Research Outputs
Evaluating the isotopic composition of leaf organic compounds in fog-dependent Tillandsia landbeckii across the coastal Atacama Desert: Implications for hydroclimate reconstructions at the dry limit
Fog is an important component of the coastal climate of northern Chile and southern Peru. Moisture and nutrients from fog maintain highly endemic vegetation (lomas) as well as unique Tillandsia landbeckii ecosystems that thrive at elevations of ca. 900–1200 m asl. Although this epiphytic CAM bromeliad is well adapted to the extreme climate, declining Tillandsia stocks observed over the past decades question the long-term survival with ongoing climate change. Here, we aim at better understanding the hydroclimatic signal encoded in the leaf organic compounds of Tillandsia landbeckii across the Atacama Desert’s coastal mountain range (ca. 18–21◦S). First, we investigate spatiotemporal patterns of fog occurrence and related moisture sources available for the plants applying a new satellite-based fog-detection approach. We then use stable carbon, oxygen and hydrogen (δ13C, δ18O, δD) isotope analysis of leaf wax n-alkanes and cellulose to identify photosynthetic pathway as well as environmental and physiological processes that shape the isotopic composition in Tillandsia landbeckii. We find that leaf wax n-alkanes and cellulose reflect the balance of climatic and physiological drivers differently. While nalkane δD values more closely follow changes in precipitation δD, evaporative enrichment seems to have a dominant influence on cellulose δ18O values. Cellulose δD values are highly enriched compared to n-alkane δD values, likely reflecting a predominant metabolic imprint on δD. δ13C signatures in the organic compounds are valid proxies for CAM activity. Our results prove the general applicability of the isotopic biomarkers for reconstructing environmental change in the coastal Atacama Desert. This approach can be extended globally to west-coast deserts that share fog as a major source of moisture.
Lake water based isoscape in central-south Chile reflects meteoric water
Warming across the globe is expected to alter the strength and amount of regional precipitation, but there is uncertainty associated with the magnitude of these expected changes, and also how these changes in temperature and the hydrologic cycle will affect humans. For example, the climate in central-south Chile is projected to become significantly warmer and drier over the next several decades in response to anthropogenically driven warming, but these anthropogenic changes are superimposed on natural climate variability. The stable isotope composition of meteoric water provides significant information regarding the moisture source, pathways, and rain-out history of an air mass, but precipitation samples suitable for stable isotope measurements require long-term placement of field equipment making them difficult to obtain. The International Atomic Energy Agency (IAEA) Global Network of Isotopes in Precipitation (GNIP) stations generate isotopic and ancillary data of precipitation from many locations around the world, but remote areas of developing countries like Chile typically have sparse networks of meteorological stations, which inhibit our ability to accurately model regional precipitation. Central-south Chile, in particular, has a sparse network of GNIP stations and, as a result, the isotopic composition of meteoric water is underrepresented in the global database complicating efforts to constrain modern day hydroclimate variability as well as paleohydrologic reconstruction for southern South America. In this study, we measured the stable isotope compositions of hydrogen (δ2H) and oxygen (δ18O) in surface lacustrine waters of central-south Chile to determine what physical and/or climatic features are the dominant controls on lacustrine δ18O and δ2H composition, assess whether or not the isotopic composition of the lakes record time-averaged isotope composition of meteoric water, and determine whether an isoscape map based on lake surface waters could predict the H and O isotope compositions of precipitation at the few GNIP stations in the region.
Development of global temperature and pH calibrations based on bacterial 3-hydroxy fatty acids in soils
Gram-negative bacteria produce specific membrane lipids, i.e. 3-hydroxy fatty acids with 10 to 18 C atoms. They have been recently proposed as temperature and pH proxies in terrestrial settings. Nevertheless, the existing correlations between pH or temperature and indices derived from 3-OH FA distribution are based on a small soil dataset (ca. 70 samples) and only applicable regionally. The aim of this study was to investigate the applicability of 3-OH FAs as mean annual air temperature (MAAT) and pH proxies at the global level. This was achieved using an extended soil dataset of 168 topsoils distributed worldwide, covering a wide range of temperatures (5 to 30 ∘C) and pH (3 to 8). The response of 3-OH FAs to temperature and pH was compared to that of established branched glycerol dialkyl glycerol tetraether (GDGT)-based proxies (MBT'5Me/CBT). Strong linear relationships between 3-OH-FA-derived indices (RAN15, RAN17 and RIAN) and MAAT or pH could only be obtained locally for some of the individual transects. This suggests that these indices cannot be used as palaeoproxies at the global scale using simple linear regression models, in contrast with the MBT'5Me and CBT. However, strong global correlations between 3-OH FA relative abundances and MAAT or pH were shown by using other algorithms (multiple linear regression, k-NN and random forest models). The applicability of the three aforementioned models for palaeotemperature reconstruction was tested and compared with the MAAT record from a Chinese speleothem. The calibration based on the random forest model appeared to be the most robust. It generally showed similar trends with previously available records and highlighted known climatic events poorly visible when using local 3-OH FA calibrations. Altogether, these results demonstrate the potential of 3-OH FAs as palaeoproxies in terrestrial settings.
Bioenergetic traits of three keystone marine species in the food web of a pristine Patagonian fjord
The Patagonian fjords are high-latitude aquatic ecosystems, highly sensitive to climate change and play a key role in the exchange of organic matter and carbon flows between terrestrial and marine environments. The bioenergetic composition of species living in these ecosystems are fundamental to understanding the distribution, seasonal variations, and exchange of organic matter within benthic communities. This study reports on the bioenergetic characteristics (lipids, protein, glucose, and energy content) of three keystone species with different life-style and feeding habits: a benthic sea star (Ctenodiscus australis); squat lobster (Munida gregaria); and a Patagonian notothenioid (Eleginops maclovinus). Samples were obtained from the Yendegaia Fjord (54°40'S - 68°50′W) in Chilean Patagonia. Our results indicate that M. gregaria has higher concentrations of lipids, proteins, glucose, and total energy compared to either E. maclovinus or C. australis. The predominance of lipids in all species is possibly related to physiological characteristics and feeding strategies. Also, may be associated with the availability of food and environmental conditions typical of a fjord ecosystem and the reproductive stage in that they were collected. These results suggest that marine animals inhabiting glacially influenced environments with low temperature and low productivity, requires a convergent physiological strategy characterized by high levels of energy storage (i.e. lipids) for metabolism and key bioenergetic processes such as growth and reproduction.
A progressively wetter climate in southern East Africa over the past 1.3 million years
African climate is generally considered to have evolved towards progressively drier conditions over the past few million years, with increased variability as glacial–interglacial change intensified worldwide1,2,3. Palaeoclimate records derived mainly from northern Africa exhibit a 100,000-year (eccentricity) cycle overprinted on a pronounced 20,000-year (precession) beat, driven by orbital forcing of summer insolation, global ice volume and long-lived atmospheric greenhouse gases4. Here we present a 1.3-million-year-long climate history from the Lake Malawi basin (10°–14° S in eastern Africa), which displays strong 100,000-year (eccentricity) cycles of temperature and rainfall following the Mid-Pleistocene Transition around 900,000 years ago. Interglacial periods were relatively warm and moist, while ice ages were cool and dry. The Malawi record shows limited evidence for precessional variability, which we attribute to the opposing effects of austral summer insolation and the temporal/spatial pattern of sea surface temperature in the Indian Ocean. The temperature history of the Malawi basin, at least for the past 500,000 years, strongly resembles past changes in atmospheric carbon dioxide and terrigenous dust flux in the tropical Pacific Ocean, but not in global ice volume. Climate in this sector of eastern Africa (unlike northern Africa) evolved from a predominantly arid environment with high-frequency variability to generally wetter conditions with more prolonged wet and dry intervals.
Distribution of branched glycerol dialkyl glycerol tetraether (brGDGT) lipids from soils and sediments from the same watershed are distinct regionally (central Chile) but not globally
Quantitative reconstructions of past continental climates are vital for understanding contemporary and past climate change. Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are unique bacterial lipids that have been proposed as universal paleothermometers due to their correlation with temperature in modern settings. Thus, brGDGTs may serve as a crucial paleotemperature proxy for understanding past climate variations and improving regional climate projections, especially in critical but under constrained regions. That said, complications can arise in their application due to varying source contributions (e.g., soils vs. peats vs. lacustrine). As such, this study investigates brGDGT distributions in Chilean lake surface sediments and corresponding watershed soils to determine the source of brGDGTs to lake sediments. Global datasets of brGDGTs in lake sediments and soils were additionally compiled for comparison. Distinct brGDGT distributions in Chilean lakes and soils indicate minimal bias from soil inputs to the lacustrine sediments as well as in situ lacustrine production of brGDGTs, which supports the use of brGDGTs in lake sediments as reliable paleotemperature proxies in the region. The ΣIIIa/ΣIIa ratio, initially promising as a brGDGT source indicator in marine settings, shows global complexities in lacustrine settings, challenging the establishment of universal thresholds for source apportionment. That said, we show that the ratio can be successfully applied in Chilean lake surface sediments. Direct comparisons with watershed soils and further research are crucial for discerning brGDGT sources in lake sediments and improving paleotemperature reconstructions on regional and global scales moving forward. Overall, this study contributes valuable insights into brGDGT variability, essential for accurate paleoreconstructions.
A molecular isotope record of climate variability and vegetation response in southwestern North America during mid-Pleistocene glacial/interglacial cycles
Climate variability during Pleistocene glacial/interglacial transitions is well documented in marine and ice-sheet isotopic records, but terrestrial records showing the continental response to these transitions are scarce, especially for earlier in the Pleistocene. Cyclic intervals of warm interglacial and cold glacial conditions preserved in terrestrial records such as lake sediments provide opportunities to probe the biosphere's response to climate change. In this study, we track climate and plant type changes, specifically the presence of C3 and C4 plants, using the abundance and δ13C signatures of leaf waxes in paleolacustrine sediments from Valles Caldera in New Mexico. Through these changes, weassess there sponse of vegetation to climate variability in southwestern North America through two mid-Pleistocene glacial/interglacial transitions (Marine Isotope Stage [MIS] 14/13 and 12/11). Leaf wax data show that the C3 forest taxa were dominant through the entire record whereas C4 plants, better adapted to warm conditions and competitive under water stress, are favored during warming and extended arid periods during interglacials. The δ13C signature in leaf wax n-alkanes suggests that C4 plants persisted in the water shed throughout the interglacials and that some summer rainfall (which is required to support C4 grasses) was maintained even during prolonged dry periods. The abundance and carbon isotope composition of leaf waxes together with new MBT/CBT (methylation index of branched tetraethers/cyclization index of branched tetraethers) temperature data confirm warmer and more arid conditions during MIS 13 than during MIS 11, in spite of relatively low greenhouse gas concentrations during MIS 13. This suggests that variations in incoming solar radiation have played a major role in regulating the surface temperature, regional hydrological systems and vegetation in southwestern North America, likely through changes in the North American Monsoon coupled with variations in the location of the mid-latitude westerlies.
FROG: A global machine-learning temperature calibration for branched GDGTs in soils and peats
Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are a family of bacterial lipids which have emerged over time as robust temperature and pH paleoproxies in continental settings. Nevertheless, it was previously shown that other parameters than temperature and pH, such as soil moisture, thermal regime or vegetation can also influence the relative distribution of brGDGTs in soils and peats. This can explain a large part of the residual scatter in the global brGDGT calibrations with mean annual air temperature (MAAT) and pH in these settings. Despite improvements in brGDGT analytical methods and development of refined models, the root-mean-square error (RMSE) associated with global calibrations between brGDGT distribution and MAAT in soils and peats remains high ( 5 °C). The aim of the present study was to develop a new global terrestrial brGDGT temperature calibration from a worldwide extended dataset (i.e. 775 soil and peat samples, i.e. 112 samples added to the previously available global calibration) using a machine learning algorithm. Statistical analyses highlighted five clusters with different effects of potential confounding factors in addition to MAAT on the relative abundances of brGDGTs. The results also revealed the limitations of using a single index and a simple linear regression model to capture the response of brGDGTs to temperature changes. A new improved calibration based on a random forest algorithm was thus proposed, the so called random Forest Regression for PaleOMAAT using brGDGTs (FROG). This multi-factorial and non-parametric model allows to overcome the use of a single index, and to be more representative of the environmental complexity by taking into account the non-linear relationships between MAAT and the relative abundances of the individual brGDGTs. The FROG model represents a refined brGDGT temperature calibration (R2 = 0.8; RMSE = 4.01 °C) for soils and peats, more robust and accurate than previous global soil calibrations while being proposed on an extended dataset. This novel improved calibra- tion was further applied and validated on two paleo archives covering the last 110 kyr and the Pliocene, respectively.
Unpacking the complexity of longitudinal movement and recruitment patterns of facultative amphidromous fish
Longitudinal movement plays fundamental role in habitat colonization and population establishment of many riverine fish species. Movement patterns of amphidromous fish species at fine-scales that would allow characterizing the direction of movement and factors associated with the establishment of specific life-history strategies (resident or amphidromous) in rivers are still poorly understood. We assess fine-scale longitudinal movement variability patterns of facultative amphidromous fish species Galaxias maculatus in order to unfold its life-history variation and associated recruitment habitats. Specifically, we analyzed multi-elemental composition along core to edge transects in ear-bones (otoliths) of each fish using recursive partitions that divides the transect along signal discontinuities. Fine-scale movement assessment in five free-flowing river systems allowed us to identify movement direction and potential recruitment habitats. As such, resident recruitment of G. maculatus in freshwater (71%) and estuarine (24%) habitats was more frequent than amphidromous recruitment (5%), and was linked to availability of slow-flowing lotic or lentic habitats that produce or retain small-bodied prey consumed by their larvae. We postulate that life-history variation and successful recruitment of facultative amphidromous fish such as G. maculatus in river systems is driven by availability of suitable recruitment habitats and natural hydrologic connectivity that allows fish movement to these habitats.
Characterization and chemo-taxonomic evaluation of plant leaf waxes (long chain n-alkanoic acids, n-alkanes and n-alkanols) as a vegetation biomarker from species of the South American temperate forest (STF)
Plant leaf waxes are used as vegetation biomarkers in several archives (i.e. soils, lake and marine sediments), study of these compounds in modern plants is needed to makes their application and interpretation more robust. However, in the South American Temperate Forest (STF), few species have been studied. The main goal of this research was to characterize twelve dominant modern species of the STF using three classes of leaf wax compounds, n-alkanoic acids, n-alkanes and n-alkanols. In addition, we evaluate the potential of leaf waxes as a vegetation and chemotaxonomic biomarker in the region, considering species that were found in different sampling sites and therefore environmental conditions. Clear differences among leaf wax abundance (μg/g) and ACL (average chain length) within and among the twelve species were found. Only the ACL of n-alkanoic acids and n-alkanols allows differentiation between leaf habit species (i.e. evergreen vs. deciduous), with high values associated with evergreen and low values with deciduous plants. This study differentiates the five species found in more than one site (i.e. different environmental condition) using different combinations of leaf waxes and in addition using only n-alkanes. It was not possible to differentiate among all sites with any combination of leaf waxes. The differences in the distribution of leaf waxes among species is an expected pattern in the study area, and it seems reliable to use the ACL as a vegetation biomarker differentiating between evergreen and deciduous species. The clear chemotaxonomic differences among the five species exposed to different and natural environmental conditions and the high preservation potential of the study area allow us to suggest that leaf waxes are likely a reliable tool to be incorporated in quantitative models to track vegetation and may be useful as a chemotaxonomic biomarker at the species level.