Sarah J. Feakins

Biomarker records of late Neogene changes in northeast African vegetation

Open savannah grasslands (dominated by C4 plants) became a significant component of northeast African vegetation during the late Neogene. We present molecule-specific carbon isotopic mea- surements of terrestrial plant biomarkers preserved in marine sed- iments off northeast Africa that allow reconstruction of orbital- scale vegetation changes in short time windows over the past 9.4 m.y. The biomarker data show large-amplitude vegetation vari- ability as early as 3.8 Ma, with the greatest C4 expansion occurring after 3.4 Ma. We sampled orbital-scale oscillations of up to 5‰, almost as large as the observed late Neogene range of 7‰, sug- gesting that large and repeated oscillations between more open and more closed landscapes were an important aspect of northeast Af- rican vegetation change during the past 4 m.y.

Northeast African vegetation change over 12 m.y.

Intense debate surrounds the evolution of grasses using the C 4 (Hatch-Slack) photosynthesis pathway and the emergence of African grasslands, often assumed to be one and the same. Here, we bring new insights with the combination of plant leaf wax carbon isotopic composition (δ 13 C wax ) and pollen data from marine sediments of the Gulf of Aden (northeast Africa), which show that C 4 biomass increases were not necessarily associated with regional grassland expansion. We find broadly opposing trends toward more enriched δ 13 C wax values and decreased grass pollen proportions between 12 and 1.4 Ma. This apparently contradictory evidence can be reconciled if a greater proportion of the Late Miocene northeast African landscape were covered by C 3 grasses than previously thought, such that C 4 grasses and shrubs replaced a C 3 ecosystem including trees and productive grasslands. In addition, δ 13 C wax and pollen both indicate that true rainforests were unlikely to have been extensive in northeast Africa at any time in the last 12 m.y., although seasonally dry forests were a significant component of the regional landscape since the Late Miocene. Here, we extend regionally integrative marine archives of terrestrial vegetation back to 12 Ma, and we evaluate them in the context of an updated compilation of pedogenic carbonate δ 13 C values from East African Rift strata. We identify two distinct phases of increasing C 4 biomass between 11 and 9 Ma (with a reversal by 4.3 Ma) and then a re-expansion between 4.3 and 1.4 Ma; surprisingly, neither was associated with grassland expansion.

Leaf wax biomarkers in transit record river catchment composition

Rivers carry organic molecules derived from terrestrial vegetation to sedimentary deposits in lakes and oceans, storing information about past climate and erosion, as well as representing a component of the carbon cycle. It is anticipated that sourcing of organic matter may not be uniform across catchments with substantial environmental variability in topography, vegetation zones, and climate. Here we analyze plant leaf wax biomarkers in transit in the Madre de Dios River (Peru), which drains a forested catchment across 4.5 km of elevation from the tropical montane forests of the Andes down into the rainforests of Amazonia. We find that the hydrogen isotopic composition of leaf wax molecules (specifically the C28 n-alkanoic acid) carried by this tropical mountain river largely records the elevation gradient defined by the isotopic composition of precipitation, and this supports the general interpretation of these biomarkers as proxy recorders of catchment conditions. However, we also find that leaf wax isotopic composition varies with river flow regime over storm and seasonal timescales, which could in some cases be quantitatively significant relative to changes in the isotopic composition of precipitation in the past. Our results inform on the sourcing and transport of material by a major tributary of the Amazon River and contribute to the spatial interpretation of sedimentary records of past climate using the leaf wax proxy.

Hydroclimate of the western Indo-Pacific Warm Pool during the past 24,000 years

Significance The Indo-Pacific Warm Pool (IPWP) is the largest source of atmospheric water vapor, and region of highest rainfall, on Earth. At irregular intervals, this high-rainfall regime weakens, causing severe droughts with massive consequences for the local population. Research into the underlying mechanisms is limited by the temporal coverage of climate data. We produced a record of rainfall over the western IPWP covering the past 24,000 years. Our data indicate that topography and coastline position govern regional IPWP hydrology on glacial−interglacial timescales. During the Holocene, western IPWP rainfall is linked to that of East Africa through a precipitation dipole. Fluctuations of this dipole identified in our study serve as an impetus for future studies to advance our understanding of IPWP dynamics. The Indo-Pacific Warm Pool (IPWP) is a key site for the global hydrologic cycle, and modern observations indicate that both the Indian Ocean Zonal Mode (IOZM) and the El Niño Southern Oscillation exert strong influence on its regional hydrologic characteristics. Detailed insight into the natural range of IPWP dynamics and underlying climate mechanisms is, however, limited by the spatial and temporal coverage of climate data. In particular, long-term (multimillennial) precipitation patterns of the western IPWP, a key location for IOZM dynamics, are poorly understood. To help rectify this, we have reconstructed rainfall changes over Northwest Sumatra (western IPWP, Indian Ocean) throughout the past 24,000 y based on the stable hydrogen and carbon isotopic compositions (δD and δ13C, respectively) of terrestrial plant waxes. As a general feature of western IPWP hydrology, our data suggest similar rainfall amounts during the Last Glacial Maximum and the Holocene, contradicting previous claims that precipitation increased across the IPWP in response to deglacial changes in sea level and/or the position of the Intertropical Convergence Zone. We attribute this discrepancy to regional differences in topography and different responses to glacioeustatically forced changes in coastline position within the continental IPWP. During the Holocene, our data indicate considerable variations in rainfall amount. Comparison of our isotope time series to paleoclimate records from the Indian Ocean realm reveals previously unrecognized fluctuations of the Indian Ocean precipitation dipole during the Holocene, indicating that oscillations of the IOZM mean state have been a constituent of western IPWP rainfall over the past ten thousand years.

3000 years of environmental change at Zaca Lake, California, USA

Climatic variations of the last few millennia can reveal patterns of variability beyond that recorded by the instrumental record. In this study we use pollen and sediments to generate a high resolution 3000 year record of vegetation and climate along the southern California coast. An increase in Pinus and Quercus pollen found in the top 100 years of the record is a result of known planting and fire suppression by the forest service. In the pre-historic record, a period of high Salix percentages and high pollen concentration from 500-250 cal yr BP represents the wettest period of the record and coincides with the Little Ice Age. We also find evidence for 3 warm periods between 1350 and 650 cal yr BP which are identified in the record by the presence of Pediastrum boryanum var. boryanum. The latter two of these periods, dating from 1070-900 and 700–650 cal yr BP correspond to Medieval Climatic Anomaly droughts identified in other records. In addition to these events, we identify a multi-centennial scale drought between 2700 and 2000 cal yr BP in Zaca Lake, corroborating evidence from across the Great Basin and extending the regional spread of this multi-centennial drought to southern California. Corresponding wetter conditions in the northwest indicate that the modern ENSO precipitation dipole also occurred during this persistent drought. Today this dipole is associated with La Nina conditions and we note a coincidence with intriguing evidence for a change in ENSO dynamics from marine records in the tropical Pacific. This dry period is remarkably persistent and has important implications for understanding the possible durations of drought conditions in the past in California.

Analytical challenges in the quantitative determination of 2H/1H ratios of methyl iodide.

RATIONALE A published method to determine the hydrogen isotope ((2)H/(1)H) ratios of methoxyl groups, cleaved from lignin by hydroiodic acid (HI), calls for the analysis of methyl iodide (CH(3)I). However, analysis of halogenated compounds by gas chromatography/pyrolysis/isotope ratio mass spectrometry (GC/P/IRMS) yields incomplete conversion into H(2) and unwanted HX products post-pyrolysis. METHODS We tested analytical capabilities when measuring a pure CH(3) I standard using a GC Isolink pyrolysis furnace at 1420 °C and a DELTA V(Plus) ™ isotope ratio mass spectrometer, both by Thermo Scientific. We added and tested the efficacy of either a secondary reduction furnace containing heated metals or a cold trap to eliminate unwanted HI prior to IRMS analysis for (2)H/(1) H determinations. RESULTS While 94% of the hydrogen in CH(3) I is converted into H(2) in GC/P/IRMS, 6% yields HI. The use of either a secondary reduction furnace or cold trap can eliminate HI. However, secondary reduction is untenable given changes in reduction/transmission efficiency and (2)H/(1) H drift. A cold trap provides a pragmatic solution: the δ(2)H values of -97 ‰ were measured from injections of 23-57 µg of CH(3)I to a precision of 4.5 ‰ (n = 26), within the error of the offline measurements. CONCLUSIONS We have refined an earlier method to measure the δ(2)H values of CH(3)I by GC/P/IRMS by adding a cold trap to remove HI. This approach should facilitate future experimentation with the measurement of CH(3)I liberated from natural samples for potentially diverse applications including forensic sourcing of wood and tree-wood-based reconstructions of source water.

Comparison of three methods for the methylation of aliphatic and aromatic compounds

RATIONALE Methylation protocols commonly call for acidic, hot conditions that are known to promote organic 1 H/2 H exchange in aromatic and aliphatic C-H bonds. Here we tested two such commonly used methods and compared a third that avoids these acidic conditions, to quantify isotope effects with each method and to directly determine acidic-exchange rates relevant to experimental conditions. METHODS We compared acidic and non-acidic methylation approaches catalyzed by hydrochloric acid, acetyl chloride and EDCI (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide)/DMAP (4-dimethylaminopyridine), respectively. These were applied to two analytes: phthalic acid (an aromatic) and octacosanoic acid (an aliphatic). We analyzed yield by gas chromatography/flame ionization (GC/FID) and hydrogen and carbon isotopic compositions by isotope ratio mass spectrometry (GC/IRMS). We quantified the 1 H/2 H exchange rate on dimethyl phthalate under acidic conditions with proton nuclear magnetic resonance (1 H-NMR) measurements. RESULTS The δ2 H and δ13 C values and yield were equivalent among the three methods for methyl octacosanoate. The two acidic methods resulted in comparable yield and isotopic composition of dimethyl phthalate; however, the non-acidic method resulted in lower δ2 H and δ13 C values perhaps due to low yields. Concerns over acid-catalyzed 1 H/2 H exchange are unwarranted as the effect was trivial over a 12-h reaction time. CONCLUSIONS We find product isolation yield and evaporation to be the main concerns in the accurate determination of isotopic composition. 1 H/2 H exchange reactions are too slow to cause measurable isotope fractionation over the typical duration and reaction conditions used in methylation. Thus, we are able to recommend continued use of acidic catalysts in such methylation reactions for both aliphatic and aromatic compounds.

High temperatures in the terrestrial mid-latitudes during the early Palaeogene

The early Paleogene (56–48 Myr) provides valuable information about the Earth’s climate system in an equilibrium high

Molecular Paleohydrology: Interpreting the Hydrogen-Isotopic Composition of Lipid Biomarkers from Photosynthesizing Organisms

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