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During past periods of advance, Arctic glaciers and ice sheets overrode soil, sediments, and vegetation and buried significant stores of organic matter (OM); these glaciers are now shrinking rapidly due to climate warming. Little is known about the biogeochemical processing of the OM buried beneath glacier ice which makes the processes associated with deglaciation difficult to predict. Subglacial sediments exposed at receding glacier fronts may represent a legacy of past biogeochemical processes. Here, we analyzed sediments from retreating fronts of 19 Arctic glaciers for their mineralogical and elemental composition, contents of major nutrients, OM biomarkers (aliphatic lipids and lignin-derived phenols), C-14 age, and microbial community structure. We show the character of the sediments is mostly determined by local glaciation history and bedrock lithology. Most subglacial sediments offer high amounts of readily bioavailable phosphorus (i.e., loose, labile, and Fe/Al P fractions) but lack readily accessible carbon substrates. The subglacial OM originated mainly from overridden terrestrial vascular plants. The results of OM biomarker analysis and C-14 dating suggest the OM substrates degrade in the subglacial environment and are reworked by the resident microbial communities. We argue the biogeochemical legacy of the perishing subglacial environments is an important determinant for the early processes of proglacial ecological succession.

WOS:000684005500003; International audience; Coastal habitats are interlinked by ecological connectivity, defined as the exchange of organic matter or organisms between habitats. The degree of this connectivity will depend in particular on the geometric properties of the landscape. The increasing anthropogenic alterations in estuaries therefore raises the need to understand how their morphological characteristics influence fluxes between habitats. We used stable isotopes to investigate the variability of ecological connectivity between three estuaries and their adjacent coastal areas by tracking the origin of the organic matter (estuarine vs coastal) underlying the diet of the migrant species Eucinostomus argenteus. The chosen estuaries were geomorphologically distinct, exhibiting, in particular, differences in their degree of connection to the sea, corresponding to the morphological features (shape, mouth width) controlling key physico-chemical variables in this habitat (e.g. salinity). The sampling of the basal food sources contributing to the food web was performed in the three estuaries and in their adjacent coasts. The variability in stable isotope ratios between estuaries was examined for both fish and sources of organic matter. In the three estuaries, Bayesian models were applied for each season (dry and rainy) to quantify the relative contribution of sources from estuarine and coastal environments supporting the diet of the silver mojarra. The share of coastal organic matter increased with the degree of sea connection, indicating that the properties of the seascape can regulate the intensity of interactions between ecosystems. Variations in ecological connectivity are likely to affect the functioning of ecosystems as they influence trophic pathways and energy flows between adjacent habitats. Morphological modifications could thus significantly disturb ecosystems by altering the structure of food web, thereby affecting certain ecosystem services such as the availability of living marine resources.

Deforestation associated with the initial settlement of New Zealand is a dramatic example of how humans can alter landscapes through fire. However, evidence linking early human presence and land-cover change is inferential in most continental sites. We employed a multi-proxy approach to reconstruct anthropogenic land use in New Zealand’s South Island over the last millennium using fecal and plant sterols as indicators of human activity and monosaccharide anhydrides, polycyclic aromatic hydrocarbons, charcoal and pollen as tracers of fire and vegetation change in lake-sediment cores. Our data provide a direct record of local human presence in Lake Kirkpatrick and Lake Diamond watersheds at the time of deforestation and a new and stronger case of human agency linked with forest clearance. The first detection of human presence matches charcoal and biomarker evidence for initial burning at c. AD 1350. Sterols decreased shortly after to values suggesting the sporadic presence of people and then rose to unprecedented levels after the European settlement. Our results confirm that initial human arrival in New Zealand was associated with brief and intense burning activities. Testing our approach in a context of well-established fire history provides a new tool for understanding cause-effect relationships in more complex continental reconstructions.

California has experienced a dry 21st century capped by severe drought from 2012 through 2015 prompting questions about hydroclimatic sensitivity to anthropogenic climate change and implications for the future. We address these questions using a Holocene lake sediment record of hydrologic change from the Sierra Nevada Mountains coupled with marine sediment records from the Pacific. These data provide evidence of a persistent relationship between past climate warming, Pacific sea surface temperature (SST) shifts and centennial to millennial episodes of California aridity. The link is most evident during the thermal-maximum of the mid-Holocene (~8 to 3 ka; ka = 1,000 calendar years before present) and during the Medieval Climate Anomaly (MCA) (~1 ka to 0.7 ka). In both cases, climate warming corresponded with cooling of the eastern tropical Pacific despite differences in the factors producing increased radiative forcing. The magnitude of prolonged eastern Pacific cooling was modest, similar to observed La Niña excursions of 1o to 2 °C. Given differences with current radiative forcing it remains uncertain if the Pacific will react in a similar manner in the 21st century, but should it follow apparent past behavior more intense and prolonged aridity in California would result.

n-Alkane distributions are frequently used as palaeoclimate proxies in ombrotrophic peat deposits. Although n-alkane distributions differ strongly between plant species, n-alkanes are not species-specific molecules. For a proper interpretation, it is important to understand the different abundances of n-alkanes in various plant species as well as the changes that occur when plant litter is transformed to peat. In particular because molecular markers are especially valuable in highly decomposed peat where plant remains are no longer recognizable, it is important to understand the effects of decomposition on n-alkane distributions. The organic matter (OM) of a high-resolution sampled, 9 m thick, ombrotrophic peat deposit from Tierra del Fuego was analysed with pyrolysis-gas chromatography/mass spectrometry (pyrolysis-GC/MS). The same samples were analysed for carbon (C) and nitrogen (N) content. Depth profiles of C:N ratio, the summed lignin and summed polysaccharide pyrolysis products, and markers specific for Sphagnum spp., Empetrum rubrum and Nothofagus antarctica, enabled a reconstruction of changes in vegetation composition to be made. This reconstruction was used to examine the validity of the n-C23 alkane to indicate Sphagnum and the summed long chain n-alkanes (C29 and C31) to reflect leaf input of the woody species E. rubrum and N. antarctica. Our results show that even in Sphagnum-dominated peat, the n-alkane distribution is not determined by Sphagnum but by leaf input of E. rubrum and N. antarctica. However, good correlations between the n-C23 alkane and the Sphagnum marker 4-isopropenylphenol, and between the summed n-C29 and n-C31 alkanes and the marker of N. antarctica support that their relative change with depth can be used to indicate the abundance of these species in Sphagnum-dominated peat. In peat with relatively low contributions of Sphagnum, both n-alkane proxies (C23 and C29 + C31) reflect the degree of decomposition. We evaluated the influence of Sphagnum dominance, decomposition, and pyrolysis on the n-alkane distributions in peat OM.

International audience; The origin of perylene, a five-ring polycyclic aromatic hydrocarbon (PAH), in sediments has been a matter of constant debate and its formation mechanisms remain unclear. A perylene record in sediment was described and could support a clear link between perylene and terrigenous organic matter (OM) input. The distributions of PAHs, the variations of organic proxies such as the ratio of terrigenous to aquatic n-alkanes (TAR(HC)) and 5a(H)-stanols/D5-sterols ratio in sediments have been investigated. Sediments were sampled from a pond artificially created during the Middle Ages from a swampy area. In the period prior to the pond creation, for which high contributions of terrigenous OM were evidenced, perylene was found to be the predominant PAH. Furthermore, perylene content was shown to increase in response to the establishment of more reducing conditions. This result supports the common idea that the main source of perylene is natural and derives from biogenic precursors under reducing conditions. The creation of the pond in this wetland is marked by the deposition of a wood rich level characterized both by more oxygenated conditions and higher concentrations of perylene. These high concentrations could result from the introduction of high concentrations of biogenic precursors of perylene under oxidative conditions. Subsequently, the progressive burial of the woody level could have allowed the establishment of oxygen depleted conditions and the formation of perylene. These results validate the use of perylene as a paleoenvironmental marker of terrigenous sources but it must be carefully used as a marker for syn- or post-depositional oxygen depleted conditions in lacustrine environments