Yongsong Huang

The Origins of C4 Grasslands: Integrating Evolutionary and Ecosystem Science

Grassland Emergence The evolution of the C4 photosynthetic pathway from the ancestral C3 pathway in grasses led to the establishment of grasslands in warm climates during the Late Miocene (8 to 3 million years ago). This was a major event in plant evolutionary history, and their high rates of foliage production sustained high levels of herbivore consumption. The past decade has seen significant advances in understanding C4 grassland ecosystem ecology, and now a wealth of data on the geological history of these ecosystems has accumulated and the phylogeny of grasses is much better known. Edwards et al. (p. 587) review this multidisciplinary research area and attempt to synthesize emerging knowledge about the evolution of grass species within the context of plant and ecosystem ecology. The evolution of grasses using C4 photosynthesis and their sudden rise to ecological dominance 3 to 8 million years ago is among the most dramatic examples of biome assembly in the geological record. A growing body of work suggests that the patterns and drivers of C4 grassland expansion were considerably more complex than originally assumed. Previous research has benefited substantially from dialog between geologists and ecologists, but current research must now integrate fully with phylogenetics. A synthesis of grass evolutionary biology with grassland ecosystem science will further our knowledge of the evolution of traits that promote dominance in grassland systems and will provide a new context in which to evaluate the relative importance of C4 photosynthesis in transforming ecosystems across large regions of Earth.

Northern hemisphere controls on tropical southeast African climate during the past 60,000 years.

The processes that control climate in the tropics are poorly understood. We applied compound-specific hydrogen isotopes (δD) and the TEX86 (tetraether index of 86 carbon atoms) temperature proxy to sediment cores from Lake Tanganyika to independently reconstruct precipitation and temperature variations during the past 60,000 years. Tanganyika temperatures follow Northern Hemisphere insolation and indicate that warming in tropical southeast Africa during the last glacial termination began to increase ∼3000 years before atmospheric carbon dioxide concentrations. δD data show that this region experienced abrupt changes in hydrology coeval with orbital and millennial-scale events recorded in Northern Hemisphere monsoonal climate records. This implies that precipitation in tropical southeast Africa is more strongly controlled by changes in Indian Ocean sea surface temperatures and the winter Indian monsoon than by migration of the Intertropical Convergence Zone.

Interplay between the Westerlies and Asian monsoon recorded in Lake Qinghai sediments since 32 ka

Two atmospheric circulation systems, the mid-latitude Westerlies and the Asian summer monsoon (ASM), play key roles in northern-hemisphere climatic changes. However, the variability of the Westerlies in Asia and their relationship to the ASM remain unclear. Here, we present the longest and highest-resolution drill core from Lake Qinghai on the northeastern Tibetan Plateau (TP), which uniquely records the variability of both the Westerlies and the ASM since 32 ka, reflecting the interplay of these two systems. These records document the anti-phase relationship of the Westerlies and the ASM for both glacial-interglacial and glacial millennial timescales. During the last glaciation, the influence of the Westerlies dominated; prominent dust-rich intervals, correlated with Heinrich events, reflect intensified Westerlies linked to northern high-latitude climate. During the Holocene, the dominant ASM circulation, punctuated by weak events, indicates linkages of the ASM to orbital forcing, North Atlantic abrupt events, and perhaps solar activity changes.

Glacial–interglacial environmental changes inferred from molecular and compound-specific δ13C analyses of sediments from Sacred Lake, Mt. Kenya

Abstract Molecular stratigraphic analyses, including lipid distributions and compound-specific δ13C measurements, have been performed at 15 levels in a sediment core from Sacred Lake, Mt. Kenya, a high-altitude (2350 m a.s.l.) freshwater lake with a record extending from the last glacial (>40,000 cal. yr BP) through the present interglacial. Terrestrial and aquatic organic-matter sources were independently assessed using source-specific biomarkers. δ13C values of long-chain n-alkyl lipids from terrestrial higher plants exhibit large glacial to interglacial shifts: those from the last glacial maximum (LGM) (−20 to −18‰) indicate a terrestrial vegetation dominated by C4 grasses or sedges, whereas those from the early Holocene (−34 to −27‰) reflect recolonization of the catchment area by C3 plants, consistent with a rapid rise in the upper treeline. Specific algal biomarkers, including five unsaturated hydrocarbons of novel structure ascribed to the microalga Botryococcus braunii, were abundant, as confirmed by scanning electronic microscopy (SEM). An extreme δ13C shift of over 25‰ is displayed by the algal biomarkers, an elevated value of −5.1‰ at the last glacial maximum (LGM) contrasting with a minimum value of −30.3‰ at the beginning of the Holocene. A major change in the molecular distributions of the algal biomarkers parallels this large δ13C shift, with acyclic isoprenoid hydrocarbons dominating the last glacial and cyclic isoprenoid hydrocarbons the Holocene. The low atmospheric partial pressure of CO2 (pCO2) at the LGM would favour photosynthetic organisms possessing CO2-concentrating mechanisms, including terrestrial C4 grasses and freshwater green algae. Hence, glacial/interglacial changes in pCO2, and in the CO2:O2 ratio in particular, had a significant impact on both terrestrial and aquatic ecosystems on Mt. Kenya, in addition to the effects of climate and local environmental factors.

Hydrogen isotope ratios of individual lipids in lake sediments as novel tracers of climatic and environmental change: a surface sediment test

We determined hydrogen isotope ratios of modern lake-waters and individual lipids from surface sediments of 36 lakes in the eastern North America. The lakes selected lie on two transects (south–north transect from Florida to Ontario and east–west transect from Wisconsin to South Dakota) and encompass large temperature and moisture gradients, and a wide range of lake water δD values (>100‰). The study allows a rigorous test of the applicability of using δD values of sedimentary lipids as paleoclimatic and paleoenvironmental proxies. We examined a range of lipids including C17n-alkane, straight chain fatty acids, phytol and sterols in both free extracts and ester-bound fractions in the solvent extracted sediments. Useful isotopic indicators are expected to show a linear correlation and constant fractionation factor between their δD values in surface sediments and modern lake water. Our results demonstrate that several lipid compounds, free and ester-bound palmitic acid (16:0), C17n-alkane, and phytol are useful candidates for paleoclimate reconstructions, in addition to two sterols that have been suggested previously (. Compound-specific D/H ratios of lipid biomarkers from sediments as a proxy for environmental and climatic conditons. Geochim. Cosmochim. Acta 65: 213–222). Authigenic or biogenic carbonate in sediments is conventional material for paleoclimatic study using ocean and lake sediments. However, because majority of lake sediments do not contain suitable carbonate materials for isotopic study, hydrogen isotope ratios of these lipids provide invaluable new sources of paleoclimatic and paleoenvironmental information.

Hydrogen isotope ratios of palmitic acid in lacustrine sediments record late Quaternary climate variations

The rich paleoclimate information preserved in lacustrine sedimentary organic matter can be difficult to extract because of the mixed terrestrial and aquatic inputs. Herein we demonstrate that compound-specific hydrogen isotope analysis of palmitic acid, (PA), a ubiquitous compound in lacustrine sediments, captures the δD signals of lake water. Samples collected across a diverse range of 33 North American lakes show a strong correlation between water and δDPA values. At Crooked Pond, Massachusetts, the δDPA changes in a 14 k.y. sediment record parallel temperature trends inferred from fossil pollen. Downcore changes reveal differences between climatic trends in New England and in Greenland that are consistent with important regional differences in climate controls.

Mapping of C4 plant input from North West Africa into North East Atlantic sediments

Abstract Mapping the abundance of 13C in leaf-wax components in surface sediments recovered from the seafloor off northwest Africa (0–35°N) reveals a clear pattern of δ13C distribution, indicating systematic changes in the proportions of terrestrial C3 and C4 plant input. At 20°N latitude, we find that isotopically enriched products characteristic of C4 plants account for more than 50% of the terrigenous inputs. This signal extends westward beneath the path of the dust-laden Sahara Air Layer (SAL). High C4 contributions, apparently carried by January trade winds, also extend far into the Gulf of Guinea. Similar distributions are obtained if summed pollen counts for the Chenopodiaceae-Amaranthaceae and the Poaceae are used as an independent C4 proxy. We conclude that the specificity of the latitudinal distribution of vegetation in North West Africa and the pathways of the wind systems (trade winds and SAL) are responsible for the observed isotopic patterns observed in the surface sediments. Molecular-isotopic maps on the marine-sedimentary time horizons (e.g., during the last glacial maximum) are thus a robust tool for assessing the phytogeographic changes on the tropical and sub-tropical continents, which have important implications for the changes in climatic and atmospheric conditions.

EVIDENCE FOR THE CLOSE CLIMATIC CONTROL OF NEW ENGLAND VEGETATION HISTORY

Sediments from lakes in the northeastern United States (“New England”) document climatic changes over the past 15 000 years that may, in turn, explain the long- term history of regional forest development. A rise in New England temperatures ∼14 600 yr BP (calendar years before present) coincided with the initial increase in spruce (Picea spp.) populations after deglaciation. Later temperature fluctuations correlated with changes in spruce forest composition until 11 600 yr BP, when evidence for a shift to warm, dry conditions agrees with the replacement of spruce by pine (Pinus spp.) populations. Raised lake levels indicate increased moisture availability by 8200 yr BP when mesic hemlock (Tsuga canadensis) and beech (Fagus grandifolia) populations replaced the dry-tolerant pines. Cooler-than-modern temperatures, however, persisted until 6000 yr BP and appear to have limited the expansion of hickory (Carya spp.) populations. Similarly, moisture- dependent chestnut (Castanea dentata) populations did not inc...

Large-scale hydrological change drove the late Miocene C4 plant expansion in the Himalayan foreland and Arabian Peninsula

Carbon isotope changes in paleosols from Siwalik, Pakistan, and marine sediments from the Bengal Fan indicate a major C 4 plant expansion in the Himalayan foreland during the late Miocene. However, the timing and mechanisms behind the C 4 plant expansion remain enigmatic. Here we present high-resolution (∼60 k.y.) biomarker and compound-specific isotope data spanning the past 11 m.y. from Ocean Drilling Program Site 722 in the Arabian Sea. An ∼5‰–6‰ increase in leaf wax δ 13 C values indicates a marked rise of C 4 plants from 10 to 5.5 Ma, with accelerated expansion from 7.9 to 5.5 Ma. A concurrent ∼50‰ rise in leaf wax δD values is attributed to a combined effect of changes in precipitation amount and evaporation, indicating that source regions for the plant waxes became progressively drier from 10 to 5.5 Ma. In contrast to earlier reports, our isotope records, biomarker abundances, alkenone U K′ 37 , and Globigerina bulloides abundance data do not suggest enhanced summer monsoon circulation during this time interval. Rather, our results suggest that large-scale hydrological changes drove the late Miocene expansion of C 4 plants in the Himalayan foreland and Arabian Peninsula.

Origin and Evolution of Prebiotic Organic Matter As Inferred from the Tagish Lake Meteorite

The study of organic matter in a well-preserved meteorite provides insight into processes that affected its parent asteroids. The complex suite of organic materials in carbonaceous chondrite meteorites probably originally formed in the interstellar medium and/or the solar protoplanetary disk, but was subsequently modified in the meteorites’ asteroidal parent bodies. The mechanisms of formation and modification are still very poorly understood. We carried out a systematic study of variations in the mineralogy, petrology, and soluble and insoluble organic matter in distinct fragments of the Tagish Lake meteorite. The variations correlate with indicators of parent body aqueous alteration. At least some molecules of prebiotic importance formed during the alteration.