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1. Organic Molecular Paleohypsometry: A New Approach to Quantifying Paleotopography and Paleorelief

   https://doi.org/10.3389/feart.2021.665324  

   Hren, Michael T.; Ouimet, William (September 2021, Frontiers in Earth Science)

   Stable isotope paleoaltimetry is one of the most commonly used approaches for quantifying the paleoelevation history of an orogen yet this methodology is often limited to arid to semi-arid climates, mountain systems with a clear orographic rainshadow and terrestrial basins. We present a new approach to reconstructing past topography and relief that uses the catchment-integrated signature of organic molecular biomarkers to quantify the hypsometry of fluvially-exported biomass. Because terrestrially-produced biomolecules are synthesized over the full range of global climate conditions and can be preserved in both terrestrial and marine sediments, the geochemistry of fluvially-transported sedimentary biomarkers can provide a means of interrogating the evolution of topography for a range of environments and depositional settings, including those not well suited for a traditional isotope paleoaltimetry approach. We show an example from Taiwan, a rapidly eroding tropical mountain system that is characterized by high rates of biomass production and short organic residence time and discuss key factors that can influence molecular isotope signal production, transport and integration. Data show that in high relief catchments of Taiwan, river sediments can record integration of biomass produced throughout the catchment. Sedimentary biomarker δ 2 H n C29 in low elevation river deposition sites is generally offset from the δ 2 H n C29 value observed in local soils and consistent with an isotope composition of organics produced at the catchment mean elevation. We test the effect of distinct molecular production and erosion functions on the expected δ 2 H n C29 in river sediments and show that elevation-dependent differences in the production and erosion of biomarkers/sediment may yield only modest differences in the catchment-integrated isotopic signal. Relating fluvial biomarker isotope records to quantitative estimates of organic source elevations in other global orogens will likely pose numerous challenges, with a number of variables that influence molecular production and integration in a river system. We provide a discussion of important parameters that influence molecular biomarker isotope signatures in a mountain system and a framework for employing a molecular paleohypsometry approach to quantifying the evolution of other orogenic systems. 

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2. Synchronous Marine and Terrestrial Carbon Cycle Perturbation in the High Arctic During the PETM

   https://doi.org/10.1029/2020PA003942  

   Cui, Ying; Diefendorf, Aaron_F; Kump, Lee_R; Jiang, Shijun; Freeman, Katherine_H (April 2021, Paleoceanography and Paleoclimatology)

   Abstract The Paleocene‐Eocene Thermal Maximum (PETM; 56 Ma) is considered to be one of the best analogs for future climate change. The carbon isotope composition (δ¹³C) ofn‐alkanes derived from leaf waxes of terrestrial plants and marine algae can provide important insights into the carbon cycle perturbation during the PETM. Here, we present new organic geochemical data and compound‐specific δ¹³C data from sediments recovered from an early Cenozoic basin‐margin succession from Spitsbergen. These samples represent one of the most expanded PETM sites and provide new insights into the high Arctic response to the PETM. Our results reveal a synchronous ∼−6.5‰ carbon isotope excursion (CIE) in short‐chainn‐alkanes (nC₁₉; marine algae/bacteria) with a ∼−5‰ CIE in long‐chainn‐alkanes (nC₂₉andnC₃₁; plant waxes) during the peak of the PETM. Although δ¹³C_n‐alkanesvalues were potentially affected via a modest thermal effect (1‰–2‰), the relative changes in the δ¹³C_n‐alkanesremain robust. A simple carbon cycle modeling suggests peak carbon emission rate could be ∼3 times faster than previously suggested using δ¹³C_TOCrecords. The CIE magnitude of both δ¹³C_n‐C19and δ¹³C_n‐C29can be explained by the elevated influence of¹³C‐depleted respired CO₂in the water column and increased water availability on land, elevatedpCO₂in the atmosphere, and changes in vegetation type during the PETM. The synchronous decline in δ¹³C of both leaf waxes and marine algae/bacteria argues against a significant contribution to the sedimentary organic carbon pool from the weathering delivery of fossiln‐alkanes in the Arctic region. 

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3. Mixed Signals From the Stable Isotope Composition of Precipitation and Plant Waxes in the Northern Tropical Andes

   https://doi.org/10.1029/2022JG006932  

   Pérez‐Angel, Lina C.; Sepúlveda, Julio; Montes, Camilo; Smith, Jamila J.; Molnar, Peter; González‐Arango, Catalina; Snell, Kathryn E.; Dildar, Nadia (December 2022, Journal of Geophysical Research: Biogeosciences)

   Abstract We evaluate the efficacy of the stable isotope composition of precipitation and plant waxes as proxies for paleoaltimetry and paleohydrology in the northern tropical Andes. We report monthly hydrogen (δ²H_p) and oxygen (δ¹⁸O_p) isotope values of precipitation for an annual cycle, and hydrogen isotope values of plant waxes (δ²H_wax) obtained from modern soils along the eastern and western flanks of the Eastern Cordillera of Colombia. δ²H_p, δ¹⁸O_p, as well as the unweighted mean δ²H_waxvalues ofn‐C₂₉,n‐C₃₁, andn‐C₃₃n‐alkanes in the eastern flank show a dependence on elevation (R² = 0.90, 0.82, and 0.65, respectively). In stark contrast, the stable isotope compositions of neither precipitation nor plant waxes from the western flank correlate with elevation (R² < 0.23), on top of a negligible (p‐value >0.05) correlation between δ²H_waxand δ²H_p. In general, δ²H_waxvalues along the eastern flank of the Eastern Cordillera seem to follow the trend of a simple Rayleigh distillation process that is consistent with studies elsewhere on the eastern side of the Andes in South America. Neither δ²H_pnor δ¹⁸O_p, and therefore δ²H_wax, offer reliable estimates of past elevations in the western flank, due perhaps to water vapor source mixing, evaporation overprint, contrasting plant communities, and/or differences in evapotranspiration. Thus, δ²H_waxis only reliable for paleohydrology and paleoaltimetry reconstructions on the eastern flank of the Andes, whereas interpretations based on δ²H_pand/or δ¹⁸O_pwest of the highest point of the Eastern Cordillera need to consider mixing of moisture sources in addition to precipitation amount. 

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4. Aquatic and Terrestrial Plant Contributions to Sedimentary Plant Waxes in a Modern Arctic Lake Setting

   https://doi.org/10.1029/2022JG006903  

   Hollister, Kayla V.; Thomas, Elizabeth K.; Raynolds, Martha K.; Bültmann, Helga; Raberg, Jonathan H.; Miller, Gifford H.; Sepúlveda, Julio (August 2022, Journal of Geophysical Research: Biogeosciences)

   Abstract Sedimentary plant waxδ²H values are common proxies for hydrology, a poorly constrained variable in the Arctic. However, it can be difficult to distinguish plant waxes derived from aquatic versus terrestrial plants, causing uncertainty in climate interpretations. We test the hypothesis that Arctic lake sediment mid‐ and long‐chain plant waxes derive from aquatic and terrestrial plants, respectively. We comparen‐alkanoic acid andn‐alkane chain‐length distributions andn‐alkanoic acidδ²H andδ¹³C values of the 29 most abundant modern plant taxa to those for soils, water filtrates, and lake sediments in the Qaupat Lake (QPT) catchment, Nunavut, Canada. Chain length distributions are variable among terrestrial plants, but similar and dominated by mid‐chain waxes among submerged/floating aquatic plants. Sedimentary wax distributions are similar to those in submerged/floating aquatic plants and toSalixspp., which are among the most abundant terrestrial plants in the QPT catchment. Mid‐chainn‐alkanoic acidδ²H values are similar in sediments and submerged/floating aquatic plants, but 50‰ lower thanSalixspp. In contrast, sedimentary long‐chainn‐alkanoic acidδ²H values fall between those for submerged/floating aquatic plants andSalixspp. We therefore infer that mid‐chain waxes in QPT are primarily from aquatic plants, whereas long‐chain waxes are from a mix of terrestrial and aquatic plants. In Arctic lakes like QPT, terrestrial wax transport via leaf litter and surface flow is limited by low‐lying topography and sparse vegetation. If these lakes also have abundant aquatic plants growing near the sediment‐water interface, the aquatic plants can contribute large portions of sedimentary waxes. 

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5. Biomarker Evidence for an MIS M2 Glacial‐Pluvial in the Mojave Desert Before Warming and Drying in the Late Pliocene

   https://doi.org/10.1029/2023PA004687  

   Peaple, Mark D.; Bhattacharya, Tripti; Tierney, Jessica E.; Knott, Jeffrey R.; Lowenstein, Tim K.; Feakins, Sarah J. (January 2024, Paleoceanography and Paleoclimatology)

   Abstract Ancient lake deposits in the Mojave Desert indicate that the water cycle in this currently dry place was radically different under past climates. Here we revisit a 700 m core drilled 55 years ago from Searles Valley, California, that recovered evidence for a lacustrine phase during the late Pliocene. We update the paleomagnetic age model and extract new biomarker evidence for climatic conditions from lacustrine deposits (3.373–2.706 Ma). The MBT′_5Metemperature proxy detects present‐day conditions (21 ± 3°C,n = 2) initially, followed by warmer‐than‐present conditions (25 ± 3°C,n = 17) starting at 3.268 and ending at 2.734 Ma. Bacterial and archeal biomarkers reveal lake salinity increased after 3.268 Ma likely reflecting increased evaporation in response to higher temperatures. The δ¹³C values of plant waxes (−30.7 ± 1.4‰,n = 28) are consistent with local C₃taxa, likely expanded conifer woodlands during the pluvial with less C₄than the Pleistocene. δD values (−174 ± 5‰,n = 25) of plant waxes indicate precipitation δD values (−89 ± 5‰,n = 25) in the late Pliocene are within the same range as the late Pleistocene precipitation δD. Microbial biomarkers identify a deep, freshwater lake and a cooling that corresponds to the onset of major Northern Hemisphere glaciation at marine isotope stage marine isotope stages M2 (3.3 Ma). A more saline lake persisted for ∼0.6 Ma across the subsequent warmth of the late Pliocene (3.268–2.734 Ma) before the lake desiccated at the Pleistocene intensification of Northern Hemisphere Glaciation. 

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