Many continental paleoclimate archives originate from wetland sedimentary sequences. While several studies have investigated biomarkers derived from peat-generating vegetation typical of ...temperate/boreal bogs (e.g., Sphagnum ), only scant information is available on emergent plants predominant in temperate/subtropical coastal marshlands, peri-lacustrine and fen environments. Here, we address this gap, focusing on two wetlands in the Mediterranean (Nisí fen and Tenaghi Philippon, Greece). We examined the concentration, homologue distribution, and hydrogen stable isotopic composition (δ 2 H) of leaf wax n -alkanes in 13 fen plant species, their surrounding soil, and surface water during the wet growing season (spring) and the declining water table period (summer). Our findings indicate that local graminoid species primarily contribute to the soil n -alkane signal, with a lesser influence from forbs, likely owing to differences in morphology and vegetation structure. The δ 2 H values of surface and soil water align with local average annual precipitation δ 2 H, reflecting winter-spring precipitation. Consistently, the average δ 2 H of local surface, soil, and lower stem water showed negligible evaporative enrichment, confirming minimal 2 H-fractionation during water uptake. We find that δ 2 H values of source water for wax compound synthesis in local fen plants accurately mirror local annual precipitation. Furthermore, despite differences between leaves and lower stems in n -alkane production rates, their δ 2 H values exhibit remarkable similarity, indicating a shared metabolic substrate, likely originating in leaves. Our net 2 H-fractionation values (i.e., precipitation to leaf n -alkanes) align with those in Chinese highlands and other similar environments, suggesting consistency across diverse climatic zones. Notably, our data reveal a seasonal decrease in the carbon preference index (CPI) in plant samples, indicating wax lipid synthesis changes associated with increased aridity. Additionally, we introduce a new parity isotopic difference index (PID) based on the consistent δ 2 H difference between odd and even n -alkane homologues. The PID demonstrates a strong anticorrelation with plant CPI, suggesting a potential avenue to trace long-term aridity shifts through δ 2 H analysis of odd and even n -alkane homologues in sedimentary archives. While further development of the PID is necessary for broad application, these findings highlight the intricate interplay between plant physiology, environmental parameters, and sedimentary n -alkanes in unravelling past climatic conditions.
The basin-scale spatial variability in lipid biomarker proxies in lacustrine sediments, which are established tools for studying continental environmental change, has rarely been examined. It is ...often implicitly assumed that a lake sediment core provides an average integral of catchment sources. Here we evaluated the distribution of lipid biomarkers in a modern ecosystem and compared it with the sedimentary record. We analyzed lipid biomarkers in terrestrial and aquatic organisms and in lake surface sediments from 17 locations within the salineaalkaline Lonar crater lake in central India. Terrestrial vegetation and lake surface sediments were characterized by relatively high average chain length (ACL) index values (29.6a32.8) of leaf wax n-alkanes, consistent with suggestions that plants in drier and warmer climates produce longer chain alkyl lipids than plants in cooler and humid areas. A heterogeneous spatial distribution of ACL values in lake surface sediments was found: at locations away from the shore, the values were highest (31 or more), possibly indicating different sources and/or transport of terrestrial biomarkers. In floating, benthic microbial mats and surface sediment, n-heptadecane, carotenoids, diploptene, phytol and tetrahymanol occurred in large amounts. Interestingly, these biomarkers of a unique bacterial community were found in substantially higher concentrations in nearshore sediment samples. We suggest that human influence and subsequent nutrient supply resulted in increased primary productivity, leading to an unusually high concentration of tetrahymanol in the nearshore sediments.
Abstract
Tectonics exerts a strong control over the morphology of Earth's surface that is apparent in active mountain belts. In lowland areas, subtle processes like lithospheric flexure and isostatic ...rebound can impact Earth surface dynamics, hydrologic connectivity, and topography, suggesting that geomorphic and hydrologic analyses can shed light on underlying lithospheric properties. Here we examine the effect of lithospheric flexure on the geomorphology, hydrology, and river water chemistry of the Rio Bermejo fluvial system in the east Andean foreland basin of northern Argentina. Results show that proximal to the mountain front, foredeep basin subsidence causes sedimentation along a braided channel belt that is superelevated relative to the surrounding flood basin. During floods, water flows from the superelevated channel into the groundwater reservoir, causing a net loss of discharge with distance downstream. Further downstream, forebulge uplift forces channel narrowing, high lateral migration rates, and incision up to 13 m into older river deposits. This incision locally allows groundwater flow into the river, causing a ∼20% increase in river solute load. Groundwater emerges from the forebulge into the backbulge, predominantly as spring‐fed channels. Here, channel migration rates decrease, suggesting a switch from net uplift to subsidence that reduces the depth to the groundwater table. This analysis shows that subtle lithospheric flexure can have significant effects on river channel morphology that determine hydrologic flow paths, and ultimately influence geochemical and ecological patterns. We suggest that these effects may elucidate lithospheric properties that are otherwise inferred from bulk geophysical observations.
Plain Language Summary
Tectonics, or the subsurface forces that deform Earth's crust, play an important role in shaping the Earth's landscape and thus determining how ecosystems evolve and where humans can live. The effect of tectonics on Earth's surface is most apparent in mountain ranges, where uplift of Earth's crust has created steep, eroding slopes and dynamic river valleys. Tectonic processes also shape lowland landscapes and influence how water is conveyed across the Earth's surface. We studied the Bermejo River in Argentina, which flows from the Andes Mountain Range across the lowland Chaco Plain. We studied topographic data, river channel geometry, river water discharge, and river water chemistry to examine the effects of tectonics on this low relief landscape. Close to the mountains, land subsidence is causing the river to deposit sediment and develop a braided channel that is prone to flooding. Further downstream, tectonic uplift forces the river to become narrow and erode its banks, creating a highly unstable river channel. Farther away from the mountains, there is a switch from uplift to subsidence, causing groundwater to emerge as springs at the ground surface. Our analysis shows that subtle tectonic processes can profoundly influence river networks and landscape evolution in lowland settings.
Key Points
Subtle lithospheric flexure can profoundly impact hydrology, geomorphology, and river water chemistry in lowland foreland basins
The Rio Bermejo in the subsiding Andean foredeep basin is superelevated, causing frequent avulsions, overspilling, and discharge loss
Forebulge uplift in the Andean foreland basin drives river incision into the floodplain and groundwater infiltration into the channel
Abstract
The mean state of the tropical Pacific ocean-atmosphere climate, in particular its east-west asymmetry, has profound consequences for regional climates and for the El Niño/Southern ...Oscillation variability. Here we present a new high-resolution paleohydrological record using the stable-hydrogen-isotopic composition of terrestrial-lipid biomarkers (δD
wax
) from a 1,400-year-old lake sedimentary sequence from northern Philippines. Results show a dramatic and abrupt increase in δD
wax
values around 1630 AD with sustained high values until around 1900 AD. We interpret this change as a shift to significantly drier conditions in the western tropical Pacific during the second half of the Little Ice Age as a result of a change in tropical Pacific mean state tied to zonal sea surface temperature (SST) gradients. Our findings highlight the prominent role of abrupt shifts in zonal SST gradients on multidecadal to multicentennial timescales in shaping the tropical Pacific hydrology of the last millennium, and demonstrate that a marked transition in the tropical Pacific mean state can occur within a period of a few decades.
Abstract
High-altitude ecosystems react sensitively to hydroclimatic triggers. Here we evaluated the ecological and hydrological changes in a glacier-influenced lake (Hala Hu, China) since the last ...glacial. Rapid fluctuations of aquatic biomarker concentrations, ratios, and hydrogen isotope values, from 15 to 14,000 and 8 to 5000 years before present, provided evidence for aquatic regime shifts and changes in lake hydrology. In contrast, most negative hydrogen isotope values of terrestrial biomarkers were observed between 9 and 7,000 years before present. This shows that shifts of vapour sources and increased precipitation amounts were not relevant drivers behind ecosystem changes in the studied lake. Instead, receding glaciers and increased meltwater discharge, driven by higher temperatures, caused the pronounced ecological responses. The shifts within phytoplankton communities in the Late Glacial and mid Holocene illustrate the vulnerability of comparable ecosystems to climatic and hydrological changes. This is relevant to assess future ecological responses to global warming.
The Himalayan mountain range produces one of the steepest and largest rainfall gradients on Earth, with >3 m/yr rainfall difference over a ∼100 km distance. The Indian Summer Monsoon (ISM) ...contributes more than 80% to the annual precipitation budget of the central Himalayas. The remaining 20% falls mainly during pre‐ISM season. Understanding the seasonal cycle and the transfer pathways of moisture from precipitation to the rivers is crucial for constraining water availability in a warming climate. However, the partitioning of moisture into the different storage systems such as snow, glacier, and groundwater and their relative contribution to river discharge throughout the year remains under‐constrained. Here, we present novel field data from the Kali Gandaki, a trans‐Himalayan river, and use 4‐year time series of river and rain water stable isotope composition (δ18O and δ2H values) as well as river discharge, satellite Global Precipitation Measurement amounts, and moisture source trajectories to constrain hydrological variability. We find that rainfall before the onset of the ISM is isotopically distinct and that ISM rain and groundwater have similar isotopic values. Our study lays the groundwork for using isotopic measurements to track changes in precipitation sources during the pre‐ISM to ISM transition in this key region of orographic precipitation. Specifically, we highlight the role of pre‐ISM precipitation, derived from the Gangetic plain, to define the seasonal river isotopic variability across the central Himalayas. Lastly, isotopic values across the catchment document the importance of a large well‐mixed groundwater reservoir supplying river discharge, especially during the non‐ISM season.
Plain Language Summary
Himalayan rivers are the essential water source for downstream communities. However, it is still not well understood where precipitation moisture comes from and how it is transferred to rivers. In particular, the role of intermediate water storage in groundwater, snow, and glacier is not well understood. To resolve these issues, we collected river water, groundwater, snow, glacier, and rain samples across Central Himalayas in Nepal and analyzed their isotopic composition. We determine characteristic signatures of the different moisture sources to trace water transfer in and out of the study area. Combining field observations with satellite observations and numerical models, we show that rainfall before the beginning of the Indian Summer Monsoon undergoes several evaporation‐precipitation cycles and is derived from the Gangetic foreland, while monsoon precipitation is derived from the Indian and Arabian Sea. During our 4‐year observation period, pre‐Monsoon precipitation resulted in the largest variation in river water isotopes. We also identify groundwater as the principal source of river discharge during the dry season. This work highlights the importance of detailed spatiotemporal isotopic analysis to determine precipitation moisture sources, how water sources change throughout the year, and how they contribute to river flow in the central Himalayas.
Key Points
The isotopic signature of rivers in the central Himalayas is strongly buffered by a well‐mixed groundwater reservoir
Moisture sources and transport processes determine distinct pre‐monsoon and monsoon rainfall isotopic signatures in the Himalayas
Water recycling in the Gangetic plain sets the isotopic composition of pre‐monsoon rainfall across the central Himalayan mountain range
Nitrogen lipid regulator (NlpR) is a pleiotropic regulator that positively controls genes associated with both nitrogen and lipid metabolism in the oleaginous bacterium
Rhodococcus jostii
RHA1. In ...this study, we investigated the effect of
nlpR
disruption and overexpression on the assimilation of
13
C-labeled glucose as carbon source, during cultivation of cells under nitrogen-limiting and nitrogen-rich conditions, respectively. Label incorporation into the total lipid extract (TLE) fraction was about 30% lower in the mutant strain in comparison with the wild type strain under low-nitrogen conditions. Moreover, a higher
13
C abundance (∼60%) into the extracellular polymeric substance fraction was observed in the mutant strain.
nlpR
disruption also promoted a decrease in the label incorporation into several TLE-derivative fractions including neutral lipids (NL), glycolipids (GL), phospholipids (PL), triacylglycerols (TAG), diacylglycerols (DAG), and free fatty acids (FFA), with the DAG being the most affected. In contrast, the
nlpR
overexpression in RHA1 cells under nitrogen-rich conditions produced an increase of the label incorporation into the TLE and its derivative NL and PL fractions, the last one being the highest
13
C enriched. In addition, a higher
13
C enrichment occurred in the TAG, DAG, and FFA fractions after
nlpR
induction, with the FFA fraction being the most affected within the TLE. Isotopic-labeling experiments demonstrated that NlpR regulator is contributing in oleaginous phenotype of
R. jostii
RHA1 to the allocation of carbon into the different lipid fractions in response to nitrogen levels, increasing the rate of carbon flux into lipid metabolism.
Leaf wax n-alkanes of terrestrial plants are long-chain hydrocarbons that can persist in sedimentary records over geologic timescales. Since meteoric water is the primary source of hydrogen used in ...leaf wax synthesis, the hydrogen isotope composition (δD value) of these biomarkers contains information on hydrological processes. Consequently, leaf wax n-alkane δD values have been advocated as powerful tools for paleohydrological research. The exact kind of hydrological information that is recorded in leaf wax n-alkanes remains, however, unclear because critical processes that determine their δD values have not yet been resolved. In particular the effects of evaporative deuterium (D)-enrichment of leaf water on the δD values of leaf wax n-alkanes have not yet been directly assessed and quantified. Here we present the results of a study where we experimentally tested if and by what magnitude evaporative D-enrichment of leaf water affects the δD of leaf wax n-alkanes in angiosperm C3 and C4 plants. Our study revealed that n-alkane δD values of all plants that we investigated were affected by evaporative D-enrichment of leaf water. For dicotyledonous plants we found that the full extent of leaf water evaporative D-enrichment is recorded in leaf wax n-alkane δD values. For monocotyledonous plants we found that between 18% and 68% of the D-enrichment in leaf water was recorded in the δD values of their n-alkanes. We hypothesize that the different magnitudes by which evaporative D-enrichment of leaf water affects the δD values of leaf wax n-alkanes in monocotyledonous and dicotyledonous plants is the result of differences in leaf growth and development between these plant groups. Our finding that the evaporative D-enrichment of leaf water affects the δD values of leaf wax n-alkanes in monocotyledonous and dicotyledonous plants – albeit at different magnitudes – has important implications for the interpretation of leaf wax n-alkane δD values from paleohydrological records. In addition, our finding opens the door to employ δD values of leaf wax n-alkanes as new ecohydrological proxies for evapotranspiration that can be applied in contemporary plant and ecosystem research.
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