Effect of precipitation regime on δ D values of soil n-alkanes from elevation gradients - Implications for the study of paleo-elevation

We measured delta D values of long chain n-alkanes isolated from 30 surface soil samples along two elevation transects on the Tibetan Plateau differing in precipitation regime and water source. The East Asian Monsoon precipitation dominates the wetter regime on the eastern slope (from 1230 to 4300 m) of Gongga Shan on the eastern Tibetan Plateau. Precipitation from the Polar Westerlies dominates the drier region on the slope from 1900 to 5000 m in the West Kunlun Shan on the northwestern Tibetan Plateau. The decrease in delta D value with elevation in the wetter region greatly exceeded that in drier region by, -1.9 +/- 0.1 parts per thousand/100 m and -1.4 +/- 1.0 parts per thousand/100 m respectively. The apparent fractionation between leaf wax and precipitation epsilon(wax-p) values in the wetter region (ca. -164 parts per thousand) were more negative than those in drier region (ca. -125 parts per thousand above 3200 m). We also measured delta D values in leaves of six common living trees (values from -287 parts per thousand to -193 parts per thousand) from Gongga Shan, ranging from about 2900-4200 m. The abundance-weighted average values of the n-alkanes (delta D(wax)) show a strong reverse correlation with sample source elevation (R(2) 0.78 for soils from Gongga Shan; R(2) 0.85 for soils from West Kunlun Shan above 3200 m), suggesting that n-alkane delta D(wax) faithfully records the precipitation delta D and that the isotopic altitude effect of precipitation controls delta D(wax) altitudinal gradients in the mountains. The data show a fairly strong monotonic dependency of n-alkane delta D values on elevation for the eastern Plateau, but a complex relationship between n-alkane delta D values and elevation for the northwestern Plateau. The delta D(wax) values at sites below 3200 m from the Kunlun Shan area exhibit an unexpected positive correlation with elevation. The study confirms the potential for using sediment delta D(wax) values to reconstruct paleo-elevation in wetter regions, but suggests caution in applying the approach to dry regions. Our results also show it is essential to consider the intricacy of the pattern of atmospheric circulation and water sources and their influence on the lapse rate of delta D values with elevation. (C) 2011 Elsevier Ltd. All rights reserved

Compound-specific δ¹³C and δ²H analyses of plant and soil organic matter: A preliminary assessment of the effects of vegetation change on ecosystem hydrology

Here we present delta C-13 and delta H-2 data of long-chained, even-numbered (C-27-C-31) n-alkanes from C3 (trees) and C4 (grasses) plants and from the corresponding soils from a grassland-woodland vegetation sequence in central Queensland, Australia. Our data show that delta C-13 values of the C4 grassland species were heavier relative to those of C3 tree species from the woodland (Acacia leaves) and woody grassland (Atalaya leaves). However, n-alkanes from the C4 grasses had lighter delta H-2 values relative to the Acacia leaves, but showed no significant difference in delta H-2 values when compared with C3 Atalaya leaves. These results differ from those of previous studies, showing that C4 grasses had heavier delta H-2 values relative to C3 grasses and trees. Those observations have been explained by C4 plants accessing the more evaporation-influenced and isotopically heavier surface water and tree roots sourcing deeper, isotopically lighter soil water ("Two-layered soil-water system"). By comparison, our data suggest that ecosystem changes (vegetation "thickening") can significantly alter the soil hydrological characteristics. This is shown by the heavier delta H-2 values in the woodland soil compared with lighter delta H-2 values in the grassland soil, implying that the recent vegetation change (increased tree biomass) in the woodland had altered soil hydrological conditions. Estimated delta H-2 values of the source-water for vegetation in the grassland and woodland showed that both trees and grasses in open settings accessed water with lighter delta H-2 values (avg. -46 parts per thousand) compared with water accessed by trees in the woodland vegetation (avg. -7 parts per thousand). These data suggest that in semi-arid environments the "two-layer" soil water concept might not apply. Furthermore, our data indicate that compound-specific delta H-2 and delta C-13 analyses of n-alkanes from soil organic matter can be used to successfully differentiate between water sources of different vegetation types (grasses versus trees) in natural ecosystems. (c) 2006 Elsevier Ltd. All rights reserved. [References: 56

Reconstruction of the depositional history of the former coastal lagoon of Vilamoura (Algarve, Portugal): A sedimentological, microfaunal and geophysical approach

The late Holocene evolution of the former coastal lagoon of Vilamoura was reconstructed according to sediment cores and geophysical profiles. According to sedimentological analyses of the cores, five palaeoenvironmental stages were defined. (1) The pretransgression stage is represented by an erosive surface formed during incision of the river into the basement because of a lower sea level. This palaeosurface was retraced by refraction seismic profiles, showing that the marine transgression took place on a wide plain with several incised channels. (2) The development of an estuary started by transgression into the river valley corresponding to the postglacial sea level rise. Radiocarbon dating indicates a sea level not lower than -4 m at a minimum age of 4716 +/- 72 Cal BP. After the transgressive maximum, infilling of the estuary started, beginning with (3) subtidal infilling related to the formation of a sandy barrier followed by (4) supratidal infilling with further accretion of the barrier, changing the previous open bay into a coastal lagoon. (5) Finally, the lagoon was fluvially filled with terrestrial sediments, changing the marine to a fluvial milieu with floodplain deposition. Analysis of benthic foraminiferal and ostracod assemblages revealed additional information about the environmental conditions during evolution of the estuary, which led to a further subdivision of the marine facies into stages with mainly estuarine, lagoonal, or marine influence. The end of the marine stage was dated at 2895 +/- 48 Cal BP, indicating a pre-Roman onset of human-induced soil erosion

Hydrological and geomorphological basin and catchment characteristics of Lake Nam Co, South-Central Tibet

For Lake Nam Co, south-central Tibetan Plateau, geomorphological features and hydrological characteristics have been compiled to provide an overview of the landscape character. Information is based on primary data, collected during field campaigns in the summers of 2005 and 2006, and secondary data as available from satellite images. Nam Co is located at the intersection of the Westerlies, the winter monsoon, the South West Asian monsoon, and the East Asian monsoon. It is situated in a graben structure. Landforms of the drainage basin are highly influenced by tectonics, superimposed by fluvial and periglacial processes, and locally by glacial and aeolian processes. Its drainage basin is endorheic. Thus, lake level is primarily controlled by the balance between input through precipitation or inflow streams and evaporation as the output. The significance of lake water evaporation is evidenced by the salinity and measurable increase of H-2 and O-18 of the lake water when compared to the inflow streams and precipitation. Maximum conductivities total 431 mu S mm(-1) for the inflowing river waters and 1920 mu S mm(-1) for the lake water. Overall, the solute contents are low and characterise Nam Co as a pristine environment. Dominant ions are calcium and bicarbonate in river waters. Sulphate contents are relatively high in Nam Co, and the lake is also characterised by high boron values. A significant decrease in lake water salinity during the past two decades is likely related to increased freshwater input. (C) 2009 Elsevier Ltd and INQUA. All rights reserved

Effect of lake evaporation on δ D values of lacustrine n-alkanes: A comparison of Nam Co (Tibetan Plateau) and Holzmaar (Germany)

Compound-specific hydrogen isotope ratio values of lacustrine n-alkanes from two contrasting ecosystems, the semiarid to arid Nam Co, Central Tibet and the humid Holzmaar, Germany, were compared in order to assess whether or not these environmental conditions are recorded in the isotopic signatures of biomarkers. Increased evaporation of lake water at Nam Co is recorded by the n-alkanes of aquatic origin. Hence, isotopic enrichment results in a difference between terrestrial and aquatic n-alkanes in the opposite direction (similar to-68 parts per thousand) from that known for humid climate conditions (similar to+30 parts per thousand) predominating at Holzmaar. Based on this isotopic difference between terrestrial and aquatic n-alkanes, evaporation to inflow ratio (E/I) values were estimated and suggest that the isotopic difference is indicative of the general hydroclimatic characteristics of a lake system. Moreover, the comparison with E/I values calculated with actual stable water isotope data showed that the delta D values of aquatic and terrestrial lacustrine n-alkanes serve as a proxy for the relative isotopic differences between lake and inflow waters and can be used to assess the proportion of water undergoing evaporation relative to the inflow, and thus to reconstruct the lake water balance in the past. (C) 2008 Published by Elsevier Ltd. [References: 68