Climate variability at glacial-interglacial timescales is not well characterized in the tropical Andes, and paleoclimate records are lacking in this region. Lake Junin, in the Peruvian Andes, offers a unique and continuous paleoclimate archive that spans the last 700,000 years. Here, we use organic compounds to characterize climate variability in the region since the Last Glacial Maximum. First, we determined the preservation of organic matter in the sediments using the Carbon Preference Index (CPI), which suggests that n-alkanes have not been altered, and their H isotope composition can be used as paleo precipitation proxies. To reconstruct the isotopic composition of lake water, biomarkers from Eustigmatophyte algae (long chain diols) and diatoms (loliolide/isololiolide) have been identified. This will allow us to better understand aridity and evaporation as well as lake water inputs through time. Additionally, we will use the changes in n-alkane chain length distributions to constrain changes in terrestrial plants (long chain n-alkanes) and aquatic macrophytes (mid-chain n-alkanes) as a potential proxy for changes in lake level as a response to climate. Finally, temperature will be reconstructed using the distributions of br-GDGTs (branched glycerol dialkyl glycerol tetraethers). Using these set of proxies, we aim to characterize climate variability during the Holocene and the end of the LGM in the context of teleconnections between the South American Summer Monsoon and global climate patterns
more »
« less
Integrating plant wax abundance and isotopes for paleo-vegetation and paleoclimate reconstructions: a multi-source mixing model using a Bayesian framework
Abstract. Plant wax n-alkane chain length distribution and isotopeshave been studied in modern ecosystems as proxies to reconstruct vegetationand climate of the past. However, most paleo-proxies focus on eitherconcentrations or isotopes, whereas both carry complementary information onthe mixing sources. We propose a multi-source mixing model in a Bayesianframework that evaluates both chain length distributions and isotopessimultaneously. The model consists of priors that include user-definedsource groups and their associated parametric distributions of n-alkaneconcentration and δ13C. The mixing process involves newlydefined mixing fractions such as fractional leaf mass contribution (FLMC)that can be used in vegetation reconstruction. Markov Chain Monte Carlo isused to generate samples from the posterior distribution of these parametersconditioned on both data types. We present three case studies from distinctsettings. The first involves n-C27, n-C29, and n-C31 alkanes in lake surface sediments of Lake Qinghai, China. The model provides more specific interpretations on the n-alkane input from aquatic sources than the conventional Paq proxy. The second involves n-C29, n-C31, and n-C33 alkanes in lake surface sediments in Cameroon, western Africa. Themodel produces mixing fractions of forest C3, savanna C3, andC4 plants, offering additional information on the dominant biomescompared to the traditional two-end-member mixing regime. The third couplesthe vegetation source model to a hydrogen isotope model component, usingbiome-specific apparent fractionation factors (εa) toestimate the δ2H of mean annual precipitation. By leveraging chain length distribution, δ13C, and δ2H data offour n-alkane chains, the model produces estimated precipitation δ2H with relatively small uncertainty limits. The new framework shows promise for interpretation of paleo-data but could be further improved by including processes associated with n-alkane turnover in plants, transport,and integration into sedimentary archives. Future studies on modern plantsand catchment systems will be critical to develop calibration datasets thatadvance the strength and utility of the framework.
more »
« less
- Award ID(s):
- 1759730
- PAR ID:
- 10440067
- Date Published:
- Journal Name:
- Climate of the Past
- Volume:
- 18
- Issue:
- 10
- ISSN:
- 1814-9332
- Page Range / eLocation ID:
- 2181 to 2210
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Paleoclimate records from the tropical Andes are scarce, and the variability of glacial-interglacial cycles is still not well characterized. Lake Junin, in the Peruvian Andes, offers a unique and continuous paleoclimate archive that spans the last 700,000 years. Here, we explore the potential of organic compounds in reconstructing Andean paleoclimate over the last 20,000 years. To address this, we first evaluated the preservation of organic matter in the lake’s sediments. The Carbon Preference Index (CPI) suggests that n-alkanes have not been altered, and their H isotope composition can be used as paleo precipitation proxies. Furthermore, biomarkers from Eustigmatophyte algae (long chain diols) and diatoms (loliolide/isololiolide) have been identified, and can be used to reconstruct the hydrogen isotopic composition of lake water. The contrast between rainfall and lake water will be a good tool for understanding lake water inputs through time as well as evaporation and aridity. Changes in n-alkane chain length will be used to identity the terrestrial plant (long chain n-alkanes) and aquatic macrophyte inputs (mid-chain n-alkanes), with potential implications for interpreting past lake level change as a function of climate. Finally, distributions of br-GDGTs (branched glycerol dialkyl glycerol tetraethers) will be used to reconstruct past temperature changes. With these proxies, we aim to characterize climate variability at the end of the Last Glacial Maximum (LGM) and the Holocene, with a focus on characterizing climate variability in the light of teleconnections between the South American Summer Monsoon and global climate patterns and their relationship with hydroclimate in the Amazon Basin.more » « less
-
Plant wax n-alkanes serve as reliable biomarkers given their abundance, stability, and distribution in the sedimentary record. As a result, their utility as isotopic indicators of vegetation and hydroclimate is well-established. A less well studied aspect of plant n-alkanes is the use of their molecular distributions, or differences in the relative abundances of homologues, for chemotaxonomy. Limited plant n-alkane datasets from southern and western Africa suggest molecular distributions can differentiate C4 grasses from C3 woody vegetation. Here we examine a suite of plants from East Africa, where almost no plant biomarkers data exists from modern plants. In this study, over 100 samples of 19 species of plants were collected monthly from the Samburu National Reserve in Kenya from October 2001 to March 2003, across multiple growing seasons; n-alkane distributions and concentrations from both individual species and designated plant functional types (PFTs) - based on both photosynthetic pathway and growth form - were investigated. Previously published n-alkane data from western and southern Africa, or the "All Africa" dataset, were examined to further understand potential spatial differences in biomarker distributions. n-alkane distributions in both datasets vary in both individual species and within PFTs. Principal Components Analysis (PCA) was used to analyze distributions of n-alkanes in individual species and in PFTs, to determine the primary sources of variability. Results indicate that n-alkane distributions can be used to separate some individual species - namely, C4 grasses - and can be used to separate PFTs. C4 grasses and C3 woody vegetation were successfully separated in both datasets. Additionally, we found that n-alkane concentrations vary by four orders of magnitude across homologues and PFTs. A compiled African plant data set shows that C31 concentration is the most representative of the plant community for C4 grasses, C3 shrubs, and C3 trees and thus, is most ideal for stable isotope vegetation reconstructions. These data suggest that an organic geochemical approach to plant taxonomy is crucial to future biomarker applications for reconstructing vegetation distribution and structure in past ecosystems.more » « less
-
Terrestrial plant biomarkers preserved in lake sediments are commonly used in paleoenvironmental reconstructions. Basin-specific transport pathways and distribution controls of plant biomarkers, however, are poorly understood. This study mapped the distribution of sedimentary n-alkanes sourced from vascular plant waxes to delineate possible transport pathways and quantified the contribution of terrestrial and aquatic input. We combine these data with existing leaf and pollen taphonomy literature and sediment focusing models to develop a better understanding of the controls on plant biomarker transport within lake basins. Here, we report the spatial distribution of sedimentary n-alkanes, the carbon isotope values and C:N ratios of bulk sediment, and percent organic matter from three lakes in the Adirondack Mountains, NY. Preliminary carbon isotope data and n-alkane concentrations within each lake suggests a large terrestrial input. Bulk sediment carbon isotope values ranged from - 26‰ to -32‰ consistent with carbon isotope values of modern terrestrial vegetation. The concentrations of long-chain n-alkanes (indicative of higher land plants), moreover, are much higher than short-chain n-alkanes (indicative of aquatic and microbial activity) by almost two times. By contrast, C:N ratios range from 11-14 indicating a mix of aquatic and terrestrial contribution to the lake’s total organic matter. We combined high-resolution sonar data with the sediment analyses to identify basin- specific controls on the distributions of n-alkanes and bulk sediment carbon isotopes. The statistical categorization of sediment zones based on relative hardness and roughness along the lake bottom delineates where organic material is concentrated. For the terrestrially sourced plant waxes, we measured low n-alkane concentrations in sandy littoral sediments relative to deeper sediments towards the main depo-center. Together, this information validates sediment focusing models and suggests that terrestrial carbon and n-alkanes are preferentially transported to the main depo-center of the lake. These observations highlight important relationships between basin-specific sediment properties and processes controlling the transport and deposition of n- alkanes.more » « less
-
Plant wax compounds preserved in lake sediments are used as proxies for paleohydrologic reconstructions. Despite their presence in lake sediments, little is known about their transport from plants to their deposition in lake sediments. By drawing on the leaf and pollen taphonomy literature combined with sediment focusing models, it is possible to develop several working hypotheses for the transport and deposition of plant waxes in lake sediments. An improved understanding of plant wax transport and deposition into lake sediments is necessary to increase the accuracy of paleohydrologic reconstructions. To better understand the controls on plant wax transport and deposition in lake sediment, we analyzed the sedimentary plant waxes from 3 lakes in the Adirondack Mountains of New York. These lakes were chosen to capture a range of basin-specific properties to evaluate their influences on the transport and deposition of plant wax compounds in surface sediments. We spatially characterized sediment properties with surface sediment samples and high-resolution underwater imaging, acoustically profiled the sub-bottom, and measured temperature profiles. From each site, we measured n-alkanes, bulk organic content (loss-on-ignition), bulk carbon and nitrogen concentrations, C:N ratios, and bulk carbon isotopes. Preliminary n-alkane concentrations and chain length distributions, as well as bulk carbon isotopes, are variable within each lake basin suggesting a mix of aquatic and terrestrial sources. The bulk carbon isotope values for two of the three lakes show a similar range of -2‰ compared to a range of -6.3‰ at the third lake. Likewise, the range of total n-alkane concentrations is much higher in the third lake suggesting that the controls on the distribution of n-alkanes and organic carbon are different between lakes. For terrestrial plant waxes, we find low n-alkane concentrations in sandy nearshore sediments relative to higher n-alkane concentrations in deeper fine-grained sediments. Combined, this information suggests that littoral processes focus organic compounds and fine sediments towards the main depo-center of the lake. These and other observations highlight important relationships between basin-specific properties and processes controlling the transport and deposition of plant wax compounds.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/cp-18-2181-2022
