different parts of the Zambezi River basin. During humid phases, such as HS 1 and the YD, sediment was mainly exported from the coastal areas, while during more arid phases sediments mirror the hinterland soil and lithological properties and are likely derived from the northern Shire sub-basin. We propose that a de-coupling of sedimentological and organic signals with variable discharge and erosional activity can occur.

Description: Changes in heat transport associated with fluctuations in the strength of the Atlantic meridional overturning circulation (AMOC) are widely considered to affect the position of the Intertropical Convergence Zone (ITCZ), but the temporal immediacy of this teleconnection has to date not been resolved. Based on a high-resolution marine sediment sequence over the last deglaciation, we provide evidence for a synchronous and near-linear link between changes in the Atlantic interhemispheric sea surface temperature difference and continental precipitation over northeast Brazil. The tight coupling between AMOC strength, sea surface temperature difference, and precipitation changes over northeast Brazil unambiguously points to a rapid and proportional adjustment of the ITCZ location to past changes in the Atlantic meridional heat transport.

Description: Precipitation isotope reconstructions derived from speleothems and plant waxes are important archives for understanding hydroclimate dynamics. Their climatic significance in East Asia, however, remains controversial. Here we present terrestrial plant-wax stable hydrogen isotope (δDwax) records over periods covering the last four interglacials and glacial terminations from sediment cores recovered from the northern South China Sea (SCS) as an archive of regionally-integrated precipitation isotope changes in Southeast China. Combined with previous precipitation isotope reconstructions from China, we find that the SCS δDwax and Southwest-Central China stalagmite O records show relatively enriched and depleted isotopic values, respectively, during interglacial peaks; but relatively similar isotopic variations during most sub-interglacials and glacial periods over the past 430 thousand years. During interglacial peaks, strong summer insolation should have intensified the convection intensity, the isotopic fractionation along moisture trajectories and the seasonality, which are all in favor of causing isotopically-depleted rainfall over the East Asian monsoon regime. These effects in combination with a relatively high proportion of Indian Ocean- versus Pacific-sourced moisture influx should have resulted in strongly depleted precipitation isotopes (stalagmite O) over most parts of China. However, Southeast China should have been affected by a relatively low ratio of Indian Ocean- versus Pacific-sourced moisture influx, which dominated over effects yielding depleted precipitation isotopes and led to enriched precipitation isotopes (δDwax). It is thus concluded that glacial boundary conditions and insolation forcing are the two most important factors for causing regional differences in precipitation isotope compositions over subtropical East Asia on orbital timescales.

Description: Atmospheric transport is an understudied mechanism for leaf wax hydrogen isotope applications that contributes to mobilizing and depositing these compounds on the surface of the Earth. While previous efforts have identified the importance of atmospheric leaf wax deposition in remote marine locations, the processes are not well constrained on land in temperate latitudes where lakes are common and sedimentary leaf wax hydrogen isotope values are an attractive tool for understanding past precipitation changes. This work presents results from a field study that was conducted in 2010 and 2011 at Hainich National Park, Germany in order to evaluate the quantity and sources of leaf waxes in the atmosphere. Aerosols were sam- pled at approximately weekly intervals inside the forest canopy, and n-alkane distributions and hydrogen isotope values were compared with those from major tree species surrounding the sampling site. Despite sampling in what was expected to be a major production center, the distribution and hydrogen isotope values of atmospheric n-alkanes bore little resemblance to those of the local vegetation. Comparison with local meteorological data and to 10-day and 36-h back air mass trajectories indicated shifting effects of winds and temperature, and that mesoscale transport processes were more important than long- range mechanisms. Back trajectories also highlighted source effects, with easterly winds coinciding with relatively lower leaf wax hydrogen isotope values from more continental regions. These results suggest that leaf wax aerosols average over spatial scales that exceed typical surface catchment areas for small lake systems, even in forested areas, yet that the area over which these compounds are derived is still relatively regional. Depositional fluxes were also estimated in order to assess the potential importance of atmospheric transport to sedimentary archives. Although difficult to constrain, these estimates suggest that atmospheric deposition may be non-negligible for lake systems in cases where inputs from rivers or surface runoff are limited. Together, these observations provide new insights on how leaf waxes from different sources are integrated during aeolian transport and the spatial scales over which these processes occur.

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 per mil) in most samples, indicating a modern forest source for most of the samples. Enriched values (-31 to -33 per mil) 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 per mil), while n-alkanes from the lowland tributaries, the Negro, Xingu and Tocantins Rivers (-142 to -154 per mil), 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: The Amazon River transports large amounts of terrestrial organic carbon (OCterr) from the Andean and Amazon neotropical forests to the Atlantic Ocean. In order to compare the biogeochemical characteristics of OCterr in the fluvial sediments from the Amazon drainage basin and in the adjacent marine sediments, we analysed riverbed sediments from the Amazon mainstream and its main tributaries as well as marine surface sediments from the Amazon shelf and fan for total organic carbon (TOC) content, organic carbon isotopic composition (d13CTOC) and lignin phenol compositions. TOC and lignin content exhibit positive correlations with Al/Si ratios (indicative of the sediment grain size) implying that the grain size of sediment discharged by the Amazon River plays an important role in the preservation of TOC and leads to preferential preservation of lignin phenols in fine particles. Depleted d13CTOC values (-26.1 per mil to -29.9 per mil) in the main tributaries consistently correspond with the dominance of C3 vegetation. Ratios of syringyl to vanillyl (S/V) and cinnamyl to vanillyl (C/V) lignin phenols suggest that non-woody angiosperm tissues are the dominant source of lignin in the Amazon basin. Although the Amazon basin hosts a rich diversity of vascular plant types, distinct regional lignin compositions are not observed. In marine sediments, the distribution of d13CTOC and Lambda8 (sum of eight lignin phenols in organic carbon (OC), expressed as mg/100mg OC) values implies that OCterr discharged by the Amazon River is transported north-westward by the North Brazil Current and mostly deposited on the inner shelf. The lignin compositions in offshore sediments under the influence of the Amazon plume are consistent with the riverbed samples suggesting that processing of OCterr during offshore transport does not change the encoded source information. Therefore, the lignin compositions preserved in these offshore sediments can reliably reflect the vegetation in the Amazon River catchment. In sediments from the Amazon Fan, low lignin content, relatively depleted d13CTOC values and high (Ad/Al)V ratios indicating highly degraded lignin imply that a significant fraction of the deposited OCterr is derived from petrogenic (sourced from ancient rocks) sources.

Description: Terrestrial signals in marine sediment archives are often used for paleoclimatic reconstructions. It is therefore important to know the origin of the different terrestrial sedimentary components. The proximity to a river mouth is often the key location to determine the source. Especially in regions with strong ocean currents, such an assumption might,however, lead to considerable misinterpretations. To investigate the source of various terrigenous sediment fractions in southeastern Africa, a region with strong sediment redistribution, we have performed an extensive comparison between terrestrial material (pollen, plant lipids, detrital modes and heavy minerals as well as bulk inorganic geochemical composition) from potential source regions and the same components in the adjacent coastal and continental shelf sediments. Onshore the proxy‐indicators reflect small‐scale diversity in sampling locations and associated environments (riverbank sediments, flood deposits, suspension loads and soils). Nevertheless, the overall trends reflect significant environmental gradients along a SW to NE transect. We note a general comparability of the studied parameters between the continental and marine sediments regardless of their specific differences in transport and depositional characteristics. We propose that the influence of the Agulhas Current affects sediment deposition and distribution only seaward of the mid‐shelf and that pockets of sediment remain preserved in the lee of coastal protrusions where they are protected from erosion. This study provides the essential prerequisite to allow the attribution of temporal variations of compositional changes in marine sediment cores to environmental changes in southeastern Africa.